Fibre laser pumped periodically poled lithium niobate based nonlinear devices
Four experiments have been demonstrated, each successfully combining PPLN crystals and fibre lasers. Firstly second harmonic generation (SHG) was achieved with an 83% conversion efficiency using an all-fibre based nanosecond pump. 60% efficiency was achieved with a simple Q-switched Erbium fibre laser. Cascading two PPLN samples created a 33% and 15% conversion of the pump to the third harmonic, for each source respectively. A second experiment used a picosecond all-fibre chirped pulse amplification source for parametric generation, and amplification of a CW diode seed. Very high gain, in excess of 90dB, was achieved, demonstrating PPLN's high nonlinearity with the powers now available from fibre lasers. A simple Q-switched Erbium fibre laser pumped by a diode was developed for the third experiment. Energies up to 180µJ were produced, which at the time was a record for this type of source. The output was frequency doubled by a PPLN sample, before pumping a second crystal for singly resonant parametric oscillation. A low threshold of 10µJ was achieved with a broad spectral tunability of 0.945 to 4.45µm, limited only by the mirror reflectivities. The final experiment was a similar parametric oscillator but with no doubling stage. This final system is simpler to tune, with a range from 2.58 to 3.99µm. The idler to signal energy ratio was now greater. Throughout the thesis, nonlinear optical interaction theories have been studied and models developed to explain the experimental observations. All the experiments have been simulated, and major factors to the results considered. A model for the parametric processes was built based on the solution to the three time independent coupled wave equations. Numerical computations were performed, such that phase-mismatching, simultaneously competing nonlinear processes and pump depletion could be considered. The effects of focusing and spatial profiles have also been discussed.