Displacement measuring apparatus, displacement measuring method, and scale-pitch measuring device

Abstract

The purpose of the present invention is to realize high resolution corresponding to a high number of optical divisions. This displacement measuring apparatus is provided with: a laser light source; a reuse-type multiple diffraction optical system that acquires diffracted light by causing light-source light emitted from the laser light source to enter a diffraction element as illumination light, and causes the diffracted light from the diffraction element to enter the diffraction element again as illumination light to obtain diffracted light; a frequency modulator that is incorporated in the optical path of the multiple diffraction optical system and performs frequency modulation on the diffracted light; an optical splitter that splits output light including diffracted light corresponding to a multiple modulation component formed as a result of passing through, a plurality of times, the frequency modulator incorporated in the optical path of the multiple diffraction optical system; and a phase detection device that extracts phase information related to the multiple modulation component from the output light split by the optical splitter.

Description

Displacement measuring device, displacement measuring method, and scale pitch measuring device

[0001] The present invention relates to a diffraction grating-based displacement measurement device and method, and an associated scale pitch measurement device, and more particularly to a displacement measurement device that uses multiple diffracted light.

[0002] Displacement measurement devices such as interferometers and grating encoders are widely used in precision industries such as semiconductors, and their measurement accuracy reaches several nanometers.

[0003] Conventional grating encoders that are commercially available acquire +1st-order diffracted light and -1st-order diffracted light, which has a phase change in the opposite direction, and then diffract these two light beams twice on the same diffraction grating surface, causing them to interfere with each other, amplifying the sine wave period of the interference wave signal to four times the original scale pitch.

[0004] As part of the development of a laboratory-level high-resolution grating encoder, an optical system has been devised in which a dedicated prism is placed in front of the diffraction grating, and the diffracted light is returned to the diffraction grating by backside reflection, thereby diffracting the incident beam four times into ±1st order diffractions (Non-Patent Document 1). In this case, each diffraction angle is divided into four parts by the four diffractions, and the +1st order diffracted light and the -1st order diffracted light have phase changes in opposite directions, so the total number of light divisions is eight.

[0005] However, the method of increasing the number of accesses to the diffraction grating by spatially separating the beams requires a complex optical system design and difficult alignment of the optical elements. Furthermore, as the number of diffractions increases, the intensity of the environmental noise relative to the intensity of the interference wave increases due to the attenuation of the light intensity associated with the diffraction, resulting in a deterioration of the signal-to-noise ratio (SNR).

[0006] Jili Deng, Xiaona Yan, et al. "Eightfold optical encoder with high-density grating", Applied Optics, Vol.57, pp2366-2375(2018)

[0007] The present invention has been made in view of the above-mentioned background art, and aims to realize high resolution equivalent to a high number of light divisions by increasing the number of times that diffracted light accesses a diffraction grating surface without spatially separating the beams using a simple optical system, while extracting such multiple diffracted light using a novel method.

[0008] In order to achieve the above object, the displacement measuring device according to the present invention includes a laser light source, a reusable multiple diffraction optical system that causes light source light emitted from the laser light source to be incident as illumination light on a diffraction element to obtain diffracted light, and causes the diffracted light from the diffraction element to be incident again as illumination light on the diffraction element to obtain diffracted light, a frequency modulator that is incorporated in the optical path of the multiple diffraction optical system and performs frequency modulation on the diffracted light, an optical branching device that is incorporated in the optical path of the multiple diffraction optical system and branches output light including multiple modulated components that are formed by passing through the frequency modulator multiple times, and a phase detection device that extracts phase information related to the multiple modulated components from the output light branched by the optical branching device.

[0009] In the displacement measurement device, the phase detection device extracts phase information related to the multiple modulation components from the output light branched by the optical branching device. This phase information reflects the phase changes that are sequentially added up, i.e., accumulated, due to multiple diffractions by the diffraction element. In other words, the relative displacement of the diffraction element can be detected with high accuracy based on phase information with a sensitivity multiplied by the number of diffractions, i.e., phase information in which the number of diffractions is the number of light divisions. Here, the multiple diffraction optical system does not need to change the optical path according to the number of diffractions to selectively extract the final diffracted light, and the structure can be easily simplified.

[0010] The displacement measurement method according to the present invention includes the steps of: using a reusable multiple diffraction optical system, irradiating light source light emitted from a laser light source onto a diffraction element as illumination light to obtain diffracted light; and irradiating the diffracted light from the diffraction element again as illumination light to obtain diffracted light; frequency modulating the diffracted light using a frequency modulator incorporated on the optical path of the multiple diffraction optical system; branching output light containing multiple modulated components formed by passing through the frequency modulator multiple times from the optical path of the multiple diffraction optical system; and extracting phase information related to the multiple modulated components from the branched output light.

