JPS6211283B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION An interferometer, preferably a Michelson type interferometer, in which a reflector associated with an interferometer arm is stationary and scanning is performed by a wedge-shaped refractive element on one of the arms. is carried out by the movement of
An interferometer is disclosed in which the direction of orientation of the refractive element and the direction of its movement are specific mathematically derived directions that minimize the translational displacement of the transmitted light beam.

The present invention relates to the field of interferometers, and more particularly to scanning interferometers adapted for use in spectroscopic measurements. More specifically, its primary focus is on the improvement of Michelson interferometers, which are increasingly being used in infrared Hurier transform spectroscopy.

The applicant's earlier U.S. patent application (application number 790457, filed April 25, 1977, titled ``Refractively Scanned Interferometer'') includes An interferometer is disclosed which is carried out by a refractive element without a wedge-shaped cross section, preferably a refractive element having a wedge-shaped cross-section, which is used in conjunction with stationary reflectors of both interferometer arms. As detailed in the specification of the above-mentioned U.S. patent application,
The major advantages of such a configuration include the greatly improved control of motion during scanning, which is primarily useful for stack monitoring, analysis of medical gases, and control of liquid and gas handling. ,
Practically used in Furier transform spectroscopy as an on-line technique such as analysis of gas chromatography sections.

The use of wedge prisms for interferometric scanning can be problematic if the prism orientation and direction of movement are not properly aligned. The problem is the translational or lateral displacement of the light beam during the scanning movement of the prism. This problem is particularly acute when a retroreflector is used as a mirror in the interferometer arm.

The main idea of the invention is to prevent lateral displacement of the light beam due to the scanning movement of the wedge. There is a second form of beam displacement, which is due to wavelength rather than wedge position. In particular, the angle at which the beam of radiation is bent as it passes through the wedge varies depending on its wavelength. This is due to the chromatic dispersion of the refractive index of the wedge. In January 1976 consultant Aaron Kassel's ``Compensated'' Wedge Design for Applicant, he proposed a double wedge configuration, which was designed to be statically compensated. includes a wedge, which has the same top angle as the moving wedge to eliminate chromatic dispersion effects. Such a double wedge configuration addresses the same problems of lateral beam displacement due to wedging motion as Applicant's single wedge design. The concepts covered by this application can be either a single, uncompensated scanning wedge design or a dual wedge design where an additional resting wedge 'compensates' for chromatic dispersion effects. I need.

The present invention achieves the following by using an optimal combination of the arrangement direction and movement direction of the wedge prism.
This substantially solves the above problems. The orientation and movement direction are mathematically derived directions that minimize the displacement of the refracted light beam. More particularly, the wedge prism is oriented and moved such that the apparent displacement points of the parallel radiation extending through the different thicknesses of the prism remain stationary. This condition is achieved by first determining the placement of an imaginary line extending through all such apparent displacement points and then translating the prism so that its tip remains on this line. Can match.

FIG. 1 shows an interferometer coupled to a Hurier transform spectrometer. In this interferometer, the radiation source 11 preferably uses an infrared light to
The radiation is directed towards a beam splitter 13 which directs the reflected beam 15 in one path and the transmitted beam 17 in the other path.

Beam 15 is reflected back toward the beam splitter by a stationary reflector 19, preferably a retroreflector. This reflector determines the path of the radiation within the interferometer arm providing an invariant path.

The other beam 17 is also reflected back towards the beam splitter by a stationary reflector 21, also shown as a retroreflector. However, the path length of the radiation in this arm of the interferometer is modified by the refractor.

The single wedge-shaped member 23 is connected to the beam splitter 1
3 and the mirror 21 as a scanning prism in the path of the beam 17. This wedge-shaped member is movable across the path of the beam in the direction indicated by arrow 24 and serves to change the length of the path by changing the thickness of the refractive material through which the radiation passes. . Suitable drive means 2
5 in combination with the wedge-shaped member 23 to move it back and forth across the path of the beam 17, thereby scanning the effective radiation path.

The reflected beams 15 and 17 are recombined at beam splitter 13 and a portion of the combined radiation is directed towards a suitable detector 27, which is responsive to the radiation intensity. . This strength is
As the refractive wedge is moved across the path of the radiation in one interferometer arm, it changes, thereby causing the radiation in that arm to
That is, it changes the phase relationship with the radiation from the interferometer arm, which does not change in length.

When the interferometer is used for spectroscopic measurements, the sample 29 of the material to be analyzed is placed between the detector 27 and the beam splitter or the light source 1 as shown.
1 and the beam splitter.

