Since the frequency response of the FBG has very narrow pass band, it can be used as a narrow band filter. As mentioned earlier, at 1550nm wavelength a band width as narrow as 0.2-0.3nm can be achieved very easily.
If a broad band or multi-wavelength signal is fed to the FBG, selectively a wavelength is reflected back with out affecting the propagation of other wavelengths.
Add-Drop Multiplexer
In a DWDM network wavelengths are used for labeling the packets. Also the network nodes are addressed by specific wavelengths. A particular node transmits and receives a particular wavelength assigned to it.
The node needs a device which facilitates it to drop the signals at a particular wavelength and at the same time allows it to add the signals at that wavelength.
The device is called the add-drop multiplexer (ADM).
FBG can be used for realizing an ADM. An FBG based ADM consists of two optical circulators and a FBG as shown in Fig.
The circulator has a property that signals can flow in one direction. The direction is indicated by a an arrow. In the Figure the signals can propagate in clockwise direction only.
The input signal gets connected to the FBG but the reflected signal from FBG is diverted to the lower port and not the input.
Since the FBG has reflecting wavelength , all wavelengths pass through the FBG except . The wavelength is dropped at the input circulator.
The signal at the output of the FBG are devoid of . However, if a new signal is fed at the input of the output circulator, the signal gets connected to the FBG and gets reflected towards the output. It then gets combined with the incoming signals which are devoid of .
All signals are then connected to the output through the circulator. The output therefore has again all the wavelengths with old replaced by new .
Instead of single FBG, if multiple FBGs of different wavelengths are placed between the circulators, multiple wavelengths would be dropped and added by the device.
, all wavelengths pass through the FBG except 
. The wavelength 
is dropped at the input circulator. 
. However, if a new signal 
is fed at the input of the output circulator, the signal gets connected to the FBG and gets reflected towards the output. It then gets combined with the incoming signals which are devoid of 
. 
replaced by new 
. 
