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.

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 photonpair 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.

Backwards Wave Optical Parametric Oscillators (BWOPOs) eliminate the need for a resonant cavity, offering a compact and robust alternative to traditional OPOs. In this study, a BWOPO using a PPKTP crystal with a 765 nm poling period was pumped by a Q-switched laser at 1030 nm, delivering 40 mu J pulses. The backward wave generated at 2.8 mu m exhibited a narrow spectral bandwidth of 1.75 GHz, despite the pump's broad 360 GHz bandwidth. The extracted pulse energy was around 5 mu J. These results highlight the BWOPO's potential for applications requiring precise wavelength control and stability.

Abstract Backward wave oscillators (BWO) represent a class of tunable sources of electromagnetic radiation that do not require a resonant cavity to satisfy the oscillation condition. Electronic BWOs are widely used as high-power sources of microwave radiation. In the optical regime the backward wave optical parametric oscillator (BWOPO) rely on a conceptually similar principle between counter-propagating electromagnetic waves, where a nonlinear interaction provides the positive feedback required for oscillation. The unique properties of the BWOPO have so far been shown in bulk second-order nonlinear crystals only, but the absence of an optical resonator makes the BWOPO concept naturally suitable for integration in a waveguide format. Here, we demonstrate the first waveguide BWOPO, showcasing an oscillation threshold nearly 20 times lower than the corresponding bulk device, and exhibiting low loss (0.2 dB/cm). The backward wave has a narrow linewidth of 21 GHz at 1514.6 nm, while the forward wave at 1688.7 nm has a broadband spectrum replicating that of the pump. A conversion efficiency of 8.4% was obtained.

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 our last numerical and experimental results on a mid-infrared source based on a tunable Yb-based hybrid MOPA pump and a Backward Wave Optical Parametric Oscillators (BWOPO). The BWOPO has a record-low oscillation threshold of 19.2 MW/cm2 and generates mJ-level output with an overall conversion efficiency exceeding 70%. The BWOPO acts a frequency shifter of the pump radiation toward the forward wave, maintaining the pump spectral properties. The demonstrated tuning range of 10 GHz is already compliant for DIAL applications. We have also developed advanced numerical modelling of the BWOPO taking into account spectral and, for the first time, spatial beam profiles.

For the first time, we show spectral and polarization indistinguishability of photon pairs generated in counter-propagating degenerate spontaneous parametric downconversion in the telecommunications band using first-order quasi-phase-matching in periodically poled Rb-KTiOPO 4 .

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.

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.

A highly efficient mirrorless OPO tunable in the mid-infrared around 2 μm has been developed and characterized in an original pumping configuration comprising a tunable high power hybrid Ytterbium laser MOPA (Master Oscillator Power Amplifier) in the nanosecond regime. The hybrid pump laser is based on a fiber laser seeder continuously tunable over several GHz at 1030 nm, which is shaped in the time domain with acousto-optic modulators (AOM), and power amplified in a dual stage Ytterbium doped fiber amplifiers, followed by two Yb:YAG bulk amplifiers. The pump delivers up to 3.5 mJ of energy within narrowband 15 ns pulses with a 5 kHz repetition rate. The output was focused into Periodically Poled KTP (PPKTP) crystals with a quasi-Phase Matching (QPM) period of 580 nm, producing Backward Optical Parametric Oscillation (BWOPO), with a forward signal wave at 1981 nm and a backward traveling idler at 2145 nm. We report significant optical to optical efficiencies exceeding 70 % depending on crystal length and input power. As theoretically expected, the forward wave could be continuously tuned over 10 GHz following the pump frequency sweep, while the backward wave remains almost stable, both being free from mode hops. These properties obtained from an optical arrangement without free-space cavities are attractive for future space Integrated Path Differential Absorption (IPDA) Lidar applications, which require robust and efficient tunable frequency converters in the mid-infrared. Additional presentation content can be accessed on the supplemental content page.

