Advances in High-Power Fiber and Diode Laser Engineering

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Contents
1 Diode laser: fundamentals and improving the brightness
	1.1 A brief history of high-power semiconductor laser: the rise of a disruptive technology
		1.1.1 The beginning of a semiconductor laser
		1.1.2 Era of high-temperature operation—birth of quantum-well gain medium
		1.1.3 Era of high-reliability operation
		1.1.4 Race for super high efficiency for defense application
		1.1.5 Era of high brightness—the rise of a disruptive technology
	1.2 High power and high brightness broad area diode lasers
		1.2.1 Fundamentals of diode lasers
		1.2.2 Optical gain medium—quantum well
		1.2.3 Optical waveguide
		1.2.4 Optical feedback
		1.2.5 Electrical-to-optical power conversion efficiency
	1.3 Mitigation of slow-axis divergence blooming in broad area diode lasers
		1.3.1 Why brightness degrades as BPP increases
		1.3.2 The origin of slow-axis divergence blooming in high-power diode lasers
		1.3.3 Mitigating slow-axis divergence blooming
			1.3.3.1 Reducing junction temperature via thermal management
			1.3.3.2 Reducing thermal gradient across the stripe-width
			1.3.3.3 Reducing the divergence by suppressing higher-order modes
		1.3.4 Fiber-coupled multi-single-emitter diode lasers
		1.3.5 Reliability of fiber-coupled multi-single-emitter diodes
	1.4 Diode laser applications
		1.4.1 Diode-pumped solid-state lasers and fiber lasers
		1.4.2 Markets and applications
			1.4.2.1 1980s: optical storage and initial niche applications
			1.4.2.2 1990s: optical networking boom
			1.4.2.3 2000s: laser as a tool
	1.5 The future prospects of diode lasers
		1.5.1 Increasing power and efficiency
		1.5.2 Reducing slow-axis BPP and increasing fast-axis brightness
		1.5.3 Increasing submount thermal conductivity
		1.5.4 Improving optical coupling scheme
	Acknowledgments
	References
2 Coherent beam combining architectures for high-power laser diodes
	2.1 Introduction
	2.2 High-power semiconductor lasers and amplifiers used for coherent beam combining
		Arrays versus individual emitters
	2.3 Principles of coherent beam combining architectures
		2.3.1 Phase locking
		2.3.2 Coherent superposition
			2.3.2.1 Tiled aperture approach: far-field superposition
			2.3.2.2 Filled aperture approach: near-field superposition
	2.4 Master-oscillator power amplification architectures
		2.4.1 Coherent beam combining of amplifier arrays
			2.4.1.1 Ridge waveguide amplifiers
			2.4.1.2 Tapered waveguide amplifiers
			2.4.1.3 Limitations related to coherently combined arrays
		2.4.2 Coherent combining of individual amplifiers
			2.4.2.1 Coherent beam combining of three high power tapered amplifiers
			2.4.2.2 Coherent beam combining module based on commercially available amplifiers
	2.5 Extended-cavity architectures
		2.5.1 Principles of operation
		2.5.2 Cavity architectures based on beam superposition
			2.5.2.1 General description
			2.5.2.2 Self-organisation and spectral filtering
			2.5.2.3 Multi-arm interferometric cavities: experimental results
			2.5.2.4 Back-side resonator configurations
		2.5.3 Parallel coupled cavities
			2.5.3.1 Cavities using near-field spatial filtering: the Talbot cavity
			2.5.3.2 Cavities using far-field angular filtering
	2.6 Conclusion
	References
3 High-power laser diodes for direct applications and laser pumping
