WO1982004311A1

WO1982004311A1 - Fiber optic interferometer

Info

Publication number: WO1982004311A1
Application number: PCT/US1982/000698
Authority: WO
Inventors: Inc Gould; Vincent J Tekippe
Original assignee: Inc Gould
Priority date: 1981-05-26
Filing date: 1982-05-21
Publication date: 1982-12-09
Also published as: EP0079945A1

Abstract

An interferometer for optically sensing displacements in a surface. Prior art interferometers use an air path to transmit the light from the source to the eventual interference target. Fluctuations in the ambient atmosphere due to local variations in temperature or pressure can introduce phase changes into the optical paths which are not the same for both the signal and the reference path. These fluctuations, which limit the ultimate sensitivity of the device, are minimized by utilizing fiber optics to direct the light within the interferometer. A first fiber optic waveguide (56) receives light from a laser (12) in one end and transports it to the other end (60) where a portion is back reflected and a portion is projected upon the surface (24). The reflected light from the surface (24) re-enters the fiber optic at the end (60) and combines with the back reflected signal to form an optical information wave. This optical information wave is evanescently coupled over a length, L, to a second fiber optic waveguide (58) which transports this wave to a detector (44). An absorber (66) is placed at one end of fiber optic waveguide (58) in order to eliminate end reflections in fiber optic waveguide (58).

Description

FIBER OPTIC INTERFEROMETER

BACKGROUND OF THE INVENTION

The present invention relates to optical inter¬ ferometers and is more particularly directed to an appa- 5 ratus that utilizes fiber optics to direct the multiple light paths within the interferometer itself.

An interferometer is a device that utilizes light in order to make precise measurements. These meas¬ urements may include measurements from very small dis- 10 tanceε to measurements of binary star separations. Also, an interferometer can be used to determine refractive in¬ dices, measure the deformation of surfaces and measure small ultrasonic vibrations in a surface.

The prior art teaches several different config- 15 urations for interferometers. Some of the more well known configurations are the Rayleigh interferometer, the Hichelson stellar interferometer, the Fabry-Perol inter¬ ferometer, and the Mach-Zehnder interferometer. All of these interferometers use an air path to transmit the 20 light from the source to the eventual interference tar¬ get. Fluctuations in the ambient atmosphere due to local variations in temperature or pressure can introduce phase changes into the optical paths which are not the same for both the signal and the reference path. These fluctua- 25 tions in phase introduce noise into the interferometer wlaich limits the ultimate sensitivity of the device.

~ OBJECTS OF THE INVENTION

An object of the present invention is to pro¬ vide an interferometer configuration that will minimize the effects due to fluctuations in the ambient atmos¬ phere.

Another object of the present invention is to provide an interferometer that utilizes fiber optic wave- 5 guides to contain the various light beams within the in¬ terferometer.

Yet another object of the present invention is to provide a fiber optic interferometer that does not re¬ quire an excessive amount of calibration and thus makes ~ the device easier to use.

The above objects are given by way of example.

Thus other desirable objectives and advantages achieved by the invention may occur to those skilled in the art.

The scope of the invention is to be limited only by the

~- ^~ appended claims.

BRIEF SUMMARY OF THE INVENTION

The above objects and other advantages are achieved by the present invention. An apparatus is pro¬ vided that utilizes fiber optic waveguides to transmit

Z ~ and carry the internal beams within an interferometer. In one embodiment of the present invention a first fiber optic carries the source beam directly to the interfer¬ ence target. A second fiber optic directs the source beam to the surface to be measured. A third fiber optic

25 receives the reflected light from the surface to be meas¬ ured and carries or transports this reflected light to the interference target. The interference target will exhibit a fringe pattern which is a function of the source beam directly carried by the first fiber optic and

30 the reflection pattern received and transported by the third fiber optic. Since all the fiber optics contained

CV-PI „_ W WIIPPOO , within the interferometer will be subject to the same en¬

* vironment, variations in the ambient atmosphere will be nullified.

In another embodiment of the present invention 5 a reference beam is derived from back reflection at the output face of a first fiber optic waveguide. Such a re¬ flection occurs naturally due to the difference in re¬ fractive indices between the glass and the air. The re¬ flection is enhanced by placing a partially reflecting

10 layer at the end of the fiber. A second light beam is obtained by reflecting the light output from the refer¬ ence fiber off of the surface to be examined and then back into the fiber. These two interfering beams trans¬ fer engery to a detecting fiber optic waveguide via eva-

15 nescent coupling therebetween.

