Butterworth, S.D., Lefort, L., Puech, K., Smith, P.G.R. and Hanna, D.C.
Efficient operation of a synchronously-pumped optical parametric oscillator in periodically-poled lithium niobate over the range 1.33µm-4.8µm
At 13th Quantum Electronics Conference (QE13), United Kingdom.
01 Sep 1997.
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Periodically-poled lithium niobate (PPLN) is proving to be a very versatile and effective nonlinear optical material. It has a high nonlinearity (20pm/V) and offers non-critical phase-matching over its entire transmission range. In the first experiments demonstrating synchronously-pumped parametric oscillation in PPLN, the tuning range achieved using a 1047nm pump (from a mode-locked Nd:YLF laser) covered 1.67µm - 2.806µm (signal+idler), using 6mm long PPLN samples that we had fabricated by electric field poling. In the experiment reported here, we have used longer samples (19mm, fabricated by Crystal Technology). This has resulted in lower thresholds, now down to as low as ~8mW of mean pump power at 1047nm. We have also used shorter grating periods, thus allowing a considerable extension of tuning range, now covering 1.33µm - 4.81µm. The efficient performance of the OPO at the long wavelength limit is particularly notable. PPLN allows extension to longer wavelengths than bulk lithium niobate since the IR absorption edge for the extraordinary polarisation (as used in PPLN) is at a longer wavelength than for the ordinary polarisation. While the infrared absorption of 0.75cm at 4.8µm is already significant for our 19mm PPLN sample, the long-wave limit for oscillation was, in fact, set by the shortest grating period available and not by the IR absorption.
It is the high-gain available from PPLN, helped by the long sample and mode-locked pump, that allows operation well into the region of IR absorption. Thus, pumping with a mode-locked 1047nm Nd:YLF laser (4ps pulses, 120MHz, ~1W mean power, Microlase DPM-1000-120), a threshold (mean incident power) of only 80mW was achieved for 4.8µm generation. An available output of 40mW was measured at 4.8µm for 800mW of incident pump power.
The observed pump depletion was ~75%, a level which was maintained or exceeded across the entire tuning range, implying that over 100mW of 4.8µm power was generated, of which some 60% was lost by IR absorption and reflection at the PPLN AR coating.
Based on these performance figures we calculate that, allowing for the increased IR absorption, this PPLN device could be usefully extended even beyond 5µm, thus covering an extensive and important range in infrared spectrum
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