What Is Fiber Bragg Grating (FBG) ?

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A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical 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 dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.

Simply put, a fiber Bragg grating is a distributed Bragg reflector. This is what a simple FBG looks like.

Normal fibers have uniform refractive index along the length. But in a FBG, the refractive index of the core changes periodically along a short length. As shown in this picture, the period is Λ (Lambda).

So what kind of effect will this have on the fiber's light guiding properties? Let's look at the next slide.

As shown here, when a light with many wavelengths is launched into a FBG, one particular wavelength is in phase with the grating period, and this wavelength is reflected back to the input end. All other wavelengths pass through to the other end since they are not in phase with the grating period.

This makes the FBG reflect a particular wavelength and transmit all others.

On this slide, it shows the spectrum response of a FBG.

The first graph is the input spectrum, the center graph is the transmitted spectrum, and the third graph is the reflected spectrum. Here is the close up view of the reflected spectrum.

Bragg grating wavelength λB is the central wavelength that is reflected back by a FBG, λB can be calculated using this equation.

We can see that λB is the function of the effective refractive index and the period of the grating.

Δλ is the bandwidth of a Bragg grating. Δλ is the wavelength spacing between the first minima, and it is given in this equation.

The previous sample shows a very basic FBG. But you can change a FBG's refractive index profile, or the grating period to achieve many different designs.

The grating period can be uniform or graded, and either localized or distributed in a superstructure as shown in the left picture.

On the other hand, you can change the refractive index profile or the offset. Typically, the refractive index profile can be uniform or apodized, and the refractive index offset is positive or zero. The right picture shows different refractive index changes.

Fiber Bragg Gratings have many applications in modern fiber optic communication systems. They have been used as Optical Add-Drop Multiplxers, Dispersion Compensators, and stress and temperature sensors.

Fiber Bragg gratings can be made using holographic approach with a bulk interferometer.

The UV beam is divided into two at a beam splitter and then brought together at a mutual angle of θ, by reflections from two UV mirrors. The two UV beams interfere and writes the pattern on the photosensitive fiber.


So there you have it.

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Colin Yao

Fiber Optics For Sale Co.