High Power, High Brightness Laser Diode Technology Berthold Schmidt ISLC 2008, Sorrento September 2008
My special thanks to Boris Sverdlov, Nobert Lichtenstein, Susanne Pawlik & Gunnar Stolze (Bookham) John Marsh, Iulian Petrescu-Prahova, Chris Baker, Stewart McDougal, Steve Gorton (Intense) Reinhart Poprawe (ILT Aachen) Toby Strite, Victor Rossin, Erik Zucker (JDS Uniphase) Friedhelm Dorsch (Trumpf) Bernd Witzigmann (ETH Zürich) Christoph Harder (Harder & Partner) Stefan Heinemann (Fraunhofer Institut USA - Visotek) David Roh (Coherent) Volker Krause (Laserline) Ed Wolak, Jim Harrison, Michael Atchely (Newport Spectra) the IEEE publishing organization .... and many more. 2 2 2
Component design for high power fiber laser and direct diode systems Introduction Global photonic market view Fields of application Underlying technologies Materials for High Power Laser (HPL) Diodes Component design for high power fiber laser and direct diode systems Epitaxial desin g Waveguide design Chip reliability and quality assessment (life testing) S ysem t d esgn i aspecs t Summary and outlook 3 3
World wide photonics market Market segments and estimated revenues Circled: Major application areas of High Power Laser Diodes 4 4
Projected Market Growth till 2015
HPL Applications at a glance material processing, analysis, measurement Production technology Optical communication • Data transmission, Optical amplification Defence & Aerospace Communication (inter satellite,..), distance measurement, countermeasure, target designation, Illumination, LIDAR,... Information Technology • Optical storage, printing, marking, display Aesthetics (Skin treatment,..), surgery, Medical therapy (PDD -Photodynamic disinfection-, PDT -therapy-,..), ophthalmology, Cancer treatment 6 6
Production Technologies: Geometric scalin of material rocessin g p g 102 Ship Building Light weight structures Automotive Remote 100 Forming Weld ing Hybrid Semi finished Prod ucts 10-2 Engraving Tuning Marking Cutti ng Surface Technology Functional Surfaces Drilling Micro welding Selective Laser Sheet Metal Micro joining Electron ics Microelectronics Melting Pol ish ing Bio-photonics 10-4 Hard Materials Electronics Micro processing Communication Data Processing Optical Memories 10-6 LIBS Computer chips Optoelectronics Micro Lithography EUV/13n m 10 -8 1970 1980 1990 2000 2010 Year Which Laser for which Application? Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) 7 7
Application example: welding of thin foils -Polypropylen (PP) transparent and black thickness 100 µm Microfluidic device - PMMA or PP sealingfoil (75 µm) P= 1,4 –5,9 W v= 50 to 250 mm/s d0= 70 µm dweld seam = 150 to 500 µm Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) 8 8
Application example: laser polishing Materials: 1.2343, 1.2344, 1.2316 Initial state: mille, d eroe, d d grinded Sharp edge maintained 1 mm 2002 2008 Ra = 0,35 µm Ra = 0,09 µm 24 µ m) Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) 9
Application Examle p SLM of ZrO2-based Ceramics Zirconium oxide (ZrO2): max. bending strength, resistance to wear and tensile strength Principle of Selective Laser Melting (SLM): Ceramic powder is fully molten (no sintering) Potential Application: Production of full-ceramic dental prostheses 10 mm SLM demo parts Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) 10 10 10
Optical Communication: E ary l driver for high power laser dioes d In Space Under the Ocean On the Ground Optical communication for inter satellite, “submarine” and terrestrial areas. Supporting phone, internet and TV related digital data traffic. g 11 11
Defence and homeland security applications: slowly evolving Laser countermeasure system against heat-seeking missiles Example: Directional Infrared Countermeasure D istance measu rement , (DIRCM) from Northrop Grumman (public Information) Target designation - Laser fuses - Illumination - Detection of chemicals Laser area defence system (LADS from Raytheon) (public Information) 12 12
optical storage, printing, display Information technology: optical storage, printing, display CTP Printing - Computer to plate (CTP) Prinng ti - Digital printing - Display (RGB) technology - Rear projection TV 13 13
Medical applications: Hair removal Ophtalmology Surgery Before After Skin Treatment: Tattoo / Hair Removal - Acne treatment - Photodynamic Therapy (PDT) - Photodynamic Disinfection (PDD) - Dental Before After 14
Why choosing high power laser systems among other production technologies Processing Speed Process Accuracy Process Consistency ... cost (reduced cost of consumables) Uptime (Reliability) System flexibility Compactness Greener Processing (low environmental impact) Enabling new fields of application 15
