EP1506448A2 - Optische anordnung zur homogenisierung eines zumindest teilweise koh renten lichtfeldes - Google Patents
Optische anordnung zur homogenisierung eines zumindest teilweise koh renten lichtfeldesInfo
- Publication number
- EP1506448A2 EP1506448A2 EP03752770A EP03752770A EP1506448A2 EP 1506448 A2 EP1506448 A2 EP 1506448A2 EP 03752770 A EP03752770 A EP 03752770A EP 03752770 A EP03752770 A EP 03752770A EP 1506448 A2 EP1506448 A2 EP 1506448A2
- Authority
- EP
- European Patent Office
- Prior art keywords
- light field
- radiation
- optical
- beam splitter
- radiation component
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/48—Laser speckle optics
Definitions
- the invention relates to an arrangement for homogenizing an at least partially coherent light field, in particular from a laser, preferably an excimer laser.
- DE 1 95 01 521 C1 describes an arrangement for reducing interference of a coherent bundle of light by reducing the time coherence. It is proposed to use microstructured phase plates through which the laser beam passes in order to reduce the time coherence, with a phase plate of this type having a phase-changing surface structure being designed to be reflective in a particular embodiment of the invention. If the laser light passes through the microstructured phase plate, the coherence of the laser light is broken.
- the invention is based on the object of creating an arrangement for reducing the undesired speckle, which achieves high efficiency with simple optical means.
- an arrangement which essentially consists of an optical circuit, at least one coupling element for coupling a light field into the optical circuit and at least one coupling element, wherein the coupled light field or a part of this light field repeats the optical circulation and the wavefront of the light field is at least partially deformed, and wherein
- Parts of the light field are coupled out if they have passed through the optical circulation once or several times.
- the light field is coupled into the optical circulation with a given wavefront.
- portions of the light field are decoupled from the cycle, the wavefronts of which differ from one another. If pulsed laser radiation is involved, the path lengths covered by the individual components of the light field are greater than the time coherence length.
- An important advantage of the arrangement according to the invention is that no radiation is directed back in the direction of the incident light field and thus a high efficiency in effect is achieved.
- the optical circulation is formed by two beam splitters.
- a first beam splitter separates the coupled light field into two subfields.
- a first of these two subfields reaches the second beam splitter via a shorter optical path, preferably directly.
- the second of these subfields reaches the second beam splitter via a longer optical path, for example deflected via reflectors.
- the optical circulation is realized with a beam splitter that separates the coupled light field into two sub-fields, after which a first sub-field is coupled out and the second sub-field is deflected via reflectors in such a way that it reaches the beam splitter again and is separated into two parts , a first part of which is coupled out and the second part is returned to the beam splitter via further reflectors.
- the coupled-in light field and the respectively returned radiation component strike the mutually opposite radiation surfaces of the splitter layer, the direction of the incident light field and the direction of the radiation component falling on the splitter layer enclosing an angle of 90 'with one another.
- a further embodiment of the arrangement according to the invention is equipped with a reflection grating which the coupled light field strikes and furthermore has a beam splitter onto which the light diffracted at the reflection grating is directed. A first portion of the diffracted light passes through the dividing surface of the beam splitter and is then decoupled from the optical circuit, while the remaining portion is returned to the reflection grating, for example via a reflector.
- the returned portion is partly diffracted at the reflection grating and directed onto the splitting surface of the beam splitter, where it is partly coupled out and partly to Reflection grating thrown back, etc., the respectively coupled radiation components mix.
- the coherence of the laser radiation is reduced on account of the multiple splitting and the path length difference of the radiation components which are respectively brought together at the splitter layer of the beam splitter.
- the optical circulation is generated by means of two reflection gratings.
- the coupled light first hits a first reflection grating.
- the light diffracted in the first order on the first reflection grating is directed onto a second reflection grating. Part of the incident light is again diffracted into the first order and decoupled from the second reflection grating.
- a plane mirror is provided to which both the radiation component not diffracted in the first order on the first reflection grating and the radiation component not diffracted in the first order on the second reflection grating is directed, and from which the radiation component coming from the first reflection grating is directed onto the second reflection grating and the radiation component coming from the second reflection grating is directed onto the first reflection grating.
- the two reflection gratings are preferably designed with an identical number of lines and with identical angular relationships. Furthermore, the efficiency of the second reflection grating is lower than the efficiency of the first reflection grating, as a result of which only a small proportion of the circulating light on the second reflection grating is diffracted into the first order and decoupled from the circulation while the Weighing, non-diffracted portion of the second reflection grating is reflected in the specular order and is deflected by means of the plane mirror in such a way that it strikes the first reflection grating at the same angle as the coupled light field, but in mirror image of the grating normal.
