GB2079000A

GB2079000A - Two-stage interferometer

Info

Publication number: GB2079000A
Application number: GB8119729A
Authority: GB
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1980-06-30
Filing date: 1981-06-26
Publication date: 1982-01-13
Also published as: FR2485718B1; FR2485718A1; GB2079000B; SU1168800A1; DD158187A3; JPS5714703A; DE3112633A1

Abstract

A two-stage inteferometer includes a laser light source (1) and a beam splitter (2) having a partially metallised surface (3) which acts as an interference plane for producing two separate light beams (4 and 5). A measuring reflector (6) and a reference reflector (7) are disposed in the paths of the beams (4 and 5), respectively. The beam splitter (2) has on each side of the surface (3) a further beam-splitting, partially metallised surface (8,9), the latter surfaces lying symmetrically of the interference plane (3). A deflecting element, which can comprise a one- part element (13) or multiple elements (11, 12-see Fig. 2 not shown), is disposed at the end of the beam splitter (2) facing the measuring and reference reflectors (6,7). The deflecting element has optically effective reflective surfaces (14, 15) which align the light beams (4,5) so that the latter are mutually parallel, and has a beam exit surface (16) extending at 90 DEG to the interference plane (3). <IMAGE>

Description

SPECIFICATION Two-stage interferometer The invention relates to two-stage interferometers, particularly for linear measurements.

A two-stage interferometer is one which operates with two interference images to determine lengths and displacements.

An optical interferometer for producing two interference images is described in East German Patent Specification 111,993, this interferometer comprising a beam splitter having on each side a further beam-splitting, partially metallised surface which is disposed substantially parallel to the beam-splitting surface of the beam splitter. These surfaces deflect light components of the two-part light beams of the interference image and cause them to interfere with one another again.

In this two-stage interferometer, which is based on the Michelson principle, a coupling prism was provided for the purpose of optimally determining the reference point of the interferometric measurement, although this prism requires a greatly asymmetric beam path in the interferometer.

In this known interferometer, the most favourable position of the reference point and the symmetry of the metrologically sensitive part of the interferometer exlude one another.

Thus, it is impossible to be able to measure with a stable zero reference point over long measuring periods in, for example, the range of resolution of 1 0-8 . 10-9 m, since the parts of the interferometer which determine the optical path permit different changes in the optical path length in the measuring and reference beams in the case of thermal interference.

An object of the present invention is to avoid the disadvantages of the prior art devices and to increase the accuracy in interferometric systems.

In accordance with the present invention, there is provided a two-stage interferometer having a light source, a beam splitter having a beam-splitting, partially metallised surface serving as an interference plane for producing two separate light beams, a measuring reflector disposed in the optical path of the first light beam, a reference reflector disposed in the optical path of the second light beam, the beam splitter having on each side of the surface forming the interference plane a further beam-splitting, partially metallised surface, the latter surfaces lying symmetrically of the interference plane, a one-part deflecting element, or a deflecting element comprising a plurality of individual prisms which are adjustable independently of one another, disposed at that end of the beam splitter which faces the measuring reflector and the reference reflector, the deflecting elemen having optically effective reflecting surfaces which align the light beams so that they are mutually parallel, and the deflecting element having a beam exit surface which extends at right angles to the interference plane.

An advantage of the invention is that it enables the path of the beams in a two-stage interferometer to be made symmetrical so as to obtain in the interferometer an optical path length which is largely independent of external influences and to achieve parallel measuring and reference beams.

Advantageously, that end of the beam splitter which faces the measuring reflector and reference reflector has a surface on which the deflecting element is arranged and which extends at right angles to the interference plane.

A solution which is technologically simple to realise is achieved when the deflecting element is formed by the combination of individual prisms which are disposed symmetrically of the interference plane, the beam exit surfaces of the individual prisms lying approximately in a plane which extends at right angles to the interference plane.

It is particularly advantageous if, in the twostage interferometer, the surface forming the interference plane and the beam-splitting surfaces extending parallel thereto are extended in the direction of the path of the measuring beam and the path of the reference beam, and if decoupling prisms which deflect the light beams and align them parallel are disposed on unmetallised portions of said extensions of the beam-splitting surfaces.

By virtue of the invention, the path of the measuring beam and the path of the reference beam extend parallel in two-stage interferometers, and symmetry between these two beam paths are obtained within the beam splitter. This results in increased accuracy of the apparata equipped with interferometers of this kind, since the optical path length in the path of the measuring beam and the path of the reference beam within the interferometer remain constant despite temperature changes which occur, even over long measuring periods. A further advantage is that the entire path of the interferometer beam is insensitive to slight tilting of the interferometer. The individual parts of the interferometer are of simple construction.

The invention will be described further hereinafter with reference to the accompanying drawings, in which: Figure 1 shows one embodiment of a beamsplitter having a one-piece deflecting prism; Figure 2 shows a beam splitter having an assembled deflecting prism; and Figure 3 shows a beam splitter having extended surfaces.

Advantageously, the two-stage interferometer illustrated in Figs. 1 and 2, such as is used particularly for measuring lengths and displacements, comprises a laser 1 serving as a light source, a beam splitter 2 having a beam-splitting, partially metallised surface 3 serving as an interference plane for producing two separate light beams 4 and 5, and reflectors disposed in the optical paths of the light beams 4 and 5. Thus, a displaceable measuring reflector 6 is disposed in the optical path of the light beam 4, and a fixed reference reflector 7 is disposed in the optical path of the light beam 5, these reflectors advantageously being in the form of triple reflectors.

