The present invention relates to optical dis¬ placement sensing apparatus and, more particularly, to such apparatus utilising the principles of a confocal Fabry-Perot inter erometer for sensing small vibrations or linear όisiilaceπcnts.

The principles of a Fabry-Perot interferoineter are v:ell nov:n and its application to the measurement of distances between two planes is veil established. HO*. ^* - ever, because of the necessity to use large and heavy optical components and light sources, its practical use has been limited. Hitherto, novn systems of this type have occupied a significant volume and, because of the ^• weight of their components, their dynamic ranges have boon limited. An object of the present invention is to provide a displacement sensing apparatus vhich utilises the high sensitivity of the Fabry-Perot interferometer to achieve improved sensitivity to small displacements. To this end, the invention consists in optical displacement sensing apparatus characterised by a confocal Fabry-Perot inter¬ ferometer in ^* .*:h.ich one of the confocal mirrors is resili- ently mounted so that the mirrors are relatively movable in response to an external stimulus applied to the inter¬ ferometer, and servo means responsive to the optical output from the interferometer to adjust an optical para¬ meter, upon displacement of the mirrors, in a manner to maintain or restore the optics.1 transmi sivity of the interferometer .and thereby monitor the displacement. The apparatus according to ^' the invention may be adapted to monitor or measure p?.rsx ^* eters of any external

physical or mechanical stimulus v.'hich may affect the light path distance between the t ^* wo confocal mirrors of the interferometer. It may be used to sense the amplitude of small periodic vibrations, over a '..'ide range of frequences, and small linear displacements, for example, of the order of 100 microns. Typically, it is euitable for use in accelerometers or pres. _' -ure measuring devices.

The main advantages of using a confocal Fabry- Perot interferometer over other optical displacement sensors is its very high sensitivitj* ^* and its immunity from mechanically induced misalignment, -v:hich hitherto has limited the use of optical sensors, for example , in εcceleromcters.

Preferably, the reflective surfaces of the confocal mirrors are formed from dielectric reflective coatings. The latter have a low absorption factor ε.nd afford the int r ex-ometer a maximum optical trεmsmissivitj' or transfer function of the order of 99^« The transfer function of the confocal Fabry-Perot interferometer varies vez-y rapidly vhen the mirror separation is substantially or close to an integral number of quarter vavelengths of the light signal illuminating the interferometer and a vax*iation of approximately 100>o in the optically trans¬ mitted signal occurs for a 0.3 nanometer change in the mirror separation. For monitoring vibrations, the spacing of the confocal mirrors is preferably set so that, in their rest position, the interferometer has a transfer function of the order of 5 ^c -.

In one embodiment of the invention, a piezoelectric element is arranged to move the other mirror relativel3 ^r

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to the resiliently mounted mirror, in order to vary t e- light path distance between the two confocal mirrors, and the servo means is responsive to the optical output from the interferometer to energise the piezoelectric element so as to maintain a substantially constant spacing between the mirrors.

The rc≤iliently mounted mirror may be supported by a resilient diaphragm attached to a body or housing of the interferometer in which is mounted the other mirror. The body may be adapted to be mechanically coupled to the external stimulus to be sensed ^nd the diaphragm may be arranged to have a relatively large inertia so that the resiliently mounted mirror remains stationary upon application of the external stimulus to the body.

Convenient13 ^* -, the apparatus may include an optical fibre for transmitting light from a suitable coherent light source to the optical input of the inter erometer. The optical output of the interferoπeter may be sensed by a photodetector v.'hich is arranged to supply a corresponding electrical signal to the servo means. Tiiis photodetector may be mounted adjacent the resiliently mounted mirror or, alternatively, the optical output from that mirror may be transmitted to a remote photodetector by an output optical fibre. The use of an optical fibre to introduce the light beam into optical input of the inter erometer removes the necessity of having the light or laser source disposed in proximity to the interferometer. Similarl}*-, the use of a second optical fibre to transmit the output of the inter erometer, enables the electronic detection

cor_τroncnt-3 to be removed to a considerable distance from the sensing position.

The servo means may be arranged to compare the optical output of the interferometer with a reference signal derived from the optical input signal so as to produce an output signal which maintains the optical transmissivity of the interferometer. This output signal from the servo means corresponds to one or more of the parameters of the external stimulus and may be processed to provide a measure of such parameter(s) .

In order that the present invention may be more readily understood, reference will now be made to the accompanying drawing whic schematically illustrates an accelero eter constructed in accordance with the invention.