Component design for high power fiber laser and direct diode systems Introduction Global photonic market view Fields of application Underlying technologies Materials for High Power Laser (HPL) Diodes Component design for high power fiber laser and direct diode systems Epitaxial desin g Waveguide design Chip reliability and quality assessment (life testing) S ysem t d esgn i aspecs t Summary and outlook 16 16 16
HPL Applications: underlin y g technoloies g Optical Pumping: DPSSL, fibre Laser Production technology (Er/Y), Disk Laser, Direct Diode Laser Systems Optical communication • DFB laser diodes, EDFAs, Raman amplifiers Defence & Aerospace – pumpng, rect oe snge YAG i Di di d ( i l element, stacks), DPSSL, fibre laser, Information Technology • Red and short wavelength diodes, direct diode systems, SHG (frequency doubling) Medical & Life science • Low energy, direct diode, DPSSL, fibre laser 17 17
Laser Market Development by Laser Type 1.400 1.200 World Market 2007 1.000 Extrapolation of current trends to 2010 800 600 Including new applications / markets 2010 400 200 Laser Type Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) 18 18
Underlying technologies: Erbium doped fibre amplifier (EDFA) 3 level energy sysem t when pumped with 980nm light 2 level system when pumped with 1480nm light 19
Dispersion Compensating Fiber Why High Power 980nm Pumps? Typical 3-stage EDFA Dispersion Compensating Fiber Gain Flattening Erbium Erbi E rbiu m Doped Fiber um Filter Doped Fiber Doped Fiber Isolator & filter Coupler Isolator Input Coupler Output 980 Pumps: Avn: d a tage 1480 Pumps (now 980nm !): Advantage: Low noise figure (3 level) Low heat load High laser efficiency Low cost coupler higher optical conversion efficiency Disadvantage Increased noise figure Un-cooled operation More expensive coupler High heat load, expensive cooling Disadvantage Lower optical power conversion efficiency High power 980 pumps enable low cost EDFAs www.bookham.com Page 20 20
14xy Pump laser for Raman amplifier Features —* Coupling to optical phonons in the glass —* max. gain at frequency shift of 13 THz in silica (60-1 00nm) Raman Gain @ pump wavelength 1455 nm —* broad (but not particularly flat) gain spectrum Advantaes g 8.0E-04 6.0E-04 4.0E-04 —* Effect present in all fibers (using installed fibers) —* Gain at any wavelength only limited by pump source —* Low noise figure because of low ASE —* Distributed amplification 2.0E-04 0.0E+00 1450 1500 1550 1600 1650 Wavelength (nm) Disadvantages —* Fast response time (no upper state lifetime buffer) —* Pump fluctuations -> gain fluctuations -> noise —* Polarization dependent gain —* High pump powers required (>200 mW) www.bookham.com
Laser system desin g principles transversally p um p ed rodlaser INNOSLAB laser All based on the same design principle requiring Mirrors (Cavity) Gainmedium Pump source Extension... amplifier technology fibre laser end-pumped thin fibre l aser disk laser “Y” -pumped Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) 22
Fibre laser pumping scheme CP-active fiber Fiber com b i ne r Multi-mode pumps FBG FBG Multi-mode pumps Cladding pumped fiber: Yb / Er in SiO2 Emission wavelength: 1.08 / 1 .53um Pump wavelength at 915, 960, 975 nm Pump source: Broad area single emitters / Bars Output power: ~100W -> xxkW Fiber laser output optic CP-active fiber Multi-mode pumps Filter Fiber laseroutput optic Single Mode Pump “Seed laser” ~1 064nm www.bookham.com Page 23
Rod- and Disklaser Beam quality Efficiency ' Arc lamps Rod Lamp pumped Rodlaser Beam quality Efficiency ' Arc lamps Rod Diode pumped Disklaser Beam quality Efficiency Laser-diodes Disk Presentation title TRUMPF abbreviation - 2007/03/25 24 Presentation title TRUMPF abbreviation - 2007/03/25 24 Presentation title TRUMPF abbreviation - 2007/03/25 24
Disklaser: TruDisk TruDisk 1000 TruDisk 8002 Presentation title TRUMPF abbreviation - 2007/03/25 25 Presentation title TRUMPF abbreviation - 2007/03/25 25
Design of a disklaser (8 kW) Resonator Presentation title TRUMPF abbreviation - 2007/03/25 26 Presentation title TRUMPF abbreviation - 2007/03/25 26
Pump- Beam 1 Pumping of a Disk Bending prisma End-mi rror Parabolic Mirror 1 Disk C avy it 8 Outcoupler 5 2 4 3 Laser beam 1 6 Actually: 20 passes of the pump light through the disk! 7 Presentation title TRUMPF abbreviation - 2007/03/25 27
> 5 kW Output Power per Disk 6000 70 60 5000 50 4000 40 3000 30 2000 20 1000 10 0 2000 4000 6000 8000 Pump Power Pp (W) Presentation title TRUMPF abbreviation - 2007/03/25 28 Presentation title TRUMPF abbreviation - 2007/03/25 28 Presentation title TRUMPF abbreviation - 2007/03/25 28
TruDisk 10003 –4 disk resonator 14000 12000 10000 8000 6000 4000 2000 0 70% 60% 50% 40% 30% 20% 10% 0% 0 5000 10000 15000 20000 Pump Power Pp [W] Presentation title TRUMPF abbreviation - 2007/03/25 29 Presentation title TRUMPF abbreviation - 2007/03/25 29
