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The ion temperature is an important parameter for ionospheric detection, yet there is limited research on ion temperatures at altitude range from 80 to 300 km. Currently, a single‐frequency Ca+ ion optical parametric oscillator (OPO) Lidar system can be used to obtain the Ca+ ion density in this region at Yanqing Station, Beijing (40.4°N, 116.0°E). In this study, the ion temperature can be obtained by extending our original lidar to include three‐frequency Ca+ Doppler detection. This development represents a pioneering step on measuring ion temperatures by lidar in the ionospheric E‐F region. Preliminary results in the E region show that lidar temperatures at 90–105 km under the condition of smooth changes in Ca+ ion morphology align reasonably with satellite‐based observations and model‐derived temperatures. Additionally, the exploratory ion temperatures of the F layer peak heights at 200–300 km were obtained. Furthermore, parameter optimization and temperature error analysis in the E‐F region are explored, providing valuable insights for the development of Ca+ Doppler lidar systems. The advent of Ca+ Doppler lidar has greatly expanded the lidar detection altitude range of temperature and the possibilities for in‐depth research on ion temperature and velocity in regions affected by complex electrodynamic effects. These findings lay the foundation for subsequent studies of coupling processes in the ionospheric E‐F region. Key Points: Innovatively modified Ca+ Doppler lidar achieves ion temperature measurements in the challenging E‐F regionThe temperatures detected by lidar in the E region are reasonable, and exploratory temperatures in the F region are also obtainedThe discussion on optimizing the Ca+ Doppler lidars provides valuable guidance for temperature measurements in the E‐F region up to 300 km [ABSTRACT FROM AUTHOR]

The synthesis of 2D MoWS2 nanosheets involved the liquid-phase exfoliation technique was explored in this paper. The nonlinear optical response of MoWS2 was characterized in the 1 µm wavelength range, and its suitability as a saturable absorber (SA) was confirmed. Experimental demonstrations were conducted by using MoWS2 as an SA in an idler-resonant intracavity KTA optical parametric oscillator (OPO) driven by a dual-loss-modulated Q-switched and mode-locked (QML) YVO4/Nd:YVO4 laser with an acousto-optic modulator (AOM). By appropriately tuning the pump power and the AOM repetition rate, the Q-switched envelope pulse widths for the signal and idler waves could be significantly reduced to be shorter than the cavity round-trip transit time, i.e., the interval between two neighboring mode-locking pulses. Consequently, this enabled the generation of sub-nanosecond single mode-locking pulses with a low repetition rate, high pulse energy, and remarkable stability. With a repetition rate of 1 kHz and maximal pulse energies of 318 µJ and 169 µJ, respectively, sub-nanosecond single mode-locking pulses of the signal and idler waves were generated. The theoretical model was established using coupled rate equations with a Gaussian spatial distribution approximation. The numerical simulation results for generating sub-nanosecond single mode-locking pulses for the signal and idler waves within their respective Q-switched envelopes aligned fundamentally with the experimental results, proving that MoWS2 can be a potential nanomaterial for further optoelectronic applications. [ABSTRACT FROM AUTHOR]

Coherent anti‐Stokes Raman scattering microscopy probing two independently selectable vibrational frequencies across the vibrational spectrum is demonstrated. A one‐box femtosecond laser source provides three synchronised 100–200 fs pulses of 77 MHz repetition rate, one at a fixed wavelength of 1025 nm from an Yb oscillator and two independently tuneable ones, over 680–960 nm and 960–1300 nm, respectively, from two optically synchronised parametric oscillators. Combined with spectral focusing, the source allows addressing the fingerprint, cell silent and C–H stretch vibrational regions simultaneously, avoiding the motion artefacts seen in sequential imaging. We demonstrate the microscopy method on water/heavy water/oil emulsions, plastic beads and deuterated lipids inside cells. [ABSTRACT FROM AUTHOR]

This work is devoted to the development of a compact source of coherent radiation with frequency-energy characteristics and a spectral generation range that allows remote determination of background concentrations of greenhouse gases in the atmosphere. The aim of this work was to create a frequency parametric converter based on ZGP, pumped by Ho:YAG laser radiation. For use as a source in a mobile lidar for remote determination of the concentration of greenhouse gases in the atmosphere. In the course of the research, a layout of an Optical parametric oscillator OPO based on a ZGP crystal with Ho:YAG laser radiation pumping was developed. The system's continuous failure-free operation time was 1.5 h at a pulse repetition rate of 10 kHz and a pulse energy of the generated radiation of 0.08 mJ. The tuning range of the OPO was from 3.3 to 5 μιτι when using a Lyot filter. The losses from the average generation power when the Lyot filter was introduced into the resonator were 30%. At the same time, it was possible to achieve a linewidth of the generated radiation of 0.7 nm. The divergence of the generated radiation did not exceed 1.5 mrad.The absorption spectrum of gases CO2, CH4, N2O, CO in a gas cell was simulated for the entire generation range of the ZnGeP2-based OPO. As a result of the simulation, the most intense absorption lines of gases CO2, CH4, N2O, CO in the OPO tuning range were revealed, the central wavelengths of the absorption lines and their spectral width were determined. [ABSTRACT FROM AUTHOR]

