Optical Parametric Oscillator Threshold Research Articles

Anneal-free ultra-low loss silicon nitride integrated photonics

Heterogeneous and monolithic integration of the versatile low-loss silicon nitride platform with low-temperature materials such as silicon electronics and photonics, III–V compound semiconductors, lithium niobate, organics, and glasses has been inhibited by the need for high-temperature annealing as well as the need for different process flows for thin and thick waveguides. New techniques are needed to maintain the state-of-the-art losses, nonlinear properties, and CMOS-compatible processes while enabling this next generation of 3D silicon nitride integration. We report a significant advance in silicon nitride integrated photonics, demonstrating the lowest losses to date for an anneal-free process at a maximum temperature 250 °C, with the same deuterated silane based fabrication flow, for nitride and oxide, for an order of magnitude range in nitride thickness without requiring stress mitigation or polishing. We report record low anneal-free losses for both nitride core and oxide cladding, enabling 1.77 dB m-1 loss and 14.9 million Q for 80 nm nitride core waveguides, more than half an order magnitude lower loss than previously reported sub 300 °C process. For 800 nm-thick nitride, we achieve as good as 8.66 dB m−1 loss and 4.03 million Q, the highest reported Q for a low temperature processed resonator with equivalent device area, with a median of loss and Q of 13.9 dB m−1 and 2.59 million each respectively. We demonstrate laser stabilization with over 4 orders of magnitude frequency noise reduction using a thin nitride reference cavity, and using a thick nitride micro-resonator we demonstrate OPO, over two octave supercontinuum generation, and four-wave mixing and parametric gain with the lowest reported optical parametric oscillation threshold per unit resonator length. These results represent a significant step towards a uniform ultra-low loss silicon nitride homogeneous and heterogeneous platform for both thin and thick waveguides capable of linear and nonlinear photonic circuits and integration with low-temperature materials and processes.

Numerical analysis and validation of an optical parametric oscillator considering crystal thermal effects

In this paper, a calculation model is proposed for the optical parametric oscillation (OPO) process considering the crystal thermal effects. Based on existing models, we combine a set of three-wave coupled equations with the Sellmeier equation. In order to optimize the calculation of the nonlinear process, a temperature variable t is introduced to describe the heat generated by the laser crystal during operation. The waveforms under different pump powers are analyzed. The effects of the reflectivity of the output mirror on the OPO threshold and inverse conversion are investigated. In addition, the optimal reflectivity under different pump powers can be estimated. Based on the simulation results, experiments are also performed in the near-infrared 1.57 µm band and mid-infrared 3.15 µm band. The experimental results are compared with the results of this model and a model that does not consider crystal thermal effects. The experimental results are consistent with the improved theoretical results, affirming that the proposed theoretical model can simulate the energy conversion process of OPO. This provides a theoretical basis for optimizing the parameters of the OPO output mirror and improving the efficiency of the parametric wave conversion.

Low-threshold dual-wavelength CW mid-IR laser from shared intracavity quasi-phase-matched OPO.

A low-threshold continuous wave dual-wavelength mid-infrared laser is demonstrated by using an intracavity optical parametric oscillator (OPO) with a dual-wavelength pump wave. To realize a high-quality dual-wavelength pump wave, a composite Nd:YVO4/Nd:GdVO4 gain medium is applied for a linear polarized state and synchronization output. With the quasi-phase-matching OPO process, it is found that the dual-wavelength pump wave shares equal signal wave oscillation and leads to a lower OPO threshold. Finally, a diode threshold pumped power of barely 2 W can be achieved for the balanced intensity dual-wavelength watt-level mid-IR laser.

Nonlinear power dependence of the spectral properties of an optical parametric oscillator below threshold in the quantum regime

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

Low Threshold, Dual-Wavelength, Mid-Infrared Optical Parametric Oscillator

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.

