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DIRECTED
ENERGY
PROFESSIONAL
SOCIETY
Abstract: 24-Symp-051
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UNCLASSIFIED, PUBLIC RELEASE
Thermal Management and the Power Scaling of Fiber Lasers for Directed Energy Applications
Yb-doped fiber lasers have experienced tremendous power scaling over the past three decades, with advertised commercial systems now exceeding 500 kW in a multimode beam. Single-mode offerings, on the other hand, namely those more suitable for directed-energy applications, are still hovering around the 10-kW mark. Due to continued technological hurdles in scaling the power produced from single-mode fiber lasers, methods to increase output, while preserving high brightness, have mainly been embodied by both coherent and incoherent beam combining approaches, which are cumbersome and bulky. However, the question remains: what is needed to scale the output power of single-mode, single-aperture fiber lasers beyond 25 kW or even 100 kW while still maintaining the size, weight, and power (SWaP) requirements for directed-energy applications? Of primary concern, as it has always been, is the thermal management in these power-scaled systems. There are several sources of heating in a fiber laser, including the quantum defect, the less-than-one quantum efficiency of the rare-earth dopant, background absorption, and imperfect splices. Although forced convective cooling via flowing water is the most endemic approach to thermal management, such cooling systems also scale in size and power with laser power, persistently challenging the low SWaP requirements of the laser. To overcome these obstacles, future systems will require several factors to come to a confluence. This includes the fiber and material it comprises, its waveguide design, and the laser architecture, including the judicious selection of pumping and lasing wavelengths. First, the fiber must be of extraordinarily high quality in terms of quantum efficiency and low absorptive background loss. To illustrate this point, an active fiber with 98% quantum efficiency versus 99% quantum efficiency in a 100-kW system represents 1000 W of additional heat that must be managed. The fiber itself must also be resistant to photodegradation, which can lead to excess absorptive loss and increased heating. Aside from lowering the quantum defect, which is done by bringing the pump and lasing wavelengths closer together, a newer approach only recently demonstrated in fiber lasers and amplifiers is the use of anti-Stokes pumping, such as in radiation-balanced lasers (RBLs). Like active water cooling, RBLs and related architectures are able to extract thermal energy from the fiber, but they do it through the emission of spontaneous photons. Such lasers are typically pumped at long wavelengths (e.g., greater than 1020 nm), thus necessitating longer gain fibers. Although this also helps to reduce the thermal load and temperature in the fiber, it also lowers the threshold for nonlinear effects. Therefore, the fiber material may be selected to possess intrinsically low nonlinearity, such as low Brillouin and Raman gain coefficients, or reduced thermo-optic coefficient for the management of transverse mode instability (TMI). In addition, it may also be one that has a much larger Yb3+ doping concentration. This is not a trivial task since increasing the active dopant concentration can lead to ion-ion interactions that deleteriously lower the quantum efficiency due to quenching, giving rise to enhanced heating. In this respect, the low inversion associated with long-wavelength pumping afforded by the RBL configuration may help mitigate some of these quenching mechanisms. Finally, it may not be possible to remove or mitigate all heat generated by a high-energy system. Accordingly, the waveguide can be designed to exhibit more favorable characteristics when operated at some predetermined elevated temperature, such as increased loss to the LP11 mode to make it effectively single-moded. Bringing these ideas together and the various tradeoffs in power scaling will be discussed at the conference.
UNCLASSIFIED, PUBLIC RELEASE
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