FBG Sensors
Sensors based on fibre Bragg grating techniques:
Principle:
By projecting a fringe of UV light on an optical fibre, it is possible to create a permanent change in the refractive index of the core of the fibre. Those “disks” are called Bragg gratings. As the light travel within the core of the fibre some of it is reflected back by the Bragg grating at a specific wavelength called the Bragg wavelength. As a measurand (temperature or strain for example) is applied to the Bragg grating, its refractive index changes and also the distance between the disk changes resulting in a change in the reflected wavelength. By detecting that change it is possible to relate temperature or strains change to a wavelength change. Current techniques are using optical spectrum analyser to detect that change however they are expensive and slow so therefore it was necessary to develop a fast and cheaper way to measure wavelength. The basic idea is to relate wavelength (change) to intensity (change). It was proven that by using standard telecom devices such as a thin film dense wavelength division demultiplexer (DWDM), wavelength division demultiplexer (WDM), Bragg grating and coupler it was possible to develop the following systems to interrogate a Bragg grating, the main one of interest (faster time response and better sensitivity) were the DWDM technique and Bragg grating technique. The systems layout is illustrated in figures 1 and 2.
Figure 1: Thin film dense DWDM system.
Figure 2: Two gratings system.
Temperature measurements:
These Bragg grating techniques were then used to monitor the temperature changes during machining process such as milling and grinding and the results were compare to each other and also to thermocouples results. Two of the fibre Bragg grating systems offered the advantages to be faster, more sensitive than the thermocouples commonly used. Figures 3 and 4 show the raw data results from the grinding tests.
Figure 3: Raw data for the two gratings and the thermocouple techniques.
Figure 4: Raw data for the DWDM and the thermocouple techniques.
Also a very significant advantage of fibre Bragg grating techniques is that they are not sensitive to noise generated by the machine itself as shown in figure 5.
Figure 5: Effect of other machines on the thermocouple signal.
Current work:
Current work includes the development of a distributed sensor using the DWDM technique. Four Bragg gratings were written on one fibre. By grinding a block and inclining the fibre inside it, it will be possible to measure temperature distribution in the workpiece.
A vibration sensor was also developed using a fibre Bragg gratings interrogated by a DWDM.
Other work includes the development of an inexpensive mechanical-optical temperature sensor based on the property of material to expend when heated up.
Frederic Bezombes.
