Pump Sources and Related Devices for High-Power Fiber Laser Systems

Pump Sources and Related Devices for High-Power Fiber Laser Systems Christoph S. Harder ETH Zürich, SWITZERLAND charder@swissonline.ch; +41 79 219 9051 OAA, Whistler Mountain, June 25, 2006

Overview Fiber Laser MOPA: Seed laser and amplifier Seed Lasers Switching speed 2 Pump Diode Yb fiber wavelength bands Diode heat removal Pump power injection Cost Reliability Diode reliability Fiber reliability System robustness Acknowledgement: B. Schmidt (Bookham), T. Strite (JDSU), K. Eberl (Lumics)

Fiber Laser: MOPA Pump Diode Dual Clad Yb PMF MM PMF 3 Seed laser Pump Diode Seed laser Fiber laser: Good spectral control Need external modulators (Lithium Niobate waveguide, etc) Diode laser: Excellent dynamic control FP laser have poor spectral control, of no concern DFB have excellent spectral and dynamic control Pumplaser Single broad area MM diode Barstack

1060nm seed laser Pump laser diodes have excellent dynamic characteristics in standard pump butterfly package www.bookham.com

1W 1064nm SEED-laser module 2.2mm Laser Ex1947 / Puls Reponse @ 100mA Bias 0.018 0.016 Data 0.014 Power (a.u.) 0.012 0.010 0.008 0.006 0.004 Simulation: fg (3dB): ~400MHz Measured data Damped RO 0.002 0.000 0.00E+00 2.00E-09 4.00E-09 6.00E-09 8.00E-09 1.00E-08 Time (s) 5ns Also available with external stabilization and khz bandwidth

Seed Lasers 1060nm DFB 2005, C. E. Zah et al, Corning DFB has stable, narrow spectrum and can be switched on and off at high speeds 6

Yb fiber wavelength bands 7 Yb: Glass fiber absorption and emission spectrum Wide pump band: 870nm to 980nm Red band (976nm): Highest absorption, narrow width Preferred for high gain amplifiers and q-switched lasers with short fiber (SBS) Pump diode challenge: Diode wavelength control (+/-2nm) necessary Blue band (915nm): Good absorption, wideband Preferred for lower power stage Pump diode requirement: Good reliability at 920nm. Possible Green band (940nm..960nm): Lowest absorption, wideband, high optical conversion Preferred for very high power stage Easiest for pump diode

Yb fiber wavelength bands 976nm band Single mode pump diode external grating stabilization Established acceptance (reliability) through tremendous effort 8 optical power (db) 0-10 -20-30 -40-50 T=-10 o C wavelength (nm ) 980.8 980.6 980.4 980.2 980.0 0 20 40 60 80 100 tem perature ( o C) 35 nm T=80 o C 960 970 980 990 1000 1010 1020 wavelength (nm) 2001, Bookham S. Mohrdiek et al. Pump diode wavelength temperature shift Free running FP diode: 0.3nm/K DFB: 0.1nm/K -> External grating for +/- 2nm stability MM external stabilization: Very active field and good progress Nevertheless: Very challenging locking range, efficiency, noise and reliability Cost ->976nm band: Most likely limited for very special applications

Yb fiber wavelength bands 920nm band SES8-9xx-01: Pump Diode Challenge: Reliability Reliability assessment at 920nm Feared: COMD degradation at high operating power COD level versus time @ 915nm wavelength Stable COD level at around 22A injection current (in average) equivalent to 17.5W out of a ~90um aperture (over 2000h test time at 9A, 25 C) 2006, Bookham, B. Schmidt et al. 920nm Reliability Challenge solved through facet passivation techniques 9 -> 920nm Band: Preferred for high absorption cross section applications

