Mid-infrared dual-rhombic air hole Ge 20 Sb 15 Se 65 chalcogenide photonic crystal fiber with high birefringence and high nonlinearity

We propose a mid-infrared dual-rhombic air hole hexagonal lattice photonic crystal fiber with high birefringence and large nonlinearity based on Ge20Sb15Se65 chalcogenide glass. The properties of birefringence, dispersion, nonlinearity, and confinement loss were investigated in the 3 µm~5 µm mid-infrared range by using the Finite Difference Time Domain (FDTD) method with perfectly matched layer (PML) absorption boundary conditions. The results indicate that for the optimized structural parameters of Λ= 2.0 µm, D= 1.932 µm, d= 0.8 µm, and H= 0.8 µm, an ultrahigh birefringence of 0.041, a very low confinement loss of 0.0013 dB/km (for x-polarization modes) and 0.0342 dB/km (for y-polarization modes), and the maximum nonlinearity coefficient of 4375 w−1km−1 (for x-polarization modes) and 3960 w−1km−1 (for y-polarization modes) were achieved, respectively. The proposed PCF has a lower confinement loss and higher birefringence than an elliptical-hole PCF with the same air-filling fraction. Thus, it will be an excellent candidate for mid-infrared optical fiber sensing, precision optical instruments and nonlinear optics

Flexible wavelength-, pulse-controlled mode-locked all-fiber laser based on a fiber Lyot filter

In this paper, we report a flexible wavelength-, pulse-controlled mode-locked all-fiber laser based on a novel fiber optic Lyot filter. The wavelength, pulse duration and spectral bandwidth of passive mode-locked lasers can be tuned by controlling the polarization controller. The proposed Lyot filter was constructed by a single-mode fiber insertion between two polarization-maintaining fibers. The filter bandwidth and laser output tunability were based on the birefringence characteristics of the polarization-maintaining fibers. This all-fiber laser is simple and stable and can be used for various applications where width-tunable or wavelength-tunable pulses are necessary

Focal shift and focal switch of broadband hollow Gaussian beams passing through an aperture-lens system

In this paper, we deduce the approximate analytical formula for the distribution of axial intensity for broadband hollow Gaussian beams (HGBs) passing through an optical system with an aperture and a lens separated based on the Collins formula. Moreover, the focal shift and focal switch effect of broadband HGBs passing through the system is investigated in detail. As demonstrated by the numerical results, whether the focal switch of broadband HGBs occurs is primarily dependent on the size of the aperture, the effective Fresnel number, as well as the relative separation between the aperture and the lens. Furthermore, the bandwidth may have effects on the behavior of the focal switch and reduce the relative focal shift. In this paper, the focal shift of HGBs through a thin lens is recognized as a special case

The Fabrication of Porous Metal-Bonded Diamond Coatings Based on Low-Pressure Cold Spraying and Ni-Al Diffusion-Reaction

A porous metal-bonded diamond grinding wheel has an excellent performance in precision grinding. In this research, a novel manufacturing process of porous metal-bonded diamond coating was presented. Firstly, the diamond/Ni/Al coatings (400–600 μm) were fabricated via low-pressure cold spraying and their microstructures were studied. The diamond particles in the feedstock had a core–shell structure. Secondly, the post-spray heat-treatments were set at 400 °C and 500 °C to produce pores in the cold-sprayed coatings via Ni-Al diffusion. The porosities of 400 °C and 500 °C heated coating were 8.8 ± 0.8% and 16.1 ± 0.7%, respectively. Finally, the wear behavior of porous heated coating was tested in contrast with cold-sprayed coating under the same condition via a ball-on-disc tribometer. The wear mechanism was revealed. The porous heated coating had better wear performance including chip space and slight clogging. The surface roughness of wear counterpart ground by the porous heated coating was smaller (Sa: 0.30 ± 0.07 μm) than that ground by cold-sprayed coating (Sa: 0.37 ± 0.09 μm). After ultrasonic clean, the average exposure height of diamond particles in the wear track of porous heated coating was 44.5% higher than that of cold-sprayed coating. The presented manufacturing process can contribute to fabricate high performance grinding wheels via cold spraying and porous structure controlling through Ni-Al diffusion–reaction

