GB2304923A

GB2304923A - Detector system for an interferometric measuring apparatus

Info

Publication number: GB2304923A
Application number: GB9617529A
Authority: GB
Inventors: Raymond John Chaney
Original assignee: Renishaw PLC
Current assignee: Renishaw PLC
Priority date: 1995-09-02
Filing date: 1996-08-21
Publication date: 1997-03-26
Also published as: GB9617529D0

Abstract

A detector system for an interferometric measuring apparatus has a plurality of detectors 36, 38, 40 and a prism for receiving a combined output beam 24 from the apparatus and for producing from it secondary beams directed towards the detectors. The prism has at least three coated external facets A, B, C from which the secondary beams are produced. The prism is designed so that no two facets lie in parallel planes, and a combination of the coatings and the angles between the facets is such as to ensure that the intensities of any stray reflected beams (if any) from the facets are insignificant, and no stray beams are directed towards the detectors or back to the laser. The beam is split at facets A and B, and transmitted through facet C.

Description

DETECTOR SYSTEM FOR AN INTERFEROMETRIC MEASURING APPARATUS FIELD OF THE INVENTION The present invention relates to a detector system for interferometric measuring systems, and to an optical component for use in such a detector.

DESCRIPTION OF THE PRIOR ART A known detector system for an interferometric measuring apparatus is described in our International Patent Publication No. W089/01612. This detector system includes two beam splitters for splitting an incoming laser beam into three measuring component beams so that the intensities of the beam at three different phase angles can be measured.

The beam splitters used in the above-mentioned detector system are thick plates having substantially parallel front and rear surfaces which cause stray reflected beams to be produced in near parallelism to the required reflected beams. These stray beams can cause undesirable interference with the measuring beams impinging on the photodiode detector and at the same time cause a loss of intensity in the transmitted beam.

SUMMARY OF THE INVENTION The detector system of the present invention is made much more immune to the effects of stray beams than the system described above. This is achieved by the use of a prism which has a plurality of external optical facets, and uses a combination of optical coatings and angles of the facets to minimise the production of stray beams and to ensure that those which may be produced have either no effect or an insignificant effect.

According to one aspect of the present invention, a prism for use in a detector system for an interferometric measuring system comprises at least three external optical facets, at least one facet having non-polarising beam splitting properties, at least one other facet having antireflective properties, the facets being arranged so that at least two of the facets lie in planes which are nonparallel.

According to another aspect of the present invention there is provided a detector system for an interferometric measuring apparatus which comprises: a plurality of optical detectors, a prism positioned to receive an incoming light beam and to split it into secondary beam portions directed towards the detectors, the prism having at least three external optical facets, at least one facet having non-polarising beam splitting properties, at least one other facet having antireflective properties, the facets being arranged so that at least two of the facets lie in planes which are nonparallel.

The properties of the facets are preferably produced by appropriate coatings.

In a preferred embodiment of the invention no two facets lie in planes which are parallel.

The number of facets required depends on number, and positioning of the associated detectors of the detector system. As a general rule, if there are n detectors in the system the prism will include n-l beam splitting facets and.

at least 1 anti-reflecting facet, but not all of the beam splitting facets have to have non-polarising properties.

Additional facets, for example coated to have reflecting properties, may need to be provided to ensure that the secondary output beam portions leave the prism in the appropriate directions and that any stray beams produced are not directed parallel to the input beam or to any of the secondary output beam portions.

In a preferred embodiment there are three detectors, and the prism has three coated optical facets. In this embodiment the angles of the coated facets of the prism relative to each other and relative to an incoming light beam can be arranged such that an incoming light beam impinging on the input facet, which is a non-polarising beam-splitting facet, is split, a reflected portion of the light beam being directed from the beam-splitting facet to a respective one of the detectors. The transmitted portion of the light beam being directed towards another beam splitting facet to produce a further transmitted beam portion directed towards a second one of the detectors.

The remaining reflected portion of the light beam being directed towards the anti-reflecting output facet of the prism through which it leaves the prism as a final secondary beam portion directed towards the third one of the detectors, and no stray light beams are directed towards any of the detectors.

The reflection and transmission coefficients of the coatings on the facts are arranged to produce as far as possible equal intensity beams at the detectors.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be more particularly described with reference to the accompanying drawings in which: Fig 1 illustrates a conventional Michelson interferometer in which the detector system of the present invention has been incorporated, Fig 2 illustrates the prism and the paths of the laser beams therethrough, Fig 3 illustrates the arrangement of the complete detector system inside a housing, and Fig 4 illustrates an alternative prism of the present invention.