Diode heat removal Diode power conversion Pout I V I V P Ith I 1 I I I leak Bandgap discontinuities 1. Thermal and vertical leakage 2. Injection barriers V 1 1 ev 1 I E h E E R R f ev Fh Fe V sh se 10 P 1 S f 1 ln( Rb ) L 2 ln( R ) ln( R ) f f Doping levels 1. Series resistance 2. Mainly free carrier absorption Material limits: Even after optimized mirror losses (S f, R f, R b ) and low threshold current. Due to limited mobility and carrier mass there are always trade-offs in doping levels (series resistance R s vs free carrier absorption) and Bandgap discontinuities (leakage losses vs injection barriers) Today s approach: InGaAlAs material system Asymmetric (thin p-region), low aluminum, low confinement LOC, low doping levels Holes have poor conductivity and high free carrier losses. Relatively low barriers for high mobility and good injection (some thermal and vertical leakage)

Diode power conversion light output power (W) 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Light output power Wall-plug Efficiency Differential Quantum Efficiency 0 1 2 3 4 5 6 7 8 9 10 11 12 injection current (A) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Efficiency CW-operation = 920 nm Bookham: SES8-9xx-01 Product >12 W @ 12 A in CW-operation @ 20 C ~67% maximum wall-plug efficiency (~65% @ 9 W) which results in 50-57% overall wall plug efficiency out of the module www.bookham.com

Diode power conversion Device Efficiency of Similar Structures at 25C, 940nm Dashed = Commecially Available Solid Red = SHEDS Design 10 80% 8 70% Output Power (W) 6 4 60% 50% Power Conv. Eff. 2 40% 0 0 2 4 6 8 10 30% Drive Current (A) Research funded by DARPA SHEDS program promises power conversion efficiency improvements in future fiber laser pumps 12 2006 JDSU. All rights reserved.

Diode heat removal Single Emitter: Heat spreading Ball Golddraht Soldered to expansion matched materials for heat spreading Mechanical pressure thermal contact to copper heatsink Chip Submount Wedge AuSn AlN CuW Cu Water CTE match CTE mismatch 13

Telecom technology Technology: Monolithic optical platform (AlN) with Laser diode, soldered with AuSn Fiber tip attached to monlolithic optical platform Monitor diode and thermistor Performance Very stable laser facet/fiber tip fixture Small size and low cost www.bookham.com

MM Uncooled Module with >14W 16 10 C 14 E x -F ib re P o w e r (W ) 12 10 8 6 45 C 25 C 4 Record Performance: >14W @ 18A and 10 C T hs Standard MU package Module fully qualified MSA with EM4 For 100um pigtail with NA=0.22 or NA=0.15 (same performance) 2 0 0 2 4 6 8 10 12 14 16 18 20 injection current (A) www.bookham.com

Pump power injection Coaxial dual cladding Coaxial cladding 400um, NA=0.46: 150 000 modes 16

Pump power injection MM Beam divergence Intensity (a.u) Intensity (a.u.) NA=0.15 NA=0.2 9A 7A 5A 17-30 -20-10 0 10 20 30 lateral far field angle ( ) Stable beam Vertical: 0.3mm mrad: Single lateral Mode Lateral: 7.5mm mrad: 25 lateral modes Overall Beamquality: 25 modes Coupled in 100um NA=0.2 fiber: 2000modes (10mm mrad) -70-50 -30-10 10 30 50 70 vertical far field angle ( )

Pump power injection Brightness-Power Diagram 18 Number of lateral modes ' 1'000'000 100'000 10'000 1'000 Brightness Power Diagram 100 10 Fiber Laser Output 1 1 10 100 Power [Watt] 400um, NA=0.22 200um, NA=0.22 100um, NA=0.22 BA100, NA=0.2, 8W Active fiber cladding 400um NA=0.46 Pump pigtail 100um, NA=0.2 Pump laser BA100, NA=0.2 Assume: 90% Pump Diode to pigtail ce 90% Fiber combiner ce 75% Fiber laser ce

