A Fiber Bragg Grating is a Bragg Grating embedded within an optical fiber. Bragg grating is a reflector used in optical fibers which are constructed using alternating material with varying refractive indices. It has a periodic or aperiodic refractive index variation, along the core for a certain length as shown in the figure below. Its design and operation are based on the principle of Fresnel reflection, i.e., the light propagating from one media to another with different refractive indices will experience both partial reflection and refraction at the interface. A Fiber Bragg Grating reflects a narrow band of wavelengths centered at the Bragg wavelength and allows transmission of all other wavelengths in the input light.
If light with a broad spectrum is input into the fiber Bragg grating, the wavelength corresponding to the Bragg wavelength will be reflected back to the input end and the remaining wavelengths will pass through it to the other end. That is, the reflection spectrum will show the presence of the Bragg wavelength alone while the transmission spectrum at the output contains the broad spectrum except for the Bragg wavelength. So, the Fiber Bragg Grating acts like a wavelength selective dielectric mirror or a narrow band filter which allows transmission of all wavelengths except for a very narrow band of wavelengths centered at the Bragg wavelength. FBG being a symmetric system reflects light at Bragg wavelength regardless of the side from which it emerges.
Ideally, the Bragg grating can be considered to have a rectangular refractive index profile. However, depending on the way the grating is marked onto the fiber core, the contrast in the refractive index profile will be less sharp and often shows a periodic variation closer to sinusoidal variation.
Considering a rectangular refractive index profile, the refractive index of the core region varies between n1 and n1 +∆n in the FBG region with a period of ∧. At each interface where the refractive index changes, a small amount of light is reflected. All these partial reflections add up in phase and result in a back reflected or diffracted wave at Bragg wavelength 𝛌B. It is determined by the period of the microstructure ∧ and its effective refractive index as,
Here, q = 1, 2, … is the diffraction order. Many Bragg wavelengths are possible for this system. These are given by each value of q. But, the deviation from the rectangular refractive index profile in real fibers ensures that only the fundamental diffraction order (i.e., q=1) will be available for practical applications.
Fabrication of FBG
FBG is usually fabricated by illuminating the fiber core with a high-intensity UV laser beam which causes a permanent change in its refractive index profile. Various techniques are used to provide the necessary illumination patterns that correspond to the microstructure of the FBG. These techniques can be classified as holographic and non-interferometric techniques.
In the holographic technique, a single UV laser beam is split into two using a beam splitter. Later, these two beams are allowed to interfere at a specific angle to produce an interference pattern at the fiber core. This angle determines the Bragg wavelength independent of the UV wavelength. In non-interferometric techniques, the optical fiber is periodically exposed to pulsed UV illumination, or an amplitude mask is used.
The changes in temperature or strain will directly modify the values of eitheror ∧ and hence cause a Bragg wavelength shift. This makes FBG a suitable candidate for sensing applications. It is also ideal for multiplexing and chromatic dispersion compensation applications.
When an FBG is stretched or compressed, it can measure strain. This is because the optical fiber experiences deformation which results in the change in the grating period ∧. Consequently, it causes a shift in the Bragg wavelength. Depending on the wavelength shift, the strain experienced by the fiber can be calculated. It will also contain some contributions from the photoelastic effect, which causes variation of refractive index due to strain.
The temperature change can cause refractive index variations in the material used to manufacture the fiber due to the thermo-optic effect. This will result in shifting the Bragg wavelength. Studying the reflection spectra will allow temperature sensing. It has a very small contribution due to thermal expansion that alters the periodicity.
FBG can be used as Optical Add-Drop Multiplexer (OADM) device in optical fiber communication systems to introduce a certain signal or channel with a particular wavelength into the stream of signals or to extract it out.
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