We demonstrate first-order quasi-phase-matched backward second-harmonic generation (BSHG) with an efficiency of 18.7%. This represents an increase by two orders of magnitude from earlier experiments employing higher-order quasi-phase-matching. The efficient BSHG is demonstrated in bulk periodically poled Rb:KTiOPO4 with a poling period of 317 nm. Using these structures, the frequency doubling in the backward direction is achieved for the fundamental wavelength of 2309 nm. Here we report on the experimental investigation of the BSHG properties such as spectral bandwidth, temperature tuning, and temperature bandwidth by employing broadband and narrowband fundamental wavelength sources. The BSHG properties are compared with those of co-propagating second harmonic generation to reveal the BSHG potential for novel applications that were proposed theoretically but have not been realized in practice so far.

We demonstrate a continuously tunable mid-infrared source that produces narrowband radiation at 1981 nm and 2145 nm based on a tunable Yb-based hybrid MOPA pump and a backward-wave optical parametric oscillator (BWOPO). The BWOPO employs a PPRKTP crystal with 580 nm domain periodicity. The BWOPO has a record-low oscillation threshold of 19.2 MW/cm2 and generates mJ level output with an overall efficiency exceeding 70%, reaching an average power of 5.65W at the repetition rate of 5 kHz. The system is mechanically robust and optical cavity-free, making it suitable for spectroscopic systems on mobile platforms. The mid-infrared signal frequency is tuned by pump tuning with a linear pump-to-signal frequency translation rate close to the predicted 1 to 1.001 Hz/Hz.

We report a self-phase-locked and degenerate backward wave optical parametric oscillator (BWOPO) generating a counterpropagating signal and an idler in the spectral band of optical communication. The device is based on a periodically poled Rb-doped KTiOPO4 crystal with a poling period of 433 nm. To the best of our knowledge, this is the shortest quasi-phase matching period reported in the bulk of a ferroelectric crystal. The spectral and coherence properties of the BWOPO at degeneracy are investigated. We observe a single pass conversion efficiency of 40%. At degeneracy, the counterpropagating signal and idler generate phase-matched sum-frequency generation in both directions. We confirm the phase-locked state at degeneracy by interfering the pump with the frequency-doubled backward wave. The phase-locked state is stable during the entire duration of the experiment without active stabilization.

Precision-tunable mJ, nanosecond pulses at 1981.1 nm and 2145.6 nm are generated at 5kHz repetition with an efficiency of 70% in a cavity-free arrangement using a PPRKTP BWOPO pumped by a narrowband DPSSL at 1030nm.

Precision-tuned nanosecond pulses at 1981.1 nm and 2145.6 nm are generated from a PPKTP BWOPO with 70% efficiency, pumped by an amplified single-frequency fiber laser at 1030 nm. It is free from mechanically-adjustable optical cavities.

Cavity-free, precision-tunable source of a nanosecond, mJ-level, pulses consisting of backward optical parametric oscillator pumped by diode-seeded laser amplifier shows a record threshold of 19 MW/cm2 and an optical-to-optical efficiency exceeding 70%.

We demonstrate 3 rd order QPM backward SHG in a bulk PPRKTP with a poling period of 317nm. This is the shortest QPM period ever fabricated. These results pave the way towards 1 st order BSHG.

We demonstrate multi-cycle terahertz (MC-THz) generation in a 15.5 mm long periodically poled rubidium (Rb)-doped potassium titanyl phosphate (Rb:PPKTP) crystal with a poling period of 300 µm. By cryogenically cooling the crystal to 77 K, up to 0.72 µJ terahertz energy is obtained at a frequency of 0.5 THz with a 3 GHz bandwidth. A maximum internal optical-to-terahertz conversion efficiency of 0.16% is achieved, which is comparable with results achieved using periodically poled lithium niobate crystal. Neither photorefractive effects nor damage was observed with up to 900 m J / c m 2 , showing the great potential of Rb:PPKTP for multi-millijoule-level MC-THz generation.