7.7 Fiber Bragg Grating Sensors
A highly successful precise wavelength selection component is afiber Bragg
grating (FBG). As Sect.4.2.1describes, a FBG is a narrowband reflection-grating
filter that is constructed within an opticalfiber core [ 3 ]. For biophotonics appli-
cations an external force, such as a strain induced on the device by the weight of a
person, will slightly stretch thefiber thereby changing the length of the FBG and
thus changing the reflected wavelength. To measure the induced strain, the FBG
sensor typically is glued to or embedded in a specimen that responds to the external
strain. The one precaution that needs to be taken when using such a sensor is to
realize that the FBG is temperature sensitive. Thus, either the substrate on which the
FBG is glued has to be temperature insensitive or some type of temperature
compensating method has to be deployed along with the strain sensor.
A wide selection of biosensor applications to diagnose or monitor the health
status in a person has been realized using FBGs [ 3 ]. Among these are the following:
- Measuring the polymerization contraction of common dental composite resins [ 52 ]
- Use of a non-invasive FBG-based optical fiber probe for carotid pulse (a
heartbeat felt through the wall of a carotid artery) waveform assessment [ 53 ] - The development of a smart-bed healthcare system for monitoring patient
movements [ 54 – 56 ] - The use of FBG sensors in biomechanics and rehabilitation applications, for
example, (a) monitoring contact stress, contact area, and joint alignment during
knee joint replacement and (b) simultaneous measurements of contact force or
stress andfluid pressure in articular joints [ 57 – 59 ] - FBG-embedded sensor pads for human-machine interface surfaces for rehabil-
itation and biomechanics applications, such as medical beds, wheelchairs, and
walkers [ 60 ] - In vivo use of a high-resolution FBG-based opticalfiber manometer for diag-
nosing gastrointestinal motility disorders [ 61 , 62 ] - Measuring the shock absorption ability of laminate mouth guards [ 63 ]
- Smart textiles for respiratory monitoring [ 64 ]
- Simultaneous pressure and temperature measurements [ 65 ]
More details for two of these examples are given below. Both applications used
wavelength division multiplexing techniques for simultaneous querying of an array
of FBG sensors along a singlefiber line with different wavelengths.
7.7.1 Smart-Bed FBG System
One healthcare implementation of an array of FBG sensors is a smart-bed or
smart-chair system for monitoring movements of patients [ 54 – 56 ]. In this
7.7 Fiber Bragg Grating Sensors 223