Acousto-optic tunable filter ( AOTF ) is an optical filter based on acousto-optic modulator. Wavelength tuning is electrically controlled by the applied radio frequency.

Principle of Operation

If a sinusoidal (fixed frequency) RF input signal is applied to the modulator, diffraction may occur only in a narrow optical frequency range, where the phase matching conditions involving light waves and acoustic waves are met. Observe the diffracted light to obtain a band-pass filter, while non-diffracted light provides a notch filter.

Through radio frequency waves of different frequencies, people can deal with different light frequency regions. For example, it is possible to use light input from argon ion lasers emitted on different laser lines, and use a tunable filter to transmit only the roots of these lines.

It is also possible to use any superposition of different RF frequencies to obtain diffraction of different optical frequencies. The diffraction efficiency of any wavelength can be controlled by the corresponding radio frequency power.

According to the design, AOTF can work in the light wavelength range of hundreds of nanometers wide. Other devices are optimized for high resolution in a narrower wavelength range. Some of them also apply to ultrashort pulses.

Acousto-optic tunable filter design

Because the requirements of different applications are very different, different types of acousto-optic filters have been developed . The collinear filter has the highest efficiency, but the angle acceptance range is narrow. These devices use collinear interactions between light waves and sound waves. For example, it is possible to use a tellurium oxide (TeO 2) crystal with a high acousto-optic quality factor, and apply an acoustic shear wave (ie, have an oscillation perpendicular to the beam direction). The diffracted light will then have a polarization direction perpendicular to the polarization direction of the linearly polarized input light. Therefore, it is easy to isolate the diffracted light behind the device with the polarizer. Such devices have a small acceptance angle and they only work with properly aligned laser beams with limited beam divergence.

There are also tunable filters based on non-collinear geometry . These usually have a larger acceptance angle, but only a very limited interaction length, so the diffraction efficiency is correspondingly reduced, or higher RF power is required. Non-collinear filters may be more suitable for applications that require a large acceptance angle.

Some devices can use non-polarized input, taking advantage of the birefringence of the crystal material used. Different filter designs may have great differences in various performance parameters, such as

· Addressable range of light wavelength

· Spectral resolution and quality of sidelobe suppression

· Diffraction efficiency and required RF power

As used RF drive requirements are largely dependent on the device design and application.

Application of acousto-optic tunable filter

The wide application of AOTF is in multispectral imaging, for example in the form of laser microscopy. The main advantage of this technology is that it can perform very fast scanning (for example, compared with a mechanically controlled spectrometer ), so that a microscope image with spectral information can be quickly acquired. In addition, the optical device can be very compact.

For example, for terrestrial observations with spectral resolution for monitoring the state of plants, the large field of view and high spatial resolution of non-collinear filters can be used. It is also possible to obtain additional information about the polarization of the light (spectral polarization method).

There are other applications for laser spectroscopy. For example, AOTF can be used to select certain wavelengths of the excitation laser, and another AOFT can be used to spectrally filter the fluorescence caused by the excitation light on certain samples.

Some flexible wavelength tunable light sources use AOTF.

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

In acousto-optic tunable filters (AOTF) , the RF driving frequency is applied to acousto-optic materials, such as tellurium dioxide (TeO2), to create a diffraction grating in which the refractive index of the crystal changes with position. When a coherent beam passes through the crystal, only a narrow frequency band will constructively interfere (that is, meet the phase matching condition) and effectively transmit to leave the crystal at an angle different from the undiffracted beam.

The selective diffraction of light with a wavelength allows the crystal to act as a tunable bandpass filter. As the RF drive frequency changes, the center wavelength of the narrow passband will also change. Compared with other wavelength selection devices, the main advantage of acousto-optic tunable filters is speed. Wavelength tuning can be completed within tens of microseconds.

Factors affecting the choice of acousto-optic tunable filter include :

· Wavelength range
· Resolution bandwidth (reported as the FWHM of the transmitted beam’s spectrum)
· Required beam size or effective aperture
· Beam collimation
· Required tuning speed
· Polarization (we recommend using polarized light for best efficiency)

Most applications of AOTF involve filtering light from broadband light sources (such as supercontinuum fiber lasers), or selecting a single wavelength from a combined beam of multiple laser wavelengths.

An AOTF with an aperture of less than 6 mm is generally used, and its resolution bandwidth at NIR wavelengths is less than tens of nanometers, and its resolution bandwidth at visible light wavelengths is less than 10 nanometers. We also provide quasi-collinear AOTF, if you use highly collimated light operation, it can provide a resolution bandwidth of <1nm.

The AOTF with larger aperture (>6mm) is a powerful spectral imaging tool that can quickly and efficiently scan the wavelength of the entire image. This is very useful in high-speed applications such as hyperspectral imaging, confocal microscopy, and online process control. For very large apertures, the cost of AOTFs increases significantly, but they provide unparalleled speed for time-sensitive multispectral measurements in industry and biotechnology, near real-time video rate spectral imaging.

Our acousto-optic tunable filters are manufactured using internally grown high-quality TeO2 crystals, polished and manufactured in accordance with strict standards. We provide wavelengths with various apertures and resolution bandwidths from 350 nm to 4.4 µm.

We can filter images up to 25 mm in size to meet extremely low driver power requirements, or design AOTF to select and transmit multiple discrete wavelengths.

Acousto-optic tunable filters usually leak light outside the resolution bandwidth of interest, usually 10-20dB below the peak power. This is due to the response function of AOTF itself, but it can be minimized using our patented technology. We provide a variety of types of sideband suppression functions. Compared with the main beam, the out-of-band sidelobes are reduced by more than 20dB.

Our AOTF product line includes specific application solutions for illumination or excitation wavelength selection and multispectral or hyperspectral imaging. For best performance, we recommend using matched RF drivers , including the latest digital frequency synthesizer (DFS) driver technology and random access wavelength control.

Application of acousto-optic tunable filter
Confocal microscope, fluorescence imaging, hyperspectral imaging, imaging spectrum, laser wavelength tuning, online process control, spectrum, wavelength selection