pp.33-36
Sandhya Sharma1, Chhavi Sharma2 1Associate Professor, Department of ECE, Gyan Vihar School of Engineering and Technology SGVU, Jaipur, Rajasthan, India 2 M.Tech Scholar-DWCE, Gyan Vihar School of Engineering and Technology, SGVU, Jaipur
Abstract: This paper proposed new class of optical fibre which is used in various applications and gives better performance as compare to conventional optical fibre called Photonic crystal fibre. It is microstructure optical fibre in which photonic crystal plays very important role runs along the entire fibre length and enhances its properties. In this paper comparative study has been done between the properties of the hexagonal shape pcf namely V-parameter, Birefringence and Confinement loss with different shapes of air hole such as circular and square shape. Properties are numerically investigated by using Finite difference time domain method and it is found that holey fibre is single mode, highly birefrigent and exhibit low confinement loss for square shaped air holes.
Keywords- Birefringence, Confinement loss, FDTD method, Hollow core, Photonic Band gap, Photonic crystal fibre, V-parameter.
INTRODUCTION
Now a days we deals with optical fibre for communication purpose and non communication field also where it shows its best performance but it has some limitations such that rigid design rules, limited core diameter, material choice etc. To overcome these limitations and enhance its characteristics, a new class is developed called Photonic crystal fibre in which photonic crystals plays very important role and its design flexibility increases its capability and working area which is impossible in classical optical fibre. Firstly, demonstrated by Yeh et.al in1978 which is Bragg Fibre after this in 1992 Philip Russell gives another idea of two dimensional photonic crystal fibre[123] with air core and in 1996 first Photonic crystal fibre was proposed with enhance its properties and easy to manipulate its design parameter such as pitch, diameter, air hole shape etc. Pcfs are usually made from pure silica, for this experiment we use holey fibre also called Photonic crystal fibre in which photonic crystal runs along the entire length of the fibre and this special arrangement of air holes provide confinement and guidance of light. We take hexagonal lattice of nine rings pcf with central defect is created by removing one or more (as per requirement) air holes from the central lattice. In this experiment holey fibre is consider in which one air hole is removing from the centre of the fibre lattice and this defect is work as core of the fibre whereas remaining part of the fibre work as cladding whose refractive index is different from core refractive index and responsible for modified total internal reflection. There are two guiding
mechanism in Pcf one is index guiding and another is photonic bandgap guiding mechanism [3].
Fig 1: Photonic Crystal Fibre
If the periodicity of the lattice or structure is broken another property of photonic crystals are created which support particular mode of frequencies falling inside the photonic band gap. Photonic crystal allows a certain set of wavelengths of electromagnetic radiation, pass through it so that the certain set of frequency which are prohibit called photonic band gap thus, photonic crystal transmit light of certain wavelength only in the certain direction. For the light confinement mechanism, index guiding pcf rely on total internal reflection to confine light in the defected region called core, capable of controlling guidance of light within the certain frequency band. In this paper, hollow core photonic crystal fibre of two dimensional hexagonal crystal lattice with nine
rings of air hole is consider where diameter of air hole is given by d and pitch (˄) is defines the distance between the air holes. For this experiment different design parameters has been taken with constant pitch and value of diameter is varied for each design parameter.
Fig 2: Structure of PCF
The proposed pcf consists of nine rings of air hole with hexagonal lattice have core and cladding structure with different refractive index. Spacing between the adjacent air holes called pitch, constant at 2.3µm and diameter varies from 1.104µm to 0.70µm with operating wavelength (λ) = 1.55µm, used to design pcf structure with circular air holes and calculate different properties of pcf and compare with square shape air holes pcf with same design parameters as used for circular shape. For calculating the different numerical values of refractive index we use finite differential time domain method.
V- PARAMETER
In this we observe that whether our fibre is single mode fibre or multimode fibre. By using this property we clarify the propagation modes of the fibre, travel in it and describe its normalized frequency or V parameter [4]. When we talking about single mode operation, it is required that value of V-parameter should be less than or equal to 2.4048 and for multimode propagation value of V is greater than 2.4048. For this we have different value of diameter and constant pitch value, used to design pcf structure with circular and square shape air holes. We calculate refractive index value of cladding structure and core is made from pure silica whose value is 1.45µm. This concept also used in conventional optical fibre where modes in the fibre are determined by using formula of normalized frequency given by
V = 2πρ/λ (^2 – ^2) ^1/2 (1)
FDTD method is used to calculate the refractive index value for circular and square shape air holes, putting all these values in above given formula and compare it and make graph between them which conclude that our fibre is single mode or multi mode fibre.
Fig 3: Hexagonal lattice PCF with circular shape of air holes and Hexagonal lattice PCF with Square shape air holes.
Fig 4: Comparison graph between normalized frequency parameter and wavelength of circular and square shape air hole PCF.
In above graph, we observe that value or refractive index is less than 2.408 which shows that our fibre is single mode fibre.
BIREFRIENGENCE
In this study, high birefringence [6, 7] pcf is investigated by using fdtd method which gives the measurement of the propagation of light that propagates into the fibre and highly birefringence property is observed in pcf as compared to conventional optical fibre. It is the optical property of a material which have refractive index that depends on the polarization and direction of propagation of light. It gives control over the propagation of light and reduces the coupling between the modes of degeneration and defined by the difference between the refractive index of the ypolarized and x- polarized fundamental mode of orthogonally polarized beam in the y and x direction respectively.
ΔN= − – − (2)
ΔN = Birefringence
Where − = effective index of the Y polarized fundamental mode
− = effective index of the X polarized fundamental mode.
A novel hollow hexagonal lattice pcf is consider which have different design parameter with circular and square shape air hole and comparison has been done by using graph between the values of birefringence and wavelength of circular and square shape of air holes.
Fig 5: Comparison graphs between birefringence (ΔN) and wavelength (λ) of circular and square shape air hole PCF.
CONFINEMENT LOSS
In this we examined the confinement loss [5] property which gives the ability to the fibre to confine light in the hollow core region. In pcf propagation of light occurs in the core region and number of air holes runs along the fibre act as cladding region which fixed in number. If leakage of the light occurs from core of the fibre to the exterior potion the fibre core, reduces its effective transmission of light or signal that is called confinement loss. We can observed confinement loss and effectively reduce its value by the proper selection of the value of diameter, pitch and number of air holes in the fibre which is not possible in conventional optical fibre. It can be calculated by the imaginary part of the refractive index, given by
Confinement loss = 8.686Im () dB/m
(3)
Where = 2π/λ
We calculate different imaginary values of refractive index for circular and square shape air holes and compare all these values, plotting graphs as given below. We conclude that square shaped cells exhibit low confinement loss as compare to circular shape air hole.
Fig 6: Comparison graphs between confinement loss and wavelength (λ) of circular and square shape air hole PCF.
CONCLUSION
In this paper we observed different properties of hexagonal lattice photonic crystal fibre with circular and square shape air hole and conclude that our fibre is single mode hollow core fibre in which its normalized frequency is less than critical value. High birefringence and confinement loss are analyzed numerically by using finite differential time domain method and gives result which shows
that fibre is highly birefringence and value increases as the size of the circular shape air hole increases. We can also conclude that the value confinement loss in square shaped air hole is low as compared to circular shape air holes.
REFERENCES
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