D I O D EL A S E R

G U I D E

SONY SEMICONDUCTORation

Notes

• Responsibility for quality assurance, defect warranties and other items relating to individual transactions shall conform to these

sales contracts and other adjunct contracts concluded between the Sony Sales Department or Sony agents and customers.

• Sony makes the utmost efforts to improve the quality and reliability of its products, but semiconductor failure of a certain

percentage is unavoidable. Therefore, we request that sufficient care be given to ensuring safe design in customer products such as

redundant design, anti-conflagration design and design for preventing misoperation in order to prevent accidents resulting in

injury or death, fire or other social damage from occurring as a result of semiconductor failure. In addition, be sure to consult your

Sony sales representative beforehand when there is a chance that customer products manufactured using Sony products may pose

a life- or injury-threatening risk or are highly likely to cause significant property damage in the event of such a failure.

• Sony reserves the right to change the contents of this “Laser Diode Guide” without notice. Be sure to check the latest informa-

tion before using Sony semiconductor products.

• Technical materials shown in this “Laser Diode Guide” are typical references illustrating operation. This information does not

convey any license by any implication or otherwise under any Sony or third party patents or other rights, and Sony cannot assume

responsibility for any problems arising out of the use of this information or for any infringement of patent or other right due to

same.

• Export of products noted in this “Laser Diode Guide” which fall under the category of regulated commodities as prescribed by

the Foreign Exchange and Foreign Trade Control Act requires approval under said Act.

• The contents of this “Laser Diode Guide” may not be reproduced or transferred in any form, in part or in their entirety, without

the express written permission of Sony Corporation.

Chapter 6 Applications

D I O D EL A S E R

G U I D E

Chapter 5 Measurement Procedure

Chapter 4 Theory Of Operation

Chapter 3 Operating Procedure

Chapter 2 Handling Precautions and Reliability

Chapter 1 Items and Definitions

CONTENTS

1

7

23

39

45

51

D I O D EL A S E R

G U I D E

Items and DefinitionsChapter 1

– 2 –

1. Items and Definitions

Absolute Maximum Ratings for Laser DiodeSpecifications are given for a case temperature or a thermister temperature of 25˚C.

For devices with an internal Peltier element, temperature (Tth) is determined by thermister.

Definition of maximum ratingItem

Po

VR

Topr

Tstg

NotesSymbol

Optical

output

Reverse

voltage

Operating

temperature

Storage

temperature

Maximum allowable output defined for continuous tempera-

ture or pulse operation.

Optical power downonly appears higherthan specified Po.

Po

0

Optical Power/Forward Current Characteristic

Opt

ic p

ower

Po

Forward current IF

Maximum permissible voltage allowed when the reverse

bias voltage for semiconductor laser or photo diode is ex-

ceeded.

Temperature limit given for case or thermister when device

is active.

Temperature limit for storing the device.

– 3 –

Laser Diode Electro-optical Characteristics

SymbolItem

Ith

Iop

Vop

λp

NotesDefinition

The diagram below can be divided into two regions a spon-

taneous emission region A and a laser oscillation region B.

The oscillation start current is the current value for starting

oscillation known as the threshold current.

Po

0Ith

A

B

Optical output vs. Forward current characteristicO

ptic

pow

er P

o

Forward current IF

Iop

Forward (excitation) current necessary for maintaining the

diode's specified optical output.

Forward voltage at specified output.

Peak oscillation wavelength for operation of laser diode at

rated optical output. For a multi-mode device, this is the

wavelength of the center line of the set of spectral lines

which fall at 1/2 of maximum intensity. Classification into

single mode or multi-mode can be made according to the

number of spectral lines.

Threshould

current

Operating

current

Operating

voltage

Oscillation

wavelength

Oscillation Spectrum Characteristic

Single mode

Multi-mode

Lum

inou

sin

tens

ityL

umin

ous

inte

nsity

λ Wavelength

λ Wavelength

– 4 –

SymbolItem NotesDefinition

Monitor

current

Current which flows when the rated reverse voltage is ap-

plied to the photo diode at the rated optical output Po.

Light emission from the laser diode is emitted as shown in

Figure (a). Measurement of the light intensity along the X

and Y axis results in the characteristic shown in Figures (b)

and (c) respectively. The horizontal direction spread angle

θ// and the vertical direction spread angle θ are given as the

width in degrees at 1/2 peak intensity (FWHM).

Laser light

Beam Spread Angle

Laser diode chip

Y

X

Rearside Frontside

(a)

(b)1

0.5

X direction

(c)1

0.5

Y direction

(d)

-7 0 7

Power

SL SR

Rad

iati

on a

ngle

Imon

θ⊥

θ//

∆Sr

Per

pend

icul

ar to

junc

tion

Par

alle

l to

junc

tion

θ sy

mm

etry

– 5 –

SymbolItem NotesDefinition

∆X

∆Y

∆Z

∆φ⊥

∆φ//

ηD

Slope

efficiency

The emission accuracy is determined

by the combination efficiency between

the reflection angle and collimator

lens. Also, the stability of other pa-

rameters affects the accuracy.

A larger sloping efficiency causes the

optical output vs. current characteristic

curve to become steeper which reduces

the separation between the rated oscil-

lation initial current and the rated oper-

ating current. For a lower efficiency,

the curve become gentler causing a

larger separation between initial and op-

eration currents necessitating a larger op-

erating current for the rated optical out-

put.

Displacement of the laser diode chip with respect to the

device package. ∆X and ∆Y are measured as the planar

displacement of the chip from the physical axis of the pack-

age, ∆Z is measured perpendicular to the reference plane.

Reference plane

Optical path

Z

X

Y

Emission accuracy

The deviation of the optical axis of the beam from the me-

chanical axis of the package, measured perpendicular to the

junction plane.

Reference plane

Optical path

The deviation of the optical axis of the beam from the me-

chanical axis of the package, measured parallel to the junc-

tion plane.

The mean value of the incremental change in laser power

output for an incremental change in forward current ; rep-

resented by the slope of the B segment in the “Optical out-

put vs. Current characteristic” figure. The larger the slop-

ing efficiency, the more steep the curve of the optical out-

put vs. current characteristic becomes. As the sloping effi-

ciency becomes smaller, the curve becomes gentler.

Pos

itio

nal a

ccur

acy

Rot

atio

nal a

ccur

acy

Em

issi

on a

ccur

acy

– 6 –

D I O D EL A S E R

G U I D E

Handling Precautions and ReliabilityChapter 2

– 8 –

Guidance for setting up Laser Diodes Procedure for Establishing Laser Diode Emission

Support devices for LD environment∗ Static prevention measures∗ Stabilization techniques for laser optics

Design of cooling systemfor package

Design of LD heat sinkfor on-chip TE cooler

Select LD Type

YES NO

Course of action forLD driver ?

Design DriverPurchaseoff-shelf driver

Please refer topage 24-27.

Double check aboveprocedures.

• Economize costs• Desire to build

yourself• Accuracy not critical• Special functions are

desired

• Get laser functioningquickly

• Need to controlwith goodaccuracy

LD mounting∗ Mount heat sink∗ Connect power supply circuit

Does thetype include an on-chip

TE cooler ?

Light emission !!!

Please refer to pages 27-32

– 9 –

2. Laser Diode Handling Precautions and Reliability

2-1. Laser diode handling precautionsLaser diodes, unlike ordinary transistors and integrated circuits, make it necessary to pay special attention to the fol-

lowing points when handled.

(1) Eye protection against laser beams

(2) Gallium Arsenide

(3) Prevention of surge current and electrostatic discharge

(4) Other precautions

All these precautions to observe are collectively described below.

When you handle a laser diode, read through the precautions and use the laser diode safely is the right way.

(1) Eye protection against laser beams

Take care not to allow laser beams to enter your eyes under any circumstances.

For observing laser beams ALWAYS use Safety goggles that block laser beams. Usage of IR scopes, IR cameras and

fluorescent plates is also recommended for monitoring laser beams safely.

Laser diodes are ranked as Class III B and IV products by the U.S. Depertment of Health, Education and Welfare. Class

III B covers laser diodes up to 500mW in optical output. 1W lasers fall in the Class IV category. (See page 10 and 11)

Preventive Measures against Injury from Laser BeamsThe standards, laws and regulations relating to laser devices in force in different countries use different criteria of laser

device classification such as AEL value, range of wavelength, and applicable bandwidth. Difference is also seen in

prescriptions of the label form (including layout, symbols and colors), the instructions and warning to be contained in the

label, and provisions relating to interlock, secondary radiation, and so on. In addition, the laws and regulations in

particular will inevitably be altered as the laser technology advances. Therefore, when manufacturers of laser devices

(including OEM products) export their products, it is essential that they take export proceedings fully consulting the

authorities of the importing countries on these problems (including the problem of safety).

Example of Label on Product Specification Sheets

Class IV

Class III B

– 10 –

Guidelines of Measures for Prevention of Injury by Laser Light PurposeThe guidelines are aimed at eliminating the possibility of injury to the workers who engage inlaser device handling or services in which they might be exposed to laser light (referred to aslaser-related work hereinafter)....(referred to as service workers).

Laser equipment classification....based on the degrees of the effects the laser equipment, categorized as defined in the

accompanying sheet on the basis of the radiation exposure limits corresponding to the wavelengths and durations of laser

light generated by the laser devices, cause on the human body. The meaning of each of the classes is defined below.

Class 1 : Laser equipment with a low output (approximately 0.39µw or less) which does not cause any injury to the

human body.

Class 2 : Laser equipment with an output of such level of visible light (400 to 700nm in wavelength) that enables the

defensive reaction of the human body to avert injury (approximately 1mW).

Class 3A : Laser equipment with an output which makes direct observation of the beam by optical means dangerous

and which is less than five times the output of class 2 (approximately 5mW or less).

Class 3B : Laser equipment with an output that can cause eye injury if exposed to directly but which does not cause eye

injury if exposed to diffused reflected (approximately 0.5W or less).

Class 4 : Laser equipment with an output which can cause eye injury even if exposed to diffused and reflected beam

(approximately more than 0.5W)

Note) 1W=103mW=106µW

• Scope of application

The guidelines apply to the laser related work performed by use of Class 3A, 3B and 4 laser devices.

For the time being, however, they do not apply to the laser related work performed by use of medical and educational

laser equipment at educational and research institutes.

– 11 –

If any part of the human body is exposed to laser light, degeneration of protein due to thermalaction, optochemical reactions with the cellular tissue, and breakdown of the tissue by impactwaves (plasma current and consequent pressure waves) occur. Such effects on the living bodyvary with laser light wavelength, output, output waveform (continuous or pulsed waves), etc.Generally, however, the eyes suffer heavier injury than the skin, and irreversible changes occurmore readily. It is necessary to pay attention not only to the direct and primary action of laser lightbut also to secondary injury that may be caused by diffusion of toxic substances resulting fromirradiation of the object being aimed or other objects around the device with laser light.(1) Eye injury (See illustration).

a. Argon laser, YAG laser, CO2 laser etc, that emit continuous waves or long pulses could cause injuries shown below

because of their thermal or optical action.

1. Laser light that has wavelengths out of the visual focusing area (ultraviolet area (200 to 400nm) and some of the

infrared area (1.400 to 106nm) could cause cornea burn or cataract involving amblyopia as the beam is absorbed by the

cornea, crystalline lens and other tissues.

