WO2005040740A1

WO2005040740A1 - High-resolution spectrometer

Info

Publication number: WO2005040740A1
Application number: PCT/EP2004/009914
Authority: WO
Inventors: Ross Helmut Becker; Stefan Florek; Günter WESEMANN
Original assignee: Gesellschaft zur Förderung angewandter Optik, Optoelektronik, Quantenelektronik und Spektroskopie e.V.; Gesellschaft zur Förderung der Spektrochemie und angewandten Spektroskopie e.V.
Priority date: 2003-10-10
Filing date: 2004-09-06
Publication date: 2005-05-06
Also published as: DE10347862A1; DE10347862B4

Abstract

The invention relates to a high-resolution spectrometer (10) comprising an entrance slit (12), a dispersing element (16), an optical camera element (14), and a detector system provided with a detector (22). The optical components are arranged in relation to each other in such a way that they are analogous to a Littrow arrangement, and radiation penetrating the spectrometer (10) through the entrance slit (12) can be guided towards the dispersing element (16), by means of the optical camera element (14), reflected back at a small angle (y) by means of the same optical camera element (14), and then focussed on the detector (22). Said spectrometer is also provided with elements (18, 20) for generating a multiple dispersion by means of the dispersing element (16). The inventive spectrometer is characterised in that the means for generating the multiple dispersion comprise at least two flat reflecting surfaces (18, 20) forming a right angle and reflecting the dispersed and focussed radiation (3) back in the direction of the other reflecting surfaces and then in the direction of the dispersing element (16). The reflecting surfaces are arranged in such a way that the entrance slit (12) is located in the line of intersection of the planes defined by the reflecting surfaces (18, 20).

Description

High resolution spectrometer

Technical field

The invention relates to a high-resolution spectrometer with an entrance slit, a dispersing element, camera optics, and a detector arrangement with a

Detector, which are arranged similar to one another in a manner similar to a Littrow arrangement so that radiation which enters the spectrometer through the entrance slit can be conducted onto the dispersing element by means of the camera optics, and then can be conducted back up to a small angle via the same camera optics and can be focused on the detector, means being provided for producing a multiple dispersion by the dispersing element.

State of the art

A high-resolution spectrometer is known from DE 199 61 908 C2, in which

Radiation from an entrance slit is directed onto an Echelle grating by means of camera optics. The Echelle grating is positioned so that the single dispersed beam from the Echelle grating first to one to create a double grating pass Plane mirror is bent. The radiation is reflected back at the plane mirror. The beam then runs again over the grating and back over the camera optics towards a detector. By slightly tilting the plane mirror, the radiation is directed out of the main plane of the beam path. As a result, the location of the spectrum, which is formed from the monochromatic images of the entrance slit, lies above or below the entrance slit. There the spectrum is registered by means of a detector. The beam path can be changed by a suitable grating rotation so that the beam runs back directly from the grating. Then a simple dispersion is realized. The minimum line spacing Δλ, at which two lines of a wavelength λ can still differentiate reliably, is called the resolution R = λ / Δλ. The theoretical resolution R in a grating spectrometer is determined by the total number of grating grooves N and the diffraction order m. With double passage over the grating, an increased spectral resolution can be achieved compared to the single passage.

From the publication "Precise measurements with a compact vacuum infrared spectrometer" by DB Braund, ARH Cole, JA Cugley, FR Honey, RE Pulfrey and GD Reece, an arrangement is known in which the radiation is guided twice over a grating double the passage of a grating, a mirror in the beam path is tilted slightly about an axis lying in the dispersion plane so that the beam runs out of this plane. The return beam then hits a plane mirror. This plane mirror is arranged somewhat above the incoming beam within the beam path The plane mirror directs the beam towards two other mirrors that form a right angle with each other. The beam travels a short way back onto the plane mirror. From there, the beam runs back to the grating. In this way, another becomes Passage realized on the grille h connected to a variety of optical components. Furthermore, the beam is deflected out of the dispersion plane, so that the imaging quality deteriorates.

An arrangement is disclosed in US Pat. No. 6,573,989 B2, in which multiple dispersion is implemented by means of a plurality of gratings. The radiation, which is simply dispersed on a first grating, is diffracted in the direction of a further grating. This further grids are arranged in a Littrow arrangement. The radiation redispersed there runs back into itself. The radiation is then dispersed a third time on the first grating. By rotating the first grating, it can be brought into a position in which the simply dispersed jet runs back directly into itself. It is therefore possible to examine two spectra sections of different widths with different resolutions. This arrangement is expensive due to the use of two grids.

