C

Laser Sources and Their Fundamental and Engineering Limits

This appendix summarizes in table form the characteristics of lasers and other light sources systems important to active EO sensing and their fundamental and practical engineering limitations. Tables C-1 to C-4 are called out in the laser discussion in Chapter 4. Tables C-5 through C-12 are summarized in Chapter 5.

TABLE C-1 Key Characteristics of Edge-Emitting Interband Diode Lasers

Material Wavelength Range (nm) Single-emitter Power Single-Emitter Efficiency (%) Bar Power (W) Bar Efficiency (%)
GaInN ~380 0.2 10-15
400, 450 1.2-1.6 25-30
~520 0.05 <<10
AlGaInP 639-690 0.75-1.5 20-25
632, 635, 638 2.5-8 W >25
675 20 W >35
GaAlAs 793, 808, 852 5-7 W 60 60-200 50-60
InGaAs 915-976 10-15 60-70 120-200 60-70
1,064 5 W 50-55 60 50-55
InGaAsP/ 1,470-1,532 5-7 W 30-45 60-100 30-40
AlInGaAs 1,600-1,700 3 W 20-25 40 20-25
AlGaInAsSb 1,900-2,100 1-2 W 10-15 15-20 W 10-20
2,300-2,500 1 W 5-10

SOURCE: Data provided by Steven Patterson, DILAS, Tucson, Ariz.



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C Laser Sources and Their Fundamental and Engineering Limits This appendix summarizes in table form the characteristics of lasers and other light sources systems important to active EO sensing and their fundamental and practical engineering limitations. Tables C-1 to C-4 are called out in the laser discussion in Chapter 4. Tables C-5 through C-12 are summarized in Chapter 5. TABLE C-1 Key Characteristics of Edge-Emitting Interband Diode Lasers Wavelength Range Single-emitter Single-Emitter Bar Efficiency Material (nm) Power Efficiency (%) Bar Power (W) (%) GaInN ~380 0.2 10-15 400, 450 1.2-1.6 25-30 -- -- ~520 0.05 <<10 -- -- AlGaInP 639-690 0.75-1.5 20-25 -- -- 632, 635, 638 -- -- 2.5-8 W >25 675 -- -- 20 W >35 GaAlAs 793, 808, 852 5-7 W 60 60-200 50-60 InGaAs 915-976 10-15 60-70 120-200 60-70 1,064 5W 50-55 60 50-55 InGaAsP/ 1,470-1,532 5-7 W 30-45 60-100 30-40 AlInGaAs 1,600-1,700 3W 20-25 40 20-25 AlGaInAsSb 1,900-2,100 1-2 W 10-15 15-20 W 10-20 2,300-2,500 1W 5-10 SOURCE: Data provided by Steven Patterson, DILAS, Tucson, Ariz. 287

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288 LASER RADAR TABLE C-2 Key Characteristics of State of-the-Art Cascade Diode Lasers QCL: InP/InGaAs/InAlAs Wavelength of best operation: 4-5 µm; output power: 5 W, CW with 21% efficiency (obtained with AlAs inserts to increase the effective barrier heights). Wavelengths between 2.9 and 4 µm and from 5 to 150 µm operate with lower performance. QCL: GaAs/GaAs/AlGaAs Wavelength of best operation: 10 µm; output power: 80 mW at 77 K; CW up to 150 K. Far-IR wavelength: 100 µm; output: 8 mW, CW at 45 K with 0.2 % efficiency. No CW above 117 K. Full wavelength range = 9-300 µm; non-competitive in mid-IR. QCL: GaSb/(InAs/AlSb) Wavelengths; 2.6 – 5 µm. CW only with TE cooler QCL: InP/InGaAs/AlAsSb To date, non-competitive with InGaAs/InAlAs at any wavelength. No CW room temperature operation. ICL on GaSb Wavelength: 3-4.2 µm; output power: 360 mW; efficiency: 15%. Wavelengths from 4.2-6 mm with lower performance. ICL on InAs Wavelength: 5.3 µm; Output Power: 40 mW at 180 K; CW up to 248 K. Wavelength range: 5.3-10.4 mm. NOTE: QCL, quantum cascade laser; ICL, interband cascade laser. SOURCE: Data provided by Jerry Meyer and Igor Vurgaftman, Naval Research Laboratory, Washington, D.C. TABLE C-3 Characteristics of Several Common Bulk Laser Materials Saturation Wavelength Storage Time Cross section Gain Linewidth Fluence Material (nm) (msec) (cm2) (nm) (J/cm2) Nd:YAG 1,064 0.24 2.8 × 10-19 0.6 0.66 Nd:vanadate 1,064 0.09 1.1 × 10-18 1.0 0.17 -19 Nd:YLF 1,047 0.485 1.8 × 10 1.0 1.0 -20 Nd:glass 1,050-1,060 0.3-0.4 3-4 × 10 20-30 4.7-6.3 -20 Yb:YAG 1,030 0.95 2.1 × 10 9 9.2 -19 Yb:YAG(77K) 1,030 0.85 1.1 × 10 1.5 1.8 Er:YAG 1,645 7.6 5.0 × 10-21 5 24 -21 Er:glass 1,550 7.9 8.0 × 10 55 16 -20 Ho:YAG 2,090 8.5 1.3 × 10 25 7.3 Ho:YLF 2,050 15 1.8 × 10-20 25 5.3 -19 Ti:sapphire 800 0.0032 3.0 × 10 225 (100 THz) 0.83 -18 Cr:ZnSe 2,450 0.006 1.3 × 10 1,000 (50 THz) 0.06