[0011] The scale pitch measuring device of the present invention comprises a laser light source that emits light source light over a predetermined wavelength band including multiple wavelengths; a reusable multiple diffraction optical system that causes the light source light from the laser light source to be incident on a diffraction element as illumination light, and the diffracted light emitted from the diffraction element at multiple diffraction angles corresponding to the multiple wavelengths to be incident again on the diffraction element as illumination light to obtain diffracted light emitted at multiple diffraction angles; an optical branching device that branches output light including multiple diffraction components formed by passing through the multiple diffraction optical system multiple times; a wavelength detection device that measures wavelength information of the multiple diffraction components from the output light branched by the optical branching device; and a processing unit that calculates the deviation of the scale pitch of the diffraction element from the wavelength information obtained by the wavelength detection device.

[0012] FIG. 1 is a conceptual diagram illustrating the basic structure of a displacement measurement apparatus according to a first embodiment. FIG. 2A is a conceptual diagram illustrating diffracted light detected by the displacement measurement apparatus shown in FIG. 1 , and FIG. 2B is a conceptual diagram illustrating diffracted light detected by a displacement measurement apparatus according to a modified example. FIG. 3 is a schematic diagram summarizing phase fluctuations of an optical signal in a displacement measurement apparatus. FIG. 4 is a conceptual diagram illustrating a displacement measurement apparatus according to a modified example. FIG. 5 is a diagram illustrating a specific configuration example of a displacement measurement apparatus. FIG. 6 is a diagram illustrating a stage drive waveform used in an experiment. FIG. 7 is a diagram illustrating the temporal change in the phase of a measured beat signal. FIG. 8 is a diagram illustrating an experiment related to the number of light circuits or the number of light divisions. FIG. 9A shows the results of measuring the frequency distribution of the beat signal intensity without amplification, and FIG. 9B shows the results of measuring the frequency distribution of the beat signal intensity with amplification. FIG. 10 is a conceptual diagram illustrating a displacement measurement apparatus according to a second embodiment. FIG. 11 shows the results of measuring the frequency distribution of the beat signal intensity with amplification. FIG. 12 is a conceptual diagram illustrating a displacement measurement apparatus according to a third embodiment. Fig. 13 is a conceptual diagram illustrating the scale pitch measurement device of the fourth embodiment. Fig. 14A shows the frequency characteristics of the light source light emitted from the laser light source, and Figs. 14B to 14D are diagrams illustrating the reduction in the wavelength width of the diffracted light depending on the number of times the light makes circuits in the multiple diffraction optical system.

[0013] First Embodiment A displacement measuring device according to a first embodiment of the present invention will be described below with reference to FIG. 1 and other figures.

[0014] 1 includes a laser light source 10, a multiple diffraction optical system 20, a frequency modulator 30, an optical branching / coupling device 40, a phase detection device 50, and a processing device 90. The displacement measurement device 100 measures the movement of the diffraction element 2 in the lateral direction DR1 by utilizing diffraction at the diffraction element 2.

