Fiber-Optic Raman Probes for Biomedical and Pharmaceutical Applications - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 32

A B

785 nm

100

(a)

(b)

60

80

(c)

40 60

(d)

40

0 °
5 °
10 °
15 °

20

20

0

0

770 775 780 785 790 795

760 780 800 820 840

Wavelength (nm)

Wavelength (nm)

Fig. 2.4. Transmission spectra of the BP filter ( A ) and LP filter ( B ) measured at
0 ◦ (i), 5 ◦ (ii), 10 ◦ (iii), and 15 ◦ (iv) of incident angle

cycles, the transmission angle increases to the point where some of the light
will pass through the second filter. Hence, attention must be paid to the stray
light reflected by the filters when multiple filters are present in a small area.

2.2.4 Spectrometer Systems

The Raman scattered light collected by the fiber-optic probe is directed into
the coupling stage, where it is collimated by a lens, L a , and filtered, then
focused on the slit of the Raman spectrometer by a second lens, L b .Toop-
timize the system eciency, the optical property of the Raman spectrometer
must be taken into consideration. Here, let us consider how to develop a high-
throughput fiber-optic probe Raman system. It is presumed that one would
like to simultaneously measure the spectral range from 0 to 2000 cm − 1 with
a spectral resolution of 10 cm − 1 . We can also assume that the f /number of
the Raman spectrometer will be 2.4 and the number of pixels and the size of
1 pixel will be 1024 and 25

m in the lateral direction. In this case, the spectral
range for a single pixel is about 2 cm − 1 and a spectral spread of 10 cm − 1 covers
5 pixels, which corresponds to a width of 125

μ

m on the CCD chip. Therefore,
the slit width of the spectrometer must be 125

μ

μ m or smaller. Many workers

use 100 m slits which are usually available. Now, we assume again that an

optical fiber with an NA of 0.2 will be employed for the collection fiber. To
catch all of the light coming out of a fiber with an NA of 0.2, the f /number of
the lens, L a , collimating the light must be smaller than 2.45. The spectrometer
governs the f /number of the lens, L b , focusing the light into the slit to be 2.0.
Therefore, to gather all of the light into the spectrometer, the core diameter
of the fiber should be 128 (= 102

μ

m or smaller. In fact, the power
density across the fiber is Gaussian. Even if the image of the fiber is a little too
large, not much light will be lost. Although this is a very rough estimation,
it illustrates the need to take the entire Raman system into consideration in
developing a high-throughput fiber-optic probe Raman system.

× 2 . 45 / 2 . 4) μ