Fiber Bragg Gratings are created by exposing a short length of the fiber to intense ultraviolet light. A mask with a periodic pattern is placed over the fiber, and the 240 to 260 nm light alters the refractive index of the fiber's core. The mask is made in relation to the wavelengths of light upon which the filter is to operate. The period of the pattern should be half the period of the light upon which filter is to act. Most light pass through the filter with ease, but the selected wavelength will be strongly reflected. The performance of the grating depends on the strength of the changes to the refractive index, the number of periods used in the grating, and the precision of the filter's creation.
Bragg Gratings used as fiber optic filters can typically have a near zero insertion loss for unfiltered wavelengths, with a 50 dB loss at the set wavelength. High quality gratings can have 0.2 nm bandwidths and reflect 99.9% of the light in that band. Other gratings can be made to reflect less light, or have a wider band, allowing wavelength-specific precision attenuation. The low insertion loss of Bragg Gratings and the narrow bandwidths they offer make them ideal for use in wavelength division multiplexing. The biggest drawbacks to these filters is that they must be kept at a consistant temperature.
To use a Bragg Grating in a WDM application, there are multiple approaches. One option is to use an optical circulator, like so:
/ \ Bragg
λ1, λ2, λ3, λ4 >--------| |----||||--> λ1, λ3, λ4
Although this approach is effective, the cost of circulators makes it often desirable to find a cheaper alternative, like the Mach-Zehnder Interferometer
. The Mach-Zehnder
approach uses a coupler
-like structure with waveguide
s. Although this method is cheaper, it is much more complex to build.
Another use for Bragg Gratings in fiber is to filter the output of the pump lasers in optical amplifiers, and to flatten the gain of the amplifier's output. A long period Bragg Grating couples the high-gain wavelengths into the cladding, causing all wavelengths to show the same gain at the output.
Yet another use in fiber optics is compensating for chromatic dispersion. By carefully varying the periods along the length of the grating, different amounts of delay can be added to different wavelengths. Done properly, this will tighten the pulse.