DE102012208906A1 - Diffraction grating for use in spectral sensing unit, has two partial lattices that are arranged with different spectral diffraction efficiencies, where lattices are arranged in direction that is arranged next to another direction - Google Patents

Abstract

The grating (2) has a planar substrate (8) comprising a surface (9), and a diffraction grating structure (5) is formed on the surface of the substrate. Two partial lattices are arranged with different spectral diffraction efficiencies. The partial lattices are arranged next to each other in one direction. The partial lattices are arranged in another direction that is arranged next to the former direction. The surface is formed in a non-planar manner. The structure of the diffraction grating and/or the substrate are provided with illustrating effect. An independent claim is also included for a spectral sensing unit.

Description

The present invention relates to an imaging diffraction grating with high spectral diffraction efficiency.

For example, imaging diffraction gratings are used for the spectral decomposition of light in a spectrometer. Frequently, the diffraction gratings are formed with a blaze profile to provide the highest possible diffraction efficiency for a given wavelength. However, in such a blazed grating, the diffraction efficiency drops sharply with increasing spectral distance to the optimum wavelength (blaze wavelength), e.g. disadvantageously leads to a sharp drop in the signal-to-noise ratio at the edges of the addressable spectral range.

From the DE 10 2009 029 324 A1 is a non-imaging diffraction grating with an extended spectral range with high diffraction efficiency known, wherein on a first sub-grating at least a second sub-grating is formed. However, such a diffraction grating is technologically difficult to produce and therefore is only produced on flat substrates.

Proceeding from this, it is therefore an object of the invention to provide an imaging diffraction grating which has an increased spectral diffraction efficiency and is easy to produce.

According to the invention, the object is achieved by an imaging diffraction grating which has a surface having a substrate and a diffraction grating structure formed on and / or in the surface of the substrate, which is partially divided into at least two sublattices with different spectral diffraction efficiencies, wherein the at least two sublattices in a first direction are arranged alternately several times side by side and / or are arranged side by side both in the first direction and in a second direction.

Such an imaging diffraction grating can be easily manufactured since e.g. conventional lithographic patterning techniques can be used and the two sublattices e.g. can be formed differently by forming the groove depth varies in size.

Furthermore, the inventive imaging diffraction grating has an improved S / N ratio for a large wavelength range and is suitable for high numerical apertures, so that a wide range of applications for the imaging diffraction grating according to the invention is open.

In the diffraction grating according to the invention, the surface may not be formed flat. In particular, the surface may be curved. Preferably, it is spherically curved.

The imaging effect of the diffraction grating according to the invention can be provided by the diffraction grating structure and / or the substrate (for example the surface).

The segmental division of the diffraction grating structure may be striped or tiled. Thus, e.g. three, four or more strips can be provided. Furthermore, the sections can be split without gaps.

The at least two sublattices may differ in their geometric design. Preferably, they differ in the furrow or groove depth.

The at least two partial gratings are preferably diffraction gratings of the same type. It can be e.g. to act on blazed grids, sinusoidal grids or even binary grids.

The imaging diffraction grating may have three, four or more sublattices.

The first direction may, for example, be a direction of a coordinate system. Thus, this direction can extend linearly, be a radial direction or even a direction in the circumferential direction of a circle. Thus, the at least two sub-grids could be arranged like a cake next to each other. Also, an arrangement in the form of concentric circles, rings or other closed forms (such as triangle, quadrilateral or other polygon) is possible.

By the feature that the at least two sub-gratings are alternately arranged side by side alternately in a first direction, it is here understood in particular that at least one of the sub-gratings is arranged in two sections (for example strips) and the other sub-grid is arranged therebetween. Of course, it is also possible that the at least two sub-gratings are arranged in several sections. If the two sublattices are labeled A and B, they can thus be arranged in the first direction in the order ABA, ABAB, ABABA, ABABAB, etc. If e.g. a third subgrid (designated C) may be provided e.g. the order ABCA, ABCAB, ABCABC, ABCABCABC, ABCACB, ABCBAC, ABCACBCBA, etc. are present.

Of course, the at least two partial gratings can also be arranged alternately several times next to one another in a second direction, which is different from the first direction. The first direction and the second direction preferably lies in the plane in which the diffraction grating structure lies, and thus preferably in the surface of the substrate or parallel to this surface.

By the feature that the at least two partial gratings are arranged next to each other both in the first and in the second direction, it is here understood in particular that e.g. a checkerboard-like arrangement is provided in which only 2 × 2 fields are provided. In the first row of these 2 × 2 fields, the sub-grids are in the order AB and in the second line, the sub-grids are arranged in the order BA. Of course, the checkered arrangement may also have more than two fields in at least one of the two directions, so that in that direction then e.g. in turn, one of the already described sequence ABA, ABAB, etc. may be present.

