Reflective Cavity Research Articles

High-efficiency PPMgLN-based mid-infrared optical parametric oscillator pumped by a MOPA-structured fiber laser with long pulse duration

We report our recent investigation on the conversion efficiency improvement of a PPMgLN-based single pump pass, singly resonant optical parametric oscillator (OPO) pumped by an acousto-optic Q-switched fiber MOPA. The impacts of the pump pulse duration and signal reflectivity on the pump-to-idler conversion efficiency were studied by numerically solving the coupled wave equations in the first place. The results revealed that longer pulse durations were beneficial to higher conversion efficiencies as long as the signal reflectivity of the OPO cavity was optimized accordingly. Experiments were carried out thereafter utilizing the optimal parameters obtained from the simulation. Idler powers of 4.7 and 3.81 W were achieved at 3.4 and 3.8 μm, respectively, under the highest pump power of 28 W with pump pulse duration of 240 ns. The experimental results were in good agreement with the calculated results. According to our simulation, higher conversion efficiency could be expected when such an OPO was pumped by pulses with even longer duration provided that the signal reflectivity of the output coupler was optimized under that pump condition.

Single-step fabrication process of 1-D photonic crystals coupled to nanocolumnar TiO2 layers to improve DSC efficiency.

The present work proposes the use of a TiO2 electrode coupled to a one-dimensional photonic crystal (1DPC), all formed by the sequential deposition of nanocolumnar thin films by physical vapor oblique angle deposition (PV-OAD), to enhance the optical and electrical performance of DSCs while transparency is preserved. We demonstrate that this approach allows building an architecture combining a non-dispersive 3 µm of TiO2 electrode and 1 µm TiO2-SiO2 1DPC, both columnar, in a single-step process. The incorporation of the photonic structure is responsible for a rise of 30% in photovoltaic efficiency, as compared with a transparent cell with a single TiO2 electrode. Detailed analysis of the spectral dependence of the photocurrent demonstrates that the 1DPC improves light harvesting efficiency by both back reflection and optical cavity modes confinement within the TiO2 films, thus increasing the overall performance of the cell.

Terahertz narrow-band filter based on rectangle photonic crystal

A terahertz filter with a channel drop cavity and a resonant reflection cavity in a two-dimensional photonic crystal is theoretically proposed. The channel drop cavity is used to trap photons at resonant frequency from the bus waveguide through coupling and emit them to a drop waveguide, while the resonant reflection cavity is used to realize wavelength selective reflection feedback in the bus waveguide. The transmission properties of the terahertz filter are simulated using the finite element method. It is found that a peak with the central frequency of 1.12 THz is existed in the transmission spectrum. The full width at half maximum of the passband is only 5 GHz, and the peak drop efficiency is up to 94.8%.

Determining the average path length of amplified spontaneous emission in a four-level laser near the 1/3 mode-degeneracy cavity configurations

We determine the average path length ls of amplified spontaneous emission (ASE) by comparing the numerical slope of a straight line with the experimental slope in the graph of the square of relaxation oscillation frequency versus normalized pump ratio. The simple method is applied in an end-pumped Nd:YVO4 laser with the 1/3 mode-degeneracy cavity having the transverse mode spacing equal to 1/3 of the longitudinal mode spacing. We find that ls is larger at the degeneracy than that far from the degeneracy. This result indicates the existence of stronger ASE at the degeneracy, which is confirmed below the threshold. This is because many spontaneous emission photons can undergo amplification not only before escaping from the gain medium but also after leaving the gain medium, owing to cavity reflection. Our method can be applied in the situations where the Auger upconversion effect is absent, weak, or well-known.

Design and fabrication of a foldable 3D silicon based package for solid state lighting applications

Miniaturization of solid state lighting (SSL) luminaires as well as reduction of packaging and assembly costs are of prime interest for the SSL lighting industry. A novel silicon based LED package for lighting applications is presented in this paper. The proposed design consists of 5 rigid Si tiles connected by flexible polyimide hinges with embedded interconnects (ICs). Electrical, optical and thermal characteristics were taken into consideration during design. The fabrication process involved polyimide (PI) application and patterning, aluminium interconnect integration in the flexible hinge, LED reflector cavity formation and metalization followed by through wafer DRIE etching for chip formation and release. A method to connect chip front to backside without TSVs was also integrated into the process. Post-fabrication wafer level assembly included LED mounting and wirebond, phosphor-based colour conversion and silicone encapsulation. The package formation was finalized by vacuum assisted wrapping around an assembly structure to form a 3D geometry, which is beneficial for omnidirectional lighting. Bending tests were performed on the flexible ICs and optical performance at different temperatures was evaluated. It is suggested that 3D packages can be expanded to platforms for miniaturized luminaire applications by combining monolithic silicon integration and system-in-package (SiP) technologies.

