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High Power, High Brightness Laser Diode Technology Berthold Schmidt ISLC 2008, Sorrento September 2008.

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Presentation on theme: "High Power, High Brightness Laser Diode Technology Berthold Schmidt ISLC 2008, Sorrento September 2008."— Presentation transcript:

1 High Power, High Brightness Laser Diode Technology Berthold Schmidt ISLC 2008, Sorrento September 2008

2 222 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.

3 33 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

4 44 World wide photonics market Market segments and estimated revenues Circled: Major application areas of High Power Laser Diodes

5 55 Projected Market Growth till 2015

6 66 HPL Applications at a glance  material processing, analysis, measurement Production technology Optical communicationData transmission, Optical amplification Defence & Aerospace Information TechnologyOptical storage, printing, marking, display  Aesthetics (Skin treatment,..), surgery, Medical therapy (PDD -Photodynamic disinfection-, PDT -therapy-,..), ophthalmology, Cancer treatment  Communication (inter satellite,..), distance measurement, countermeasure, target designation, Illumination, LIDAR,...

7 77 Production Technologies: Geometric scalin of material rocessin gpg Year Which Laser for which Application? Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) Drilling Hard Materials Engraving Tuning Marking Micro joining Electronics Sheet Metal Cutti ng Micro welding Forming Weld ing Electron ics Microelectronics Semi finished Prod ucts Optoelectronics Micro Lithography Communication Data Processing Optical Memories Surface Technology Functional Surfaces Micro processing Light weight structures Automotive Ship Building Selective Laser Computer chips Remote Hybrid Melting Pol ish ing Bio-photonics LIBS EUV/13n m

8 88 Application example: welding of thin foils 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 d 0 = 70 µm d weld seam = 150 to 500 µm Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008)

9 Application example: laser polishing Materials: , , Initial state: mille, d eroe, d d grinded Sharp edge maintained 1 mm Ra = 0,35 µmRa = 0,09 µm 24 µ m) Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) 9

10 10 Application Examle p SLM of ZrO 2 -based Ceramics 10 mm  Zirconium oxide (ZrO 2 ): 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 SLM demo parts Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008)

11 11 Optical Communication: E ary l driver for high power laser dioes d Under the OceanOn the Ground Optical communication for inter satellite, “submarine” and terrestrial areas. Supporting phone, internet and TV related digital data traffic. g In Space

12 12 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 area defence system (LADS from Raytheon) (public Information) - Laser fuses -Illumination - Detection of chemicals

13 13 Information technology: optical storage, printing, display CTP Printing- Computer to plate (CTP) P r i nng ti -Digital printing - Display (RGB) technology - Rear projection TV

14 14 Medical applications: Surgery BeforeAfter Skin Treatment: Tattoo / Hair Removal - Acne treatment - Photodynamic Therapy (PDT) - Photodynamic Disinfection (PDD) - Dental BeforeAfter Hair removal Ophtalmology

15 15 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

16 16 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

17 17 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 Information Technology Red and short wavelength diodes, direct diode systems, SHG (frequency doubling) Medical & Life science Low energy, direct diode, DPSSL, fibre laser  – pumpng, rect oe snge YAGiDidi d ( i l element, stacks), DPSSL, fibre laser,

18 18 Laser Market Development by Laser Type Laser Type Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008) World Market 2007 Extrapolation of current trends to 2010 Including new applications / markets 2010

19 19 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

20 Why High Power 980nm Pumps? Typical 3-stage EDFA E rbiu m Doped Fiber Dispersion Compensating Fiber Gain Flattening ErbiumErbi Filter Doped Fiber um Isolator & filter CouplerIsolator Input Coupler 980 Pumps:  Avn : d a tage 1480 Pumps (now 980nm !):  Advantage: Output  Low noise figure (3 level)  Low heat load  High laser efficiency  Low cost coupler  Un-cooled operation  Disadvantage  Lower optical power conversion efficiency  higher optical conversion efficiency  Disadvantage  Increased noise figure  More expensive coupler  High heat load, expensive cooling High power 980 pumps enable low cost EDFAs

