• A three part circulator is shown in Fig. 7.5.1. Signals of different wavelengths are entered at a port and sends them out at next port.

  • All the wavelengths are passed to port-2. If port-2 absorbs any specific wavelength then remaining wavelengths are reflected and sends them to next port-3.
  • Circulators are used to implement demultiplexer using ∂ fiber Bragg grating for extracting a desired The wavelength satisfying the Bragg condition of grating gets reflected and exits at next port. Fig. 7.5.2 illustrates the concept of demultiplexer function using a fiber grating and an optical circulator. Here, from all the wavelengths only λ3 is to be extracted.

  • The circulator takes four wavelengths λ1, λ2, λ3 and λ4 from input port-1 tunable filter operates on similar principle as passive devices. It operates over a range of frequencies and can be tuned at only one optical frequency to pass through it. Fig. 7.6.1 illustrates concept of tunable filter.


  • The system parameters for tunable optical filters are –
  • Tuning rage (∆v)
  • Channel spacing (δv)
  • Maximum number of channels(N)
  • Tuning


1.  Tuning Range (∆v)


  • The range over which filter can be tuned is called tuning Most common wavelength transmission window is 1300 and 1500 nm, then 25 Hz is reasonable tuning range.

2.  Channel spacing (δv)


  • The minimum frequency separation between channels for minimum cross talk. The cross talk from adjacent channel should be 30 dB fro desirable

3.  Maximum number of channels (N)


  • It is maximum number of equally spaced channels that can be packed into the tuning range maintain an adequately low level of cross-talk between adjacent channels. It is defined as the ratio of the total tuning range ∆v to channel spacing δv.


4.  Tuning speed


  • Tuning speed specified how quickly filter can be reset from one frequency to another.





Tunable Filter Types


  • Tunable filters with fixed frequency spacings with cannel separations that are multiples of 100 GHz (δv ≤ 100 GHz) are used in WDM systems
  1. Tunable 2 x 2 directional couplers
  2. Tunable Mach-Zehnder interferometers
  3. Fiber Fabry-perot filters
  4. Tunalbe waveguide arrays
  5. Liquid crystal Fabry-perot filters
  6. Tunable multigrating filters
  7. Acousto-optic tunable filters (AOTFs)