A simple temperature-insensitive fiber Bragg grating displacement sensor
Introduction
FBG sensors have been used in strain and temperature sensor applications [1]. The main feature of FBG for sensing is that they perform a direct transformation of the sensed parameter into a shift in the wavelength. The wavelength-interrogated FBG sensors need sophisticated equipment such as spectrum analyzers to detect wavelength changes or demodulators in order to convert the wavelength changes to power or current changes. Most of the reported schemes for wavelength interrogation are either using a tunable laser [2] or using a broadband source combines with a Fabry–Perot (FP) tunable filter [3]. These interrogators are usually expensive and their measurement speed is usually limited by the scanning speed of tunable filter or tunable laser. Moreover, they require post data manipulation (e.g. peak search algorithm) to determine the wavelength shift that may even slow down their response speed.
Another problem of wavelength-interrogated FBG sensors is the discrimination of temperature and strain responses. A number of techniques for overcoming this limitation have been reported and demonstrated. For example, the dual wavelength technique involves writing two superimposed FBG with large Bragg centre wavelength separation (850–1300 nm) that requires two broadband sources to address the sensors [4]. Cancellation of the thermal response of the gratings has been reported using two FBGs that are mounted on opposite sides of a bend surface, such that the gratings have equal, but opposite strain [5]. Another example is a two grating sensors with different fiber diameters, which have same temperature property, to discriminate temperature and strain induced wavelength shift [6]. The above described sensors can discriminate the two effects, but their structures are complex and they need two FBGs to achieve the purpose. Moreover, these sensors need expensive and slow response interrogator to detect wavelength changes.
Section snippets
Proposed sensor structure and prior arts
A need therefore exists to solve the response speed and cost issue faced by the wavelength interrogator and its measurement must be temperature-independent. Instead of using wavelength interrogated FBG, we proposed a FBG sensor module that can translate displacement into the variation of the FBG bandwidth. Some researchers have reported their ideas to achieve the similar results. Xu et al. [7] has demonstrated a temperature-independent strain sensor using a chirped FBG in a tapered optical
Experimental results and discussion
Fig. 2 shows the experimental setup of the proposed FBG based displacement sensor. A superluminescent light emitting diode (SLED) was used as the light source because its ripple of power level is only about 0.2 dB over a wavelength range of 10 nm around 1556 nm. Its flat optical power spectrum reduces the fluctuation of the FBG reflected power when there is a temperature induced wavelength shift. The FBG reflected power and spectrum were measured by a PD and an optical spectrum analyzer (OSA),
Conclusions
We have proposed and demonstrated a novel and simple FBG sensor module that can translate force into the variation of the FBG bandwidth and hence the displacement measurement simply involves monitoring the back-reflected power from the FBG. The FBG is embedded into low-cost material (such as fiber-reinforced composite) and it is well protected and hence has higher mechanical strength. Unlike Kim’s idea [8], the dynamic range of the sensor module is not limited by half of the FBG 3 dB bandwidth
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