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We present a tunable (6.62-11.34 µm), singly-resonant, cascade optical parametric oscillator with intracavity pumping of BaGa2GeSe6 in the second stage and spectral narrowing realized by a Volume Bragg Grating acting on the signal wave of the first stage which serves as a pump for the second stage. The maximum energy achieved near 8 µm in the narrowband regime is 1.1 mJ at 100 Hz (spectral width: ∼20 cm−1, pulse duration: ∼7 ns). The overall conversion efficiency from 1 to 8 µm for broadband and narrowband operation is 4.0% and 3.1%, respectively, corresponding to quantum efficiencies of 31% and 23%., QC 20240202

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-KTiOPO4., Part of ISBN 978-1-957171-39-5QC 20240906

Spectral narrowing of the first stage signal wave in a cascade optical parametric oscillator (OPO) using a volume Bragg grating produced narrowband ∼0.6-mJ idler pulses at 9.245 mu m from the second stage based on CdSe at a repetition rate of 100 Hz. The CdSe crystal is pumped intracavity by the signal wave from the periodically poled KTiOPO4 based first stage, and both stages operate under noncritical phase-matching conditions. The ∼8-ns-long mid-IR output pulses have a spectral bandwidth of 8.4 cm-1. An overall quantum conversion efficiency of 14.2% from 1.064 mu m is achieved with a simple, compact, and robust all-solid-state design., QC 20240923

Efficiencies of nonlinear optical-to-terahertz (THz) conversion below one percent remain a limiting factor for applications of multicycle THz radiation like THz-driven acceleration and inspired the use of multi-line pump spectra. To overcome the difficulty of phase stabilization of multiple narrowband sources required by the multi-line approach, we exploit its temporal analog, i.e., regular pulse trains with THz repetition rate, in which the THz waves generated by rectifying the individual pulses add coherently. The optical setup producing the pulse trains consists of motorized interferometers and enables precise control over the pulse train parameters like pulse spacing and amplitude. It is operated with a laser providing 400 fs pulses and energies of up to 110 mJ, which is the highest yet attempted for a pulse-train-type experiment. Opposed to earlier work, pulse division is done after amplification making the system more flexible in terms of tuning the pulse number. We present initial results of an experimental campaign of multicycle THz generation in custom periodically-poled crystals with large-apertures up to 10x20 mm(2). The available pump energy allows filling these apertures at high fluences, promising increased THz yields. We investigate the dependence of the conversion efficiency on the single pulse duration and aim to find the optimum pulse number for different crystal lengths to determine the efficiency limitations in a regime avoiding laser-induced damage. Since crystal length and pulse number define the bandwidth of the THz pulses, this work demonstrates a path to an optimized THz source tunable to different requirements of applications., QC 20240617

We investigate a new method of coercive field engineering for periodic poling of RbKTiOPO4 (RKTP). By ion exchanging RKTP in a molten salt containing 7 mol% Ba(NO3)2 and 93 mol% KNO3 we achieve more than an order of magnitude difference in polarization switching time between the exchanged and non-exchanged regions. This method is used to fabricate periodic gratings of 2.92 µm in 1 mm thick bulk RKTP for second harmonic generation at 779 nm with a normalized conversion efficiency of 2%/Wcm. We show that the poled domain structures are stable at 300 °C, and that there is no bulk refractive index modification associated with the periodic ion exchange., QC 20240429

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., Not duplicate with DiVA 1755027QC 20240102

We experimentally investigate using backward-wave spontaneous parametric downconversion for frequency translation, where spectral characteristics of the pump wave are transferred to the signal wave., Part of ISBN 9781957171258QC 20231124

The development of reliable periodic poling methods that allow for sub-µm quasi-phase matched (QPM) gratings and, at the same time, allow for waveguide implementation, is of paramount importance for a large number of applications. For instance, backward-wave optical parametric oscillators [1] are only viable if the QPM period is on the same order of magnitude as the wavelengths of the interacting waves. Furthermore, the integration of such QPM devices in a waveguide format would unveil countless possibilities in quantum optics employing the crystal as an ultrabright bi-photon source with unique spectral characteristics., Part of ISBN 9798350345995QC 20231116

QC 20231025

A non-resonant optical parametric oscillator based on 5-mm-thick Rb-doped periodically-poled KTiOPO4 (PPKTP) generates narrow-band (

We present a tunable (6.62-11.34 µm), singly-resonant, cascade optical parametric oscillator with intracavity pumping of BaGa2GeSe6 in the second stage and spectral narrowing realized by a Volume Bragg Grating, providing millijoule output at 100 Hz., QC 20240531Part of ISBN 978-1-957171-23-4

Large aperture periodically-poled Rb:KTP crystals designed for optical parametric amplifiers in 2 µm LIDAR systems were radiation hardness tested by exposure of proton beams at 10 MeV and 60 MeV energies. An irradiation dose of 55 Gy was used to commensurate the crystals’ estimated exposure on board a mission in the low-Earth orbit. The irradiation effects were investigated by comparing optical transmission spectra and 2D effective nonlinearity mapping in a 2 µm OPO setup before and after irradiation. The results reveal that the periodically poled structure remained intact after irradiation, and the changes in the optical transmission and nonlinear properties were close to the measurement uncertainty. This investigation is essential for realizing efficient frequency converters for space applications, such as spaceborne active greenhouse gas monitoring LIDAR instruments or correlated photon-pair sources., QC 20230529

