Proposal for a Quad-Elliptical Photonic Crystal Fiber for Terahertz Wave Guidance and Sensing Chemical Warfare Liquids

Abstract

A porous-core photonic crystal fiber based on a cyclic olefin homopolymer (Zeonex) is proposed; it shows high birefringence, high core power fraction, low losses, and near-zero flat dispersion. The fiber’s core was designed with quad-elliptical (QE) air holes with its center occupied by bulk background material. The superiority of the QE design over the commonly adopted tri- and penta-elliptical (TE and PE) core designs is demonstrated. The presence of the bulk material at the core center and the geometrical configuration cause a broad contrast in phase refractive indices, thereby producing high birefringence and low transmission losses. A high birefringence of 0.096 was obtained at 1.2 THz, corresponding to a total loss of 0.027 cm−1 and core power fraction of approximately 51%. The chromatic dispersion and effective area of the reported fiber were also characterized within a frequency range of 0.4–1.6 THz. The QE air holes were then filled with chemical warfare agents, namely, tabun and sarin liquids. Then, the relative sensitivity, confinement loss, fractional power flow, and effective material loss (EML) of the sensor were calculated. Nearly the same relative sensitivity (r = 64%) was obtained when the QE core was filled with either liquid. Although the obtained EML for tabun was 0.033 cm−1 and that for sarin was 0.028 cm−1, the confinement loss of the fiber when it was immersed in either liquid was negligible. The proposed fiber can be fabricated using existing fabrication technologies. Moreover, it can be applied and utilized as a THz radiation conveyor in a terahertz time domain spectroscopy system for remote sensing of chemical liquids in the security and defense industries.

Modelling and simulation of novel liquid‐infiltrated PCF biosensor in Terahertz frequencies

“…In the past, photonics engineers have used various dielectric materials as the substrate for designing THz PCF sensors. Some of these substrate materials include Polymethyl‐methacrylate, Tetrafluoroethylene and cyclic olefins (Topas and Zeonex) [6, 12]. Cyclic olefins have flat refractive index profile ( n ≃ 1.53) within a wide terahertz frequency range, resulting in negligible material dispersion.…”

“…Furthermore, optical properties of PCFs can be adapted to various applications by fine‐tuning the shape, size and position of the air hole discontinuities. Thus, PCFs have found applications in many areas, including biomedicine [10], spectroscopy [11], security [12], telecommunication [13] etc.…”

“…Recently, various researchers have proposed PCF‐based sensors for operation in the terahertz regions; with improved losses and sensitivity profiles. Various research groups have proposed PCF‐based sensors for THz sensing applications [12, 19–21]. Islam et al [19] proposed an ethanol sensor consisting of kagome‐cladding lattice and a rectangular slotted core, and have shown birefringence of 5 × 10 −3 , the high relative sensitivity of 85.7% and confinement loss of 1.7 × 10 −9 cm −1 .…”

“…Nevertheless, the authors have not thoroughly considered the impact of air holes size and position on the obtained transmission and sensitivity profiles during parameter optimisation. An elliptical core PCF was proposed for sensing liquid warfare agents [12]. The sensor showed with relative sensitivity, confinement loss and EML of 64%, 10 −12 and 0.028 cm −1 , respectively.…”

Design and simulation of rotated hexagonal porous core photonic crystal fibre with improved effective material loss and dispersion properties

“…Although chromatic dispersion is a combination of material and waveguide dispersions, material dispersion is discarded for the proposed fibre as TOPAS has negligible material dispersion in THz frequencies. Waveguide dispersion or group velocity dispersion, measured in ps/THz/cm, can be calculated using the following expression [25]; (8) where n eff denotes the effective refractive index, ω is angular centre frequency and c is the velocity of light in free space. Near zero ultra-flattened dispersion allows THz sub-pulses to be transmitted simultaneously with nearly equal pulse spreading.…”