When talking about the role of optical switches, it could be said that if there is no switch, there are no communication networks. Although this view of optical switches is a bit exaggerated, it fully proves the importance of the optical switches. From the first generation of telecommunication networks, namely the telephone switching system, a large number of switches are used to form a switching unit that satisfies the circuit-switching between users. However, today, with the development of DWDM technology, all optical network is gradually developed as the next generation communication network which is paid more and more attention of people and become a hot spot of research. In the DWDM based all optical network, it is inevitable to use optical switched to convert and transfer different wavelengths of optical signals in the network. Optical switch is the core component to complete switching and even protecting the circuit. Optical switched are widely used and play an irreplaceable role in optical networks.
Technical Indexes of Optical Switches
When evaluating of a new optical switch, we must consider the following seven technical indexes.
- Highly stable and reliable for long-term large capacity requirements of communication systems.
- Low insertion loss and high coupling efficiency.
- Low cross talk and high extinction ratio. Cross talk can directly affect the signal transmission quality, typical isolation for 40 and 50dB.
- With low drive and temperature characteristics.
- Speed switching can be regulated for different applications.
- Working bandwidth of the optical switch is corresponding to the fiber, optical filter and amplifier of DWDM working window 1300nm-1650nm, and optical switch must be in accordance with it.
- The cost and extensions of the optical switch, also an important consideration.
Types of Optical Swiches
An optical switch has one or more inputs ports and two or more output ports that we usually call 1xN or NxN optical switch. Different principles and technologies of optical switch are different characteristics and suitable for different occasions. Depending on its fabrication technology, optical switch can be divided into mechanical optical switch, opto-micro-mechanical optical switch, MEMS (Micro-Electro-Mechanical Systems) optical switch and other switches. Among them, mechanical optical switch and MEMS optical switch are the most mature and commonly used switches in the field. In addition, there are also Liquid crystal optical switch, thermal optical switch, acousto-optic switch, waveguide optical switch, solid-state optical switch and magneto-optic switch etc. On the other hand, based on its application, optical switches can be divided into mechanical optical switch, rack mount optical switch, bench top optical switch and so on. Now, we are going to give a brief introduction to some commonly used optical switches in the following.
Opto-Mechanical Optical Switch
Mechanical optical switch has developed for a long time and it is the most widely deployed at the time. These devices achieve switching by moving fiber or other bulk optic elements by means of stepper motors or relay arms. Benefits of traditional mechanical optical switch are low insertion loss (<2db), high isolation (>45dB), and not influenced by polarization and wavelength. In general, opto-mechanical optical switches collimate the optical beam from each input and output fiber and move these collimated beams around inside the device. This allows for low optical loss, and allows distance between the input and output fiber without deleterious effects. The defects of traditional mechanical optical switch are the long time to turn on or turn off, also devices have more bulky compared to other alternatives. Thus it is not easy to make large optical switch matrix. However, with development of technology, the new micro-mechanical devices overcome this. The new generation of opto-micro mechanical optical switch is with the characteristics of the wider broadband, compact structure and small size which can significantly reduce the optical switch elements in a matrix and the corresponding number of drives.
MEMS Optical Switch
MEMS (Micro-Electro-Mechanical Systems) optical switch is a micro-optical switch in free space which is composed of the semiconductor material. It is an advanced technology of optical switch and currently attracted a wide attention in the world. MEMS optical switch is compact, lightweight and easy to expand as well as the combination of the advantages of mechanical optical switch and waveguide optical switch while overcomes their defects. It is because of combining the electrical, mechanical and optical integration as a whole, so it can transparently transmit different rates and different business services and now have been widely used in industry.
Thermal optical switch
This kind of technology is commonly used to make miniature optical switch. In general, thermo-optic switches are normally based on waveguides made in polymers or silica. For operation, they rely on the change of refractive index with temperature created by a resistive heater placed above the waveguide. Their slowness does not limit them in current applications. It mainly has two basic types: digital optical switch (DOS) and interferometer optical switch.
In this kind of switch, acoustic wave is utilized to control the deflection of light. Because there are no moving parts, it is more reliable to do it. In general, the loss of 1×2 acousto-optic switch is lower than 2.5db.
