Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 13

a b

0.17 200

150

Incident
beam

0.16

Beam stop

100

f

D n

0.15

50

Acoustic
grating

0.14 0

500 600 700 800 900 1000

Electrode

Wavelength (nm )

Fig. 1.7 ( a ) Operation of an AOTF filter. A grating is generated by the RF acoustic wave applied
through the two electrodes. ( b ) A typical dispersion of a TeO 2 crystal and its corresponding
wavelength-frequency dependence

(RF) acoustic waves. AOTF is a tunable band-pass filter. The central wavelength
and the applied RF frequency are governed by

1
;

f D V ˛ .n e n o / (1.5)

where V is the velocity of the acoustic wave in the AO crystal and n e and n o are the
birefringence of the AO crystal. ˛ is a parameter dependent on the design. The most
commonly used AO crystal is TeO 2 [ 22 ]. The dispersion of TeO 2 and the frequency
and wavelength relations are shown in Fig. 1.6 b. Note that the dispersion (n e n o )
is wavelength dependent.
In general, AOTF has three outputs: one zero order and two first orders ( C 1
and 1). The zero-order beam has the same polarization as the incident beam, and
the two first-order beams are cross polarized. In practice, the zero order is usually
blocked by a beam stop. Polarizers are also often used to block the zero-order beam
and one of the first-order beams.
The main advantages of AOTF filters are their extremely fast switching time,
broad spectral range, and random accessibility. The response time of the AOTF is
about 100 times faster than an LCTF filter (s vs. ms). The limitations of AOTFs
are that they have relatively lower rejection ratio ( 10 3 ) and they are sensitive to
the incident angle which may cause image blurring and signal-to-noise problems.

1.2.5 Detectors

There are various types of detectors for biomedical spectroscopy systems. They can
be classified as single-channel detectors and multichannel detectors. Choice of the
detectors depends on the application, including the wavelength range, the signal
level, the system size, and the data acquisition speed.