DESCRIPTION OF THE DRAWINGS

Figure 1 is a generally schematic diagram of one of the embodiments of the present invention;

Figure 2 is a generally schematic diagram of 20 another embodiment of the present invention;

Figure 3 is a generally schematic diagram of another embodiment of the present invention; and

Figure 4 is a generally schematic diagram of yet another embodiment of the present invention.

25 DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description of the invention follows refer¬ ring to the drawings in which like reference numerals de¬ note like .elements of structure in each of the several figures. In general, the terms "optical waveguide" or 30 "fiber optic waveguide" will be used herein to refer to a glass transmission line having a core member with clad¬ ding members concentrically surrounding the core for transmission by internal reflection at the core-clad in¬ terface of electromagnetic radiation which lies in the 5 optical portion of the electromagnetic spectrum between microwaves and x-rays and including the ultra-violet, visible and infra-red regions.

Turning now to the figures showing the various embodiments of the present invention, Figure 1 shows a 0 generally schematic diagram of interferometer 10. A light source 12 preferably a monochromatic light source such as a laser directs a beam of light 14 toward focus¬ ing lenses 16a and 16b. The focusing lenses are used to broaden the parallel path of the light beam 14. The -5 light is directed toward two fiber optic waveguides 18 and 20. Fiber optic waveguide 18 carries the incoming parallel light beam 14 through its core and projects the beam upon surface 22 thus defining a reference beam. Optical waveguide 20 also receives light beam 14 and car¬ o ries the beam through its core and projects the beam upon a surface 24 which is to be measured. One contemplative - measurement that can be made of" surface 24 is surface vibrations. The projected light 26 from optical wave¬ guide 20 strikes surface 24 and reflects off of a sur- 5 face. The reflected light 28 enters optical waveguide 30. It will be appreciated by those skilled in the art that only a small amount of the reflected light 28 actu¬ ally enters the optical waveguide 30. Optical waveguide 30 thus carries the reflected light 28 through its core 0 and projects it also upon surface 22. Optical waveguides 20 and 30 are in juxtaposition with optical waveguide 18 such that the optical waveguides 20 and 30 are parallel with optical waveguide 18. The two projected beams from optical waveguide 18 and optical waveguide 30 projected upon surface 22 causes an interference pattern which will fluctuate as a function of the surface vibrations of sur¬ face 24. Since the reference beam 14 and the reflected signal from the surface to be measured 28 is carried through fiber optic waveguides, the interferometer 10 is less sensitive to ambient variations. The fluctuations due to ambient variations caused by light travelling in air paths in prior art interferometers is substantially eliminated. Figure 2 shows another embodiment of the pres¬ ent invention in which interferometer 32 has two fiber optic waveguides 34 and 36. A monochromatic light source 12 directs a beam of light 14 toward fiber optic 34. The core 35 of fiber optic 34 carries the optical energy toward end 38 of fiber optic 34. A reference beam is de¬ rived within fiber optic waveguide 34 by back reflection at surface 38 of fiber optic 34. Such reflection occurs naturally due to the difference in the refractive index between the glass core and the air. Back reflection can be enhanced by placing a partially reflecting layer at the end 38 of fiber optic waveguide 34. Fiber optic waveguide 36 is connected to fiber optic waveguide 34 over a finite length L such that evanescent wave coupling occurs between core 35 of fiber optic 34 and core 40 of fiber optic 36 over this finite length L. A finite amount of optical energy will pass through surface 38 of fiber optic 34 and through surface 42 of fiber optic 36 and will project upon surface 24 to be measured. A quan¬ titative amount of light reflected off a surface 24 will again enter fiber optic 34 and 36 at surface 38 and 42 respectively. This reflected light will combine with the initial reference back reflection beam discussed supra. This combined signal which is due to the back reflection which is evanescently coupled to the fiber optic core 40 within fiber optic waveguide 36 and the reflected signal from surface 24 will be carried along fiber optic 40 and will be received by an analyzing device 44. The optical energy received by 44 will vary in intensity as a func- tion of the surface vibrations of surface 24. Fiber op¬ tic 36 defines the detecting leg of the interferometer 32. This embodiment of the present invention is also substantially insensitive to the ambient environment fluctuations since the reference signal and the reflected light travel through the same fiber optic waveguide. Also the evanescent wave coupling action between the two fibers also eliminates phase noise caused by motion sen¬ sitivity between the light paths. Critical alignment of the interferometer itself is also eliminated. Figure 3 shows another embodiment of the pres¬ ent invention which again .has two- fiber optic waveguides 46 and 48. The monochromatic light source 12 projects a light beam 14 to fiber optic waveguide 46. Core 50 and fiber optic 46 carries the optical energy to end surface 52 of fiber optic waveguide 46. Fiber optic waveguide 46 and fiber optic waveguide 48 are in juxtaposition such that evanescent wave coupling occurs over a distance L which is not near the ends of the fiber optic wave¬ guides. This leaves the fiber optic waveguide end 52 of fiber optic waveguide 46 and fiber optic waveguide end 54 of fiber optic waveguide 48 free to vary their respective orientations above surface 24 to be measured. This will enhance the amount of light that will be received by re¬ flection off of surface 24. The combination of the ref- erence signal due to back reflection and the detection signal which is reflected off of surface 24 works in a similar manner as is described with respect to the embod¬ iment shown in Figure 2. Figure 4 shows yet another embodiment of the present invention in which there are two fiber optic waveguides 56 and 58. The monochromatic light source 12 projects a light beam 14 toward fiber optic waveguide 56. Fiber optic waveguide 56 and fiber optic waveguide 58 are in juxtaposition so as to have evanescent coupling over a finite length L. In this configuration, the light emerges from surface 60 of fiber optic waveguide 56 and is received back only through fiber optic waveguide 56. Therefore the signal travelling back up through core 62 of fiber optic waveguide 56 carries the combined signal of both the back reflection signal off of surface 60 and the reflected signal off of surface 24. This combined signal is then evanescently wave coupled to core 64 of fiber optic 58. An absorber 66 is placed at one end of fiber optic 58 in order to eliminate end reflections in the fiber optic 58. The combined signal is again pro¬ jected from fiber optic waveguide 58 to a detection or analyzing device 44. This type of configuration is used when the amount of reflected light off of surface 24 is of sufficient intensity to provide sufficient measurement information.