2. Laser light that has wavelengths in the visual focusing area (visible area (400 to 780nm) and some of the infrared area

(780 to 1.400nm) could cause the following injuries as it converges on the retina rather than the optical system of the

eyes (the cornea, crystalline lens) and has roughly 105 times larger density.

i) Continuous wave laser light absorbed by the retina (at its center or vicinity) could cause retina burn due primarily to

thermal action.

ii) Visible laser around 430nm in wavelength (absorbed by the retinal pigment of retinal photoreceptive cell) could cause

retina injury due primarily to opto-chemical action.

b. YAG (Q-switch) laser, CO2 laser, etc. that emit short pulse high peak power could cause retina burn, fundus bleeding,

etc.due to impact waves, which often involve high level amblyiopia.

(2) Skin injuries

Excessive exposure to high output laser light could cause the skin to anywhere light red spots, blisters, thermocoagula-

tion to carbonization.

light absorptionby the eye

CIE's wavelengthbands (nm)

Results onthe eyes, injuries

C o r n e a i n f l a m m a t i o n involving severe pain due to opto-chemical and thermal action

Cataract due to thermal action

Retina injury caused by opto-chemical action of visible lightD a m a g e t o r e t i n a b y o p t o - c h e m i c a l a c t i o n , thermal action and impact waves

Cornea and cataract by thermal action

CIE is an abbreviation for Commission Internationaie de Eniuminure.Fig. Effects Caused on Eyes by Excessive Exposure to Laser Light

UV-C

UV-B

UV-A

IR-A

IR-B

IR-C

200

280

315

400

780

1.400

3.000

106

Vis

ual f

ocus

ing

area

s

Infr

ared

Vis

ible

Ult

ravi

olet

– 12 –

(2) Gallium Arsenide

Laser diodes use gallium arsenide (GaAs). This is not a problem for normal use, but GaAs vapors may be potentially

hazardous to the human body. Therefore, never crush, heat to the maximum storage temperature or higher, or place the

product in your mouth.

In addition, the following disposal methods are recommended when disposing of this product.

1. Engaging the services of a contractor certified in the collection, transport and intermediate treatment of items

containing arsenic.

2. Managing the product through to final disposal as specially managed industrial waste which is handled separately

from general industrial waste and household waste.

(3) Prevention of surge current and electrostatic discharge

Laser diodes are most sensitive to electrostatic discharge among semiconductors. When a large current is passed through

the laser diode for even an extremely short time, the strong light emitted from the laser diode promotes deterioration and

then destruction of the laser diode. Therefore, note that surge current should not flow to the laser diode driving circuit

from switches and others. Also, if the laser diode is handled carelessly, it may be destroyed instantly because electrostatic

discharge is easily applied by a human body. Therefore, be extremely careful about overcurrent and electrostatic dis-

charge.

Also, use the power supply that was designed not to exceed the optical power output specified at the absolute maxi-

mum ratings. The following section lists the methods to prevent static electricity discharge.

(a) Protective clothing for static electricity

Personnel handling laser diodes should wear clothing as described in the figure below. Nylon fabrics are known to be a

source of static discharge should be avoided.

Wrist strap

Electrostatic countermeasure clothing

Gloves and finger sacks

Electrostatic countermeasure floor

Electrostatic countermeasure shoes

Fig.1 Human Body (Worker) Countermeasures

(b) Laser diode handling

Do not directly touch the leads of the laser diode. Wear cotton finger socks or cotton gloves ESD protection gloves and

handle the laser diode as shown in Figure. 2

Fig.2 Holding of Laser diode

– 13 –

A grounded wrist strap should be connected to the wrist.

Snap

Cotton glove

Grounding wire

Wrist strap

Fig.3 Attachment of a grounded wrist strap

Conductive sheets, and stainless steel or aluminum plates on the workbench should be grounded.

1MΩ

Grounding wire

Stainless steel or aluminum plate

Fig.4 Workbench Grounding

Other equipment and worktools should be grounded also.

(c) Soldering

• The metal part of the solder iron tip should be grounded and if solder iron is used more than 5 min. an isolation

resistor of at least 10MΩ should be used.

• Keep the solder iron (30W) tip temperature less than 260°C (200°C for B block) and do not keep in contact for more

than 10 seconds.

• After the connections are brought in place apply solder promptly.

Grounding wire

1MΩ

Fig.5 Solder Iron Tip Grounding

– 14 –

(d) Procedure for Cutting Leads

• Perform work over conductive sheet.

• Worker should wear grounding band.

• Do not use air nippers as they are a source of static electricity.

Example of static electricity testAn example of a static electricity strength test circuit and an example of the results of a test are shown below.

Voltage generator

C = 200pFLD Device under test

Fig. 6 Example of static electricity breakdown strength test

Reverce direction Forward direction

Survival rate (%)

Applied voltage (V)

For exsample : SLD203V-3

-1000 -800 -600 -400 -200 0 200 400

100

80

60

40

20

0

Fig. 7 Example of laser diode static electricity breakdown strength

A laser diode is vulnerable particularly in the forward direction where the possibility of static electricity breakdown is

high. Special anti-static measures must be taken for reliable operation.

(4) Other precautions

Note that deposits or dust on the window glass of a laser diode could cause deterioration of the laser diode performance.

The following practices are recommended.

1. Do not touch the window glass with bare hands.

2. Use care to prevent damage to the window glass by tools or other objects.

3. When dust deposited on the window glass is to be removed with an air gun, short circuit the leads or use a moss pack

covered with a conductive bag.

– 15 –

2-2.

a) Fundamental Approach to Reliability

Beginning with a firm grasp of the customer's requirements, a conscience effort is made to achieve quality and reliabitility

in design, development, and all phases of production. This challenge in met through quality control in selection of

materials and every step of fabrication along with regular testing of shipments. The entire process is carefully monitored

to ensure consistency so that the customer's satisfaction is guaranteed. As the state of the art and the range of applications

take quantum leaps forward so must the quality and reliability steadily increase to meet the custmer's needs. The follow-

ing activities are performed to implement these goals.

(1) Reliability management from planning to development to production setup.

(2) A high level of automated production to provide consistent quality.

(3) Small work groups are formed to raise reliability awareness of each technician.

(4) Reliability information and field data is analyzed and fedback into the production cycle.

The flowchart for the reliability assurance methodology is given in Figure 8.

– 16 –

• Quality Assurance System for the Development of New Devices

Customer Sales Department

Planning Control Department

Technology Department

Manufacturing Department

Quality Assurance Department

IPQC : In Process Quality Control QAT : Quality Assurance Test

Needs Market research Product planning

Product planning review

Determination of target specifications

Development plan

Development design

Design review

Prototype manufacture

Characteristics evaluation Reliability

evaluation

Quality certification

Design verification

Sales plan

Materials purchasing

Production plan

Shipping plan

Material incoming inspection

Wafer process

Assembly process

Final inspection

QAT

IPQC

Inventory

Shipping

Acceptance by customer

TroubleComplaint reception

Reception

Investigation, analysis and corrective actionResponse

Order reception

Plan

nin

g stag

eD

evelop

men

t and

d

esign

stage

Prototype manufacture stage

Mass p

rod

uctio

n stag

eU

sage stag

e

Fig.8 Quality Assurance System of Semiconductor Products

– 17 –

b) Reliability Theory

The well known bathtub curve describes the breakdown probability for most electrical devices (Figure. 9). In the intial

stage the breakdown probability starts off low and steadily increases. In the following stage, random breakdown, the

breakdown probability is constant. The last stage is the wearout period in which the breakdown probability steadily

increases. The lifetime of equipment using semiconductors is much shorter than the devices themselves which usually do

not arrive at the wearout stage. Through the years improvements in laser diode technology have extended average life-

times to a level of typical semiconductor devices. More importantly, the breakdwon probability in the initial stage has

been reduced by upgrading fabrication materials, improvements in processing, and the use of screening techniques.

Initialbreakdown

m>1m=1m<1

Drive time (t)

Random breakdownWearoutbreakdown

Bre

akdo

wn

ratio

λ (t)

Fig.9 Breakdown ratio vs. Time for Parts

When the light emission element of laser diodes is operated in the forward direction, a current unrelated to the genera-

tion of light increases causing the emission characteristics to change as time goes by. This current increase also has a big

affect on the driving circuits which in turn draws more current from the power supply. Therefore, the life of the light

emission element is conventionally determined by the optic power vs. current characteristics with time as a parameter.

These relationships are illustrated in Figure 10. Even by using an APC (Automatic Power Control) circuit to hold the

optical output to a fixed value, generation of the rated optical output Po becomes impossible at time ts.

At Sony, in order to protect the system from this runaway current, first the lifetime of the device is defined as the time

at which the current reaches 1.2x the initial value (Figure 10 (b)). This point of operation is still well within the capability

of the device to generate light as can be seen from the graphs of Figure 10.

Opt

ic p

ower

out

put

Supp

ly c

urre

nt

Po

0 I0 I1 I2 I3 I4 t0 t1 t2 t3 t4

I0I1

I2

I3

I4t0 t1 t2 t3

t4

t5

t6

Current I

Optical output power Po : Fixed

Time t

1.2 × lo

lo

Fig.10 Optic Output Operating Current Characteristics as Function of Time

– 18 –

To explain the characteristics of the laser diode the actual operation of a small package laser for compact discs which

operates at 70°C with a 25mW optical output will be described.

Temperature is one of the primary factor affecting reliability for semiconductor laser diodes. As the temperature of the

diode and the surrounding area increases during use, the rate of degradation (given in terms of drive current per unit time,

∆Iop/∆t) increases exponentially and can be described by the following formula.

( )τ∝ ∝ exp–EakT

∆Iop∆t

Ea : Activation energy (eV)k : Boltzman's constant (=8.616×10-5eV/K)T : Absolute temperature (K)

From the general relationship of temperature vs. drive current the rate of increase for semiconductor lasers is given as

Ea=0.7eV

For every 10°C increase in room temperature lifetime decreases by about a factor of 1/2. Since the recent trend is

towards products with small package ICs, when designing laser diode products care should be taken to keep the rate of

temperature increase down.

The average lifetime of the laser diode is derived from data obtained through accelerated high temperature testing and

the average breakdown time (MTTF : Mean Time to Failure) calculated from a Weibell Chart.

An explanation of Weibell Chart and average breakdown time is given below. The bathtub curve of Figure 9 can be

described by a Weibell distribution function resulting in the following probability distribution function : f (t)

f (t) = m • • exp( – )t 0

t m–1

t 0t m

From this the cumulative failure rate is given by :

F (t) = 1 – exp (– )t 0t m

The failure probability is thus :

λ (t) = m • t 0t m–1

MTTF is represented by to.

The breakdown rate is given for m<1, m=1, or m> 1, which corresponds to a decreasing, constant, or increasing

relationship relative to time. The breakdown probability for the initial stage decrease with time, for the random stage is

constant, and for the wearout stage increases with time. The distribution depends on the size of m (shaping factor).

Devices which have cleared screening by a fixed method are then subjected to a lifetime test to confirm that they have

cleared the initial breakdown stage and will be able to reach their average lifetimes. The resulting data can be used to

derive a relationship between drive time and cumulative breakdowns.