The significance of intelligent sensor systems has grown across diverse sectors, including healthcare, environmental surveillance, industrial automation, and security. Photoacoustic gas sensors are a promising type of optical gas sensor due to their high sensitivity, enhanced frequency selectivity, and fast response time. However, they have limitations such as dependence on a high-power light source, a requirement for a high-quality acoustic signal detector, and sensitivity to environmental factors, affecting their accuracy and reliability. Machine learning has great potential in the analysis and interpretation of sensor data as it can identify complex patterns and make accurate predictions based on the available data. We propose a novel approach that utilizes wavelet analysis and neural networks with enhanced architectures to improve the accuracy and sensitivity of photoacoustic gas sensors. Our proposed approach was experimentally tested for methane concentration measurements, showcasing its potential to significantly advance the field of gas detection and analysis, providing more accurate and reliable results. [ABSTRACT FROM AUTHOR]

We describe an efficient long-wave infrared optical parametric oscillator based on ZnGeP2 crystal pumped by a diode-pumped AO Q-switched Ho:YAG laser in this paper. The wavelength of the laser pulse generated by Ho:YAG laser is 2.09- μιτι, and the maximum output average power is 25.3 W at repetition rate of 10 kHz. When pumped by such Ho:YAG laser, the maximum average output power of 0.66 W at 9.8 μ!Γ is achieved, corresponding to slope efficiency of 5.0% and optical efficiency of 2.6%. Furthermore, the beam quality factor of the longwave infrared beam is measured to be approximately 1.3. To the authors' best knowledge, this is the best performance reported for a long-wave infrared ZGP-OPO pumped by a diode-pumped AO Q-switched Ho:YAG laser. [ABSTRACT FROM AUTHOR]

In this paper, we present a picosecond pulsed, synchronously pumped optical parametric oscillator producing vortex beam output with tunable wavelengths in the near- to mid-infrared range. The system utilizes a Nd:YVO4 picosecond pulsed solid-state laser emitting at a wavelength of 1.064 µm to pump a Z-shaped, singly resonant OPO which contains a MgO:PPLN crystal with a fan-shaped grating. The wavelength tuning characteristics of the OPO output are examined both as a function of the MgO:PPLN grating period and crystal temperature. The orbital angular momentum of the pump field can be selectively transferred to either the signal or idler fields by appropriately adjusting the location of the MgO:PPLN crystal within the OPO cavity. The maximum output power of the signal and idler vortex fields are 5.12 W and 3.46 W, respectively, for an incident pump power of 19 W. [ABSTRACT FROM AUTHOR]

We report the development of a combined optical parametric oscillator based on a fan-out MgO:PPLN crystal and a HgGa2S4 crystal, providing smooth wavelength tuning in the spectral range from 2.5 to 10.8 μm. A transversely pumped Nd:YAG laser with a pulse energy of ∼4 mJ and a spatial beam distribution described by a second-order Gaussian function along the x axis and a fourth-order Gaussian function along the y axis is used as a pump source. A maximum achievable radiation energy of the optical parametric oscillator is at a level of 360 μJ for a wavelength of 3.3 μm, 130 μJ for a wavelength of 4.73 μm, and 110 μJ for a wavelength of 7.40 μm. The developed radiation source is used to record absorption spectra of gas mixtures of methane, propane and nitrogen dioxide. [ABSTRACT FROM AUTHOR]

We present a high-power, widely tunable, synchronously pumped picosecond pulsed optical parametric oscillator (OPO) generating emissions in the near to mid-infrared wavelength ranges. The OPO is pumped using a Nd:YVO4 picosecond pulsed laser and utilizes a fan-shaped, multi-grating MgO doped PPLN crystal (MgO:PPLN). The system generates near to mid-infrared output across the wavelength range 1.3–5 µm, with a high overall conversion efficiency of 75.15%. [ABSTRACT FROM AUTHOR]

Lasers operating around 2-µm have been extensively researched for various applications, such as LiDAR, remote sensing, optical communication, and medicine. One of the approaches to generate 2-µm emission is to employ an optical parametric oscillator pumped by 1-µm laser; however, improving the 2-µm laser conversion efficiency of optical parametric oscillators is still a challenge. In this study, we exploit the high nonlinear coefficient of the KTP crystal at the 2-µm band to realize a high-efficiency optical parametric oscillator. TheOPOis driven by a laser diode (LD) side-pumped, electro-optic Q-modulated Nd:YAG laser. The cavity was designed and optimized based on mode matching to increase the peak power density of the pump laser. Different powers were obtained for different pump laser focusing conditions, and the maximum power was obtained when the KTP crystal was placed in front of the pump laser focus. The output characteristics of the KTP at different phase-matching angles were also investigated. An output power of 2.44Wand an output pulse width of 9.04 ns were obtained at 2154.4 nm with an optical-to-optical conversion efficiency of 23.39% and a slope efficiency of 36.09%. [ABSTRACT FROM AUTHOR]

Spectra of rare gas (Rg) – D2O dimers (Rg = Ar, Kr, Xe) are studied in the region of the D2O ν3 asymmetric stretch (≈2800 cm−1), using a rapid-scan tunable infrared optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. Three bands are observed in each case, labelled according to the D2O rotational transition with which they correlate as: Σ(101) ← Σ(000), Π(101) ← Σ(000), and Σ(000) ← Σ(101). The Ar-D2O bands exhibit various perturbations and line broadening effects. For Kr-D2O, the Π(101) ← Σ(000) and Σ(000) ← Σ(101) bands are simple and relatively unperturbed with almost no resolved Kr isotope structure, while the Σ(101) ← Σ(000) band is highly perturbed and shows large isotope splittings. This latter perturbation is analyzed in detail with good results. For Xe-D2O, the experimental spectra are unfortunately weaker. Analysis of the Π(101) ← Σ(000) and Σ(000) ← Σ(101) bands is again relatively straightforward, but the Σ(101) ← Σ(000) band is even more perturbed than for Kr-D2O and its analysis proves impossible. Line broadening (for Ar-D2O) and vibrational frequency shift effects are described. [ABSTRACT FROM AUTHOR]

