Variable Optical Attenuators
Author: the photonics expert Dr. Rüdiger Paschotta
Acronym: VOA
Definition: optical attenuators with a variable degree of attenuation
More general term: optical attenuators
DOI: 10.61835/sbb Cite the article: BibTex plain textHTML Link to this page LinkedIn
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, or 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 kind of problems cannot occur.
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 of 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
More to Learn
Encyclopedia articles:
Suppliers
The RP Photonics Buyer's Guide contains 40 suppliers for variable optical attenuators. Among them:
EKSMA OPTICS
EKSMA Optics has variable attenuators for high power Nd:YAG laser or femtosecond Ti:sapphire laser applications.
AMS Technologies
AMS Technologies carries a broad range of variable fiber-optic attenuators (VOAs), for manual operation as well as driven by stepper motors, MEMS mirrors or shutters, or based on liquid crystals:
- manually operated, PCB mountable, blocking VOAs
- manual VOAs with neutral density filter for MM
- manual VOAs, reflection mode
- motor driven VOAs
- MEMS-based VOAs, coaxial-style, miniature or ultra-small
- single or dual MEMS-based VOAs, reflection or transmission mode
- MEMS-based VOA arrays, 4 to 40 channels
- micromechanical shutter VOAs
AeroDIODE
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.
LASEROPTIK
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°.
Quantifi Photonics
Quantifi Photonics' offers a range of extremely compact, cost-effective variable optical attenuators designed for streamlined testing and characterization of optical communication systems and optical components. They provide fast attenuation speed with low insertion loss and built-in power monitoring, with fixed attenuation or constant output power modes and supports single-mode, multi-mode and polarization-maintaining fibres.
Questions and Comments from Users
Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.
Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.
By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.
Share this with your network:
Follow our specific LinkedIn pages for more insights and updates: