Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 11

Band-pass filters include narrowband-pass filter and broadband-pass filter.
Narrowband-pass filter is usually used to reject side bands of a laser beam for
fluorescence or Raman excitation. Broadband-pass filters are usually used for
detection purposes such as the fluorescence emission. Notch filter is the reverse
of the band-pass filter in which a central band is blocked, allowing the signal
with shorter or longer wavelength to pass. Notch filters are commonly used in
fluorescence and Raman spectroscopy and imaging.
Most of the filters are based on the properties of the substrate materials and sur-
face coatings, such as absorption, interference, or birefringence. Absorption filters
include neutral density filter, short-pass filters, long-pass filters, and broadband-
pass filters. Because they are based on the absorption properties of the materials,
absorption filters are usually less dependent on the incident angle of the light. This
is a major advantage of the absorption filters. However, because it depends on the
absorption of the materials, absorption filters are usually thicker and heavier.
Interference filters are usually narrowband-pass filters made from the interfer-
ence effect of thin films of dielectric or metallic layers on the substrate surface.
The films are arranged such that the light within the band is transmitted through
constructive interference, while the light out of the band is rejected through destruc-
tive interference. Interference filters can be made very thin and light. The rejection
edge of interference filter is very steep, so it is particularly suitable for rejection
of side bands for laser excitations. The main disadvantage of interference filters is
its dependence on the incident angle of light beam. A few degree difference in the
incident angle may change the rejection ratio by a few orders. Birefringence filters
are based on the birefringence effect of the material, usually used with polarizers.
Choice of the filters depends on the applications. For certain applications, the
combination of absorption and interference filters shows superior advantages over
only a single type of filters.
There are also two types of tunable band-pass filters commercially available for
biomedical applications: liquid crystal tunable filter (LCTF) and acousto-optical
tunable filter (AOTF). The mechanism of LCTF and AOTF is fundamentally differ-
ent. Both tunable filters have been used for fluorescence and Raman spectroscopy
[ 19 - 21 ].

1.2.4.1 Liquid Crystal Tunable Filters

Liquid crystal tunable filter works as a tunable band-pass filter by electrically
controlling the transmission of certain wavelength and blocking of the others. The
passband can be as narrow as 1 nm with a rejection ratio up to 10,000:1. The most
commonly used materials for LCTF are ferroelectric liquid crystal or nematic liquid
crystal wave plates. LCTF is comprised of several cascaded stages of birefringent
elements based on the design of Lyot filter as shown in Fig. 1.6 . A fixed retarder
and liquid crystal retarder are positioned between two parallel linear polarizers.
The exit polarizer for a given stage works as the input polarizer of the following
stage. Because of the birefringent effect of the liquid crystal, the incident linearly