Three-wavelength interferometric measurement device and method - Patents.com

Abstract

The present disclosure relates to an interferometric measurement device (10) for measuring a surface (2, 4) or a profile of an object (1), the measurement device (10) comprising: - a beam generating unit (20) operable to generate a measurement beam (BM) having a spectral content at a wavelength less than 550 nm; - a splitter (55) for splitting an object beam (BO) and a reference beam (BR) from the measurement beam (BM); - a measurement probe (50) connected to the beam generating unit (20) and configured to direct the object beam (BO) towards the surface (2, 4) and to capture a portion of the object beam reflected from the surface (2, 4) as a signal beam (BS); and a signal analyzer (80) operable to derive a distance (D) between the measurement probe (50) and the surface (2, 4) based on signals acquired from first and second detectors (71, 72) connected to the detector unit (70) and operable to detect first and second partial beams (BP1, BP2) from an analysis beam (BA), which is a recombination of the signal beam (BS) and the reference beam (BR).

Description

[Technical field]

[0001] The present disclosure relates to an interferometric measurement device for measuring a surface or profile of an object, such as an optical element. In a further aspect, the present disclosure relates to a method for measuring a surface or profile of an object. In another aspect, the present disclosure relates to an interferometric measurement device and respective methods for measuring a surface or profile of an object, the respective surface comprising or covered by a coating. [Background technology]

[0002] Non-contact measurement of the surface or profile of an object, e.g. an optical element such as a lens, is per se well known and established in the art. An interference mechanism splits a measurement light beam into a reference beam and an object beam, and the object beam is directed to the surface of the object and reflected from the surface of the object as a signal beam. When the signal and reference beams are recombined, optical interference can be observed that indicates the runtime difference, and therefore the path difference, between the reference and signal beams.

[0003] The measurable phase shift between the signal and reference beams is only evident within a range defined by half the wavelength. When the path difference between the signal and reference beams is greater than half the wavelength of the measurement beam, the respective interference patterns generated by the recombination of the signal and reference beams repeat. Thus, the absolute distance range of such interference mechanisms is rather generally limited. To increase the spatial measurement range, longer wavelengths can be used, for example wavelengths in the infrared spectral range.

[0004] Furthermore, if the object, e.g. an optical element, comprises a coating on the surface to be measured, this coating may be relatively thin and may include a thickness in the range of only a few micrometers or even less than 1 μm. Then, when the coating is substantially transparent to the wavelength of the measurement beam, a first reflection will occur at the outer surface of the coating and a second reflection will occur, e.g., at or from an inner or lower surface of the coating that is in direct contact with the outer surface of the object. It is very difficult, if not impossible, for interferometric measurement devices and methods to distinguish between these first and second reflections arising from both sides of such a coating. Summary of the Invention [Problem to be solved by the invention]

[0005] It is therefore desirable to provide an improved interferometric measurement device and an improved method for measuring the surface or profile of an object that allows for precise surface or profile measurements even when the object has a coating on its surface on which measurements are to be made. Furthermore, the interferometric measurement device may have to operate with a measurement beam within the visible spectral range. It is therefore desirable to provide a cost-effective, reliable, long-lasting and robust solution for enabling interferometric measurements of coated surfaces. At the same time, the improved interferometric measurement device should provide a relatively large measurement range for measuring absolute distances between the device and the surface of the object.

[0006] Interferometric measurement devices should be broadly applicable and easily and unambiguously reconfigurable to match or accommodate a variety of variable measurement conditions. Interferometric measurement devices and methods should also be robust to environmental influences and variations. [Means for solving the problem]

[0007] These needs are generally solved by an interferometric measuring device and a method for measuring a surface or a profile of an object according to the subject matter of the independent claims. Numerous embodiments are subject to the dependent claims.

[0008] In one aspect, an interferometric measurement device is provided for measuring a surface or profile of an object, such as an optical element, for example a lens or mirror. The interferometric measurement device includes a beam generating unit. The beam generating unit includes at least a first light source. The beam generating unit, and thus the first light source, is operable to generate a measurement beam. The measurement beam includes spectral components with wavelengths less than 550 nm. Typically, the measurement beam includes a central wavelength in a range less than 500 nm and typically greater than 380 nm. The wavelength of the measurement beam is typically within the visible spectrum less than 550 nm.

[0009] The interferometric measurement device further includes a splitter that splits the object beam and the reference beam from the measurement beam. In some examples, the splitter can be configured or operable to split the measurement beam into the object beam and the reference beam. The measurement device further includes a measurement probe optically coupled to the beam generating unit. In some examples, the measurement probe is optically coupled to the beam generating unit by at least one optical fiber. In other examples, the measurement probe is optically coupled to the beam generating unit by an optical device that provides free propagation of the measurement beam from the beam generating unit to the measurement probe.

[0010] The measurement probe is configured to direct an object beam to a surface of the object and capture a portion of the object beam reflected from the surface as a signal beam.

[0011] Typically, the signal beam comprises, or is a portion of, the object beam reflected from the surface of the object to be measured. The interferometric measurement device further comprises an optical recombiner operable to recombine the signal beam and the reference beam into an analysis beam. Typically, the analysis beam comprises a superposition of the reflected signal beam and the reference beam. The analysis beam undergoes interference and exhibits interference fringes or an interference pattern, which indicates a path difference between the reference beam and the signal beam and therefore the portion of the object beam reflected by the surface and captured by the measurement probe.

[0012] In some examples, the optical beam recombiner is integrated into the splitter and / or other splitters are integrated into the optical beam recombiner. Thus, a single optical component or optical fiber can be provided that provides both the splitter and the optical beam recombiner. In some examples, a fiber end face of an optical fiber provides both the splitter and the optical beam recombiner. Typically, the fiber end face that terminates inside the measurement probe acts as a splitter, generates a reference beam and an object beam from the measurement beam, and recombines the signal beam reflected from the object surface with the reference beam.

[0013] The interferometric measurement device further includes a beam splitter unit optically coupled to the optical beam recombiner and / or the measurement probe. In some examples, the beam splitter unit is optically coupled to the optical beam recombiner and / or the measurement probe by at least one optical fiber. In other examples, the beam splitter unit is optically coupled to the optical beam recombiner and / or the measurement probe by an optical device that provides free propagation of the respective optical beams.

[0014] The beam splitter unit is operable to extract a first partial beam of a first central wavelength from the analysis beam and to extract a second partial beam of a second central wavelength from the analysis beam. Typically, the second central wavelength is distinct from the first central wavelength. The interferometric measurement device further comprises a detector unit optically coupled to the beam splitter unit, for example through at least one optical fiber. In another example, the detector unit is optically coupled to the beam splitter unit by an optical arrangement providing free propagation of the respective light beams. The detector unit comprises at least a first detector for detecting the first partial beam and further comprises at least a second detector for detecting the second partial beam.

[0015] Typically, at least one of the first central wavelength and the second central wavelength is in a spectral range less than 550 nm, since the measurement beam provided and generated by the beam generating unit includes a spectral component with a wavelength less than 550 nm. In some examples, both the first central wavelength and the second central wavelength are in a spectral range less than 550 nm.

[0016] Finally, the interferometric measurement device includes a signal analyzer connected to the detector unit in signal communication, the signal analyzer operable to derive or calculate a distance between the measurement probe and the surface based on signals acquired from the at least first detector and the at least second detector.

[0017] Typically, each of the first and second detectors is separately connected to a signal analyzer. Thus, the signal analyzer includes a first input connected to the first detector and further includes a second input connected to the second detector. The first input of the signal analyzer receives a measurement signal from the first detector indicative of the interference signal measured for the first central wavelength, whereas the signal analyzer includes a second input connected to the second detector to analyze a signal provided by the second detector upon detecting a respective optical signal of the second central wavelength.

[0018] The interferometric measurement device provides a multi-wavelength interferometric measurement of a surface of an object. By using a first wavelength and a second wavelength, the overall measurement range of the measurement device can be increased.

[0019] In effect, by using a second beam or by using a signal analysis based on at least two optical frequencies, a synthetic wavelength defined by the two measurement wavelengths can be created and thus the spatial range of the interferometric measurement device can be increased. By using a larger (synthetic) wavelength, the range for unambiguous and therefore absolute distance measurements can be increased at the expense of spatial resolution and measurement accuracy.

[0020] Furthermore, by having separate signal analysis based on the first and second central wavelengths, a synthetic wavelength larger or much larger than the first and second wavelengths can be provided to increase the apparent measurement range of the interferometric measurement device. Furthermore, since the measurement beam includes spectral components with wavelengths less than 550 nm, the interferometric measurement device operates within the visible spectral range. When the optical coupling between any of the beam generating unit, the measurement probe, the beam splitter unit, and the detector unit is provided by at least one optical fiber, each optical fiber is specifically configured for optical beam propagation and beam delivery for this particular wavelength or spectral range.

[0021] It is beneficial in many respects for an interferometric device to operate within the visible spectrum. Because the measurement beam, and thus the reference and object beams, are within the visible spectrum, an operator of the interferometric device can visually inspect whether and to what extent the object beam has correctly hit the surface of the object to be measured. The use of visible radiation thus provides an easy and straightforward control of the operation of the interferometric device.

