Varied fiber-optic switches serve diverse applications
More than a half-dozen optical switch technologies continue to compete successfully as multiple component choices allowing the device to match the requirement
Gerry pesavento and
steven poncelet
dicon fiberoptics inc.
Although optical switches are fundamental components used in fiber-optic communications systems, they are often misunderstood. This problem probably arises because of the proliferation of research in optical switching and the variety of available technologies.
The leading optical switch categories are optomechanical, electro-optic (integrated waveguide), acousto-optic, magneto-optic, thermo-optic and photonic (all optical). No particular optical switch technology will take over the market, because different technologies are better suited to different applications. At this time, optomechanical switches are in the forefront.
As fiber continues its deployment into telecommunications, data communications and video networks, and as fiber finds newer uses (instrumentation, sensing, lighting, etc.), more applications for fiber-optic switches will emerge. Fiber-optic switches are being used in applications that were not anticipated just a few years ago. Because they are passive, fiber-optic switches are truly general-purpose components.
Following the price trend of fiber couplers and other passive components, prices for fiber-optic switches have fallen gradually. This trend should continue, thanks to manufacturing improvements and volumes. Today, switch prices range from $200 (multimode) to $300 (singlemode) per channel; these prices have proven cost-justifiable for many applications. Lower costs are predicted with improvements in automated alignments, micro-machining and new material developments.
Versatile switching
Fiber-optic switches connect input fibers to output fibers on a one-to-one basis. They also establish and release connections among fiber paths. As optically passive components, these switches are versatile products. The fiber-optic switches available today are all bit-rate transparent; that is, they pass optical data independent of data rate or data format. Moreover, they have little more effect on the optical signal than do fiber-optic connectors.
Optical switch configurations have become similar to those available for electrical relays: on/off, 1ٴ, 2ٶ, 2ٴ, 1¥n, 2¥n and m¥n. In fact, mechanical fiber-optic switches are sometimes referred to as fiber relays, and they accommodate both singlemode and multimode fibers. Many 1¥n switches can handle 100 channels.
Consider the switch technologies that are available off-the-shelf today. Studies show that optomechanical switches dominate the market today. In operation, they use a moving mechanism to either reposition a fiber or an optical element. The mechanism is typically a solenoid, piezoelectric material or stepper motor. Actuation is either manually or electrically induced. The resulting specifications for these types of optomechanical technologies are similar and feature low insertion loss and millisecond switching speeds.
Optomechanical switch performance has improved markedly during the past few years. Consider these typical data-sheet specifications of singlemode and multimode switches: 0.5-decibel insertion loss; -60-dB backreflection; 0.005-dB repeatability; 80-dB isolation; 10-million-cycle durability; and 20-millisecond switching speed. These specifications are on par with even rugged fiber-optic components.
Optomechanical switch technology is mature. Major technology revolutions are not expected. However, step-by-step improvements continue to evolve, and several advances have taken place during the past few years. For example, progress in manufacturing technology has permitted more accurate fiber alignments. The result: Switches possess losses of less than 0.4 dB--almost as low as fiber-optic connectors.
For another example, enhanced slant-angle techniques and antireflective coatings are producing switches with low backreflection, wide wavelength operating ranges and flat spectral response. High-precision stepper motors, similar to those used in computer disk drives, are being used to produce miniature, printed-circuit-board mountable 1¥n switches. Furthermore, all-manual switches that require no electrical power have been developed. All-optical m¥n matrix switches can now handle gigabit-per-second data for network reconfiguration. In addition, fiber-optic switches are available with manual, DC, TTL, RS-232, GPIB and VXI control options.
Many industry analysts have predicted that optomechanical fiber-optic switches will be replaced in the future by faster integrated waveguide or photonic switches. To date, this substitution has not happened. Just as electromechanical relays exist despite the proliferation of solid-state devices, optomechanical switches (relays) are expected to perform alongside integrated optical components for the long term.
Although they are receiving little media attention, optomechanical switches continue to satisfy non-critical applications, such as switching test equipment among several test points, bypassing and reconfiguring networks, and manual switching security units. In addition to impressive cost-performance ratios, optomechanical switches have a proven track record with an estimated 50,000 switches in field use today.
The reasons behind the quest for higher speed optical switching is obvious: The potential market for lightwave switching in telecommunications, information processing and computing appears enormous. Photonic switching would preserve the end-to-end continuity of the optical path in the fiber network.
On the other hand, no matter how reliable they are, optomechanical switches will always provide millisecond switching speed. They are not suitable for high-speed networks that require nanosecond or picosecond switching. Consequently, as evident in the technical literature, most switch research efforts focus on fast integrated optical and photonic switching. Both bit-rate transparent and bit-sensing switch technologies are also under development.
Electro-optic switches are bit-rate transparent components that offer nanosecond switching speeds. They employ an electro-optic material that alters its refractive index in the presence of an electric field. Several switches have been developed from silicon and lithium niobate waveguide technology.
Packaging and fiber pigtailing of these devices have also been improved. The remaining improvement hurdles for electro-optic switches are their relatively high insertion loss (5 dB), high crosstalk (20 dB) and high cost.
Acousto-optic elements use sound waves to generate a sinusoidal refractive index wave-grating to manipulate light, a principle known as Bragg diffraction. This effect is used to control the intensity and position of a laser beam. Several materials exhibit acousto-optic effects, including fused quartz, flint glass, germanium, indium phosphide, lithium niobate and tellurium dioxide.
Acousto-optic components are used mostly as laser beam modulators, deflectors and frequency shifters. For switching applications, both time-division and frequency-division multiplexing are available, with fast switching speeds possible. However, to date, they have not been successfully used in fiber-optic switching because of inefficient light coupling to fiber resulting from beam divergence and fiber-positioning accuracy.
Other devices considered for fiber-optic switching are magneto-optic materials, liquid crystal light modulators, holographic elements and photo-refractive materials. However, for low cost, low insertion loss, high repeatability, low backreflection and precise repeatability, none have shown as much promise or success as optomechanical technologies.
Diverse applications
For applications, fiber-optic switches are used extensively in telecommunications (mostly with singlemode fiber), data communications (mostly with multimode fiber) and fiber sensing (mostly with large-core fiber). Most switches are being used for network security switching, fiber and component testing and fiber monitoring.
Network security switching includes bypass switching and computer security switching. For example, 2ٴ switches are used as bypass switches for fiber distributed data interface dual-attachment stations (concentrators, bridges, routers, etc.). The bypass switch contains two 2ٴ optical bypass switchesone for the FDDI primary ring and one for the FDDI secondary ring. The dual bypass switch protects the network by disconnecting an FDDI dual attach station, while maintaining ring integrity, in the event of station failure, loss of power or routine station removal.
On/off and 1ٴ switches are being used extensively for fiber security switching in classified networks. Some computer networks used by banks and military groups require special access authorization that can be more readily provided by fiber security switches. For example, an on/off fiber-optic switch in a secure key-switch actuated box can be used only by authorized personnel to activate computer access to certain areas.
Fiber-optic switches are also being used to distribute test equipment heads to several measurement points. Using 1¥n and 2¥n switches improves both test automation and test data accuracy. For example, Hewlett-Packard Co. is supplying HPIB and VXI controllable 1¥n and 2¥n fiber-optic switches to complement its line of optical test equipment. For another example, 1¥n and 2¥n lightwave switches can be combined with an optical power meter for an automated n-channel measurement system for fiber and component testing. u
Gerry Pesavento is director for sales and marketing, and Steven Poncelet is product marketing manager at Dicon Fiberoptics Inc. in Berkeley, CA.