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The Researches and Investigations of Pulsed Laser in NIR Eye-safe Wavelength 脈衝式近紅外人眼安全雷射研究及探討

The Researches and Investigations of Pulsed Laser in NIR Eye-safe Wavelength 脈衝式近紅外人眼安全雷射研究及探討. 張漢龍 指導教授：陳永富 Department of Electrophysics National Chiao Tung University , TAIWAN. 1. Outline. Introduction, Background, and Motivation

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The Researches and Investigations of Pulsed Laser in NIR Eye-safe Wavelength 脈衝式近紅外人眼安全雷射研究及探討

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  1. The Researches and Investigations of Pulsed Laser in NIR Eye-safe Wavelength脈衝式近紅外人眼安全雷射研究及探討 張漢龍 指導教授：陳永富 Department of Electrophysics National Chiao Tung University , TAIWAN 1

  2. Outline Introduction, Background, and Motivation Intracavity Optical Parametric Oscillator (OPO) Pumped by Nd:doped Laser Self-Stimulated Raman Scattering (Self-SRS) Passively Q-switched Erbium/Ytterbium Fiber Laser PCF Laser pumped OPO Optically Pumped Semiconductor Laser (OPSL) Conclusion：Contribution and Future Work 2 2

  3. Eye-safe wavelength UV VIS 100-280nm 400-700nm 315-400nm 280-315nm IR NIR < 1400nm NIR > 1400 nm 3

  4. Background and Motivation High pulse energy High rep rate Wavelength tunable 4

  5. Conventional methods for generating NIR Eye-safe laser • Nonlinear conversion process • Optical Parametric Oscillator • Optical Parametric Amplification • Stimulated Raman Scattering • Rare-earth-ion-doped materials • Er3+, Tm3+, Ho3+ • Semiconductor Laser • InGaAsP, AlGaInAs, GaInAsSb 5 5

  6. Outline Introduction, Background, and Motivation Intracavity Optical Parametric Oscillator (OPO) Pumped by Nd:doped Laser Self-Stimulated Raman Scattering (Self-SRS) Passively Q-switched Erbium/Ytterbium Fiber Laser PCF Laser pumped OPO Optically Pumped Semiconductor Laser (OPSL) Conclusion：Contribution and Future Work 6

  7. Optical Parametric Oscillator GM NLC signal pump Idler External cavity Intra-cavity NLC pump signal Idler OPO Cavity • Lower threshold • Higher efficiency • Dynamic pulse • Higher threshold • Lower cavity stability requirement

  8. NIR Eye-safe Laser with Intra-cavity OPO Requirement Application Laser Range Finder • High pulse energy up to ~mJ • High peak power • 0.x~ x10 Hz Rep rate 8

  9. Passively Q-switched Intra-cavity OPO 1064 nm  1572 nm KTP OPO 3-bars • x-cut KTP: • Temp. insensitive • Large nonlinear coeff. • Non-walk-off • High damage threshold High power QCW LD ASAP simulation

  10. Output coupler with 40% ~ 60% reflectivity is commonly used to optimize energy conversion efficiency. Appl. Phys. B 79, 823-825 (2004) Appl. Opt. V45 N25, 6007-6015 (2006) • How to optimize peak power? • Loss vs. threshold

  11. Threshold of IOPO Photon density of threshold in a passively Q-switched laser …JJ Degan (1995) ψf,max~1.5 x 1017 cm-3 Photon density of threshold in a SRO …Brosnan and Byer (1979) ψf,th = 6 x 1015 cm-3 ~ 6 x 1016 cm-3 The threshold of an intracavity OPO is determinedby the bleach of the saturable absorber. ff,max >> ff,th

  12. Experimental Result • Lasing threshold is nearly constant for different output coupler • There is an individual optimum value of pulse energy and peak power.

  13. Pulse profile Eopo=3.7 mJ Peak power=0.5 MW Eopo=4 mJ Peak power=0.7MW Eopo=3.3 mJ Peak power=1.5MW

  14. Experimental Result Enlarge cross section to enable higher pump power Output energy : 10.8 mJ Opt. Express 15, 4902-4908 (2007) ※ OPO back conversion

  15. Thermally induced birefringence effect Depolarization Nd:YAG • Portion of electric field transfer into another orthogonal axis. Energy feedback 15

  16. Theoretical analysis of Passively Q-switched IOPO Depolarization term OPO loss term 16

  17. Pulse profile 20 ns/div 20 ns/div 10 ns/div 10 ns/div Rs=9% Rs=16% Rs=34% Rs=50% 0.9 MW 0.68 MW 0.62 MW 0.56 MW • Good coincidence between theory and experimental result. • Output coupler with higher reflectivity gives higher feedback of energy and results in more fluctuation in pulse dynamics. • Optimized peak power still holds in lower reflectivity. 17