[0022] In a further aspect, when the object is coated with a coating comprising at least one of, for example, silicon or diamond-like carbon (DLC), it is particularly beneficial to use radiation in the spectral range below 550 nm, since the coating is substantially absorptive for this particular spectral range. Unwanted or undesirable reflections from the interface between the coating and the respective outer surface of the object can then be substantially suppressed and weakened or attenuated.

[0023] The signal beam reflected by the object is provided or constituted primarily by reflections occurring at the outer surface of the coating: the portion of the object beam that enters or propagates through the coating is substantially not reflected by the underlying object surface because the coating is substantially absorptive for this particular wavelength or spectral range.

[0024] Today, the availability of light sources operating in the desired spectral range below 550 nm that can be used in interferometric applications is limited. The light source should or must provide sufficient spatial coherence to enable interferometric measurements between the reflected signal and reference beams. Furthermore, a light source for interferometric applications should or must be extremely stable with respect to its central wavelength. Furthermore, the light source should be commercially available at a moderate or low cost.

[0025] Generally, only one light source can be provided to the beam generating unit. Here, a broader band light source can be implemented and used. The beam splitter unit can then be operable to extract a first partial beam with a first central wavelength and a second partial beam with a second central wavelength and provide the first and second partial beams separately to the detector unit. In this way, a signal analysis can be performed and executed based on the first and second central wavelengths, thus different central wavelengths, thus making it possible to increase the measurement range of the interferometric device.

[0026] According to a further example, the splitter is operable to split the object beam and the reference beam from the measurement beam and includes or is formed by a fiber end face, for example provided in or on the measurement probe.

[0027] In some examples, the measurement probe is connected to the beam generating unit through at least one optical fiber. Here, a fiber end face of the optical fiber located in the measurement probe can act as a splitter. A part of the measurement beam propagating through the optical fiber is emitted through the fiber end face and propagates towards the object as an object beam. Another part of the measurement beam is reflected as a reference beam by internal reflection at the fiber end face.

[0028] In some examples, the fiber end face is provided with or coated with a reflective coating to increase the intensity of the reference beam reflected at the fiber end face. In a typical usage scenario, when the surface of the object is highly absorbing for the object beam and / or the object includes a relatively large refractive index, the intensity of the signal beam reflected at the surface of the object can be relatively large. In order to obtain a good signal-to-noise ratio for the interference of the analysis beam, it is particularly beneficial to increase the intensity or proportion of the reference beam compared to the object beam.

[0029] In some examples, with respect to at least one of the first and second central wavelengths, the coating provided on the fiber end face and / or on or within the splitter is configured and / or effective to generate a reference beam comprising an intensity of 5% to 60% of the intensity of the measurement beam, to generate a reference beam comprising an intensity of 17.5% to 45% of the intensity of the measurement beam, to generate a reference beam comprising an intensity of 23.25% to 37.5% of the intensity of the measurement beam, or to generate a reference beam comprising an intensity of 25% to 35% of the intensity of the measurement beam.

[0030] In this way, the intensity of the reference beam reflected by the coated reflector, e.g., the coated fiber end face, can be increased compared to a solution without a reflective coating, so that the intensity of the reference beam can be closely matched to the intensity of the signal beam reflected by the object's surface and captured by the measurement probe.

[0031] According to a further example, the beam generating unit includes a second light source. The second light source is operable to emit electromagnetic radiation comprising a spectral component at a second central wavelength. Typically, when the beam generating unit is provided with at least two light sources, a first light source of the beam generating unit can be operable to emit electromagnetic radiation comprising a spectral component at a first central wavelength. Typically, in a further example, the first and second light sources of the beam generating unit can be configured or operable to emit electromagnetic radiation at respective first and second central wavelengths, respectively.

[0032] Thus, the spectral characteristics of the first and second light sources can be applied to the beam splitter unit and closely matched to the beam splitter unit, whereby first and second partial beams of first and second central wavelengths are extracted from the analysis beam.

[0033] In other examples, it is also conceivable that the first and second light sources are implemented as broadband or narrowband light sources and that the respective first and second partial beams having the first and second central wavelengths are specifically extracted by a beam splitter unit.

[0034] Nevertheless, with the emission spectra of the first and second light sources closely matched to the beam splitter unit operable to extract respective spectral components from the analysis beam, the entire beam guidance system of the interferometric measurement device may also be particularly adapted and configured to provide less attenuated and loss-free beam propagation, for example through various optical fibers coupling or connecting the beam generating unit, the splitter, the measurement probe, the optical beam recombiner, the beam splitter unit and the detector unit.

[0035] According to a further example, the beam splitter unit is operable to extract a third partial beam of a third central wavelength from the analysis beam. The third central wavelength is different from the first central wavelength. The third central wavelength is also different from the second central wavelength. The difference between the first, second and third central wavelengths is well defined and known to the signal analyzer to provide a precise signal analysis and distance calculation between the measurement probe and the surface of the object to be measured.

[0036] When the beam splitter unit is operable to extract the first, second and third partial beams, the detector unit optically coupled to the beam splitter unit also includes a third detector for detecting the third partial beam. Similarly, the third detector is also connected to the signal analyzer. In this example, the signal analyzer is individually connected or coupled to the first, second and third detectors of the detector unit so as to enable wavelength-selective or wavelength-specific interferometric analysis of the analysis beam.

[0037] In yet another embodiment, the beam splitter unit is operable to extract a fourth partial beam of a fourth central wavelength from the analysis beam. The fourth central wavelength is different from the first central wavelength, the second central wavelength, and the third central wavelength. Again, the detector unit includes a fourth detector for detecting the fourth partial beam. In general, the multi-wavelength interferometric measurement device is not limited to only two, three, or four individual wavelengths. The multi-wavelength interferometric measurement device can also be implemented based on 5, 6, 7, 8, or up to 10, or even up to 12, 14, 16, or 20 different individual wavelengths for the signal analysis of the analysis beam.

[0038] According to another example, the beam generating unit includes a third light source. The third light source is operable to emit electromagnetic radiation that includes a spectral component of a third central wavelength. Similar to the first and second light sources, the third light source can also be one of broadband light sources that have or exhibit a substantial light intensity of the third central wavelength. Alternatively, the third light source can be a narrowband light source, such as a laser, that emits a substantial beam intensity of the third central wavelength.

[0039] In a further example of the measurement device, the first light source includes one of a laser and a superluminescent diode SLD. Typically, when implemented as a laser, the first light source provides a measurement beam with a smaller spectral range. Here, the measurement beam exhibits a longer coherence length, for example in the range of several meters. When implemented as a laser, for example a solid-state laser or a semiconductor laser, the first light source provides a substantial light intensity of the first central wavelength.

[0040] When implemented as a broadband light source, for example as a superluminescent diode, the first light source emits a broader spectrum compared to a laser light source. Here, the spatial coherence length is much shorter compared to a light source implemented as a laser. Broadband light sources such as SLDs are available in the desired spectral range below 550 nm at reasonable cost. Both laser light sources as well as superluminescent light sources can provide sufficient time stability over several hours of operation. Typically, light sources used or usable with the present interferometric measurement device exhibit a long-term stability or long-term drift of the central wavelength of less than 5 pm in 8 hours, preferably less than 3 pm in 8 hours, or even less than 2 pm in a time interval of at least 8 hours.

[0041] According to another example, the second light source includes one of a laser and a luminescent diode. To that extent, when the beam generating unit includes a first light source and a second light source, either light source can be implemented as a laser, the first laser operable to emit radiation at a first central wavelength, and the second laser operable to emit electromagnetic radiation at a second central wavelength. In some examples, the first light source can be implemented as a laser and the second light source can be implemented as a superluminescent diode.

[0042] Here, the first light source can be operable to emit electromagnetic radiation at a first central wavelength and the second light source emits electromagnetic radiation within a spectral range that includes a spectral component at the second central wavelength. In some examples, the roles of the first and second light sources can be switched, such that the first light source is implemented as a superluminescent diode and the second light source is implemented as a laser.

[0043] According to a further example, the third light source includes one of a laser and a superluminescent diode, where when implemented as a laser, the third light source is operable to emit electromagnetic radiation at a third central wavelength, and when implemented as a superluminescent diode, the third light source is operable to generate a measurement beam having a spectral component at the third central wavelength.

[0044] In some examples, the first, second, and third light sources are each implemented as a laser operable to emit electromagnetic radiation, for example, at the first, second, and third central wavelengths, respectively. In some examples, one of the first, second, and third light sources is implemented as a laser. The other two light sources are implemented as individual superluminescent diodes. In further examples, it is contemplated that at least two of the light sources of the beam generating unit each include a laser for emitting electromagnetic radiation, for example, at the first and second central wavelengths, respectively. The third light source can be implemented as a superluminescent diode.

[0045] In yet another example, the first, second, and third light sources are each implemented, for example, as individual superluminescent diodes, each of which can exhibit a different or characteristic spectral range compared to the others of the superluminescent diodes.

[0046] Depending on the particular configuration of the beam generating unit comprising at least one, two or three individual light sources and the respective configuration of the respective light sources implemented, for example, as lasers or superluminescent diodes, the beam generating unit may comprise at least one or several optical couplers by means of which the respective electromagnetic radiation generated by the individual light sources can be combined in a single optical fiber to transmit respective multi-component measurement beams towards the measurement probe.

