It is known from United Kingdom Patent No. 2,296,766 to provide a laser interferometer in which the interference between the measuring and reference beams is arranged to produce a spatial fringe pattern in the form of a series of fringes transverse to the direction of propagation of the combined beams.

In the particular embodiment described the interferometer included a polarising beam splitter to generate beams of orthogonal polarisation states in the measuring and reference arms of the interferometer.

The arrangement of the optical elements of this interferometer is such that the re-combined measuring and reference beams are not co-axial but remain parallel and partially overlap. Upon returning to the laser head the beams are passed through a birefringent prism which converges the two beams. The convergent beams are passed through a polaroid which mixes their polarisation states to produce interference between the beams, and because of their convergence a spatial fringe pattern is generated in a plane transverse to the direction of propagation of the beams. This fringe pattern is projected onto an electrograting photodetector having detecting elements of a width and spacing substantially the same as the width and spacing of the fringe pattern.

In an alternative embodiment proposed in this patent specification it is suggested that the beam splitter is non-polarising, which eliminates the need for a

birefringent prism and polariser thereby producing a cheaper and less complex system. It is proposed to produce interference of the beams through the construction and geometry of one of the retroreflectors or the beam splitter. No detail is provided however, as to how this can be achieved.

The present invention provides a simple and convenient arrangement of optical components which produce a spatial interference pattern in an interferometer and in which the spatial interference pattern lies in a plane transverse to the direction of propagation of the combined beams and is matched with the width and spacing of the detecting elements of an electrograting.

According to the present invention an interferometer comprises : a coherent light source for producing a light beam; a plurality of optical components, including a beam splitting device, which generate from the light beam measurement and reference beams which are reflected and re-combined at the beam splitting device; and a detector which receives the re-combined beam and generates measurement information therefrom; characterised in that a reflecting surface of at least one of the optical components is slightly angularly misaligned relative to the other optical components of the interferometer whereby the re- combined reference and measuring beams are non-parallel and overlap at least at the detector so that a spatial fringe pattern is produced which is transverse to the general direction of propagation of the re-combined

beam and is sensed at the detector.

In one embodiment the reference arm of the interferometer includes a retroreflector, one of the reflecting surfaces of which is not quite at 90° to the other surface. For example the two surfaces could be formed from two plane mirrors positioned at 91° to each other.

In another embodiment the beam splitter is a plate beam splitter at least one of the front or rear surfaces of which may be non-planar, and may be divided into two portions with an included angle of, for example 179° between them.

The invention will now be more particularly described by way of example only with reference to the accompanying drawings in which: Fig 1 shows the invention applied in a retroreflector in a Michelson type of interferometer; Fig 2 shows the manner in which the interference fringes produced by the interferometer react with the electrograting; Fig 3 shows an alternative embodiment in which the invention is applied to the beam splitter of a Michelson type of interferometer; Fig 4 shows the invention applied in an alternative type of interferometer; Fig 5 shows the invention using a modified cubic beam splitter; and Fig 6 shows a further embodiment using a further alternative beam splitter.

Referring now to Fig 1 there is shown a Michelson type

of interferometer in which a light source 10 produces a coherent light beam 12 directed towards a beam splitting device in the form of a plate beam splitter 14 which lies at an angle of 45° to the direction of propagation of the light beam. The beam splitter 14 produces from the light beam 12 a first, transmitted beam 16a which in this example forms the measuring arm of the interferometer, and is directed towards a retroreflector 18 attached to a moving object (not shown) the position of which is to be measured by the interferometer. The retroreflector is a standard retroreflector which returns the light beam parallel to itself as beam 16b, but transversely displaced, to the beam splitter. The return beam 16b is transmitted through the beam splitter and passes onwards towards a detector system 20.

The beam splitter 14 also produces a second, reflected beam 22 from its front surface 14a. The reflected beam is directed orthogonally to the transmitted beam towards a second retroreflector 24. This retroreflector consists of two plane mirrors disposed at not quite 90° to each other. For example they may be at 91°. The result of this is that the beam 22 is reflected as beam 22b which is not quite parallel to outgoing beam 22.

On its return to the beam splitter 14 the beam 22b strikes the front surface 14a of the beam splitter at a point slightly displaced from the position of the transmitted beam 16b, and at an angle which is not quite at 45° to the beam splitter surface. Thus the beam 22b is reflected from the beam splitter surface in a direction which is not quite parallel to the beam 16b

but is also directed towards the detector system 20.

However, the arrangement is such that the two beams 22b and 16b largely overlap, so that they form a spatial fringe pattern in the areas of overlap between the two beams.

The system detector 20 is placed in the path of the overlapping beams to receive the interference fringe pattern. A lens may be placed in the path of the overlapping beams to focus the fringe pattern onto the detector.

The detector used is an electrograting. Such a detector is known from our European Patent No.

