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An efficient continuous-wave (CW) mid-infrared laser with a tunable wavelength range from 3 to 4.5 μm is demonstrated by using intracavity optical parametric oscillator (ICOPO). An optically pumped semiconductor laser (OPSL) is applied as the parent pumped laser gain medium to eliminate self-modulation effect caused by Nd-doped gain medium with ICOPO for fulfilling true CW emissions. Simultaneously, the OPSL gain chip with flip-chip packaging method can avoid mode hopping resulted from internal heat spreader etalon when strong depletion in ICOPO was achieved. The diode pump criteria of the OPSL is first investigated to prevent thermal roll over. By observing the spectral and temporal dynamics of the parent pumped OPSL with and without OPO operation, the true CW and mode-hop-free quality of the laser is verified. A low diode pump threshold of 2.67 W is obtained for the mid-IR idler emission and a total extracted idler power is 250 mW at a diode pump power of 8 W. Finally, the wavelength tunability is confirmed by adjusting the horizontal location of the multiple channel MgO:PPLN crystal, which indicated the feasibility of practical application in such a compact configuration.

We report the generation of widely-tunable, narrow-linewidth midinfrared laser emission from an Yb: KYW-based intracavity singly-resonant optical parametric oscillator (ICSRO). The wavelength-tuning scheme is based on exploiting the wide emission-bandwidth of Yb: KYW crystals. Birefringent filters were used in the ICSRO cavity for linewidth narrowing and wavelength tuning. The benefit of this tuning scheme is that it does not rely on the temperature-tuning mechanism or the usage of customized fanout PPLN crystals. By using 6 different poling-periods of a 25 mm-long PPLN crystal, and synchronizing the tuning of the pump and signal waves, we obtained midinfrared idler light with a tuning range over 2600-4030 nm, having an estimated linewidth of less than 0.8 cm-1.

Spectroscopic stimulated Raman scattering (SRS) imaging has become a useful tool finding a broad range of applications. Yet, wider adoption is hindered by the bulky and environmentally sensitive solid-state optical parametric oscillator (OPO) in a current SRS microscope. Moreover, chemically informative multiwindow SRS imaging across C-H, C-D, and fingerprint Raman regions is challenging due to the slow wavelength tuning speed of the solid-state OPO. In this work, we present a multiwindow SRS imaging system based on a compact and robust fiber laser with rapid and wide tuning capability. To address the relative intensity noise intrinsic to a fiber laser, we implemented autobalanced detection, which enhances the signal-to-noise ratio of stimulated Raman loss imaging by 23 times. We demonstrate high-quality SRS metabolic imaging of fungi, cancer cells, and Caenorhabditis elegans across the C-H, C-D, and fingerprint Raman windows. Our results showcase the potential of the compact multiwindow SRS system for a broad range of applications.

Optoacoustic imaging is a relatively novel biomedical imaging modality that relies on the absorption of light to create pressure transients that can be detected ultrasonically. In most scientific communications, the source of tissue contrast has been described as primarily optical. However, the thermomecahnical properties of tissue, as expressed through the Gruneisen coefficient, also affect the optoacoustic signal. To investigate the effect of thermomechanical tissue properties short pulses (~ 6.5 ns) from an optical parametric oscillator at 750 nm were used to irradiate coagulated and uncoagulated tissue-mimicking albumen phantoms, to emulate normal tissue and tissue that has been heated. The phantoms respond to the laser-induced stress by thermoelastic expansion. This thermomechanical behavior of the samples was assessed using an interferometric system capable of measuring transient displacements with a temporal resolution of less than 10 ns and a spatial resolution of < 10 nm. The experimental measurement allowed determination of the Gruneisen coefficient which is an important thermo-mechanical sample property that can affect generation of optoacoustic signals. An increase in the value of Gruneisen coefficient of 65% was measured when phantoms were coagulated compared to uncoagulated phantoms, consistent with the stiffening of the tissue mimicking material. This suggests that for thermal therapy the changes in the Gruneisen coefficient are also an important source of optoacoustic contrast.

In this paper, we report an external-cavity 2128 nm narrow-linewidth optical parametric oscillator involving a reflective volume Bragg grating. Based on the Sellmeier equation, we introduce the thermal expansion coefficient together with the thermally-induced output wavelength shift. When the pump power is 12.91 W, with the repetition rate being 70 kHz, a 2128 nm laser with an output power of 3.85 W is obtained, having the optical-to-optical conversion efficiency equal to 30% and the linewidth equal to 108 nm. Moreover, the output power is 2.71 W at 2128 nm and the output linewidth is compressed to 0.19 nm, when a reflective volume Bragg grating is used instead of a total reflection mirror, with the corresponding optical-to-optical conversion efficiency being 21% and the beam quality factor being $$ {\mathrm{M}}_x^2 $$ = 1.92 and $$ {\mathrm{M}}_y^2 $$ = 1.98. As a result, a 2128 nm narrow-linewidth laser is implemented.

We demonstrated highly-efficient pulsed mid-infrared generation based on intracavity difference frequency mixing inside a continuous-wave (CW) PPLN optical parametric oscillator (OPO). The near-infrared pulsed fiber source was located at 1120 nm with variable pulse width and repetition rate. The pump source of OPO was a CW linearly-polarized high-power 1060 nm fiber laser. The 1120 nm and 1060 nm pump beams were simultaneously incident into the OPO cavity. The 1060 nm pump beam built parametric oscillation firstly and generated high-power resonant signal beam inside the cavity. By controlling the PPLN temperature properly, the phase-matched difference frequency generation (DFG) occurred between the signal beam and 1120 nm pulsed pump beam in the PPLN crystal. Finally, the pulsed 1120 nm pump beam was successfully transformed to 3593 nm mid-infrared radiation at every power level and temporal characteristic. Both the slope efficiency and pump-to-idler conversion efficiency of the intracavity DFG process reached over 20%. In addition, the pulse shape of newly generated 3593 nm idler beam was basically the same as 1120 nm pump beam. The experimental results exhibited huge potential in frequency down conversion of low-power laser sources with special temporal characteristics.

