Analysis of proposed PCF with square air hole for revolutionary high birefringence and nonlinearity

https://doi.org/10.1016/j.photonics.2021.100896Get rights and content

Highlights

  • Novel structure of PCF composed with square air holes in the cladding is proposed.

  • Central slot has been doped with highly nonlinear refractive index material gallium phosphide.

  • A comprehensive analysis of optimized PCF is descripted.

  • Very high values of core power fraction and nonlinearity are achieved.

  • Very high value birefringence is achieved with very low effective material loss (EML).

Abstract

A novel square shaped air hole based photonic crystal fiber (PCF) is proposed with three symmetrical rectangular slots in the core region. A highly nonlinear material Gallium phosphide (GaP) is doped in the middle rectangular slot to achieve high nonlinearity and birefringence as well. Finite element method (FEM) is used for numerical investigation of the structure. Different optical parameters of the proposed PCF are calculated by varying its geometrical parameters. The optical parameters are optimized for the core pitch (ΛC) value between the rectangular slots as 0.18 μm. From simulation results, it can be concluded that an ultra-high nonlinear coefficient and birefringence values of 0.645 × 105 W−1 km−1 and 0.385 are achieved along with very low confinement loss and effective area of 2.10 × 10-08 dB/m and 4.18 × 10−13 m2, respectively at the centre of communication wavelength 1.55 μm. The core power fraction (CPF) is also found high as 98.056195% with extremely low effective material loss (EML) of 0.018 cm-1. Moreover, scattering loss, beat length, numerical aperture (NA), V-parameter and spot size have also been calculated. Therefore, the proposed PCF can be effectively used in single mode polarization splitter, polarization maintaining fiber, solitons generator, four wave mixer, optical signal processing, bio-medical imaging and also in sensing applications.

Introduction

Photonic crystal fiber (PCF) is researcher’s field of interest due to its exceptional properties compared to conventional optical fiber. High nonlinearity and birefringence, amendable dispersion and high-power handling capabilities are some exceptional properties of the PCF [1]. PCF possess such mentioned features due to its array of holes in the cladding region around the core that run along its length [2]. In PCF, light can be guided by two mechanisms either by total internal reflection (TIR) or by photonic band gap (PBG). In PBG, light can be guided in low refractive index (RI) core by mirroring effect of cladding while in TIR light can be guided in core by making core RI higher than cladding RI [3,4]. However, PBG-PCFs bear narrower transmission range compared to TIR based PCFs [5]. Due to high design versatility of core and cladding, PCF can be effectively utilized in dispersion compensation, polarization sustaining fiber, supercontinuum generation, high nonlinear as well as in terahertz applications [[6], [7], [8], [9], [10]]. High birefringence as well as high nonlinearity is required for the PCF to be used in sensing applications [11,12]. In order to achieve high birefringence and nonlinearity, PCF structure should have high RI contrast (between core and cladding) as well as asymmetric core or cladding. Moreover, elliptical shape of core and cladding with different orientation also produces high birefringence [13]. In 2007, D. Chen et al. achieved high Birefringence with hexagonal shaped PCF [14]. In this sequence, Kim et al. in 2012 showed that higher birefringence and flattened dispersion can be achieved with PCF structure having elliptical air holes [15]. In 2016, Bo et al. demonstrated that birefringence can also be enhanced using a micro structured hybrid PCF [16].

It has been often seen that pure silica-based PCF structure gave very low value of nonlinear coefficient due to its low nonlinear refractive index [17]. To overcome this issue, highly nonlinear materials such as SF-57, Bismuth and, GaP etc. can be used to obtain high nonlinearity as these materials have high nonlinear RI. Apart from these nonlinear materials, intrusion of liquids in the core slots also showed high value of nonlinearity [18]. In 2014 S. Wei et al. presented that PCF having elliptical core with telluride doping results high value of nonlinearity equals to 3400 W−1Km−1 [19]. In 2015, it is reported that elliptical core made up with SF-57 material produced high value of nonlinearity [20]. In 2016, Amin et al. reported high value of nonlinearity valued equals to 104 W−1Km−1 by doping gallium phosphide (GaP) in the symmetrically slotted core of PCF [21]. A very high value of both non linearity and birefringence has also been reported for elliptical core PCF doped with As2S3 by Zhang et al. [22]. In 2018, a manuscript showed that Si-nc doped core also produced high value of nonlinearity [23]. Anas et al. in his manuscript also showed that high value of nonlinearity, birefringence and numerical aperture could be achieved with Gap doped rectangular slotted core [17]. In 2018, Paul et al. showed that high nonlinearity value 4.72 × 104 W−1Km-1 can be achieved with chalcogenide doped elliptical core [24]. In 2019, Paul et al. also delineated that doping of Si3N7 in the elliptical core resulted high value of nonlinearity as 4.9 × 104 W−1Km−1 [25]. Moreover, they have also calculated core power fraction, scattering loss and numerical aperture (NA) in his manuscript.

In this article, a rectangular slotted PCF core doped with GaP is proposed. Three rectangular slots have been introduced in the core in order to improve birefringence. For high nonlinearity, GaP is filled in the middle of three rectangular slots. Moreover, these slots have been taken in hexagonal manner for the ease in fabrication. The cladding region is delineated with circular rings of square shaped air holes. The proposed structure provides the high nonlinear coefficient and power fraction values of 1.73 × 105 W−1Km−1 and 98.056195% respectively, at 1.00 μm. Additionally, a high numerical aperture (NA) value of 0.797 is observed at 1.8 μm wavelength which makes this structure more suitable for imaging applications. Other properties like scattering loss, confinement loss (CL), V-parameter, spot size, mode field diameter (MFD) and effective material loss (EML) have been also calculated for the proposed structure.

Section snippets

Structure of the proposed Fiber

Fig. 1 displays the cross section of proposed PCF with magnified core structure view. Proposed PCF has five circular rings of square shaped air holes in cladding region. Here, we have chosen square shaped airhole as it bears larger hole packing density (HPD) compared to circular air holes [26]. In order to prove this fact, the HPD ratio of both square and circular shaped airholes has been compared. The HPD ratio (S/ Λ) of square and circular airholes are estimated using Eqs. (1) and (2)

Results and discussion

This section of the manuscript bears analysis of optical properties of the PCF such as core power fraction (CPF), effective area (Aeff), nonlinear coefficient (γNL), birefringence, confinement loss (CL), scattering loss, numerical aperture (NA), mode field diameter (MFD) and beat length. Fabrication tolerance has been taken more than ±2% due to variation in designing parameters during the fabrication process. Hence, effect of varying core pitch (ΛC) on above said parameters are studied with the

Conclusion

In short, the optical properties of the proposed PCF are investigated for large range of wavelength variation using FEM. Various optical parameters such as wideband birefringence, nonlinear coefficient, power fraction, beat length, effective material loss, scattering loss and confinement loss are calibrated against wavelength by changing pitch (ΛC) of the core. Therefore, the propounded PCF achieved an ultra-high value of nonlinear coefficient as 1.73 × 105 W−1Km−1 as well as exceptionally high

Declaration of Competing Interest

The authors report no declarations of interest.

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