• 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)