High-speed electro-optic modulator integrated with graphene-boron nitride heterostructure and photonic crystal nanocavity.

Nanoscale and power-efficient electro-optic (EO) modulators are essential components for optical interconnects that are beginning to replace electrical wiring for intra- and interchip communications.1-4 Silicon-based EO modulators show sufficient figures of merits regarding device footprint, speed, power consumption, and modulation depth.5-11 However, the weak electro-optic effect of silicon still sets a technical bottleneck for these devices, motivating the development of modulators based on new materials. Graphene, a two-dimensional carbon allotrope, has emerged as an alternative active material for optoelectronic applications owing to its exceptional optical and electronic properties.12-14 Here, we demonstrate a high-speed graphene electro-optic modulator based on a graphene-boron nitride (BN) heterostructure integrated with a silicon photonic crystal nanocavity. Strongly enhanced light-matter interaction of graphene in a submicron cavity enables efficient electrical tuning of the cavity reflection. We observe a modulation depth of 3.2 dB and a cutoff frequency of 1.2 GHz.

基于纳米颗粒溶液散射的激光平场系统

To meet the demand of the high precision correction requirements of an array detector for photo-electronic response errors, a laser flat-field system based on diffuse scattering effect of nanoparticle solution was established to generate a reference light field with a high flatness. The structure and principle of the flat-field system were introduced and some key techniques involved in the design of optical system were also given. An optical fiber was used to deliver the laser into solution efficiently to reduce the backscattering light loss on the interface. By optimizing the fiber's position and structure parameters of the cavity, the utilization of light energy was improved with a stable light field uniformity. Using improved Monte Carlo method, the effect of diffuse reflector cavity on system transmission was studied numerically. Finally, a laser flat field system was established by using Polytetrafluoroethylene (PTFE) materials with high reflectivity. Results show that the optimal position of the fiber depends on the cavity reflectivity. For high reflectance material, the optic port of the fiber near the cavity back can effectively enhance forward scattering. The laser flat field system provides the light field with a nonuniformity better than 0.3%, which basically meets the requirements of precision instruments for detector error correction, such as a long trace profiler. ©, 2015, Chinese Academy of Sciences. All right reserved.

Observation of Kerosene Injected from a Cavity into a Supersonic Airstream

Kerosene-PLIF and high-speed schiliren are used to visualize flow-field of kerosene injected from a single injector at low plate and cavity walls. PLIF and high-speed schliren images are obtained. The results show that vortex or three-dimensional surface waves appear along boundary of fuel jet plume. With increasing dynamic ratio qj, start point of surface waves move upwards to the injector. In this paper, flapping flows at rear ramp of a cavity lead to oscillation of shock waves generated by shear layer upwards the cavity and separation bubble near shock reflection point on upper walls. Wavy rear ramp of cavity reduces shock strength and separation bubble disappears. PLIF images provide accurate distribution of fuel mixing and penetration depth in contrast to high-speed photography and schliren.

Structural, Raman spectroscopic and microwave dielectric studies on Ni1-x(Zn1/2Zr1/2)xW1-xNbxO4 ceramic compounds with wolframite structure.

Ni1-x(Zn1/2Zr1/2)xW1-xNbxO4 (x = 0.0-1.0) compositions were synthesized via conventional solid-state reaction method. Structural and lattice vibrational characteristics of these compositions were studied with the help of powder X-ray diffraction and Raman spectroscopic measurements. Rietveld refinements confirm the formation of all these compositions in monoclinic wolframite structure with P2/c space group. When moving towards Ni(2+)-poor compositions, splitting in the X-ray reflections was observed and is explained with lattice parameter variation. With increasing value of x, Raman spectra show two additional Raman active modes and the possible reasons for observing these modes are discussed in terms of electronegativity difference of the randomly distributed cations in B-site of these compositions. X-ray photoelectron spectroscopic measurements were done to understand the chemical bonding states of different elements in these compositions. Surface morphology studies reveal that the average grain size increases with increasing x. Microwave dielectric properties such as dielectric constant and quality factor were measured using Hakki-Coleman and reflection cavity techniques and enhancement in these values with x is correlated with intrinsic parameters such as polarizability and 3d electrons present in the constituent ions of these compositions. Temperature coefficient of resonant frequency was measured using an invar cavity attached to a programmable hot plate and is explained with B-site octahedral distortion of these compositions. Well dense Ni1-x(Zn1/2Zr1/2)xW1-xNbxO4 (x = 0.0-1.0) compositions possess good microwave dielectric properties.

