Nanosecond Optical Parametric Oscillator Research Articles

Application of a pulsed OPO seeded by a CW OPO to investigation of linewidth effects on TALIF excitation efficiency

In this article we present a widely tunable laser architecture for high-resolution nonlinear spectroscopy which incorporates a continuous wave optical parametric oscillator (OPO) for injection-seeding of a singly resonant actively-locked β-Ba(BO2)2 nanosecond OPO. Such an instrumental configuration is designed to provide near-transform-limited pulses over 450–650 nm signal, and 225–325 nm after signal doubling. Novel features that enable frequency agility and ramp-lock injection-seeding over a wide frequency range are described. System performance was demonstrated through investigations of two-photon laser induced fluorescence (TALIF) and the influence of excitation linewidth on the signal strength of the 5p6S0 - 6p[3/2]2 transition of atomic xenon at 252.49 nm. Measurements conducted at xenon partial pressures of 1×10-1-4×10-4 Torr, with and without active cavity locking and injection seeding, resulted in a 17× increased TALIF fluorescence yield during injection-seeded operation. The results are found to be consistent with modeling, taking into account the effects of lineshape, energy, and beam spatial cross-section. Modeling was informed by direct single-shot measurements of the signal beam lineshape in seeded and free-running operation using a virtually imaged phased array (VIPA) spectrometer.

Nanosecond optical parametric oscillator with midinfrared intracavity difference-frequency mixing in orientation-patterned GaAs.

We report on efficient midinfrared difference-frequency generation (DFG) in orientation-patterned GaAs by intracavity mixing the signal and idler pulses of a narrowband nanosecond optical parametric oscillator based on periodically poled LiNbO3. The maximum average DFG output power reached 215 mW at 8.15 µm for a repetition rate of 35 kHz. The temperature tuning range spanned over 8026-8710 nm. The maximum overall conversion efficiency from 1 to 8 µm amounted to ∼1.3%.

All-fibre supercontinuum laser for in vivo multispectral photoacoustic microscopy of lipids in the extended near-infrared region

Among the numerous endogenous biological molecules, information on lipids is highly coveted for understanding both aspects of developmental biology and research in fatal chronic diseases. Due to the pronounced absorption features of lipids in the extended near-infrared region (1650−1850 nm), visualisation and identification of lipids become possible using multi-spectral photoacoustic (optoacoustic) microscopy. However, the spectroscopic studies in this spectral region require lasers that can produce high pulse energies over a broad spectral bandwidth to efficiently excite strong photoacoustic signals. The most well-known laser sources capable of satisfying the multi-spectral photoacoustic microscopy requirements (tunability and pulse energy) are tunable nanosecond optical parametric oscillators. However, these lasers have an inherently large footprint, thus preventing their use in compact microscopy systems. Besides, they exhibit low-repetition rates. Here, we demonstrate a compact all-fibre, high pulse energy supercontinuum laser that covers a spectral range from 1440 to 1870 nm with a 7 ns pulse duration and total energy of 18.3 μJ at a repetition rate of 100 kHz. Using the developed high-pulse energy source, we perform multi-spectral photoacoustic microscopy imaging of lipids, both ex vivo on adipose tissue and in vivo to study the development of Xenopus laevis tadpoles, using six different excitation bands over the first overtone transition of C–H vibration bonds (1650−1850 nm).

Design and analysis of 90° image rotating four-mirror non-planar ring resonator based on mid-infrared optical parametric oscillator beam quality optimization

