Compact low repetition rate optical parametric oscillators

Abstract

Optical parametric oscillators (OPO) offer a potential route to a cost effective low repetition rate ultrafast infrared laser sources. The current favoured technology is optical parametric amplifiers (OPA), which require high powered pump laser sources to generate sufficient pulse energies, but these are large and expensive systems. For an OPO to achieve the same level of performance it would require overcoming some demanding engineering challenges. The cavity length of the OPO typically must match that of its pump laser, meaning to achieve sub-50 MHz repetition rates the cavity length of the OPO needs to be >6 m and for 1 MHz the OPO cavity would need to be 300 m long. This thesis explores OPO cavity designs that target the construction of a low repetition rate OPO within a compact footprint. The primary method investigated utilises an intracavity Herriott cell to store a large portion of the cavity length within a compact footprint. A Herriott cell is a type of multipass cell which is made up of two opposing spherical mirrors usually with a hole machined in one or both mirrors to allow a beam to enter and exit the cell. Once the beam enters the cell it is reflected multiple times, forming an elliptical pattern on the mirrors, with the number of reflections being determined by the separation distance of the mirrors. Incorporating a Herriott cell into a OPO cavity presents challenges for optimising the stable resonating mode, maintaining a Boyd-Kleinman focusing ratio near 1, and achieving the cavity length required for synchronous pumping. This was demonstrated in a synchronously pumped 49.16 MHz Herriott cell OPO producing femtosecond pulses from 1440 nm to 1530 nm with average signal powers up to 312.6 mW when pumped with 1.8W from the Yb:fiber pump laser, and extended to 12.29 MHz in a 12.2-m cavity. The next method demonstrated was a harmonically pumped idler resonant OPO, in which the OPO cavity mirrors are coated so that the longer wavelength idler now resonates in the cavity, and the signal leaves the cavity immediately after the nonlinear crystal. The cavity is made compact by constraining the cavity length to be a harmonic of the pump cavity length. This increases the repetition rate of the resonant idler, but the signal remains at the repetition rate of the pump laser. A 294.96 MHz idler resonant cavity was demonstrated producing femtosecond signal pulses with average powers up to 88 mW when pumped with 1.7 W of pump power. This is reduces the cavity to just 1/6th the size of a synchronously pumped OPO. Finally, fiber feedback OPOs have been demonstrated as a method to achieve a compact low repetition rate OPO, however the additional dispersion from the fiber can limit operation to the picosecond range. To address this modelling work was done investigating cascaded fiber systems where the fibers have complimentary dispersion coefficients, minimising the pulse broadening. A combination of SMF-28 and UHNA7 was used to show that at key wavelengths of 1550 nm, 1700 nm and 2090 nm a 50 fs pulse propagating thorough 1 m of this cascaded system sees minimal broadening with the shortest pulse seen for 1700 nm at just 59.7 fs.