Development of multipoint strain measurement systems using small-diameter fiber Bragg gratings

摘要

We have been studying optical sensing technologies using fiber Bragg gratings (FBGs) for health-monitoring systems in the fields of constructions, civil engineering, aerospace and so on. In these fields, various kinds of sensing techniques such as strain, temperature, vibration and crack detection are required. To meet these needs, we have fabricated FBGs with precise photo-induced refractive index modulation or the grating period along a fiber. Also, as embedded sensors, we have developed small-diameter FBGs which can be embedded in fiber reinforced plastics (FRP) composite materials without inducing any mechanical deterioration. In this paper, first we present the progress of our small-diameter FBGs for various sensing applications. Next, we propose a new wavelength detection technique for FBG sensors using a wavelength division multiplexing (WDM) coupler. The reflected light is divided in the ratio depending wavelength through the WDM coupler. Especially, we adopted a PLC-type WDM coupler which has the advantage in low polarization sensitivity and integration compared with a fused-optical fiber-type WDM coupler. This technique is useful in high-speed detection for vibration or impact damage. Another application is a multipoint measurement system by using together with optical time domain reflectometry (OTDR) method. In this system, pulses of light incident into an optical fiber return from the FBGs to a detector with different arrival time. As a result, we can use FBGs with the same Bragg wavelength, because measuring delay time of the pulses enables to distinguish each FBG location. In addition, using together with wavelength division multiplexing within wavelength region for optical communications can increase the number of FBG sensors. The experimental results of the performance in this system showed that it is very promising for multipoint strain measuring systems.