Interferometer for Fourier transform infrared spectrometry

Abstract

An interferometer wherein an incident beam from a radiation source hits a beam splitter at a first oblique angle of incidence and is split into a first, reflected partial beam, and a second, transmitted partial beam, that subsequently travel along separate arms of the interferometer. The first and second partial beams are respectively intercepted, reflected, and re-split to form returning beam portions and reflected and transmitted exit beam portions. A second terminal mirror and a folding mirror, which intercepts the second partial beam at a second oblique angle of incidence, are associated with the second interferometer arm and positioned orthogonal to the reference plane and on opposite sides of the exit path, so that a section of the second partial beam from the folding mirror to the terminal mirror and back to the folding mirror crosses the exit beam twice.

Claims

1. An Interferometer, comprising: a body supporting stationary parts of the interferometer, a movable scanner assembly, and a motor; said stationary parts comprising a radiation source, a collimator arrangement, a beam splitter, a folding mirror, a first terminal mirror, and a second terminal mirror; said movable scanner assembly comprising a carrier part drivable by the motor in reciprocating guided movement relative to the body; and a retro-reflector supported by the carrier part; wherein, radiation originating from the radiation source and collimated into a beam of substantially parallel rays will meet an optical surface of the beam splitter at a first oblique angle of incidence to form an incident beam; wherein the incident beam will be split into a first, reflected partial beam that will continue along a first arm of the interferometer and a second, transmitted partial beam that will continue along a second arm of the interferometer; wherein the incident beam and a surface normal of the beam splitter define a reference plane; wherein along the first arm, the scanner assembly is arranged with the retro-reflector positioned to intercept the first partial beam and fold it towards the first terminal mirror such that the first partial beam will meet and be reflected by the first terminal mirror with substantially perfect normal incidence, and the reflected first partial beam will return through the same path along the first arm to the retro-reflector and from there to the beam splitter, where the returning first partial beam will be split into a first reflected portion returning to the collimator and a first transmitted portion continuing on an exit path; wherein along the second arm, the folding mirror is positioned to intercept the second partial beam at a second oblique angle of incidence and reflect it to the second terminal mirror, which is positioned to intercept and reflect the second partial beam with substantially perfect normal incidence, such that the reflected second partial beam will return through the same path along the second arm to the folding mirror and from there to the beam splitter, where the returning second partial beam will be split into a second transmitted portion returning to the collimator and a second reflected portion continuing on the exit path; and wherein the folding mirror and the second terminal mirror are positioned orthogonal to the reference plane and on opposite sides of the exit path, so that a section of the second partial beam from the folding mirror to the second terminal mirror and back to the folding mirror lies in the reference plane and will cross the exit beam twice.

2. The interferometer of claim 1, wherein reciprocating movement of the scanner assembly with the retro-reflector will cause a corresponding variation of the optical path length of the first partial beam between its departure from and return to the beam splitter, which will result in a variable retardation of the first partial beam relative to the second partial beam at their recombination, so that optical interference therebetween will produce an exit beam of varying intensity, which is measurable and recordable as an interferogram.

3. The interferometer of claim 1, wherein the retro-reflector is selected from the group consisting of roof reflectors and corner-cube reflectors.

4. The interferometer of claim 1, wherein the first oblique angle of incidence is between 25 and 35 from the surface normal.

5. The interferometer of claim 1, wherein the reciprocating guided movement of the scanner assembly is a rotary swivel movement about a pivot axis thereof.

6. The interferometer of claim 5, wherein the pivot axis of the movable scanner assembly is located in a plane that extends orthogonal to the first arm.

7. The interferometer of claim 5, wherein the pivot axis is oriented perpendicular to the reference plane.

8. The interferometer of claim 5, wherein the first partial beam, after interception by the retro-reflector, will be folded back to the first terminal mirror along a fold-back path that lies in the reference plane.

