Device for the interferometric checking of optical surfaces

Abstract

The invention relates to a device (1) for the interferometric checking of optical surfaces (2), comprising a light source (4) designed to generate an exposure beam (5), wherein at least one conversion element (6), in particular a conversion crystal, is designed to convert the exposure beam (5) into a measurement beam (7) and a reference beam (8), wherein the device (1) has a light-guiding element (9), in particular a beam splitter, which is designed to guide the reference beam (8) into a reference path and the measurement beam (7) into a measurement path of the device (1), wherein the device (1) has a wave front adjustment apparatus (11) in the measurement path.

Description

[0001] Device for interferometric testing of optical surfaces

[0002] The invention relates to a device for the interferometric testing of optical surfaces, comprising a light source designed to generate an exposure beam.

[0003] Devices for the interferometric testing of optical surfaces are generally known from the prior art. Such interferometric testing is typically used to measure optical surfaces or to determine deviations of an optical surface, for example a lens surface, from a desired shape. Interferometric surface testing works very well using radiation in the visible spectrum, although applications in the so-called NIR spectral range (near-infrared spectrum) are also possible. Applications at longer wavelengths are considerably more complex. This is because the sensitivity of the measuring system suffers and the required available detectors are less sensitive and significantly more expensive. A further disadvantage is that light sources for this spectral range are usually less developed than their equivalents in the visible range.

[0004] If the optical surface to be tested is an asphere or aspherical, a device is usually used that is designed to test the optical surface using a wavefront adaptation device, specifically with a null test device in an interferometric null test, in order to meet the high accuracies of today's manufacturing processes such as magnetorheological polishing. Since the null test, for example, adapts an incident plane wave to the (known) ideal surface shape of the asphere, only deviations from the ideal target shape become visible in the form of interference fringes, for which the entire sensitivity range of the interferometer is available. The adaptation is usually achieved using diffractive null elements that are manufactured lithographically. However, this does impose limitations on the aspherical shape.Steep aspheres lead to structural sizes in the zero element used that are difficult to manufacture.

[0005] Alternatively, aspheres can also be measured using radiation with higher wavelengths, for example radiation in the infrared range of the spectrum, although the disadvantages described above with regard to the generation of the radiation and the detection of the signal must be accepted.

[0006] The invention is based on the object of providing an improved device for the interferometric testing of optical surfaces. This object is achieved by a device having the features of claim 1. Advantageous embodiments are the subject of the dependent claims.

[0007] As described above, the invention relates to a device for the interferometric testing of optical surfaces. The device comprises a light source which is designed to generate an exposure beam. As explained above, in principle any exposure beams or radiation in any form can be generated as an “exposure beam”, wherein, depending on the detector used and, if appropriate, in coordination with the conversion element described below, exposure beams can be selected or the exposure beam and the light source which generates the exposure beam can be selected such that radiation suitable for the detector is generated. In particular, the exposure beam can be generated such that the radiation which falls on the detector has a suitable spectrum orspectral range, so that a comparatively less complex detector can be used and a good signal-to-noise ratio can be achieved.

[0008] The invention is based on the finding that a conversion element, in particular a conversion crystal, is provided which is designed to convert the exposure beam into a measurement beam and a reference beam. The conversion element can in principle be designed to generate a measurement beam and a reference beam from the exposure beam by interaction, in particular by parametric fluorescence (also called "parametric down conversion", PDC), which both have different wavelengths. The device further comprises a (first) light-guiding element, for example a beam splitter, which is designed to guide the reference beam into a reference path and the measurement beam into a measurement path of the device, wherein the device has a wavefront adaptation device, in particular a zero test device, in the measurement path.Essentially, the exposure beam is guided onto or through the conversion element and used to convert the exposure beam into a measurement beam and a reference beam based on the mechanism of the conversion element. Specifically, a pump beam is used as the exposure beam, which passes through the conversion element and, as described, generates the measurement beam and the reference beam through parametric fluorescence. The terms "exposure beam" and "pump beam" can therefore be used analogously or interchangeably in the following description.