	3.1 Introduction
	3.2 High power broad area lasers
		3.2.1 Motivation
		3.2.2 Device configurations and performance comparison
		3.2.3 Challenge 1: efficiency and power
		3.2.4 Challenge 2: beam quality
		3.2.5 Challenge 3: external stabilization
	3.3 High power laterally single mode lasers
	3.4 High power lasers with monolithic grating stabilization
		3.4.1 Overgrown gratings
		3.4.2 Surface gratings
		3.4.3 Comparison
	3.5 Seed lasers
		3.5.1 Gain switching
		3.5.2 Q-switching
		3.5.3 Mode locking and pulse picking
		3.5.4 Pulse gating
	3.6 Wavelength limits on GaAs-based high radiance quantum well lasers
		3.6.1 Introduction
		3.6.2 Short wavelength limit
		3.6.3 Long wavelength limit
	3.7 Conclusions and path forward
	Acknowledgments
	References
4 Quantum cascade lasers
	4.1 Introduction
	4.2 Governing equations for pulsed QCL operation
	4.3 Laser core design
	4.4 Waveguide design
	4.5 CW power scaling
	4.6 Beam combining
	4.7 External cavity QCLs
	4.8 Distributed feedback QCLs
	4.9 Conclusion
	References
5 Diode pumped high power lasers
	5.1 Material selection
		5.1.1 Laser properties
		5.1.2 Thermal properties
		5.1.3 Nonlinear properties
		5.1.4 Gain bandwidth
		5.1.5 Comparison
	5.2 Laser amplifiers
		5.2.1 Regenerative amplifiers
		5.2.2 Multipass amplifiers
		5.2.3 Chirped pulse amplification
	5.3 Geometry of the active medium in high power amplifier
		5.3.1 Rod-type amplifiers
		5.3.2 Fiber amplifiers
		5.3.3 Thin-disk and active mirror amplifiers (incl TRAM)
		5.3.4 Slab (zig-zag, multislab, innoslab)
	5.4 Thin-disk high power system
		5.4.1 Yb: YAG active medium
		5.4.2 Pump geometry for thin-disk lasers
		5.4.3 Zero-phonon-line pumping
		5.4.4 Thin-disk module manufacturing
		5.4.5 High power regenerative amplifiers
		5.4.6 Thin-disk-based multipass amplifier
	5.5 Multislab high power system
		5.5.1 Modeling [ 103,104]
		5.5.2 System layout
			5.5.2.1 Front-end
			5.5.2.2 10 J main preamplifier
			5.5.2.3 100 J power amplifier
		5.5.3 Output parameters
	References
6 High average power large mode area (LMA) fiber amplifiers
	6.1 A brief history of fiber lasers
	6.2 Advantages of fiber lasers
	6.3 Rare-earth-doped fibers
		6.3.1 Fundamentals of optical fibers
		6.3.2 Design of fiber amplifiers
	6.4 Limitations of high average power Yb-doped fiber amplifiers
	6.5 Overcoming the limitations of power scaling in combinable LMA fibers
		6.5.1 Mitigating SBS
		6.5.2 Mitigating TMI
		6.5.3 Chirally coupled core fiber technology
		6.5.4 Conclusions
	Acknowledgments
	References
7 Optical fibers for high-power operation
	7.1 A brief historical overview
	7.2 High-power fiber laser systems: performance against the odds
		7.2.1 Stimulated Brillouin scattering
		7.2.2 Stimulated Raman scattering
		7.2.3 Self-phase modulation
		7.2.4 Self-focusing
		7.2.5 Mode shrinking
		7.2.6 Transverse mode instabilities
	7.3 Optical fibers for high-power operation
		7.3.1 The fiber core: guiding mechanisms
			7.3.1.1 Step-index fibers
			7.3.1.2 Photonic-crystal fibers
			7.3.1.3 General considerations
		7.3.2 The fiber cladding: added functionality
			7.3.2.1 Single-clad fibers
			7.3.2.2 Double-clad fibers
			7.3.2.3 Triple-clad fibers
		7.3.3 The fiber material: laser properties
			7.3.3.1 Laser active ions
			7.3.3.2 General considerations
	7.4 Outlook: multicore fibers
	References