This relationship is called the Weibell Chart and the m value (shaping parameter) which determines the intersection of

the lines can be used to calculate the average lifespan. By the time the random breakdown stage (m=1) is entered, 63.2%

of the entire sample has reached breakdown. Figure 11 illustrates the shaping parameters of this Weibell plot.

– 19 –

1000

Drive time (H)

Cum

ulat

ive

brea

kdow

ns (

%)

0.1

1.0

10.0

63.2

10000

m=2.0

m=1.0

m=0.5

Fig. 11

c) Reliability Testing

To ensure that Sony's laser diode products are performing according to specifications as given by document JIS-C-702,

regular inspections are performed on random samples of devices. Items checked include the device's environmental

tolerance, ability to withstand stress from equipment, and operational lifetime.

Fig. 12 gives the parameters for reliability testing and Fig. 13 shows the breakdown criteria standard for reliability

testing.

d) Handling Precautions

A high-speed response characteristic greater than 1GHz and an operating voltage less than 2V makes the semiconduc-

tor laser very susceptible to surge currents.

If the operating current exceeds its rated value by just a small margin a runaway current will result causing the optical

power output to be exceeded and allow degradation. If static electricity builds up in the circuit given in Figure 14 (a), a

voltage will be induced which results in a relationship between optical power output and operating current as given in

Figure 14 (b). If the surge current becomes large or if there is a pulse of surge current greater than a few usec, a large

degradation occurs. Not only is the optical output/operating current relationship upset, the long-sighted resolution of the

laser deteriorates and the light continuity is affected. Even if a relatively weak surge current occurs, the laser must be

inspected to confirm the remaining operating lifetime. When handling take static electricity discharge precautions. A

design which does not induce or allow an excessive power current is necessary for high output laser systems to maintain

a good transient response from the drive circuits and reduce noise in the power supply.

– 20 –

Fig.12 Environment and Tolerance Testing

Test item

Operational lifetime

Max. storage temperature

Min. storage temperature

Tolerance to humidity

Temperature cycle

Tolerance to soldering heat

Natural fall∗

Vibration

Pin strength Pulling Bending

Airtightness

Gross leak

Fine leak

Test condition

Fixed operational output

Optical output POMAX mW

Case temperature TOPMAX °C

Ta=85°C

Ta=–40°C

Ta=85°C 85%RH

Low temperature –40°C High temperature 85°C

Idle time 30 minutes

Transition time 5 minutes

Temperature 260 _ 5°C

Height 75cm Maple plank strike surface

100Hz to 2kHz to 100Hz/4 min.

Acceleration 20G

Flouranet 125°C

He process method PUMPING 1h

BOMBING 3h

FLUSHING 30min

Test time

1000h

1000h

1000h

1000h

100°C

5S

3 times

X, Y, Z/

each 16min

1min

Test method

JIS C7021

B-10

B-12

B-11

A-4

A-1

A-8

A-10

A-11 Method I

A-11 Method III

A-6 Method III

A-6 Method I

+

∗ SLD300, 400 series are not applied.

– 21 –

Po (mW)

Po

IthI (mA)

Iop

Fig.13 Breakdown Criteria Standard for Reliability Testing

0 50 100

1

2

3

4

5

6

IF – Forward Current (mA)

Results from Electrostatic Destruction Test

Po

– O

ptic

al p

ower

out

put (

mW

)

50V surge

60V surge

Normal

(b)(a)

Test Circuit Electrostatic Destruction

C

C=200pF

R=0Ω

R

LASERDIODEEa

Fig. 14 E.S.D Test

Test method

JIS C 5943Item

Threshold currentOperating current

Test condition

Optical output ____mW

Allowable tolerance

Min. Max.

S×1.2

S×1.2

Unit

mAmA

S : Initial value

– 22 –

D I O D EL A S E R

G U I D E

Operating ProcedureChapter 3

– 24 –

3. Operating Procedure

3-1. APC (automatic power control) circuitSince the optical power output vs. forward current characteristics of a laser diode are temperature dependent, an APC is

generally employed to make sure that the optical power remains constant as the temperature varies. Fig. 1 shows an

example of an APC circuit for SLD200 and SLD300 series laser diodes. Because the another APC circuit must be used

which features a ±5V noise free power supply. If a voltage regulator is used, confirm that no noise is present at the

output.

Due to an exceptionally high giga-hertz response time along with low impedance, laser diodes may be damaged by a

minute amount of noise in an instant. C6 and D in the APC circuit which filter out the noise should be connected as near

as possible to the laser diode.

VR1 and VR2 must be adjusted according to the procedure in Figure 1-1. VR1 compensates the sensitivity of the

monitor diode and VR2 establishes the level of optical power output. Simply stated, adjustment starts by turning VR2 all

the way up (to VR2-5V potential) and adjusting VR1 so that maximum optical power output is obtained. Then adjust

VR1 to establish the desired optical output.

The basic operation of this circuit will now be explained. A current corresponding to the optical output flows from

photo diode PD to laser diode LD. This current is then I-V (current to voltage) converted by IC1 and output to TP1. The

voltage is then reverse amplified at IC2 using the optical output set voltage VR2 (TP2) as a bias. The IC2 amp is output

to TP3. The TP3 output is input to transistor Q whose current output drives the laser diode.

At this point, the laser diode is operating at it's normal emission level. If the output increases for whatever reason, the

photo diode current will increase followed by a fall in voltage at TP1. Since the TP1 output is sent to the non-inverting

input of IC2, the TP3 voltage also falls. Following this the base current to Q drops and the current flowing to the laser

diode decreases thus lowering the optical output. This design enables the optical output to be fixed by adjusting the level

to VR2.

The collector current capacity of the drive transistor should be larger than LD's operational current Iop and hFE should

be greater than 100µ.

For turning the LD ON/OFF (low frequency pulse driving in Hz range) with this circuit (Fig. 1-1), attach a circuit

equivalent to that shown in Fig. 1-2 to TP2. For turning the LD OFF, 1V or greater is input to TP2 ; for turning ON leave

open.

The time constant of C4 and R6 of circuit 1-1 is 10 msec which is appropriate for slow starts. For higher speeds (in the

hundreds of KHz range) the ON/OFF circuit is not effective unless the time constant is lowered. However, beware that

a low time constant makes the circuit more vulnerable to noise.

For pulse operation the duty cycle is variable and sometimes short which causes the APC function to be ineffective. A

peak-hold circuit with appropriate I-V characteristics should be inserted to rectify.

The XT, and ZT, YT packages are fitted with an on-chip TE cooler for water cooling and a thermister for temperature

sensing. The ATC (Automatic Temperature Control) driver circuit for these packages is illustrated in Figure 2. A ther-

mistor, which can be seen in middle of diagram, is employed with constants B of 3450k and Ro of 10KΩ (@TO=25°C).

This circuit can be replaced with another providing the same function if so desired.

For given temperatures T (K) and To (K), the resistance Rth can be determined by the value for resistance Ro as

expressed by the following formula.

Rth=Ro ¥ exp [B(1/TÐ1/To)]

The 10V power supply for this circuit should be current limited to a current value as specified by the maximum ratings

for the TE cooler. There are 3 adjustment points for this circuit, VR1 for zero adjustment, VR2 for gain adjustment, and

VR3 for temperature adjustment. After all three have been adjusted once according to adjustment procedures, only

adjustments to VR3 are necessary to accommodate various temperatures.

Since the control unit of this circuit is just a simple proportional contrl circuit, the response time and accuracy may be

– 25 –

insufficient for some users. For the heat sink, care should be taken to ensure a sufficient capacity to prevent heat racing.

The sink must cover the FET for the driver which consumes as much current as the TE cooler. An FET type should be

chosen with a maximum rated current larger, a Vgs (off) smaller, a gm larger (greater than 1S), and Vds (on) smaller (less

than 1.5Ω) than that for the TE cooler.

Utilizing a thermal shut-down circuit is recommended to prevent the temperature from exceeding maximum ratings

and to cut-off laser emission in case the temperature does go over.

+IC1-

+IC2-

C20.01µ

C4100µ/16V

–5V

R7

Qt

*

37

4

8

*

6

5 2 R6100

TP3

+5V

GND

1S1555

TP2

TP1

VR1R1

LM358

PD LD

D

VR25k

C1470P

LM358

* Note : Heat sink employed

2SC2987

+++

+

C51000µ

16V

C310µ16V

C62.2µ10V

R25.1k

R31k

R5100k

R45.1k

1. Preparations before connecting power supply.

VR1 : Resistor value at VR1 should be as large as possible.

VR2 : Adjust to GND potential.

2. Adjustment of maximum optical power output.

a. While referencing the optical power meter, rotate VR2 so that the TP2 potential is negative and a fixed optical power

output is attained.

b. If a fixed optical output cannot be achieved in (a), adjust VR1 to attain the fixed output.

Now adjustment of VR1 is complete. If the current has stopped flowing, bring TP2 to GND potential by rotating

VR2.

3. Be careful not to allow the laser output to exceed the absolute maximum rating during these procedures.

4. After the laser light turns back on, turn on the power and adjust VR2 until a fixed optical output is reached. Care

should be taken to make sure that the laser power does not exceed the absolute maximum ratings.

Fig. 1-1 APC Circuit Example for SLD200,SLD300, and SLD320 Series

Model No. R1 R7 VR1

SLD201 7.5k 7.2(0.5W) 200k

SLD202 6.2k 7.2(0.5W) 200k

SLD201-3 3k 7.2(0.5W) 100k

SLD202-3 3k 7.2(0.5W) 100k

SLD301/SLU301 10k 5(2W) 100k

SLD302 5.1k 3(2W) 50k

SLD322 2k 1(3W) 20k

SLD304/SLD323 1k 0.5(3W) 10k

SLD324 650k 0.3(6W) 3k

– 26 –

TP2

2SC945

+5V

1k

1k

OFF

ON

+1V 5V

–5V

Fig. 1-2

+

+

+

+10V

Th

–5V

2SK405, 2SK1529

LM358

LM358TP1

Vo

TP2

R6150k

R56.2kR4

15k

R312k

R133k

VR25k

VR15k

VR31k

R28.2k

78L05

Heat sink employed

Regulator

Peltier element heatsink employed

C41000µ16V

C3470µ16V

C2470µ/16V

C10.18µ

–

+

–

+

Adjustment Procedure

1. Replace Th with a 28k ohm resistor and adjust VR1 so that Vo reaches 0V.

2. Replace Th with a 10k ohm resistor and adjust VR2 so that Vo reaches 2.5V.

3. Connect the Peltier element and thermistor.

4. Adjust VR3 so that a fixed Vo is reached. In this condition Vo=0V for 0°C and Vo=2.5V for 25°C. Within this range

an approximately linear T-V rate of change of 0.1V/°C is exhibited.

Precautions

1. Use a heat sink of sufficient thermal capacity on the laser diode board.

Fig. 2 ATC Circuit Example

3-2. ACC (Automatic Current Control) circuitA constant current circuit shown in Fig.3 is used to ensure that a constant current flows through the laser diode. This

circuit is known as a suction type constant current source.

The current Iop which flows through the laser diode is determined by reference voltage Vr and resistance R1 as given

by the folowing equation.

Iop = (Vr+6) /R1…q

Raising the potential Vr thus increases the current.

To set up the circuit, first calculate R1 for the desired Iop from the equation R1=1/Iop and then set R2 and R3 to achieve

the Vr necessary according to 1. A drive transistor which possesses a larger collector current than the working current Iop

of the laser diode and parameter hFE greater than or equal to 100 should be used.