An optical circuit is designed for the ring cavity of an optical parametric oscillator (OPO) based on two series-mounted active elements—mercury thiogallate crystals (HgGa2S4) with the radiation tunable in the near- and mid-IR ranges (4.75–9.07 μm). The dependences of the radiation idler wavelength on the phase-matching angle and of the distribution of the IR OPO output radiation intensity on the idler wavelength are derived. The possibility of narrowing the spectral width of the output radiation by introducing a dispersing element (Fabry–Perot etalon) into the cavity is considered.

A novel scheme to realize widely tunable optical parametric oscillator (OPO) based on an x-cut KTP crystal is demonstrated. The tuning range from 1572.5 nm to 1697 nm was enabled by a 1:1 imaging optical system with a variable deflection mirror to couple the 1064-nm laser pulses into the OPO cavity in the noncollinear phase-matching (PM) configuration. Benefitting from varying the pump direction rather than the cavity axis, the OPO cavity was unaffected so that the stability of the output signal energy and beam direction could be maintained, which greatly facilitates subsequent applications. The change of pump threshold, output energy, linewidth and temporal pulse shapes during wavelength tuning were also discussed.

Although higher order Poincare (HOP) beams are attractive due to their unique advantages and various efforts have been made for the generation of HOP sphere beams, the current generation technologies suffer from both limited output power and narrow bandwidth, preventing further practical applications. In this paper, we report on a femtosecond optical parametric oscillator (OPO) which can deliver high average power, on-demand arbitrary HOP sphere beams tunable across a wide spectral range in the near-infrared. The desired beam mode can be achieved by manipulating the geometric phase inside the OPO cavity to map the polarization to orbital angular momentum (OAM). Pumped by a Gaussian beam from an Yb-fiber laser, the generalized vector vortex femtosecond signal beams with spectral coverage from 1367 to 1650 nm are obtained. The device can produce over 200 mW structured beam signal power cross the complete tuning range with as much as 377 mW at 1480 nm for 2 W of input pump power. The proposed configuration, which shows superiorities of high average output power, flexible wavelength tunability and complete mode controlling property, may have direct implications in applications such as quantum optics and material processing.

A watt-level continuous wave (CW) orange-red laser with wavelength tunable properties is experimentally implemented based on sum-frequency generation (SFG) in a composite cavity. The cavity is composed of an 880 nm laser diode (LD) side-pumped Nd: GdVO4 p-polarized 1062.9 nm cavity and a single-resonant optical parametric oscillator (SRO) of signal light using a magnesium oxide-doped periodically-poled LiNbO3 (MgO: PPLN) crystal with a poling period of 29.1 μm. In the overlap region of the two cavities, orange-red laser is generated by using a type-II critical phase-matched potassium titanyl phosphate crystal (KTP) crystal. In the temperature tuning range of the MgO: PPLN crystal (from 30 °C to 200 °C), the CW orange-red laser beams are generated in a tunable waveband from 612.24 nm to 620.72 nm (Δλ = 8.48 nm), corresponding to the mid-infrared idler light is also obtained with tunable wavelength from 4027.5 nm to 3708.2 nm (Δλ = 319.3 nm). At the lowest tuning temperature of 30 °C, the maximum CW output power of the orange-red laser at 612.24 nm and the idler light at 4027.5 nm are obtained, which are 1.145 W and 2.83 W, respectively.

A recently introduced technique based on MALDI with laser-induced postionization (PI), also named MALDI-2, increases the ion yields for numerous classes of lipids, metabolites, and carbohydrates in MALDI-MS imaging experiments under certain experimental conditions. Here, we used a semiautomatic LabVIEW-based protocol to investigate and optimize the efficiency of the PI process dependent on four relevant input parameters and a dense parameter grid: pulse energies of the two lasers, delay between the laser pulses, and buffer gas pressure in the ion source. All experiments were conducted with a modified MALDI-2 Synapt G2-S mass spectrometer (Waters) and use of a focal spot size on the sample of 15-17 μm. A wavelength-tunable optical parametric oscillator (OPO) laser served for PI at 260 or 280 nm. The investigated MALDI matrices were: 2,5-dihydroxybenzoic acid (positive ion mode, +), 2,5-dihydroxyacetophenone (+), α-cyano-4-hydroxycinnamic acid (+), norharmane (negative-ion mode, -), and 1,5-diaminonapthalene (-). A porcine brain extract served as lipid standard. In the positive-ion mode, a maximum boost for the generated [M + H]+ species was found with a N2 buffer gas pressure of ∼2 mbar and a delay between the laser emissions of ∼10 μs. Higher optimal delay settings of several 10 μs were registered for the two studied matrices in negative-ion mode. With regard to the laser fluences, best PI efficiencies were reached using maximum available ablation and PI laser pulse energies of up to 25 and 160 μJ, respectively. For analytes not profiting from MALDI-2, best ion signal yields were recorded for ablation laser pulse energies of around 7 μJ, depending on the MALDI matrix. At higher laser pulse energies, sizable fragmentation is observed for these ions. The PI laser pulse energy did not have any influence on the ion signals of these species. For optimal ion yield of all analyte species, best results were obtained with an ablation laser pulse energy of ∼7 μJ and a PI laser pulse energy of ∼160 μJ. Our comprehensive data set provides valuable insight into the mechanisms underlying the MALDI-2 processes and could help to further optimize this emerging technique.

