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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Degenerate Mirrorless Optical Parametric Oscillator

Anne‐Lise Viotti, Patrick Mutter, Andrius Žukauskas, Valdas Pašiškevičius, Carlota Canalias

Published Jun 1, 2019• 1 citation
Publisher

Abstract

A mirrorless optical parametric oscillator (MOPO) is a very special class of optical parametric oscillators, where the feedback for oscillation is realized by counter-propagating second-order nonlinear interaction without a need for external mirrors. This arrangement makes the MOPO a very simple, self-aligned and robust oscillator. The MOPO was first proposed theoretically in 1966 however its realization was only possible using the quasi-phase matching (QPM) approach due to requirement for compensation of large momentum mismatch in such interactions. Realization of first-order counter-propagating parametric oscillation requires structures with periodicity in the sub-pm range, which poses substantial fabrication challenges. In addition to its self-aligned nature, MOPO offers unique spectral properties, whereby the spectral contents of the pump are transferred to the forward wave, while the backward wave remains narrowband. So far, only nondegenerate MOPO oscillation has been demonstrated. However, it has been theoretically proposed that a degenerate MOPO could be used to realize a high-brightness biphoton source for quantum optics applications.

DOI
10.1109/cleoe-eqec.2019.8871602

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A mirrorless optical parametric oscillator (MOPO) is a very special class of optical parametric oscillators, where the feedback for oscillation is realized by counter-propagating second-order nonlinear interaction without a need for external mirrors. This arrangement makes the MOPO a very simple, self-aligned and robust oscillator. The MOPO was first proposed theoretically in 1966 however its realization was only possible using the quasi-phase matching (QPM) approach due to requirement for compensation of large momentum mismatch in such interactions. Realization of first-order counter-propagating parametric oscillation requires structures with periodicity in the sub-pm range, which poses substantial fabrication challenges. In addition to its self-aligned nature, MOPO offers unique spectral properties, whereby the spectral contents of the pump are transferred to the forward wave, while the backward wave remains narrowband. So far, only nondegenerate MOPO oscillation has been demonstrated. However, it has been theoretically proposed that a degenerate MOPO could be used to realize a high-brightness biphoton source for quantum optics applications.
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.
Mirrorless optical parametric oscillator (MOPO) is a consequence of intrinsic feedback provided by the nonlinearity in a medium due to the interaction of a pair of strong counterpropagating fields. As the name suggests, the device does not require a cavity for lasing other than the nonlinear medium. Here we report the demonstration of MOPO under the effect of an all-optical waveguide. The efficient four-wave mixing process due to counterpropagating pump and control fields interacting with a multilevel atomic system facilitates the generation of mirrorless Stokes and anti-Stokes fields counterpropagating to each other. The maximum generated laser power could rise up to mW with pump conversion efficiency more than 30%. Furthermore, the cross-phase modulation due to the strong fields creates all-optical waveguides for the generated fields and hence induces correlated spatial modes in the Stokes and the anti-Stokes fields.
We experimentally demonstrate a dissipative phase transition in a mirrorless optical parametric oscillator (MOPO) using a double-$\mathrm{\ensuremath{\Lambda}}$ system in thermal atomic vapor. Bistable behavior is observed in the output power of the beams generated via the MOPO with the variation of the input power of the driving fields. We show that the long-lived hyperfine ground-state coherence induced by the Gaussian pump and the generated fields leads to the observation of optical bistability in the MOPO threshold. We further study the critical slowing-down of nonequilibrium dynamics in the vicinity of the forward threshold where the switching time of the generated fields diverges with the critical exponent, $\ensuremath{\alpha}=\ensuremath{-}0.86\ifmmode\pm\else\textpm\fi{}0.02$. Our observations pave the way to understanding and characterizing the phase transitions in driven-dissipative systems using a simple nonlinear optical system like the MOPO.
A nonlinear process based on backward quasi-phase matching (BQPM) can be used to realize mirrorless optical parametric oscillation, the generation of paired photons with a separable joint spectral amplitude and narrow wavelength bandwidth, and the preparation of counterpropagating polarization-entangled photons, which shows distinct advantages over some applications based on forward quasi-phase matching. In this work, three types of BQPM in a bulk periodically poled potassium titanyl phosphate crystal with a single period are theoretically analyzed. Experimentally, the harmonic wave generated by second-harmonic generation in type 0 and type I exhibits a narrow bandwidth of 15.5 GHz. Furthermore, photon pairs generated by spontaneous parametric downconversion in all types of BQPM (type 0, type I, and type II) at 7th order are observed and characterized. Their coincidence-to-accidental ratios are all greater than 5 × 10 3 in the pump power range from 10 mW to 500 mW. This research lays the foundation for further applications of BQPM in nonlinear optics, quantum optics, and quantum information processing.
This work describes the continuous variable entanglement of the counter-propagating twin beams generated in a Mirrorless Optical Parametric Oscillator (MOPO) below threshold, encompassing both their quadrature and photon-number correlation. In the first case, a comparison with the single-pass co-propagating geometry outlines a completely different stability of the two sources with respect to the phase-angle. In the second case, stimulated by the critical divergence of the correlation time evidenced by Corti et al., we address the issue of the temporal bandwidth of the intensity squeezing.
Distributed positive feedback established by a counterpropagating three-wave mixing process in a submicrometer structured second-order nonlinear medium leads to mirrorless parametric oscillation with unusual spectral properties. In this work, we demonstrate experimentally and theoretically that the phase modulation of the pump is coherently transferred to the co-propagating parametric wave, while the counterpropagating wave retains a narrow bandwidth and high coherence. The main mechanisms responsible for these properties are the maximized convective separation between the counterpropagating waves and the associated phase-locking between the pump and the co-propagating parametric wave. This suggests that mirrorless optical parametric oscillators can be pumped with incoherent light and still generate highly coherent backward-propagating radiation.