GB2076558A

GB2076558A - Differential Delay Interferometers

Info

Publication number: GB2076558A
Application number: GB8115915A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1980-05-22
Filing date: 1981-05-22
Publication date: 1981-12-02
Also published as: GB2076558B

Abstract

An interferometer for viewing a time varying object in which first and second light beams 31 and 32 illuminate the object 34 and in which the return paths from the object 31' and 32' to a detector 35 for each beam are different in length thus optically eliminating any unwanted D.C. or low frequency effects. Two embodiments described are the use of the interferometer in an optical fibre hydrophone and as a surface acoustic wave probe. <IMAGE>

Description

SPECIFICATION Improvements In or Relating to Interferometers The present invention relates to Interferometers and more particularly to the adaption of interferometric techniques for viewing time varying objects.

Known interferometric methods for viewing such time varying objects are to apply A.C.

detection techniques to the output of a conventional interferometer or by the use of two different optical frequencies in the interferometer arms. Both of these methods generally seek to eliminate the D.C. or low frequency signal due to the structure of the object or other spurious effects, leaving only the required A.C. signal. This required A.C. signal is extracted electrically from an optical output in which both are present.

The present invention has for an object a method and apparatus in which the unwanted D.C. or low frequency signals are eliminated optically.

According to the present invention a differential delay interferometer includes means for generating first and second light beams and means for interfering said light beams and including means for delaying the first light beam with respect to the second light beam prior to reception of the beams by a detector.

According to a further aspect of the present invention there is provided a method of viewing time varying objects in which the object is illuminated by first and second beams and in which the reflected beams corresponding to said first and second beams are both collected by a detector, the path of said second beam between said object and the detector being longer than the path of said first beam between said object and said detector and in which prior to the detector the first and second beams interfere.

The system of the present invention offers three main advantages over previously known systems. Firstly no high frequency electronics is required, secondly the C.W. optical signal is reduced with considerable noise improvements and thirdly the object appears in both arms of the interferometer allowing minimal sensitivity to, for example, lens quality.

Embodiments of the present invention will now be described, by way of example with reference to the accompanying drawings in which: Figure 1 shows the known Michelson interferometric technique, Figure 2 shows the known shear interferometric technique, Figure 3 illustrates the general method of the present invention using a pulsed light beam, Figure 4 shows in a diagrammatic form apparatus for the application of the method of the present invention to an optical fiber hydrophone and Figure 5 shows in a diagrammatic form apparatus for the application of the method of the present invention to a Surface Acoustic Wave probe system.

Referring now to Figure 1 of the drawings the known Michelson interferometer system is shown in which a light illuminating beam 10 is split into two light beams 11 and 12 by a splitter 13. The splitter can be any conventional light beam splitting means. In Figures, 1 2 3 and 4 the optical paths are purely diagrammatic.

The first beam 11 is directed onto an object 14 which is being observed and the second beam 12 is used as a reference beam and is directed on to a reference mirror 1 5. Reflected beams 11' and 121 are directed via means (not shown) onto a detector 1 6. In the detector the phase of the beam 111 reflected from the object is compared with that of the reflected reference beam 121.

Figure 2 shows the Shear technique. Again a beam 20 is split into two beams 21,22 by a splitter 23. The first beam 21 is reflected from a first point on the object 24 to form a reflected beam 211 which is directed by a series of reflective surfaces (not shown) onto a detector 25. The second beam 22 is directed by means not shown on to a second point on the object 24. The reflected beam 221 is directed by means not shown onto the detector 25. In the detector the phases of the two input signals 211 and 221 are compared to produce the required output.

The configuration according to the present invention is shown in its general principles in Figure 3. An input light beam 30 is split into two beams 31 and 32 by a beam splitter 33. The first beam 31 is directed onto a first point on an object 34 and the reflected beam is directed by means not shown to a detector 35. The second beam 32 is directed onto the same point on the object 34 and the reflected beam 321 is directed by means not shown to the detector 35. The return path 321 is however substantially longer than the return path 311. Thus the object under examination is placed in both arms of the interferometer but one arm is longer than the other.

The delay difference in the differential delay interferometer should occur between the object 34 and the detector 35-a delay difference before the object has a rather complex effect involving the coherence of the input wave.