Fiber, Disk or Direct Diode Laser? 1000 Ha rd en ing Polymer weldingB razi ng Welding, sintering, surface treatment Cladding D ee-en etration p p we l ding Metal cutting 100 10 1 Sold eri ng Non-Metal cutting Pri nti ng Marking Drilling ‘SM’ fiber 1 10 100 1000 10000 Sources: •P. Loosen, Fraunhofer Inst. Aachen Laser output power (W) 30 30 30
Fiber, Disk or Direct Diode Laser? .... many applications don’t need highest beam quality (diffraction limited beam). 1000 Hardening Polymer welding Brazing 1 500um,0.22 Cladding 60um,0.22 100 Welding, sintering, surface treatment Sold eri ng 200um0.22 , D ee-en etration p p we l ding Metal cutting Non-Metal cutting 10 P 50um .0.22 ri nti ng Marking 1 Drilling ‘SM’ fiber 1 10 100 1000 10000 Sources: P. Loosen, Fraunhofer Inst. Aachen C. Harder LEOS 2006 Laser output power (W) 31 31
Developing trends for Lasers and their applications 2003 2008 1.000 2w0 F#4 focusing optics (NA 0,12) transformation hardening melting, cleaning plastics welding f brazing 100 Diode lasers (2003) soldering selective laser powder remelting deep penetration welding metals metal see h t cutting 10 Printing therm. marking CO2-laser BPP = θ · w0 1 lamp pumped Nd :YAG-laser DPSSL 0.1 1 10 100 1.000 10.000 Laser Power P [W] Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) 32 32
Component design for high power fiber laser and direct diode systems Introduction Global photonic market view Fields of application Underlying technologies Materials for High Power Laser (HPL) Diodes Component design for high power fiber laser and direct diode systems Epitaxial desin g Waveguide design Chip reliability and quality assessment (life testing) S ysem t d esgn i aspecs t Summary and outlook 33 33
Materials for Semiconductor Laser Processing 34 34
Direct Diode Applications Wavelength Range for major industrial applications / technologies requiring (red / MIR) HPL 600nm 700nm 800nm 900nm 1000nm 1100nm 1400nm 1600nm GaP Capability GaAs Capability InP Capability DPSS Med ical Printing Optical storage Telco Pumps Display,... Laser TV Pumps & SHG LIDAR Fiber Laser Pumps Fiber Laser Seeds Direct Diode Applications Nd YAG Replacement 35 35
Pearls of Laser History Herbert Nelson, RCA Liquid Phase Epitaxy 1962 Quantum Dots, Quantum Cascade Lasers, new materials, new process technologiesnew integration technologies intercavity Lasers (SHG) monolithic Integration MOPA, SEDFB, α-DFB Tunable l asers FBG-Stabilization Flared / Taper Design Efficiency I mprovements New Waveguide Designs Materials Laser bars / Stacks 1917 1950s 1960s 1970s 1980s 1990s 2000s 2010s 36 36
Component design for high power fiber laser and direct diode systems Introduction Global photonic market view Fields of application Underlying technologies Materials for High Power Laser (HPL) Diodes Component design for high power fiber laser and direct diode systems Epitaxial desin g Waveguide design Chip reliability and quality assessment (life testing) S ysem t d esgn i aspecs t Summary and outlook 37 37
System Design Requirements Majority of diode based laser systems have common d esgn i requiremens: t Efficient coupling into passive optics elements or an optical fiber (with low NA) High wall plug efficiency (low power consumption) Good system reliability Cost competitive Simple to use (cooling, turn on time, robustness,...) From a diode perspective this relates to various design objectives.... 38 38
Laser diode design targets Common design requirements for high power laser (HPL) diodes: High output power High brightness High wall-plug and coupling efficiency (low power consumption) High reliability + robust Design capable for high volume manufacturing 39 39
Challenges for the design of HPL Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) – Quality assessment Waveguide Design Epitaxial Structure Output Power, Capacity 40 40
Challenges for the design of HPL: Wall-plug efficiency – Broad area single emitter laser Key measure for high power laser diodes: Epitaxy Design parameters: bandgap profile, carrier confinement d/gamma, confinement factors doping profile, growth process ... η = WP h I I νη νη νηνη ( ) d th d th = ⎛ −⎛ − − ⎞ ⎜⎝ 1 ⎠⎟ h I qIV qV I Key characteristics: internal losses, internal quantum efficiency, series resistance, laser voltage thermal resistance (wall-plug efficiency) Waveguide characteristics: Optical Confinement, Vertical far field, Coupling efficiency, Others: defect density, material properties (i.e. expansion match) Output Power, Capacity 41 41
• Device characteristics show importance of a balanced desin aroach: Similar laser structures with varying η ηWP 0.7 0.6 0.5 0.4 14 12 10 8 0.3 6 WPE 1 WPE 2 P 1 P 2 0.2 4 0.1 2 0.0 2 4 6 8 10 12 14 current (A) • Device characteristics show importance of a balanced desin aroach: g pp – Similar differential quantum efficiency – different terminal (operating) voltages => different doping profiles impacting wall-plug efficiency (5-6%) www.bookham.com Page 42