  18. Summary in IOPO • Up to 10-mJ pulse energy eye-safe laser is achieved with IOPO. • The threshold of passively Q-switched IOPO is dominated by the bleach of saturable absorber. • The birefringence effect induced from thermal effect in Nd:YAG gives rise to parasitic pulse in time domain. Y. P. Huang, H. L. Chang, et al. “Subnanosecond mJ eye-safe with an intracavity optical parametric oscillator in a shared resonator” Opt. Express 17, 1551-1556 (2009) 18

  19. Outline Introduction, Background, and Motivation Optical Parametric Oscillator (OPO) Self-Stimulated Raman Scattering (SRS) Passively Q-switched Erbium/Ytterbium Fiber Laser PCF Laser pumped OPO Optically Pumped Semiconductor Laser (OPSL) Conclusion：Contribution and Future Work 19

  20. Raman Scattering • First discovered by C. V. Raman in 1928. • Third-order nonlinear process. (Four-wave mixing) • Inelastic collision • No consideration of phase-matching 20

  21. Conventional crystals for stimulated Raman scattering A. A Kaminskii et. al. Opt. Com. (2001)

  22. Nd:YVO4 used as a self-SRS crystal Nd:YVO4 can be used to serve as a gain medium and Raman medium simultaneously. Self-stimulated Raman scattering crystal • Heat generation resulting from quantum defect restricts the available output power “Diode-pumped actively Q-switched c-cut Nd:YVO4 self-Raman laser, “Y. F. Chen, V 29. No. 11 Opt. Lett. 1251 (2004) “Compact efficient all-solid-state eye-safe laser with self-frequency Raman conversion in a Nd:YVO4 crystal,” Y. F. Chen, V 29. No. 18 Opt Lett 2172-2174 (2004)

  23. Thermal effect • The influence of thermal effect • Thermal lens • Mode matching • Cavity stability • Conversion efficiency • Beam quality

  24. Thermal effect W. Webber et. al. IEEE J. Quantum Electron 34 (1998) Uniform doped Nd:YAG Undoped YAG-bundled 15W 15W Temp dist. Stress dist. 24 24

  25. Thermal lens in fundamental cavity • Thermal diffuser • Raman gain medium 0.3-% doped Nd:YVO4 2mm 10mm 8mm Nd:YVO4 YVO4 YVO4 20W R=92% • The thermal lens effect is reduced by 1.6 times 25 25

  26. Actively Q-switched intra-cavity Self-SRS Limited by critical power • Lasing threshold decreased. • Maximum power of 2.23 W was obtained. • Conversion efficiency was enhanced from 8.9% to 13% with double-end crystal. 26

  27. 40 kHz 20 kHz Pulse energy : 56 uJ Peak power : 17 kW Pulse energy : 86 uJ Peak power : 22 kW

  28. Summary in Self-SRS A double-end diffusion bond Nd:YVO4 crystal was firstly used to be a self-stimulated Raman scattering crystal in eye-safe wavelength. Thermal effect ↓ (1.6X) Critical pump power ↑ Raman power ↑ Conversion efficiency↑ (9% 13%) Y. T. Chang, K. W. Su, H. L. Chang, et al. “Compact efficient Q-switched eye-safe laser at 1525 nm with a double-end diffusion-bonded Nd:YVO4 crystal as a self-Raman medium” Opt. Express 17, 4330-4335 (2009)

  29. Outline Introduction, Background, and Motivation Optical Parametric Oscillator (OPO) Self-Stimulated Raman Scattering (Self-SRS) Passively Q-switched Erbium/Ytterbium Fiber Laser PCF Laser pumped OPO Optically Pumped Semiconductor Laser (OPSL) Conclusion：Contribution and Future Work 29

  30. Inner cladding (silica) Outer cladding (polymer) Single-mode core (doped) core 1st clad Double-Clad Fiber • Double clad fiber • Low NA ~ 0.07 (Single mode ) • Large mode area • High pump absorption : 3dB/m 30

  31. 4FI9/2 Energy transfer 2F5/2 4FI11/2 Pump 976 nm 4FI13/2 Lasing 1540 nm 2F7/2 Er3+ Yb3+ Er/Yb codoped material • Broad Yb Absorption relaxes pump wavelength constraints • 100X increase in ‘practical’ pump absorption 31

  32. Passively Q-switched Er/Yb Fiber Laser Fiber-coupled LD @976 nm cavity 20W Er/Yb doped double-clad fiber; clad/core: Dia. 300/25 μm NA : >0.46 /<0.07 HT@976 nm HR@1530~1600 nm HR @ 1530~1600 nm SA R~4% 7m Laser output

  33. InGaAsP/InP AlGaInAs/InP SA • Conventional saturable absorber at 1.5 um • Co2+:MgAl3O4, Cr2+:ZnSe, Co2+:ZnS, Co2+:ZnSe • Semiconductor saturable absorber at 1.5 um • Quantum wells: InGaAsP, AlGaInAs • AlGaInAs SA was firstly proposed in eye-safe fiber laser 33

