Jamin-type interferometers and components therefor

• US 20020015153A1
• Filed: 07/23/2001
• Published: 02/07/2002
• Est. Priority Date: 07/27/2000
• Status: Active Grant

• Current Assignee: Angela Christine Downs, Qioptiq Photonics Ltd. (Excelitas Technologies Holdings LLC), Michael John Downs
• Original Assignee: Angela Christine Downs, Michael John Downs
• Inventors: Downs, Michael John
• Granted Patent: US 6,678,056 B2
• Time in Patent Office: Days
• US Class Current: 356/450
• CPC Class Codes: G01B 2290/45 Multiple detectors for detecting interferometer signals
• CPC Class Codes: G01B 9/02056 Passive reduction of errors
• CPC Class Codes: G01B 9/02081 simultaneous quadrature detection, e.g. by spatial phase shifting
• CPC Class Codes: G02B 27/1073 characterized by manufacturing or alignment methods
• CPC Class Codes: G02B 27/144 using partially transparent surfaces without spectral selectivity G02B27/147 takes precedence
• CPC Class Codes: G02B 27/145 having sequential partially reflecting surfaces
• CPC Class Codes: G02B 5/122 cube corner, trihedral or triple reflector type

Abstract

A retroreflector has three mutually-orthogonal reflective surfaces arranged around an optical axis. The reflective surfaces stop short of the optical axis to provide a central region of the retroreflector which transmits incident light and a peripheral region of the retroreflector which retroreflects incident light. When the reflector is used in a Jamin-type interferometer with another reflector, this enables the interferometer to be used for measuring displacement between the reflectors.

In the interferometer, a projected beam is disposed between a pair of return beams and/or one of the return beams is disposed between a pair of the projected beams. This enables a first contiguous area of a face of a beam splitter to be provided with a phase-shifting coating to produce a phase quadrature relationship between a pair of interferogram beams. This simplifies the masking required when applying the coating.

In manufacture of the beam splitting member, a thin-film, beam-splitting, metal coating is applied to the member, and the member and coating are baked so as to modify the phase shift produced by the coating to enable the phase quadrature relationship. During baking a beam of light is projected at the coating with an angle of incidence of substantially π/4 radians so that the beam is split into a transmitted component and a reflected component. The intensities or phases of the transmitted and reflected components are monitored during baking, and the baking is terminated when the monitored intensities or phases have a predetermined relationship. This improves the reliability and/or accuracy of the resulting phase shift.

Claims

1. A retroreflector having three mutually-orthogonal reflective surfaces arranged around an optical axis, wherein the reflective surfaces stop short of the optical axis to provide a central region of the retroreflector which transmits incident light and a peripheral region of the retroreflector which retroreflects incident light.

2. A retroreflector as claimed in claim 1, and comprising a body of optical material which provides the reflective surfaces by internal reflection in the body.

3. A retroreflector as claimed in claim 2, and in the form of a solid cube comer having a first transmitting surface for incident light and a second transmitting surface which truncates the cube comer.

4. A retroreflector as claimed in claim 3, wherein the first and second transmitting surfaces are generally, but not exactly, parallel.

5. A retroreflector as claimed in claim 2, and in the form of a solid cube comer having a passageway extending therethrough generally in the direction of the optical axis to provide the central region.

6. A retroreflector as claimed in claim 1, and comprising three plane mirror elements arranged around the optical axis, each providing a respective one of the reflective surfaces.

7. A retroreflector as claimed in claim 6, and in the form of a hollow truncated cube comer.

18. A retroreflector as claimed in claim 3, the retroreflector forming part of an optical apparatus having an optical axis, and the first transmitting surface being generally, but not exactly, orthogonal to the optical axis of the apparatus.

8. A reflector system comprising first and second relatively movable reflectors, wherein:

the first reflector is a retroreflector having three mutually-orthogonal reflective surfaces arranged around an optical axis, wherein the reflective surfaces stop short of the optical axis to provide a central region of the retroreflector which transmits incident light and a peripheral region of the retroreflector which retroreflects incident light; and

the reflectors are arranged so that light which is transmitted through the central region of the first reflector is reflected by the second reflector and transmitted back through the central region of the first reflector.

