Applications of microlens-conditioned laser diode arrays

Abstract

The ability to condition the radiance of laser diodes using shaped-fiber cylindrical-microlens technology has dramatically increased the number of applications that can be practically engaged by diode laser arrays. Lawrence Livermore National Laboratory (LLNL) has actively pursued optical efficiency and engineering improvements in this technology in an effort to supply large radiance-conditioned laser diode array sources for its own internal programs. This effort has centered on the development of a modular integrated laser diode packaging technology with the goal of enabling the simple and flexible construction of high average power, high density, two-dimensional arrays with integrated cylindrical microlenses. Within LLNL, the principal applications of microlens-conditioned laser diode arrays are as high intensity pump sources for diode pumped solid state lasers (DPSSLs). A simple end-pumping architecture has been developed and demonstrated that allows the radiation from microlens- conditioned, two-dimensional diode array apertures to be efficiently delivered to the end of rod lasers. This architecture enables the generation of pump bemas that are scalable in absolute power with intensities approaching 100 kW/cm<SUP>2</SUP>. To date, pump powers as high as 2.5 kW have been delivered to 3 mm diameter laser rods. Such high power levels are critical for pumping solid state lasers in which the terminal laser level is a Stark level lying in the ground state manifold. Previously, such systems have often required operation of the solid state gain medium at low temperature to freeze out the terminal laser Stark level population, so as to minimize losses resulting from reabsorption of the laser radiation. The necessity of low temperature operation has rendered such systems impractical for many applications. Our recently developed high intensity pump sources overcome this difficulty by effectively pumping to much higher inversion levels, allowing efficient operation at or near room temperature. Because the end-pumping technology is scalable in absolute power, the number of rare-earth ions and transitions that can be effectively accessed for use in practical DPSSL systems has grown tremendously. Unique laser systems for applications in fields such as medicine and remote sensing can now be simply realized. We have also been involved in programs to evaluate the use of direct diodes for material processing applications. Here, diode radiation from an extended two-dimensional microlens-conditioned array is focused and delivered directly onto a work piece. Systems based on this concept can be utilized in the heat treating and hardening of metals. Another application of microlens-conditioned laser diode arrays is in the direct coupling of their radiation to optical fibers. Direct diode-to-fiber coupling has recently been demonstrated for a medical application in which 22 W of cw 690 nm radiation was delivered from a microlens-conditioned stack of AlGaInP laser diode bars through a 1 mm core fused silica fiber. This approach used a simple and inexpensive 1 cm focal length lens to direct the microlens-conditioned radiation from the diode stack into the optical fiber.