GB2136117A

GB2136117A - Interferometer Spectrometer

Info

Publication number: GB2136117A
Application number: GB08404808A
Authority: GB
Inventors: Jack G Kisslinger; David S Calhoun; David I Plaut; Warren D Vidrine
Original assignee: Nicolet Instrument Corp
Current assignee: Thermo Electron Scientific Instruments LLC
Priority date: 1983-03-04
Filing date: 1984-02-23
Publication date: 1984-09-12
Also published as: CA1210608A; DE3344545A1; FR2542084A1; GB8404808D0; JPS59164926A

Abstract

Analytical radiation from a source 10 enters a Michelson-type interferometer including a beam splitter 13 and a movable reflecting member 17. Quadrature phase discrimination is employed to provide signals indicative of the position and direction of movement of a portion of member 17 within the analytical radiation path, using a beam from laser 25 directed onto beam splitter 13 and, after reflection by fixed mirror 15 (via retardation plate 27) and movable mirror 17, passed coaxially with beam 11 to detector 28. Multiple quadrature phase discrimination systems may be employed to indicate of the position and direction of movement of three portions of a flat reflecting member. One portion may be within the analytical radiation path. An initial reference indicative of the position of the movable reflecting member independently of the beam splitter and an additional quadrature phase discrimination system may be provided. The reference system may provide redundantly an indication of position and direction of movement of member 17. <IMAGE>

Description

SPECIFICATION Interferometer Spectrometer Background of the Invention Interferometer spectrometers are known to the prior art. Within that context, Michelson-type interferometers have gained wide acceptance, particularly in the form that employs a beam splitter to establish two independent optical paths with at least one path having a movable reflecting element. Inducing the desired movement of the reflecting member and locating its position in space are primary problems with such instruments.

Modern interferometer spectrometers employ coherent radiation from a laser within the interferometer with the movable reflecting element producing "fringes" which are counted as the reflecting element moves. However, this basic method does not establish the position of the movable reflecting element, only the amount of its movement. For some measurements, this is adequate while others require more.

Instrument sensitivity can be improved by repeating a measurement. However, measurement repetition requires some referencing. In the prior art, this has been accomplished by what is known as the "white light" method which is based on the fact that there is a maximum signal when the arms of the interferometer are equal. While this has been useful, it is not totally satisfactory for several reasons. Among those reasons are the fact that physically separate interferometers are required, one for generating the "white light" signal and one for generating the analytical spectrum.

Coupling of two interferometers in a manner that eliminates positional errors between them is difficult. Another of the reasons is that the fringe counter loses count when the moving reflecting element changes direction. Thus, referencing must be done at the beginning of each measurement while bidirectional scanning is rendered impractical. In addition, the "white light" system requires a high mechanical precision which increases costs and establishes a practical requirement that a single point be chosen at the start of a measurement scan. The point chosen is a compromise which degrades performance.

Summary of the Invention The present invention provides an absolute position sensing method based on a laser which has particular application to determining the position and direction of movement of a movable reflecting member within the interferometer of an interferometer spectrometer. This results in a position reference which can be retained over many scans while the position referencing or gauging system can be incorporated into the interferometer which generates the spectrum.

Bidirectional referencing is provided which allows bidirectional scanning, while a start of measurement "marker" can be set at any point in the interferogram which allows any part of the interferogram to be scanned. Additionally, the number of reference marks provided by the present invention is double that of "white light" systems which allows synchronized sampling over twice the spectral range.

The present invention has particular application to an interferometer spectrometer of the type employing a Michelson interferometer.

For the purpose of this specification and claims, a Michelson or Michelson-type interferometer is one wherein a beam splitter is employed to establish first and second interferometer paths, at least one of those paths having a movable reflecting element for altering the optical length of that path. Typically, the reflecting element will be a planar member referred to herein as a mirror.

However, other reflecting elements, such as corner cubes or other types of retro reflectors, may be employed within the scope of the present invention. Specifically, the present invention provides an improvement in the gauging of the position of the movable mirror.

Typical prior art interferometer spectrometers include a source of analytical radiation, a Michelson-type interferometer operative on that radiation, including a beam splitter and at least one movable mirror, and a system for gauging the position of the movable mirror. The improvement of the present invention comprises quadrature phase discrimination means having an optical path which includes the beam splitter and movable mirror to provide signals indicative of the position and direction of movement of that portion of the movable mirror within the path of the analytical radiation. The optical path of the quadrature phase discrimination means may be made coaxial with the analytical radiation path associated with the movable mirror.In this manner, the mirror position gauged by the quadrature system is an average position of that portion of the mirror upon which the analytical radiation impinges. Alternatively, the position of three portions of the mirror may be determined to establish the plane of the mirror. From this, the average position of that portion of the mirror within the analytical radiation beam may be determined. In this last instance, one of the positions determined can be a position coaxial with the analytical radiation beam.