Referring now to Fig 1 a conventional Michelson interferometer is shown which consists of a housing 10 in the top part of which is disposed a laser beam generator for producing a laser beam 12 directed towards a beam splitter 14. The beam splitter produces from the beam 12 a reference beam 16 and a measuring beam 18.

The reference beam 16 is directed towards a fixed reference reflector 20 and back to the beam splitter to form a socalled reference arm of the interferometer. The measuring beam 18 is directed towards a movable reflector 22 carried by an object the movement of which is being measured, and is returned to the beam splitter 14 to form a so-called measuring arm of the interferometer.

The returning measuring and reference beams are combined at the beam splitter to form a combined beam 24, and by suitable use of polarising optical components (not shown but known per se) it is arranged that the measuring and reference beam components which make up the combined beam are polarised in different polarisation states so that they do not interfere in the combined beam. Movement of the reflector 22 causes a relative phase change between the two parts of the combined beam reflected from the measuring and reference arms. The combined beam is directed into the lower part of the housing 10 in which is disposed a detector system (Fig 3). Optical components are included in the path of the combined beam, either in the detector or elsewhere, for changing the polarisation states of the two component beams causing them to interfere. The movement of..

the movable reflector can then be determined by the detector from a determination of the changes in the relative phase between the two beams.

The detector system of the present invention is suitable for use in such an interferometer in place of conventional detector systems.

It is to be understood that the above-described Michelson interferometer is purely illustrative of one application of the present invention and is not intended to limit the scope of the invention. The laser may be of any type including but not limited to a HeNe laser, or a laser diode, and is preferably of single frequency.

The type of interferometer with which the detector of the present invention may be used is not limited to the Michelson type illustrated in Fig 1 for measuring distances. Another suitable type of interferometer would be a Mach Zehnder interferometer. The measurements made may additionally include, but are not limited to measurements of angle, squareness or straightness, or even air pressure.

The reflectors used are shown as rooftop retroreflectors but again other types of reflectors may be used including but not limited to, corner cube reflectors or plane mirrors.

Referring now to Fig 2, the prism of the detector of the present invention is shown in greater detail. The prism has three external optical facets A,B and C on which light beams impinge, and two other external surfaces D and E being provided simply as a means for mounting the prism in a housing. The three optical facets produce from an incoming beam 50 three secondary beam portions 30,32 and 34 directed towards detectors shown diagrammatically at 36,38 and 40 respectively.

All three optical facets of the prism are coated in this example with non-polarising coatings. In this example the coating on facet C is simply an anti-reflection coating which only allows transmission of light impinging thereon so that facet C is a transmission facet only.

The coatings on facets A and B however, have beam-splitting properties which cause part of light beams striking them to be reflected and part to be transmitted. These facets thus become beam-splitting facets.

The proportions of the light reflected and transmitted by the beam-splitting facet of the prism are a matter of choice. In the preferred embodiment of the invention, the proportions are chosen such that the three secondary beam portions 30,32 and 34 emerging from the prism are of equal intensity.

The incoming light beam 50 is split by facet A (the input facet) into a transmitted secondary beam portion 52 and a reflected secondary beam portion 30 which is directed towards detector 36. The coating on facet A has a reflection coefficient of 1/3 and a transmission coefficient of 2/3 so that the transmitted beam 52 has twice the intensity of the reflected measuring beam 30.

The transmitted beam is deflected by the facet A and proceeds to the second beam splitting facet B at which it is split into a reflected portion 54 and a transmitted portion 32 which is deflected by facet B and is directed towards detector 38. The coating on facet B has a reflection coefficient of h and a transmission coefficient of X, whereby the intensities of beams 32 and 54 are each 1/3 of the intensity of the original beam 50, and thus equal to the intensity of beam portion 30.

Finally the reflected beam portion 54 is directed towards the transmission-only facet C, and is transmitted after deflection at the surface to form beam portion 34 directed towards detector 40 with an intensity equal to the other two beam portions 30 and 32.

It can be seen that the prism of the present invention has no parallel surfaces from which stray reflections can be produced to interfere with the various secondary beam portions. The only source of stray reflection is a reflected beam 58 from the facet C where the beam portion 54 impinges on the facet. Because of the chosen angles of the facets such a reflected beam 58 is directed away from the detectors and the lasers, and since facet C is coated with an anti-reflection coating, the intensity of beam 58 will be insignificant.

Because of the difficulty of getting the same coefficient of transmission or reflection for the two polarisation states in the combined beam at anything other than low angles of incidence, the disposition of the prism relative to the incoming beam 50, and the angles of the facets of the prism are arranged in accordance with a further novel aspect of the present invention so that the angles of incidence of the various beams on the facets A,B and C of the prism are small (for example in the range 20 to 30 ).

