WO2008077533A1

WO2008077533A1 - Arrangement of two diffractive optical elements

Info

Publication number: WO2008077533A1
Application number: PCT/EP2007/011116
Authority: WO
Inventors: Hans-Jürgen DOBSCHAL
Original assignee: Carl-Zeiss Ag
Priority date: 2006-12-22
Filing date: 2007-12-18
Publication date: 2008-07-03
Also published as: DE102006061066B4; DE102006061066A1

Abstract

The invention relates to an arrangement of diffractive optical elements, comprising two diffractive structures (1, 4). The invention is characterized in that the grating of a first DOE1 (1) is applied to a light entrance surface of a transparent carrier (2) and the grating of a second DOE2 (4) is applied to a light exit surface of a transparent carrier, wherein the first DOE1 and the second DOE2 are spaced from each other. The gratings of the first DOE1 and the second DOE2 have the same line density of lines per mm in the center of the optical path, which is a carrier frequency and the line density of one or of both of the gratings increases toward the edge of the optical path. Thus, a course of overall frequencies is obtained, wherein the overall frequency for the first DOE1 is obtained from the sum of the carrier frequency and the modulation frequency of the grating of the first DOE1, and the overall frequency for the second DOE2 from the sum of the carrier frequency and the modulation frequency of the grating of the second DEO2.

Description

ARRANGEMENT OF TWO DIFFERENTIAL OPTICAL ELEMENTS

The invention relates to a diffractive optical element arrangement, which is composed of two diffractive structures.

For some time, diffractive optical elements (DOEs) have increasingly been used for imaging systems. The goal here is to reduce the number of optical components, the construction volume and the costs. This exploits the fact that DOEs have an abnormal dispersion behavior compared to refractive elements and thus achromatization is possible in broadband applications with relatively little effort. Also other aberrations such. Opening errors can be minimized at the same time. However, such elements have not yet been sufficiently successful on the market. The main reasons for this are that DOEs have a strong wavelength-dependent diffraction efficiency. In general, the profile shape of the grid is designed so that in a VIS application in green, a diffraction efficiency of> 95% is achieved and the spectral edges towards blue or red only 75-80% can be achieved. The problem here is less the absolute level of efficiency than the fact that non-first order diffracted light in the zero order of diffraction and in several higher diffraction orders (greater than 1 and less than 0) reduces the contrast as a stray light. Due to the correction, these DOEs have only a relatively low number of lines, which means that the unwanted diffraction orders have only a slightly different diffraction angle than the desired order. In connection with the overall optical system, this results in several longitudinally offset foci, which are relatively close to the desired focus and thus cause a clear blurring of the image. The proportion of stray light here is so high that the contrast reduction of the optical system at the spectral edges is no longer tolerable, at least in more demanding systems. There are several solutions to this problem.

It is known from DE 195 33 951 A1 a multi-layer grating whose layer system is adapted so that the phase deviation when passing through the DOEs remains almost constant and thus one always lies in the vicinity of the optimum diffraction efficiency. The disadvantage here is that the production of such a multilayer system is relatively difficult to master and also suitable material combinations are not freely available.

The object of the invention is to provide a diffractive optical element arrangement which has a simple structure and is inexpensive to produce and has high efficiency over a broad spectral range and, in particular, has low stray light losses.

The object of the invention is achieved with the features of the main claim 1. The claims 2 to 12 are advantageous embodiments of the main claim.

The basic idea of the invention is to place two DOEs at a distance from one another and to combine their structures in such a way that a double DOE arrangement is produced, which causes a strong separation of the undesired orders, which then on further passing through the optical system in the light propagation direction of the useful light be blocked by lens frames and / or aperture or arrive at least when passing through the optical system outside of the intended image area used. The basic principle of the diffractive-optical element arrangement can be compared with the known from the broadcasting technology modeling a NF-audio signal to an RF frequency. By analogy, a first DOE1 represents the modulator and a second DOE2 represents the demodulator.

First, a high-frequency "carrier frequency" is introduced for the two grids of DOE1 and DOE2, that is, both high-frequency grids have a radial equidistant line-number distribution of, for example, 1000 L / mm, which corresponds to a diffractive axicon initially equal to zero, because the diffraction for the first order is canceled out for all colors.

One or both of the high-frequency gratings is further superimposed on a lower frequency, which substantially corresponds to the frequency of a known single-layer DOEs, wherein the low frequency is either modulated only on one of the two grids or can be distributed to both grids. At a carrier frequency of 1000 L / mm on the two DOEs, the

Modulation frequency on one of the DOEs from 0 in the beam center (inside) to the

Edge towards, for example, 50, that is, there is an overall frequency profile of 1000 L / mm to 1050 L / mm.