The interferometer shown in Figure 1 is a “dual detector”.
such a device is possible by using a retroreflector rather than a flat mirror, using a second detector 31 to detect approximately the source. direction, which is parallel to, but not coincident with, the radiation from the source.The separation of the beam from the source and the beam 33 causes the second detector 31 to If the electrical signals from both detectors are properly measured and summed, the net signal can be zero (i.e., if the zero output is equal to the path length of the detector and the sample is When the sample is placed between the beam splitter and one of the detectors, a signal is obtained that depends only on the properties of the sample. This "dual detector" method This greatly reduces the dynamic range required for the Fuierier transform calculation device.

FIG. 1 also schematically illustrates the components of an electronic system used in a Feller transform spectrometer, which may include a summing amplifier 35, a computer 37, and a spectral display 39.

Next, the essence of the invention will be explained, which is to consider the optimal orientation of the wedge refractive element 23 and the desired direction of movement during scanning.

Second, if the wedge refractive element 23a of the shape shown were to move in the direction of arrow 24a, it would not be properly aligned. The two positions of the wedge 23a during scanning are shown by solid and dashed lines. The solid and broken line beams 41a and 43a are
It shows the displacement of the beam (or light ray) due to a change in the position of the wedge. In this figure, the stationary reflectors are shown as flat mirrors 19a and 21a. With such a flat reflector, beam displacement is generally not a significant problem since each of the rays is directed back along the path of incidence as shown.

However, significant problems arise when using retroreflectors. For reasons detailed in Applicant's earlier U.S. application cited above, the use of retroreflectors is highly desirable, especially when combined with Applicant's wedge refractive elements. desirable. FIG. 3 illustrates the beam displacement problem caused by the use of retroreflectors in combination with refractive scanning elements. Here, the displaced ray 43b (indicated by a broken line) is
After reflection, it follows a different path and does not match the corresponding beam 15b of the other interferometer arm at the beam splitter 13b.

First of all, as can be seen from Figure 3, the situation is not very good, since there is still an overlapping region of the two beams at the beam splitter due to the extended wave front. However, the efficiency of the interferometer becomes progressively worse as the displacement becomes worse. This is especially true if the wave front is curved, as is usually the case. Curved solid line and bent broken line 45 in Figure 3
and 47 indicate a surface of constant optical phase.
With proper alignment, the constant phase surfaces of the beams coming from the two arms are parallel and constructive interference occurs simultaneously across the entire wavefront. On the other hand, if one of the beams is displaced, the surfaces are no longer parallel and an alternative configuration of lines and harmful interference occur. This significantly reduces the size of the resulting interferogram.

The present invention minimizes beam displacement during wedge movement by using an optimal combination of wedge placement and movement directions. The following theoretical process will help achieve a general set of conditions for optimal operation.

FIG. 4 shows that two parallel rays are solid lines 51 and 5.
3 shows a general prism following the path indicated by 3; Each of these lines has three parts,
They are named a, b and c, respectively. The points where the lower line 51 enters and exits the prism are named X ₁ , Y ₁ and X ₂ , Y ₂ . 2
The deflection angles in the two planes are β ₁ and β ₂ .
These can be calculated using Snell's law.

For reference, the center of the coordinate system is the tip of the prism 5.
5, and the X axis 57 is a parallel line 51b inside the prism.
Let's assume that it is parallel to the traveling direction of 53b and 53b. The first prism surface 59 and the second prism surface 61 make angles α ₁ and α ₂ with this X-axis.

If we extend the plotted lines of incidence and rays 51a and 51c with dotted lines 51d and 51e, it will be seen that these lines intersect at an apparent deflection denoted P(X,Y).

If it is desired to avoid displacement of the beam during transmission through the prism, a direction of movement must be found that does not change the apparent deflection point of the individual lines. It should first be noted that if a number of parallel lines are drawn, they will all have an apparent deflection point drawn on a certain line 63. Furthermore, if we draw a line through the prism closer to the tip, we will see that line 63 must pass through the tip.

The manual task is to find the trajectory of the series of points P(X,Y), ie to find the equation of the line 63.
Once this line has been determined, it will be clear that as long as the prism is moved in such a way that the locus of the apparent deflection point is kept fixed within a given space, the exit path will not change. . This will be the case as long as the trajectory is straight, the prism does not rotate, and the movement is such as to keep the tip on the initial trajectory 63.

The solution is as follows. First, note that the equations for the incident ray and radiation are as follows.

(y−Y ₁ )=tanβ ₁ (x−X ₁ ) (1) and (y−Y ₂ )=tanβ ₂ (x−X ₂ ) (2) “P” is the only one common to the two lines. Therefore, by solving (1) and (2), we can simultaneously obtain X and y using other parameters. Each of these parameters can be initial conditions or obtained from Snell's law.

The condition is simplified by noting that the X-axis 57 is parallel to the optical path within the prism. In this case Y ₁ =Y ₂ =Y and equations (1) and (2) are as follows.

y-Y=B ₁ (x-X ₁ ) (3) and y-Y=B ₂ (x-X ₂ ) (4) where B ₁ = tanβ ₁ and B ₂ = tanβ ₂ .

