Interrogation techniques for FBG Sensor Arrays

FBG sensors are very suitable for sensing and data acquisition, where sensor arrays can be multiplexed using similar techniques that have been applied to fiber-optic sensors like wavelength-division multiplexing (WDM), spatial-division-multiplexing (SDM), and time-division-multiplexing (TDM) as they can be directly implemented in the fiber without changing the diameter of the fiber. This feature makes FBG sensors suitable for a wide range of applications.

The main problem with the TDM system is that the sensors must be placed sufficiently far apart because the pulse returning from the adjacent sensors must be able to reach and get detected separately. In WDM systems, different sensors have the nominal central wavelength, and other sensors are separated by a few nanometers. WDM interrogation is available in two topologies i.e., series and parallel. The parallel approach is easier to implement but the series topology allows the optical power from the sensing FBG array to be used much more efficiently than parallel topology.

The number of sensors that you can incorporate within a single fiber depends on the wavelength range of operation of each sensor and the total available wavelength range of the interrogator. Because typical interrogators provide a measurement range of 60 to 80 nm, each fiber Bragg grating array of sensors can usually incorporate anywhere from one to more than 80 sensors – as long as the reflected wavelengths do not overlap in the optical spectrum. Be careful when selecting the nominal wavelengths and ranges for the FBG sensors in an array to ensure that each sensor operates within a unique spectral range.

Major limitations in interrogating FBG sensor arrays are the cross-talk, spectral shadowing, and interference. For all the interrogation approaches, some crosstalk between adjacent sensors seems to be unavoidable. The use of a serial array of FBG sensors with the same central wavelength results in the crosstalk between sensors. The amount of light reflected by the FBG sensors located nearest to the source will affect the amount of the optical power reaching and be returned from gratings further from the source. The lower the peak reflectivity of the FBGs is, the smaller the effect is. Another source of the crosstalk in a TDM serial array of identical FBG sensors arises from multiple reflections between FBGs. This can lead to pulses arriving simultaneously at the detector having undergone a direct reflection from a sensor element and also having experienced a number of multiple reflection paths between FBGs.