Structural phase transformation and microwave dielectric studies of SmNb(1-x)(Si(1/2)Mo(1/2))(x)O(4) compounds with fergusonite structure.

Temperature- and composition-induced phase transition in SmNbO4 was studied by differential scanning calorimetry, Raman spectroscopy and high-temperature powder X-ray diffraction measurements. In situ X-ray diffraction studies revealed that SmNbO4 possesses a monoclinic fergusonite crystal structure at ambient temperature and transforms to a tetragonal scheelite structure above the transition temperature (To ≥ 800 °C). The second-order nature of this transition was confirmed by observing a linear relationship between the spontaneous strain (es) of SmNbO4 and the Landau order parameter (η) around the phase transition temperature. We stabilized this high-temperature tetragonal scheelite phase at ambient temperature by substituting Si(4+) and Mo(6+) into the Nb site of SmNbO4. The SmNb1-x(Si1/2Mo1/2)xO4 (x = 0.0-0.69) ceramic compositions were prepared by the conventional solid-state reaction method. Rietveld refinement was carried out on all the compositions to examine the phase purity, and the compositions where x < 0.06 all formed a monoclinic fergusonite structure (I2/a space group, Z = 2). Both the X-ray diffraction and Raman spectroscopy measurements revealed that increasing the concentration of x transformed the structure from monoclinic fergusonite to tetragonal scheelite (I41/a space group, Z = 4) at a critical concentration (xc). Both the monoclinic and tetragonal phases coexisted in the composition range of 0.06 ≤ x < xc. The Hakki-Coleman and reflection cavity techniques were used to measure the dielectric constant and quality factor of these stabilized phases, respectively. The temperature coefficient of the resonant frequency was measured by using an invar cavity attached to a programmable hot plate. The high-density samples possessed good microwave dielectric properties.

Light source system for high-precision flat-field correction and the calibration of an array detector

Signal distortion due to the non-uniform response of the detector degrades the measurement accuracy of most metrology instruments. In this Letter, we report a newly developed calibration source system for reference-based non-uniformity correction using a laser source, a fiber, and a diffusive module. By applying the Monte Carlo simulation, we show that the transmittance of the system highly depends on the cavity reflection of the diffusive module. We also demonstrate the use of this system to achieve a flat field at a very low non-uniformity (less than 0.2%) with proper illumination intensity, which most costly commercial integrating sphere systems traditionally cannot provide.

All-periodically poled, high-power, continuous-wave, single-frequency tunable UV source.

We report on experimental demonstration of an all-periodically poled, continuous-wave (CW), high-power, single-frequency, ultra-violet (UV) source. Based on internal second-harmonic-generation (SHG) of a CW singly resonant optical parametric oscillator (OPO) pumped in the green, the UV source provides tunable radiation across 398.94-417.08 nm. The compact source comprising of a 25-mm-long MgO-doped periodically poled stoichiometric lithium tantalate (MgO:sPPLT) crystal of period Λ(SLT)=8.5 μm for OPO and a 5-mm-long, multi-grating (Λ(KTP)=3.3, 3.4, 3.6 and 3.8 μm), periodically poled potassium titanium phosphate (PPKTP) for intra-cavity SHG, provides as much as 336 mW of UV power at 398.94 nm, corresponding to a green-to-UV conversion efficiency of ∼6.7%. In addition, the singly resonant OPO (SRO) provides 840 mW of idler at 1541.61 nm and substantial signal power of 108 mW at 812.33 nm transmitted through the high reflective cavity mirrors. UV source provides single-frequency radiation with instantaneous line-width of ∼18.3 MHz and power >100 mW in Gaussian beam profile (ellipticity >92%) across the entire tuning range. Access to lower UV wavelengths requires smaller grating periods to compensate high phase-mismatch resulting from high material dispersion in the UV wavelength range. Additionally, we have measured the normalized temperature and spectral acceptance bandwidth of PPKTP crystal in the UV wavelength range to be ∼2.25°C·cm and ∼0.15 nm·cm, respectively.

