Quasi-Phase-Matched Four-Wave Mixing Enabled by Grating-Assisted Coupling in a Hybrid Silicon Waveguide

We propose the idea of using a grating structure for solving the problem of the incomplete quasi-phase matching (QPM) period and demonstrate the simulation results of phase compensation in achieving efficient round-trip optical parametric oscillation in periodically poled LiNbO/sub 3/ (PPLN). The layout of a monolithic optical parametric oscillator (OPO) with two surface gratings at both ends is shown. The two gratings are chirped to guarantee that their phase variations over the Bragg reflection windows are larger than 2/spl pi/. One of them has a length of 1 mm and the other has a length of 0.8 mm. The pump input is applied from the long grating side and the signal output is obtained from the short grating side. The whole PPLN length is 5 mm. For our numerical study, we consider the pump wavelength at 1064 nm and signal wavelength at 1550 nm. The idler wavelength is hence at 3.393 /spl mu/m. The corresponding QPM period of PPLN is 29.6 /spl mu/m. The central grating period is 361.83 nm, which corresponds to the Bragg wavelength 1550 nm. To obtain numerical results, we solved the coupled equations for the forward and backward pump complex amplitudes, the forward and backward signal complex amplitudes, and the forward and backward idler complex amplitudes.

The evolution characteristics of the degenerate four-wave mixing (DFWM) effect in the proposed tellurite photonic crystal fiber (PCF) are investigated systematically. The influence of the pitch distance and diameter-to-pitch ratio on the phase-matching condition is analyzed numerically. The effects of fiber length, signal wavelength, idler wavelength, initial pump power and initial signal power on the signal gain and idler conversion efficiency are investigated. Based on the DFWM occurring in the proposed tellurite PCF, the signal (anti-Stokes) wave is efficiently amplified and the idler (Stokes) wave is efficiently generated at the mid-infrared (mid-IR) waveband with the pump wavelength of 1.553 μm. The signal gain can be increased by increasing the initial pump power. The idler conversion efficiency can reach ~ 26%. The simulated results will be instructive for the experimental FWM process to generate mid-IR optical parametric oscillators.

The influence of the core diameter and pump wavelength on the phase-matching conditions when different signal vector beams are injected is analyzed numerically in a suspended-core ${{\rm As}_2}{{\rm Se}_3}$As2Se3 microstructured optical fiber (MOF). The idler wavelengths satisfying the phase-matching conditions with different signal vector beams can reach wavelengths of 4.8–5.9 µm in the ${{\rm As}_2}{{\rm Se}_3}$As2Se3 MOFs with different core diameters. By changing the signal vector mode field and adjusting the corresponding signal wavelength, the idler wavelength can be tuned in 754.8 nm from 4.9182 to 5.6730 µm with the pump wavelength changing from 2.80 to 2.90 µm. The influence on signal gain and the idler conversion efficiency is calculated in the ${{\rm As}_2}{{\rm Se}_3}$As2Se3 MOFs with different lengths when injecting different signal vector beams. The peak gain of signal and idler conversion efficiency can reach $\sim{28}\,\, {\rm dB}$∼28dB and $\sim{24}\% $∼24%, respectively, when the different signal vector beams are injected. The simulated results demonstrate that the optical parametric oscillation with wavelengths longer than 5 µm can be realized in chalcogenide fibers with signal vector beam injection.

A multiwavelength midinfrared quasi-phase-matched (QPM) difference frequency generation (DFG) scheme is proposed based on a segmented temperature controlling technique for the uniform grating periodically poled lithium niobate (PPLN). The QPM DFG output performances have been investigated under the fixed pump and signal wavelength conditions, respectively. Our theoretical results show that when the pump wavelength is fixed at 1.08 μm, three idler QPM bands, located at 3.46, 3.50, and 3.57 μm, have been obtained, where the temperatures of the three PPLN segments with the same length are set at 20°C, 70°C, and 100°C. However, when the signal wavelength is fixed at 1.58 μm, six idler QPM bands, located at 2.95, 3.06, 3.14, 3.63, 3.72, and 3.83 μm, have been achieved with the same crystal temperature distribution of 20°C, 70°C, and 100°C.

The output characteristics of light in two parametric generation regimes [parametric superluminescence and parametric generation in a singly resonant optical parametric oscillator (OPO) based on a 50-mm long PPLN crystal, synchronously pumped by a 50-ps Nd : YAG laser] are compared. The total average output power at the wavelengths of signal (λ = 1475 nm) and idler (λ = 3820 nm) waves in the superluminescence regime is ∼120 mW (peak power ∼120 kW). The maximum total conversion efficiency with respect to the absorbed power in the superluminescence regime is ∼60%. The maximum conversion efficiency into the idler wave with respect to the absorbed pump power is 15% and 25% (quantum efficiencies of 54% and 90%) for the superluminescence and singly resonant OPO regimes, respectively. The maximum values of the average output power at the idler wavelength are ∼30 and ∼60 mW at degrees of pump depletion of 47% and 55% for the superluminescence and singly resonant OPO regimes, respectively.