[0047] According to a further example, the at least one light source includes a superluminescent diode. The measurement device then further includes an optical delay unit coupled to the at least one light source in a light-transmitting manner, for example through an optical fiber or by an optical device providing free beam propagation. The optical delay unit is operable to impart a variable and adjustable-sized phase shift to at least one of the measurement beam, the signal beam, the object beam, and the reference beam.

[0048] Typically, when using superluminescent diodes as light sources in the beam generating units, each measurement beam can exhibit a shorter coherence length. In some examples, the coherence length of each measurement beam can be shorter than the path difference between the splitter and the optical beam recombiner, such that the recombination of the signal beam and the reference beam cannot interfere with each other.

[0049] The delay unit can split at least one of the measurement beam, the signal beam, the object beam, and the reference beam into a non-delayed beam portion and a delayed beam portion, and impart a variable and adjustable-sized phase shift to only the delayed beam portion. Typically, an auxiliary optical path can be provided to the delayed beam portion compared to the non-delayed beam portion. The delayed beam portion and the non-delayed beam portion then co-propagate through their respective optical fibers.

[0050] The measurement beam is split into delayed and non-delayed beam portions to generate corresponding object, reference, signal, and analysis beams, each having a non-delayed and a delayed beam portion, where a low-coherence light source allows the non-delayed portion of the signal beam to interfere with the delayed portion of the reference beam, and the non-delayed and delayed portions of the signal and reference beams experience the same or substantially the same optical path length and are therefore capable of interfering with each other.

[0051] The delay provided in one of the signal beam, the object beam, and the reference beam can be selected and tuned to correspond to the path difference between the reference beam and the signal beam. In this way, a measurement beam that initially exhibits lower coherence or includes a relatively short coherence length, for example, less than 10 cm, less than 1 cm, less than 1 mm, or even in the range of only a few hundred micrometers, can still be used as a light source. The optical delay unit can be suitably configured to compensate for the run-time difference between the reference beam and the signal beam.

[0052] In some examples, the optical delay unit is provided upstream of the measurement probe, while in other examples the delay unit is provided downstream of the measurement probe with respect to the propagation of the measurement beam and any further beams derived therefrom.

[0053] In general, the optical delay unit may be located at various positions in the interferometric measurement device. The optical delay unit may be located between the beam generating unit and the splitter. The optical delay unit may be located between the splitter and the measurement probe. The optical delay unit may also be located between the measurement probe and the beam splitter unit.

[0054] According to a further example of the measuring device, the first central wavelength λ1 and the second central wavelength λ2 are expressed by the formula:

number

[0055] In some examples, 5≦n, m≦35. In further examples, 10≦n, m≦20.

[0056] Typically, in some instances, n and m are integers. SMdefines a first synthetic wavelength that is artificially generated by the signal analyzer when analyzing the first and second signals of the first and second detectors, respectively. Typically, the first synthetic wavelength is about 1-50 times larger than either the first central wavelength λ1 or the second central wavelength λ2. In this way, the measurement range for the absolute distance measurement provided by the interferometric measuring device can be increased.

[0057] Preferably, the first synthetic wavelength is about 5 to 35 times larger than either the first central wavelength λ1 or the second central wavelength λ2. Such a selection of n and m allows the synthetic wavelength to be slightly reduced, with the benefit of improving robustness and reducing measurement variance and measurement error.

[0058] Most preferably, the first synthetic wavelength is about 10-20 times greater than either the first central wavelength λ1 or the second central wavelength λ2. Such a selection of n and m allows the synthetic wavelength to be further reduced, with the benefit of further improving robustness and further reducing measurement variance and measurement error.

[0059] The signal analyzer is operable to determine a relative phase between the signal beam and the reference beam for at least one of the first central wavelength and the second central wavelength. This type of analysis provides the highest measurement accuracy but with the shortest absolute measurement range. When performing the signal analysis for the first synthetic wavelength, the absolute measurement range can be increased at the expense of measurement resolution or measurement accuracy.

[0060] Typically, in some examples, the signal analyzer is operable to derive a distance between the measurement probe and the surface based on an interference signal of the analysis beam for at least one of the first central wavelength and the second central wavelength, and the signal analyzer is further operable to determine a distance between the measurement probe and the surface of the object by evaluating the interference signal of the analysis beam for the first synthetic wavelength.

[0061] Separate measurements and separate signal analyses performed on the one hand based on the first synthetic wavelength and on the other hand based on at least one of the first central wavelength and the second central wavelength provide an increase in the absolute measurement range without losing measurement accuracy or measurement resolution.

[0062] According to a further example of the measuring device, the third central wavelength λ3 and one of the first and second central wavelengths λ1 or λ2 are expressed by the formula:

number

[0063] Here, the third central wavelength λ3 is located relatively close to at least one of the first and second central wavelengths. Thus, the second synthetic wavelength λ SG is generated, and a second synthetic wavelength λ SG is the first synthetic wavelength λ SM In some examples, the second synthetic wavelength, λ SG is the first synthetic wavelength λ SM This second synthetic wavelength is approximately 10 to 20 times larger than the reference wavelength. In this way, the absolute measurement range of the distance measurement provided by the interferometric measuring device can be further increased. This second synthetic wavelength can be of the order of magnitude of a few centimeters or even meters.

[0064] When at least two of the first, second, and third central wavelengths are located closer to one another, for example when the first and third central wavelengths differ by less than 5 nm, less than 3 nm, or even less than 2 nm, a larger second synthetic wavelength can be provided, thereby increasing the range of absolute distance measurements provided by the interferometric measurement device to a range of millimeters, centimeters, or even meters.

[0065] Preferably, the second synthetic wavelength is about 50 to 400 times greater than either the first central wavelength λ1 or the second central wavelength λ2. By such selection of k and l, the second synthetic wavelength can be increased, and thus the overall absolute measurement range of the measurement device can be increased, respectively.

[0066] Most preferably, the second synthetic wavelength is about 50 to 500 times greater than either the first central wavelength λ1 or the second central wavelength λ2. By such selection of k and l, the second synthetic wavelength can be further increased, thus providing a respective increase in the overall absolute measurement range of the measurement device.

[0067] According to a further example of the measurement device, the beam splitter is operable to extract a fourth partial beam of a fourth central wavelength λ4 from the analysis beam. Additionally or alternatively, the beam generating unit may include an auxiliary light source operable to emit electromagnetic radiation comprising a spectral component of the fourth central wavelength λ4.

[0068] Typically, by way of further example, the fourth central wavelength λ4 and one of the first, second, and third central wavelengths λ1, λ2, and λ3 are represented by the formula:

number

[0069] Here, the fourth central wavelength λ3 is located relatively close to at least one of the first, second, and third central wavelengths. Thus, the third synthetic wavelength λ SL is generated, and the third synthetic wavelength λ SLis the first synthetic wavelength λ SM and the second synthetic wavelength λ SG In some examples, the third synthetic wavelength, λ SL is the second synthetic wavelength λ SL This second synthetic wavelength is approximately 10 to 20 times larger than the reference wavelength. In this way, the absolute measurement range of the distance measurement provided by the interferometric measuring device can be further increased. This second synthetic wavelength can be of the order of magnitude of a few centimeters or even meters.

[0070] According to another example, the first central wavelength, the second central wavelength, and the third central wavelength are: - Within the spectral range of 380-490 nm, - Within the spectral range of 400nm~460nm, - Within the spectral range of 404nm~455nm, - Within the spectral range of 449nm~511nm, - Within the spectral range of 449-489 nm, or - Located in the spectral range of 404-475 nm.

[0071] Typically, the selection and / or combination of the individual first, second, and optional third, or even fourth light sources will depend on the measurement requirements, as well as the quality and cost-effectiveness of commercially available light sources.

[0072] Preferably, according to a further example, the first, second and third central wavelengths are all in the blue spectral range. These central wavelengths are located in the visible blue spectral range. Using at least the first and second central wavelengths of the measurement beam for interferometric measurement of the surface or profile of an object in the blue spectral range, i.e. below 511 nm or below 500, is particularly useful for measuring an object with a coating that is substantially absorbing in this particular spectral range.

[0073] In some examples, when an SLD is implemented as the first light source and a laser is used as the second light source, the first central wavelength can be about 450 nm and the second central wavelength can be about 473 nm.

[0074] In another example, when the first and second light sources are both implemented as SLDs, the first central wavelength can be about 405 nm and the second central wavelength can be about 450 nm. Alternatively, the first central wavelength can be about 450 nm and the second central wavelength can be about 510 nm.

[0075] Further alternatively, when the first light source includes an SLD and the second and third light sources each include a laser, the first central wavelength can be about 450 nm, the second central wavelength can be about 473 nm, and the third central wavelength can be about 480 nm.

[0076] Alternatively still, when the first and second light sources are both implemented as SLDs and the third light source is implemented as a laser, the first central wavelength is about 405 nm, the second central wavelength is about 450 nm, and the third central wavelength is about 473 nm.

[0077] When three separate light sources are used and one of these sources is implemented as an SLD, four or even five different central wavelengths can be easily derived for signal detection and subsequent signal analysis.

[0078] According to further examples, the object to be measured by the measuring device comprises a material at its surface that is substantially absorptive to electromagnetic radiation at at least one of the first and second central wavelengths. Typically, the material of the object is highly absorptive to electromagnetic radiation at at least one of the first, second, and third central wavelengths. In some examples, the object comprises an optical element coated with a coating or coating layer. Typically, the coating of the object can comprise a layer thickness in the range of 1 μm to 10 μm, 10 μm to 100 μm, 20 μm to 80 μm, or 30 μm to 50 μm.