Enhancement of second-harmonic generation based on the cascaded second- and third-order nonlinear processes in a multimode optical microcavity

Optical microcavities are often used to realize enhanced nonlinear optical interactions for highly efficient second-harmonic generation. With increased pump power, the efficiency of nonlinear frequency conversion can be increased further, while some other unwanted nonlinear effects will also emerge, leading to complicated dynamics or instability. Here, we study the interplay between cascaded second- and third-order nonlinear processes and investigate their impact on the second-harmonic generation in microcavities. It is found that the non-degenerate optical parametric oscillation (OPO) appears and the presence of $\chi^{(3)}$ process can modify the OPO threshold significantly when the multimode cavity is strongly pumped at the fundamental optical mode. One can even break the efficiency limitation of the second-harmonic mode restricted by the OPO by utilizing the interference between the OPO and the four-wave mixing. The present coherent interplay between nonlinear optical processes in microcavities is conducive to exploring new physics in the cavity nonlinear photonics.

Investigation of Adhesive-free bonded Potassium Titanyl Phosphate crystal based optical parametric oscillator for generation of 2.1 µm wavelength at high repetition rate

In this paper an application of newly available adhesive-free bonded Potassium Titanyl Phosphate crystal is reported. An optical parametric oscillator (OPO) based on a pair of adhesive-free bonded (AFB) Potassium Titanyl Phosphate (KTP) crystals in a walk-off compensated configuration has been demonstrated and investigated. A maximum average output power of 3 W has been achieved at a pulse repetition rate of 10 KHz using Nd:YVO4 as a pump source. The OPO generates near degenerate wavelengths at 2.127 µm and 2.131 µm with a small spectral separation of 4 nm. The results show a highly stable output with a slope efficiency of 23.6% and OPO threshold as low as 32 MW/cm2. The OPO output stability and threshold achieved is significantly better as compared to conventional KTP OPO.

Quantum correlations across two octaves from combined up- and down-conversion

We propose and analyse a cascaded optical parametric system which involves three interacting modes across two octaves of frequency difference. Our system, combining degenerate optical parametric oscillation (OPO) with second harmonic generation (SHG), promises to be a useful source of squeezed and entangled light at three differing frequencies. We show how changes in damping rates and the ratio of the two concurrent nonlinearities affect the quantum correlations in the output fields. We analyse the threshold behaviour, showing how the normal OPO threshold is changed by the addition of the SHG interactions. We also find that the inclusion of the OPO interaction removes the self-pulsing behaviour found in normal SHG. Finally, we show how the Einstein-Podolsky-Rosen correlations can be controlled by the injection of a coherent seed field at the lower frequency.

Low-loss silicon platform for broadband mid-infrared photonics

Broadband mid-infrared (mid-IR) spectroscopy applications could greatly benefit from today’s well-developed, highly scalable silicon photonics technology; however, this platform lacks broadband transparency because of its reliance on absorptive silicon dioxide cladding. Alternative cladding materials have been studied, but the challenge lies in decreasing losses while avoiding complex fabrication techniques. Here, in contrast to traditional assumptions, we show that silicon photonics can achieve low-loss propagation in the mid-IR from 3 to 6 μm wavelength, thus providing a highly scalable, well-developed technology in this spectral range. We engineer the waveguide cross-section and optical mode interaction with the absorptive cladding oxide to reduce loss at mid-IR wavelengths. We fabricate a microring resonator and measure an intrinsic quality (Q) factor of 106 at wavelengths from 3.5 to 3.8 μm. This is the highest Q demonstrated on an integrated mid-IR platform to date. With this high-Q silicon microresonator, we also demonstrate a low optical parametric oscillation threshold of 5.2 mW, illustrating the utility of this platform for nonlinear chip-scale applications in the mid-IR.

High-pulse-energy mid-infrared optical parametric oscillator based on BaGa4Se7 crystal pumped at 1.064 μm

We have demonstrated a high-pulse-energy nanosecond mid-infrared optical parametric oscillator (OPO) based on the nonlinear crystal BaGa4Se7 pumped by 1.064 µm Nd:YAG laser. The experimental OPO threshold of 7.97 MW/cm2 was in good agreement with the theoretical calculation of 8.05 MW/cm2. The maximum pulse energy of the idler wavelength was 2.56 mJ at 4.11 µm wavelength when the pump energy was 61.6 mJ, corresponding to an optical-to-optical conversion efficiency of 4.16%. The idler wavelength can be continuously tuned in the range from 3.12 to 5.16 µm.

