Butterworth, S.D., Lefort, L. and Hanna, D.C.
Compressed 200fsec pulses from a synchronously pumped optical parametric oscillator in periodically-poled lithium niobate
At National Quantum Electronics Conference (QE13), United Kingdom.
08 - 11 Sep 1997.
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A cw synchronously-pumped optical parametric oscillator (SPOPO) based on periodically-poled lithium niobate (PPLN) has produced a factor of 20 pulse compression, resulting in signal pulses as short as 200fs, when driven by 4ps pump pulses. Furthermore these compressed pulses extract most of the energy from the pump pulses so that the output signal pulses have higher peak power than the pump pulses. The principle of this pulse compression in a SPOPO has previously been demonstrated in BBO, using very intense (~1MW) Q-switched mode-locked pulse trains. The requirements for pulse compression were identified as high gain (10 quoted in ref. ) and large group-velocity walk-off between pump and resonated wave. Thus when the OPO resonator is biassed longer than the length for exact synchronism, the resonated signal pulse arrives at the nonlinear crystal late relative to the pump pulse and then 'walks-through' the pump pulse over the crystal length. The high gain allows the leading edge of the signal pulse to deplete the entire pump pulse so that essentially all of the pump pulse energy is extracted into the compressed pulse.
We have shown that this principle can be extended to the case of cw mode-locking in PPLN, since high gains can be achieved for much lower pump powers. In fact we also find that the required gain in the cw mode locked case is only ~10.
In the first experiment on a 1047nm pumped PPLN OPO, we used a 6mm sample that we had fabricated by electric-field poling. For these compression experiments we used a longer crystal (19mm, fabricated by Crystal Technology). The length was chosen to provide a group delay between pump and signal which was comparable to the 4ps pump pulse duration. A 4 mirror OPO resonator was used, either as a standing wave resonator or as a ring. The output mirror was an uncoated flat of LiNbO3 or ZnSe, providing a 15% reflectivity, hence forcing operation in the high-gain regime. The OPO was pumped by a diode-pumped Nd:YLF mode-locked laser (4psec, 120MHz, mean power 1W, Microlase DPM-1000-120). With the resonator set for exact synchronism, the signal pulses (tunable over 1.7-1.9µm) were 4psec in duration and the average power 300mW. With the resonator length increased by 50µm, compressed pulses of 200-250fsec were obtained at an average power of ~220mW. These compressed pulses were extremely stable, and had a time bandwidth product of ~0.33. The idler pulses, in the 2.3 to 2.7µm range, also showed compression, although to a smaller extent.
These compression results confirm PPLN as a material well suited to ultrashort uses and offering a convenient route to broad tunability in the femtosecond regime
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