The fiber Bragg grating sensors work by sending the light into the fiber, where it is reflected back from the FBGs. The light that has been reflected travels back to the photodetectors of the instrument, where it is compared to wavelength reference artifacts. During this process the fiber Bragg grating interrogator evaluates the position of the center wavelength of the FBG; this information is later converted to engineering units. The gage factor supplied with the FBG sensor helps to determine and translate the data obtained during the measurements.
The described principle is true when one FBG sensor is present on a fiber. However, if the particular application requires multiple FBG sensors, like FBG sensor pipeline monitoring, fiber optic well monitoring, FBG temperature sensing, etc., the interrogators use one of the discriminating schemes in order to discriminate between one FBG sensor and the next. There are a couple of discrimination methods that are used in FBG interrogators. The first one is referred to as time division multiplexing utilizes the known speed of light in the fiber to discern which signal is reflected from which FBG along the fiber path. Around 100 FBG sensors can be interrogated with this method.
The second method, wavelength division multiplexing, is the most utilized one. As FBG sensors are at distinctly different nominal center wavelengths from their neighbors, the FBG interrogator uses the wavelengths of the sensors to track them along the fiber. The range of this method is large due to the developments in the fiber optic technology.
Other approaches to FBG sensor interrogating, some of which include:
- Broadband source, Dispersive element, Diode Array;
This method is less reliable than the aforementioned ones due to limited resolution, which is a result of inherent limitations of commercially available diodes.
- Broadband source, Optical Spectrum Analyzer/Multi-line wavelength meter;
The optical spectrum analyzers are large and expensive, which makes them less desirable in a laboratory setting. They are also not able to perform optimally under some temperatures.
- OTDR/TDM systems;
The system cannot handle a large number of sensors on the fiber as its data acquisition rates scale down with increasing sensor counts.
- External Cavity Tunable Laser, Power Meter, Wavelength Meter;
External cavity tunable lasers have low speed and do not have a wide operating temperature range. Moreover, they are expensive and do not have the required mechanical robustness.
Optromix interrogators can control up to 8 optical channels. The interrogator operates with the 20 maximum sensors per channel. The device is controlled by the PC with the specialized software for sensors monitoring. The system contains a broadband source of radiation and it can carry out spectrum analysis.
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