Fiber-optic Switches

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Fiber-optic switches are optical switches in the context of fiber optics. The simplest device is an on/off switch with one input and one output, which allows light to pass with low insertion loss when open, and blocks it completely (or at least causes high insertion loss) when closed. There can also be multiple outputs, and the light from one input can be sent to any of the outputs. As a further generalization, there can also be multiple inputs; for example, a 2×2 switch has 2 inputs and 2 outputs, and there are even large matrix switches like 64×64.

The inputs and outputs are often provided with fiber connectors for individual fibers, but there are also switches connected with fiber arrays.

The switching is in most cases controlled through an electronic interface rather than manually.

Going beyond ordinary fiber-optic switches are reconfigurable add/drop multiplexers (ROADM), where only light in specific wavelength regions is switched.

Optical Switch Technologies

There is a wide range of different technical realizations of fiber-optic switches, varying in performance and cost, and being suitable for a very different fields of application.

Mechanical Switches

Mechanical switches involve physically moving parts. For example, tiny mirrors or prisms can be flipped into the beam path, or fiber ends can be shuffled into different positions.

Simple switches may be operated manually; others are motorized with some kind of actuators, which may be based on different technologies. Some examples:

• An electromagnet can move a part (such as a mirror) in a linear or rotating fashion. A special form is a voice coil actuator.
• A piezo stack allows very fast and accurate motion, but requires a high drive voltage.
• Stepper motors provide reliable and accurate positioning.
• Optical MEMS (= MOEMS) technology (micro-opto-electro-mechanical systems) involves chip-level micro-scale elements and allows for large switching matrices and high speed operation. They typically have substantial insertion losses and handle only quite limited optical powers.

With such technologies, switching is typically possible on a millisecond time scale; with MEMS, microsecond response times can be possible.

Electro-optic Switches

Devices based on electro-optic modulators can be extremely fast, with switching times in the nanosecond region. However, they are expensive and require high drive voltages. Also, they are usually polarization-sensitive.

Thermo-optic Switches

Thermo-optic switches are usually chip-level devices, where a waveguide can be heated to change the optical phase, and an interferometer turns a phase change into a change in transmission. They can work with relatively low power and operate on a millisecond or even microsecond timescale.

Liquid Crystal Devices

Liquid crystal modulators can cause phase and polarization changes, and in combination with polarizers also changes in transmission. They require low drive power and act on a millisecond timescale.

Performance Figures

There are a number of performance metrics that can be relevant to applications:

• The switching time is essentially the time required to switch between two states. Due to mechanical vibrations, the system may not be fully settled after that time, e.g. still show a damped oscillation on the insertion loss.
• The insertion loss should be low in the “on” state and very high in the “off” state. The return loss may also be relevant in feedback-sensitive systems.
• There is a maximum allowed optical power. Exceed this may cause temporary malfunction or permanent damage.
• Switches will only work properly in a limited wavelength range; at a minimum, the insertion loss may increase outside the recommended range.
• Typically, fiber-optic switches should not be polarization-sensitive, since most fibers are not polarization-maintaining, so that the occurring polarization state is essentially random.

Several other aspects may need to be considered:

• size and weight, mounting options
• environmental robustness, reliability and lifetime
• allowed ambient temperature range
• required drive power
• digital interface
• monitoring features
• scalability of input/output count

Applications of Fiber-optic Switches

Communications and Data Centers

The main application area is optical fiber communications, where fibers carry telecommunications signals and fiber-optic switches are used mainly for routing optical signals. Data centers have similar requirements, using fibers for transmitting large amounts of information, in that case over shorter distances.

During normal operation, such a switch may remain in a certain state for a long time, but a sufficiently fast reconfiguration of the signal path may be required, e.g. if some components fail or if the traffic requirements change substantially.

There are also high-capacity optical cross-connects (OXC) with large fiber counts for dynamic network provisioning, traffic aggregation and disaster recovery routing. For example, multi-subscriber video events can trigger such an OXC to reallocate resources. Switching can also be combined with wavelength conversion, amplification, or regeneration devices.

Switches may also be used for simple test purposes, e.g. for temporarily sending an optical signal to some diagnostic device.

Fiber-optic Sensor Systems

Fiber-optic switches are also used in the context of fiber-optic sensors, which are used in many fields, such as infrastructure monitoring, manufacturing and robotics. For example, one can use them for multiplexing: Multiple fiber sensors can be interrogated by a single device when they are connected to it via a multi-port switch. This can be cost-saving, as interrogators tend to be expensive.

Other Purposes

Fiber-optic switches can be useful for general testing purposes in fiber optics. For example, instead of manually reconnecting fiber-optic connectors too often, one should install a switch where this can be done more conveniently and without wearing out the connectors. One may then e.g. sequentially test many devices under test over many periods.

Another possibility is to switch between different light sources, e.g. between multiple redundant sources in case of a failure, or to select one of several mode-locked fiber lasers operating with different wavelengths or pulse repetition rates.

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