The synthesis of 2D MoWS2 nanosheets involved the liquid-phase exfoliation technique was explored in this paper. The nonlinear optical response of MoWS2 was characterized in the 1 µm wavelength range, and its suitability as a saturable absorber (SA) was confirmed. Experimental demonstrations were conducted by using MoWS2 as an SA in an idler-resonant intracavity KTA optical parametric oscillator (OPO) driven by a dual-loss-modulated Q-switched and mode-locked (QML) YVO4/Nd:YVO4 laser with an acousto-optic modulator (AOM). By appropriately tuning the pump power and the AOM repetition rate, the Q-switched envelope pulse widths for the signal and idler waves could be significantly reduced to be shorter than the cavity round-trip transit time, i.e., the interval between two neighboring mode-locking pulses. Consequently, this enabled the generation of sub-nanosecond single mode-locking pulses with a low repetition rate, high pulse energy, and remarkable stability. With a repetition rate of 1 kHz and maximal pulse energies of 318 µJ and 169 µJ, respectively, sub-nanosecond single mode-locking pulses of the signal and idler waves were generated. The theoretical model was established using coupled rate equations with a Gaussian spatial distribution approximation. The numerical simulation results for generating sub-nanosecond single mode-locking pulses for the signal and idler waves within their respective Q-switched envelopes aligned fundamentally with the experimental results, proving that MoWS2 can be a potential nanomaterial for further optoelectronic applications.

Laser pulse amplification by a high-pressure CO2 amplifier in the long-wave infrared (LWIR) spectral range is a feasible technology for strong-field physics research. Crystals such as ZnGeP2 (ZGP) have high nonlinear coefficients and transmittance in the LWIR region, with spectral widths of generated pulses closely matching the gain spectrum of high-pressure CO2 amplifiers. Therefore, ZGP optical parametric oscillation (OPO) may allow higher-efficiency energy extraction in amplifiers, improving the output characteristics of LWIR amplification systems. In this study, the gain measurement of ZGP OPO pulses amplified by a high-pressure CO2 amplifier was carried out for the first time. Single-detector acquisition was utilized to achieve a unified sensor responsivity, and a laser signal-triggered function generator was used to synchronize the seed pulse and amplifier. Six-pass amplification was performed successively, yielding an amplification factor of 4.5 for the peak power and a maximum coefficient of 0.42% cm−1 for the small-signal gain. The gain and loss effect during small-signal amplification were discussed. The potential capability of acquiring ultra-short pulses with ZGP OPO pulses was also explored with the FFT function of MATLAB software. [ABSTRACT FROM AUTHOR]

This work is devoted to the development of a compact source of coherent radiation with frequency-energy characteristics and a spectral generation range that allows remote determination of background concentrations of greenhouse gases in the atmosphere. The aim of this work was to create a frequency parametric converter based on ZGP, pumped by Ho:YAG laser radiation. For use as a source in a mobile lidar for remote determination of the concentration of greenhouse gases in the atmosphere. In the course of the research, a layout of an Optical parametric oscillator OPO based on a ZGP crystal with Ho:YAG laser radiation pumping was developed. The system’s continuous failure-free operation time was 1.5 h at a pulse repetition rate of 10 kHz and a pulse energy of the generated radiation of 0.08 mJ. The tuning range of the OPO was from 3.3 to 5 μm when using a Lyot filter. The losses from the average generation power when the Lyot filter was introduced into the resonator were 30%. At the same time, it was possible to achieve a linewidth of the generated radiation of 0.7 nm. The divergence of the generated radiation did not exceed 1.5 mrad.The absorption spectrum of gases CO2, CH4, N2O, CO in a gas cell was simulated for the entire generation range of the ZnGeP2-based OPO. As a result of the simulation, the most intense absorption lines of gases CO2, CH4, N2O, CO in the OPO tuning range were revealed, the central wavelengths of the absorption lines and their spectral width were determined.

We describe an efficient long-wave infrared optical parametric oscillator based on ZnGeP2 crystal pumped by a diode-pumped AO Q-switched Ho:YAG laser in this paper. The wavelength of the laser pulse generated by Ho:YAG laser is 2.09-μm, and the maximum output average power is 25.3 W at repetition rate of 10 kHz. When pumped by such Ho:YAG laser, the maximum average output power of 0.66 W at 9.8 μm is achieved, corresponding to slope efficiency of 5.0% and optical efficiency of 2.6%. Furthermore, the beam quality factor of the long-wave infrared beam is measured to be approximately 1.3. To the authors’ best knowledge, this is the best performance reported for a long-wave infrared ZGP-OPO pumped by a diode-pumped AO Q-switched Ho:YAG laser.

