Investigations on Fiber Bragg Gratings for Fiber Optic Communication Systems

Abstract

This thesis presents the Investigations on Fiber Bragg Gratings for Fiber Optic Communication Systems. A Fiber Bragg Grating (FBG) is a periodic or aperiodic perturbation of the effective absorption coefficient or the effective refractive index of an optical waveguide. A Bragg Grating can reflect a predetermined narrow or broad range of wavelengths of light incident on the grating, while passing all other wavelengths of the light. FBG can be fabricated by exposing a photosensitized fiber with an intensive pattern of excimer laser radiation. Fiber Bragg Gratings have emerged as important components in a variety of applications. Their unique filtering properties and versatility as in-fiber devices is illustrated by their use in Wavelength-Stabilized Lasers, Fiber Lasers, Remotely Pump Amplifiers, Raman Amplifiers, Phase Conjugators, Wavelength Converters, Passive Optical Networks, Wavelength Division Multiplexers (WDM’s), Demultiplexers, Add/drop Multiplexers, Dispersion Compensators, and Gain Equalizers. They are also stimulating growth in fiber optic applications outside of telecommunications, such as nonlinear frequency conversion, spectroscopy, and remote sensing. Since the output of a Bragg grating sensor is present in the wavelength domain it requires sophisticated circuits to measure wavelength shift. One way of interrogating a Bragg grating sensor is to use a tunable filter. One option is the Fabry–Perot interferometer with a cavity length controlled by a piezoelectric actuator. Although this approach has a good resolution and is independent of light intensity variations, it is relatively slow and expensive. Chirped Fiber Bragg Grating (CFBG) can also be used as the wavelength-sensitive component. A chirped Bragg grating can be used as a device that converts wavelength into delay. By measuring this delay, the wavelength of light corresponding to the delay can be found. Very fast electronic circuitry is required for resolving this delay and it proves to be very expensive. An interferometer may be used to convert wavelength shifts into phase shifts, which can be detected by measuring variations in the light intensity as the path difference in the interferometer is varied. This technique potentially allows for very high sensitivity, but the equipment to do it is expensive and prone to environmental interference. A sloped optical filter, which may be another Bragg grating, can be used to convert wavelength shifts directly into intensity changes. If the filter is designed to have a known pass/reject ratio which varies with wavelength, then the wavelength of a narrowband reflection from a single grating can be determined simply by measuring and comparing the passed and rejected intensities. This technique referred to as edge filtering technique is the simplest and most economical way of demodulating FBG. The accuracy in this technique is impaired by microbend losses that occur in the fiber connecting the Fiber Bragg Grating Interrogator to the Fiber Bragg Grating. If a portion of fiber is deformed, the fiber would exhibit excess light loss. Such perturbation of fiber axis results in redistribution of guided power between modes of the fiber and also coupling of the fiber from one mode/mode group to another. This thesis presents the technique to circumvent this problem. Thus this thesis presents the Investigations on Fiber Bragg Gratings for Fiber Optic Communication Systems. The simulation of Fiber Bragg Gratings has been carried out and the results presented have been validated using theoretical and experimental work. A marginal change in the model of FBG has been proposed to account for the loss in accuracy due to non-identical refractive index modulation in the FBG. The wavelength shift in Fiber Bragg Gratings on simulated external perturbation using a wideband source has also been observed and finally a Fiber Bragg Grating Interrogator for Sensor/Communication systems has been designed. A new technique for mitigating the errors due to effect of microbending in the fiber lead from FBG to the interrogator has also been incorporated in the design of FBG interrogator. Most of the research findings of this thesis have been published in various international referred journals (Chapters 4, 5, 6, 7 and 8) as per the list at page (201).