Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry - Perot wavelength filter.

There has been considerable interest recently in the development of fiber-optic sensors based on in-fiber Bragg gratings (FBG's), which can be written into Ge-doped fibers by use of a transverse holographic method.' The use of such elements for distributed strain or temperature sensing in advanced composite or other structural elements and their use as inline reflectors in interferometric systems has been discussed in several recent articles.2 4 One of the key issues we and others have addressed is related to the problem of detecting small shifts in the Bragg wavelength of FBG sensor elements; schemes based on simple broadband optical filtering,' interferometric approaches,6 7 and fiber-laser approaches8 9 have been described. These approaches permit varying degrees of resolution and dynamic range and should be suitable to some of the application areas of interest for FBG sensors. In this Letter we describe results obtained with a fiber Fabry-Perot (FFP) filter as a demodulator for FBG sensors. The system can be operated either in a closed-loop tracking mode for use with a single sensor element or in a scanning mode for use with multiple sensors. In the latter case a derivative form of signal detection is used to permit higher resolution to strain-induced shifts in the Bragg wavelengths of the sensor elements. Figure 1(a) shows the concept for a single FBG element. Light from a broadband source is input into the system, and the component reflected by the FBG is directed via a coupler to a tunable FFP filter, which has a bandwidth comparable with that of the FBG and a free spectral range (FSR) larger than with the operational wavelength domain of the FBG (typically less than ±5 nm). The narrow passband of the FFP filter is locked to the narrow-band FBG return signal, R, with a simple feedback-loop arrangement to the tuning mechanism of the FFP (e.g., with piezoelectric adjustment of the cavity spacing). To accomplish this, we modulate the transmission wavelength of the FFP slightly (-0.01 nm) by dithering the tuning elements at a frequency fd. This results in a modulation in the optical output of the FFP, which, in general, will contain components at the fundamental and harmonics of fd. When the wavelengths of the FBG return signal and FFP transmission peak are aligned, the amplitude of the fundamental is nulled. The amplitude of the modulation component at the fundamental of the dither frequency thus serves as an error signal that can be fed via a simple integrator circuit to the FFP tuning elements to lock the FFP passband wavelength to the Bragg wavelength, AB, of the sensor return signal. Consequently, the FFP control voltage (feedback voltage) is a measure of the mechanical or thermal perturbation of the FBG. One obvious limitation of this approach is that it can only be used to interrogate a single FBG, and multiplexing of the sensors is not possible in this locked mode. Operating the FFP in a wavelength-scanning mode, however, provides a means for addressing several FBG elements. In this case, as shown in Fig. 1(b), several FBG sensor elements are placed along a fiber path (a star or branching system would be equally applicable). The nominal Bragg wavelengths and operational wavelength domains of the FBG's are chosen not to overlap, and all fall within the spectral envelope of the source. Furthermore, the FSR