Agile tuning and long-wavelength tuning of synchronously pumped optical parametric oscillators
This thesis reports investigations of several configurations of high pump power, cw-mode-locked synchronously pumped optical parametric oscillator (SPOPO). The focus is on singly resonant, picosecond-domain systems, including: (1) a SPOPO based on periodically poled lithium niobate (PPLN), equipped with a diffraction grating to tune within the gain bandwidth; (2) a PPLN SPOPO designed to operate at long idler wavelengths, where the idler experiences strong attenuation; (3) a SPOPO based on cadmium selenide (CdSe), pumped in tandem by a 1-μm-pumped PPLN SPOPO. The primary pump source used is a Nd:YLF oscillator-amplifier system (λ = 1.047 μm), delivering ~ 4-ps pulses at 120-MHz repetition rate to the SPOPOs. Details of this source, used in conjunction with a standard configuration of PPLN SPOPO, are presented, including modifications to the amplifier stage to extract higher output power. Several aspects of the SPOPO performance related to later work are examined, including generation of extra spectral content, idler characterisation and cavity-length-dependent pulse compression. The diffraction grating provides a means of agile tuning, as well as providing general benefits to SPOPO operation, including suppression of extra spectral content in the signal, suppression of cavity-length dependent effects such as pulse compression and frequency drift, and a significant increase in cavity-length tolerance. Asymmetry of the tuning curves is examined and attributed to non-collinear phase-matching components. The long-idler-wavelength operation of a PPLN SPOPO is then presented, a strategy for this regime being minimisation of signal losses in the cavity. Preliminary results are presented, including tuning out to ~ 7.3 μm, and ~ 0.5 mW of average power measured at 7-μm. Finally, a tandem-pumped, noncritically phase-matched CdSe SPOPO is demonstrated. By varying the pump wavelength, idler tuning was achieved over the range 9.1-9.7 μm and average output powers of up to 40 mW were recorded, with significant scope for wider tuning and power scaling.