Optical Parametric Oscillator Research Articles

Near to Mid-Infrared (1.3–5 μm) Widely Tunable, High Power Picosecond Pulsed Laser

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%.

Highly efficient and widely tunable Si3N4 waveguide-based optical parametric oscillator.

We demonstrate an efficient and widely tunable synchronously pumped optical parametric oscillator (OPO) exploiting four-wave mixing (FWM) in a silicon nitride (Si3N4) waveguide with inverted tapers. At a pump pulse duration of 2 ps, the waveguide-based OPO (WOPO) exhibited a high external pump-to-idler conversion efficiency of up to -7.64 dB at 74% pump depletion and a generation of up to 387 pJ output idler pulse energy around 1.13 μm wavelength. Additionally, the parametric oscillation resulted in a 64 dB amplification of idler power spectral density in comparison to spontaneous FWM, allowing for a wide idler wavelength tunability of 191 nm around 1.15 μm. Our WOPO represents a significant improvement of conversion efficiency as well as output energy among χ3 WOPOs, rendering an important step towards a highly efficient and widely tunable chip-based light source for, e.g., coherent anti-Stokes Raman scattering.

Mean-field equation for phase-modulated optical parametric oscillator

The widely established techniques for the generation of ultrashort optical pulses rely on passive mode locking of lasers, with the output pulse duration and emission spectrum determined by the intrinsic lifetime of laser transition in the gain medium. Due to the instantaneous nature of nonlinear gain, optical parametric oscillators (OPOs) are capable of generating optical radiation in all timescales from continuous-wave (cw) to ultrashort femtosecond regime, if driven by laser pump sources in the corresponding time domain. In the ultrashort timescale, operation of OPOs conventionally relies on mode-locked pump lasers, with the concomitant disadvantages of large footprint and high cost. At the same time, the lack of gain storage mandates the use of synchronous pumping, resulting in increased complexity. In this paper, we present the concept of phase-modulated OPO driven by cw pump laser. The approach overcomes the traditional drawbacks of ultrafast OPOs, enabling femtosecond pulse generation without the need for synchronous pumping, resulting in a simplified, compact, and cost-effective architecture using cw input pump lasers. We derive a mean-field equation for a degenerate χ(2) OPO driven by a cw laser with intracavity electro-optic modulator (EOM), and also including dispersion compensation. The equation predicts the formation of stable femtosecond pulses (<200fs), in both normal and anomalous dispersion regimes, with a controllable repetition rate determined by the frequency of the EOM. The remarkable functionality of the proposed scheme paves the way for the development of an alternative class of widely tunable coherent femtosecond light sources in both bulk and integrated format based on χ(2) OPOs using cw pump lasers. Published by the American Physical Society 2024

Narrowband-seeded PPLN non-resonant optical parametric oscillator

A PPLN non-resonant optical parametric oscillator injection-seeded by narrowband sub-100-mW CW radiation at the signal wavelength produces > 3 W idler average power at 2376 nm for a 20-kHz repetition rate, with a sub-2-nm spectral linewidth. The maximum quantum efficiency reaches 39.5%, roughly 1.4 times higher compared to narrowband operation achieved with a volume Bragg grating at the same pump level. Seed levels as low as 40 mW are sufficient to produce the desired spectral narrowing effect.

High-efficiency narrow-bandwidth KTP optical parametric oscillator for kHz-MHz planar laser-induced fluorescence.

The electronic excitation of key combustion species or flow tagging of chemical species requires a narrowband tunable UV source. In this work, a potassium titanyl phosphate (KTP) burst-mode optical parametric oscillator (OPO) pumped by a 532 nm laser is developed to generate a spectrally narrow signal and an idler output with 1.48 ± 0.19 cm-1 bandwidth without the need for injection seeding. The idler (1410-1550 nm range) is further mixed with 355 or 266 nm to generate 284 or 226 nm for OH or NO planar laser-induced fluorescence (PLIF), respectively, with up to 1.9% conversion efficiency from 1064 nm to the UV. MHz-rate burst profiles are reported, and OH and NO PLIF are demonstrated in a rotating detonation combustor at rates up to 200 kHz.

Integrated frequency-modulated optical parametric oscillator.