Lasers operating around 2-µm have been extensively researched for various applications, such as LiDAR, remote sensing, optical communication, and medicine. One of the approaches to generate 2-µm emission is to employ an optical parametric oscillator pumped by 1-µm laser; however, improving the 2-µm laser conversion efficiency of optical parametric oscillators is still a challenge. In this study, we exploit the high nonlinear coefficient of the KTP crystal at the 2-µm band to realize a high-efficiency optical parametric oscillator. The OPO is driven by a laser diode (LD) side-pumped, electro-optic Q-modulated Nd:YAG laser. The cavity was designed and optimized based on mode matching to increase the peak power density of the pump laser. Different powers were obtained for different pump laser focusing conditions, and the maximum power was obtained when the KTP crystal was placed in front of the pump laser focus. The output characteristics of the KTP at different phase-matching angles were also investigated. An output power of 2.44 W and an output pulse width of 9.04 ns were obtained at 2154.4 nm with an optical-to-optical conversion efficiency of 23.39% and a slope efficiency of 36.09%.

Abstract The widely tunable and high resolution mid-infrared laser based on a BaGa4Se7 (BGSe) optical parametric oscillator (OPO) was demonstrated. A wavelength tuning range of 2.76–4.64 μm and a wavelength tuning resolution of about 0.3 nm were obtained by a BGSe (56.3°, 0°) OPO, which was pumped by a 1064 nm laser. It is the narrowest reported wavelength tuning resolution for BGSe OPO, and was obtained by simultaneously controlling the angle and temperature of BGSe. Graphical Abstract

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A widely tunable 4.16–7.31 μm high-resolution mid-infrared optical parametric oscillator with a type-I BaGa4Se7 (BGSe) crystal pumped by a 1064 nm laser was demonstrated for the first time. We have achieved a wavelength tuning accuracy of less than 5 nm through simultaneously performing temperature tuning and angular tuning on the BGSe (46°, 0°) Optical Parametric Oscillation (OPO). The wavelength resolution will be less than 1 nm when using commercial temperature control equipment. [ABSTRACT FROM AUTHOR]

The widely tunable and high resolution mid-infrared laser based on a BaGa4Se7 (BGSe) optical parametric oscillator (OPO) was demonstrated. A wavelength tuning range of 2.76-4.64 μm and a wavelength tuning resolution of about 0.3 nm were obtained by a BGSe (56.3°, 0°) OPO, which was pumped by a 1064 nm laser. It is the narrowest reported wavelength tuning resolution for BGSe OPO, and was obtained by simultaneously controlling the angle and temperature of BGSe. [ABSTRACT FROM AUTHOR]

We report an all-solid-state narrowband lidar system for the simultaneous detection of Ca and Ca+ layers over Yanqing (40.41°N, 116.01°E). The uniqueness of this lidar lies in its transmitter, which is based on optical parametric oscillation (OPO) and optical parametric amplification (OPA) techniques. The injection seeded OPO and the OPA are pumped by the second harmonic of an injection-seeded Nd:YAG laser, which can generate coherent light at the wavelength of 786 nm or 846 nm lasers, whose second harmonics in turn generate the 393 nm or 423 nm pulses, respectively, for the detection of thermospheric and ionospheric Ca+ and Ca layers. Compared to the conventional dye-based system, this lidar transmitter is a narrowband system (bandwidth < 200 MHz), which has produced a factor of two more output power with higher stability and reliability. The lidar system in Yingqing demonstrated Ca+ detection sensitivity of 0.1 atoms-cm−3 for long-term observation and reached a height of ~300 km. Potential applications and further improvements in this lidar technique are also discussed in this paper. [ABSTRACT FROM AUTHOR]

In this paper, we present a picosecond pulsed, synchronously pumped optical parametric oscillator producing vortex beam output with tunable wavelengths in the near- to mid-infrared range. The system utilizes a Nd:YVO4 picosecond pulsed solid-state laser emitting at a wavelength of 1.064 µm to pump a Z-shaped, singly resonant OPO which contains a MgO:PPLN crystal with a fan-shaped grating. The wavelength tuning characteristics of the OPO output are examined both as a function of the MgO:PPLN grating period and crystal temperature. The orbital angular momentum of the pump field can be selectively transferred to either the signal or idler fields by appropriately adjusting the location of the MgO:PPLN crystal within the OPO cavity. The maximum output power of the signal and idler vortex fields are 5.12 W and 3.46 W, respectively, for an incident pump power of 19 W.

We present a high-power, widely tunable, synchronously pumped picosecond pulsed optical parametric oscillator (OPO) generating emissions in the near to mid-infrared wavelength ranges. The OPO is pumped using a Nd:YVO4 picosecond pulsed laser and utilizes a fan-shaped, multi-grating MgO doped PPLN crystal (MgO:PPLN). The system generates near to mid-infrared output across the wavelength range 1.3–5 µm, with a high overall conversion efficiency of 75.15%.

