Ranging device

Abstract

This ranging device comprises: a first light source that emits a first laser beam; a second light source that emits a second laser beam; at least one interferometer that outputs a first signal by means of first interference based on a plurality of laser beams, the laser beams including the first laser beam and having mutually different wavelengths, and outputs a second signal by means of second interference based on the second laser beam, and that includes a reference surface; a head of which the distance to a target object can be varied; and a processing circuit that calculates a first distance from the head to the target object on the basis of the first signal, and calculates a second distance from the reference surface to the head on the basis of the second signal.

Description

distance measuring device

[0001] The present disclosure relates to a distance measuring device.

[0002] Optical interference using laser light is widely used as a means for acquiring information indicating the distance and / or shape of an object without contact. For example, a frequency-modulated continuous wave radar (FMCW) LiDAR (Light Detection and Ranging) is known as a three-dimensional measurement device with millimeter accuracy. Optical interference using optical coherence tomography (OCT) or optical comb is known as a means for measuring with micrometer accuracy. These methods are widely used in the medical and / or industrial fields.

[0003] Furthermore, by controlling the optical interference phenomenon with even greater precision, measurements with nanometer accuracy become possible. For example, measurements using a Michelson interferometer with a single-wavelength laser are one method for measuring distance differences on the order of nanometers as light intensity.

[0004] Optical measurements with nanometer precision, such as homodyne optical interferometry, are capable of non-contact, highly accurate measurements, but have the drawback of being limited to a measurement range of sub-micrometers, which is half the wavelength. This makes it difficult to measure samples that have both nanometer-scale structures and structures on the order of several tens of micrometers.

[0005] Multi-wavelength interference, which is an optical interference technique using two or more single-wavelength lasers, is expected to solve this problem. Multi-wavelength interference can eliminate the trade-off between measurement range and measurement accuracy, which has been a problem in the past, and can simultaneously achieve a long measurement range and high measurement accuracy.

[0006] Non-Patent Document 1 discloses a method for three-dimensionally measuring the surface shape of a sample, such as a lens, by scanning a non-contact probe using multi-wavelength interference. Specifically, the distance from a reference frame to the non-contact probe and the distance from the non-contact probe to the sample are measured to generate three-dimensional data. In this case, to perform three-dimensional measurement with high accuracy on the nanometer scale, it is necessary to measure the distance from the reference frame to the non-contact probe with high accuracy on the nanometer scale.

[0007] J. Petter et al., “Non-contact profiling for high precision fast asphere topology measurement”, Proc. SPIE 8788, Optical Measurement Systems for Industrial Inspection VIII, 878819 (13 May 2013)

[0008] There is room for improvement in the accuracy of distance measurement compared to the technique disclosed in Non-Patent Document 1.

[0009] Therefore, the present disclosure provides a distance measuring device that can measure distance with high accuracy.

[0010] A distance measuring device according to one aspect of the present disclosure includes a first light source that emits a first laser light, a second light source that emits a second laser light, at least one interferometer that includes the first laser light, outputs a first signal by first interference based on a plurality of laser lights having different wavelengths, and outputs a second signal by second interference based on the second laser light, and includes a reference surface, a head whose distance to an object is variable, and a processing circuit that calculates a first distance from the head to the object based on the first signal, and calculates a second distance from the reference surface to the head based on the second signal.

[0011] According to the present disclosure, distance can be measured with high accuracy.

[0012] FIG. 1 is a diagram illustrating an overview of a distance measuring device according to a first embodiment. FIG. 2A is a diagram illustrating the principle of distance measurement using single-wavelength interference. FIG. 2B is a diagram illustrating the principle of distance measurement using multi-wavelength interference. FIG. 3 is a block diagram illustrating the configuration of a distance measuring device according to the first embodiment. FIG. 4 is a diagram illustrating a specific configuration of an interferometer provided in the distance measuring device according to the first embodiment. FIG. 5 is a flowchart illustrating the operation of the distance measuring device according to the first embodiment. FIG. 6 is a block diagram illustrating the configuration of a distance measuring device according to a second embodiment. FIG. 7 is a diagram illustrating an overview of a distance measuring device according to a first modification of the embodiment. FIG. 8 is a diagram illustrating an overview of a distance measuring device according to a second modification of the embodiment. FIG. 9 is a diagram illustrating an overview of a distance measuring device according to a third modification of the embodiment.

[0013] (Summary of the present disclosure) First, definitions of key terms used in this specification are provided below.

[0014] "Measurement accuracy" refers to the degree of accuracy when measuring distance. In other words, measurement accuracy is a measure of how accurately distance information can be obtained. Therefore, the higher the measurement accuracy, the more accurate the distance measurement.

[0015] The "measurement range" represents the range in the distance direction in which unique distance information can be acquired. That is, the measurement range represents the range in which distance measurement (i.e., distance measurement) is possible.

[0016] In this specification, both the measurement accuracy and the measurement range are expressed in the same dimension as the distance. Specifically, the units of the measurement accuracy and the measurement range are both expressed in nanometers (nm), micrometers (μm), millimeters (mm), etc. Therefore, "high measurement accuracy" is synonymous with "short measurement accuracy" expressed in the dimension of distance. "low measurement accuracy" is synonymous with "long measurement accuracy" expressed in the dimension of distance. Furthermore, in this specification, measurement accuracy may be simply referred to as "accuracy." Measurement range may be simply referred to as "range."

[0017] Distance measurement (i.e., distance measurement) within the measurement range is called “absolute distance measurement.” For example, a distance measurement with an accuracy of 10 nm and a measurement range of 1 mm is an absolute distance measurement that can distinguish a difference of 10 nm within a range of 1 mm.

[0018] Aspects of the distance measuring device according to the present disclosure are as follows.

[0019] A distance measuring device according to a first aspect of the present disclosure includes a first light source that emits a first laser beam, a second light source that emits a second laser beam, at least one interferometer that includes the first laser beam, outputs a first signal by first interference based on a plurality of laser beams having different wavelengths, and outputs a second signal by second interference based on the second laser beam, and includes a reference surface, a head whose distance to an object is variable, and a processing circuit that calculates a first distance from the head to the object based on the first signal, and calculates a second distance from the reference surface to the head based on the second signal.

[0020] When measuring distances using the interference of laser light, it is desirable that the frequency of the laser light is stable. However, light sources that emit laser light with stable frequencies have low wavelength selectivity. Poor wavelength selectivity may prevent measurement depending on the material of the target object, resulting in a distance measuring device with low versatility. Light sources that emit laser light with stable frequencies are also expensive. For this reason, it is difficult to use light sources that emit laser light with highly stable frequencies for all light sources. The more locations where interference based on multiple laser lights is used, i.e., the more distances to be measured, the greater the number of light sources, which increases the difficulty. For this reason, it is necessary to effectively utilize interference based on a single laser light, as well as interference based on multiple laser lights.

[0021] Furthermore, for example, when the distance measuring device according to this aspect measures the surface shape of an object, the distance from the head to the object (i.e., the first distance) changes depending on the surface shape of the object. The change in the first distance may be continuous or discontinuous. On the other hand, the change in the distance from the reference surface to the head (i.e., the second distance) is often continuous and rarely discontinuous.

[0022] A signal obtained by interference based on multiple laser beams having different wavelengths (i.e., the first signal) is suitable for measuring distances that vary discontinuously, whereas a signal obtained by interference based on a single laser beam (i.e., the second signal) is not suitable for measuring distances that vary discontinuously.

[0023] Therefore, as described above, the distance measuring device according to this aspect uses the first signal to measure the first distance, which may change discontinuously, and the second signal to measure the second distance, which is less likely to change discontinuously, thereby enabling the distance measuring device according to this aspect to measure distances with high accuracy.

[0024] A distance measuring device according to a second aspect of the present disclosure is the distance measuring device according to the first aspect, and in the distance measuring device according to the second aspect, the frequency of the second laser light may be more stable than the frequency of the first laser light.

