| Mid-infrared (IR) laser sources around3-5μm, covering the high transparency atmospheric window and the intrinsic absorption bands of a large number of molecules and chemicals, have versatile applications including mid-IR counter-measurement, LIDAR and medical diagnostics which makes them gain growing attention in the past decades. To generate such mid-IR laser emission, optical parametric oscillators (OPO) are commonly adopted which can convert shorter wavelength radiation from sophisticated lasers to the desired mid-IR output. Among all the nonlinear materials for parametric conversion, the quasi-phase-matched (QPM) periodically poled and aperiodically poled magnesium oxide doped lithium niobate (PPMgLN/APMgLN) crystals are the most promising candidates due to their large effective nonlinear coefficient, collinear interaction scheme, high damage threshold, wavelength switchable operation and design flexibility. Therefore methods to generate high power, high efficiency and practical mid-IR radiation, optimization of OPO cavity design and construction of their pump lasers were investigated in this dissertation. Three parts of work are mainly included in the research. Firstly, MOPA-structured, linearly-polarized, acousto-optic Q-switched Ytterbium-doped fiber lasers (YDFL), served as the pump sources for OPOs, were experimentally studied. Secondly, fiber-laser-pumped PPMgLN-based OPOs were investigated both theoretically and experimentally. Then investigations on the APMgLN based cascaded OPOs were carried out in order to improve their conversion efficiencies further.Both randomly-polarized and linearly-polarized (LP) acousto-optic (AO) Q-switched YDFLs were employed as the master oscillators, respectively, while large mode area polarization-maintaining (PM) Ytterbium-doped fibers were utilized as the power amplifiers of the MOPA-structured fiber lasers. Stable pulse trains with average power of2W and repetition rate of65kHz were produced from the randomly-polarized laser seed and they were then boosted to an average power of35W with polarization extinction ratio (PER) over12dB by a single-stage fiber amplifier after passing through a fiberized isolator. The LP fiber seed laser was constructed with all PM fibers and with novel semi-fiber-coupled design. More than2.5W average power at repetition rate of65kHz with PER greater than22dB was obtained from the LP seed laser and average power over30W together with PER over18dB could be extracted from the succeeding PM fiber amplifier. Such MOPA-structured, LP YDFLs with high average power and high brightness could be excellent pump sources for OPOs.The two typical OPOs, single-pass, singly-resonant (SPSR) and double-pass, singly-resonant (DPSR), were investigated both theoretically and experimentally. The theoretical work was focused on the numerical calculation of the several crucial parameters of both types of OPOs including thresholds, conversion efficiencies and optimized signal reflectivity while experiments were carried out on the home-made PPMgLN crystals pumped by the self-constructed YDFLs. A fiber-laser-pumped multi-channel PPMgLN-based OPO in a DPSR configuration generating idler output power of4.7,4.3,4.1,3.3, and2.1W at the wavelengths of3.43,3.63,3.72,3.83, and3.99μm was demonstrated in experiment. From which conclusion was reached that strong idler absorption at longer wavelength is likely responsible for the efficiency roll-off and power instability of such OPOs, especially at high pump rates. An OPO specifically configured in SPSR structure to improve the long-term stability pumped by a MOPA-structured YDFL seeded by a LP fiber laser was constructed thereafter. At the highest pump power of25W, an idler average output power of3.27W with one-hour peak-to-peak instability of5.2%was recorded. The measured M2factors were1.98and1.44for horizontal and vertical axis, respectively. Commercialized products were made accordingly.The pump-to-idler conversion efficiencies of conventional OPOs are limited by the Manley-Rowe relations while those of cascaded OPOs can be enhanced by recycling part of the unwanted signal wave from the primary OPO process into the desired idler output via difference frequency generation (DFG) process. As a consequence, APMgLN crystals were designed to compensate the phase mismatch between the OPO process and the DFG process simultaneously. A solid state laser with high peak power and a fiber laser with modest peak power, both working at1.06μm, were employed in the APMgLN-based cascaded OPO experiments, respectively. When pumped by the solid state laser, such OPO could produce more than2W at3.8μm under the average pump power of11W with calculated pump-to-idler conversion efficiency of18.5%and slope efficiency of21.5%. In the fiber laser pumping scenario,4.35W idler power at wavelength of3.81μm was achieved under the average pump power of24.8W. The corresponding pump-to-idler conversion efficiency and slope efficiency were17.5%and21.3%. Both the high and modest peak power pumping cases revealed the feasiablity of such cascaded OPOs for high power efficient mid-IR generation. |
