Biophotonics_Concepts_to_Applications

application, a mattress pad or a chair cushion can have a series of embedded FBG
sensors with different Bragg wavelengths cascaded along a singlefiber, as is shown
in Fig.7.25. In one setup, a line of twelve FBG sensors was mounted on the surface
of a bed to form a 3 by 4 matrix array, which then was covered by a standard
mattress [ 3 , 55 ]. Each FBG sensor was specially packaged into an arc-shaped
elastic bending beam using a carbonfiber reinforced plastic material, which ensures
excellent sensitivity and good linear translation from a lateral force exerted to the
apex of the sensor into axial strain of the FBG when a subject is on the bed.
Thereby patient movements and respiratory rate cause different pressures on indi-
vidual FBG sensors, so that changes in the Bragg wavelength of specific FBG
sensors allows the monitoring of both healthy and abnormal conditions of a patient.

7.7.2 Distributed FBG-Based Catheter Sensor

A second example of an FBG-based biosensor is a catheter that does distributed
in vivo sensing of pressure abnormalities due to motility disorders in the gas-
trointestinal tract [ 3 , 61 ]. The catheter was formed from a serial array of twelve
FBG sensors that were fabricated into a continuous length of single modefiber. As
shown in Fig.7.26, each FBG was attached to a localized pressure-sensitive
structure consisting of a rigid metallic substrate and a flexible diaphragm.
Each FBG element was 3 mm long and had a full-width half-maximum spectral
response of 0.6 nm for Bragg wavelengths spaced 1.3 nm apart in the 815–850 nm
sensing range. The FBG sensor elements were spaced 10 mm apart, thereby
resulting in a catheter with a 12-cm sensing length. The device was designed to
measure pressure changes between−50 and +300 mmHg, which adequately covers
the range of pressures normally encountered in gastrointestinal tracts. As shown in
Fig.7.26, a circulator was used to insert light from a broadband optical source into
the sensor array. Optical signal variations returning from the sensor array reentered
the circulator and were sent to an optical detector from the third port of the
circulator.

To light sources

and detectors

Serial FBG sensor array

Sensor pad with N FBG elements

FBG 3 FBG 2 FBG 1

FBG 4 FBG 5 FBG 6

FBG N

Fig. 7.25 FBG pressure-pad
sensor array for patient
condition monitoring

224 7 Optical Probes and Biosensors