0543513 B1, and is shown in Fig 2. The electrograting consists of a semiconductor substrate 50 upon which a plurality of elongate, substantially parallel photosensitive elements 52 are provided. The elements 52 are divided into a plurality of sets depending on the number of phase-shifted signals required from transverse displacement of the spatial fringe field falling on the detector due to movement of the retroreflector 18 in the measuring arm of the interferometer. In the present example the elements 52 are divided into four sets, each individual element in a set being connected together and the set being denoted by the connections 52A, 52B, 52C and 52D.

As can be seen in Fig 2 the spacing of the fringe pattern produced by the overlapping beams is arranged to be substantially equal to the spacing of the detecting elements of the electrograting, so that the intensity of the illumination of the elements in each

set caused by the fringe pattern is the same. If the spacing of the fringe pattern is less than the spacing of the sets of sensing elements, the pitch of the elements 52 of the electrograting (i. e. the spacing between like elements) can be effectively decreased by rotating the electrograting about an axis A until the pitch of the sensing elements 52 is equal to that of the fringes in the fringe pattern.

Greater flexibility can be achieved by mounting the electrograting at a small angle of incidence to the incoming beam (say up to 10°) so that by rotating it in one direction about axis A the pitch of the elements 52 effectively increases, and by rotating it in the opposite direction about the axis A the pitch of the elements effectively decreases.

One of the main benefits of the invention is that it is no longer necessary to have the reflecting surfaces of the retroreflector in the reference arm of the interferometer aligned relative to each other within a tolerance of less than an arc second. This significantly reduces the cost of this retroreflector.

Fig 3 shows an alternative embodiment of the invention applied to a Michelson type of interferometer in which similar elements to those shown in Fig 1 are given the same reference numerals.

In this embodiment, both of the retroreflectors 18,24 are standard retroreflectors, and the beams 16b and 22b respectively transmitted through, and reflected from, the beam splitter, are made to overlap and diverge by arranging for the front reflecting surface 14a of the

beam splitter to consist of two reflecting surfaces which are slightly misaligned so as to lie at an included angle of, for example 179°. This again will produce a spatial fringe pattern in the interfering beams which can be detected by the electrograting detector 20.

Clearly the same effect can be achieved by making the rear surfaces of the beam splitter misaligned relative to each other.

In a third embodiment shown in Fig 4, another independently novel and inventive arrangement is illustrated in which the separate beam splitter 14 is dispensed with and two retroreflectors 60 and 62 are used which are placed in line in the direction of propagation of the light beam 12. In this embodiment the retroreflector 62 is a standard retroreflector but the retroreflector 60 is made from two glass plates 60a and 60b which are not quite at 90° to each other.

Beam splitting coatings are applied to a surface of each of the plates 60a and 60b so that the retroreflector acts as a beam splitting device. The beam 12 from the source 10 impinging on plate 60a is split, part of the beam 12a being transmitted to the second retroreflector 62, the other part 12b being reflected towards plate 60b from which it is reflected as beam 12d in a direction towards the detector 20.

The beam 12a is reflected from retroreflector 62 as beam 12c which is parallel to, but spaced transversely from, beam 12a and impinges on the plate 60b of the retroreflector 60. Beam 12c is transmitted through the

plate 60b where it overlaps beam 12d. The overlapping beams 12c and 12d are convergent and produce a spatial fringe pattern in a direction transverse to their direction of propagation. An electrograting detector is positioned to detect the fringe pattern as described above.

Referring now to Fig 5 the arrangement is similar to that of Fig 1, and similar elements of the optical components are given the same reference numerals.

The beam splitting device in this embodiment is a combination of a standard cubic beam splitter 70 and a modified retroreflector 72. The beam splitter has a beam splitting surface 74 which produces from the outgoing laser beam 12 a transmitted beam 16a and a reflected beam 22. The transmitted beam 16a forms the measuring arm of the interferometer and is directed towards a standard retroreflector 18 on the object the position of which is to be measured, and is reflected back to the beam splitting surface 74 as return beam 16b. The reflected beam 22 is directed towards the retroreflector 72 which consists of two plane mirrors which are not quite at 90° to each other, so that the beam 22 returns to the beam splitting surface 74 as a beam 22b which is not quite parallel to the outgoing reflected beam 22. Thus the beam 22b strikes the beam splitting surface 74 at a slightly displaced position and is directed towards detector 20 in a direction which is not quite parallel to the beam 16b returning from retroreflector 18. The two beams 16b and 22b are arranged to overlap to form a spatial fringe pattern at the detector.

In the embodiment shown in Fig 6 the beam splitting device is in the form of two glass plates 90,91 disposed at not quite 90° to each other. In this embodiment instead of being joined together in the normal way to form a retroreflector they are shown mounted independently on a base plate 92, at right angles to each other.