Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 29

substantially minimized. The elastically backscattered laser light is also minimized
through these two-stage filtering (long-pass filter on the fiber tips and two long-
pass filters in the filter module). The filtered signal was coupled into another fiber
bundle for delivery to the spectrograph. The bundle consisted of 54 fibers of 100 m
in diameter, packed in a round geometry at the filter module end and spread out
into a parabolic arc at the spectrometer end, which replaced the standard entrance
slit. All fibers used in this system are ultralow-OH optical fibers for high infrared
transmission.

1.3.2.3 Spectrograph and Detectors

The system was equipped with a f /2.2 transmissive imaging spectrograph
( HoloSpec , Kaiser Optical Systems, Ann Arbor, MI) with two interchangeable holo-
graphic gratings: one covering a low-frequency (LF) range of Raman shifts ( 200 to
2;000 cm 1 ) and the other for a higher-frequency (HF) range (1,500-3;400 cm 1 ).
Tissue Raman spectra were measured with an NIR-optimized back-illuminated,
deep-depletion, liquid nitrogen-cooled CCD array (400 1;340, 20 20 mper
pixel, model: Spec-10:400BR/LN, Princeton Instruments, Trenton, NJ).
Similar to earlier discussions, straight entrance slit will result in curved image
on the CCD, which will lead to lower signal-to-noise ratio (S/N), the spectral
resolution, and the throughput of the system. Therefore, the 54 fibers were specially
aligned for hardware binning. Following the previous work, the exact shape of the
parabolic image was determined by measuring the curvature of the image of the
standard slit. For this system, it was found to be

x D 1:12824 10 5 y 2 C 1:26767 10 3 y C 1:80834; (1.10)

where x was the displacement along the horizontal direction and y was the
distance of the slit away from the center of the slit along the vertical direction,
both in m. When such an arrangement of fibers replaced the entrance slit with
orientation opposite to the image curvature, it resulted in a straight line image at
the center of the CCD. Hardware binning with such an arrangement led to an S/N
increase up to 20 times of the software binning.

1.3.2.4 Raman Data Acquisition and Processing

The system was wavelength calibrated using a Hg(Ne) lamp (Newport Corporation,
Stratford, CT). The spectral resolution was estimated to be 9 cm 1 for both LF
and HF gratings, which was in agreement with the manufacturer's specification
(10 and 8:5 cm 1 for the LF and HF gratings, respectively, assuming a 100-m slit
width). Spectral response calibration was implemented using a standard halogen
calibration lamp (RS-10, Gamma Scientific, San Diego, CA) and an integrating