In many commercial interferometer spectrometers, the choice of beam splitter is dependent upon the analytical radiation desired.

Since the quadrature system described above employs a beam splitter in common with the analytical radiation, changing of the beam splitter for a different analytical radiation will result in a loss of reference by the quadrature system. In this instance, the present invention provides an initial reference indicative of the position of the movable mirror which is independent of the beam splitter and by which the quadrature system initial conditions may be established. In a preferred embodiment, the initial reference may be established by a further quadrature phase discrimination system. Indeed, the further quadrature phase discrimination system may be employed redundantly with the primary quadrature system which employs a common beam splitter with the analytical radiation, with an appropriate system for arbitrating between any variation in their position indications.

Qudarature systems for distance measurement are known to the prior art. An example of one such system is disclosed in U.S. Pat. 3,409,375 issued Nov. 5, 1968 to Hubbard for "Gauging Interferometer Systems", which is hereby incorporated by reference.

Brief Description of Drawings Fig. 1 illustrates the interferometer portion of a prior art interferometer spectrometer employing a Michelson interferometer with planar reflecting surfaces; Fig. 2 illustrates the incorporation of a quadrature phase discrimination system within the optical system of the prior art spectrometer illustrated in Fig. 1; Figs. 3-5 illustrate preferred embodiments of some of the optical elements of the prior art spectrometer illustrated in Fig. 1, specifically adapted to embodiments of the present invention; and Figs. 6 and 7 illustrate modifications to a portion of the embodiment of Fig. 2.

Detailed Description of Drawings Fig. 1 diagrammatically illustrates an interferometer of the Michelson-type employed within an interferometer spectrometer known to the prior art. A source of analytical radiation, including associated optical elements for collimating and directing the analytical radiation to the interferometer, is illustrated at 1 0. The path of the analytical radiation through the interferometer is shown by a single line 11 with a circle 12 around the line 11 being used to represent the fact that the collimated beam of analytical radiation has a significant dimension.

Within many commercial instruments of the type being discussed, the collimated radiation beam may have a diameter on the order of two or more inches. Analytical radiation follows the direction of the arrowhead on line 11 to a beam splitter 13 which divides the beam to follow a first path 14, to and from a fixed mirror 15, and along a second path 16, to and from a movable mirror 1 7. The direction of movement of the movable mirror 1 7 is indicated by the arrows 18 while the mechanism for supporting and imparting motion to the mirror 17 is designated generally at 19.

Reflected analytical radiation from the mirrors 1 5 and 17 is recombined by the beam splitter 13 to exit the interferometer along the line 20, the extent of the beam following the line 20 again being indicated by the circle 12. Analytical radiation following the line 20 is directed to a sample compartment, in known manner and for well recognized purposes.

Fig. 2 illustrates the optical elements 13, 1 5 and 1 7 of Fig. 1 together with their adapation in accordance with the present invention. A laser 25 has its output directed to the beam splitter 13 by a reflecting element 26. The beam splitter 13 divides the laser output into the two arms of the interferometer (along two optical paths one of which includes the moving mirror 17 and the other of which includes the fixed mirror 15). Laser radiation reflected by the mirrors 1 5 and 1 7 is recombined by the beam splitter to travei along a path indicated at 27, to a detector 28. A circle 29 is illustrated in Fig. 2 to show the size of the laser beam relative to the analytical radiation beam indicated by a circle 12.

Typical beam splitters are linearly dichroic. The effects of beam splitter dichroism can be "nulled" by proper laser selection and orientation in a manner known to the prior art. However, a retardation is induced in one polarization of the laser output by a retardation plate positioned in one arm of the interferometer. In the embodiment of Fig. 2, the retardation plate is illustrated at 27 with its thickness being selected to establish the desired retardation. The detector 28 includes a polarizing beam splitter which separates the polarizations of the combined beams and a detector for each polarization.

The system shown in Fig. 2 illustrates a quadrature system by which phase discrimination may be employed to establish the position and direction of movement of the mirror 17, relative to the mirror 1 5, through the use of suitable counters associated with the detector 28. For example, if the retardation plate 27 is a 1/8 wave plate, the output from the two detectors within detector 28 will be sine waves displaced from each other by approximately 900. With one direction of mirror travel, one detector output will lag the other but will lead the other during mirror movement in the opposite direction. It is within the skill of one ordinarily skilled in the art to employ these signals and phase relationships to discriminate between travel directions and to employ a bidirectional counter to maintain a count indicative of the position of the movable mirror 1 7.

