A synchronously pumped optical parametric oscillator, based on periodically poled lithium niobate, has operated at idler wavelengths out to 7.3 microm . Directly measured idler output powers (average) are 4.2 mW at 6.1 microm, 0.5 mW at 7.0 microm, and 0.04 mW at 7.25 microm . Characterization of the idler output indicates essentially bandwidth-limited and diffraction-limited operation.

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The very high parametric gain achievable in periodically poled lithium niobate (PPLN), when pumped by intense short pulses, has allowed synchronously pumped oscillation at wavelengths where very strong idler absorption is present.122 With appropriate design practical sources operating at idler wavelengths well beyond 6 pm are possible. A general design strategy is outlined and results from its implementation are presented.An element in the design strategy includes careful minimisation of signal losses, since these strongly affect performance. For this reason a ring configuration is preferable. The strategy also favours short pulses, and tight focussing at the exit end of the crystal, to maximise signal and pump intensities over the last idler extinctionlength of the crystal. Despite the strong idler absorption coefficient a, it is not essential to use a crystal length L which makes a,L small, and we have operated successfully with a,L -30.For operation in the picosecond regime an APM mode-locked Nd:YLF laser (Microlase DPM-1000-120) followed by an amplifier (QPeak MPS-1047 CW-IO) has been used as the pump, providing 4 ps pulses at 120 MHz, with a maximum average power of 3.6 W delivered to the PPLN crystal. A four-mirror ring resonator was used, with all mirrors highly reflecting over the signal wavelength range 1.2 pm-1.4 pm. AR coatings on the PPLN crystal had an estimated loss of -0.5% per surface over this signal wavelength range. Representative results include: for 5.3 pm (a, -4 cm-I), 30 mW measured average power leaving the output mirror (35 mW incident on the mirror); for 6.1 pm (a, -8 cm-', corresponding to a local minimum in the IR absorption), 4 mW average o/p power (8 W peak pulse power). The available PPLN gratings limited the longest idler wavelength to 6.6 pm for which the threshold pump power was -2.6 W.To investigate the use of shorter pump pulses we have used a tandem OPO scheme, in which a PPLN SPOPO operating at -1.5 pm, producing compressed pulses3 (down to -300 fs) pumps a second PPLN OPO (with 19 mm long crystal). The first SPOPO can provide higher peak pump powers than those available from the Nd:YLF laser. Initial results show a reduced threshold for operation at an idler wavelength of 5.3 pm. Further results from this system at longer wavelengths will be reported, providing guidance on the expected performance of a long-wavelength PPLN SPOPO using an EDFA source in place of the Nd:YLF pump.