Demodulation of fiber optic interferometric sensors

A demodulation system comprising:

• a laser coupled to a first optical fiber;
• multiple arms splitting from the optical fiber;
• an optical frequency shifter connected to at least one of the arms configured to generate different laser wavelengths at quadrature positions of interferometric fringes;
• an interferometric sensor receiving a combination of a frequency-shifted laser beam from the shifter and an unshifted laser beam wherein a difference of the frequency shifted laser beam and the unshifted laser beam corresponds to phase difference of (m+1/4)π, where m is an integer, of spectral fringes of the sensor so that frequency shifted laser beam and the unshifted laser beam are at different quadrature points of fringes;
• and
• a photodetector receiving light from the sensor.

A demodulation system comprising:

• a single frequency laser emitting a laser beam into an optical fiber;
• a fiber-optic split coupler dividing the laser beam into a first beam in a first arm and a second beam in a second arm;
• a first intensity modulator modulating the intensity of the laser beam in the first arm at a first modulation frequency to form a first laser beam portion;
• a second intensity modulator modulating the intensity of the laser beam in the second arm at a second modulation frequency different than the first frequency;
• a frequency shifter shifting the frequency of the laser beam in the second arm to form a second laser beam portion;
• a coupler coupling the first laser beam portion and the second laser beam portion;
• an interferometric sensor configured to receive the modulated laser beam light from the at least second optical fiber, the modulated laser beam light sent to the interferometric sensor including two laser lines whose difference corresponds to a phase difference of (m+1/4)π, where m is an integer, of spectral fringes of the sensor so that the first line and second line are at different quadrature points, such that the interferometric sensor is configured to provide a response to a spectral shift therein due to ultrasonic acoustic signals;
• a photodetector configured to receiving light from the sensor;
• at least one electronic filter coupled to the photodetector;
• and
• at least one envelope detector coupled to the filter.