Degenerate Backward wave Optical Parametric Oscillator

Abstract

Backward wave optical parametric oscillators (BWOPO) are a special class of nonlinear optical devices with unique properties, due to the fact, that the pump photon is split in two photons propagating in opposite directions.The counter-propagating nature of the interaction automatically establishes positive feedback, without any external cavity, making the system very robust and insensitive to misalignment.Furthermore, the forward wave inherits the phase modulation of the pump, while the backward generated wave is inherently narrow [1].Although the concept of BWOPO was proposed in the 1960s [2], the fist experimental demonstration had to wait until 2007 [3].This long delay is ascribed to fact that quasi-phase matched (QPM) structures with sub-um periodicities are required to compensate for the large phase-mismatch between the counter-propagating waves.Thanks to the recent progress in periodic poling based on coercive field engineering, several BWOPO interactions with ps and ns pump pulses have been achieved, demonstrating efficient single pass configuration with conversion efficiencies up to 50% at a mJ output level [4].BWOPO operating at degeneracy, especially in the telecommunications band, is of special interest due to the possibility to have phase locking inherent to degenerate parametric processes which can be used for generation of biphoton states, a resource for quantum communications and quantum sensing [5,6].Efficient BWOPO operation in the spectral range of 1.5 µm -1.6 µm, requires first-order QPM, which can be realized in periodic nonlinear structures with a periodicity of ~400 nm, i.e., close to half of the pump wavelength in vacuum.In this work, we take advantage of our coercive-field engineering technique [7] to fabricate 435 nm-period in 1 mm thick Rb-doped KTiOPO4 (PPRKTP) crystals.The resulting domain pattern shows excellent uniformity over the entire crystal aperture and a duty cycle close to 50% (see Fig. 1a)) To the best of our knowledge, this poling period marks the shortest QPM period ever fabricated in ferroelectric crystals of comparable aperture.The crystals are used to demonstrate the first realization of a degenerate BWOPO, generating counter-propagating signal and idler in the spectral band of optical communications.