US4572949A — Fiber optic sensor for detecting very small displ…

Claim 1

Independent Claim 159 words

An apparatus for detecting small displacements of a surface comprising: a first directional coupler for dividing a single coherent light signal into first and second coherent light signals; a first delay line for delaying said first coherent signal relative to said second coherent light signal by a first delay period to provide a delayed first light signal; a waveguide for directing each of said delayed first and said second light signals onto said surface to produce a first reflected light signal corresponding to said delayed first light signal and a second reflected light signal corresponding to said second light signal; a second delay line for delaying said second reflected light signal by said first delay period to produce a second delayed reflected light signal; and a detector for measuring the phase difference between said first reflected light signal and said second delayed reflected light signal to measure variations in the geometric path length between said waveguide and said surface.

Claim 2

Independent Claim 159 words

An apparatus for detecting small displacements of a surface comprising: a first directional coupler for dividing a single coherent light signal into first and second coherent light signals; a first delay line for delaying said first coherent signal relative to said second coherent light signal by a first delay period to provide a delayed first light signal; a waveguide for directing each of said delayed first and said second light signals onto said surface to produce a first reflected light signal corresponding to said delayed first light signal and a second reflected light signal corresponding to said second light signal; a second delay line for delaying said second reflected light signal by said first delay period to produce a second delayed reflected light signal; and a detector for measuring the phase difference between said first reflected light signal and said second delayed reflected light signal to measure variations in the geometric path length between said waveguide and said surface.

Claim 3

Dependent Claim 39 words

An apparatus as defined in claim 1, wherein said single coherent light signal which is divided by said first directional coupler is a continuous wave of light having a coherence time which is less than said first delay period.

Claim 4

Dependent Claim 44 words

An apparatus as defined in claim 1 wherein said single light signal which is divided by said first directional coupler is comprised of a coherent light pulse said pulse having a duration less than said first delay period imposed by said first delay line.

Claim 5

Dependent Claim 35 words

An apparatus as defined in claim 2 or 3 wherein said first directional coupler is a 3 dB coupler for evenly slitting said first coherent light signal into said first and second coherent light signals.

Claim 6

Dependent Claim 71 words

An apparatus as defined in claim 1 wherein said first delay line is a first fiber optic waveguide and wherein said second coherent light signal is guided to said waveguide by a second fiber optic waveguide which is shorter than said first fiber optic waveguide by an amount sufficient to cause said first coherent light signal to be delayed relative to said second coherent light signal by said first delay period.

Claim 7

Dependent Claim 42 words

An apparatus as defined in claim 5 wherein said first and second fiber optic waveguides are coupled together by a second directional coupler and wherein said waveguide is coupled to said first and second fabric optic waveguides by said second directional coupler.

Claim 8

Dependent Claim 31 words

An apparatus as defined in claim 6 wherein said second directional coupler recombines said delayed first light signal and said second coherent light signal into two light signals in said waveguide.

Claim 9

Dependent Claim 93 words

An apparatus as defined in claim 7 wherein said second directional coupler is bidirectional in that it splits said first reflected light signal into a third reflected light signal which is coupled into said second fiber optic waveguide and a fourth reflected light signal which is coupled into said first fiber optic waveguide and such that said second reflected light signal is split into a fifth reflected light signal which is coupled into said second fiber optic waveguide and a sixth reflected light signal which is coupled into said first fiber optic waveguide.

Claim 10

Dependent Claim 37 words

An apparatus as defined in claim 7 wherein said first directional coupler is bidirectional such that it couples part of each of said third, fourth, fifth and sixth reflected light signals to the input of said detector.

Claim 11

Dependent Claim 30 words

An apparatus as defined in claim 9 wherein said detector is a photodetector for detecting the intensity of the combined signal resulting from said third and sixth reflected light signals.

Claim 12

Dependent Claim 58 words

An apparatus as defined in claim 5 or 6 or 7 or 8 or 9 or 10 further comprising a phase modulator means located in either said first or said second fiber optic waveguides for causing a continuous periodically time varying phase difference in each of the light signals travelling in said first and second fiber optic waveguides.