[0079] In a further example, the object to be measured comprises a surface coated with a coating comprising a layer thickness of less than 10 μm. The material of the coating can be substantially absorptive for at least one of the first central wavelength, the second central wavelength, and the optional third central wavelength. Typically, the selection of the first, second, and optional third central wavelengths depends on the type of material of the coating of the object to be measured. For a given object with a given coating, at least the first and second central wavelengths are selected such that the respective spectral components are substantially absorbed by the coating. Typically, by the term "substantially absorptive" for electromagnetic radiation of a particular central wavelength, it is meant that the intensity of the electromagnetic radiation of the respective central wavelength after propagating 1 μm through the respective material or coating is reduced to at least 50%, at least 20%, or at least 10%, or even less than 10% of the initial intensity.

[0080] By using a first central wavelength and a second central wavelength in the analysis of the analysis beam, where the coating material of the object is substantially absorptive, any possible reflections at the interface between the coating and the outer surface of the coated object can be substantially avoided. The signal beam reflected from the surface of the object is substantially reflected only from the outer surface of the coating. The portion of the object beam that propagates into the coating is substantially absorbed and cannot be further reflected.

[0081] Therefore, choosing an appropriate wavelength depending on the absorption characteristics of the coating of the coated object is ultimately beneficial to avoid any reflection from the underside of the coating that is in direct contact with the outer surface of the coated object.

[0082] According to a further example, the measurement device includes a coupler unit optically coupled to the beam generating unit, for example through an optical fiber. The coupler unit can be further connected to the measurement probe through another optical fiber and to the splitter unit through a further optical fiber. The coupler unit is configured to direct the measurement beam provided by the beam generating unit from the beam generating unit to the measurement probe and to transmit or direct any captured beam from the measurement probe to the splitter unit. Typically, in some examples, the measurement probe is connected or coupled to the beam generating unit through the coupler unit and the measurement probe is connected or coupled to the beam splitter unit through the coupler unit. In other examples, at least one of the optical couplings between the coupler unit, the measurement probe and the splitter unit is implemented as an optical path supporting free beam propagation. This optical path may not include an optical fiber and can support free beam propagation.

[0083] In some examples, the splitter as well as the optical beam recombiner are implemented in or on the measurement probe.

[0084] If the analysis beam is generated directly within the measurement probe, the coupler unit is configured to direct the analysis beam from the measurement probe to the splitter unit. In another example, when the analysis beam is generated downstream of the measurement probe, the optical unit can be configured to direct the signal beam from the measurement probe to the splitter unit. The coupler unit can include a fiber optic coupler, whereby the measurement beam can be directed to the measurement probe and any beam captured by the measurement probe can be forwarded or directed to the splitter unit.

[0085] In some examples, the optical unit is implemented as an optical circulator. In further examples, the optical unit is specifically adapted to combine and redirect different beams of electromagnetic radiation of a given central wavelength. Typically, the coupler unit is optimized to combine and redirect electromagnetic radiation of at least a first and a second central wavelength, and possibly a third central wavelength, from one optical fiber or optical element to another optical fiber or optical element.

[0086] According to a further example of the measurement device, the splitter unit includes at least one of a wavelength division multiplexer WDM and a fiber splitter. The fiber splitter includes a first output connected to a first optical filter. The fiber splitter further includes a second output connected to a second optical filter. In some examples, the splitter unit consists essentially of a wavelength division multiplexer. Such a wavelength division multiplexer is operable to split the analysis beam into different spectral components, for example into a first spectral component of a first central wavelength and a second spectral component of a second central wavelength. In some cases, the wavelength division multiplexer can also be operable to split the analysis beam into three partial beams of a first central wavelength, a second central wavelength, and a third central wavelength, respectively.

[0087] The wavelength division multiplexer allows the intensity loss to be reduced to a minimum, while providing the first and second detectors of the detector unit downstream of the splitter unit with the maximum available spectral intensity of the analysis beam.

[0088] When implemented as a fiber splitter, with the first output connected to a first optical filter and the second output connected to a second optical filter, each optical filter can include a transmittance matching the first and second central wavelengths. Typically, the first optical filter transmits only the first central wavelength. The second optical filter transmits only the second central wavelength. The fiber splitter is operable to split an incoming beam into at least a first partial beam and a second partial beam, the first and second partial beams including some identical or substantially equal spectral distributions.

[0089] Another fiber splitter can further be provided, by which the incoming analysis beam provided by the measurement probe or coupler unit is split into first, second and third partial beams. The first, second and third partial beams can then be directed to respective first, second and third optical filters. By means of the respective optical filters, a more precise central wavelength can be selected from the incident electromagnetic radiation. The use of optical filters entails a substantial reduction in the electromagnetic intensity. However, the use of optical filters provides a shorter bandwidth and a precise central wavelength for further signal analysis.

[0090] According to another aspect, the present disclosure relates to a method for measuring a surface or profile of an object, e.g., an optical element, that can be coated with a coating. The method includes a step of providing an object, the object including a surface material. In some examples, the surface material includes a coating provided on a surface of the object. In a further step, a measurement beam is generated by a beam generating unit. An object beam and a reference beam are branched from the measurement beam. The object beam is then directed to the surface of the object by an optical probe.

[0091] For a given wavelength of at least one of the object beam and the measurement beam, the surface material includes an optical absorption or optical penetration depth of less than 100 μm, less than 50 μm, less than 20 μm, less than 10 μm, less than 5 μm, less than 2 μm, less than 1 μm, less than 0.5 μm, or less than 0.1 μm, where the optical penetration depth is defined by the thickness of the material where the beam intensity has dropped to 1 / e, i.e., about 36% of the initial beam intensity.

[0092] Alternatively, according to another example, the surface material includes an absorption coefficient AC, AC being greater than or equal to 500,000 cm for at least one of the object beam and the measurement beam. -1 ~5,000cm -1 (i.e. 1cm -1 (500,000 to 5,000 per person).

[0093] The part of the measurement beam reflected by the surface of the object as a signal beam is further captured. The signal beam reflected by the surface of the object, for example the outer surface of a coating of an optical element, is then recombined with the reference beam. The recombined signal beam and the reference beam constitute an analysis beam. The interference of the analysis beams is then detected by a detector unit, and the distance between the optical probe and the surface is derived by analyzing the signal of the detector unit.

[0094] Typically, and by way of further example, the wavelengths of the object beam and / or the measurement beam are selected such that the intensity of the signal beam reflected and at least partially absorbed at the bottom surface of the object facing away from the interferometric measuring device is less than 5%, less than 2%, less than 1%, or less than 0.5% of the intensity of the signal beam reflected at the top surface of the object.

[0095] Here, an at least partially absorbed signal beam enters the surface material from a top surface of the object and undergoes propagation through the surface material, where the signal beam is internally reflected at a bottom surface of the surface material facing away from the interferometric measurement device and propagates again through the surface material or re-enters the measurement probe as an at least partially absorbed signal beam.

[0096] Typically, for a given surface material and coating of the surface material on the surface of the object, at least one of the first and second central wavelengths of the measurement beam is selected such that radiation reflected by or from the top surface of the object's surface material is favored over the signal beam re-entering the measurement probe, such that the portion of the object beam that enters the surface material is substantially absorbed and the reflected signal beam portion does not interfere with or overlap the signal beam portion reflected at the top surface of the surface material.

[0097] In a further aspect, a method is provided for measuring the surface and / or profile of an object coated with a coating, the coating comprising a surface material. The method for measuring the surface is an interferometric method, in which an object beam and a reference beam are split off from a measurement beam. The central wavelength of the object beam is selected such that the object beam is substantially absorbed by the surface material of the object. In this way, it can be ensured that only light reflected from the top or outer surface of the coating of the object re-enters the measurement probe for further interference signal analysis.

[0098] To that extent, the method is characterized in that a measurement beam is selected or provided with a desired central wavelength at which the surface material of the object is substantially absorbent. In this way, when the coating of the surface material is thinner or when the object to be measured is to be relatively thin, for example when the object is to include a thickness of less than 10 mm, less than 5 mm, or less than 1 mm, the central wavelength of the measurement beam is selected with respect to the absorption coefficient or light penetration depth of the surface material. This is particularly beneficial, since any reflection from the underside of the coating or the underside of the object to be measured can be substantially avoided or suppressed.

[0099] In some examples, the above-mentioned methods are to be implemented by the above-mentioned interferometric measuring device. To that extent, all configurations, effects, and benefits described above in relation to the interferometric measuring device are equally applicable to the method of measuring the surface or profile of an object. Vice versa, the above-mentioned measuring device can also be configured and applied to implement the above-mentioned method of measuring the surface or profile of an object.

[0100] According to a further example, the object to be measured comprises a coating on a surface. Here, the coating of the object comprises the surface material. Typically, the entire coating of the object is made from the surface material. In another example, an outer or upper layer of the coating is made from the surface material. When the object beam is directed to the surface of the object, a large portion of the object beam that enters the coating is absorbed by the coating. In this way, any reflections from the underside of the coating or from the underside of the object facing away from the measurement probe can be substantially avoided and suppressed.