[0025] This improves the accuracy of measuring the second distance using the second laser light. Furthermore, a laser light source that emits laser light with low frequency stability, such as a laser light source with wide wavelength selectivity, can be used as the first light source. Because the first laser light emitted by the first light source is light that is irradiated onto an object, improving the wavelength selectivity of the first laser light increases the variety of objects that can be used as objects. This makes it possible to realize a highly versatile distance measuring device.

[0026] A distance measuring device according to a third aspect of the present disclosure is a distance measuring device according to the first or second aspect, and in the distance measuring device according to the third aspect, the second light source may be fixed to a member different from the head.

[0027] This allows the head to be made smaller and lighter. For example, if the head is movable, it is possible to suppress misalignment and vibration of the optical system within the head that accompanies movement, thereby improving measurement accuracy.

[0028] A ranging device according to a fourth aspect of the present disclosure is a ranging device according to any one of the first to third aspects, and in the ranging device according to the fourth aspect, the second light source may be a HeNe laser light source or a frequency-locked laser light source having a gas cell.

[0029] This increases the stability of the frequency of the second laser light emitted by the second light source. For example, a HeNe laser light source or a frequency-locked laser light source having a gas cell has little frequency fluctuation even when the light emission period is long. This makes it possible to realize a distance measuring device capable of long-term measurement.

[0030] A ranging device according to a fifth aspect of the present disclosure is a ranging device according to any one of the first to fourth aspects, and in the ranging device according to the fifth aspect, the first light source may be a distributed feedback (DFB) laser light source.

[0031] This improves the accuracy of measuring the first distance using the first laser light from the first light source. Furthermore, since a DFB laser light source has higher wavelength selectivity than a gas laser light source, the wavelength selectivity of the first laser light is also improved. This increases the variety of objects that can be used as targets, making it possible to realize a highly versatile distance measuring device.

[0032] A distance measuring device according to a sixth aspect of the present disclosure may be a distance measuring device according to any one of the first to fifth aspects, further comprising an optical fiber that guides the first laser light or the second laser light to the at least one interferometer.

[0033] This allows the use of optical fiber to reduce laser light loss, improving the energy efficiency of the distance measuring device. Also, since high-intensity laser light can be used for measurement, the effects of noise can be reduced, improving measurement accuracy.

[0034] A ranging device according to a seventh aspect of the present disclosure is a ranging device according to any one of the first to sixth aspects, and in the ranging device according to the seventh aspect, the at least one interferometer may be configured to utilize homodyne interference.

[0035] This allows for improved wavelength selectivity compared to when heterodyne interference is used. For example, improved wavelength selectivity of the first laser light increases the variety of objects that can be used as targets. This allows for a highly versatile distance measuring device to be realized.

[0036] A ranging device according to an eighth aspect of the present disclosure is a ranging device according to any one of the first to seventh aspects, and in the ranging device according to the eighth aspect, the first signal may be generated by multi-wavelength interference of the multiple laser beams.

[0037] This makes it possible to achieve both a long measurement range and high measurement accuracy when measuring the first distance.

[0038] A ranging device according to a ninth aspect of the present disclosure is a ranging device according to any one of the first to eighth aspects, and in the ranging device according to the ninth aspect, the at least one interferometer may include a first interference system for causing the first interference and a second interference system for causing the second interference, and the first interference system may be fixed to the head, and the second interference system may be fixed to a member different from the head.

[0039] As a result, for example, if the head is movable, the second interferometry system is not affected by the movement of the head, and the accuracy of measuring the second distance can be improved.

[0040] A ranging device according to a tenth aspect of the present disclosure is a ranging device according to any one of the first to ninth aspects, and in the ranging device according to the tenth aspect, the processing circuit may calculate the distance from the reference surface to the object based on the first distance and the second distance.

[0041] This makes it possible to obtain the absolute position of the object relative to the reference surface, thereby enabling measurement of the surface shape of the object with high accuracy.

[0042] A ranging device according to an eleventh aspect of the present disclosure is a ranging device according to any one of the first to tenth aspects, and in the ranging device according to the eleventh aspect, the timing of outputting the first signal and the timing of outputting the second signal may be synchronized.

[0043] This synchronizes the timing for measuring the first distance and the timing for measuring the second distance, making it possible to measure the distance from the reference surface to the object with high accuracy.

[0044] A ranging device according to a twelfth aspect of the present disclosure is a ranging device according to any one of the first to eleventh aspects, and in the ranging device according to the twelfth aspect, the head may include a reflector, and the processing circuit may calculate the second distance as the distance from the reference surface to the reflector based on the second signal.

[0045] This allows the second laser beam to be efficiently reflected by the reflector, and since high-intensity reflected light can be used for measurement, the influence of noise can be suppressed and measurement accuracy can be improved.

[0046] A ranging device according to a thirteenth aspect of the present disclosure is a ranging device according to any one of the first to twelfth aspects, and in the ranging device according to the thirteenth aspect, the wavelength of the second laser light may be shorter than the wavelength of the first laser light.

[0047] This allows the accuracy of measurement of the second distance to be improved since the shorter the wavelength of the laser light, the more accurate the measurement becomes.

[0048] A distance measuring device according to a fourteenth aspect of the present disclosure is a distance measuring device according to any one of the first to thirteenth aspects, wherein in the distance measuring device according to the fourteenth aspect, the at least one interferometer may include a first interference system for causing the first interference and a second interference system for causing the second interference, and the distance measuring device may further include a first optical fiber for guiding the first laser light to the first interference system and a second optical fiber for guiding the second laser light to the second interference system.

[0049] This allows the first laser light to be transmitted with little loss from the first light source to the first interference system via the first optical fiber. Similarly, the second laser light to be transmitted with little loss from the second light source to the second interference system via the second optical fiber. This improves the energy efficiency of the distance measuring device. Furthermore, since high-intensity first and second laser lights can be used for measurement, the effects of noise can be suppressed, improving measurement accuracy.

[0050] A distance measuring device according to a fifteenth aspect of the present disclosure is a distance measuring device according to any one of the first to fourteenth aspects, and in the distance measuring device according to the fifteenth aspect, the at least one interferometer may output at least one third signal due to a third interference based on at least one of a plurality of branched laser beams generated by branching the second laser beam, and the processing circuit may measure the two-dimensional position or three-dimensional position of the head based on the second signal and the at least one third signal.

[0051] This makes it possible to measure the surface shape of the object.

[0052] A distance measuring device according to a sixteenth aspect of the present disclosure may be the distance measuring device according to any one of the first to fifteenth aspects, further comprising a third light source that emits a third laser beam having a wavelength different from that of the first laser beam. In the distance measuring device according to the sixteenth aspect, the plurality of laser beams may further include the third laser beam, and the at least one interferometer may output the first signal by the first interference based on the first laser beam and the third laser beam.

[0053] As a result, the first laser beam and the third laser beam are used for the first interference to output the first signal, and the second laser beam is used for the second interference to output the second signal. That is, the first laser beam, the second laser beam, and the third laser beam are each used for only one of the first interference and the second interference, making it possible to optimize the wavelengths of each laser beam according to the measurement conditions. This allows for improved accuracy of each of the first signal and the second signal, and therefore improved measurement accuracy.

[0054] A distance measuring device according to a seventeenth aspect of the present disclosure is a distance measuring device according to any one of the first to fifteenth aspects, and in the distance measuring device according to the seventeenth aspect, the plurality of laser beams may further include the second laser beam, and the at least one interferometer may output the first signal by the first interference based on the first laser beam and the second laser beam.

[0055] This allows the second laser light to be used for both interference for outputting the first signal and interference for outputting the second signal, thereby reducing the number of light sources included in the distance measuring device and realizing a smaller and lighter distance measuring device.