Claim 13

Dependent Claim 54 words

An apparatus as defined in claim 11 wherein said detector generates an output signal component which has an amplitude which is proportional to the amplitude of the surface displacement and which amplitude varies in time at the difference frequency between the frequency of the surface displacement and the frequency of said phase modulator means.

Claim 14

Dependent Claim 42 words

An apparatus as defined in claim 12 wherein said detector includes a means for filtering out output signal components at frequencies other than said difference frequency and for integrating said difference frequency component over a plurality of periods of said difference frequency.

Claim 15

Dependent Claim 26 words

An apparatus as defined in claim 3 wherein said pulses are also of a duration less than the period of the surface displacement to be sensed.

Claim 16

Dependent Claim 25 words

An apparatus as defined in claim 14 wherein said detector has a bandwidth greater than the center frequency of the surface displacement to be sensed.

Claim 17

Dependent Claim 77 words

An apparatus as defined in claim 6 or 7 or 8 or 9 or 10 or 14 or 15 further comprising a polarization controller means located in either said first or said second fiber optic waveguide between said first directional coupler and said second directional coupler for causing the portions of said first and second reflected light signals which travel the same geometric path length between said couplers to arrive at said detector in the same polarization.

Claim 18

Dependent Claim 76 words

An apparatus as defined in claim 16 wherein said first light signal has a coherence length such that light travelling the longest geometric path to said surface and back to said detector does not interfere with light travelling the shortest geometric path, and wherein said polarization controller means causes the portions of said first and second light signals which travel and same geometric path to said detector arrive there shifted in relative phase by π/2 radians.

Claim 19

Independent Claim 96 words

A method of sensing small movements of a surface comprising the steps of: directing first and second optical signals to a surface, said first signal preceding said second signal by a time T; receiving third and fourth optical signals reflected from said surface in response to said first and second optical signals, said third signal preceding said fourth signal; delaying said third optical signal by a time T, thereby providing a delayed third optical signal; and causing interference between said delayed third optical signal and said fourth optical signal to determine the relative phase shift therebetween.

Claim 20

Independent Claim 96 words

A method of sensing small movements of a surface comprising the steps of: directing first and second optical signals to a surface, said first signal preceding said second signal by a time T; receiving third and fourth optical signals reflected from said surface in response to said first and second optical signals, said third signal preceding said fourth signal; delaying said third optical signal by a time T, thereby providing a delayed third optical signal; and causing interference between said delayed third optical signal and said fourth optical signal to determine the relative phase shift therebetween.

Claim 21

Dependent Claim 24 words

The method as defined by claim 18 and further including the step of detecting the amplitude of the combined signal caused by said interference.

Claim 22

Dependent Claim 16 words

The method as defined by claim 18 wherein said first and second optical signals are pulses.

Claim 23

Dependent Claim 25 words

The method as defined by claim 18 wherein said first and second optical signals are continuous wave signals generated by a short coherence length source.

Claim 24

Dependent Claim 64 words

The method of claim 19 further comprising the steps of: phase modulating the first and second optical signals at a modulation frequency which is related to the frequency of movement of said surface such that the difference frequency between the modulation frequency and the surface movement frequency lies within a predetermined range; and detecting the amplitude of said combined signal at said difference frequency.

Claim 25

Dependent Claim 53 words

The method of claim 19 further comprising the steps of: intensity modulating said first and second optical signals at a modulation frequency related to the frequency of movement of said surface such that the difference frequency lies within a predetermined range; and detecting the amplitude of said combined signal at said difference frequency.

Claim 26

Dependent Claim 110 words

The method of claim 22 further comprising the steps of: intensity modulating said first and second optical signals at a second modulation frequency; detecting the amplitude of said combined signal at either the sum or difference frequency between said modulation frequency and said second modulation frequency to provide a first detected signal; detecting the amplitude of said combined signal at the difference frequency between said modulation frequency and the frequency of movement of said surface to provide a second detected signal; and dividing said second detected signal by said first detected signal to provide a measure of the amplitude of the surface movement which is substantially independent of sample reflectivity.