Photoluminescence Investigation of Porous Anodic Alumina Films

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Monitoring Roadbed Stability in Permafrost Area of Qinghai–Tibet Railway by MT-InSAR Technology

Permafrost areas pose a threat to the safe operation of linear projects such as the Qinghai–Tibet railway due to the repeated alternating effects of frost heaving and thawing settlement of frozen soil in permafrost area. Time series InSAR technology can effectively obtain ground deformation information with an accuracy of up to millimeters. Therefore, it is of great significance to use time series InSAR technology to monitor the deformation of the permafrost section of the Qinghai–Tibet railway. This study uses multi-time InSAR (MT-InSAR) technology to monitor the deformation of the whole section of the Qinghai–Tibet railway, detect the uneven settlement of the railway roadbed in space, and detect the seasonal changes in the roadbed in the time domain. At the same time, the local deformation sections over the years are compared and discussed. The time series deformation monitoring results of the permafrost section Sentinel-1 data in 2020 show that the length of the railway roadbed from Tanggula station to Za’gya Zangbo station (TZ) section is approximately 620 m, the deformation of the east and west sides is uneven, and the average annual deformation difference is 60.68 mm/a. The impact of frozen soil in WangKun station to Budongquan station (WB) section on railway roadbed shows the distribution characteristics of high in the middle and low at both ends, and the maximum annual average settlement can reach −158.46 mm/a. This study shows that the deformation of permafrost varies with different ground layers. The impact of human activities on frozen soil deformation is less than that of topography and hydrothermal conditions. At the same time, the study determined that compared with other sections, the roadbed deformation of TZ and WB sections is more obvious

A Novel Mode-Division Multiplexer/Demultiplexer with Ultra-Large Bandwidth and Ultra-Low Insertion Loss Based on Five-Core Photonic Crystal Fiber

A novel mode-division multiplexer/demultiplexer (MUX/DMUX) based on a five-core photonic crystal fiber (PCF) is proposed in this study. The structural parameters of MUX/DMUX were optimized using finite-difference eigenmode (FDE) and eigenmode expansion methods. The numerical simulation results show that the device can simultaneously multiplex five modes of LP01, LP11, LP21, LP31, and LP12 in the main core with an ultra-low insertion loss. At 1.55 μm, the mode conversion efficiency and insertion loss of the five modes were greater than 93.5% and less than 0.29 dB, respectively. The proposed MUX/DMUX is compact, with a length of only 1.84 mm. In addition, the device can operate efficiently with crosstalk of less than -11.34 dB over an ultra-wide bandwidth of 620 nm (from 1.33–1.95 μm, covering E-, S-, C-, L-, U-bands), offering great potential in future mode-division multiplexing systems

Design of a dispersion-engineered broadband Ge11.5As24Se64.5-Si3N4 strip–slot hybrid waveguide with giant and flat dispersion over 350 nm for on-chip photonic networks

We propose a new strip–slot hybrid waveguide with extreme large and flat dispersion over broad wavelength range. The strong resonance coupling between the strip and slot modes has been employed to obtain a high dispersion at desired wavelength. The properties of dispersion are analyzed using the finite-difference time-domain method. All numerical simulation results reveal that for the optimized waveguide structure, a maximum dispersion of ps nm km and dispersion full width at half-maximum of 2.5 nm at m are obtained. By cascading the strip–slot hybrid waveguides with varied width and height, a large and flattened dispersion of ps nm km covering 350 nm is achieved. Moreover, dispersion compensation of 100 Gbit/s return-to-zero on-off-keying optical-time-division-multiplexing signals after 50-km single mode fiber transmission, and after 7-km hollow-core photonic band-gap fiber transmission are demonstrated, separately. Such a waveguide will find widespread applications in on-chip all-optical signal processing