[0056] A distance measuring device according to an eighteenth aspect of the present disclosure is a distance measuring device according to any one of the first to seventeenth aspects, and in the distance measuring device according to the eighteenth aspect, the head may be movable.

[0057] This allows distance measurement and surface shape measurement to be performed by simply moving the movable head, without moving the object. This eliminates the need for displacement or vibration of the object due to movement, improving measurement accuracy.

[0058] A distance measuring device according to a nineteenth aspect of the present disclosure may be the distance measuring device according to any one of the first to eighteenth aspects, further comprising a movable stage for supporting the object.

[0059] This allows the head to vary the distance to the object by moving the object together with the movable stage, enabling distance measurement and surface shape measurement. Because the head can be kept fixed, there is no displacement or vibration of the optical system that accompanies head movement, which improves measurement accuracy.

[0060] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0061] The embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, component placement and connection configurations, steps, and step order shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, among the components in the following embodiments, components not described in the independent claims are described as optional components.

[0062] Furthermore, each figure is a schematic diagram and is not necessarily an exact illustration. Therefore, for example, the scales of the figures do not necessarily match. Furthermore, in each figure, substantially the same components are given the same reference numerals, and redundant explanations are omitted or simplified.

[0063] Furthermore, in this specification, terms indicating the relationship between elements, such as parallel or perpendicular, terms indicating the shape of elements, and numerical ranges are not expressions that only express a strict meaning, but are expressions that also include a substantially equivalent range, for example, a difference of about a few percent.

[0064] Furthermore, in this specification, ordinal numbers such as "first" and "second" do not refer to the number or order of components unless otherwise specified, but are used for the purpose of avoiding confusion and distinguishing between components of the same type.

[0065] (First embodiment) [1. Overview] First, an overview of a distance measuring device according to a first embodiment will be described with reference to Fig. 1. Fig. 1 is a diagram showing an overview of a distance measuring device 100 according to this embodiment.

[0066] The distance measuring device 100 shown in Fig. 1 measures the distance from the distance measuring device 100 to an object 102 in a non-contact manner. The object 102 is, for example, an industrial product such as a lens or a mold. The distance measuring device 100 can measure the shape of the surface of the object 102 by measuring the distance. The distance measuring device 100 can be used, for example, as an inspection device that inspects whether the object 102 is suitable for use as a product.

[0067] The distance measuring device 100 includes a head 104 whose distance to the object 102 is variable. The head 104 is also called a probe. In this embodiment, the head 104 is movable. Specifically, the head 104 is movable along a distance measurement direction as shown by the outline arrow in FIG. 1 . The distance measurement direction is a direction parallel to the line segment representing the distance d1 shown in FIG. 1 . The distance measuring device 100 measures the distance d1 from the head 104 to the object 102 and the distance d2 from a reference plane 106 to the head 104.

[0068] The distance measuring device 100 measures distances optically and non-contactly using laser light. Specifically, the distance measuring device 100 measures distance d1 by irradiating a predetermined position on the surface of the object 102 with a first laser light and detecting the light reflected from the predetermined position. The distance measuring device 100 also measures distance d2 by irradiating a head 104 with a second laser light and detecting the light reflected from the head 104. By scanning the irradiation position of the laser light on the surface of the object 102, the shape of the surface of the object 102 can be measured three-dimensionally.

[0069] The distance measuring device 100 measures the distance d1 by using interference of a plurality of laser beams including the first laser beam. Specifically, the distance measuring device 100 measures the distance d1 by using multi-wavelength interference (MWI).

[0070] The distance measuring device 100 uses interference of the second laser light to measure the distance d2. Specifically, the distance measuring device 100 measures the distance d2 using single-wavelength interference (SWI).

[0071] In this way, the distance measuring device 100 uses a combination of SWI and MWI to measure distance, which improves the accuracy of distance measurement. The specific reasons for this will be explained below.

[0072] [2. SWI and MWI] First, the specific measurement principles and characteristics of SWI and MWI will be described with reference to FIGS. 2A and 2B.

[0073] 2A is a diagram for explaining the principle of distance measurement by single wavelength interference (SWI). In the following, an example using a Michelson interferometer will be described.

[0074] Light L emitted from a light source (not shown) is split (specifically, intensity-divided) into two beams of light by a beam splitter 182. One of the two beams is reflected by a mirror 184 and then re-enters the beam splitter 182, with a portion of it passing through the beam splitter 182 and traveling toward a photodetector 186 as a reference beam. The other of the two beams is reflected by the object 170a and then re-enters the beam splitter 182, with a portion of it being reflected by the beam splitter 182 and traveling toward the photodetector 186 as a measurement beam. The photodetector 186 detects light resulting from interference between the reference beam and the measurement beam, and outputs a signal corresponding to the intensity of the light. The photodetector 186 includes a photoelectric conversion element, such as a photodiode.

[0075] In the case of SWI, a single wavelength laser beam is used as the light L. The wavelength of the single wavelength laser beam is λ ０Then, the wavelength of both the reference light and the measurement light is λ ０ At the photodetector 186, the same wavelength λ ０ The interference light generated by the homodyne interference between the reference light and the measurement light is detected.

[0076] The intensity P of the signal output from the photodetector 186 ＰＤ is expressed by the following equation (1).

[0077]

[0078] In formula (1), L － =L ｘ -L ｙ It is. ｘ is the distance from the beam splitter 182 to the reflecting surface of the mirror 184. ｙ is the distance from the beam splitter 182 to the object 170a. ｋ is the wavelength of the single-wavelength laser light. Here, k=0. λ ｋ is the desired wavelength for the light source. ｘ and λ ｋ are all known values ​​to the processing circuit that calculates the distance. ＰＤ Based on this, the distance L from the beam splitter 182 to the object 170a is ｙ can be calculated.

[0079] Measurement using SWI is characterized by high measurement accuracy and a short measurement range.

[0080] In Fig. 2A, objects 170b and 170c respectively represent object 170a located at different positions. Fig. 2A shows a graph in which the horizontal axis represents the absolute distance with a predetermined position as the reference point, and the vertical axis represents the calculated distance. As shown in Fig. 2A, the distance from beam splitter 182 to object 170a can be calculated within a predetermined measurement range. As can be seen from equation (1), the measurement range is determined by dividing the wavelength of the single-wavelength laser light by λ ０ If so, the half wavelength (λ ０ / 2).

[0081] In the SWI, if the measurement range is exceeded, the absolute distance cannot be calculated. For example, in the example shown in Fig. 2A, the distance from beam splitter 182 to object 170a, the distance from beam splitter 182 to object 170b, and the distance from beam splitter 182 to object 170c are all calculated as the same distance.

[0082] The wavelength λ of the single-wavelength laser light ０ is, for example, a wavelength in the near-infrared light band or the visible light band. The near-infrared light band is a wavelength band of approximately 700 nm or more and approximately 2500 nm or less. The visible light band is a wavelength band of approximately 380 nm or more and approximately 780 nm or less. In this case, the measurement range of SWI is approximately 190 nm or more and approximately 1250 nm or less. That is, the measurement range of SWI is on the order of several hundred nanometers to several micrometers. Thus, the measurement range of SWI is relatively narrower than the measurement range of MWI, which will be described below.

[0083] 2B is a diagram for explaining the principle of distance measurement by multi-wavelength interference (MWI). As in the case of FIG. 2A, an example using a Michelson interferometer will be described below.

[0084] In the case of MWI, a light source emits multiple laser beams with different wavelengths. Each of the multiple laser beams is a single-wavelength laser beam. Here, the wavelength λ １ Single wavelength laser light L1 and wavelength λ ２ It is assumed that the single wavelength laser light L1 and the single wavelength laser light L2 are emitted.

[0085] Both single-wavelength laser beams L1 and L2 emitted from a light source (not shown) are split into two beams (specifically, intensity-divided) by a beam splitter 192. One of the two beams is reflected by a mirror 194 and then re-enters the beam splitter 192, and a portion of it passes through the beam splitter 192 and travels to a photodetector 196 or 197 as a reference beam. The reference beam has a wavelength λ １ and λ ２ The reference light is divided into two light components of wavelength λ by a wavelength separation element 198 such as a dichroic mirror. １and wavelength λ ２ The wavelength of the reference light is λ １ The light component of the reference light of wavelength λ ２ The light component is reflected by the mirror 199 and then travels toward the photodetector 197. The mirror 199 does not necessarily have to be provided.

[0086] The other of the two beams split by the beam splitter 192 is reflected by the object 170a and then re-enters the beam splitter 192, and a part of it is reflected by the beam splitter 192 and travels as measurement light toward the photodetector 196 or 197. Like the reference light, the measurement light has a wavelength λ １ and λ ２ The wavelength separation element 198 separates the measurement light into light components of wavelength λ １ and wavelength λ ２ The wavelength of the measurement light is λ １ The light component of the wavelength λ of the measurement light is directed to the photodetector 196. ２ The light component is reflected by a mirror 199 and then directed to a photodetector 197 .

[0087] The photodetector 196 detects the wavelength λ of the reference light. １ and the wavelength λ of the measurement light. １ The interference light generated by the interference of the light components of the reference light λ is detected, and a signal corresponding to the light intensity is output from the photodetector 196. ２ and the wavelength λ of the measurement light. ２ Interference light generated by the interference of the light components is detected, and a signal corresponding to the light intensity is output from the photodetector 197. Each of the photodetectors 196 and 197 includes a photoelectric conversion element such as a photodiode.

[0088] Note that the configuration for detecting light is not limited to the above example as long as light can be received for each wavelength. For example, after splitting the light from the beam splitter 192 heading toward the photodetector 196 or 197 into two beams by intensity division, a specific wavelength component (specifically, λ １ and λ ２The light may be passed through a filter having a passband. The filter may be, for example, a bandpass filter, but may also be a lowpass filter, a highpass filter, or the like.

[0089] Furthermore, heterodyne interference may be used in MWI. In this case, the wavelength separation element 198 may not be provided, and the number of photodetectors may be one.

[0090] 2B shows two graphs in which the horizontal axis represents absolute distance with a predetermined position as a reference point, and the vertical axis represents calculated distance. Of the two graphs, the upper graph represents distance calculated based on a signal obtained from photodetector 196. Of the two graphs, the lower graph represents distance calculated based on a signal obtained from photodetector 197.

[0091] If you use the two graphs individually, the measurement range will be λ １ / 2 or λ ２ / 2, the order of the measurement range is almost the same as in the case of SWI. In MWI, the measurement range can be extended by combining two graphs.

[0092] Specifically, the distances calculated for the objects 170a, 170b, and 170c corresponding to the upper graph are almost the same. However, the distances calculated for the objects 170a, 170b, and 170c corresponding to the lower graph are different. Therefore, by combining the two calculation results, λ １ / 2 and λ ２ / 2.

[0093] The measurement range of the MWI is half the beat wavelength of the two single-wavelength laser beams L1 and L2. For example, if the wavelengths of the two single-wavelength laser beams are λ １ and λ ２ Then, the beat wavelength Λ １２ is expressed by the following equation (2).

[0094]

[0095] Beat wavelength Λ of the beat light obtained by interference １２ For example, λ １ and λ ２ and are 1550 nm and 1551 nm, respectively, the beat wavelength Λ １２ The distance measurement range in the case of single wavelength interference is about 775 nm, which is on the order of nanometers, whereas the distance measurement range in MWI is expanded to the order of millimeters.

[0096] As described above, MWI has a longer measurement range than SWI, making it advantageous for measuring discontinuous changes in distance. Note that a change in distance refers to a change in the distance between two consecutive measurement positions when scanning the measurement positions. For example, the distance changes when the surface shape of the object is curved or when there are irregularities or steps. A change in distance equal to or greater than a predetermined value can be considered a discontinuous change, and a change less than the predetermined value can be considered a continuous change. The predetermined value is, for example, half the wavelength of the single-wavelength laser light emitted by the laser light source used.

[0097] If the surface of the object has steps that cause discontinuous changes in distance measurement, the SWI may exceed the measurement range and may not be able to measure the distance accurately. In contrast, with the MWI, the distance often falls within the measurement range, making accurate distance measurement possible. For example, if the object is a lens with a stepped structure, the size of the optical steps or foreign matter such as dust that may be trapped is at most about 1 mm. Therefore, if the measurement range of the MWI is about several mm, it is possible to measure the distance with high accuracy.

[0098] From the above, when comparing SWI and MWI, the following characteristics can be seen: - MWI has a better length measurement range than SWI. - SWI has better length measurement accuracy than MWI. - MWI is better suited to measuring discontinuous changes in distance than SWI. - SWI is better suited to measuring continuous changes in distance than MWI.

[0099] When the distance measuring device 100 shown in FIG. 1 measures the object 102, a discontinuous change may occur in the distance d1 depending on the surface shape of the object 102. Therefore, by using the MWI, the distance d1 can be measured with high accuracy. In contrast, the movable head 104 is less likely to experience discontinuous changes. Therefore, by using the SWI, the distance d2 can be measured with high accuracy.

[0100] Furthermore, in order to improve the accuracy of measuring the distances d1 and d2, it is possible to use a laser light source with high frequency stability as the light source. Frequency stability is expressed as the amount of fluctuation in the frequency of the laser light with respect to changes over time. If σ is the standard deviation statistically calculated for the average (or median) frequency of the laser light over a finite period of time, frequency stability can be expressed, for example, by σ. In this case, high frequency stability is synonymous with a small value of σ. However, frequency stability may be expressed not only in frequency units but also in wavelength units, or in other units correlated with frequency.

[0101] When the frequency is stable, the wavelength λ in Eq. (1) ｋ Therefore, the fluctuation of the wavelength λ used for calculation becomes small. ｋ and the wavelength of the light actually used for measurement becomes smaller, so that the accuracy of measurement can be improved.

[0102] As a laser with high frequency stability, a HeNe laser light source or a frequency-locked laser light source having a gas cell can be used. However, a laser light source with high frequency stability has low wavelength selectivity. Poor wavelength selectivity may make it impossible to perform measurement depending on the material of the target object 102, resulting in a distance measuring device with low versatility. For example, in the case of a HeNe laser light source, the wavelength of the emitted laser light is 632.8 nm.

[0103] The surface of the lens is often provided with an AR (Anti-Reflection) coating that suppresses reflection of visible light. In this case, if light in the visible light range, such as HeNe laser light, is used for measurement, the intensity of the light reflected from the lens decreases, resulting in a lower S / N ratio at the detector. In other words, a light source with low wavelength selectivity, such as HeNe laser light, is not suitable for measuring the distance d1 from the head 104 to the object 102.

[0104] In contrast, materials that highly efficiently reflect the wavelength components of the light used for measurement can be used for the reference surface 106 and the head 104. Therefore, to measure the distance d2 from the reference surface 106 to the head 104, a light source with low wavelength selectivity, such as a HeNe laser light source, or a laser light source with high frequency stability, such as a frequency-locked laser light source with a gas cell, can be used.

[0105] As described above, the distance measuring device 100 according to this embodiment uses the MWI to measure the distance d1, which may change discontinuously, and the SWI to measure the distance d2, which is less likely to change discontinuously. This allows the distance measuring device 100 to measure distances with high accuracy.

[0106] Note that the change in distance d2 accompanying the movement of head 104 is usually a continuous change. This change amount can be up to about 1 m. In other words, a length measurement range of 1 m may be required for SWI. Furthermore, to improve the accuracy of measurement of object 102, a length measurement accuracy on the order of nanometers is required for SWI, as with MWI.