Spectroscopy in the terahertz (THz) band has been discussed as a promising tool for identifying substances such as mold in food, narcotics, or explosive materials. Other than the technological limitations, the most important difficulty is the presence of water vapor in the atmosphere, which affects THz measurements. In this paper, we present a systematic approach to the challenging subject of remote identification. We start with a brief analysis of the technical capabilities of the THz components and report the choice of devices for designing an experimental setup for reflection spectroscopy. We follow with the presentation of the transmission THz spectrometer working in an open atmosphere. Research conducted on the transmission configuration provides findings that are implemented in the experimental setup working in a reflective configuration. The final phase is an experiment providing data measured in the reflection configuration with the presence of water vapor, allowing the use of spectra in the identification of the measured samples. [ABSTRACT FROM AUTHOR]

High-power optical parametric oscillators (OPOs), as mature radiation sources in mid-infrared (MIR), degenerate gradually with wavelength increase, mainly above 3700 nm. Using a periodically poled magnesium-oxide-doped lithium niobate (MgO:PPLN) as the nonlinear crystal, we build a high-power signal-resonant OPO pumped by ytterbium-doped fiber laser (YDFL). To improve the OPO's output power at ~3.8 μm, the parameters, such as the pump beam's waist diameter and location, the curvature radius of the output coupler and the length of MgO:PPLN, are discussed in detail. When pump power is 79 ± 4 W with a repetition rate of 200 kHz, the OPO provides up to 8 ± 0.4 W average power in beam quality with M2 factors of ~1.84 and ~1.69 in the two axes. Under the highest output power, the center wavelength of the idler beam is 3768.4 nm with a full-width at half-maximum (FWHM) bandwidth of ~18.6 nm. When the output power reaches ~6.3 W, its power stability is 1.6% root mean square (RMS) over 7 h. Further analysis of the factors affecting OPO's performance and simple structure are critically essential for compact OPO prototypes with a capacity of high output power. [ABSTRACT FROM AUTHOR]

Quasi-phase matching optical parametric oscillators (OPOs) are widely used for mid-infrared laser generation. However, the traditional OPO bulk material using high voltage electric field poling limits the flexibility of the poling period. In this paper, we propose a novel OPO design based on all-optical poled silicon nitride, allowing for adjustable poling period. Two OPO design schemes are presented, each offering unique advantages in terms of operational characteristics and performance. The output characteristics of the OPO are investigated in three modes: OPO pump tuning, poling fundamental wave (PFW) tuning, and OPO pump tuning and PFW tuning with the same laser. For OPO pump tuning, the OPO achieves output wavelengths of 3.319- $5.265 ~\mu \text{m}$ and 2.225- $3.769 ~\mu \text{m}$ with PFW wavelengths of 1450 nm and 1700 nm. For PFW tuning, the OPO produces output wavelengths of 5.502- $3.722 ~\mu \text{m}$ and 5.497- $3.577 ~\mu \text{m}$ using OPO pump wavelengths of 1310 nm and 1654 nm, respectively. Furthermore, for OPO pump tuning and PFW tuning with the same laser, the OPO generates output wavelengths ranging from $5.504 ~\mu \text{m}$ to $3.941 ~\mu \text{m}$ with OPO pump and PFW wavelengths ranging from $1.420 ~\mu \text{m}$ to $1.610 ~\mu \text{m}$ . This new OPO design offers enhanced flexibility in controlling the poling period for efficient mid-infrared laser generation.

We introduce an infrared laser generation scheme by cascaded difference frequency generation (CDFG) combined with optical parametric oscillator (OPO). An inventive infrared laser generator is proposed that enable cascaded optical waves, initially generated by CDFG in an aperiodically poled lithium niobate crystal, then continuously and repeatedly transferred to high-order Stokes waves by oscillations of cascaded optical waves, and finally brought out infrared laser from the high-order Stokes waves with specially designed OPO. We explain the physical mechanism of the above infrared laser generation process and demonstrate wide frequency tuning characteristics and high conversion efficiency characteristics of the infrared laser generator by providing theoretical research. The frequency tuning from pump frequency to 60 THz with a high conversion efficiency are realized by numerical calculations. The infrared laser generator exhibits physics distinctly different from lasers by atomic transitions, optical parametric oscillation or difference frequency generation. We consider that the infrared laser generator based on CDFG combined with OPO is promising for achieving high-efficiency and high-power infrared laser. [ABSTRACT FROM AUTHOR]

We present a theoretical investigation, based on the tight-binding Hamiltonian, of efficient electric-field-induced three-waves mixing (EFIM) in an undoped lattice-matched short-period superlattice (SL) that integrates quasi-phase-matched (QPM) SL straight waveguides and SL racetrack resonators on an opto-electronic chip. Periodically reversed DC voltage is applied to electrode segments on each side of the strip waveguide. The spectra of χxxxx(3) and of the linear susceptibility have been simulated as a function of the number of the atomic monolayers for “non-relaxed” heterointerfaces, and by considering all the transitions between valence and conduction bands. The large obtained values ofχxxxx(3) make the (ZnS)3/(Si2)3 short-period SL a good candidate for realizing large effective second-order nonlinearity, enabling future high-performance of the SLOI PICs and OEICs in the 1000-nm and 2000-nm wavelengths ranges. We have made detailed calculations of the efficiency of second-harmonic generation and of the performances of the optical parametric oscillator (OPO). The results indicate that the (ZnS)N/(Si2)M QPM is competitive with present PPLN technologies and is practical for classical and quantum applications.

Laser pulse amplification by a high-pressure CO2 amplifier in the long-wave infrared (LWIR) spectral range is a feasible technology for strong-field physics research. Crystals such as ZnGeP2 (ZGP) have high nonlinear coefficients and transmittance in the LWIR region, with spectral widths of generated pulses closely matching the gain spectrum of high-pressure CO2 amplifiers. Therefore, ZGP optical parametric oscillation (OPO) may allow higher-efficiency energy extraction in amplifiers, improving the output characteristics of LWIR amplification systems. In this study, the gain measurement of ZGP OPO pulses amplified by a high-pressure CO2 amplifier was carried out for the first time. Single-detector acquisition was utilized to achieve a unified sensor responsivity, and a laser signal-triggered function generator was used to synchronize the seed pulse and amplifier. Six-pass amplification was performed successively, yielding an amplification factor of 4.5 for the peak power and a maximum coefficient of 0.42% cm−1 for the small-signal gain. The gain and loss effect during small-signal amplification were discussed. The potential capability of acquiring ultra-short pulses with ZGP OPO pulses was also explored with the FFT function of MATLAB software.

