Optical switches are an integral part in fiber optic transmission systems and contribute to the development of the “all-optical” network. An optical switch is simply a switch which accepts a photonic signal at one of its ports and send it out through another port based on the routing decision made.
There are two basic types of optical switches – the O-E-O (Optical to Electrical to Optical) and the O-O-O, or all-optical switch. Each has its place in fiber optic systems with their unique features and capabilities.
The OEO switch is a technology already used in networks today. The problem with this technology is that may not be able to keep up with the speed of optical transmission in the future. OEO switches are also bit rate and protocol dependant, which means that protocols must be used and bits from the transmitted frames must still be processed (unlike with OOO switches). They have the ability to process header information, and are able to make routing decisions based on this. The OOO switch holds the promise of the future AON -All Optical Networks,, but it is still an emerging technology and cannot adequately function without the intelligence that OEO switches currently hold.
Protocols and Standards
OEO switches are already being used within the confines of SONET Synchronous Optical Networks or SDH.- synchronous Digital Hierarchy. SONET and SDH are multiplexing protocol standards which were setup to support the very fast data rates required in optical networks.
Optical switching networks makes use of Dense Wave Division Multiplexing (DWDM). This is a multiplexing technique where multiple signals can be shared on a single optical fiber, with each signal sharing a different wavelength (ie. a different spectrum of light). DWDM systems today carry up to 160 different signals on a single fiber. This greatly enhances bandwidth of networks today. Optical networks use the standard protocol of traditional networks, such as IP and Ethernet.
The Optical switch will soon be operating within a network architecture known as GMPLS (Generalised MPLS).
Prime applications are optical protection, test systems, and remotely reconfigurable add-drop multiplexers.
NxM Matrix Switch – This switch uses standard telecom optical SFP (Small Form Pluggable) transceivers on the inputs to convert the incoming optical signal to its native electrical digital data stream. Similar optical transceivers are used on each of the corresponding outputs to convert the switched electrical data stream back to an optical signal for further transmission. The switch matrix is electrical and includes other functions such as reclocking and retiming to help clean up the signal and return it to its original condition, discounting any conversion-related anomalies.
Passive protection switching for service restoration following a disruption, such as a fiber cut. At the transmit location the signal is split into two redundant signals and sent over two diverse optical paths. This switch design accepts these two optical signals from the same transmitter via the different fiber paths and will monitor the optical activity on each fiber. One path is set as the primary optical path and will automatically switch to the redundant fiber path if this primary path were to be interrupted or if the signal level falls below a pre-determined optical threshold.
One common application for switches is in Remote Fiber Test Systems (RFTSs) that can monitor and locate a fault on a fiber transmission line.
An emerging application of optical switches is optical cross-connection. Optical cross-connects utilize optical switching fabrics to establish an interconnection between multiple optical inputs and outputs. Optical Cross Connects are similar of electronic routers which forward data using switches. An OXC may contain a whole series of Optical Switches.
Types of Optical Switches MEMS
The Micro Electrical Mechanical System (MEMS) was the first all optical device to be developed into a physically feasible product and is now the most common wavelength switching technique without initial electronic conversion. These devices are normally miniscule mechanisms made from silicon, with many moving mirrors ranging from a few hundred micrometers to a few millimeters. These mirrors exist on a silicon wafer and are packed as an array. The switch works by deflecting light waves from one port to another through these mirrors.
Liquid Crystal Switches
The Liquid Crystal Switch makes uses of the polarisation effects of light in liquid crystals (similar to the type used for laptop screens) to switch light. The advantages of the liquid crystal switch lies in its low power consumption..
A bubble based switch, named the Photonic Switching Platform has been developed by Agilent Technologies Inc, using technology similar to that which is used in inkjet printers. This switch is capable of using 32×32 switches without the moving parts of MEMS.
These switches are normally small in scalability, from 1×2 to 6×6 switches. There are two main types of switches: digital optical switches (DOS) and interferometric switches. The DOS works by changing the refractive index of light. Using a 1×2 Y switch, light travels through both arms of the switch. One of the arms is heated and the light will be blocked in the switch.