Biophotonics_Concepts_to_Applications

9.6 Raman Spectroscopy

Raman spectroscopyis a non-invasive, label-free biomedical optics tool that is used
for evaluating the chemical composition of biological tissue samples [ 2 ]. Detailed
compositional information is obtained by analyzing photons that have been scattered
inelastically from vibrational and rotational level transitions in the chemical bonds of
the tissue sample. A number of variations of the basic Raman spectroscopic method
have been investigated, such as surface-enhanced Raman scattering, coherent
anti-stokes Raman scattering, stimulated Raman scattering, time-resolved spec-
troscopy, polarization modulation, and wavelength-modulated Raman spectroscopy.
Thefirst three of these techniques are discussed in Sects.9.7through9.9. Designs of
miniaturizedfiber-optic Raman probe can be made from a simple single opticalfiber,
twofibers with lenses for better sensitivity, and coherentfiber bundles that have
opticalfiltering modules integrated on the bundle ends. The biomedical Raman
spectroscopy applications include in vivo examinations of organs such as the
bladder, breast, colon, esophagus, larynx, lung, oral cavity, and stomach [ 36 – 40 ].
The possible molecular transitions in a Raman inelastic scattering event are
shown in Fig.9.14and are compared with Rayleigh scattering. During the Raman
interaction of a photon with a molecule either a small amount of energy is trans-
ferred from the photon to the molecule, or the molecule can transfer some energy to
the photon (see Sect.6.3.4). Thefirst transfer process (from the lowest molecular
vibrational energy level to a higher vibrational state) is calledStokes scattering.If
the incoming photon interacts with a molecule that already is in a higher vibrational
state, the molecule can transfer some of its energy to the photon during the scat-
tering event. This process is calledanti-Stokes scattering.
In a Raman spectroscopy system,first laser light emerging from an excitation
fiber is scattered by the tissue and then a fraction of the scattered light is collected
by a detectionfiber [ 2 ]. In Raman spectroscopy only inelastically scattered photons
are of interest. Therefore the elastically scattered photons are suppressed by long-
pass or bandpass opticalfilters, which are located in the detection channel (see
Sect.5.6). At the analysis end of the collectionfibers, the inelastically scattered
photons are separated according to wavelength in a spectrograph (an instrument that

Virtual

energy states

Vibrational

energy states

Rayleigh

scattering

Stokes

scattering

Anti-Stokes

scattering

0

1

2

3

4

Fig. 9.14 State transitions in
Rayleigh elastic and Raman
inelastic scattering

276 9 Spectroscopic Methodologies