A picosecond synchronously pumped terahertz optical parametric oscillator based on silicon membrane waveguide (Si‐based WTPO) is proposed and numerically simulated. Through designing the structure parameters of the silicon membrane waveguide, a broadband terahertz frequency range is obtained by satisfying the phase matching condition. The parametric oscillation process and output characteristics of Si‐based WTPO are numerically analyzed by solving the four‐wave mixing coupled‐wave equations, and the result has indicated that the parametric oscillation within the resonant cavity can apparently enhance the output terahertz wave power. Meanwhile, high bandwidth and efficiency output terahertz wave can be obtained, and too high pump power will lead to a reaction of reverse conversion. Finally, the threshold pump power is calculated. The Si‐based WTPO system we proposed is expected to make some contributions to the study of compact and efficient terahertz wave sources. [ABSTRACT FROM AUTHOR]

High-power and widely tunable continuous-wave optical parametric oscillators (cw OPOs), as mature radiation sources in the mid-infrared range, are limited by their frequency fluctuation. We built a cw OPO working at the near- and mid-infrared ranges and discuss the extent to which its frequency stability is affected by the mechanical temperature and the air pressure of the cavity. In addition, we artificially provoked mode hops by changing the above two factors to analyze the thermal-induced refractive index variation of the crystal. An inequality between the spacing of the hops and the free spectrum range (FSR) was observed. The wavelength stability of the cw OPO was guaranteed passively, and the longest mode-hopping-free time exceeded 13 h through sealing the cavity and thermal control. Further analysis of factors affecting frequency drift is critically essential for the reliability of cw OPO, particularly in environments outside the laboratory, and may provide a new method for high-precision tuning wavelengths. [ABSTRACT FROM AUTHOR]

• Designed and assembled a high-quality, fully solid-state 1064 nm laser using a passive Q-switching crystal. • Theoretically analyzed the impact of various factors on efficiency improvement during the optical parametric oscillation process. • In the design for compact devices, we effectively balanced the OPO oscillation threshold and walk-off effects by optimally selecting the crystal length. A 100 Hz compact 2 μm optical parametric oscillator (OPO) based on a type II non-critically phase-matched KTiOPO4 crystal (KTP) was reported. The monolithic KTP crystal was pumped with a passively Q-switched 1064 nm Nd:YAG laser. At a repetition rate of 100 Hz and a single pulse energy of 5.8 mJ of pump light, a 2128-nm laser output was achieved with a maximum of 1.17 mJ of parametric light at the degenerate point, with a parametric light pulse width of approximately 10.5 ns, corresponding to a pump to parametric light conversion efficiency of 20.1 %. [ABSTRACT FROM AUTHOR]

A widely tunable 4.16–7.31 μm high-resolution mid-infrared optical parametric oscillator with a type-I BaGa4Se7 (BGSe) crystal pumped by a 1064 nm laser was demonstrated for the first time. We have achieved a wavelength tuning accuracy of less than 5 nm through simultaneously performing temperature tuning and angular tuning on the BGSe (46°, 0°) Optical Parametric Oscillation (OPO). The wavelength resolution will be less than 1 nm when using commercial temperature control equipment.

We report an all-solid-state narrowband lidar system for the simultaneous detection of Ca and Ca+ layers over Yanqing (40.41°N, 116.01°E). The uniqueness of this lidar lies in its transmitter, which is based on optical parametric oscillation (OPO) and optical parametric amplification (OPA) techniques. The injection seeded OPO and the OPA are pumped by the second harmonic of an injection-seeded Nd:YAG laser, which can generate coherent light at the wavelength of 786 nm or 846 nm lasers, whose second harmonics in turn generate the 393 nm or 423 nm pulses, respectively, for the detection of thermospheric and ionospheric Ca+ and Ca layers. Compared to the conventional dye-based system, this lidar transmitter is a narrowband system (bandwidth < 200 MHz), which has produced a factor of two more output power with higher stability and reliability. The lidar system in Yingqing demonstrated Ca+ detection sensitivity of 0.1 atoms-cm−3 for long-term observation and reached a height of ~300 km. Potential applications and further improvements in this lidar technique are also discussed in this paper.

