Fiber Bragg Gratings (FBG)

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Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. The main benefits of fiber are its exceptionally low loss, allowing long Distances between amplifiers or repeaters; and its inherently high data carrying Capacity , such that thousands of electrical links would be required to replace a Single high bandwidth fiber cable. Another benefit of fibers is that even whenrun alongside each other for long distances, fiber cables experience effectively no crosstalk, in contrast to some types of electrical transmission lines.

Fiber can be installed in areas with high electromagnetic interference (EMI),(along the sides of utility lines, power-carrying lines, and railroad tracks). The optical fibers are simple, flexible and have low cost. So they widely replaced the copper wire communication.

The discovery of photosensitivity in the fibers made a new path way to the development of fiber Bragg grating.By fiber Bragg grating we can integrate optical devices inside optical fiber.Fiber Bragg grating is a perturbation in the uniform refractive index of the fiber core. This is permanent due to the effect photosensitivity. They reflect a narrow band of optical field incident on it. This property of the fiber can be used in a filter application.As the filter is designed inside the fiber insertion loss of the device can be reduced to a very low value.The design of the filter is not complicated as well.

The reflected band changes when there is a change in temperature, pressure or strain.So they can be used in the sensing application. Fiber Bragg grating can be used as a chemical sensor also.They are also used in the biomedical application .Researches are going in the field of fiber Bragg grating which may lead to discovery of new devices.

What is Fiber Bragg Grating?

Fibre Bragg grating is an integrated optical device fabricated by a periodic Perturbation of refractive index along the photosensitive optical fiber. As we Know, in an optical fiber there is a core and a cladding having different refractive indices. But in Bragg grating we make the uniform refractive index of the core perturbed. So that it will not transmit all the frequencies of the incident optical field. The perturbation may be uniform or non-uniform.

This refractive index change is written by the irradiation of interfering uv Lights inside the fiber core. The magnitude of the refractive index change obtained depends on several different factors such as the irradiation conditions (wavelength, intensity, and total dosage of irradiating light), the composition of Glassy material forming the fiber core and any processing of the fiber prior to irradiation. A wide variety of different continuous wave and pulsed laser light sources with wavelengths ranging from the visible to the vacuum ultraviolet have been used to photo-induce refractive index changes in optical fibers.

In practice , the most commonly used light sources are KrF and ArF excimer lasers that generate , respectively , 248 and 193 nm optical pulses (pulsewidth 10 ns) at pulse repetition rates of 50 to 75 pulses/s. The typical irradiation conditions are an exposure to the laser light for a few minutes at intensities ranging for 100 to 1000 mJ/cm2.

When ultraviolet light radiates an optical fiber , the refractive index of the Fiber is changed permanently; the effect is termed photo sensitivity. The change in refractive index is permanent in the sense that it will last for decades (life times of 25 years are predicted) if the optical wave guide after exposure is annealed appropriately, that is by heating for a few hours at a temperature of 50 degree above its maximum operating temperature. Photo sensitivity is also observed in a wide variety of different fibers, many of which did not contain germanium as a dopant. Nevertheless, optical fiber having a germanium doped core remains the most important material for the fabrication of devices.

How Fiber Bragg Grating works ?

Fiber Bragg Gratings are based on the principle of Bragg reflection. When Light propagates through periodically alternating regions of higher and lower Refractive index, it is partially reflected at each interface between those regions. If the spacing between those regions is such that all the partial reflections add up. In phase when the round trip of the light between two reflections is an integral number of wavelengths, the total reflectivity can grow nearly 1, even if the individual reflections are very small. Of course, that condition will hold only for specific wavelengths. For all other wavelengths, the out of phase reflections end up cancelling each other, resulting in high transmission. The condition for high reflection is known as Bragg condition.

Types of Fiber Bragg Gratings

The optical properties of a fiber grating are essentially determined by the variation Of the induced index change along the fiber axis .There are several types of Bragg gratings according to the index change like uniform, apodized, chirped,phase shifted etc. Among them, the most commonly used gratings are uniform, apodized and chirped.

Uniform Fiber Bragg Grating

Here the grating is uniform throughout the core. This is the simplest type of Bragg grating. The transitivity function of a uniform grating has in general large and densely located out–of–band side-lobes. So in dense WDM (DWDM) systems where the channels are to be placed as closely as permitted by data bandwidth, the side lobes of the grating response introduces crosstalk as well as group velocity dispersion between the WDM channels. This is the major disadvantage of uniform Bragg grating.

Apodized Fiber Bragg Grating

Here the grating of the refractive index approaches to zero at the end of grating. That is there is a variation of intensity of induced refractive index change .The maximum index change is at the centre of Bragg grating. Apodization offers significant improvement in side lobe suppression. They are widely used in sensing application.

Chirped Fiber Bragg Grating

There is a linear variation in the grating period called chirp. Here the reflected Wavelength changes with grating period. So the reflected spectrum is more complicated than the other two types. A grating possessing a chirp has the property of adding dispersion namely, Different wavelengths reflected from the grating will be subject to different delays. This property has been used in the development of phased-array antenna Systems and polarization mode dispersion compensation, as well.

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