  34. ΔEC Eg,barrier ΔEV AlGaInAs ΔEC ΔEV InGaAsP Semiconductor saturable absorber at 1.5 μm InGaAsP v.s. AlGaInAs • AlGaInAs vs. InGaAsP • Higher conduction band-offset  better confinement • Higher modulation depth • Larger absorption cross section 34

  35. ΔR~50% Semiconductor saturable absorber • Periodic structure with half wavelength of lasing mode  Damage avoided. • High modulation depth results in high pulse energy 35

  36. Laser Performance of Er/Yb Fiber Laser 200 ns/div Q-switching efficiency > 85 % Pulse energy ~ 105 μJ Higher efficiency than bulk SA Higher pulse energy than InGaAsP[1] [1] “Passively Q-switched 0.1-mJ fiber laser system at 1.53 um,” R. Pasbotta, et. al. Opt. Lett V24. 388 (1999)

  37. Summary in EYDFL An AlGaInAs/InP semiconductor absorber was firstly used in eye-safe laser region. No active cooling. High pulse energy up to 100-μJ. Good beam quality was obtained with M2< 1.5 J. Y. Huang, S.C. Huang, H. L. Chang, et. al “Passive Q-switching of Er-Yb fiber laser with semiconductor saturable absorber” Opt. Express, V16, 3002-3007 (2008)

  38. Outline Introduction, Background, and Motivation Intracavity Optical Parametric Oscillator (OPO) Pumped by Nd:doped Laser Self-Stimulated Raman Scattering (Self-SRS) Passively Q-switched Erbium/Ytterbium Fiber Laser Widely tunable eye-safe laser by a PCF laser and OPO Optically Pumped Semiconductor Laser (OPSL) Conclusion：Contribution and Future Work 38 38

  39. Pump Source of External-cavity OPO Yb doped Photonic Crystal Fiber AlGaInAs/InP 70/200 • Single mode • Extreme large mode area • Polarization maintain • High pump absorption: 30 dB/m • Periodic structure with half wavelength of lasing mode • High modulation depth • High absorption cross section 39

  40. External cavity OPO – pump source Yb-doped PCF Passively Q-switched Laser 40 40

  41. Performance of PCF Laser Low power pumping • Central wavelength : 1029~1031 nm • Optical-to-optical conversion efficiency ~ 37 % • Rep rate: 1k ~ 6 kHz • Maximum output peak power : 170 kW High power pumping 41 41

  42. External-cavity OPO L=2.0 cm R=55% • PPLN : • High nonlinear coefficient : 15pm/V (5 x of KTP) • Large phase matching wavelength coefficient (~0.5 nm/℃) 42

  43. Performance of external-cavity OPO pump 20ns/div signal Conversion efficiency ~ 35% Maximum pulse energy : 138 uJ Maximum peak power : 19 kW Pth=0.12W Round trip loss 43 43

  44. Wavelength vs. Temperature Wavelength tuning range: 1513 to 1593 nm 44 44

  45. Summary in PCF pumped OPO H. L. Chang, W. Z. Zhuang, W. C. Huang, et. al “Widely tunable eye-safe laser by a passively Q-switched Photonic crystal fiber laser and an external-cavity optical parametric oscillator” Laser Phys. Lett. To be published. A 750 uJ PCF fiber with AlGaInAs semiconductor saturable absorber was used to be a 1030-nm OPO pump source. Wavelength tuning range up to 80 nm and pulse energy of 138 uJ in eye-safe region from 1513 to 1593 nm was obtained by a PPLN-OPO. 45 45

  46. Outline Introduction, Background, and Motivation Optical Parametric Oscillator (OPO) Self-Stimulated Raman Scattering (SRS) Passively Q-switched Erbium/Ytterbium Fiber Laser PCF Laser pumped OPO Optically Pumped Semiconductor Laser (OPSL) Conclusion：Contribution and Future Work 46 46

  47. Optically Pumped Semiconductor Laser • Advantages: • Uniform distribution of pump power over large active region • Excellent beam quality • Wide wavelength range • Concerning issue • Thermal management • Heat sink/spreader • Pumping style 47

  48. Optically Pumped Semiconductor Laser Barrier QW Barrier Barrier QW Barrier CB CB 1.342μm 1.064μm 1.57μm 1.57μm VB VB In-well pumping Barrier pumping Barrier pumping and in-well pumping Transmission spectrum • The quantum defect of in-well pumping is reduced from 32% to 14% compared with barrier-pumping

  49. Barrier-pumping Q-switched Nd:GdVO4 1064–nm laser Focusing lens 30 groups AlGaInAs QWs Beam expander 1.56μm output Reflection mirror HR at 1.56μm (R>99.8%)HT at 1.06μm (R>80%) PR at 1.56μm (R=95%) PR at 1. 06μm (R=60%) Fluorescence spectrum

  50. The laser performance 30kHz 10°C • The performance of output power depends on the temperature • Output power saturates at 135 mW at a repetition rate of 30 kHz 50

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