9. The interferometer of claim 5, wherein the first partial beam, after interception by the retro-reflector, will be folded out of the reference plane and back to the first terminal mirror along a fold-back path that extends parallel to, and at a distance from, the reference plane.

10. The interferometer of claim 5, wherein a swivel pivot of the movable scanner assembly is configured as an arrangement of crossed flexures defining the pivot axis of the scanner assembly.

11. The interferometer of claim 10, wherein the arrangement of crossed flexures comprises a pair of spaced-apart cross-flexure pivots.

12. The interferometer of claim 1, wherein the second oblique angle of incidence is between 25 and 35 from a surface normal of the folding mirror.

13. The interferometer of claim 1, further comprising a compensator plate located in the second arm of the radiation path between the beam splitter and the second terminal mirror.

14. The interferometer of claim 1, wherein the motor comprises a permanent magnet assembly and a cylindrical coil arranged in a cylindrical gap of the magnet assembly, wherein the coil is installed in a stationary position on the body, while the permanent magnet assembly is attached to the movable scanner assembly.

15. The interferometer of claim 14, wherein the scanner assembly with the retro-reflector and the permanent magnet assembly is balanced relative to a pivot axis of the scanner assembly, so that the mass of the permanent magnet assembly substantially counterbalances the mass of the retroreflector.

16. The interferometer of claim 2, further comprising: a reference source located in the area of the incident beam upstream of the beam splitter; and a laser detector installed on the body in the area of the recombined exit beam downstream of the beam splitter; wherein the reference source is positioned and aimed to emit a reference beam parallel to the incident beam such that the reference beam will fall on an area of the beam splitter having a suitable coating for the reference wavelength, where the reference beam will be split into two arms, analogous to the incident beam, and the recombined reference beam will be received by the laser detector.

17. The interferometer of claim 16, wherein the reference source is a diode laser arranged behind a paraboloid-shaped second reflector with the reference beam aimed through an opening in the second reflector, while the laser detector is arranged in the area of a sampling interface.

18. The interferometer of claim 16, wherein the reference source is a diode laser arranged in the area of a sampling interface, while the laser detector is arranged behind a paraboloid-shaped second reflector so as to receive the reference beam through an opening in the second reflector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The interferometer according to the invention will be described hereinafter through embodiments shown schematically in the drawings, wherein

(2) FIG. 1 illustrates the principle of a scanning interferometer;

(3) FIG. 2 illustrates a scanning interferometer according to a first embodiment of the invention;

(4) FIG. 3 illustrates a scanning interferometer according to a second embodiment of the invention in a perspective view;

(5) FIG. 4 illustrates the scanner assembly of an interferometer according to the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

(6) A scanning Michelson interferometer 1 as illustrated schematically in FIG. 1 includes a beam splitter 5 with a beam splitter coating 11, a compensator plate 12, a moving mirror 6 terminating a first arm A.sub.1 of the interferometer 1, and a fixed mirror 7 terminating a second arm A.sub.2 of the interferometer 1. The reciprocating scanning movement of the mirror 6 which causes a periodic variation in the length of the first arm A.sub.1 is indicated by the double arrow S. Radiation 3 from a source 2 is collimated by a collimating mirror 4 and directed to the beam splitter 5, which is positioned to receive the collimated incoming radiation A.sub.in at an angle of incidence of 45. The beam splitter 5 divides the incoming radiation A.sub.in into two partial beams propagating along the two arms A.sub.1 and A.sub.2, i.e. a first, reflected partial beam A.sub.1 and a second, transmitted partial beam A.sub.2 which propagate in directions 90 apart from each other to the two mirrors 6 and 7, respectively, where they are reflected as A.sub.1 and A.sub.2 and return along the same paths to the beam splitter 5. Each of the two returning partial beams A.sub.1 and A.sub.2 is again divided by the beam splitter 5 into a reflected portion and a transmitted portion. At the same time, the transmitted portion of A.sub.1 and the reflected portion of A.sub.2 are recombined in the exit beam A.sub.out, while the reflected portion of A.sub.1 and the transmitted portion of A.sub.2 are recombined in the returning beam A.sub.rt which travels along the same path as the incoming beam A.sub.in, but in the opposite direction. The radiation path of the interferometer 1 with the incoming beam A.sub.in, the two arms A.sub.1 and A.sub.2 and the exit beam A.sub.out lies in the plane of incidence of the incoming radiation A.sub.in, i.e. the reference plane defined hereinabove which contains the surface normal of the beam splitter 5 and the propagation vector A.sub.in.