When superimposed on the analytical radiation system of Fig. 1 , the referencing or gauging system of Fig. 2 has the laser beam of Fig. 2 and analytical beam of Fig. 1 generally coaxial with each other. In this manner, the output signals from the detector 28 are indicative of the position and direction of movement of that portion of the mirror 1 7 within the analytical radiation beam.

That is, assuming that the mirror 17 is planar, the geometrical center of the analytical beam will be at the "average" position of that portion of the mirror upon which the analytical beam falls. Thus, any "tilt" in the mirror 1 7 may be compensated for by gauging the position of the geometrical center of the analytical beam on the mirror. This is accomplished with a coaxial system.

It has been found that the small size of the laser beam relative to the analytical radiation beam does not unduly impact upon the sensitivity of the analytical instrument. The detector 28 and reflecting element 26 may be configured to largely restrict laser radiation to the interferometer portion of the instrument, while the analytical radiation "passes around" them.

The retardation plate 27 may be positioned in either arm of the interferometer and supported in any desired manner. In the illustrated embodiment, the plate 27 is directly secured to the mirror 15, as by gluing, which eliminates any supporting structure and its obstruction of the analytical beam.

A desired analytical radiation may require a change in beam splitter. Since the quadrature phase discrimination system of Fig. 2 employs a common beam splitter with the analytical radiation of the instrument, a change in beam splitter results in a loss of reference in the quadrature phase discrimination system shown in Fig. 2. A beam splitter configuration to overcome this problem, together with a modification of the quadrature system of Fig. 2, is illustrated in Fig. 3 wherein the beam splitter is formed of two beam splitters designated at 30 and 31. Beam splitter 30 corresponds to beam splitter 1 3 of Figs. 1 and 2 and is a common beam splitter for the analytical radiation and laser radiation illustrated in those Figures.The beam splitter 31 is fixed within the instrument to remain in place on removal of the beam splitter 30 and substitution of another.

Thus, the beam splitter 31 may be employed to provide a stable reference with a dual or redundant quadrature phase discrimination system. This may be implemented as illustrated in Fig. 6 with the output of the laser 25 being bifurcated by a beam splitter 32 with one portion passing directly to a reflecting element 33 and the other portion passing to a reflecting element 34 via a reflecting element 35. Laser radiation reflected from the reflecting element 33 in the direction of the arrow 36 may correspond directly to the laser radiation illustrated in Fig. 2, while laser radiation reflected from the reflecting element 34 in the direction of arrow 37 may be directed to the beam splitter 31 into the two arms of the interferometer of Fig. 2.In this instance, the mirror 17 may be configured as illustrated at 40 in Fig. 4 with that portion of the mirror 40 within the analytical path being illustrated by the dashed line 41 and the region of interaction between the laser radiation 36 and mirror 40 being indicated as being coaxial with the region 41 at 42. That is, the laser radiation directed in the direction of arrow 36 (see Fig. 6) impinges on the mirror at 42 after being divided and directed into the interferometer arm containing the mirror 40. A portion of the laser radiation indicated at arrow 37 is directed by the beam splitter 31 to impinge on the mirror 40 as indicated at 43. A mirror arrangement for the "fixed" arm of the interferometer is illustrated in Fig. 5 including a first mirror 45 commonly supported with a second mirror 46.Mirror 45 carries the retardation plate 27 within the path of the analytical radiation which is indicated by the dashed line 47. The point of impingement of the laser radiation on mirror 45 is indicated at 48.

That portion of the laser radiation at 37 that is directed, by beam splitter 31, into the fixed arm of the interferometer impinges the mirror 46 at 49.

The configuration illustrated in Fig. 5 allows the retardation plate 27 to be affixed to the mirror 45 and aligned or oriented relative to the laser radiation by rotation of the mirror 45, without displacement of a reflecting surface from the region of mirror 46. The single mirror construction of Fig. 4 ameliorates positional errors that might result from dual movable mirrors. In some instances, the nature of the analytical radiation, or the beam splitter it requires, will restrict the utility of a laser quadrature phase discrimination system in common with that beam splitter. In this instance, the beam splitter may be provided with a region specially adapted to function as a laser beam splitter with the surrounding area serving as a beam splitter for the analytical radiation. Such a region is illustrated at 50 in Fig. 3.Of course, a separate detector, such as that illustrated at 28 in Fig. 2, will be required for the laser radiation forming the separate quadrature system associated with the beam splitter 31 including appropriate processing circuitry.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, the two quadrature systems represented by the modified embodiment of Fig. 6 may be employed to provide an initial reference via the quadrature system associated with laser radiation 37 with the quadrature system associated with laser radiation 36 providing the primary gauging function.