This is necessarily a compromise between having the angles of incidence as low as possible so that the cost of the coatings is not prohibitive, and providing sufficient angular separation of the secondary beam portions 30 and 34 to enable the detectors to be mounted in the housing without interference and without the housing becoming impractically long.

This is particularly important when the laser is a laser diode chosen to take advantage of its small size, so that the complete system can be made small and compact.

We have found that an acceptable compromise between getting a small, compact system, and having efficient operation of the coatings at small angles of incidence is achieved when the angle of the beams with the normals to their respective facets is chosen to be 20'.

In the example illustrated, to achieve the 20' angles for the beams, the incoming beam 50 is incident at 20 on facet A and the angle between facets A and C is approximately 128'.

Referring now to Fig 3 the complete detector system is shown inside the lower part of the housing 10 as described with reference to Fig 1. The reference numerals used in Figs 1 and 2 are used in Fig 3 to described the same components for continuity.

In this example the optical components required for causing interference of the components of the combined beam 24 are mounted at the front of the detector.

In another novel feature according to the present invention the combined beam 24 coming from the interferometer is passed through a half-wave plate 60 and a quarter-wave plate 62 at the entrance to the housing to produce the incoming beam 50. Thus the incoming beam 50 prior to impinging on the prism has the two components of the combined beam 24 circularly polarised in opposite senses.

Thus in the beam 50 the two component beams produce a linear vector the angle of which is dependent on the phase difference between the two component beams.

The combination of both a half-wave plate and a quarterwave plate (or three individual quarter-wave plates) is a device known as an achromatic waveplate which need only be used when the frequency uncertainty and/or the tolerance on the waveplate are such that a single waveplate will not produce adequately circularly polarised light. Such achromatic waveplates work over a greater range of frequencies and according to another novel feature of the invention can be tuned by rotating the individual plates relative to each other about the axis of the beam 24.

The three detectors (photodiodes) 36,38,40 are shown in position in slots 64,66 and 68 respectively in the housing, and polaroids 36A,38A and 40A are mounted ahead of the detectors to ensure that only light from the measuring beams which is polarised at a specific angle, which is different for each detector, reaches the detector. For example, the relative planes of polarisation of the polaroids are chosen in this embodiment to be at 0, 45O and 90 to each other, resulting in electrical signals being produced having relative phases of 0', 90' and 180'.

The detector system of the present invention thus provides the following advantages alone or in combination over conventional detector systems.

1. There are no stray beams directed near enough to any of the detectors to affect the signals they generate.

Nor are any reflected beams directed back towards the laser.

2. Use of low angles of incidence enable the coatings to be produced in which the transmission coefficients for the two polarisation states of the beam 50 are equal, and the reflection coefficient for the two polarisation states of the beam 50 are equal.

3. The prism and detectors form a small compact unit.

The prism described with reference to Figs 2 and 3 has three facets, but if it is desired to have another number of detectors, for example four, then the prism could be made with at least four coated optical facets. It would be possible to eliminate the polaroid 38A by making the coating on facet B a polarising coating. It may also be possible to make the coating on facet C a polarising coating as well as having a polarising coating on facet B, but in this case it should be polarising in the opposite sense to coating B so that it transmits the polarised light reflected from facet B.

Depending on the number and positioning of the detectors, and the direction of the incoming beam the number of optical facets and the properties of the coated optical facets may be varied.

For example, Fig 4 shows a prism suitable for use with two detectors but which has three coated external optical facets F,G and H and two other external surfaces J and K for use in mounting the prism in a detector system. In use this prism is situated in the path of an incoming beam 50 which strikes an input facet F at a low angle of incidence.

The facet F is coated with an anti-reflection coating whereby substantially the whole of the beam is transmitted into the prism. This beam strikes facet G at a low angle of incidence, and facet G is coated in the area where the beam strikes with a non-polarising beam splitting coating which produces a transmitted secondary beam portion 60 directed towards a first detector (not shown) and a reflected secondary beam portion 62. The transmission and reflection coefficients of this coating on facet G are each The reflected secondary beam portion 62 meets facet H at a small angle of incidence. Facet H is coated with a reflecting coating so that all of the beam portion 62 is reflected back towards facet G at an area which is coated with an anti-reflection coating. Thus substantially all of the reflected beam portion 62 is transmitted through facet G towards the second detector (not shown).

Thus it can be seen that there is a possibility that stray reflected beams 64 and 66 may be generated at the positions where the incoming beam 50 strikes the facet F, and where the reflected beam 62 meets the facet G, but at both of these positions the anti-reflecting coatings on the facets minimise the intensities of these beams and the angles at which these beams leave the facets are such that there can be no interference with either the incoming beams 50, or the secondary output beam portions 60 and 62.