Fiber combiner (6+1)*1, (2+1)*1 Free space combiner 19 (6+1)*1: Fused (2+1)*1: Proximity Fiber combiner of pigtailed single emitters Space multiplexer for signal/pump separation Free space combiner of pump diode stack Wavelength multiplexer for signal/pump separation

Pump power injection Fiber combiner Brightness Power Diagram 20 Number of lateral modes ' 1'000'000 100'000 10'000 1'000 100 10 1 1 10 100 1000 Power [Watt] BA100, NA=0.2, 8W 6 BA100, NA=0.2, 8W 21 BA100, NA=0.2, 8W Fiber combiner modal window gets smaller with increased bundle size

Pump power injection Polarisation combiner, both side pumping Brightness Power Diagram 21 Number of lateral modes ' 1'000'000 100'000 10'000 1'000 100 10 1 1 10 100 1000 Power [Watt] 21 BA100, NA=0.2, 8W 21 BA100, NA=0.2, 8W, *2 Pol *Both sides To inject 84 pump diodes through 21 fiber bundle: polarisation combining in pump diode package and co and counter propagating scheme

Pump power injection Series and parallel addition of fiber amplifiers Single stage ~ Scaling: Cost proportional to power Serial ~ Parallel ~ 22

Pump power injection Pump Diode: $/Watt Ultimately 10$/W as a goal for very large volume Still need factor of approximately 5 from today (to be a good business by itself) Learning experience from telecom pumps to reduce cost Large fully absorbed fabs, large sunk R&D cost Manufacturing experience: Volume: One platform for all parts Hybrid assembly: Automatic and manual At 20% improvement per year: Need another 7 years 23 Pigtailed package: For 200 to 300$? Need 20W to 30W in pigtail Increase pump power per chip Fundamental brightness limits? Not reached yet! Thermal limits can be streched, (longer laser chips) Task for fiber community: Find best match between pump diode pigtails and fiber combiners Standard today: 100um, NA=0.22 Move to 100um, NA=0.15 > larger combiners Move to 200um or even 400um, NA=0.15: Higher pump power per package From fibers to waveguide tapes?

In search of fundamental limits Power, W 35 30 25 20 15 10 5 0 Th=15C 32 W pulsed powerpulsed at 40A 100um stripe 17.8 W CW roll-over power cw, 100um stripe cw, 11.550um W CW roll-over power 50um stripe 0 10 20 30 40 Current, A With thermal limit removed, broad-area heroes hit single-mode telecom 980nm pump rated power density 24 2006 JDSU. All rights reserved.

Brightness limit: Not reached yet 30 25 p o w e r ( W ) 20 15 10 5 current (A) 30 20 10 current power 40x10-3 30 power (a.u.) 20 10 0 0 10 20 30 current (A) 0.5ms 40A pulse: 30Watt from 90um BA single emitter 0-2 0 2 t (s) 4 0.5ms 6 8x10-4 0 Improve CW power by better thermal performance Longer chip Higher efficiency www.bookham.com

Pump power injection Low NA, wide single emitters Brightness Power Diagram 26 Number of lateral modes ' 1'000'000 100'000 10'000 1'000 100 10 1 1 10 6 BA100, NA=0.2, 100 8W 1000 Power [Watt] 6 BA200, NA=0.15, 11W 6 BA300, NA=0.15, 13W 6 BA400, NA=0.15, 15W Increase power of single BA emitter by increasing emission width (6+1)*1 combiners commercially available for 100 and 200um fibers 300 and 400um should be feasible

Pump power injection Improve match of pump diode and waveguide Brightness Power Diagram 27 Number of lateral modes ' 1'000'000 100'000 10'000 1'000 100 10 1 1 10 100 1000 6 BA400, NA=0.15, Power 15W [Watt] 6 BA400, NA=0.15, 15W, Fiber Tape 10*400um, NA=0.46 Innovation needed: Improve match by going from round pigtail to high NA fiber tape pigtail (e.g. 10*400um, NA=0.46)

