Bernd Sumpf

Ferdinand-Braun-Institut

Lichtenwalde, October 18, 2012

Diode lasers for sensor applications

Outline

1. Diode Lasers – Basic Properties

2. Diode Lasers for Sensor Applications

Diode lasers with internal grating

Diode lasers in external cavities

Diode lasers as pump sources for non-linear frequency conversion

Hybrid integrated laser module for ps- and ns-pulses

3. Summary

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Diode lasers – Features

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Wide spectral range: 0.34 nm ... 33 µm

FBH: 630 nm ... 1.2 µm

High wall-plug efficiency

Easy excitation

Direct Modulation

Small size

Mechanical robustness

Lifetime (> 107 h)

Tuneability

Current

Temperature

External grating

PbxSn1-xSe

PbSxSe1-x

PbxCd1-xS

InPxAs1-x

GaxIn1-xAs

AlxGa1-xAs

GaxIn1-xP

ZnxCd1-xS

AlxGa1-xN

0.5 0.3 1 2 5 3 7 20 10 30

Wavelength / µm

Compound Semiconductors

Laser diodes: High Output Power

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Laser diode

P = 20 W

200 µm x 2 µm

Power density

p = 5 MW / cm2

Coal-fired power plant

600 MW

Same power density in a

12 cm cable

Surface of the sun

6 kW / cm2

Laser Diodes: High efficiency

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Efficiency 73%

0 10 20 30 40 50 60 700

10

20

30

40

50

60

70

80

90

ou

tpu

t p

ow

er

P / W

current I /A

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

vo

lta

ge

U / V

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

C = 73 %

co

nve

rsio

n e

ffic

ien

cy

c

Light bulb:

< 5%

Energy-saving lamp:

< 20%

Laser Diodes: Narrow spectral linewidth

min 10-6 µm

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0.9758 0.9760 0.97620.0

0.2

0.4

0.6

0.8

1.0

optical pow

er

P / a

rb. units

wavelength / µm

T = 25°C

P = 400 mW

30 µm

880 km

Paris Berlin

3 cm

Overview: Fabrication Process of a Laser Diodes

Deposition of very thin crystalline layers on a GaAs substrate

Epitaxy

Structuring of devices on the wafer

Processing

Separation of single devices

Cleaving

Mounting of devices on heat sinks

Housing of devices

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Schematic view of a high-power DFB laser

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Design epitaxial structure

First epitaxy

Manufacturing

of the grating

e.g. using holographic

eposure

Second epitaxy

Process

Facet coating

Mounting

Bragg Grating

Period = 150 … 300 nm

Coupling coefficient

= 1 … 10 cm-1

Ridge waveguide (RW)

WRW = 2 ... 4 µm

neff 3x10-3

7 … 9°

Resonator length

L = 0.75 … 3 mm

HR coating

AR coating

Rf < 10-3

21°

active layer

grating

940 nm DFB lasers for H2O absorption spectroscopy

Transmission measurement

Tuneable Laser

Lambert-Beers-Law

Threshold current Ith = 35 mA

Slope efficiency S = 0.9 W/A

Maximum output power

Pmax 500 mW

Dips in the characteristic due to water absorption

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Laser Medium - 0 Det.

LII )(exp)( 0

0 200 400 600 8000

100

200

300

400

500

T = 20°C

Pow

er

P / m

W

Current I / mA

940 nm DFB laser:

Absorptions spectroscopy of water vapour

Spectra calculated based on Lambert-Beers-law

Comparison of calculated spectra to the data

from the HITRAN database

Excellent agreement

Continuous tuning over 5 nm at one temperature

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938 940 942 944

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Laser at 50°C

Laser at 20°C

HITRAN

Rela

tive inte

nsity / a

rb. units

Wavelength / nm

Diode lasers in external cavities for Raman spectroscopy

Raman measurement

Fixed frequency laser (e.g. 488 nm, 671 nm, 785 nm)

Spectral emission width: 10 cm-1

Spectral stability 1 cm-1

Well-established contact free method

for material analysis, food safety control, clinical diagnostic.

Excitation in the visual spectral range

Advantages:

Higher Raman signals due to a -4 – dependence

Resonance Raman

Stokes lines in the maximum of the sensitivity of CCDs

Disadvantages:

Possible fluorescence background

Shifted excitation Raman difference spectroscopy

Spectral distance for SERDS SERDS ≈ 10 cm-1

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Laser Raman R Det.

