The effect of under sea level on the transmission of signals for FBG
Abstract
The transmission and the reflection of Fiber Bragg grating (FBG) are calculated theoretically under ocean. The nonlinear effects appear on the reflectivity and the transmitivity of traveling signals. A full study for the performance of the nine apodization profiles for the FBG under the effect of temperature, pressure and water depth are investigated. Then, schedules have been done for the optimum values for the reflectivity of all types, selecting the appropriate profile which is the Sinc one giving about 99% reflectivity for the grating.
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Introduction
An FBG is an optical fiber, in which the refractive index in a part of its core is perturbed forming a periodic or quasi-periodic index modulation profile. A narrow band of the incident optical field within the fiber is reflected by successive, coherent scattering from the index variations. When the reflection from a crest in the index modulation is in phase with the next one, we have maximum mode coupling or reflection. Then, the Bragg condition is fulfilled. By modulating the quasi-periodic index perturbation in amplitude and (or) phase, we may obtain different optical filter characteristics. The formation of permanent gratings by photosensitivity in an optical fiber was first demonstrated by Hill el al. in 1978 [1].
Because an FBG can be designed to have an almost arbitrary, complex reflection response, it has a variety of applications, well described by Hill and Meltz among others [2]. For telecommunications, the probably most promising applications have been dispersion compensation [3] and wavelength selective devices [4] depending on its reflectivity. Examples of the latter are filters for WDM systems [5].
Any change in the fiber properties, such as strain, temperature, or pressure which varies the modal index or grating pitch, will change the Bragg wavelength. Therefore, this change is studied in the following, including undersea fiber cables that use WDM techniques. Both linear and nonlinear fibers are taken into consideration.
Conclusion
The reflectivity of chirped FBG changes according to the apodization profile. A comparison between the different profiles leads to choose the best one in the wavelength selection performance. Sinc profile is the profile having the greatest reflectivity (R≈0.99 in the nonlinear case), while the lowest one is in Hamming profile (R≈0.85 in the nonlinear case) under the ocean depth effect. The obtained results show that the nonlinearity acts as a parameter that will help in the reflectivity increase by the increasing in the refractive index of the chirped FBG.