Oscillators are one of the key elements in various applications as a signal source to generate periodic oscillations. Among them, an optical parametric oscillator (OPO) is a driven harmonic oscillator based on parametric frequency conversion in an optical cavity, which has been widely investigated as a coherent light source with an extremely wide wavelength tuning range. However, steady oscillation in an OPO is confined by the cavity delay, which leads to difficulty in frequency tuning, and the frequency tuning is discrete with the minimum tuning step determined by the cavity delay. Here, we propose and demonstrate a counterpart of an OPO in the optoelectronic domain, i.e., an optoelectronic parametric oscillator (OEPO) based on parametric frequency conversion in an optoelectronic cavity to generate microwave signals. Owing to the unique energy-transition process in the optoelectronic cavity, the phase evolution in the OEPO is not linear, leading to steady single-mode oscillation or multimode oscillation that is not bounded by the cavity delay. Furthermore, the multimode oscillation in the OEPO is stable and easy to realize owing to the phase control of the parametric frequency-conversion process in the optoelectronic cavity, while stable multimode oscillation is difficult to achieve in conventional oscillators such as an optoelectronic oscillator (OEO) or an OPO due to the mode-hopping and mode-competition effect. The proposed OEPO has great potential in applications such as microwave signal generation, oscillator-based computation, and radio-frequency phase-stable transfer., A new oscillator featuring phase-controlled oscillation Parametric oscillators are driven harmonic oscillators that widely used in various areas of applications. In the past, parametric oscillators have been designed in the pure optical domain or the electrical domain, which are both delay-controlled oscillators with a steady oscillation confined by the cavity delay. Ming Li from the Chinese Academy of Sciences in Beijing and his colleagues have now developed a brand-new parametric oscillator in the microwave photonics domain, i.e., a hybrid optical-electrical oscillator. Owing to the unique parametric process in the optoelectronics cavity, the oscillation in the optoelectronic parametric oscillator is a phase-controlled operation, leading to a steady oscillation that is not bounded by the cavity delay. Continuously tuneable single frequency oscillation and stable multimode oscillation are produced by the new optoelectronic parametric oscillator, which are hard or even impossible to achieve in traditional delay-controlled oscillators.

The dynamics of dipolariton states in a planar microcavity under pumping into the middle dipolariton branch by two laser pulses with close frequencies is investigated. It is shown that both aperiodic and periodic modes of the conversion of pump dipolaritons into idler- and signal-mode dipolaritons can take place under the conditions of exact resonance.

In this article, we propose the design and analyze a plasmonic-organic-hybrid (POH) waveguide structure for efficient forward and backward second harmonic generation (SHG) to bridge the mid-infrared (IR) to near-IR. The required phase matching is satisfied by quasi-phase matching (QPM) technique through periodically altering the material of the metal-insulator-metal slot region between a nonlinear polymer (with second-order nonlinear coefficient of χ (2) ≠ 0) and Al2O3 (with χ (2) = 0) leading to modulation of χ (2) coefficient along the propagation direction. The strong field confinement offered by surface plasmon polaritons (SPPs) significantly enhances the normalized efficiency up to 1.75 × 106 W−1cm−2 in the case of backward SHG over a short device length of 17 μm with only 20 mW input power. Also, a slightly lower efficiency is achieved for forward configuration with broadband response, high fabrication tolerance, and small temperature-variation sensitivity. Furthermore, our proposed structure is based on standard nanofabrication technologies and can be used for all-optical signal processing, low-noise detection of a mid-IR signal in the near-IR windows, and (mirrorless) optical parametric oscillator.

In this paper, a novel high efficient dual-wavelength mid-infrared optical parametric oscillator (OPO) was demonstrated, which was pumped by a dual-wavelength Raman fiber oscillator for the first time. The adoption of oscillator structure reduced the waste of power, increased Raman pump power and promoted its frequency conversion. The dual-wavelength pump source was fixed at 1070 nm and 1120 nm and the Raman conversion efficiency was greatly enhanced from 26.7% to 72.1%. The high Raman efficiency and low Raman threshold enabled the Raman laser to build optical parametric oscillation easily and the pump-to-idler conversion efficiency of Raman laser improved greatly from 4.2% to 13.3%. The generated dual-wavelength mid-infrared laser was fixed at 3272 nm and 3663 nm, with a maximum power of 6.87 W, indicating a 13.9% pump-to-idler conversion efficiency. The highest mid-infrared output power generated by optical parametric oscillation of Raman laser was also achieved, which is almost ten times than before.

An efficient intracavity-frequency-doubled singly resonant optical parametric oscillator (SR-OPO), pumped by a high-maturity 532 nm laser, tunable in the yellow wavelength region is demonstrated. An elaborate V-shaped idler cavity design is employed for a potassium titanyl phosphate (KTP) OPO nonlinear crystal to achieve a 5.06 W idler output at 1151.2 nm at a 10 kHz pulse repetition frequency. Using a second KTP crystal to intracavity frequency double the circulating idler wave, a 2.37 W yellow output at 575.6 nm is generated. High optical conversion efficiency (25.4% and 11.2%), 0.5 nm spectral linewidth, and 32 ns pulse duration of the 575.6 nm yellow output are achieved.

Optical carried microwave radar (OCMR) has better resistance to turbulence and scattering than conventional laser radar for using intensity modulated laser beam as detection medium. Intensity modulated mid-infrared source is the key to mid-infrared OCMR system. A frequency chirped intensity modulated mid-infrared light source with tunable wavelength is presented. Single frequency output from a non-planar ring oscillator (NPRO) was modulated via a Mach-Zehnder electro-optic modulator (EOM). The modulation frequency was tuned from 10 MHz-2.1 GHz. The modulated light was amplified via a two-stage ytterbium-doped fiber amplifier (YDFA). The amplified 1064 nm laser was used to pump a signal light resonance optical parametric oscillator (OPO). The nonlinear crystal was a 50 mm long magnesium oxide doped periodically-poled lithium niobite (MgO:PPLN) crystal. Single frequency oscillation was realized by adding a 0.2 mm-thick Fabry-Perot (F-P) etalon in the resonator. When the pump power was 15.2 W, the idler output power at mid-infrared was 2.16 W. The maximum pump-idler conversion efficiency was 16.5%. The wavelength of the idler light was tuned from 3.1 μm to 3.8 μm by changing the temperature of the MgO:PPLN crystal. The frequency chirping linearity with respect to time was 0.9937 on average with modulation frequency tuned from 10-350 MHz. The frequency instability was less than 1.5 Hz in 200 seconds at a modulation frequency of 300 MHz.