– 27 –

0.1µ

100µ

100µ

1S1555

R3

27Q

IC

Vr

+

+

2SC2987

Laser diode

* Note : Heat sink employed

R1

IOP

LM358

GND

–6V

R2

–

+

Fig. 3 ACC Circuit Example

Note 1. The power supply used for both APC and ACC circuits should be a regulated power supply free from any noise.

2. Whenever a laser diode emits light, heat is generated. High temperatures will cause degradation of the laser

diode and it's permormance ; therefore, proper heat dissipation and cooling are required for both the laser diode

and drive transistor.

3. The laser power should not be allowed to exceed the absolute maximum rating.

3-3. MountingThermal Dissipation considerations

High power laser diodes obviously require proper heat dissipation to maintain the chip temperature within rated levels

and assure a long lifetime. Since the optical conversion efficiency of a laser diode is around 30%, a substantial portion of

the DC power is dissipated as heat from the package.

Sony's medium and high power lasers are currently mounted in the 9mim. U/V package and the special square shape

XT, ZT, YT package. The V package has the following approximate thermal resitance from the laser cavity to the

package's external surface (including the laser chip's thermal resistnace).

The values have been measured by means of the standard delta-Vf method.

V package : 42°C/W (SLD200 series)

– 28 –

2mm

44mm

Upper holdingplate

Laser diode

Lower holding plate

2-M2.6 5 screw

3mm

26mm

Material : Copper

Fig. 4

Since most of the heat dissipated from the laser chip is drained from the underside of the package, make sure that the

heat sink makes good contact with the bottom flange of the package.

The characteristic values of a laser diode are rated in terms of the case temperature Tc (thermistor reading for packages

with a built-in TE cooler).

For example, when the SLD323V is operated to provide an optical power output Po=1W, approx. 2W of heat is

generated. A heat sink capable of cooling to the operating temperature of Tc =Ð10 to +30°C can be attached to provide for

forced air or water cooling, of coding the TE cooler.

In the next section the procedure for heat sinking with an external TE cooler and heat sinking for on-chip TE cooler

packages shall be presented.

3-3-2. Procedure for heat sinking with an external TE coolerWhile various methods can be employed for heat dissipation of a laser diode, the method for heat dissipation by use of

a peltier element is described below.

In cooling a laser diode, its operating temperature should be considered. The operating temperature of a laser diode is

specified in terms of its case temperature.

It is therefore important to determine whell the case temperature should be set at.

Let the case temperature be Tc, and :

Ta : Atmospheric temperature (°K)

P : Calorific value (W) generated by laser diode

Pp : Power consumption of peltier element

To begin with, the heat absorbed by the peltier element,

Qab, is given by the following equation.

Qab = PLD +x

– 29 –

(The x is the calorific value absorbed from the atmospheric air on the cooling side.)

The peltier element's cool side temperature TCOOL is given by :

TCOOL=TcÐ(H × PLD)

H : cool side conductance

The peltier element's hot side temperature, THOT' is given by the following equation.

Qab = nS TCOOL Ip Ð 1/2Ip2R Ð K ( THCO Ð TCOOL )

S : Mean Seebeck coefficient (V/°K) of peltier element

n : Number of semicondutor elements in peltier element

Ip : Current (A) that flows through peltier element

R : Internal resistance of peltier element (ohm)

K : Overall heat transfer coefficient (W/°K) of peltier element

Therefore

THOT = TCOOL + Ð Ip2R ÐnS TCOOL Ip

K1

2KQab

K

the calorific value dissipated by the peltier element, QD, is given by :

QD=Qab+Pp(W)

Form the results described above, the heat sink conductance H is given by :

H = ( W/ºC)QD

TH Ð Ta

* However, the thermal resistances of the laser diode and cooling block, the cooling block and peltier element's cool side,

and the Peltier element's hot side and heat sink have been ignored.

For example, assume that the SLD323V is driven as in the diagram shown above.

(Iop=1.4A Vop=2.1V Po=1.0W)

Case temperature (Tcase)=15°C, Ta=25°C,

Peltier element current Ip=2A, Vp=3V,

nS=0.023 (V/°K)

K=0.29 (W/°K)

PLD

TCOOL

THOT

QD

Qab

Ta

TCX

Laser diode

Peltier element

Heat sink

¥ Calorific value absorbed by Peltier element

Qab = (Iop×VOÐPO) +x

= 1.94+x (W)

where the x is the absorbed calorific value taking into consideration the thermal conductance of the laser diode and

peltier element cool side. Let x=1.06W for example, and we get : Qab=3 (W)

¥ Peltier element cool side temperature

TCOOL = TcaseÐ (Qab/H)

= 15 Ð (0.5×3) 13.5°C = 286.5(°K)

(However, the H depends on the methed for thermal coupling of the laser diode and peltier element's cool side.) Here the

1/H is assumed to be 0.5 (°C/W)

– 30 –

¥ Peltier element's hot side temperature

THOT = TCOOL + Ð Ip2R ÐnS TCOOL Ip

KQab

K1

2K

0.023 286.5 20.29

12 0.29

= 286.6 + Ð 22 1.15

Ð = 313.7K = 40.7 ( C)

∆T = 40.7 Ð 13.5 = 27.2 (K )

30.29

The Vp (Peltier element voltage) is obtained from the Peltier element's characteristic graph (shown below).

Vp=3 (V) From the result mentioned above,

Qp=Qab+Ip×Vp= (3+2×3) =9 (W)

Accordingly, the heat sink conductance H is given by :

H = = = 0.57 ( W/ )QD

TH Ð Ta9

40.7 Ð 25

Therefore, a heat sink that has a larger value than this is required.

Peltier element voltage vs. Temperaturedifferential characteristics

Tc = 15ûC

2A

00

1

2

3

4

5

6

10 20 30 40 50

Fig. 5

3-3-3. Procedure for heat sinking for packages with on-chip TE coolerInterpretation of TE cooler characteristics graph and method of selecting heat sink.

A graph of the TE cooler characteristics for a given set of parameters is shown in Fig.6. Fig. 7. illustrates the heat sink

for XT packages.

The temperature differential ∆T between the thermistor (laser diode chip) temperature Tth and the case temperature Tc

(Tth < Tc) is given along the horizontal axis. The absorbed heat capacity Q (heat generated from laser diode) is given on

the left vertical axis and the voltage VT generated with respect to the TE cooler is given on the right vertical axis. The

temperature used for deriving the charateristics is shown above the graph on the right side. The specifications list

temperatures Tc and Tth for a given set of conditions at recommended operating temperature.

Design of heat sink device and determination of heat resistor using this graph (Tth is fixed).

– 31 –

1. Deriving the generated heat Q

Q can be derived from the operation voltage Vop, the operation current Iop, and the generated optical power output

of the laser diode by the following relation.

Q(W)=Vop(V) × Iop(A)ÐPo(W)

For the SLD304XT package this would result in :

Q (W) =2.1V×1.4AÐ0.9W

=2.0W

(The operating voltage and current are standard values. For the actual generated power, contact a Sony representative.)

2. Deriving ∆Tvs from Q and the maximum allowable case temperature Tc.

From 1 we have an absorbed heat Q of about 2W (neglecting heat generated from currents inside package). The

intersection of the characteristics curve when the maximum rated current IT of the TE cooler is 2.5A and the straight

line for Q=2W results in a T of 47°C. …q

Following this the maximum allowable value of Tc, Tcmax, becomes :

IT = 2.5A

Q VT

Tc max(ûC)=47û+15ûC=62ûC

∆T

2

1

Fig. 6

Tth

Ta

TcTE Cooler

Copper Block

XT Package

Laser Diode Chip

Heat sink

Fig. 73. Deriving generated heat capacity Qc of entire package Qc can be determined by following relationship.

Qc (W) =VT (V) × IT (A) +Q (W)

VT is taken from the ∆T vs. V characteristics. …w

When IT and ∆T are determined Qc is resolved.

(Note : VT is considered as the voltage generated to the pin rather than as an induced voltage. In other words, since

the TE cooler is current controlled, a sufficient induced voltage must be applied.)

For the SLD304XT package this would result in :

Qc (W) =6.3V×2.5A+2.0W

=17.8W

– 32 –

4. Determination of the required maximum heat resistance for the heat sink.

θ h can be found by the formula :

θ h (°C/W) = (Tcmax Tamax) (°/Qc (W))

Tamax is the maximum allowable ambient temperature.

For a Tamax of 45°C with the SLD304 package the following result is gained :

θ h (°C/W) = (62°C-45°C) /17.8W

= 0.96°C/W

Therefore, a heat sink with a heat resistance smaller than this value must be chosen.

These characteristics may vary depending on the placement of the heat sink and the ambient temperature, so the design

should allow for a margin of error. When there are restrictions on the placement and available space countermeasures,

include cooling or using a lower Tamax design parameter.

For a given ∆T vs. Q for the TE cooler characteristics, a characteristics curve for MAX IT in the left region should be

used. In the right region temperarure control becomes difficult. If IT must be chosen near the right region, reliable

operation can be maintained by lowering the laser output or using a lower Tamax design parameter.

The required heat sink capability for the SLD300 Series is given in the table below. Remember that this table just serves

as a standard and the actual values depend on actual operating and environment temperatures.

SLD301XT 3.1/W25

SLD302XT 2.9/W 45

SLD304XT 0.96/W 15

Product NameConditions

Tth TamaxRequired Maximum

Heat Resistance

3-4. System opticsThe basics of the optical system for a semiconductor laser is briefly described below. The fundamentals for a compact

disc are described but we believe that they provide good guidance in other applications as well.

1. Basic parameters of lens

a. Focal lenght f

When parallel incident rays are being propagated, the point where it looks as if there were a thin lens is called the

primary point. ÒFocal lengthÓ refers to the distance from the primary point where parallel incident rays are brought

in to the position. where the rays converge into an image.

Parallel incident rays

Focalpoint

Focal planePrimary point

Focal length f

Optical axis

Fig. 8

– 33 –

b. NA (Numerical Aperture)

The NA is the parameter that determines the resolution of a lens and is expressed by the following equation.

NA = n sin =　　　　 r

r2 +f 2

f

rOptical axis

r : Effective luminous flux radiusn : Refractive index

Fig. 9c. Focal depth L

Focal depth refers to the distance over which the imaging plane can be moved, while satisfying the required resolu-

tion. This is expressed by use of laser diode oscillation wavelength and lens NA.

L =(NA)2

λ

Focal plane

Required resolution

Focal depth

Fig. 10d. Transverse magnification α and perpendicular magnification β Generally, the imaging and related items of a lens

comply with Newton's law.

a × b = – f 2

Transverse magnification α denotes the size of an object, Y, and size of its image, Y', and can be stated as described

below.

= = – =Y'Y

fa

bf

When an object slightly moves in the direction of the optical axis, the amount the image moves is called perpendicu-

lar magnification β which can be approximated by squaring the transverse magnification α.

= = ∆X'∆X

Y

a f f b Y'

∆X'∆X

Fig. 11

– 34 –

e. OTF (Optical Transfer Function)

The OTF is the parameter which represents the resolution of a lens and includes contrast MTF (Modulation Transfer

Function) phase component PTF (Phase Transfer Function).

