Low-threshold femtosecond optical parametric oscillators based on chirped-pulse frequency conversion in long aperiodically-poled crystals

We describe an high-gain optical parametric oscillator incorporating highly chirped pump-pulses and a 20mm aperiodically-poled crystal of potassium titanyl-phosphate. Results include a threshold pump power of 14.4mW and a signal slope efficiency of 37%. 02002 Optical Society of America Ocis Codes: (190.4410) Non-linear optics, parametric oscillators, (190.4970) Parametric oscillators,( 190.71 10) Ultrafast non-linear optics, (320.1 590) Chirping, (320.2250) Femtosecond phenomena, (999.9999) Quasi-phasematching. In femtosecond optical parametric oscillators (OPOs), the finite bandwidth of the pump pulse places an upper limit on the size of crystal that can be used. Under conventional phasematching, bandwidth depends inversely on the crystal length and consequently only short crystals are used in femtosecond OPOs as those of excessive length show strong back-conversion and low overall efficiency. On the other hand, the limited single-pass gain available in short crystals requires high pump threshold powers. Conventional crystals are therefore incapable of simultaneously providing low pump thresholds and high conversion efficiencies due to the conflicting length criteria imposed by the requirement of a broad bandwidth and a high single-pass gain [I]. Theoretical results indicate that by chirping the crystal poling periods [2,3] it should be possible to reduce threshold values while maintaining the crystal phasematching bandwidth, even for very long crystals. The ability to reduce operational thresholds in femtosecond OPOs is of particular interest as one practical consequence would be the possible use of low-power diode-based femtosecond oscillators such as Cr3':LiSAF as pump sources. This report describes an experimental validation of our earlier work in which an OPO containing a 20mm crystal of aperiodically-poled potassium titanyl-phosphate (APPKTP) [4] is pumped by chirped pulses derived from a femtosecond Tksapphire laser oscillator. In order to achieve parametric conversion using an aperiodically-poled crystal, the pre-chirped pump pulses must be matched to the chirp of the quasi-phasematched grating. Fig. I illustrates the situation adopted in our experiment where an unchirped signal pulse walks through a chirped pump-pulse as the two pulses co-propagate through the aperiodically-poled crystal. The chirped pump-pulse entering the crystal sees its longer wavelengths, present in the trailing edge of the pulse, being converted to signal pulses by the larger local grating period at the entrance of the crystal. The progressively longer wavelengths seen across the pump-pulses are converted to signal pulses by the progressively shorter local gratings within the crystal until the shorter wavelengths present on the leading edge of the pump-pulses are converted to signal pulses by the shortest local grating period seen at the exit of the crystal. The wavelength and bandwidth of the pump-pulse automatically define the mean crystal period and aperiodicity, and for a particular crystal length the optimum pump chirp is determined by the group-velocity dispersion properties of the crystal.