A large communication network can be pictured as having two main parts: a transmission plant and switching facilities. The first transports traffic between network nodes, while second routs traffic over the transmission plant to get it from the source to destination. In recent years optical transmission technology has progressed very faster.
Transparent switches are the switches in which optical switches are routed without intermediate conversion into electronic form.
These switches are also called photonic or transparent switches .Of course these switches are cheap and capable of dealing with thousand of inputs and outputs that traditional electronic switches handle so well.
Several approaches are being explored for making these devices. These include array of tiny movable mirrors, known as microelectromechanical systems, or MEMS, and unit based on holographic crystal, liquid crystal total internal reflection and polarization dependent materials. The problem is to figure out which all-optical switching technology to use in what application. Optical switches are sometimes referred as O-O-O switches. Unlike O-E-O switches, present all optical switches are not capable of separately routing each of low data streams carried by a single input wavelength.
O-E-O Switches:
The equipments available for switching optical signal today is almost all of hybrid optical electronic optical (O-E-O) type which is expensive to build integrate and maintain. As a result these switches have not been widely deployed.
An optical electronic optical (O-E-O) switch separates incoming optical signals into individual wavelengths (optical demultiplexing) .Then each wavelength is converted into single high speed electronic data stream ,and the high speed data stream is demultiplexed into low speed channels . Then route each channel path digitally ,combining groups of low speed channels into high speed stream and modulating each high speed stream into an optical wave length. Finally, through optical multiplexing (wave length division) they place many of optical wave length onto an optical fiber .These switches are called opaque switches.
Advantages of this technique are powerful .Since each data stream has been converted into electronic form, so each stream can be monitored and can be routed independent of all the other .But drawbacks are equally formidable. Not only they are expensive, they are also incapable of handling signals that do not conform to standard data rates .They consumes kilowatts of power .these switches also required.
Transparent switches:
Now telecommunication industry is crying out for all-optical switches in which optical signals are routed without inter mediate conversion into electronic form. These switches are also called photonic or transparent switches .Of course these switches are cheap and capable of dealing with thousand of inputs and outputs that traditional electronic switches handle so well.
Several approaches are being explored for making these devices. These include array of tiny movable mirrors, known as microelectromechanical systems, or MEMS, and unit based on holographic crystal, liquid crystal, total internal reflection and polarization dependent materials. The problem is to figure out which all-optical switching technology to use in what application. Optical switches are sometimes referred as O-O-O switches. Unlike O-E-O switches, present all optical switches are not capable of separately routing each of low data streams carried by a single in out wavelength. Fortunately this capability is not an intermediate requirement of many applications.
Comparing technologies –
Many possible technologies are being applied to create optical switching systems. In fact, any physical process that will affect some property of light without causing too much loss can be used.
Affected property can include propagation speed, polarization and, direction. Any changes in them are exploited to redirect light beams as desired from an input to an output.
Precision Bulk Optics:
The most mature approach available is precision bulk optics which creates robust connection. This technologies have many forms- for example, having a motor move a precision mirror surface to direct an input light beams as desired from input to an out put .Example are Lucent technologies’ original direct beam steering technology .
These switches can have exception optical performance (low loss, reflection, and crosstalk) because they rely on high manufacturing technique .Yet there are three pronounced limitations in bulk optics that prevents technology from sweeping all optical infrastructure. They are too expensive, too large, and too slow.
Mach-Zender interferometers:
Mach-Zender interferometers are the base of next most mature O-O-O switching technology. This method splits the in coming light beams into two beams and, routing each beam along a different path, then recombining them to form two out puts.
If the phase is varied on one of two paths by changing the speed of light along that path, fraction of input light sent to each of outputs can be controlled. Changing the phase from 0 to 180 degrees shifts all the light output port to another. By having the path traverse a material in which speed of light is a function of temperature or the strength of applied electric field .There are several advantages of using MZI switch. It is reliable, fast and it integrates well with other functions. On the market today are many of two inputs, two output (2X2) switches based on MZI.
Yet draw backs of MZIs have serious implication for future use in optical infrastructure.
Speed of light can be changed only slightly (less than .01%) by reasonable change in electric field strength or temperature. So this restricts technology’s scalability about 40 ports.
Most waveguide devices are also polarization dependent. The use of dielectric waveguide leads to losses and coupling issues.
Microelectromechanical Systems :
Microelectromechanical systems (MEMS) are small mechanical devices. These devices are built using semiconductor fabrication technologies that provide small size , precision ,repeatability, and low cost in high volume. All-optical switches can be built using MEMS in many configuration .The simplest use a single microscopic moving mirrors to redirect the light. This creates a single pole, double through (1X2) switch. These can be implemented in two ways:
(1).By covering and uncovering the beam path using a sliding, fixed orientation mirror
(2) by swinging a titling mirror between two precision angular stops.