We present a mid-infrared optical parametric oscillator (OPO) with a high pump-to-idler conversion efficiency from $1.06~\mu \text{m}$ to $3.8~\mu \text{m}$ , which was quasi-synchronously pumped by a burst-mode fiber laser. The pump pulse was specially designed with a rectangular profile in temporal, which was composed of hundreds of ultrashort sub-pulses. By applying the quasi-synchronous pumping scheme, the buildup time of the parametric signal pulse could be minimized. No back-conversion effect was observed when the pump peak power was increased. A maximum pump-to-idler conversion efficiency of 19.6% was achieved when the number of intra-burst sub-pulses was set to be 240. By increasing the number of intra-burst sub-pulses to be about 480, the maximum average output power of 7.9 W at $3.8~\mu \text{m}$ was realized under pump power of 45.3 W. The corresponding pump-to-idler conversion efficiency was computed to be about 17.5%. The experimental results show an improvement of 40% in pump-to-idler conversion efficiency for this quasi-synchronously pumped OPO when compared with the conventional long laser pulse pumped OPO.

Nowadays, the Fabry–Perot etalon (F–P) has been widely utilized in the optical parametric oscillator (OPO) to improve the filtering performance. In this paper, we reported an F–P etalon composed of two ultra-thin silicon wafers spaced with the air. The linewidth of the signal laser and the threshold are 0.03 nm and 0.6 W, respectively when the proposed etalon is employed to a OPO system based on the MgO-doped LiNbO3 (MgO: PPLN). A stabilized output at 1492.4 nm is obtained, and a tunable, high-precision filtering performance can be achieved by varying the gap distance of the F–P etalon arbitrarily due to its ultra-thin thickness. In addition, the F–P etalon can work on a very wide bandwidth due to its weak absorption during the infrared and terahertz waveband. The high-precision tuning capability and wide-band function of proposed etalon may benefit many applications, including spectroscopy, filtering, and optical communication.

Numerical simulation of the fiber optic parametric oscillator aiming the goal to produce picosecond narrowband pulses for CARS has been performed in an extremely wide range of parameters, such as a pump pulse duration, parametric frequency shift, spectral bandwidth of the pump and the parametric pulses. It required extremely large calculation window both in time (3.5 ns) and spectral (from 433 nm to 3100 nm) domains. We managed to speed up simulation in fifty times by graphic processor unit, which allowed to define the areas of stability for different lengths of standard passive and photonic-crystal fibers used in the external linear cavity of oscillator. The cavity length reached a value of 100 meters that was resulted in parametric pulses with the energy up to 40 nJ and peak power up to 1 kW at a wavelength about 800 nm.

A widely tunable eye-safe noncollinear phase-matched (PM) KTP optical parametric oscillator (OPO) with fixed output direction was proposed. Based on a novel confocal optics system, the input pump beam from a pulsed 1064 nm Nd:YAG laser could be deflected into the OPO with a tunable and agile noncollinear angle while maintaining the resonator unaffected. As a result, stable OPO operation with a wide tuning range of 124 nm was achieved easily with a beam scanner. The pump threshold, output energy, linewidth and temporal pulse shapes during wavelength tuning were also measured and discussed.

We present multicolor coherent Raman imaging (CRI) with enhanced sensitivity by a novel fiber optical parametric oscillator (FOPO). The FOPO combines a rapid and wide tunability for accessing Raman bands between 700 and 3500 cm-1 within only 5 ms with a frequency modulation (FM) at 20 MHz for enhancing CRI sensitivity by at least two orders of magnitude.

A Dual differential absorption LIDAR (DIAL) based on two stage tandem OPO midinfrared (IR) tunable Optical Parametric Oscillator (OPO) has been developed and characterized. The first OPO stage, built around a nonlinear KTP crystal in an unstable resonator, produces 40 mJ of 1.57 μm radiation when pumped by a Q-switched Nd:YAG laser. Silver gallium selenide (AgGaSe2) crystal forms the OPO second stage which generates 0.9 mJ of Idler peak energies within 5.0 to 11.0 μm tuning region with typical spectral bandwidth values of the mid-IR radiation of 7-8 cm-1 which is well-matched with many complex molecules in the atmosphere which have bandwidths larger than 15 cm-1. The receiver is composed of a gold-plated primary mirror of a 300 mm forms the Cassegrain telescope, two channel detection system and control and amplifying electronics. Preliminary path-averaged ammonia concentration measurements are presented and analyzed.

We proposed a compact experimental scheme for a low repetition rate, high-energy, nanosecond laser at 604 nm based on Pr: YLF crystal at room temperature. The thermal effect of Pr: YLF crystal was simulated, which demonstrated that high energy 604 nm laser line at room temperature can be generated with appropriate parameters such as crystal and the laser output coupling. Then a stable 604 nm orange laser pumped by a pulsed 444 nm optical parametric oscillator was developed experimentally. A high pulse energy of 63 μJ with 30 ns pulse duration at a repetition rate of 10 Hz was obtained. To the best of our knowledge, this is the first report of such a high-energy 604 nm laser using a gain switched pump source at room temperature.