9. A system as claimed in claim 8, wherein the second reflector is a retroreflector.

10. A system as claimed in claim 9, wherein the second reflector has three mutually-orthogonal reflective surfaces arranged around a second optical axis.

11. A system as claimed in claim 10, wherein each of the reflective surfaces of the second reflector is arranged parallel to a respective one of the reflective surfaces of the first reflector.

12. A system as claimed in claim 10, wherein the second reflector comprises a second body of optical material which provides the reflective surfaces by internal reflection in the second body.

13. A system as claimed in claim 12, wherein the second body has a transmitting surface for incident light which is generally, but not exactly, parallel to the second transmitting surface of the first reflector.

14. A system as claimed in claim 10, wherein the second reflector comprises three plane mirror elements arranged around the second optical axis.

15. A system as claimed in claim 8, wherein the first and second reflectors are joined by a piezo electric material.

16. A system as claimed in claim 8, wherein the second reflector comprises a plane mirror, the system further including a lens between the first and second reflectors.

17. A system as claimed in claim 8, wherein the second reflector comprises a polarising beam splitter, quarter-wave retardation plate, plane mirror and second retroreflector arranged such that:

light which is transmitted through the central region of the first reflector is transmitted through the beam splitter and quarter-wave plate to the mirror;

the light which is thus reflected by the mirror is transmitted through the quarter-wave plate, is reflected by the beam splitter, and is directed to the second retroreflector;

the light which is thus reflected by the retroreflector is reflected by the beam splitter and transmitted through the quarter-wave plate to the mirror; and

the light which is thus reflected by the mirror is transmitted through the quarter-wave plate, the beam splitter and the central region of the first reflector.

23. A reflector system as claimed in claim 8, and forming part of a Jamin-type interferometer.

24. A reflector system as claimed in claim 8, in combination with a beam splitter, wherein:

the beam splitter is arranged to split an incident beam of light into first and second generallyparallel, spaced-apart, projected beams;

the reflector system is arranged to reflect the first and second projected beams to produce first and second return beams, respectively, which are spaced apart from and generally parallel to each other and the first and second projected beams; and

the beam splitter is arranged to enable the first and second return beams to be superposed to produce at least one interferogram;

so as to form a Jamin-type interferometer.

19. A method of manufacturing a retroreflector, comprising the steps of:

providing a cube corner retroreflector comprising a body of optical material, the body having three mutually-orthogonal reflective surfaces arranged around an optical axis, the reflective surfaces providing internal reflection in the body, and the body having a first transmitting surface for incident light; and

cutting the body in a plane generally orthogonal to its optical axis to form a second transmitting surface which truncates the cube corner so that the reflective surfaces stop short of the optical axis and so that a central region of the retroreflector is provided that transmits incident light and a peripheral region of the retroreflector is provided that retroreflects incident light.

20. A method of manufacturing a retroreflector, comprising the steps of:

providing a cube comer retroreflector comprising a body of optical material, the body having three mutually-orthogonal reflective surfaces arranged around an optical axis, the reflective surfaces providing internal reflection in the body, and the body having a first transmitting surface for incident light; and

forming a passageway through the body generally in the direction of the optical axis so that the reflective surfaces stop short of the optical axis and so that a central region is provided that transmits incident light and a peripheral region of the retroreflector is provided that retroreflects incident light.

21. A method of manufacturing a retroreflector, comprising the steps of:

providing three plane mirrors each having a pair of mutually orthogonal edges;

cutting each mirror along a line intersecting each of its mutually orthogonal edges to form a respective first mirror and a respective second mirror; and

assembling the first mirrors mutually-orthogonally to form a retroreflector having a central region that transmits incident light and a peripheral region of the retroreflector that retroreflects incident light.

22. A method as claimed in claim 21, further including the step of assembling the second mirrors mutually-orthogonally to form a second retroreflector.

26. An interferometer as claimed in claim 25, wherein a first contiguous area of the face of the beam splitter is provided with a phase-shifting coating, one of the projected beams being projected from, and one of the return beams returning to, the first area.