[0101] According to a further example, the coating of the object comprises or consists of silicon or diamond-like carbon DLC. When the object, for example in the form of an optical element such as a lens or mirror, is coated with, for example, a silicon layer, the method and the measurement device are typically implemented to provide a measurement beam that includes substantial, i.e. non-negligible, spectral components at wavelengths below 550 nm. Preferably, the measurement beam includes several spectral components, for example at a first central wavelength and a second central wavelength, both of which are below 550 nm.

[0102] In a typical example, when the object to be measured is silicon coated or DLC coated, the measurement beam includes spectral components at first and second central wavelengths, and optionally a third central wavelength, the first, second, and third central wavelengths being between 380 nm and 480 nm. In this way, the portion of the object beam that enters the coating is substantially absorbed, and any reflection from the underside of the coating can be substantially reduced or avoided.

[0103] In the following, a number of examples of measuring devices and methods for measuring the surface and / or profile of an object, such as an optical element, will be described in more detail by reference to the drawings. [Brief description of the drawings]

[0104] [Figure 1] FIG. 1 illustrates an example of an interferometric measurement device according to the present disclosure. [Diagram 2] FIG. 13 illustrates a further implementation of an interference measuring device. [Diagram 3] FIG. 2 illustrates an embodiment of a beam generating unit comprising a combiner unit. [Figure 4] FIG. 13 illustrates a further example of a beam generating unit and a combiner unit in a further example. [Diagram 5] FIG. 2 shows an optical delay unit. [Figure 6] FIG. 2 is a diagram illustrating an example of a measurement probe. [Figure 7] FIG. 2 is a diagram showing the light absorption of the surface material of an object. [Figure 8] FIG. 2 illustrates another example of a beam generating unit. [Figure 9] FIG. 2 illustrates a further example of a beam generating unit. [Figure 10] FIG. 2 shows the spectral distribution of a measurement beam. [Figure 11] FIG. 2 shows a combination of a beam splitter unit and a detector unit. [Figure 12] FIG. 13 illustrates another example of the spectral distribution of the measurement beam. [Figure 13] FIG. 2 illustrates an example of an optical diode arrangement. [Figure 14] 1 is a flow diagram of a method for measuring a surface with an interferometric measuring device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0105] In Fig. 1 an interferometric measuring device 10 for measuring a surface 2, 4 or a profile of an object 1 is shown diagrammatically. The interferometric measuring device 10 comprises a beam generating unit 20 including at least a first light source 21. The first light source 21 is operable to generate a measuring beam BM having a spectral component at a wavelength below 550 nm. Typically, the first light source 21 is operable to generate a measuring beam in the visual spectrum range below 550 nm and above 380 nm.

[0106] The interferometric measuring device further comprises a measurement probe 50 connected to the beam generating unit 20 through at least one optical fiber 91, 92, 93, 94. As shown in more detail in Fig. 6, the measurement probe 50 comprises a splitter 55, by which the measurement beam BM provided by the beam generating unit 20 is split into an object beam BO and a reference beam BR.

[0107] Only the object beam BO is directed towards the surfaces 2, 4 of the object 1. There, i.e. at the surfaces 2, 4 of the object 1, the object beam BO is reflected as a signal beam BS and recombined with the reference beam BR by a recombiner 56. The signal beam BS recombined with the reference beam BR constitutes or forms the analysis beam BA. The analysis beam BA is transmitted from the measurement probe 50 towards a beam splitter unit 60 as shown in FIG.

[0108] The splitter unit 60 is connected to the measurement probe 50 through at least one optical fiber 94, 95 and is operable to extract a first partial beam BP1 of a first central wavelength λ1 and to derive a second partial beam BP2 of a second central wavelength λ2 from the analysis beam BA. The first and second partial beams BP1 and BP2 are provided and transmitted separately to a first detector 71 and a second detector 72 of the detector unit 70, where wavelength-selective detection of interference of the analysis beam BA can be provided.

[0109] The individual detectors 71, 72 of the detector unit 70 are connected to a signal analyzer 80 via a transmission line 98. The signal analyzer 80 is configured to derive or calculate a distance D between the measurement probe 50 and the surfaces 2, 4 based on signals acquired from the at least first detector 71 and the at least second detector 72.

[0110] In the example shown in FIG. 6, the fiber end face 52 of the optical fiber 94 connecting the measurement probe 50 to the beam generating unit 20 acts as a splitter 55. Thus, a part of the measurement beam BM provided by the beam generating unit 20 is reflected, i.e. internally reflected from the fiber end face 52 in the optical fiber 94. Another part of the measurement beam BM propagates from the fiber through the optical element 51 and is directed towards the surfaces 4, 2 of the object 1 as the object beam BO. Typically, the object beam BO is focused on a part of the respective surface 2, 4. In order to increase the internal reflectivity of the fiber end face 52, the fiber end face 52 and thus the splitter 55 can be provided with a coating 53. In this way, the intensity of the reference beam BR can be increased at the expense of the intensity of the object beam and thus of the signal beam.

[0111] A portion of the object beam BO is reflected from the surfaces 2, 4 to be measured and enters the measurement probe 50 again as a signal beam BS. Typically, the signal beam BS propagates in the opposite direction to the object beam BO. The signal beam BS enters the optical fiber 94 and is recombined with the reference beam BR. Accordingly, a runtime difference occurs between the reference beam BR and the signal beam BS, which are reflected with a phase offset. This runtime difference is directly correlated to the optical path difference between the splitter 55 and the recombiner 56, and thus between the splitter 55 and the surfaces 4, 2 of the object 1, and thus the distance D.

[0112] Due to the variable distance, the resulting interference signal of each of the analysis beams BA undergoes a measurable change, which can be detected by respective detectors 71, 72, and the signals from the detectors 71, 72 then undergo further signal analysis, thereby deriving or calculating the distance D between the measurement probe 50 and the surfaces 4, 2 of the object 1.

[0113] In some examples, the object 1 to be measured is provided with a coating 3 or a surface material 5. The coating 3 and / or the surface material 5 can comprise silicon or diamond-like carbon DLC. The coating 3, and thus the layer of the surface material 5, facing the measurement probe 50 can comprise a thickness in the range of just a few micrometers, such as 5-10 μm, 10-20 μm, 10 μm-100 μm, 50 μm-150 μm, 100 μm-200 μm, 150 μm-250 μm, or 100 μm-300 μm. Using a measurement beam BM in the infrared spectral range can lead to a number of problems, since the coating 3 is practically transparent to electromagnetic radiation in this spectral range. Reflections can then occur not only on or from the outer surface 4 of the coating 3, but also from the inner surface 2 of the coating 3, which is in contact with and directly adjacent to the outer surface of the object 1 or the optical element. It is then more difficult to distinguish between the signal beam BS reflected from the outer surface 4 and the signal beam BS′ reflected from the lower surface 2 or the interface between the object 1 and the coating 3 .

[0114] By selecting or designing the measurement beam to include spectral components with wavelengths less than 550 nm, the respective object beam BO propagating into and through the coating 3 is substantially absorbed. In this way, the intensity of the signal beam BS' reflected at the interface between the object 1 and the coating 3 may be reduced to a minimum and not interfere with the detection and analysis of interference in the analysis beam BA resulting from the recombination of the reflected signal beam BS and reference beam BR.

[0115] Typically, the beam splitter unit 60 is operable to generate or derive first and second partial beams BP1, BP2, each of which comprises a first and second central wavelength λ1, λ2 between 380 nm and 550 nm. In this way, it can be ensured that only such object beams reflected from the outer surface 4 of the coating 3 of the object 1 are subjected to detection and subsequent signal analysis. Internal reflections from the inner surface 2 of the coating are substantially suppressed.

[0116] In some examples, the measurement probe 50 includes a housing 54. The housing 54 serves as a mount for the optical element 51, e.g. a collimating or focusing optical lens. The fiber end face 52, and thus the entire fiber 94, may be mounted and fixed in the housing 54. Optionally, a transducer 58, such as a piezoelectric transducer, or some other type of phase modulator, may be provided, which may periodically modify the phase of the measurement beam BM. Such a periodic modulation of the phase of the measurement beam, and thus the reference and signal beams, is beneficial for precise measurement of the relative phase between the reference beam BR and the signal beam BS.

[0117] The interferometric device 10 shown in FIG. 1 is implemented by fiber optic components. The individual components of the interferometric device 10 are optically connected through individual optical fibers 91, 92, 93, 94, 95, 96, 97, 99. The optical fiber implementation and the optical fiber connections of each of the individual components make the entire interferometric device more robust and stable. The interferometric device typically includes an optical coupler unit 40, which is connected to the beam generating unit 20 through an optical fiber 93, to the measurement probe 50 through another optical fiber 94, and to the splitter unit 60 by a further optical fiber 95. The coupler unit 40 can be implemented as an optical circulator. In some embodiments or examples, the coupler unit 40 includes an optical fiber coupler, such as an X-coupler.

[0118] The electromagnetic radiation provided by the optical fiber 93 is transmitted through a further optical fiber 94 to the measurement probe 50. The analysis beam BA captured and provided by the measurement probe 50 is transmitted through the same optical fiber 94, propagating in the opposite direction. The analysis beam BA is redirected by the coupler unit 40 to the beam splitter unit 60.

