Laser Diode Efficiencies Slope Efficiency. Laser Diode Efficiencies External Quantum Efficiency.

Laser Diode Efficiencies

ththresholdaboveslope II

P

I

P oo

Slope Efficiency

thresholdabovecurrent input in Increase

poweroutput optical in Increaseslope


IE

eP

eI

hP

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 Increase

secondunit per diode from photonsoutput ofnumber in IncreaseEDQE

External Differential Quantum Efficiency

thslopeEDQE /

/

II

P

E

e

h

e

eI

hP o

g

o


nrr

r

/1/1

/1IQE

Internal Quantum Efficiency

secondunit per diode into electrons injected ofNumber

secondunit per internally generated photons ofNumber IDQE

tr = Radiative recombination time

tnr = Nonradiative recombination time


secondunit per diode into electrons injected ofnumber in Increase

secondunit per internally generated photons ofnumber in IncreaseIDQE

Internal Differential Quantum Efficiency

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

The increase in the number of photons generated internally is then

hIDQE × DI/e


Extraction efficiency=

(Loss from the exit cavity end) / (Total loss)

hEE = (1/2L)ln(1/R1) / at


Po = hEEhIDQEhu(I - Ith)/e

hslope = DPo/ DI = hEEhIDQEhu/e

hEDQE = (DPo/ hu) / (DI/e)

= (Po/ hu) / [(I - Ith)/e] = hEEhIDQE


Power Conversion Efficiency

eV

E

IV

P goEQEPCE powerinput Electrical

poweroutput Optical

eV

E

IV

P goEQEPCE


LD Po (mW)

l(nm)

Ith

(mA)

I

(mA)

V

(V)

q q// hslope

(mW/mA)

hPCE

%

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 (hslope ) and power conversion efficiency (hPCE) 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),

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

From Eq. (4.12.8), PCE ishPCE = 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 hu, which is hc/l. In eV,h u (eV) = 1.24 / l (mm) = 1.24 /0.488 = 2.54 eV

EQE is given by Eq. (4.12.3)hEQE = (Po /hu) / (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 LD

Solution (continued)

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

hEDQE = (DPo /hu) / (DI /e ) (Po /hu) / [( I - Ith )/e )]

= [(60×10-3)/(2.54×1.6×10-19)] / [(100×10-3-30×10-3)/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.

EXAMPLE: Laser diode efficienciesConsider an InGaAs FP semiconductor laser diode that emits CW radiation at 1310 nm. The cavity length (L) is 200 mm. The internal loss coefficient as = 20 cm-1, R1 = R3 0.33 (cleaved ends). Assume that internal differential quantum efficiency, IDQE, is close to 1. The threshold current is 5 mA. What is the output power Po at I = 20 mA? The forward voltage is about 1.3 V. What is the EDQE and conversion efficiency?

SolutionFrom the definition of IDQE in Eq. (4.12.6), the number of internal coherent photons generated per second above threshold is hIDQE(I - Ith)/e. Thus,

Internal optical power generated = hu ×hIDQE(I - Ith)/e

The extraction efficiency hEE then couples a portion of this optical power into the output radiation. The output power Po is then hEE × hu ×hIDQE(I - Ith)/e. Thus,

Po = hEEhIDQEhu(I - Ith)/e Output power vs current (4.12.9)

The slope efficiency from Eq.(4.12.2) ishslope = DPo / DI = hEEhIDQE(hu/e ) Slope efficiency (4.12.10)

EXAMPLE .3: Laser diode efficienciesSolution (continued)

Further, from the definition of EDQE and Eq.(4.12.9) is

hEDQE = (DPo / hu) / (DI /e) = (Po / hu) / [(I - Ith)/e] = hEEhIDQE

External differential quantum efficiency (4.12.11)

We can now calculate the quantities needed. The total loss coefficient is

at = as + (1/2L) ln (1 / R1R2)

=2000 + (2×200×10-6)-1 ln(0.33×0.33)-1 = 7543 m-1

The extraction efficiency is

hEE = (1/2L)ln (1/R1) / at = (2×200×10-6)-1 ln(1/0.33) / (7543) = 0.37 or 37%

Thus, using I = 20 mA in Eq. (4.12.9),

Po = (0.37)(1)[(6.62×10-34)(3×108)/(1310×10-9)][(0.02 - 0.005) / (1.6×10-

19)] = 5.2 mW

The slope efficiency from Eq. (4.12.10) is

hslope = DPo / DI = (5.2 mW - 0) / (20 mA - 5 mA) = 0.35 mW mA-1

The EDQE from Eq. (4.12.11) is

hEDQE = hEEhIDQE = 0.37 or 37%

The power conversion efficiency hPCE = Po / IV = 5.2 mW / (20 mA × 1.3 V)

= 0.20 or 20%

EXAMPLE: Laser diode efficienciesSolution (continued)

Laser DiodeEquation

A highly simplified and idealized description of a semiconductor laser diode for deriving the LD equation. (a) The heterostructure laser diode structure. (b) The current I injects electrons in the conduction band, and these electrons recombine radiatively with the holes in the active region. (c) The coherent radiation intensity across the device; only a fraction G is within the active region where there is optical gain. (d) Injected electron concentration n and coherent radiation output power Po vs. diode current I. The

current represents the pump rate.

