Biophotonics_Concepts_to_Applications

The excitation light can come from any one of the diverse optical sources that
are described in Chap. 4 , which include light-emitting diodes that emit optical
power either in a broad white-light spectral band or in a narrow spectral band, an
arc lamp, a laser diode, or afiber laser. A dielectric optical bandpassfilter or a
monochromator can be used to select a very narrow spectral width for light delivery
to a tissue sample. Candidate photodetection instruments that interpret the returned
light include a viewing scope, a photomultiplier tube (PMT), a camera, or an optical
spectrum analyzer. Anoptical spectrum analyzer(OSA), which also is called a
spectrometer, is an instrument that measures some property of light (e.g., intensity
per wavelength) contained within a selectable wavelength range.
For the example single-fiber setup shown in Fig.7.3a, a dichroic opticalfilter
reflects a specific spectral band from a light source into thefiber, which sends the
light to the tissue target. As described in Sect.5.6, a dichroicfilter separates spectral
bands by reflecting and transmitting light as a function of wavelength. The trans-
mission and reflections are 50 % at the cutoff wavelengthλc, that is, 50 % of the
light below a specific wavelength is reflected from thefilter and the other 50 %
above the cutoff wavelength is transmitted through thefilter. Thus, the configura-
tion shown in Fig.7.3a can be used in spectroscopy applications wherein the light
is absorbed in a narrow spectral band (or at a specific wavelength, e.g., atλ 1 )bya
tissue sample and is remitted at a longer wavelengthλ 2 (that is, at a lower energy).
In this case, thefilter reflects the source spectral band that is centered atλ 1 , and the
remitted light centered atλ 2 passes through thefilter and falls on a photodetector.
In Fig.7.3b the link uses an optical circulator in place of the opticalfilter. For
this configuration, the light from an optical source enters the optical circulator at
port 1 and exits into the transmissionfiber at port 2. After being reflected or
remitted from the tissue sample, the light travels back along the transmissionfiber,
reenters port 2, and emerges from port 3 toward a photodetector. In this setup, for
spectroscopic applications a narrow-pass opticalfilter is needed after port 3 in order
to suppress backscattered light from the source that can interfere with the desired
optical signal from the tissue.

(b)

Target tissue sample

Light source

Photodetection

instrument

Bidirectional

optical fiber probe

Dichroic filter

Target tissue sample

Light source

Photodetection

instrument

Bidirectional

optical fiber probe

Fiber-coupled circulator

2 1

3

(a)

1

2

Optical coupling elements

1

2

Fig. 7.3 Example single-fiber probe configurations usingaa dichroicfilter andbafiber-coupled
optical circulator

7.2 Optical Fiber Probe Configurations 201