Kerr cavity solitons (CSs) are pulses that propagate unperturbed in an optical resonator. They were first observed in fiber resonators [1] and subsequently in microresonators [2], where they are often called dissipative Kerr solitons (DKSs). So far, the focus has been on CSs driven at their natural oscillation frequency, i.e. with a driving laser at the carrier frequency of the soliton. However, CSs can also be parametrically driven by a laser at twice their carrier frequency. This parametric Kerr cavity soliton (PCS) is solution of the parametrically driven nonlinear Schrodinger equation (PDNLSE) [3]-[5], which describes a singly-resonant optical parametric oscillator:\begin{equation*}{t_R}\frac{{\partial A}}{{\partial T}} = \left( { - \frac{{{\Lambda _e}}}{2} - i\frac{{{\beta _2}L}}{2}\frac{{{\partial ^2}}}{{\partial {\tau ^2}}} - i{\delta _0}} \right)A + \kappa {B_{in}}{L_1}{A^{\ast}} + i\gamma {L_2}|A{|^2}A\tag{1}\end{equation*}where A is the intracavity electric field envelope, Λ e is the effective loss, L is the total length of the resonator and $\underline {{\beta _2}}$ its average group velocity dispersion. T=nt R , where n is an integer, is a slow time and τ is a time reference traveling at the group velocity. δ 0 the phase detuning from the closest cavity resonance. κ and ɣ are the second-and third-order nonlinear parameters and B in is the driving amplitude. PCSs differ from CSs in their lack of homogeneous background, but also in their multiplicity as two different solitons of the same amplitude but opposite phase, may exist for the same set of parameters. To demonstrate the existence of this new soliton as well as its multiplicity, we built the experimental set-up depicted in Fig. 1a. It is a resonator made of three different fibers. A periodically poled fiber (PPF, L 1 = 30cm) and a single-mode fibre (SMF, L 2 = 21m) provide the second- and third-order nonlinearity, respectively. In addition, a short piece of erbium doped fiber (EDF, 50cm) is used for loss compensation [6]. Using 650-ps flat top pulses at 775 nm (P p =|B in |2=10W), we manage to excite a single PCS. Detailed temporal and spectral analysis (not shown), show the presence of a 5W, 3.6 ps long, background-less sech-shaped pulse circulating inside the cavity for δ0=0.03, in perfect agreement with Eq. (1). To demonstrate their multiplicity, we perform a coherent detection. We extend the pump pulses duration to 1 ns to generate several PCSs and imprint a phase modulation to resolve them individually on the oscilloscope [5]. Once PCSs are excited, we send half of the cavity output power $P_s^{out}$ to a fast photodiode for direct measurement (Fig. 1b) and make the other half beating with a local oscillator (Fig. 1c). The direct measurement clearly shows that the two solitons have the same amplitude. On the other hand, the result of the interference confirms that they have two different phases. Since there are only two distinct phases [4] and the probability of having one or the other is the same, PCSs can be used to generate random bits. As a proof of principle, we generate four PCSs and associate the result of the interference to a binary value. This leads to the generation of random numbers (Figs. 1d,e). Optical parametric oscillator have already been used for random bits generation [7] and Ising machines [8]. Yet, complex set-ups had to be used to force the bits localization. Our demonstration therefore opens up new applications for Kerr cavity solitons.

Ultrafast optical parametric oscillators (OPOs), providing tunable pico- or femto-second pulses in the near- and mid-inrared (mid-IR), have attracted massive attentions and been used in various applications. Traditionally, these OPOs are synchronously pumped by mode-locked lasers and thus have repetition rates typically at tens or hundreds of MHz level. Further increase of the OPO repetition rate to multi-GHz level can be achieved by increasing the pump repetition rate [1] , [2] , which usually involves challenging cavity design, insufficient output power as well as relatively long picosecond pulses. Alternatively, one can use a high harmonic OPO cavity [3] , which, however, will inevitably create an uneven envelope of the GHz pulse train due to round-trip loss in the cavity. Here, leveraging the recently emerging cavity-less GHz electro-optic (EO) frequency comb technique, we demonstrate a new kind of GHz-repetition-rate OPO synchronously pumped by an EO comb.

High-power wavelength-agile fiber sources are now competitive for many widespread applications such as non-linear imaging or coherent Raman scattering spectroscopy. Soliton self-frequency shift, self-mode conversion and self-phase modulation are commonly used to obtain the necessary wavelength shift but are limited in peak-power scalability due to intensity-related damages. However, it is possible to overcome this limitation by exploiting the broad tunability offered by degenerate four-wave mixing in optical fibers along with chirped-pulse amplification (CPA), then allowing the generation of high-energy fs pulses at different wavelengths [1] - [4] . Moreover, the exploitation of this principle in a fiber optical parametric oscillator (FOPO) with normal dispersion can enable record performances in terms of pulse energy [5] . Combining both concepts to build a fiber optical parametric chirped-pulse oscillator (FOPCPO) is then attractive to enable a full control of the output wavelength and bandwidth, as recently shown in a numerical study [6] . Thereby, we experimentally demonstrate here a high-energy FOPCPO, numerically analyze its operation, and discuss its potential for further energy scaling [7] .

The recent emergence of real-time temporal and spectral characterization techniques such as time lenses [1] and dispersive Fourier transformation (DFT) [2] has opened new horizons by unveiling the shot-to-shot dynamics of ultrafast optical systems. Especially, the ability of DFT to record large ensembles of single-shot spectra has enabled the use of statistical metrics to describe the dynamical behaviours of various optical phenomena. In this frame, spectral correlations have been used for understanding the complex processes involved in supercontinuum generation, modulation instability, four-wave mixing or random lasing. The shaping of spectral correlations has been experimentally demonstrated in fiber systems [3] , [4] and could now drive many quantum optics applications. However, to date, such wavelengths correlations have never been studied within fiber optical parametric oscillators (FOPO), which are now mature and widespread wavelength-agile sources.

We explored possibility to enhance spectroscopic capabilities of mid-IR optical parametric oscillators (OPOs). Gas analysis based on the principles of photoacoustic spectroscopy (PAS) [1] is known to be an efficient approach to measure concentration of certain gases (e.g., methane) in the atmosphere. Laser radiation source for PAS gas analyzer should provide high-energy pulsed output with narrow optical spectrum. There should be also possibility for fine tuning of its wavelength to a strong absorption line of the detected gas. Herein, we report an original high-energy pulsed OPO, in which the fan-out periodically poled lithium niobate (PPLN) and injection seeding technique were employed for spectral narrowing and fine wavelength tuning.

