DD228352A5 - Interferometer - Google Patents

Abstract

In an interferometer operating according to the Michelson principle, a rotating return emitter (RS) whose rotation axis (DA) and its axis of symmetry (SYA) are either parallel to one another or inclined relative to one another is used as the movable element, so that the two axes (DA, SYA ) do not coincide, that neither of the two axes (DA or SYA) coincides with the optical axis (OA) of the interferometer (IF), and further that one or both of the axes of the retroreflector (RS) are inclined or inclined towards the optical interferometer axis (OA) you are parallel. Further, in the interferometer, as the refractive element, there is provided a non-movable, fixed wedge (K) having a refractive index (nK) which is different from the refractive index (nL) of air. In this case, the fixed wedge (K) between the beam splitter (ST) the retroreflector (RS) and a second fixed mirror is arranged so that in each position of the Rücklestrahlers (RS) coming from the beam splitter (ST) Strahlbuendel (SB) the wedge ( K), which is incident on the return emitter (RS), laterally deflected by the latter, once again passes through the wedge (K) and impinges perpendicularly on the second mirror (S2), reflects therefrom and passes through it on the same back to the beam splitter (ST), where the ray beam interferes with the beam coming from a first fixed mirror (S1). Fig. 2

Description

Title of the invention: 25

interferometer

Field of application of the invention:

The invention relates to an interferometer according to the Michelson principle.

Λ fr · , An "- I i> it II t ~ l

^ Characteristic of the known technical solutions:

Interferometers with refractive elements are known,. in which a path difference is generated by a back and forth wedge of a wedge or two wedges or a prism or two prisms in one arm or in both arms of the interferometer. In Fig. 1 the basic structure of such a device is shown. Here are with

51 and S2 denotes two fixed mirrors of a known InterfelOrometers, which may be formed as a plane mirror or as a triple mirror (reflector). K1 and K2 are two identical wedges (or prisms)., Which are made of a material having a refractive index η which is different from the refractive index η.

15's of air. ST denotes a beam splitter which can be applied as a coating on the rear side of one of the wedges K1 or K2 or which is arranged between the two, mutually opposite surfaces of the wedges K1 and K2. Q denotes a radiation source,

Whose radiation is to be brought into interference. While D denotes a detector with which the interfering radiation is measured.

The optical path through the two interferometer arms is. 25gleich when the distances from the fixed mirrors S1 and

52 are each equal to the beam splitter ST and at the same time the two wedges K1 and K2 are not shifted from each other, i. are arranged mirror-symmetrically to the beam splitter ST. If now one of the wedges, at-

By way of example, the wedge K2, as indicated by dashed lines in FIG. 1, is displaced along the beam splitter in the direction of the wedge tip, for example, then the radiation in both arms of the interferometer traverses paths of different lengths through the air and the wedge material; hereby

Different optical paths then result as long as the refractive index n _{v of} the wedge material is not equal to the refractive index η of air. Thus, by back and forth

Moving one of the wedges K1 or K2 in the manner described different optical paths are generated in the two interferometer without the geometric paths are changed. In these considerations, it has been assumed that the following applies to the optical path d_ through a material with the refractive index η:

^d o; ⁿ · ^d g

where d is the geometric path.

Various embodiments of such interferometers with refractive elements are known; In this case, the path difference is always generated by a reciprocation of one or more optical elements. This movement (s) must (must) be carried out with great precision, which is why a great effort in the storage and the drive of the moving elements is required.

Be ± .gegenwärtig verwen- Interferometry in Practice deterministic methods and apparatus with refractive elements is therefore considered to be disadvantageous in that

a) floats are performed,

b) the measuring speed is limited for this reason, c) continuous measurements are not possible, and

d) a relatively large effort is needed.

To explain the above-mentioned points a) to

c) the moving elements must be constantly accelerated alternately and then decelerated again to a standstill. Another disadvantage of the conventional methods and devices is that due to the necessary storage of the moving elements in general, only one operation of the interferometer in a horizontal position is possible, but at least an operation in any position is not possible.

- 4 - Aim of the invention;

The invention is therefore an interferometer after the. Michelson principle be improved as much as possible avoid the disadvantages and difficulties mentioned in such a way that with less effort temporally gapless and continuous spectral measurements at a very high speed in any position of the interferometer are feasible without back-and-forth HerbewelOgungen are required ,

Explanation of the essence of the invention;

In such a Michelson principle interferometer is used as a moving element, a rotating reflector whose axis of rotation and its axis of symmetry are either parallel to each other or inclined against each other, so that the two axes do not coincidej and that neither of the two axes with the optical axis of the interferometer coincides. Further, one or both of the axes of the retroreflector must be inclined or parallel to the optical axis of the interferometer and the axis of rotation of the retroreflector must pass through its reflective area. In addition, the optical