Differential quantum efficiency = ⎛ ⎜ ⎝ 2 α i L ⎞ ⎟ 1 ⎠ 1 + η η η d i ln 1 1 2 ( ) R R ηi − − injection efficiency, defined as fraction of injected carriers which are converted into optical radiation by the stimulated emission; − physical effects controlling it are not very well established; − difficulty: carrier loss rate increases with increase of carrier concentration (quasi-Fermi level separation), but both stabilize at threshold α − distributed loss α = α + α + α i , i FC scatt. coupl. Free carrier absorption αp = 7 - 14⋅10-1 8p; αn = 3 - 6⋅ 10-18n 43 43 43
Hole Quasi- Fermi Level Bandgap engineering example: Graded Index carrier confinement (GRICC) 0.5 1.0 InGaAs/AlGaAs GRICC design Electron Quasi- Fermi Level provides efficient 0.8 carrier confinement 0.0 -0.5 Conduction Band stable control of vertical waveguide low internal losses -1 (below 1cmreported) 0.6 -1.0 0.4 Hole Quasi- Fermi Level -1.5 0.2 Valence Band -2.0 0.0 Vertical Position (a. u.) B. Schmidt et al. Proceedins g of SPIE Vol. 5248, SPIE, Bellinham g , WA, 2003, 42-54. 44 44
Low loss laser waveguides: strong guided structures The main idea - to minimize overlap between optical mode and regions of high doping to reduce internal absorption losses Very strong confinement undoped waveguide M. Gokhale et al., JQE, v.33, p.2236 (1997) 45 45
Shifting mode into n material Low loss waveguides: Shifting mode into n material 2. A symmerc t i wavegues id Mode maximum shifted from the QW position: lower FC absorption in QW -1 Values of aFC as low as 0.4 cmwere reported Free carrier absorption in p material is higher than in n material: - higher absorption cross section; - higher doping for comparable conductivity required; => The design idea is to shift optical mode from p- to n-type material In addition higher order modes can be suppressed M. Buda et al., JSTQE, v.3, p.173 (1997) C.M. Stikley et al., SPIE Proc., v.6104, (2006) 46 46
Minimizing laser diode voltage Laser voltage V = Vj +IRs+ Vhb: * * Vj - voltage on pn junction; stabilizes at threshold: V j = F n − F p V hb - voltage drop on (isotype) heterojunctions throughout laser structure:usually minimized by graded hetero-interfaces R s - series resistance: compromise has to be found to reconcile requiremens t of l ow d opng i f or high ηd and high d opng i f or l ow R s M. Kanskar et al., Electron Lett, v.41, (2006) 47 47
Example: Multi-mode single emitter diode for fiber laser pumping 793nm pump wavelength for Thulium based fiber lasers Waveguide with ~90 um aperture 3.5 mm chip length Achieving around 10-11 W roll over at 25 C (CW) High wall plug efficiency 48 48
Challenges for the design of HPL Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) – Quality assessment Waveguide Design Epitaxial Structure Output Power, Capacity 49 49
Optical Waveguide & Substrate modes: GaN* laser diode SiC Al2O3 Strain % 0.0 3.4 16 Refr. Index 2.52 2.75 1.78 SNOM GaN-Substrate, dtot = 100μm vertical section Potential Impact of Substrate Modes: 2 µm Simulation - otical loss p - gain oscillations ... *Source: Prof. B. Witzigmann et al., IEEE JQE 2007 50 50
Challenges for the design of HPL: Waveguide Design – single mode ridge laser gain profile SQW - GRICC Structure InGaAs/AlGaAs intensity profile Waveguide Design impacted by length scaling, spatial hole burning, local heating, carrier injection refractive index step (etch depth), => slow axis diverence linear ower near field attern g , p , p Output Power, Volume www.bookham.com Page 51
2D simulation (*) of GaAs-based high-power SM-Laser Diodes - Most often 1 – 2D simulation - Solving wave equation with changing parameters, i.e. at increased drive currents (time consuming) - Input Parameters often not fully explored Pictures: Boundaries and simulation domains for modeling high power laser diodes. *Source: Prof. B. Witzigmann, ETH-Zurich 52
Wave guiding impacted by free carriers, etch depth and temperature... Ridge High waveguide stability is essential for every laser diode used for telecom- Active region and non-telecom applications Influenced by > Refractive index step > free carrier density > temperature variations (thermal blooming,..) Temperature induced index change B.Schmidt et al., Proceedings ISLC 2002 53
Lateral Waveguide stabilization for high power spatial SM laser diodes High power SM operation > Linear Powers up to 1.9W achieved (*) > Low divergence leaky waveguide > Stabilization of fundamental mode by increasing losses of higher order modes (significant higher relative increase of optical losses of 1st, 2nd and.. order modes as compared to the zero order mode) => suppression of coherent coupling with higher order modes (higher linear power) * Wenzel et al., “Fundamental-Lateral Mode Stabilized RWG Lasers”, IEEE Phot. Tech. Let., Vol.20, No.3, 2008 54 54 54