Laser Diode Equation

phNCnτ

n

eLWd

I

r

Rate of coherent photon loss in the cavity

= Rate of stimulated emissionsphph

ph NN

Cn

Photon cavity lifetime

Radiative lifetime

Rate of electron injection by current I

= Rate of spontaneous emissions

+ Rate of stimulated emissions


rτ

n

eLWd

I thth Threshold

phth

1

Cn Threshold

phthth NCn

τ

n

eLWd

I

r

)( thph

ph IIeLWd

N

Substitute back into steady state rate equation


)( thph

ph IIeLWd

N

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

1

RN

t

Po

)(2

)1(th

ph2

IILe

hcPo

n

R

)(2

)1(th

ph2

JJde

hco

n

RI

Light intensity = Po / A

Threshold Gain

21th

1ln

2

1

RRg

Lst

G= 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 G 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 > Eg.

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

(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.)

EXAMPLE: Threshold current and optical output power from a Fabry-Perot (FP) heterostructure laser diodeConsider GaAs DH laser diode that lases at 860 nm. It has an active layer (cavity) length L of 250 mm. The active layer thickness d is 0.15 mm and the width W is 5 mm. The refractive index is 3.6, and the attenuation coefficient as inside the cavity is 103 m-1. The required threshold gain gth corresponds to a threshold carrier concentration nth 2×1018 cm-3. The radiative lifetime tr in the active region can be found (at least approximately) by using tr = 1/Bnth, where B is the direct recombination coefficient, and assuming strong injection as will be the case for laser diodes [see Eq. (3.8.7) in Chapter 3]. For GaAs, B 2×10-16 m3 s-1. What is the threshold current density and threshold current? Find the output optical power at I = 1.5Ith, and the external slope efficiency hslope. How would G = 0.5 affect the calculations?

SolutionThe reflectances at the each end are the same (we assume no other thin film coating on the ends of the cavity) so that R = (n-1)2/ (n+1)2 = 0.32. The total attenuation coefficient at and hence the threshold gain gth, assuming G = 1 in Eq. (4.13.9), is

= 55.6 cm-1

From Figure 4.48(b), at this gain of 56 cm-1, nth 2×1018 cm-3. This is the threshold carrier concentration that gives the right gain under ideal optical confinement, with G = 1.

)32.0)(32.0(

1ln

)cm102502(

1)cm10(

41

th tg

EXAMPLE: Threshold current and optical output power from a Fabry-Perot (FP) heterostructure laser diodeSolution (continued)

The radiative lifetime tr = 1/Bnth = 1/[2×10-16 m3 s-1)( (2×1024 m-3)] = 2.5 ns

Since J = I/WL, the threshold current density from Eq. (4.13.4) is

=

= 1.9×107 A m-2 or 1.9 kA cm-2 or 19 A mm-2.The threshold current itself is,

Ith = (WL)Jth = (5×10-6 m) (250×10-6 m)(1.9×107 A m-2) = 0.024 A or 24 mA

The photon cavity lifetime depends on at, and is given bytph = n/(cat) = 3.6 / [(3×108 m s-1)(5.56×103 m-1)]

= 2.16 psThe laser diode output power is

rτ

ednJ th

th )s105.2(

)m1015.0)(C106.1)(m102(9

619324

)()10250)(10860)(6.3)(106.1(2

)32.01)(1016.2()103)(10626.6()(

2

)1(th6919

122834

thph

2

IIIILe

hcPo

n

R


That is Po = (0.35 W A-1)(I – Ith ) = (0.35 mW mA-1)(I – 24 mA)When I = 1.5Ith = 36 mA,

Po = (0.35 mW mA-1)(36 mA-24 mA) = 4.2 mWThe slope efficiency is the slope of the Po vs. I characteristic above Ith,

= 0.35 mW mA-1

We can now repeat the problem say for G = 0.5, which would give Ggth = at, so that gth = 55.6 cm-1 / 0.5 = 111 cm-1. From Figure 4.48 (b), at this gain of 111 cm-1, nth 2.5×1018 cm-3. The new radiative lifetime,

tr = 1/Bnth = 1/[2.0×10-16 m3 s-1)(2.5×1024 m-3)] = 2.0 nsThe corresponding threshold current density is

Jth = nthed/tr = (2.5×1024 m-3)(1.6×10-19 C)(0.15×10-6 m)/(2.0×10-9 s) = 30 A mm-2

and the corresponding threshold current Ith is 37.5 mA

Le

hc

I

Po

n

R

2

)1(ph2

slope


There are several important notes to this problem

• First, the threshold concentration nth 2×1018 cm-3 was obtained graphically from Figure 4.48 (b) by using the gth value we need.