Lens

Sinusoidal wave chart

Fig. 12

Assume that the image of the sinusoidal gratings of predetemined spatial frequencies is formed. Let the amplitude of

the original intensity be A, and the amplitude of the intensity measured throuth a lens be B.

MTF = BA

Their phase difference is PTF.

PTF

B A

Measured intensity distribution

Ideal intensity distribution

Intensity

Fig. 13f. Beam spot diameter d (1/e2 of peak intensity)

Light having an ideal wave surface can be concentrated to the size expressed by the following equation.

d = 1.22NAλ

Intensity

1

0

d1/e2

Fig. 14

– 35 –

2. Diffraction grating

If light is passed through a thin slit, the light is diffracted so that ±1st, ±2nd,....order beams traveling at different angles

from 0-order beam can be obtained.

The 3-beam technique for CD uses the 0-order beam to read pit data and uses the ±1st order beams to apply tracking

servo.

2nd-order

1st-order

0th-order

Ð1st-order

Ð2nd-order

Fig. 153. Polarizing Beam Splitter (PBS)

The polarizing Beam Splitter is used to separate the light emitted from a laser diode from the light reflected back from

the disc. The separation is accomplished by combined use of a quarter wave plate. The splitter allows light to pass

straight or reflects it in the 90° direction,depending on the polarizing direction of light.

LD PBS DISCQuater wave plate

Fig. 164. Quarter wave plate

The quarter wave plate is an anisotropic crystal (quartz crystal) whose refractive index is different in the X and Y

directions and is an optical part for converting linearly polarized light to circularly polarized light.

In the CD, light travels both ways. So circularly polarized light is reconverted to linearly polarized light. At the same

time, the polarization plane is rotated 90°. As a result, the forward light can be separated from the backward light by the

polarizing Beam Splitter mentioned above.

Incident light(Linearly polarized light)

Anisotropic crystal

combined waveform

d

A

B

C

Fig. 17

– 36 –

The incident light from A has X and Y direction components different in refractive index, so the traveling speeds of the

electromagnetic waves differ in the individual directions. As a result, they go out of phase. If the crystal thickness is

large, repetitive (if the crystal is constant) twisted polarized light is produced. If there is no crystal at point B (thickness

d), they come out as electromagnetic waves 90° out of phase and become sin and cos waves, resulting in circularly

polarized light. At point C, they are not out of phase, so linearly polarized light is restored. Elliptically polarized light

will be produced if the thickness is other than described above.

5. Cylindrical lens

This lens and four-division detector are used to detect whether the focus of laser beam is formed on the disc.

Fig. 18 shows how the laser beam is focused by the cylindrical lens.

If the laser beam whose cross section is a true circle enters the cylindrical lens, it will be focused in the transverse

direction only. As a result, the laser beam passed through the lens will be elliptical in cross section.

The degree of ellipse will vary with the distance from the lens. Whether the focus of laser beam is formed on the disc

can be detected by use of a detector capable of detecting the degree of ellipse. The focus of laser beam can always be

formed on the disc by moving the pickup lens nearer to the disc if the disc is too far away or by moving it away if the disc

is too close.

Cylindrical lens

Top

Side

Fig. 18

Why slanted cap package is provided

The SLD100 series, SLD200 series offer a slanted cap package called the “U package”. The “U package” is designed

for compensating the astigmatic distance* of a laser diode.

It is well known in optics that when the luminous flux radiated from a spot light source obliquely enters parallel flat

glass plates, the rays in XY plane and the rays in YZ plane have different positions of apparent light emitting points after

passing through the glass plates, producing an astigmatic distance.

In the U package specification laser diode, the light emitting point located in the rear is moved forward by utilizing this

principle.

In other words, the original light emitting point is point A in Fig. 19, but the luminous flux passed through the slanted

glass appears as if its light emitting point were located at point B, eliminating the astigmatic distance between θ⊥ and θ//.

– 37 –

LD chip

A : Original light emitting point

B : Virtual light emitting point of luminous flux afterpassing through oblique glass

BY

Z XA

Fig. 19

Astigmatism

The two beam components θ// (theta parallel) and θ⊥ (theta perpendicular) of a laser diode's beam originate at different

points. The parallel component originates at the laser chip's facet, while the perpendicular component originates within

the chip, typically 3 to 7 microns for index-guided lasers and 20 to 30 microns for gainguided lasers, as shown in Fig.20.

This difference in focal point results in astigmatism.

LD chip

Single mode laser Multimode laser Single mode laser,Multimode laser

Fig. 20

– 38 –

D I O D EL A S E R

G U I D E

Theory Of OperationChapter 4

– 40 –

4. Principle of Semiconductor Laser Action

4.1 Principle of light emissionThe energy levels of electrons of the molecules and atoms of the substances that constitute lasers are positioned as

shown in Fig.1.

Excited levels

Energy levels

E4

E3

E2

E1

E0 Ground levelE

nerg

y

Fig. 1 Energy Level Diagram

The electrons absorb or emit the rays of a wavelength proportional to the energy difference between energy levels as

they move (transition) between the energy levels. The wavelenth of light emitted can be expressed by the following

equation.

λ = =C

E2 – E1 /h1.2398E2 – E1

C : Velocity of light (2.998 108m/sec)h : Planck's constant (6.626 10-34 J.S)E1 : Energy before transitionE2 : Energy after transition

The transition processes include three types of processes; induced adsorption, spontaneous emission, and stimulated

emission.

(Absorption)

EH

EL

Incident light

Before transition

EH

EL

After transition

In the absorption process the electrons positioned at a lower energy level (EL) absorb the incident light and are excited

to a higher energy level.

(Spontaneous emission)

EH

EL

EH

ELEmitted light

Before transition After transition

In the spontaneous emission process the electrons positioned at a higher energy level (EH) transition to a lower energy

level (EL) and emits light by converting the energy difference (EH-EL) encountered at the time of the transition to light.

Since the rays emitted in this cane are the rays produced by random transitioning of electrons, they are not in phase in

time and space.

Such emission is called incoherent light.

EH

EL

EH

EL

Emitted lightIncident lightIncident light

Before transition After transition

– 41 –

The electrons are forced to jump from a higher energy level (EH) to a lower energy level (EL) by producing light. This

process may be regarded as the reverse of the absorption process. So the emitted rays are rays in phase in time and space

that are of the same wavelength with the same phase relationships as the incident rays. Such emission is called coherent

light.

Laser light refers to the coherent light produced by stimulated emission. In a normal equilibrium state, more electrons

are collected at a lower energy level, creating a state where absorption occurs more readily than does emission. There-

fore, current is supplied to a create a special condition for collection of more elections at a higher energy level. Such a

condition is known as inverted population and readily causes emission of light. Incidence of light in this condition

produces more rays than absorbed and amplifies the light intensity.

For laser emission, not only the amplification function but also the feedback function must be provided. The feedback

function is shown below in terms of the Fabry-Perot resonator which is most commonly used.

Reflecting mirror Reflecting mirrorInverted populationregion

Resonator length L

Fig. 2 Fabry-Perot resonator diagram

The Fabry-Perot resonator basically comprises two reflecting mirrors arranged in such a way as to face the cleavage

plane of crystal with stimulated emitted rays confined between the reflecting mirrors. The stimulated emitted rays travel

toward the reflecting mirrors (Fig.2) and are returned to the same position by reflection from the reflecting mirrors.

During the period some of the rays are lost because they are transmitted through the reflecting mirrors or are absorbed or

diffused in the resonator. The current value at which the loss and the gain by current amplification become equal is

referred to as the threshold current value, and emission begings at the value.

• Structure of AlGaAs double hetero-junction laser

A laser diode essentially comprises a double hetero junction. The junction consists of a very thin activation layer of

GaAs sandwiched between two cladding layers of n-AlGaAs and p-AlGaAs.

QQ¢¢n-AIGaAs

Insertion Injection

Cladding layer

P-AIGaAs

( – ) ( + )

GaAs active layer

Fig. 3 Cross-Sectional View

– 42 –

Application of - and + forward bias respectively to the cladding layers n-AlGaAs and p-AlGaAs on both sides causes

them to start injecting carriers into the active layer. At this point, the cladding layers n-AlGaAs and p-AlGaAs have a

larger band gap than the active layer GaAs, providing hetero barriers. As a result, the carriers are confined within the

active layer, and inverted population readily occurs. (Fig. (b)) The active layer is so thin that a high excitation effect can

be produced by a small current. It is therefore possible to obtain a high intensity of emitted light.

An advantage of this structure is that since GaAs is approx. 6% higher in refractive index than AlGaAs, the laser light

internally produced cannot readily spread transversely, making it possible to confine the light. (Fig. (d)) For this reason,

it would be safe to say that the double-hetero junction is a very convenient structure for efficient production of laser light.

(b) Carrier confinement effect

(c) Gain distribution

(d) Refrective index distribution

(e) Light intensity distribution

(Electrons)

Conductionband

Hetero barrier

Recombination

Valence band (Holes)

Fig. 4 Theory of Operation ofDouble Hetero Junction Laser Diode

– 43 –

(Transverse mode)

The transverse mode is a mode determining radiation strength distribution in a plane perpendicular to the emitted beam

and is normally expressed by near and far field patterns (NFP and FFP). It includes perpendicular and parallel transverse

modes.

(Perpendicular transverse mode)

The rays are confined in the activation layer because of the difference in refractive index as described above. The

quantity of light confined varies with the thickness of the active layer. The thicker the active layer, the larger the quantity

of light confined. If the active layer is thin, the light will escape. Generally, in a semiconductor laser, the laser light

distribution width in the element is equivalent to or less than the wavelength, and therefore makes the external laser light

emission surface wider.

(Parallel transverse mode)

The rays are confined in horizontal direction of the active layer with a stripe structure. The structure is sandwiched by

the current blocking area. Ion implantation method or epitaxy makes the current blocking area. The stripe width is

determined the resonator size. The radiation angle of the parallel transverse mode is greater than the radiation angle of

the perpendicular transverse mode because the ray is enough narrower than the laser’s wavelength.

Longitudinal mode

Parallel transverse mode

Perpendiculartransversemode

Laser light

emission region

Perpendicular transverse mode

Parallel transverse mode

n-GaAs (substrate)

fo

p-GaAs (Cap layer)p-AIxGa1-xAs (Cladding layer)p-AIyGa1-yAs (Active layer)n-AIxGa1-xAs (Cladding layer)

Reverse direction

NFP

FFP

TE

TE

IF

TM

TM

Fig. 5 Semiconductor Laser Structure Example

– 44 –

(Longitudinal mode)

The longitudinal mode expresses the wavelength spectrum components of emitted beam.

With laser in emission, the reflected light in the resonator suffers interference as it goes around, and the standing waves

of light develop.

The resonator length L is q (an integral number) times half the wavelength of light. If this relation is expressed by an

equation, the following equation holds.

L = q λ2n

Since q is a very large value, even a slight change of wavelength will cause emission.

Normally, emission occurs at a wavelength where the gain reaches a maximum.

Refrective index

L

1 2 3 q

Fig. 6

D I O D EL A S E R

G U I D E

Measurement ProcedureChapter 5

– 46 –

5. Laser Measuring Procedure

5-1. Optical power output-forward current characteristics (L-l)The optical power output-forward current characteristics of a laser diode is measured by supplying current in the

forward direction of the laser diode and detecting the generated laser light with a light receiving element that has a large

receiving port diameter.