Length scaling broad area laser Both electrical and thermal resistance of the chip are inversely proportionalto the its waveguide active area; broad area laser V. Gaontsev et al. Proc. o SPIE v. 6104 2006 p , f ( injected current (mA) 55 55
( ) z ( ) ( ) Optical intensity distri ( ) z V ( ) ( ) e Asymmetric power/current distribution along the wavguide Power distribution inside laser cavity Optical intensity distri facetRate equations refl 1.0 j ( ) z =+ R R 0.8 Rfront= 0.1% Rback = 100% ed sp stim 0.6 Γ ( ) z g = hcS ) [ ( ) ( )] P z P z + + − 0.4 Rstim (z 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 25 20 cavity length (a.u.) Voltage on laser ΔF * ( ) z V + j z z ( ) ( ρ ) 15 e I = ∫ L j z dz 0 ( ) 10 5 567 2 3 4567 2 3 4567 Δ F * z ⇒ N z ⇒ ( ) ( ) ( ) z 0.1 1 10 g front facet reflectivity (percent) =>> refractive index and gain profile changes along laser cavity 56
Challenges for the design of HPL: Waveguide Design – single mode ridge laser G08 junction side up CW-operation = ° T 25C >100MW/cm2 1600 1400 G07 G06 1200 1000 800 G05 600 400 G03 200 -200 0 500 1000 1500 2000 2500 inected current mA j ( ) Waveguide Design Length Scaling, Spatial hole burning, local heating, Carrier injection Refractive index, slow axis divergence Output Power, Volume www.bookham.com
Challenges for the design of HPL Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) – Quality assessment Waveguide Design Epitaxial Structure Output Power, Capacity 58 58 58
Long term reliability & COMD protection Catastrophic Optical Mirror Damage (COMD) Free carrier absorption Stimulated EmissionAbsorption: e-h Non-rad iative recombination via surface states Facet Degradation leads to formation surface states Surface states cause non-radiative recombination Non-radiative recombination leads to a local increase of current injection This leads to an increase of the local temperature Which causes a further shrinkage of the bandgap and thus an increase of free carriers Additional generation of e-h pairs leads to further absorption of stimulated laser light causing an acceleration of the thermal run away .... COMD 59 59
Avoiding COMD Avoid or reduce formation of non-radiative recombination states E2 -> Cleaving in high vacuum and in-situ passivation of the cleaved surface Use of InGaAsP based barrier materials -> reduced oxidation (“Al-free”) -> re-growth covering cleaved facets possible Remove formation of non-radiative recombination states “Cleaving on air” -> Dry etching in vacuum -> in-situ nitridation or sulphation “Cleaving on air” -> low energy hydrogen plasma or ion beam cleaning -> in situ passivation (ZnSe, Si,...) M. Gasser, E.E. Latta, "Method for mirror passivation of semiconductor laser diodes," U.S. Patent No. 5063173 M. Hu, L.D. Kinney, M. Pessa, et al.,"Aluminium-free 980-nm laser diodes for Er-doped optical fiber amplifiers," SPIE 1995, vol. 2397, 333-341 K. Hausler, N. Kirstaedter, "Method and device for passivation of the resonator end faces of semiconductor lasers based on III-V semiconductor material," U.S. Patent No. 7033852 L .. K Lindsrom, t et al."Meo th d to obtan i contamnaon i ti free laser mirrors and passivaon tiof these," U .. S Patent No. 6812152 H. Kawanishi, et al."Semiconductor laser device with a sulfur-containing film provided between the facet and the protective film," U.S. Patent No. 5208468 E.C. Onyiriuka, M.X. Ouyang, C. E. Zah, "Passivation of semiconductor laser facets," U.S. Patent No. 6618409 P. Ressel et al. "Novel Passivation Process for the Mirror Facets of Al-Free Active-Region High-Power Semiconductor Diode Lasers," PTL 2005, vol. 17, no. 5, 962-964 60 60
Long term reliability & COMD protection E2 by Bookham: Stress test of first generation 980nm SM laser diodes 330 310 290 270 250 230 210 190 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Time (years) www.bookham.com Page 61
Avoiding COMD Reduce number of free carriers at the laser facet Reduce direct carrer i injecon ti without changng i the bandgap (at wafer level process) Front section isolation NAM (non absorbing mirrors) to reduce absorption, diffusion and thermalized carriers (at wafer level process) Zn diffusion Etching and subsequent re-growth of a III-V window Si- doped and disordered windows Vacancy induced windows (QWI) B. Schmidt et al., US Patent 6782024 - High power semiconductor laser diode H.0. Yonezu, M. Ueno, T. Kamejima and I. Hayashi, "An AIGaAs Window Structure Laser," JQE 1979, vol. 15, no. 8, 775-781 J. Ungar, N. Bar-Chaim and I. Ury, "High-Power GaAlAs Window Lasers," EL 1986, vol. 22, no. 5, 279-280 R .. L Thornon, t D .. F W ec, l h R .. D B urnam, h T L. Paoli and P .. S Cross, "High power (2.1 W) 10-srpe t i AlGas Alaser arrays with Si disorere d d facet windows," Appl Phys Lett 1986, vol. 49, no. 23, 1572-1574 S. Yamamura, K. Kawasaki, K. Shigihara, Y. Ota, T. Yagi and Y. Mitsui, "Highly Reliable Ridge Waveguide 980nm Pump Lasers Suitable for Submarine and Metro Application," OFC 2003, vol. 1, 398-399 J.H. Marsh, C.J. Hamilton, "Semiconductor laser," U.S. Patent No. 6760355 62 62 62
