High-speed rail catenary fails easily due to foreign objects intrusion. Foreign objects are identified by machine vision now. However, extreme temperature changes can degrade the imaging quality of the optical system, leading to increased difficulty in foreign object identification at high altitudes. We design different types of optical systems for targets at 2 km away by Zemax and carried out thermal analysis to obtain the variation pattern of system imaging quality at different temperatures for telephoto structures and Schmidt-Cassegrain systems. The results show that Schmidt-Cassegrain structures are better adapted to wide temperature environments than telephoto structures. After correction for defocus, the Schmidt-Cassegrain structure is almost unaffected by temperature, and the telephoto structure also has good image quality within a certain temperature range. It shows that the Schmidt Cassegrain structure is more suitable for inspection along the Sichuan-Tibet Railway.
We report a type of single-hole core photonic crystal fiber for low-loss polarization-maintaining terahertz (THz) wave guidance. Simulation results show that high birefringence at a level of 10 − 2 can be obtained by a design of minor position adjustment of the central air hole. Low effective material loss can be achieved because of the introduced central air hole. The strategy of the central air hole movements is also applicable for the three-hole core THz photonic crystal fibers. Other transmission characteristics including single-mode condition, power fraction, confinement loss, and dispersion were discussed in detail. It is quite clear that the proposal facilitates the fabrication process due to the simple structure.
A type of high-birefringent terahertz (THz) photonic crystal fiber (PCF) with all circle air holes is proposed. The characteristics including birefringence, dispersion, and confinement loss are numerically analyzed in detail by using the finite element methods. Simulation results show that the proposed THz PCFs exhibit high birefringence on the level of 10−2 in the frequency range of 2 to 4 THz, which is realized by the minor position adjustment of air holes in the first ring of the cladding. We believe that the proposed THz PCFs can be fabricated without complications due to their simple structure. In addition, two porous-core THz PCFs are proposed and the birefringence property is investigated.
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