In particular, the frequency of one of the grids DOE1 or DOE2 or both gratings DOE1 and DOE2 is in the range of 1000 L / mm in the middle up to 1030

L / mm at the edge.

The grating of the first DOE1 is on the light entrance surface of a plane plate and the

Grating of the second DOE2 is applied to the light exit surface of a plane plate.

In the simplest case, these are the front and back of a plane parallel plate. In the further case, a plan plate is used for each DOE, wherein the

Planplatten have an air gap or a cemented surface.

The advantage of this methodology is that all higher diffraction orders are clearly separated from the desired first order of diffraction, with the effect that they are either blocked at the lens frames or appear outside the image field used.

If the DOEs are e.g. 80% in the 1st order and 20% in the 0th order, after passage through the two DOEs are 64% in the 1st order and 4% in the 1st order

O.Ordnung, which corresponds to a false-light ratio of 1:16; opposite one

Ratio 1: 4 for a single DOE (idealized, assuming that no

Absorption occurs).

The only remaining false light arises in the event that diffracted at the first DOE1 and the second DOE2 each with the same diffraction order. Due to the relatively high carrier frequency, only the 2nd order is possible, except for the O.order, since all other higher orders do not arise.

Therefore, the invention also proposes to minimize the remaining residual scattered light by further measures. In the following four variants are given:

A first variant provides that by creating a distance between the two DOEs a cavity is formed, in which only the O.Ordnung is and you completely blocked by a Zentralabschattungsblende on the second DOE this light. The useful light runs on a ring around the blocked light on the outside. A second variant provides that by applying prisms on both inner sides of the DOEs with a suitable choice of the prism angle a total reflection of the O.Ordnung is achieved. The O. order is mirrored backwards out of the system.

A third variant provides that a layer designed especially for the direction of the first order after the DOE1 reaches a partial filtering of the O. order between the two DOEs.

A fourth variant provides that the higher orders (second and possibly third, if present) on the rear side of the carrier of DOE 1 are totally reflected by applying the two DOEs to separate plates provided with an air gap

The variants 1 and 2 are particularly suitable for the variants in which the stray light is predominantly in the O.order, while variant 4 is particularly suitable for the case that the stray light occurs mainly in the higher orders.

The grating profile of the DOEs can be optimized in such a way that the resulting stray light is conducted in substantial proportions either in higher orders or in the O.order. Test calculations have shown that the O. order diffraction efficiency per DOE can be reduced below 1%, so that the O. order overall efficiency does not exceed 0.01%. The higher orders would then be totally reflected in variant 4.

Advantageously, the profile of the DOEs is designed so that the stray light evenly distributed to the O.Ordnung and the 2nd order, each with 10%. Then only 2% of scattered light will result for the above example.

Of course, the "carrier frequency" is chosen not to be higher than absolutely necessary because high frequency gratings are more difficult to produce, and in the case where the two DOEs are close to one aperture, it is sufficient to make all the misalignments with an angle larger This may well be already 200 L / mm Furthermore, the structure that forms the "carrier frequency" does not necessarily have to be radially symmetric. It can also be formed like a normal linear grating known from spectroscopy, to which the modulation frequency is superimposed, so that the splitting takes place only in one plane. The invention will be described below with reference to figures. s show:

Figure 1: Schematic representation of a classical Achromats

FIG. 2: Beam deviation of the classical achromat

Figure 3: Schematic representation of an achromatic in combination with a

Double DOE arrangement Figure 4: Beam deviation of the combination of Achromat and a

Double DOE Arrangement FIG. 5: Schematic representation of a double DOE arrangement on two

Support plates Figure 6: Schematic representation of a double-DOE arrangement on a

Support plate Figure 7: Top view of the first DOE and the second DOE with the

Carrier frequency Figure 8: Top view of the second DOE with the superposition of

Carrier Frequency and Correction Distributions on the Second DOE Figure 9: Carrier frequency distribution for the first DOE and the second DOE of a 20mm diameter double DOE assembly Figure 10: Correction distribution on the second DOE for the imaging

Effect Figure 11: Superposition of the Carrier Frequency and Correction Distributions on the Second DOE Figure 12: Double DOE Arrangement with Central Shading of the O. Order by a Central Gate Figure 13: Double DOE Arrangement with Total Reflection of the O.order

microprisms

FIG. 14: Double DOE arrangement with total reflection of the higher orders FIG. 15: Double DOE arrangement with filter layer between the two DOEs

The invention will be described by examples of a collimator for white light.