[0119] The splitting of the measurement beam BM into the object beam BO and the reference beam BR is performed in the measurement probe 50, and the recombination of the signal beam BS and the reference beam BR is performed at a fiber end face 52 of the measurement probe 50. In other examples, the splitting of the measurement beam BM and the recombination of the signal beam BS and the reference beam BR can be performed by other components of the interferometric measurement device 10. For example, the splitting of the measurement beam BM into the object beam BO and the reference beam BR can be performed by one of the beam generating unit 20 and the coupler unit 40. In other examples, the recombination of the reference beam BR and the signal beam BS can be performed by the coupler unit 40 or the beam splitter unit 60.

[0120] Typically, the beam generating unit 20 includes at least one light source 21 that includes a spectral component with wavelengths less than 550 nm. In some examples, the light source 21 is a broadband light source, providing a larger spectral distribution of electromagnetic radiation. The beam splitter unit 60 is then operable to derive or extract first and second partial beams BP1, BP2 of first and second central wavelengths from the spectral distribution provided by the first light source 21.

[0121] In another example, the beam generating unit 20 comprises a first light source 21 operable to emit electromagnetic radiation comprising a spectral component at a first central wavelength λ1 and further comprises a second light source 22 operable to emit electromagnetic radiation comprising a spectral component at a second central wavelength λ2. In this way, the spectral distribution of the electromagnetic radiation provided or generated by the beam generating unit 20 is closely adapted to and can be closely matched to the first and second central wavelengths of the first and second partial beams BP1, BP2 extracted by the beam splitter unit 60.

[0122] Preferably, the first light source 21 is operable to generate and emit electromagnetic radiation at a first central wavelength and the second light source 22 is operable to generate and emit electromagnetic radiation at a second central wavelength. In this way, the signal to noise ratio at the detector unit 70 can be improved and the overall efficiency and effectiveness of the interferometric measuring device 10 can be increased.

[0123] As further shown in Fig. 1, the beam generating unit 20 may include or be connected to a combiner unit 25. As shown in Fig. 1, the first light source 21 is connected to the combiner unit 25 by or through an optical fiber 91. The second light source 22 is individually and separately connected to the combiner unit 25 through another optical fiber 92. Within the combiner unit 25, the separate light beams generated by the first and second light sources 21, 22 and provided by the respective optical fibers 91, 92 are combined within a single optical fiber 93 that enters the coupler unit 40.

[0124] In a further example shown in Fig. 2, the setup of the interferometric measuring device 10 according to Fig. 1 is extended to a third central wavelength λ3, where the detector unit 70 comprises a first detector unit 71 for the first central wavelength λ1, a second detector unit 72 for the second central wavelength λ2 and a third detector unit 73 for the third central wavelength λ3, the respective central wavelengths being different with respect to each other.

[0125] The individual detectors 71, 72, 73 can be connected separately or together to the signal analyzer 80 through electrical signal lines 98, and the electronic or electrical signals generated by the individual detectors 71, 72, 73 are transmitted by the electrical signal lines 98 to the electronic signal analyzer or electronically implemented signal analyzer 80.

[0126] As further evident from Fig. 2, in order to provide a multi-wavelength selective analysis of the analysis beam BA, the beam splitter unit 60 is operable to extract not only the first and second partial beams but also the third partial beam BP3 from the analysis beam provided by the optical fiber 95. The three partial beams BP1, BP2 and BP3 are transmitted individually through separate optical fibers 96, 97, 99 to the respective first, second and third detectors 71, 72, 73. Depending on the type of light sources 21, 22, 23 implemented by the generation unit 20, different central wavelengths can be extracted from the analysis beam BA.

[0127] In the example of Figure 2, the beam generating unit 20 comprises three individual light sources 21, 22, 23. Here, the first light source 21 is operable to generate and emit electromagnetic radiation of a first central wavelength, the second light source 22 is operable to generate and emit electromagnetic radiation of a second central wavelength, and the third light source 23 is operable to generate and emit electromagnetic radiation of a third central wavelength.

[0128] The separate light sources 21, 22, 23 are individually coupled to the coupler unit 25 by respective optical fibers 90, 91, 92. Within or by the combiner unit 25, the electromagnetic radiation provided by the separate light sources and the three separate optical fibers 90, 91, 92 are combined into a single optical fiber 93, and the combined or superimposed electromagnetic radiation is transmitted by the optical fiber 93 to the coupler unit 40.

[0129] The optical combiner unit 25 may include an optical coupler 15 implemented, for example, as an optical fiber combiner 14 as shown in Figure 8. The combiner unit, and thus the optical combiner 14, allows the electromagnetic light intensities provided by the first and second optical fibers to be combined in a single output fiber.

[0130] In some examples, the individual light sources 21, 22, 23 are implemented as laser light sources 27 or superluminescent diode SLDs 26. In the example of Figure 1, light sources 21, 22 are both implemented as laser light sources 27. Similarly, in the examples of Figures 2 and 4, at least one of light sources 21, 22, 23 is implemented as a laser 27. Alternatively, at least one of light sources 21, 22, 23 can be implemented as an SLD or a white light source.

[0131] In the example of Fig. 3, the first light source 21 and the second light source 22 are both implemented as SLDs. Here, radiation generated by the first light source 21 is transmitted through an optical fiber 91 to the coupler unit 25. Light generated by the second SLD light source 22 is transmitted through another optical fiber 92 into and towards the coupler unit 25. Within the coupler unit 25, the respective beams provided by the separate optical fibers 91, 92 are combined and co-propagate through a further optical fiber 91a into the optical delay unit 30.

[0132] The optical delay unit 30 is an optional component of the interferometric device 10 and is typically provided in examples of the device 10 in which at least one of the light sources 21, 22, 23 is implemented as a relatively low spatial coherence light source such as an SLD. The delay unit 30 is further connected to the coupler unit 40 via the optical fiber 93 mentioned above.

[0133] In a further example shown in Fig. 4, the beam generating unit 20 includes a first light source 21 implemented as a laser light source 27. The beam generating unit 20 further includes a second light source 22 implemented as an SLD 26. Here, the output of the first light source 21 is directly connected to the combiner unit 25 through an optical fiber 91. The output of the second light source 22 is connected to the delay unit 30 via an optical fiber 92. The output of the delay unit 30 is connected to the combiner unit 25.

[0134] The delay unit 30 serves to impart an artificial phase or delay to the spectral components of the measurement beam BM generated by the low-coherence light source 22. Typically, when using low-coherence light, the optical path difference D between the reference beam BR and the signal beam BS can be made larger than the coherence length of the electromagnetic radiation provided by the respective light sources.

[0135] The two beams would then no longer interfere and interferometric distance measurements would no longer be possible. An optical delay unit 30, shown in more detail in Figure 5, provides an artificial, adjustable delay to the reference beam such that the artificially delayed part of the reference beam substantially matches the measured distance D between the measurement probe 50 and the surfaces 2, 4.

[0136] The delay unit 30 shown in Fig. 5 includes an input 31, which is connected to an optical fiber 91a as shown in Fig. 3, or to an optical fiber 92 as shown in Fig. 4. Inside, the delay unit 30 includes an optical fiber coupler 33 by which the incoming light is guided into a delay head 35. The delay head 35 may include an optical element 37, such as a focusing lens, by which the incoming light is guided to a reflector 34, implemented for example as a mirror or a retroreflective element. The delay head 35 is connected to the input 31 and / or the optical fiber coupler 33 by another optical fiber 39. Between the delay head 35 and the reflector 34, the respective light beam undergoes free propagation.

[0137] The fiber end face 36 of the optical fiber 39 terminating in the delay head 35 is fixed to the delay head 35. The delay head 35 is movable along its optical axis or relative to the reflector 34. Again, similar to what has been described above in relation to the measurement probe 50, the end face 36 of the optical fiber 39 acts as a splitter for the electromagnetic radiation provided by the optical fiber 39. A portion of the electromagnetic radiation is retroreflected by the fiber end face 36, and another portion of the respective electromagnetic radiation is reflected by the reflector 34 and captured by the delay head 35. Optionally, the delay head 35 can be subjected to periodic modulation by a transducer 38 or any other type of phase modulator. When the measurement beam BM propagates completely through the optical delay unit 30, the transducer 38 or phase modulator can replace or be substituted for the transducer 58 or phase modulator 58 of the measurement probe 50.

[0138] In this way, an adjustable and modifiable delay Δd can be imparted to the reference beam BR. The light reflected from the reflector 34 is picked up by the delay head 35 and re-enters the coupler 33. From there, the picked up light propagates from the coupler 33 towards the output 32 which is optically connected to the optical fiber 93 and thus directly connected to the coupler unit 40 as shown in FIG.

[0139] Alternatively, as shown in FIG. 4, the output 32 of the delay unit 30 can be connected to an optical fiber 91a and enters a coupler unit 25 to be recombined with further spectral components provided by the laser source 27. In FIG. 7, a graph 100 is shown diagrammatically illustrating the absorption of a coating 3 provided on a surface 2 of an object 1. The coating 3 includes a surface material 5. The surface material 5 includes a layer having a thickness C. The layer thickness C can be in the range of a few micrometers. In some examples, the layer thickness is less than 5 μm, less than 10 μm, less than 20 μm, less than 50 μm, or less than 100 μm. In some examples, the thickness C of the coating 3 can even be less than 1 μm.

