Three-wavelength interferometry device and method

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] This disclosure relates to an interferometric device for measuring the surface or profile of an object, such as an optical element. In a further embodiment, this disclosure relates to a method for measuring the surface or profile of an object. In another embodiment, this disclosure relates to an interferometric device and a method for measuring the surface or profile of an object, wherein the respective surface comprises or is covered by a coating. [Background technology]

[0002] Non-contact measurement of the surface or profile of an object, such as an optical element like a lens, is well known and established in the art. Through an interference mechanism, the measurement light beam is split into a reference beam and an object beam; the object beam is guided to the surface of the object and reflected from the surface as a signal beam. When the signal beam and the reference beam are recombined, optical interference can be observed, indicating the runtime difference, and therefore the path difference, between the reference beam and the signal beam.

[0003] The measurable phase shift between the signal beam and the reference beam is evident only within the range defined by half the wavelength. When the path difference between the signal beam and the reference beam is greater than half the wavelength of the measurement beam, the respective interference patterns produced by the recombination of the signal beam and the reference beam are repeated. Therefore, the absolute distance range of such interference mechanisms is rather generally limited. To increase the spatial measurement range, longer wavelengths, such as those within the infrared spectral range, can be used.

[0004] Furthermore, if an object, such as an optical element, has a coating on the surface to be measured, this coating may be relatively thin, with thicknesses ranging from just a few micrometers to less than 1 μm. In this case, if the coating is substantially transparent to the wavelength of the measurement beam, a first reflection will occur on the outer surface of the coating, and a second reflection will occur on or from the inner or underside of the coating, for example, in direct contact with the outer surface of the object. For interferometric measurement devices and methods, distinguishing between these first and second reflections arising from both sides of such a coating is extremely difficult, if not impossible. [Overview of the project] [Problems that the invention aims to solve]

[0005] Therefore, it is desirable to provide improved interferometric devices and improved methods for measuring the surface or profile of an object, enabling precise surface or profile measurements even when the object has a coating on its surface to be measured. Furthermore, the interferometric device may need to operate with a measurement beam within the visible spectral range. Therefore, it is desirable to provide a cost-effective, reliable, and long-lasting robust solution to enable interferometric measurements of coated surfaces. At the same time, the improved interferometric device should provide a relatively large measurement range for measuring the absolute distance between the device and the surface of the object.

[0006] Interferometry devices should be widely applicable to accommodate or adapt to a variety of variable measurement conditions, and should be easily and clearly reconfigurable. Interferometry devices and methods should also be robust to environmental influences and variations. [Means for solving the problem]

[0007] These requirements are generally addressed by interference measuring devices and methods for measuring the surface or profile of an object, as described in the independent claims. Numerous embodiments are provided for in accordance with the dependent claims.

[0008] In one embodiment, an interferometric measuring device is provided for measuring the surface or profile of an object, such as an optical element, such as a lens or mirror. The interferometric measuring device includes a beam generating unit. The beam generating unit includes at least a first light source. The beam generating unit, and therefore 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 is less than 500 nm and typically has a central wavelength in the region greater than 380 nm. The wavelength of the measurement beam is typically within the visible spectral range less than 550 nm.

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

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

[0011] Typically, the signal beam includes, or is a portion of, the object beam reflected from the surface of the object to be measured. The interferometric measurement device further includes an optical recombiner that can operate to recombine the signal beam and the reference beam into an analysis beam. Typically, the analysis beam includes the superposition of the reflected signal beam and the reference beam. The analysis beam is interfered with and exhibits interference fringes or interference patterns, which indicate the 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 with the splitter, and / or the splitter is integrated with 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, the fiber end face of the optical fiber provides both the splitter and the optical beam recombiner. Typically, the fiber end face terminating inside the measurement probe acts as a splitter, generating a reference beam and an object beam from the measurement beam, and recombining the signal beam reflected from the surface of the object with the reference beam.

[0013] The interferometric measurement device further includes a beam splitter unit coupled to an optical beam recombiner and / or measurement probe so as to transmit light. In some examples, the beam splitter unit is optically coupled to the optical beam recombiner and / or 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 measurement probe by an optical device that provides free propagation of each optical beam.

[0014] The beam splitter unit is operable to extract a first partial beam with a first central wavelength from the analysis beam and a second partial beam with a second central wavelength from the analysis beam. Typically, the second central wavelength is distinctly different from the first central wavelength. The interferometric device further includes a detector unit coupled to the beam splitter unit, for example, through at least one optical fiber to transmit light. In other examples, the detector unit is optically coupled to the beam splitter unit by an optical device that provides free propagation of each light beam. The detector unit includes at least a first detector for detecting the first partial beam and further includes at least a second detector for detecting the second partial beam.

[0015] Typically, since the measurement beam provided and generated by the beam generation unit contains spectral components with wavelengths below 550 nm, at least one of the first and second central wavelengths is within the spectral range below 550 nm. In some examples, both the first and second central wavelengths are within the spectral range below 550 nm.

[0016] Finally, the interferometry device includes a signal analyzer connected to a detector unit to transmit signals. The signal analyzer is operable to derive or calculate the distance between the measuring probe and the surface based on signals obtained from at least a first detector and at least a second detector.

[0017] Typically, each of the first and second detectors is connected separately to the signal analyzer. Thus, the signal analyzer includes a first input connected to the first detector and a second input connected to the second detector. The first input of the signal analyzer receives a measurement signal from the first detector indicating the interference signal measured with respect to a first center wavelength, while the signal analyzer includes a second input connected to the second detector to analyze the signal provided by the second detector when it detects the respective optical signals of the second center wavelength.

[0018] An interferometry device provides multi-wavelength interferometry for the 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] Substantially, by using a second beam, or by using signal analysis based on at least two optical frequencies, a synthetic wavelength defined by the two measurement wavelengths can be generated, thereby increasing the spatial range of the interferometry device. By using a larger (synthetic) wavelength, the range for unambiguous and thus absolute distance measurement can be increased at the expense of spatial resolution and measurement accuracy.

[0020] Furthermore, by having separate signal analysis based on the first central wavelength and the second central wavelength, a synthetic wavelength greater or much greater than the first and the second wavelengths can be provided to increase the unambiguous measurement range of the interferometry device. Furthermore, since the measurement beam comprises spectral components with a wavelength of less than 550 nm, the interferometry device operates within the visible spectral range. When optical coupling between any of the beam generation unit, the measurement probe, the beam splitter unit and the detector unit is provided by at least one optical fiber, each respective optical fiber is particularly configured for light beam propagation and beam transmission for this specific wavelength or spectral range.

