Variable Optical Attenuators
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Acronym: VOA
Definition: optical attenuators with a variable degree of attenuation
- optical elements
- optical attenuators
- variable optical attenuators
- fiber-optic attenuators
- optical attenuators
Related: optical attenuatorsfiber-optic attenuatorsvariable reflectivity mirrors
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DOI: 10.61835/sbb Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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What are Variable Optical Attenuators?
Optical attenuators are devices for reducing the optical power of a light beam, for example a free-space laser beam or a telecom signal sent through an optical fiber. Some attenuators have a degree of attenuation which is either manually adjustable or can be controlled with an electrical signal. They can be based on different operation principles, which are explained in the following sections. Depending on the requirements, which vary substantially between different applications, different technological approaches can be appropriate.
Variable optical attenuation can be used for various purposes; some examples:
- Signal levels in optical fiber communications systems may be adjusted.
- One may test the bit error rate of a telecom system as a function of signal power level at the receiver.
- One may use a variable laser power in various applications, where direct adjustment of the laser output e.g. via the pump power is not practical.
Bulk Attenuators
Filter Wheels
A simple approach would be to use fixed attenuators which can easily be exchanged and possibly combined. However, it is more convenient to use a filter wheel, e.g. of neutral density filter type, where a variable degree of attenuation can be adjusted simply by rotating the wheel to another position. The rotation may be done manually or with a motorized drive, possibly with computer control. Some variable reflectivity mirrors are also suitable for such applications.
The attenuation may vary in steps or continuously; in the latter case, one will of course have some variation of attenuation across the beam profile.
Absorbing filters are quite limited in terms of power handling capability.
Rotated Dielectric Mirrors
For high-power applications, one will usually use a device where a variable fraction of the incident power can be sent into a robust beam dump. One such possibility is to use rotated dielectric mirrors.
For an input with constant wavelength, one may use an edge filter with a variable tilt angle, so that the filter edge can be varied. While the reflected beam will have a substantially varying propagation direction, the transmitted beam exhibits only a variable parallel offset, which may either be small enough to be tolerated (if a thin substrate is used) or can somehow be compensated, e.g. by using two such devices in a sequence, which are rotated in synchronism.
The attenuation of such devices will typically be polarization-dependent, which however is often not a problem when using such devices with linearly polarized laser beams.
Rotated Polarization Devices
If the input beam is linearly polarized, one may first apply a variable rotation of the polarization direction with a rotatable half-waveplate and then use a polarizer. This type of device may also be able to handle very high optical powers; the rejected beam can be sent into a beam dump.
Liquid Crystal Attenuators
Liquid crystal modulators can be used for obtaining electrically adjustable attenuation. The liquid crystal device actually only modifies the polarization state, which causes variable attenuation in a subsequent polarizer. Usually, linear polarization at the input is expected, although polarization-independent designs are possible. The power handling capability is modest.
Fiber-optic Attenuators
Variable optical attenuation for fiber devices can be introduced in different ways, for example via
- variable (mis)alignment of fiber ends in the longitudinal or transverse direction
- via variable bending, causing bend losses
- For multimode fibers, such approaches could lead to strongly mode-dependent losses, but many fiber-optic attenuators are actually of the single-mode type, where such problems cannot occur.
- with MEMS (Micro-Electro-Mechanical systems), using a tilting micromirror or a shutter to partially block or divert the light beam coupling into an output fiber
- Magneto-optic attenuators: These exploit the Faraday effect to rotate the polarization state of light, which is then passed through a polarizer. By varying the magnetic field (via a current through a coil), the transmission is controlled. These devices contain no moving parts and can offer reasonably high speed (response times in the microsecond range) and high reliability.
For obtaining variable losses in multimode devices while avoiding mode-dependent and polarization-dependent losses, one may have to use some kind of bulk-optical variable attenuator between two fiber collimators.
Fiber-optic attenuators can be inline devices, incorporated into fiber patch cables.
See the article on fiber-optic attenuators for details.
Performance Factors
Various kinds of performance figures may be relevant for variable optical attenuators:
- the range in which the attenuation can be adjusted (e.g. 0.5 to 10 decibels)
- the accuracy and reproducibility
- polarization dependence
- in the case of multimode fibers, the mode dependence of the attenuation
- in the case of bulk devices, the uniformity of attenuation across the beam profile, the introduced wavefront distortions and possibly changes in beam direction
- the maximum allowed optical input power
- the range of operation wavelengths with reasonably constant attenuation (e.g. the telecom C band, 1530–1565 nm), and the maximum difference in attenuation within that wavelength band
- for electrically controlled attenuators, the speed of adjustment
- the power handling capability
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is a variable optical attenuator?
A variable optical attenuator is a device for reducing the optical power of a light beam by an amount that can be adjusted, either manually or via an electrical signal. They are used to set signal levels in optical fiber communications or to control laser power in various applications.
What are the common principles for bulk variable optical attenuators?
Common methods for bulk attenuators include using a filter wheel with different neutral density filters, rotating a dielectric mirror (like an edge filter) to change its reflection/transmission properties, or using a combination of a rotatable half-waveplate and a polarizer for linearly polarized light.
How can high optical powers be attenuated variably?
For high-power laser beams, variable attenuation is often achieved by devices that divert a controllable fraction of the power to a beam dump. Examples include rotating a dielectric mirror or using a rotatable half-waveplate in front of a polarizer.
What are the key performance criteria for variable optical attenuators?
Important performance factors include the adjustable attenuation range (in decibels), power handling capability, wavelength dependence, polarization dependence, and adjustment speed. For fiber devices, mode dependence can also be critical.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains 39 suppliers for variable optical attenuators. Among them:
LASEROPTIK can provide coatings for variable optical attenuators. One can change the transmission by tilting an edge filter, with highest transmission at normal incidence and typical operation for angles of incidence between 0° and 45°.
AeroDIODE has developed the variable optical attenuator — 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.
EKSMA Optics has variable attenuators for high power Nd:YAG laser or femtosecond Ti:sapphire laser applications.
Thorlabs designs and manufactures a wide selection of fiber optic attenuators, providing fixed or variable attenuation for single mode, polarization-maintaining, or multimode fibers. Our variable optical attenuators (VOAs) are available with manual, digital, or electronic control of the attenuation.