AOTF Buyer’s guide

An acousto-optic tunable filter (AOTF) is a device that uses acoustic waves in a crystal to select a specific wavelength (or wavelengths) of light that pass through the crystal. AOTFs can come in a number of shapes and sizes, depending on the desired application. The design of an AOTF can have a significant effect on performance and price, so it is important to make sure that you get the right AOTF for your application.

It is worth remembering that the AOTFs shown on our website are only an overview of what is available. There are many other designs/specifications that we offer and we can also provide a custom design service.

Here are a few questions and comments that may help you chose the right AOTF.

How much does an AOTF cost? Essentially the ultimate Price (assuming largish quantities) of an AOTF will be determined by the cost of the raw material; ie the size of the AO cell. In other words, economies of scale can only drive the cost down so-far.

What wavelength range do you need? AOTFs are almost exclusively made from TeO2 which has a transmission window from 390nm to 4·4µm, within that range the AOTF wavelength range is generally slightly less than one octave; meaning that the maximum wavelength is slightly less than double the minimum wavelength. This is due to the bandwidth of the ultrasonic transducer. It is sometimes possible to bond two transducers onto a single AOTF cell to increase the range, but this inevitably results in cost increase and is not always practical.

What diffraction efficiency do you need? Diffraction efficiency is the amount of light of the desired wavelength and polarization that is isolated by the AOTF. It is given as a percentage of the light at that wavelength and polarization that passes straight through the AOTF when it is turned off. The value quoted in spec sheets is the maximum.

Can an AOTF control diffracted intensity? By changing the applied RF power the diffracted optical intensity can be controlled. The relationship between applied RF power and diffracted optical intensity is not linear, but can be calculated.

Is the light polarised? The useful light out of an AOTF is polarised and the quoted diffraction efficiency is based on using the correct polarisation input. If the light entering is un-polarized then the quoted diffraction efficiency will only apply to 50% of the light.

What light source are you using? AOTFs work best with a well-collimated and polarised beam. They will work with other light sources, but the optical system will need to be designed to match the field of view of the AOTF to obtain optimum performance.

What is the beam diameter? The beam diameter determines the AOTF aperture and crystal size, which is a large component of the cost. If you want to reduce the price of the AOTF, reducing the beam diameter is a good place to start.

Are you imaging through the AOTF? Hyperspectral imaging is an increasingly popular technique. When imaging through an AOTF a larger aperture and field of view is required. This requires a larger crystal than is used in many of our other AOTFs, and this can push the price higher. Some people prefer to design their hyperspectral system with an AOTF on the illumination source and a grayscale camera. This allows them to collimate the light through a smaller AOTF aperture, which requires a lower cost AOTF.

We have a range of AOTFs available for imaging spectroscopy and we also offer an integrated hyperspectral camera.

What optical throughput do you require? This is the key question for most spectral imaging applications where a perfectly collimated and polarized beam is not possible. The field of view (FOV) is set by the AO design, so to increase throughput one has to increase the aperture. This is why there has been a trend towards large-aperture AOTFs over recent years. Clearly this will impact on the cost. Ultimately aperture size is limited by the availability of large pieces of single-crystal TeO2. There will inevitably be a balance between aperture, FOV, resolution, range etc. Don’t forget the intensity of the diffracted beam can be controlled by varying the RF power from the driver. It is therefore a good idea to make sure you have more light available than you need as you can always turn the diffraction efficiency down.

What pass band resolution do you require? The resolution (passband linewidth) is determined by the interaction-length within the AOTF and also the crystallographic orientation. A typical AOTF might have a resolution of about 0·5% of wavelength. To achieve a tighter resolution the interaction-length needs to be increased. Ultimately the material size will limit the resolution (although other factors come into play). We have built “conventional AOTFs” with resolution down to about 0·1%. Clearly, a larger piece of TeO2 will cost, and this will set the limit on the unit cost, even for large quantities. The pass band resolution is fundamental to the design of the AOTF, but an AOTF can be simultaneously be driven with a number of pass bands, so it is possible to build up a boarder pass band from several narrower ones.

Quasi-Collinear AOTFs

As well as our conventional AOTFs we also offer other configurations. Generally, these will have a restricted field of view and can only be used with diffraction limited / fiber coupled source. For this reason they are not suitable for applications where an image is formed through the AOTF or lamp light passes through the AOTF. They can, however, feature narrower pass bands.

Switching Speed

The switching speed is essentially determined by the time taken for the acoustic wavefront to traverse the optical beam. Typically this is <10µs, making them very fast compared to competing technologies however the speed of the driver may be slower. If speed is an issue please contact us to discuss your needs.

Movement of Spectral Image (Spectral Scene-Shift)

As the AOTF tunes, the deflection-angle of the diffracted order will change. However, we use the inherent dispersion of the TeO2 to correct for this by polishing the output face at the appropriate wedge-angle. This normally means that the image does not “shift pixel”.

If you would like us to design a custom AOTF for you we will need to know the wavelength range, resolution, aperture and field of view. If you are not sure of all of these values, feel free to get and touch to discuss the options.