(7) The function of the compensator plate 12 is best understood by counting the number of times the radiation path traverses the thickness of either the glass substrate of the beam splitter 5 or the equal thickness of the compensator plate 12. The radiation coming from the source makes a first traverse of one glass plate thickness through the glass substrate of the beam splitter 5. After reflection at the beam splitter coating 11, the continuing first partial beam A.sub.1 makes a second traverse through the beam splitter substrate, the returning first partial beam A.sub.1 makes a third traverse through the beam splitter substrate, and the recombined outgoing beam A.sub.out completes a fourth traverse of one glass plate thickness through the compensator plate. At the same time, after the first traverse of the incoming beam A.sub.in through the beam splitter substrate and transmission through the beam splitter coating 11, the continuing second partial beam A.sub.2, the returning second partial beam A.sub.2, and the recombined outgoing beam A.sub.out make, respectively, a second, third, and fourth traverse of one glass plate thickness through the compensator plate.

(8) The phase difference between the returning partial beams A.sub.1 and A.sub.2 at their recombination into the exit beam A.sub.out is dependent on the difference between the respective path lengths travelled by A.sub.1, A.sub.1 and A.sub.2, A.sub.2 between the beam splitter 5 and the terminal mirrors 6, 7 as well as on the wavelength of the radiation. As described previously herein, due to the optical interference between A.sub.1 and A.sub.2 at their recombination, the intensity of the recombined beam A.sub.out varies as a function of the difference (commonly referred to as retardation ) between the variable path length travelled by A.sub.1, A.sub.1 and the fixed path length travelled by A.sub.2, A.sub.2. In interferometric spectrometry (FT-IR spectrometry), the exit beam A.sub.out generally passes through (or is interacted with) a sample 9 in a sample container 8 and subsequently focused on an electro-optical sensor 10 which produces an electrical signal representing the light intensity I of the recombined beam A.sub.out as a function of the retardation , which is referred to as interferogram I().

(9) FIG. 2 schematically illustrates the conceptual layout of a scanning interferometer 101 in accordance with the present invention, whose components are mounted on a supporting structure 100, referred to herein as the body 100. As in FIG. 1, radiation 103 from a source 102 is collimated by a parabolic mirror 104 and directed to the beam splitter 105, which splits the incoming beam A.sub.in into the two arms A.sub.1 and A.sub.2. Likewise, analogous to FIG. 1, the drawing plane coincides with the plane of incidence of the incoming radiation A.sub.in, i.e. the plane which contains the surface normal of the beam splitter 105, the propagation vector A.sub.in and the surface normals of mirrors 106, 107 and 112. However, in contrast to FIG. 1, the incoming beam A.sub.in meets the beam splitter 105 at an angle of incidence <45, so that the incoming beam A.sub.in and reflected partial beam A.sub.1 enclose an angle of less than 90.

(10) Along the first arm A.sub.1 of the interferometer 101, the scanner assembly with the retro-reflector 111 is arranged to intercept the first partial beam A.sub.1 and fold it onto a parallel path to the first terminal mirror 106. The first partial beam A.sub.1 meets the first terminal mirror 106 with essentially perfect normal incidence, so that the returning first partial beam A.sub.1 travels the exact same path as A.sub.1 in the opposite direction back to the retro-reflector 111 and from there to the beam splitter 105, where the returning first partial beam A.sub.1 is split into a first reflected portion returning to the collimator and a first transmitted portion continuing on an exit path of the interferometer.

