查看更多>>摘要：? 2022 Elsevier B.V.Due to the significant advancement of transmission networks and related technologies, optical communication with a large capacity is essential. In this study, a few-mode PbS quantum dot-doped fibre amplifier (FM-PQDFA) was developed and numerically studied. The amplifier can effectively amplify the linear polarisation (LP) signal modes, including LP01, LP11, and LP21, in the range 1100–1350 nm. The influences of the quantum dot (QD) doping concentration, fibre length, and signal power on the gain and noise figure (NF) of FM-PQDFA were investigated. In addition, the pump influenced the gain, NF, and differential mode gain (DMG). The results revealed that the gain, NF, and DMG can be optimised by adopting an appropriate pump light mode. When the signal and pump light modes were both LP01, the highest gain of 31.6 dB and lowest NF of 3.25 dB were achieved. LP11 pump mode was the optimal pumping mode to obtain the high gain, low NF and small DMG simultaneously.
查看更多>>摘要：? 2022 Elsevier B.V.This article describes the change in geometric parameters of the quasi-periodic structure of the micro cavities inside the optical fiber. The quasi-periodic structure is a chain of localized cavities filled with oxygen. This structure is produced by the melting of the core of the fiber with a moving plasma spark, which appeared because of optical breakdown. While the quasi-periodic structure is being recorded, the fiber sample is placed between the poles of a permanent magnet with a magnetic field induction of 11.7 T. As a result of the effect of the magnetic field on the moving plasma, there is a change in the parameters of the quasi-periodic structure of the micro cavities. In the region with the highest magnetic field gradient, a 7% increase in the transverse dimensions of the cavities and a 9% increase in the longitudinal dimensions were observed. It was depending on the direction of plasma propagation. In the magnetic field, the effect of dividing the cavities into two unequal parts was observed. The main cavity was located at the axis of the fiber, an additional smaller cavity-satellite appeared at some distance from it in the radial direction.
查看更多>>摘要：? 2022 Elsevier B.V.The traditional acoustic signal detection structure uses a diaphragm or a short optical fiber as its acoustically sensitive moving unit. However, the acoustically sensitive moving unit limits the acoustic pressure detection range, sensitivity range, or detectable frequency range. In order to improve this limitation, this study presents a high sensitivity acoustic signal detection structure based on Fabry–Pérot etalon (FPE). The FPE is composed of two Plano-Concave Lenses, and the reflectivity of its cavity mirrors exceeds 99%. According to the COMSOL 5.6 simulation, 1 Pa acoustic pressure can cause a change in the refractive index of 2.6511 × 10?9 and a drift of the resonance frequency of 1.5464 × 10?6nm. An acoustic signal detection system based on phase modulation spectrum technology is constructed. The experimental results of this study show that the high Q value of FPE is 107. In the range of 31.5 Hz–10 kHz, the acoustic detection system has a frequency response with a sensitivity of 2.5873 V/Pa and a total harmonic distortion (THD) of 0.97%. Using an acoustic reflector to converge the acoustic pressure, and then optimize the FPE acoustic detection structure, the sensitivity can be increased by 2.24 times, which is consistent with the simulation results. The excellent performance of the acoustic reflector structure will benefit the future research of photo-acoustic imaging.
查看更多>>摘要：? 2022Sequence coding is beneficial for reducing range ambiguity and acquisition time in single-photon lidar (SPL) ranging. In SPL data acquisition, dead time is usually regarded as an adverse condition. However, properly utilizing echo-induced dead time in sequence coding may allow it to be useful. In this study, we propose a dead-time-based sequence coding method. We set the time intervals of the subpulses to be slightly longer than the dead time, such that the detection of a subpulse can ‘block’ incoming noise detections until the arrival of the next subpulse. Starting with a sequence of three subpulses, we determined the optimal pulse-interval criteria of ‘≈ dead time + pulse width’ when the subpulses are identical in shape and intensity. A method to extract the range with few accumulations was proposed (FARE method), incorporating the maximum cross-correlation and threshold filtering. Simulation and experiment results demonstrated that our ‘optimal interval + FARE’ method is more efficient and less ambiguous than the sequence coding method without the merit of dead time, and the conventional ‘single laser pulse’ method. Our method is promising for low range ambiguity and fast ranging, and helps reduce laser energy consumption without wasting transmitting pulses.
查看更多>>摘要：? 2022 Elsevier B.V.In this paper, the security of the optical image cryptosystem based on interference and an amplitude mask has been evaluated. The silhouette problem existing in the conventional interference-based cryptosystem is removed by eradicating the dependence of two phase-only masks (POM), which fixed the security leak. Moreover, the key space is enlarged by generating the amplitude mask in the encryption path as an additional private key. Since the number of unknown keys increases, the improved cryptosystem could be immune to the phase-retrieval technique-based iterative attack which the classical interference-based scheme is vulnerable to. However, the random phase mask (RPM) used as phase lock is predefined in the encryption path and irrelative to the plaintext. According to this finding, hybrid algorithms including a known-plaintext attack (KPA) and phase-retrieval technique-based iterative processes with different constraints have been proposed to crack the security-enhanced cryptosystem based on interference and an amplitude mask. Numerical simulations have been carried out to demonstrate that the silhouette problem existing in this enhanced scheme could be released by our proposed attacks.