Disclosure of the invention

The object of the invention is a high-resolution spectrometer of the type mentioned

To create a way with which an improved resolution compared to the prior art can be achieved with the same size of the optical components.

According to the invention the object is achieved in that the means for producing the multiple dispersion comprise at least two reflective, flat surfaces which form a right angle with one another and which the dispersed and focused radiation first in the direction of one of the other reflecting surfaces and then in the direction of reflect the dispersing element back, and which are arranged relative to the entry slit in such a way that the entry slit is in the intersection of the planes defined by the reflecting surfaces.

With this arrangement, the incoming beam runs from the entrance slit onto camera optics. The camera optics generate a parallel beam. The beam passes over the dispersing element and is dispersed there for the first time. The dispersed radiation runs from the dispersing element back to the camera optics, which focuses the radiation in the plane of the entrance slit. The returning jet does not run back exactly from itself from the dispersing element, but forms a small angle with the incoming jet within the dispersion plane. The dispersion level is defined by the incoming and the deflected beam at the dispersing element. The angle is so small that the aberrations remain small. Within the dispersion plane there is a reflecting surface on both sides of the entrance slit, for example two small plane mirrors. Due to the small angle between the incoming and the returning beam of a wavelength at the dispersing element, the returning beam does not run back to the entrance slit, but hits one of the reflecting surfaces. The reflective surfaces form a right angle with each other. The returning beam is therefore deflected by the first reflecting surface before the focus and strikes the second reflecting surface. There the beam is redirected again. The image is reversed. After the twofold deflection, the beam runs back into itself a small distance offset from one another.

The parallel incoming jet hits the dispersing element for a second time, where it is redispersed. Via the camera optics, the beam that returns again is then directed past the edge of the reflecting surface next to the entrance slit onto a detector which is located directly next to the reflecting surface in the plane of the entrance slit.

Depending on the position of the dispersing element and the length of the reflecting surface, further passes can be realized. For this purpose, the angle of deflection at the dispersing element is reduced so that the first returning beam strikes the reflecting surface closer to the entrance slit. The second and each further returning beam strikes the reflecting surface somewhat offset further away until the last returning beam falls past the reflecting surface onto the detector as described above.

The arrangement with image reversal has the effect that the one realized on the dispersing element

Path difference between the marginal rays, which determines the resolution, adds up. The resolution can therefore be increased significantly. The size of the components does not change.

In contrast to arrangements with multiple gratings, only two reflecting surfaces, for example low-cost plane mirrors, are used here. The arrangement is also flexible since the resolution can be set by simply rotating the dispersing element. This has the advantage of being a spectral one Area can also be viewed in a larger width and with somewhat lower resolution. A spectral section of smaller width can be examined with high resolution without the structure changing significantly.

Preferably, the optical beam path for all passes between the

Entry slit and the detector arranged in one plane. This results in a compact arrangement with few elements and low imaging errors.

The dispersing element can be formed by a diffraction grating, in particular an Echeller grating. However, it is also conceivable to use a prism. The reflecting surfaces can be formed by mirrors. Instead of two mirrors arranged at right angles to each other, the reflecting surfaces of a prism can also be used, which form a right angle. By flattening the edge on which the right angle is formed, the light can enter the spectrometer through the prism. This flattening then takes up the entry gap or it is formed directly by it.

In a; According to an embodiment of the invention, a further reflecting surface is provided, and the dispersing element is arranged between the camera optics and this further reflecting surface in such a way that the dispersed light from the dispersing

Element is first directed directly onto the reflective surface, then back onto the dispersing element and only then back onto the camera optics. Then a double pass on the dispersing element is realized in one cycle.

The camera optics is preferably formed by a parabolic mirror. This results in a compact arrangement of high image quality without chromatic errors and with low loss n.

In a preferred embodiment of the invention, the detector arrangement comprises an optical system for enlarged imaging of the spectrum on the picture elements of the detector.

This optic can be formed by two cylindrical lenses or mirrors. With such an optical system, the high-resolution spectrum is enlarged, that is, in particular with the height remaining the same, "pulled apart" in width. Then the spectrum can be used larger detector elements are recorded without the resolution deteriorating.

