Fiber-Optic Raman Probes for Biomedical and Pharmaceutical Applications - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 29

435

(a)

481

(b)

554

593

666

(c)

789

1039

1500 1000 500

Raman Shift (cm -1 )

Fig. 2.2. FSB spectra of GeO 2 -doped fused silica ( a ), low-OH fused silica (b ), and
normal fused silica (c ) fibers measured with 785 nm excitation (modified from [8])

increased with a decrease in core diameter. The FSB intensity was estimated
in a different way, as they measured the laser transmission power and Raman
background intensity separately.
Figure 2.2 depicts the typical FSB spectra for GeO 2 -doped fused silica
(E-190, Mitsubishi; a), low-OH fused silica (FIP200220240, Polymicro Tech-
nology; b), and normal fused silica (SF-180, Sumitomo; c) fibers. The bands
at 1039, 789, 593, 481, and 435 cm − 1 are assignable to silica. The FSB value
of the pure silica fiber was slightly higher than that of the low-OH silica fiber
at low wave numbers, but not above 500 cm − 1 [28]. Since it is dicult to
remove the FSB where it overlaps with sample Raman spectrum, the use of
pure silica fiber is recommended. It is well known that pure silica fiber has a
weak absorption band above 700 nm due to the OH group in the fiber. The
maximum loss is only about 1% per meter, and is negligible for laboratory
or clinical use, where fiber lengths rarely exceed designing a few meters. The
user should be aware that there is a strong wavelength dependence to FSB,
and the values of Table 2.1 may not be useful at much lower or higher excita-
tion wavelengths. Therefore, researchers who want to fabricate a miniaturized
Raman probe must take into account the excitation wavelength to be used.
Polymer clad fibers, plastic fibers, and multicomponent glass fibers are
inappropriate Raman probes because these materials generate intense Raman
scattered light and fluorescence. Although the core material is fused silica for
some of these fibers, the light penetrates into the cladding material and the
generated FSB is propagated in the fiber. The FSB spectra of graded-index
fibers closely resemble that of GeO 2 -doped fused silica fibers. Single-mode