The Skinny on Switching

Jung-Chih Chiao and Amy Dugan

Here's a primer on the strengths--and weaknesses - of seven types of optical switching designs.

Just as electrical switching replaced mechanical relays of the past, optical switching is on the verge of replacing some of today's electrical switching functions. Although new approaches to optical switching are constantly being developed, optical switch designs can be roughly classified into seven categories: optomechanical, thermo-optical, liquid crystal, micro-electrical mechanical, gel/oil-based, electro-optical, and others such as acousto-optic, semiconductor optical amplifier (SOA) and ferro-magnetic. SWITCHING TECHNOLOGIES In evaluating the performance of different optical switches, the following should be considered: reliability, energy usage, port configurations and scalability, optical insertion loss, cross-talk, temperature resistance, and polarization-dependent loss characteristics. OPTOMECHANICAL Optomechanical switches employ electromechanical actuators to redirect light beams. One type of optomechanical switch inserts and retracts a reflective surface into a light stream to redirect it to another port. Another architecture redirects the light stream by bending a grating-written fiber. In terms of optical insertion loss and switching speed, performance characteristics of optomechanical switches vary according to architecture. However, the universal drawback is the durability and cycle limitation of the mechanical actuator.

Research Projects May 2001

THERMO-OPTICAL

Planar lightwave circuit thermo-optical switches are usually polymer-based or silica on silicon substrate. Thermo-optical switches use temperature control to change index of refraction properties of Mach-Zehnder interferometer-based waveguide arms on the substrate. The light is processed by waveguide interaction and is guided through the appropriate path to the desired port. Thermo-optical switches are small but have high power requirements and optical performance issues.

LIQUID CRYSTAL

Liquid crystal switches work by processing light polarization states. If voltage is applied, the liquid crystal element allows one polarization state to pass through. If no voltage is applied the liquid crystal element passes through the orthogonal polarization state. These polarization states are steered to the desired port, processed and recombined to recover the original signal's properties. Because it has no moving parts, liquid crystal is highly reliable and has good optical performance, but can be affected by extreme temperatures if not properly designed.

MICRO-ELECTRO-MECHANICAL SYSTEM (MEMS)

MEMs (micro-electrical mechanical machines) can be considered a subcategory of optomechanical switches. MEMs use tiny reflective surfaces to redirect light beams to a desired port by either ricocheting the light off neighboring reflective surfaces or steering the light beam directly to a port. Analog-type (3D) MEMs have reflecting surfaces that pivot on axes to guide the light. Digital-type (2D) MEMs have reflective surfaces that "pop up" and "lay down" to redirect the light beam. The reflective surfaces' actuators consist of everything from electrostatically driven hinges that bend and straighten to coils that create electromagnetic fields that distort a torsion bar.

MEMS easily scale to large port counts but can be a challenge to package due to the density and microscopic size of the light paths entering and exiting the substrate.

GEL/OIL-BASED

Index-matching gel-and oil-based optical switches can be classified as a subset of thermo-optical technology because the switch substrate needs to heat and cool to operate. However, heating a portion of the switch causes an index of refraction-changed atmosphere to form at the waveguide junctions. This index-of-refraction-changed "bubble" or liquid redirects the light stream through the appropriate waveguide path to the desired port. This technology has been compared to inkjet printer technology and can achieve good modular scalability. There are still questions regarding long-term reliability and optical insertion loss.

ELECTRO-OPTICAL

Electro-optical switches use highly birefringent substrate material and electrical fields to redirect light from one port to another. A popular material used in an electro-optical switch is lithium niobate. An electrical signal is fed as the control into the device's substrate. This electrical field changes the substrate's index of refraction, which manipulates the light through the appropriate waveguide path to the desired port. Opto-electrical switches are fast and reliable, but have high insertion loss and possible polarization dependence.

OTHER TECHNOLOGIES

Acousto-optic, SOA (semiconductor optical amplifier) and ferro-magnetic are other types of switches. Acousto-optic switches receive acoustic-wave-induced pressure from a RF-fed piezoelectric transducer to generate fine gratings in optical waveguides. The gratings diffract lights to the desired port. SOA technology uses semiconductor-based amplifiers that operate in the gain-clamped mode with some types of interferometric switch geometries to optically switch. Ferro-magnetic technology uses magneto-optical Faraday effect in which electromagnetic waves interact each other directly to create extremely fast switches (femto-seconds).

NETWORK APPLICATIONS

Switch technology and system application compatibility depend on parameters chosen by the system designer. The column at the right of Table 1 suggests the appropriate applications for various switching technologies.

