Opportunities in Intense Ultrafast Lasers Reaching for the Brightest Light (2018) / Chapter Skim
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3 Current and Future Intense Source Technology
3 Current and Future Intense Source Technology
Pages 49-70
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From page 49... ...
Appendix B1 provides a background for solid-state lasers. Appendix B2 covers the nonlinear optics related to optical parametric amplifiers, particularly the broadband optical parametric chirped-pulse amplifiers (OPCPAs)
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From page 50... ...
The committee also refers the reader to appendixes that describe various specific systems, including brief summaries of critical supporting technologies that are used to facilitate or enhance system operation. 3.1 CURRENT PETAWATT-CLASS SOLID-STATE LASERS AND OPTICAL PARAMETRIC CHIRPED-PULSE AMPLIFIERS Table B.1 in Appendix B1 lists important characteristics of many common laser materials.
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From page 51... ...
It should be noted that this table includes only the range of parameters associated with PW-class systems and not the wide range available from these technologies in general. 3.1.1 Glass-based Systems The first laser to deliver petawatt performance was the "Nova Petawatt," based at the Nova Facility at the Lawrence Livermore National Laboratory (LLNL)
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From page 52... ...
For example, the Vulcan laser at the UK's Central Laser Facility (CLF) has a repetition rate of 1 shot every 20 minutes.
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From page 53... ...
Their designs underscore three limitations to this technology: (1) The gain bandwidth limits output pulses to 0.5 ps in single-glass amplifiers in Nd:glass; therefore, large pulse energies are required and large diffractive optics must be designed to handle the energy (Figure 3.2)
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From page 54... ...
period, the stored energy from the pump laser can be efficiently extracted by a large-bandwidth, stretched pulse. An important point is that multiple pump lasers can be used, as long their beams all overlap inside the active volume of the laser material, and thus energy scaling of the Ti:sapphire laser is not limited by the energy available from a single pump source.
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From page 55... ...
Provided one can find large-enough-aperture nonlinear crystals, sufficiently broad-bandwidth chirped signal pulses, and high-energy pump lasers, OPCPA technology can be scaled up in energy to reach the PW peak-power level. At present there are three published examples of such systems, all employing frequencydoubled Nd:glass lasers as pump sources.
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From page 56... ...
3.2 FUTURE INTENSE SOURCE TECHNOLOGY AND SYSTEMS All current PW systems utilize power-amplifier technologies such as flashlamp pumping that are both well-developed and, at the industrial level, close to being obsolete. Nd:glass lasers, because of their limited average power, find little use in applications other than high-energy systems for ICF research or intense sources 7 R
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From page 57... ...
. 3.2.1 Technical Advances In Appendix B3, the committee describes some technologies, such as diode pumping, that are already being explored for application to intense sources and can be the basis for future systems with performance that goes beyond the limits currently set by flashlamp-pumped, Nd-doped lasers.
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From page 58... ...
Since the gain medium in an FEL is the relativistic electron beam, there are some unique features compared to conventional gain media. The gain bandwidth is limited only by the radiation per undulator period, since the electrons slip one X-ray cycle on every undulator wiggle.
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From page 59... ...
Appendix B3 provides more details on the technology of fiber lasers, along with examples of the current state of the art. 3.2.3 State of Future Intense Source Technology This section briefly assesses and summarizes where future source technology is headed.
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3, More advanced cooling of large-aperture laser materials is being employed to increase the pulse rate of high-peak-power systems, featuring either gas or liquid flow between relatively thin disks of gain media.
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With cryogenic cooling, Yb:YAG lasers have generated 100-J-level pulses with >20 percent optical efficiency. One system, in the planning stage, seeks to generate 25 kW of average power (250-J pulses)
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From page 62... ...
Concerns such as the pulse rate are important in terms of the productivity of the facility. In the facilities under construction in Europe and, to a lesser extent, in Asia, multiple sources based on different technologies are being deployed to, in principle, increase the breadth of science to be done and also increase productivity.