Raman

PRaman

671 nm microsystem light source

Gain medium:

Broad area laser

w = 30 µm, 60 µm, 100 µm; L = 2 mm

Output power up to 1.5 W

Resonator

Front facet of the diode laser and

Reflection Bragg Grating

Emission width below 100 pm (10 cm-1)

Active adjustment necessary

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RBG

Microoptical bench (13 mm x 4 mm)

Diode Laser

with Rr ≤ 0.1%

and Rf = 1%

CuW-Submount

Microoptics Microoptics

FAC SAC

FAC SAC

BA-Laser

RBG

FAC + SAC

FAC + SAC

IEEE Phot. Tech. Lett. Vol. 20(19), pp. 1627 (2008).

671 nm module – application in Raman-Spectroscopy

Ethanol as analyte (A):

Raman-signals spectrally resolved

Fluorescent interference introduced (B)

Laser dye Cresyl violet

Application of SERDS successful (C)

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A B C

Diode lasers in external cavity for interferometry

Absolute distance interferometry (ADI) with 10-6

accuracy requires tunable red emitting diode lasers:

Preferred wavelength ≈ 633 nm

Single-mode operation

Tuning range ≥ 50 pm (40 GHz)

Determines the smallest measurement distance

about 4 mm

Narrow spectral line width ≤ 10 MHz (0.015 pm)

Determines the maximal measurement distance

about 15 m

Output power ≥ 5 mW

Current tunable, no moving parts

Solution:

ECDL with mode spacing larger the spectral width of the RBG RBG 50 pm

n L ≤ 4 mm FP 50 pm - within RBGs spectral width only one mode!

Narrow line width due to high quality resonator (High facet reflectivity)

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Scheme of the external cavity laser

Single mode operation

34 pm, i.e. 25 GHz

Emission line width (self-delayed heterodyne)

Between mode hops smaller 10 MHz

coherence length of 30 m

At mode hop increase to about 15 MHz

Side mode suppression ratio: Better than 25 dB

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Diode lasers as pump sources

for non-linear frequency conversion, e.g. SHG

Low power application (25 mW) for Raman spectroscopy

Non-linear frequency conversion – Second Harmonic

Generation (SHG)

Pump source

Distributed Feedback (DFB) RW Laser

SHG-crystal

periodically poled MgO:LiNbO3 for 488 nm at 25°C

RW-SHG-Waveguide (3 µm x 5 µm x 11.5 mm)

higher efficiency

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Microoptics Diode laser WG-SHG-crystal Microoptics

CuW-submount

Microbench (25 mm x 5 mm)

Diode lasers for the generation of ps- and ns-pulses

Methods:

Gain switching

Current injection

Pulse length: 1 ns – 1 s

Q-switching

Changing the properties of the laser cavity

E.g. implentation of an absorber section

Pulse length: 50 ps – 150 ps

Rep. Rate: up to 0.5 GHz

Mode locking

Coupling of longitudinal modes

Passively (saturable absorber section)

Actively

Pulse length: 1 – 20 ps

Resonator length determines the rep. rate in the

GHz-range

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400 - 600µm

120

µm

300 - 3300µm

Ridge waveguide laser

Ridge

0 1 2 3 4

0

1

2

L = 2 mm

W = 6 m

Ith = 50 mA

S = 0.70 W/A

puls

e p

ow

er

P / W

pulse current I / A

pulse

= 10 ns

frep

= 1 MHz

Gain Switching – DFB laser

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Geometry: L = 2 mm, W = 6 µm

Pulse length 10 ns … 1 ms; Rep. rate 1 MHz

DFB laser with Popt = 2 W at I = 4 A

MOPA system with up to Popt = 10 W

0

25

50

0

25

50

0

25

50

0 5 10 15 20

0

25

50

75

I = 0.80 A

P = 0.52 W

I = 2.80 A

P = 1.53 W

I = 1.80 A

P = 1.05 W

I = 4.00 A

P = 2.02 W

time / ns

inte

nsity /a.u

.

Q-Switching – Multi-Section RW- and DBR Lasers

e.g. 3 – section DBR laser with gain, absorber, and grating section – length 1.5 … 4.0 mm

Current through gain section varied

Output power can be modulated using the absorber section VSAB = -2.0 V; tmod = 1 ns; frep = 40 MHz

Pulse length 100 ps

Pulse power 300 mW

Amplified power: 20 W

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Gewinn-SektionDBR-Sektion

Absorber SektionWiderstand zum Heizen

1500µm

200µm

80 - 300µm

0.00

0.05

0.10

0.00

0.05

0.10

0.00

0.05

0.10

0.00

0.05

0.10

0.0 0.5 1.0 1.5 2.0

0.00

0.05

0.10

72ps

100 mA

82ps

200 mA

72ps300 mA

84ps

400 mA

inte

nsity / a

.u.

