The second device that we are developing, depicted in Fig. 41, is a simpler Bragg-grating filter. The gratings in this device are long, uniform structures without quarter-wave shifts. In this implementation, each of the Bragg gratings acts like a wavelength-selective reflector. The two identical Bragg gratings are integrated in a Mach-Zehnder interferometer, which separates the signals reflected from the gratings from the input signal.
Light is launched in the upper left port of the device, and split equally by the coupler.
A portion of the light is reflected by the identical Bragg gratings located in the arms of the interferometer. Provided the arm lengths are matched, these reflected signals recombine and emerge in the lower left port of the device. Depending upon the characteristics of the Bragg grating, the filter can be configured to perform many different functions. For example, by appropriately selecting the length and depth of the Bragg grating, the reflection spectral response can be made to have a bandpass shape. The same channel may be simultaneously added by launching it in the upper right port.