Effect of front section isolation *( ) Standard Contact Truncated Contact Reduction of current injection into front section (reduced local heating) Reduction of free carriers (impact on the gain profile) *Source: Prof. B. Witzigmann, ETH-Zurich 63 63
Transparent output waveguide Avoiding COMD: QWI (quantum well intermixing) Uncoated 830 nm lasers Test conditions: Initial CW measurement followed by a pulsed L-I NAM 45 nm shift 350 NAM 65 nm shift 300 NAM 0 nm shift 250 QWI regions 200 150 100 Transparent output waveguide 50 0 250 500 750 1000 1250 1500 1750 Current (mA) Increase of the bandgap close to the facet region Reduction of free carriers Conventional Laser Reduction of local absorption 64 64
QWI allows bandgap tuning in selected areas of the chip QWI process concept As-grown bandgap Intermixed bandgap Al Ga AlGaAs GaAs AlGaAs AlGaAs QWI allows bandgap tuning in selected areas of the chip 65 65
High Temperature Anneal QWI Process Steps Intense QWI technology enables high power/brightness lasers to be produced in a manufacturing environment Dielectric caps are deposited on surface of wafer W aer f i s anneae l d Quantum wells intermix with adjacent material altering the bandgap wavelength Wavelength change depends on properties of dielectric cap QWI works in a variety of materials and wavelength s 808 nm SQW material used in this work Photoluminescence spectra 160 140 120 Suppressed Intermixed Intermixing cap Suppressing cap 100 80 60 QW 40 20 High Temperature Anneal 700 720 740 760 780 800 820 840 860 Wavelength, nm 66 66
Life testing array with 64 single emitters (QWI – technology) 64 emitter life test Each device operates at 808nm with >200mW output power, room temp. 67 67
Challenges for the design of HPL Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) – Quality assessment Waveguide Design Epitaxial Structure Output Power, Capacity 68
Quality assessment and life test strategies Challenge for quality control of high power laser diodes for industrial laser systems No clear industrial standards defined, yet. (in comparison to Telcordia standards for fibre networks -GR-468-) Different system requirements in different industries (from commodity to space application) Comparison of varying technologies for similar applications Single emitter high power laser diodes (spatial multi-mode or single mode) 1 cm High Power Laser bars with varying fill factors “Mini-Bars” and related devices Differn e t worin d g to describe reliability (FIT, MTBF, MTTF, end of life,...) Additional influence factors Mounting (Indium or hard solder (AuSn, SnAg, SnAgCu,...)) Cooling (passive cooling, active cooling (TEC, micro-channel, macro- channel) Environment (hermetic, non-hermetic, ....) Operating regime (CW, quasi CW, pulsed) Cost of life test equipment and test 69 69 69
Classical life test strategy: single emitter devices Assumptions Known failure modes Laser diode lifetime follows “bath tube” curve Infant mortality rate is vanishing or can be screened out by burn-in Constant failure rate (intrinsic period) is determined by sudden death (or a short wear out period followed by sudden death) Wear out is expece t d to kick in after guaranee t d d evce i life time ∝ x y − Constant failure rate can be described by: FR I P exp(E a kbT) tonset 70 70
Example: Reliability testing of 980nm telecom chip Stress cells with different conditions (acceleration of failure rate) high number of stress cells and devices similar failure mode as at “field condition” failure rate for each condition (must be constant) Determination of the acceleration factors in the reliability model with maximum likelihood approach (Ea , x , y) Prediction of reliability at use conditions: 30 - 500 FIT 1400 25 °C Testi ng 1200 45 °C 1000 65 C ° 800 85 °C 600 Use 400 200 0 500 1000 1500 2000 Current (mA) www.bookham.com Page 71
Challenge of qualifying industrial devices High Power (single emitter) Laser Diodes are expected to operate closer to 1 – 5kFIT at standard operating condition (in comparison to below 100FIT for telecom “submarine” applications) Acceleration of failure rates is more challenging since devices operate closer to the thermal roll over point at standard operating condition Cost of life testing can be significant if devices have to be tracked in situ and separately operated / cooled for conditions ranging from (10-20A, 25°-75°C heat sink temperatures) Operating conditions might vary severely (CW, quasi CW, pulsed) 72 72 72
Central life test philosophy 73 73
Example: JDSU L4 module performance & testing Laser chip 25 75% InAlGaAs – ex-ace f t ex-fiber – 880-1000nm – 100μm aperture 20 60% – 4.1mm cavity – AuSn solder 15 45% Fiber-coupled package 10 30% – 105μm diameter – 0.15 or 0.22NA – Rth= 2.2oC/W – rae pow 10W t d 5 15% er 15C – 50% wall plug 0 0% 0 5 10 15 20 25 Current (A) 74 © 2008 JDSU. All rights reserved. SSDLTR June 2008
