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Intensity Interrogating Method with Multiple Wavelengths

Multiple wavelengths from one or more cw light sources can be employed as a way of overcoming the sensitivity nulls and direction of change ambiguities that occur when interferometers are interrogated using a single light source. Using a 3-nm wavelength separation between two modes of an AlGaInP laser, Potter et al. [4] achieved quadrature optical outputs from a 20-mm-long FFP cavity. A similar approach using spectrally filtered optical signals from FFP sensors monitored with a low-coherence superluminescent diode (SLD) light source has been reported in which two wavelengths [5] are simultaneously monitored. In another experiment, two wavelengths from an Erbium-doped fiber broadband source are selected by an FBG after reflection from an FFP sensor to provide quadrature optical signals [6]. A three-wavelength-based passive quadrature digital- phase-demodulation scheme is developed for readout of vibration, acoustic, and strain sensors based on FFP [7].

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