Novel configurations for pulsed optical parametric oscillators and their pump sources

Abstract

The development of all-solid-state, diode-laser pumped neodymium (Nd) lasers and optical parametric oscillators (OPOs) is described, which realise practical sources of coherent radiation with a high degree of frequency agility, are efficient, reliable and potentially compact. A comparison of various neodymium doped host materials reveals yttrium lithium fluoride (YLF) to be an appropriate replacement for the more widely known host yttrium aluminium garnet (YAG) in diode-laser pumped devices. The development of an end-pumped Nd:YLF laser that utilises a 12-mJ, 60W, quasi-CW diode-laser bar is initially described. Multilongitudinal-mode, TEM00 pulse energies of greater than 2 mJ have been observed, with corresponding peak output powers in excess of 118 kW. The incorporation of a novel pre-lase Q-switching technique has realised single-longitudinal-mode peak powers in excess of 90 kW continuing to be achieved. Further, the development of a more powerful end- pumped Nd:YLF laser utilising 2, 3-bar diode-laser arrays, each providing 72-mJ of pump energy is described. In this case, Q-switched, multilongitudinal-mode, TEM00 pulse energies of greater than 11 mJ are reported, with the clear potential for increasing this to greater than 20 mJ, based on measured fixed-Q pulse energies of greater than 30 mJ. Complementing the development of these diode-laser pumped solid-state lasers is the development of optical parametric oscillators based on the nonlinear materials lithium triborate (LBO) and beta-barium borate (?-BBO). Pumped by the frequency up-converted (third harmonic) output of the mid laser, such optical parametric oscillators introduce extensive frequency agility spanning a spectral range from the deep blue (0.4 mum) to the mid-infrared (2.5 mum). Initially, the development of an LBO based device is reported, which in a type I critical phase- match (CPM) geometry has a measured oscillation threshold of <0.3 mJ, when pumped by the frequency tripled output of the 144-mJ diode-laser pumped Nd:YLF laser at 0.349 mum. Observed pump depletions are as high as 35%. A similar CPM geometry is reported in beta-BBO, in this case pumped by the frequency tripled and amplified output of a diode-laser pumped Nd:YAG laser at 0.355 mum. This is a more energetic device with thresholds of >5 mJ, but through the introduction of interferometric, dispersive and injection seeding techniques made to operate on a single axial mode. Near transform limited linewidths are reported in devices which continue to have modest pump thresholds and broad tunability. The parametric generation of broad spectral bandwidths (polychromatic) by the use of suitable phase-matching geometries is also reported. Greater than 100 nm simultaneous bandwidth in the visible spectrum is generated in a collimated signal-wave from a novel, noncollinear phase-matching geometry in a beta-BBO optical parametric oscillator, which is pumped by the collimated output of frequency tripled diode-laser pumped Nd:YAG laser. The device is demonstrated to be efficient, having a similar pump threshold and efficiency to that of the well known collinear phase-matched tunable device, and to continue to encompass a degree of tunability allowing the large simultaneous bandwidth to be tuned across the entire visible spectrum. Dispersive cavity tuning of the optical parametric oscillator by the use of a Littrow-mounted grating or acousto-optic tuning filter, with a static crystal and pump configuration, is also described.