High average output power femtosecond sources operating across the 1.5 µm telecommunication band are restricted mainly to colour-centre lasers, Kerr-lens modelocked Cr+4:YAG oscillators and optical parametric oscillators (OPOs). The need for cryogenic conditions limits the versatility of colour-centre-based souces and practical difficulties have made self modelocked operation of Cr−4:YAG lasers near the 1.4 µm water absorption band very difficult to sustain. In contrast, the OPO that we describe in this paper operates at room temperature and is continuouslv-tunable over 200 nm from 1.375 to 1.575 µm using only a single cavity mirror set. The OPO is based on periodically poled RbTiOAsO4 (PPRTA)1. The crystal used had a grating period of 30 µm and dimensions 5 mm long × 0.5 mm width × 5 mm high2. The crystal was antireflection coated for a wavelength of 1.1 µm and we estimate that the loss of each surface at a wavelength of 1.5 µm to be ~1 %. The OPO was synchronously pumped by a modelocked Ti:sapphire laser at a repetition rate of 84 MHz. with output pulses of 100 fs duration and centred at a wavelength of 850 nm. We used a 90 mm focal-length lens to focus the pump beam in to the PPRTA crystal. The polarisation of the pump, signal and idler wavelengths were all horizontal in the cavity and parallel to the poling direction which lay along the crystalographic c axis. The dispersion-compensated OPO cavity consisted of two SF14 prisms, two curved mirrors (r = 100 mm) and one flat mirror as the output coupler. All the mirrors were high-reflecting at the central wavelength of 1.5 µm.

Biphotons from spontaneous parametric down conversion with broad bandwidth are highly wanted in many quantum technologies. However, achieving broad bandwidth in both frequency and momentum while keeping a high rate remains a challenge for both conventional nonlinear crystals and recently emerging nonlinear metasurfaces. Here, we address this challenge by introducing a periodically poled Brillouin zone folding (PP‐BZF) metasurface incorporating a periodically perturbed silica grating atop a periodically poled lithium niobate film. PP‐BZF metasurfaces support high‐Q guided mode resonances at pump, signal and idler wavelengths while enabling quasi‐phase matching between three guided modes in a broad frequency/momentum range. The predicted biphoton rate is over 100 MHz/mW with a frequency bandwidth of 20.5 THz (1472.5–1637.8 nm) and a momentum bandwidth of , improving the state‐of‐the‐art for metasurface biphoton sources by over three orders of magnitude in rate and one order of magnitude in bandwidths. This ultrathin broadband bright biphoton source could stimulate system‐level miniaturization of various free‐space quantum photonic technologies.

This article shows a dual-frequency OPO with multi-grating (28.5-31.5 μm) periodically poled MgO:LiNbO₃ (MgO:PPLN) pumped by a dual-frequency continuous wave at 1.064 μm. The wavelengths of idler and signal varying versus temperature at different periods of inverted domains were numerical simulated. It proves that as the temperature rises, or as the poling period increases, the idler wavelength shortens and signal wavelength lengthens. The pump is a 30 W dual-frequency fiber laser MOPA with beat note frequency varying from 125 MHz to 175 MHz. The pump threshold of the bow-tie ring cavity OPO was 3 W. An average dual-frequency idler output power of 2.6 W was obtained when the pump power was 17.2 W at 45 °C. The idler wavelength was 3.4 μm when the poling period was 30.5 μm. The idler wavelength could be tuned from 2.9 μm to 3.9 μm by changing the temperature and the poling period, and the beat note frequency was proved to be equal to that of the pump.

The parametric generation in a nonlinear PPLN crystal synchronously pumped by a -switched mode-locked Nd : YAG laser with a pulse duration of is studied. The output pump intensity in the nonlinear crystal reaches . At a pulse repetition rate of , the average output power at the idler wavelength is , the peak power is , and the conversion efficiency (with respect to the absorbed power) is . The radiation linewidth at the signal wavelength is . Along with the signal and idler waves, the output emission spectrum contains lines at wavelengths of 822, 754, 624, 532, 463, 442, 392 and . The tunable radiation with wavelengths in the vicinity of 392, 463 and is observed for the first time. The tuning ranges for the new lines are measured and their origin is explained.

We experimentally demonstrate on-chip all-optical wavelength conversion of 10-Gbit/s (9.35-Gbit/s net rate) 4-level pulse amplitude modulation (PAM-4) signal by exploiting degenerate four-wave mixing (FWM) in a silicon waveguide. The measured optical signal-to-noise ratio (OSNR) penalty of wavelength conversion is ~1 dB at a bit-error rate (BER) of 2 × 10-3. Moreover, the use of wavelength conversion for PAM-4 signal regeneration is also demonstrated in the experiment.