In each position of the rotating reflector, the 25 axis of the interferometer impinge on a reflecting surface of the reflector. As a refractive element in this interferometer, a non-movable, stationary wedge is provided with a refractive index, which is not equal to the refractive index of air. Finally, a stationary wedge between the beam splitter, the rotating reflector and the second fixed mirror is still arranged so that in each position of the rotating reflector a beam coming from the beam splitter passes through the wedge, then impinges on the reflector, laterally offset from this reflected once again at another point passes through the wedge and perpendicular to the second fixed

Mirror hits, reflected from this and then backwards through the same way, the beam splitter, where then the beam interferes with the coming of a first solid mirror beam. 5

Furthermore, the wedge acts like a plane-parallel plate when using a triple mirror as a retroreflector for every two successive passes of the beam. In a parallel arrangement of the three axes, namely the axis of rotation and the axis of symmetry of the retroreflector and the optical axis of the interferometer, in each position of the rotating retroreflector, the polarization of the reflected radiation caused by this is the same.

The invention will be explained in detail with reference to embodiments with reference to the accompanying drawings. Show it:

Fig.1.1 is a schematic sectional view of a

conventional interferometer with refractive elements;

25Fig. 2 is a schematic sectional view of a

preferred embodiment of an interferometer according to the invention;

Fig. 3 is a schematic plan view of the embodiment of Fig. 2 and

Fig. 4 in plan view a further embodiment of the invention.

Embodiment:

According to a preferred embodiment of the invention is in conjunction with a conventional beam splitter ST, a fixed plane mirror Sl and a second fixed plane mirror S2 as a movable mirror of an interferometer IF a retroreflector RS, for example in the form of a triple mirror, a retroreflector, etc. used; this retroreflector RS is designed to rotate about a rotation axis DA at a predetermined desired speed; the axis of rotation DA of the retroreflector RS extends parallel to the optical axis OA of the interferometer IF and is offset laterally relative to this (OA). In addition, the axis of rotation DA of the retroreflector RS also runs parallel to an axis of symmetry SYA of the retroreflector RS and is offset laterally to this (SYA). The parallelism of the three axes DA, OA and SYA is convenient for a simple construction and to simplify and to increase the clarity of the representation; but it is not necessary for the function of the interferometer. However, it is necessary that the three axes DA, OA and SYA or at least two of them do not coincide.

As an element producing a path difference, between the retroreflector RS and the second fixed mirror S2, there is a wedge K (a prism) of refractive material whose refractive index n "is different from the refractive index n _L. This wedge K is expediently arranged so that its plane of symmetry SE, ie the plane which is formed by its refractive edge BK and its perpendicular projection onto the base surface B of the wedge K or the prism, is perpendicular to the plane passing through the optical plane Axis OA of the interferometer IF and the axis of rotation DA of the retroreflector RS is clamped when both - as in the present case - are prallel, and which thus lies parallel to the second fixed mirror S2. For non-parallel course of the axes OA and DA should

the plane of symmetry of the wedge K is perpendicular to the optical axis OA of the interferometer IF and thus parallel to the second fixed mirror S ". In principle, it does not matter whether the refractive edge BK, as is the case in FIGS of the wedge K is opposite to the beam splitter ST.

By an appropriate arrangement of the axes OA, DA and SYA and the other parts is to ensure above all that in Figure 2, a beam SB, which comes from the beam splitter ST in the arm with the wedge K and the retroreflector RS,

a) completely passes through the wedge after leaving the beam splitter ST,

b) laterally staggered by the retroreflector RS for the second time through the wedge K, - wherein the beam SB after the second pass through the wedge is again parallel to the optical axis OA of the interferometer IF, so that the distraction caused by the first pass through the second passage has been reversed and thus impinges perpendicular to the second fixed mirror, and

c) is reflected from there, so that the beam (SB) in the same way as from the beam splitter ST to the fixed mirror S2 now runs back in the reverse direction from the fixed mirror S2 on the retroreflector RS to the beam splitter ST. The bundle of rays must therefore pass through the wedge K twice on the "way out" and on the "way back".

After the retroreflector RS (for example a triple mirror) reflects the beam SB (with respect to all sides) in a reversed manner, this arrangement and the two passes through the wedge K in each case ensure that all parts of the beam SB, irrespective of whether they are pointed tapering or wider end of the wedge K impinges, the same distance through the Keilma-.

return the material; the beam SB "sees" in a sense a plane-parallel., Plate, which passes through it twice in four passes through the wedge K (sie., He Fig.2 with respect to the beam SB). In addition, each cause the first two and the last two consecutive passes through the wedge K that the deflection caused by the refraction of the beam SB is reversed, so that therefore the beam SB parallel to the optical axis OA and thus perpendicular to the mirror S2 hits, if this is correct, ie perpendicular to the "fixed mirror Sl and is adjusted at 45 ° to the beam splitter ST, and then returns to its starting point on the beam splitter ST.

