DE4136300C1 - Michelson type interferometer using single rotating retroreflector - with aperture receiving split light beam halves deflected against each other by mirrors - Google Patents

Abstract

A Michelson interferometer contains two plane mirrors (2a,2a';2b,2b') with a rotating retroreflector (1) and associated drive motor, two deflection mirrors (3a,3b'), a beam splitter (4), a collimating lens (5), a detector (6) and a laser reference unit. The retroreflector's rotation axis is laterally offset w.r.t. its triple point. The single retroreflector for both interferometer branches is arranged so that the half beams (100a', 100b') from the splitter are deflected into regions of its aperture which are opposite each other relative to the retroreflector's axis of rotation. The optical axes (10a',10b') of both beam halves are inclined w.r.t. each other by twice the inclination angle (alpha) of each w.r.t. the retroreflector's axis. USE/ADVANTAGE - Fourier transformation spectroscopy. High resolution achieved at relatively low equipment cost.

Description

The invention relates to an interferometer according to Michelson according to the preamble of the claim. Such Michelson interferometers are used in Fourier transform spectroscopy (FTS) used in which optical path differences due to rotating retroreflectors be generated.

These are Michelson interferometers, which are also called "Fourier spectrometers" are known, in which optical Path differences generated by rotating retroreflectors be (for example, DE 40 05 491 A1). It will be eccentric and inclined "nutating" retroreflectors used. For generating larger optical path differences, So a higher spectral resolution, two or more more retroreflectors in one - based on the optical Path change in both interferometer arms - asynchronous rotation offset, with a fixed phase relationship of the various Rotational movements must be complied with each other.

The well-known Michelson interferometers with rotating retroreflectors  However, they have the following disadvantages:

• 1. To generate a high spectral resolution, the Interferometers are not operated with just one reflector, but there are two or more of these relatively expensive ones Elements necessary.
• 2. Complex measures must be taken to ensure compliance with the fixed phase relationship of the various rotational movements of the two or more retroreflectors. A very advantageous solution is the drive of each retroreflector with its own stepper motor and the "electrical coupling" of all motors with each other by means of only one common control clock for all engines. However, this in turn has the following disadvantages: The mechanical vibrations caused by the stepping operation of the motors are transmitted to the entire structure in a manner which interferes with the measuring signal; These vibrations must therefore be remedied by additional complex, mechanical damping measures. Furthermore, the entire effort is increased by each additional drive motor. In addition, stepper motors are generally more expensive than comparable DC motors, which are sufficient for operation with only a retroreflector.
  When two or more retroreflectors are driven by only one motor, a clutch via a transmission is required. As a result, in turn, the effort is greater and by a not completely excludable gear play and vibration additional sources of interference in the structure.
• 3. Through each additional retroreflector perform its optical Error to an additional deterioration of quality the optical signal of the device. This could only be be avoided by retroreflectors extremely high quality; The result would be an extremely expensive device.  
• 4. To the known interferometers with only one rotating Retroreflector undesirably large, mechanical dimensions to avoid the device is also in the second interferometer installed a fixed retroreflector (however), through which especially the optical path folded is, and thus allows a small mechanical size becomes. This, however, all result in the above already described disadvantages, which are due to the use a second retroreflector are due.

The invention is based on the object to create an interferometer to Michelson, that with relatively little technical effort high resolution allows.

According to the invention, this is an interferometer according to the preamble of the claim by the Characteristics achieved in its characterizing part.  

The inventive embodiment of an interferometer contains thus only a rotating retroreflector; the height, achieved with a low technical effort, spectral Resolution corresponds to that, which so far only with two rotating retroreflectors was reached. Despite the low Efforts are also the possible sources of interference reduced. Since both halves of the beam through the same reflector run, they are "optically coupled". Therefore, the drive can with a simple DC motor without any interference causing gearbox or any faults Stepping controls are done and is therefore also technical less expensive.

The invention will be described below with reference to preferred embodiments with reference to the accompanying drawings explained in detail. It shows

Fig. 1 in plan view a schematic representation of an embodiment of the interferometer according to the invention, and

Fig. 2 also in plan view a schematic representation of another embodiment of an interferometer according to the invention with optimized beam path.

