Description:
OBJECTIVE: Demonstrate a wavelength-stabilized diode laser system for pumping high-power fiber laser amplifiers consisting of diode laser emitters that are at least ten times brighter than conventional broad-stripe emitters. DESCRIPTION: High average and peak power fiber lasers and amplifiers offer an attractive combination of high efficiency, near diffraction-limited beam quality, low phase noise, and reliable operation. They have found wide use in industrial and scientific applications ranging from cutting and welding to gravitational wave detection, and their small size makes them promising candidates for defense applications such as laser-based weapons and long-range lidar on airborne platforms. Fiber laser and amplifier systems can also be scaled to even higher power using coherent or spectral beam combining [1], but two competing nonlinear processes limit the power available from a single continuous-wave fiber amplifier and, by extension, the power from a beam-combined system. To achieve good efficiency, both coherent and spectral beam combining require the fiber lasers and amplifiers to have a narrow spectral bandwidth, but these narrow-band systems are very susceptible at high powers to stimulated Brillouin scattering (SBS), which is a nonlinear process that can scatter significant power backwards into the laser system. Several approaches have been used to suppress SBS, but the most common is to utilize short fibers with large cores to reduce the interaction length and lower the Brillouin gain [2]. Recently, a new modal instability has been identified that drastically reduces the output beam quality and limits the useful power from high-power beam-combinable amplifiers [3]. Experimental data show that a significant amount of signal power is coupled into higher-order optical modes of the fiber core and/or cladding when the average amplifier power exceeds a threshold on the order of 1 kW. Theoretical investigations into the mode-coupling mechanism and ways to mitigate it are not yet conclusive [4]. Smaller cores with fewer modes would reduce this instability but at the expense of higher Brillouin gain. One approach to reducing both SBS and modal instabilities is to use extremely short fibers with narrow cores that guide only a few modes, at most. However, short double-clad fibers require extremely bright pump lasers that are spectrally narrowed and locked to match the gain fiber's absorption peak in order to efficiently absorb the pump light. Currently, state-of-the-art fiber-coupled diode pump lasers are limited to an ex-fiber brightness of ~25 MW/cm2sr, corresponding to 100 W from a fiber with a 105-µm core and 0.12 NA (numerical aperture) without wavelength stabilization [5], but this fiber-coupled spatial brightness is significantly lower than the record of 1 GW/cm2sr for a single diode laser [6,7]. This SBIR topic seeks innovative approaches to realizing a high-power wavelength-stabilized fiber-coupled diode laser system that employs extremely bright emitters to achieve an ex-fiber brightness>100 MW/cm2sr. The resulting pump laser module could be transitioned to multiple government-funded high-power laser programs or commercialized as a part of systems targeting industrial laser cutting applications. PHASE I: Demonstrate a single diode laser operating at ~976 nm with output power>10 W, spatial brightness>1 GW/cm2sr, and electrical-to-optical efficiency>52%. All three performance metrics should be achieved simultaneously on a single device. Develop a concept to package several of these emitters into a single wavelength-stabilized module that can achieve the Phase II performance metrics. PHASE II: Construct and demonstrate a prototype laser system suitable for pumping high-power fiber lasers based on the Phase I module concept and diode emitters. The key performance goals are: 1) fiber-coupled power>500 W continuous-wave, 2) ex-fiber spatial brightness>100 MW/cm2sr, 3)>42% ex-fiber electrical-to-optical efficiency, 4)<0.25 nm full-width half-maximum output spectrum*, 5)Δλ/ΔT<0.07 nm/°C, 6)Δλ/ΔP<0.03 nm/W, and 7) specific weight<1 kg/kW of fiber pump power delivered. Conduct a preliminary reliability assessment. The final Phase II system should be at Technology Readiness Level 6. *The narrow spectral width is to allow the future potential for spectral beam combining to even higher spatial brightness within the narrow absorption peak of Yb-doped silica (~7 nm bandwidth). PHASE III: Industrial applications include metal cutting, welding, and marking. A laser module meeting the Phase II metrics would have sufficient power and brightness for entry-level cutting applications, and several Phase II modules could be spectrally combined into a single kW-class fiber-coupled cutting system. Direct-diode lasers are of significant industrial interest because of their potential for higher reliability, better efficiency, and lower complexity than competing solid-state and fiber lasers [8]. Military applications include lidar and directed-energy weapons, and the Phase II technology could be readily transitioned to multiple government directed-energy programs, including ongoing high-power fiber laser programs funded by DARPA and HEL-JTO, such as Excalibur or RELI. Once delivered, a fiber-coupled Phase II module could be readily spliced into an existing high-power fiber amplifier system, and new laser systems could be designed to exploit the brightness of these pump lasers. Since light-weight packaging would be developed during Phase II, Phase III development activities might include increasing output power, improving efficiency, and/or modifying the module for alternative thermal management techniques (e.g. phase change materials or spray cooling).
