System and method for monitoring Fabry-Pérot cavity displacements

A system for measuring variations in time of a length of a Fabry-Perot cavity (FPC), the system comprising:

• an optical interrogator apparatus configured to
• interrogate the FPC with light,
• produce a periodic spectral interference pattern from the light that interrogated the FPC, wherein the periodic spectral interference pattern spans over an operational spectral range {λmin, λmax}, and wherein locations of peaks of the periodic spectral interference pattern vary in time in accordance with the variations of the FPC length, and
• output a signal of the peak locations;
• and
• a data processing apparatus communicatively coupled with the optical interrogator apparatus and configured to
• receive, from the optical interrogator apparatus, the signal of the peak locations,
• determine a reference length of the FPC as a function of at least one FPC length corresponding to a reference instance of the signal of the peak locations,
• for each instance of the signal of the peak locations following the reference instance,
• determine a current length of the FPC as a statistical analysis of FPC lengths corresponding to a current instance of the signal of the peak locations,
• predict peak locations corresponding to the current instance of the signal of the peak locations based on the current length, the reference length, and the reference instance of the signal of the peak locations,
• identify as corrected peak locations the corresponding ones from among the peak locations of the current instance of the signal within the operational spectral range that are nearest to the predicted peak locations, and
• estimate a current FPC length variation as a statistical analysis of FPC length variations corresponding to the corrected peak locations, the reference instance of the signal of the peak locations, and the reference length, and
• output a signal of the FPC length variations.

A method for monitoring Fabry-Perot cavity (FPC) displacement, comprising the steps of:

• interrogating the FPC with light by a light source emitting light over an operational spectral range {λmin, λmax};
• obtaining a spectral interference pattern representative of light returning from the FPC, the returning light being reflected from at least two different reflective surfaces, wherein locations of peaks of the periodic spectral interference pattern vary in time in accordance with the variations of the FPC length;
• producing a signal including data associated with a plurality of peaks in the spectral interference pattern;
• determining a reference length of the FPC as a function of FPC lengths corresponding to a reference instance of the signal of the peak locations
• for each instance of the signal of the peak locations following the reference instance,
• processing the signal using a period tracking algorithm to produce a prediction for a peak location based on the data associated with the plurality of peaks;
• identifying as correct one of the plurality of peaks, the correct peak being selected as one of the plurality of peaks closest to the predicted peak location;
• estimating a plurality of FPC length variations using a phase tracking algorithm;
• determining the current FPC length variation as a median of previously estimated FPC length variations over a moving time window of predetermined duration;
• and
• output a signal of the determined FPC length variations.