[0107] In this case, the frequency stability of the light source used for SWI is 10 －６ For example, stability is required such that the amount of wavelength fluctuation per 24 hours is 3 fm or less, or stability is required such that the amount of wavelength fluctuation per 8 hours is 1 fm or less.

[0108] The greater the optical path length difference in the interference system, the greater the deterioration in measurement accuracy due to wavelength fluctuations of the laser light. Therefore, the longer the measurement range, the more a laser with high wavelength stability is required. Examples of light sources that have such stability and can be used for SWI include a HeNe laser light source and a frequency-locked laser light source with a gas cell.

[0109] On the other hand, the measurement range required for MWI is about several mm. In this case, the frequency stability of the light source used for MWI is 10 －３ The required stability is in the order of nm. For example, the required stability is such that the wavelength fluctuation per 24 hours is 2 pm or less, or such that the wavelength fluctuation per 8 hours is 0.6 pm or less. Note that these numerical values ​​are merely examples and are not limiting. A light source having such stability and usable for MWI is, for example, a DFB laser light source.

[0110] [3. Specific Configuration] Next, a specific configuration of the distance measuring device 100 will be described with reference to Fig. 3 and Fig. 4. Fig. 3 is a block diagram showing the configuration of the distance measuring device 100 according to this embodiment. Fig. 4 is a diagram showing the specific configuration of an interferometer provided in the distance measuring device 100 according to this embodiment.

[0111] 3, the distance measuring device 100 includes single-wavelength laser light sources 110 and 112, a wavelength combining unit 114, optical fibers 116 and 122, a frequency-stabilized laser light source 120, interference systems 130 and 140, photodetectors 150 and 152, a signal receiving unit 160, a memory 162, and a processing circuit 164. The interference system 130 is fixed to the head 104. The interference system 140 is fixed to a member different from the head 104. The interference system 130 and the photodetector 150 constitute a first interferometer. The interference system 140 and the photodetector 152 constitute a second interferometer.

[0112] The first interferometer and the second interferometer are included in at least one interferometer included in the distance measuring device 100. The at least one interferometer is configured to utilize homodyne interference. In this embodiment, each of the first interferometer and the second interferometer is configured to utilize homodyne interference.

[0113] The first interferometer includes a first laser beam and outputs a first signal by interference based on a plurality of laser beams having different wavelengths. The first interferometer is an interferometer utilizing MWI. The first signal is generated by the MWI of the plurality of laser beams.

[0114] The second interferometer is an interferometer that utilizes SWI and outputs a second signal by interference based on the second laser light. The second signal is generated by SWI of the second laser light.

[0115] The single-wavelength laser light source 110 is an example of a first light source that emits a first laser beam. The single-wavelength laser light source 112 is an example of a third light source that emits a third laser beam that has a wavelength different from that of the first laser beam. Both the first laser beam and the third laser beam are single-wavelength laser beams. The wavelength of the first laser beam is λ １ and the wavelength of the third laser beam is λ ２ Let wavelength λ １ and λ ２ The wavelengths λ are, for example, 1550 nm and 1551 nm, but are not limited to these. １ and λ ２ is included in the near-infrared light band, but depending on the type of object 102, it may be included in the visible light band or the ultraviolet light band.

[0116] The single-wavelength laser light sources 110 and 112 are, for example, DFB laser light sources, but are not limited to this. At least one of the single-wavelength laser light sources 110 and 112 may be a semiconductor laser light source other than a DFB laser light source, or a device that combines an absorption line of a gas cell as a reference frequency with a semiconductor laser light source. Furthermore, at least one of the single-wavelength laser light sources 110 and 112 may be a HeNe laser light source depending on the type of the target object 102.

[0117] The wavelength combining unit 114 combines the first laser light and the third laser light, and is, for example, a Dense Wavelength Division Multiplexing (DWDM) element or a holographic optical element.

[0118] The wavelength combining unit 114 is connected to each of the single-wavelength laser light sources 110 and 112 by optical fibers. １ The first laser light having a wavelength λ 1 emitted from the single-wavelength laser light source 112 is transmitted through the optical fiber and enters the wavelength combining unit 114. ２ The third laser light is transmitted through the optical fiber and enters the wavelength combining unit 114. The wavelength combining unit 114 combines the incident first laser light and third laser light, and outputs the combined light to the interference system 130.

[0119] The optical fiber 116 is a first optical fiber, and guides the first laser light and the third laser light to the interference system 130. Specifically, the optical fiber 116 causes the first laser light and the third laser light combined by the wavelength combining unit 114 to enter a beam splitter 132 of the interference system 130.

[0120] In this embodiment, the single-wavelength laser light sources 110 and 112 and the wavelength combining unit 114 are all fixed to members (not shown) different from the head 104. In other words, the single-wavelength laser light sources 110 and 112 and the wavelength combining unit 114 are not fixed to the head 104. This makes it possible to reduce the weight and size of the head 104. Also, for example, the light-emitting end of the optical fiber 116 is fixed to the head 104. Fixing the light-emitting end of the optical fiber 116 to the head 104 makes it possible to suppress deviation of the optical path.

[0121] The frequency-stable laser light source 120 is an example of a second light source that emits a second laser beam. ０ For example, the wavelength λ of the second laser beam is ０ is the wavelength λ of the first laser beam１ In this embodiment, the wavelength λ of the second laser light is different from ０ is the wavelength λ of the first laser beam １ Shorter.

[0122] The second laser light has a more stable frequency than the first laser light. That is, the frequency-stable laser light source 120 is a light source having a higher frequency stability than the single-wavelength laser light source 110. For example, the frequency-stable laser light source 120 is a HeNe laser light source or a frequency-locked laser light source having a gas cell.

[0123] In this embodiment, the frequency-stabilized laser light source 120 is fixed to a member (not shown) different from the head 104. In other words, the frequency-stabilized laser light source 120 is not fixed to the head 104. This allows the head 104 to be made lighter and smaller.

[0124] The optical fiber 122 is a second optical fiber that guides the second laser light to the interference system 140. Specifically, the optical fiber 122 inputs the second laser light from the frequency-stable laser light source 120 to a beam splitter 142 of the interference system 140.

[0125] The interference system 130 is an example of a first interference system for causing interference based on a plurality of laser beams having different wavelengths. Specifically, the interference system 130 causes interference based on a first laser beam and a third laser beam. The interference system 130 is an optical system that utilizes Michelson interference. The interference system 130 is fixed to the head 104. As shown in FIG. 3 , the interference system 130 includes a beam splitter 132 and a reference surface 108.

[0126] The beam splitter 132 is an optical element that splits incident light into multiple beams by intensity division and emits each of the multiple beams in different directions. The beam splitter 132 is, for example, a half mirror, and splits the incident light into transmitted light and reflected light so that each has the same intensity. Note that the intensity ratio between the transmitted light and reflected light does not have to be 1:1. The beam splitter 132 corresponds to the beam splitter 192 in FIG. 2B.

[0127] The reference surface 108 is an example of a reflector included in the head 104. In the present embodiment, as shown in FIG. 4 , the reference surface 108 includes a reference surface 108a and a reference surface 108b. The beam splitter 132, the reference surfaces 108a and 108b, and the photodetector 150 are all fixed to the head 104, and their relative positional relationships are fixed. Note that the photodetector 150 may be fixed to a member different from the head 104.

[0128] The reference surface 108a corresponds to the mirror 194 in Fig. 2B. In the interferometer 130 shown in Fig. 4, the positional relationship between the reference surface 108a corresponding to the mirror 194 and the object 102 corresponding to the object 170a is reversed compared to the interferometer of the MWI shown in Fig. 2B, but either configuration may be used.

