1 Fatima Hussain

Introduction The unprecedented demand for optical network Capacity has fueled the development of long-haul optical network systems which employ wavelength- division multiplexing (WDM) to achieve tremendous capacities. Such systems transport tens to hundreds of wavelengths per fiber, with each wavelength modulated at 10 Gb/s or more. Up to now, the switching burden in such systems has been laid almost entirely on electronics. In every switching node, optical signals are converted to electrical form (O/E conversion), buffered electronically, and subsequently forwarded to their next hop Electronic switching is a mature and sophisticated technology. However, as the network capacity increases, electronic switching nodes seem un able to keep up. Apart from that, electronic equipment is strongly dependent on the data rate and protocol Fatima Hussain 2

1. Insertion loss: This is the fraction of signal power that is lost because of the switch. 2. Crosstalk: This is the ratio of the power at a specific output from the desired input to the power from all other inputs. 3. Extinction ratio (ON–OFF switches): This is the ratio of the output power in the on-state to the output power in the off state. 4. Polarization-dependent loss (PDL): If the loss of the switch is not equal for both states of polarization of the optical signal, the switch is said to have polarization-dependent loss. 3 Fatima Hussain

1)Optomechanical Switches: Optomechanical technology was the first commercially available for optical switching. optomechanical switches, the switching function is performed by some mechanical means. These mechanical means include prisms, mirrors, and directional couplers. Advantages: 1. low insertion losses. 2. low polarization-dependent loss. 3. low crosstalk. 4. low fabrication cost. Disadvantages: 1. Switching speeds are in the order of a few milliseconds. 2. The lack of scalability. 3. Optomechanical switch configurations are limited to 1 ×2 and 2× 2 port sizes. 4 Fatima Hussain

2)Electrooptic Switches: A 2× 2 electrooptic switch uses directional coupler whose coupling ratio is changed by varying the refractive index of the material in the coupling region. One commonly used material is lithium niobate LiNbO. A switch constructed on a lithium niobate waveguide is shown in Fig. 1. An electrical voltage applied to the electrodes changes the substrate’s index of refraction. The change in the index of refraction manipulates the light through the appropriate waveguide path to the desired port. Advantages: Electrooptic switch is capable of changing its state extremely rapidly, typically in less than a nanosecond. This switching time limit is determined by the capacitance of the electrode configuration. Electrooptic switches are also reliable. Larger switches can be realized by integrating several 2 ×2 switches on a single substrate Disadvantages: 1. high insertion loss. 2. Polarization independence is possible but and possible polarization dependence.. 3. higher driving voltage, which in turn limits the switching speed. 5 Fatima Hussain

Fig. 1. An electrooptic directional coupler switch. 6 Fatima Hussain

3)Microelectromechanical System Devices Although Microelectromechanical system (MEMS) devices can be considered as a subcategory of optomechanical switches, they are presented separately mainly because of the great interest that the telecommunications industry has shown in them, but also because of the differences in performance compared with other optomechanical switches 4)Thermooptic Switches The operation of these devices is based on the thermooptic effect. It consists in the variation of the refractive index of a dielectric material, due to temperature variation of the material itself. 7 Fatima Hussain

8 5)Liquid-Crystal Switches The principle of operation is as follows the birefringent plate at the input port manipulates the polarization states to the desired ones. Birefringent materials have different refractive indexes along two different directions (for example Х, the and У axes). Without applying a bias, the input signal passes through the liquid-crystal cell and polarization beam splitter with the same polarization. Liquid-crystal switches have no moving parts. They are very reliable, and their optical performance is satisfactory, but they can be affected by extreme temperatures if not properly designed.. A 1×2 liquid-crystal optical switch structure is shown in Fig. 2.

fig.2. Scheme of 1×2 liquid-crystal optical switch 9 Fatima Hussain

10 Fatima Hussain 7)Acoustooptic switches the input signal is split into its two polarized components (TE and TM) by a polarization beam splitter (Fig. 3). Then, these two components are directed to two distinct parallel waveguides. 6)Semiconductor Optical Amplifier Switches Semiconductor optical amplifiers (SOAs) are versatile devices that are used for many purposes in optical networks. An SOA can be used as an ON–OFF switch by varying the bias voltage. If the bias voltage is reduced, no population inversion is achieved, and the device absorbs input signals. If the bias voltage is present, it amplifies the input signals. The combination of amplification in the on-state and absorption in the off-state make this device capable of achieving very high extinction ratios

Fatima Hussain 11 Fig. 3. Schematic of a polarization independent Acoustooptic switch.