Novel methods for simultaneous strain and temperature measurements with optical fiber Bragg gratings

We report on feasibility studies of two new approaches for simultaneous strain and temperature measurements with optical fiber Bragg gratings. In sensor applications fiber Bragg gratings cannot be thermally isolated. Therefore temperature has to be compensated or measured simultaneously with the measurand of interest. In strain sensing this is often done by measuring the wavelength response of a second Bragg grating attached to an unstrained sample specimen and comparing its response to response of the strained sample. This method is very accurate. However, often it is not possible or difficult to have unstrained samples, e.g., in rockbolt anchors with embedded fiber Bragg gratings. We propose two new simultaneous measurement concepts and compare them to known concepts. The first investigated method uses the different sensitivities to strain and temperature of the main peak of a Bragg grating in transmission compared to the back-reflected cladding modes. We will present first results of simultaneous strain and temperature measurements over a range from 2 degrees C to 95 degrees C up to 3500 mu m/m in strain. This method leads to a combined resolution in temperature and strain of 4 degrees C and 35 mu m, respectively. The second method is based on two low reflectivity gratings. They are in close vicinity to each other in the fiber. Both have a small refractive index modulation of about 2-10(-4), but one was homogeneously post-illuminated and has an increased mean refractive index. We will present first results indicating that the additional UV-illumination changes temperature-and strain-sensitivity of the post-illuminated Bragg grating slightly.