Development of High Brightness High Power Fiber Laser Pump Sources

摘要

High power fiber lasers have strong potential for use in both commercial and military applications. Improved wall plug efficiency over Nd:YAG and CO: lasers combined with up to a 10-fold improvement in beam quality, make fiber lasers extremely attractive for industrial applications such as welding and cutting. In military applications, fiber lasers offer a simplified logistic train, a deep magazine limited only by electric power, and a compact footprint, allowing theater defense and self-protection of combat platforms with speed of light engagement and flexible response. Commercial viability of these systems, however, is limited by the availability of compact, cost effective, and reliable diode laser pump sources in the multi-kilowatt regime. The relatively low brightness of diode laser sources has complicated the task of building high power pumps at a reasonable cost. The slow axis beam parameter product for a lensed, commercially available, 1 cm wide bar is on the order of 220 mm-mrad while the beam parameter product of a 400 μm, 0.22 N.A. fiber is approximately 44 mm-mrad. The optical systems required to couple these arrays into the fiber are very complex and expensive. Additionally, the number of bars which can be coupled into the fiber is small because of the need for the outputs from the emitters to be rearranged in order to compensate for the astigmatic output beam from the bars. Clearly, there is a need for high power diode laser bars with much higher brightness than currently achievable with bars of broad area, multi-mode emitters. Efforts have been made to produce high power single-mode emitters capable of producing greater than 1 Watt per emitter. Single lateral mode output powers of 500 mW and up to 700 mW have been reported at 980nm with output power as high 1 Watt reported at 1480 nm. Laser designs incorporating a large optical cavity have been shown which allow for higher output power with high catastrophic optical damage (COD) thresholds. Higher COD is achieved by delivering a larger spot size at the facet, decreasing the optical power density. However, because of numerous dynamic waveguiding effects, including thermal lensing, carrier induced index suppression, and spatial hole burning,it has been very difficult to attain stable single lateral mode operation throughout a wide operating range. Additionally, this single mode performance must be repeatable across many samples to allow for production of large quantities of these devices if they are to be used in high power systems.