The dispersing element is preferably rotatable about an axis perpendicular to the dispersion plane. The rotation can be carried out using suitable means, for example a

Stepper motor and a computer can be automated. Then the change of the inspection area, i.e. of the spectral range under consideration can be carried out in a particularly simple manner. For example, if the spectral environment of a line is to be examined, the inspection area can be enlarged by placing the grating or prism on a single pass. The returning beam runs from

Grid aμs via the camera optics directly to the detector arrangement. Then the spectrum is recorded with a correspondingly lower resolution. If a line profile is to be examined with as high a resolution as possible, the grating or prism is adjusted in such a way that two or more passes are made. The beam then revolves several times before it hits the detector array. Then the inspection area is correspondingly smaller.

In one embodiment of the invention, the reflecting surfaces each have an angle of 45 degrees to the optical axis of the light beam entering through the entrance slit. Then the arrangement is arranged symmetrically about the optical axis and the imaging errors are small. A slight deviation due to misalignment

Rotation around an axis that is perpendicular to the dispersion plane does not lead to a disturbing change in the beam paths.

The spectrometer is particularly well suited for use in determining spectral profiles of laser radiation. Laser radiation is generally narrow-band and therefore requires particularly high-resolution spectrometers for profile measurement. The spectral profile can also be monitored and stabilized by means of such a spectrometer by adapting the operating parameters of the laser.

Embodiments of the invention are the subject of the dependent claims. Some

Execution examples are explained below with reference to the accompanying drawings. Brief description of the drawings

Fig.l is a schematic representation of a high-resolution grating spectrometer with retroreflectors, in which the course of the center beam is drawn for a double grating pass.

FIG. 2 is a schematic illustration of the spectrometer from FIG. 1, in which the course of the marginal rays is drawn for a simple grating passage.

3 is a schematic illustration of the spectrometer from FIGS. 1 and 2, in which the course of the center beam for a simple grating passage is shown.

4 shows a spectral line with a simple lattice passage

5 shows the spectral line from FIG. 4 with a double lattice passage

Fig. 6 shows the beam path in the area of the rescue reflectors with five times the grating

7 is a schematic representation of an alternative high-resolution grating spectrometer, in which the beam is first dispersed on a plane mirror and then passes over the grating again.

8 is a schematic illustration of the spectrometer from FIGS. 1 to 3, in which the course of the center beam for a triple lattice passage is drawn

Description of an embodiment

A high-resolution spectrometer 10 is shown schematically in FIG. The spectrometer 10 comprises an entrance slit 12, a camera mirror 14, an echelle Grating 16, two plane mirrors 18 and 20, and a detector 22. In FIG. 1, the center _j beam 24 of the radiation entering through the entrance slit 12 is shown. Arrows 1-8 indicate the direction of travel of the center beam, the sequence of the beam path corresponding to the numbering 1-8.

The radiation emitted from a light source, not shown, enters the spectrometer 10 through the entrance slit 12. This is shown by an arrow 1. It then strikes the camera mirror 14. The camera mirror 14 is designed as an off-axis parabolic mirror. Due to the parabolically shaped reflecting surface 26, the divergent radiation is parallelized and deflected in the direction of the Echelle grating 16. This is shown by an arrow 2. The surface 26 is provided with a mirror coating, which also has a high reflectivity even in the UV range below 200 nm.

2, the marginal rays V and 2 'are shown. There you can see the effect of

Parabolic mirror 14 on the beam. The incoming, parallel beam hits the grating 16 and is dispersed there. The grid 16 is rotatably supported about an axis 28. The grating 16 is designed as an Echelle grating. A large blaze angle at the Echelle grating 16 produces a large path difference 38 between the marginal rays 2 'and 2 ". This causes the radiation to be highly dispersed in a high diffraction order. In this way, a high resolution is achieved in one pass.

