Acousto-optic tunable filters (AOTFs) are solid state wavelength tunable optical filters. Modern AOTFs are constructed by attaching piezoelectric transducers to an appropriate crystalline material. By driving the transducers at the appropriate frequency, a series of perturbations traverse the material. Interactions of photons with these perturbations allow the AOTF to selectively diffract a single narrow-bandpass wavelength. The wavelength may be varied by changing the applied frequency.
Principle of Operation of Brimrose’s Hyperspectral Imaging System (HSI)
GREENOPTICS'S Hyperspectral Imaging System (HSI) is based on an Acousto-Optic Tunable Filter (AOTF). The AOTF acts as an electronically tunable spectral band pass filter. It is a solid state electro-optical device with no mechanical moving parts. Brimrose AOTFs are non-collinear type and made out of Tellurium Dioxide (TeO2) crystals. An RF transducer bonded to one side of the TeO2 crystal emits acoustic waves. The wavelength of light selected is a function of the frequency of the acoustic wave traveling through the crystal. Thus, by varying the frequency of the acoustic wave, the wavelength of the separated or filtered light can be varied. The AOTF only diffracts one specific wavelength of light, so that it acts more like a filter than a diffraction grating.
In the AOTF device by definition the filtered light is diffracted into two first order beams, orthogonally polarized to each other. For randomly polarized input light the P-Polarization components are diffracted into the (+) order and the S-Polarization components are diffracted into the (-) order. The (+) order is SPolarized and the (-) order is P-Polarized due to 90 deg polarization rotation from AO interaction.
A beam stop is used to block the un-diffracted zero order, (broadband light) and the (+) and/or (-) monochromatic light is directed to the camera. The bandwidth of the selected light depends on the device and the wavelength of operation. Transmission efficiencies are high--up to 90%--with the intensity divided between the (+) and (-) beams.
For a given AOTF geometry, the transmitted wavelength is determined only by the frequency of the applied RF signal, which can be generated with digital precision. The typical Brimrose TeO2 AOTF has a wavelength repeatability that is less than 0.05nm.
The utilization of the AOTF into the imaging system offers unmatched spectral flexibility in terms of scanning the full range or random hopping on wavelengths of interest and wavelength scanning speed. Using Brimrose standard software, the target or area of interest can be viewed at a user specified wavelength in real-time. The outline of how our system operates is shown below:
AOTF Hyperspectral Imager
Powered by brimrose
• Up to 2,000 frames / second*
• Data Cube Collection < 1 second*
• High-Spatial & Spectral Resolution
• Real-Time Image Display at user specified λ
• Data Cube Collection of stationary or moving target at an angle
• Dual Use: Multispectral or Hyperspectral Imaging
• Spectral Transmission & Reflectance Imaging
• 16-bit PNG file, Text Image File, or BSQ
• ENVI Software Compatible
What makes AOTF - Hyperspectral Imagers unique?
1. Large field of view (FOV) angle and aperture
Unlike conventional Bragg Cells, AOTFs are capable of a large acceptance angle (as large as ~ 10 degrees). Input aperture of 10 mm x 10 mm is typical for AOTF, compared to a typical value of 0.5 mm slit for grating based spectroscopic instrument. The combination of large angle and aperture result to high etendue of AOTF spectral imaging system and optical throughput.
2. Random wavelength access
The passband wavelength can be tuned to any values within the operation range within just a few microseconds.
3. Broad spectral range
400 nm to 1000 nm or 900 nm to 1700 nm
4. Practically unlimited tuning finesse
Control the center wavelength with resolution of less than 1 Hz (tuning resolution of less than 0.1 ppm of center wavelength of the passband). This provides ultimate flexibility in positioning the location of wavelengths during spectral data acquisition to obtain optimal signal congruence.
5. Increasing reliability and ruggedness
The battery powered compact solid-state device with no moving parts is immune to mechanical vibration and shock. Operate over extreme temperature ranges by virtually eliminating temperature change induced shift of wavelengths.
6. Optimizable Signal to Noise Ratio
It is very easy to improve the SNR for a specific wavelength or spectral region that contains the most important spectral signature by extending acquisition time at those wavelengths.
7. Accurate special information
The spatial information of the scene is accurately captured, especially for changing scenes.