The next level of complexity is built using a two dimensional array of these mirrors to form a matrix switch, with rows of inputs or columns of outputs (or vice -versa).( SEE NEXT FIGURE)
Switches are with eight inputs and eight outputs are readily implemented using this technique. This can also be extended to about 64X64. In these cases of, control of mirror is digital – that is, mirror is swung between fixed stops, and tight control of its motion is not needed.
However precision manufacturing and packaging are required to ensure that stops are positioned properly.
Different lengths of optical paths through various switch configurations limit the scaling. The approach leads to a very cost-effective medium-scale matrix switch, as all of the packaging is planner.
The optical paths between individual mirrors can be through free space or waveguides. Combination of microelectomechanical systems and wave guides are extremely promising for next generation
of medium scale switch .these switches are currently being investigated by several companies such as Nanovation Technologies and Kymata.
More complex switches are 3-D switches. 3-D switches are built using two axis mirrors to steer the optical beams. (SEE NEXT FIGURE).
These require extremely fine analog control to align their optical beams because the beams must be accurately directed along two angles and then stop at precise intermediate positions, not just at fixed end points. Three-dimensional switches scale well because the number of mirrors required equals just the total no. of ports.
It appears today that sizes are as high as 4096X4096 are feasible and could become available as soon as the economical climate improves enough for demand to develop.
Another key contributor to the scaling is that the optical path length depends little on which ports are connected ,as opposed to 2-D matrix switches , leading to more uniform switch behavior. Limits to the scaling include the diameter of the mirrors and their maximum tilt angle. The mirrors should be about fifty percent bigger than optical beams to avoid excessive loss, tilt is limited by both the methods used to build the switch and technique used to actuate the mirrors .Another challenge is the electrical drive for mirrors .
At least four electrical connection per mirror are required. Thus thousand of electrical interconnects must com off the MEMS chip unless the addressing, control and drive electronics can be integrated under the mirrors. That integration will be far from easy, because high voltage circuitry that must function with great precision is required.
In their present form these switches are limited to large ports counts because their costly 3-D packaging makes them too expensive for smaller switches.
Although suitable tests are not available for establishing 20 years life telecommunication application , non contact MEMS have proven a track record of high reliability in many industrial and consumer application ,such as air bag acceleratometers ,pressure sensor and inkjet printheads .
The robustness and reliability of MEMS switches was recently proven when switches from OMM Inc. passed the demanding Telcordia qualification test.
Many other MEMS approaches to switching are being investigated, including moving fiber, bending wave guide and sliding shutter, and curling mirrors. Each approach has unique that have promise in application for small and medium scale switching.
Liquid Crystals-
Another method of achieving all-optical switching is by means of liquid crystal (LC) technology .Liquid crystal technology can be used to control the polarization of alight beam. Liquid crystals are used in conjunction with polarization dependent material, which absorb or reflect light with specific polarization. When a voltage is applied to a LC device, the individual crystal elements align with the applied electric field. If elements are orthogonal to optical beam polarization, light will reflected. On other hand if elements are lined up with optical, light passes through the liquid crystal.
While liquid crystal technology is well characterized and has been proven reliable in many years of display applications. It has three noted disadvantages. It is fairly slow especially at low temperature, where switching time can be hundreds of millisecond .It is difficult to integrate with other optical components.
It has relatively high light losses from liquid crystal itself ,the polarization splitter and imperfections in fairly complex optical path.
One of the most challenging aspects of applying liquid crystal technology to optical switching directly relates to their use of polarization. The optical polarization 0of any input signal is completely uncontrolled. Therefore the signal must be split into two known orthogonal polarization using polarization splitters and switching done separately on each. The result is then recombined to form the output.
This approach is troublesome and costly to implement and could cause unacceptable polarization mode dispersion (PMD), in which short pulses are spread out in time because different component of the pulses propagate at different speed depending on their polarization. In addition, compensating for the liquid crystal’s temp. dependence renders it too costly for all optical switching needs in a metro and access networks.
Total internal reflection-
Total internal reflection- known as TIR, the phenomenon that makes light propagates down an optical fiber –can, with an added twist, also serves as the basis of switch. The way the principle works, if light attempts to cross from a medium of higher refractive index (Dielectric1) to one of the lower refractive index (Dielectric 2) at too shallow an angle, all of the light is reflected from the interface back into the higher index medium (see next fig.).