The grating 16 is positioned analogously to a Littrow arrangement in such a way that the center beam, designated 3 in FIG. 1, has a wavelength down to a small angle γ. In practice, the angle γ is much smaller than shown here. The

Angles in Fig.l are drawn larger for illustration only. After a further reflection on the camera mirror 14, the returning beam is directed back towards the exit slit. This is shown in Fig. 1 by an arrow 4. The focus lies in the plane 30 of the entry slit 12. However, due to the angle γ, the focus is not exactly in the entry slit 12, but somewhat next to it. There is just next to that

Entry slit 12 arranged in the beam path a first plane mirror 18. The plane mirror 18 is arranged at an angle of + 45 ° (clockwise) to the optical axis corresponding to the center beam 14. The returning beam is therefore deflected at right angles upwards in FIG. 1. The focus is then on the optical axis of the input beam (arrow 1). A further plane mirror 20 is arranged on the other side of the entry gap 12. This plane mirror is arranged at an angle of -45 ° (clockwise) to the optical axis. The mirror planes of the mirrors 18 and 20 form a right angle and intersect along an intersection line that coincides with the center line of the entry slit 12. The beam is deflected at a right angle again at this mirror 20. After the double deflection at the mirrors 18 and 20, the beam travels back by a distance 32 in the direction of the camera mirror 14. This is shown by an arrow 5.

The image is reversed during the double reflection at the plane mirrors 18 and 20. This means that the path difference 38 of the marginal rays on the grid is added. As a result, the resolution is maximally increased by the second grating passage that now follows. The incoming beam 5 is again parallelized by the camera mirror 14 (FIG. 2) and reflected as a parallel beam 6 in the direction of the grating 16.

The twice-dispersed beam 7 runs back to the camera mirror 14. The beam 7, which is again reflected back, now has an angle difference of 3γ with respect to the beam 6. Accordingly, the beam strikes another point on the camera mirror 14 and becomes at a further offset point in the Level of the entrance gap 12 focused. At this point there is a schematically represented detector arrangement 22. Due to the further displacement of the beam 8 with respect to the first returning beam 4 by a distance 3γ, the beam this time passes the outer edge 36 of the mirror 18 and strikes the detector arrangement 22.

The detector arrangement 22 can consist of a simple line or area detector. Any common detector such as a photodiode array, a CCD or a CID detector is suitable here. However, light guides can also be arranged there, or a magnifying optic consisting of two cylindrical lenses or mirrors with which the image of the entrance slit is enlarged. The enlarged image is then recorded using one of the detectors listed above. This has the advantage that commercially available detectors with larger detector elements can be used without losing their resolution. The grid 16 is arranged rotatable about an axis 28. By rotating it into a suitable position, the beam can be aligned so that the angle γ between the incoming and outgoing beam becomes somewhat larger. Then the returning beam is no longer incident on the mirror 18, but directly on the detector 22. This case is shown in FIG.

The effect on the resolution is shown in FIGS. 4 and 5. A laser line 40 consisting of two peaks is not resolved in FIG. 4. For this purpose, with a limited detector size, a larger area is recorded, in which another line 42 lies. Such a 'spectrum is obtained when the grating is shown in FIG. 3

Position. 5 shows a spectrum for the same laser line with higher resolution. Line 42 from FIG. 4 cannot be detected because the inspection area has shrunk. The line components 44 and 46 of line 40 are now completely separated. The profile can be determined with sufficient accuracy. The higher resolution is achieved by the grid 16 in a

Position is brought, at which the angle γ is so small that a two or more grid pass is achieved, as shown in Fig. 1.

If the resolution is to be increased further, the grating 16 is rotated into a position in which the angle γ is even smaller. The beam then travels back and forth between the mirrors 18, 20 and the grating until it passes the mirror 18 and hits the detector 22. 6 shows a situation for a 5-fold lattice passage. The beam 50 runs from the entrance slit 12 to the grating 16, is dispersed there and runs back offset by a small angle. The simply dispersed, returning beam 52 is focused by the camera mirror 14 and on

Mirror, 20 reflected. The focus 53 lies on the optical axis of the beam 50 halfway between the mirrors 18 and 20. The beam is reflected again at the mirror 18 and runs back to the grating. The returning beam is labeled 54.

The beam is redispersed on the grating. The returning beam 56 forms an even larger angle than the beam 50, so that it strikes the mirror 20 further out. There are just enough revolutions until the beam hits the edge 58 of the mirror 20 runs past on the detector. In the example of Fig. 6, 5 grid passes are realized.

Depending on the desired resolution, the number of grid passes can therefore be set by rotating the grid. In any case, the aberration caused by the wobble γ is small, since the detector is arranged directly next to the entrance slit and the mirrors. By using highly reflective layers on the camera mirror and on the plane mirror, the reflection losses can be kept low. To minimize the imaging errors, the parabolic mirror is rotated about an axis perpendicular to the dispersion plane. However, this is generally not necessary.

