Biophotonics_Concepts_to_Applications

In the return path, a MMF is attached to the DCF coupler at port 4 to collect

the light from the multimode cladding and to deliver it to a photodetection

instrument.

Instead of using a single-fiber probe, influorescence spectroscopy instruments
often a more advantageous or necessary setup is to use separate opticalfibers for the
illumination and light collection functions, as is shown in Fig.7.6. The excitation
fiber typically would be a conventional single-modefiber or some type of photonic
crystalfiber. The collection and returnfiber choice can be an opticalfiber bundle, a
SMF, a MMF, a polarization-preservingfiber, a PCF, or a DCF [ 3 ]. In the setup in
Fig.7.6, it is desirable to have a selectable wavelength for the incident radiation and
to have the optical detector be capable of precise spectral manipulation and signal
assessment. Therefore, a monochromator or an optical spectrum analyzer is pro-
vided for both the selection of the excitation light and the analysis of the optical
emission from the sample. The function of amonochromatoris to separate and
transmit a selectable narrow spectral band of light from an optical signal that
contains a wider range of wavelengths.
If the system can accommodate a larger diameter probe, then the efficiency of the
illumination and collection functions can be increased by using more than two

2×2 DCF

coupler

Port 2

Target tissue sample

Bidirectional

DCF probe

MMF to

photodetector

SMF from

laser source

Photodetection

instrument

Port 4 Port 3

Port 1

Blocked

Fig. 7.5 Single-fiber probe
based on a DCF coupler and a
DCF probe

Light

source

Monochromator

or optical filter

Photodetection

instrument

Illumination

optical fiber Collection

optical fiber

Target tissue sample

Optical probe

housing

Fig. 7.6 Schematic of a
generic probe system
consisting of one or more
illumination and
light-collection channels
(J. Biomed. Opt. 19(8),
080902 (Aug 28, 2014).
doi:10.1117/1.JBO.19.8.
080902 )

7.2 Optical Fiber Probe Configurations 203