laser modified 20012015

OPTOELECTRONICS 20

Principle of Laser Diode

Population Inversion in Homojunction Laser Diode

degenerately doped direct bandgap

p-n junction diode

� EFn > Ec , EFp < Ev

forward bias with eV > Eg

�population inversion at the junction

incoming phonon hυ = Eg in active region

� more likely to cause stimulated emission

than being absorbed � optical gain


Formation of Cavity in Laser Diode

reflectors formed by cleaved surfaces ( ~ 30% reflecting )

mode of the cavity : , ( n : refractive index )


Threshold Current

pumping mechanism : forward diode current

at I = Itrans ( transparency current )

� stimulated emission balances counter absorption

at I > Ith ( threshold current ) � optical gain g reached gth

� optical gain overcome photon losses from the cavity

� optical gain reached gth �lasing emission

Jth in homojunction laser diode is too high for practical uses ( can operate

only at very low temp. )

Prof. Wei-I Lee

Heterostructure Laser Diodes


to reduce Ith �need better (1) carrier confinement (2) photon confinement

improved carrier confinement in DH structure

�easier to achieve population

inversion in narrow Eg active layer

� Ith ↓

narrow Eg semiconductor usually

has higher refractive index

� better photon confinement

narrow Eg active region

photon conc. ↑

stimulated emission rate ↑

in

�

�

�Ith ↓

advantage of the AlGaAs DH laser

� lattice matched to substrate

24 Prof. Wei-I Lee


Gain Guided Laser Diode

Ex. stripe geometrical AlGaAs/GaAs/AlGaAs DH laser diode

current flow confined to between path 2 & 3

J at path 1 > path 2 & 3 , region where J > Jth defines active region

width of the active region decided by J and hence the optical gain

�gain guided laser

advantages of stripe geometry :

1. reduced contact area

� Ith ↓

2. reduced emission area

�easier coupling to

optical fibers

typical W ~ a few μm

� Ith ~ tens of mA

poor lateral optical

confinement of photons


Index Guided Laser Diode

Ex. buried double heterostructure laser diode

good lateral optical confinement by lower refractive index material

�stimulated emission rate ↑

active region confined to the waveguide defined by the refractive index

variation��index guided laser diode

buried DH with right dimensions compared with the

�only fundamental

mode can exist

�single mode laser

diode

λ of radiation

DH AlGaAs/GaAs LD

�~ 900 nm LD

DH InGaAsP/InP LD

�1.3/1.55 μm LD

S.O. Kasap, Optoelectronics and Photonics: Principles and Practices, Second Edition, © 2013 Pearson Education© 2013 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the

publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

LEFT: The laser cavity definitions and the output laser beam characteristics. RIGHT: Laser diode output beam astigmatism. The beam is elliptical, and is characterized by two angles and //.

Elementary Laser Characteristics

Prof. Wei-I Lee

Elementary Laser Diode Characteristics

Output Modes of LD

output spectrum depends on

1. optical gain curve of the active medium

2. nature of the optical resonator

L decides longitudinal mode separation

W & H decides lateral mode separation

with sufficiently small W & H

�only TEM00 lateral mode will exist

( longitudinal modes depends on L )

( from Kasap Ex. 4.5.1 : Number of laser modesdepends on how the cavity modes intersect the

optical gain curve. )

diffraction at the cavity ends

�laser beam divergence

( aperture ↓ �diffraction ↑)


Current Dependence of Power Spectrum

output spectrum depends on

(1) optical gain curve of the active medium, and

(2) nature of the optical resonator

output spectrum depends on pumping current

Ex. output spectrum from an index guided LD

low current � multimode

high current � single mode

spectrum of most gain guided LD remain multimode even at high

diode current

level




Output spectra of lasing emission from an index guided edge emitting LD. At sufficiently high diode currents corresponding to high optical power, the operation becomes single mode. (Note: Relative power scale applies to each spectrum individually and not between spectra.)


Temperature Dependence of Ith and λ

Tj ↑� Ith ↑

Tj ↑ � Eg ↓ , n ↑ , cavity length ↑ �λ0 ↑

in single mode LD :

when shift of peak gain causes mode

change to an adjacent longer λ mode

�mode hopping

to restrict mode hopping �� design the

device structure to keep modes sufficiently separated

Tj depends on

(1) ambient

temperature

(2) operation

current




Peak wavelength o vs. case temperature characteristics. (a) Mode hops in the output spectrum of a single mode LD. (b) Restricted mode hops and

none over the temperature range of interest (20 40 C ). (c) Output spectrum from a multimode LD.


