The Study Of Low-loss Broadband Terahertz Wave Photonic Crystal Fibers

Terahertz(THz) wave is a hotspot in current researches, and plays an important role among the electromagnetic spectrum, which is called to be "the light of life". Terahertz wave has wide application prospects. By far, it has been applied in imaging, medical, monitoring, communication and so on. The structural design of Photonic Crystal Fiber is flexible and it owns good transmission performance, so Photonic Crystal Fiber(PCF) is a preferred medium for the transmission of terahertz wave. Previous research shows that the transmission loss of cyclic olefin copolymer in the terahertz wave band is very low, and is also easier to be controlled than that of the quartz. In this paper, we took the ring olefin copolymer(COC) as the substrate material, and two new types of terahertz photonic crystal fiber( THz-PCF) are be designed. We deeply studied the transmission characteristics of terahertz wave band. The research results provide a theoretical reference for the production of optical fiber. The main content is summarized as follows:1. In order to achieve long range broadband transmission in terahertz band, a low absorption loss of cyclic olefin copolymer(COC) was designed as the base material of the double cladding terahertz photonic crystal fiber. Based on the full vector finite element method and the mode selection theory, the single mode transmission range, the limiting loss, the dispersion and the effective mode field area of the optical fiber are numerically simulated. The results show that the optical fiber in the terahertz wave has excellent transmission characteristics, which can makes fundamental mode confinement loss much smaller than 0.1d B/m in the frequency range 1 ~ 10 THz, and make the second order mode confinement loss more than 1d B/m, through reasonable structural parameter design and optimization. Therefore, we can obtain a wide band single mode transmission of 1Thz to 10 THz, and the group velocity dispersion can be controlled within ±0.1ps/(THz·cm) from 1.5THz to 10 THz.2. A new type of single polarization single-mode(SPSM) photonic crystal fiber was designed, which could realize the high birefringence by adding the non symmetrical diamond shaped air hole. The results show that with the increase of the terahertz frequency, a polarization state of the base mode will not meet the core transfer condition and then change to the leaky mode, which can not transmit in the fiber core, and the single polarization single- mode transmission of THz wave can be realized. The photonic crystal fiber can not only realize the broadband SPSM transmission, but also reduce the absorption loss of the fiber substrate material. At the same time, the structural symmetry of the cladding layer effectively limits the leakage of the optical fiber, and a lower limit loss can be achieved.