- Symmetrical gratings should be used as reflection gratings, the grating constant of which is of the order of magnitude of the wavelength of the injected light.
- a phase front-changing element is arranged in the optical circulation, which element impresses a phase front change of approximately half a wavelength on the light field passing through.
- the following can be provided as elements that change the phase front: static elements such as wedge plates, diffusing disks, lens or mirror arrays, diffractive elements, holographic elements, light mixing rods; or rotating elements such as rotating prisms or Faraday rotators.
- Fig.l a first outgassing variant with two beam splitters in the optical
- FIG. 2 shows a second outgassing variant with two beam splitters in the optical circulation
- FIG. 3 shows a first outgassing variant with a beam splitter in optical circulation
- FIG. 4 shows a second outgassing variant with a beam splitter in optical circulation
- FIG. 5 shows an outgassing variant with a reflection grating and another element for beam splitting in optical circulation
- FIG. 6 shows an outgassing variant with two reflection gratings in the optical
- FIG. 7 shows an outgassing variant with a phase-changing element in circulation
- 8 shows the intensity distribution within the beam cross section of a laser beam emanating from an excimer laser
- FIG. 9 shows the noise reduction from 1 6% to 2.5% after 1 2 rotations
- Fig.l 0 in curve a a normalized intensity distribution and in curve b the difference between two normalized independent intensity distributions.
- an incident light field 1 strikes a first beam splitter 2 which has a splitter layer 3.
- the light field 1 is split into a portion 4 which passes through the divider layer 3 and into a portion 5 which is deflected by the divider layer 3 in the direction of a reflector 6.
- a second beam splitter 7 with a splitter layer 8 is arranged downstream of the first beam splitter 2.
- the radiation component 4 strikes the irradiation surface 8.1 of the divider layer 8, partly passes through the divider layer 8 and leaves the optical arrangement in the radiation component 9, while the remaining part of the radiation component 4 is deflected by the divider layer 8 in the direction of a reflector 10.
- the radiation component 5 When the radiation components 4 and 5 reach the second beam splitter 7, the radiation component 5 has covered a larger optical path than the radiation component 4.
- the difference in the optical path length is greater than the time coherence length of a laser pulse in the incident light field 1.
- a portion of the radiation portion 5 passes through the divider layer 8 and then mixes with the light of the radiation portion 4 deflected by the divider layer 8.
- the light of the radiation portion 5 deflected on the irradiation surface 8.2 of the divider layer 8 mixes with the light through the divider layer 8 - Incoming light of the radiation component 4 and leaves the optical arrangement with the radiation component 9.
- the radiation component 1 2, in which light from the radiation components 4 and 5 is mixed, is returned via the reflector 1 0 and a further reflector 1 3 to the beam splitter 2, where it strikes the radiation surface 3.2 of the splitter layer 3, which is the radiation surface 3.1 lies in parallel.
- part of the light of the radiation component 1 2 is directed from the splitting layer 3 in the direction of the radiation component 4 and mixes with it, while the remaining part passes through the splitting layer 3, mixes with the radiation component 5 and with it reaches the second beam splitter 7 again via the reflectors 6 and 11.
- the splitting takes place as already described, so that different radiation fractions pass through the circulation at different times and are combined with one another after their decoupling, changes in the wave fronts of the individual radiation fractions due to the multiple splitting and the optical path length difference and, as a result, a reduction in the coherence result in the laser light emerging from the arrangement.
- FIG. 2 The principle of a further exemplary embodiment is shown in FIG. 2, in which two beam splitters are also used to reduce coherence.
- the incident light field 1 strikes the incident surface 1 5.1 of the splitter layer 1 5 of a first beam splitter 1 4.
- the incident light field 1 is split into a radiation component 1 6, which passes through the splitter layer 1 5 and is directed onto a reflector 17, and a radiation component 1 8, which is directed to a reflector 1 9.
- the two radiation components 1 6 and 1 8 are merged with the splitter layer 21 in a second beam splitter 20 after reflection at the reflectors 1 7 and 1 9, respectively.
- the radiation component 16 impinges on the irradiation surface 21 .1 of the divider layer 21, passes through the divider layer 21 with a radiation component 22, while the rest of the radiation component 23 is deflected by the divider layer 21 and emerges from the optical arrangement.
