查看更多>>摘要：? 2021 Elsevier B.V.A combined sensor for simultaneous measurement of directional torsion and displacement is proposed and experimentally demonstrated. The sensor is constructed with a helical long-period fiber grating (H-LPG) and an air-cavity fiber-optic Fabry–Perot interferometer (FPI) in series. By investigating the resonant wavelength shift of the H-LPG and the FSR variation of the FPI in the transmission spectra of the sensor, the applied torsion and displacement can be unambiguously discriminated and determined simultaneously. The experimental results show that the sensor achieves to the directional torsion responsivity of ?2.693 nm/rad/m, and the displacement responsivity of 0.0275 nm/μm in ranges of ?240 to 240 degrees and 0 to 200μm, respectively.
查看更多>>摘要：? 2021 Elsevier B.V.We theoretically investigate the angular displacement measurement based on a hybrid interferometer combining two optical parametric amplifiers (OPAs), which are used to amplify the phase-sensing intensity and squeeze shot noise. We obtain significant improvement in sensitivity for this modified hybrid interferometer over the Mach–Zehnder interferometer (MZI) combining two OPAs. Finally, the effect of losses on sensitivity is discussed and we find that our scheme has more robust compared with the modified MZI. Our results provide a more accurate angular displacement estimation which has potential applications in precision measurements and quantum sensing.
查看更多>>摘要：? 2021 Elsevier B.V.Active illumination time-of-flight single photon imaging has the advantage of high photon sensitivity which is suitable for imaging in long-distance and low light scenarios. The problem of missing information in depth profile of single photon imaging will deteriorate the imaging quality. The noise which comes from the scene reflected of the ambient light has always been ignored or excluded as strong interference of the active signal. In this letter, an ambient noise guided missing data filling method is proposed to improve the single-photon imaging quality. This method takes the ambient noise which was considered as interference in the past as the guidance for the filling of missing information in depth image. Both simulated and experimental verification are implemented to demonstrate the effectiveness of the proposed method. The observed experimental root mean square error has been improved from 1.97 m to 0.38 m and the structural similarity has been improved from 0.28 to 0.81 with the signal to background noise ratio of 0.15.
查看更多>>摘要：? 2021 Elsevier B.V.In order to characterize biological tissue or to model light propagation in tissue medium it is necessary to define four key optical parameters, namely, absorption coefficient μa, reduced scattering coefficient μs’, anisotropy factor g (or scattering phase function), and refractive index n. Successful derivation of these valuable parameters can provide comprehensive information regarding the condition of tissue during disease pathogenesis and therapy and help to build accurate light propagation simulator. Usually, absorption and reduced scattering coefficients are investigated while the other two are assuming to be constant factors (g ≈ 0.9, n ≈ 1.4) across the near-infrared region. In order to quantify the g and n spectrum of a sample, we propose the use of spatial frequency domain (SFD). In SFD, periodic illumination patterns (structured illumination) at different spatial frequencies and wavelengths are serially projected onto the sample surface. Then, the collected diffusely reflected light is analyzed via the diffusion equation (or Monte Carlo simulation) in SFD to separately recover the sample's absorption and scattering properties over a wide field of view. This work aimed to use the solution of the diffusion equation (DE) developed for the diffuse reflectance in SFD to estimate the wavelength-dependent variability of the g and n parameters of various turbid samples in the near-infrared spectral region. Since the solution of DE contains four unknown parameters, we use four different spatial frequencies to extract these four components; four equations at four different frequencies with four unknown parameters are solved at each of the four discrete wavelengths used. During data processing of n, its wavelength-dependence was fitted using the dispersion model of Sellmeier. While there are several approaches to retrieving information about g and n, the work in SFD offers an easier and more straightforward framework and grants expanded capability to this domain by enlarging its measurement range.
查看更多>>摘要：? 2021 Elsevier B.V.In self-mixing interferometry, the fast and accurate estimation of fundamental feedback parameters, such as optical feedback coupling factor C and linewidth enhancement factor α, is critical for high-resolution real-time displacement sensing using an SMI sensor. In this work, a Marquardt's algorithm (MA) based fast and highly accurate method is proposed to estimate C and α. The proposed method utilizes the SMI signal phase and applies a simple numerical analysis to estimate C and α. The proposed method can accurately estimate C and α for all three major feedback regimes (i.e., week, moderate and strong). A detailed comparison is also made with previously proposed algorithms to show the superiority of the proposed algorithm in terms of initial conditions, number of iterations, accuracy, and ability to work under all feedback regimes. The results have shown that our method has a negligible average error of around 0.0023 and 0.0006 for C and α estimation, respectively. The proposed method only requires approximately five iterations for accurate estimation. The method's simplicity, accuracy, and fast nature enable compact and cost-effective SMI sensors with high-resolution real-time displacement sensing.
