查看更多>>摘要：? 2021For the quasi-steady-state (QSS) self-focusing case, the imaging and focusing characteristics of partially coherent light pulses (PCLPs) propagating through a thin lens in nonlinear self-focusing media are studied. The analytical formulae of the image distance and the image size are derived. It is shown that the time delay of the minimum and maximum of the image distance depends on the light spatial coherence, while the time delay of the maximum of the image size is independent of the light spatial coherence. On the other hand, the analytical formula of the focal length of PCLPs is derived. It is found that, when the value of the pulse peak power is fixed, a PCLP has more advantages to avoid the optical damage of materials as the incident distance or the thin lens focal length increases or the initial pulse width decreases.
查看更多>>摘要：? 2021 Elsevier B.V.We propose a joint modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) estimation method via analyzing the trajectory information of the received adjacent symbols. A uniform grid in the first quadrant of constellation diagram is designed for converting the trajectory information of the received adjacent symbols to the adjacent matrix of corresponding mapped graph structure. Then, we translate the OSNR-independent MFI scheme to an appropriate canonical vector searching optimization problem via improving on the classical cosine similarly algorithm, where the eigenvector associated with the largest eigenvalue of the adjacent matrix is served as discriminated-feature to identify five commonly-used modulation format types, i.e., polarization division multiplexing (PDM)-quadrature phase shift keying (QPSK), PDM=8 quadrature amplitude modulation (QAM), PDM-16QAM, PDM-32QAM, and PDM-64QAM. After exact modulation format type is obtained, we build the relationship between the second-largest eigenvalue of the adjacent matrix and the OSNR value via polynomial regression method, and the least-squares principle (LSP) is utilized to find the best fit line for obtaining the optimal polynomial coefficient. The proposed method is verified by 28 GBaud PDM-QPSK, PDM-8QAM, PDM-16QAM, PDM-32QAM, and PDM-64QAM optical signal numerical simulations, and the results show that could achieve excellent performance on various parameter analysis. Besides, a proof-of-concept experiment is also conducted. More specifically, the proposed method only need one training process and one matrix decomposition, which would provide a superior convenience for future optical performance monitoring.
查看更多>>摘要：? 2021 The AuthorsIn this paper, to solve the problem of uneven distribution of far-field intensity of Flash Lidar, a method of amplitude modulation for optical phased array (OPA) is proposed, which makes the far-field intensity distribution close to the uniform rectangular distribution. We use the impulse response sequence of Finite Impulse Response (FIR) digital filter designed by the window functions to modulate the amplitude of the OPA. This kind of uniform intensity distribution can simultaneously detect a wide range with the same intensity. The modulation sequence can also be extended to two-dimensional situation. In the case of considering the radiation pattern of optical waveguide, the amplitude modulation sequence can also be modified to obtain the far-field pattern that is close to the uniform rectangular distribution.
查看更多>>摘要：? 2021 Elsevier B.V.The collection of low-frequency vibration signals is very important for the monitoring research in various fields such as national defense engineering, bridges and dams, aerospace, and the earthquake resistance of civil building structures. During the structural design of existing cantilever Fiber Bragg Grating (FBG) acceleration sensors, the tilt angle of the cantilever is often ignored, therefore, aiming at this problem, this paper proposed a new FBG acceleration sensor with double titled cantilevers, and used formula derivation and ANSYS software to optimize the structural parameters of the proposed sensor and conduct simulation analysis. Then, according to the simulation results, two sensor prototypes were fabricated, one with tilted cantilevers and the other with horizontal cantilevers, then, the performance of the two was tested, compared, and analyzed to verify the impact of the tilt angle of cantilevers on the performance of the sensor. The experimental results showed that, the natural frequency of the proposed sensor was about 46 Hz, which can be used to monitor vibration signals with the 6–30 Hz frequency range; the sensitivity of the proposed sensor was about 4.16 pm/m s?2; and its degree of lateral an-interference was less than 5.62%. Compared with the sensor with horizontal cantilevers, the flat response area of the sensor with tilted cantilevers increased by about 30%, while its sensitivity reduced by about half. The research findings of this paper provide a useful reference for the research on similar type sensors and further expansion of the measurement range of FBG acceleration sensors.