- the radiation component 1 8 is also split up by the divider layer 21, a portion passing through the divider layer 21, mixing with the radiation component 23 and leaving the optical arrangement with it, while the remaining radiation component on the irradiation surface 21 .2 of the divider layer 21 into the Direction of the radiation portion 22 is deflected and mixes with it.
- These mixed radiation components 22 are returned to the beam splitter 14 via reflectors 24, 25, 26 and 27 and hit the irradiation surface 1 5.2 there, which lies opposite the irradiation surface 1 5.1 on the splitter layer 15 in parallel.
- the direction in which the radiation component 22 strikes the divider layer 15 forms an angle of 90 'with the direction in which the light field 1 falls on the divider layer 15.
- the radiation component 22 partly passes through the splitter layer 15 and reaches the second beam splitter 20 again via the reflector 19, where it is partly deflected at the irradiation surface 5.2 in the direction of the radiation component 16 and mixed with it reaches via the Reflector 1 7 the irradiation surface 21 .1 of the divider layer 21.
- this already mixed radiation partly passes through the splitter layer 21 and is returned via the reflectors 24 to 27 to the first beam splitter 14, partly deflected in the direction of the radiation component 23 on the incident surface 21 .1 and mixed with this leaves the optical arrangement.
- FIG. 1 Another embodiment is shown in FIG.
- the incident light field 1 first strikes the irradiation surface 30.1 of a divider layer 30 and is thereby split into a radiation component 31 and a radiation component 32.
- the radiation component 31 strikes a reflector 33 and is again reflected towards the divider layer 30, strikes the irradiation surface 30.2 and is included in the process deflected a radiation component 34, which leaves the optical arrangement and passes with the rest of the radiation component 35 through the divider layer 30 and thereby meets reflectors 36, 37, 38 and 39, through which it is deflected and now directed onto the irradiation surface 30.2 of the divider layer 30 is.
- This radiation component 35 is split again, the radiation component 35 being deflected partly in the direction of the radiation component 31 and mixing with it, the rest of it passes through the splitter layer 30 and then onto a reflector 40 and from there again onto the irradiation surface 30.1 Divider layer 30 is directed.
- This part of the radiation part 35 mixed with the radiation part 32 partly passes through the divider layer 30, mixes with the radiation part 34, is partly deflected at the irradiation surface 30.1, mixes with the radiation part 35 and reaches via the reflectors 36, 37, 38 and 39 repeats the divider layer 30.
- FIG 4 shows an embodiment of the arrangement according to the invention with only one beam splitter.
- the incident light field 1 strikes the incident surface 43.1 of the splitter layer 43 of a beam splitter 44, a radiation component 45 passing through the splitter layer 43 and leaving the optical arrangement.
- a second radiation component 46 is deflected by the irradiation surface 43.1 and reaches the beam splitter 44 again via reflectors 47, 48, 49 and 50, but in this case strikes the irradiation surface 43.2, which is aligned parallel to the irradiation surface 43.1 of the divider layer 43.
- the radiation component 46 partially passes through the divider layer 43 and mixes with the radiation component 46, which is likewise deflected in this direction by the incident light field 1, and is partly in the radiation surface 43.2 in the Deflected in the direction of the radiation component 45, mixes with this radiation component 45 and emerges from the optical arrangement with it.
- FIG.5 Another embodiment is shown in Fig.5.
- the incident light field 1 strikes a reflection grating 53, a radiation component 54 being diffracted in the direction of a beam splitter 55.
- the radiation component 54 is divided into a component 57, which passes through the splitter layer 56 and leaves the optical arrangement, and into a component 58, which is deflected at the splitter layer 56 in the direction of a reflector 59. split.
- the reflector 59 is oriented such that the portion 58 hits the reflection grating 53 again, the direction of irradiation including the same angle ⁇ with the normal to the reflection grating as the direction of irradiation of the incident light field 1.
- the incident light field 1 and the portion 58 strike the reflection grating 53 in mirror symmetry from different directions.
- a laser radiation suitable for illumination purposes is available at the output of the optical arrangement, which is composed of different radiation components that have passed through different optical path lengths.
- the incident light field 1 first strikes a reflection grating 62, from which, for example, a radiation component 63 diffracted in the first order is directed onto a second reflection grating 64, while a reflected second radiation component 65 reaches a reflector 66 who is so Tet is that the radiation portion 65 hits the reflection grating 64 opposite to the direction of incidence of the radiation portion 63.