Laser diodes are classified into DC (CW) drive and pulse types, depending on their drive circuits.

The light receiving elements in general use are silicon PIN diodes calibrated with a reference phtoto diode APDs

(Avalanche Photo Diodes), etc.

LD

Senser head

Power meterEG&G Model 460

x-y recordermW

Po

L

IIF

A

Fig. 1

5-2. Longitudinal mode spectrumA longitudinal mode spectrum shows the results of laser light separated into components by a monochromator, optical

spectrum analyzer, etc., and expressed in terms of wavelength vs. light intensity distribution.

Spectrum where many laser emission modes are excited are called axial multimode emission, whereas spectrum where

only a single laser emission mode is excited are called longitudinal single mode emission.

The emission wavelength of a laser diode shifts toward the longer wavelength side as the ambient temperature rises.

For high accuracy spectrum measurement, therefore, the temperature of the laser diode must be controlled by a Peltier

element and the like to maintain it constant.

Peltier element

Chopper

Monochrometer

Lock-in amplifier

Photomultiplier

LD

Variable slit Variable slit

Optical poweroutput

Wavelengthx-y recorder

Fig. 2

– 47 –

5-3. FFP (Far Field Pattern)A far field pattern is measured by detecting via a pinhole or slit the light intensity distributions vertical and horizontal

to junction of a laser diode.

The spatial resolution of a far field pattern image depends on the distance between the laser diode and light receiving

element and the aperture diameter of the pinhole.

LD

15cm

0.1mmPD

Pinhole

x-y recorder

Fig. 3

5-4. AstigmatismThe difference between the virtual light source positions of beams parallel and perpendicular to the junction is the

astigmatism. The simplest method for detecting the virtual light source position is by measuring the beam diameter at a

point closet to the laser end, while moving the laser diode, and by finding the position where the beam diameter is

reduced to a minimum.

The astigmatism of a laser diode produces large effects on its imaging performance. Therefore, it constitutes an impor-

tant item particularly for the real write operations of magnetic-optical disks, etc. that offer a high recording density.

LD

Long focus objectivelens Close-up lens

Oscilloscope Monitor

ND filter TV camera

Fig. 4

– 48 –

5-5. Polarization ratioThe polarization ratio of a laser can be measured by converting the light emitted from the laser diode into parallel rays

with a collimator lens and by turning a polarized beam splitter to find the ratio of the maximum to minimum value of the

quantity of light.

The polarization ratio generally has different values, depending on the aperture diameter of the collimator lens and the

optical power output of the laser diode. It is therefore necessary that the measuring conditions that match the operating

conditions are set.

LTE

TM

IFI

Po

A

LD

PD

PBSB 1100NA = 0.1

Collimator lensPolarizied beamsplitter

Fig. 5

5-6. Frequency responseThe output beam of a laser diode can be directly modulated by current modulation. To obtain a good frequency re-

sponse, the modulated current signal is superimposed on a DC bias current.

BIASCIRCUIT

LASERDIODE

TRACKINGGENERATOR

PHOTO DIODE(APD)

SPECTRUMANALYZER

Fig. 6 Configuration of PulseResponse Measuring System

Frequency response characteristics (SLD202V)

Ou

tpu

t (d

B)

0

0 1 2

10

20

Frequency f (GHz)

ATTEN:10dB10dB/d i v

Fig. 7

– 49 –

Rectangular wave response waveform (SLD202V)

Op

tica

l po

we

r o

utp

ut

(10

0m

V/d

iv)

Time (1ns/div)

Fig. 8

– 50 –

D I O D EL A S E R

G U I D E

ApplicationsChapter 6

– 52 –

6. ApplicationsThis section provides a brief introduction to the typical fields where laser diodes find application.

6-1. CD

Fig. 1 Typical Optical System

Table top typeItem Portable

Operating current IOP

Wavelength λ • The coating on optical parts is designed such that the transmittance, reflectivity, etc. will be

satisfied for a given range of wavelengths. If wavelengths are out of the range, therefore re-

quired power may not be available.

• Allowable degree of skew of disc, L

L∝(NA)3

λ

Allowable degree : Extent of skew that is toleratedwithout degrading playback quality

• Allowable degree of unevenness in thickness of disc, M

M∝(NA)4

λ

NA : Numerical aperture of lens

Disc

Laser diode

Optical system

• Because of battery drive, the operating current

produces direct effects on playback time.

• The smaller, the better.

• Because of household power supply, the ef-

fects are not so grave as in the case of the por-

table type.

– 53 –

θ//, θ⊥ • Coupling efficiency ηc

Since a minimum quantity of light for reading signals from a disc is fixed, the quantity of light

emitted from the laser diode has to be increased if the NA of the collimator lens is small or the

radiation angles (θ//, θ⊥ ) are large. This results in a shorter life.

The coupling efficiency, which serves as an index to indicate how effectively the light quantity

of laser diode is in use, is determined by the NA of the lens and the radiation angles (θ//, θ⊥ ) of

the laser diode.

Po

PoPt

Pt

100 (%)

Po : Total quantity of light emitted from laser diodePt : Quantity of parallel emerging from lens

ηc=

The smaller the θ// and θ⊥ , larger the ηc.

• Imaging performance

Radiation angle large Equivalent NA large

Radiation angle small Equivalent NA small

spot size SP = k k : ConstantNAλ

• The spot size is related to the NA. If the radiation angles (θ//, θ⊥ ) become too small, the equiva-

lent numerical aperture NA decreases, whereas the spot size increases.

Item

– 54 –

• The S/N ratio of laser is determined by its ability to correct errors in digital signals.

• Block error rate (BLER)

The error correction system CIRC (Cross Interleave Read Solomon Code) has 7350 unit blocks

in a second. If any of the blocks that has even a bit of error is to be treated as an error block, the

BLER is defined by:

BLER = Number of error blocks in a second7350

B BLER value specified in CD

BLER<3 ×10-2 (Standard value)

Eye pattern diagram

S/N

Item

– 55 –

S/N • Spatial frequency characteristics of MTF (Modulation Transfer Function)

MT

F

νCO0 500 1000

500 1000 1500

B : Channel bit rate 4.3218Mbit/secV : Line velocity 1.25m/sec

Longest pit

11T

3.18 m

1571p/mm

196KHz

Shortest pit

3 T

0.87 m

5741p/mm

720KHz

Opticalcutoff

frequency

–

–

12001p/mm

1500KHz

Channel bitlength N

Pit length

Spatial frequency

Frequency

PN =

fN =

VB N

2 PNνN =

1

2 N( νN V)

B

µµ

Spatial frequency ν(Line pair/mm)frequency f (kHz)

νCO = 2NAλ

• Nine types of pit lengths are employed for CD. The features of the longest and shortest pit

lengths are shown above.

• As is evident from the eye pattern, the shorter the pit length, the smaller the amplitude in terms of

electrical signal. This is because the MTF of the lens has spatial frequency characteristics. The

strictest S/N ratio requirement has to be met when the amplitude is the smallest, or the pit length

the shortest (f=720kHz).

Item

– 56 –

S/N

Astigmatic distance

As

• S/N and C/N

* C/N (Carrier to noise ratio)

S/N for a specific carrier frequency In LD, S/N is defined by:

S/N=10 log 1000(dBm)–N(dBm)VDC

50On the other hand, C/N is defined by:

C/N=10 log 2

1000(dBm)–N(dBm)

MTF VDC

2 50

2

In CD, MTF=0.4 when f=720kHz.

So the following relation holds.

S/N=C/N+17 (dB)

The standard value of C/N, because of its correlation with BLER,

1000

500

100

50

10

5

155

C/N (dB)

BL

ER

(bl

ock/

sec)

50 45 43 40 35

CD standard value(220)

C/N ≥ 43dB Thus,

S/N = C/N+17 ≥ 43+17 = 60dB is required.

In CD or any other applications where concentration up to almost the diffraction limit is neces-

sary, the astigmatism is a parameter that must always be taken into consideration. The parameter

varies among different optical systems. In the CD applications, if the difference is less than

15µm, it is tolerated.

(It depends on the superposition of the perpendicular magnification and focal depth of the lens

system.)

Item

– 57 –

6-2. Optical discs (except CD and DVD)The MD (Mini Disc), CD-R, CD-RW are typical of the optical discs except CD and DVD. Both are designed primarily

for recording digital data and are used for storage of document files and computer data.

A major difference from CD and DVD is in that “write” is made.

The following “write” techniques are employed.

(1) Pits physically formed on recording material (CD-R)

(2) Reflectivity of recording material changed (CD-RW)

(3) Magnetization of recording material changed (MD)

All of the techniques employ “heat”. Therefore, a large output laser that allows concentration to a spot of 1 to 2µm is

required.

For example, Fig.2 shows a functional diagram of MD.

Weak magnetic field

Optical system

(a) In record mode

(b) In read mode

(C) Behavior of reflected light on polarizing plate B

Polarizing plate BPolarizing plate A

Polarizing platetransmission axis

0°

0°

0°

Recording layer(Vertically magnetized film)

Fig. 2 Principle of MD

– 58 –

(a) Record mode

With bias field applied in the direction that the disc is to be magnetized, the desired portion to be recorded is exposed

to laser light and the temperature raised up to the Curie point where the magnetism transition occurs.

(b) Reproduction mode

The Kerr effect is employed.

Kerr effect : When linearly polarized ligh is reflected from a magnetic pole, the polarization plane is rotated according

to the derection of magnetization.

For removal of the LED component of the laser diode, polarizing plate A is inserted to bring linearly polarized light to

the disc. The reflected light from the disc, because of the Kerr effect, has a polarization plane different from that of the

incident light. Let the polarization plane of the incident light be 0˚. Then the polarization plane of the reflected light from

downward magnetization changes by θk, whereas that of the reflected light from upward magnetization varies by-θk.

If polarizing plate B is placed in a direction 90˚ to the reflected light θk, then the reflected θk is not transmited, whereas

only sin2θk portion of the reflected lightθk is transmitted as the signal from the disc. (C)

It follows from the above that the following characteristics are required for laser.

An output of 10mW is required.The larger the output, the higher the write speed.

5mW

On MO in particular, because of use of polarizing plates, the larger the polarization ratio, the smaller the laser output and the longer the life.

Capability must be provided to allow concentration up to nearly the diffraction limit. Has direct bearing on recording density. Astigmatism should be less than 10µm.

Recordmode

Reproductionmode

Output

Polarizationratio

Convergingperformance(mainlyastigmaticdistance)

6-3. Bar code scannerFig.3 shows a functional diagram of the bar code scanner.

Optical systemSemiconductor laser

Collimator Scanning mirror

PD

AD conversion

Analog signal

Digital signal

Fig. 3 Functional diagram of bar code reader

– 59 –

The laser light emitted from the He-Ne laser travels through the optical system and is reflected by the optical system to

scan the bar code label. The light brought to the bar code label is absorbed by the black bars and reflected by the white

spaces as it comes back to the photodiode where it is converted into analog signals snatching the pattern of the bar code.

Thereafter, the analog signals are converted into digital signals by the AD converter and decoded by the decoder.

In the past the He-Ne laser and LED were used as the light source of the bar code scanner but they have the disadvan-

tages described below.