• Second is that, at best, the calculations represent rough values since we also need to know how the mode spreads into the cladding where there is no gain but absorption and, in addition, what fraction of the current is lost to nonradiative recombination processes. We can increase as to account for absorption in the cladding, which would result in a higher gth, larger nth and greater Ith. If tnr is the nonradiative lifetime, we can replace tr by an effective recombination time t such that , which means that the threshold current will again be larger. We would also need to reduce the optical output power since some of the injected electrons are now used in nonradiative transitions.

• Third, is the low slope efficiency compared with commercial LDs. hslope depends on tph, the photon cavity lifetime, which can be greatly improved by using better reflectors at the cavity ends, e.g. ,by using thin film coating on the crystal facets to increase R.

111 nrr

Distributed Bragg Reflector (DBR) LDs

(a) The basic principle of the Distributed Bragg Reflection (DBR) laser. (b) Partially reflected waves at the corrugations can only constitute a reflected

wave when the wavelength satisfies the Bragg condition. Reflected waves A and B interfere constructively when q(lB/n) = 2L. (c) Typical output

spectrum. SMSR is the side mode suppression ratio.

Distributed Feedback (DFB) LDs

LEFT: Distributed feedback (DFB) laser structure. The mode field diameter is normally larger than the active layer thickness and the radiation spreads into the guiding layer.

RIGHT: There are left and right propagating waves, partial reflections from the corrugation, and optical amplification within the cavity, which has both the active layer and the guiding layer.

Distributed Feedback (DFB) LDs

LEFT: Ideal lasing emission output has two primary peaks above and below lB. RIGHT: Typical output spectrum from a DFB laser has a single narrow peak with a dl typically very narrow, and much less than 0.1 nm

)1(2

2

mL

BBm n

Distributed Bragg Reflector (DBR) and Distributed Feedback (DFB) LDs

LD l0(nm)

du, dl

SMSR

dB

Po

mW

I

mA

hslope

mA

Comment

DBRa 1063 2 MHz,8 fm

45 80 200 0.8 GaAs DBR LD for spectroscopy and metrology, includes monitor current, TEC and thermistor.

DFBb 1063 2 MHz,8 fm

45 80 190 0.2 GaAs DFB LD for spectroscopy and metrology, includes monitor current, TEC and thermistor

DFBc 1550 10 MHz,0.08 pm

45 40 300 0.3 Pigtailed to a fiber, includes monitor current, TEC and thermistor. CW output for external modulation. For use in long haul DWDM.

DFBd 1653 0.1 nm 35 5 30 0.23 Pigtailed to a single mode fiber, includes monitor current, TEC and thermistor. Mainly for fiber optic sensing.

ECe 1550 50 kHz;0.4 fm

45 40 300 0.2 Pigtailed. Tunable over Du = 3 GHz. Mainly for communications

Selected properties of DBR, DFB and external cavity (EC) laser diodes Note: fm is 10-15 s; du and dl are spectral widths (FWHM). SMSR is the side

mode suppression ratio, TEC is a thermoelectric cooler

aEagleyard, EYP-DBR-1080-00080-2000-TOC03-0000; bEagleyard, EYP-DFB-1083-00080-1500-TOC03-0000; cFurukawa-Fitel, FOL15DCWD; dInphenix, IPDFD1602; eCovega SFL1550S, marketed by Thorlabs.

Example: DFB LD wavelengthConsider a DFB laser that has a corrugation period L of 0.22 mm and a grating length of 400 mm. Suppose that the effective refractive index of the medium is 3.5. Assuming a first order grating, calculate the Bragg wavelength, the mode wavelengths and their separation.

SolutionThe Bragg wavelength is

= 1.5400 mm.

and the symmetric mode wavelengths about lB are

so that the m = 0 mode wavelengths are l0 = 1.53915 or 1.54085 mm.

The two are separated by 0.0017 mm, or 1.7 nm. Due to a design asymmetry, only one mode will appear in the output and for most practical purposes the mode wavelength can be taken as lB. Note: The wavelength calculation was kept to five decimal places because lm is very close to lB..

1

μm)(3.5) 22.0(22

qB

n

2 2(1.5400 μm)( 1) 1.5400 (0 1)

2 2(3.5)(400 μm)B

m B mL

n

External Cavity Laser Diodes (ECLD)

A simplified diagram of an external cavity diode laser (ECDL), which uses an angled interference filter (IF) to select the wavelength lo (depends on the angle of the IF), and the

optical cavity has a GRIN lens with one end coated for full reflection back to the LD. The output is taken from the left facet of the LD.

External Cavity Laser Diodes (ECLD)

LEFT: A commercial external cavity diode laser, based on the principle shown on the right. (US Patent 6,556,599, Bookham Technology). The output is a single mode at 785 nm ( 1.5 pm) with a linewidth less than 200 kHz, and coupled into a fiber. The output power is 35 mW, and the SMSR is 50 dB. (ECDL, SWL-7513-P. Courtesy of Newport, USA)

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Laser Diode Efficiencies Slope Efficiency. Laser Diode Efficiencies External Quantum Efficiency.

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