Critical behavior of coherence and correlation of counterpropagating twin beams

This work analyzes the temporal coherence and correlation of twin beams generated in a quasi-phase-matched nonlinear crystal in a counterpropagating configuration, ranging from the low-gain regime, where counterpropagating photon pairs are generated spontaneously, to the regime of stimulated pair production, close to the mirrorless optical parametric oscillator threshold. Here, we show a critical divergence of the correlation time and slowing down of quantum fluctuations originating from the feedback mechanism responsible for the MOPO threshold.

High-efficiency CW all-fiber parametric oscillator tunable in 092-1 μm range

Continuous tuning over 80 nm is demonstrated for the anti-Stokes wave generated in an optical parametric oscillator (OPO) based on a birefringent photonic crystal fiber pumped by a CW Ytterbium-doped fiber laser tuned around 1.05 μm (within 4 nm only). An influence of the pump laser linewidth and polarization state on the fiber OPO threshold and efficiency is studied. Slope efficiency of parametric generation at 931 nm reaches 19% for ~0.1 nm linearly polarized pump with threshold power of ~5W. At that, the generated linewidth amounts to about 1 nm.

A continuous-wave optical parametric oscillator around 5-μm wavelength for high-resolution spectroscopy

We present a continuous-wave optical parametric oscillator (OPO) capable of high resolution spectroscopy at wavelengths between 4.8 μm and 5.4 μm. It is based on periodically poled lithium niobate (PPLN) and is singly resonant for the signal radiation around 1.35 μm. Because of the strong absorption of PPLN at wavelengths longer than 4.5 μm, the OPO threshold rises to the scale of several watts, while it produces idler powers of more than 1 mW and offers continuous tuning over 15 GHz. A supersonic jet spectrometer is used in combination with the OPO to perform measurements of the transient linear molecule Si(2)C(3) at 1968.2 cm(-1). Fifty rovibrational transition frequencies of the ν(3) antisymmetric stretching mode have been determined with an accuracy on the order of 10(-4) cm(-1), and molecular parameters for the ground and the v(3) = 1 state have been determined most precisely.

Low Threshold and High Conversion Efficiency Nanosecond Mid-Infrared KTA OPO

Based on a Type II non-critically phase-matched KTA crystal, a low-threshold and high conversion efficiency mid-infrared optical parametric oscillator (OPO) pumped by a diode-end-pumped Nd:YVO4 laser is demonstrated. The OPO threshold is only 0.825 W. The maximum output power of 435mW at 3.47 μm is achieved with the repetition rate of 30 kHz, corresponding to an optical-to-optical conversion efficiency of 4.4%. The photon conversion efficiency is as high as about 64%. The pulse width is 3.5 ns with a peak power of 4kW for the maximum output power.

Backward three-wave optical solitons

Backward symbiotic solitary waves in quadratic media with absorption losses are generated through the nonlinear non-degenerate three-wave interaction. We study these solitary waves in the particular case of a doubly backward quasi-phase matching configuration. The same mechanism responsible for nanosecond solitary wave morphogenesis in the c.w. pumped Brillouin-fiber-ring laser may act for picosecond pulse generation in a quadratic c.w. pumped optical parametric oscillator (OPO). The resonant condition is automatically satisfied in stimulated Brillouin backscattering when the fiber-ring laser contains a large number of longitudinal modes beneath the gain curve. However, in order to achieve quasi-phase matching between the three optical waves in the Χ(2) medium, a nonlinear susceptibility inversion grating of sub-µm period is required. Such a quadratic medium supports solitary waves that result from energy exchanges between dispersionless waves of different velocities. We show, by a stability analysis of the non-degenerate backward OPO in the QPM decay interaction between a c.w. pump and backward signal and idler waves that the inhomogeneous stationary solution exhibits a Hopf bifurcation with a single control parameter. Above OPO threshold, the nonlinear dynamics yields self-structuration of a backward symbiotic solitary wave, which is stable for a finite temporal walk-off (i.e. different group velocities) between signal and idler waves. We also study the dynamics of singly backward mirrorless OPO’s (BMOPO’s) pumped by an incoherent field, in line with the recent experimental demonstration of this OPO configuration. We show that this system is characterized, as a general rule, by the generation of a highly coherent backward field, despite the high degree of incoherence of the pump field. This remarkable property finds its origin in two distinct phase-locking mechanisms that originate respectively in the convection and the dispersion properties of the fields. In both cases we show that the incoherence of the pump is transferred to the co-moving field, which thus allows the backward field to evolve towards a highly coherent state. We propose realistic experimental conditions that may be implemented with currently available technology and in which backward coherent wave generation from incoherent excitation may be observed and studied.