Mid-infrared optical parametric oscillator (OPO) operating in the mid-infrared transmission window (3—5 μm wavelength range) is one of hot issues in the field of laser system. It has many applications in environmental detection, remote sensing, and medicine. Besides, this laser system is used as a key component of infrared countermeasures. The optical damage limit of nonlinear crystal is a great challenge to the mid-infrared OPO which is pumped by a nanosecond laser source. Therefore, the pump beam diameter should be appropriately increased to avoid damaging the crystal when scaling a nanosecond OPO to high pulse energy. The result of this design is that the Fresnel number in the cavity is increased and the beam quality is deteriorated. In order to improve the beam quality of mid-infrared OPO laser, a 90<sup>°</sup> image-rotating four-mirror non-planar ring resonator structure is designed. The advantages of this design include the general ring resonators, such as greatly reduced feedback into the pump laser and the avoidance of optical damage caused by standing wave cavity structure. Most importantly, the image rotating cavity can uniform the beam in the cavity and improve the beam quality. In this paper, the equivalent sphere representation of a four-mirror nonplanar ring resonator is established, and the image rotation angle of this special cavity structure is calculated. Based on this method, the parameters related to the 90<sup>°</sup> image rotating resonator structure suitable for mid-infrared OPO operation are designed. The self-reproduction of the transverse mode in the axially-asymmetric resonator is further established. It is found that the transverse mode in the resonator is gradually uniformed as the rotation angle of the image changes from 0<sup>°</sup> to 90<sup>°</sup>. When the rotation angle is 90<sup>°</sup>, the fundamental mode and the high-order mode both exhibit very good central symmetry. Finally, the mid-infrared ZnGeP2 OPO laser with the 90<sup>°</sup> image rotating resonator structure is used to verify the improvement of beam quality. The beam quality of <inline-formula><tex-math id="Z-20190909022329-2">\begin{document}$M_X^2=1.81 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20182001_Z-20190909022329-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20182001_Z-20190909022329-2.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="Z-20190909022441-3">\begin{document}$M_Y^2=1.61$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20182001_Z-20190909022441-3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20182001_Z-20190909022441-3.png"/></alternatives></inline-formula> are achieved. It can be proved that the 90<sup>°</sup> rotating four-mirror non-planar ring resonator has a significant effect on the optimization of the output beam quality of the mid-infrared OPO laser system.

High repetition rate nanosecond optical parametric oscillator pumped by the third harmonic of a DPSSL

A high repetition rate (5 kHz), nanosecond, type-I phase matched β-BaB2O4 (BBO) crystal-based optical parametric oscillator (OPO), pumped by the third harmonic (355 nm) of a diode-pumped solid state laser, is demonstrated for the first time. The high threshold pump pulse energy, which impedes the high repetition rate operation of OPO, is reduced to ≤ 1 mJ using cylindrical focusing of the pump beam. The performance of the OPO is compared for different gain lengths of the type-I BBO crystal and the optimum length of the crystal for efficient operation of the OPO is determined. We have obtained a stable output power of ~ 2.6 W at a signal wavelength of 492 nm, and at a conversion efficiency of 26%, when operated with pump laser at 5 kHz. Furthermore, the narrowband (~ 0.15 nm) operation of the OPO at a repetition rate of 2 kHz was achieved by employing type-II phase-matched BBO crystal in double pass pump beam configuration. Thermal loading in the BBO crystal arising due to high repetition rate operation was studied by measuring the temperature of the crystal for determining the average power scaling limit of visible nanosecond OPOs.

Time-dependent spatial intensity profiles of near-infrared idler pulses from nanosecond optical parametric oscillators

We report on an experimental investigation of the time-dependent spatial intensity distribution of near-infrared idler pulses from an optical parametric oscillator measured using an infrared (IR) camera, in contrast to beam profiles obtained using traditional knife-edge techniques. Comparisons show the information gained by utilizing the thermal camera provides more detail than the spatially- or time-averaged measurements from a knife-edge profile. Synchronization, averaging, and thresholding techniques are applied to enhance the images acquired. The additional information obtained can improve the process by which semiconductor devices and other IR lasers are characterized for their beam quality and output response and thereby result in IR devices with higher performance.

Contribution to the development of low frequency terahertz coherent Raman micro-spectroscopy and microscopy

We report the construction and characterization of a coherent Raman tabletop system utilizing a novel astigmatic optical focusing geometry, a broadband nanosecond optical parametric oscillator and volumetric Bragg filters assisting 3CBCRS measuring system for the first time. In order to illustrate the versatility of the measurements and reveal the molecular information obtainable, two well-characterized chemicals were selected. Polarization sensitive epi-detected 3CBCRS spectra of liquid CCl4 and calcite crystal were recorded and analyzed. An unexpected polarization dependence of the signals of the lowest frequency modes of CCl4 was observed. The 1122 third order susceptibility component was phase flipped. The non-resonant susceptibility normalized 1122 component was found to be larger than the 1111 component for the lowest vibrational modes. This anomalous comportment was attributable to the anisotropy Raman tensor invariant in the third order nonlinear susceptibility tensor.

