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Acousto-Optic Tunable Filters (AOTF)
Acousto-optic tunable filters (AOTF) are used to rapidly and dynamically select a specific wavelength from a broadband or multi-line laser source. As the applied RF frequency is varied, the transmitted wavelength changes, “tuning” the wavelength of the beam or image in tens of microseconds or less.We offer an extensive line of AOTFs for wavelength regions from the UV through mid-IR, with resolution bandwidths of less than 1nm. We also offer options such as large-aperture imaging filtering and sideband suppression. Fiber-coupled AOTF devices are available upon request.
In an acousto-optic tunable filter (AOTF), an RF drive frequency is applied to an acousto-optic material such as tellurium dioxide (TeO2) to create a diffraction grating in which the refractive index of the crystal varies with position. As a coherent optical beam passes through the crystal, only a narrow band of frequencies will interfere constructively (i.e., meet the phase-matching condition) and be transmitted efficiently to exit the crystal at an angle that differs from the undiffracted beam.
The selective diffraction of light with wavelength allows the crystal to act as a tunable bandpass filter. As the RF drive frequency is varied, the center wavelength of the narrow passband changes. The primary advantage of an acousto-optic tunable filter over other wavelength selection devices is speed. Wavelength tuning can be accomplished in tens of microseconds.
Factors that influence choice of an acousto-optic tunable filter include:
- Wavelength range
- Resolution bandwidth (reported as the spectral FWHM for the transmitted beam)
- Beam size or active aperture needed
- Degree of beam collimation
- Desired tuning speed
- Polarization (we recommend polarized light for best efficiency)
Most applications of AOTFs involve filtering of light from a broadband source, such as a supercontinuum fiber laser, or selecting a single wavelength from a combined beam of multiple laser wavelengths.
AOTFs with apertures less than 6 mm are typically used that have less than tens of nm of resolution bandwidth at NIR wavelengths and less than ten nm at visible wavelengths. We also offer a quasi-collinear AOTF which can deliver < 1 nm resolution bandwidth if operated with highly collimated light.AOTFs with larger apertures (> 6 mm) are a powerful tool for spectral imaging, rapidly and efficiently scanning an entire image in wavelength. This is of use in high-speed applications like hyperspectral imaging, confocal microscopy, and on-line process control. The cost of AOTFs increases significantly for very large apertures, but they deliver unmatched speed for time-sensitive multispectral measurements in industry and biotech, approaching real-time video rate spectral imaging.
Our acousto-optic tunable filters are manufactured using high quality TeO2 crystals grown in-house, polished and fabricated to rigorous standards. We offer wavelengths from 350 nm to 4.4 µm in a wide variety of apertures and resolution bandwidths.
We can filter images up to 25 mm across, meet exceptionally low driver power requirements, or design an AOTF to select and transmit multiple discrete wavelengths.
Acousto-optic tunable filters often exhibit light leakage outside the resolution bandwidth of interest, typically at 10-20 dB below peak power. This is due to the response function of the AOTF itself, but can be minimized using our patented techniques. We offer sideband suppression in several models, reducing out of band side lobes by greater than 20 dB relative to the primary beam.
Our AOTF product family includes application-specific solutions for illumination or excitation wavelength selection, as well as multispectral or hyperspectral imaging. For best performance, we recommend a matched RF driver, including the latest digital frequency synthesizer (DFS) driver technology and random access wavelength control.
Applications of acousto-optic tunable filters
Confocal microscopy, fluorescence imaging, hyperspectral imaging, imaging spectroscopy, laser wavelength tuning, on-line process control, spectroscopy, wavelength selection