[0129] In the interference system 130 shown in FIG. １ and λ ２ Each of the laser beams is split (specifically, intensity-divided) into two beams by the beam splitter 132. One of the two beams is reflected by the reference surface 108a and then re-enters the beam splitter 132, and a part of it is reflected by the beam splitter 132 and travels to the photodetector 150 as a reference beam. The reference beam has a wavelength λ １ and λ ２ The other of the two beams is incident on a predetermined position on the surface of the object 102 by the condenser lens 134. After being reflected at a predetermined position on the object 102, the other of the two beams is incident on the beam splitter 132 again, and a part of it passes through the beam splitter 132 and travels toward the photodetector 150 as measurement light.

[0130] Incidentally, the interference system 130 includes a condenser lens 134, which can improve the lateral resolution and increase the allowable slope of the sample surface that can be measured. Meanwhile, the distance over which light returns in the height direction is limited within the Rayleigh length. Therefore, the distance between the object 102 and the head 104 is essentially kept constant by moving the head 104 in accordance with the surface shape of the object being measured. If the surface shape of the object 102 changes discontinuously, the movement speed of the head 104 cannot keep up. However, this problem can be solved by expanding the measurement range using MWI.

[0131] The photodetector 150 outputs a first signal generated by interference between the measurement light and the reference light. １ Measurement light and wavelength λ １ The photodetector 150 detects interference light generated by homodyne interference with the reference light of wavelength λ and outputs a signal corresponding to the intensity of the interference light as a first signal. ２ Measurement light and wavelength λ ２ The optical detector 150 detects interference light generated by homodyne interference with the reference light and outputs a signal corresponding to the intensity of the interference light as a first signal. The optical detector 150 may include a DWDM and detectors for the number of wavelengths, and may detect multiple homodyne interferences, but this is not limited thereto. For example, the optical detector 150 corresponds to a configuration including optical detectors 196 and 197, a wavelength separation element 198, and a mirror 199 shown in FIG. 2B.

[0132] Interference system 140 is an example of a second interference system for causing interference based on the second laser light. Specifically, interference system 140 causes interference based on the second laser light emitted from frequency-stable laser light source 120. Interference system 140 is an optical system that utilizes Michelson interference. Interference system 140 is fixed to a member different from head 104. As shown in FIG. 3 , interference system 140 includes a beam splitter 142 and a reference surface 106.

[0133] The beam splitter 142 is an optical element that splits incident light into multiple beams by intensity division and emits each of the multiple beams in different directions. The beam splitter 142 is, for example, a half mirror, and splits the incident light into transmitted light and reflected light so that each has the same intensity. Note that the intensity ratio between the transmitted light and reflected light does not have to be 1:1. The beam splitter 142 corresponds to the beam splitter 182 in FIG. 2A.

[0134] The reference surface 106 is fixed to a member different from the head 104. The reference surface 106 corresponds to the mirror 184 in FIG. 2A. Compared to the SWI interferometry system shown in FIG. 2A, the interferometry system 140 shown in FIG. 4 has the reference surface 108b of the head 104 disposed as the object 170a. That is, the interferometry system 140 is an interferometry system for measuring the distance from the reference surface 106 to the reference surface 108b as the distance d2 from the reference surface 106 to the head 104. As shown in FIG. 4, a mirror 144 is provided to adjust the optical path from the beam splitter 142 to the reference surface 108b, but the mirror 144 need not be provided.

[0135] In the interference system 140 shown in FIG. ０ The laser light is split (specifically, intensity-divided) into two beams by the beam splitter 142. One of the two beams is reflected by the reference surface 106 and then re-enters the beam splitter 142, with a portion of it passing through the beam splitter 142 and traveling as reference light toward the photodetector 152. The other of the two beams is incident on the reference surface 108b via the mirror 144, is reflected by the reference surface 108b, and then re-enters the beam splitter 142, with a portion of it being reflected by the beam splitter 142 and traveling as measurement light toward the photodetector 152.

[0136] The photodetector 152 outputs a second signal generated by interference between the measurement light and the reference light. ０ Measurement light and wavelength λ ０ The photodetector 152 detects interference light generated by homodyne interference with the reference light, and outputs a signal corresponding to the intensity of the interference light as a second signal. For example, the photodetector 152 corresponds to the photodetector 186 shown in FIG. 2A.

[0137] The SWI interferometer 140 and photodetector 152 are fixed to a member different from the head 104, and measure the distance d2 from the reference surface 106 to the reference surface 108b of the head 104. As the surface area of ​​the object 102 increases, the moving distance of the head 104 increases, and therefore the measurement range widens, although the distance d2 changes continuously. By using a frequency-stabilized laser with extremely high frequency stability, such as a HeNe laser light source, as the frequency-stabilized laser light source 120, it is possible to measure a long measurement range with high accuracy.

[0138] The signal receiving unit 160 acquires the first signal output from the photodetector 150 and acquires the second signal output from the photodetector 152. The signal receiving unit 160 stores the acquired first and second signals in the memory 162.

[0139] The memory 162 is a storage device such as a non-volatile memory and / or a volatile memory. The first signal and the second signal are stored in the memory 162. The memory 162 may also store programs and data for operating the components of the distance measuring device 100, as well as distance information obtained by signal processing.

[0140] The processing circuit 164 is a signal processing circuit that processes the signal output from the interferometer. Specifically, the processing circuit 164 calculates the distance d1 from the head 104 to the object 102 based on the first signal output from the photodetector 150. The processing circuit 164 also calculates the distance d2 from the reference surface 106 to the head 104 based on the second signal output from the photodetector 152.

[0141] For example, the processing circuit 164 processes the first signal output from the photodetector 150 based on a predetermined algorithm to obtain the position of the object 102 as phase information. A typical phase estimation algorithm that can be used is a 4-step phase-shifting algorithm. The processing circuit 164 can calculate a distance d1 from the head 104 to the object 102 based on the phase information. Similarly, the processing circuit 164 can calculate a distance d2 from the reference surface 106 to the head 104 based on the second signal output from the photodetector 152. Specifically, the processing circuit 164 calculates the distance d2 from the reference surface 106 to the reference surface 108b of the head 104 based on the second signal.

[0142] Furthermore, the processing circuitry 164 calculates the distance from the reference surface 106 to the object 102 based on the distances d1 and d2. Both the reference surface 106 and the object 102 are fixed and do not move during measurement. By scanning the measurement position on the surface of the object 102, the processing circuitry 164 can obtain the shape of the surface of the object 102 based on the distance from the reference surface 106 to the object 102.

[0143] The processing circuit 164 is realized by an integrated circuit such as an LSI (Large Scale Integration). For example, the processing circuit 164 may be realized by a dedicated hardware configuration and calculate the distance from the reference plane 106 to the object 102. Alternatively, the processing circuit 164 may include a processor and calculate the distance from the reference plane 106 to the object 102 by causing the processor to execute a program stored in the memory 162. Alternatively, the processing circuit 164 may be a programmable FPGA (Field Programmable Gate Array) or a reconfigurable processor in which the connections and settings of circuit cells in an LSI can be reconfigured.

[0144] In this embodiment, the timing at which the first signal and the second signal are output is synchronized. If the timing of the MWI measurement and the SWI measurement differ, there is a problem that the error in the distance measurement increases and the accuracy decreases. For this reason, the timing of the MWI measurement and the SWI measurement are synchronized. In this case, the sampling rates may differ because the measurement principles of SWI and MWI are different. The distance measuring device 100 according to this embodiment is equipped with a function that enables sampling synchronization. For example, the respective signals may be collected using the same FPGA, or if different FPGAs are used for MWI and SWI, a configuration may be provided to synchronize the respective FPGAs.

[0145] [4. Operation] Next, the operation of distance measuring device 100 according to this embodiment will be described with reference to Fig. 5. Fig. 5 is a flowchart showing the operation of distance measuring device 100 according to this embodiment. Fig. 5 mainly shows the operation of processing circuit 164.