Accelerated life test examples Chip Life Test – 9 ., 3W 13A – Tcase = 70oC – Tj= 117oC – 28 lasers – 0 failures Packae Test g – 10W, 12A – T case = 35oC 10% 5% 0% – 14 lasers – 0 failures -5% -10% 0 1000 2000 3000 4000 , , , , Time (hours) 75 © 2008 JDSU. All rights reserved. SSDLTR June 2008
L4 Package qualification and robustness tests 500G Mechanical Shock + 20G Vibration • Reference Telcordia GR-468 – ero pacage aures n u sute Z k f il i f ll i – Proves robustness of design 10% 5% 0% -5% 12 units -10% Time Zero Mech Shock Vibration 85°C / 85% RH Damp Heat 10% -40°C to +85°C Temperature Cycling 5% 10% 27 units 0% 5% 0% -5% 18 units -5% -10% -10% 0 100 200 300 400 500 0 1000 2000 3000 4000 5000 Time (hours) No of Cycles 76 © 2008 JDSU. All rights reserved. SSDLTR June 2008
Pump power trends – commercially available 10 L46398 L3-6 397 L3-63 96 L3-6 390 L2-6380 • • • 9XXnm 105μm diameter fiber “Reliable” rated power 15% annual increase in reliable power imilar trend or S f – 8XXnm – Singlemode lasers – Multi mode lasers – Bars 1 1992 1994 1996 1998 2000 2002 2004 2006 2008 Year 77 © 2008 JDSU. All rights reserved. SSDLTR June 2008
Life t esng ti of laser dioe d bars Important to understand cooling type fill factor (FF) of bars operational mode (CW, pulsed, QCW) expected degradation scheme (gradual degradation, sudden failure or a combination) and expected failure mode (stress induced failure, contamination induced failure, COMD, bulk fail or combinaon,... ti ) 78 78 78
Types of coolers for hard solder bar mounting (*) - Passive Cooling (copper heat sink with heat exchanger -> Water or Air cooled) - A cve ti C oong li of high power bars (micro, meso or macro channel cooer l ) - Active cooling of single emitter devices (TEC and heat exchanger) * Christoph Harder; “Chapter: Pump Diode Lasers”, Optical Fiber Telecommunications V A (Fifth Edition), Components and Subsystems, Editor: Ivan P. Kaminow, Tingye Li andAlan E. Willner, pp. 107-144. 79 79
Rth = 0.23 C/W L = 3.5 mm P = 200 W (FF=50%) P = 207 W (FF=33%) Results – Compare FF = 50% to FF = 33% Microchannel Cooler (MCC) Rth = 0.23 C/W L = 3.5 mm P = 200 W (FF=50%) P = 207 W (FF=33%) 300 70 270 MCC 60 240 Tj = 60 C 210 50 180 40 150 30 120 90 20 FF=50% Model FF=50% Data FF=33% Model FF=33% Data 60 10 30 0 50 100 150 200 250 Current (A) Current (A) Current (A) Presentation title TRUMPF abbreviation - 2007/03/25 80 Presentation title TRUMPF abbreviation - 2007/03/25 80 Presentation title TRUMPF abbreviation - 2007/03/25 80
Rth = 0.38 C/W L = 3.5 mm P = 127 W (FF=50%) P = 151 W (FF=33%) Results – Compare FF = 50% to FF = 33% Mesochannel Compact Heat Exchanger (MesCHE) Rth = 0.38 C/W L = 3.5 mm P = 127 W (FF=50%) P = 151 W (FF=33%) 240 60 220 MesCHE 200 50 180 160 Tj = 60 C 40 140 120 30 100 80 20 FF=50% Model FF=50% Data FF=33% Model FF=33% Data 60 40 10 20 0 20 40 60 80 100 120 140 160 180 200 220 Current (A) Current (A) Presentation title TRUMPF abbreviation - 2007/03/25 81 Presentation title TRUMPF abbreviation - 2007/03/25 81
Life testing of laser diode bars: modes of operation Short pulse operation I ntermittent-CW operation Current (us) Current (s) GaAs α = 6 ppm/ °K Optical pulse Optical pulse CuW α = 6.5ppm/ °K Cu α = 16.5ppm/ °K Semiconductor / Mount Temp Semiconductor / Mount Temp Mechanical stress Mechanical stress - Short pulse operation with strong variations in duty cycle and pulse length (ns - ms.... ) Application oriented (highest optical stress per pulse) - Intermittent-CW (normally ~1 .3Hz) creates highest mechanical stress 82 82 82
Example: QCW-Stack HERMES QCW Stack 10 Bar QCW Stacked Diode Arrays 200W bars @ 808nm 250u sec x 20Hz Hard solder MIL-SPEC 3000 2500 12 % 9 % 9 % 2000 1500 Onoin life test g g 1000 500 -5.0E+07 0.0E+00 5.0E+07 1.0E+08 1.5E+08 2.0E+08 2.5E+08 3.0E+08 3.5E+08 Number of Shots 83 83
808 BAR: High Performance in CW 160 2.5 100 2.5 140 90 80 120 2.0 2.0 70 100 60 80 1.5 50 1.5 60 40 30 40 1.0 1.0 20 20 10 0.5 0 0.5 0 20 40 60 80 100 120 140 0 10 20 30 40 50 60 70 80 90 Drive Current [A] Current [A] • Improved optical and electrical design – Low drive current: 60W @ 63A (50%FF) / 40W @ 42A (30%FF) – High wallplug efficiency: 53% @ 60W, 57% max WP – Improved far field pattern: 7° x 55° (90% power) www.bookham.com Page 84 www.bookham.com Page 84 www.bookham.com Page 84
808 BAR: Reliability assessment (Intermittent operation) 1.1 1.1 1 1 0.9 0.9 0.8 0.8 0.7 0.7 75W / 75A 0.6 105W / 100A 0.6 0.5 0.5 500 1000 1500 2000 2500 3000 3500 4000 0 500 1000 1500 Burn in Time (h] Time (h] Multi-cell accelerated long-term aging test – Hard-pulse (1.3 Hz, 50% duty-cycle, full ON-OFF) Average power wear-out <1% / 1000h (gradual degradation) – Stability of design and proven benefits of hard solder technology www.bookham.com Page 85 www.bookham.com Page 85