Protection switching allows the completion of traffic transmission in the event of system or network-level errors. Optical protection switching usually requires optical switches with smaller port counts of 1x2 or 2x2. Protection switching requires switches to be extremely reliable, since sometimes they are single points of failure in the network. Optical component switch speed for DWDM, SONET transport and cross-connect protection is important, but not critical; other processes in the protection scheme take longer than the optical switch.

It is desirable in the protection application, however, to optically verify that the switch has been made. Some MEMs verify optical path connection by bouncing a small portion of the optical signal off the semireflective, coated mirror backside into a monitor port. The other switch types can electrically verify that a switch signal was given, but do not inherently optically verify that the switch was completed. To optically verify path connection within these switches, optical taps that direct a small portion of the optical signal to a separate monitoring port can be placed at each of the switch's output ports.

Optical cross-connects groom and optimize transmission data paths. While the industry push is toward evolving incumbent electrical switch fabrics to optical switch matrices, this is difficult because network designers still require the performance monitoring and high port counts afforded by electrical matrices. Optical matrices allow processing transmissions in the optical domain and isolate the system from set data rates, which eliminates the need to accommodate multiple traffic protocols.

Avoiding electrical-based traffic protocols at the cross-connect level eliminates OEO (optical-electrical-optical) conversion, which is costly and cumbersome to network expansion. Unfortunately, to reach the level of monitoring network designers require while reducing the OEO-induced cost, an opaque cross-connect switching fabric may have to be deployed. Opaque cross-connects are mostly optical at the switching fabric, but still rely on a limited subset of surrounding electronics to monitor system integrity.

Optical switch requirements for optical cross-connects include scalability, high port count switches, the ability to switch without sticking, low loss, good uniformity of optical signals independent of path length, and the ability to switch a specific optical path without disrupting others.

OADM (optical add/drop multiplexing) resides at network junctions and nests within networks. At the correct processing node, an OADM extracts optical wavelengths from the transmission stream and inserts them in another optical transmission stream before the processed transmission stream exits the same node.

n a long-haul, WDM-based network, OADM requires the optical power level of the added optical signal to resemble the dropped optical signal to prevent the amplifier profiles from being altered. This power stability requirement between the add and drop channels creates uniformity across a wavelength range.

n a long-haul, WDM-based network, OADM requires the optical power level of the added optical signal to resemble the dropped optical signal to prevent the amplifier profiles from being altered. This power stability requirement between the add and drop channels creates uniformity across a wavelength range. In a metro DWDM network, low insertion loss and a small physical size of the OADM subsystem optical switch are important because of shorter unamplified network spans and smaller system physical size. The optical switches required for any OADM subsystem are a series of low-port-count switches or a modular medium-port-count optical switch.

OSM (optical spectral monitoring) receives a small optically tapped portion of the aggregated WDM signal, separates the tapped signal into its individual wavelengths and monitors each channel's optical spectra for wavelength accuracy, power level and cross-talk. OSM usually wraps software processing around optical switches, filters and OEO converters.

The optical switch size that supports the OSM depends on the system wavelength density and desired monitoring thoroughness and ranges from a series of small port count optical switches to a medium size optical switch. In the OSM application, it is important that the optical switch employed has a high extinction ratio (low interference between ports), low insertion loss and good uniformity because the tapped optical signal is very low in power.

The network of the future is heralded as being all-optical, data-rate independent and end-to-end compatible. Optical switches will be firmly entrenched in this vision. Optical switches already provide signal protection, allow provisioning, increase network capacity, and generally improve system cost. As networks continue to mature from electrical-based to all-optical, different optical switch types will address shifting priorities.

REFERENCES

1. Grout, S. "Proposition of All-optical Switching and Routing: an Overview of Optical Switching Technologies by Comparison with the Liquid Crystal Optical Switch," Optical Switching Conference, London, UK, May 2000.

2. Perrier, P.A., "Position, Functions, Features, and Enabling Technologies of Optical Cross-Connects in the Photonic Layer," NFOEC'99.

3. Mossberg, Thomas, et al., "Lightwave CDMA as a New Enabler of Optimal WDM System Design," NFOEC'99.

4. Schrope, Mark, "Tiny Bubbles," Business 2.0, pp. 190-191, July 25, 2000

5. Eldada, Louay, et al, "Thermo-optically Active Polymeric Photonic Components", OFC'2000.

6. Enguang, Dai, et al, "High Speed Integrated Acousto-optic Switch with High Extinction Ratio", OFC'2000.

7. Makihara, Mitsuhiro, et al, "Strictly Non-blocking NXN Thermo-Capillary Optical Matrix Switch using Silica-based Waveguide," OFC'2000.

Created by J.C. Chiao