Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 1

J.H. Abeles and V.B. KhalfinPhotonic ICs and Components Organization

Sarnoff Corporation201 Washington Rd., Princeton, NJ 08543 USA

jabeles@sarnoff.com, (609) 734-2571

High Average Power Solid-state Laser (HAPSL)

DARPA/MTO Diode Pump WorkshopDr. M. Stickley, Program Manager

Booz, Allen & Hamilton, 3811 Fairfax Dr., Arlington, VA 22203

June 27, 2001

Considerations for Realization of High Power Conversion Efficiency in

Diode Lasers

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 2

Lexicon

• Power Conversion Efficiency

• Potential Defect

• Non-ohmic Resistance

• Series Resistance = Ohmic Contact Resistance + Non-Ohmic Resistance*

• Voltage = Bandgap + Potential Defect+ Current Series Resistance

V = Eg +Ud + I Rs

* ”Non-Ohmic” in “Non-Ohmic Resistance” signifies that

resistance does not scale inversely with area;is not constant with voltage

… with area it goes down, at most, only logarithmically …

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 3

The Sarnoff Perspective

• 1960s & 70s Basic Materials

• 1980s-1994 CDH-LOC; CSP MIC Arrays (PILOT)

• 1989-95 MOPA Tapered Amplifier MOPA

• 1994-2000 BW

RCA/Sarnoff and SRI/Sarnoff Pioneers Have Set Standards for Laser Performance at Wavelengths from 0.8 to 3 m

A Brief History of Laser Diode Materials

First Era (1964-1982):Increase Temperature

Second Era (1982-2000):Increase Power

Third Era (2000-2018?):Increase Efficiency

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 4

D. Garbuzov, R. Menna, A. Komissarov, M. Maiorov,V. Khalfin, A. Tsekoun, S. Todorov and J. Connolly, OFC ‘01

Air Force Funding for High Power Lasers Recent Result: 1000 mW Fabry-Perot (Sarnoff & Princeton Lightwave)

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 5

DARPA Support for High Power DFB Lasers (1996):Recent Result: 400 mW (Sarnoff & Princeton Lightwave)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.0

0.1

0.2

0.3

0.4

0.5

1550 1552-60

-50

-40

-30

-20 440 mW

Sig

na

l, d

BWavelength, nm

power in fiber

laser output power

coupling efficiency

TC = 18oC, CWL = 2 mm

In p-side down

Ou

tpu

t P

ow

er,

W

Current, A

60

65

70

75

80

85

90

Co

up

ling

Eff

icie

ncy

, %

R. Menna, A. Komissarov, M. Maiorov, V. Khalfin, L. DiMarco, J. Connolly and D. Garbuzov, CLEO ‘01

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 6

STANDARD DESIGN

p-cladding

SCH

3 QW, 2 barriers

SCH

n-cladding

transverse optical mode

Conduction Band

BROADENED WAVEGUIDE

Broadened Waveguide Lowers Cladding-Layer Losses

• Broadened Waveguide Design:Broadened Waveguide Design:— Significantly reduces optical losses Significantly reduces optical losses — Increases slope efficiencyIncreases slope efficiency

• Example of the Sarnoff Track Record Example of the Sarnoff Track Record Innovating New Paradigms for Laser Performance OptimizationInnovating New Paradigms for Laser Performance Optimization

Example: Broadened Waveguide Design ... Garbuzov, Abeles, Connolly U.S. Patent 5,818,860 ...

5% 5%

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 7

Stripe Lasers Are The Baseline… in the conventional wisdom of Eras 1 & 2 …

• Stripe Lasers Offer Minimization of interfaces Two facets Wide stripe

• Arrays Stabilization of wide stripes by trenches, surface implantation VCSELs

N.B.: Ebeling et al., Novalux, etc.