A variety of prism designs can be provided in accordance with the invention in order to produce at least two output beams from a single input beam with no significant interference from stray reflected beams by adopting the following design criteria: i) using only external facets of the prism as optical facets, ii) having at least three optical facets, iii) ensuring that no two optical facets lie in parallel planes, and iv) providing coatings of different optical properties on the facets to ensure that the secondary output beam portions are directed towards the detectors, while any stray reflected beams are of insignificant intensity, and are directed away from the detectors and the laser.

A beneficial feature would be to ensure that the signals produced by the detectors are of equal intensity. Although the reflection and transmission coefficients of the various coatings are designed to produce beams of equal intensities on the detectors, this may not happen in practice if the coefficients cannot be made exactly right. Any differences in the signal intensities can be adjusted on set up of the apparatus using two polarisers ahead of each detector, one of which is used to set the relative phase angle for the detector, the other of which is mounted for rotation about the axis of the beam and is rotatable relative to the first to vary the intensity of the light passing through them.

Claims

1. A prism for use in a detector system for an interferometric measuring apparatus, the prism comprising at least three external optical facets at least one facet having non-polarising beam splitting properties, at least one other facet having anti-reflective properties, the facets being arranged so that at least two of the facets lie in planes which are non-parallel.

2. A prism according to claim 1 and wherein the nonpolarising beam splitting properties and the antireflective properties of the facets are produced by appropriate coatings on the facets.

3. A prism according to claim 2 and wherein the third facet is coated to provide the coated facet with beam splitting properties.

4. A prism according to claim 2 in which the third facet is coated to provide the coated facet with reflecting properties.

5. A prism according to any preceding claim wherein no two optical facets lie in planes which are parallel.

6. A detector system for an interferometric measuring apparatus, the detector system comprising: a plurality of optical detectors, a prism positioned to receive an incoming light beam and to split it into secondary beam portions directed towards the detectors, wherein the prism is a prism according to any one of claims 1 to 5.

7. A detector system according to claim 6 wherein the system has three detectors and the prism has three coated facets, the prism being positioned in an orientation such that the incoming light beam impinges first on the coated facet having non-polarising beam splitting properties which produces a reflected secondary beam portion directed towards a first one of the detectors and a transmitted secondary beam portion directed towards a second one of the facets which is coated so as to have beam splitting properties, said second one of the facets producing a transmitted secondary beam portion directed towards a second one of the detectors and a reflected secondary beam portion directed towards the third one of the facets which is coated so as to have anti-reflective properties, said third one of the facets transmitting the reflected secondary beam portion from the second facet to the third detector.

8. A detector system according to claim 6 wherein the beam splitting coating on the facet on which the incoming light beam impinges has a coefficient of transmission of 3 and a coefficient of reflection of , and the beam splitting coating on the second beam splitting facet has transmission and reflection coefficients of h.

9. A detector system according to claim 6 wherein the system has two detectors and the prism has three coated facets, the prism being positioned in an orientation such that the incoming light beam impinges first on the coated facet having anti-reflective properties and is transmitted towards an area of a second one of the facets which is coated so as to provide the area with non-polarising beam splitting properties, said area of the second one of the facets producing a transmitted secondary beam portion which is directed towards a first one of detectors, and a reflected secondary beam portion which is directed towards - the third one of the facets which is coated so as to have reflecting properties, said third one of the facets reflecting the reflected secondary beam portion back towards the second facet at an area coated so as to have anti-reflective properties which transmits said reflected secondary beam portion towards the second detector.

10. A detector system according to claim 9 wherein the beam splitting coating on the second facet has transmission and reflection coefficients of h.

11. A detector system according to any one of claims 6 to 10 wherein the relative orientation of the optical facets of the prism, and the orientation of the prism itself relative to the incoming beam, is such that the incoming bean and the secondary beam portions impinge on the respective facets at small angles of incidence.

12. A detector system according to claim 11 wherein the angles of incidence at which the incoming beam and the secondary beam portions impinge on the respective facets lie in the range 20 to 30".

13. A detector system according to any one of claims 6 to 12 wherein the interferometric measuring apparatus produces an output combined beam directed towards the detector system, the detector system includes a tunable achromatic wave plate through which the combined beam passes to produce said incoming beam of the detector system.

14. A detector system according to any one of claims 6 to 13 wherein polarising means are positioned ahead of each detector and through which the secondary beam portions directed towards the detector must pass to enter the detector, said polarising means defining the relative phase angle for the detector.

15. A detector system according to claim 14 wherein the polarising means includes two polarisers which are positioned along the axis of the secondary beam portion and which are relatively rotatable about the axis of the beam to vary the intensity of the light reaching the detector.