The at least two partial gratings can each occupy the same area fraction of the diffraction grating structure.

Furthermore, in the diffraction grating according to the invention, the surface portions of the diffraction grating structure occupied by the at least two sublattices may be selected such that a predetermined spectral diffraction efficiency is achieved. It is therefore possible, based on a given spectral diffraction efficiency or a given spectral diffraction efficiency profile, to choose the corresponding area proportions for each of the sublattices so that this predetermined spectral diffraction efficiency is achieved.

Preferably, the diffraction grating structure is formed reflective. However, it is also a transmissive training possible.

Furthermore, a spectral sensor unit with an imaging diffraction grating according to the invention and a detector is provided, wherein the diffraction grating spectrally splits a spectrally analyzed beam and images it on the detector.

The spectral sensor unit may be used e.g. have an entrance slit. The spectral sensor unit may be vacant between the entrance slit and the diffraction grating from a beam splitter and / or be free from a spectral splitting unit of the beam (before impinging on the diffraction grating).

In particular, the spectral sensor unit can be designed as a spectrometer.

The diffraction grating according to the invention is designed in particular for a wavelength range of nm or from 300 nm to 1000 nm.

The detector can be designed, for example, as a line detector.

The spectral sensor unit can be constructed inexpensively, since no additional optical components are necessary. If the surface of the substrate is curved, it can be used to reduce optical aberrations. In particular, the surface may be spherically concave curved.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the specified combinations but also in other combinations or alone, without departing from the scope of the present invention.

The invention will be explained in more detail for example with reference to the accompanying drawings, which also disclose characteristics essential to the invention. Show it:

1 a schematic representation of an embodiment of the spectral sensor unit according to the invention;

2 a schematic plan view of the diffraction grating structure 5 of the diffraction grating 2 in 1 ;

3 a schematic sectional view for explaining the grid structure in the two grid areas 6 and 7 according to 2 ;

4 a diagram for explaining the spectral diffraction efficiency of the diffraction grating according to the invention 2 , and

5 a schematic plan view of the diffraction grating structure 5 according to a further embodiment.

At the in 1 In the embodiment shown, the spectral sensor unit according to the invention comprises 1 an inventive imaging diffraction grating 2 , a detector 3 , which is designed here as a line detector, and an entrance slit 4 ,

The imaging diffraction grating 2 is designed so that it is the entrance slit 4 on the detector 3 maps. Electromagnetic radiation, as indicated by the arrow P1 is indicated by the entrance slit 4 as a divergent beam on the imaging diffraction grating 2 therefore, is wavelength dependent at different locations on the detector 3 shown as schematic for two different wavelengths by the solid lines and the dashed lines is indicated.

In order to increase the diffraction efficiency compared to conventional imaging diffraction gratings, in particular at the edges of the detected spectral range, the diffraction grating has 2 a schematically illustrated diffraction grating structure 5 which is divided in sections into two sub-grids with different diffraction efficiencies. This is in the schematic plan view of the diffraction grating structure 5 in 2 shown, wherein the diffraction grating structure 5 strip-shaped into first grid areas 6 (hatched) and second grid areas 7 (not shown hatched) is divided. In both grid areas 6 and 7 In each case, a blazed diffraction grating is formed, as shown for example in a sectional view in 3 is indicated.

The blazed grid for the first grid areas 6 is optimized for a wavelength of about 300 nm and the blazed grating of the second grating areas 7 is optimized for a wavelength of nm. The stripe width of the two grid areas 6 . 7 is chosen here so that the smallest possible for the particular application of the sensor unit spot on the diffraction grating structure 5 both grid areas 6 and 7 covered, so that for the spot almost one from both grids areas 6 . 7 averaged diffraction grating is present. This leads to a diffraction efficiency as shown in FIG 4 in which along the abscissa the wavelength in μm and along the ordinate the diffraction efficiency in% is plotted. The curve K1 shows the diffraction efficiency of the first grating regions 6 which are optimized for a wavelength of 300 nm and have a usable range of about 210 to about 700 nm. The curve K2 shows the diffraction efficiency of the second grating regions 7 which are optimized for a wavelength of about 850 nm and have a useful wavelength range of about 550 nm to at least 1000 nm. The curve K3 shows the diffraction efficiency of the diffraction grating structure according to the invention 5 which provides a usable range of about 210 nm to at least about 1000 nm.