27. An interferometer as claimed in claim 25, wherein the coating produces a phase shift such that there is a phase difference of substantially π

/2 radians between the two interferogram beams.

28. An interferometer as claimed in claim 25, wherein the coating comprises a thin metal film.

29. An interferometer as claimed in claim 25, wherein a second contiguous area of the face of the beam splitter is devoid of any coating providing any substantial phase-shift, the other projected beam being projected from, and the other return beam returning to, the second area.

30. An interferometer as claimed in claim 25, wherein:

the reflector system comprises first and second relatively movable reflectors;

the first reflector is a retroreflector having three mutually-orthogonal reflective surfaces arranged around an optical axis;

the reflective surfaces stop short of the optical axis to provide a central region of the retroreflector which transmits incident light and a peripheral region of the retroreflector which retroreflects incident light; and

the reflectors are arranged so that light which is transmitted through the central region of the first reflector is reflected by the second reflector and transmitted back through the central region of the first reflector.

31. An interferometer as claimed in claim 25, further comprising a dual-chamber gas or liquid refractometer arranged so that the first projected and return beams pass through one chamber of the refractometer and the second projected and return beams pass through the other chamber of the refractometer.

32. An interferometer as claimed in claim 25, wherein the reflector system comprises first and second retroreflectors rigidly joined together adjacent each other, the first retroreflector being arranged to retroreflect the first projected beam to produce the first return beam, and the second retroreflector being arranged to retroreflect the second projected beam to produce the second return beam.

33. An interferometer as claimed in claim 25, further including means for modulating the optical path length of one of the beams, or one of the projected beams and its respective return beam.

34. An interferometer as claimed in claim 33, wherein the modulating means comprises an optically-transmitting, varying-thickness plate disposed in the path of the beam(s) to be modulated, and means for rotating the plate.

35. A interferometer as claimed in claim 33, wherein the modulating means comprises a layer of optically-transmitting, flexible material sandwiched between a pair of optically-transmitting plates disposed in the path of the beam(s) to be modulated, and means for modulating the spacing of the plates.

36. An interferometer as claimed in claim 33, wherein the modulating means is disposed adjacent the beam splitter.

37. An interferometer as claimed in claim 33 wherein:

the first and second reflectors are joined by a piezo electric material; and

the modulating means includes means for modulating the thickness of the piezo electric material to modulate the spacing between the reflectors.

39. A method as claimed in claim 38, wherein it is the intensities which are monitored.

40. A method as claimed in claim 39, wherein the baking step is terminated when the monitored intensities are generally equal.

41. A method as claimed in claim 39, wherein the intensity of the transmitted component is monitored after transmission through another surface of the member, and the baking step is terminated when the monitored intensity of the transmitted component is substantially equal to a predetermined proportion of the monitored intensity of the reflected component.

42. A method as claimed in claim 38, wherein the coating is of aluminium.

43. A method as claimed in claim 42, wherein the baking temperature is between 450 and 480°

C.

44. A method as claimed in claim 38, wherein the intensities of the polarisation components of the projected beam in the perpendicular and parallel directions are generally equal to each other.

25. A Jamin-type interferometer, wherein:

a beam splitter is arranged to split an incident beam of light into first and second generallyparallel, spaced-apart, projected beams;

a reflector system is arranged to reflect the first and second projected beams to produce first and second return beams, respectively, which are spaced apart from and generally parallel to each other and the first and second projected beams;

the beam splitter is arranged to enable the first and second return beams to be superposed to produce at least one interferogram beam;

the projected beams are projected from, and the return beam returns to, a single face of the beam splitter; and

one of the projected beams is disposed between the return beams and/or one of the return beams is disposed between the projected beams.

38. A method of manufacture of a beam splitting member, comprising the steps of:

applying a thin-film, beam-splitting, metal coating to at least part of a surface of the member;

projecting a beam of light at the coating with an angle of incidence of substantially π

/4 radians so that the beam is split into a transmitted component and a reflected component;

baking the member and coating so as to modify the phase shift produced by the coating;

monitoring the intensities or phases of the transmitted and reflected components during the baking step; and

terminating the baking step when the monitored intensities or phases have a predetermined relationship.