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APPENDIX C 289 TABLE C-4 Properties of Hybrid Lasers Wavelength Pulse Energy Pulsewidth Pulse Rate Material (nm) (mJ) (ns) (Hz) Er:YAGa 1,617 30 42 30 b Er:YAG 1,645 4.2 100 1,000 Er:YAGc 1,645 1.6 1.1 10,000 d Er:YAG 1,645 60 W CW e Ho:YLF 2,050 170 20 100 Ho:YLFf 2,050 100 20 1,000 g Ho:YLF 2,050 115 W CW h Ho:YAG 2,090 125 20 100 i Ho:YAG 2,090 22 70 1,000 j Ho:YAG 2,090 1.7 50 35,000 a J.W. Kim, J.I. Mackenzie, J.K. Sahu, and W.A. Clarkson, “Hybrid fibre-bulk erbium lasers—Recent progress and future prospects,” 7th EMRS DTC Technical Conference, Edinburgh, 2010. b D.Y. Shen, J.K. Sahu, and W.A. Clarkson, “Highly efficient in-band pumped Er:YAG laser with 60 W of output at 1645 nm,” Opt. Lett. 31:754, 2006. c R.C. Stoneman, R. Hartman, E.A. Schneider, A.I.R. Malm, S.R. Vetorino, C.G. Garvin, J.V. Pelk, S.M. Hannon, and S.W. Henderson, “Eye-safe 1.6-µm Er:YAG transmitters for coherent laser radar,” Proceedings 14th Coherent Laser Radar Conference, July 8-13, 2007, Snowmass, Colo. d D.Y. Shen, J.K. Sahu, and W.A. Clarkson, “Highly efficient in-band pumped Er:YAG laser with 60 W of output at 1645 nm,” Opt. Lett. 31:754, 2006. e A. Dergachev, “45-dB, Compact, Single-Frequency, 2-µm Amplifier,” paper FTh4A.2 in Lasers, Sources, and Related Photonic Devices, OSA Technical Digest (CD), Optical Society of America, 2012. f Ibid. g Ibid. h K. Schmidt, C. Reiter, H. Voss, F. Maßmann, and M. Ostermeyer, “High Energy 125mJ Ho:YAG (2.09 um) MOPA Double Pass Laser System Pumped by CW Thulium Fiber Laser (1.9 um),” paper CA3_4 in CLEO/Europe and EQEC 2011 Conference Digest, OSA Technical Digest (CD), OSA, 2011. i Ibid. j A. Hemming, J. Richards, A. Davidson, N. Carmody, S. Bennetts, N. Simakov, and J. Haub, “99 W mid-IR operation of a ZGP OPO at 25 percent duty cycle,” Opt. Express 21:10062, 2013.