Stable high-energy Q-switched resonantly diode-pumped Er:YAG laser at 1645 nm

We demonstrate the generation of a stable high-energy Q-switched resonantly diode-pumped Er:YAG laser at 1645nm in passive and active operation modes. For the passively Q-switched Er:YAG laser, the bi-layer graphene saturable absorber (SA), which was transferred onto the highly reflective cavity mirror by an improved method, was applied to deliver a stable pulse train with per-pulse energy of about 0.1mJ. While in the active operation mode, the voltage-ON-type rubidium titanyl phosphate (RTP) Pockels cell as an electro-optic (EO) Q-switcher allows for the generation of a stable Q-switched pulse with per-pulse energy of up to 7.5mJ. Our work may provide a basis for the development of a high-energy Er:YAG laser at 1645nm to fulfill specific applications.

Reflective vertical cavity quantum-well saturable absorber as an all-optical nonlinear phase-shifting element

A scheme of an all-optical nonlinear phase-shifting element is proposed, based on a reflective vertical cavity semiconductor (quantum-well) saturable absorber (R-VCSSA). The nonlinear round-trip phase shift in a low-intensity resonant VCSSA is obtained at an input intensity due to the saturating nonlinearity in index change and optically induced thermal effects. An enhanced large nonlinear positive or negative phase shift incurred in a reflected signal is investigated analytically with the wavelength tuning of the probe signal around the low-intensity resonant wavelength of a Fabry&#x2013;Perot cavity. The device may be useful in compensating the phase of an input signal, either positive or negative, in long-haul fiber communications. A representative InGaAs/InP quantum-well-based VCSSA is considered to demonstrate the phase shift in the pump&#x2013;probe configuration. With a device speed of 10&#xA0;GHz, maximum phase shifts of &#x2212;0.24 and +0.14&#x2009;&#x2009;rad at pump power of 19.2&#xA0;mW are observed for the operations at probe wavelengths of 1555.6 and 1557.8&#xA0;nm, respectively.

Auto-tuning systems for J-PARC LINAC RF cavities

The 400-MeV proton linear accelerator (LINAC) at the Japan Proton Accelerator Research Complex (J-PARC) consists of 324-MHz low-β and 972-MHz high-β accelerator sections. From October 2006 to May 2013, only the 324-MHz low-β accelerator section was in operation. From the summer of 2013 the J-PARC LINAC was upgraded by installing the 972-MHz high-β accelerator section, and the proton beam was successfully accelerated to 400MeV in January 2014. Auto-tuning systems for the J-PARC LINAC RF cavities have been successfully developed. A first generation design, an auto-tuning system using a mechanical tuner controller, was developed and operated for the first 3 years. Then the second-generation auto-tuning system was developed using a new approach to the RF cavity warm-up process, and this was applied to the accelerator operation for the subsequent 4 years. During the RF cavity warm-up process in this system, the mechanical tuner is constantly fixed and the input RF frequency is automatically tuned to the cavity resonance frequency using the FPGA (Field-Programmable Gate Array) of the digital feedback RF control system. After the input power level reaches the required value, input RF frequency tuning is stopped and it is switched to the operation frequency. Then, the mechanical tuner control begins operation. This second-generation auto-tuning system was extremely effective for the 324-MHz cavity operation. However, if we apply this approach to the 972-MHz RF cavities, an interlock due to the RF cavity reflection amplitude occasionally occurs at the end of the warm-up process. In order to solve this problem a third generation novel auto-tuning system was successfully developed in December 2013 and applied to the operation of the J-PARC LINAC, including the 972-MHz ACS RF cavities. During the warm-up process both the mechanical tuner controller and the input RF frequency tuning are in operation, and good matching between the input RF frequency and the RF cavity is obtained during the entire warm-up process. As a result of their excellent performance, 972-MHz ACS cavity operation and the J-PARC LINAC 400MeV upgrade have been successfully implemented. In this paper, the development of the auto-tuning systems and their applications will be described in detail.