Slope Efficiency

slope efficiency ηslope :

ηslopetypical < 1 W/A

ηslopePo = ( I – Ith )

Determines the optical power Po

of the output coherent

radiation in terms of diode current above the threshold Ith

Depends on LD structure as well as the semiconductor packaging

Conversion efficiency

gauges the overall efficiency of the

conversion from the input of electrical

power to the output of the optical power

Modern LDs as high as 30-40%




Output optical power vs. diode current at three different temperatures. The threshold current shifts to higher temperatures.

Ith = Aexp(T/To)



EXAMPLE: Laser output wavelength variation with temperatureThe refractive index n of GaAs is approximately 3.6 and it has a temperature dependence d n/dT 2.0 10-4 K-1. Estimate the change in the emitted wavelength at around 870 nm per degree change in the temperature for a given mode.

SolutionConsider a particular given mode with wavelength m,If we differentiate mwith respect to temperature,

where we neglected the change in the cavity length with temperature. Substituting for L/m in terms of m ,

0.048 nm K-1.

Note that we have used n for a passive cavity whereas n above should be the effective refractive index of the active cavity which will also depend on the optical gain of the medium, and hence its temperature dependence is likely to be somewhat higher than the d n/dT value we used.It is left as an exercise to show that the changes in m due to the expansion of the cavity length with temperature is much less than that arising from d n/dT. The linear expansion coefficient of GaAs is 6 10-6 K-1.

Lm m

n2

dTd

mLL

mdTd

dTd m nn 22

) K102(3.6

nm 870 1-4dTd

dTd mm n

n



Laser Diode Efficiencies

ththresholdaboveslope II

PIP oo

Slope Efficiency

thresholdabovecurrent input in Increasepoweroutput optical in Increase

slope




IEeP

eIhP

g

oo //

EQE

External Quantum Efficiency

secondunit per diode theinto electrons injected ofNumber secondunit per diode thefrom photonsoutput ofNumber

EQE




secondunit per diode into electrons injected ofnumber in Increasesecondunit per diode from photonsoutput ofnumber in Increase

EDQE

External Differential Quantum Efficiency

thslopeEDQE /

/II

PEe

he

eIhP o

g

o




nrr

r

/1/1/1

IQE

Internal Quantum Efficiency

secondunit per diode into electrons injected ofNumber secondunit per internally generated photons ofNumber

IDQE

r = Radiative recombination time

nr = Nonradiative recombination time




secondunit per diode into electrons injected ofnumber in Increasesecondunit per internally generated photons ofnumber in Increase

IDQE

Internal Differential Quantum Efficiency

If the current increases by I above threshold, increase in the injected electrons is I/e

The increase in the number of photons generated internally is then

IDQE × I/e



Laser Diode EfficienciesExtraction efficiency

=(Loss from the exit cavity end)

/ (Total loss)

EE = (1/2L)ln(1/R1) / t




Po = EEIDQEh(I Ith)/eslope = Po/ I = EEIDQEh/e

EDQE = (Po/ h I/e) = (Po/ h (I Ith)/e] = EEIDQE




Power Conversion Efficiency

eVE

IVP go

EQEPCE powerinput Electricalpoweroutput Optical

eVE

IVP go

EQEPCE




LD Po(mW)

(nm)

Ith

(mA)I(mA)

V(V)

// slope

(mW/mA)PCE

%Red 500 670 400 700 2.4 21 10 1.0 30Red 100 660 75 180 2.5 18 9 1.0 22Red 50 660 60 115 2.3 17 10 0.90 19Red 10 639 30 40 2.3 21 8 1.0 11Violet 400 405 160 390 5.0 45 15 1.7 21Violet 120 405 45 120 5.0 17 8 1.6 20Violet 10 405 26 35 4.8 19 8.5 1.1 6.0

Typical characteristics for a few selected red and violet commercial laser diodes. All LDs are MQW structures and have FP cavities. Violet lasers are based on InGaN/GaN MQW, and red

LEDs use mainly AlGaInP/GaInP MQW.




Typical values for the threshold current Ith, slope efficiency (slope ) and power conversion efficiency PCE) for 36 commercial red LDs with different optical output powers from 3 mW 500 mW.