Figure 1 shows a schematic diagram of a classical Achromats, consisting of NSK2 and F2. The system data for the known achromat are:

Area Radius Distance Medium

5 34.85 (ex) 3.0 NSK2

6 16.7 (ex) 2.0 F2

7 266.36 (ex) 50.0

The diameter of the parallel entrance bundle on the surface 5 is 10.0 mm.

FIG. 2 shows the beam deviations of the classical achromat according to FIG. 1.

FIG. 3 shows a schematic illustration of an achromat in combination with a double DOE arrangement according to the invention, consisting of a combination of a first DOE1 and a second DOE2, which are at a distance from one another. The system data for the combination of achromatic and double DOE arrangement are:

Area Radius Distance Medium

1 plan (with DOE 1) 0.5 NBK7

2 plan 0.1

3 plan 0.5 NBK7

4 plan (with DOE 2)

5 29.35 (ex) 3.0 NSK2

6 18,595 (cx) 2.0 F2

7 441.5 (CC) 50.0

The diameter of the parallel entrance bundle on the first DOE1 is 10.0 mm

FIG. 4 shows the beam deviations of the combination of achromatic and double DOE arrangement. Compared to Figure 2 can be seen the significant reduction of the error rates.

FIG. 5 shows a double DOE arrangement according to FIG. 3 in detail, wherein in each case a DOE is applied to a carrier plate and the two carrier plates are at a fixed distance from one another. The first DOE1 and the second DOE2 are respectively mounted on the outer sides 1 and 4 of the plates separated by a space. The distance may be an air gap or a cement layer, wherein one or both of the opposing optically active surfaces may have further optical effects.

FIG. 6 shows a further double-DOE arrangement whose DOE1 and DOE2 are applied on the two outer surfaces of a plane-parallel plate. Figure 7 shows the image of a DOE surface of the first DOE1, with the same carrier frequency applied to the first DOE1 and the second DOE2.

FIG. 8 shows an illustration of a DOE surface, with the carrier frequency of the first DOE1 being additionally superimposed on the second DOE2 and a modulation frequency being applied.

The phase functions of the two DOEs of a double DOE arrangement according to the invention are described as follows: n

Phase (r): = ai -r +

[ai-r ^{2 (H)} ]

¹ ^ and derived from it the number of lines per

millimeter

a _] + ± [2 (i - \) a, r « ^ι -» - ^> ] number of lines) = -

r is the radial height with respect to the center of the DOE; the special phase coefficient a1 determines the high-frequency component (carrier frequency) while the sum component by the coefficients a, - (i> 1) represents the low-frequency correction phase (modulation frequency); λo is the reference wavelength in mm. The number n is sufficiently dimensioned with 8.

In the example, the modulation frequency is non-zero only for the DOE2: phase distribution of the first DOE1: a1 = 4.0000E-01

Reference wavelength λ ₀ : 457.9 nm

Phase distribution of the second DOE2: a1 = -4.0000E-01 a2 = -4.4960E-06 a3 = 7.1669E-07 a4 = 4.7936E-09 a5 = -9.4819E-11

Reference wavelength: 457, 9nm

FIG. 9 shows the same carrier frequency on the two DOE1 and DOE2, which is 1000 over the entire extent. FIG. 10 shows the correction distribution which is additionally applied only on the second DOE2.

Figure 11 shows the resulting overall distribution on the second DOE2.

FIG. 12 shows a first embodiment of the double DOE arrangement according to the invention on a plane-parallel plate 2, with the first DOE1 on the optical surface 1 and the second DOE2 on the optical surface 4, in which the remaining O. diffraction order is represented by a central annular diaphragm 8 is blocked. The annular aperture 8 is applied in the central region of the surface 4, on which the second DOE2 is otherwise applied. The optical path is limited to the outside by the annular diaphragm 4, which is applied to the optical surface 4. In this example, the grids are dimensioned so that technically only the O.-diffraction order and the 1st-diffraction order arise (the other diffraction orders are practically negligible). It is a grid combination used, which is described in Figures 6 to 11. The diameter of the incident light beam is 5 mm, the thickness of the plane-parallel plate is 10 mm, the pinhole 8 has a diameter of 10 mm.