He-Ne laser

(1) Large overall dimensions

(2) Low electricity to light conversion efficiency

LED

Since the LED does not allow concentration up to the diffraction limit, it has a large reading error rate, resulting in

many wrong readings.

To solve the disadvantages of both, a bar code scanner using a semiconductor laser began to be commercialized.

Bar code Scanners are available in two types; [a] the hand held type and [b] desk top type. The characteristics impor-

tant for both of them are shown below.

The bar code ink is wavelengthdependent (Fig. 4). This contrast directly governs the error rate in the read mode. The shorter the wavelength, the better.

[a] Hand held type [b] Desk top typeItem

Important characteristics of bar code Scanner

No special restrictions imposed because of single direction scanning.

Because of multidirection scanning, the closer to true circle (1:1), the better.

//

(Aspect ratio)

1.0

0.5

0600 700 800

White bar

Black bar

Ref

lect

ivity

Wavelength λ (nm)

Fig. 4 Spectral reflectance of bar code label

– 60 –

6-4. Laser beam printerThe laser beam printer is attracting wide attention as an ideal printer, because it provides very clear high printing

quality, allows high speed printing, and operates silently without producing noise.

The laser diode is finding wide use as the laser light source for the printer, because it meets the small size and low power

consumption requirements.

1. Basic configuration

Fig.5 shows a basic configuration of the printer.

The printer as a whole consists of a signal control system, optical system and recording system.

laser diode

Collimatorlens

Polygon mirrorf lens

Cylindrical lens

Photo-sensitivematerial drum

Fig. 5

a. Signal control system

The laser diode, modulated by the record signal, gives the pattern of dot signals to be printed in light ON/OFF form.

b. Optical system

In the optical system, the laser light from the laser diode is converted into parallel beams by the collimator lens. The

beams are horizontally and vartically scanned by the polygon mirror and photosensitive material drum in rotation at a

low speed.

c. Recording system

What role the laser diode plays in the recording system is described below, while referring to Fig.6.

Toner

Ordinary paper

r Thansferred

q Charged

w Exposed tolaser beem

e Developed

t Fixedy Cleaned

u Discharged

Photo • sensitivematerial drum

Fig. 6

– 61 –

q The surface of the photo-sensitive material drum is uniformly charged.

w Then the drum is exposed to laser beam. The charge in the exposed portion escapes, so an electrostatic latent image

is formed.

e The toner pre-charged to reverse charge is brought into contact with the photo-sensitive material on which the electro-

static latent image was formed for development.

r A voltage is applied for discharge to transfer the toner to ordinary paper.

t Fixing is done by pressurization and heating.

y The remaining toner is removed.

u The surface of the photo-sensitive material is discharged.

6-5. Light source for optical pumping of solid-state laserThe laser diode (LD) can be used as a light source for optical pumping of solid-state laser. Since an LD excited solid-

state lasers offer many advantages listed below, it began to attract wide attention in recent years. Intensive efforts are now

being made for development and commercialization.

(1) Provides overall high power efficiency.

(2) Features small size and light weight, making it possible to reduce power supply.

(3) Can operate without water cooling.

(4) Offers longer life and higher reliability than the lamps, contributing to a substantial reduction in maintenance cost.

(5) Can be used for optical pumping of various types of solid-state lasers 770 to 840nm in absorption wavelength.

The following table shows a comparison of the LD with the conventional excitation techniques in efficiency, etc.1)

Electric input power

Optical power

Laser power

Overall powerefficiencyOptical pumpinglight sourcelife expectancy

Inert gaslamp

Tungstenlamp

Laser diode

Nd : YAG laser excitation light source comparison

2kw

100W

8W

0.4%

400h

500W

5W

0.2W

0.04%

100h

1W

0.2W

0.06W

6%

500h

– 62 –

To explain the excitation technique dependent differences in efficiency, let's compare the light emission spectra of inert

gases such as xenon and krypton and those of laser diode with respect to the absorption spectra of Nd : YAG.

• Nd ; YAG and Nd: Glass, as shown below in Figs

. (a) and (a'), have sharp absorption bands, including 810nm. If they are excited by use of a xenon or krypton lamp shown

below in Figs. (b) and (b'), poor excitation efficiency will result, because such inert gas lamps have a wide light emission

wavelenth range in which even the wavelengths that are not absorbed by Nd : YAG and Nd : Glass are included. The

laser diode, on the other hand, has sharp light emission spectrum and offers a high pumping efficiency, because it allows

tuning of wavelengths to only the 810nm absorption band.

100

80

60

40

20

00.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

100

80

60

40

20

00.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

70

60

50

40

30

20

10

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10

70

60

50

40

30

20

10

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10

500mmHg

2400A/cm2

1200A/cm2

(b) Xenon lamp light emission spectra (b') Krypton lamp light emission spectra

(a) Nd3+ : YAG absorption spectra (a') Nd3+ : Glass absorption spectra

Wavelength λ [ µm ] Wavelength λ [ µm ]

Wavelength λ [ µm ] Wavelength λ [ µm ]

750mmHg

2400A/cm2

Abs

olut

e qu

antit

y of

ligh

t [ m

W/c

m2 p

er10

0

]

Abs

olut

e qu

antit

y of

ligh

t [ m

W/c

m2 p

er10

0

]

Tra

nsm

ittan

ce [

%]

Tra

nsm

ittan

ce [

%]

Fig. 7 Comparison of light emission spectra

– 63 –

Excitation of solid-state laser by LD can be accomplished by two methods shown below in Figs.8 and 9.

The end excitation method achieves excitation by converging output light from the laser diode on the end of the solid-

state crystal, making it possible to obtain high-quality single-frequency laser emission whose laser emission threshold

value is low.

The side excitation method is used for excitation by converging the output light from the laser diode on the side of the

solid-state crystal. It is suited for use in obtaining large output laser oscillations.

Laser diode

Collimatorlens

Focus lens808nm Nd:YAG

1.06µm

Fig. 8 End excitation method

Collimator lens

Laser diode

Nd:YAG

808nm

1.06µm

Fig. 9 Side excitation method

Development and practical utilization of LD excited solid-state lasers, which began with Nd : YAG, are now evolv-

ing in the following directions as well.

(1) Development of crystals suitable for LD excitation

Crystals like YVO4 having a wide light absorption band

(2) Development of light emission centers other than Nd

Eye-safe lasers like Ho

(3) Production of larger output by side excitation meshod

Larger output by use of flat plate, larger aperture diameter, etc.

(4) Generation of higher harmonics by non-linear opticalcrystals

Nd: YAG (1.06µm) and KTP combined to produce 530nm green laser with SHG

For detailed information, refer to the section on laser display.

(5) Development of various emission operations

Optical modulation elements are inserted into resonators to have various emission operations performed by the Q

switch method or mode lock method.

– 64 –

The mother crystals developed so far and laser emission wavelengths by light emission centers are summarized in

the following table. In the subsequent sections (1) through (3), the examples of developments are introduced, while

referring to the '87 CLEO, etc.

• The most widely used solid-state laser, because it provides monochromatic light and also facilitates laser emissions.

• Used for large output laser because optically uniform large size material can be obtained at low price.

• Polarized light emissions possible

• Thermal lens effect small

• Polarized light emissions possible

• Light absorption band wide

Laser light wavelengths (µm)Light emission

center

MaterialsNd Ho Er Tm

YAG

Glass

LiYF(YLF)

YVO4

0.9461.0641.319

2.09 2.9 1.9

1.06

1.05

1.06

2.06 1.5

2.8

Main applications

Nd YLF Master emitter for nuclear fusion

Ho YLF Eye-safe laser

Er Glass Optical fiber sensor laser

Er YAG Surgical removal of living tissue and boring

– 65 –

Let's make a little more detailed study of the relationships between excitation and laser transition, taking the Nd : YAG

as an example.

The strong absorption band around 810nm observed in Fig.10 corresponds to the transition of the Nd from its base state

of 4I9/2 to 4F3/2. When a laser diode is used, it is tuned to this strong absorption band. The transition from the 4F3/2 to the

laser emission level of 4F3/2 occurs without radiation. The Nd : YAG, as shown in Fig.10, emits more than 20 rays. In

most of them laser emission, are observed. The main transition among them is from 4F3/2 to 4F11/2, providing 1.06µm

emissions. In addition, 1.32µm emissions by the transition from 4F3/2 to 4F3/2, and 0.946µm emissions by the transition

from 4F3/2 to 4I9/2 are also included.

20

18

16

14

12

10

8

6

4

2

0

x103cm-1

@@@@@@ÀÀÀÀÀÀ@@@@@@ÀÀÀÀÀÀ@@@@@@ÀÀÀÀÀÀ@@@@@@ÀÀÀÀÀÀ

@@@@@@ÀÀÀÀÀÀ@@@@@@ÀÀÀÀÀÀ@@@@@@ÀÀÀÀÀÀ@@@@@@ÀÀÀÀÀÀ Inert gas lamp excitation band

LD excitation band

Transition withoutradiation

Lasertransiton

4F5/2

4F3/2

4I15/2

4I13/2

4I11/2

4I9/2 4I9/2

4I11/2

4I13/2

4I15/2

4F3/2

0.81µm

1.06µm0.946µm

1.32µm

L level fragmented by spin-orbit interaction

Base state

(1) (2) (4) (6)

(3) (5) (7) (8) 11502cm-1 R2

11414 R1

6000cm-1

4000cm-1

25262473

21462111

20292001

848

311197

1340

Levels Stark Splitted by crystalline field

Fig. 10

– 66 –

(1) Examples of developments of crystals suitable for laser diode excitation2)

The Nd: YVO4 has a wider absorption band than the Nd: YAG. As shown in Fig.11, therefore, it provides higher slope

efficiency. It also provides a high optical power output over a wide excitation wavelength range as shown in Fig.12.

140

120

400 500 600 700 800 900 1000 1100

100

80

60

40

20

0

Nd: Bel

Nd: YAG

Nd: YVO4

Diode Input Electrical Power (mW)

Out

put O

ptic

al P

ower

(m

W)

(optical slope effciency percent)

a) Measured solid-state laser output as a function of pump laser diode electrical input.

Optical efficiency percents are in parenthses (YAG and YVO4 at 97% and Bel at 99% output couplers).

Fig. 11

100

80

60

40

20

803 804 805 806 807 808 809 810 811 8120

Pump Diode Wavelength (nm)

Out

put O

ptic

al P

ower

(m

W)

Nd: YVO4

Nd: YAG

Nd: Bel

b) Measured solid-state laser output as a function of the center wavelength (approximate) of the diode pump laser array.

Optical pump power was held constant at 167mW.

Fig. 12

(2) Example of larger output achieved by side excitation method

In a slab (parallel flat plate), the input breakdown limit is prportional to the length and lateral width of the slab, and is

inversely proportional to the thickness of the slab. Compared with a rod of the same length, the slab allows a larger

breakdown limit input and therefore has the possibility of making a larger output available. For slab excitation, use of

fiber collected laser diodes as the optical pumping light source is proposed.4)

– 67 –

SLAB

FIBER COUPLERS

LASER DIODEPOWER BANK

Fig. 13

Laser diodes used for solid-state laser excitation are selected on the basis of required power and wavelengths. Many of

the Sony laser diodes ranging from the 25mW SLD202 to 1W SLD304 can be used in a wide range of applications.

