acousto-optic tunable filters (original) (raw)

Acronym: AOTF

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

Alternative term: acousto-optic programmable dispersive filters

Category: photonic devices

• optical modulators
  • acousto-optic modulators
    * acousto-optic deflectors
    * acousto-optic frequency shifters
    * acousto-optic Q-switches
    * acousto-optic tunable filters

Related: acousto-optic modulatorsoptical filterswavelength-tunable light sources

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Contents

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

What are Acousto-optic Tunable Filters?

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 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, an 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 crystal material used.

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

RF Drivers for Acousto-optic Tunable Filters

In contrast to an acousto-optic Q-switch, for example, a tunable filter requires an RF driver with constant drive power but variable frequency. It typically contains a voltage-controlled oscillator (VCO), the frequency of which can be adjusted with an analog input drive signal. For higher precision, a Direct Digital Synthesizer (DDS) can be used.

See also the article on acousto-optic modulator drivers.

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 AOTF for spectrally filtering fluorescence light which the excitation light has caused on some sample.

Some wavelength-agile 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.

Frequently Asked Questions

What is an acousto-optic tunable filter (AOTF)?

An acousto-optic tunable filter is a type of optical filter based on an acousto-optic modulator. Its filtered wavelength can be rapidly changed by electronically tuning the frequency of an applied radio-frequency (RF) signal.

How does an AOTF work?

An AOTF uses an RF signal to generate an acoustic wave in a crystal. This wave diffracts light only within a narrow band of optical wavelengths where a specific phase-matching condition is met, thus filtering the light.

Does an AOTF have different types of optical outputs?

Yes. The diffracted light constitutes a bandpass filter output, while the undiffracted light that passes straight through provides a notch filter output.

Can an AOTF select multiple wavelengths at the same time?

Yes. By applying a signal containing a superposition of different RF frequencies, an AOTF can simultaneously diffract and select multiple optical wavelengths. The diffraction efficiency for each wavelength is controlled by the power of the corresponding RF component.

What are the main differences between collinear and non-collinear AOTFs?

Collinear AOTFs offer high diffraction efficiency due to a long interaction length but have a small acceptance angle. Non-collinear designs provide a much larger acceptance angle but have a shorter interaction length, which can result in lower efficiency or require higher RF powers.

What are typical applications for acousto-optic tunable filters?

AOTFs are widely used for rapid wavelength selection in applications like multispectral imaging, laser microscopy, and spectroscopy. They are also used in wavelength-agile light sources and for wavelength division multiplexing in fiber-optic communications.

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