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A structure-based lead optimization procedure is an essential step to finding appropriate ligand molecules binding to a target protein structure in order to identify drug candidates. This procedure takes a known structure of a protein-ligand complex as input, and structurally similar compounds with the query ligand are designed in consideration with all possible combinations of atomic species. This task is, however, computationally hard since such combinatorial optimization problems belong to the non-deterministic nonpolynomial-time hard (NP-hard) class. In this paper, we propose the structure-based lead generation and optimization procedures by a degenerate optical parametric oscillator (DOPO) network. Results of numerical simulation demonstrate that the DOPO network efficiently identifies a set of appropriate ligand molecules according to the Boltzmann sampling law.

Photon pairs and heralded single photons, obtained from cavity-assisted parametric down conversion (PDC), play an important role in quantum communications and technology. This motivated a thorough study of the spectral and temporal properties of parametric light, both above the optical parametric oscillator (OPO) threshold, where the semiclassical approach is justified, and deeply below it, where the linear cavity approximation is applicable. The pursuit of a higher two-photon emission rate leads into an interesting intermediate regime, between above OPO threshold and very low pump power, where the OPO still operates considerably below the threshold but the nonlinear cavity phenomena cannot be neglected anymore. Here, we investigate this intermediate regime and show that the spectral and temporal properties of the photon pairs, as well as their emission rate, may significantly differ from the widely accepted linear model. The observed phenomena include frequency pulling and broadening in the temporal correlation for the down-converted optical fields. These factors need to be taken into account when devising practical applications of the high-rate cavity-assisted spontaneous PDC sources.

Based on a photonic crystal fibre (PCF) pumped by an ytterbium-doped fibre laser, a high-efficiency all-fibre optical parametric oscillator (OPO) is demonstrated. The high conversion efficiency is guaranteed by the relatively low intra-cavity loss of the all-fibre configuration. In the experiment, an internal conversion efficiency of up to 36% is achieved, which is believed to be the largest value for PCF-based OPOs.

Parametric down-conversion in a nonlinear crystal is a widely employed technique for generating quadrature squeezed light with multiple modes, which finds applications in quantum metrology, quantum information and communication. Here we study the generation of temporally multimode femtosecond pulsed squeezed light in a synchronously pumped optical parametric oscillator (SPOPO) operating below the oscillation threshold, while considering the presence of non-compensated intracavity group-velocity dispersion. Based on the developed time-domain model of the system, we show that the dispersion results in mode-dependent detuning of the broadband supermodes of the pulsed parametric process from the cavity resonance due to temporal Gouy phase, as well as linear coupling between these supermodes. With perturbation theory up to the second order in the coupling coefficients between modes, we obtained a solution for the amplitudes of multiple supermodes given an arbitrary sub-threshold pump level. The dispersion affects the quantum state of the supermodes by influencing their squeezing level and the rotation of the squeezing ellipse. It also affects the entanglement among the supermodes, leading to reduced suppression of shot noise level as measured in the balanced homodyne detection scheme. Furthermore, our study highlights the potential of SPOPO with group-velocity dispersion as a testbench for experimental investigations of multimode effects in linearly evanescent coupled parametric oscillators.

Laser frequency combs based on optical parametric oscillators provide widely tunable sources of regularly spaced frequencies from the visible to the mid-infrared, with potential applications in precision metrology, spectroscopy and astronomy. Reported is the extension of the comb mode spacing into the GHz domain by using a Fabry-Pérot cavity, the length of which was electronically stabilised to a harmonic of the original pulse repetition frequency via a dither-locking technique. A 1500 nm femtosecond optical parametric oscillator frequency comb, the repetition frequency and carrier-envelope offset frequency of which were locked to 280 MHz and 10 MHz, respectively, was filtered by a high-finesse cavity to generate a pulse sequence at 1.4 GHz, corresponding to a factor of five increase in the mode spacing of the comb.

A periodically poled LiNbO3 (PPLN) non-resonant optical parametric oscillator injectionseeded by narrowband sub-50-mW CW radiation at the signal wavelength produces a >3 W average idler power at 2376 nm for a 20 kHz repetition rate, with a ~2 nm spectral linewidth. Seed levels as low as 5 mW are sufficient to produce the desired spectral narrowing effect, and spectral tuning is possible by changing the seed wavelength and simultaneously adjusting the crystal temperature. The spectral features are in good agreement with numerical simulations based on the plane wave approximation.