[0015] In the displacement measurement device 100, the laser light source 10 emits a source light SL for illuminating the diffraction element 2. The source light SL is coupled to the optical branching / coupling device 40 via an optical fiber 8a. The source light SL is maintained at a constant wavelength and intensity during measurement. The multiple-diffraction optical system 20 is a reusable multiple-diffraction optical system that reuses the diffracted light DL from the diffraction element 2 as the illumination light RL. That is, the multiple-diffraction optical system 20 acquires the diffracted light DL by making the source light SL emitted from the laser light source 10 incident on the diffraction element 2 as the illumination light RL, and acquires the diffracted light DL by making the diffracted light DL from the diffraction element 2 incident on the diffraction element 2 again as the illumination light RL. Here, the diffracted light DL is, for example, +1st-order diffracted light. In the multiple-diffraction optical system 20, an optical path from an inlet 21b, where the diffracted light DL is acquired, to an outlet 21a, where the illumination light RL is emitted, is connected by an optical fiber 22. An exit collimator lens 23 is provided at the exit 21a facing the diffraction element 2, and an entrance collimator lens 24 is provided at the entrance 21b facing the diffraction element 2. The diffracted light DL emitted from the diffraction element 2 may be −1st-order diffracted light. The frequency modulator 30 is incorporated in the optical path of the multiple diffraction optical system 20 and performs frequency modulation on the diffracted light DL. The frequency modulator 30 is provided in the optical path of the reusable multiple diffraction optical system 20, and the diffracted light DL circulating through the multiple diffraction optical system 20 is converted into multiple modulated components by passing through the frequency modulator 30 multiple times. These multiple modulated components are generated by multiplying the basic phase difference generated by passing the diffracted light DL once by the number n of passes through the frequency modulator 30, i.e., the number n of passes through the diffraction element 2. Here, the number n of passes corresponds to the number of turns through the multiple diffraction optical system 20. The optical branching / coupling device 40 is incorporated in the optical path of the multiple diffraction optical system 20 and branches output light ML including diffracted light DL out of the multiple diffraction optical system 20. The output light ML is incident on the phase detection device 50 via an optical fiber 8b. In the example shown, the optical branching / coupling device 40 includes an optical coupler 41 and an optical splitter 42 as an optical multiplexer / splitter, but these can be replaced with a two-branch coupler having an integrated function. The optical coupler 41 is part of the multiple diffraction optical system 20.The phase detection device 50 extracts phase information PI of the diffracted light DL from n-times diffracted light DL contained in the output light ML branched outside the multiple diffraction optical system 20 by the optical branching / coupling device 42 of the optical branching / coupling device 40, or from the interference light IL between the source light SL and the diffracted light DL. The arithmetic processing device 90 is a computer, and determines the relative displacement amount of the diffraction element 2 with respect to the displacement measurement device 100 based on changes in the phase information PI detected by the phase detection device 50. Here, the displacement amount of the diffraction element 2 is not limited to a one-dimensional linear displacement, but may also be converted into a rotation angle. In other words, the displacement measurement device 100 of this embodiment can be implemented in various precision instruments as a rotary encoder, not limited to a linear encoder.

[0016] In the above displacement measurement apparatus 100, the displacement measurement method includes the steps of: using a reusable multiple diffraction optical system 20, causing the light source light SL emitted from the laser light source 10 to be incident as illumination light RL on the diffraction element 2 to obtain diffracted light DL; and causing the diffracted light DL from the diffraction element 2 to be incident again as illumination light RL on the diffraction element 2 to obtain diffracted light DL; frequency-modulating the diffracted light DL using a frequency modulator 30 incorporated in the optical path of the multiple diffraction optical system 20; branching output light ML containing multiple modulated components formed by passing through the frequency modulator 30 multiple times from the optical path of the multiple diffraction optical system 20; and extracting phase information PI related to the multiple modulated components from the branched output light ML. When performing the above displacement measurement method, calibration can be performed to compensate for errors in measurement by the displacement measurement apparatus 100, using errors in the diffraction element 2 obtained by a scale pitch measurement apparatus (a partially modified version of the displacement measurement apparatus 100) described below.

[0017] FIG. 2A is a diagram illustrating the diffracted light DL detected by the phase detection device 50 of the displacement measurement device 100 shown in FIG. The horizontal axis represents frequency, i.e., the reciprocal of wavelength, and the vertical axis represents intensity. The diagram illustrates the light source light SL, the diffracted light DL1 after one revolution, the diffracted light DL2 after two revolutions, the diffracted light DL3 after three revolutions, and the diffracted light DL4 after four revolutions. If the frequency of the light source light SL is f, the frequency of the diffracted light DL1 is f+Δf, the frequency of the diffracted light DL2 is f+2Δf, and the frequency of the diffracted light DL3 is f+3Δf. In other words, the frequency of the diffracted light DLn having the desired number of revolutions or number of passes through the diffraction element 2 is f+nΔf, and a frequency shift of nΔf occurs in the diffracted light DLn. By utilizing this frequency shift or frequency difference, only the diffracted light DLn having passed n can be extracted, and the phase information of the diffracted light DLn includes a multiplied phase difference. In the above, Δf is extremely small compared to the light source frequency f, and has almost no effect on the measurement results using the multiple diffraction optical system 20.