21 14xy Pump laser for Raman amplifier Features —  Coupling to optical phonons in the glass —  max. gain at frequency shift of 13 THz in silica ( nm) —  broad (but not particularly flat) gain spectrum Advantaes g —  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 0.0E E E E E-04 Raman pump wavelength 1455 nm 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)

22 22 Laser system desin g principles transversally p um p ed rodlaser INNOSLAB laser fibre laser end-pumped thinfibre l disk laser aser “Y” -pumped All based on the same design principle requiring  Mirrors (Cavity)  Gainmedium  Pump source Extension... amplifier technology Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008)

23 23 Fibre laser pumping scheme CP-active fiber Multi-mode pumps Fiber com b i ne r 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 Single Mode Pump “Seed laser” ~1 064nm Fiber laseroutput optic Multi-mode pumps Filte r

24 Presentation titleTRUMPF abbreviation /03/2524 Rod- and Disklaser  Lamp pumped Rodlaser Beam quality Efficiency  Arc lampsRod  Diode pumped Disklaser Laser- diodes Disk Beam quality Efficiency

25 Presentation titleTRUMPF abbreviation /03/2525 Disklaser: TruDisk TruDisk 1000TruDisk 8002

26 Presentation titleTRUMPF abbreviation /03/2526 Design of a disklaser (8 kW) Resonato r

27 Parabolic Mirror Outcoupler Bending prisma Laser beam Actually: 20 passes of the pump light through the disk! Pumping of a Disk 1 Disk 1 Pump- Beam C avy it End- mi rror Presentation titleTRUMPF abbreviation /03/2527

28 Presentation titleTRUMPF abbreviation /03/2528 > 5 kW Output Power per Disk Pump Power P p (W)

29 Presentation titleTRUMPF abbreviation /03/2529 TruDisk –4 disk resonator % 60% 50% 40% 30% 20% 10% 0% Pump Power Pp [W]

30 30 Fiber, Disk or Direct Diode Laser? 1000 Ha rd en ing Polymer weldingB razi ng Sold eri ng ‘SM’fiber Welding, sintering, surface treatment Cladding D ee-en etration p p we l ding Metal cutting Non-Metal cutting Marking Drilling Pri nti ng Sources: P. Loosen, Fraunhofer Inst. Aachen Laser output power (W)

31 31 Fiber, Disk or Direct Diode Laser?.... many applications don’t need highest beam quality (diffraction limited beam). Sources:  P. Loosen, Fraunhofer Inst. Aachen  C. Harder LEOS 2006 Laser output power (W) Hardening Polymer welding Cladding Brazing D ee-en etration p p we l ding Metal cutting 200um0.22, 60um, Sold eri ng Welding, sintering, surface treatment 10P 50um.0.22 ri nti ng Non-Metal cutting Marking Drilling 1 ‘SM’fiber 1 500um,0.22

32 32 2w 0 f BPP =  · w 0 Developing trends for Lasers and their applications Printing therm. marking Diode lasers (2003) F#4 focusing optics (NA 0,12) soldering lamp pumped Nd :YAG-laser plastics welding selective laser powder remelting CO 2 -laser brazing metal see h t cutting DPSSL transformation hardening melting, cleaning deep penetration welding metals Laser Power P [W] Source: Prof. Dr. Reinhart Poprawe, ILT (AKL 2008)

33 33 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

34 34 Materials for Semiconductor Laser Processing

35 35 Wavelength Range for major industrial applications / technologies requiring (red / MIR) HPL 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 600nm 700nm 800nm 900nm 1000nm 1100nm 1400nm 1600nm GaP Capability GaAs Capability InP Capability

36 36 Pearls of Laser History MOPA, SEDFB, α-DFB Tunable l Quantum Dots, Quantum Cascade Lasers, new materials, new process technologiesnew integration technologies intercavity Lasers (SHG) monolithic Integration asers FBG-Stabilization Flared / Taper Design Efficiency I mprovements New Waveguide Designs Materials Laser bars / Stacks s1960s1970s1980s1990s2000s2010s Herbert Nelson, RCA Liquid Phase Epitaxy 1962

37 37 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

38 38 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....