[0140] Typically, in some examples, the measurement beam BM has a central wavelength, or at least a spectral component, of wavelengths less than 550 nm. Within this spectral range, the surface material 5 is substantially absorptive for the respective electromagnetic radiation. As shown in the graph 100 of FIG. 7, the light intensity transmitted through the layer thickness C drops to about 1% after propagating about 1 μm through the surface material 5. In effect, by using a measurement beam having a spectral component of wavelengths less than 550 nm, the coating 3 and the respective surface material 5, for example in the form of silicon or DLC, are substantially absorptive, and therefore the contribution of light reflected from the lower surface 2 of the coating 3 as the signal beam BS′ is negligible.

[0141] Figure 8 shows the embodiment according to figure 4 in a more detailed view. Here, the beam generating unit 20 comprises a first light source 21 implemented as a laser 27. The beam generating unit 20 comprises a second light source 22 implemented as an SLD 26. The SLD 26 is connected to an optical isolator or photodiode 24 to avoid any reflection or backscattering into the light source 22. The output of the photodiode 24 is connected to an optical delay unit 30. Here, the light beam generated by the second light source 22 undergoes a variable and tunable optical delay as described above in relation to figure 5.

[0142] The first light source 21 is a laser light source 27. The first light source 21 is connected to an optical fiber 91 by an optical fiber coupler 11. At the output of the delay unit 30 an optical fiber 92 is provided. The two optical fibers 91, 92 enter a coupler unit 25. The coupler unit 25 comprises an optical fiber coupler 15, for example implemented as an optical fiber coupler 14. The optical fiber coupler 15 is typically implemented as a coupler and can be asymmetric with respect to the degree of crosstalk or optical fiber coupling due to the different types of light sources 21, 22.

[0143] In this example, the light intensity provided by the first light source 21 is about 10 times greater than the light intensity provided by the second light source 22. The mixing ratio of the optical fiber coupler 14 is appropriately designed and configured such that the spectral components of the individual light sources 21, 22 are substantially equally distributed in the output optical fiber 93, which is connected to the output of the optical combiner unit 25 through or by another optical fiber coupler 11, according to the different light intensities of the various light sources 21, 22.

[0144] In a further example shown in Fig. 9, the beam generating unit 20 comprises a first light source 21 and a second light source 22, both of which are implemented as SLDs 26. Here, each individual light source 21, 22 is equipped with a photodiode 24, which is provided between the coupler unit 25 and the respective light source 21, 22. The photodiode 24 is implemented by an optical fiber. The photodiode 24 is connected to the optical fiber of the light source 21, 22 as well as to the optical fiber of the coupler unit 25 by the respective optical fiber coupler 11.

[0145] An output of the combiner unit 25 is connected to a delay unit 30 including a coupler 33 implemented for example as an optical fibre x-coupler 15. An output of the coupler 15 is connected to a delay head 35 via an optical fibre 39. A further output of the coupler 15 may be connected to an optical fibre 93 which is connected or connectable to the coupler unit 40 and / or the measurement probe 50. A further output of the coupler 15 may be provided with a beam trap 12.

[0146] The specific choice of the different light sources 21, 22, 23 and their mutual optical coupling depends on the type of coating 3 of the object 1 to be measured. This specific choice may further depend on the availability of commercially available light sources as well as optical components for guiding the projection light, e.g. optical fibers, couplers, and combiners.

[0147] In the example of Fig. 10, a graph 110 is shown illustrating the spectrum of the measurement beam when using a superluminescent diode 26 as the first light source 21 and a laser 27 as the second light source 22. As shown, the superluminescent diode 26, and therefore the first light source 21, provides a broader spectrum in the region of 550 nm. The spectral width of the electromagnetic radiation generated and provided by the SLD can be about 6 nm at 3 dB.

[0148] In comparison, the second light source 21 is configured to emit electromagnetic radiation at a second central wavelength, for example 472.9 nm. The radiation emitted by a laser light source is of relatively long coherence and exhibits a smaller bandwidth (FWHM), for example less than 1 MHz.

[0149] Since the first light source 21 provides a broader optical signal, the first and third central wavelengths can be selected from the emitted spectrum. This can be done by suitable filters having respective central wavelengths, for example 448.2 nm and 450.4 nm, thus defining the first and third central wavelengths. Here, the third central wavelength is located closer to the first central wavelength, so that a relatively large synthetic wavelength is composed of the first and third central wavelengths.

[0150] The second central wavelength is provided at a well-defined spectral distance from at least one of the first and third central wavelengths, whereby another synthetic wavelength, for example based on the first and third central wavelengths, can be artificially generated for an interferometric analysis of the analysis beam BA.

[0151] In Fig. 11 an example of a splitter unit 60 implemented especially for wavelength splitting of a measurement beam composed of the electromagnetic radiation of the superluminescent diode 26 and the electromagnetic radiation generated and emitted by the laser 27 is shown. Here, the splitter unit 60 comprises a wavelength division multiplexer 61 having a first output 69a and a second output 69b. The first output 69a is connected to an optical filter 68 configured to transmit radiation of the second central wavelength, and thus the radiation provided by the laser 27. Downstream of the optical filter 68 a second detector 72 of the detector unit 70 is provided.

[0152] The second branch or second output 69b of the WDM 61 is connected by a further optical fiber, via the optical fiber coupler 11, to an optical fiber splitter 62, which is operable to split the beam provided by the second output 69b into equal or different branches or portions. Here, a first output of the splitter 62 is directed to a filter 65 and then to a first detector 71. A second output 64 of the splitter 62 is directed to a filter 66, further to a filter 67 and finally to a third detector 73. A filter 68 located before the second detector 72 is implemented as a protection filter for the detector 72. The filter 68 is operable and / or configured to suppress the spectral components generated by the SLD. Similarly, the filter 65 located before the first detector 71 is operable to suppress the spectral components of the laser source 27. The illustrated cascade connection of optical filters 66, 67 located before the third detector 73 serves to protect the respective detector from any disruption. Here, an optical filter 66 is implemented to suppress the spectral components of the laser source 27 because the third central wavelength is closer to the second central wavelength of the laser source than the first central wavelength.

[0153] The further optical filter 67 is then configured to suppress any further spectral components of the light outside the third central wavelength.

[0154] A number of graphs 111, 112, 113, and 114 provide examples of transmission and filter efficiency of the various filters 65, 66, 67, and 68 described above in connection with FIG. 11. Filter 65 is operable to suppress spectral components at wavelengths greater than 450 nm. Filter 66 is operable to suppress electromagnetic radiation at wavelengths in the spectral region around 420 nm. Optical filter 67 is operable to transmit spectral components in the range of 450 nm to 490 nm. Optical filter 67 is operable to provide and transmit a third partial beam at a third central wavelength, e.g., around 450 nm.

[0155] The optical filter 68 shown in graph 114 is particularly operable to suppress any spectral components with wavelengths less than 460 nm.

[0156] In a further example shown in FIG. 12, two separate first and second light sources 21, 22, both implemented as SLDs, are provided. Here, the first light source 21 is operable to emit electromagnetic radiation in the spectral region around 405 nm. The second SLD light source 22 is operable to emit electromagnetic radiation at about 450 nm. As shown in graph 120 of FIG. 12, the first central wavelength is about 405 nm, the second central wavelength is about 449 nm, and the third central wavelength is about 451 nm. Here, the second and third central wavelengths are operable to generate a slightly larger synthetic wavelength, thus increasing the absolute measurement range of the interferometric measurement device.

[0157] The arrangement of the beam splitter unit 60 and the beam generating unit 20 shown in Figure 12 can be somewhat identical to the schematic diagram of Figure 11. Of course, the respective optical filters are selected to correspond closely to the central wavelength of the beam generating unit 20.

[0158] The superluminescent diode SLD 26 described and proposed herein is more susceptible to internal reflections that may occur within the interferometric measurement device 10. In order to suppress any reflections or internal reflections propagating back into the SLD, an optical diode arrangement 85 as shown in FIG. 13 may be provided. The optical diode arrangement 85 may be provided between the beam generating unit 20 and any one of the measurement probe 50 and the coupler unit 40. The optical diode arrangement 85 may include a wavelength division multiplexer 61, which may split the incoming light into a first optical fiber branch and a second optical fiber branch. The first branch may include a photodiode 24 and the second branch may include another photodiode 24′.

[0159] The photodiodes 24, 24' are connected to a wavelength division multiplexer 61, which separates at least two spectral components of the measurement beam BM into two separate optical fibers and respective optical fiber couplers 11. The outputs of the photodiodes 24, 24' are recombined by another WDM 61'. The output of the WDM 61' can be coupled or connected through optical fibers 93, 94 to one of the coupler unit 40 and the measurement probe 50. The optical diode arrangement 85 shown in FIG. 13 is particularly beneficial because the incoming light is separated into a first spectral component and a second spectral component, the first spectral component propagating through the first branch and the first photodiode 24, and the second optical component propagating through the second branch and therefore the second photodiode 24'.

[0160] In this manner, a wavelength selective optical diode arrangement is provided which serves to prevent any back reflection or any back scattering within the optical fiber system for each of the spectral components.

[0161] 14 shows a schematic diagram of a method for measuring a surface 2, 4 or a profile of an object 1. Here, in a first step 200, a measurement beam BM is generated by the above-mentioned beam generating unit 20. In a further step 202, an object beam BO and a reference beam BR are branched off from the measurement beam provided in the previous step 200.

