optical switches (original) (raw)

Definition: photonic devices for controlling the flow of light

Category: photonic devices

• optical switches
  • fiber-optic switches
  • Q-switches
  • cavity dumpers
  • pulse pickers
  • photoconductive switches
  • electro-optic switches
  • acousto-optic switches
  • mechanical switches
  • thermo-optic switches
  • (more topics)

Related: fiber-optic switchesQ-switchescavity dumpingoptical choppers

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Contents

What Are Optical Switches?

Optical switches are photonic devices that control the flow of light. At their simplest, they operate as on/off gates, allowing light to pass with low insertion loss in the open state and blocking transmission (causing high insertion loss) when closed. However, more advanced devices can route one optical input to any of multiple outputs — or vice versa. 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 64×64 matrix switches, which are used for complex signal routing tasks.

Continuous power control is usually not what switches provide; that would be the domain of more general optical modulators.

Optical switches may interface with free-space beams (like laser beams) or with guided light in optical fibers or other types of waveguides. Many are fiber-coupled — see the article on fiber-optic switches –, while others are realized on photonic integrated circuits with waveguide interfaces.

Control modalities for optical switches include:

• Electrical actuation: Most switches are controlled electrically, for example, through electro-optic or thermo-optic effects.
• All-optical control: Some advanced switches use intense light pulses and optical nonlinearities to perform ultrafast switching without electronics.
• Mechanical actuation: Switching can also be mechanical, using movable beam blocks or reflectors — for example, MEMS micro-mirrors. Even a motorized optical chopper is, in essence, a periodic optical switch.

Applications of Optical Switches

The main application fields of optical switches are:

Laser Technology

In laser technology, optical switches can perform various functions:

• Q-switching of lasers: For generating nanosecond light pulses, one can use a switch inside a laser resonator which remains closed during pumping of the laser gain medium and is then suddenly turned to its “open” (light-transmitting) state to generate a pulse. Such devices are called Q-switches. In most cases, they are of acousto-optic or electro-optic type.
• Cavity dumping is another application, where even faster (ns or sub-ns) switching is required to extract laser pulses by suddenly redirecting the circulating optical energy.
• In ultrafast laser systems, one requires fast switches as pulse pickers and in regenerative amplifiers. Between amplifier stages, optical switches can be used to suppress ASE.

Optical Communications

Optical communications requires many optical switches:

• Signal routing: Large-scale optical switches flexibly connect data paths in fiber-optic networks, including the backbone of the internet, data centers, and fiber-to-the-home deployments.
• Automated redispatching: Used in telecom to reroute light paths rapidly in case of fiber cuts, equipment failure or overload of fiber-optic links.

See also the articles on fiber-optic networks and fiber shuffles.

Photonic Computing

Optical switches are foundational for photonic logic gates, interconnects, signal processing, and optical computing architectures, leveraging the ultrafast and interference-free nature of light.

Sensing and General Instrumentation

There is a wide range of applications of optical switches in various technological areas; some examples:

• Optical test and measurement: Switches select among multiple sources or detectors for automated optical testing setups or optical spectrum analyzers.
• Multiplexed sensor arrays: In fiber-optic sensors, switches address individual sensors across wide networks for efficient data acquisition.
• Multiplexing in biosensing: In chip-based optical biosensors, switches can scan through many detection spots sequentially.

Types of Optical Switches

Optical switches differ widely in physical implementation and performance. Key types include the following:

Electro-optic Switches

Many fast optical switches are based on electro-optics. The central component is usually a Pockels cell, containing an optical crystal with electrodes attached. Essentially, one can modify the refractive index (with a polarization dependence) with the applied electric voltage. Typical switching times are in the nanosecond domain, or even below 1 ns. The insertion loss can be very low.

Typical disadvantages of electro-optic devices are the required high control voltage and high cost.

Electro-optic switches are usually bulk-optical devices, but it is possible to realize waveguide-based solutions, e.g. on LiNbO3 PICs. This is favorable when the inputs and outputs are in waveguide form to avoid the free-space-to-waveguide coupling. Such devices can also work with substantially lower drive voltages.

See the articles on electro-optic modulators for details. Specially used for Q-switching are electro-optic Q-switches.

Acousto-optic Switches

Acousto-optic devices, essentially acousto-optic modulators, can be used for optical switching of light in the form of low-divergence light beams. They exploit diffraction of light at an acoustic wave, the creation of which requires an RF driver with substantial power. Due to the small deflection angle, light with higher divergence could not be switched. The switching speed is limited by the propagation time of the acoustic wave.

See the articles on acousto-optic modulators for details. Specially used for Q-switching are acousto-optic Q-switches.

Mechanical Switches

Switching can be accomplished with moving beam blocks or reflectors.

An extremely compact form of optical switch with possibly many inputs and outputs can be realized with micro-electro-mechanical systems (MEMS) as used in telecommunications, for example. Here, micro-mirrors fabricated on silicon chips can quickly be tilted, in many cases using electrostatic comb drive actuators. Hundreds or thousands of switch ports can be realized on a single chip.