Operating regimes of an optical system consisting of a compact pulsed master diode-pumped Q-switched Nd:YAG laser and triple-crystal (KTP) ring cell of an optical parametric oscillator have been investigated. It was experimentally shown that the frequency-selective properties of a passive Q-switched unit in natural mode selection are enhanced by creating a Lyot polarization interference filter with the phase plate in the form of a laser active element with thermally induced birefringence and a polarizer. Such Lyot filter appears in the Nd:YAG laser cavity during operation at relatively high energy and repetition rate of radiation pulses (60–100 mJ, 20 Hz). The combined action of the natural mode selection process and the Lyot filter ensures stable operation of a passive Q-switched Ng:YAG laser in single-frequency lasing mode (the lasing bandwidth is <57 MHz). The master single-frequency pulsed Nd:YAG laser (λ = 1.06 μm) allows the energy of pulses applied to the input of the optical parametric oscillator to be reduced by greater than 1.5 times while maintaining the specified energy level of the output pulses (30 mJ, λ = 1.57 μm). An additional increase in the efficiency of the optical parametric oscillator conversion is achieved by introducing a two-lens 1.3× telescope into the master laser cavity. [ABSTRACT FROM AUTHOR]

Powerful and efficient optical frequency combs in the mid‐infrared (MIR) spectral region are highly desirable for a broad range of applications. Despite extensive efforts utilizing various techniques, MIR frequency comb sources are still lacking power, efficiency, or bandwidth for many applications. Here, the generation of an intrinsically locked frequency comb source centered at 4.18 µm from an optical parametric oscillator (OPO) operating in the simulton regime is reported, in which formation of purely quadratic solitons leads to enhanced performance. Advantages of operation in the simulton regime in direct experimental comparisons to the conventional regime are shown, which are also supported by simulation and theory. 565 mW of average power, 900 nm of instantaneous 3 dB bandwidth, 350% slope efficiency, and 44% conversion efficiency are achieved, a performance that is superior to previous OPO demonstrations and other sources in this wavelength range. Here, a new avenue toward MIR frequency comb generation with high power and efficiency is opened, and the great potential of soliton generation based on quadratic nonlinearity in the MIR spectral region is suggested. [ABSTRACT FROM AUTHOR]

Most of intracavity pumped optical parametric oscillators (OPO) are made nowaday according to a scheme with a single-resonance OPO located in the cavity of a pump laser. Usually the cavities of the pump and OPO (signal) emission have different values of round-trip time (delay). Aim of the study is therefore to build up the mathematical model of intracavity optical parametric oscillator (ICOPO) considered as a time-delay dynamic system with two values of delay in both cavities (the pump and signal). Methods. The model allows to analyze the steady state (equilibrium point) of the dynamic system and its stability with the help of characteristic equation’s solution. Results. Countless set of the characteristic equation roots is shown to consist of complex-conjugate pairs with imaginary parts which are nearly multiples of intermode beat frequencies in the pump and signal cavities. The diagram of stability depending on the position of nonlinear crystal in the resonator was built on the parameter plane. Features of the plane partition into the areas of stability/instabilty vs behavior of the characteristic equation roots are examined. Discussion. The results of the study allow to consider an ICOPO as the time-delay dynamic system thus adding to the physical picture of intracavity parametric oscillators.

Spectroscopy in the terahertz (THz) band has been discussed as a promising tool for identifying substances such as mold in food, narcotics, or explosive materials. Other than the technological limitations, the most important difficulty is the presence of water vapor in the atmosphere, which affects THz measurements. In this paper, we present a systematic approach to the challenging subject of remote identification. We start with a brief analysis of the technical capabilities of the THz components and report the choice of devices for designing an experimental setup for reflection spectroscopy. We follow with the presentation of the transmission THz spectrometer working in an open atmosphere. Research conducted on the transmission configuration provides findings that are implemented in the experimental setup working in a reflective configuration. The final phase is an experiment providing data measured in the reflection configuration with the presence of water vapor, allowing the use of spectra in the identification of the measured samples.

High-power optical parametric oscillators (OPOs), as mature radiation sources in mid-infrared (MIR), degenerate gradually with wavelength increase, mainly above 3700 nm. Using a periodically poled magnesium-oxide-doped lithium niobate (MgO:PPLN) as the nonlinear crystal, we build a high-power signal-resonant OPO pumped by ytterbium-doped fiber laser (YDFL). To improve the OPO’s output power at ~3.8 μm, the parameters, such as the pump beam’s waist diameter and location, the curvature radius of the output coupler and the length of MgO:PPLN, are discussed in detail. When pump power is 79 ± 4 W with a repetition rate of 200 kHz, the OPO provides up to 8 ± 0.4 W average power in beam quality with M2 factors of ~1.84 and ~1.69 in the two axes. Under the highest output power, the center wavelength of the idler beam is 3768.4 nm with a full-width at half-maximum (FWHM) bandwidth of ~18.6 nm. When the output power reaches ~6.3 W, its power stability is 1.6% root mean square (RMS) over 7 h. Further analysis of the factors affecting OPO’s performance and simple structure are critically essential for compact OPO prototypes with a capacity of high output power.

A tunable and compact mid-infrared optical parametric oscillator (OPO) based on BaGa4Se7 (BGSe) crystal with a repetition rate up to 250 Hz was demonstrated. A high energy and more approachable side-pumped Q-switched Nd:YAG laser was employed as the pump for the BGSe OPO. Due to the pump double-pass single-resonant oscillator (DP-SRO) configuration, the maximum average power of 250 mW and the maximum pulse energy of 1.28 mJ at 4.06 μm was achieved with the repetition rate of 250 Hz and 100 Hz, respectively. The tunable mid-infrared output from 3.42–4.73 μm was obtained. The influence of repetition rate and cavity length was studied and the thermal effect was analyzed. [ABSTRACT FROM AUTHOR]