Waveguide optical switch
Waveguide optical switch is new kind of optical switch which is a waveguide structure. In addition, electro-optic, acousto-optic, thermal-optic, and magneto-optic effect are also used in waveguide optical switch. Thanks to its small size, waveguide optical switch has a large-scale application in OXC.
The principle of magneto-optic switch is the Faraday rotation effect. Compared to the traditional mechanical optical switch, magneto has the benefits of faster switching speed and higher stability. In addition, compared to the other non-mechanical optical switches, it has a lower driving voltage and little cross talk. Therefore the magneto-optic switch will be a very competitive type of optical switch in the future.
Liquid crystal optical switch
The working principle of the liquid crystal optical switch is based on polarization control, ie. one patch, light is reflected by the polarization while the other path, light can go through. Because the optic coefficient of liquid crystal electro is high that comes to be the most effective photoelectric material. Additionally, the switching speed of liquid crystal optical switch can reach the degree of sub-microsecond. And with the increasing progress of technology, it may achieve the speed nano-second degree in the future.
Optical Bypass Switch
After talking about those several commonly used kinds of optical switches, you may have a deeper understanding of the optical switch. In addition, there is an optical switch which is called optical bypass switch. Users are often confused by its name. optical bypass switch is an optical switch which has the protection switching function. It is typically used for network failure recovery. It provides a permanent and trouble-free access port for in-line network security and monitoring devices. The optical bypass switch automatically switches network traffic through added in-line devices or bypasses devices that are about to be removed. With a heartbeat, the optical bypass switch protects network traffic against both signal and power loss on the attached in-line device. This kind of optical switching technology is used in many other devices and it is widely used in opticalline protection of PDH, SDH, C/DWDM, power communication and CATV systems etc.
Application & Prospect of Optical Switches
Optical switch plays a very important role in optical network, which is not only as the switching core of the key equipment in WDM network, but also as key components in optical networks. The main applications of optical switches as following:
- Protection switching function – optical switches are usually used for network fault recovery. When fiber break or other transmission failure occurs, optical switch will switch signal from the primary to the alternate route. For this application, the simplest kind of 1X2 optical switch seems to be the most commonly used one. In addition, optical switch with bypass function can be widely used in various types of gateway devices in a network. It protects against network failure and simplifies network maintenance by ensuring network integrity during power loss. When in-line devices on a gigabyte fiber network lose power or need to be removed from the network, the optical bypass switches automatically switch network traffic to bypass devices that have less power.
- Network monitoring – In the remoter test point, several fibers can be connected to an ODTR with the 1xN optical switch. Via switching of the optical switch can achieve the monitoring for all fibers. In addition, optical bypass switch supports fail-open monitoring with any fiber in-line device when it shares the same power source as the in-line appliance. While the optical bypass switch is receiving power, it diverts network traffic to attach in-line devices. In this state, all in-line traffic is routed directly to the device connected to the optical bypass switch.
- Optical testing – Connecting the optical components which are going to be tested with fiber optics, through 1xN optical switches, these optical components can be tested by monitoring the signal of each channel of the optical switches.
- Applied to OADM and OXC – OADM is mainly used in loop MAN (metropolitan area network) which allows to add one or more new wavelength channels to an existing multi-wavelength WDM signal, and/or to drop (remove) one or more channels, passing those signals to another network path. Using optical switches, it can greatly increase the flexibility of network configuration. OXC is consisted of optical switch matrices. It is mainly used for cross connection in backbone optical network that realizes the fault protection, dynamic light path management and the flexible services for the networks.
With the development of optical transport network technology, new optical switch technologies are constantly emerging while the original optical switch technologies are continuing to improve. As the optical transmission network developed in the direction of ultra high-speed and large-capacity, network survivability, network protection switching and recovery may be the critical issues. Optical switch plays an important role in protection and recovery of this field and fills the gap. In order to better fit the new and increasing network update, the size of the switching matrix of the optical switches may continue to grow in the future. In addition, the switching speed of optical switches will need higher requirements. In a word, a large-capacity, high-speed, low-loss optical switch will be needed in the future network and plays a more important role in the development of optical network.
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