Optical frequency combs have revolutionized precision measurement, time-keeping and molecular spectroscopy1-7. A substantial effort has developed around 'microcombs': integrating comb-generating technologies into compact photonic platforms5,7-9. Current approaches for generating these microcombs involve either the electro-optic10 or Kerr mechanisms11. Despite rapid progress, maintaining high efficiency and wide bandwidth remains challenging. Here we introduce a previously unknown class of microcomb-an integrated device that combines electro-optics and parametric amplification to yield a frequency-modulated optical parametric oscillator (FM-OPO). In contrast to the other solutions, it does not form pulses but maintains operational simplicity and highly efficient pump power use with an output resembling a frequency-modulated laser12. We outline the working principles of our device and demonstrate it by fabricating the complete optical system in thin-film lithium niobate. We measure pump-to-comb internal conversion efficiency exceeding 93% (34% out-coupled) over a nearly flat-top spectral distribution spanning about 200 modes (over 1 THz). Compared with an electro-optic comb, the cavity dispersion rather than loss determines the FM-OPO bandwidth, enabling broadband combs with a smaller radio-frequency modulation power. The FM-OPO microcomb offers robust operational dynamics, high efficiency and broad bandwidth, promising compact precision tools for metrology, spectroscopy, telecommunications, sensing and computing.

Genetic algorithm-enhanced microcomb state generation

Microcavities enable the generation of highly efficient microcombs, which find applications in various domains, such as high-precision metrology, sensing, and telecommunications. Such applications generally require precise control over the spectral features of the microcombs, such as free spectral range, spectral envelope, and bandwidth. Most existing methods for customizing microcomb still rely on manual exploration of a large parameter space, often lacking practicality and versatility. In this work, we propose a smart approach that employs genetic algorithms to autonomously optimize the parameters for generating and tailoring stable microcombs. Our scheme controls optical parametric oscillation in a microring resonator to achieve broadband microcombs spanning the entire telecommunication C-band. The high flexibility of our approach allows us to obtain complex microcomb spectral envelopes corresponding to various operation regimes, with the potential to be directly adapted to different microcavity geometries and materials. Our work provides a robust and effective solution for targeted soliton crystal and multi-soliton state generation, with future potential for next-generation telecommunication applications and artificial intelligence-assisted data processing.

Broadband detection of 18 teeth in an 11-dB squeezing comb

Squeezed vacuum is a versatile resource of bipartite entanglement for quantum metrology and information. Optical parametric oscillators generate these states in a frequency comb structure. We show the simultaneous detection of 18 teeth with 11 dB of squeezing each. We built a 1.5-m-long optical resonator to increase the spectral teeth density and significantly increase the detection bandwidth by employing unbalanced homodyne detection instead of balanced. Our approach can be a crucial step in using the scaling potential of high-frequency squeezed states in quantum information. Published by the American Physical Society 2024

Efficient 10 ns-scale 2-μm optical parametric oscillator based on 1064 nm pump source

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%.

Direct sampling of femtosecond electric-field waveforms from an optical parametric oscillator

Detecting the electric-field waveform of an optical pulse from the terahertz to the visible spectral domain provides a complete characterization of the average field waveform and holds great potential for quantum optics, time-domain (including frequency-comb) spectroscopy, high-harmonic generation, and attosecond science, to name a few. The field-resolved measurements can be performed using electro-optic sampling, where a laser pulse is characterized through an interaction with another pulse of a much shorter duration. The measured pulse train must consist of identical pulses, including their equal carrier-envelope phase (CEP). Due to the limited coverage of broadband laser gain media, creating CEP-stable pulse trains in the mid-infrared typically requires nonlinear frequency conversion, such as difference frequency generation, optical parametric amplification, or optical rectification. These techniques operate in a single-pass geometry, often limiting efficiency. In this work, we demonstrate field-resolved analysis of the pulses generated in a resonant system, an optical parametric oscillator (OPO). Due to the inherent feedback, this device exhibits a relatively high conversion efficiency at a given level of input power. By electro-optic sampling, we prove that a subharmonic OPO pumped with CEP-stable few-cycle fiber-laser pulses generates a CEP-stable mid-infrared output. The full amplitude and phase information renders dispersion control straightforward. We also confirm the existence of an exotic “flipping” state of the OPO directly in the time domain, where the electric field of consecutive pulses has the opposite sign.

Thin flexible photoacoustic endoscopic probe with a distal-driven micro-step motor for pump-probe-based high-specific molecular imaging.

