a. Optical Isolator
In a number of applications it is desirable to have a passive optical device that is nonreciprocal; that is, it works differently when its inputs and outputs are reversed. Two examples of such a device are isolators and circulators
Optical isolators
Optical isolators are devices that allow light to pass through them in only one direction. This is important in a number of instances to prevent scattered or reflected light from traveling in the reverse direction.
One common application of an optical isolator is to keep such light from entering a laser diode and possibly causing instabilities in the optical output.
Many design configurations of varying complexity exist for optical isolators. The simple ones depend on the state of polarization (SOP) of the input light as shown in figure 3.7.
Figure 3.7 Principle of operation of isolator for particular state of polarization of the input signal
However, such a design results in a 3-dB loss (one-half the power) when unpolarized light is passed through it, since it blocks one-half of the input signal.
In practice, the optical isolator should be independent of the SOP since light in an optical link normally is not polarized.
Figure 3.8 shows a design for a polarization-independent isolator that is made of three miniature optical components. The core of the device consists of a 45° Faraday rotator that is placed between two wedge-shaped birefringent plates or walk-off polarizers. These plates could consist of a material such as YVO4 or $TiO_2$.
Figure 3.8 A polarization independent isolator. Spatial walk off polarizer used at the input and output a) Propagation from left to right b) Propogation from right to left
Light traveling in the forward direction (left to right in Fig. 3.8) is separated into ordinary and extraordinary rays by the first birefringent plate. The Faraday rotator then rotates the polarization plane of each ray by 45°.
After exiting the Faraday rotator the two rays pass through the second birefringent plate. The axis of this polarizer plate is oriented in such a way that the relationship between the two types of rays is maintained.
Thus when they exit the polarizer, they both are refracted in an identical parallel direction.
Going in the reverse direction (right to left), the relationship of the ordinary and extraordinary rays is reversed when exiting the Faraday rotator due to the nonreciprocity of the Faraday rotation.
Consequently, the rays diverge when they exit the left-hand birefringent plate and are not coupled to the fiber anymore.
b .Optical Multiplexer & Filters
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Optical filters are essential components in transmission systems for at least two applications: to multiplex and demultiplex wavelengths in a WDM system—these devices are called multiplexers/ demultiplexers—and to provide equalization of the gain and filtering of noise in optical amplifiers.
A simple filter is a two-port device that selects one wavelength and rejects all others. It may have an additional third port on which the rejected wavelengths can be obtained.
Multiplexer combines signals at different wavelengths on its input ports onto a common output port, and a demultiplexer performs the opposite function. Multiplexers and demultiplexers are used in WDM terminals as well as in larger wavelength crossconnects and wavelength add/drop multiplexers.
Figure 3.9 Different applications for optical filters in optical networks. (a) A simple filter, which selects one wavelength and either blocks the remaining wavelengths or makes them available on a third port. (b) A multiplexer, which combines multiple wavelengths into a single fiber. In the reverse direction, the same device acts as a demultiplexer to separate the different wavelengths.
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 3.10 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.
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