EXAMPLE: Laser diode efficiencies for a sky blue LDConsider a 60 mW blue LD (Nichia SkyBlue NDS4113), emitting at a peak wavelength of 488 nm. The threshold current is 30 mA. At a forward current of 100 mA and a voltage of 5.6 V, the output power is 60 mW. Find the slope efficiency, PCE, EQE and EDQE.

SolutionFrom the definition in Eq. (4.12.2),

slope = Po / (I Ith)= (60 mW) / (100 30 mA) = 0.86 mW/mA-1

From Eq. (4.12.8), PCE isPCE = Po / IV

= (60 mW) / [(100 mA)(5.6 V)] = 0.11 or 11%We can find the EQE from Eq. (4.12.3) but we need h, which is hc/. In eV,

h (eV) = 1.24 / m= 1.24 /0.488 = 2.54 eV

EQE is given by Eq. (4.12.3)EQE = (Po /h) / (I /e )

= [(60×10-3)/(2.54×1.6×10-19)]/[(100×10-3) /(1.6×10-19)] = 0.24 or 24%



EXAMPLE: Laser diode efficiencies for a sky blue LDSolution (continued)

Similarly, EDQE is given by Eq. (4.12.4b) above threshold,

EDQE = (Po /h) / (I /e ) (Po /h) / [( I Ith )/e )]

= [(60×10-3)/(2.54×1.6×10-19)] / [(100×10-330×10-3)/1.6×10-19)]

= 0.34 or 34%

The EDQE is higher than the EQE because most injected electrons above Ith are used in stimulated recombinations. EQE gauges the total conversion efficiency from all the injected electrons brought by the current to coherent output photons. But, a portion of the current is used in pumping the gain medium.



Laser DiodeEquation

A highly simplified and idealized description of asemiconductor laser diode for deriving the LDequation. (a) The heterostructure laser diode structure.(b) The current I injects electrons in the conductionband, and these electrons recombine radiatively withthe holes in the active region. (c) The coherentradiation intensity across the device; only a fraction is within the active region where there is optical gain.(d) Injected electron concentration n and coherentradiation output power Po vs. diode current I. Thecurrent represents the pump rate.

Prof. Wei-I Lee

Steady State Semiconductor Rate Equations

Rate Equations and Laser Diode Equation I

Only half of the photons in the cavity would be moving towards the output face of the crystal at any instant

Only (1-R) fraction of the radiation power will escape

Δt = nL/c seconds for photon to cross the laser cavity length L

at steady state :

( neglecting nonradiative recombinations )

at I = Ith , n = nth, Nph ≈0

�

Ithτsp� nth = / edLW �

Consider DHLD under FB

Rate of electron injection by current I

= Rate of spontaneous emissions

+ Rate of stimulated emissions



Laser Diode Equation

phNCnτn

eLWdI

r

Rate of coherent photon loss in the cavity = Rate of stimulated emissionsph

ph

ph NN

Cn

Photon cavity lifetime

Radiative lifetime

Rate of electron injection by current I= Rate of spontaneous emissions + Rate of stimulated emissions




rτn

eLWdI thth Threshold

phth

1C

n Threshold

phthth NCnτn

eLWdI

r

)( thph

ph IIeLWd

N

Substitute back into steady state rate equation

Steady State Semiconductor Rate Equations

Rate Equations and Laser Diode Equation II

at steady state :

( τph : average time for a photon to be

lost due to transmission through the

end-faces, scattering and absorption )

� nth = 1 / C τph

�

�

( laser diode equation )

Rate of coherent photon loss in the cavity

= Rate of stimulated emissions




)( thph

ph IIeLWd

N

)1(energy)oton Volume)(Ph)(Cavity ( ph2

1

RN

t

Po

)(2

)1(th

ph2

IILe

hcPo

nR

)(2

)1(th

ph2

JJde

hco

nR

I



Threshold Gain

21th

1ln21

RRg

Lst

= Fraction of the coherent optical radiation within the active region

The gain g works on the radiation within the cavity, which means that we must multiply g with to account for less than perfect optical confinement



Optical Gain Curve

(a) The density of states and energy distribution of electrons and holes in the conduction and valence bands respectively at T > 0 in the SCL under forward bias such that EFn EFp > g.

Holes in the VB are empty states. (b) Gain vs. photon energy (h).