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From page 63... ...
, LCLS-II (SLAC) .a NOTE: FL, Flashlamps; DP, Diode lasers; FL-Nd:YAG, Flashlamp-pumped Nd:YAG; FL-Glass, Flashlamppumped Nd:glass; DP-Glass, Diode-pumped Nd:glass; DP-Yb:YAG, Diode-pumped Yb:YAG; OP, Operational; UC, Under construction; Prop, Proposed or notional.
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From page 64... ...
It is, for example, used for shot peening turbine blades in commercial jet aircraft engines.10 Flashlamp pumping is giving way to diode pumping as the commercial cost of diodes comes down. LLNL has shown ICF power-plant designs that could be based on diode-pumped Nd:glass, in the event that ICF can eventually prove to be feasible.11 There could be a problem with future viability of the technology, however, if key commercial suppliers of large glass slabs and flashlamps decide to exit the business.
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From page 65... ...
The lack of inherent heating in the OPCPA process promises future scaling to high average powers provided appropriate pump lasers can be developed. On the other hand, besides the need for precise control of the pulsewidth and timing, OPCPAs do place a burden on the pump laser in terms of its beam quality.
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From page 66... ...
They do provide the highest average powers for ultrafast systems, at very limited pulse energies. The major driving force for the technology is the promise of high efficiency combined with a high average power and pulse rate, a requirement for future ap plications for intense sources, such as drivers for advanced particle accelerators.
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From page 67... ...
TABLE 3.3 Comparison of Technologies for High-Intensity Lasers Technology Pros Cons Nd:glass High energy pulses Low pulse rate Maturity Long pulses Relative low cost Supply chain Ti:sapphire Highest peak power Heat dissipation Good conversion of pump to output ASE Flexibility, simplicity of pump lasers Ultimate energy limit from crystal size OPCPA No ASE Burdensome pump requirements Large-aperture media Crystal size limits for non-KD* P No inherent heating systems Potential for fs-duration pulses Untested at highest energies, rates Yb-doped bulk Simplicity for direct lasers Not yet at PW level as direct sources Potential for high efficiency Long pulsewidths High average powers as pumps Low efficiency to date as direct sources Yb-doped fibers Promise of high efficiency Many orders of magnitude from PW Promise of high average power, PW Technology of massive beam Now high average ultrafast source combination not at hand Linac-based Shortest wavelength (X-ray FELs)
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From page 68... ...
Mirrors are an option, but dielectric-coated mirrors face limits on obtaining a con stant-phase, high-reflectivity over a large wavelength region, and metallic mirrors have inherent absorption that can limit operation at high pulse rates. Discussions of future spectrally combined sources that produce several-fs-duration pulses require accompanying ultra-broadband focusing optics to reach high intensities.
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From page 69... ...
The list includes the following: • Nearly all of the solid-state lasers that have been applied, including Nd:glass, Nd:YAG, Yb:YAG, and Ti:sapphire; • The CPA technique that was critical to the overall approach; • The lamp-pumped, high-energy Nd:glass systems that were used in the first PW system and are still being used to scale to higher powers; • Large-aperture gratings that also enabled the first PW system; • High-power diode lasers and diode-pumped solid-state lasers, including low-heat-generation Yb-doped lasers; • Nonlinear optics, notably harmonic generation and parametric devices; • Cladding-pumped fiber lasers that have allowed scaling of fiber-geometry systems to high average powers; and • The original free-electron laser concept and the extension to self-amplified spontaneous emission (SASE) that made X-ray FELs possible.
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From page 70... ...
Otherwise, companies in the United States, such as Coherent, Spectra-Physics, and KMLabs, have concentrated on lower-power ultrafast sources, primarily based on Ti:sapphire, to address the still-large scientific market for these devices. It is to be expected that the ELI pro gram in the UK, besides providing business to the French companies Thales and Amplitude Technologies, will create EU-based spin-off companies that specialize in high-peak-power laser systems.
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