Optical Parametric Conversion IN KTiOPO 4 (KTP)

Abstract

R. Burnham, M. L. Dlabal , W. KoechnerFIBERTEK, Inc.510 -A Herndon ParkwayHerndon, VA 22070(703) 471-7671R. StolzenbergerPhilips LaboratoriesNorth American Philips Corporation345 Scarborough RoadBriarcliff Manor, NY 10510andAlbert PintoUS Army Center for Night Vision and Electro- OpticsFort Belvoir, VA 220601. INTRODUCTIONThe atmospheric transmission window between 3.4 and 4.212m is an important infrared band for many optical remote sensingapplications. Not only is the atmospheric attenuation minimal, but several detectors (e.g. InAs, InSb, PbSe) have excellentsensitivity in this spectral region. Unfortunately active system development has been impeded by the lack of a convenient,efficient laser source in this spectral range. Only the DF chemical laser has capability for high power output in this region,but the complications associated with storage and handling of fuel, and the limited operating life have curtailed the applicationof these devices.An alternative to the development of in -band lasers with the required efficiency, power and reliability, is the conversion inwavelength of suitable sources in adjacent spectral regions using non -linear optical materials. Processes using the secondorder non -linear properties of crystals provide particularly efficient optical conversion.1 In particular, optical parametricconversion has been used to convert the output of shorter wavelength lasers to longer wavelengths of interest. A great deal ofactivity has been devoted to the conversion of the fundamental and harmonic outputs of solid -state Nd lasers to the nearinfrared and visible using non -linear parametric materials such as lithium niobate, lithium iodate, proustite, KDP, ADP,2 andmore recently urea 3 and barium borate .4 While the process is conceptually simple and elegant, many difficulties have beenencountered in realizing a practical system. Among the most serious has been optical damage to the non -linear crystal, causedby the high electric fields necessary for non -linear conversion. Typically damage thresholds for infrared non -linear materialsrange between 10 and 200 MW /cm2 for 10 ns laser pulses.A second drawback to parametric conversion for generation of tunable mid -infrared (MIR) radiation has been the low efficiencyof the pump sources themselves. Typical wall -plug efficiencies for lamp- pumped Nd:YAG lasers are around a few tenths of apercent for Q- switched fundamental mode operation. Thus with even a 50% conversion efficiency for the non -linear stage, thesystem efficiency is unacceptably low.Recent advances in laser and material technology have considerably improved the prospects for efficiently generating MIRradiation using an optical parametric converter pumped by a solid state laser. These developments include: