Monochromator arrangement

A monochromator apparatus comprises a monochromator (9) and a reversing device (2) which, with the aid of four reversing mirrors (10, 11, 17, 18) shifts the exit ray (13), of a monochromator (9) introduced in an intermediate focus of an external path of rays (4, 5), in the external path of rays (5) so that the external path of rays (4, 5) will not be influenced by the presence or absence of the monochromator apparatus (2, 9).

The invention concerns a monochromator arrangement with an adjustable grid 
monochromator whose path of rays features an entrance focus and an exit 
focus which relative to the entrance focus is offset sideways. 
Known from the German patent application No. 34 00 299 C 2 and Fresenius Z 
Anal. Chemi (1984) 317:347-349, such monochromator arrangements are 
employed in the use of adjustable infrared diode lasers for 
high-resolution spectroscopy, for instance in the quantitative gas 
analysts, in order to separate the spectrum emitted by the diode laser in 
individual laser modes respectively laser frequencies while blanking out 
the remaining modes. High-resolution grid monochromators are used for mode 
selection. 
In prior monochromator arrangements it is for adjustment purposes necessary 
to move a plane mirror within the monochromator arrangement before the 
diffraction grid. Since such a mirror is of necessity arranged before the 
diffraction grid, the illumination of the following lenses will change 
when the mirror is moved in. An optimization of the lighting is thus not 
achievable for all cases. 
Basing on this prior art, the problem underlying the invention is to 
provide a monochromator arrangement which retroactively can be inserted in 
an external path of rays of a spectrometer that features an intermediate 
focus, after the spectrometer has already been adjusted, where this 
adjustment is fully retained. 
This problem is inventionally solved in that with the grid monochromator 
there is coordinated a reversing device that features a flat reversing 
mirror through which the entrance focus and the exit focus are reproduced 
in one and the same intermediate focus of an external path of rays that 
extends transverse to the connecting line between the entrance focus and 
the exit focus of the grid monochromator. 
In the inventional monochromator arrangement, adjustment and operation of 
the entire spectrometer arrangement are possible without monochromator. 
In the inventional monochromator arrangement, the entrance and exit ray are 
so reversed by the reversing mirror that the entrance focus and the exit 
focus will coincide, with the entrance and exit directions being 
identical. In this way it is possible to operate the monochromator 
arrangement in transparency in an intermediate focus of an optical laser 
spectrometer setup, without influencing the external path of rays. 
In accordance with the instant invention, the diffraction grid is utilized 
twice, and under a large angle in the monochromator arrangement, for 
increasing the dispersion. Accomplished thereby is a sufficiently high 
resolution for mode selection also with a small grid of only about 30 mm 
edge length. Owing to the possibility of requiring only a small grid, the 
monochromator arrangement can be built very small, enabling a closely 
spaced serial arrangement. An extra-axial parabolic mirror is used in the 
monochromator as a collimator in order to obtain a reproduction that is 
limited in diffraction and free of astigmatism. 
Owing to the reversing device of the monochromator arrangement it is 
possible to move the entrance focus of the monochromator at the place of 
an intermediate focus of the external path of rays and to reproduce the 
exit focus of the monochromator, by reflection, as well in the same 
intermediate focus in such a way that the monochromator will restore the 
external path of rays and appears from outside the same as an aperture 
with a spectral filter effect. The reversing device is an integral part of 
the monochromator arrangement. By means of a centering pin it is possible 
to introduce the monochromator arrangement in reproducibly accurate 
fashion in a free intermediate focus of the external spectrometer 
arrangement. 
Suitable developments and designs of the invention are characterized in the 
subclaims.

The monochromator arrangement illustrated in FIG. 1 comprises a 
monochromator housing 1 and a reversing device housing 2 which is designed 
as a plug-in part and provided, on its bottom end in FIG. 1, with a 
centering pin 3. 
To the left and right of the reversing device housing 2, a converging and 
entering external path of rays 4 and a diverging exiting external path of 
rays 5 can be seen. The converging external path of rays 4 originates from 
the light of an adjustable infrared laser, which is not illustrated in the 
drawing and is used in high resolution spectroscopy, for instance in the 
quantitative gas analysis. The light of the laser not illustrated in the 
drawing is focused on an intermediate focus 6 which is coordinated with 
the converging external path of rays 4 and the diverging external path of 
rays 5. The latter proceeds to a measuring section not illustrated in the 
drawing and finally to a detector not illustrated in the drawing. 
The intermediate focus 6 is located within the reversing device housing 2, 
as can be seen best from FIGS. 2 and 3, in the opening of the entrance 
aperture 7 of the monochromator arrangement. The opening of the entrance 
aperture 7 likewise forms the entrance focus 8 for the monochromator 9 
arranged in the monochromator housing 1. The longitudinal axis of the 
centering pin 3 extends through the intermediate focus 6 and the entrance 
focus 8. 
When the monochromator arrangement illustrated in FIG. 1 is removed from 
the path of rays of the not illustrated laser spectrometer, such will not 
change the converging external path of rays 4 illustrated in FIG. 1, 
bottom, and the diverging external path of rays 5 in its position. The 
monochromator arrangement illustrated in FIG. 1 acts therefore the same as 
an aperture with spectral filter effect. 
Once the light of the converging external path of rays 4 has passed the 
entrance aperture 7 in the reversing device housing 2, the light in the 
diverging external path of rays 5 leaving the entrance aperture 7 proceeds 
in the direction of propagation of the light, shortly behind the entrance 
aperture 7, onto a first reversing mirror 10 which, in FIG. 1, together 
with a second reversing mirror 11 has in side view the shape of the letter 
X and can be seen more clearly in FIGS. 2 and 3. The first reversing 
mirror 10 deflects the laser light, in FIGS. 1 and 2, by 90.degree. 
upward, the deflected ray being marked 12 in FIGS. 1 and 2. The deflected 
ray 12 forms the entrance ray for the monochromator 9 arranged in the 
monochromator housing 1. The exit ray 13 of the monochromator 9 is 
illustrated in FIGS. 1, 2 and 4 in a fashion overlapping the entrance ray 
12 although, as can be seen best from FIGS. 3 and 5, the exit ray 13 is 
offset transverse to the direction of propagation of the external path of 
rays 4, 5, relative to the entrance ray 12, as it leaves the monochromator 
9, so that the exit ray 13 impinges on the second reversing mirror 11 
which, as illustrated in FIG. 3, is offset sideways and transverse to the 
direction of propagation 14 of the external path of rays 4, 5 by an amount 
V. 
Corresponding to the lateral offset of the second reversing mirror 11, an 
exit aperture 15 is provided in the housing 2 of the reversing device, 
offset relative to the entrance aperture 7. As can be seen from FIG. 3, 
the exit aperture 15 is offset relative to the entrance aperture 7, 
transverse to the direction of propagation 14, by the same amount V as the 
second reversing mirror 11. Besides, the exit aperture 15 is shifted by 
the amount A in the direction of propagation 14, the amount A equaling the 
offset V. 
The exit focus 16 of the monochromator 9, in properly adjusted condition, 
is located exactly in the opening of the exit aperture 15. 
As can be seen best from FIG. 3, the second reversing mirror 11, which 
relative to the first reversing mirror 10 is tilted by 90.degree., 
deflects the exit ray 13 of the monochromator 9 to a third reversing 
mirror 17 which reflects the exit ray 13 of the monochromator 9 opposite 
to the direction of the offset of the second reversing mirror 11 relative 
to the first reversing mirror 10. 
A fourth reversing mirror 18 is arranged in the reversing device housing 2 
in the extension of the diverging beam impinging on the first reversing 
mirror 10 and coinciding in its position with the diverging external path 
of rays 5, said mirror deflecting the beam leaving the third reversing 
mirror 17 by 90.degree. in the direction of propagation 14, so that the 
beam leaving the fourth reversing mirror 18 has the same position as the 
diverging external path of rays 5. The mirror arrangement in the reversing 
device housing 2 thus causes, for one, that the entrance focus of the 
monochromator 9 is being moved to the location of the intermediate focus 6 
of the external path of rays 4, 5 and, for another, that the exit focus 16 
of the monochromator 9 is being reproduced by mirror reflection on the 
reversing mirrors 11, 17 and 18 as well as on the same intermediate focus 
6, so that the monochromator arrangement leaves the external path of rays 
4, 5 unchanged. 
Following the description of the reversing device contained in the 
reversing device housing 2, the structure of the monochromator 9 in the 
housing 1 shall be discussed now. To that end, reference is made, in 
addition to FIG. 1, to FIGS. 4 and 5. 
The entrance ray 12 of the monochromator 9 proceeds first to a folding 
mirror 19 which in the monochromator housing 1 is inclined by 45.degree. 
relative to the longitudinal axis of the centering pin 3. Subsequent to 
the reflection on the folding mirror 19, the ray proceeds to an 
extra-axial parabolic mirror which is provided as a collimator mirror and 
mounted in the housing 1 with the aid of an adjustable three-point support 
21. The parabolic mirror 20 collimates the rays to a parallel ray 22 that 
proceeds then to the grid 23. From the grid 23, which by a sine drive 24 
can be tilted about an axis 25 perpendicular to the direction of 
propagation in the external path of rays 4, 5, the ray is diffracted 
toward a plane mirror 26 which with the aid of an adjustable threepoint 
support 27 is mounted inside the housing 1. The plane of dispersion of the 
grid 23 extends parallel with the drawing plane extending between the 
entrance focus 8 and the exit focus 16. In the adjustment illustrated in 
FIG. 1, the incidence on the plane mirror 26 is essentially below the 
drawing plane, which mirror reflects the incident rays obliquely in the 
direction above the drawing plane. Subsequent to the reflection on the 
plane mirror 26, the ray 28 is diffracted a second time on the grid 23 and 
then focused by the parabolic mirror 20 in the exit focus 16, which in 
FIG. 4 is located above the drawing plane, whereas the entrance plane 8 is 
located below the drawing plane in FIG. 4. 
For clarification of the position of the entrance focus 8 and exit focus 
16, FIG. 5 shows an illustration that derives from FIG. 4 viewed in the 
direction of an arrow 28'. 
With a proper grid position, the ray is diffracted toward the parabolic 
mirror 20 already in the first diffraction on the grid 23. But the 
parabolic mirror 20 is so adjusted that the incidence of the focused ray 
can then not be in the regular exit focus 16, thus avoiding an erroneous 
measurement of the laser wave-length. In a regular passage, the ray is 
focused toward the exit focus 16 by alignment of the plane mirror 26. 
As will be evident to the expert from the above description, the entrance 
ray 12 and the exit ray 13 are offset only transverse to the plane of 
dispersion of the grid 23.