The trick to exploiting the phenomenon in the switch is to turn the effect off (or on) by replacing (or not replacing) the second medium with one whose index of refraction matches that of the first.
The best known product based on this phenomenon is the AGILENT CHAMPAGNE switch, in which sections of waveguide intersect with the fluid filled channels (see fig.). The fluid has nearly the same index of refraction as the waveguide, enabling the light to cross the intersections with fairly low loss. When a bubble (vaporized fluid) is introduced at the intersections, its low refractive causes the light to be reflected or switches onto another waveguide because of total internal reflection.
The Agilent design builds on company’s unparalleled experience in in-jet printing and has great promise for low cost manufacturability.
How ever the waveguide intersections carries several challenges .In unswitched, when fluid fills intersection, the wave guide cross-section is not maintained perfectly across the intersection because losses occur.
If there were only single intersection then this would present no problem, but light beam may have to cross many intersection in TIR switch (possibly as many as total number of ports) and losses are cumulating. Also some the lost light finds its way in output waveguide, so this causes cross talk. To minimize these detrimental effect intersections should be kept as small as possible .
In the unswitched case, when bubble is present, a different problem rears its head. Light reflects off the bubble into output waveguide, but owing the nature of TIR, also extends some distance into the bubble. To ensure a very little continues across the intersection the bubble should be made as large as possible. Designer of TIR switches are faced with two conflicting requirements low loss must be traded off against high isolation.
An addition problem with TIR switches is that reflected wave undergoes a wavelength dependent phase shift because energy storage in bubbles. This causes amplitude variation and dispersion switches output .
This variation in amplitude and dispersion lowers its usefulness for some applications .
Creating and removing the fluid in the intersection can be done in two basic ways either by vaporizing some of fluid to create bubble and then condensing it to reverse the process , or simply by moving a liquid air interface into and out of the intersection . A bubble can be created in few milliseconds by applying heat and then continuously applying just enough heat to maintain the bubble. The reverse condensation process is slower .The moving interface approach has the potential for being faster than bubble approach, but has yet to be realized in real practice.
To make the larger switches, waveguides are arranged to form a matrix of switching intersections. Note that this a matrix switch, the number of intersections equals the products of the number of inputs and the number of outputs.
As mentioned above, each intersection traverse by the light contributes to the loss and crosstalk, limiting the scaling of matrix to less than 100 ports because the number of intersections to be crossed by the light in worst case may equal to total number of ports.
Emergent Electrography:
Electrography is newest all-optical switching technology. This method features a solid state switch matrix created rows and columns of ferroelectric crystals such as lithium tantalite nibote. (see next figure) .
Rows corresponds to individual fiber , and each column is for different wavelength .Each crystal is laser etched with Brag grating to create a hologram in which crystal’s optical properties are changed when it is energized .For example by the application of electric field.
In current implementation individual crystal are manually assembled and thus must be greater than 1 mm on side.
As technology evolves the holographic elements may be able to be written more densely into a single crystal. Then patterning will be required only for through energizing electric fields are applied to each crystal or holographic elements.
When crystals are not energized, light goes through it .Energized crystals deflect a controllable portion of the incident light to appropriate fiber. Holographic switches are quite faster and claim instant restoration. They, along with other switches made from electroptical materials, will be fast enough for long term of optical packet switching. Because it is an emerging technology no data about its long term reliability is available, but past holographic applications like high density storage have shown life time issues with the holograms themselves.
On the plus side, electroholographic switches may be easily integrated with other network functions like equalization and monitoring. They consume negligible power. The technique allows a single crystal to be used for switching and for variable attenuation since the fraction of light reflected is controlled by applied signal.
Yet from an application point of view, the technology is not the ideal solution it is sometimes represented to be. The approach is that of wavelength selective matrix switch. The hologram blocks are analogous to the mirrors in 2-D MEMS switch. The number of matrix
element in a electrographic switch increases as the product of the numbers of inputs and output ports , and will not scale well. As switch matrix size is increased to the sizes needed for the core network switching, the required optical beam size will expand and optics for collimating and focusing the beams will be required.
Non energized blocks in optical path will contribute the loss and cross talk of the switch . Also holograms are diffractive that are inherently polarization and wavelength dependent which leads to dispersion and polarization dependent loss (PDL) issues.
Conclusion
All thing considered, mirror based MEMS approach seems to be best poised to fill the near term need for large optical switches, first in long-haul (core) networks, then in metropolitan area networks and later perhaps at access level. With its low loss, adequate switching speed ability to scale to a large port counts, and high reliability. MEMS technology offers best combination of crucial qualities needed to produce an effective transparent switch.