Further beam-splitting, partially metallised surfaces 8 and 9 are provided one on each side of the interference plane 3 in the beam splitter 2, the latter surfaces 8, 9 extending parallel to the interference plane 3 and being disposed symmetrically thereto. A one-part deflecting element 1 3 (Fig. 1), or a deflecting element 1 3 comprising a plurality of individual prisms 11 and 1 2 adjustable independently of one another (Fig. 2), is disposed on a sectional surface 10 provided on that end of the beam splitter 2 which faces the measuring reflector 6 and reference reflector 7.The deflecting element 1 3 has optical effective reflecting surfaces 14 and 1 5 which form preferably the same angle with the interference plane 3. These angles are dimensioned such that the path of the measuring light beam 4 and the path of the reference light beam 5 extend parallel. In the embodiment of Fig. 2, it is possible to adjust the reflecting surfaces 1 4 and 1 5 within certain limits, the thickness of the cement layer between the individual prisms 11 and 1 2 and between the sectional surface 10 and the individual prisms 11 and 12 being used for this purpose. The deflecting element 1 3 has a beam exit surface 1 6 which extends at right angles to the interference plane 3.In the arrangement of Fig. 2, the beam exit surfaces 1 7 and 1 8 of the individual prisms 11 and 1 2 are disposed such that they lie approximately in a plane which extends at right angles to the interference plane 3.

A particularly advantageous construction of a beam splitter 1 9 of the two-stage interferometer is illustrated in Fig. 3. As will be seen in Fig. 3, the beam splitter 1 9 is advantageously composed of two plane parallel plates 20 and 21, the surfaces 22, 23 and 24 being partially metallised on specific parts thereof determining the beam splitter. These surfaces 22,23 and 24 are extended in the direction of the measuring reflector 6 and the reference reflector 7, these extensions not being metallised. Decoupling prisms 27 and 28 which deflect the light beams and align them so as to be parallel are located on the extensions of the surfaces 22 and 24. The surface 23 constitutes the interference plane analogously to Figs. 1 and 2.

In the embodiment of Fig. 3, the optical paths of the measuring beam path and the reference beam path can be adjusted and symmetrised within the interferometer by adjusting the decoupling prisms 27 and 28 relative to one another by displacing these prisms along the extensions of the surfaces 22 and 24, and tilting them by utilising the thickness of the cement layer.

The function of the two-stage interferometer will be described with reference to Fig. 3, the function being analogous in the embodiment of Figs. 1 and 2. The collimated light beam emanating from the light source 1, a laser, is introduced into the top portion of the beam splitter 2, 1 9 by way of a coupling prism 29 arranged on the beam splitter. The coupling prism 29 is provided in order to make the directions of the incident light beam and the light beams 4 and 5 uniform, this being advantageous for many measuring purposes.

On the other hand, in principle, the light beam can be introduced into the beam splitter, 2, 1 9 by way of the prism 30. The beam introduced passes through the divider layer,of the surface 23 where it is split up into two beams 4 and 5 which constitute the measuring beam and the reference beam. These light beams 4 and 5 leave the interferometer by way of the decoupling prisms 27 and 28 and are displaced into the bottom portion of the beam splitter 2, 1 9 by the measuring reflector 6 and the reference reflector 7 and are brought to interference in the divider layer of the surface 23 which constitutes the interference plane. In accordance with the known two-stage principle, the partial beams, which already contain the interference information, pass from the interference plane by way of the divider layers of the surfaces 22 and 23 and the prisms 30 and 31 to signal receivers 32 and 33 or as reflecting light components from the divider layers of the surfaces 22 and 24 to the second interference in the plane of the surface 23 and then to the signal receivers 34 and 35.

Claims

1. A two-stage interferometer, particularly for linear measurements, having a light source, a beam splitter having a beam-splitting, partially metallised surface serving as an interference plane for producing two separate light beams, a measuring reflector disposed in the optical path of the first light beam, a reference reflector disposed in the optical path of the second light beam, the beam splitter having on each side of the surface forming the interference plane a further beam-splittingtx partially metallised surface, the latter surfaces lying symmetrically of the interference plane, a one-part deflecting element, or a deflecting element comprising a plurality of individual prisms which are adjustable independently of one another, disposed at that end of the beam splitter which faces the measuring reflector and the reference reflector, the deflecting element having optically effective reflecting surfaces which align the light beams so that they are mutually parallel, and the deflecting element having a beam exit surface which extends at right angles to the interference plane.

2. A two-stage interferometer as claimed in claim 1 wherein said light beams form equal angles with the interference plane.

3. A two-stage interferometer as claimed in claim 1 or 2, wherein the beam splitter has at its end facing the measuring reflector and the reference reflector a surface on which the deflecting element is disposed and which extends at right angles to the interference plane.

4. A two-stage interferometer as claimed in claim 1, 2 or 3, wherein the deflecting element is formed by the combination of individual prisms which are disposed symmetrically of the interference plane, the beam exit surfaces of the individual prisms lying approximately in a plane which extends at right angles to the interference plane.

5. A two-stage interferometer as claimed in claim 1, wherein the surface forming the interference plane and the beam-splitting surface extending parallel thereto are extended in the direction of the path of the measuring beam and the path of the reference beam, and wherein decoupling prisms which deflect the light beams and align them parallel are disposed on unmetallised portions of said extensions of the beam-splitting surfaces.

6. A two-stage interferometer substantially as hereinbefore described with reference to Fig. 1, Fig. 2 or Fig. 3 of the accompanying drawings.