查看更多>>摘要：? 2022 Elsevier B.V.A fractal shaped metasurface inspired graphene-based absorber is proposed in this paper for THz range application. The space-filling property of the Haferman carpet pattern is utilized to realize the shape of the graphene layer deposited on a dielectric layer with a metallic ground that is found to exhibit improved absorbance compared to the Euclidean? shaped absorber counterparts. The resonating structure is made up of a graphene layer based on a second iteration Haferman Carpet fractal geometry that constitutes the unit cell of the metasurface. The absorbance is polarization independent for normal incidence. The efficacy of the absorber has been studied for various chemical potentials that render the tunability. Comprehensive discussions on the obtained results are presented by using numerical simulation to determine the electric field distributions and the absorption spectra. Over 90% absorbance is observed for the frequency range of 7.66 THz to 9.025 THz, resulting in a bandwidth of 1.365 THz. Therefore, these graphene-based Haferman carpet fractal shaped THz absorbers can be vastly used in high-performance THz devices, electro-optical switches and bio-sensors.
查看更多>>摘要：? 2022 Elsevier B.V.Subaperture stitching interferometry (SSI) is an essential method for the map testing of large-aperture optical components. The surface map of a reference surface is a typical error source. In this study, we propose a hybrid self-calibration method to eliminate the reference surface error in SSI by combining the modified shift-rotation method and the maximum likelihood method. The traditional shift-rotation method is a general full-aperture absolute interferometric measurement that can retain localized irregularities. The shift-rotation operations are leveraged to generate a couple of subapertures covering the surface under test, whereby a ring of subapertures and a central subaperture are acquired by rotations and a lateral shift, respectively. The modified shift-rotation method is proposed to obtain the rotationally asymmetric components of the test surface in the ring of subapertures. The same components within the central subaperture are retrieved using the maximum likelihood method. Then, the rotationally symmetric components of the test surface are acquired using the least squares method, utilizing the measured data before and after the shift. Reference surface maps are sufficiently eliminated from the measured data. High-frequency components of the test surface are also retained, which engender high-accuracy SSI. Simulations are conducted to verify the proposed method. The positioning errors of the proposed method are analysed and discussed. Subaperture testing experiments of a 100-mm aperture flat are performed and compared with full-aperture absolute measurement results. The stitched errors with 0.018 λ PV and 0.003 λ RMS are obtained.
查看更多>>摘要：? 2022 Elsevier B.V.A phase measuring profilometry (PMP) based on binary grating projection for fringe order self-extraction is proposed. In traditional PMP based on sinusoidal grating projection, the phase is wrapped between (?π, π] due to the arctangent operation. And wrapped phase needs to be unwrapped using relative phase unwrapping algorithms such as diamond-type phase unwrapping algorithm in which the accumulation of phase errors is easy to occur or even lead to wrong phase unwrapping; or with the assistance of some additional structured light patterns such as Gray code, the fringe order may be determined to achieve phase unwrapping without the phase error accumulation. In the proposed method, three-dimensional measurement is achieved by just projecting a set of phase-shifted binary gratings, which can not only avoid the gamma non-linearity influence of the projector, but also increase the refresh frequency of the projector by more than ten times. At the same time, by using the image characteristics of the binary grating itself, the fringe order can be extracted without increasing any additional structured light pattern. So that the phase unwrapping can be achieved by combining the phase of extracted fringe order with the wrapped phase without the phase error accumulation. The experimental results verify the feasibility and effectiveness of the proposed method, and the solution result is in good agreement with the solution result of the diamond algorithm without cumulative errors.
查看更多>>摘要：? 2022 Elsevier B.V.Exploring the influence of atmospheric turbulence on remote sensing based on filament-induced fluorescence is still challenging. In this study, we applied artificial turbulence during remote filamentation and recorded the enhanced backward fluorescence of nitrogen. At the strongest turbulence, the nitrogen fluorescence signal was higher than that obtained when no turbulence was applied. The air turbulence introduces random noise in the laser profile, which is subsequently amplified by self-focusing effect and causes multiple filamentation. Moreover, by lowering the threshold for multiple filaments generation, turbulence led to an increase in the number of filaments, thereby enhancing fluorescence.
查看更多>>摘要：? 2022 Elsevier B.V.The systematic studies of Ti Laser-Induced Plasma Lasers (LIPL) are presented. Many generation lines in the blue–green spectral range are found. Ti LIPLs are generated according to the quasi-three-level (direct lasing) scheme where generation transitions occur from the pumped to the intermediate level. Most Ti LIPL generation lines are strongly linearly polarized with polarization vector Eg normal or parallel to the pumping polarization vector Ep depending on energy states involved in the generation cycle. External magnetic fields directed parallel to the generation direction strongly reduce the generation lines polarization and reduce line intensity. It is proposed that generation lines polarization at zero magnetic fields, and their depolarization in the external magnetic field may be explained by Hanle zero-field level-crossing effect, which needs to be modified for a direct generation scheme.
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