Low Effective Material Loss TOPAS Based Single-Mode Photonic Crystal Fiber with High Core Power Fraction in the THz Waveguiding

In this study, five layers of hexagonal cladding and two elliptical air holes based on photonic crystal fiber are discussed highly for many communication areas by decreasing different types of losses such as effective material loss (EML), scattering loss, and conﬁnement loss in the terahertz (THz) wavegu iding. Our suggested fiber (H-PCF) and all simulation results are obtained with the finite element method (FEM) and the perfectly matched layer (PML) boundary conditions based COMSOL Multiphysics software have been used to design in the THz region. After investigating all the graphical results, this optical communication-related H-PCF fiber discloses an extremely low effective material loss (EML) of 0.0184 cm −1 , with an effective area of 7.07×10-8 m 2 and flow of power in the core region of 88% at 1 terahert z (THz). Here, other simulation parameters such as conﬁnement loss, scattering loss, and V-parameter are also presented with a proper graph. So, we can easily say that the reported H-PCF fiber is strongly appropriate for different types of short and long-distance communication applications in the terahertz (THz) wave pulse region.


Introduction
Nowadays, researchers are trying to investigate the high-level research work on wireless communication systems at THz frequency to increase more capacity compared to the previous system. Terahertz (THz) radiation which changing from 0.1 to 10 THz has gained considerable interest due to its numerous functional uses related to electromagnetic radiation [1][2]. The assortment of THz frequency exists between the microwave and infrared radiation (IR) in the electromagnetic range. The THz frequency range shows the fascinating development in the field of sensing [3], spectroscopy [4], pharmaceutical drug testing [5], biomedical sensing [6], telecommunications [7], weapons in a non-destructive way [8], DNA Hybridization [9], etc. The leading sources of THz radiation are high-frequency Gunn diodes, far-infrared (FIR) gas laser, quantum cascade laser (QCL), free-electron laser, etc. Moreover, Schottky barrier diodes, pair braking detectors, hot electron mixers, field-effect transistor detectors, Bolometers, etc. are the fundamental T-ray detector. The necessity of high bandwidth in wireless schemes has been increased due to the unoccupied bandwidth for the current communication scheme. The transmission scheme of THz frequency largely depends on the free space medium but most of the THz waveguides experience an unavoidable absorption loss, path loss, difficult integration with other components during free-space propagation [10].
Over the last few years, some metallic and dielectric waveguides for the THz spectrum have been reported such as Bragg fibers [11], dielectric metal-coated hollow glass tubes [12], bare metal wires [13], parallel-plate waveguides [14], sub-wavelength porous fibers [15], single metallic wires [16], plastic ribbon waveguides [17]. Recently, porous core photonic crystal fibers (PCFs) have obtained significant interest because of their versatility in structural nature and desirable optical guiding properties such as high core power fraction, lower effective material loss, lower dispersion, highly birefringence, high nonlinearity, lower bending loss. Total internal reflection (TIR) and photonic bandgap (PGB) are two basic optical guiding properties are found in PCF. If the light is confined in a higher region of the refractive index in solid-core PCF then the total internal reflection can be optimized. Numerous polymers have been used as background materials in microstructure core PCF to control the optical guiding properties such as TOPAS, Tellurite, Zenox, poly methyl-methacrylate (PMMA), Graphene, Teflon [18][19][20][21][22][23], etc.
To get a higher sensitivity across the EM spectrum, many standard articles regarding PCFs have already been investigated. Islam et al. [24] anticipated a porous-core spiral shape photonic crystal fiber (PCF). Their proposed model obtained the EML and EA of 0.1 cm −1 and 1.82 × 10−7 m2 accordingly at 1 THz frequency. But their proposed model showed higher EML. In 2016, Hasan et al. [25] explored hexagonal PCFs that gained EML of 0.089 cm −1 at 1 THz frequency. Saiful et al. [26] suggested a rotated hexagonal porous core with circular shape cladding and obtained an EML of 0.053 cm −1 with a dispersion of 0.25 ps/THz/cm. In 2018, Rana et al. [27] proposed a hexagonal-shaped hole combined within the core of a Kagome lattice PCF. Their proposed model displays EML of 0.029 cm −1 and core power fraction of 33% at 1.3 THz frequency. In the same year, Sultana et al. [28] designed a hexagonal shape cladding with elliptical core PCF obtain EML of 0.05 cm −1 and very high birefringence of 0.086. From the previous background, we have got comparably higher EML and some important aspects such as power fraction (PF) and bending loss were not explored. An elliptical core PCF with kagome lattice cladding where EML of 0.056 cm -1 and lower dispersion of 0.27±0.18 ps/THz/cm 1 THz operating frequency was anticipated in 2019 by Saiful [29].
In this paper, we have designed a TOPAS based hexagonal shape of PCF with an elliptical core has been introduced in the THz regime. The proposed model shows a low EML of 0.0184 cm-1 with 80% core power fraction and a large effective area of 7.07×10-8 m2 at 1 THz optical frequency.