[0021] The operation of the interferometry device within the visible spectral range is beneficial in many aspects. Since the measurement beam, and thus the reference beam and the object beam, are within the visible spectral range, an operator of the interferometry device can visually check whether, and to what extent, the object beam accurately hits the surface of the object to be measured. Therefore, the use of visible radiation provides easy and clear control of the operation of the interferometry device.

[0022] In a further aspect, when an object is coated with a coating containing at least one of, for example, silicon or diamond-like carbon DLC, it is particularly advantageous to use radiation within a 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 mainly provided or constituted by reflections occurring at the outer surface of the coating. The portion of the object beam that enters the coating or propagates through the coating is barely reflected by the surface of the underlying object, since the coating is substantially absorptive for this particular wavelength or spectral range.

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

[0025] In general, only one light source can be provided for a beam generation unit. A broader-band light source can be implemented and used herein. In this case, the beam splitter unit can be operable to extract a first partial beam having a first center wavelength and a second partial beam having a second center wavelength, and separately provide the first and second partial beams to the detector unit. In this way, signal analysis can be implemented and performed based on the first and second center wavelengths, and thus different center wavelengths, thereby making it possible to increase the measurement range of the interferometric measurement device.

[0026] In a further example, a splitter is operable to split the measurement beam into an object beam and a reference beam, and includes or is formed by fiber end faces, and is provided, for example, within or on a measurement probe.

[0027] In some examples, the measurement probe is connected to the beam generation unit through at least one optical fiber. Here, the fiber end face of the optical fiber located within the measurement probe can act as a splitter. The portion of the measurement beam that propagates through the optical fiber is emitted through the fiber end face and propagates toward the object as the object beam. Another portion of the measurement beam is reflected as a reference beam by internal reflection at the fiber end face.

[0028] In some cases, a reflective coating is provided on the fiber end face, or the fiber end face is coated with a reflective coating, to increase the intensity of the reference beam reflected from the fiber end face. In typical use scenarios, when the surface of an object has high absorptiveness to the object beam and / or the object has a relatively large refractive index, the intensity of the signal beam reflected from the surface of the object can be relatively large. Increasing the intensity or proportion of the reference beam compared to the object beam is particularly beneficial to obtain a good signal-to-noise ratio for interference of the analysis beam.

[0029] In some examples, a coating provided on the fiber end face and / or on or within the splitter is configured to generate a reference beam containing an intensity of 5% to 60% of the intensity of the measurement beam, a reference beam containing an intensity of 17.5% to 45% of the intensity of the measurement beam, a reference beam containing an intensity of 23.25% to 37.5% of the intensity of the measurement beam, or a reference beam containing an intensity of 25% to 35% of the intensity of the measurement beam, and / or is effective in generating such a reference beam.

[0030] In this way, compared to solutions without reflective coating, the intensity of the reference beam reflected by a coated reflector, such as a coated fiber end face, can be increased. As a result, the intensity of the reference beam can be closely matched to the intensity of the signal beam reflected by the surface of the object and captured by the measuring probe.

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

[0032] Therefore, 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, thereby extracting the first and second partial beams of the first and second central wavelengths from the analysis beam.

[0033] In other examples, the first and second light sources may be implemented as broadband or narrowband light sources, and the first and second partial beams, each having a first and second center wavelength, may be specifically extracted by a beam splitter unit.

[0034] Nevertheless, by closely matching the emission spectra of the first and second light sources to a beam splitter unit that can operate to extract their respective spectral components from the analysis beam, the entire beam guidance system of the interferometric measurement device can also be specifically adapted and configured to provide less attenuated, lossless beam propagation through various optical fibers that couple or connect, for example, the beam generation unit, splitter, measurement probe, optical beam recombiner, beam splitter unit, and detector unit.

[0035] In a further example, a beam splitter unit can be operated to extract a third sub-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. To provide precise signal analysis and distance calculations between the measurement probe and the surface of the object to be measured, the differences between the first, second, and third central wavelengths are clear and known to the signal analyzer.

[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 a signal analyzer. In this example, the signal analyzer is individually connected to or coupled to the first, second, and third detectors of the detector unit to enable wavelength-selective or wavelength-specific interferometry of the beam being analyzed.

[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 distinct from the first, second, and third central wavelengths. Again, the detector unit includes a fourth detector for detecting the fourth partial beam. In general, multi-wavelength interferometry devices are not limited to just two, three, or four distinct wavelengths. Multi-wavelength interferometry devices can also be implemented based on five, six, seven, eight, or up to ten, or even up to 12, 14, 16, or 20 different distinct wavelengths for signal analysis of the analysis beam.

[0038] In another example, the beam generation unit includes a third light source. The third light source is operable to emit electromagnetic radiation containing a spectral component of a third central wavelength. Like the first and second light sources, the third light source can also be one of a broadband light source having or exhibiting substantial light intensity at the third central wavelength. Alternatively, the third light source can be a narrowband light source, such as a laser, emitting substantial beam intensity at the third central wavelength.

[0039] In further examples of measurement devices, the first light source includes one of a laser and one of 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, within a region of several meters. When implemented as a laser, for example, a solid-state laser or a semiconductor laser, the first light source provides substantial optical intensity at a 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 at a reasonable cost for the desired spectral range below 550 nm. Both laser and superluminescent light sources can provide sufficient time stability over several hours of operation. Typically, light sources used or available with this interferometric device exhibit long-term stability or long-term drift of the center wavelength of less than 5 pm over 8 hours, preferably less than 3 pm over 8 hours, or even less than 2 pm over time intervals of at least 8 hours.

[0041] In another example, the second light source includes one of a laser and one of a luminescent diode. To that extent, when the beam generation unit includes a first light source and a second light source, both light sources can be implemented as lasers, the first laser can be operated to emit radiation of a first central wavelength, and the second laser can be operated to emit electromagnetic radiation of 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 configured 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 a second central wavelength. In some examples, the roles of the first and second light sources can be switched, so that the first light source is implemented as a superluminescent diode and the second light source is implemented as a laser.

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

[0044] In some examples, the first, second, and third light sources are each implemented as lasers capable of emitting electromagnetic radiation at, for example, 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 separate superluminescent diodes. In further examples, at least two of the light sources in the beam generation unit may each include lasers for emitting electromagnetic radiation at, for example, 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 separate superluminescent diodes. Typically, each of the superluminescent diodes may exhibit a different or characteristic spectral range compared to the others.

[0046] Depending on the specific configuration of the beam generation unit, which includes at least one, two, or three separate light sources, and the respective configurations of each light source, which may be implemented as, for example, a laser or a superluminescent diode, the beam generation unit may include at least one or more optical couplers, which can couple the respective electromagnetic radiations generated by the separate light sources within a single optical fiber to transmit each multi-component measurement beam toward the measurement probe.

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

[0048] Typically, when using superluminescent diodes as light sources within a beam generation unit, 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. In this case, the recombination of the signal beam and the reference beam becomes unable to interfere with each other.

[0049] The delay unit allows at least one measurement beam, signal beam, object beam, and reference beam to be split into a non-delayed beam portion and a delayed beam portion, with a variable and adjustable phase shift being applied only to the delayed beam portion. Typically, an auxiliary optical path can be provided to the delayed beam portion compared to the non-delayed beam portion. In this case, the delayed beam portion and the non-delayed beam portion copropage through their respective optical fibers.

[0050] The measurement beam is divided into a delayed beam portion and a non-delayed beam portion to generate corresponding object beams, reference beams, signal beams, and analysis beams, each of which has a non-delayed portion and a delayed beam portion. In this case, a low-coherence light source allows the non-delayed portion of the signal beam to interfere with the delayed portion of the reference beam. The non-delayed and delayed portions of the signal beam and reference beam, respectively, have the same or substantially identical optical path lengths and are therefore capable of interfering with each other.

[0051] The delay provided by one of the signal beam, object beam, and 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 one with relatively short coherence lengths, such as less than 10 cm, less than 1 cm, less than 1 mm, or even just a few hundred micrometers, can still be used as a light source. The optical delay unit can be suitably configured to compensate for the runtime difference between the reference beam and the signal beam.

[0052] In some examples, the optical delay unit is located upstream of the measurement probe. In other examples, the delay unit is located downstream of the measurement probe, for the propagation of the measurement beam and all further beams derived therefrom.

[0053] In general, optical delay units can be installed at various locations within the interferometric measurement device. An optical delay unit can be placed between the beam generation unit and the splitter. An optical delay unit can be placed between the splitter and the measurement probe. An optical delay unit can also be placed between the measurement probe and the beam splitter unit.

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

number

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

[0056] Typically, in some examples, n and m are integers. SMThis defines a first combined wavelength artificially generated by the signal analyzer when the first and second signals from the first and second detectors are analyzed, respectively. Typically, the first combined wavelength is about 1 to 50 times larger than either the first central wavelength λ1 or the second central wavelength λ2. In this way, the measurement range for absolute distance measurements provided by the interferometric measurement device can be increased.

[0057] Preferably, the first composite wavelength is about 5 to 35 times greater than either the first central wavelength λ1 or the second central wavelength λ2. Such selections of n and m allow the composite wavelength to be slightly reduced for the benefit of improving robustness and reducing measurement variability and measurement errors.

[0058] Most preferably, the first composite wavelength is about 10 to 20 times greater than either the first central wavelength λ1 or the second central wavelength λ2. Such selections of n and m allow the composite wavelength to be further reduced for the benefit of further improving robustness and further reducing measurement variability and measurement errors.

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

[0060] Typically, in some examples, the signal analyzer can be operated to derive the distance between the measurement probe and the surface based on the interference signal of the analysis beam with respect to at least one of a first central wavelength and a second central wavelength. The signal analyzer can further operate to determine the distance between the measurement probe and the surface of the object by evaluating the interference signal of the analysis beam with respect to a first combined wavelength.

[0061] An increase in the absolute measurement range is provided without loss of measurement accuracy or measurement resolution by separate measurements and separate signal analyses, performed on the one hand based on a first composite wavelength and on the other hand based on at least one of the first and second central wavelengths.

[0062] According to a further example of a measuring device, the third central wavelength λ3 and one of the first and second central wavelengths λ1 or λ2 are given 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 composite wavelength λ SG This is generated, and the second composite wavelength λ SG is the first composite wavelength λ SM Larger. In some examples, the second composite wavelength λ SG The first composite wavelength λ SM This is approximately 10 to 20 times larger. In this way, the absolute measurement range of distance measurement provided by the interference measuring device can be further increased. This second combined wavelength can be as large as a few centimeters or even a few meters.

[0064] When at least two of the first, second, and third central wavelengths are located closer to each other, 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 composite wavelength can be provided, thereby increasing the range of absolute distance measurement provided by the interferometric measurement device to several millimeters, several centimeters, or even several meters.

[0065] Preferably, the second combined wavelength is about 50 to 400 times greater than either the first central wavelength λ1 or the second central wavelength λ2. Such selections of k and l can increase the second combined wavelength, and thus provide an increase in the overall absolute measurement range of the measuring device.

[0066] Most preferably, the second combined wavelength is about 50 to 500 times greater than either the first central wavelength λ1 or the second central wavelength λ2. Such selections of k and l can further increase the second combined wavelength, thus providing an increase in the overall absolute measurement range of the measuring device.

[0067] In a further example of a measurement device, a beam splitter may be operable to extract a fourth partial beam with a fourth central wavelength λ4 from the analysis beam. In addition, or alternatively, a beam generation unit may include an auxiliary light source operable to emit electromagnetic radiation containing a spectral component of the fourth central wavelength λ4.

[0068] Typically, according to further examples, the fourth central wavelength λ4 and one of the first, second, and third central wavelengths λ1, λ2, and λ3 are given 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 composite wavelength λ SL This is generated, and the third composite wavelength λ SLis larger than any of the first synthetic wavelength λ SM and the second synthetic wavelength λ SG . In some examples, the third synthetic wavelength λ SL is approximately 10 to 20 times larger than the second synthetic wavelength λ SL . In this way, the absolute measurement range of distance measurement provided by the interference measurement device can be further increased. The second synthetic wavelength can be on the order of several centimeters or even several meters.

[0070] According to another example, the first center wavelength, the second center wavelength, and the third center wavelength are: - within the spectral range of 380 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 - within the spectral range of 404 to 475 nm.

[0071] Typically, the selection and / or combination of individual first, second, and optionally third, or even fourth light sources depends on 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 center wavelengths are all within the blue spectral range. These center wavelengths are located within the visible blue spectral range. Using at least the first and second center wavelengths of the measurement beam for interference measurement of a surface or profile of an object within the blue spectral range, i.e., less than 511 nm or less than 500 nm, is particularly advantageous for measuring an object provided with a coating that is substantially absorptive for this specific 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 central wavelength of the first can be set to approximately 450 nm and the central wavelength of the second can be set to approximately 473 nm.

[0074] In other examples, when both the first and second light sources are implemented as SLDs, the central wavelength of the first can be approximately 405 nm and the central wavelength of the second can be approximately 450 nm. Alternatively, the central wavelength of the first can be approximately 450 nm and the central wavelength of the second can be approximately 510 nm.

[0075] Alternatively, when the first light source includes an SLD and the second and third light sources each include a laser, the central wavelength of the first can be set to approximately 450 nm, the central wavelength of the second to approximately 473 nm, and the central wavelength of the third to approximately 480 nm.

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

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

[0078] In a further example, an object to be measured by a measuring device has a material on its surface that is substantially absorbent to at least one of the first and second central wavelengths of electromagnetic radiation. Typically, the material of the object has high absorbency to at least one of the first, second, and third central wavelengths of electromagnetic radiation. In some examples, the object includes optical elements coated with a coating or coating layer. Typically, the coating of the object can have layer thicknesses 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 includes a surface coated with a coating having a layer thickness of less than 10 μm. The coating material can be substantially absorbent to at least one of a first central wavelength, a second central wavelength, and an optional third central wavelength. Typically, the selection of the first, second, and optional third central wavelengths depends on the type of coating material on 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 their respective spectral components are substantially absorbed by the coating. Typically, the term "substantially absorbent" to electromagnetic radiation at a particular central wavelength means that the intensity of electromagnetic radiation at each central wavelength after propagation of 1 μm through the respective material or coating is reduced to at least 50%, at least 20%, at least 10%, or even less than 10% of the initial intensity.

[0080] By using a first and second central wavelength in the analysis of an analytical beam where the object's coating material is substantially absorbent, any reflections that may occur at the interface between the coating and the outer surface of the coated object can be substantially avoided. The signal beam reflected from the object's surface 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 reflected any further.

[0081] Therefore, selecting an appropriate wavelength according to the absorption characteristics of the coating on the coated object is ultimately beneficial in avoiding any reflections from the underside of the coating that are in direct contact with the outer surface of the coated object.

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

[0083] In some cases, the splitter and optical beam recombiner are implemented within or on the measurement probe.

[0084] When the analysis beam is generated directly within the measurement probe, the coupler unit is configured to guide the analysis beam from the measurement probe to the splitter unit. In other examples, when the analysis beam is generated downstream of the measurement probe, the optical unit can be configured to guide the signal beam from the measurement probe to the splitter unit. The coupler unit may include an optical fiber coupler, which can guide the measurement beam to the measurement probe and transfer or guide any beam captured by the measurement probe 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 applied to couple and re-induce different beams of electromagnetic radiation at a given central wavelength. Typically, the coupler unit is optimized to couple and re-induce electromagnetic radiation at at least first and second central wavelengths, and optionally third central wavelength, from one optical fiber or optical element to another optical fiber or optical element.

[0086] In a further example of a 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 substantially of a wavelength-division multiplexer. Such a wavelength-division multiplexer can be operated to split the analysis beam into different spectral components, for example, a first spectral component at a first central wavelength and a second spectral component at a second central wavelength. In some cases, the wavelength-division multiplexer can also be operated to split the analysis beam into three subbeams at a first central wavelength, a second central wavelength, and a third central wavelength, respectively.

[0087] A wavelength-division multiplexer can minimize intensity loss. In this configuration, the first and second detectors of the detector unit located downstream of the divider unit can be provided 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 may have transmittances that match the first and second center wavelengths. Typically, the first optical filter transmits only the first center wavelength, and the second optical filter transmits only the second center wavelength. The fiber splitter can be operated to split the incoming beam into at least a first and a second partial beam, the first and second partial beams having more or less identical or substantially equal spectral distributions.

[0089] An additional fiber splitter can be provided, thereby splitting the incoming analysis beam provided by the measurement probe or coupler unit into first, second, and third partial beams. These first, second, and third partial beams can then be directed to first, second, and third optical filters, respectively. Each optical filter allows for the selection of a more precise center wavelength from the incident electromagnetic radiation. The use of optical filters inevitably involves a substantial reduction in electromagnetic intensity. However, the use of optical filters provides a shorter bandwidth and more precise center wavelength for further signal analysis.

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

[0091] For a given wavelength of at least one of the object beam and measurement beam, the surface material includes optical absorption or penetration depths 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. Here, the optical penetration depth is defined by the thickness of the material at which the beam intensity is reduced to 1 / e, i.e., about 36% of the initial beam intensity.

[0092] Alternatively, according to another example, the surface material contains an absorption coefficient AC, and AC is 500,000 cm for at least one of the object beam and the measurement beam. -1 ~5,000cm -1 (That is, 1 cm -1 The odds are 500,000 to 5,000.

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

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

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

[0096] Typically, for a given surface material and a coating of the surface material applied to the surface of an object, at least one of the first and second central wavelengths of the measurement beam is selected such that radiation reflected by the upper surface of the object's surface material takes precedence over the signal beam re-entering the measurement probe. In this way, 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 with the signal beam portion reflected by the upper surface of the surface material.

[0097] In a further embodiment, 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 interference method, in which an object beam and a reference beam are branched from the 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 enters the measurement probe again for further interference signal analysis.

[0098] To that extent, this method is characterized by selecting or providing a measurement beam having a desired central wavelength in 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 should be relatively thin, for example when the object should have 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 because any reflections 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 methods described above will be carried out by the interferometric measuring devices described above. To that extent, all the configurations, effects, and benefits described above in relation to the interferometric measuring devices can also be equally applied to methods for measuring the surface or profile of an object. The reverse is also true; the measuring devices described above can also be configured and applied to carry out the methods for measuring the surface or profile of an object described above.

[0100] In a further example, the object to be measured has a coating on its surface. Here, the object's coating consists of a surface material. Typically, the entire coating of the object is made from the surface material. In other examples, the outer or upper layer of the coating is made from the surface material. When the object beam is guided to the object's surface, most of the object beam entering the coating is absorbed by the coating. In this way, any reflections from the underside of the coating or reflections from the underside of the object that are facing away from the measuring probe can be substantially avoided and suppressed.

[0101] In a further example, the coating of an object may include or consist of silicon or diamond-like carbon (DLC). For example, when an object in the form of an optical element such as a lens or mirror is coated with, for example, a silicon layer, the method and measuring device are typically implemented to provide a measuring beam that includes substantial spectral components, i.e., non-negligible spectral components, at wavelengths less than 550 nm. Preferably, the measuring beam includes, for example, several spectral components at a first central wavelength and a second central wavelength, both of which are less than 550 nm.

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

[0103] Numerous examples of methods for measuring the surface and / or profile of objects such as measuring devices and optical elements will be described in more detail below with reference to the drawings. [Brief explanation of the drawing]

[0104] [Figure 1] This figure shows an example of an interference measurement device according to this disclosure. [Figure 2] This figure shows further implementations of the interferometry device. [Figure 3] This figure shows one embodiment of a beam generation unit equipped with a coupling unit. [Figure 4] This figure shows further examples of beam generation units and coupling units in further examples. [Figure 5] This is a diagram showing an optical delay unit. [Figure 6] This figure shows an example of a measuring probe. [Figure 7] This figure shows the light absorption of the surface material of an object. [Figure 8] This figure shows another example of a beam generation unit. [Figure 9] This figure shows a further example of a beam generation unit. [Figure 10] This figure shows the spectral distribution of the measurement beam. [Figure 11] This diagram shows the combination of the beam splitter unit and the detector unit. [Figure 12] This figure shows another example of the spectral distribution of the measurement beam. [Figure 13] This figure shows an example of an optical diode arrangement. [Figure 14] This is a flowchart illustrating a method for measuring a surface using an interferometric measurement device. [Modes for carrying out the invention]

[0105] Figure 1 schematically shows an interferometric measurement device 10 for measuring the surface 2, 4 or profile of object 1. The interferometric measurement device 10 includes a beam generation unit 20 which includes at least a first light source 21. The first light source 21 is operable to generate a measurement beam BM having spectral components with wavelengths less than 550 nm. Typically, the first light source 21 is operable to generate a measurement beam within the visual spectral range of less than 550 nm and greater than 380 nm.

[0106] The interferometry device further includes a measurement probe 50 connected to the beam generation unit 20 through at least one optical fiber 91, 92, 93, 94. As shown in more detail in Figure 6, the measurement probe 50 includes a splitter 55, and the measurement beam BM provided by the beam generation unit 20 is separated by the splitter 55 into an object beam BO and a reference beam BR.

[0107] Only the object beam BO is guided toward surfaces 2 and 4 of object 1. There, that is, at surfaces 2 and 4 of object 1, the object beam BO is reflected as a signal beam BS and recombined with the reference beam BR by the 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 toward the beam splitter unit 60, as shown in Figure 1.

[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 with a first central wavelength λ1 from the analysis beam BA and derive a second partial beam BP2 with a second central wavelength λ2. The first and second partial beams BP1 and BP2 are separately supplied and transmitted to the first detector 71 and the second detector 72 of the detector unit 70. This allows for wavelength-selective detection of interference in the analysis beam BA.

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

[0110] In the example shown in Figure 6, the fiber end face 52 of the optical fiber 94 connecting the measurement probe 50 to the beam generation unit 20 acts as a splitter 55. Thus, a portion of the measurement beam BM provided by the beam generation unit 20 is reflected, i.e., internally reflected from the fiber end face 52 within the optical fiber 94. Another portion of the measurement beam BM propagates from the fiber through the optical element 51 and is guided toward the surfaces 4, 2 of the object 1 as an object beam BO. Typically, the object beam BO is focused toward a portion of each of the surfaces 2, 4. To increase the internal reflectivity of the fiber end face 52, a coating 53 can be provided on the fiber end face 52, and therefore on the splitter 55. In this way, the intensity of the reference beam BR can be increased at the expense of the intensity of the object beam, and therefore the signal beam.

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

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

[0113] In some examples, the object 1 to be measured is provided with a coating 3 or surface material 5. The coating 3 and / or surface material 5 may include silicon or diamond-like carbon (DLC). The layer of coating 3, and therefore surface material 5, facing the measurement probe 50 may have a thickness in the range of just a few micrometers, such as 5-10 μm, 10-20 μm, 10-100 μm, 50-150 μm, 100-200 μm, 150-250 μm, or 100-300 μm. Using a measurement beam BM in the infrared spectral range can lead to numerous problems because the coating 3 is substantially transparent to electromagnetic radiation in this spectral range. In this case, reflections may 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 that is in direct contact with and adjacent to the outer surface of the object 1 or optical element. In this case, it is 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, each object beam BO propagating through coating 3 into coating 3 is substantially absorbed. In this way, the intensity of the signal beam BS' reflected at the interface between object 1 and coating 3 is reduced to a minimum, which may prevent interference with the analysis beam BA caused by the recombination of the reflected signal beam BS and the reference beam BR.

[0115] Typically, the beam splitter unit 60 is operable to generate or derive first and second partial beams BP1, BP2, each partial beam containing first and second central wavelengths λ1, λ2 in the range of 380 nm to 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 object 1 are subject to detection and subsequent signal analysis. Internal reflections from the inner surface 2 of the coating are substantially suppressed.

[0116] In some examples, the measuring probe 50 includes a housing 54. The housing 54 acts as a fixture for optical elements 51, such as a sighting or focusing optical lens. The fiber end face 52, and thus the entire fiber 94, can be mounted and fixed to the housing 54. Optionally, a transducer 58, such as a piezoelectric transducer, or some other type of phase modulator, can be provided, which can periodically correct the phase of the measuring beam BM. Such periodic modulation of the phases of the measuring beam, and thus the reference beam and signal beam, is beneficial for precise measurement of the relative phase between the reference beam BR and the signal beam BS.

[0117] The interference measurement device 10 shown in Figure 1 is implemented using optical fiber components. The individual components of the interference measurement device 10 are optically connected through individual optical fibers 91, 92, 93, 94, 95, 96, 97, and 99. The implementation of optical fibers and the optical fiber connections of each individual component make the entire interference measurement device more robust and stable. The interference measurement device typically includes an optical coupler unit 40, which is connected to a beam generation unit 20 through an optical fiber 93, to a measurement probe 50 through another optical fiber 94, and to a 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 to the measurement probe 50 through a further optical fiber 94. The analysis beam BA, captured and provided by the measurement probe 50, is transmitted through the same optical fiber 94 and propagates in the reverse direction. The analysis beam BA is re-guided to the beam splitter unit 60 by the coupler unit 40.

[0119] The splitting of the measurement beam BM into the object beam BO and the reference beam BR is performed within the measurement probe 50. Recombination of the signal beam BS and the reference beam BR is performed at the 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 generation 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] Generally, the beam generation unit 20 includes at least one light source 21 containing spectral components 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. In this case, the beam splitter unit 60 is operable to derive or extract first and second partial beams BP1, BP2 with first and second central wavelengths from the spectral distribution provided by the first light source 21.

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

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

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

[0124] In a further example shown in Figure 2, the mechanism of the interference measurement device 10 according to Figure 1 is extended to a third central wavelength λ3. Here, the detector unit 70 includes a first detector unit 71 for a first central wavelength λ1, a second detector unit 72 for a second central wavelength λ2, and a third detector 73 for a third central wavelength λ3, where the respective central wavelengths are different from each other.

[0125] Individual detectors 71, 72, and 73 can be connected individually or together to the signal analyzer 80 via an electrical signal line 98, and the electronic or electrical signals generated by the individual detectors 71, 72, and 73 are transmitted via the electrical signal line 98 to an electronic signal analyzer or an electronically implemented signal analyzer 80.

[0126] As is further evident from Figure 2, in order to provide 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 to the first, second, and third detectors 71, 72, and 73, respectively, through separate optical fibers 96, 97, and 99. Depending on the types of light sources 21, 22, and 23 implemented by the generation unit 20, different central wavelengths can be extracted from the analysis beam BA.

[0127] In the example shown in Figure 2, the beam generation unit 20 includes three separate light sources 21, 22, and 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, and 23 are individually coupled to the coupler unit 25 by their respective optical fibers 90, 91, and 92. Within the coupler unit 25, or by the coupler unit 25, the electromagnetic radiation provided by the separate light sources and the three separate optical fibers 90, 91, and 92 is coupled within a single optical fiber 93, and the coupled or superimposed electromagnetic radiation is transmitted through the optical fiber 93 to the coupler unit 40.

[0129] The optical coupler unit 25 may include an optical coupler 15 implemented as an optical fiber coupler 14, for example, as shown in Figure 8. The coupler unit, and therefore the optical coupler 14, can combine the electromagnetic light intensities provided by the first and second optical fibers within a single output fiber.

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

[0131] In the example shown in Figure 3, both the first light source 21 and the second light source 22 are implemented as SLDs. Here, the radiation generated by the first light source 21 is transmitted to the coupler unit 25 through the optical fiber 91. The light generated by the second SLD light source 22 is transmitted towards the coupler unit 25 through another optical fiber 92. Within the coupler unit 25, the respective beams provided by the separate optical fibers 91 and 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 interferometry device 10 and is typically provided in an example of the device 10 in which at least one light source 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 aforementioned optical fiber 93.

[0133] In a further example shown in Figure 4, the beam generation unit 20 includes a first light source 21 implemented as a laser light source 27. The beam generation 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 coupler unit 25 via 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 coupler 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 greater than the coherence length of the electromagnetic radiation produced by each light source.

[0135] At this point, the two beams will no longer interfere with each other, and it should no longer be possible to measure the interference distance of each beam. The optical delay unit 30, shown in detail in Figure 5, can artificially and adjustably delay the reference beam so that the portion of the reference beam to which the artificial delay is applied substantially matches the distance D between the measured measurement probe 50 and surfaces 2 and 4.

[0136] The delay unit 30 shown in Figure 5 includes an input 31, which is connected to an optical fiber 91a as shown in Figure 3, or to an optical fiber 92 as shown in Figure 4. Internally, the delay unit 30 includes an optical fiber coupler 33, which guides the incident light into the delay head 35. The delay head 35 may include an optical element 37, such as a focusing lens, which guides the incident light to a reflector 34, which is implemented as, for example, a mirror or 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, each light beam undergoes free propagation.

[0137] The fiber end face 36 of the optical fiber 39, which terminates within the delay head 35, is fixed to the delay head 35. The delay head 35 is movable along the optical axis or relative to the reflector 34. In this case, as 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 each electromagnetic radiation is reflected by the reflector 34 and captured by the delay head 35. If applicable, the delay head 35 may be subjected to periodic modulation by a transducer 38 or any other type of phase modulator. When the measurement beam BM has fully propagated through the optical delay unit 30, the transducer 38 or phase modulator may replace or substitute for the transducer 58 or phase modulator 58 of the measurement probe 50.

[0138] In this way, an adjustable and correctable delay Δd can be applied to the reference beam BR. The light reflected from the reflector 34 is captured by the delay head 35 and enters the coupler 33 again. From there, the captured light propagates from the coupler 33 to the output 32, which is connected to an optical fiber 93 to allow light to pass through, and thus directly connected to the coupler unit 40 as shown in Figure 3.

[0139] Alternatively, as shown in Figure 4, the output 32 of the delay unit 30 can be connected to the optical fiber 91a and enter the coupler unit 25 to recombine with further spectral components provided by the laser light source 27. Figure 7 schematically shows a graph 100 illustrating the absorption of a coating 3 provided on the surface 2 of 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 several 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 be even 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 absorbent to each electromagnetic radiation. As shown in Graph 100 of Figure 7, the light intensity transmitted across the layer thickness C decreases to about 1% after propagating through the surface material 5 for about 1 μm. Substantially, by using a measurement beam with spectral components of wavelengths less than 550 nm, the coating 3 and each surface material 5, for example in the form of silicon or DLC, are substantially absorbent, and therefore the contribution of light reflected from the underside 2 of the coating 3 as the signal beam BS' becomes negligibly small.

[0141] Figure 8 shows a more detailed illustration of the embodiment shown in Figure 4. Here, the beam generation unit 20 includes a first light source 21 implemented as a laser 27. The beam generation unit 20 also includes 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. An optical fiber 92 is provided at the output of the delay unit 30. The two optical fibers 91 and 92 enter a coupler unit 25. The coupler unit 25 includes an optical fiber coupler 15, which is implemented as, for example, an optical fiber coupler 14. The optical fiber coupler 15 is typically implemented as a coupler and can be asymmetrical in terms of the degree of crosstalk or optical fiber coupling because the types of light sources 21 and 22 are different.

[0143] In this example, the light intensity provided by the first light source 21 is approximately 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 and 22 are distributed substantially equally within the output optical fiber 93 connected to the output of the optical coupler unit 25 via or by another optical fiber coupler 11, according to the different light intensities of the various light sources 21 and 22.

[0144] In a further example shown in Figure 9, the beam generation unit 20 includes 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 and 22 is equipped with an optical diode 24, which is located between the coupler unit 25 and the respective light sources 21 and 22. The optical diode 24 is implemented using optical fibers. The optical diode 24 is connected to the optical fibers of the light sources 21 and 22 and the optical fiber of the coupler unit 25 by their respective optical fiber couplers 11.

[0145] The output of the coupler unit 25 is connected to a delay unit 30 which includes a coupler 33 implemented as, for example, an optical fiber xcoupler 15. The output of the coupler 15 is connected to a delay head 35 via an optical fiber 39. Another output of the coupler 15 can be connected to an optical fiber 93, which is connected to or can be connected to the coupler unit 40 and / or the measurement probe 50. A beam trap 12 can be provided at a further output of the coupler 15.

