Biomedical Engineering Reference
In-Depth Information
not have satisfactory optical specifications for Raman applications, although
the technology is advancing rapidly.
Although the limitations of UV Raman for 'normal' samples are severe,
where the excitation of the laser overlaps with an absorption band of the
molecule, a huge enhancement of the Raman signal can occur, referred to as
the resonance Raman effect [13]. These absorptions may occur in the visible
region (such as hemoglobin at 550-600 nm [14]); however, they are ubiqui-
tous in the UV, and enable sensitive, highly selective probing of the physical
state of molecules [15]. This has proven especially useful in the elucidation of
macromolecular structure; however, because it is important to be able tune
the excitation wavelength to make full use of the power of this technique, tun-
able OPO or multiwavelength doubled Kr/Ar ion lasers are typically used,
which are both large and costly [16]. The spectra are also currently dicult
to interpret, with limited numbers of reference spectra. Although this is an
important emerging technology, the cost and complexity of the equipment will
likely prohibit its wide emergence outside the academic lab in the near future.
One further scheme for reducing fluorescence in Raman spectroscopy
should be noted here, which is temporal filtering. The Raman signal is gen-
erated instantaneously after interaction with a photon, while fluorescence is
delayed by several nanoseconds. If a very short pulse laser is used to gener-
ate the Raman signal, and an appropriately fast gated detector is used, the
Raman signal can be obtained before the fluorescence signal has appeared
[17]. This is a temporal corollary to the spectral window leveraged in deep
UV Raman. Such systems have been demonstrated with good success on a
variety of highly fluorescent samples. However, these systems are costly and
complicated compared to widely available commercial systems; they also ex-
hibit lower spectral and spatial resolution, coupled with lower tolerance of
scattering samples [18]. Advances in pulsed laser technology and detection
will almost certainly improve the attractiveness of these systems. However,
currently the cost of the system coupled with the relatively small number of
critical samples that cannot be measured using other techniques limits this
technique's widespread applicability.
This highlights the importance of size and cost of the laser devices in
emerging applications. Many of the measurements described in this topic were
shown to be possible by studies occurring as early as the 1970s (e.g., teeth
[19], connective tissue [20], and ocular tissues [21]); the principle differentiating
factor between these proof-of-principle studies and the potentially transfor-
mative technologies described in this topic is the size and complexity of the
Raman system, of which the laser makes up a large proportion. Diode lasers
may be two orders of magnitude smaller than their gas laser counterparts
from 20 years ago. Because many of the innovations applied to Raman in-
strumentation are technologies borrowed from the broader industrial market,
the commoditization of laser diodes has also driven down the cost of Raman
lasers. Solid-state diode pumped lasers, as well as stand-alone laser diodes can
now be manufactured inexpensively with a wide range of output wavelengths
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