In Fig. 1, a rotatable retroreflector 1 is shown in two extreme, solid or dashed reproduced rotational positions in plan view, the axis of rotation 7 is offset relative to the triple point of the retroreflector 1 laterally by a distance L. Further, two plane mirrors 2 a and 2 b, each with a concentric bore 2 a₁ and 2 b₁ and two deflecting mirrors 3 a and 3 b shown. In addition, optical axes 10 a and 10 b are registered, as well as a beam splitter 4 , a converging lens 5 and a detector 6 reproduced. The angle of inclination of the optical axes 10 a and 10 b relative to the axis of rotation 7 is denoted by α.

The interferometer in Fig. 1 also has the following, for reasons of clarity there not reproduced components: a (not shown) drive motor which rotates the retroreflector 1 in a known manner about the rotation axis 7 , and a known laser reference, the a laser, for example in the form of a HeNe laser and a laser detector in the form of a radiation-sensitive silicon diode.

The incident on the beam splitter 4, for example, at 45 ° radiation is split into two to the two optical axes 10 a and 10 b respectively symmetrical halves 100 a and 100 b, which impinge on the plane mirror 3 a and 3 b are reflected by them through the holes 2 a₁ and 2 b₁ of the plane mirror 2 a and 2 b pass and enter on each of different sides of the axis of rotation 7 in the reflector 1 . The beam halves leave the retroreflector each parallel to the direction of entry, meet perpendicular to the plane mirror 2 a and 2 b, are reflected back by these in itself and therefore run on the same path back to the beam splitter. 4

At the beam splitter 4, the two beams recombine in a known manner and their sum is focused by the converging lens 5 on the detector 6 . Path differences between the two reference surfaces of the beam halves come about by the fact that change the path lengths for the two beam halves 100 a and 100 b in opposite directions by the rotation of the retroreflector 1 , wherein the mirror surfaces 2 a and 2 b serve as reference surfaces.

The arrangement in Fig. 2 corresponds in all details to that in Fig. 1 except for the following modification:

In Fig. 2, an incident beam impinges on the beam splitter 4 at an angle of 30 ° to the solder. Therefore, the optical axes 10 a 'and 10 b' of the two beams when leaving the beam splitter 4 form an angle of 120 °. Therefore, the beams 100 a 'and 100 b' at an angle which is steeper by 7.5 ° than in the embodiment of Fig. 1, on the deflecting mirrors 3 a 'and 3 b' to meet at the angle α the axis of rotation 7 enter the retroreflector 1 . In this way, smaller deflecting mirrors 3 a 'and 3 b' can be used and the polarizations can be reduced at these.

In the arrangements shown in FIGS. 1 and 2, the spectral resolution can be adjusted mechanically by changing the lateral offset L of the axis of rotation 7 relative to the triple point of the retroreflector 1 . When changing the inclination angle α for adjusting the spectral resolution, the other components must be readjusted, in particular the optical axes 10 a, 10 b or 10 a ', 10 b' must in any case be perpendicular to the mirror surfaces of the deflecting mirrors 2 a, 2 b or 2 a ', 2 b' stand.

Below is an example of the sizing of the given the most important parameters:

Usable aperture of the retroreflector | 12.7 cm Lateral offset L of the axis of rotation 21 mm Angle α between the optical axes and the axis of rotation 18 ° Achieved maximum path difference about 10 cm Achieved spectral resolution better than 0.1 cm ^-1

Claims (1)

Michelson interferometer, comprising two plane mirrors ( 2 a, 2 b; 2 a ', 2 b') with a rotating retroreflector ( 1 ) with associated drive motor, the retroreflector axis of rotation ( 7 ) being laterally opposite the triple point of the retroreflector ( 1 ) is offset, with two deflection mirrors ( 3 a, 3 b, 3 a ', 3 b'), a beam splitter ( 4 ), a converging lens ( 5 ), a detector ( 6 ) and with a laser reference unit with laser and laser detector, characterized in that the rotating retroreflector ( 1 ) is arranged as a single retroreflector for both interferometer branches in such a way that both divided by the beam splitter ( 4 ) and each on a deflecting mirror ( 3 a, 3 b, 3 a ', 3 b ') incident beam halves ( 100 a, 100 b, 100 a', 100 b ') in the aperture portions of the single retroreflector ( 1 ) are directed, with respect to the retroreflector axis of rotation ( 7 ), facing each other, wherein the optical axes ( 10 a, 10 b, 10 a ' , 10 b ') of both beam halves ( 100 a, 100 b; 100 a ', 100 b') are inclined relative to each other by an angle of 2α and relative to the retroreflector axis of rotation ( 7 ) by an inclination angle α.