Design Methodology
The cross views of H-PCF are exposed in Fig. 1. where Λ1 and d1 are defined by the pitch and diameter of our design concept. The constraints d1/Λ1 is called the air filling ratio and this ratio tries to guard against collapse between two AHs and the background material of TOPAS helps to reduce many types of losses. Λc, da and db constraints are called the pitch and diameters of the two elliptical AHs similarly. Here, we find the numerical properties such as effective area, scattering loss, V-parameter, EML, fiber power fraction, and confinement loss of the fiber in the THz region with the COMSOL Multiphysics software. The optimum constraints are cladding diameter d1 = d2 = d3 = d4 = d5 = 300 μm, cladding pitch Λ1 = Λ2 = Λ3 = Λ4 = Λ5 = 400 μm, core diameter da = 52 μm, db = 167 μm and core pitch Λc = 100 μm.

Numerical Analysis:
Background material (TOPAS) has been used of this suggested H-PCF fiber to reduce the effective material loss (EML). Here, EML α eff is premeditated by [31]: Where, α mat is the bulk material absorption loss and n mat is the RI of the material. Ɛ 0 is the relative permittivity and the permeability of free space is µ 0 . Sz = SL of H-PCF fiber is thought-out by the subsequent equation [32]: Where, C R is called the scattering coefficient.
The low confinement loss-based PCF fiber is highly used for different types of communication sectors. Here, the confinement loss L c is calculted the equation [33]: Where, K 0 = ( f c ) is the free wave number, f is the frequency and c are the speed of photon. Im [n eff ] is the imaginary part of ERI.
In H-PCF fiber, the principal part is formed by the effective mode area (EMA). Here, the EMA is figured by [34]: Where, Aeffective is the EMA and I(rr) = |Ert| 2 is the cross-sectional electric field intensity.
Power fraction (PF) is resulted by the total power through the H-PCF fiber. So, the PF η is intended by [34]: V-parameter describes the mode propagation of the H-PCF structure. So, V-parameter is restrained by the following equation [35]: V= 2 √n 2 co − n 2 cl ≤ 2.045 (6) Where, the core radius is r, nco and ncl are signed by the EMI of the core and cladding area.

Analysis of Numerical Results and Discussions:
COMSOL Multiphysics software has been used to compute entirely optical properties and graphical results from Fig. 2 to Fig. 9 of this the recommended H-PCF are premeditated from 0.8 to 3 THz frequency range. The effective area of the designed PCF is illustrated in figure 2 according to the frequency changing from 1.00 THz to 3.00 THz for 60%, 70%, and 80% porosities. The effective mode area is pragmatic to be decreased gradually that shown in Fig. 2.
EML due to the changes in core diameter (Dcore) of proposed model with 60%, 70% and 80% porosities have been shown in Fig. 5 at 1 THz frequency. The increases of core diameter the EML is decreasing gradually at optimum design parameter. In our proposed PCFs Dcore = 324 μm the EML is about 0.0184 cm −1 for 80% core porosity which is optimum value and not production any complexity in fabrication. At a constant value of Dcore, the proposed model shows the different values of EML for different porosities.   6 indicates the distribution of power across the core, cladding and materials in accordance with frequency at a fixed Dcore = 324 μm. The experimental frequency varieties within 0.08 THz to 3 THz in electromagnetic spectrum. As it was found that, 80% optical power generated through the fiber core at frequency 1 THz which means maximum light contact with analytes in the core region. Furthermore, the air holes in cladding region induced light waves to pass within the core and provide maximum core power fraction. The pragmatic power fraction is significantly higher than the previously stated article.  Scattering loss is an important parameter because it contributes the total losses of the fiber. Scattering loss is increasing with the increases of frequency within 0.08 to 3 THz range appeared in Fig. 7 where's the Dcore= 324 μm. The gained scattering loss of proposed PCF is 1.236×10 -10 dB/km at optical wavelength 1 THz which is negligible.   Veff is explored as the function of frequency for optimum design constraint at Dcore= 416 um which has been revealed in Fig. 9. Her, the optimum constraints are cladding diameter d1 = d2 = d3 = d4 = d5 = 300 μm, cladding pitch Λ1 = Λ2 = Λ3 = Λ4 = Λ5 = 400 μm, core diameter da = 52 μm, db = 167 μm and core pitch Λc = 100 μm. The designed H-PCF shows outstanding EML, confinement loss, core power fraction, and effective area belongings than other designed PCFs at 1 THz functional frequency as providing in Table 1.

Conclusion:
An excellent design of five layers hexagonal cladding area based CAHs and two elliptical AHs in the core region are offered for communication applications with decreasing different types of losses such as EML, confinement loss, and scattering loss. TOPAS has been used as background material to remove different losses compare to the previous research work. Moreover, our designed H-PCF structure are designed with the procedure of FEM and PML conditions based on COMSOL Multiphysics software to get the simulated data. The graphical results of this H-PCF fiber show an ultra-low effective material loss (EML) of 0.0184 cm −1 , the larger effective area of 7.07×10 -8 m 2 , power moving in the core region of 88%, low confinement loss of 3.36 ×10 -15 dB/m, and scattering loss of 1.236×10 -10 dB/km respectively at 1 THz. So, after investigating all the simulation results, we can say strongly that our H-PCF fiber will be highly appropriate for numerous communication areas in the terahertz (THz) regime.