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290 LASER RADAR TABLE C-5 Fundamental Limits of Diode Lasers: Interband, Edge-Emitting Property Value Limit Reason Comments Electrical 100% Fundamental One photon per one injected carrier efficiency energy (theory) conservation Electrical 40-70% Multiple device Ohmic losses, injected carrier spreading away from lasing efficiency issues region, active region absorption loss, Auger losses (at long (actual) wavelengths) Wavelength >380 nm Materials Limit of GaAlN material, no other large-bandgap II-VI has allowed PN junction fabrication <520 nm Materials Limits of GaAlN material >630 nm Materials Lack of semiconductors with bandgaps in green-yellow-red region that can form PN junctions <2,500 nm Materials Limits on active materials and DBR structures Power out 0.1-0.5 W pn junction Height of emitting region limited to 0.5 µm by junction (1 emitting physics, facet height, width limited by multi-mode operation, intensity facet, 1 TM) damage limited by facet damage levels Power out 15 W (at pn junction Same height limit as above, width of emitting region limited (1 emitting 9xx µm, less physics, by lasing in transverse direction, intensity limited by facet facet, Multi- at others) transverse damage TM) lasing, facet damage Power out 200 W (at Device Efforts to improve heat removal are underway, to allow limit (1-cm-long, 9xx µm, less temperature, to become facet damage, but cost and reliability are linear bar, at others) limited by heat challenges. Efficiency improvements will allow bars to multiple removal generate higher powers emitters) Spectral Several Coupling of Can be reduced to the kHz range through use of an external linewidth MHz diode current cavity (single TM) fluctuations to cavity refractive index NOTE: TM = transverse mode. Refer to Table C-1 for device properties as a function of wavelength.

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APPENDIX C 291 TABLE C-6 Diode Lasers Vertical Cavity (VCSEL) Property Value Limit Reason Comments Electrical 100% Fundamental One photon per one injected carrier efficiency energy (theory) conservation Electrical 40-60% Multiple device Ohmic losses, injected carrier spreading away from lasing efficiency (8xx-9xx issues region, absorption loss in active region, Auger losses (at long (actual) nm) wavelengths) Wavelength >410 nm Materials Limit of GaAlN material, no other large-bandgap II-VI has allowed PN junction fabrication <503 nm Materials Limits of GaAlN material >630 nm Materials Lack of semiconductors with bandgaps in green-yellow-red region that can form PN junctions <2,000 nm Thermal The smaller bandgaps can be bridged with thermally activation activated carriers Power out (1 5-15 mW Active region Diameter of 1 TM limited to ~4 µm on chip by optical emitting facet, heating cavity, limits power for a given current 1 TM) Power out 5W Active region Done with 300-µm emitting region diameter (1 emitting heating region facet, MTM) Power out 230 W Temperature Power from 0.22 cm2 area, can scale further by increasing (VCSEL (maximum limited by heat the area. Low-duty cycles increase peak power to nearly 1 arrays) at 9xx nm) removal kW from 5 × 5 mm area. Spectral Several Coupling of Can be reduced to the kHz range through use of an external linewidth MHz diode current cavity (1 TM) fluctuations to cavity refractive index

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292 LASER RADAR TABLE C-7 Diode Lasers: Quantum Cascade (QCL) Property Value Limit Reason Comments Electrical Variable % Fundamental Each laser transition efficiency limited by energy of efficiency energy photon divided by bandgap of base material, but multiple (theory) conservation transitions in series (typ. 25-75) increase the efficiency by this factor. Electrical 21 %, cw, RT, Multiple device Ohmic losses, injected carrier spreading away from efficiency (40-50% issues lasing region, absorption loss in active region, losses (actual) pulsed, 160 K) through the injector regions. Wavelength = 4.9 µm. Wavelength >3.8 µm Materials Limits to depth of quantum well in InP structures, but (cw) efficiency is low on the short-wavelength end. Pulsed operation to ~3 µm <13 µm Materials Typical limit for room-temperature, cw operation. (cw) Pulsed/cryogenic operation to 30 µm >60 µm Materials Phonon absorption in InP-based devices prevents cryogenics for coverage of 30-60-µm wavelengths, cw THz Power Out 5 W (4.9 µm) Thermal Achieved in high-efficiency (21%) devices Single device heating Spectral Sub-MHz Current and 1/f Can be reduced to the kHz range with current feedback. linewidth noise Intrinsic noise is several hundred Hz. (1 TM) NOTE: Refer to Table C-2 for device properties as a function of semiconductor material system. TABLE C-8 Diode Lasers: Interband Cascade (ICL) Property Value Limit Reason Comments Electrical Variable % Fundamental: Eacc laser transition efficiency limited by the energy of the efficiency energy photon divided by the bandgap energy of the base (theory) conservation semiconductor material, but ICLs employ multiple transitions in series so efficiency is increased Electrical 15% Multiple device Ohmic losses, injected carrier spreading away from lasing efficiency (3.7 µm, cw, issues region, absorption loss in active region, losses through the (actual) RT) injector regions Wavelength >3 µm, cw Materials Limited depth of interband quantum well <6 µm cw Materials Too high current densities at longer wavelengths Power out 0.36 W (3.7 Thermal heating Achieved in high-efficiency (15%) devices single device µm) NOTE: Refer to Table C-2 for device properties as a function of semiconductor material system.