Widely tunable (2.6–10.4 μm) BaGa4Se7 optical parametric oscillator pumped by a Q-switched Nd:YLiF4 laser

We report on the first BaGa4Se7 nanosecond optical parametric oscillator pumped by Q-switched Nd:YLiF4 laser at 1053 nm. The oscillator exhibits a pump threshold energy as low as 0.25 mJ. Mid-infrared (MIR) idler wave tuning from 2.6 μm to 10.4 μm is demonstrated with an angle-tuned type-I (o-ee) y-cut sample, highlighting the superior performance of this novel large bandgap chalcogenide nonlinear crystal to generate tunable coherent radiation over its full MIR transparency range (0.47–18 μm). The phase-matching data are used to identify the most accurate dispersion relations of this compound among three recently published Sellmeier equations. Damage threshold measurements yielded values as high as 2.04 J cm−2 at 100 Hz pulse repetition rate, one of the largest among existing MIR χ(2)nonlinear materials.

Optical parametric oscillator pumped at ~1 µm with intracavity mid-IR difference-frequency generation in OPGaAs

Intracavity difference-frequency generation (DFG) between signal and idler pulses is investigated in orientation-patterned GaAs inside the cavity of a ~1 µm pumped nanosecond optical parametric oscillator (OPO). Using two different samples and temperature tuning in the non-critical configuration, tunability between 7 and 9.2 µm is demonstrated. The superior thermo-mechanical properties of OPGaAs enabled also for the first time operation of this cascaded scheme at kilohertz (1-3 kHz) repetition rates reaching average powers ~10 mW in the mid-IR.

Widely tunable in the mid-IR BaGa4Se7 optical parametric oscillator pumped at 1064 nm.

A BaGa4Se7 nanosecond optical parametric oscillator (OPO) shows extremely wide idler tunability in the mid-IR (2.7-17μm) under 1.064μm pumping. The ∼10 ns pulses at ∼7.2 μm have an energy of 3.7mJ at 10Hz. The pump-to-idler conversion efficiency for this wavelength reaches 5.9% with a slope of 6.5% corresponding to a quantum conversion efficiency or pump depletion of 40%.

Numerical study of high-energy optical parametric oscillator with top-hat pump intensity profile and quasi-phase-matched crystal with wedge-shaped domain

We present a numerical study for the design of efficient MgO :PPLN based pulsed nanosecond optical parametric oscillator (OPO), with input pump pulses of top-hat transverse intensity profile, and a double-pass pump cavity configuration. In particular, we investigate the effect of domain shape on the conversion efficiency. We considered two types of domain structures: "perfect vertical-wall" and "wedge-shaped" domains and our numerical results show that at higher pump pulse energies, the conversion efficiency saturates, and the saturated efficiency is almost independent of the domain shapes. Based on our study, we also propose a new scheme for development of compact high-power OPO sources using MgO :PPLN chips of relatively short lengths and large cross-sectional area.

Mercury thiogallate nanosecond optical parametric oscillator continuously tunable from 4.2 to 10.8 μm

We demonstrate an optical parametric oscillator based on two HgGa2S4 crystals with an extremely wide tuning range from 4.2 to 10.8 μm. The HgGa2S4 optical parametric oscillator was pumped by a Q-switched nanosecond Nd : YLF laser at 1.053 μm. The absorption spectrum of ammonia was presented to demonstrate the feasibility of the developed optical parametric oscillator system for spectroscopic measurements and gas detection.

