US 11,747,133 B2
Demodulation of fiber optic interferometric sensors
Ming Han, Okemos, MI (US)
Assigned to Board of Trustees of Michigan State University, East Lansing, MI (US)
Filed by Board of Trustees of Michigan State University, East Lansing, MI (US)
Filed on Apr. 22, 2021, as Appl. No. 17/237,438.
Claims priority of provisional application 63/016,319, filed on Apr. 28, 2020.
Prior Publication US 2021/0333089 A1, Oct. 28, 2021
Int. Cl. G01D 5/353 (2006.01); G01B 9/02015 (2022.01); G01B 9/02003 (2022.01); G02B 6/293 (2006.01)
CPC G01B 9/02028 (2013.01) [G01B 9/02003 (2013.01); G01D 5/35316 (2013.01); G02B 6/29347 (2013.01); G01B 2290/70 (2013.01)] 24 Claims
OG exemplary drawing
 
1. 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 reflecting the first laser beam portion as a first line of light and the second laser beam portion as a second line of light, wherein a difference of the first line of light and the second line of light 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 of fringes;
a photodetector receiving light reflected from the sensor; and
an envelope detector detecting variations in reflected power in the first laser line and the second laser line which are at the quadrature positions of the spectral fringes, such that the interferometric sensor provides a sensitive response to a spectral shift.