[0018] Returning to FIG. 1 , the phase detection device 50 specifically includes an optical detector 51, a frequency filter 52, and a phase meter 53. The optical detector 51 detects the interference light IL emitted from the extraction port P4 of the optical branching / coupling device 40 and converts it into an electrical signal. The frequency filter 52 extracts a beat signal component related to the diffracted light DLn corresponding to the target multiple modulation component from the electrical signal obtained from the interference light IL contained in the output light ML. The phase meter 53 extracts phase information PI related to the diffracted light DLn, i.e., the multiple modulation component, from the beat signal extracted by the frequency filter 52 and outputs it to the arithmetic processing device 90. As will be described in detail later, this phase information PI has sensitivity multiplied by the number of diffractions and corresponds to phase information with a shortened spatial period, with the number of diffractions being the number of light divisions. Note that the beat signal resulting from interference between the source light SL of frequency f and the diffracted light DLn corresponding to the target multiple modulation component is a signal E having different frequencies f and f+nΔf. ｆ , E ｆ＋ｎΔｆ The interference intensity between the two is I, and E ｆ = A １ expi(2πft+θ １ ) E ｆ＋ｎΔｆ = A ２expi(2π(f+nΔf)t+θ ２ I=A １ ２ +A ２ ２ +2A １ A ２ cos(2πnΔft+θ １ -θ ２ ) and has a frequency nΔf that is the difference between the two light waves, and a phase difference θ １ -θ ２ It has the following characteristics.

[0019] The phase information PI regarding the diffracted light DLn is converted into the relative displacement amount of the diffraction element 2 by the calculation processing device 90. However, since the phase information of the diffracted light DLn includes information regarding the multiplied phase difference, the detection of the displacement amount of the diffraction element 2 is significantly more accurate than, for example, measuring the displacement amount of the diffraction element 2 based on the diffracted light DL1 after one revolution.

[0020] The arithmetic processing device 90 has an arithmetic processing circuit 91, an interface circuit 92, a storage device 93, and a user interface device 94. The arithmetic processing circuit 91 is composed of a central processing unit (CPU) and the like, the interface circuit 92 includes a communication circuit and the like, the storage device 93 is a semiconductor storage device including RAM, ROM, flash memory and the like, and the user interface device 94 includes a display, a keyboard and other input / output devices. The arithmetic processing circuit 91 reads out a program stored in the storage device 93 and executes the program. Specifically, the arithmetic processing circuit 91 executes a program that determines the number of passes or the number of revolutions n of the diffracted light DL based on a change in the phase information PI detected by the phase detection device 50 and calculates the amount of displacement of the diffraction element 2 relative to the displacement measurement device 100.

[0021] 3 is a conceptual diagram summarizing the phase fluctuation of the optical signal in the displacement measurement device 100. The amount of phase change occurring in the path PA0 from the laser light source 10 to the optical branching / coupling device 40 is represented as φ ０ and the amount of phase change occurring on the path PA1 from the optical branching / coupling device 40 to just before the diffraction element 2 is φ １ The phase change amount occurring before and after diffraction by the diffraction element 2 is expressed as φ ｇand the amount of phase change occurring on the path PA2 from immediately after the diffraction element 2 through the frequency modulator 30 to the optical branching / coupling device 40 is φ ２ The phase change amount occurring on the path PA3 from the optical branching / coupling device 40 to the phase detection device 50 is φ ３ Let's say.

[0022] The signal phase of the zero-circulation light is Ω ０ As, Ω ０ =φ ０ +φ ３ and the signal phase of one round trip light is Ω １ As, Ω １ =φ ０ +φ ｇ +φ １ +φ ２ +φ ３ and the signal phase of the double circular light is Ω ｎ As, Ω ｎ =φ ０ +n(φ ｇ +φ １ +φ ２ ) + φ ３ For the beat signal, the signal phase of the frequency Δf after one revolution is expressed as ω １ As, ω １ =Ω １ -Ω ０ =φ ｇ +φ １ +φ ２ and the signal phase of the n-cycle frequency nΔf is ｎ As, ω ｎ =Ω ｎ -Ω ０ = n (φ ｇ +φ １ +φ ２ ) that is, the phase of the beat signal is determined by the phase change amount φ ｇ and the phase change amount φ is determined according to the frequency nΔf of the beat signal. ｇ is amplified by n times. In other words, the number of light divisions increases in proportion to the number of revolutions of the diffracted light DL, and the device can function as a highly accurate grating encoder.

[0023] In the following, the characteristics of the beat signal detected by the phase detection device 50 will be mathematically and strictly evaluated.