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APPENDIX C 293 TABLE C-9 Solid State Lasers: Bulk Format Property Value Limit Reason Comments Optical Quantum defect Fundamental: energy Example: 76% for 1,064-nm Nd:YAG laser efficiency ≡ ratio of laser conservation pumped at 808 nm. Violated when interaction (theory) wavelength to average between active ions allows >1 excited state per pump wavelength pump photon Optical < quantum defect Multiple Poor spatial overlap of pump and lasing regions efficiency in material, incomplete absorption of pump, (actual) reflection of pump from material surface, losses in laser material, excited state absorption of pump or laser power, upconversion from upper laser level Electrical Pump electrical Fundamental: Energy Example: Pump diodes with 60% efficiency at efficiency efficiency times conservation 808 nm with Nd:YAG laser at 1,064 nm  46% (theory) quantum defect electrical efficiency Electrical < pump electrical Multiple Optical efficiency < quantum defect; pump light efficiency efficiency X quantum loss in transport to laser material. Example: (actual) defect typical diode-pumped Nd:YAG lasers are 20- 25% electrically efficient. Wavelength >286 nm Materials Transparency of host crystal Wavelength <7,150 nm Materials Long-wavelengths have multi-phonon decay, requires low-phonon hosts; may be impractical for high-power. Better hosts (e.g. YLF) can be used for <4,300-nm lasers Average power 2 kW (rods) Material fracture, thermal 1,060-nm, diode-pumped systems. Higher (single device) 10 kW (disks) effects powers with multiple active media: >100 kW is 15 kW (slabs) record (Nd:YAG). Yb:YAG used for thin disks Average power 50 W (rods) Thermal distortion of laser 1,060-nm region, diode-pumped systems. Higher (1 device, 1 kW (disks) material powers possible with multiple active media, diffraction- 15 kW (slabs) >100 kW is present record, using Nd:YAG slab. limited) Yb:YAG used for thin disks Spectral Several Hz Fundamental: Schawlow- Set by spontaneous emission of gain medium linewidth Townes limit into the laser mode. True for all lasers. (theory) Spectral Several kHz Technical noise Fluctuations in optical cavity length from linewidth coupling between pump power and gain medium (actual, ms time refractive index, acoustic noise, other cavity scale) perturbations. External stabilization can reduce technical noise to the Hz level. Spectral 10-50 MHz Environmental drift Slow change in laser cavity temperature linewidth (long term) Mode-locked ∼3.5 fs Fundamental: laser material Value is for Ti:sapphire at 800 nm, Cr:ZnSe is 7 pulsewidth gain-bandwidth. fs at 2500 nm, Nd:YAG is 2 ps at 1,064 nm (theory) Mode-locked ∼4.5 fs Dispersion in optical cavity, Cr:ZnSe is around 50 fs pulsewidth Ti-sapphire at 800 nm mirrors’ spectral response, (actual) nonlinearities.