In this work, a unique pentagonal TMD material, palladium disulfide (PdS 2) nanosheets were fabricated by the liquid-phase exfoliation (LPE) process, and the physical characteristics were also investigated. The nonlinear transmittance is measured by the power scanning method and the modulation depth is fitted to be 15.3%. Based on the transmittance curve, the saturable absorption parameters of PdS 2 are calculated, including 5.85 × 10−19 cm2 ground-state absorption cross-section, 1.63 × 10−19 cm2 excited-state absorption cross-section, and 683.5 μs excited-state lifetime. By employing both PdS 2 as a saturable absorber (SA) and an acousto-optic modulator (AOM), a doubly Q-switched and mode-locked (QML) laser-pumped intracavity KTiOPO4 (KTP) optical parametric oscillator (OPO) was realized. The pulse energy and the pulse width of the Q-switched envelope for signal wave have been measured. At higher pump power, the high-peak-power low-repetition-rate sub-nanosecond mode-locking pulse of signal wave is generated, in which there is only one mode-locked pulse underneath a Q-switched envelope. The measured shortest signal pulse duration is 530 ps, corresponding to a peak power of 337 kW, at an incident pump power of 9.81 W and an acousto-optic modulator (AOM) repetition rate of 1 kHz. In addition, by considering the Gauss distribution of intracavity photon density, a set of coupled rate equations for a doubly QML laser-pumped intracavity OPO was given and the numerical simulations agreed with the experimental results. These results indicate that PdS 2 is a promising nonlinear optical material for optical applications in the near-infrared (NIR) region. • The PdS 2 nanoflakes was fabricated by liquid-phase exfoliation and characterized. • Palladium disulfide nanomaterials performed well for pulsed laser generation. • Applying the PdS 2 SA, a dual-loss-modulated QML IOPO was realized. • The rate equations for IOPO with PdS 2 were numerically solved. [ABSTRACT FROM AUTHOR]

• The Bi 2 Te 3 nanoflakes was fabricated by liquid-phase exfoliation and characterized. • Applying the Bi 2 Te 3 SA, a dual-loss-modulated QML IOPO laser was realized, resulting in the generation of sub-nanosecond single mode-locking pulses. • The rate equations for IOPO driven by the QML laser with Bi 2 Te 3 SA were numerically solved. By utilizing the ultrasound-assisted liquid phase exfoliation process, Bi 2 Te 3 nanosheets are synthesized and deposited onto a quartz plate to form a saturable absorber (SA), in which nonlinear absorption characteristics around 1 μm are investigated with a home-made Q-switched laser. With the as-prepared Bi 2 Te 3 SA employed, a sub-nanosecond KTiOPO 4 (KTP) based intracavity optical parametric oscillator (IOPO) pumped by a dual-loss-modulated Q-switched and mode-locked (QML) laser is successfully realized. With a short duration of sub-nanosecond and a repetition rate of several kilohertz (kHz), the signal wave's single mode-locking pulse existed underneath a Q-switched envelope can be formed. At an incident pump power of 8.92 W and an acousto-optic modulator (AOM) repetition rate of 2 kHz, a signal wave's single mode-locking pulse underneath a Q-switching envelope with a measured minimum pulse duration of 520 ps is accomplished. It is the first demonstration of such Bi 2 Te 3 SA utilized in a solid-state QML IOPO laser, to the best of our knowledge. According to the experimental results, this provides an efficient and straightforward approach to acquire sub-nanosecond OPO signal wave pulses. Additionally, the relevant coupled rate equations are established, and the simulation results match the experimental results, revealing the potential of Bi 2 Te 3 as a nanomaterial for optoelectronic applications. [ABSTRACT FROM AUTHOR]

Mid-infrared optical parametric oscillators (OPOs) offer a compelling route for accessing the ‘molecular fingerprint’ region and, thus, can find intensive applications such as precision spectroscopy and trace gas detection. Yet it still remains rather a challenge to realize broadband mid-infrared OPOs within a single cavity, usually limited by strict phase-matching conditions for wide spectral coverage and available pump power for adequate frequency generation. Here, we report the mid-infrared parametric oscillation spanning from 3.4 to 8.2 μ m, based on four-wave mixing in a high- Q MgF2 microresonator with optimized dispersion. The center wavelength at 4.78 μ m is determined by the continuous tunable quantum cascade laser source, which contributes to effective expansion towards longer wavelength, as well as systemic miniaturization with smaller pump module. Such results could not only shed light on new ultimates of crystal and other microresonators, but also inspire explorations on their growing potentials in near future. [ABSTRACT FROM AUTHOR]

A 6.2 nm-thickness platinum disulfide (PtS2) film was prepared by electron beam evaporation with post vulcanization. The nonlinear transmittance was measured by power scanning method and the modulation depth is fitted to be 13%. Based on the transmittance curve, saturable absorption parameters of PtS2 are calculated with inhomogeneously broadening mechanism, including 6.4298 × 10−19 cm−2 ground-state absorption cross-section, 2.5927 × 10−19 cm−2 excited-state absorption cross-section, and 1.043 ms excited-state lifetime. The PtS2 film combined with active time management was implemented to modulate the fundamental light of optical parametric oscillator (OPO). Owing to the nonlinear absorption property of PtS2, the operation of Q-switched OPO was optimized in both the experiment and dynamical theory. In particular, the conversion efficiency was experimentally improved by 13.2%. The pump-to-signal conversion efficiency went up to 3.29%, which is the highest conversion value reported so far. The theoretical values fit the experiment well, which are from the Gaussian rate equations with PtS2's saturable-absorption characteristic. [ABSTRACT FROM AUTHOR]