Fiber optic sensing system

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Sep 7, 1989 - [45] Date of Patent: Feb. 26, 1991. FIBER OPTIC SENSING SYSTEM. Inventor: Grigory Adamovsky, Solon, Ohio. Assignee: The United States of ...
United States Patent

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Adamovsky

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FIBER OPTIC SENSING SYSTEM Inventor:

Grigory Adamovsky, Solon, Ohio

Assignee:

The United States of America as represented by the administrator of the National Aeronautics and Space Administration, Washington, D.C.

Appl. No.: 404,291 Filed:

Int.

Sep. 7, 1989

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G02B 6/02; G02B 6/16; GOlB 9/02 US. C1. ................................. 350/96.29; 356/345; 250/227 Field of Search ............... 350/96.15, 96.20, 96.21; 356/345, 350, 351, 352; 250/227 (3.5

References Cited U.S. PATENT DOCUMENTS 3,542,472 11/1970 Smith-Vaniz ........................... 356/4 3,970,389 7/1979 Mendrin et al. ................ 356/106 R 4,492,464 1/1985 Bourdet et ai. ...................... 356/4.5

356/345 4,627,728 12/1986 Willson ............................... 4,644,556 2/1987 Petuchowski et ai. ........... 372/97 X

Patent Number: Date of Patent:

4,738,527 4,753,529 4,768,880 4,789,240 4,873,989

4/1988 6/1988 9/1988 12/1988 10/1989

4,995,697 Feb. 26, 1991

McBrien .................................

365/5 Latton ............................ 250/227.27 Tur et al. ............................ 356/345 Bush .................................... 356/345 Einzig ............................. 356/352 X

Primary Examiner-Akm Ullah Attorney, Agent, or Firm-Gene E. Shook; John R. Manning; James A. Mackin

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A.BSTRACT

A fiber optic interferometer utilizes a low coherence LED laser as a light source which is filtered and driven at two R F frequencies, high and low, that are specific to the initial length of the resonator chamber. A displacement of a reflecting mirror changes the length traveled by the nonreferencing signal. The low frequency light undergoes destructive interference which reduces the average intensity of the wave while the high frequency light undergoes constructive interference which increases the average intensity of the wave. The ratid of these two intensity measurements is proportional to the displacement incurred.

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ing systems based on phase modulation. In such applicaFIBER OPTIC SENSING SYSTEM tions optical frequency is usually employed, and a phase difference between recombining signals is utilized to ORIGIN O F T H E INVENTION determine the measured parameter. As the phase difference changes either in the space, time, O r frequency The invention described herein was made by an emdomains, the resultant signal changes its value from ployee of the United States Government and may be some maximum when the phase difference is a multiple manufactured and used by or for the Government for of 2n to some minimum when the phase difference is an governmental purposes without the payment of any odd multiple of r. royalties thereon or therefor. 10 The swing from a maximum to a minimum value is TECHNICAL FIELD known from optical interferometry as fringe visibility This invention is directed to an improved sensing or fringe contrast. Among the parameters that affect system. The invention is particularly concerned with an fringe visibility are the amplitudes of the interfering interferometer which employs frequency domain refersignals or intensities of the interfering light beams. encing. Equal amplitudes of the interfering signals or equal A classical interferometer is based on generating a intensities of the interfering beams give the highest phase difference between two or more coherent fields. fringe visibility assuming that the other factors affecting The fields may be optical, electrical, magnetic, gravitathe visibility remain unchanged. tional, or other. The recombination of the fields proA disadvantage of the referencing technique using duces an interferencepattern, and the process Of recorn- 2o temporarily separated signal and reference channels lies bination is called interference. in the practical limits of a pulsed source and a fiber optic An interference pattern comprises alternating zones loop. The optical length of the loop L and the duration with low and high field intensities. The alternation may of the pulse t are related. For a double pulse generated occur either in space, in time, or in frequency domains. L>2ct Or t