Fiber Bragg Gratings

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Fiber Bragg grating is one of the key optical components, which are gaining increasing attention in different fields of optical technologies including optical fiber communication and sensing applications. The report entitled ?Recent Trends in the Fiber Bragg Grating Based Technologies? focuses the recent developments and advancements in the applications and fabrication methods of Fiber Bragg based optical systems. It also highlights the innovative technological developments and the limitations of fiber Bragg Gratings. A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core, which generates a wavelength specific mirror. The ability to filter out a specific wavelength made it possible to use it as sharp wavelength filter which extensively found applications in the field of optical communication as multiplexers, demultiplexer and add-drop multiplexer etc. The use of the fiber Bragg grating as the dispersion compensation element made revolutionary developments in the field of fiber optic communication..The Bragg wavelength as a function of grating pitch also made it possible to build transducers for precisely measuring many physical quantities like strain, temperature, acceleration etc.


2. BASIC THOERY OF FIBER BRAGG GRATING

A fiber Bragg grating is a periodic or aperiodic perturbation of the effective refractive index in the core of an optical fiber. Typically, the perturbation is approximately periodic over a certain length of e.g. a few millimeters or centimeters, and the period is of the order of hundreds of nanometers. This leads to the reflection of light (propagating along the fiber) in a narrow range of wavelengths, for which a Bragg condition is satisfied. This basically means that the wave number of the grating matches the difference of the wave numbers of the incident and reflected waves. (In other words, the complex amplitudes corresponding to reflected field contributions from different parts of the grating are all in phase so that can add up constructively; this is a kind of phase matching.) Other wavelengths are nearly not affected by the Bragg grating, except for some side lobes which frequently occur in the reflection spectrum (but can be suppressed by apodization). Around the Bragg wavelength, even a weak index modulation (with amplitude of e.g. 10-4) is sufficient to achieve nearly total reflection, if the grating is sufficiently long (e.g. a few millimeters).
The reflection bandwidth of a fiber grating, which is typically well below 1 nm, depends on both the length and the strength of the refractive index modulation. The narrowest bandwidth values, as are desirable e.g. for the construction of single-frequency fiber lasers or for certain optical filters, are obtained for long gratings with weak index modulation. Large bandwidths may be achieved with short and strong gratings, but also with aperiodic designs. As the wavelength of maximum reflectivity depends not only on the Bragg grating period but also on temperature and mechanical strain, Bragg gratings can be used in temperature and strain sensors. Transverse stress, as generated e.g. by squeezing a fiber grating between two flat plates, induces birefringence and thus polarization- dependent Bragg wavelengths.

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