A high-power and high-repetition KTiOAsO4 (KTA) optical parametric oscillator (OPO) was established in this study, with the adoption of plane-parallel and ring cavities. The pump was a high-power Nd:YAG master oscillator power amplifier (MOPA) system with a pulse repetition frequency (PRF) of 300 Hz, and the corresponding beam quality factors were Mx2 = 3.4 and My2 = 3.2. In the plane-parallel cavity experiment, powers of 51.1 W (170 mJ) and 15.9 W (53 mJ) in the signal and idler were obtained, respectively. In terms of the average power of 1 μm of a pumped KTA OPO, to our knowledge, this is the highest average power for KTA OPO. The ring cavity was constructed to achieve lasers with both high power and beam quality. The output powers of the ring cavities for the signal and idler were 33.9 W (113 mJ) and 8.7 W (29 mJ), respectively, and the corresponding beam quality factors of the signal were Mx2 = 5.3 and My2 = 7.9. The 300 Hz 100 mJ class 1.54 μm laser with a beam quality factor of less than 10 is an ideal eye-safe light detection and ranging (LiDAR) source.

The wavefront aberrations (WAs) of a laser beam produced by an optical parametric oscillator (OPO) have been measured using a Hartmann–Shack sensor. The OPO tuning operation requires changes in the device that might affect the shape of the wavefront beam as the illumination wavelength is being modified. Different output wavelengths in the range 550–850 nm were systematically analyzed in terms of WAs. The WA laser beam was fairly stable with time (changes of about 1%), independently of the wavelength. Moreover, WAs were non-negligible and nearly constant between 600 and 800 nm, but they noticeably increased for 550 (~90%) and 850 nm (~50%), mainly due to a higher astigmatism influence. The contributions of other higher-order terms such as coma and spherical aberration also present particular spectral dependences. To our knowledge, this is the first report of a spectral OPO laser beam characterization in terms of optical aberrations. It addresses a gap in OPO laser characterization of WAs and offers actionable insights for multi-wavelength applications. These results might be useful in applications ranging from micromachining procedures to biomedical imaging, where an optimized focal spot is required to increase the efficiency of certain physical phenomena or to enhance the quality of the acquired images.

This paper introduces a high single-pulse energy, narrow-linewidth mid-infrared self-optical parametric oscillator (mid-IR SOPO) with a cavity length of 120 mm and a Nd:MgO:PPLN crystal. To achieve high single-pulse energy and high peak power in mid-IR light sources, a LiNbO 3 electro-optic Q-switch (EOQ) is introduced for the first time in a mid-IR SOPO. A narrow-linewidth EOQ-SOPO rate equation is formulated, and experiments are conducted using a single Fabry–Pérot etalon. At a 500 μs pump pulse width, a 4.71 mJ single-pulse idler light at 3838.2 nm is achieved, with a linewidth of 0.412 nm, single-pulse width of 4.78 ns and peak power of 985 kW. At 200 μs, the idler light at 3845.2 nm exhibits a minimum linewidth of 0.212 nm.

We report on the investigation of a high–efficiency, widely tunable femtosecond optical parametric oscillator (OPO) based on a multi–period MgO–doped periodically poled lithium niobite (MgO: PPLN) crystal, pumped by an all–solid–state femtosecond mode–locked Yb: KGW laser at 1030 nm providing 100 fs pulses. With 6 W pump power, the OPO generates 2.68 W of signal power at 1540 nm and 1.2 W of idler power at 3110 nm, which corresponds to the total conversion efficiency adding up to 67.4%. To the best of our knowledge, this is the highest conversion efficiency of a femtosecond OPO. Meanwhile, in order to obtain a broad optical spectrum range, both the grating period and working temperature are tuned, resulting in tunable signals of 1.43–1.78 µm and idlers of 2.44–3.68 µm. This source will be used to generate a femtosecond mid–infrared laser of wavelength range 3.7–6.5 µm and tens milliwatts average power through difference frequency generation (DFG).