A new technique for simultaneous strain and temperature sensing is demonstrated. The approach employs two different types of photogenerated fiber grating, namely, a fiber Bragg grating and a fiber polarization-rocking filter. The method relies on the different dependencies of the fiber refractive index and birefringence on strain and temperature. Both of these measurands can be determined from the effect that they have on the resonant wavelength of each grating. The information is provided in the frequency domain, avoiding the problem of limited unambiguous signal range associated with the use of competing optical fiber interferometers.
High-spatial-resolution measurements of axial-stress profiles of UV-irradiated fibers are reported, providing diagnostic information and a technique for the direct observation of UV-written grating structures. Measurements have been made with a spatial resolution of approximately 0.3 microm, which is capable of resolving detail within the pitch of the gratings.
IntroductionIn recent years, several readout techniques for Fibre Bragg grating (FBG) sensors have been proposed [l-3]. In this paper, we present a robust readout system capable of repeatable, high resolution, measurements developed in the framework of the NOSOST project for aerospace application. In such applications, the user may be prevented from using mechanically scanned instrumentation or passive optical filters that are highly susceptible to environmental fluctuations.The continuing advances of the CCD technology and its compatibility with digital computers is a driving force towards their application in conjunction with bulk-optic diffraction gratings as spectrometers for wavelength measurements. The use of spectrometers has previously been reported for demodulating fibre optic Fabry-Perot sensors [4-61. A scheme for addressing narrow gratings (0.04 nm) using a spectrometer has also been reported [7]. The spectrometer returns the wavelength with a resolution equal to the product of the grating's linear dispersion at the detector plane with the pixel width. For low cost spectrometers, this resolution is typically 0.1 nm, however, when the bandwidth of the measured wave exceeds one pixel, the use of some processing techniques can result in improved wavelength resolution. Initial results obtained in our laboratories indicate that a resolution of -1 pm can easily be achieved when using a spectrometer with a -0.1 nm pixel resolution. This translates into a sensing strain resolution of -lp& for in-house written FBG's in the 820 nm spectral region. Algorithms A number of algorithms may be used including the two-pixel-ratiometric technique, whereby the grating spectrum is divided into two areas along a vertical line. The ratio of the two areas is then used to track the evolution of the peak wavelength. This technique however is optimum when the gratings bandwidth covers a maximum of two pixel's spectral windows and the wavelength shifts to be monitored are smaller than the pixel spectral window.The centroid detection algorithm (CDA) described in [7] uses the algorithm in equation (l), where ij and ;zi represent the intensity and the centre wavelength of the j" pixel respectively and where ;18 is the Bragg wavelength.
Cl -This expression thus represents a moving average that points to the maximum of the FBG to within a fraction of the pixel spectral window. The number of pixels chosen to perform the processing should offer both stable wavelength resolution inside the pixels as well as prevent discontinuities at the pixel boundaries. The improvement in wavelength resolution is essentially limited by the optical power available from the grating, hence FBGs with different reflectivities will exhibit different accuracies.Another algorithm for resolving the Bragg wavelength is the use of a least squares method (LSQ) which fits a quadratic polynomial to sequential pixel outputs. In this case, the peak region of the FBG reflection is approximated to a polynomial of second order (&,hjk) such that the error q in approximat...
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