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.  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  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 . A three-wavelength-based passive quadrature digital- phase-demodulation scheme is developed for readout of vibration, acoustic, and strain sensors based on FFP .