Nonlinear Cherenkov radiations (NCR) have attracted increasing research interest in the past years. Similar to the traditional Cherenkov radiation effect in particle physics, NCR requires that the nonlinear polarization wave propagates faster than the harmonic waves. Cherenkov frequency up-conversions can be automatically realized in normal dispersion materials and extensive work on Cherenkov second-harmonic generation, sum-frequency generation and high-order harmonic generation have been reported in both bulk and waveguide systems. However, Cherenkov frequency down-conversion process is still a challenge for the restriction of phase velocity threshold. Recently, we have proposed two schemes to realize the degenerated Cherenkov difference-frequency generation (CDFG) by modulating the phase velocity of the nonlinear polarization wave using both quasi-phase-matching (QPM) and birefringence-phasematching (BPM) respectively. For the first scheme, we used the QPM method. The reciprocal vectors provided by the domain engineered structure can modulate the phase velocity of the nonlinear polarization wave, and particularly, the backward vectors can accelerate the nonlinear polarization wave. If the backward reciprocal vector is large enough, Cherenkov DFG can be realized. In experiment, a proton exchanged LiTaO 3 planar waveguide with a poling period of 4.5 µm was used and the wavelengths of pump and signal were 532 and 1064nm respectively. At a radiation angle close to the theoretical one, the infrared CDFG spot can be observed with a CCD. For the second scheme, the birefringence property of KTP was utilized. For birefringent crystal, light with different polarization has different refractive index. By proper choice of the polarization states of the interacting waves, the refractive index of the signal wave can be larger than that of the pump wave, thus the phase velocity of the nonlinear polarization wave of the DFG process will exceed that of the harmonic wave. In experiment, we set the polarization of 532-nm pump and the 1064-nm signal waves along x and z axis of KTP respectively. CDFG was realized and some cascaded Cherenkov processes were observed as well. The method based on BPM can reduce the requirements for fabricating short-period domain structures and can be extended to mode-matching in waveguides for realizing nonlinear Cherenkov radiations.

Here we present an analysis of a fully planar optical sensor based on ARROW waveguides. We consider a device geometry that consists of two intersecting ARROW waveguides, one with solid and one with liquid core. The pump wavelength is input from the solid core waveguide and penetrates through the wall into the liquid core of the other waveguide where the fluorescence is excited in the sample material. Then it is captured by the ARROW waveguide and guided to a detector at the end. Pump and signal wavelength can be guided with low loss through the solid and hollow core respectively. At the same time, high loss discrimination inside the core can be obtained by tailoring the thickness of ARROW layers, leading to efficient filtering. The pump wave can be transmitted efficiently in and out of the core, allowing for multiple intersections. In the following experimental part, we investigate the waveguide loss of the liquid core as a function of core width, we measure values as low as 1.7cm-1 that can be further reduced by improving the thickness control of ARROW layers in the fabrication process. Finally, we report fluorescence experiments on Alexa dye molecules. We demonstrate fluorescence detection at concentrations as low as 10-8 mol/l from a detection volume of 21pl. Photo-bleaching is observed and discussed as a function of input power intensity.

We exploit nondegenerate four-wave mixing to generate narrow-bandwidth visible light from blue to orange in microstructured optical fiber. The process is excited by nanosecond dual-wavelength pump pulses (532/1064 nm) produced by a frequency-doubled passively Q-switched Nd:YAG laser. A linearly polarized, fundamental mode signal wave is generated in the visible, accompanied by an idler wave in the near-infrared. The signal and idler wavelengths are adjustable by the design of the microstructure.

High-speed and random-access tuning of output wavelengths was achieved in an optical parametric oscillator (OPO) pumped with an electronically tuned Ti:sapphire laser, which was fully under computer control. The signal and idler wavelengths in 90° phase-matched KTiOPO4 (KTP) were scanned without mechanical rotation of crystals or mirrors. Signal waves in the range from 1.06 to 1.31 μm and idler waves in the range from 2.27 to 2.97 μm were accessed in random order by setting the pumping wavelengths between 713 and 923 nm.

All-optical wavelength conversion between ps-pulses based on cascaded sum- and difference frequency generation (SFG+DFG) is proposed and experimentally demonstrated in periodically poled LiNbO3 (PPLN) waveguides. The signal pulse with 40-GHz repetition rate and 1.57- ps pulse width is adopted. The converted idler wavelength can be tuned from 1527.4 to 1540.5nm as the signal wavelength is varied from 1561.9 to 1548.4nm. No obvious changes of the pulse shape and width, also no chirp are observed in the converted idler pulse. The results imply that single-to-multiple channel wavelength conversions can be achieved by appropriately tuning the two pump wavelengths.