Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 23

Single fiber to diode laser

BP filter

Collimator

Lens

Lens

Skin

Fiber bundle to spectrometer

Lens Window LP filter

Fiber bundle
cross-section

Fig. 1.11 Block diagrams of the specially designed skin Raman probe

the surface skin irradiance is 1:56 W=cm 2 , which is lower than the ANSI maximum
permissible exposure (MPE) limit of 1:63 W=cm 2

for a 785-nm laser beam with 1 s

integration time [ 31 ].
The collection arm has a double-lens (FPX11685/102, JML, Rochester, NY)
configuration, with the first lens for Raman signal collection and beam collimation
and the second lens for focusing the Raman signal to the fiber bundle. Both
lenses have a diameter of 25.4 mm with 90% of clear aperture and an effective
focal length of 50 mm, which matches the numerical aperture of the fiber for
high optical throughput. The focal point of the collection lens overlaps the focal
point of the illumination lens on the illumination arm. The diameter of the fiber
bundle determines the measurement size (1.3 mm on the skin surface). In between
the two collection lenses lies a high-transmission interference 785-nm long-pass
filter (LP01-785RU-25, Semrock, Rochester, NY). Because the rejection by the
interference LP filter depends on the incident beam angle, two 17.7-mm aperture
windows are used before the LP filter to reject stray light. This arrangement more
effectively rejects the laser line and allows us to record Raman shifts down to
500 cm 1

for in vivo skin tissue.

1.3.1.4 Raman Signal Collection

The raw signal, which is composed of Raman scattering and the tissue autofluo-
rescence background, is collected by the probe and transmitted to the spectrometer
detection unit through a fiber bundle. The fiber bundle is composed of fifty-eight
100-m core-diameter low-OH fibers. The number of fibers is determined by the
height of the CCD detectors (6.9 mm). The end of the fiber bundle that connects
the Raman probe is packed into a 1.3-mm diameter circular area, which also defines
the measurement spot size at the skin surface. There is an additional single fiber