While the technology has been around about 10 years, its commercial application has only recently occurred. A Bragg grating can be built directly into a fiber to act as a wavelength-selective filter. An FBG is made by subjecting a short length of fiber to strong ultraviolet light. A mask is used to form a pattern through which the UV light passes. The strong light, typically in the 240 to 260-nm region, creates interference patterns that permanently alter the refractive index of the fiber core at precisely defined intervals. The mask is precisely made in relation to the wavelengths of light it will deal with. The patterns of altered refractive indexes are periodic, that is, they repeat in regular patterns. Different periodic patterns can be combined to work on a variety of wavelengths. How It Works

When a stream of light reaches the FBG, most wavelengths pass through with little loss and no disruption to the signal. However, certain wavelengths will be strongly reflected. The wavelengths reflected depend on the period of the grating: the grating will reflect wavelengths that are twice the grating period. In other words, to reflect a 1550-nm wavelength, the grating should have a period of 775 nm. The operation of the FBG depends on three things: the strength of the changes in refractive index, the length of the grating (how many periods), and the precision that the gratings in which formed.

A FBG can be used as a narrowband filter. A single wavelength is strongly reflected, while other wavelengths pass through with next to no insertion loss. FBGs can be constructed with many different properties to meet application needs. Some select the narrowest of wavelength bands and offer extremely strong reflections. FBGs are available with bandwidth of under 0.2 nm and the ability to reflect over 99.9% of the light. In this case, bandwidth refers to the width of the light reflected. For a 1550-nm signal, it will strongly reflect light from 1549.9 to 1550.1 nm. Others work over a wider band and have weaker reflections or even couple light from the core into the cladding. For example, you may want to flatten the power in one wavelength compared to another. In this case, you might want the FBG to reflect only 10% of the energy at a specific wavelength and pass the rest. The precision with which an FBG can be built to select only very narrow wavelengths makes it popular in WDM and other applications that require very narrow wavelength selection.

Applications

FBGs have been used for the following:

Wavelength multiplexers and Demultiplexers in dense wavelength-division multiplexing.

FBGs can have a resolution of well under 1 nm. The figure below shows two approaches to using an FBG for DWDM applications. One uses an optical circulator, which is a device that sequentially presents the light to each optical port. At port 2, all the wavelengths pass except the selected one, which is reflected. This wavelength then appears as output on port 3. The circulator-based approach is simple, but the cost of circulators tend, at this point, to be high.

The second approach is based on what is known as a Mach-Zehnder interferometer, which uses a coupler-like structure using waveguides. It is more complex, but avoids the circulator.

Dispersion compensation:

a chirped FBG can be used to recompress pulses back to their original shape. Essentially, the FBG negates material or chromatic dispersion. It does so by varying the period along the length of the grating. While a WDM grating reflects only one wavelength, an FBG for dispersion compensation reflects different wavelengths along the length of the grating. This introduces differing amounts of delay for different wavelengths. By carefully gauging the delays needed in terms of the dispersion, you can build a filter that "tightens" the pulse.

Wavelength locking of pump lasers in optical-fiber amplifiers.

The FBG ensures the narrow spectral width of the output.

Gain-flattening filter in optical-fiber amplifiers.

This ensures that all wavelengths are amplified an equal amount to the same amplitude. A long-period FBG couples light from higher-gain wavelengths into the cladding. All wavelengths show the same gain at the output.

FBGs offer excellent wavelength selectivity and filter shape; that is, they cleanly distinguish between different wavelengths. In addition, the technology is very flexible and allows great latitude in choosing the type of filter created for different needs. Drawbacks include temperature dependence, which requires the device to be maintained at a consistent temperature through heating or cooling. In addition, filtering a number of wavelengths can create complex structures.