Switches

Optical Switches

Here we will treat an optical switch as a device that diverts or changes the entire aggregated multiwavelength optical signal from one path to another. This is in contrast to couplers, which passively diverge or combine signals (and are usually not selectable or programmable), and also does not include more sophisticated devices such as add-drop multiplexers or wavelength converters or 'steering units'.

Configurations

General Characteristics

Switch Types and Specific Characteristics

Optical Switches -- Configurations

An optical switch can exist in 1x2, 2x2, 1xN, or NxM configurations. They all share the need for at least one control signal (shown below, dashed), informing the switch to change state, and they all have some impact on the quality of the optical signal being switched, including (but not limited to):

Single-Pole Single-Throw (rare)

SinglePole Double-Throw (1 x 2)

The SPST "interrupter" is sometimes used as a modulator, for example as an on-off keyer.

SPDT/Combiner Bypass Example

A 1 x 2 switch on input and a 2:1 combiner on output is a common configuration for bypassing around a failed component. Also known as "Automatic Protection Switching" in SONET systems.

2 x 2 Ring Bypass

This is a typical ring-bypass switching mechanism, using a 2 x 2 switch arrangement. ["Synchronous" bypass elements.]

Optical Switches -- General Characteristics

Switching time. This has several aspects: control signal propagation, time to start of switching (initial datapath unavailable), potential switch 'bounce' or signal partial degradation, time to new datapath available.

Type of control signal: Manual/Mechanical, electrical, optical. In-band or out-band.

Efficiency, power required to perform switching.

Loss (attenuation)

Losses due to Configuration (i.e., if an input signal arrives at 2 destinations, there will necessarily be a theoretical best-case loss of ~3 dB due to half of the signal power going to each destination.)

Insertion Loss due to excess losses such as splice or connector interfaces, modal displacement, or other 'filtering' effects depending on the type and configuration.

Wavelength-dependent transmission effects. The loss vs. wavelength profile will in general not be perfectly flat, and will hence introduce differential loss across the channels carried.

Reflection coefficient. The non-ideal insertion of the signal into the switch will be accompanied by a return loss.

Insertion Delay.

This can be interpreted as the excess signal propagation time compared to continuous propagation on the fiberoptic channel.

Wavelength-dependent delay characteristics. Similarly, the delay vs. wavelength profile will not be perfectly flat, leading to differential delay across the channels carried.

Envelope/Bandwidth/Modal distortion ....

Optical Switches -- Types

At least 5 kinds of optical switch can be envisioned:

1. Manual/Mechanical.

An example of a manual or mechanical switch is a fiberoptic connector that, when physically removed, causes the light path to automatically bypass the fiber that was connected. Their function is activated over a time-period of milliseconds to seconds, and is generally confined to long-term physical configuration of the cable plant. I am unable to find any currently available examples of this type of switch.

2. Electro-mechanical,

As an example, several FDDI bypass switches, in a 2x2 configuration, have used electromechanical relays with a fiber strapped to the relay armature. When the relay is in its default state, the fiber is aligned with the default channel. When the relay is activated, the fiber is moved into conjunction with an alternate output fiber channel. This is known as a "moving fiber" construction.

Similarly, some electromechanical optical switches utilize a moving mirror or prism to redirect the input 'beam' to the output channel.

All of these switches suffer from problems of the inertia associated with the physical mechanism, wear, overshoot / backlash, alignment stability over time, temperature and in the presence of vibration, and losses and reflections due to the (usually air) mismatches in the optical path. There may also be excess magnetic fields present due to the solenoid actuators within the relay.

These switches typically operate with switching times on the order of several milliseconds, and can experience switch-bounce and/or intermittency during switching. Hence, the application of these switches is limited in optical networking to bypass switching under serious fault conditions. (see APS.)

New work in micro-mirrors: EPFL (Switzerland), EPFL

3. Acousto-optic.

These devices utilize the principle that sound (or ultrasound) waves of pressure can influence the propagation characteristics of an optical signal through a crystal.

Piezoelectric crystals

SAW (Surface acoustic wave)

Acousto-optic switches can be made with low insertion loss, but their switching times are on the order of hundreds of microseconds to milliseconds.

4. Electro-optic.

Electro-optic switches fall into ? main categories, including:

Induced electro-optic effect devices

Induced interferometric switches