A stable, narrow-bandwidth (274 MHz) backward wave optical parametric oscillator (BWOPO) generating mJ-level backward signal at 1885nm and forward idler at 2495 nm is presented. The BWOPO was pumped by a single-longitudinal mode, Q-switched Nd:YAG high-energy laser at 1064 nm. We show that multi-transversal mode pumping leads to the spectral broadening of the BWOPO backward signal and the generation of nanosecond pulses 2.7 times above the Fourier transform limit. We demonstrate over 100 GHz continuous tuning of the parametric output by adjusting the temperature of the BWOPO crystal, showcasing the significant role of thermal expansion in tuning performance. The BWOPO signal was used as a seed for a single-stage PPRKTP optical parametric amplifier (OPA) to boost the narrowband signal and idler energies to 20 mJ. This combination of mJ-level BWOPO seed with a single-stage PPRKTP OPA comprises a simple concept that would benefit long-range differential absorption lidar (DIAL) in the near and mid-infrared regions., QC 20230815

We demonstrate periodic poling of large aperture Rb-doped KTP (RKTP) crystals with a QPM period of 3.43 µm via coercive field engineering. The periodically-poled RKTP has excellent pattern-fidelity, with a second harmonic generation (SHG) conversion efficiency of 1.4%/Wcm at 405 nm. We use ion-exchange (IE) to fabricate grating of high-and-low coercive fields and establish that the depth of the in-diffused Rb+ is the main parameter to achieve sufficiently large coercive field contrast that enables uniform periodic poling in the thick crystal. Furthermore, we demonstrate that our robust coercive field grating allows multiple cycles of polarization-switching which can be employed to decrease the refractive-index change along the polar axis, induced during the IE process., QC 20230907

QC 20230530

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 mag-nitude 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 band-width, temperature tuning, and temperature bandwidth by employing broadband and narrowband fundamental wave-length 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., QC 20230425

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., QC 20230316

A Nd:YVO4 laser operating at 1064 nm generating a stable mode-locked train of 10 ps-long dark pulses with a 211 MHz repetition rate is presented. The mode-locking relies on a periodic loss modulation produced by intra-cavity sum-frequency mixing with a synchronous bright-pulse train from a mode-locked femtosecond Yb:KYW laser at 1040 nm. A modulation depth of 9050 was achieved for the dark pulses, confirmed by cross-correlation measurements. The ultrafast loss modulation injects power into the Nd:YVO4 laser cavity modes beyond the laser gain bandwidth. At proper laser cavity length, the detuning interaction of these modes with the lasing modes leads to the generation of periodic ultra-fast transients at frequencies above 1.5 THz., QC 20220328

Nonlinear optical interactions involving counter-propagating photons are of great interest for both classical and quantum optical applications. However, their use is sparsely spread due to the fact that they require quasi-phase-matched devices with sub-µm periods. A recent breakthrough has been the fabrication of bulk sub-µm domain gratings in Rb-doped KTiOPO4 by creating a grating of low- and high- coercive field regions in the crystal via periodic ion exchange, so-called coercive-field engineering. Here, we investigate the physical mechanisms behind this method and study the interplay between the ion-exchanged grating properties and the ferroelectric domain dynamics. Furthermore, we investigate the scalability of the method by studying the domain morphology of sub-µm periodically poled crystals with periods ranging from 755 to 433 nm and correlating it to that of the ion-exchanged regions. We show that the formation of sub-µm domain gratings is governed by the depth, shape, and critical ion-concentration of the ion-exchanged volume and that it is independent of the poling period. These findings are crucial for further scaling the coercive field engineering technique to even shorter poling periods and larger aperture periodically poled crystals., QC 20221124

We present a narrowband, non-resonant optical parametric oscillator based on 5-mm thick Rb-doped periodically-poled KTiOPO4 (PPKTP) operating in the high-energy/low repetition-rate regime. An uncoated volume Bragg grating (VBG) is employed as one of the cavity mirrors reflecting only the signal whereas the other cavity mirror is reflecting only the idler. Pumping by a Nd:YAG laser at 1.0642 mu m in a double-pass, the signal plus idler output energy reached almost 5 mJ at a repetition rate of 100 Hz corresponding to a conversion efficiency of similar to 26%. Both signal and idler are narrowband with full width at half maximum (FWHM) of 0.5 nm at 1942 nm and 0.76 nm at 2355 nm, respectively., QC 20220812Part of proceedings: ISBN 978-1-5106-4842-5; 978-1-5106-4841-8

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%., Part of ISBN 9781557528209QC 20230801

We observed backward-wave spontaneous parametric downconversion in subµm periodically poled Rb-doped KTP. Pumped at 800 nm, forward-wave signal at 1400 nm and backward-wave idler at 1868 nm were obtained., Part of ISBN 9781557528209QC 20230802