A Novelty Analysis Method of Solar-Pumped Laser with Fresnel Lens and Cr/Nd:YAG Ceramic

In this paper, the simple and effective experimental method is proposed for studying on the influence of tracing deviation on solar-pumped laser output performance. The threshold pumping power is obtained by the novelty method. The experimental setup of solar-pumped laser employed Fresnel lens of 1.4×1.05m2 and diffuse reflective conical cavity focusing solar radiation on the size of 5mm×60mm Cr/Nd:YAG ceramic. The effective threshold pumping power calculated is 190W, which is about 30% lower than reported results.

Optical cavity for improved performance of solar receivers in solar-thermal systems

A principal loss mechanism for solar receivers in solar-thermal systems is radiation from the absorbing surface. This loss can be reduced by using the concept of directional selectivity in which radiation is suppressed at angles larger than the incident angle of the sunlight striking the absorber. Directional selectivity can achieve efficiencies similar to high solar concentration, without the drawbacks associated with large heat fluxes. A specularly reflective hemispherical cavity placed over the absorber can reflect emitted radiation back to the absorber, effectively suppressing emission losses. An aperture in the cavity will still allow sunlight to reach the absorber surface when used with point focus concentrating systems. In this paper the reduction in radiative losses through the use of a hemispherical cavity is predicted using ray tracing simulations, and the effects of cavity size and absorber alignment are investigated. Simulated results are validated with proof of concept experiments that show reductions in radiative losses of more than 75% from a near blackbody absorber surface. The demonstrated cavity system is shown to be capable of achieving receiver efficiencies comparable to idealized spectrally selective absorbers across a wide range of operating temperatures.

Modeling of two wavelength switching using a reflective vertical cavity semiconductor saturable absorber

Wavelength switching between two optical signals has been studied numerically utilizing the concept of cross-absorption modulation and partial cross-phase modulation in a reflective vertical cavity semiconductor saturable absorber (R-VCSSA). The switching performance of an R-VCSSA designed with high power impedance-matched top mirror reflectivity is studied by fixing a control beam (CB) at low power cavity resonance wavelength (λres) and a signal beam on the short wavelength side of λres, with a fixed low input power. On gradually increasing the CB input power, the carrier concentration within the cavity increases which modifies the absorption and the refractive index of the saturable absorber. This results in a change in the output power of both the beams. In the analysis we have considered a 40Gb/s input signal. The high intra-cavity power introduces thermal effects within the VCSSA cavity due to which the effective cavity length of the device changes. The change in cavity length leads to a red-shift of the cavity resonance wavelength. This effect has also been incorporated in the model. The advantage of a VCSSA, used for switching is owing to its comparatively small carrier recombination time (~5ps or less), which could be utilized for the high speed optical system.

All-optical XNOR/NOT logic gates and LATCH based on a reflective vertical cavity semiconductor saturable absorber.

This work proposes a scheme of all-optical XNOR/NOT logic gates based on a reflective vertical cavity semiconductor (quantum wells, QWs) saturable absorber (VCSSA). In a semiconductor Fabry-Perot cavity operated with a low-intensity resonance wavelength, both intensity-dependent saturating phase-shift and thermal phase-shift occur, which are considered in the proposed logic operations. The VCSSA-based logics are possible using the saturable behavior of reflectivity under the typical operating conditions. The low-intensity saturable reflectivity is reported for all-optical logic operations where all possible nonlinear phase-shifts are ignored. Here, saturable absorption (SA) and the nonlinear phase-shift-based all-optical XNOR/NOT gates and one-bit memory or LATCH are proposed under new operating conditions. All operations are demonstrated for a VCSSA based on InGaAs/InP QWs. These types of SA-based logic devices can be comfortably used for a signal bit rate of about 10GHz corresponding to the carrier recovery time of the semiconductor material.

Study of surface acoustic waves in SiO2/LiNbO3 layered-structure phononic crystals

Phononic crystal (PnC) in half-space allows anisotropic propagation and band gaps for surface acoustic wave (SAW) and promise applications such as filters and reflectors. For the feasible MEMS fabrication, a PnC comprised of cylindrical holes in a SiO2 layer covering a LiNbO3 substrate was proposed and analyzed numerically. Dispersion curves of SAWs in the PnC were calculated by using finite element method and then the band gap range for SAW was indicated. The variation of band gaps by modifying the layer thickness and cylinder size was reported. Simulation shows that SAW propagation could be stopped by thirty rows of PnCs. Then reflective gratings and resonant cavities for SAW were designed accordingly. The result showed feasibility of applying PnC in acoustic electronics devices.