31
May
Red laser beam against a black background
PAT Roberts

All About Acousto-Optic Tuneable Filters

Acousto-Optic Tuneable Filters, or AOTFs, are devices used to filter light with the ability to control the filter via a radio frequency (RF) input.

These handy optical filters have a wide range of applications and have seen growing demand in recent years – the future is bright for AOTFs.

What is an acousto-optic filter?

In general, an acousto-optic filter works in the presence of an RF input signal. When a signal is applied to the modulator, diffraction is limited to a narrow waveband, which meets both acoustic and optical phase-matching conditions.

The filter essentially works in two ways: the non-diffracted light acts as a notch filter, while the diffracted light functions as a bandpass filter.

Filter Designs

There are several common filter designs of AOTFs. Examples include:

  • Collinear AOTFs with a low angular acceptance range but high efficiency
  • Non-collinear AOTFs with a wider acceptance angle but lower efficiency

Filter design can also determine whether the input light must be polarised or not. Birefringent crystal materials can allow for the construction of filters with no polarisation requirement, which can be a significant advantage in some applications.

Tuneable Filters

By adjusting the applied RF frequency, it is possible to tune the filter’s output to achieve different results.

For example, if using a laser source with multiple emission lines, the tuneable filter can be restricted to transmit only the desired laser line at any given time.

Multiple RF frequencies may be superposed to enable diffraction at multiple optical frequencies, with the power of each RF signal used to tune the diffraction efficiency of each wavelength independently.

This allows a single AOTF to be tuned in quite complex ways, resulting in a very specific bandpass spectrum with tuneable amplitudes.

How does it work?

AOTFs have been around since the early 1990s and contain an optically transparent crystal, which can be excited using acoustic waves.

The frequency of the acoustic waves – the RF input signal applied to the crystal – will determine the wavelength of light that is distracted when passing through the crystal.

All unaffected wavelengths pass directly through the crystal, allowing the filter to function as a dichroic beam splitter with tuneable wavelength profile.

What is acousto-optic modulation?

An acousto-optic modulator, also known as a Bragg cell, uses an input signal to generate oscillating sound waves in the filter crystal.

These periodic expansions and contractions alter the effective refractive index of the filter, leading to Brillouin scattering of the incoming light and interference akin to Bragg diffraction.

Acousto-optic modulation is the process by which AOTFs influence the light they transmit and, by altering the characteristics of the modulation, this enables the filters to be tuneable too.

Applications

Forecasts from Market Study Report put the global acousto-optic devices market at just over $400 million in 2020 and predict a compound annual growth rate of 7.24% to take the industry to nearly $660 million by 2027.

This growth is driven by demand for laser applications in the scientific sector, as well as laser-based applications in healthcare and manufacturing.

“The increase in shipments indicates a rise in demand for growing production capacity, which necessitates the usage of laser-based devices, driving market growth,” the analyst reported.

To find out more about optical filters or make an enquiry about bespoke filters for your upcoming research and development, please contact Envin Scientific today.