Enhanced fiber coupled laser power and brightness for defense applications through tailored diode and thermal design

Advances in both diode laser design and packaging technology, particularly thermal management, are needed to enhance the brightness of fiber coupled diode lasers while maintaining the small size and light weight required for defense applications. The principles of design for high efficiency fiber coupling are briefly covered. Examples are provided of fielded and demonstrated 100 and 200 micron diameter fiber coupled packages ranging in output from a few hundred to kW-class units in fibers, to include sub-kg/kW capabilities. The demand for high-power and high-brightness fiber coupled diode laser devices is mainly driven by applications for solid-state and fiber laser pumping. The ongoing power scaling of fiber lasers requires scalable fiber-coupled diode laser devices with increased power and brightness. A modular diode laser concept combining high power, high brightness, wavelength stabilization and low weight, which is considerable concern in the SWaP trades needed to field defense systems, has been developed. In particular the defense technology requires robust but lightweight high-power diode laser sources in combination with high brightness. The heart of the concept is a specially tailored diode laser bar, with the epitaxial and lateral structures designed such that only standard fast-and slow-axis collimator lenses in combination with appropriate focusing optics are required to couple the beam into a fiber with a core diameter of 200 mu m and a numerical aperture (NA) of 0.22. The spectral quality, which is an important issue especially for fiber laser pump sources, is ensured by means of Volume Holographic Gratings (VHG) for wavelength stabilization. This paper presents a detailed characterization of different diode laser sources based on the scalable modular concept. The optical output power is scaled from 180 W coupled into a 100 mu m NA 0.22 fiber up to 800W coupled into a 400 mu m NA 0.22 fiber. In addition we present a lightweight laser unit with an output power of more than 300 W for a 200 mu m NA 0.22 fiber with a weight vs. power ratio of only 0.9 kg/kW.