Alternatively, the two systems may be employed redundantly with appropriate arbitration approaches being employed in the event of variations between them. Of course, an initial reference may be established in a manner other than through the use of a second quadrature system. As a further alternative, the average position of the movable mirror portion within the optical path of the analytical radiation may be established by determining the position of three points on that mirror and establishing an average position relative to the analytical radiation path.

One or more of the laser radiation paths may lie within the analytical radiation path or all may lie without it. One of the three laser radiation paths may be coaxial with the analytical radiation path.

A fourth laser radiation path may be provided to cooperate with a distinct beam splitter, such as that illustrated at 31 in Fig. 3, with the other three laser radiation paths each providing a distinct quadrature system path and sharing a common beam splitter. The implementation of such a system is illustrated in Fig. 7 which includes beam splitters 51-53 and reflecting elements 54-57.

The laser radiation from elements 53-55 may be employed within separate quadrature systems having a common beam splitter with the analytical radiation, while a quadrature system based on the laser radiation from optical element 56 may employ a separate beam splitter. When multiple quadrature systems are employed, multiple lasers may be also employed. However, for reasons well known to the prior art, a single laser and beam splitters is preferred. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

1. In an interferometer spectrometer of the type having a source of analytical radiation, having optical elements forming a Michelson-type interferometer operative on said radiation, said optical elements including beam splitter means for directing said analytical radiation along first and second interferometer paths and movable reflecting means within one of said first and second paths for altering the optical length of said one path, and having means for gauging the position of said movable reflecting means, the improvement wherein said gauging means comprises quadrature phase discrimination means having an optical path including said beam splitter and movable reflecting means for providing signals indicative of the position and direction of movement of that portion of said movable reflecting means within said one path.

2. The interferometer spectrometer of Claim 1 wherein said quadrature phase discrimination means comprises means having an optical path at least a portion of which is generally co-axial with said one path.

3. The interferometer spectrometer of Claim 2 wherein said gauging means comprises further means for providing signals indicative of the position and direction of movement of said movable reflecting means independently of said beam splitter.

4. The interferometer spectrometer of Claim 3 wherein said further means comprises quadrature phase discrimination means.

5. The interferometer spectrometer of Claim 2 wherein said gauging means comprises further means for providing an initial reference indicative of the position of said movable reflecting means independently of said beam splitter.

6. The interferometer spectrometer of Claim 5 wherein said further means comprises quadrature phase discrimination means.

7. The interferometer spectrometer of Claim 1 wherein said quadrature phase discrimination means comprises means for providing signals indicative of the position and direction of movement of portions of said movable reflecting means outside that portion of said movable reflecting means within said one path.

8. The interferometer spectrometer of Claim 7 wherein said quadrature phase discrimination means comprises means having an optical path, at least a portion of which is generally co-axial with said one path.

9. The interferometer spectrometer of Claim 8 wherein said gauging means comprises further means for providing signals indicative of the position and direction of movement of said movable reflecting means independently of said beam splitter.

1 0. The interferometer spectrometer of Claim 9 wherein said further means comprises quadrature phase discrimination means.

11. The interferometer spectrometer of Claim 7 wherein said gauging means comprises further means for providing an initial reference indicative of the position of said movable reflecting means independently of said beam splitter.

12. The interferometer spectrometer of Claim 11 wherein said further means comprises quadrature phase discrimination means.

1 3. The interferometer spectrometer of Claim 7 wherein said signal providing means comprises means for providing signals indicative of the position and direction of movement of three portions of said movable reflecting means.

14. The interferometer spectrometer of Claim 1 3 wherein said gauging means comprises further means for providing signals indicative of the position and direction of movement of said movable reflecting means independently of said beam splitter.

1 5. The interferometer spectrometer of Claim 1 4 wherein said further means comprises quadrature phase discrimination means.

1 6. The interferometer spectrometer of Claim 1 3 wherein said gauging means comprises further means for providing an initial reference indicative of the position of said movable reflecting means independently of said beam splitter.

1 7. The interferometer spectrometer of Claim 1 6 wherein said further means comprises quadrature phase discrimination means.

1 8. The interferometer spectrometer of Claim 13 wherein said three movable reflecting means portions are outside said one path.

1 9. The interferometer spectrometer of Claim 18 wherein said gauging means comprises further means for providing an initial reference indicative of the position of said movable reflecting means independently of said beam splitter.

20. The interferometer spectrometer of Claim 1 9 wherein said further means comprises quadrature phase discrimination means.