A passive signal processing technique for miniature interferometric sensors is reported. It is based on phase stepping techniques and utilizes four adjacent axial modes from a multimode laser diode to provide four phase shifted intensity outputs. It is shown that the effects of multiple reflections on the recovered phase are reduced by approximately two orders of magnitude compared to methods employing two and three phase shifted outputs. Demultiplexing is accomplished using a monochromator and line scan CCD combined with digital processing. Demonstration as a vibration and temperature sensor for a 174 µm low finesse fibre Fabry-P érot cavity produced 50 dB dynamic range and 9 mrad Hz −1/2 sensitivity at 40 Hz, and 70 • C range and 0.07 • C resolution respectively. Systematic errors are also discussed.
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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