118ps500mA

time t / ns

Monolythic devices for mode locking

4 section DBR-Laser

DBR grating determines wavelength

Fast saturable absorber for mode locking

Passive and active mode locking possible

Round trip: ~ 230 ps, rep. rate ~ 4.3 GHz

Pulse length ~ 8 ps, Jitter: < 1 ps

Peak power: 1 W at 8 ps Pulse length

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Gewinn-SektionDBR-Sektion Absorber Sektion

10000µm

200µm 8100 -8850 µm 200µm 750 -1500 µm

Kavität

FrontfacetteRückfacette

0 2 40

10

20

time / ns

po

we

r / a

.u.

f active ML = 4.324GHz

0 30 60 90 120 1500.0

0.2

0.4

0.6

0.8

1.0

FWHM; sech

~8.4ps

T = 20.°C

Uabs

= -1.3V

Icav

= 180mA

Igain

= 500mA

inte

nsity / a

.u.

ACF

~13.0ps

time / ps

MOPA system with tapered amplifier

for Q-switched ps-pulses

Master Oscillator: Q-switched DBR

Power Amplifier: Tapered Laser

Amplification of short pulses

Maintaining Beam Quality

Seperate Excitation of RW

and Tapered Section

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Generation of current pulses

MO <1 ns, 1 A

PA < 2 ns, 20 A

Electronics also developed at FBH

XXXXXFBH 1

PRE1 TPA1

DC

RW

SAB1

SAB2

RWG1

DBR2

DBR1

DC

RWPuls

0.5 A

ECL

DBR Laser mit GaN Transistor Trapezverstärker mit GaN Transistor

ECL

Puls

16A

t< 2nst 0.5-1ns

50mW...

200mW

100W

GaN Transistor5x8x250

DBR GaN Transistor5x8x500

TPA

RW-Sektion Trapez-Sektion

Results: MOPA system for Q-switched ps-pulses

Pulse length (FWHM) = 73 ps

Pulse power: 20 W

Wavelength defined by MO with emission width (FWHM ~ 0.2 nm)

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-90

-60

-30

-90

-60

1020 1040 1060 1080 1100-90

-60

DBR: tpuls,

~ 0.9ns

tperiod

= 25ns

Ig,cw

= 300mA

TPA: tpuls,

~ 1.0ns

tperiod

= 400ns

ITpa,puls

= 16A

IRW,Tra

= 50mA

Pgentec

= 2.13mW

Ppulse

= 12.2W

without seed pulse

with seed pulse

in

tensity / d

B

IRW,Tra

= 100mA

Pgentec

= 2.76mW

Ppulse

= 15.8W

wavelength / nm

IRW,Tra

= 200mA

Pgentec

= 3.53mW

Ppulse

= 20.2W

42dB

0

50

100

150

0

50

100

150

0.0 0.5 1.0 1.5 2.00

50

100

DBR: tpuls,

~ 0.9ns

tperiod

= 25ns

Ig,cw

= 300mA

TPA: tpuls,

~ 1.0ns

tperiod

= 400ns

ITpa,puls

= 16A

IRW,Tra

= 50mA

in

ten

sity /

a.u

.

IRW,Tra

= 100mA

time / ns

IRW,Tra

= 200mA

73ps

Pulse picking using tapered amplifier

Basic principle:

RW-section of TPA as selector

Transparent or absorbing

Controlled with GaN-HF-transistor

Selectable duty cycle

Amplification in tapered section

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high frequency pulses (GHz)

HF transistor

low frequency pulses 1kHz - 100MHz

U

ampI

rwI

master oscillator

pulse picker element

DBR

cavity

gain

absorber 1cm

G D S

amplifier section

RW-section for gating

0 2 40

10

20

time / ns

pow

er

/ a.u

.

f active ML = 4.32399GHz

0 5 10 15 200

5

10

15

po

we

r / a

.u.

time / ns

pulse picker f/64 ~ 67MHz

1cm DBR Laser Pulspicker mit HF Transistor Auskoppeloptiken

HF Ansteuerung PulspickerHF Ansteuerung Modenkopplung

Pulse picker – optical micro bench

Integeration of

optical elements

high-frequency electronics

500 mA current pulses

200 ps pulse width

Adjustable rep.rate

1 kHz – 333 MHz

Jitter smaller 25 ps

Small inductivity – short wires

18/10/2012 24

Pulspicker

GaN high

electron mobility

transistor HEMT

Summary and Acknowledgments

Diode lasers:

Compact, reliable, high-power light sources for different applications

Features can be optimized with respect to the application:

Wavelength

Power

Emission width

Beam quality

Pulse parameter

Acknowledgments:

All colleagues at the FBH

Colleague at the Technische Universität Berlin (Agr. Laserspektroscopy)

Financial Support:

Zukunftsfond Berlin

Deutsche Forschungsgemeinschaft

Bundesministerium für Bildung und Forschung

Europäische Gemeinschaft

18/10/2012 25