For the satellite-based methane lidar instrument MERLIN a reliable laser source is needed that emits laser pulses at two wavelengths of around 1645 nm to measure the methane concentration of earth’s atmosphere with an Integrated Path Differential Absorption LIDAR (IPDA). To generate those pulses, the laser (LASO) consists of a seeded, actively qswitched, diode pumped Nd:YAG master oscillator power amplifier (MOPA) and a subsequent seeded and frequencycontrolled optical parametric oscillator (OPO). Due to the passive thermal control of the instrument the laser has to withstand a large non-operational and operational temperature range and also high mechanical loads while at the same time a compact envelope is required. Together with the demanding requirements on the laser performance a very robust optical design is needed. To meet those requirements, Fraunhofer Institute for Laser Technology (ILT) uses optomechanical mounts that were developed in a previous project and show very low tilting over a large operational temperature range, even after nonoperational temperature cycling and applying mechanical loads. The mounts are soldered and free of organic substances as the LASO is enclosed in a pressurized housing (LASH). Any outgassing could lead to a decay or damage of the optics and thus a failure of the laser. During the development of the optomechanical mounts many tests were performed to quantify the statistical behavior under mechanical and thermal loads. Based on those results and additional mechanical simulations, Monte-Carlo-Analyses have been performed to analyze the performance of the laser and to verify the fulfilment of the requirements.

Optical parametric oscillators (OPOs) are ubiquitous sources for efficient frequency conversion and have evolved into robust fiber technology [1] . However, integrated waveguides offer in contrast to photonic crystal fibers a higher nonlinearity resulting in lower necessary pump energies and shorter interaction lengths [2] . Therefore, OPOs would require smaller footprints on a chip with additionally improved stability [3] . Here, we present a waveguide-based OPO (WOPO) exploiting four-wave mixing (FWM) in silicon nitride (Si 3 N 4 ) waveguides, with the potential to be fully integrated.

Optical parametric oscillator based on a periodically poled MgO:LiNbO3 crystal pumped by a repetitevely pulsed Tm3+:Lu2O3 ceramics laser at 1966 nm was studied. The OPO average output power up to 530 mW at wavelength of 3.7–4.2 μm was achieved at the ceramics laser power of 7.9 W. The optical-to-optical efficiency of OPO energy conversion was up to 6.7%, and the slope efficiency was up to 8%.

A degenerate optical parametric oscillator (DOPO) has unique characteristics available for quantum-mechanical applications, whose behavior is conventionally analyzed based on a short-cavity laser model. This paper presents a traveling-wave model for a pulse-pumped DOPO in a long ring cavity, where an isolated pulse goes round the cavity while alternatively amplified and attenuated. Such a DOPO system is analyzed based on a multistage-amplification model, the results of which reasonably show the lasing behavior of a DOPO.

Laser-induced fluorescence (LIF) on ITER will be used for local measurement of helium density nHe and ion temperature Ti in divertor. The LIF diagnostic is combined with the divertor Thomson scattering (DTS) via common laser injection and signal collection optics. Collisional-radiative model developed for helium ions is used to estimate laser parameters required for robust measurement of laser-induced signals against plasma background radiation. The estimations demonstrate feasibility of measuring nHeII with laser parameters provided by available optical parametric oscillator pumped by Nd:YAG. New approach for measuring Ti is proposed and the expected errors are estimated using numerical simulations.

Stimulated Raman scattering (SRS) imaging is a fast, label-free, and sensitive technique to map the distribution of a vibrational species in a microscopy setting. It has great potential for applications in many fields, such as lipid imaging in biomedicine. However, depth penetration of the light into the sample is an issue with any light-based technique, especially with multiphoton techniques such as SRS. Using longer wavelengths allows deeper penetration into densely scattering materials, but applying wavelengths above 1,500 nm is challenging technically. We have built a flexible SRS microscope system capable of imaging with a combination of 1,064 nm and wavelengths over 1,500 nm, using the idler output of an optical parametric oscillator (OPO). For comparison, the same system was also operated in the more common configuration, using 1,064 nm in combination with the OPO signal output around 800 nm. With the long-wavelength settings, we show improved depth penetration in polyethylene plastic material and in a silicone phantom with embedded polymer microbeads, and we report images of lipid structure in biological tissue. These results demonstrate the technical feasibility of using these long wavelengths for SRS imaging. Disadvantages such as poorer spatial resolution and lower signal strength are also discussed. The application of this new approach to SRS microscopy can allow greater insight into deep-lying structures in a non-invasive way.

With the pump of a KTP optical parametric oscillator (OPO) at 1150 nm, a gain-switched Ho,Pr:LLF laser was experimentally demonstrated in this paper. The maximum single pulse energy of 98.8 μJ with the minimum pulse width of 22 ns was achieved at the pulse repetition frequency (PRF) of 1 Hz, corresponding to a peak power of 4.5 kW. The central wavelength of the gain-switched laser was 2958 nm with a full width at half maximum (FWHM) of 4.5 nm.

We have designed and tested a broadband fan-out PPLN-based optical parametric oscillator (OPO) pumped by a Q-switched Nd:YAG laser (1.064 mm) and operating in the spectral range 2.6 – 4.2 mm. The optical scheme of the OPO utilises a cylindrical pump beam expander for increasing the effective aperture of the PPLN structure and obtaining broadband generation.

In this letter, we demonstrated a dual-wavelength (DW) mid-infrared optical parametric oscillator (OPO) that was pumped by two independent fiber lasers fixed at 1018 nm and 1080 nm, and realized phase-matched signal beam coupling by tuning grating period of PPLN. Based on signal beam coupling, two interesting phenomena were observed for the first time. One is that the OPO threshold for 1018 nm and 1080 nm pump beams could be drastically lowered from 15.4 W to 266 mW and from 13.5 W to 1.38 W separately. The other is that the total idler power obtained when the OPO was simultaneously pumped by two fiber lasers was higher than sum of idler power obtained when the OPO was independently pumped by two fiber lasers, revealing obvious parametric gain enhancement. It also indicated that the two independent OPO processes became coherent when two signal beams were coupled. The results exhibit great potential in practical applications such as frequency down-conversion of some special near-infrared laser sources such as pulsed single-frequency sources, spectrum high-speed swept sources, and ultra-short pulse sources with low duty cycle.