[0162] In step 204, the object beam BO is directed to the surface 2, 4 of the object 1 to be measured. In step 206, the portion of the object beam BO that propagates into the surface material 5 of the object 1 is absorbed, and in step 208, only the portion of the object beam BO that is reflected from the outer surface of the object 1 or coating 3 enters the measurement probe 50 again. Then, in step 210, the reflected portion of the object beam BO that enters the measurement probe again as a signal beam BS is recombined with the reference beam BR and then used in an interference signal analysis to determine or derive the distance D between the optical probe 50 and the surface of the object. [Explanation of symbols]

[0163] 1 object 2 surface 3. Coating 4 Coating surface 5 Surface material 10 Measuring Devices 11 Coupler 12 Beam Trap 14 Combiner 15 Coupler 20 Beam generating unit 21 Light source 22 Light source 23 Light source 24 Photodiode 25 Combiner Unit 26 Superluminescent Diode 27 Laser 30 Optical Delay Unit 31 Input 32 Output 33 Coupler 34 Reflector 35 Delay Head 36 Fiber End Face 37 Optical elements 38 Transducer 39 Optical Fiber 40 Coupler unit 50 Measuring Probes 51 Optical elements 52 Fiber end face 53 Coating 54 Housing 55 Splitter 56 Recombiner 58 Transducer 60 Divider Unit 61 WDM 62 Splitter 63 Output 64 Output 65 Filters 66 Filters 67 Filters 68 Filters 69 Output 70 Detector Unit 71 Detector 72 Detector 73 Detector 80 Signal Analyzer 85 Optical Diode Array 90 Optical Fiber 91 Optical Fiber 92 Optical Fiber 93 Optical Fiber 94 Optical Fiber 95 Optical Fiber 96 Optical Fiber 97 Optical Fiber 98 Transmission lines, electrical signal lines 99 Optical Fiber 100 Graphs 110 Graph 111 Graph 112 Graphs 113 Graph 114 Graphs 120 Graphs

Claims

1. An interferometric measuring device (10) for measuring a surface (2, 4) or a profile of an object (1), comprising: a beam generating unit (20) including at least a first light source (21), the beam generating unit (20) being operable to generate a measurement beam (BM) having a spectral content at wavelengths less than 550 nm; A splitter (55) for splitting an object beam (BO) and a reference beam (BR) from the measurement beam (BM); a measurement probe (50) coupled to the beam generating unit (20) in a light-transmitting manner and configured to direct an object beam (BO) to the surface (2, 4) and to capture a portion of the object beam reflected from the surface (2, 4) as a signal beam (BS); an optical beam recombiner (56) operable to recombine the signal beam (BS) and the reference beam (BR) into an analysis beam (BA); A beam splitter unit (60) coupled to the recombiner (56) for splitting a first central wavelength λ 1 from the analysis beam (BA). １ and extracting a first partial beam (BP1) of a second central wavelength λ from the analysis beam (BA). ２ and operable to extract a second partial beam (BP2) having a second central wavelength λ ２ is the first central wavelength λ １ a beam splitter unit (60) different from a detector unit (70) coupled to the beam splitter unit (60) and including at least a first detector (71) for detecting the first partial beam (BP1) and including at least a second detector (72) for detecting the second partial beam (BP2); and a signal analyzer (80) connected to the detector unit (70) and operable to derive a distance (D) between the measurement probe (50) and the surface (2, 4) based on signals acquired from the at least a first detector (71) and the at least a second detector (72).

2. The beam generating unit (20) includes a second light source (22) having a second central wavelength λ ２ The measurement device (10) of claim 1, operable to emit electromagnetic radiation having spectral components of:

3. The beam splitter unit (60) splits the third central wavelength λ 1 from the analysis beam (BA). ３ and operable to extract a third partial beam (BP3) having a third central wavelength λ ３ is the first central wavelength λ １ Unlike the second central wavelength λ ２ 3. The measuring device (10) according to claim 1 or 2, wherein, distinct from said first partial beam (BP3), said detector unit (70) comprises a third detector (73) for detecting said third partial beam (BP3).

4. The beam generating unit (20) includes a third light source (23), which emits a third central wavelength λ ３ 4. The measurement device (10) of claim 3, operable to emit electromagnetic radiation having spectral components of:

5. The measuring device (10) according to any one of the preceding claims, wherein the first light source (21) comprises one of a laser (27) and a superluminescent diode SLD (26).

6. The measurement device (10) of any one of claims 1 to 5, wherein the at least one light source (21, 22, 23) comprises a superluminescent diode (27), and the measurement device (10) further comprises an optical delay unit (30) coupled to the at least one light source (21, 22, 23), the optical delay unit (30) operable to impart a phase shift of a variable and adjustable size to at least one of the measurement beam (BM), the signal beam (BS), the object beam (BO), and the reference beam (BR).

7. First central wavelength λ １ and a second central wavelength λ ２ is the formula: [0010] The measuring device (10) according to any one of claims 1 to 6, wherein n, m are 1≦n, m≦50.

8. Third central wavelength λ ３ and the first and second central wavelengths λ ｉ One of the formula: [0025] The measuring device (10) according to any one of claims 3 to 7, wherein i=1 or 2, 10≦k, l≦2,500.

9. First central wavelength λ １ , the second central wavelength λ ２ , and a third central wavelength λ ３ teeth: Within the spectral range of 380 nm to 490 nm; Within the spectral range of 400 nm to 460 nm; Within the spectral range of 404 nm to 455 nm; Within the spectral range of 449 nm to 511 nm; Within the spectral range of 449 to 489 nm, or A measuring device (10) according to any one of claims 3 to 8, in the spectral range of 404 to 475 nm.

10. The object (1) has first and second central wavelengths λ １ , λ ２ A measuring device (10) according to any one of the preceding claims, comprising at its surface (2, 4) a material which is substantially absorptive for at least one of the electromagnetic radiations.

11. 11. The measuring device (10) of claim 1, further comprising a coupler unit (40) coupled to the beam generating unit (20) in an optically transparent manner, coupled to the measurement probe (50) and coupled to the splitter unit (60), the coupler unit (40) configured to direct a measurement beam (BM) from the beam generating unit (20) to the measurement probe (50) and to direct an analysis beam (BA) from the measurement probe (50) to the splitter unit (60).

12. The divider unit (60) comprises: i) a wavelength division multiplexer WDM (61); and ii) at least one of a fiber splitter (62) having a first output (63) connected to a first optical filter (65) and a second output (64) connected to a second optical filter (66).

13. The measuring device (10) of any one of claims 2 to 4, 8 and 9, wherein the beam splitter unit (60) is operable to extract a fourth partial beam of a fourth central wavelength from the analysis beam (BA), the fourth central wavelength being different from the first central wavelength, the second central wavelength and the third central wavelength, and the detector unit (70) comprises a fourth detector for detecting the fourth partial beam.

14. Fourth central wavelength λ ４ and first, second, and third central wavelengths λ １ , λ ２ and λ ３ One of the formula: [0030] 14. The measuring device (10) of claim 13, wherein i=1, 2, or 3 and 100≦h, j≦250,000 or 1000≦h, j≦5,000.

15. First, second, and third central wavelengths λ １ , λ ２ and λ ３ The measuring device (10) according to any one of claims 2 to 4, 8, 9, 13 and 14, wherein two of the central wavelengths differ by less than 5 nm, less than 3 nm or less than 2 nm.

16. The first and second light sources (21, 22) are both implemented as superluminescent diodes; First central wavelength λ １ is about 405 nm, and the second central wavelength λ ２ is about 450 nm, or First central wavelength λ １ is about 450 nm, and the second central wavelength λ ２ The measuring device (10) according to any one of claims 2 to 4, wherein .lambda. is about 510 nm.

17. First central wavelength λ １ is about 405 nm, and the second central wavelength λ ２ is about 449 nm, and the third central wavelength λ ３ The measuring device (10) according to claim 16 and according to claim 3 or 4, wherein .lambda. is about 451 nm.

18. 1. A method for measuring a surface (2, 4) or a profile (P) of an object (1), comprising: Providing an object (1) comprising a surface material (5); generating a measurement beam (BM) by a beam generating unit (20); A step of splitting an object beam (BO) and a reference beam (BR) from a measurement beam (BM) and directing the measurement beam (BM) to a surface (2, 4) of an object (1) by an optical probe (50), wherein the surface material is one of the object beam (BO) and the measurement beam (BM). a light penetration depth of less than 100 μm, less than 50 μm, less than 20 μm, less than 10 μm, less than 5 μm, less than 2 μm, less than 1 μm, less than 0.5 μm, or less than 0.1 μm for each of the surfaces; capturing the portion of the measurement beam (BM) reflected by the surface (2, 4) as a signal beam (BS); recombining the signal beam (BS) and the reference beam (BR) into an analysis beam (BA); detecting the interference of the analysis beam (BA) by a detector unit (70) and deriving the distance (D) between the optical probe (50) and the surface (2, 4) by analyzing the signal of the detector unit (70).

19. 20. The method of claim 18, wherein the object (1) comprises a coating (3) on its surface (2) comprising the surface material (5), and a large portion of the object beam (BO) entering the coating (3) is absorbed by the coating (3).

20. 20. The method of claim 19, wherein the coating (3) comprises or consists of silicon or diamond-like carbon DLC.

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