Disadvantages of this technology include substantial coupling losses (particularly when interfacing to waveguides), vibration sensitivity and mechanical wear-out.

Thermo-optic Switches

Thermo-optic switches are waveguide devices containing a Mach–Zehnder interferometer. By locally heating a waveguide with a thin-film electric heater, one can control the relative optical phase shift and thus the interference conditions in a coupler, which determine the power throughput. Such devices are mostly realized on photonic integrated circuits. With microseconds to milliseconds switching times, they are not particularly fast, but are a practical solution for various on-chip switching needs, requiring low voltage and power (few milliwatts).

All-optical Switches

Optical nonlinearities can be utilized for switching light with other light. For example, the Kerr effect can cause cross-phase modulation, and phase changes can be used to get power changes by interference.

As light pulses can have extremely short durations and optical nonlinearities can work on femtosecond time scales, all-optical switches can be realized with extremely short switching times. For example, one can obtain picosecond or femtosecond time gating of light controlled with optical input pulses. This opens interesting prospects for ultrafast all-optical logic and next-generation computing. However, such techniques are still at the experimental or pilot stage due to challenges like high power thresholds and integration complexity.

Performance Characteristics

The most important performance characteristics of optical switches are:

• Switching speed: Depending on the application, low or very high switching speeds may be required. For example, cavity dumping of lasers requires very fast switching (order of 1 ns), while telecom signal routing may tolerate far lower speeds.
• Insertion loss is the loss of optical power (often measured in decibels) in the “on” state. For some applications, it needs to be very low.
• Blocking loss: In the “off” state, an optical switch may still exhibit a small amount of light transmission. A high blocking loss, minimizing that transmission, is often essential.

Various other properties can also be relevant:

• Open aperture: For switches with free-space interface, the open aperture size is an important parameter.
• Cross-talk: In switches with multiple inputs and outputs, undesirable cross-talk can occur, i.e., light which gets to output ports where it should not go.
• Beam fidelity: For free-space switches, the beam quality of transmitted light might be degraded, e.g. due to clipping, non-ideal optical quality, or thermal effects.
• Power handling: A switch is always limited in terms of the optical power which it can handle without being damaged or exhibiting deteriorated performance.
• Polarization dependence: Some switches work only with polarized light, while others are polarization-independent.
• Drive requirements: Some optical switches require quite high electric voltages, but little current (except when the voltage changes quickly). Others operate with low voltage, but possibly higher currents. Acousto-optic devices require RF drivers (→ modulator drivers).
• Scalability: Some types of switches (e.g. MEMS-based ones) are particularly suitable for scaling, i.e., for realizing many switches in a small volume with consistent performance.
• Environmental tolerance: It may be important that switches can be reliably operated in a wide temperature range or under conditions of strong vibrations and mechanical shock (e.g., for aerospace or field-deployed sensors).
• Durability: While mechanical switches may allow only a quite limited number of switching cycles, other solutions are often extremely durable.

Different types of optical switches vary enormously in performance parameters and also in cost and applicability in various fields. New types of optical switches may need to be developed for future applications — for example, extremely low loss switches for quantum networks operating with single photons.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 38 suppliers for optical switches. Among them:

⚙ hardware

AeroDIODE has developed the fiber modulator — a high-speed intensity modulator and optical switch based on a semiconductor optical amplifier (SOA). It is available over a wide wavelength range from 750 to 1650 nm. Key features are high speed (down to 1 ns rise/fall time), high dynamic range (>48 dB), high extinction ratio (>50 dB) and an easy to use graphical user interface with multiple software libraries (LabVIEW, Python etc.). AeroDIODE also offers the Semiconductor Optical Amplifier alone or the SOA driver in either CW or pulsed configuration.

AeroDIODE also offers a wide range of fiber coupled AOMs (Acousto Optic Modulators) with various types of digital (TTL) or analog RF drivers.

See also our white paper/tutorial on fiber-coupled modulators.

⚙ hardware

AMS Techno­logies carries a broad range of MEMS-based fiber optic switches in miniature, small or larger module and benchtop form factors. Latching or non-latching optical MEMS fiber optic switches are available with single mode (SM), multi mode (MM) or polarization maintaining (PM) fibers, suitable for wavelengths from 600 nm to 1600 nm.

Customers can choose among various port configurations beginning with 1×1. Our MEMS-based fiber optic switches’ extremely high reliability matches with demanding applications like telecom, datacom, sensor networks, instruments or test and measurement.

⚙ hardware

The PSW-LN and PSC-LN are two compact and high speed electro-optic polarization switches and scramblers. These integrated-optic devices feature a low-loss single-mode waveguide and are capable of modulating the polarization at frequencies ranging from DC to more than 10 GHz. They are available for the C and O bands.

⚙ hardware

Quantifi Photonics offers a wide range of fast and reliable optical switches to streamline fibre optic test procedures for wavelengths from 800 nm to 1590 nm. They can be customized with a wide range of switch configurations, fiber types and connectors including MPO for high-channel-count applications.

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