Preferring to the drawing, the accelerometer basically comprises a confocal Fabry—Perot interferometer 1 including a body or housing 2 of bell-shaped configu¬ ration. The input spherical mirror 3 of the interfero— meter is disposed at the closed end of the bell-shaped housing 2. It is mounted at th.e inner end of a cylindrical piezoelectric element h for varying the position of the mirror. The opposite end of this piezoelectric element is mounted in an aperture 5 in the closed end of the housing. The output spherical mirror 6 is supported at the inner end of a cylindrical mounting member 7 arranged at the centre of a rigid diaphragm 8 which is attached to the housing 2 adjacent its outh. The thiclmess, tension and density of the diaphragm S determine the resonance frequency of the accelerometer and can be

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tailored for any specific npj-.l ca. on. The dia]*hr.-;;m is positioned so that the reflective surface separation of the two coaxial spherical mirrors ^~ \ , 6 is equcl to the r-ε-dius of each mirror which may, for example, be 1 cm. This radius is arranged to be clos.e to an integral number of quarter wavelengths of the light signal illuminating the interferometer. U e the reflective surfaces are cpaced an integral number of quarter vavelengths apart, they foi-m an optical resonance cavity which, wit refloc- tive surfaces formed from dielectric reflective coatings having 99 ^~ i- reflectivity, afford the interferometer a maximum optical transmissivity of the order of 99>-"*- The spacing of the reflective surfaces of the mirrors is arranged to be fractionally less than an integral number of quarter wavelengths so that, in the rest position of the mirrors, the transmissivity is of the order of 0 .

A large mass is also disposed on the cylindrical mounting member 7. In use, the housing 2 of the inter- ^' ferometer is mechanically coupled, for example, via its end surfaces 10 adjacent the diaphragm 8, to a periodi¬ cally vibrating test object, the acceleration of whose vibrations is to be measured, and the purpose of the mass is to provide a large amount of inertia for the diaphragm mounted mirror so that, when the housing 2 moves in synchronism with the test object, the mirror 6 remains stationary.

The coherent light source for illuminating the interferometer 1 is a laser light source 11 and, in order to ma e the accelerometer as versatile as possible, the

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output be m from this laser light source is co; ^* -__5c to the inter erometer via a single mode optical fibre 12. This enables the laser source to be operated at a position remote from the interferometer, for example at distances up to 1 kilometer away from the interferometer. The output from the optic ^* *-! fibre 12 is focussed into the input of the interferometer by a lens 1 disposed in the ajertux-e 5 in the housing 2. The optical signal trans¬ mitted by the interferometer is detected at its output by a photodiode 1 h , -. * . The latter 1 h i tty be mounted in the cylindrical mounting member. 7 εnd supply an electrical signal, via an electrical lead 15 _> to a remote location or, alternatively, may itself be disposed at the remote location and the output optical signal may be transmitted from the output of the interfex-omc-ter to the photodiode i h - by an optical fibre 15.

16 is an electronic servo unit. The output of this servo is connected to the piezoelectric element h, and the servo is responsive to the electrical output signal produced by the photodiode 1•_ ,1•■_ ^• - . so as to supply a voltage signal to the piezoelectric element which varies the latters length, in a manner such that it opposes the acceleration induced separation of the mirrors 3j6, that is, the separation of the mirrors is maintained constant or at a mean spacing whatever the value of the acceleration of the vibrations sensed by the inter erometer. The servo output voltage is produced by comparing the output signal produced by the photodiode 1 ,1 ' and corresponding to the optical signal transmitted through the interferometer, with a reference

signal derived from the laser light source 11. iϊence , the light beam from the laser is directed into a beam splitter 17 haying one output coupled to the optical fibre 12 and its other output detected by a photodiode 18. The signal produced by the photodiode 18 is supplied to the servo 16 which derives therefrom a reference signal corresponding to half the maximum optical signal trans¬ mitted by the inter erometer 1. This is compared with the signal produced by the photodiode l4,l ^! - to produce the voltage signal controlling the piezoelectric element h„ The resulting voltage output signal from the servo 16 is directly proportional to the acceleration of the periodic vibrations being sensed and can be processed to compute this acceleration. ^" _. ⁷ ith the above described accelerometer, only the inter erometer 1 is required to be coupled to or mounted on the test object and the laser source 11 and control and measuring components 11',16,17 can be disposed at a remote location. The interferometer can be of compact construction with a high sensitivity and large dynamic range. For example, it has been found that a sensitivit3 ^r

_o of 10 -" g can be achieved v.'ith a d3* ^* namic range of six orders of magnitude. Alternatively, other configurations are possible which will enable it to operate with a lower absolute sensitivit3 ^r , the d3 ^* namic range remaining constant. The design of the apparatus is such that it can be readily incorporated into a triple axis accelerometer.

Uhilst a particular embodiment has been described, it v.'ill be understood that modifications can be made with- out departing from the scope of the invention as defined D ^T the appended claims.