Pump power injection Bar stacks: Why? Stacks to further reduce $/Watt! 28 Stacks Open package Free space optic combiner Opto-mechanical precision High heat density MCC coolers with high water flow velocity Bar needs to be solderred to MCC Water and bar at same voltage potential. Bars and MCC in series Reliability MCC limits lifetime (degradation by flow-erosion, cavitation and electro-erosion) MCC has different cte than bar. Strain for hard solders or unstable joint for soft solders

Super-efficient 480W stack Stack Serial #: A668 Current (A) Voltage (V) Corr. Power (W) E-to-O Eff. 6 0 10 8.310 27 32.5% 20 8.480 96 56.6% 30 8.630 167 64.5% 40 8.750 233 66.6% 50 8.870 297 67.0% 60 8.980 367 68.1% 70 9.080 431 67.8% 77 9.155 480 68.1% 80 9.170 498 67.9% Output Power (W) 500 400 300 200 100 0 0 20 40 60 80 Current (A) 9.5 9.2 8.9 8.6 8.3 8.0 Voltage (V) Water Temp ~20C Ith (A) 6.0 Eff per Bar (W/A) 1.11 I at 480W 77 V at 480W 9.16 Peak PCE 68.1% Peak Wav (nm) 937.2 Relative Intensity Spectra at 80A, 20C Peak 937.2nm FWHM 4.7nm 925 930 935 940 945 950 Wavelength (nm) Power conversion efficiency (PCE) of >68% with good FWHM in 20 C water-cooled six bar stack 29 2006 JDSU. All rights reserved.

9xxnm 120W Bar Performance Electro-Optical 175 150 P-I curve at 25C up to 200W: Power (W) Power: 120W @ 140A Threshold: 14A Slope Eff.: 1W/A 200 125 100 75 50 25 Reliability 0 100 200 Current (A) Condition 120W pulsed (1.33Hz, 0<->140A) 1.1 1 Rel. power (a.u.) 5 200h at 120W lifetest data at 1.33Hz full on/off pulsed conditions available The extrapolated median lifetime is above 80 000hrs or 350 MShots, less than 1% fails after 120 MShots. No open fails 0 0.9 0.8 0.7 0.6 0.5 0 1000 2000 3000 Time (h) www.bookham.com 4000 5000 6000

Pump power injection Bar stacks vs single emitters Stacks Open package Free space optic combiner 31 MCC coolers with high water flow velocity Bar needs to be solderred to MCC Water and bar at same voltage potential. Bars and MCC in series Reliability Opto-mechanical precision High heat density Single pigtailed emitters Hermetically sealed package Fiber combiners Fiber combiner: Cost and reliability Distributed heat Robust water coolers Galvanic isolation Built on telecom technology Need MCC limits lifetime (degradation by flow-erosion and electro-erosion) MCC has different cte than bar. Strain Need 1. Ultra high efficient bar to reduce heatload 2. Macro Channel coolers which are cte matched and galvanically isolated 3. Optomechanical precision at low cost 1. Ultra high brightness chip to increase power per pigtailed package 2. Ultra high efficient chip to reduce heatload 3. Fiberoptics and fiber combiners which are matched to pump diode

Fiber Laser Reliability Pump Diode reliability Chip: Methodology known from telecom 32 FMEA, Multi-cell testing Apply to drive up power levels from single emitters Package Single emitters: Known from telecom Bar stacks: Opto-mechanics and cooling system: Need to bring in FMEA and multi cell/damage limit testing methodology for stacks Fiber Passive fiber: Active fiber: Well understood from telecom Photodarkening at high power operation. Understood by some manufacturers Fiber combiners Need to increase power capability together with single emitters Fiber Laser System Need to control fiber laser system aspects