FIG. 13 shows a further variant of a double DOE arrangement according to the invention, in which the DOEs are respectively applied to the surfaces 1 and 4 of two plane-parallel plates 2 and 3. The plates 2 and 3 have an air gap in which two prism arrays 11 and 13 are arranged. The first prism array 11 lies with a base on the plane surface 10 of the plane-parallel plate 2. This prism array 11 totally reflects the O. diffraction order and breaks the 1st order of diffraction toward the second DOE2. The second prism array 13 lies with a base on the plane surface 12 of the plane-parallel plate 3. This prism array 13 deflects the 1st diffraction order toward the second DOE2. The prism arrays 11 and 13 are of isosceles prisms and are staggered relative to each other so that the tips of one prism array protrude into the valleys of the other prism array. Between the prism arrays 11 and 13 is an air gap of 0.2 mm; with a leg length of the prisms of 1mm.

FIG. 14 shows an arrangement with one DOE each on one of the plane-parallel plates 2 and 3, wherein the plane surfaces 10 and 12 have an air gap. At the plane surface 10, a total reflection of the higher takes place Diffraction orders 14 (greater the 1st diffraction order) back. The transmission of the 0.-order of diffraction is less than 0.01%.

FIG. 15 shows an arrangement with a respective DOE on one of the plane-parallel plates 2 and 3, the plane surfaces 10 and 12 having no air clearance. Between the plane surfaces 10 and 12 there is a filter layer 15 which essentially filters out the 0th diffraction order and the higher diffraction orders 14 (greater than the 1st diffraction order). The transmission of the 0th diffraction order and that of the 2nd diffraction order is less than 0.01%.

LIST OF REFERENCE NUMBERS

1 optical surface for the first DOE1

2 plane-parallel plate

3 plane-parallel plate

4 optical surface for the second DOE2

5 optical surface

6 optical surface

7 optical surface

8 ring aperture

9 pinhole

10 plane surface

11 prism array

12 plane surface

13 prism array

14 reflected higher diffraction order

15 filter layer

ai specific phase coefficient, which describes the height of the carrier frequency ai coefficients, which describe the modulation frequency r radial height coordinate, relative to the center of the DOE λ ₀ reference wavelength

Claims

claims

1. Diffractive optical element arrangement, which is composed of two diffractive structures, which are aligned to the center of a beam path, characterized in that the lattice of a first DOE1 on a light entrance surface of a transparent support and the lattice of a second DOE2 on a light exit surface of a transparent Carrier are applied, wherein the first DOE 1 and the second DOE2 have a distance from each other, further the grids of the first DOE1 and the second DOE2 in the middle of the beam path have a same line density lines per mm, which is a carrier frequency and the line density of one or both the grating increases toward the edge of the beam path, resulting in a history of total frequencies, wherein the total frequency for the first DOE1 from the sum of the carrier frequency and the modulation frequency of the grating of the first DOE1 and the total frequency for the second DOE2 from the sum of the carrier frequency and d he modulation frequency of the grating of the second DOE2 arise.

2. Diffractive-optical element arrangement according to claim 1, characterized in that the modulation frequency is determined by a continuous compression of the lines of the grating from a center to an edge of the carrier.

3. Diffractive-optical element arrangement according to claim 1, characterized in that line density according to the function

Line number (r) = -

K to take.

4. Diffractive-optical element arrangement according to claim 1, characterized in that the carrier is a single plate for the first DOE1 and the second DOE2.

5. A diffractive optical element arrangement according to claim 1, characterized in that the carrier is a first plate for the first DOE1 and a second plate for the second DOE2.

6. Diffractive-optical element arrangement according to claim 4 or 5, characterized in that the plate is plane-parallel.

7. Diffractive-optical element arrangement according to claim 4 or 5, characterized in that the plate has an additional refractive effect.

8. Diffractive optical element arrangement according to claim 1, characterized in that the increase in the line distribution from the center to the edge of the beam path is between 0.5% and 5%.

9. Diffractive-optical element arrangement according to claim 1, characterized in that the carrier frequency is between 500 and 2000 lines / mm.

10. diffractive optical element arrangement according to claim 1, characterized in that for the wavelength range of 450 nm to 650 nm, the line density between 0 and 50 lines increases from the center to the edge of the beam path, which is the modulation frequency.

11. Diffractive-optical element arrangement according to claim 1, characterized in that the carrier frequency is formed by radially symmetrical grooves or a line grid.

12. Diffractive-optical element arrangement according to claim 1, characterized in that the modulation frequency is formed by radially symmetrical grooves or a line grid.