[0009] The procedure of generating a measurement beam and a reference beam from the exposure beam using a conversion element, for example, a conversion crystal, by means of PDC is generally known in the context of quantum OCT, i.e., a quantum-based optical coherence tomography method. Since such methods, in particular quantum OCT, are already known from the prior art, the details of such methods will not be discussed in detail here. In the following description, particularly in places where the conversion element and the conversion into a measurement beam and reference beam or signal beam are described in more detail, the necessary details will be discussed where necessary.

[0010] As described, in addition to the light source and the conversion element, which is designed to convert the exposure beam generated or emitted by the light source into the measuring beam and the reference beam, the device comprises a wavefront adaptation device, for example a zero test device. The wavefront adaptation device is designed to test the optical surface as part of an adapted interferometric test. This means, in particular, that the optical surface is not tested in its entirety, but rather the radiation incident on the optical surface, in particular the measuring beam, is modified such that the changes made to the measuring beam are adapted entirely to the desired shape of the optical surface in the case of the zero test or a zero test device, or generally only partially.

[0011] The incident radiation can be selected or adjusted as a plane wave and adjusted to the (known) ideal surface shape of the optical surface of the test piece or measurement object by placing a suitable object in the beam path of the wavefront adjustment device. If the adjustment to the desired shape is completed, the wavefront adjustment device is referred to as a zero test device, or the wavefront adjustment as a zero test. This means that only deviations from the ideal desired shape become visible in the form of interference fringes on the detector, for which the entire sensitivity range of the interferometer is available.

[0012] In other words, with a perfectly manufactured optical surface that corresponds to its ideal target shape, the zero test achieves complete compensation for the change in the measuring beam deliberately introduced in the zero test device, so that the measuring beam and reference beam do not exhibit any deviations from each other. As a result, the zero test only determines deviations in the shape of the optical surface from its target shape, rather than determining the absolute shape of the optical surface. This advantageously allows the measuring range of the zero test device and thus the measuring range of the device to be fully used for deviations from the target shape. If a general change that deviates from complete compensation is carried out instead of the zero test, this is referred to as general wavefront adaptation.The device is thus designed to provide an exposure beam through a light source, which is converted or split into the measurement beam and the reference beam by means of a conversion element. A (first) light-guiding element, for example a beam splitter, guides the reference beam into the reference path, for example to a reference mirror, and the measurement beam into the measurement path and thus onto the optical surface to be tested, for example an optical measurement object or test piece. Since a wavefront adaptation device is arranged in the measurement path of the device, the optical surface, i.e. the optical measurement object to be tested, can be tested as described.

[0013] The wavefront adaptation device can have at least one diffractive optical element for adapting the incident measuring beam to the optical surface, wherein the diffractive optical element guides the measuring beam onto the measurement object. As described, the diffractive optical element can be matched to the optical surface to be tested in such a way that, with a perfectly manufactured measurement object, the changes in the measuring beam introduced by the diffractive optical element are just compensated for by the optical surface to be tested, so that the measuring beam reflected at the optical surface and exiting through the wavefront adaptation device exactly corresponds to the measuring beam incident on the wavefront adaptation device. Deviations in the optical surface result in the outgoing measuring beam deviating from the incident measuring beam.The deviations can thus be detected, allowing the optical surface to be characterized based on the detected deviations. In other words, the diffractive optical element modifies the wavefront of the measuring beam in such a way that, when the modified measuring beam is reflected from the measurement object, the introduced changes are compensated for, corresponding to the desired shape of the test object. Remaining deviations indicate a deviation of the optical surface from the desired shape.