(11) The retro-reflector 111 in the first arm A.sub.1 of the interferometer 101 is schematically represented as a roof reflector, i.e. two mirrors 114, 115 joined along a ridge 113 and set at a 90 angle to each other. As the two mirror surfaces of the roof reflector 111 in FIG. 2 are oriented perpendicular to the drawing plane (which coincides with the aforementioned plane of incidence of A.sub.in), the folded segments of the radiation path A.sub.1, A.sub.1 remain in the same plane. As a result, the output beam also lies in the same plane, so the folded arm must reach a terminal mirror lying in the same plane. For this embodiment, it is critical that the pivot axis be exactly normal to the plane containing the surface normal of mirrors 106, 107 and 112. In a preferred alternative embodiment, the retro-reflector is a corner-cube reflector, in which three mirror surfaces are joined together like the sides of a cube. The use of corner-cube reflectors maintains high-quality interferometric alignment even when the flexures have undesired deviations. A roof reflector also may be used to fold the beam up to a terminal mirror, but then a different pivot axis is required, one that is parallel to the reference plane containing the aforementioned surface normals.

(12) Along the second arm A.sub.2 of the interferometer 101, the folding mirror 112 is positioned to intercept the second partial beam A.sub.2 at a second angle of incidence 13 and reflect it to the second terminal mirror 107, which is positioned to intercept and reflect the second partial beam A.sub.2. It is essential that the beam arrive at terminal mirror 107 with perfect normal incidence and to thereby cause the returning second partial beam A.sub.2 to travel along the same path as A.sub.2 in the opposite direction back to the folding mirror 112 and from there to the beam splitter 105, where the returning second partial beam A.sub.2 is split into a second transmitted portion returning to the collimator and a second reflected portion continuing on an exit path A.sub.out of the interferometer. As the mirror surface of the folding mirror 112 is further oriented perpendicular to the drawing plane of FIG. 2, which coincides with the aforementioned plane of incidence of A.sub.in, the folded segments of the radiation path A.sub.2, A.sub.2 remain in the same plane.

(13) As described above for the interferometer 1, the portion of A.sub.1 that is transmitted and the portion of A.sub.2 that is reflected by the beam splitter 105 are recombined in the exit beam A.sub.out, while the transmitted portion of A.sub.2 and the reflected portion of A.sub.1 are recombined in the returning beam A.sub.rt which travels along the same path as the incoming beam A.sub.in, but in the opposite direction. Also, as described previously herein, optical interference occurs between the recombined beam portions, so that an interferogram I() can be registered by an optical sensor, also referred to as radiation detector (not shown in FIG. 2).

(14) Further in contrast to FIG. 1, the compensator plate 108 in the embodiment of FIG. 2 is placed in the second arm A.sub.2 of the radiation path between the folding mirror 112 and the terminal mirror 107. Following the radiation path on the one hand along the segments A.sub.in, A.sub.1, A.sub.1, A.sub.out, and on the other hand along the segments A.sub.in, A.sub.2, A.sub.2, A.sub.out, one finds that, either way, the radiation passes through only three glass plate thicknesses. The resultant advantageous reduction in transmission and reflection losses in comparison to the conventional compensator arrangement of FIG. 1 has been discussed previously herein.

(15) The two-dimensional schematic layout of the interferometer 101 of FIG. 2 illustrates three essential features through which the invention meets its primary objective of a compact design:

(16) 1) the light source, the collimator and the beam splitter are arranged in such a way that the incoming radiation beam A.sub.in meets the beam splitter surface at an angle of incidence that is significantly smaller than 45, for example 30 or less;

(17) 2) the first, scanning arm A.sub.1 is parallel-folded with a retroreflector used as scanner, such as a trihedral (corner-cube) or dihedral (roof-shaped) reflector; and

(18) 3) the second, fixed arm A.sub.2 is folded by a folding mirror that directs the reflected beam across a space that is also simultaneously traversed by the exit beam A.sub.out of the interferometer.

