Page 628 - Introduction to Information Optics

612 10. Sensing with Optics

results obtained with conventional resistant strain gauges [44]. Thus, indeed,
fiber Bragg grating-based quasi-distributed fiber sensors may be used in
real-world sensing.

10.3.2.1.4. Multiple-Parameter Measurements Using Fiber Bragg
Gratings Written in Highly Birefringent
Polarization-Preserving Fibers
In real-world applications, strain and temperature perturbations may add
to the fiber Bragg gratings simultaneously. To avoid the use of additional
sensors to separate influence from strain and temperature changes, the tech-
niques of using double Bragg gratings written at widely spaced wavelengths at
the same location in a polarization-maintaining (PM) fiber were developed
[45,46]. In this case, the spectrum reflected from one location contains four
peaks. Thus, in principle, one can determine axial strain, two components of
transverse strain, and temperature change in that location based on these four
peaks, as given by

= K (10.42)
Ax
A

where a and b designate peak wavelength measurements for Bragg gratings
written at wavelengths a and b, A/l la represents the wavelength shift from
horizontal polarization direction at wavelength a, AA 2a represents the wave-
length shift from vertical polarization direction at wavelength a, A/t lb repre-
sents the wavelength shift from horizontal polarization direction at wavelength
b, Al 2fc represents the wavelength shift from vertical polarization direction at
wavelength b, K is a 4 x 4 matrix the elements of which may be determined
by separate experimental calibrations of sensor response to transverse strain,
axial strain and temperature change, e^ e 2, and % are axial, two transverse
strains, respectively, and AT represents the temperature change. This technique
is still under development and some promising results have been achieved [46].

10.4. SUMMARY

This chapter provided a brief summary on sensing with optics. First, it
introduces the mathematical description of the light field, which includes
amplitude, polarization, phase, and frequency. Since all these parameters of the
light field may change with external perturbations, sensing may be performed