The present invention has proved to have suffi¬ cient sensitivity as to observe displacements of 5 x 10^-7 centimeters with no spectral analysis or narrow band fil¬ tering. Displacements of the order of 10^-9 centimeters could be easily detected by using spectral analysis.

This invention has been described with refer¬ ence to preferred embodiments. Obvious modifications and alterations will occur to others upon reading and under¬ standing the specification. The intent is to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalent thereof.

Claims

What is claimed is:

1. An interferometer for measuring displace¬ ments of a surface, said interferometer comprising: a first fiber optic waveguide for transporting a first coherent optical signal from an optical energy source and projecting the transported first coherent op¬ tical signal? a second fiber optic waveguide for transporting a second coherent optical signal preferably from the same energy source as said first coherent optical signal and projecting said second coherent optical signal upon the surface to be measured; a third fiber optic waveguide for receiving at least a portion of optical energy reflected from the sur¬ face to be measured due to the projecting of said second coherent optical signal upon the surface to be measured and transporting the reflected light received and pro¬ jecting the transported reflected light; said first fiber optic waveguide and said third fiber optic waveguide being in such spatial relation as to have the projected first coherent optical signal from said first fiber optic waveguide and the projected re¬ flected light from said third fiber optic to interfere with each other optically and thus define an interference pattern, said interference pattern varying as a function of the displacement in the surface to be measured.

2. An interferometer for measuring displace¬ ments of surface, said interferometer comprising: a light source; a detector; a first fiber optic waveguide to transport light from said light source to said detector;

OMPI

< a second fiber optic waveguide to transport light from said light source and project it upon the sur¬ face to be measured; a third fiber optic waveguide to receive a por¬ tion of light reflected off of said surface to be meas¬ ured and transport said light reflected to said detector, the light received by said detector from said first and third fiber optic waveguide causing an optical interfer- ence pattern which varies as a function of the surface displacements.

3. The interferometer of Claim 2 wherein said light source is a laser.

4. The fiber optic interferometer substan¬ tially as hereinbefore described with reference to and as illustrated in the accompanying drawings.