Challenges for the design of HPL Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) – Quality assessment Waveguide Design Epitaxial Structure Output Power, Capacity 86 86 86
Various pumping/coupling schemes end pumping with fiber combiner laser bar lens End pumping with collimated bar, Direct applications step mirror 87 87
Cladding pumped active fiber (CP-fiber) Special fibers for hih g power fiber laser Low index coating (polymer with low index) •Multi-mode waveguide (normally with high NA ~0.46 to provide strong guiding of the pump light) The high NA of the double clad fiber allows multiple low NA fiber ports to be coupe l d into one fiber and to maintan ithe brightness Rare earth doped waveguide (normally 10-30um with 0.07 NA) 88 88
P Coupling pump radiation into fiber laser P R ≅ α D NA D NA Pump radiation is usually supplied through multimode fibers with core diameter D = 50 - 105 mm and numerical aperture NA = 0.15 -0.22 P end pumping with fiber combiner side pumping In other words: it is impossible to increase radiance by combining radiation of Th t d h pw p ni rea p ui slid agl i ns d several uncorrelated but otherwise identical sources Fiber radiance Max. No. of pumps per node 2 ⎛ laser laser D NA clad clad ⎜ ⎟ × ⎞ pump pump ⎝ D NA core core ⎠ × R ≅ α P ( )2 D NA core × Key objective: Optimize brightness per pump source! 89
Example: Second-Generation Fiber Pump Modules Characteristics – Multiple emitters (e.., g a single mini-bar) – Micro-optics for beam conditioning – More power in (e.., g higher current at std voltage) Features – Enhanced brightness per fiber channel – Reduced thermal and electrical resistance (higher power at rollover) – CoS with single-emitter economies, no smile – Independent dropouts, reduced facet loading, enhance reliability – Highly scalable at module level H2 short presentation 90 90
Mini-Bar Fiber Pump Module • - Brightness of industrial modules now exceeds 1 MW/cm2 sr OrionTM series - 20W, 105um core, 0.20NA 3.8cm - 915nm, 940nm, 976nm 30 0.6 - mini-bar architecture 25 0.5 20 0.4 - very high reliability 15 03 Specified Operating Pt 10 0.2 Power PCE 5 0.1 0 5 10 15 20 25 30 35 Current (A) H2 short presentation 91
Mini-Bar Reliability: verification of emitter independence CoS: 5-element mini-bar on CT (AuSn solder on CuW heatsink) single-emitter failures Multi-Stripe Modules as Ensembles of Semi-Independent Emitters: failures are dominated by random, sudden failures of individual emitters the failure of an individual emitter only impacts other emitters by an increase in ensemble drivecurrent (for constant power) and warming of the other stripes on the same mini-bar all assumptions are consistent with test data giving over 300,000 hrs MTBEF (mean time between emitter failure) at the specified operating point H2 short presentation 92 92
Bar Based Direct Diode Laser Systems Material Processing requires very high powers and power densities! To achieve the required power levels the following technologies are applied on system level: Bar multiplexing with passive optical elements Polarization combining Wavelength multiplexing 93 93 93
Bar multiplexing to achieve highest optical power densities for direct application Picture with courtesy of 94 94
Challenges for the design of HPL: Bar Bonding – Low Smile and High Current Capability Subsystem design Submount material (expansion matched) Robust cooler design (avoid corrosion) Strain – Stress (reduced at all interfaces) low Smile (hard solder, low smile) Passive optics design (efficient) Fiber diameter (low core, low NA) Picture with courtesy of 95 95
Wavelength and Polarization multiplex Stack 1 (interleaved 2 x 10 bars) Stack 2 λ1 optics optics λ2 λ3 Polarizationcombining Output Power System: 2 X Pstack X 3 = 10kW- 12kW BPP ~ BPP (Stack) ~ 90-200 mm*mrad 96 96
Challenes g for the desin g of HPL Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) – Quality assessment Waveguide Design Epitaxial Structure Output Power, Capacity 97 97
Efficient design for robust volume manufacturing: one design platform for multiple packaging formats TO Can for smaller arrays Laser Array Laser Array on Ceramic Carrier Low cost Butterfly Package for larger arrays Marking Module 98 98
45+ years of semiconductor laser development has Summary and outlook 45+ years of semiconductor laser development has brought the technology to high level in terms of efficiency, achievae bl power l eves, l waveguide design and reliability Current challenges are more in the interaction with the system design level to optimize efficiency, brightness and robustness of those systems Future developments will lead to a broadening of the wavelength range, strengthening of the direct diode laser system desins g and exploration of new fields of application 99 99
End Berthold_Schmidt@intenseco.com 100