Numerical analysis for single-frequency nanosecond optical parametric oscillators and optical parametric amplifiers

We numerically analyze single-frequency nanosecond optical parametric oscillators and amplifiers. It shows that a single-frequency optical parametric oscillator requires a single-frequency pump wave and a seed source for injection locking. The power of the injection seed should be increased dramatically to maintain single-frequency output as pump energy increasing. The results numerically verify that high energy and single-frequency output waves can hardly be reached simultaneously by optical parametric oscillators. Multistage optical parametric amplifiers are helpful to meet the shortcoming of optical parametric oscillators. During the amplification, the pump energy should match the signal energy in optical parametric amplifiers to obtain high conversion efficiency.

Synthesis and optical power limiting properties of heteroleptic Mo3S7 clusters.

Substitution of the halide ligands in (Bu4N)2[Mo3S7X6] (X = Cl, Br) by diimine ligands, such as 4,4'-dimethyl-2,2'-bipyridine (dmbpy), 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen), affords the neutral heteroleptic clusters Mo3S7Cl4(dmbpy) (), Mo3S7Br4(dmbpy) (), Mo3S7Br4(bpy) (), and Mo3S7Br4(phen) (). Further substitution of the halide ligands in Mo3S7Br4(diimine) clusters by dmit (1,3-dithiole-2-thione-4,5-dithiolate) allows the preparation of the mixed diimine-dithiolene neutral cluster complexes Mo3S7(dnbpy)(dmit)2 (, dnbpy = 4,4'-dinonyl-2,2'-bipyridine), Mo3S7(dcmbpy)(dmit)2 (, dcmbpy = 4,4'-dimethoxycarbonyl-2,2'-bipyridine), and Mo3S7(dcbpy)(dmit)2 (, dcbpy = 2,2'-bipyridine-4,4'-dicarboxylic acid). The optical limiting properties of complexes have been assessed by the open-aperture Z-scan technique at 570 nm, employing a nanosecond optical parametric oscillator. In order to investigate the effect of increasing the π-system, complexes , with the general formula Mo3S7X4(diimine), (X = Cl, Br), were compared to clusters , containing the dmit ligand. The influence of the metal content on the optical power limiting properties was also investigated by comparing the trinuclear series of complexes prepared herein with the bis(dithiolene) dinuclear cluster (Et4N)2[Mo2O2S2(BPyDTS2)2], which has been recently prepared by our group. All trinuclear clusters are efficient optical limiters (σeff > σ0) with the threshold limiting fluence F15% decreasing on proceeding from dinuclear to trinuclear clusters and, generally, on extending the π-system.

Mid-infrared rotated image singly resonant twisted rectangle optical parametric oscillator based on HgGa(2)S(4) pumped at 1064 nm.

We compare linear, planar ring, and rotated image singly resonant twisted rectangle (RISTRA) type nanosecond optical parametric oscillator cavities using HgGa2S4 nonlinear crystal pumped by 8ns pulses at 1064nm from a low beam quality pump source. The input-output characteristics and the output idler beam quality at 6300nm are compared for two values of the pump beam diameter presenting different cavity Fresnel numbers and magnitudes of the spatial walk-off effect due to birefringence. The RISTRA cavity ensures in all cases a circular output beam profile but is advantageous in terms of beam quality with respect to the planar ring only at a large pump beam diameter.

Optical damage studies of mercury thiogallate nonlinear crystals for use in 1-μm pumped optical parametric oscillators

We investigated optical damage (surface and bulk) in one of the most promising wide bandgap nonoxide nonlinear crystals, HgGa2S4, that can be used in ∼1-μm pumped optical parametric oscillators (OPOs) and synchronously pumped OPOs (SPOPOs) for generation of idler pulses above 4 μm without two-photon absorption losses at the pump wave- length. The optical damage has been characterized at the pump wave- length for different repetition rates using uncoated and antireflection- coated (mainly with a single layer for pump and signal wavelengths) sam- ples. HgGa2S4 is the most successful nonlinear crystal (both in terms of output energy and average power) for such OPOs, but optical damage inside the OPO has a lower threshold and represents at present the prin- cipal limitation for the achievable output. It is related to peak pulse and not to average intensity, and bulk damage in the form of scattering centers occurs before surface damage. Such bulk damage formation is faster at higher repetition rates. Lower repetition rates increase the lifetime of the crystal but do not solve the problem. The safe pump fluence in extrac- avity measurements is <1 J∕cm 2 , which corresponds to ∼100 MW∕cm 2 for the 8-ns pulse duration (both values peak on-axis). In the OPO, how- ever, peak on-axis fluence should not exceed 0.3 J∕cm 2 limited by the formation of bulk scattering centers in orange-phase HgGa2S4. In the nanosecond OPO regime, the damage resistivity of Cd-doped HgGa2S4 is higher and that of the almost colorless CdGa2S4 is roughly two times higher, but the latter has no sufficient birefringence for phase-matching. In SPOPOs operating in the ∼100 MHz regime, the dam- age limitations are related both to the peak pulse and the average inten- sities, but here HgGa2S4 seems the best nonoxide candidate to obtain first steady-state operation with Yb-based mode-locked laser pump sources. © 2013 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10