Conventional photoacoustic endoscopy (PAE) is mostly for structural imaging, and its molecular imaging ability is quite limited. In this work, we address this issue and present the development of a flexible acoustic-resolution-based photoacoustic endoscopic (AR-PAE) probe with an outer diameter of 8 mm. This probe is driven by a micro-step motor at the distal end, enabling flexible and precise angular step control to synchronize with the optical parametric oscillator (OPO) lasers. This probe retains the high spatial resolution, high penetration depth, and spectroscopic imaging ability of conventional AR-PAE. Moreover, it is capable for background-free high-specific photoacoustic molecular imaging with a novel pump-probe detection technique, as demonstrated by the distribution visualizing of the FDA approved contrast agent methylene blue (MB) in an ex-vivo pig ileum. This proposed method represents an important technical advancement in multimodal PAE, and can potentially make considerable contributions across various biomedical fields.

Energy scaling of a narrowband, periodically poled LiNbO3, nanosecond, nonresonant optical parametric oscillator

We demonstrate that 3-mm-thick, periodically poled LiNbO3 enables energy scaling of a nonresonant optical parametric oscillator operated in the narrowband mode with a volume Bragg grating at the signal wavelength. Utilizing the full available pump power at 1064 nm, we obtained maximum average powers of 2.25 and 2.08 W for the signal (1.922 µm) and idler (2.383 µm) pulses at 10 kHz, at a total conversion efficiency of 32.8%, which represents a fourfold increase in terms of peak powers over our previous work. The signal and idler spectral linewidths were ∼1nm, with pulse lengths of ∼6ns and an idler beam propagation factor of ∼5.

Continuous-wave operation of a tandem optical parametric oscillator up to 5.19 µm based on periodically poled LiNbO3.

A continuous-wave, tandem optical parametric oscillator (TOPO) based on a MgO-doped periodically poled LiNbO3 (MgO:PPLN) is demonstrated. Because the MgO:PPLN is tandemly pumped by the OPO's signal beam, it outputs simultaneously two groups of signal and idler with a single pump source. The entire range spans from 1398 to 1490 nm, 1914 to 2107 nm, 3720 to 4444 nm, and 4849 to 5190 nm, which is limited by periods of the MgO:PPLN and cavity mirror coatings. The TOPO, whose oscillation threshold of pump power exceeds 7 W, can be easily triggered by marginally increasing the pump power as long as the OPO process occurs. The maximum idler powers are respectively 2.6 W (at 3896 nm) and 34 mW (at 4863 nm), and the corresponding signal powers are both nearly 100 mW.

Multiline molecular tagging velocimetry of nitric oxide at 100 kHz using an injection-seeded burst-mode OPO.

An injection-seeded, burst-mode optical parametric oscillator (OPO) operating at a repetition rate of 100 kHz is used to demonstrate the multiline molecular tagging velocimetry of an underexpanded jet using nitric oxide fluorescence. The very narrow linewidth of the OPO system, along with the relatively high pulse energies of the burst-mode system, enables efficient single-photon excitation of nitric oxide along multiple laser beam lines at a high repetition rate. Simultaneous one-dimensional velocity profile measurements were obtained of an underexpanded jet system at six different locations using a reference initial image and single-shot delayed images. A methodology for calculating the uncertainty of single-shot velocity is also described. Mean and root-mean-square velocity profiles are obtained at multiple locations simultaneously over a sampling time of 1 ms. The high-repetition-rate velocity measurements also appear to capture the onset of velocity oscillations and has the potential to reveal velocity frequency content occurring in the tens of kHz. The demonstrated velocimetry technique could be paired with other emerging burst-mode laser capabilities for a quantitative multiparameter gas property or multicomponent gas velocity measurements for supersonic and hypersonic flows, especially within ground test facilities that are limited to very short run durations.

Gain Measurement of ZnGeP2 Optical Parametric Oscillator Pulses in a High-Pressure CO2 Amplifier

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.

Quantitative photothermal analysis and multispectral imaging of dental structures: insights into optical and thermal properties of carious and healthy teeth.

