> µ ò · 2017. 3. 14. · WK WK 3 KY 3 KY 7 7, , T , , T K K G H[S G 7 7 7K K K § · ¨ ¸¨ ... Microsoft PowerPoint - Lecture 6 Author: cheng wang Created Date: 3/14/2017 9:44:41

Lecture 6 1

Characterization of semiconductor lasers

Internal loss and quantum efficiency 2

The internal loss αi and quantum efficiency ηi can be determined by measuring

the external differential quantum efficiency ηd .

This can be realized by measuring identical lasers of different cavity lengths.

1

1ln

1ln

i

d

R

L

1

1

2

2

1ln

1ln

1ln

i

i

d

i

LR

R

LR

2 1

1 1 2 2

1 21 2

1 1 2 2

1lnd d

id d

i d dd d

L L R

L L

L L

Internal loss and quantum efficiency 3

However, it is more reliable to determine the unknowns by fitting a curve to a

number of measured data points.

1 1

ln(1/ )i

d i i

LR

Material gain 4

Once the internal loss is found, the modal gain versus current density can be

determined from the threshold gain values of different cavity lengths.

1 1lnth ig

L R

i th

th

IJ

wL

0 lntr

Jg g

J

P77, example 2.4

The internal parameters for

VCSELs can be obtained from

the diagnostic in-plane lasers

Wall-plug efficiency 5

Wall-plug efficiency: the output optical power relative to the electrical power

0

2

in

in s d s

P

P

P I R IV IV

R is the series resistance Rs is the series resistance

Vs is a current independent series voltage

Vd is the ideal diode voltage, which is equal to the

quasi-Fermi level separation. This voltage is clamped

at its threshold value above threshold.

Heat flow 6

The temperature rise is 0

is the thermal impedance

D T

D in

T

T P Z

P P P

Z

(1-D flow)

T

hZ

A ln(4 / )

(line: w<<h<w )

T

s

h wZ

l

1

2

(small disk)

TZ s

ξ is the thermal conductivity, it is 0.45 for GaAs, and 0.68 for InP.

Temperature dependence of laser output 7

With the increased temperature, the threshold usually increases, while the output

power and the differential quantum efficiency decreases.

Temperature dependence of threshold 8

The threshold increases due to

The gain coefficient g0 decreases;

The internal loss increases;

The transparency carrier density increases;

The Auger recombination increases;

The carrier leakage increases;

0

1

C exp( )

exp( )

i

tr

C

l l

gTT

N T

T

R T

The carrier leakage increases;

The internal efficiency decreases;

exp( )l lR T

2 3

0 0

( ) ( )exp 2 exp 3i m i m

th tr tri

qVI BN CN

g g

Temperature dependence of threshold 9

The relation between the threshold and the temperature is given by

0 0exp( / )thI I T T

T0 is the characteristic temperature in K,

a larger T0 indicates a weaker dependence

on the temperature

1 1 2

2 0

expth

th

I T T

I T

on the temperature

0

th thdI I

dT T

Differential quantum efficiency dependence 10

For a fixed output power, the required bias current is also temperature

dependent, but not as strong as the threshold current.

0 exp( / )th pI I I T T

1 1 2 21 1 2 2

/ ( ) / ( ); ;

( ) / ( ) /d dth th

P hv P hvT T

I I q I I q

1 2 1

2

exp ;d

d

T T

T

2d

Temperature dependence 11

P82, example 2.5

Derivative analysis 12

The derivative helps to identify the threshold current and the differential efficiency.

The kinks in the L-I curve indicates a switching between lateral or axial modes, or an additional parasitic mirror in the device.

Premature saturation of output power indicates current leakage paths, or excessive heating of the gain materialheating of the gain material

The slope of the IdV/dI-I curve provides the value of the series resistance R.

Because the voltage across the junction (Vd) clamps at threshold, there is a kink in the derivative of IdV/dI.

(Below threshold)

dV nkTI IRdI q

(Above threshold)

dVI IRdI

L-I and V-I curves 13