[0146] The specific selection of different light sources 21, 22, 23 and their mutual optical coupling depends on the type of coating 3 on the object 1 to be measured. This specific selection can further depend on the availability of commercially available light sources and optical components that guide the projected light, such as optical fibers, couplers, and coupling devices.

[0147] In the example in Figure 10, graph 110 shows the spectrum of the measurement beam when a superluminescent diode 26 is used as the first light source 21 and a laser 27 is used 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 550 nm region. The spectral width of the electromagnetic radiation generated and provided by the SLD can be approximately 6 nm at 3 dB.

[0148] In contrast, 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 the laser light source has a relatively long coherence and exhibits a smaller bandwidth (FWHM), for example, less than 1 MHz.

[0149] The first light source 21 can select a first and a third central wavelength from the emitted spectrum to provide a broader optical signal. This can be done by using appropriate filters that define the first and third central wavelengths, each having, for example, 448.2 nm and 450.4 nm. Here, the third central wavelength is located closer to the first central wavelength, and therefore a relatively large combined wavelength is composed of the first and third central wavelengths.

[0150] The second central wavelength is provided at a distinct spectral distance from at least one of the first and third central wavelengths. Here, for interferometry of the analytical beam BA, another composite wavelength based, for example, the first and third central wavelengths can be artificially generated.

[0151] Figure 11 shows an example of a splitter unit 60 specifically implemented for wavelength splitting of a measurement beam consisting of electromagnetic radiation from a superluminescent diode 26 and electromagnetic radiation generated and emitted by a laser 27. Here, the splitter unit 60 includes 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 a second central wavelength, and therefore radiation from the laser 27. Downstream of the optical filter 68 is a second detector 72 of a detector unit 70.

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

[0153] In this case, the additional optical filter 67 is configured to suppress any further spectral components of light outside the third central wavelength.

[0154] Numerous graphs 111, 112, 113, and 114 provide examples of the transmission and filtering efficiencies of the various filters 65, 66, 67, and 68 described above in relation to Figure 11. Filter 65 is operable to suppress spectral components with 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 central wavelength, for example, a third partial beam around 450 nm.

[0155] The optical filter 68 shown in Graph 114 can be specifically configured to suppress any spectral components with wavelengths below 460 nm.

[0156] In a further example shown in Figure 12, two separate first and second light sources 21 and 22 are provided, both implemented as SLDs. 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 approximately 450 nm. As shown in Graph 120 of Figure 12, the center wavelength of the first is approximately 405 nm, the center wavelength of the second is approximately 449 nm, and the center wavelength of the third is approximately 451 nm. Here, the second and third center wavelengths are operable to produce a slightly larger combined wavelength, and thus it is possible to increase the absolute measurement range of the interference measurement device.

[0157] The mechanisms of the beam splitter unit 60 and the beam generation unit 20 shown in Figure 12 can be made to some extent identical to the schematic diagram in Figure 11. Naturally, each optical filter is selected in close correspondence to the central wavelength of the beam generation unit 20.

[0158] The superluminescent diode SLD26 described and proposed herein is more susceptible to internal reflections that may occur within the interferometric measurement device 10. To suppress any reflections or internal reflections that propagate backward into the SLD, an optical diode arrangement 85, as shown in Figure 13, can be provided. The optical diode arrangement 85 can be provided between the beam generation unit 20 and one of the measurement probe 50 and the coupler unit 40. The optical diode arrangement 85 may include a wavelength division multiplexer 61 that can separate the incoming light into a first optical fiber branch and a second optical fiber branch. The first branch may include an optical diode 24, and the second branch may include another optical diode 24'.

[0159] The photodiodes 24 and 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 their respective optical fiber couplers 11. The outputs of the photodiodes 24 and 24' are recombined by another WDM 61'. The output of the WDM 61' can be coupled or connected to one of the coupler unit 40 and the measurement probe 50 via optical fibers 93 and 94. The optical diode arrangement 85 shown in Figure 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 way, a wavelength-selective optical diode arrangement is provided that prevents any back reflection or back scattering within the optical fiber system for each of the spectral components.

[0161] Figure 14 schematically illustrates a method for measuring the surface 2, 4 or profile of object 1. Here, in the first step 200, the measurement beam BM is generated by the beam generation unit 20 described above. In a further step 202, the object beam BO and the reference beam BR are branched from the measurement beam provided in the previous step 200.

[0162] In step 204, the object beam BO is guided to the surfaces 2 and 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 reflected from the outer surface of the object 1 or coating 3 enters the measurement probe 50 again. Subsequently, 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 for 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. Coated surface 5 Surface material 10 Measuring Devices 11 Couplers 12 Beam Trap 14 Combiner 15 Couplers 20 Beam generation unit 21 Light source 22 Light source 23 Light source 24 Photodiodes 25 Coupling Unit 26 Superluminescent Diode 27 Lasers 30 Optical Delay Units 31 Inputs 32 outputs 33 Couplers 34 Reflector 35 Delayed Head 36 Fiber end face 37 Optical elements 38 Transducers 39 Optical Fiber 40 Coupler Units 50 measuring probes 51 Optical elements 52 Fiber end face 53 Coating 54 Housing 55 Splitter 56 Recombiner 58 Transducers 60-part divider unit 61 WDM 62 Splitter 63 Output 64 outputs 65 filters 66 filters 67 filters 68 filters 69 Output 70 Detector Unit 71 Detectors 72 detectors 73 Detectors 80 Signal Analyzer 85 Optical Diode Arrangement 90 Fiber Optics 91 Optical Fiber 92 Optical Fibers 93 Optical Fiber 94 Optical Fibers 95 Optical Fiber 96 Optical Fiber 97 Optical Fiber 98 Transmission lines, electrical signal lines 99 Optical Fibers 100 graphs 110 Graphs 111 Graph 112 Graphs 113 Graph 114 Graphs 120 Graphs