[0146] 5, first, the distance d2 between the reference surface 106 outside the head 104 and the reference surface 108b inside the head 104 is measured (S10). ０ The second laser beam is emitted and made incident on the interference system 140. The interference system 140 detects the reflected light (i.e., the wavelength λ) from the reference surface 106. ０ the reference light) and the reflected light from the reference surface 108b (i.e., the wavelength λ ０ The optical detector 152 generates homodyne interference with the measurement light (measurement light of the optical detector 152). The optical detector 152 outputs a second signal due to the homodyne interference in the interference system 140. The processing circuit 164 calculates the distance d2 based on the second signal acquired via the signal receiving unit 160.

[0147] Next, the distance d1 between the reference surface 108a in the head 104 and the object 102 is measured (S12). １ The first laser beam has a wavelength λ ２ The third laser beam is emitted and made incident on the interference system 130. In the interference system 130, the reflected light (i.e., the third laser beam having a wavelength λ１ and λ ２ the reference light) and the reflected light from the object 102 (i.e., the reference light of wavelength λ １ and λ ２ The optical detector 150 generates homodyne interference with the measurement light (measurement light of the optical detector 150). The optical detector 150 outputs a first signal due to the homodyne interference in the interference system 130. The processing circuit 164 calculates the distance d1 based on the first signal acquired via the signal receiving unit 160.

[0148] The calculation of the distance d1 may be performed before the calculation of the distance d2, or may be performed simultaneously in parallel. At this time, the output timings of the first signal and the second signal are synchronized.

[0149] Next, the processing circuitry 164 calculates the position information of the object 102 from the distances d1 and d2 (S14). Specifically, the processing circuitry 164 calculates the distance from the reference plane 106 to the measurement position of the object 102.

[0150] Thereafter, the measurement of the distances d1 and d2 and the calculation of the position information are repeated until the measurement of the object 102 is completed (No in S16).

[0151] Second Embodiment Next, a second embodiment will be described.

[0152] The main difference between the second embodiment and the first embodiment is that a frequency-stabilized laser light source is used for both SWI and MWI. The following description will focus on the differences from the first embodiment, and the description of the commonalities will be omitted or simplified.

[0153] Fig. 6 is a block diagram showing the configuration of a distance measuring device 200 according to this embodiment. The distance measuring device 200 shown in Fig. 6 differs from the distance measuring device 100 according to the first embodiment in that it does not include a single-wavelength laser light source 112, and the second laser light emitted by the frequency-stabilized laser light source 120 is used for MWI.

[0154] The frequency-stabilized laser light source 120 and the wavelength combining unit 114 are connected via an optical fiber. The second laser light emitted from the frequency-stabilized laser light source 120 is split into two beams and enters the wavelength combining unit 114 and the interference system 140. Both of the two split beams have wavelengths λ ０ The wavelength combining unit 114 combines the second laser beam with the wavelength λ ０ and the second laser light of wavelength λ emitted from the single-wavelength laser light source 110. １ The first laser beam is multiplexed with the second laser beam and output to the interference system 130.

[0155] The operation of the interference system 130 and the photodetector 150 is the same as in the first embodiment, except that the homodyne interference of the second laser beam is used instead of the third laser beam. That is, in this embodiment, the first interferometer including the interference system 130 and the photodetector 150 outputs a first signal due to interference based on the first laser beam and the second laser beam.

[0156] According to the distance measuring device 200 of this embodiment, the second laser light can be used for both MWI and SWI, which allows the number of light sources included in the distance measuring device 200 to be reduced, thereby enabling the distance measuring device 200 to be made smaller and lighter.

[0157] (First Modification of the Embodiment) Next, a first modification of the embodiment will be described.

[0158] The main difference between Modification 1 and Embodiments 1 and 2 is that multiple reference surfaces are provided on the outside of the head, and the distance from each of the multiple reference surfaces to the head is measured. The following explanation will focus on the differences from Embodiments 1 and 2, and explanation of the commonalities will be omitted or simplified.

[0159] 7 is a diagram showing an overview of a distance measuring device 300 according to this modification. As shown in FIG. 7, distance measuring device 300 differs from distance measuring device 100 according to the first embodiment in that it further includes a reference surface 306.

[0160] The reference surface 306 is disposed at a position different from the reference surface 106. Specifically, the reference surface 306 is disposed in an orientation perpendicular to the reference surface 106.

[0161] The distance measuring device 300 measures the distance d3 from the reference surface 306 to the head 104 in the same manner as measuring the distance d2 from the reference surface 106 to the head 104. Specifically, the distance measuring device 300 further includes a third interferometer having a configuration similar to that of the second interferometer including the interference system 140 and the photodetector 152. In the first modification of the embodiment, the head 104 is movable along the measurement direction of the distance d2 and the measurement direction of the distance d3, as indicated by the open arrows in FIG.

[0162] The second laser beam emitted from the frequency-stable laser source 120 is split into two branched laser beams, which are then incident on the second and third interferometers. The second interferometer outputs a second signal due to interference of one of the two branched laser beams. The third interferometer outputs a third signal due to interference of the other of the two branched laser beams. Specifically, the third interferometer outputs the third signal using SWI.

[0163] Based on the second signal and the third signal, the processing circuit 164 measures the two-dimensional position of the head 104. Specifically, the processing circuit 164 calculates the distance d2 based on the second signal, and calculates the distance d3 based on the third signal.

[0164] It is to be noted that a plurality of reference surfaces 306 may be provided. For example, two reference surfaces 306 may be provided so as to be orthogonal to each other and to the reference surface 106. This allows the processing circuitry 164 to measure the three-dimensional position of the head 104.

[0165] Furthermore, although the second laser light emitted from the frequency-stabilized laser light source 120 is used to measure the distances d2 and d3, this is not limiting. The distance measuring device 300 may also include a dedicated light source used only for measuring the distance d3.

[0166] (Second Modification of the Embodiment) Next, a second modification of the embodiment will be described.

[0167] The main difference between Modification 2 and Embodiments 1 and 2 is that a movable stage is provided between the head and the reference surface, and the distance from the reference surface to the movable stage and the distance from the movable stage to the head are measured. The following explanation will focus on the differences from Embodiments 1 and 2, and explanation of the commonalities will be omitted or simplified.

[0168] Fig. 8 is a diagram showing an overview of a distance measuring device 400 according to this modification. As shown in Fig. 8, distance measuring device 400 differs from distance measuring device 100 according to the first embodiment in that it further includes a movable stage 404.

[0169] The stage 404 is disposed between the reference surface 106 and the head 104. The stage 404 is movable in a direction parallel to the movement direction of the head 104, as indicated by the white arrow in Fig. 8. The distance measuring device 400 measures a distance d1 from the head 104 to the object 102, a distance d21 from the reference surface 106 to the stage 404, and a distance d22 from the stage 404 to the head 104. In other words, the distance measuring device 400 measures the second distance from the reference surface 106 to the head 104 by dividing it into the distance d21 and the distance d22.

[0170] The distances d21 and d22 are measured in the same manner as in the distance measuring device 300 according to Modification 1. Specifically, the distance measuring device 400 further includes a third interferometer having a configuration similar to that of the second interferometer including the interference system 140 and the photodetector 152. The second interferometer measures the distance d21, and the third interferometer measures the distance d22. Note that one of the distances d21 and d22 may be measured using MWI.

[0171] According to this modification, the provision of the movable stage 404 makes it possible to reduce the amount of movement of the movable head 104. By reducing the amount of movement of the head 104 equipped with the interferometer 130, it is possible to suppress a decrease in measurement accuracy that accompanies movement. Even if the movable range of the head 104 is narrow or the accuracy of the movement of the head 104 is low, it is possible to perform measurements with high accuracy by moving the movable stage 404.

[0172] (Third Modification of the Embodiment) Next, a third modification of the embodiment will be described.