[0014] By splitting or converting the exposure beam into the measurement beam and the reference beam, it is achieved in particular that the measurement beam can have a wavelength suitable for the optical zero test, while the reference beam can have a different wavelength, which, for example, simplifies detection. As described, in the first case, the use of long-wave radiation outside the visible spectrum is particularly advantageous in order to keep the required diffractive optical elements as simple and inexpensive to manufacture as possible. In particular, structure sizes, for example a period, of such diffractive optical elements must meet less stringent requirements, so that they can be manufactured with less effort. In the second case, however, visible light is preferable in order to enable signal detection with low technological effort and a high signal-to-noise ratio.

[0015] The conversion element can fundamentally be designed to generate a signal beam through interaction, in particular through parametric fluorescence, with the measurement, reference, and exposure beams reflected by the zero test configuration and the reference mirror. The described signal beam can have the same wavelength as the reference beam and can be guided to the detector, for example by means of a (second) light-guiding element, where it can therefore interfere with the reflected reference beam. In particular, the information encoded in the measurement beam regarding the surface quality of the test specimen is transferred to the signal beam. The (second) light-guiding element can in turn be a beam splitter. The measurement beam is, as described, reflected by the optical element to be measured or the optical surface under test and guided back to the conversion element.Accordingly, the reference beam is guided from the conversion element into the reference path, where the reference beam is reflected, for example at a plane mirror, and also guided back to the conversion element.

[0016] The conversion element thus receives the reflections of the measurement beam, the reference beam, and the pump beam, so that, based on the parametric fluorescence, a signal beam of the same wavelength as the reference beam can be generated. This signal beam is guided through the (second) light-guiding element to the detector and detected there. Specifically, the interference pattern generated by the reference beam and signal beam is detected. This pattern contains information about the surface deviations of the test object and can be further processed using conventional optical metrology methods.

[0017] As described, the conversion element generates a reference beam and a measurement beam. The reference beam and the measurement beam form entangled photon pairs in the sense of quantum OCT, with one photon (usually in the MIR (mid-infrared) or even the THz range) interacting with the measurement object as the measurement beam, while the other photon is guided into the reference path as the reference beam. In the second pass, the measurement beam (usually in the visible or NIR) transfers its information to a signal beam, which is also generated via PDC but now has a wavelength in the visible range. It is guided to the detector and superimposed there with a photon of the same wavelength from the reference beam.

[0018] The pairs are generated via the quantum effect of spontaneous parametric fluorescence in a nonlinear interferometer, where the photons generated from the exposure beam via PDC are referred to as s- and i-photons. Interference is observed between two s-photons provided they share a common i-beam. In this case, the role of the interfering s- and i-photons is embodied by the signal and reference beams, as well as the measurement beam. This technique is based on the phenomenon of quantum coherence and allows for metrological acquisition in "difficult" spectral ranges (MIR or THz), while detecting in a suitable range, particularly the visible or NIR.

[0019] The high-gain PDC regime is achieved by strong pumping, typically using ps pulses in conversion elements, especially conversion crystals a few mm long. The signal and idler photon rates of the twin beams scale exponentially with the pump power. Typical values ​​for parametric amplification extend up to a gain value of G = 10, realized by focusing the pump beam into 2-3 mm long crystals, resulting in an increase in the input intensity by a factor of sin h 2 (G) leads.

[0020] High-gain PDC is advantageous for nonlinear interferometry. It allows high detector power, allowing conventional detectors to be used in the visible range without reducing the signal-to-noise ratio.

[0021] In summary, it can thus be advantageously achieved that the measuring beam and the reference beam have different wavelengths. In particular, the measuring beam can also have a different wavelength than the signal beam used for interference. The measuring process, i.e. the interaction with the measurement object or the optical surface to be tested and the detection of the signal on the detector can thus take place in different wavelengths or spectral ranges. This allows a suitable spectral range to be selected for the individual processes, namely the detection of the signal and the interaction with the measurement object. Infrared radiation in particular can be selected for the interaction with the measurement object, i.e. for the measuring beam, whereby a wavelength in the visible spectral range or in the NIR spectral range is selected for the detection of the signal or as the reference beam or signal beam.