X ₁ and X ₂ are known, and X ₁ = Y/A ₁ and X ₂ =
Y/A, where A ₁ = tanα ₁ and A ₂ = tanα ₂ .

Therefore, equation (3) becomes:

y-Y= _B1 (x-Y/ _A1 ) or y-Y= _-YB1 / _A1 + _B1x or Y( _B1 / _A1-1 )= _B1x-y .

If we form equation (4) in a similar way, divide it, and transform it, we get the following.

x(B ₁ −B ₂ C)=y(1−C) (5) Define C as follows.

C=( _B1 / _A1-1 )/( _B2 / _A2-1 ).

Since B ₁ , B ₂ , A ₁ , and A ₂ are known, equation (5) shows a straight line through the origin. This is the locus of the apparent deflection point.

A useful example is the case of symmetry between the lines where the angle of incidence and the angle of emission and the first prism surface and the second prism surface are equal. Again, X
If we choose the axes parallel to the direction of travel in the prism, then β ₁ = 180° − β ₂ , α ₁ = 180° − α ₂ , and therefore B ₁ = −B ₂ and A ₁ = −A _{2 .} be. C
=1, so X=0. The trajectory will therefore be the Y-axis, in this case the centerline of the prism. This is shown in FIG. 5, where the locus of the apparent deflection point P is on the center line of the wedge prism.

Simply put, the locus of the apparent deflection point is a straight line passing through the tip of the prism. The radiation beam remains stationary as long as the prism is moved in such a way that this trajectory remains fixed within a given space. This could be achieved if the prism was made to move its tip continuously on this trajectory in a pure translational motion. That is, the direction of motion must be along the trajectory. If the tip of the wedge is truncated, the tip is, of course, the point where the extended sides meet.

Returning to the complete interferometer shown in FIG. 1, the orientation and direction of movement of wedge prism 23 is such that the wedge prism 23 is positioned within a predetermined space as it is moved in the direction of arrow 24 for scanning. It is adapted to maintain the apparent deflection point of the prism at substantially the same point. This is ensured by arranging the prism tip to move along a line defined by a series of apparent deflection points of the beam 17 within the moving prism.

The following claims cover not only the specific embodiments described above, but also the inventive concepts described herein in the fullest breadth and interpretation permissible with respect to the prior art. It is something to do.

Certain terms in the claims are used with the following meanings. The term "uncompensated" means that the oppositely angled wedge need not bend the light to its initial direction. The term "wedge" is meant to broadly cover any refractive element that changes the length of the optical path as the "wedge" moves across the optical path. The term ``retro-reflector'' refers to a reflector, regardless of its orientation, such as a ``cube corner,''``cats-eye,'' or ``roof top'' reflector. means a reflector having the property of returning the incident beam in a direction parallel to the direction of incidence.

[Brief explanation of the drawing]

FIG. 1 is a simplified top view of an interferometer incorporating the present invention. FIG. 2 is a schematic diagram showing the displacement of the optical beam that occurs when a wedge refractive element is used for scanning without applying the present invention. FIG. 3 is a diagram similar to FIG. 2, except showing the increased scope of the problem when a retroreflector is used instead of a flat mirror. FIG. 4 explains the mathematical basis of the invention,
Top view of a wedge prism. FIG. 5 is a diagram illustrating a special case of mathematical solution in which the optical angle of incidence and the optical angle of emission of a wedge prism are equal. 11... Reflection source, 13... Beam splitter, 19
... stationary reflector, 21 ... stationary reflector, 23 ... wedge refractive element, 25 ... drive means, 27 ... first detector, 29 ... sample, 31 ... second detector.

Claims (1)

Claims: 1. In a scanning interferometer of the type in which an interference pattern is generated by comparing components of light traveling along a first fixed path and a second variable path, respectively: a first retrostatic reflector that determines the length of the path; a second retrostatic reflector provided at the end of the second path; intersecting the second path; a wedge-shaped prism movable across such a path for scanning a path; the orientation of the prism and the direction of the scanning movement are such that
A scanning interferometer characterized in that the apparent deflection point of each of the light rays traveling through the prism is maintained at substantially the same position over the entire scanning movement of the prism. 2 the angle of incidence and the angle of emission of a beam of light traveling through the prism are equal, the angle of incidence is the angle between the light entering the prism and a line perpendicular to the entrance surface of the prism, and the angle of emission is 2. A scanning interferometer as claimed in claim 1, wherein the angle is between the light exiting the prism and a line perpendicular to the surface of the prism from which the light exits. 3. A claim in which the direction of scanning of the prism is along the center line of the prism, i.e. along an imaginary line through the prism that is equally angularly spaced from the light-incidence surface and the light-emission surface of the prism. The interferometer according to item 2. 4. The scanning interferometer of claim 1, wherein the wedge prism is a single, uncompensated refractive element.