- portions of the two radiation portions 63 and 65 striking the reflection grating 64 from opposite directions are diffracted and mixed to form a radiation portion 67 which emerges from the optical arrangement and is available as illumination light.
- the part of the radiation component 63 which is not diffracted at the reflection grating 64 is directed onto the reflector 66 in exactly the opposite direction to the direction of the incoming radiation component 65 and is directed back onto the reflection grating 62 by the latter, furthermore opposite to the radiation direction of the radiation component 65.
- This direction of irradiation with the normal to the reflection grating 62 includes the same angle ⁇ as the direction of irradiation of the incident light field 1 with the normal to the reflection grating 62, but the directions of incidence run in opposite directions.
- the reflection grating 62 there is in turn diffraction of the light incident in the opposite direction to the radiation component 65 to the reflection grating 64, this part mixing with the radiation component 63 and also with the simultaneously arriving and diffracted part of the incident light field 1.
- This optical circulation is constructed without transmissive elements and can therefore be used advantageously for wide wavelength ranges of the illuminating light. This is of particular interest for those wavelengths at which no or insufficiently transmissive materials are available for the construction of beam splitters, such as at the wavelengths of deep UV light.
- a special feature of this exemplary embodiment is furthermore that, owing to the odd number of optical elements involved, the wavefront is flipped over per revolution, which leads to additional mixing of the individual radiation components.
- the reflection gratings 62 and 64 can be understood in this exemplary embodiment as triple beam splitters. Since both the specular and the diffracted orders lie geometrically within the optical orbit, several passes can be realized, which results in a further improvement in the signal-to-noise ratio in an optical image, for which the illumination light which is homogenized in this way or which is reduced in terms of coherence is used.
- the reflection gratings 62 and 64 are designed symmetrically, and their grating constants are each in the order of magnitude of the wavelength of the incident light field 1.
- a special feature is that the incident light field 1 is irradiated into the reflection grating 62 at an angle ⁇ in such a way that the angle ⁇ d of the first diffracted order is just 0 and thus coincides with the grating normal.
- the second diffracted order is therefore always in autocollimation, ie it coincides with the angle of incidence.
- the reflection grating 62 is formed with a relatively high efficiency. Light from the incident light field 1 is diffracted into the first order and thus coupled into the circulation.
- the second reflection grating 64 arrives.
- the number of lines and the angular relationships on the reflection grating 64 are the same as those on the reflection grating 62, but the efficiency of the reflection grating 64 is designed to be lower.
- part of the incident light is diffracted into the first order and mixed into the radiation component 67.
- the greater part, however, is reflected in the specular order and deflected with the aid of the reflector 66 so that it falls on the reflection grating 62 at the same angle ⁇ as the incident light field 1, but on the opposite side of the grating normal. Because of the grating symmetry, depending on the grating efficiency, light is again diffracted into the first order, which geometrically corresponds to the first order of the incident light field 1. This closes the optical circuit.
- the light diffracted into the second order depending on the efficiency distribution at the reflection gratings 62 and 64 passes through the circulation in the opposite direction Direction and until the interference in the radiation component 67 or until the coupling out of the optical arrangement according to the invention changes in the optical path lengths which are different from the actual circulation. This results in an even stronger mixing of the phase fronts and thus increases the effect of avoiding speckle.
- the optical path length of the orbit should be as large as possible than the coherence length of the light, as a result of which the decoupled sub-fields add up in the intensities, but not in the amplitudes, so that undesired interference phenomena do not occur.
- An optical element influencing the phase front of the light can be placed in the beam path between the reflection grating 62 and the reflection grating 64, which element is passed by the light with each revolution, so that a different phase front is established with each revolution.
- the incident light field 1 passes through an element 69 that changes the phase front and is then branched into a radiation component 70 and a radiation component 71.
- the branching can take place with a beam splitter 72.
- the radiation component 70 While the radiation component 70 emerges from the optical arrangement, the radiation component 71 is returned (for example via reflectors 73, 74), coupled back into the incident light field 1 by means of a beam splitter 75 and passes through the phase front-changing element 69 together with it.
- phase front-changing element 69 is thus traversed several times and different times by different radiation components, which increases the efficiency of the speckle reduction.
- the speckle pattern of the individual pulses is smeared with one another in lighting systems that work with pulse-powered lasers, and the pulses are then averaged.
- the arrangement according to the invention with which it is achieved that the speckle pattern “smears” within a single pulse. This makes it possible to increase the signal-to-noise ratio even in single-pulse operation.