[0022] As described, the measuring beam should have a wavelength that is longer and thus above the visible spectrum, in particular also above the near-infrared spectrum, preferably >3 pm. As described, the interaction with the optical surface to be tested can take place in the infrared range, so that the requirements for the diffractive optical element of the wavefront adaptation device, which changes the wavefront of the measuring beam, can be reduced, particularly with regard to manufacturability. In this regard, it has been found in particular that radiation in the infrared range or a range with longer wavelengths than the visible spectrum is advantageous for diffraction at the grating of the diffractive optical element. In particular, a spectral range that is greater than 4 pm, in particular between 4 and 5 pm, can be used here.

[0023] The reference beam and / or the signal beam can have a different wavelength. The signal beam and / or the reference beam can have a wavelength in the visible spectrum. This fundamentally simplifies the acquisition or detection of the signal. Simplified detectors can be used for this purpose, particularly simplified compared to a detector operating in the IR spectral range. For example, standard CCD or CMOS detectors can be used, allowing the device to be designed more simply and signal detection to be improved, for example, in terms of the signal-to-noise ratio.

[0024] The conversion element can be designed, in particular, to generate the signal beam with a defined number of photons and / or a defined power on the detector, for example, depending on at least one object parameter of an optical element to be tested. The object parameter of the optical element to be tested or of the optical measurement object that has the optical surface to be tested can, for example, define the intensity that the measurement object can tolerate. Advantageously, the intensity of the measurement beam can be selected so that it lies at the upper range of what the measurement object can tolerate. This can be controlled, for example, via the intensity of the pump beam.

[0025] As described above, the conversion element can convert the exposure beam, for example, a pump beam, into a measurement beam and a reference beam, or generate a measurement beam and a reference beam with the pump beam through the conversion element. The conversion element can specifically be designed as a conversion crystal or comprise such a conversion crystal, in particular ß-barium borate or lithium niobate. Depending on the desired mechanism or the desired measurement beam and reference beam, various conversion crystals can be used as the conversion element.

[0026] As explained at the beginning, the device described can be used, in particular, to test aspheres or aspheric optical surfaces or aspheric optical elements. Specifically, the device can be designed to detect at least one parameter of optical elements or measurement objects with a numerical aperture greater than 0.3-0.5, in particular greater than 0.7. The at least one parameter can, in particular, be a deviation from a desired shape. The described optical elements or optical surfaces are characterized by a comparatively large numerical aperture, so that the device can be designed, in particular, to measure comparatively steep optical surfaces.

[0027] Furthermore, the invention relates to a method for the interferometric testing of optical surfaces by means of a device, in particular a device described above, wherein the exposure beam is converted into a measuring beam and a reference beam by means of a conversion element, in particular a conversion crystal, which is illuminated with an exposure beam, wherein the reference beam is guided into a reference path and the measuring beam is guided into a measuring path of the device which has a wavefront adaptation device, in particular a zero test device, and at least one parameter of an optical element arranged in the measuring path and having the optical surface is determined, in particular based on an interference of a signal beam generated by the conversion element with the reference beam.

[0028] All advantages, details and features described with regard to the device are fully transferable to the process.

[0029] The invention is explained below using an exemplary embodiment with reference to the figure. The figure is a schematic representation of a device for the interferometric testing of optical surfaces.

[0030] The figure shows a device 1 for the interferometric testing of an optical surface 2 of a measurement object 3 or a test piece. The measurement object 3 can in particular be an asphere, i.e. an optical element with an aspherical optical surface 2, for example with a numerical aperture greater than 0.3-0.5, in particular greater than 0.7. The device 1 has a light source 4 which is designed to generate an exposure beam 5, in particular a "pump beam". In this exemplary embodiment, the exposure beam 5 is guided onto or through a conversion element 6, for example a conversion crystal, in particular made of ß-barium borate or lithium niobate.

