a. Fabry-Perot Filters:

A Fabry-Perot filter consists of the cavity formed by two highly reflective mirrors placed parallel to each other, as shown in Figure. This filter is also called a Fabry-Perot interferometer or etalon. The input light beam to the filter enters the first mirror at right angles to its surface. The output of the filter is the light beam leaving the second mirror.

Principle of Operation: The principle of operation of the device is illustrated in Figure above. The input signal is incident on the left surface of the cavity. After one pass through the cavity, as shown in Figure, a part of the light leaves the cavity through the right facet and a part is reflected. A part of the reflected wave is again reflected by the left facet to the right facet. For those wavelengths for which the cavity length is an integral multiple of half the wavelength in the cavity—so that a round trip through the cavity is an integral multiple of the wavelength—all the light waves transmitted through the right facet add in phase. Such wavelengths are called the resonant wavelengths of the cavity.

This is a classical device that has been used widely in interferometric applications. Fabry-Perot filters have been used for WDM applications in several optical network testbeds. There are better filters today, such as the thin-film resonant multicavity filter. These latter filters can be viewed as Fabry- Perot filters with wavelength-dependent mirror reflectivities.

b. Arrayed Waveguide Grating:

Ans:

Arrayed waveguide grating (AWG) is used for demultiplexing. These are based on diffraction principle. The input consists of several channels carrying different wavelengths. Light propagating in the input waveguides are coupled to the arrayed waveguides after propagating through a free space region (FPR). The AWG consists of an array of curved waveguides. Each curved waveguide has a length ΔL = m λc /n which is more than the array element immediately below it, where λc is the central operating wavelength, n is the effective refractive index and m is an integer. Thus at the central wavelength, the light will focus at the centre of the image plane. Different wavelengths have different phase differences and the focal point for each will be shifted. A second coupler will deliver different wavelengths to different waveguides.

c. Optical Switch

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• Switching is an essential operation in communication networks. It is also a basic operation in digital computers and signal processing systems. The current rapid development of high-data-rate fiber-optic communication systems has created a need for high-capacity repeaters and terminal systems for processing optical signals and, therefore, a need for high-speed photonic switches.

• A switch is characterized by the following parameters:

  • Size (number of input and output lines) and direction(s), i.e., whether data can be transferred in one or two directions.

  • Switching time: (time necessary for the switch to be reconfigured from one state to another).

  • Propagation delay time: (time taken by the signal to cross the switch).

  • Throughput: (maximum data rate that can flow through the switch when it is connected).

  • Switching energy: (energy needed to activate and deactivate the switch).

  • Power dissipation: (energy dissipated per second in the process of switching).

  • Insertion loss: (drop in signal power introduced by the connection).

  • Crosstalk :( undesired power leakage to other lines).

  • Physical dimensions. This is important when large arrays of switches are to be built.

• Optical signals may be switched by the use of electronic switches: the optical signals are converted into electrical signals using photodetectors, switched electronically, and are converted into electrical signals using photodetectors, switched electronically, and then converted back into light using LEDs or lasers (Fig. 4.10).

• These optical/electrical/ optical conversions introduce unnecessary time delays and power loss

Figure 4.10 An optoelectronic 8*8 crossbar switch. Eight optical signals carried by eight optical fibers are detected by an array of photodetectors, switched using an 8 X 8 electronic crossbar switch, and regenerated using eight LEDs (or diode lasers) into eight outgoing optical fibers. The data rates that can be handled by silicon switches are currently a few hundred Mb/s, while GaAs switches can operate at rates exceeding 1 Gb/s.

• Optical Switching Technologies

  • Optomechanical Switches

  • Micro electrical mechanical system devices

  • Electro optic switches

  • Thermo optic switches

  • Liquid Crystal switches

  • Bubble switches

  • Acousto optic switch

  • Semiconductor amplifier switch