8 High power fiber lasers
	8.1 Introduction
		8.1.1 Diode-laser pumped solid-state laser media
	8.2 High power Yb fiber lasers
		8.2.1 Yb: silica spectroscopy
		8.2.2 1 micron fiber lasers
		8.2.3 Fiber laser architectures and fiber design
			8.2.3.1 High power fiber designs
			8.2.3.2 High power fiber laser architectures
		8.2.4 Nonlinear optical loss mechanisms
			8.2.4.1 Stimulated Raman scattering
			8.2.4.2 Stimulated Brillouin scattering
			8.2.4.3 Transverse mode instability
	8.3 High power TM fiber lasers
		8.3.1 Spectroscopic properties
		8.3.2 Cross-relaxation pumping with 790 nm diodes
		8.3.3 In-band pumping with 1,550—1,950 nm sources
	8.4 Other fiber laser media
		8.4.1 Er-fiber lasers
			8.4.1.1 Concentration quenching
			8.4.1.2 Er: Yb codoped fibers
			8.4.1.3 Yb-free Er-doped fiber
	8.5 High power Raman lasers
		8.5.1 Raman fiber lasers for wavelength conversion
		8.5.2 Raman fiber lasers for brightness enhancement
	8.6 Conclusions
	References
9 Beam combinable, kilowatt all-fiber amplifiers for directed energy
	9.1 Introduction
	9.2 Time-dependent nonlinear SBS theory and model
	9.3 Phase modulation in kW class all-fiber amplifiers
		9.3.1 WNS and PRBS SBS suppression comparison
		9.3.2 WNS and PRBS coherent beam combining analysis
		9.3.3 Filtered PRBS phase modulation
			9.3.3.1 Filtered PRBS: coherent combining
			9.3.3.2 Filtered PRBS: SBS suppression
		9.3.4 PRBS re-coherence
	9.4 Multi-kW coherent beam combining of PRBS modulated fiber amplifiers
	9.5 Laser gain competition of all-fiber amplifiers
		9.5.1 Laser gain competition (two-tone): power scaling
		9.5.2 Laser gain competition (two-tone): beam combining
	9.6 Conclusion
	Acknowledgments
	References
10 Applications of high-power 2 μm thulium fiber lasers in materials processing
	10.1 Introduction
	10.2 Interaction of 2-μm laser light with materials
		10.2.1 Polymers
		10.2.2 Semiconductors
			10.2.2.1 Absorption in semiconductors in the infrared
			10.2.2.2 Nonlinear material response in semiconductors
			10.2.2.3 Alternative material modification mechanisms
			10.2.2.4 Temperature dependence of absorption processes
		10.2.3 Infrared optical materials
	10.3 Joining of polymers
		10.3.1 Experimental details
		10.3.2 Butt-welding experiments
		10.3.3 Transmission welding experiments
	10.4 Processing of semiconductors
		10.4.1 Experimental details
		10.4.2 Processing of uncoated semiconductor surfaces
		10.4.3 Processing of coated semiconductor surfaces
	10.5 Processing of chalcogenide glasses
		10.5.1 Experimental conditions
		10.5.2 Characterization of the film composition and morphology
		10.5.3 Refractive index changes
	References
11 High-power GHz linewidth diode lasers and their applications
	11.1 GHz linewidth high-power diode laser sources
		11.1.1 Introduction
		11.1.2 Volume Bragg gratings in PTR glass [9]
			11.1.2.1 Volume Bragg gratings—description and properties
			11.1.2.2 Holographic recording materials
		11.1.3 100 W 20 GHz spectral width laser diode system operating at 1,550 nm
		11.1.4 250 W 10 GHz laser diode system operating at 780 nm
	11.2 Applications of narrow-line high-power diode laser systems
		11.2.1 Spin-exchange optical pumping (SEOP)
		11.2.2 Diode-pumped alkali laser
		11.2.3 Rare gas lasers applications
	11.3 Conclusion remarks
	References
Index
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