[0031] In the conversion element 6, the exposure beam 5 is converted into a measuring beam 7 and a reference beam 8. In other words, the exposure beam 5 in the conversion element e stimulates the generation of a measuring beam 7 and a reference beam 8. The conversion of the exposure beam 5 into the measuring beam 7 and the reference beam 8 is based on spontaneous parametric fluorescence or "parametric down conversion" (PDC). After the conversion element 6, the reference beam 8 is guided through a light-guiding element 9, for example a beam splitter, to a reference mirror 10 arranged in a reference arm of the device 1, at which the reference beam 8 is reflected and guided back to the conversion element 6 along the same path.

[0032] The measuring beam 7 is guided from the conversion element 6, for example, through the light-guiding element 9, along a measuring path of the device 1 into a wavefront adjustment device 11, which is designed, for example, as a zero test device. In this exemplary embodiment, the wavefront adjustment device 11 comprises a beam expander 12 and a diffractive optical element 13. The diffractive optical element 13 is inclined to the optical axis to suppress reflections from the measurement object 3.

[0033] The diffractive optical element 13 is manufactured in coordination with the optical surface 2 of the measurement object 3, so that the diffractive optical element 13 guides the expanded measurement beam 7 in a modified manner onto the optical surface 2. The change in the wavefront introduced by the diffractive optical element 13 should be partially or completely adapted to the shape of the measurement object 3 or, in the case of a zero test, precisely compensate for this. In other words, the measurement beam 7 is guided by the diffractive optical element 13 onto the optical surface 2 and reflected off the optical surface 2. If the optical surface 2 is manufactured according to its desired shape, the changes introduced by the optical surface 2 and the diffractive optical element 13 partially or completely compensate each other, so that the outgoing measurement beam 7 should ultimately exhibit partial or no changes compared to the incoming measurement beam 7.In the case of a complete adaptation, the incoming measuring beam 7 would exactly correspond to the outgoing measuring beam 7 or the reflected measuring beam 7, which is guided from the measuring object 3 back to the conversion element 6.

[0034] Deviations from the desired shape of the optical surface 2 can thus be identified, since they are not compensated for by the diffractive optical element 13. In other words, the wavefront adjustment device 11 allows deviations from the desired shape of the optical surface 2 to be identified, so that the device 1 is limited to deviations across the entire sensitivity range and does not have to determine the overall shape of the optical surface 2.

[0035] As described, the measuring beam 7 is guided from the measurement object 3, in particular the optical surface 2, back to the conversion element 6. The information carried in the measuring beam 7 can be transferred to a signal beam 14. In other words, the reflected exposure beam 5 and the measuring beam 7 reflected by the optical surface 2 and the reference beam 8 reflected by the reference mirror 10 interact, thus generating the signal beam 14, which can be guided by the conversion element 6 to a detector 15, in particular reflected by a second light-guiding element 16, which can also be a beam splitter.

[0036] The signal beam 14, together with the reference beam 8 reflected by the reference mirror 10, can be guided to the detector 15 and thus interfere with it. Based on the interference pattern on the detector 15, it is thus possible to analyze or determine the deviations from the desired shape of the optical surface 2. During spontaneous parametric fluorescence, a photon pair is generated, in which one photon is guided as the measuring beam 7 onto the optical surface 2 and thus interacts with the measurement object 3, while the other photon can be guided as the reference beam 8 onto the reference mirror 10. As also described, a photon pair is again generated by the reflected portion of the measuring beam 7, the exposure beam 5 or "pump beam," and the reflected reference beam 8 in the conversion element e, one photon of which is guided as the signal beam 14 to the detector 15.Interference can be observed on the detector 15 between the two photons, i.e. the reference beam 8 and the signal beam 14, provided that their counterparts are identical, which is the case here.