(19) The interferometer 201 in FIG. 3 differs from the embodiment of FIG. 2 in that the first arm A.sub.1 of the radiation path in FIG. 3 is folded back into a parallel plane which in the perspective drawing of FIG. 3 extends vertically above the plane of incidence of A.sub.in. This folding of the first arm A.sub.1 is achieved by flipping the roof reflector 111 of FIG. 2 by 90 about the axis B of the beam segment A.sub.1 arriving from the beam splitter 205. Analogous to the arrangement of FIG. 2, the folded-back beam A.sub.1 is intercepted and reflected by an appropriately positioned terminal mirror 206.

(20) The roof reflector 211 could be tilted at any angle (not limited to 90) about the axis B of the incident beam path. The general requirements for the positioning of a roof reflector to perform its function of folding an incoming beam into a parallel outgoing beam are that the ridge line needs to be perpendicular to the incoming radiation path and the two intersecting mirror surfaces need to enclose a right angle. This relationship must be maintained to interferometric precision. Placement of the scanner pivot axis and motion quality is much more forgiving with a hollow cube corner retroreflector than the roof reflector, because of the more complete tilt compensation.

(21) The reciprocating movement of the retroreflector which is symbolized by the double arrow S in FIGS. 2 and 3 can be a translatory movement where the retroreflector 111, 211 is guided for example by a slide track or by a so-called porch-swing mechanism. However, in a preferred arrangement according to FIG. 4, the retroreflector 311, which is shown here symbolically as a roof reflector but could also be configured as a corner cube reflector, is part of a scanner assembly 320 which is guided by a cross-flexure pivot 321 in a rotary swivel movement symbolized by the rotary double arrow R. The axis C of the reciprocating rotary movement R is oriented at a right angle to the drawing plane of FIG. 4 and (for small amplitudes of the reciprocating swivel movement) essentially coincides with the intersection of the flexure leaf springs of the pivot 321. The reciprocating swivel movement of the scanner assembly 320 comprising a carrier part 325 is driven by a motor 322 which includes a stationary coil 323 attached to the body of the interferometer and a permanent magnet assembly 324 attached to the scanner assembly 320.

(22) In the swiveling scanner assembly 320, the permanent magnet assembly 324 can be arranged so that its mass almost exactly counterbalances the mass of the retroreflector 311. In other words, the combined center of gravity of the scanner assembly lies on the rotary axis C of the swivel movement R. This mass-balancing of the scanner assembly largely immunizes the interferometer against the effects of acceleration, position changes, shocks and vibrations.

(23) According to the invention, an interferometer of the general configuration illustrated in FIGS. 2 and 3 is preferably equipped with the mass-balanced, swivel-pivoted scanner assembly 320 of FIG. 4. If the retroreflector 311 is a roof reflector (as illustrated in the drawings), the line of intersection of the two mirror surfaces has to be exactly parallel to the rotary axis C of the swivel movement R, and the axis C has to be orthogonal to the incident beam of radiation A.sub.1 and to the surface normals of mirrors 106, 107, 112 in FIG. 2 or 206, 207, 212 in FIG. 3. If the retroreflector 311 is a corner-cube reflector, on the other hand, it will work in any orientation as long as its trihedral concavity catches the incident beam and directs the reflected beam A.sub.1 to the terminal mirror 106, 206.

(24) While the invention has been described through the presentation of several specific embodiments, it is considered self-evident that numerous additional variants are encompassed by the teachings of the present invention, for example by combining the features of the individual embodiments with each other and/or by exchanging individual functional units of the embodiments against each other, and that such combinations are included in the scope of the invention.