39 39 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

40 40 Challenges for the design of HPL Output Power, Capacity Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) – Quality assessment Epitaxial Structure Waveguide Design

41 41 Challenges for the design of HPL: Wall-plug efficiency – Broad area single emitter laser Output Power, Capacity Key measure for high power laser diodes:   WP hI I       dthdth −  −  − h I  1 qIVqVI (wall-plug efficiency) Others: defect density, material properties (i.e. expansion match) Epitaxy Design parameters:  bandgap profile, carrier confinement  d/gamma, confinement factors  doping profile,  growth process Key characteristics:  internal losses,  internal quantum efficiency,  series resistance, laser voltage  thermal resistance Waveguide characteristics:  Optical Confinement,  Vertical far field,  Coupling efficiency,...

42 42 Similar laser structures with varying  WP WPE 1 WPE 2 P 1 P 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%)

43 43 Differential quantum efficiency  i  −  −  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    di ln  R 1  2  i L 11  −  distributed loss  +  +  i, iFCscatt.coupl. Free carrier absorption  p =  p;  n =  n

44 44 Bandgap engineering example: Graded Index carrier confinement (GRICC) InGaAs/AlGaAs GRICC design  provides efficient 0.8 carrier confinement  stable control of vertical waveguide  low internal losses (below 1cmreported) Hole Quasi- Fermi Level -1.5 Valence Band -2.0 Vertical Position (a. u.) B. Schmidt et al. Proceedins g of SPIE Vol. 5248, SPIE, Bellinham g, WA, 2003, Electron Quasi- Fermi Level Conduction Band

45 45 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)

46 46 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 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)

47 47 Minimizing laser diode voltage Laser voltage V = V j +IR s + V hb :*  V j - 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  d and high d opng i f or l ow R s M. Kanskar et al., Electron Lett, v.41, (2006)

48 48 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 W roll over at 25 C (CW)  High wall plug efficiency

49 49 Challenges for the design of HPL Output Power, Capacity Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) – Quality assessment Epitaxial Structure Waveguide Design

50 50 Optical Waveguide & Substrate modes: GaN* laser diode GaNSiCAl2O3 Strain % Refr. Index SNOM Simulation GaN-Substrate, d tot = 100  m 2 µm Potential Impact of Substrate Modes: vertical section... - otical loss p - gain oscillations *Source: Prof. B. Witzigmann et al., IEEE JQE 2007

51 51 Challenges for the design of HPL: Waveguide Design – single mode ridge laser Output Power, Volume SQW - GRICC Structure InGaAs/AlGaAs intensity profile gain 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

52 52 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

53 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,..) B.Schmidt et al., Proceedings ISLC 2002 Temperature induced index change 53

54 (significant higher relative increase of optical losses of 1 st, 2 nd and.. order modes as compared to the zero order mode) => suppression of coherent coupling with higher order modes (higher linear power) 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 * Wenzel et al., “Fundamental-Lateral Mode Stabilized RWG Lasers”, IEEE Phot. Tech. Let., Vol.20, No.3, 2008

55 55 Length scaling Both electrical and thermal resistance of the chip are inversely proportional to the its waveguide active area; broad area laser V. Gaontsev et al. Proc. o SPIE v p,f( injected current (mA)

56 ( ) z g Asymmetric power/current distribution along the wavguide Power distribution inside laser cavity Optical intensity distri R front = 0.1% R back = 100% cavity length (a.u.) front facet reflectivity (percent) =>> refractive index and gain profile changes along laser cavity Δ  F * z  N z ( ) ( ) z * ) V ΔFΔF  j z z ( ) (  ( ) z  R spstim j (z(z R stim Γ ( ) zg  hcS )  ( )( )  P z  − facet Rate equations refl ed Voltage on laser e I  L j z dz 0 ( )

57 Challenges for the design of HPL: Waveguide Design – single mode ridge laser inected current mA j () Output Power, Volume junction side up CW-operation = ° T 25C G03 G05 G07 G06 G08 >100MW/cm 2 Waveguide Design Length Scaling, Spatial hole burning, local heating, Carrier injection Refractive index, slow axis divergence

58 58 Challenges for the design of HPL Chip Reliability (Bulk / Facet) – Quality assessment Output Power, Capacity Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Epitaxial Structure Waveguide Design