Claims

1. An interferometric measuring device (10) for measuring the surface (2, 4) or profile of an object (1), wherein: A beam generation unit (20) including at least a first light source (21) including a superluminescent diode (27), the beam generation unit (20) is operable to generate a measurement beam (BM) having spectral components with wavelengths less than 550 nm, A splitter (55) that branches the object beam (BO) and the reference beam (BR) from the measurement beam (BM), A measuring probe (50) is coupled to a beam generation unit (20) to transmit light, and is configured to focus the object beam (BO) onto the surface (2, 4) and capture the portion of the object beam that is reflected from the surface (2, 4) as a signal beam (BS). A light beam recombiner (56) capable of recombining the signal beam (BS) and the reference beam (BR) into the analysis beam (BA), A beam splitter unit (60) coupled to the recombiner (56), The beam splitter unit (60) is: i) Wavelength-dividing demultiplexer WDM(61) and ii) comprising at least one of the fiber splitters (62), the fiber splitter having a first output (63) connected to a first optical filter (65) and a second output (64) connected to a second optical filter (66), The beam splitter unit (60) divides the analysis beam (BA) into a first central wavelength λ 1 The first partial beam (BP1) is extracted, and the second central wavelength λ is obtained from the analysis beam (BA). 2 It is operable to extract the second partial beam (BP2), Second central wavelength λ 2 The first central wavelength is λ 1 Unlike, A detector unit (70) coupled to the beam splitter unit (60) includes at least a first detector (71) for detecting a first partial beam (BP1) and at least a second detector (72) for detecting a second partial beam (BP2), A signal analyzer (80) connected to the detector unit (70) and capable of deriving the distance (D) between the measuring probe (50) and the surface (2, 4) based on signals acquired from at least a first detector (71) and at least a second detector (72), An optical delay unit (30) coupled to at least one light source (21, 22, 23), Includes, The optical delay unit (30) is operable to impart a variable and adjustable phase shift to at least one of the measurement beam (BM), signal beam (BS), object beam (BO), and reference beam (BR). The aforementioned measuring device.

2. The beam generation unit (20) includes a second light source (22), the second light source (22) having a second central wavelength λ 2 The measuring device (10) according to claim 1, which is operable to emit electromagnetic radiation containing spectral components.

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

4. The beam generation unit (20) includes a third light source (23), the third light source (23) having a third central wavelength λ 3 The measuring device (10) according to claim 3, which is operable to emit electromagnetic radiation containing spectral components.

5. The measuring device (10) according to any one of claims 1 to 4, wherein the first light source (21) includes one of a laser (27) and a superluminescent diode SLD (26).

6. First central wavelength λ 1 and the second central wavelength λ 2 The formula is: [Mathematical Formula 1] A measuring device (10) according to any one of claims 1 to 5, satisfying the following conditions, where 1 ≤ n and m ≤ 50.

7. The third central wavelength λ 3 as well as the first and second central wavelengths λ i One of them is the formula: [Math 2] A measuring device (10) according to any one of claims 3 to 6, satisfying the following conditions, where i = 1 or 2, 10 ≤ k, and l ≤ 2,500.

8. First central wavelength λ 1 , the second central wavelength λ 2 , and the 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 7, wherein the spectral range is 404 to 475 nm.

9. Object (1) has first and second central wavelengths λ 1 , λ 2 A measuring device (10) according to any one of claims 1 to 8, comprising a material on its surface (2, 4) that is substantially absorbent to at least one of the electromagnetic radiations.

10. The measuring device (10) according to any one of claims 1 to 9, further comprising a coupler unit (40) coupled to a beam generation unit (20) so as to transmit light, coupled to a measurement probe (50), and coupled to a beam splitter unit (60), wherein the coupler unit (40) is configured to guide the measurement beam (BM) from the beam generation unit (20) to the measurement probe (50) and guide the analysis beam (BA) from the measurement probe (50) to the beam splitter unit (60).

11. 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, second, and third central wavelengths, and the detector unit (70) includes a fourth detector for detecting the fourth partial beam, according to any one of claims 2 to 4, 7, and 8, the measuring device (10).

12. The fourth central wavelength λ 4 Furthermore, the first, second, and third central wavelengths λ 1 , λ 2 and λ 3 One of them is the formula: [Math 3] The measuring device (10) according to claim 11, satisfying the following conditions, where i = 1, 2, or 3, and 100 ≤ h, j ≤ 250,000 or 1000 ≤ h, j ≤ 5,000.

13. The 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, 7, 9, 11 and 12, wherein two of the central wavelengths differ by less than 5 nm, less than 3 nm, or less than 2 nm.

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

15. First central wavelength λ 1 The second central wavelength is approximately 405 nm, and λ 2 It is approximately 449 nm, and the third central wavelength λ 3 The measurement device (10) according to claim 14 and / or claim 3 or 4, wherein the wavelength is approximately 451 nm.

16. A method for measuring the surface (2, 4) or profile (P) of an object (1), the following: A step of providing an object (1) containing a surface material (5), A step of generating a measurement beam (BM) using a beam generation unit (20) including a superluminescent diode (27), A step of branching an object beam (BO) and a reference beam (BR) from a measurement beam (BM), and focusing the measurement beam (BM) onto the surface (2, 4) of an object (1) using an optical probe (50), wherein the surface material has a thickness of less than 100 μm, less than 50 μm, less than 20 μm, less than 10 μm, or less than 5 μm relative to one of the object beam (BO) and the measurement beam (BM). A process including a light penetration depth of less than 2 μm, less than 1 μm, less than 0.5 μm, or less than 0.1 μm, A process of capturing the portion of the measurement beam (BM) that is reflected as a signal beam (BS) by the surface (2, 4), The process involves recombining the signal beam (BS) and the reference beam (BR) into the analysis beam (BA), i) Wavelength-dividing demultiplexer WDM(61) and ii) A beam splitter unit (60) comprising at least one of fiber splitters (62), wherein the fiber splitter has a first output (63) connected to a first optical filter (65) and a second output (64) connected to a second optical filter (66), The process involves extracting a first partial beam (BP1) with a first central wavelength λ1 from the analytical beam (BA), and extracting a second partial beam (BP2) with a second central wavelength λ2 from the analytical beam (BA), The second central wavelength λ2 is different from the first central wavelength λ1. The detector unit (70) detects interference in the analysis beam (BA) by detecting a first partial beam (BP1) with the first detector (71) of the detector unit (70) and a second partial beam (BP2) with the second detector (72) of the detector unit (70), and derives the distance (D) between the optical probe (50) and the surface (2, 4) by analyzing the signals from at least the first detector (71) and at least the second detector (72) of the detector unit (70). The method, comprising the step of using an optical delay unit to impart a variable and adjustable transient constant phase shift to at least one of a measurement beam, a signal beam, an object beam, and a reference beam.

17. The method according to claim 16, wherein the object (1) has a coating (3) comprising a surface material (5) on its surface (2), and the majority of the object beam (BO) entering the coating (3) is absorbed by the coating (3).

18. The method according to claim 17, wherein the coating (3) comprises or consists of silicon or diamond-like carbon DLC.

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