查看更多>>摘要：? 2021 The Author(s)We observed conditional cross-phase modulation (C-XPM) applicable to quantum phase gate. Combinations of two probe field's polarizations conditioned XPM of two probe fields in the corresponding nonlinear optical system. With optimized parameters of multi-photon transition, we could increase C-XPM more than 1 rad. with small probe absorption. Group velocity matching ratio between probe pulses was estimated to 1.2 in the optical loop system. Because multi-photon interference depends on the ratio between the Rabi frequencies of the probe fields, not the individual intensity, we expect the same phase shift between few photons level's probe pulses in our scheme.
查看更多>>摘要：? 2021 Elsevier B.V.Spectroscopic data often suffer from spectral distortion and random noise. In this paper, a novel bandwidth function matrix-based spectral deconvolution with the alternate minimization method is proposed to recover the degraded spectra. Different from traditional spectral deconvolution methods which only employ or estimate one bandwidth function, the proposed method introduces a bandwidth function matrix composed of multiple bandwidth functions corresponding to the whole wavelength range of the spectrometer, and applies the alternate minimization method to alternately estimates the real spectrum and bandwidth function matrix until convergence. As a result, the correction error caused by the differences in bandwidth functions in the whole spectral band could be effectively eliminated. A series of experiments was carried out for various simulated and real measured spectra, and comparative results demonstrate the superior performance of the proposed method comparing with the traditional methods.
查看更多>>摘要：? 2021 Elsevier B.V.Honeycomb photonic crystal (PhCs) also known as photonic graphene exhibits salient features such as common complete photonic bandgap, topological states, and degrees of freedom to manipulate local density of states for efficient light–matter interactions. In this work, we report the cavity confinement offered by the six different point defect geometries of honeycomb PhCs. It is found that the reported point defect geometries are capable of supporting high confinement with low effective mode area of the order of 10?4 m2. To make use of this cavity confinement, a Fano-coupler is designed based on waveguide–cavity (WG-C) interactions in honeycomb PhC. The transmission/reflection profiles of a WG-C geometry shows typical Fano resonances with the quality factor of the order of 104. Comparing square and triangular PhCs, honeycomb WG-C shows a relatively stable Fano-lineshape resonance for a variation in imaginary part of the dielectric permittivity (?r′′) of an unknown dielectric material loaded at the center of the cavity from 0 to 0.01. The proposed WG-C Fano-coupler is suitable for dielectric loss sensing over a conventional cavity perturbation method, in which Fano resonance is sustained with Q factor variation of 25,805 to 2431 for ?r′′ variation from 0 to 0.01, respectively. Due to the nature of PhC cavity decay dynamics, two different linear sensitive regimes with the dielectric loss sensitivity of 96.901/?r′′ (for ?r′′=0 to 0.005) and 26.911/?r′′ (for ?r′′=0.005 to 0.01) are observed. We anticipate that the cavity confinement and Fano-interactions reported in honeycomb PhCs might be useful not only to explore sensing elements but also for unraveling light–matter interactions in cavity quantum electrodynamics, quantum optics and lasing systems.
查看更多>>摘要：? 2021 Elsevier B.V.Thulium-doped YCa4O(BO3)3 (Tm:YCOB) is a novel nonlinear optical crystal, which has potential applications in quasi-parametric chirped pulse amplification (QPCPA). However, the nonlinear characteristic and damage behavior of Tm:YCOB crystal was seldom investigated. Here, Raman spectra was used to analyze the functional groups that cause the nonlinear characteristics of Tm0.1Y0.9COB. The nonlinear refraction and weak absorption of Tm0.1Y0.9COB crystals were studied from the aspects of electronic effect and thermal effect. The nonlinear refractive indices of the crystal in the three directions are 5.61–6.01 × 10?7 cm2/GW (@ 1064 nm), which is less than that of YCOB crystal. The weak absorption values at 1064 nm of the crystal were measured to be ～8 × 104 ppm/cm (X), ～3 × 104 ppm/cm (Y) and ～5 × 104 ppm/cm (Z), respectively. We studied the relationship between the differences of weak absorption values in three optical axis and the crystal structure. The damage threshold of Tm0.1Y0.9COB at 1064 nm was measured to be 5.31 GW/cm2, which was twice that of YCOB. The reason for the damage threshold of Tm0.1Y0.9COB higher than that of YCOB was also discussed.
查看更多>>摘要：? 2021 Elsevier B.V.In this paper, an improved Volterra nonlinear equalizer with low computational complexity and good performance is proposed to compensate the distortions caused by bandwidth limitation and optical devices. The performance of the equalizer is improved by introducing one-hot encoding, and the complexity is reduced by using absolute operations and weight sharing based on k-means clustering. We employ the k-means clustering algorithm to replace some weights that are close in value with another similar weight, which greatly reduces the multiplications of this equalizer. Finally, we experimentally demonstrate a 50 Gb/s PAM4 IM/DD transmission with 10G-class optics using this proposed equalization scheme. The results show that, compared with the conventional 3rd-order Volterra nonlinear equalizer, the sensitivity of the proposed scheme is improved by > 2 dB, and the number of multiplication operations is reduced by ～81.2%.
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