New in-line fiber band pass filters using high silica dispersive optical fibers
Introduction
With rapid growth of wavelength division multiplexing (WDM) optical communication systems, various fiber filters are being intensively developed to manipulate optical channels in the spectral domain. Fiber-based filters have drawn attention due to their low insertion loss, low cost, and flexible design in spectral responses. Currently, fiber filters based on the mode coupling mechanism are being deployed in communication systems. Among those, fiber Bragg gratings (FBGs) [1] and fiber long-period gratings (LPGs) [2] are well-established technologies, where the spectral responses could be tailored by control of the mode coupling defined by the fiber waveguides and refractive index grating structures inscribed by UV exposure. Fiber acousto-optic tunable filter (AOTF) [3] is another type of fiber filter based on the mode coupling, which induced by a tunable flexural acoustic wave along the optical fiber. Bandwidths of these fiber filters, however, are limited by the phase matching conditions and the overlap integrals between the coupled modes.
Morishita et al. [4], [5] reported a different class of broadband fiber filters by using highly dispersive characteristics of the core and the cladding made of multi-component glasses (MCG). In the devices, two different types of glasses whose dispersion curves, refractive index versus wavelength, cross each other are used for the core and the cladding, so that the refractive index difference between them could change as shown in Fig. 1. Those dispersive fibers could inherently function as either a short-pass or a long-pass filter depending on the glass composition of the waveguides. Despite their simple structures, dispersive MCG fibers have not been readily applicable to practical optical communications and sensor systems due to fundamental problems in splicing with conventional single mode fibers (SMFs). First of all, dispersive MCG fiber could not be fusion spliced to SMF using conventional electric arc technologies due to the large differences in melting points and thermal expansion coefficients between MCG and high silica in SMF. Although mechanical butt-coupling might be attempted between those glasses, it induces a large insertion loss due to the mismatch in refractive indices and subsequent Fresnel reflection loss. Furthermore, MCG dispersive fibers suffer from a large scattering loss because they are fabricated by rod-in-tube (RIT) technique that tend to induce irregularities and imperfections at the core-cladding interface.
In this paper, we propose a new dispersive optical fiber made of high silica that is compatible to conventional SMF. Highly dispersive characteristics of fiber were obtained by using borosilicate glass and fluorosilicate glass as its constituents [6], [7]. This fiber can be fabricated using conventional vapor precursors such as BCl3 and SiF4 along with modified chemical vapor deposition (MCVD) system that would significantly reduce the fiber attenuation and fusion splice loss with SMFs. Spectral responses of the proposed fiber were theoretically analyzed and a short-pass filter fiber was experimentally demonstrated.
Section snippets
Dispersive characteristics of B2O3–SiO2 and F–SiO2 glass systems
The dispersive characteristics of binary silica glass that is doped with dopants such as GeO2, P2O5, F and B2O3 have been precisely calculated using the Sellmeier equation for given ranges of concentrations. Sellmeier equation [8] for the refractive index nj is given aswhere the suffix j refers to the concentration of dopants, and λ is the optical wavelength in μm. Sellmeier coefficients, denoted by aij and lij, for binary silica doped with F and B2O3 [8] are
Results and discussions
In order to implement the proposed short-pass filter in silica optical fiber, a preform was fabricated using MCVD system. Flow of BCl3 and SiF4 vapors were added with precise control along with SiCl4 during deposition of the core and the cladding layers, respectively. The final preform consisted of three layers, pure silica substrate, F-doped silica inner cladding, and B2O3-doped silica core. The refractive index profile measured at 0.6328 μm is shown in Fig. 8. The relative index difference, Δ
Conclusions
Utilizing dispersive characteristics of SiO2–B2O3 and SiO2–F glass system, in-line band pass filters were theoretically proposed and demonstrated. It is found that a short-pass filter could be achieved by the structure of SiO2–B2O3 core and SiO2–F cladding, while a long-pass filter with SiO2–F core and SiO2–B2O3 cladding. Precise adjustment of the cross-wavelength can be controlled by dopant concentrations during fabrication procedure. In experiment a short-pass filter with 25 dB band rejection
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