• Era 3: Series of gain regions Design option

to improve efficiency

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 8

Year 2002 State-of-the-Art in

Power Conversion Efficiency: 65%*

• Internal losses > 1 cm-1

— 1.18 Watts/Amp @9 40 nm— Differential quantum efficiency >90%

• Voltage = Bandgap + 10 kT/q = 1.32 + 0.25 = 1.57 Volts

• Contact Specific Resistance = 5 x 10-5 -cm2

• These factors account for the observed maximum of ~65%

* Fraunhofer Institute (Mikula et al.) “nearly 60%” {26th Int’l Symposium on Compound Semiconductors, 1999}

Ferdinand Braun Institute (Erbert et al.) “about 60%” {IEEE JSTQE 7(2), 143, 2001}

Ioffe Physicotechnical Institute (Mikhrin et al.) “conversion efficiency of 59%” {Semiconductor Science & Tech 15(11), 1061, 2000}

Lebedev Phys. Institute (Basov et al.) “maximum level of 75%” {25th European Conf. on Laser Interaction with Matter 1998}

Semiconductor Laser Inc. (Wang et al.) “65.5% maximum” {SPIE 3419, 377, 1998}

OptoPower (He et al) “efficiency as high as 59%” {Electronics Letts. 34(22), 2126, 1998}

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 9

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.00 5.00 10.00 15.00 20.00 25.00

current (amperes)

po

we

r co

nve

rsio

n e

ffic

ien

cy (

%)

(a)(b)(c)(d)

(e)

Ideal Laser PCE* = 100%; Real Laser PCE = 65% *Power Conversion Efficiency

J0 = 50 A/cm2

G0 = 10 cm-1

int = 100%T = 300° K r1 = 100%r2 = 3%

L = 0.10 cmw = 0.01 cm = 0.94 mint = 0 cm-1

Vb = 0 VoltsR = 0 -cm2

int = 1 cm-1

Vb = 0 VoltsR = 0 -cm2

int = 0 cm-1

Vb = 0.25 VoltsR = 0 -cm2

int = 1 cm-1

Vb = 0.25 VoltsR = 0 -cm2

int = 1 cm-1

Vb = .25 VoltsR = 5E-5 -cm2

(a)

(b)

(c)

(d)

(e)

Calculation assumes

G = G0 ln(J/J0) = mirror + int

(logarithmic gain);

mirror = (0.5/L) ln(1/(r1r2));

ext = int (mirror / (mirror + int))

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 10

Potential Defect: A Challenge… and the opportunity ...

• A 0.25 volt potential defect represents only 10 kT at 300° K

• But it brings maximum wallplug efficiency from 100% to 85%

• Barrier cannot be eliminatedConfinement of minority carriersWaveguiding

• Barrier is enhanced in high power broadened waveguide (owing to ambipolar diffusion effect)

• Interesting: A shorter wavelength laser (e.g., 400 nm) may permit greater efficiency -- owing to the fact that the potential defect is expressed in kT which is 0.025 eV regardless of .`

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 11

Three Sources of Potential Defect

Electron thermionic + tunnelingcurrent

Hole thermionic + tunnelingcurrent

~1.32 eV

electron quasi-Fermi level

holequasi-Fermilevel

Salient features:

1. In waveguide, ambipolar diffusion @ ~200 cm2/sec(n = p = ~3E17 cm-3 at extremum)contributes to voltage defect

2. Homobarriers at waveguide edges(provide index guide and barrier to escape of minority carriers)contributes to voltage defect

3. Quantum well states requires several kT of confinementcontributes to voltage defect

Jun. 27, 2002 HAPSL Workshop: Sarnoff Presentation (6-27-02) 12

Conclusions

• Achieving 85% power conversion efficiency at 940 nm can only be achieved by fundamental new advances in laser design lower losses alone won’t do the job lower contact resistance alone won’t do the job as long as voltage defect is 10 kT,

maximum theoretical efficiency is only ~82% N.B.: at 380 nm could achieve 90%

• Any successful approach must optimize transport of carriers into the quantum well while preserving other necessary features Systematic Design for Minimization of barriers? Material which provides no barrier to majority carriers but

high barrier for minority carriers (c.b./v.b. offset engineering) Bandgap engineering (miniband as in q. cascade)? The third dimension (lateral current injection)? Matter waves?