With the arrangement shown, an extremely high resolution with a very small number of components can be achieved. Only the grid is rotatable. All other components can be arranged in a fixed manner. The arrangement has the advantage that the costs of the components and the adjustment effort remain low. Compared to a second grating, plane mirrors are very inexpensive and easy to adjust. They have a high reflectivity and thus enable a high light throughput ,

7 shows an embodiment of the above exemplary embodiment. Here a plane mirror 62 is arranged behind the grating. In such an arrangement, the beam first runs from the grating 16 to the plane mirror 62 and from there back to the grating 16. There the radiation is diffracted again. There is therefore a double pass at the grating each time before the beam runs back in the direction of the mirrors 18, 20 or the detector 22. The advantage of this arrangement is that with only one additional, comparatively inexpensive, optical component, the total number of reflections can be reduced with the same number of grating passes. Dust or scratches on the mirrors then cause less stray light.

With this arrangement, too, the resolution can be set by rotating the grating. The grating can be brought into the position in which there is no passage through the mirror 62. In addition, the number of passes can be adjusted via the number of reflections on the mirrors 18 and 20. 8 shows the case in which a triple lattice passage is realized. The beam runs according to the numbers 1-12 to the grating and back to the spatulas 18 and 20. It can be seen in comparison to the arrangement in Fig.l that only the grating position of the grating 16 has changed in the arrangement. All other optical components remain the same. The grid 16 was rotated about the axis 28 by a small angle. This rotation can be done manually or computer controlled with a stepper motor.

Claims

claims

1. High-resolution spectrometer (10) with an entrance slit (12), a dispersing element (16), camera optics (14), and a detector arrangement with a detector (22), which are arranged in a manner similar to a Littrow arrangement so that Radiation, which enters the spectrometer (10) through the entry slit (12), onto the dispersing element (16) by means of the camera optics (14), and then back into itself via the same camera optics (14.) Up to a small angle (γ) ) is conductive and focusable on the detector (22), whereby means (18, 20) are provided for producing a multiple dispersion by the dispersing element (16), characterized in that the means for producing the multiple dispersion have at least two reflecting, flat surfaces ( 18, 20), which form a right angle with one another and which the dispersed and focused radiation (3) first in the direction of one of the other reflecting surfaces and then reflect back towards the dispersing element (16), and which are arranged so that the entry gap (12) is in the intersection of the planes defined by the reflecting surfaces (18, 20).

2. Spectrometer according to claim 1, characterized in that the optical beam path between the entrance slit (12) and the detector (22) is arranged in one plane.

3. Spectrometer according to claim 1 or 2, characterized in that the dispersing element is formed by a diffraction grating (16).

4. Spectrometer according to claim 3, characterized in that the diffraction grating (16) is an Echelle grating.

5. Spectrometer according to one of the preceding claims, characterized in that the reflecting surfaces are formed by mirrors (18, 20).

6. Spectrometer according to one of claims 1 to 4, characterized in that the reflecting surfaces (18, 20) are the sides of a prism.

7. Spectrometer according to one of the preceding claims, characterized in that a further reflecting surface (62) is provided, and the dispersing element (16) between the camera optics (14) and this further reflecting surface (62) is arranged such that the dispersed radiation from the dispersing element (16) is first directed directly onto the reflecting surface (62), then back into the dispersing element (16) and only then back onto the camera optics (14).

8. Spectrometer according to one of the preceding claims, characterized in that the camera optics (14) is formed by a parabolic mirror.

9. Spectrometer according to one of the preceding claims, characterized in that an optical system is provided for re-enlarging the image of the entry slit in the exit plane.

10. Spectrometer according to claim 9, characterized in that the optics are formed by two cylindrical lenses.

11. Spectrometer according to claim 9, characterized in that the optics is formed by two cylindrical mirrors.

12. Spectrometer according to one of the preceding claims, characterized in that the dispersing element (16) is rotatable about an axis (28) perpendicular to the dispersion plane.

13. Spectometer according to one of the preceding claims, characterized in that the reflecting surfaces (18, 20) have an angle of 45 degrees to the optical axis (1) of the beam entering through the entry slit (12).

14. Use of a spectrometer according to one of the preceding claims for determining spectral profiles of laser radiation.

15. Use of a spectrometer according to one of claims 1 to 13 for the stabilization of spectral profiles of laser radiation.