[0173] The main difference between Modification 3 and Embodiments 1 and 2 is that Modification 3 includes a movable stage that supports an object. The following description will focus on the differences from Embodiments 1 and 2, and explanation of commonalities will be omitted or simplified.

[0174] Fig. 9 is a diagram showing an overview of a distance measuring device 500 according to this modification. As shown in Fig. 9, the distance measuring device 500 includes a head 504, a movable reference surface 506, and a movable stage 508. In the distance measuring device 500 according to this modification, unlike the distance measuring device 100 according to the first embodiment, the head 504 is fixed and does not move, but the stage 508 that supports the object 102 and the reference surface 506 move.

[0175] The relative positional relationship between the stage 508 and the reference surface 506 is fixed. In other words, the distance from the reference surface 506 to the stage 508 is maintained constant. As a result, by measuring the distance from the reference surface 506 to the object 102 supported by the stage 508, it is possible to measure the shape of the surface of the object 102, as in the first embodiment. The positional relationship between the stage 508 and the reference surface 506 does not have to be fixed.

[0176] While the distance measuring device according to one or more aspects has been described above based on the embodiments, the present disclosure is not limited to these embodiments. As long as they do not deviate from the gist of the present disclosure, various modifications conceivable by a person skilled in the art to the present embodiments and embodiments constructed by combining components of different embodiments are also included within the scope of the present disclosure.

[0177] For example, in the above-described embodiment and modified example, the processing circuitry 164 calculates the distance from the reference surface 106 to the object 102. However, this is not limiting. After calculating the distances d1 and d2, the processing circuitry 164 may output the distances d1 and d2 to another device. For example, the processing circuitry 164 may transmit the distances d1 and d2 to another computer, causing the other computer to calculate the distance from the reference surface 106 to the object 102. Alternatively, the processing circuitry 164 may transmit the distances d1 and d2 to a display and display them on the display, or may output them to a printer and print them on a medium such as paper. This allows the distances d1 and d2 to be presented to a user, allowing the user to manually calculate the distance from the reference surface 106 to the object 102. In this way, the processing circuitry 164 does not need to calculate the distance from the reference surface 106 to the object 102.

[0178] Furthermore, for example, at least one of the interference systems 130 and 140 may be an optical system that utilizes Fizeau interference or Mach-Zehnder interference.

[0179] Furthermore, the distance measuring device does not necessarily have to include one of the single-wavelength laser light sources 110 and 112. For example, instead of including the single-wavelength laser light source 110, the distance measuring device may include a beam splitter disposed between the single-wavelength laser light source 112 and the wavelength combining unit 114, and a frequency shifter that shifts the frequency of the incident laser light. The beam splitter splits (intensity-divides) the laser light emitted from the single-wavelength laser light source 112 into two beams, and causes one of the two beams to enter the wavelength combining unit 114, while causing the other of the two beams to enter the wavelength combining unit 114 via the frequency shifter. This allows multiple laser beams with different frequencies (and wavelengths) to be multiplexed and output by the wavelength combining unit 114. For example, an acousto-optic modulator (AOM) can be used as the frequency shifter.

[0180] The distance measuring device may also include a light source that emits laser light containing multiple single modes instead of the single-wavelength laser light sources 110 and 112. For example, the laser light containing multiple single modes may be, for example, an optical frequency comb laser light. An optical frequency comb laser light is a laser light whose spectrum is composed of discrete, evenly spaced frequency lines.

[0181] Furthermore, the general or specific aspects of the present disclosure may be realized as a system, an apparatus, a method, an integrated circuit, or a computer program. Alternatively, the general or specific aspects may be realized as a computer-readable non-transitory recording medium such as an optical disk, a HDD, or a semiconductor memory on which the computer program is stored. Alternatively, the general or specific aspects of the present disclosure may be realized as any combination of a system, an apparatus, a method, an integrated circuit, a computer program, and a recording medium.

[0182] Furthermore, various modifications, substitutions, additions, omissions, etc. can be made to the above-described embodiments within the scope of the claims or their equivalents.

[0183] The present disclosure can be used as a distance measuring device that can measure distance with high accuracy, and can be used, for example, in a surface shape inspection device.

[0184] 100, 200, 300, 400, 500 Distance measuring device 102, 170a, 170b, 170c Object 104, 504 Head 106, 108, 108a, 108b, 306, 506 Reference surface 110, 112 Single wavelength laser light source 114 Wavelength combining unit 116, 122 Optical fiber 120 Frequency stabilized laser light source 130, 140 Interference system 132, 142, 182, 192 Beam splitter 134 Condenser lens 144, 184, 194, 199 Mirror 150, 152, 186, 196, 197 Photodetector 160 Signal receiving unit 162 Memory 164 Processing circuit 198 Wavelength separation element 404, 508 Stage

Claims

1. A distance measuring device comprising: a first light source that emits a first laser beam; a second light source that emits a second laser beam; at least one interferometer that includes a reference surface and outputs a first signal based on a first interference of a plurality of laser beams having different wavelengths from each other and including the first laser beam, and outputs a second signal based on a second interference of the second laser beam; a head whose distance to an object is variable; and a processing circuit that calculates a first distance from the head to the object based on the first signal and calculates a second distance from the reference surface to the head based on the second signal.

2. The distance measuring device according to claim 1, wherein the frequency of the second laser beam is more stable than the frequency of the first laser beam.

3. The distance measuring device according to claim 1, wherein the second light source is fixed to a member different from the head.

4. The distance measuring device according to claim 1, wherein the second light source is a HeNe laser light source or a frequency-locked laser light source having a gas cell.

5. The distance measuring device according to claim 1, wherein the first light source is a distributed feedback type laser light source.

6. The distance measuring device according to claim 1, further comprising an optical fiber that guides the first laser beam or the second laser beam to the at least one interferometer.

7. The distance measuring device according to claim 1, wherein the at least one interferometer is configured to utilize homodyne interference.

8. The distance measuring device according to claim 1, wherein the first signal is generated by multi-wavelength interference of the plurality of laser beams.

9. The distance measuring device according to claim 1, wherein the at least one interferometer includes: a first interference system for causing the first interference; and a second interference system for causing the second interference, the first interference system being fixed to the head, and the second interference system being fixed to a member different from the head.

10. The distance measuring device according to claim 1, wherein the processing circuit calculates the distance from the reference surface to the object based on the first distance and the second distance.

11. The distance measuring device according to claim 1, wherein the timing of outputting the first signal and the timing of outputting the second signal are synchronized.

12. The head includes a reflector, and the processing circuit calculates the distance from the reference plane to the reflector as the second distance based on the second signal. The distance measuring device according to claim 1.

13. The wavelength of the second laser light is shorter than the wavelength of the first laser light. The distance measuring device according to claim 1.

14. The at least one interferometer includes a first interference system for causing the first interference and a second interference system for causing the second interference. The distance measuring device further includes a first optical fiber for guiding the first laser light to the first interference system and a second optical fiber for guiding the second laser light to the second interference system. The distance measuring device according to claim 1.

15. The at least one interferometer outputs at least one third signal by the third interference based on at least one of the plurality of branched laser lights generated by branching the second laser light, and the processing circuit measures the two-dimensional position or three-dimensional position of the head based on the second signal and the at least one third signal. The distance measuring device according to claim 1.

16. The distance measuring device further includes a third light source that emits a third laser light having a wavelength different from that of the first laser light. The plurality of laser lights further includes the third laser light, and the at least one interferometer outputs the first signal by the first interference based on the first laser light and the third laser light. The distance measuring device according to any one of claims 1 to 15.

17. The plurality of laser lights further includes the second laser light, and the at least one interferometer outputs the first signal by the first interference based on the first laser light and the second laser light. The distance measuring device according to any one of claims 1 to 15.

18. The head is movable. The distance measuring device according to any one of claims 1 to 15.

19. The distance measuring device further includes a movable stage for supporting the object. The distance measuring device according to any one of claims 1 to 15.

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