Claim 27

Dependent Claim 99 words

A surface acoustic wave sensor comprising: a light source; a first optical fiber connected to receive light from said light source; light detector means; a second optical fiber connected to transmit light to said light detector means; a first directional coupler through which said first and second optical fibers pass; a second directional coupler through which said first and second optical fibers pass; and a segment of optical fiber extending from said second directional coupler for directing light from said second directional coupler to a sample and for returning reflected light from said sample to said second directional coupler.

Claim 28

Dependent Claim 99 words

A surface acoustic wave sensor comprising: a light source; a first optical fiber connected to receive light from said light source; light detector means; a second optical fiber connected to transmit light to said light detector means; a first directional coupler through which said first and second optical fibers pass; a second directional coupler through which said first and second optical fibers pass; and a segment of optical fiber extending from said second directional coupler for directing light from said second directional coupler to a sample and for returning reflected light from said sample to said second directional coupler.

Claim 29

Dependent Claim 31 words

The surface acoustic wave sensor as defined by claim 25 wherein said first optical fiber is a different length than said second optical fiber between said first and second directional couplers.

Claim 30

Dependent Claim 31 words

The surface acoustic wave sensor as defined by claim 26 wherein said one of said first and second optical fibers between said first and second directional couplers includes a phase modulator.

Claim 31

Dependent Claim 17 words

The surface acoustic wave sensor as defined by claim 27 wherein said detector includes a lockin amplifier.

Claim 32

Dependent Claim 32 words

The surface acoustic wave sensor as defined by claim 26 or 27 wherein one of said first and second optical fibers between said first and second directional couplers includes a polarization controller.

Claim 33

Dependent Claim 17 words

The surface acoustic wave sensor as defined by claim 29 wherein said detector includes a lockin amplifier.

Claim 34

Independent Claim 85 words

A method of sensing a surface acoustic wave comprising the steps of: directing first and second optical signals to a surface, said first signal preceding said second signal by a time, T; receiving third and fourth optical signals reflected from said surface in response to said first and second optical signals, respectively; delaying said third optical signal by a time, T, thereby providing a delayed third optical signal; and combining said delayed third optical signal and said fourth optical signal thereby providing a combined signal.

Claim 35

Independent Claim 85 words

A method of sensing a surface acoustic wave comprising the steps of: directing first and second optical signals to a surface, said first signal preceding said second signal by a time, T; receiving third and fourth optical signals reflected from said surface in response to said first and second optical signals, respectively; delaying said third optical signal by a time, T, thereby providing a delayed third optical signal; and combining said delayed third optical signal and said fourth optical signal thereby providing a combined signal.

Claim 36

Dependent Claim 17 words

The method as defined by claim 31 and further including the step of detecting said combined signal.

Claim 37

Dependent Claim 29 words

The method as defined by claim 32 wherein said first and second optical signals are pulses and said step of detecting includes detecting the amplitude of said combined signal.

Claim 38

Dependent Claim 25 words

The method as defined by claim 32 wherein said first and second optical signals are continuous wave signals generated by a short coherence length source.

Claim 39

Dependent Claim 19 words

The method as defined by claim 34 and further including the step of intensity modulating said continuous wave signals.

Claim 40

Dependent Claim 31 words

The method as defined by claim 34 and further including the steps of detecting at least two signals and dividing said signals to minimize dependence on the reflectivity of said surface.

Annuity Stage	Window Opens	Payment Deadline	Surcharge Period Until	Estimated Fee	Status
3.5 Years (Year 4)	1989-02-24	1989-08-26	1990-02-25	$2000.00	Window Closed / Lapsed
7.5 Years (Year 8)	1993-02-24	1993-08-26	1994-02-25	$3760.00	Window Closed / Lapsed
11.5 Years (Year 12)	1997-02-24	1997-08-26	1998-02-25	$7700.00	Window Closed / Lapsed

Fiber optic sensor for detecting very small displacements of a surface

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