[0037] Interaction of the measuring beam 7 with the signal beam 14 on the detector 15 is therefore not necessary. The detector 15 can therefore be aligned exclusively to the radiation or the wavelength of the reference beam 8 or the signal beam 14. This can, in particular, be in the visible range, so that the detector 15 can be designed as a CCD or CMOS sensor or can have one. This advantageously allows radiation with a different wavelength than the radiation for the reference beam 8 or the signal beam 14 to be used as the measuring beam 7.

[0038] Specifically, radiation in the mid-infrared range up to the terahertz range is used as measuring beam 7, particularly radiation with a wavelength greater than 3 pm, especially between 4 and 5 pm. In contrast, light with a wavelength in the visible spectrum can be used as signal beam 14 or reference beam 8 to improve detection. REFERENCE SYMBOL LIST

[0039] Device optical surface

[0040] Measurement object

[0041] light source

[0042] Exposure beam

[0043] Conversion element

[0044] measuring beam

[0045] Reference beam

[0046] Light guide element

[0047] Reference mirror

[0048] Wavefront adjustment device

[0049] Beam expansion diffractive optical element

[0050] signal beam

[0051] detector

[0052] Light guide element

Claims

PATE N CLAIMS Device (1) for the interferometric testing of optical surfaces (2), comprising a light source (4) designed to generate an exposure beam (5), wherein at least one conversion element (6), in particular a conversion crystal, is designed to convert the exposure beam (5) into a measuring beam (7) and a reference beam (8), wherein the device (1) has a light guide element (9), in particular a beam splitter, which is designed to guide the reference beam (8) into a reference path and the measuring beam (7) into a measuring path of the device (1), wherein the device (1) has a wavefront adaptation device (11), in particular a zero test device, in the measuring path.Device (1) according to claim 1, characterized in that the wavefront adaptation device (11) has at least one diffractive optical element (13), wherein the diffractive optical element (13) guides the measuring beam (7) onto the measurement object (3). Device (1) according to claim 1 or 2, characterized in that the conversion element (6) is designed to generate a signal beam (14) by interaction, in particular parametric fluorescence, of the reflected reference beam (8) with the measuring beam (7) reflected from the measurement object (3). Device (1) according to one of the preceding claims, characterized in that the measuring beam (7) and the reference beam (8) have different wavelengths. Device (1) according to one of the preceding claims, characterized in that the measuring beam (7) has a wavelength above the visible spectrum, in particular above the near-infrared spectrum, preferably >3 pm.Device (1) according to one of the preceding claims, characterized in that the second signal beam (14) and / or the reference beam (8) has a wavelength in the visible spectrum. Device (1) according to one of the preceding claims, characterized in that the conversion element (6), in particular depending on at least one object parameter of an optical measurement object (3) to be tested, is designed to generate the second signal beam (14) with a defined number of photons and / or a defined power on a detector (15). Device (1) according to one of the preceding claims, characterized in that the conversion element (6) is designed as a conversion crystal or comprises at least one conversion crystal, in particular Ba borate or Li niobate. Device (1) according to one of the preceding claims, characterized in that the device (1) is designed to detect at least one parameter of optical measurement objects (3) with a numerical aperture >0.3 - 0.5, in particular >0.7.Method for the interferometric testing of optical surfaces (2) by means of a device (1), in particular a device (1) according to one of the preceding claims, wherein the exposure beam (5) is converted into a measuring beam (7) and a reference beam (8) by means of a conversion element (6), in particular a conversion crystal, which is illuminated with an exposure beam (5), wherein the reference beam (8) is guided into a reference path and the measuring beam (7) is guided into a measuring path of the device (1) which has a wavefront adaptation device (11), in particular a zero test device, and at least one parameter of an optical measurement object (3) arranged in the measuring path and having the optical surface (2) is determined, in particular based on an interference of a second signal beam (14) generated by the conversion element (6) with the reference beam (8).

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