Optical waveguide vibration sensor and method

• US 5,497,233 A
• Filed: 07/27/1994
• Issued: 03/05/1996
• Est. Priority Date: 07/27/1994
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Litton Systems Incorporated
• Inventors: Meyer, A. Douglas
• Primary Examiner(s): Turner, Samuel A.
• Assistant Examiner(s): MERLINO, AMANDA H
• Application Number:
  US08/281,338
• Time in Patent Office: 587 Days
• Field of Search: 356/345, 73/657, 205/227.19, 205/227.27, 385/12, 385/14
• US Class Current: 356/477
• CPC Class Codes: G01D 5/35329 using interferometer with two arms in transmission, e.g. Mach-Zender interferometer
• CPC Class Codes: G01H 9/00 Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means

Abstract

An optical sensor that is particularly useful for measuring ground borne vibrations employs a planar optical waveguide structure to form a sensor for detecting ground born velocities and accelerations. An optical interferometer is used to detect phase shifts in optical signals guided by the waveguide caused by external vibrations. A pair of phase modulator electrodes is placed adjacent one of the legs of the interferometer and a coil of wire is connected between the pair of electrodes. A sensing element that produces a voltage in response to vibrations is connected across the electrodes. The voltage produced by the sensing element is applied to the pair of electrodes to cause phase modulation of optical signals guided by the leg of the interferometer adjacent the electrodes. The optical signals output from the interferometer are then processed to determine the velocity and acceleration associated with the vibration.

Claims

1. A sensor for measuring parameters of vibrations, comprising:

an interferometer including a first leg having a first optical path length and a second leg having a second optical path length formed such that the first optical path length is longer than the second optical path length;

an optical signal generator connected to the interferometer to apply a frequency modulated optical signal input thereto;

a first pair of electrodes placed adjacent one of the legs of the interferometer to form a first phase modulator;

a coil of wire connected between the pair of electrodes;

a magnet arranged to have freedom of motion longitudinally inside the coil in response to externally applied forces, thereby inducing a voltage in the coil, the voltage being applied to the first pair of electrodes, which causes phase modulation of optical signals guided by the leg of the interferometer adjacent the first pair of electrodes; and

apparatus for processing optical signals output from the interferometer to determine the velocity and acceleration associated with the vibration.

2. The sensor of claim 1, further including a second pair of electrodes placed adjacent the other leg of the interferometer to form a second phase modulator, the coil being connected to the first and second phase modulators such that optical signals guided by the two legs of the interferometer receive phase opposite phase shifts in response to motion of the magnet.

3. The sensor of claim 1, further including an amplifier connected between the coil and one of the electrodes.

4. A sensor for measuring parameters of vibrations, comprising:

an interferometer including a first leg having a first optical path length and a second leg having a second optical path length formed such that the first optical path length is longer than the second optical path length;

an optical signal generator connected to the interferometer to apply a frequency modulated optical signal input thereto;

a phase modulator arranged for modulating the phase of optical signals guided by the first leg of the interferometer;

a piezoelectric crystal arranged to receive vibrations to be measured and produce an electrical signal in response to stresses caused by the ground-borne vibrations, the electrical signal being connected to the first phase modulator such that the optical signals are modulated by a signal having the frequency of the vibrations being measured; and

apparatus for processing optical signals output from the interferometer to determine the velocity and acceleration associated with the vibration.

5. The sensor of claim 4, further including an amplifier connected between the piezoelectric crystal and the phase modulator.

6. A sensor for measuring parameters of vibrations, comprising:

a planar optical waveguide structure;

means for introducing an optical signal into the optical waveguide;

means for detecting vibrations due to an external field and producing an electrical signal indicative of the vibrations; and

a phase modulator connected to receive as an input the electrical signal indicative of the vibrations for causing a phase shift in the optical signal guided by the optical waveguide such that the phase shift is a known function of the magnitude of the vibration.

7. A method for forming a sensor measuring parameters of vibrations, comprising the steps of:

forming an interferometer including a first leg having a first optical path length and a second leg having a second optical path length formed such that the first optical path length is longer than the second optical path length;

connecting an optical signal generator to the interferometer to apply a frequency modulated optical signal input thereto;

placing a first pair of electrodes adjacent one of the legs of the interferometer to form a first phase modulator;

connecting a coil of wire between the pair of electrodes;

arranging a magnet to have freedom of motion longitudinally inside the coil in response to externally applied forces, thereby inducing a voltage in the coil, the voltage being applied to the first pair of electrodes, which causes phase modulation of optical signals guided by the leg of the interferometer adjacent the first pair of electrodes; and

processing optical signals output from the interferometer to determine the velocity and acceleration associated with the vibration.

8. The method of claim 7, further including the step of:

placing a second pair of electrodes adjacent the other leg of the interferometer to form a second phase modulator; and

connecting the coil being to the first and second phase modulators such that optical signals guided by the two legs of the interferometer receive phase opposite phase shifts in response to the vibration.

9. The method of claim 7 further including the step of connecting an amplifier between the coil and one of the electrodes.

10. A method for forming a sensor measuring parameters of vibrations, comprising the steps of:

forming an interferometer including a first leg having a first optical path length and a second leg having a second optical path length formed such that the first optical path length is longer than the second optical path length;

connecting an optical signal generator to the interferometer to apply a frequency modulated optical signal input thereto;

arranging a first phase modulator for modulating the phase of optical signals guided by one leg of the interferometer;

arranging a piezoelectric crystal to receive vibrations to be measured and produce an electrical signal in response to stresses caused by the ground-borne vibrations, the electrical signal being connected to the first phase modulator such that the optical signals are modulated by a signal having the frequency of the vibrations being measured; and

processing optical signals output from the interferometer to determine the velocity and acceleration associated with the vibration.

11. The method of claim 10, further including the step of connecting an amplifier between the piezoelectric crystal and the phase modulator.

12. A method for forming a sensor measuring parameters of vibrations, comprising the steps of:

forming a planar optical waveguide structure;

introducing an optical signal into the optical waveguide;

detecting vibrations due to an external field and producing an electrical signal indicative of the vibrations; and

connecting a phase modulator to receive as an input the electrical signal indicative of the vibrations for causing a phase shift in the optical signal guided by the optical waveguide such that the phase shift is a known function of the magnitude of the vibration.