High Power multi-kW class fiber lasers have become a leading technology in Directed Energy applications. With Direct Energy weapons and countermeasures moving closer to a deployable technology, industry players are now looking to ensure the components within their systems can withstand the harsh environments in which they will be used. With limited power available in the field, efficiency is a key criterion for these systems and there is a careful balance for diode laser pumps as a piece of the overall system. Increasing the cooling capacity delivered to the diode pumps will increase their Electrical-to-Optical efficiency, but requires more energy be consumed in the cooling loop through lowering the coolant temperature or increasing the pump speed to increase flow rate. In this paper, Coherent|DILAS aims to map these uncharted waters for its Low SWaP diode lasers by exploring trade space for the parameters that are critical to the overall system efficiency. By changing coolant types from water to glycol mixes, coolant freezing can be eliminated while the effects of coolant viscosity are explored. Additionally, direct changes to the coolant temperature and flow rates further explore cooling/efficiency trade space. Experiments are then repeated with an external grating to lock the center wavelength at the 976nm absorption band. The range at which locking is maintained and the efficiency change will be explored for various coolant, flow, and temperature configurations. With a large web of interacting processes being explored, Coherent|DILAS aims to enable further overall system optimization within Directed Energy community.
Recent developments in fiber lasers show the field has reached a high level of maturity, and several demonstration programs have shown successful scaling of output power into the 30-50kW regime while maintaining good beam quality. Despite these successes, much work remains before fibers lasers are ready for the range of field applications currently envisioned. Constraints set by small system size, limited power availability, and harsh environmental conditions demand that novel modes of operation be considered. The variety of use conditions for which high power fiber lasers are being considered poses additional challenges to the system architect, and the full trade-space is not yet clear.
In advance of full system definition, Coherent|DILAS has continued to develop technologies that will extend the trade-space available to the system designer and facilitate transition to the field. We will report on a variety of efforts to extend the use range of existing, SWaP optimized fiber pump modules into territory appropriate for the more demanding of these applications.
Of particular concern are the system cooling architectures needed to support diode pump modules, which in the case of large systems, comprise a significant size and power demand. While efforts to improve SWaP of cooling systems generally have negative effects on diode performance, here we show that negative effects resulting from coolant system design can be mitigated. Operational results that pair existing lightweight, high power modules with non-standard cooling architectures, pulse schemes, and wavelength stabilization will be discussed.
DILAS offers a variety of high power pump diode lasers, optimized for different gain media. Systems optimized for DPAL pumping at 766nm will be discussed, including results demonstrating precise wavelength and spectral width control necessary to optimal overlap with atomic lines. In addition, pump modules optimized at 793 nm for Tm fiber laser pumping have been demonstrated, including a low SWaP module targeted for airborne applications. Lastly, DILAS’ line of high-efficiency/low-SWaP pump at 976nm for Yb fiber laser will be presented. Starting with the 330W IS46 module, DILAS has demonstrated >53% efficiency, and has now increased brightness up to 625W from a 225 um/ 0.22 NA fiber. Developments towards a module with >900W output power will also be shown.
DILAS has leveraged its industry-leading work in manufacturing low SWaP fiber-coupled modules extending the wavelength range to 793nm for Tm fiber laser pumping. Ideal for medical, industrial and military applications, modules spanning from single emitter-based 9W to TBar-based 200W of 793nm pump power will be discussed. The highlight is a lightweight module capable of <200W of 793nm pump power out of a package weighing < 400 grams. In addition, other modules spanning from single emitter-based 9W to TBar-based 200W of 793nm pump power will be presented. In addition, advances in DPAL modules, emitting at the technologically important wavelengths near 766nm and 780nm, will be detailed. Highlights include a fully microprocessor controlled fiber-coupled module that produces greater than 400W from a 600 micron core fiber and a line width of only 56.3pm. The micro-processor permits the automated center wavelength and line width tuning of the output over a range of output powers while retaining excellent line center and line width stability over time.
DILAS Diode Laser, Inc. continues to improve and optimize high-brightness fiber-laser pump modules. Highlights include a 330W module weighing in at 300 grams, achieving greater than 55% electrical-to-optical efficiency at the operating power from a 225micron/0.22NA fiber and a power-scaled version capable of >600 W, >50% efficiency and weighing in at less than 400 grams. The macro-channel coolers enabling these modules eliminate the need for microchannels and deionized water and reduce pressure drop across the system. A road map to modules with >900W of output power will also be presented.
Electrically-injected vertical external cavity surface emitting laser (VECSEL) arrays are an attractive source for lowcost, high-brightness applications. Optical pumping can be used to investigate the emission properties of such devices without undergoing complex device fabrication. The design of such arrays is based on a single VECSEL chip, a 2D lens array, and a flat output coupling dichroic mirror. In this work, we report on the demonstration of an optically pumped, coherently-coupled VECSEL array. The array achieves a maximum total output power of >60 mW and lasing spectrum indicates single-mode operation. Near-field characterization reveals 37 individual lasing elements in a hexagonal array. Far-field measurements show an interference pattern which is consistent with inphase coherent coupling, with >60% of the total output power present in the on-axis central lobe. The physical origin of coherent coupling is attributed to diffractive coupling. The simplicity of the optical cavity design suggests scalability to much larger arrays, making the result of particular interest to the development of low-cost, highbrightness diode sources.