3 rd Annual NASA Earth Science Technology Conference, Adelphi, Maryland, June 24-26, 2003. Advancement of High Power Laser Diodes for Pumping 2-micron Solid State Lasers Farzin Amzajerdian, Byron Meadows, Michael Kavaya, Upendra Singh NASA Langley Research Center, MS 468, Hampton, Virginia 23681 Nathaniel Baker Lockheed Martin Engineering and Sciences Company, Hampton, Virginia Vikas Sudesh Science and Technology Corporation, Hampton, Virginia Abstract - The reliability and lifetime demanded by space- based applications of 2-micron solid state lasers are beyond the capability of currently available laser diode arrays. This paper provides the status of an ongoing technology advancement effort toward long-lifetime high power laser diode arrays suitable for pumping Thulium and Holmium based solid state lasers. I. INTRODUCTION Laser diode array is a critical component of any solid state laser systems and have been identified by NASA as a major risk area in deployment of laser remote sensing instruments in space. Laser diode arrays (LDAs) are used as the pump source for energizing the solid state lasing media to generate an intense coherent laser beam with a high spatial and spectral quality. The lifetime and reliability of laser remote sensing instruments are basically established by their pump laser diodes. Therefore, any improvement in their reliability will have a major impact on mission lifetime, risk, and cost. Unfortunately, limited commercial availability combined with lack of statistical data required for screening and predicating the reliability of high power laser diode arrays presented many challenges for past NASA laser missions. This led to establishing a task, addressing the laser diode issues, under the Laser Risk Reduction Project funded by NASA’s Earth Science and Aerospace Enterprises. The laser diode task is being performed jointly by NASA/GSFC focusing on laser diodes used for pumping 1-micron solid state lasers and NASA/LaRC working on the laser diodes for 2-micron lasers. The solid state laser design and the characteristics of its lasing materials define the operating wavelength, pulse duration, and power of the laser diodes. The pump requirements for high pulse energy 2-micron solid state lasers are substantially different from those of more widely used Neodymium based 1-micron lasers and in many aspects more challenging [1]. The 2-micron lasers require much longer pump pulse duration than 1-micron lasers, which causes the laser diode active material to experience drastic thermal cycling. This translates to a much shorter lifetime compared to the laser diodes used for 1-micron lasers. In addition to the need for more reliable LDAs with longer lifetime, further improvement in the operational parameters of high power quasi-cw LDAs, such as electrical efficiency, brightness, and duty cycle, are also necessary for developing cost-effective 2-micron solid state laser instruments meeting the stringent size, heat dissipation, and power constraints of space. This paper discusses the current state of the 792 nm LDA technology and the technology areas being pursued toward improving their performance and reliability. This paper also reports the development of a characterization facility for addressing the specific issues associated with the LDAs for pumping 2-micron solid state lasers and provides the results of measurements to date on different laser diode packages. II. 792 nm LDA TECHNOLOGY REQUIREMENTS Recent advances in the development of high peak power quasi-CW LDAs in conductively-cooled packages will likely ease engineering problems resulting from physical and environmental constraints of space-based solid state lidar instruments. However, despite these advances, the LDAs meeting the pump requirements of Thulium and Holmium based solid state lasers still suffer from lifetime and reliability issues. High pulse energy 2-micron solid state lasers require high power quasi-CW LDAs with minimum pulse durations of one millisecond at 792 nm wavelength. Yet it is this relatively long pulse duration that is one of the main causes of limited lifetime for these arrays. Such relatively long pulse duration causes the laser diode active regions to experience high temperatures and drastic thermal cycling. Thermal cycling of the active regions is considered the primary reason for rapid rate of reduction of the LDA’s power, and the excessive temperature rise is the leading suspect of premature failure [2]. The extreme temperature rise during the pulse can create considerable stress within the individual bars due to localized heating and various thermal mismatches between the bars, the substrate, and the bonding materials leading to premature failure. The thermally-induced rapid degradation can be improved to some extend by careful design of the