FBG Sensor Multiplexing Techniques On WDM System


Fiber Bragg Grating (FBG) is a simple and low-cost filter built into the core of a wavelength-specific fiber cable. FBGs are used as inline optical filters to block certain wavelengths, or as wavelength-specific reflectors.

In many applications a large number of sensors need to be used to achieve a distributed measurement of the parameters. In particular, using sensors in smart structures is of interest where sensor arrays are bonded or embedded into the materials to monitor the health of the structure. FBG sensors have a distinct advantage over other sensors because they are simple, intrinsic sensing elements that can be written into a fiber, and many sensors can be interrogated through a single fiber.

The most straightforward multiplexing technique for FBG sensors is wavelength division multiplexing (WDM), utilizing the wavelength-encording feature of an FBG-based sensor. The WDM technique is based on spectral splicing of an available source specturm. Each FBG sensor can be encoded with a unique wavelength along a single fiber. Since we are operating in the wavelength domain, the physical spacing between FBG sensors can be as short as desired to give accurate distributed information of measurands.

A parallel topology is used to allow simultaneous interrogation of all the sensors in WDM, as shown in Figure 4.15. A1 x N fiber optic splitter is used to divide the optical reflection into N channels, In each channel a matched fiber grating detects the wavelength shift from a specific FBG sensor.

Fiber splitter

In the parallel scheme each filter receives less than 1/2N of optical power as a result of using 1 x N fiber splitter and fiber coupler. More FBG sensors lead to a larger power penalty. An improved scheme using a serial matched FBG array is reported by Brady et al, as shown in Figure 4.15(b). This scheme is claimed to allow the optical power to be used more efficently than in the parallel topology. As can be seen, however, a large power penalty still exists through the use of the reflection of matched fiber gratings. A revised verison of the serial scheme is proposed, in which the transmisson of the matched FBG is used to monitor the wavelength shift from the corresponding sensing FBG. This reduces the power penalty of 6 dB.