Quasi-distributed sensing using WDM techniques

Due to their multiplexing capabilities, FBG sensors offer the potential for applications in the field of quasi-distributed sensing in smart structures. Multiplexed FBG sensor arrays can be attached to, or embedded into, structures for multipoint monitoring of

physical parameters such as strain, stress, temperature, cracks and vibrations at arbitrary locations. Figure 2.4 shows the basic structure of a quasi-distributed FBG sensor array, where each FBG has a different resonance wavelength (Tjin et al., 2002). Many applications of FBG sensors in quasi-distributed sensing schemes have been reported. This section briefly reviews the applications of FBGs and chirped gratings in quasi- distributed sensing.

Figure 2.4 Basic FBG-based quasi-distributed sensing system (Tjin et al., 2002).

The use of FBGs in structural health monitoring to provide real-time data of civil structures subjected to environmental stress is useful for preventive maintenance in structures. Multiplexed FBG based strain sensors embedded into carbon composite materials (CCMs) were mounted onto concrete structures to determine the strain changes at different locations within the structure during cyclic tests (Tjin et al., 2002). The FBG sensors were shown to have good linearity and repeatability, with a resolution of 1 με and an accuracy of 5 με after temperature compensation. Temperature resolution of 0.04 °C with an accuracy of 0.2 °C was obtained from a FBG sensor array mounted alongside the strain sensors.

Distributed strain sensing with multi-element FBG arrays embedded in fibre-resin composite panels by the continuous resin transfer mouldingTM (CRTM)TM process have been reported (Friebele et al., 1994). The composite panel was subjected to three-point bending test under various weights and the strain distribution was estimated from the shifts in λB of the FBG arrays. The measured strain compared well with that calculated

from beam theory and that determined from nonlinear finite element analysis. A strain error of 17 με was obtained.

Another example of quasi-distributed strain sensing used extremely short length FBGs with well-defined spectra (Wu et al., 2002). The use of short length gratings with spatial width as small as few hundred micrometre overcame the problem of distortion of FBG spectra due to localised nonuniform strain. This sensor system has applications in aerospace. An improved quasi-distributed strain sensing utilising apodised FBG arrays reduced crosstalk between sensing arrays (Chisholm et al., 1998). However apodisation resulted in increased grating bandwidth, thus reducing the resolution of the system. The trade-offs between apodisation, grating bandwidth, source bandwidth and spatial resolution in the design of grating arrays for application in quasi-distributed sensing was investigated.

An important feature of any highly distributed FBG based sensing system is the much larger number of measurements possible at multiple locations along a single optical fibre with reduced installation cost compared to conventional strain gauge technology. This high density of measurements is useful to test engineers for the visualisation of strain contours along the structure. One such strain sensing system, employing FBGs at thousands of locations along a composite wing structure, used four 8 m long optical fibres with 800 FBG arrays per fibre, interrogated by an optical frequency domain reflectometer (OFDR) (Childers et al., 2001). The large number of FBG sensors generated two dimensional mapping of the strain fields. In another application, four sets of an array of five FBGs were embedded into an aerospace carbon fibre composite material to determine strain distribution during three point bending tests (Dockney et al., 1997). Each grating in the array was 2 mm long with a nominal Bragg wavelength separation of 4 nm. The spatial separation of arrays was (50.0 ± 0.1) mm.

Recently a quasi-distributed strain sensor employing chirped Moiré fibre Bragg gratings (CMFBG) has been demonstrated (Gillooly et al., 2005). CMFBG is a superposition of two CFBGs of different periods. A dual scan phase mask fabrication technique was used to inscribe the grating. Initially a chirped grating was written in the fibre followed by a second identical inscription over the same position under strain application. This creates a beat effect in the refractive index profile along the grating. At each node within a particular beat effect, a passband is created, which corresponds to a unique position along the grating. The strain at a discrete point along the grating was measured

by monitoring the wavelength shift of the passbands. An average spatial resolution of 164 μm over the grating length and a strain sensitivity of (0.8±0.01) pm/με was obtained. Such a sensing system has application in mapping strain distributions around a miniature deformation or crack in a surface.

A number of applications of quasi-distributed temperature sensing have also been reported. Smith et al. (2002) and Patterson et al. (2001) reported developing a quasi- distributed temperature sensor consisting an array of FBGs, illuminated by a superluminescent diode (SLD) and interrogated by a fibre Fabry-Perot (FFP) tunable filter. The sensing array consisted of seven gratings, each 1 mm long. The sensor has been tested for hyperthermic in-vivo treatment of tumours in rabbit livers.

In summary, quasi-distributed sensing utilises FBG sensor arrays positioned at strategic locations along a structure to map strain/temperature distribution along the structure. The technique, however, provides only an average of the measurand along the length of each grating by tracking the peak reflection Bragg wavelength of each grating sensor. Thus it is unsuitable for applications where a continuous profile of the measurand, as a function of position along the grating length, with millimetre position resolution is required (e.g. to pinpoint hot spots or detect cracks). The technique works well for large structures where spatial resolution is not a priority but is not appropriate when high spatial resolution is required.