In a rotation of the retroreflector RS about its axis of rotation DA, this, with respect to the optical axis OA of the interferometer IF, laterally offset, whereby the beam SB course also undergoes a displacement and thus the second and third passes through the wedge K respectively depending on the position of the retroreflector RS due to its rotation take place elsewhere, so that thus the beam SB has to go through a different wedge thickness. During the rotation of the retroreflector RS, which can take place continuously and expediently should be continuous, thus the beam SB passes constantly in continuous change up to a maximum increasing and then steadily decreasing to a minimum wedge thicknesses and thus correspondingly different optical paths. Because of the two passes through the wedge K, the beam SB "sees" a plane-parallel plate with periodically continuously changing thicknesses. Between maximum and minimum or between minimum _{> (} and maximum, one side of the interferogram (symmetric about maximum-minimum-maximum) is then registered, digitized and converted into the spectrum with the aid of a Fourier transformation in a conventional manner.

From the foregoing, is based on Figures 2 and can readily be seen that major _T ch, the arrangement and dimensioning of the components must be ensured as follows. that is

a) the beam SB after the first pass and the deflection by the wedge K is not reflected back into itself, i. e. the optical axis of the beam SB may not run in any position of the rotating reflector RS through its center of symmetry SZ, (wherein the center of symmetry SZ is the point of the retroreflector RS, which reflects back into itself a ray incident parallel to the symmetry axis SYA); in the case of a triple mirror, is the center of symmetry its apex?

b) in the case of a parallel arrangement of the axes DA, OA and SYA, the optical axis of the radiation beam SB after the first passage through the wedge K must not coincide with the axis of rotation DA. (The optical axis of the beam SB may rather coincide only with the axis of rotation DA, when the axis of rotation DA and the axis of symmetry SYA of the reflector RS are inclined against each other);

c) the wedge K used is so large that each reflected by the retroreflector RS beam must pass through him, and

d) the fixed mirror S2 is so extended that it in turn reflects each reflected by the retroreflector RS beam.

After the above can be derived among other two possible construction designs, which are shown in Figure 3 and 4 in plan view. In FIGS. 3 and 4, USZ _ti denotes a circle on which the center of symmetry SZ of the retroreflector RS revolves on rotation; with ERS is a corner of the retroreflector RS farthest from the axis of rotation DA and UERS is a circle on which the corner ERS rotates on rotation.

- 10 -

In Figure 3, a beam SB is to impinge on the retroreflector RS outside of the circulation circle USZ of the symmetry ZZ SZ. Since the projection of each reflected beam at the retroreflector RS (for example a triple mirror) passes through its center of symmetry SZ, this results in the range which the beam SB can reach at most and which is bounded by the dashed lines BSB; this area must therefore be covered by the second fixed mirror S2; In addition, the wedge K must also cover the area delimited by the lines BSR and additionally also the area covered by the beam SB at the entrance.

4, a beam SB is to impinge on the retroreflector RS within the circulating circle USZ of the center of symmetry SZ. In this case, during rotation of the retroreflector RS, the bundle of rays reflected by it runs, as it were, around the axis of rotation DA and around the incident bundle SB. The fixed mirror S2 and the wedge K thus have to cover the areas around the incident beam SB, the fixed mirror S2 having to be provided with an opening at the point through which the beam SB can also enter; In this case, the opening must have exactly the same diameter as the beam SB. The orientation of the base surface B and the breaking edge BK of the wedge K in the individual arrangements is in principle arbitrary and has only on rotation influence on the dependence of the maximum or minimum of the optical path of the respective position of the retroreflector.

In all embodiments, the two arms of the interferometer IF, namely those with the fixed mirror Sl and those with the rotating reflector RS, in a known manner coordinated so that the path through the two arms is the same length when the path in the arm with the rotating reflector RS is minimal, or the arm with the fixed mirror Sl can be some wavelengths,

- 11 -

and of the largest wavelengths studied, be longer than the minimum of the car by the arm with the rotating reflector RS. The second case is more common, because at the beginning of the measurement an interferogram is obtained on both sides of the symmetry point of the interferometer due to the same path length through both arms of the interferometer, which is used in a known manner for phase correction in the calculation of the spectrum. The path lengths can also be tuned so that a completely symmetric interferogram is obtained.