(b)(a)



Optical Gain Curve

Optical gain g vs. photon energy for an InGaAsP active layer (in a 1500 nm LD) as a function of injected carrier concentration n from 1×1018 to 3×1018 cm-3. (The model described in Leuthold et al, J. Appl. Phys.,87, 618, 2000 was used to find the gain spectra at different carrier concentrations.) (Data combined from J. Singh, Electronic and Optoelectronic Properties of Semiconductor Structures, Cambridge University Press,

203, p390; N.K. Dutta, J. Appl. Phys., 51, 6095, 1980; J. Leuthold et al, J. Appl. Phys., 87, 618, 2000.)



Optical Gain Curve

The dependence of the peak gain coefficient (maximum g) on the injected carrier concentration n for GaAs (860 nm), In0.72Ga0.28As0.6P0.4 (1300 nm), and In0.60Ga0.40As0.85P0.15 (1500 nm) active layers. (Data

combined from J. Singh, Electronic and Optoelectronic Properties of Semiconductor Structures, Cambridge University Press, 203, p390; N.K. Dutta, J. Appl. Phys., 51, 6095, 1980; J. Leuthold et al, J.

Appl. Phys., 87, 618, 2000.)



Semiconductor Optical Amplifiers

Covega semiconductor optical amplifier (SOA) (Courtesy of Thorlabs)




Covega semiconductor optical amplifier (SOA) for use as a

booster amplifier in the O-band (around 1285 nm). The small signal

gain is 27 dB and the NF is 7 dB (Courtesy of Thorlabs).

A Semiconductor Optical Amplifier (SOA) for use at 1050 nm.




A semiconductor optical amplifier for use in optical

communications (Courtesy of Amphotonix, Glasgow, UK)




(a) Traveling wave (TW) and (b) Fabry-Perot (FP) semiconductor optical amplifier (SOA). (c) Gain vs. output power for a 1500 nm TW SOA (GaInAsP). SOAs exhibit saturation at high output

powers; saturation lowers the gain. Higher gain (25 dB) is achieved by passing a higher current through the SOA. (Selected data used from T. Saitoh and T. Mukai, IEEE J. Quantum Electron.,

QE23, 1010, 1987)



TW SOATraveling wave (TW) semiconductor optical amplifier: The ends of the optical cavity have antireflection (AR) coatings so that the optical cavity does not act as an efficient optical resonator, a condition for laser-oscillations. Laser oscillations are therefore prevented. Light from an optical fiber is coupled into the active region of the laser structure. As the radiation propagates through the active layer, optically guided by this layer, it becomes amplified by the induced stimulated emissions, and leaves the optical cavity with a higher intensity. The device must be pumped to achieve optical gain (population inversion) in the active layer.

Random spontaneous emissions in the active layer feed “noise” into the signal and broaden the spectral width of the passing radiation. This can be overcome by using an optical filter at the output to allow the original light wavelength to pass through. Typically, such laser amplifiers are buried heterostructure devices and have optical gains of ~20 dB depending on the efficiency of the AR coating.



FP SOA

The FP SOA is basically a regenerative amplifier in which positive feedback; reflections back into the cavity from the end mirrors

Polarization dependent gain, Gp, is defined as the difference between maximum and minimum gain for different orientations of the optical field (TE and TM waves), and is quoted in dB



SOA: Characteristics

nm

Gain

dB

BW

nm

IF

mA

VF

V

Posat

dBm

NF

dB

Gp

dB

Comment

850 25 40 150 2.2 8 7 Pigtailed (fiber coupled)a

1310 22 45 250 ~2 10 7 0.5 Pigtailed, inline amplifierb

1550 20 35 200 ~2 11 6 0.5 Pigtailed preamplifierc

Various properties of three selected TW SOAs. BW is the bandwidth of the amplifier, wavelengths over which it amplifies the optical signal, IF and VF are the forward current and voltage under normal operation with the given gain, Gp is the polarization dependent gain.

aSuperlum Diodes, SOA-372-850-SM; bInphenix, IPSAD1301; cAmphotonix (Kamelian) C-band, preamp.

Light Emitters For Optical Fiber Communications

Light Emitters for Optical Fiber Communications

LED adv. : simpler to drive,

more economic,

longer lifetime

disadv. : wider output spectrum,

less power

� usually used with multimode

graded index fibers for short haul appl.

LD adv. : narrow linewidth, high output

� wide bandwidth long haul appl.

rise time : the time

for light output to

rise from 10% to

90% of the final

value ( with a step

input )

power

laser modified 20012015

Documents

laser diodereflectors

laser cavity definitions

algaas dh laser lattice

active region photon

cavity optical gain

emission rate active

gth optical gain

active layer ith narrow