A novel hybrid hexagonal photonic crystal fibre for optical fibre communication

Highlights

• A novel hybrid hexagonal Photonic Crystal Fibre is proposed.

• Imparts an additional degree of freedom in the design by exploiting the aspect ratio.

• Design is optimized to the aspect ratio of 1.618 (golden ratio).

• Versatile design with D = −270 ps/nm.km, B = 2.3 × 10⁻², γ = 50.58 W⁻¹km⁻¹ & NA = 0.49 at 1.55 μm.

• Influence of the structural parameters on the characteristics of the PCF is studied.

Abstract

In this paper, a novel hybrid hexagonal Photonic Crystal Fibre(PCF), which harnesses the golden ratio for its elliptical core design, is proposed. This work aspires to exploit circular air holes in the construction of PCF. It aids not only to achieve the desired properties but also to curtail the extravagant efforts needed for the fabrication. The Finite Element Method is used to analyze the properties of the proposed PCF. The proposed PCF possesses a negative dispersion of −270 ps/nm.km, a very high birefringence of 2.3 × 10^-2, a high numerical aperture of 0.49, and a large nonlinear coefficient of 50.58 W^-1km⁻¹ at the desired wavelength of 1.55 μm used for optical communication. The fabrication tolerance of ± 1–2% variation in air hole diameter is also investigated. The proposed PCF with very high birefringence and a large nonlinear coefficient will be an inevitable candidate for optical broadband communication.

Introduction

Photonic Crystal Fibre (PCF) is an intriguing research area that is booming at a fast pace. It has unrivalled advantages over conventional optical fibre. This micro-structured fibre consists of a solid core surrounded by the cladding with air holes that runs along the entire length of the fibre [1]. It has the flexibility to vary its design parameters so that the inherent characteristics such as dispersion, birefringence, nonlinearity, etc. can be engineered at will. In Literature, studies have been carried out for different PCF lattice structures, such as circular [2], square [3], pentagon [4], hexagon, octagon [5], decagon [6], dodecagon, spiral [7], etc. Diverse hybrid structures ([8], [9], [10], [11], [12], [13], [14], [15]) have also been proffered to ameliorate the efficacy. This research work is restricted to a naïve hexagonal geometry since it has a unique attribute of covering the entire structure without any gap and has great potential for tight light confinement [16]. High birefringence is elicited by inculcating asymmetry into the core region. Elliptical air holes are introduced in the hexagonal lattice structure to procure high birefringence in the order of 10^-3 and, its structure is optimized for two zero-dispersion points [17]. Three elliptical air-hole rings are augmented into the cladding region of Hexagonal PCF. It has been reported that the birefringence surges with the number of elliptical rings, and an ultra flattened dispersion over a spectral range of 540 nm is realized [18]. A squeezed triangular lattice PCF composed of the elliptical air holes is experimented and high negative dispersion of −1343.46 ps/nm.km with a high birefringence of 3.72 × 10^-2 is achieved [19].

Even though the fabrication of intricate structures is feasible through the sol–gel technique, it is infelicitous for large-scale manufacturing, which is essential for long-distance fibre-optic communication [20]. Some research works concentrated on solely using circular air holes for its design and attained desirable characteristics such as high negative dispersion, large nonlinearity, and high birefringence. Compressed hexagonal PCF, a simple structure consists of circular air holes has been investigated. With this structure high negative dispersion of − 135.2 ps/nm.km, high birefringence of 1.59 × 10^-2, and nonlinear coefficient of 42.58 W⁻¹ km⁻¹ at the wavelength of 1.55 μm are obtained [21]. Bored core hexagonal PCF with circular air holes is constructed [22]. This work reported an ultra-high negative dispersion of – 2102 ps/nm.km and a large nonlinearity of 111.6 W^-1km⁻¹ simultaneously. The birefringence is improved in hybrid hexagonal PCF by varying the size of the air holes in the cladding and inserting small air holes in the core. The zero-dispersion wavelength of 1.30 μm and an ultra-low confinement loss of 6 × 10⁻⁴ dB/km at 1.55 μm is achieved [23]. All the reported works are focused on improving particular characteristics for a specific application.

The design of PCFs with different characteristics have been predominantly studied for years. However, there is still scope for a versatile design with high birefringence, large nonlinearity, and negative dispersion. In this research work, a novel hybrid hexagonal lattice PCF structure with the aforementioned characteristics is designed to find suitable applications in optical communication. The asymmetry in the core region is achieved by arranging the circular air holes within the elliptical lattice instead of using the elliptical air hole to reduce the adversities during the fabrication process. In addition to the conventional design parameters of PCF, such as the diameter of the air hole and pitch, the proposed PCF structure imparts an additional degree of freedom in the design by exploiting the aspect ratio. Most of the reference literature have not discussed much on the subsidiary characteristics such as the numerical aperture and V-number. However, those characteristics are well taken into consideration and studied in the proposed PCF. The design is optimized by varying the aspect ratio of the ellipse in the core region, and the optimum value of the aspect ratio is found to be the golden ratio(1.618), which is significant in nature. The effect of other design parameters such as the diameter of the air holes and pitch on the characteristics of the proposed PCF is also analyzed. A very high birefringence of 2.3 × 10^-2, less effective area of 2.4 μm², a large nonlinear coefficient of 50.58 W^-1km⁻¹, negative dispersion of −270 ps/nm.km, and numerical aperture of 0.491 at the wavelength of 1.55 μm are achieved in the proposed design. The fabrication tolerance of ± 1–2% variation in air hole diameter is also investigated.