- a major advantage of the optical arrangement according to the invention is that mechanically moved optical elements are not required. For example, there is no need to rotate a lens
- the number of transmissive optical elements can be reduced in the construction of the arrangement according to the invention by using diffractive optical elements; This has the advantageous consequence that such arrangements are also suitable for wavelengths in which little or no low-absorption materials are available, such as in particular in the deep UV range.
- An optical circulation as shown in FIG. 7, can be understood as an optical resonator, which acts as an optical filter and thus reduces the spectral density of the frequency noise in the transmitting field.
- the bandwidth of the transmitted field results from the folding of the bandwidth of the primary field, that is. of the incident light field, with the transfer function of this filter.
- the bandwidth of the filter is wide compared to the bandwidth of the field, there is no unwanted reduction in the spectral density of the frequency noise of the light field. This can result from the relatively high coupling rates of the resulting low finesse.
- the circulation can be technically realized in such a way that no stable resonator is created and thus no resonator mode can develop.
- phase front-changing element 69 can be provided as the phase front-changing element 69 in FIG.
- the phase front changing element 69 is run through at least once per revolution, so that after each revolution the phase front of the light field is different from the phase front of the previous revolution.
- the various subfields or radiation components circulate different times after they have been decoupled, they have different phase fronts or polarization states from one another.
- the result of this is that the speckle pattern is smeared within an image, the decoupled laser light of which is used, and the signal-to-noise ratio and, as a result, the image quality also increase.
- FIGS 8, 9, 10 and 11 serve to document the efficiency of the optical arrangement according to the invention.
- FIG. 8a shows the intensity distribution within the beam cross section of a light field 1 emanating from an excimer laser and incident in the optical arrangement according to the invention.
- FIG. 8b shows the intensity distribution along a dash line.
- FIG. 9 in particular in comparison of the diagrams a and b, shows the noise reduction from 1 6% to 2.5% after 1 2 cycles.
- Fig. 10 shows with curve a a normalized intensity distribution and with curve b the difference between two normalized independent intensity distributions.
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- Optics & Photonics (AREA)
- Lasers (AREA)
Abstract
Description
Claims
Applications Claiming Priority (5)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
DE10223106 | 2002-05-22 | ||
DE10223106 | 2002-05-22 | ||
DE10322806 | 2003-05-16 | ||
DE10322806A DE10322806B4 (de) | 2002-05-22 | 2003-05-16 | Optische Anordnung zur Homogenisierung eines zumindest teilweise kohärenten Lichtfeldes |
PCT/EP2003/005275 WO2003098316A2 (de) | 2002-05-22 | 2003-05-20 | Optische anordnung zur homogenisierung eines zumindest teilweise kohärenten lichtfeldes |
Publications (1)
Publication Number | Publication Date |
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EP1506448A2 true EP1506448A2 (de) | 2005-02-16 |
Family
ID=29550944
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP03752770A Withdrawn EP1506448A2 (de) | 2002-05-22 | 2003-05-20 | Optische anordnung zur homogenisierung eines zumindest teilweise koh renten lichtfeldes |
Country Status (2)
Country | Link |
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EP (1) | EP1506448A2 (de) |
WO (1) | WO2003098316A2 (de) |
Family Cites Families (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US6191887B1 (en) * | 1999-01-20 | 2001-02-20 | Tropel Corporation | Laser illumination with speckle reduction |
US6956878B1 (en) * | 2000-02-07 | 2005-10-18 | Silicon Light Machines Corporation | Method and apparatus for reducing laser speckle using polarization averaging |
-
2003
- 2003-05-20 EP EP03752770A patent/EP1506448A2/de not_active Withdrawn
- 2003-05-20 WO PCT/EP2003/005275 patent/WO2003098316A2/de not_active Application Discontinuation
Non-Patent Citations (1)
Title |
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See references of WO03098316A3 * |
Also Published As
Publication number | Publication date |
---|---|
WO2003098316A3 (de) | 2004-09-16 |
WO2003098316A2 (de) | 2003-11-27 |
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Inventor name: ECKARDT, PETER Inventor name: DARSCHT, MAXIM Inventor name: DOBSCHAL, HANS-JUERGEN Inventor name: STEINER, REINHARD Inventor name: ZOELLNER, FRIEDRICH Inventor name: TRAEGER, STEFAN Inventor name: KOWARSCHIK, RICHARD Inventor name: BISCHOFF, JOERG Inventor name: RUDOLF, KLAUS Inventor name: BRUNNER, ROBERT |
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