Acousto-optical Tunable Filters,

Acousto-optic Filters. The newest type of spectrometer to become commercially available is the acousto-optic tunable filter (AOTF). An AOTF is a sohd-state, electronically tunable bandpass filter based on the diffraction of optical waves by acoustic waves in an optically anisotropic crystal. 

Fig. 2. Stmctures of acousto-optic tunable filters (a) collinear and (b) noncollinear.

Because NIR was initially used for food and agriculture products, it has evolved as a technique for complex matrices. Many types of hardware have become available for NIR work interference filters, gratings, interferometers, diode arrays, and acousto-optic tunable filters. And, as it was originally developed for complex mixtures, chemometrics has been an integral part of any NIR analysis for the last few decades. NIR practitioners are quite comfortable with multivariate equations and development of equations for complex matrices. 

The acousto-optic tunable filter (AOTF) analyzer... 

AOTF acousto-optical tunable filter CPAC Center for Process Analytical... 

IR spectroscopy is not a very sensitive analytical tool and is, therefore, not well suited to the detection of small amounts of material. If, however, intermediates have intense and well-resolved IR absorptions, the progress of their chemical transformation can be followed by IR spectroscopy [83,88,91-93], Near-infrared spectroscopy, in combination with an acousto-optic tunable filter, can be sufficiently sensitive to enable the on-bead identification of polystyrene-bound di- and tripeptides, even if the peptides have very similar structures (e.g., Leu-Ala-Gly-PS and Val-Ala-Gly-PS) or differ only in their amino acid sequence (e.g., Leu-Val-Gly-PS and Val-Leu-Gly-PS) [94]. Special resins displaying an IR and Raman barcode have been developed, which may facilitate the deconvolution of combinatorial compound libraries prepared by the mix-and-split method [48]. 

There are several measurement techniques that can be considered for use in IR-based instrumentation in the NIR and the mid-IR spectral regions, and these are summarized in Table 4.1. Some of these techniques are classical, as in the case of optical filter-based instruments and scanning monochromators. Others, such as acousto-optically tunable filter (AOTF), have been considered for 15+ years, but are mainly applied to NIR... 

Wang, X. Soos, J. Li, Q. 8t Crystal, J., An Acousto-Optical Tunable Filter (AOTF) NIR Spectrometer for On-line Process Control Process Contr. Qual. 1993, 5, 9-16. 

Wang, X., Acousto-Optic Tunable Filters Spectrally Modulate Light Laser Focus World 1992, 28, 173-180. 

Fig. 1 Schematic drawing of a wide-field single-particle tracking fluorescence microscope equipped. Several lasers are used as excitation source for different fluorophores with fast selection hy an acousto-optical tunable filter (AOTF). The collimated laser light is coupled into the objective such that only the observed area is illuminated. The emission light is separated from the excitation light by a dichroic mirror. In the case of multi-color imaging, the emission light is separated by...

A modification of the Debye-Sears effect, i.e., the modulation of the refractive index by acoustical waves functioning as tunable grating, is now applied to fast scanning NIR spectroscopy The Acousto-Optic Tunable Filter (AOTF), (Harris and Wallace, 1969 Chang, 1981). In combination with array detectors, chemical imaging is possible (Treado et ah," 1992). 

Figure 9.1. Schematic of Raman spectrometer based on an acousto-optic tunable filter (AOTF). BP, bandpass filter BR, band reject filters APD, avalanche photodiode. (Adapted from Reference 9, with permission.)...

Acousto-optic tunable filters (AOTF) are solid-state, rapidscanning instruments. The heart of the instrument is an anisotropic crystal (Fig. 4) capable of splitting a particular wavelength of light into its ordinary and extraordinary (clockwise and counter-clockwise rotated) rays. One face (orthogonal to the light beam) has a transducer bonded to it. 

Figure 4 An acousto-optic tunable filter [a, tellurium dioxide (Te02) crystal b, incident or input beam c, acoustic transducer d, rf input e, monochromatic light (ordinary beam) f, nonscattered light beam g, monochromatic light (extraordinary beam) h, acoustic wave absorber].

In one paper by Kirsch and Drennen [62], three experiments were performed on coated tablets. In the first experiment, intact theophylline tablets coated with an ethylcellulose polymer were analyzed by acousto-optic tunable filter (AOTF) spectrometers. Tablets were coated with increasing levels of ethylcellulose to vary the dissolution profiles. After spectra were collected, the dissolutions were run on a U.S.P. Type II dissolution apparatus. The time required for 50% of the drug to enter solution was used as the measure of dissolution rate. Principal component (PC) regression was used to develop the calibration. This gave a SEE of 2.8 min, a coefficient of variation of 0.977, and a SEP of 6.6 min the time to 50% dissolution ranged from 48 to 93 min. 

Numerous styles and brands of instruments and sample cells have been used for the analysis of tablets. The authors currently use several brands of instrumentation for tablet analysis, including filter-, diffraction grating-, and acousto-optic tunable filter-based instrumentation. Detector configurations are evolving slowly toward an optimal design however, the designs of most manufacturers are suitable for many applica-... 

Keywords Hyperspectral imaging spectropolarimetric imaging acousto-optic tunable filter AOTF ... 

N. Gupta, R. Dahmani, and S. Choy, Acousto-optic tunable filter based visible-to-near-infrared spectropolarknetric imager, Opt. Eng. 41, p. 1033-1038,2002. 

N. Gupta, D. R. Suhre, and M. Gottlieb, LWIR spectral imager with an 8-cm passband acousto-optic tunable filter, Opt Eng. 44, p. 094601 1-7, (2005). 

N. Gupta, Acousto-optic tunable filters for Infrared Imaging, in Acousto-optics and Photoacoustics, R. Reibold, ed., Proc SPIE 5953, 595300 1-10 (2005). 

N. Gupta and V. B. Voloshinov, Hyperspectral imaging performance of a Te02 imaging acousto-optic tunable filter in the ultraviolet region, Opt Lett. 30, p. 985-987 (2005). 

V. B. Voloshinov and N. Gupta, Investigation of magnesium fluoride crystals for imaging acousto-optic tunable filter applications, Opt. 45, p.3127-3135 (2006). 

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