High-power, high-repetition-rate picosecond optical parametric oscillators for the visible to mid-infrared

Abstract

This thesis describes the design, configuration and operation of picosecond optical parametric oscillators (OPOs) tunable from the visible to mid infrared. These systems were based on the materials LiB3O5 (LBO) and KTiOAsO4 (KTA), and were pumped by a self-mode-locked Ti:sapphire laser at a repetition rate of 81 MHz. The initial design of the picosecond parametric oscillator was based on a 16 mm long crystal of LBO. This system produces transform-limited signal pulses with durations of -720 fs. Total average output powers of up to 90 mW over a signal (idler) tuning range of 1.374-1.530 mum (1.676-1.828 mum) have been generated at 1.3 times the 900 mW threshold. The system performance was improved by the use of a new LBO crystal of length 30 mm. This system was continuously tunable from 1.160 to 2.185 mum. Up to 690 mW of output power has been generated for 2 W of input pump power at 5 times threshold. For this output power a depletion of 52 % was achieved with a corresponding external extraction efficiency of 34.5 %. Picosecond pulse generation in the visible by external single-pass frequency- doubling of the LBO OPO to provide picosecond pulses in the 584-771 nm range has been demonstrated. Conversion efficiencies as high as 18 % have been demonstrated, with output powers in excess of 65 mW being measured, when utilising a combination of type I and type II temperature-tuned non-critical phase-matching in LBO. The pulse width of the second harmonic was in the region of 840-880 fs. A further new source of tunable high-repetition-rate picosecond pulses for the visible has also been demonstrated, which is based on an internally-doubled, Ti:sapphire-pumped OPO that uses temperature-tuned LBO both as the OPO and SHG crystal. Oscillation has been obtained for an input pump power of 700 mW with output powers in excess of 320 mW being generated, representing conversion efficiencies of as much as 16 %. The system is continuously tunable from 584 to 771 nm and can provide transform-limited visible pulses with durations of 840-880 fs across the available range. The ability to tune beyond wavelengths of 2.5 mum was also required. To this end a new source of tunable picosecond pulses for the near - to mid - infrared has been developed which is based on the material KTA. Oscillation has been obtained for input pump powers as low as 230 mW. The system produces total output powers in excess of 403 mW with conversion efficiencies of 31 % at 5.2 times threshold. Transform-limited signal (idler) pulses of 1.02 (2.9) ps have been generated over the tuning range 1.139-1.281 (2.377-3.160) mum.