Biomedical Engineering Reference
In-Depth Information
A similar approach has recently been introduced that uses an InGaAs
detector as a photocathode, but with electron bombardment as the gain
mechanism [54]. The detector is specifically designed to have high sensitiv-
ity/quantum eciency in the NIR between 950 and 1650 nm where FT-Raman
instruments classically operate, but with some gating capacity since a cathode
intensifier is used. Although the Raman signal will be very weak in this region,
amplification of the signal coupled with the multiplex advantage and signal
gating may prove a useful addition for sensitive, highly fluorescent materials
such as biological samples.
It should also be noted that although a great majority of Raman spec-
trometers are equipped with multichannel detectors, the single channel de-
tector still has a variety of applications which resist replacement by the now
common CCD. Various applications have been discussed above for which mul-
tichannel detectors have no advantage, such as AOTF spectrometers. Where
very fast time gating is required and simultaneous wavelength measurement
does not offer an advantage (many CARS imaging applications, for example),
PMTs are a viable option, offering high sensitivity, single photon detection,
and relatively low cost (in spite of their sensitivity to high light loads and the
need for high-voltage power supplies). Avalanche photodiodes, when operated
under the appropriate conditions, may also offer many of the same speed and
gain advantages of the PMT, although typically with a much smaller active
area [55].
1.7 Imaging and Microscopy Systems
The system components described above can be combined into a wide array of
sampling configurations suitable for measurements as diverse as characteriza-
tion of the euent emitted from oceanic vents, to identification of explosives
at a standoff of tens of meters, to determining the provenance of historical
works of art. One of the areas of greatest Raman activity is in microscopy and
microscopic imaging; this will be described briefly here as an example of how
the various components are integrated into a complete system.
The schematic presentations in Fig. 1.9 represent three common configu-
rations for Raman microscope signal collection [56] with dispersion elements
implied but not shown for clarity. One of the principle advantages of Raman
spectroscopy is the ability to make measurements on regions as small as the
laser focus. Although the laser may be focused to a diffraction limited spot,
the system will likely accept signal from other areas as well; at the very least
on-axis light that is above or below the nominal focal position can be ac-
cepted, but depending on the system configuration and the degree of diffuse
scattering from the sample, variety of off-axis light may be accepted as well.
The slit (or fiber probe) will act as a first stage of spatial rejection; insertion
of a pinhole in the optical train will further reject out-of-focus signal, as well
as spurious light from other sources. As the input to the spectrometer is a
Search WWH ::
Custom Search