Examples of the measurement technique according to the present invention are shown in Figures 4 and 5.

Figure 4 illustrates an optical fibre hydrophone system which measures hydrostatic pressure.

The hydrostatic pressure on an optical fibre produces a small change in phase delay which can be used to measure the hydrostatic pressure, in particular if the pressure is due to an acoustic wave. There is a problem with this system because there is a relatively slow drift in phase delay due to, for example, temperature changes that is much larger than the required signal phase change, causing fading. This effect could be eliminated by also running the reference beam through the fibre; but in a conventional system this eliminates the signal as well as the fading. By delaying the reference with respect to the signal, however, (DDI) a signal is still obtained providing that the phase delay is changing rapidly, as it will be for an acoustic input. Various configurations are possible to achieve this--one is shown in Figure 4.

The system comprises an optical fibre cable 40 which is wound to form a reference coil 41 and a signal coil 42. The coil is in use suspended below the water line 43 by a ship (not shown) in which the reference coil and other associated equipment is installed (or the reference coil is acoustically isolated in some other way). The signal beam 44 is directed into the fibre cable 40 by a suitable optical arrangement 44 and a reference beam 45 which is delayed with reference to the signal beam is inserted into the optical fibre cable 40 in the opposite direction.

The second example is the use of the differential delay interferometer technique to a surface acoustic wave (SAW) probe system.

It is possible to 'see' surface acoustic waves (SAW) by reflecting a light wave from the surface and detecting the rather small phase changes resulting from the surface deformation. This generally requires a microscope. DDI is applied to this, the delay being equivalent to (or less than) about 2 cycle at the appropriate acoustic frequency. A possible configuration is shown in Figure 5. In this system the beams separate AFTER the objective. This is equivalent to placing the objective in both arms. The insensitivity to overall phase delay means that the system is (to first order) insensitive to objective quality and focus.

Referring now to Figure 5 a light beam 50 is reflected by a first double prism 51 onto the surface of a S.A.W. device via a focussing lens arrangement 52. The reflected beam 501 passes through lens 52 and prism 51 to a further double prism 53 where it is split into two beams 54 and 55. The beam 54 is deflected onto a prism 56 where it is reflected twice back towards a further double prism 57. In the double prism 57 the beam 55 is rejoined to the beam 54, thus causing the required interference. The distance between double prism 53 and prism 56 may be varied to alter the delay given to the beam 55 respective to the beam 54.

In general it is desirable that the beams should interfere destructively at the output in the absence of a Surface Acoustic Wave.

A variant of this using an excellent reiay lens in path 55 would allow the surface to be imaged.

The phase of the S.A.W. could be visualized by strobing the input signal at the acoustic frequency.

Claims

1. A differential delay interferometer including means for generating first and second light beams and means for interfering said light beams and including means for delaying the first light beam with respect to the second light beam prior to reception of the beams by a detector.

2. A differential delay interferometer as claimed in claim 1 in which the means for generating first and second light beams includes an optical beam splitter

3. A differential delay interferometer as claimed in claim 1 or claim 2 in which the means for delaying the first light beam with respect to the second light beam includes a plurality of prisms, said first light beam being reflected via a greater number of prisms then said second light beam to produce said delay.

4. A differential delay interferometer substantially as described with reference to Figure 3 of the accompanying drawings.

5. An optical fibre hydrophone including a differential delay interferometer as claimed in any one of claims 1,2 or 3.

6. An optical fibre hydrophone substantially as described with reference to Figure 4 of the accompanying drawings.

7. A surface Acoustic Wave probe system including a differential delay interferometer as claimed in any one of claims 1,2 or 3.

8. A Surface Acoustic Wave probe system substantially as described with reference to Figure 5 of the accompanying drawings.

9. A method of viewing time varying objects in which the object is illuminated by first and second beams and in which the reflected beams corresponding to said first and second beams are collected by a detector, the path of said second beam between said object and said detector being longer than the path of said first beam between said object and said detector and in which prior to the detector the first and second beams interfere.

1 0. A method of viewing time varying objects substantially as described with reference to Figures 3, 4 and 5 of the accompanying drawings.