Encyclopedia … combined with a great Buyer's Guide!

Acousto-optic Tunable Filters

Acronym: AOTF

Definition: acousto-optic devices which can be used for filtering light, controlled with an RF input

German: akusto-optische abstimmbare Filter

Category: photonic devicesphotonic devices


Cite the article using its DOI: https://doi.org/10.61835/nu0

Get citation code: Endnote (RIS) BibTex plain textHTML

An acousto-optic tunable filter (AOTF) is a kind of optical filter which is based on a kind of acousto-optic modulator. Wavelength tuning is electrically controlled through the applied RF frequencies.

Operation Principle

The basic operation principle is explained in the following. If a sinusoidal (fixed-frequency) RF input signal is applied to the modulator, diffraction is possible only in a narrow range of optical frequencies, where a phase matching condition involving both optical and acoustic waves is fulfilled. Looking at the diffracted light, one obtains a bandpass filter, while the non-diffracted light provides a notch filter.

With RF waves of different frequencies, one can address different regions of optical frequencies. One may, for example, use an optical input from an argon ion laser emitting on different laser lines, and with the tunable filter one can transmit just one of those lines at a time.

It is also possible to use any superposition of different RF frequencies in order to obtain diffraction for different optical frequencies. The diffraction efficiency at any wavelength can be controlled via the corresponding RF power.

Depending on the design, a AOTF may work over an optical wavelength range which is hundreds of nanometers wide. Other devices are optimized for high resolution in a narrower wavelength range. Some of them also work with ultrashort pulses.

Acousto-optic Filter Designs

Because the requirements for different applications are quite diverse, different types of acousto-optic filters have been developed.

Some devices use a collinear interaction between light and sound waves. For example, one may use a tellurium oxide (TeO2) crystal, having a high acousto-optic figure of merit, and apply acoustic shear waves (i.e., with the oscillation perpendicular to the beam direction). The diffracted light will then have a polarization direction which is perpendicular to that of the linearly polarized input light. It is thus easy to isolate the diffracted light after the device with a polarizer. Such devices have a small acceptance angle, i.e., they work only with an appropriately aligned laser beam having a limited amount of beam divergence.

There are also tunable filters based on a non-collinear geometry. These typically have a much larger acceptance angle, but only a quite limited interaction length, so that the diffraction efficiency is correspondingly reduced, or higher RF powers are required.

Some devices can work with an unpolarized input, exploiting the birefringence of the used crystal material.

While many such filters are realized with bulk optical elements, there are also filters implemented as waveguide devices, suitable for photonic integrated circuits.

Different filter designs can differ greatly in terms of various performance parameters, for example

  • the addressable range of optical wavelengths,
  • the spectral resolution and quality of sidelobe suppression, and
  • the diffraction efficiency and required RF power.

The requirements of the used RF driver also depend very much on the device design and its application.

Applications of Acousto-optic Tunable Filters

A widespread application of AOTF is in multispectral imaging, e.g. in the form of laser microscopy. The essential advantage of this technology is that very rapid scanning (e.g. compared with mechanically controlled spectrometers) is possible, allowing for a fast acquisition of microscope images with spectral information. Also, the optical setup can be quite compact.

For terrestrial observations with spectral resolution for monitoring the status of plants, for example, one may exploit the large field of view and high spatial resolution of a non-collinear filter. It is also possible to acquire additional information on the polarization of light (spectropolarimetry).

There are other applications in laser spectroscopy. For example, one may use an AOTF for selecting certain wavelengths of an excitation laser and another AOFT for spectrally filtering fluorescence light which the excitation light has caused on some sample.

Some wavelength-agile wavelength-tunable light sources use an AOTF.

The ability to rapidly select a certain wavelength region can also be used for optical fiber communications based on wavelength division multiplexing.

