Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 22

Fig. 1.10 Block diagrams of
the integrated real-time
Raman spectrometer system
for in vivo skin disease
diagnosis and evaluation

PC Computer

Diode
Laser

Single Fiber

CCD Camera

Spectrograph

VPT TM
Grating

Fiber
Bundle

Raman
Probe

Calibration
Fiber

Skin

1.3.1.2 Excitation Light Delivery

Optical fibers are most commonly used for light delivery in medical applications.
The main considerations for choosing optical fibers are numerical aperture, core
diameter, core material, and its transmission properties. Multimode 100-200-m
core diameter fibers are commonly used for excitation light delivery in biological
Raman measurements. The choice of the numerical apertures (0.22 or 0.37) depends
on the collection capability (numerical aperture matching) of the laser system and
the lens system. The transmission properties depend on the core and cladding
materials. High-OH fibers have high-UV and visible wavelength transmission, while
low-OH fibers are preferred for NIR and IR wavelength range (Sect. 1.2.2.3 ). Raman
signals may arise from the fiber's core material itself and contaminate the tissue
signals [ 30 ]. Choosing the right optical fiber is therefore important for in vivo Raman
spectroscopy. Our system uses a 200-m core-diameter low-OH single fiber for
laser beam delivery because of its high NIR transmission.

1.3.1.3 Raman Probe Design

The optical layout of our skin Raman probe is schematically shown in Fig. 1.11 .
It is designed to maximize the collection efficiency of the tissue Raman scattering
and reduce the interference of the backscattered laser light, fiber fluorescence,
and silica Raman signals. It consists of two arms: a 1.27-cm (0.5-in.) diameter
illumination arm and a 2.54-cm (1-in.) diameter signal collection arm. In the
illumination arm, the laser beam illuminates a 3.5-mm spot on the skin surface at a
40 ı incident angle after having passed through a collimating lens, a band-pass filter
(785 ˙ 2:5 nm; model HQ785/5x, Chroma, Rockingham, VT), and a focusing lens.
The band-pass filter can effectively reject Raman scattering and fluorescence that
may arise from within the delivery fiber. The laser intensity is controlled so that