This study focuses on the thermal characteristic of an end-pumped MgO: PPLN crystal by solving the transient temperature evolution model in a high power optical parametric oscillator. In this model, an approach based on contact mechanics theory is proposed to quantitatively calculate the heat transfer between rough contacting surfaces of crystal and heat sink. Based on the model, a parametric study has been performed to investigate the influence of the heat sink property, pump laserand crystal dimension on the thermal characteristic of the laser crystal. Obtained results reveal a closed correlation between the mentioned parameters and the thermal characteristic of the crystal indicating that a proper selection of the parameters is essential to control the crystal temperature and improve the performance of the laser.

We present a miniaturized idler-resonant all-fiber-laser-pumped optical parametric oscillator (OPO) based on MgO:PPLN. The conversion-efficiency and beam-quality performances of the idler- and signal-resonant OPOs with flat-flat cavity are first compared. The idler-resonant OPO MIR beam quality is better than that of the signal-resonant one, but the idler conversion efficiency is lower. Next, we examine the influence of the output-coupler curvature radius, pump-beam-waist radius, and cavity length on the MIR conversion efficiency and beam quality of the idler-resonant OPO. Post optimization of these factors, we report a maximum MIR power of 5.84 W at 3.76 µm, conversion efficiency >14.0%, and M2 factors of 1.57 and 1.49 along the horizontal and vertical directions, respectively.

Fiber-feedback optical parametric oscillators (OPOs) incorporate intracavity fibers to provide a compact high-energy wavelength-tunable laser platform, however, dispersive effects can limit operation to the sub-picosecond regime. In this research article, we modeled pulse propagation through systems of cascaded fibers, incorporating SMF-28 and ultra-high numerical aperture (UHNA) fibers with complementary second-order dispersion coefficients. We found that the pulse duration upon exiting the fiber system is dominated by uncompensated third-order effects, with UHNA7 presenting the best opportunity to realise a cascaded-fiber-feedback OPO.

New optical sources in the nearand mid-infrared (NIR/MIR respectively) have recently attracted a growing attention for potential applications in telecommunication systems (Yoo, 1996), spectroscopy (Chen et al., 1999; Arie et al., 2002), gas sensing (Lancaster et al., 1999), and quantum information (Gisin et al., 2002; Sergienko & Jaeger, 2003). In this respect, guided-wave frequency conversion is an appealing solution, due to its efficiency, compactness and tunability of the output wavelength. In this process, three guided modes coupled by the material optical nonlinearity exchange power during propagation; provided that their phase-velocity mismatch is absent or cancelled, such interaction allows the efficient transfer of power from the injected pump modes to a new-frequency generated mode, according to energy conservation (Boyd, 2008). After the demonstration of a few original phase-matching schemes, nonlinear waveguides based on gallium arsenide (GaAs) have carved out a prominent position in the panorama of integrated frequency converters. Among their main assets, in comparison with alternative material systems: a higher nonlinear coefficient, a wider infrared transparency range, and potential monolithic integration with a laser diode pump source. On the other hand, conversion efficiency in such devices is still affected by non-negligible scattering loss, in spite of recent technological developments. Due to this limitation, no optical parametric oscillator has been reported to date in GaAs waveguides, whereas it was demonstrated long ago in lithium niobate (LiNbO3) waveguides (Bortz et al., 1995). In this chapter we focus on GaAs/AlAs nonlinear waveguides, where phase-matched threewave mixing can be performed thanks to form birefringence. This is obtained by oxidizing a few AlAs thin layers in the waveguide core, with AlAs being transformed into a low-index non-stoichiometric aluminium oxide (hereafter referred to as AlOx). Although all different χ(2) processes have been reported so far, these devices especially lend themselves to parametric down-conversion towards mid-infrared, and are promising candidates as compact sources for infrared spectroscopy. In the first section we briefly summarize the state-of-the art of midinfrared tunable coherent sources, focusing on frequency converters. At present, none of these sources fully meets the main requirements of practical spectroscopic systems. In section two, we describe the phase-matching principle and the design guidelines of GaAs/AlOx waveguides, while the fabrication process and its crucial issues are detailed in section three.

10-kHz hydroxyl radical (OH) two-color planar laser-induced fluorescence (TC-PLIF) thermometry was demonstrated with a single burst-mode optical parametric oscillator (OPO) and a single camera. A fast, dual-wavelength switched seed laser enabled a high-energy, high-repetition-rate burst-mode laser to generate two 10-kHz pulse trains at wavelengths of ∼ 354.8 n m . The two pulse trains are colinear with 3 µs time interval between the pulse pairs. The injection-seeded OPO efficiently converts the burst-mode laser output to 285.62 and 285.67 nm to excite the Q 2 ( 12 ) and P 1 ( 8 ) OH transitions. PLIF images were collected from each of the two excitation transitions, and intensity ratios from the images were used to determine local temperatures. The development of fast, dual-wavelength switching, burst-mode OPO technology significantly reduces the experimental complexity of the high-speed TC-PLIF thermometry and simplifies its implementation in harsh combustion and flow test facilities.

We investigated high energy, near and mid-infrared optical vortex lasers formed by a 1 μm optical vortex-pumped KTiOAsO4 (KTA) optical parametric oscillator. The orbital angular momentum (OAM) of the pump beam can be selectively transferred to the signal or idler output by changing the reflectivity of the output coupler. With this system, 1.535 µm vortex signal output with an energy of 2.04 mJ and 3.468 µm vortex idler output with an energy of 1.75 mJ were obtained with a maximum pump energy of 21 mJ, corresponding to slope efficiencies of 14% and 10%, respectively. The spectral bandwidth (full width at half maximum, FWHM) of the signal and idler vortex outputs were measured to be Δλs ~ 1.3 nm (~ 5.5 cm−1) and Δλi ~ 1.7 nm (~ 1.4 cm−1), respectively.

