Phase modulation transfer in mirrorless optical parametric oscillators

It has been predicted theoretically [1] that counter-propagating parametric interactions, where the signal and idler waves are propagating in opposite directions, will establish a distributed feedback mechanism and thus optical parametric oscillation without the need to apply mirrors or external feedback cavity. Such a device, the mirrorless optical parametric oscillator (MOPO), has been recently realized by employing engineered nonlinear crystals, namely, periodically poled KTiOPO4 (PPKTP) with the periodicity of the structure of 800 nm [2]. One of the remarkable properties of the MOPO was a strong asymmetry in spectral content of the signal and idler pulses, where signal pulse bandwidth was of the same order as that of the pump while the counter-propagating idler was about two orders of magnitude narrower than the signal. In this work we provide experimental and theoretical investigation on the temporal phase transfer characteristics in MOPO. In particular we show that the phase modulation is transferred primarily to the signal wave while the idler wave remains almost phase-modulation-free. This property allows a simple means for generating transform-limited pulses in mid-infrared spectral region regardless of phase modulation of the pump. Theoretical simulation reveals that the amount of remaining phase modulation in the idler wave depends on the group-velocity mismatch between the co-propagating pump and signal waves.