Lasers, Spectrographs, and Detectors - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 15

into separate wavelengths [33, 34]. As shown in Fig. 1.6, the interferogram
is generated by changing the path length of the signal by varying the posi-
tion of the moving mirror. This position is monitored very accurately using
the interference fringes from the HeNe reference laser. The detected signal is
then transformed into frequency, and a spectrum generated that is essentially
equivalent to the dispersive response. The limiting resolution of the FT in-
strument is the distance that the moving mirror travels; even a fairly modest
mirror movement of a few centimeters can give tenths of a wave number res-
olution. Because the full spectrum is contained in the interferogram, a wide
spectral window can be obtained at high resolution. The spectrum also has
inherent wavelength accuracy, since the spectrum is internally calibrated by
the reference laser. Multivariate models can be more reliably applied to FT
data, and the data between instruments will be much more consistent. More
importantly, FT-Raman can work more eciently using a long-wavelength
1064 nm excitation laser, which effectively eliminates fluorescence. This can
be important advantage when working with biological samples, although typi-
cal FT-Raman laser powers can be well above the damage threshold for many
samples [35].
Unfortunately, Fourier Transform instrumentation has been overshadowed
to some extent by the advances made in dispersive technology. This is in no
small part due to the ability of dispersive manufacturers to leverage advances
in the rapidly evolving optoelectronics market, while FT-Raman depends
heavily on already mature FT-IR platforms. Although better detectors and
laser rejection filters have improved the noise characteristics of the system,
the fundamental throughput advantage that made FT a universally adopted
approach in the mid-IR was never suciently compelling to engender a sim-
ilar dominance in Raman, principally because of the availability of low-noise
CCD detectors in the visible. The almost total suppression of background
fluorescence is a great advantage; however, the recent introduction of low-
noise multichannel detectors for the near-IR region (950-1650 nm) has enabled
the development of dispersive systems operating with higher eciency with
1064 nm excitation. In spite of FT-Raman's intrinsic advantages, it is unclear
how well it will compete with the small, cheaper, more sensitive, flexibly con-
figured, and all-solid-state dispersive instruments as an enabling technology
driving the emerging applications described in this topic.
Another wavelength selection mechanism of note is electronically tun-
able bandpass filter. There are two commercially available technologies, the
acousto-optic tunable filter (AOTF) and the liquid crystal tunable filter
(LCTF). Each uses different underlying mechanisms. The AOTF uses an
acoustic wave generated in an optically clear crystal to change the angle of
the input beam based on wavelength (essentially a variable transmission grat-
ing) [36], while the LCTF uses oriented liquid crystals to selectively retard
the beam, causing destructive interference for all wavelengths except the de-
sired passband (referred to as a Lyot filter) [37]. There are a variety of other
practical differences which impact the implementation of each technology; for