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294 LASER RADAR TABLE C-10 Solid State Lasers: Fiber Format Property Value Limit Reason Comments Optical Quantum defect = Fundamental: Example: 95% for 1,030-nm Yb:fiber laser efficiency ratio of laser energy pumped at 976 nm. Violated for a few systems (theory) wavelength to conservation when interaction between active ions allows more average pump than one excited state per pump photon wavelength Optical < quantum defect: Multiple Poor spatial overlap of pump and lasing regions in efficiency 88% slope material, incomplete absorption of pump, losses in (actual) efficiency for laser material, excited state absorption of pump or Yb:fiber laser power, upconversion from upper laser level. Electrical Pump electrical Fundamental: Example: Pump diodes with 65% efficiency at 976 efficiency efficiency times energy nm with Yb:fiber laser at 1,030 nm ->62% (theory) quantum defect conservation electrical efficiency Electrical < above Multiple Actual optical efficiency lower than quantum efficiency defect, loss of pump light in transport from diode (actual) facet to pump cladding. Example: Yb:fiber at 1,030 nm -> 40% electrical efficiency Wavelength >248 nm Materials Transparency limit of fiber (up-conversion laser operation in ZBLAN fibers Wavelength <3,900 nm Materials Multi-phonon relaxation limits operation at longer wavelengths Average power 20 kW Stimulated Raman 1,030-nm Yb:fiber laser pumped by multiple 1,018- (single fiber, scattering nm Yb:fiber lasers 1,000-nm) Average power 1 kW Available pump Tm:fiber laser pumped by 790-nm diode lasers (single fiber, power 2,000-nm) Average power 100 W (5 kHz Stimulated Yb:fiber lasers. Removed SBS by … (single fiber, 1 linewidth) Brillouin scattering 100-W result: thermal gradient along length of frequency, 1 kW (3 GHz fiber. 1,000-nm) linewidth) 1 kW result: frequency modulated source Average power 600 W Stimulated Tm:fiber laser (single fiber, 1 (<5 MHz Brillouin scattering frequency, linewidth) 2,000-nm) Peak power 0.45 MW Simulated Raman Yb:silica, rod-type fibers. (single fiber, (27 mJ in 60 ns) scattering, optical Flexible fibers typically generate 25 kW peak ns pulses) breakdown 4.5 MW powers in 400 ns (4.3 mJ; <1 ns) Peak power 3.8 GW Simulated Raman Yb:silica, employs chirped-pulse amplification (single fiber, (2.2 mJ in 0.5 ps) scattering, optical (CPA) to avoid nonlinear effects, rod-type fibers. ps range) breakdown, self- Flexible fibers with CPA operate around 100 MW phase modulation of peak power

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APPENDIX C 295 TABLE C-11 Nonlinear-Optics-Based Sources: Harmonic Generation Property Value Limit Reason Comments Conversion 100% Fundamental: For plane waves efficiency Energy (theory) conservation Second- 90% for flat-profile, high- Multiple De-phasing from finite beam width and heating harmonic energy beams (e.g., NIF) in crystal due to background and multi-photon conversion absorption and creation of color centers, losses 60-70% for Gaussian- efficiency at entrance/exit faces, crystal, material profile beam (actual) imperfections, optical damage. Third- 90% for flat-profile, high- Multiple De-phasing from finite beam width and heating harmonic energy beams (e.g., NIF) in crystal due to background and multi-photon conversion absorption and creation of color centers, losses 50% for Gaussian-profile efficiency at entrance/exit faces, crystal, material beam (actual) imperfections, optical damage. Average Unlimited Process does Nonlinear process has limited power density due power output not dissipate to optical damage. (theory) heat Average Widely variable Background, LBO material has the lowest absorption of power output multi-photon common nonlinear materials, and is limited by (actual) and coating absorption from coatings on the surface. absorption, Multiple hundreds of W for second-harmonic of color-center Nd- or Yb-doped lasers. formation. Shortest 176 nm Phase-match, 176 nm for KBBF crystals. wavelength, vanishing For more readily available BBO crystals, about second nonlinear 205 nm harmonic coefficients

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296 LASER RADAR TABLE C-12 Nonlinear-Optics-Based Sources: Optical Parametric Generation Property Value Limit Reason Comments Conversion 100% Fundamental: For plane waves efficiency from energy pump to signal + conservation idler (theory) Conversion Pump ÷ Signal Fundamental: For plane waves efficiency from wavelengths photon pump to signal conservation power (Manley-Rowe) Conversion Pump ÷ Idler Fundamental: For plane waves efficiency from wavelengths photon pump to idler power conservation (Manley-Rowe) Conversion 90% for cw Multiple De-phasing from finite beam size, de-phasing due efficiency from to heating in crystal from background absorption 50% typical for pump to signal + or multi-photon absorption and creation of color pulsed idler (actual) centers, losses at entrance/exit faces, material Buildup time imperfections, optical damage. reduces efficiency Average-power Unlimited Process does not Ultimate limit due to optical damage. output dissipate heat (theory) Average-power Widely variable Absorption: from See effects for conversion efficiency output coatings, multi- Levels currently below 100 W, but have been (actual) photons, parasitics, limited more by the pump laser power. Multiples color-center of 100 W should be possible in near-IR with formation. materials like LBO