The different diffraction efficiencies of the two grating areas 6 and 7 here are the structure depth h of the blazed grids ( 3 ), wherein in both grating areas 6 and 7 the grating period g is the same. For the first grid areas 6 the structure depth h is about 130 nm and for the second grating areas 7 the structure depth h is approximately 360 nm. If the grating period g is kept constant, this automatically leads to different blaze angles θ.

The imaging diffraction grating according to the invention 2 can be prepared, for example, by means of lithographic patterning methods, wherein the interference lithography for non-planar substrates 8th especially good.

So one becomes on the substrate 8th applied photoresist holographically exposed in the same manner as in the realization of a homogeneously structured grating. To the different grid areas 6 and 7 form a masked locally differently designed pre and / or post exposure of the photoresist. This leads to a certain variation in the pattern depth between the additionally exposed areas and the non-additionally exposed areas, since the photoresist is subjected to a different removal in the additionally exposed areas in the wet-chemical development process compared to the inscribed interference pattern of the holographic exposure.

Alternatively, it is possible to perform a downstream dry etching process (eg, ion etching or reactive ion etching) to increase the texture depth using a special mask. Thus, e.g. in a first step, the unmasked areas are transmitted to the substrate with predetermined etching parameters. Following this, either only the previously covered areas or all areas can be transmitted with a predetermined etching parameter. To increase the phase equality (height jumps) between the two grating areas, the first variant is to be preferred.

The resulting diffraction grating structure 5 then, in contrast to a conventional embodiment with homogeneous structuring the two strip-shaped alternately arranged grid areas 6 and 7 on, which adjoin each other gapless and have different spectral diffraction efficiencies.

Of course, more than two different grid areas can be provided. Also, another division of the diffraction grating structure 5 possible. In 5 is the same as in 2 a division of the diffraction grating structure 5 shown, this division is tile-shaped. In the 5 division shown can also be referred to as a checkerboard.

A preferably uniform distribution of the at least two grid areas is preferred 6 and 7 , Then it is advantageously achieved that also an uneven illumination of the pupil (input angle spectrum) of the system or the sensor unit 1 despite the local wavelength-dependent different diffraction efficiencies of the diffraction grating structure does not adversely affect the measured signal.

In the embodiments described so far, the imaging diffraction grating is 2 designed as a reflective diffraction grating. Of course, it can also be designed as a transmissive diffraction grating.

The substrate 8th has a non-planar surface in the embodiments described so far 9 which is preferably spherically curved here. Of course it is also possible that the surface 9 just trained.

The grids in the grid areas 6 . 7 do not have to be designed as a blaze grating, but can also be designed differently. They may, for example, have a sinusoidal structure or a binary structure.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102009029324 A1 [0003]

Claims (14)

Imaging diffraction grating with a surface ( 9 ) having substrate ( 8th ) and one on and / or in the surface ( 9 ) of the substrate ( 8th ) diffraction grating structure ( 5 ), which is partially divided into at least two sub-grids with different spectral diffraction efficiencies, wherein the at least two sub-gratings are alternately arranged side by side in a first direction and / or arranged side by side both in the first direction and in a second direction. A diffraction grating according to claim 1, wherein the surface ( 9 ) is not-trained. Diffraction grating according to one of the preceding claims, in which the diffraction grating structure ( 5 ) and / or the substrate ( 8th ) provide the imaging effect. A diffraction grating according to any one of the preceding claims, wherein the sectional division comprises three, four or more strips. A diffraction grating structure according to any one of the preceding claims, wherein the sectional division is tile-shaped. Diffraction grating according to one of the above claims, in which the segmental division is present without gaps. Diffraction grating according to one of the above claims, in which the at least two partial gratings ( 6 . 7 ) differ in their geometric design. Diffraction grating according to one of the above claims, wherein the at least two partial gratings differ in their groove depth. A diffraction grating according to any one of the preceding claims, wherein the at least two sublattices are of the same lattice type. A diffraction grating according to one of the preceding claims, in which the at least two sublattices have a total of the same areal components of the diffraction grating structure ( 5 ) occupy. Diffraction grating according to one of the above claims, in which the area fractions of the diffraction grating structure respectively occupied by the at least two sublattices ( 5 ) are selected so that a predetermined spectral diffraction efficiency is achieved. A diffraction grating according to any one of the preceding claims, wherein the diffraction grating structure is reflective. Spectral sensor unit with an imaging diffraction grating ( 2 ) according to one of the above claims and a detector ( 3 ), wherein the diffraction grating ( 2 ) Spectrally splits a spectrally analyzed beam and on the detector ( 3 ) maps. Spectral sensor unit according to claim 13, having an entrance slit and is free between the entrance slit and diffraction grating of a beam splitter and / or is free from a unit for spectral splitting of the beam.

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