In the embodiments described above, therefore, without moving part of the apparatus back and forth, a continuous change in the path in an arm of the interferometer is achieved solely by performing a rotary movement of the retroreflector RS. In the interferometric measuring method according to the invention, therefore, the breaking wedge K does not need to be constantly stopped and accelerated again, but the retroreflector RS rotates continuously with a uniform rotational speed. In the apparatus according to the invention, therefore, not only a technically simple and yet precise storage of the. moving mirror, i. to be offset in rotation to be reflected reflector RS with reduced effort, but also the drive and the preferably electronic control of the mirror run with a much lower cost is feasible.

Compared to the known interferometers described at the outset, the essential advantages of the Benfer interferometer according to the invention can be seen in the fact that a) continuous measurements can be carried out,

b) both slow and in particular very fast measurements are feasible,

c) with reduced design effort, a vibration

- 12 -

and shock-resistant interferometer is executable,

d) by the lower cost of electronics and mechanics, a compact, small interferometer can be produced, which can be formed in particular together with a suitable microprocessor as a portable, compact spectrometer system, and

e) because of the simple universal storage of the moving mirror in the form of the rotating reflector RS operation of the interferometer in any position in space is possible;

f) in parallel arrangement of the three axes DA, SYA, OA in each position of the rotating retroreflector RS, the beam SB at the same (or close to the same) angle on the reflecting surface meets, and therefore in each position caused by this polarization of the Radiation is the same (or nearly the same).

As the material for the breaking wedge K, the common materials can be used; however, the spectral range of the particular application must be taken into account, for example, by using optical glasses in the visible range, CaF, KBr, Irtran, etc. in the infrared, etc.

Of course, the interferometer according to the invention can also be formed in accordance with the modifications described in the literature according to the Michelson principle, for example as a polarizing interferometer. An adjustment of the fixed mirror, a path measurement of the mirror position, u.a. can also be carried out here in a known manner; For example, the path measurement can be performed by laser and white light with appropriate detectors in the beam path or by forming a corresponding reference interferometer.

- 13 -

The nature of the retroreflector, the wedge and the second fixed mirror, their geometric dimensions, the inclination and offset of the three axes and the surface quality of the individual elements are to be adapted to the Meßaufgäbe in a conventional manner. The same applies to the mirror storage, the rotation speed and the associated electronics. The wedge K should be operated with respect to the optical axis OA expediently (but not necessarily) at the minimum of the deflection.

Also, the solid mirror Sl may be formed in a known manner as a solid triple mirror (reflector). Basically, the interferometer according to the invention can be used in all previously used interferometer, in which the change in the path difference in any form by reciprocating is achieved. The mirror Sl may also be formed as a combination of a rotating reflector and a refracting wedge with a fixed plane mirror; In this case, possible aberrations can be compensated and larger path length differences can be achieved. Furthermore, the method described may be used to construct any other type of spectrometer, if changing path lengths are required. In general, .dures that the rotating reflector is to be balanced accordingly.

Claims (3)

Invention claim: 1. interferometer according to the Michselson principle / with at least one refractive element, with a first fixed mirror (S1), with a second fixed mirror (S2), with a beam splitter (ST) and with a movable element, characterized in that movable element is a rotating reflector (RS) is used, the axis of rotation (DA) and its axis of symmetry (SYA) are either parallel to each other or inclined to each other - 14 - are such that the two axes (DA, SYA) do not coincide, that neither of the two axes (DA or SYA) coincides with the optical axis (OA) of the interferometer (IF), that further one or both of the axes (DA, SYA ) of the retroreflector (RS) are inclined or parallel to the optical axis (OA) of the interferometer (IF), in that the axis of rotation (DA) passes through the reflecting region of the retroreflector (RS), and in that the optical axis (OA) of the interferometer (IF) in each position of the rotating retroreflector (RS) on a reflective surface of the retroreflector (RS) encounters that is further provided as a refractive element, a non-movable, stationary wedge (K) with a refractive index (n "j, which is different from the refractive index (n _T ) of air, and that the fixed wedge (K) between the beam splitter (ST), the rotating reflector (RS) and the second fixed mirror (S2) is arranged so that in each position of the rotating Reflector ( RS) from the beam splitter (ST) coming beam (SB) acting as a prism wedge. (K) passes, then hits the retroreflector (RS), reflected laterally from this laterally passes through the wedge (K) and perpendicular to the second, fixed mirror (S2), reflected by this (S2) and traverses back to the beam splitter (ST) on the same path where it (SB) interferes with the beam coming from the first fixed mirror (S1). 2. Interferometer according to claim 1, characterized g e k e η n-30zeichnet that the wedge (K) when using a triple mirror acts as a retroreflector (RS) for each two successive passes of the beam as a plane-parallel plate. 3. Interferometer according to claim 1, characterized in that in parallel arrangement of the three axes (DA, SYA, OA) in each position of the rotating - 15 - 1 reflector (RS) is the same caused by this polarization of the reflected radiation. - For this 4 sheets drawings -