Investigations on the beam pointing stability of a pulsed optical parametric oscillator

Although the beam pointing stability of optical parametric oscillators and amplifiers is important for various applications few results on this parameter have been published. Here, we investigate the beam pointing stability of an injection-seeded, nanosecond optical parametric oscillator, compare it to its pump laser, and measure correlations between them. Although correlation between both quantities are found, the beam pointing stability of the OPO is significantly better that the one of its pump. Furthermore, the concept of the Allan variance is applied to analyze the temporal components of the pointing stability.

Tunable mid-infrared optical parametric oscillator with intracavity parametric amplification based on a dual-grating PPLN crystal

Intracavity amplification of the idler radiation of a nanosecond optical parametric oscillator (OPO) is achieved within a single dual-grating PPLN crystal com- posed of a uniform-grating section followed by a fan-out grating section. While all the OPO wavelength-tuning capabilities are preserved, this configuration leads to a 60 % increase of the 4-lm idler power and a significant beam quality improvement when compared to a conven- tional singly resonant OPO.

Multiple-wavelength quasi-phase-matching for efficient idler generation in MgO:LiNbO3 based nanosecond optical parametric oscillator

We present numerical results for optimization of the overall idler conversion efficiency of a nanosecond optical parametric oscillator (OPO), wherein the signal generated in the OPO process is also used as the pump for a difference frequency generation (DFG) process in a quasi-periodic MgO:LiNbO(3) crystal. The phase-matching conditions are considered such that the generated idler frequencies in both the processes (i.e., OPO and DFG) coincide. Optimization for the idler generation has been performed with respect to the different parameters, such as input pump power, pump pulse duration, and the output coupler reflectivity, for quasi-phase-matched interaction in MgO:LiNbO(3). Wavelength of the pump, signal, and idler waves considered in the optimization are 1.064 μm, 1.456 μm, and 3.95 μm, respectively. A maximum overall idler generation efficiency of ≈33% could be obtained in the simultaneous OPO+DFG process for a pump pulse duration of 72 ns and output coupler reflectivity (R(s)) of 90%, whereas for the stand-alone OPO process, the maximum idler generation efficiency was found to be ≈15%. The optimization has been illustrated for an average pump power of 8 W at a pulse repetition frequency (PRF) of 10 kHz. This approach of simultaneous OPO+DFG process can be employed to significantly enhance the idler generation efficiency of nanosecond OPOs.

A nanosecond optical parametric oscillator in the mid-IR region with double-pass pump

An optical parametric oscillator (OPO) with double-pass pump based on MgO:PPLN and PPLN periodic structures is described. A compact nanosecond Nd:YLF laser has been used as a pump source at 1.053 μm (the pumping pulse duration is 5–7 ns at a maximum pulse energy of 300 μJ at frequencies of 1–7 kHz). The oscillation threshold of the OPO based on MgO:PPLN was varied in a range of 11–28 μJ at wavelength of 2.1–4.3 μm. The conversion efficiency from the pump wave to an idler wave decreased from 8.6 to 2.5% in the range of 2.0–4.3 μm. For PPLN-OPO the measured threshold was 36 μJ at 4.2 μm and 49 μJ at 4.7 μm. The conversion efficiency of the pump energy into the energy of an idler wave was 3.3μ-0.4% at wavelengths of 4.2–4.7 μm.