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10.1. Sensing with Fiber Bragg Gratings > 10.1.11 Chirped Grating Sensors - Pg. 471

Sensing with Fiber Bragg Gratings 471 higher orders in the reflection spectrum as the pressure grating induces a sam- pled FBG. As the loading on the metal grating increases, so does the reflectivity of the first and higher orders of the sampling period. Using this system, pressure may be measured; however, it is difficult to conclude from the results what is exactly the sensitivity of the sensor. It is not clear if the loading on the grating allows for the fact that two fibers support the weight, in which case the loading should be halved. However, with a suitable normalization, it would be conceiv- able that such a sensor could be used in pressure sensing, although its long-term mechanical survivability may be of concern. 10.1.11 Chirped Grating Sensors These sensors use the property of a chirped grating in several ways. The use of a chirped FBG (CFBG) in a loop mirror, as shown in Fig. 10.28, was first demonstrated by Margulis [69]. A chirped grating placed in a loop mirror has a periodic reflection and transmission spectra as a function of wavelength, simi- lar to those shown in Figs. 10.28 and 10.29, because the Michelson interfermo- ter with oppositely chirped gratings in each arm is equivalent to the Sagnac loop mirror with a chirped grating in the middle of the loop. If the CFBG is sub- jected to strain or temperature, the spectrum shifts. Thus, this arrangement