EE 230: Optical Fiber Communication Lecture 9

From the movieWarriors of the Net

Light Sources

Conditions for gain (lasing)

• E2-E1<Fc-Fv (population inversion)

• g(1/L)ln(1/R)+ (net gain)

=2nL/p, p an integer (phase coherence)

Reflectivity

2

1

1

n

nR

Longitudinal mode spacing

nL2

2

Laser Diode Structure and Optical modes

Conditions for continuous lasing (steady state)

• Net rate of change of density of conduction band electrons is zero (injection minus recombination and depletion)

• Net rate of change of density of photons created is zero (stimulated emission minus leakage and spontaneous emission)

Laser Electrical Models

Laser Pad Capacitance

Package Lead Inductance

Package Lead Capacitance

Bond wireInductance

Laser contactresistance

Laser Junction

Assume that the light output is proportional to the current through the laser junction

Use a large signal diode model for the laserjunction, this neglects the optical resonance

Simple large signal model

(Hitachi)

More exactly the laser rate equations can be implemented in SPICE to give the correct transient behavior under large signal modulation

Small signal model

Steady-state lasing conditions

N

NNNb

de

J

dt

dN

)(2

N

s

P

NfNNb

dt

d

Turn-on delay

th

bNd JJ

JJt ln

Turn-on Delay

Input Current

Output Light Signal

d

I p

b

For and applied current pulse of amplitude

the turn on delay is given by:

ln

with a bias current I applied:

ln

where is the delay at threshold (2ns

p

pd th

p th

pd th

p b th

th

I

II I

II I I

Typ.)

To reduce the turn on delay:• Use a low threshold laser and make Ip large• Bias the laser at or above threshold

Ib=0

Ib=0.9Ith

Ib=0.5Ith

Tur

n on

Del

ay (

ns)

Relaxation oscillation

Decays as e-t/2, where

and with a freqency , where

N

b

1

thNP

thth J

JJNNb

Modulation frequency

Difference between optical output at modulation frequency m and steady-state output is proportional to

2222

1

mmr

Resonance Frequency

20

2 20

0

Photon Density ( ) (0)Laser Small Signal Frequency Response=

Excitation Current ( ) (0)

1 1 1f =Resonance frequency

2 22

d

thstim

p pp e

d

fs f s

i f i f f jff

where

gS gI I

f

220

=Damping frequency 2

= f =Frequency of peak response 4

g=differential gain S= photon lifetime carrier lifetime

p

d

stimth p e

S

ffp

I I1

gS

Semiconductor lasers exhibit an inherent second order response due to energy“sloshing” back-and-forthbetween excited electrons and photons

Large Signal Transient Response

Effects of current and temperature

• Applying a bias current has the same effect as applying a pump laser; electrons are promoted to conduction band. Fc and Fv get farther apart as well

• Increasing the temperature creates a population distribution rather than a sharp cutoff near the Fermi levels

Fabry Perot Laser Characteristics

(Hitachi Opto Data Book)

Quantum efficiency

• Internal quantum efficiency i, photons emitted per recombination event, determined empirically to be 0.650.05 for diode lasers

• External quantum efficiency e given by

th

thie g

g

Total quantum efficiency

Equal to emitted optical power divided by applied electrical power, or he/qV

For GaAs lasers, TQE 50%

For InGaAsP lasers, TQE 20%

Chirping

Current modulation causes both intensity and frequency modulation(chirp)

As the electron density changes the gain (imaginary part of refractive index ni) and the real part of the refractive index (nr) both change.

The susceptability of a laser to chirping is characterized by the alpha parameter.

n rNniN

where N is the electron density. Large implies lots of chirping.

v(t ) 4

P / tP

P

vP

jf

2P0 for P= P0 Pe

jt

1-3 is expected for only the very best lasersChirping gets worse at high frequenciesRelaxation oscillations will produce large dp/dt which leads to large chirpingDamping of relaxation oscillations will reduce chirpCorrectly adjusting the material composition and laser mode volume can reduce

Reflection Sensitivity

R. G. F. Baets, University of Ghent, Belgium

Problem

Solution

Example

A GaInAs diode laser has the following properties:

• Peak wavelength: 1.5337 m

• Spacing between peaks: 1.787x10-3 m

• J/Jth=1.2

What are the turn-on delay time, the cavity length, the threshold electron density, and the threshold current?