59 59 Long term reliability & COMD protection Catastrophic Optical Mirror Damage (COMD) Free carrier absorption Stimulated Emission Absorption: 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

60 60 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 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, 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 L.. K Lindsrom, tet al."Meo th d to obtan i contamnaon i ti free laser mirrors and passivaon tiof these," U.. S Patent No H. Kawanishi, et al."Semiconductor laser device with a sulfur-containing film provided between the facet and the protective film," U.S. Patent No E.C. Onyiriuka, M.X. Ouyang, C. E. Zah, "Passivation of semiconductor laser facets," U.S. Patent No 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,

61 61 Long term reliability & COMD protection E2 by Bookham: Stress test of first generation 980nm SM laser diodes Time (years)

62 62 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 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, J. Ungar, N. Bar-Chaim and I. Ury, "High-Power GaAlAs Window Lasers," EL 1986, vol. 22, no. 5, R.. L Thornon, tD.. F W ec, l h R.. D B urnam, hT 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, 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, J.H. Marsh, C.J. Hamilton, "Semiconductor laser," U.S. Patent No

63 63 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

64 64 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 NAM 0 nm shift Current (mA) Increase of the bandgap close to the facet region Reduction of free carriers Reduction of local absorption NAM 65 nm shift QWI regions Transparent output waveguide Conventional Laser

65 65 QWI process concept As-grown bandgapIntermixed bandgap Al Ga AlGaAsGaAsAlGaAsAlGaAs QWI allows bandgap tuning in selected areas of the chip

66 66 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 workPhotoluminescence spectra Intermixing capSuppressing cap QW Wavelength, nm 40 Suppressed Intermixed High Temperature Anneal

67 67 Life testing array with 64 single emitters (QWI – technology) 64 emitter life test Each device operates at 808nm with >200mW output power, room temp.

68 68 Challenges for the design of HPL – Epitaxial Structure Waveguide Design Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) Quality assessment Robustness for Volume Manufacturing Output Power, Capacity

69 69 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

70 70 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 a k b T) ∝  x y −  Constant failure rate can be described by: FRI P exp(E t onset

71 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: FIT Current (mA) °C Testi ng °C Use °C ° 65 C

72 72 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)

73 73 Central life test philosophy

74 er – 0.15 or 0.22NA – R th = 2.2 o C/W –rae pow 10W t d 74 © 2008 JDSU. All rights reserved.SSDLTR June 2008 Example: JDSU L4 module performance & testing  Fiber-coupled package – 105  m diameter – 50% wall plug Current (A)  Laser chip InAlGaA s – – nm – 100  m aperture 20 – 4.1mm cavity – AuSn solder % ex-ace f t ex-fiber 15C 45% 30% 60% 15% 0% 10 5

75 10% 5% 0% -5%  Packae Test g – 10W, 12A – T case = 35 o C – 14 lasers – 0 failures -10% ,,,, Time (hours) 75 © 2008 JDSU. All rights reserved.SSDLTR June 2008 Accelerated life test examples  Chip Life Test – 9., 3W 13A – T case = 70 o C – T j = 117 o C – 28 lasers – 0 failures

76 Time Zero Mech Shock 85  C / 85% RH Damp Heat Vibration 76 © 2008 JDSU. All rights reserved.SSDLTR June 2008 L4 Package qualification and robustness tests  Reference Telcordia GR-468 – ero pacage aures n u sute Zkf ilif lli – Proves robustness of design 500G Mechanical Shock + 20G Vibration -10% 10% -5% 5% 0% 18 units Time (hours) -40  C to +85  C Temperature Cycling -10% 10% -5% 5% 0% 27 units No of Cycles 10% 5% 0% -5% -10% 12 units

77 Pump power trends – commercially available 10 1 L46398 L L L L  9XXnm 105  m diameter fiber “Reliable” rated power Year  15% annual increase in reliable power  imilar trend or Sf – 8XXnm – Singlemode lasers – Multi mode lasers – Bars 77© 2008 JDSU. All rights reserved.SSDLTR June 2008

78 78 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)

79 79 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 liof 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