Documents referred to:

1) T.M. Baer Laser Focus, June P82 (1986)

2) R.A6 Fields, M.Birnbaum, C.L. Fincher, CLEO'87 Tech. Digest P.344

3) R.Lz Byer, Private communication

• Wavelength tuning procedure

There are cases where adjustment of laser diode emission. wavelength to a specific wavelength is desirable as in

excitation of solid-state lasers like YAG and YLF. In such a case, the laser diode, as one of its advantages, allows use of

the tusing method taking advantage of the wavelength temperature characteristics.

Example) YAG excitation

• Absorption band of YAG : λ = 807nm

• Laser diode : SLD326YT-25

Specification λ = 810 ± 3nm

(Tth=25˚C

: Thermistor temperature)

The actually supplid data is assumed to be λ = 810nm.

• Wavelength temperature coefficient: 0.3nm/˚C

For tuning to absorption band wavelength λ = 807nm,

∆λ = 810nm – 807nm = 3nm

∆T = 3nm ÷ 0.3nm/˚C = 10˚C

Therefore, ∆T = 10˚C cooling must be done.

That is, the temperature must be lowered to:

Tth = 25˚C – 10˚C = 15˚C

Therefore, adjust the current of the T.E. cooler so that the thermistor resistance RTH found from the graph will be:

RTH = 15kΩ

– 68 –

Thermistor characteristics

50

10

5

0-10 0 10 20 30 40 50 60 70

Tth-Thermistor temperature (˚C)

Rth

-The

rmis

tor

Res

ista

nce

(kΩ

)

Fig. 14

6-6. Material ProcessingSony's interopto device will be used as an application example to describe material processing.

Picture 1 is of a soldering device exhibited in 1987. The 10W required for soldering is generated by focusing the

optical output of 20 individual 500mW laser diodes on a spherical surface. Each laser diode is mounted with a Selfox lens

to maintain a convergent beam. The laser diode optical sources are fixed and the IC package to be soldered is moved.

A 48-pin IC with a 0.8mm pitch can be soldered completely in 1 minute. This demonstration attracted attention at the

exhibit because it gave rise to the idea of using a semiconductor laser for material processing.

Picture 2 illstrates a soldering device brought out in 1988 which illustrates the realization of a functional system. The

system brings together the beams from 19pcs laser diodes in a bundle of fibers which is focused through a lens to a 2mm

minimum diameter. This fiber bundle is shown in Fig.16. The high accuracy and controllability of the system allowed

this apparatus to be marketed. The table on the next page gives the specifications of the laser oscillation unit.

Picture. 1 Soldering Devise Presented at Interopto 1987

– 69 –

Laser Beam

Focal Point

20 individual 500mWLaser Diodes providedwith Selfox lens

Fig. 15 10W optical Output LD Module

Picture.2 Soldering Device Utilizing 1WLDs

LD

Bundle fiberLens

Lens

Fig. 16 Optical System of Fiber Bundle

– 70 –

Laser

Opticalsystem

Cooling

Another

Supportdevises

Specifications of Laser Oscillaton Unit for Soldering Device(Picture 2)

Processing

Light guidance

Maximum outputoscillation

Minimum focusedlight diameter

Fiber 19 lines (Bundle construction)

Light radiation

Temperature 15˚CPower supplyPackage outline

Weight ApproxProtractor

Cooling apparatus

Near-infrared semiconductorlaser810nm wavelengthVisible semiconductor laser670nm wavelength

Greater than 10W

Continuous (CW)

Focul distance 50mm(Typical lens)

Self water coolingSystem

Approx. 1mm(Using attached lens)

AC100V, 10A (3-phase)500(W)×300(D)×150(H)mm

15kgCutter for processing laserPower supply : AC100V, 4.5APackage outline : 325(W)×430(D)×485(H)mmWeight : 32kg

6-7. Laser displaySemiconductor lasers have been finding application in new fields as their outputs grow higher. A typical field is laser

displays.

For laser displays the three primary colors, red (R), green (G) and blue (B), are required but the semiconductor lasers

currently available are limited to the red to near infrared (λ = 670 to 840nm). Considerable difficulties are encountered

in development of green and blue ones because of problems associated with materials.

However, all of the three colors R, G and B can be obtained by increasing the output of semiconductor laser.

The procedure is by use of YAG (solid-state laser) + SHG (Second harmonic generator crystal). Crystals now under

research and development include KTP, ß-Ba B2O4, etc. Use of these makes it possible to produce R, G and B as shown

below.

1.32µm 0.66µm

Red

R

Laser diode

1.064µm 0.532µm

Green

G

Laser diode

Nd:YAG

Nd:YAG

Nd:YAG

KTP

KTP

KTP

0.946µm 0.473µm

Blue

B

Laser diode

Fig. 17

– 71 –

Procedure for production of three primary colors by use of Semiconductor lasersThe following table lists the luminances of R, G and B required for configuration of a laser display.

Red 300 500

Green 150 250

Blue 300 500

Laser outputfrom SHG (mW)Luminance (1m)

Luminance required for laser display

The losses in the optical system following the SHG are not covered, but actually the losses must be taken into consid-

eration. The main governing factors, however, are the following two.

1) Laser diode → YAG laser conversion efficiency

2) SHG conversion efficiency

Since (2) in particular is heavily power dependent (almost proportional to P2), the lager the output of laser diode, the better.

6-8. Communication laserHere is a brief account of how communication systems can be be configured by use of short wavelength laser diodes

and what are the key characteristics that deserve special attention in selecting communication laser diodes.

(1) Application

The communication systems now nearing practical utilization, where short wavelength laser diodes can find applica-

tion, include the subscriber systems in public communications, inter-and intra-office transmission city and toll commu-

nications, closed-circuit TV, CATV, optical LAN computer links, intra-and inter-building communications, and intra-

aircraft and-vessel communications. These communications systems are categorized as near-and medium-distance (less

than 10km) and medium-speed (up to 400Mbps) communications.

(2) Optical communications system

An optical communications system comprises three components shown below.

1. Light emitting element

(laser diode (LD), light emitting diode (LED))

2. Transmission path

(Fiber, through atmosphere, deep space)

3. Light receiving element

(Avalanche photodiode (APD), PIN photodiode (PIN-PD))

The performance of a communications system depends on the overall characteristics of these three elements. Special

atention, therefore, must be paid to assure alignment of the characteristics of the individual elements when selecting the

elements.

(3) AlGaAs type laser diode

The AlGaAs type laser diode used as a light emitting element has frequency characteristics that extend to 1GHz,

exhibiting high speed response. The silicon ADP used as a light receiving element also has response characteristics that

extend to more than 1GHz.

The transmission path that is available for coupling these low cost light emitting and receiving elements includes (1)

silica glass fiber and (1) free space (atmosphere and deep space).

If silica glass fiber is combined, the transmission loss in the short wavelength band (0.8µm band) of silica glass fiber is

as low as 2 to 3bB/km, whereas its transmission band is as wide as 0.1 to 5GHZ. km. The optical communications system

that can be configured by the laser diodes will be a large capacity communications system. So the laser diodes can be

readily used for the application systems described before.

For transmission through free space, on the other hand, a 1Gbps channel can be immediately implented by installing a

communications unit, in which the abovementioned light emitting and receiving elements are accommodated, between

– 72 –

buildings and between satellites. The laser diode provides a very effective means for short distance communications over

the land as between buildings. In free space, no transmission loss need be considered. Optical transmission is therefore

considered as the best communication measns between satellites.

(4) AlGaln type laser diode

As a visible light laser diode, the AlGaIn type laser diode is expected to find application in various fields, In the

communications field, attempts are being made to use the laser diode in a short haul wideband transmission system by

combined use of low cost plastic fiber (transmission loss reduced to minimum at 570nm). If a communications system

now in service is using the LED as a light emitting element, replacement of the LED by the AlGaIn P type laser diode will

make it possible to radically increase the communication capacity of the system.

(5) Characteristics

The characteristics that have to be evaluated for use of the laser diode as a light emitting element for optical communi-

cations include:

1. Pulse response characteristics

Supply DC bias current to the laser diode to be evaluated. Then superpose pulse current to measure the light emission

rise and fall times of the laser diode. Relaxation emissions can also be evaluated.

Fig.18 shows an evaluation set up.

Fig.19 shows an example of measurement.

PULSEGENERATOR

LD

(LD)VBIAS

(PD)VBIAS

OSCILLOSCOPE

AMP.

APD

Fig. 18 Evaluation setup

SLD202V

Pulse response characteristics (1ns./div.)

Pulsewidth : 5ns

Fig. 19 Example of measurement

2. Frequency response characteristics

Supply DC bias current to the laser diode to be evaluated. In addition, superpose and sweep a small AC signal, detect

its optical power output with a photodiode, and measure the frequency response by aspectrum analyzer.

Evaluate the expansion and elongation of the frequency band of the light emitting diode.

Fig.20 shows an evaluation setup. Fig.21 shows

Input

Optical poweroutput

– 73 –

SIGNALGENERATOR

LD

(LD)VBIAS

AMP.SPECTRUMANALYZER

APD

RF

(PD)VBIAS

Fig. 20 Evaluation setupR

espo

nse

(dB

)

SLD202V

Frequency (GHz)0 1.0 2.0

Res

pons

e (d

B)

SLD202V

0 1.0 2.0

Frequency (GHz)

5dB/div. Po=7mW

5dB/div. Po=14mW

Fig. 21. Examples of measurements

3. High-speed digital modulation characteristics

Supply DC bias current to the laser diode to be evaluated. In addition, superpose the random pattern pulse current

generated by a signal generator to let the laser diode emit light.

The light should be coupled with the fiber of a given length, and an APD put at its end to receive the light. Evaluate the

eye pattern with an oscilloscope. If the light is directly brought to the APD, not through the fiber, the high-speed digital

modulation characteristics of the laser diode in the standalone state can be eveluated. If the eye pattern opens, the digital

modulation characteristics are generally considered acceptable.

– 74 –

Fig.22 shows an evaluation setup. Fig.23 shows examples of measurements. The error rate of the laser diode can also

be evaluated by inputting the APD output to an error meter. For a communications system, the error rate is generally

required to be less than 10.

PULSEGENERATOR

LD

L

OPTICALFIBER

Fig. 22 Evaluation setup

400psec

800psec

Input signal

AMP.

APD

OSCILLOSCOPE

(LD)VBIAS

(PD)VBIAS

Fig. 23 Examples of measurements

4. Higher harmonic distortion

Supply DC bias current to the laser diode to be evaluated. In addition, superimpose the small sinusoidal signal gener-

ated by a signal generator, let an APD receive its optical power output, and evaluate the frequency response by a spectrum

analyzer. Evaluation of the frequency response is accomplished by measuring how much secondary and tertiary higher

harmonic components are included in the fundamental wave components. The secondary and tertiary higher harmonic

components, resulting from the non-linearity of the optical power output and current characteristics, produce consider-

able effects on analog transmission. When the higher harmonic components are to be measured with high accuracy,

provision must be made eliminate the higher harmonic components included in the signals from the signal generator. The

higher harmonic components that can be tolerated for analog transmission are generally said to be less than-60dB.

A setup used for evaluation is the same as the one used for evaluation of the frequency response characteristics.

SLD202V

L = 0km

SLD202V

L = 1km

© 2000 Sony Corporation

SONY SEMICONDUCTOR

LASER DIODE GUIDE