Backward wave optical parametric oscillators (BWOPO) represent a class of nonlinear optical devices where the signal and idler photons are generated in strictly opposite directions [1]. Realizing such an oscillation requires a quasi-phase-matching (QPM) approach with sub-µm-periodicity structures [2]. Theoretically and experimentally, the temporal phases of the forward and backward waves generated in BWOPO have been shown to be well determined [3]. In particular, the forward-generated wave inherits the temporal phase modulation of the pump [4]. In contrast, the temporal phase of the backward-generated wave remains fixed as required by the general phase relations in the parametric interaction. Spatially structured beams have been receiving increasing interest due to substantial advantages offered for many applications, including optical communications [5]. The synthesis of parametric beams with spatially and temporally designed phases at any wavelength allowed by the transparency of the QPM material would be of great interest. So far, there has been no theoretical or experimental investigation on generating spatially structured beams in BWOPO.

Microlaser-pumped subnanosecond pulse duration optical parametric generators have great potential to establish themselves as a compact, simple, and cost-effective option for various tasks that do not require high temporal resolution. However, fundamental limitations of co-propagating three-wave interaction, including broad bandwidth, non-transform-limited pulses, and suboptimal spatial coherence of the output, restrict their suitability for demands of high spectral purity and beam quality. In this report, we address these shortcomings and present the first Backward Wave Optical Parametric Oscillator (BWOPO) pumped by transform-limited subnanosecond pulses from Nd:YAG microlaser. This paper reports a thorough performance investigation of BWOPO setups pumped by the first (1064 nm) and second (532 nm) harmonics of the microlaser. Based on a periodically poled Rb-doped KTiOPO4 crystal with a 427 nm grating period, BWOPO achieved nearly transform-limited pulses and diffraction-limited beams of the signal and idler waves in the near and mid-infrared with 50% conversion efficiency. A degenerate BWOPO operation is also presented. 792.5 nm wavelength pumped, temperature-tuned degenerate BWOPO generated a narrowband counter-propagating signal and idler centered at 1585 nm. This experiment also revealed thermal instability of the ferroelectric domain structure, resulting in a twofold decrease of the crystal’s effective nonlinearity.

We demonstrate the generation of counterpropagating, twin-photon pairs in the optical communication band, utilizing periodically poled Rb-doped KTiOPO4. The spectral and polarization indistinguishability of the photon pairs is confirmed through a Hong-Ou-Mandel interference measurement. At degeneracy, the photon-pair generation exhibits a broad angular distribution. Notably, the forward-propagating photon tunes with the pump frequency while the backward-propagating photon frequency remains nearly independent of the pump wavelength. 

This work demonstrates an up-conversion imaging system using silicon sensors and commercial optics. A 1550 nm laser and Nd:YVO4 laser mix in a PPKTP crystal, achieving a 61 mrad FOV and 0.96 mrad resolution.

Thanks to recent progresses in the fabrication of sub-µm domain gratings in Rb-doped KTiOPO<inf>4</inf> [1], backward quasi-phase matching can be precisely engineered to produce specific wavelengths for absorption spectroscopy applications [2]. As a consequence, the Backward Wave Optical Parametric Oscillator (BWOPO) is raising interest for application requiring precisely controlled narrow-line infrared radiation such as Differential Absorption LIDAR (DIAL). In this research work we study the fine wavelength and linewidth properties of a BWOPO designed to address the CO<inf>2</inf> R30 absorption line at 2051 nm, which is one of the most suited ones for CO<inf>2</inf> lidar probing from space [3]. For this purpose, we implemented the experimental set-up described in Fig.1(a) hereafter.

In this work, a novel 2.7 µm source used for CO2 and H2O vapor spectroscopy using the backward propagating wave of a backward wave optical parametric oscillator (BWOPO) is demonstrated for the first time to our knowledge. The unique properties of BWOPOs eliminate the need for additional spectral narrowing or wavelength stabilization, enabling the use of a multi-longitudinal mode Q-switched pump laser centered around 1030 nm. A full characterization of the source is presented, revealing a central output at 2712 nm, showcasing a temperature tuning of −1.77 GHz/K, and achieving an output pulse energy of 2.3 µJ. Novel methods are introduced for measuring the linewidth and wavelength stability using the ambient laboratory air. These approaches demonstrate a narrow output of 43 pm and establish an upper limit of stability at 65 MHz, with no active means of stabilization. These findings underscore the potential of BWOPOs as a robust platform for future differential absorption lidar (DIAL) systems.

We present backward wave optical parametric oscillator waveguides implemented in periodically poled Rb-doped KTP. The waveguides demonstrated low loss (0.16 dB/cm) and exhibited an oscillation threshold 19 times lower than the corresponding bulk device.

We present backward wave optical parametric oscillator waveguides implemented in periodically poled Rb-doped KTP. The waveguides demonstrated low loss (0.16 dB/cm) and exhibited an oscillation threshold 19 times lower than the corresponding bulk device.

The first demonstration of a 2.7 µm CO2 gas sensing source exploiting a backward wave optical parametric oscillator (BWOPO). Transmission measurements of the backward wave are demonstrated through air with good agreement with simulations.