查看更多>>摘要：? 2021 Elsevier B.V.In this work we study the transverse spatial correlation of the pair of photons generated via the process of spontaneous parametric frequency down-conversion, in periodically poled non-linear crystals illuminated by a pulsed laser beam. It is well known that the two-photon state generated in quasi-phase matching (QPM) configurations depends explicitly on the characteristics of the pump beam, on the crystal modulated non-linearity, and on the detection geometry. This has allowed the development of several techniques for controlling the biphoton spectral and spatial properties. Here we discuss another technique for implementing the spatial entanglement modification in QPM gratings. We show, theoretically and experimentally, that in nearly collinear geometries, the spatial shape modulation of the pump beam allows for the control of the biphoton transverse spatial correlation.
查看更多>>摘要：? 2021 Elsevier B.V.We report a 1064 nm Nd:YAG solid-state laser Q-switched by the dynamic thermal lens effect. The feasibility of using the thermal lens effect to realize Q-switch was proved based on the theoretical analysis of the dynamic thermal lens effect and the simulation of the Q-switched rate equation. Further, a thermal lens Q-switched experimental device with an 808 nm pumped Nd:YAG laser was constructed. The output laser beams with a repetition rate of 100 Hz, pulse width of 15.02 ns, single pulse energy of 11.9 mJ, and working wavelength of 1064 nm were realized by thermal lens Q-switch under the pump power of 412 W and pump pulse width of 80μs. The beam quality factors of the output pulse in x and y directions were 2.63 and 2.94, respectively. The results demonstrate that using a thermal lens Q-switch can generate laser beams with high energy and short pulse widths.
查看更多>>摘要：? 2021 Elsevier B.V.The SFG spectroscopic imaging has been demonstrated as a valuable tool for extracting spatial distributions of the chemical properties in surface chemistry. However, an inherently low signal-to-noise ratio demands pixel binning for extracting reliable chemical information through spectral curve fitting. Consequently, the imaging resolution is reduced. This issue can be circumvented by utilizing Neural Networks (N.N.s). This article demonstrates the utility of the N.N.s by deploying them for chemical identification in the SFG imaging of Self Assembled Monolayers (SAMs) on gold. SAMs are organic molecules that have spontaneously assembled onto a substrate in a single-molecule-thick layer. This new approach solves the chemical identification of the pixel spectra through classification. The method shows high accuracy even with extremely noisy spectra. The resulting chemical labels for the pixels are utilized to reliably generate spatial distributions of the SAMs without pixel binning, thus, enabling full-resolution and fast SFG imaging.
查看更多>>摘要：? 2021 Elsevier B.V.All-optical switching in a V-type three-level atomic medium is studied and controlled by spontaneously generated coherence (SGC), relative phase of applied fields and incoherent pumping rate. By simultaneously numerical solving the coupled Maxwell–Bloch equations for atom and field on a spatio-temporal grid, we have observed a continuous-wave probe field is switched ON or OFF by periodically modulating the coupling field intensity or the relative phase of applied fields. It also is shows that although the SGC can distort the switching probe pulse, however, under the SGC the response of the medium depends sensitively on the relative phase, and the probe field can be switched synchronous or anti-synchronous with the relative phase. On the other hand, in order to overcome the switching pulse distortion caused by SGC, we have included an incoherent pumping field. By choosing the appropriate phase and pumping rate, the fluctuations of switching probe pulse can be easily overcome. The proposed model can be useful for the design of optical switches and optical storage devices as well as possible experimental implementations.
查看更多>>摘要：? 2021 Elsevier B.V.Micro-bending is a well-known source of loss in optical waveguides. By treating the micro-bending as a stochastic process, the problem of loss mitigation can be modeled in terms of a Fokker–Planck equation. Given an initial refractive index profile, and taking micro-bending into account, we develop a formalism to derive a new refractive index profile which potentially results in less loss. Our formalism is based on applying the techniques of Supersymmetric Quantum Mechanics to a Fokker–Planck equation that is associated with a particular refractive index profile. We derive a non-linear differential equation, whose solutions determine whether an index profile can undergo a supersymmetric transformation that results in less loss. As an explicit example, we consider a monomial index profile. We show that there exists a range of values for the monomial exponent which results in the new index profile having less loss.
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