A low threshold, efficient optical parametric oscillator (OPO) based on the material Potassium Titanyl Phosphate (KTP) and pumped by a diode-laser-pumped, Q-switched Nd:YLF laser has been demonstrated and investigated. This all-solid-state device was operated in a non-critical phase match (NCPM) geometry converting the 1 mum pump light to output wavelengths of 1.54 and 3.28 mum, and has potential as an 'eyesafe' laser source with scaling to higher powers. A major contributing factor to the success of this work was the extension of the steady state theory of the singly resonant OPO to include the build-up time effects that are dominant in the pulsed regime. A number of diode pumped lasers were constructed, allowing a comparison to be made between side- and end-pumping geometries, and also between the materials Nd:YAG and Nd:YLF. The end-pumping geometry in conjunction with the higher absorption and longer upper state lifetime in Nd:YLF made it the design of choice for the case of low pump pulse energies (~ 12 mJ at 797 nm). Anamorphic expansion of the laser mode in the plane parallel to the diode laser junction was employed to achieve TEM00 operation of this laser. Subsequent Q-switching with a polariser and LiNbO3 Pockels cell combination produced 2.2 mJ at 1.047 mum in an 18 ns pulse. Investigation of the dynamic loss of the Q-switch (which is due to the elasto-optic effect) allowed improvement of laser performance. The established model for a pulsed singly resonant OPO which describes the case for a plane-plane resonator was inappropriate in this work and so the steady state focused beam theory was extended to include time dependence. Fair agreement was found between the computer model and the experimental results, where the effects of pump and signal focusing, and output coupling were investigated. The high conversion efficiency of 30% for converting the 1 mum pump light to the eyesafe wavelength of 1.54 mum is superior to the present alternative source of the Er:glass laser. Pump energy thresholds of less than 0.5 mJ were obtained, along with internal conversions approaching 50 %. An empirical relation describing pump depletion was derived which showed good agreement with experiment. A high resolution investigation of the spectral properties of the OPO identified the roles of resonant reflection and doubly resonant behaviour on the mode structure of the output. The former suggests a way in which single mode operation could be achieved without the use of additional intracavity elements, or a seeding source.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:750777 |
| Date | January 1994 |
| Creators | Terry, Jonathan A. C. |
| Contributors | Dunn, Malcolm H. |
| Publisher | University of St Andrews |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10023/15032 |
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