Turn-on delay time

=3.7 ln(1.2/1.2-1) = 6.63 ns

th

bNd JJ

JJt ln

Cavity length

L = (1.5337)2/(2)(3.56)(1.787x10-3)

= 184.9 m

nL2

2

Threshold electron density

R = 0.3152

g(1/L)ln(1/R)+

gth=1/.01849 ln(1/.3152)+100=162.4 cm-1

From figure, N=1.8x1018 cm-3

2

1

1

n

nR

Threshold current

J/2de = I/2deLW

Ith=(0.5x10-4)(1.6x10-19)(1.8x1018)(.01849)(4x10-4)/(3.7x10-9)

Ith=29 mA

N

NNNb

de

J

dt

dN

)(2

Laser Diode Structures

Most require multiple growth stepsThermal cycling is problematic for electronic devices

Laser Reliability and Aging

Power degradation over time

Lifetime decreases with current density and junction temperature

DtePP /0

Problems with Average Power Feedback control of Bias

Ligh

t

Current

Average Power

Ideal L-I Characteristic

Ligh

t

Current

Average Power

L-I Characteristic with temperature dependent threshold

Turn on delay increasedFrequency response decreased

Ligh

t

Current

Average Power

L-I Characteristic with temperature dependent threshold and decreased quantum efficiency

Output power, frequency response decreased

Average number of 1s and Os (the “Mark Density”) is linearlyrelated to the average power. If this duty cycle changes then the bias point will shift

Problem: L-I curves shift with Temperatureand aging

-

+Data

Laser Monitor Photodiode

Vref

-5V

Light Emitting Diodes

An Introduction to Fiber Optic Systems-John Powers

LED Output Characteristics

An Introduction to Fiber Optic Systems-John Powers

Typical Powers •1-10 mW

Typical beam divergence•120 degrees FWHM – Surface emitting LEDs•30 degrees FWHM – Edge emitting LEDs

Typical wavelength spread•50-60 nm

Distributed Feedback (DFB) Laser Structure

•Laser of choice for optical•fiber communication

•Narrow linewidth, low chirp for direct modulation

•Narrow linewidth good stability for external modulation

•Integrated with Electro-absorption modulators

As with Avalanche photo-diodesthese structures are challenging enoughto fabricate by themselves without requiringyield on an electronic technology as well

Hidden advantage: the facet is not as criticalas the reflection is due to the integratedgrating structure

Distributed FeedBack (DFB) Laser Distributed Bragg Reflector(DBR) Laser

Bragg wavelength for DFB lasers

k

nB

2

nL

mBB 2

2/12

Thermal Properties of DFB Lasers

Agrawal & Dutta 1986

Light output and slope efficiency decrease at high temperature

Wavelength shifts with temperature•The good: Lasers can be temperature tuned for WDM systems•The bad: lasers must be temperature controlled, a problem for integration

VCSELs

• Much shorter cavity length (20x)

• Spacing between longitudinal modes therefore larger by that factor, only one is active over gain bandwidth of medium

• Mirror reflectivity must be higher

• Much easier to fabricate

• Drive current is higher

• Ideal for laser arrays

Choosing between light sources

• Diode laser: high optical output, sharp spectrum, can be modulated up to tens of GHz, but turn-on delay, T instability, and sensitivity to back-reflection

• LED: longer lifetime and less T sensitive, but broad spectrum and lower modulation limit

• DFB laser: even sharper spectrum but more complicated to make

• MQW laser: less T dependence, low current, low required bias, even more complicated

• VCSEL: single mode and easy fabrication, best for arrays, but higher current required