80 Current (A) Presentation titleTRUMPF abbreviation /03/2580 Current (A) Presentation titleTRUMPF abbreviation /03/2580 Current (A) Presentation titleTRUMPF abbreviation /03/2580 Results – Compare FF = 50% to FF = 33%  Microchannel Cooler (MCC) R th = 0.23 C/W L = 3. 5 mm P = 200 W (FF=50%) P = 207 W (FF=33%) MCC Tj = 60 C FF=50% Model FF=50% Data FF=33% Model FF=33% Data

81 Current (A) Presentation titleTRUMPF abbreviation /03/2581 Current (A) Presentation titleTRUMPF abbreviation /03/2581 Results – Compare FF = 50% to FF = 33%  Mesochannel Compact Heat Exchanger (MesCHE) R th = 0.38 C/W L = 3.5 mm P = 127 W (FF=50%) P = 151 W (FF=33%) MesCHE 160 Tj = 60 C FF=50% Model FF=50% Data FF=33% Model FF=33% Data

82 82 Life testing of laser diode bars: modes of operation Short pulse operationI ntermittent-CW operation Optical pulse Current (us) Semiconductor / Mount Temp Mechanical stress Current (s) GaAs α = 6 ppm/ °K Semiconductor / Mount Temp Mechanical stress CuW α = 6.5ppm/ °K Optical pulse Cu α = 16.5ppm/ °K - 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

83 83 Example: QCW-Stack HERMES QCW Stack  10 Bar QCW Stacked Diode Arrays  200W 808nm  250u sec x 20Hz  Hard solder  MIL-SPEC % 9 % Onoin life test g g 9 % E+070.0E+005.0E+071.0E+081.5E+082.0E+082.5E+083.0E+083.5E+08 Number of Shots

84 Drive Current [A] Current [A] Improved optical and electrical design – Low drive 63A (50%FF) / 42A (30%FF) – High wallplug 60W, 57% max WP – Improved far field pattern:7 ° x 55 ° (90% power) 808 BAR: High Performance in CW

85 85 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 808 BAR: Reliability assessment (Intermittent operation) W / 75A 105W / 100A

86 86 Challenges for the design of HPL Output Power, Capacity Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Chip Reliability (Bulk / Facet) – Quality assessment Epitaxial Structure Waveguide Design

87 87 Various pumping/coupling schemes end pumping with fiber combiner laser barlens step mirror End pumping with collimated bar, Direct applications

88 88 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)

89 Coupling pump radiation into fiber laser Pump radiation is usually supplied through multimode fibers with core diameter D = mm and numerical aperture NA = P end pumping with fiber combinerside pumping In other words:it is impossible to increase radiance by combining radiation of Thtdh pw p ni rea p ui slid agl i nsd several uncorrelated but otherwise identical sources Fiber radiance R  P  2 D NA core  Max. No. of pumps per node 2laser D NAclad pump  D NA  corecore Key objective: Optimize brightness per pump source! 89

90 90 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

91 91 Mini-Bar Fiber Pump Module - Brightness of industrial modules now exceeds 1 MW/cm 2 sr OrionTM series - 20W, 105um core, 0.20NA 3. 8cm - 915nm, 940nm, 976nm mini-bar architecture very high reliability 1503 Specified Operating Pt Power PCE Current (A) H2 short presentation

92 92 Mini-Bar Reliability: verification of emitter independence 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 single-emitter failures CoS: 5-element mini-bar on CT (AuSn solder on CuW heatsink) H2 short presentation 92

93 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

94 Bar multiplexing to achieve highest optical power densities for direct application Picture with courtesy of

95 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

96 Wavelength and Polarization multiplex Stack 1 (interleaved 2 x 10 bars) Output Power System: 2 X Pstack X 3 = 10kW- 12kW BPP ~ BPP (Stack) ~ mm*mrad  opticsoptics Stack 2 Polarization combining  

97 Challenes g for the desin g of HPL Robustness for Volume Manufacturing Subsystem Design (Cost, Brightness, Reliability) Output Power, Capacity Chip Reliability (Bulk / Facet) – Quality assessment Epitaxial Structure Waveguide Design

98 Efficient design for robust volume manufacturing: one design platform for multiple packaging formats TO Can for smaller arrays Laser Array on Ceramic Carrier Low cost Butterfly Package for larger arrays Marking Module Laser Array

99 99 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

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