Acousto-optical tunable filters (AOTF)

 

The acousto-optic tunable filter (AOTF) is an all-solid-state optical filter that operates on the principle of acousto-optic dif fraction in an anisotropic medium. The center wavelength of the filter passband can be rapidly tuned across a wide spectral range by changing the frequency of the applied radio frequency (RF) signal. In addition to the electronic tunability, other outstanding features of the AOTF include: large angular aperture while maintaining high spectral resolution, inherent intensity, and wavelength modulation capability.

 

Acousto-optic tunable filter find many applications:

- Rapid-scan Spectrometers. The rapid, random-access tuning of the AOTF makes it well-suited to rapid-scan spectrometer applications, such as time-resolved spectra analysis.

- Multispectral Imaging. Use of noncollinear AOTF imaging in astronomy has been demonstrated by spectropolarimetry of stars and planets

- Fiber Optic Communication. One important application where the AOTF has shown great potential is in the area of fiber-optic communication. Due to its capability of selecting a narrow optical band over a wide spectral range within a time duration of microseconds, the AOTF appears to be well-suited to perform wavelength division multiplexing (WDM) networks.

- Spectropolarimeter and fluorescence spectrometer. The great advantage using an AOTF is to be  capable of measuring spatial, spectral and polarization characteristics of a target, in real time, without moving parts, only with a single instrument.

 

AOTFs use the slow-shear acousto-optic interaction. This is polarization sensitive. For an optical input along the z-axis (θi=0) the polarization should be circular for 100% (relative) efficiency, or along the t-axis (θi=90º) linearly polarized. For directions in-between, the input should be some elliptical state for maximum diffraction efficiency. In practice, this will approximate to a linear state for θi>10º i.e. far enough away from the optic axis to ignore optical activity.

 

 

Depending on field of application it is possible to use incoming beam with requested polarization shutting excess and passed non-diffracted beam by absorber. Second variant is polarizing beam separation using polarizer and analyzer at the input and output. In hyper-spectral polarimetry both diffracted beams are used and incoming beam polarization components are defined by their ratio.

 

To have higher flexibility in the process of AOTF designing possibility to choose incidence angle θi and interaction length L is very important. Variations in the effective photoelastic coefficient can be reached by varying θi and will lead to the complicating design. Angular dependence of sound velocity and power flow direction of TeO2 crystals may effect on RF power, tuning relation, crystal sizes and shapes. Complete design can be held on the evaluating following characteristics:

- tuning relation;

- spectral resolution;

- polar & azimuthal angular aperture;

- RF power requirements;

- sound velocity;

- power flow angle;

- separation of incident & diffracted beams.

 

For certain applications such as lidar receiver systems it is desirable to significantly narrow the passband of the AOTF. However, there exists a basic tradeoff relation between the angular and spectral bandwidth. For instance, the angular aperture of a TeO2 AOTF with 0.1 percent bandwidth (e.g.,5 Å at 0.5 mm) the angular is only ±3˚. In principle, increased spectral resolution can be obtained by using long interaction length. In practice AOTF transducer sizes are about several centimeters (while, for instance, AO Deflector is about several millimeters) but we have keep in mind that AOTF’s important performance is light gathering power that scales with optical aperture. Relatively big sizes also lead to the transducer matching to the electrical part. One of the way from this problem is transducer segmenting to get nominal 50Ω. Also with power increasing we get crystal heating that leads to the temperature drift of interaction characteristics. This can solved by forced cooling or for precision spectrometers by crystal placing on thermoelectric cooler (TEC).

 

АОTF main characteristics

Typical values for TeO2 AOTFs

Tuning Range

450-750nm, 900-1200nm, 1200-2500nm, 2500-5000nm

Bandwidth

0.5 nm - 15 nm

Operating Mode

Slowshear, noncollinear propagation

Angular aperture

2-10 degrees

Optical Aperture

3x3 mm – 30x30 mm

Diffraction Efficiency

70-85 %

RF power

1-10 Wt

 

 

 

 

 

 

 

 

 

 

 

AOTF crystals typical geometrical sizes, Blank for imaging AOFT with aperture 20mm

 

We offer cells’ blanks for deflectors according to your specific requirements

 Also we’d like to offer elements with AR coating and one golden electrode ready for bounding.

 Note

- Upon your request we can supply blanks with any linear dimensions up to 70mm

- Upon your request we can offer AR or protective coating.

- Blanks for AOM & AOD are also available

- Blanks for transducers from LiNbO3 crystals with different orientations are also available

Reference list:

[1] Xu J and Stroud R 1992 Acousto-Optic Devices (New York:Wiley)

[2] Handbook of optics. CHAPTER 12 ACOUSTO-OPTIC DEVICES AND APPLICATIONS I. C. Chang

[3] Goutzoulis A and Pape D 1994 Design and Fabrication of Acousto-Optic Devices (New York: Dekker)

Events/News

14th - 16th March 2017 - visit us at "Laser World of Photonics China 2017" fair in Shanghai, China. Hall 1 Booth 1735.

22 - 25 June 2015

June 2015 - visit us at "Laser World of Photonics 2015" fair in Munich, Germany.

Booth B1.439.

 

January 2015 - new edition of OptoIndex catalogue was published. You can find our company page here.