Detailed theoretical investigation of bending properties in solid-core photonic bandgap fibers

Abstract

Abstract:In this paper, detailed properties of bent solid-core photonic bandgap fibers (SC-PBGFs) are investigated. We propose an approximate equivalent straight waveguide (ESW) formulation for photonic bandgap (PBG) edges, which is convenient to see qualitatively which radiation (centripetal or centrifugal radiation) mainly occurs and the impact of bend losses for an operating wavelength. In particular, we show that cladding modes induced by bending cause several complete or incomplete leaky mode couplings with the core mode and the resultant loss peaks. Moreover, we show that the field distributions of the cladding modes are characterized by three distinct types for blue-edge, mid-gap, and red-edge wavelengths in the PBG, which is explained by considering the cladding Bloch states or resonant conditions without bending. Next, we investigate the structural dependence of the bend losses. In particular, we demonstrate the bend-loss dependence on the number of the cladding rings. Finally, by investigating the impacts of the order of PBG and the core structure on the bend losses, we discuss a tight-bending structure.

Understanding formation of photonic bandgap edge for maximum propagation angle in all-solid photonic bandgap fibers

“…The structural parameters are chosen as d=Λ× ðn 2 high − n 2 low Þ 1=2 , being constant in order to compare the PBG edges emerging at the same λ=Λ, in accordance with the ARROW theory for high-index rods [which can be explained with the aid of Eq. (4)] [21]. Both of them agree fairly well, and we can see the validity of the proposed formula.…”

“…Among all of the odd-order PBGs, the third-order PBG supports a lower confinement loss property than the first-order PBG [5] because the propagation angle for the core modes is lower in the third-order PBG; in other words, the core radius relative to the wavelength is larger in most cases. However, we have recently shown that the first-order PBG exhibits a lower bending loss property than the third-order PBG [21], and the exploitation of the first-order PBG has also attracted attention in recent years, by resolving the issue of the high confinement loss property [22].…”

“…In order to understand the properties of all-solid PBGFs, an important objective has been construction of a theory for the wavelength and propagation constant (propagation angle) of light with out-of-plane propagation at which a PBG emerges [23], because the wavelength is directly related to the transmission band, and the propagation constant to the number of allowed propagation modes [24,25] and the impact of bending losses [20,21,26]. When the PBG depth, defined as the difference between the effective indices of silica and the PBG edge, increases, a large number of guided modes are to be supported and, on the other hand, this also leads to a tightbending structure if the effective index of the core mode is constant [27].…”

Improved bending property of half-filled photonic crystal fiber

“…Moreover, a lower effective index mismatch between the fundamental and cladding modes could result in a larger bend loss in a bent fiber [13][14][15][16]. A different loss mechanism occurs in all-solid bandgap fibers in which the bend loss will arise mainly at the 'blue' or 'red' wavelength edge of the low-loss windows [15,16], depending on which of the downward and upward effective index mismatches, ∆n -and ∆n + , is smaller [13,14]. Our fully-filled PCF may be regarded as a so-called all-solid bandgap fiber.…”

“…Such a transformation in the fiber types may lead to various promising devices such as optical switches [4][5][6][7], tunable filters [8,9], attenuators [10], and dispersion compensators [11,12]. Compared with the indexguiding pure-silica PCFs, the bandgap-guiding fluid-filled PCFs are more susceptible to bend due to the low effective index mismatch between the fluid rod and the background silica [13][14][15][16]. As a result, the bandgaps of the fluid-filled PCFs could obviously be changed, even disappear, with the decease of the bend radius.…”