More to Learn

Encyclopedia articles:


[1]S. E. Harris and R. W. Wallace, “Acousto-optic tunable filter”, J. Opt. Soc. Am. 59 (6), 744 (1969); https://doi.org/10.1364/JOSA.59.000744
[2]S. T. K. Nieh and S. E. Harris, “Aperture–bandwidth characteristics of the acousto-optic filter”, J. Opt. Soc. Am. 62 (5), 672 (1972); https://doi.org/10.1364/JOSA.62.000672
[3]H. Herrmann, K. Schäfer and W. Sohler, “Polarization independent, integrated optical acoustically tunable wavelength filters/switches with tapered acoustical directional coupler”, IEEE Photonics Technology Letters 6 (11), 1335 (1994); https://doi.org/10.1109/68.334831
[4]G. Hugh Song, “Proposal for acousto-optic tunable filters with near-ideal bandpass characteristics”, Appl. Opt. 33 (31), 7458 (1994); https://doi.org/10.1364/AO.33.007458
[5]K. McCallion, W. Johnstone and G. Fawcett, “Tunable in-line fiber-optic bandpass filter”, Opt. Lett. 19 (8), 542 (1994); https://doi.org/10.1364/OL.19.000542
[6]D. O. Culverhouse et al., “Low-loss all-fiber acousto-optic tunable filter”, Opt. Lett. 22 (2), 96 (1997); https://doi.org/10.1364/OL.22.000096
[7]H. Herrmann, K. Schäfer and Ch. Schmidt, “Low-loss tunable integrated acoustooptical wavelength filter in LiNbO3 with strong sidelobe suppression”, IEEE Photon. Technol. Lett. 10 (1), 120 (1998); https://doi.org/10.1109/68.651131
[8]H. Gnewuch et al., “Broadband monolithic acousto-optic tunable filter”, Opt. Lett. 25 (5), 305 (2000); https://doi.org/10.1364/OL.25.000305
[9]N. Gupta and V. Voloshinov, “Hyperspectral imager, from ultraviolet to visible, with a KDP acousto-optic tunable filter”, Appl. Opt. 43 (13), 2752 (2004); https://doi.org/10.1364/AO.43.002752
[10]V. B. Voloshinov and N. Gupta, “Investigation of magnesium fluoride crystals for imaging acousto-optic tunable filter applications”, Appl. Opt. 45 (13), 3127 (2006); https://doi.org/10.1364/AO.45.003127
[11]Y. S. Dobrolenskiy et al., “Nonreciprocity of acousto-optic interaction in collinear tunable acousto-optic filters”, Appl. Opt. 48 (7), C67 (2009); https://doi.org/10.1364/AO.48.000C67
[12]W. Yang et al., “Wavelength-tunable erbium-doped fiber ring laser employing an acousto-optic filter”, J. Lightwave Technol. 28 (1), 118 (2010)
[13]K. J. Lee et al., “Analyses of unintentional intensity modulation in all-fiber acousto-optic tunable filters”, Opt. Express 18 (5), 3985 (2010); https://doi.org/10.1364/OE.18.003985
[14]K. J. Lee et al., “Sidelobe suppression in all-fiber acousto-optic tunable filter using torsional acoustic wave”, Opt. Express 18 (12), 12059 (2010); https://doi.org/10.1364/OE.18.012059
[15]W. Zhang et al., “All-fiber acousto-optic tunable notch filter with a fiber winding driven by a cuneal acoustic transducer”, Opt. Lett. 36 (2), 271 (2011); https://doi.org/10.1364/OL.36.000271
[16]G. E. Romanova, V: I Batshev and A. S. Beliaeva, “Design of an optical illumination system for a tunable source with acousto-optical filtering”, J. Opt. Technol. 88 (2), 66 (2021); https://doi.org/10.1364/JOT.88.000066

(Suggest additional literature!)

Questions and Comments from Users


How is the output angle of an AOTF able to remain constant? In an AOM, different wavelengths produce different Bragg angles (so the first order beam is at a different angle if the input wavelength is different). Is that not true for an AOTF? Why?

The author's answer:

You are right that generally the output angle depends on various factors such as drive frequency and optical input frequency. However, you may use the non-diffracted light as the output. Also, the range of output angles may be small enough to not matter much in some cases.

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; we would otherwise delete it soon. (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.

Spam check:

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.