Observation of Long-Lived Polariton States in Semiconductor Microcavities across the Parametric Threshold

The excitation spectrum around the pump-only stationary state of a polariton optical parametric oscillator in semiconductor microcavities is investigated by time-resolved photoluminescence. The response to a weak pulsed perturbation in the vicinity of the idler mode is directly related to the lifetime of the elementary excitations. A dramatic increase of the lifetime is observed for a pump intensity approaching and exceeding the optical parametric oscillator threshold. The observations can be explained in terms of a critical slowing down of the dynamics upon approaching the threshold and the following appearance of a soft Goldstone mode in the spectrum.

GaAs optical parametric oscillator with circularly polarized and depolarized pump

We demonstrate an optical parametric oscillator (OPO) based on GaAs pumped with linearly polarized and circularly polarized light and show that the relative OPO thresholds agree with theoretical expectations. For the circularly polarized pump, the threshold was as low as for the [111]-linearly polarized pump case. The pump was also passed through a Lyot depolarizer to produce pseudo-depolarized light, and the OPO threshold in this case was only 22% higher than that for [001]-linearly polarized pump.

Low threshold characteristic of pulsed confocal unstable optical parametric oscillators with Gaussian reflectivity mirrors

An iterative threshold model for a pulsed singly resonant Gaussian-reflectivity-mirror (GRM) confocal unstable optical parametric oscillator (OPO) has been proposed. It is found that OPO threshold is determined by important parameters such as GRM central reflectance, Gaussian reflectivity profile, cavity magnification factor, cavity physical length, crystal length, pump pulsewidth. It is demonstrated that this model can be extended to plane-parallel resonator or uniform-reflectivity-mirror (URM) unstable resonator when some specific values are taken. Experimental results show excellent agreement with values calculated from theoretical model. Both theoretical calculations and experimental data illustrate that GRM is a useful solution to reduce threshold of unstable OPO.

Experimental investigation and numerical simulation of the spatio-temporal dynamics of the light-pulses in nanosecond optical parametric oscillators

This paper reports on the experimental investigation and the numerical simulation of the spatio-temporal dynamics of the pulse formation in nanosecond optical parametric oscillators. The temporal evolution of the spatial intensity distribution is measured with a fast two-dimensional CCD-camera. The measurements are performed for a signal-resonant nanosecond pulsed optical-parametric-oscillator (OPO) which consists of a 12 mm long, critically phase-matched beta-barium-borate (BBO)-crystal in a 40 mm long cavity of two plane mirrors. The OPO was pumped by the third harmonic of a flashlamp-pumped Nd:YAG-Laser at a repetition rate of 10 Hz. At pump energies close to the OPO threshold the emitted OPO-radiation has an almost gaussian intensity distribution throughout the entire pulse. The beam quality factor M2 remains below 2.2. At high pump energies the OPO oscillation also starts with an almost gaussian beam-profile. During the build-up and parametric amplification of the pulse the fields experience, however, a spatially inhomogeneous gain, caused by walk-off in the birefringent crystal, pump-depletion, and back-conversion. The spatial intensity distribution thus becomes asymmetric and the M2 value increases. The measurements are compared with the results of detailed numerical calculations. The model takes the amplification of the OPO-radiation in the nonlinear crystal, and the properties of the OPO cavity as well as the diffraction of the beams during propagation into account. The spatio-temporal dynamics of the pulse formation predicted by the numerical model are in good agreement with the experimental results.

Mid-infrared generation based on a periodically poled LiNbO 3 optical parametric oscillator

We report a nanosecond Nd:YVO4-pumped optical parametric oscillator (OPO) based on periodically poled LiNbO3 (PPLN). Tuning is achieved in this experiment by varying the temperature and period of the PPLN. The design of double-pass singly resonant oscillator (DSRO) and confocal cavity enables the OPO threshold to be lowered considerably, resulting in a simple, compact, all-solid-state configuration with the mid-infrared idler powers of up to 466mW at 3.41μm.