Krypton (Kr)-based tagging velocimetry is demonstrated in a K r / N 2 jet at 100 kHz repetition rate using a custom-built burst-mode laser and optical parametric oscillator (OPO) system. At this repetition rate, the wavelength-tunable, narrow linewidth laser platform can generate up to 7 mJ/pulse at resonant Kr two-photon-excitation wavelengths. Following a comprehensive study, we have identified the 212.56 nm two-photon-excitation transition as ideal for efficient Kr-based velocimetry, producing a long-lived ( ∼ 40 µ s ) fluorescence signal from single-laser-pulse tagging that is readily amenable to velocity tracking without the need for a second “read” laser pulse. This long-lived fluorescence signal is found to emanate from N 2 —rather than from Kr—following efficient energy transfer. Successful flow velocity tracking is demonstrated at multiple locations in a high-speed K r / N 2 jet flow. The 100 kHz repetition rate provides the ability to perform time-resolved velocimetry measurements in high-speed and even hypersonic flow environments, where standard velocimetry approaches are insufficient to capture the relevant dynamics.

Stimulated Raman scattering (SRS) microscopy is a powerful technique for label-free identification of molecules based on their intrinsic vibrational spectrum. We present a novel approach to broadband SRS microscopy based on a recently developed balanced low-noise integrated-circuit multichannel lock-in amplifier with 10-μs integration time and 32 channels working in parallel, enabling multiplex SRS detection at speed faster than one hyperspectral frame per second. The system is powered by a narrowband Stokes pulse at 1040 nm and a broadband (approx. 500 cm^-1) pump pulse generated by a home-built low-noise optical parametric oscillator with up to 100-mW average power, covering the whole C-H stretching band. We measure stimulated Raman loss on the broadband pump pulse employing an in-line balanced detection approach to suppress the laser fluctuations and achieve close to shot-noise-limited sensitivity. We will show the system performances in high-resolution cell imaging.

NASA demands the special laser transmitter for the lidar system to detect water-ice on the Moon and other planetary bodies. Based on the data from the Moon Mineralogy Mapper (M3) instrument, the water ice was on the Moon has been claimed, but such a claim was disputed because OH- and/or H2O-bearing materials share the absorption line at the wavelength range of 2.8-3 μm. Lunar Flashlight, another mission to explore the surface of Moon (the launch date delays to this year), allows scientists map the minerals on dark side of the Moon, but it still has difficult to resolve the ambiguity mentioned above. The absorption line around 6.08 μm uniquely associated with the bending resonance of H2O has not any comparable vibration in confounding OH-bearing materials. However, a 6.08 μm laser in the gap between the atmospheric windows, middle-wave infrared (3-5 μm) and long-wave infrared (8-12 μm), has not been commercially available. Our approach is a Q-switched Ho:YLF laser pumped the orientation-pattern Gallium Arsenide optical parametric oscillator (OP-GaAs OPO) for generating high-energy laser pulses at the wavelength of 6.08 m. In the current design, a 1.94 μm Tm:fiber is used as the pump source of Ho:YLF laser. In the compact design, a 1.94 μm laser diode will replace the Tm:fiber laser as the pump source. The combination of proposed 6.08 μm laser and the latest HgCdTe avalanche photodiode (APD) array allow us to design a lidar capable of unambiguously identifying water ice on the Moon and Mars from their respective orbits, enabling novel science and in-situ resource utilization. Our instrument is an enabling technology for the Artemis program and future missions.

We present updated Sellmeier equations for β-BaB2O4 based on measurements of the phase-matching angles for second-harmonic (SHG) and sum-frequency generation (SFG) of a Nd:YAG laser-pumped 90° phase-matched RbTiOAsO4 optical parametric oscillator (OPO) and a Nd:YAG laser in the 0.6407 - 3.1392 μm range. The phase-matching angles predicted by this index formula agree well with the recently published data points of SFG between a Ti:Al2O3 laser and Nd:YAG laser-pumped KTiOAsO4 OPO in the 0.40 -5.3 μm range {G. Tamosauskas et al., Opt. Mater. Express 8, 1410 (2018)] as wellas those for SFG below 0.2048 μm thus far reported in the literature.

We describe a compact mid-IR source utilizing an intracavity, non-critically phase matched potassium titanyle arsenate (KTA) optical parametric oscillator (OPO) placed inside a passively Q-switched (PQS), 1.064 µm Nd:YAG laser resonator. A 45-degree dichroic beam splitter was employed to split the 1.064 µm resonator leg from the 1.5 μm/3.5 μm KTA OPO cavity that was singly resonant for the 1.5 μm signal. The 20 mm long KTA crystal was placed in the shared-path section of both cavities, with the KTA OPO output coupler partially reflective at 1.5 μm, and highly transmitting at 3.5 μm. With the Nd:YAG pumped by a 3-λ 12-bar diode stack at 5 Hz PRF, the KTA OPO generated 23.1 mJ signal pulses at 1.5 μm and 9.8 mJ idler pulses at 3.5 μm. To further increase the 3.5 μm energy, a cadmium silicon phosphide (CSP) optical parametric amplifier (OPA), phase matched for 1.5 μm-pumped amplification of 3.5 μm radiation, was placed immediately after the KTA OPO output coupler. A maximum 3.5 μm pulse energy of 13.2 mJ was measured after the OPA, with an additional 4.2 mJ generated at 2.8 μm. The 3.5 μm pulses had a measured temporal duration (FWHM) of 27 ns, corresponding to a peak-power of approximately 490 kW. This paper will detail the Nd:YAG pumped intracavity KTA OPO / CSP OPA and optimization of its performance as a pulsed midIR source.

Visualizing activity patterns of distinct cell types during complex behaviors is essential to understand complex neural networks. It remains challenging to excite multiple fluorophores simultaneously so that different types of neurons can be imaged. In this Letter, we report a multicolor fiber-optic two-photon endomicroscopy platform in which two pulses from a Ti:sapphire laser and an optical parametric oscillator were synchronized and delivered through a single customized double-clad fiber to excite multiple chromophores. A third virtual wavelength could also be generated by spatial-temporal overlapping of the two pulses. The performance of the fiber-optic multicolor two-photon endomicroscope was demonstrated by in vivo imaging of a mouse cerebral cortex with “Brainbow” labeling.