In the analysis of two-layered turbid dental tissues, the outer finite-thickness layer is modeled by an optical transport coefficient distinct from its underlying semi-infinite substrate layer. The optical and thermophysical parameters of healthy and carious teeth across the various wavelengths were measured leading to the determination of the degree of reliability of each of the fitted parameters, with most reliable being thermal diffusivity and conductivity, enamel thickness, and optical transport coefficient of the enamel layer. Quantitative pixel-by-pixel images of the key reliable optical and thermophysical parameters were constructed. We introduced a theoretical model of pulsed photothermal radiometry based on conduction-radiation theory and applied to quantitative photothermal detection and imaging of biomaterials. The theoretical model integrates a combination of inverse Fourier transformation techniques, avoiding the conventional cumbersome analytical Laplace transform method. Two dental samples were selected for analysis: the first sample featured controlled, artificially induced early caries on a healthy tooth surface, while the second sample exhibited natural defects along with an internal filling. Using an Nd:YAG laser and specific optical parametric oscillator (OPO) wavelengths (675, 700, 750, and 808nm), photothermal transient signals were captured from different points on these teeth and analyzed as a function of OPO wavelength. Measurements were also performed with an 808-nm laser diode for comparison with the same OPO wavelength excitation, particularly for the second sample with natural defects. The findings demonstrated that the photothermal transient signals exhibit a fast-decaying pattern at shorter wavelengths due to their higher scattering nature, while increased scattering and absorption in the carious regions masked conductive and radiative contributions from the underlayer. These observations were cross-validated using micro-computed tomography, which also enabled the examination of signal patterns at different tooth locations. The results of our study showed the impact of optical and thermal characteristics of two-layered turbid dental tissues via an inverse Fourier technique, as well as the interactions between these layers, on the patterns observed in depth profiles.

Single-frequency optical parametric oscillator intracavity-pumped by a visible VECSEL for low-noise down-conversion to 1.55 µm.

We report, to the best of our knowledge, the first optical parametric oscillator (OPO) pumped by a visible AlGaInP-based vertical-external-cavity surface-emitting laser (VECSEL). Tunable emission over 1155-1300 nm in the signal and 1474-1718 nm in the idler are observed by temperature adjustment of a 40 mm-long 5%-MgO:PPLN crystal intracavity-pumped at 690 nm. When optimized for low oscillation threshold, and by implementing resonant idler output-coupling (TOC = 1.7%), extracted output powers of 26.2 mW (signal) and 5.6 mW (idler; one-way) are measured, corresponding to a total down-conversion efficiency and extraction efficiency of 70.2% and 43%, respectively. Further, a total down-conversion efficiency of 72.1% is achieved in the absence of idler output-coupling. Of particular interest for high-precision applications, including quantum optics experiments and squeezed light generation, high stability and single-frequency operation are also demonstrated. We measure RMS stabilities of 0.4%, 1.8% and 2.3% for the VECSEL fundamental, signal and idler, with (resolution-limited) frequency linewidths of 2.5 MHz (VECSEL) and 7.5 MHz (signal and idler).

Continuous-wave pumped dispersion-compensated ultrashort-pulse doubly resonant oscillator studies

We have reported theoretical analysis of broadband phase-coherent spectral generation with stable short pulse output based on optical parametric oscillator (OPOs) pumped by low-cost single-mode continuous-wave (cw) laser considering the incorporation of mode-locking element in the cw dispersion-compensated OPO in doubly-resonant oscillator (DRO) configuration. Exploiting the control of dispersion in the cw mode-locked cavity, the intracavity round-trip phase shift, phase coherence among resonating waves, carrier envelope offset phase of mode-locked output and enhancement of efficiency of system using active mode-locking element driven at high modulation frequency has been studied. The study paves the way of realization of compact, low-cost, efficient ultrashort pulse OPO under cw pumping.

Residual optical absorption from native defects in CdSiP2 crystals

CdSiP2 crystals are used in optical parametric oscillators to produce tunable output in the mid-infrared. As expected, the performance of the OPOs is adversely affected by residual optical absorption from native defects that are unintentionally present in the crystals. Electron paramagnetic resonance (EPR) identifies these native defects. Singly ionized silicon vacancies (VSi−) are responsible for broad optical absorption bands peaking near 800, 1033, and 1907 nm. A fourth absorption band, peaking near 630 nm, does not involve silicon vacancies. Exposure to 1064 nm light when the temperature of the CdSiP2 crystal is near 80 K converts VSi− acceptors to their neutral and doubly ionized charge states (VSi0 and VSi2−, respectively) and greatly reduces the intensities of the three absorption bands. Subsequent warming to room temperature restores the singly ionized charge state of the silicon vacancies and brings back the absorption bands. Transitions responsible for the absorption bands are identified, and a mechanism that allows 1064 nm light to remove the singly ionized charge state of the silicon vacancies is proposed.

Narrowband, intracavity-pumped, type-II BaGa2GeSe6 optical parametric oscillator.

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