What is a multi-cell test? 1.0000 1.0000 0.9000 0.9000 0.8000 0.8000 0.7000 Parallel lifetests varying key parameters: Temperature Optical Power and/or Drive Current 0.7000 0.6000 Normalize d0.6000 0.5000 Failure Rate Normalize d 0.5000 0.4000 Failure Rate 0.4000 0.3000 90 9070 Junction Te mpe rature (C) Junction 70 50Te mpe rature (C) 50 10 10.5 10.5 9.5 9.5 9 e x-face t Powe r (W) e x-face t Powe r (W) 10 9 8.5 8 8.5 7.5 8 6.5 7 7.5 Intended deployment condition 7 6 Multi-cell test conditions 6.5 0.1000 0.0000 0.0000 6 0.3000 0.2000 0.2000 0.1000 Example for multicell test design Reliable InAlGaAs lasers follow: E 1 1 P F T j, P, I Fop exp A k B T j Top Pop (Fop, EA, m, n) determined from best fit of multi-cell data 33 2006 JDSU. All rights reserved. n I I op m

SES8-9xx-01: Reliability assessment Iop= 9 A, Pop~ 8.2 W, Tjct ~ 111 C Multi-Cell test Iop: 9 A -13 A (Pout: 8 W- 11 W) Tjct: 100 C -140 C Acceleration model: FR ~ IxPy exp(-ea/kbt), x = 0, y = 5, Ea = 0.45 ev Reliability < 3 kfit @ 8.2 W, 25 C (heat sink temperature) www.bookham.com

Fiber Laser Reliability System GaAs Chip facet AuSn solder CuW submount 2005, A. Jakubowicz, Bookham 35 Damaged broad area pump chip by optical back-travelling pulse in fiber amplifier (Er) Add protection by isolators?

Outlook MOPA arrangement 940/960nm and 920nm bands have robust pump diodes 36 976nm is very challenging from wavelength stability requirement Reliability has to include diodes, fiber and system Diode seed lasers are easily modulated Cascade of fiber power amplifiers for easy power scalability for higher power chips: Methodology kown For higher power active fibers: Need methodology (and more manufacturers) Reliability trade-offs of system needed to optimize fiber laser costs Pump diode cost reduction through Evolution (takes time) Inventions Pigtailed single emitters optimally matched to fiber system Stacks on galvanically isolated, expansion matched macro channel coolers High volume by a few lead suppliers

Addendum 37

Super efficient 80W 940nm bar 150 80% 125 75% 100 70% 75 65% 50 60% 25 55% 0 50% 120 0 20 40 60 80 100 Power Conversion Efficiency Output Power (W) Performance of JDSU/SHEDS 80W Bars Drive Current (A) Power conversion efficiency (PCE) of >75% in 80W, 20 C water-cooled 1cm 940nm bars 38 2006 JDSU. All rights reserved.

MU7-9xx-01: NA 0.15 vs. NA 0.22 10 9 light output power (W) NA 0.15 NA 0.22 8 7 6 5 4 3 2 1 0 0 2 4 6 injection current (A) Same performance for NA 0.15 and for NA 0.22 MM fiber. www.bookham.com 8 10

915nm pump laser Pump Power to 105µm Fiber (W) 12.0 Power out of 105µm fiber, NA=0.15 Power without thermal roll-over 10.0 8.0 6.0 4.0 2.0 0.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 Laser Diode Current (A) Optical output power (ex-fiber) versus current for the 915nm TO-220 module Currently Lumics sells 4W versions of 808, 915 and 975nm 7W is announced for Q3 2006

9xx Laser Diode Bars 915nm 940nm 980nm Bar on MCC Bar on MCC Base&Cover 50%FF 80W 80W BAC80C-9xx-01 BAC80C-9xx-02 120W 120W 2400W BAC120C-9xx-01 BAC120C-9xx-02 VBA2400C-9xx-01 50W 50W 50W BAC50C-9xx-01 BAC50C-9xx-02 BPC50C-9xx-01 50W 50W 50W BAC50C-9xx-03 BAC50C-9xx-04 BPC50C-9xx-02 50%FF 30%FF 20%FF www.bookham.com Vertical Bar on MCC Stack MM Bar on Cu Block