Filter, tunable

A variety of devices have been designed by using transparent electro-optical ceramic materials, including variable optical attenuators (VOA), polarization controllers (PC), sinusoidal filters, dynamic gain flattening filters, tunable optical filters, and (2-switches, which have been described in detail in Ref. [129]. A brief description is given for some devices as follows. 

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

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.

The potential of a tunable dye laser should not be overlooked. A tunable dye laser, employing an organic dye as lasing material allows one to choose any suitable excitation line within a particular region. This is in contrast to the case of a gas ion laser which has a limited number of emission lines at fixed wavelength. Nevertheless, a tunable dye laser has significant drawbacks such as poor resolution imposed by the dye laser linewidth (1.2 cm-1) and a continuous background spectrum which requires the use of a tunable filter 15-18). 

The IR probe radiation, provided by a liquid N2-cooled line-tunable CO laser, is passed through the cell collinearly or almost collinearly with the uv photolysis radiation. The probe beam is then directed onto an IR detector using a filter for either selectively blocking the photolysis beam or... 

Historically, this has been the most constrained parameter, particularly for confocal laser scanning microscopes that require spatially coherent sources and so have been typically limited to a few discrete excitation wavelengths, traditionally obtained from gas lasers. Convenient tunable continuous wave (c.w.) excitation for wide-held microscopy was widely available from filtered lamp sources but, for time domain FLIM, the only ultrafast light sources covering the visible spectrum were c.w. mode-locked dye lasers before the advent of ultrafast Ti Sapphire lasers. 

Gat N., Imaging Spectroscopy Using Tunable Filters A Review, Proc. SPIE, 2000 4056 50-64. 

However, the applications of the phenomenon of the modal transition are not limited to the chemical sensing field. Indeed, cladding etching and an HRI layer of sputtered indium tin oxide were exploited to achieve an LPG-based tunable filter with a large tuning range and with limited reduction in the loss peak65. 

In addition to interference filters, NIR manufacturers use holographic gratings (moving and fixed), interferometers, polarization interferometers, diode arrays, acoustic-optic tunable filters, as well as some specialty types. 

Fig. 10.9 A MEMS IR gas spectrometer, which measures the IR absorption of a gas at different wavelengths selected by a tunable interference filter. After [Lai 1 ].

Fig. 10.10 SEM micrograph of a tunable IR Bragg filter (1.1 xl. 85 mm) of density-modulated PS, suspended at two microactuator arms. Its rest position is more than 180°...

Figure 7.3. Block diagram of a near-lR optical fiber deleclor. A/D = analog-to-digital A OIF = acoustic Optical tunable filter.

C. D. Tran and R. J. Furlan, Spectrofluorometer Based on Acoustic-Optic Tunable Filters for rapid scanning and multicomponent sample Analyses, Anal. Chem. 65, 1675-1681 (1993). 

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. 

Grover, R., Ibrahim, T.A., Kanakaraju, S., Lucas, L., Calhoun, L.C., and Ho, P.-T., 2004, A tunable GalnAsP-InP optical microring notch filter, IEEE Photon. Techno Lett. 16(2) 467-469. 

Rabiei, P., and Steier, W.H., 2003, Tunable polymer double micro-ring filters, IEEE Photon. Technol. Lett. 15( 9) 1968-1975. 

Savchenkov, A.A., Ilchenko, V.S., Matsko, A.B., and Maleki, L., 2005, High-order tunable filters based on a chain of coupled crystalline whispering gallery-mode resonators, IEEE Photon. Technol. Lett. 17(1) 136-138. 

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

D.L. Wetzel, A.J. Eilert and J.A. Sweat, Tunable filter and discrete filter near-infrared spectrometers, in Handbook of Vibrational Spectroscopy, J.M. Chalmers and P.R. Griffiths (eds), vol 1, John Wiley Sons, New York, 2002. 

Broadband tunable filters (examples AOTF, LCTF) X ... 

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