[0024] Let f be the frequency and φ be the initial phase. ０ The light wave with E(f,φ ０ ), a light wave E(f+nΔf,φ) with a frequency of f+nΔf and an initial phase φn ｎ ) is defined as follows on a fixed spatial coordinate system: The interference wave intensity I of these light waves ０，ｎ can be expressed as follows: The third term in equation (3) is a time-varying periodic signal, with a frequency of nΔf and an initial phase of φ ０ -φ ｎ When the displacement measurement device 100 acquires +1st-order diffracted light, the signal sequence E(f,φ ０ ), E(f+Δf,φ １ ), E(f+2Δf,φ ２ ) ... are detected. Therefore, a combination of two of these signals generates beat signals with frequencies Δf, 2Δf, 3Δf .... Therefore, beat signals of each frequency are derived. First, the beat signal output with frequency Δf is expressed as E ｎ When written as ０ , E １ ), an interference wave consisting of (E １ , E ２ ), an interference wave consisting of (E ３ , E ４ ) interference wave... (E ｍ , E ｍ＋１ There are countless combinations such as an interference wave consisting of a light wave E that has circulated n times through the multiple diffraction optical system 20. ｎ (P ３ ) is the initial transport ０ As E n (P3)=A n,3 exp{i(2π(f+nΔf)t+Φ0+φ0+nφ1+nφg+nφ2+φ3)}, so the interference wave I ｍ，ｍ＋１ That is, the strength is as follows: At any number of revolutions m, the initial phase of the beat signal is φ １ +φ ２ +φ ３Therefore, the signal intensity I of the component with frequency Δf among the outputs from the +1st-order diffracted light acquisition unit is Δｆ Since is the sum of these beat signals, the constant j Δｆ , k Δｆ can be expressed as follows using Similarly, the signal output with frequency 2Δf is the light wave (E ｍ , E ｍ＋２ ) and the interference intensity I ｍ，ｍ＋２ is generally The sum of the beat signal outputs with a frequency of 2Δf is I ２Δｆ is the constant j ２Δｆ , k ２Δｆ can be expressed as follows using Moreover, the output strength of a signal having a frequency nΔf can generally be expressed as follows: From the above, when the displacement measurement device 100 acquires +1st-order diffracted light, signals with frequencies Δf, 2Δf, 3Δf, . . . consisting of beat signals are output, and each signal has a signal phase (φ １ +φ ｇ +φ ２ ), 2(φ １ +φ ｇ +φ ２ ), 3(φ １ +φ ｇ +φ ２ )... φ １ , φ ｇ , φ ２ are the phase change that occurs when the light wave travels from the optical branching / coupling device 40 to the diffractive element 2, the phase change that occurs before and after the +1st-order diffraction at the diffractive element 2, and the phase change that occurs when the light wave travels from the diffractive element 2 to the optical branching / coupling device 40, respectively. ｇ From equation (8), it is necessary to measure the phase of each signal output having frequencies Δf, 2Δf, 3Δf, . . . ｇ The magnitude of the linearly contained light beam is doubled, tripled, etc., and it is therefore shown that the linearly contained light beam contains displacement information of the number of light beam divisions corresponding to the number of revolutions.

[0025] 4 is a conceptual diagram illustrating a modified displacement measurement apparatus 100. In this case, the displacement measurement apparatus 100 further includes an optical amplifier 60 that is incorporated in the optical path of the multiple diffraction optical system 20 and amplifies the diffracted light DL. In this case, the optical amplifier 60 amplifies the intensity of the diffracted light DLn corresponding to the target multiple modulation component without changing its frequency. In this case, the optical amplifier 60 simply applies a fixed phase offset, and information regarding the multiplied phase difference of the diffracted light DLn is substantially maintained.

[0026] Referring to FIG. 2B, the diffracted light DL detected by the phase detection device 50 of the displacement measurement device 100 shown in FIG. 4 will be described. In this case, diffracted light DL1, DL2, ..., DLn are illustrated, and the intensities of the diffracted light DL1, DL2, ..., DLn are greater than those in FIG. 2A. As a result, even diffracted light DL10 that passes through the diffraction element 2, for example, 10 times, has sufficient intensity. In this way, by inserting the optical amplifier 60 into the optical path of the multiple diffraction optical system 20, the phase detection device 50 can easily extract diffracted light DL with a large number of diffractions, thereby enabling the relative displacement of the diffraction element 2 to be detected with higher accuracy.

[0027] FIG. 5 illustrates a specific example configuration of a displacement measurement device 100. In this example, the displacement measurement device 100 includes an acousto-optic modulator (AOM) 31 and a modulation driver 32 as the frequency modulator 30, and an optical detector 51 and a lock-in amplifier 55 as the phase detection device 50. The lock-in amplifier 55 receives a beat signal from the optical detector 51 as a measurement signal IS and a drive signal from the modulation driver 32 as a reference signal RS. The diffraction element 2 is supported on a stage 81, which also serves as an experimental stage, and can be moved by the stage 81 at a desired speed in the scanning direction (lateral direction DR1). The acousto-optic modulator 31 can be replaced with an electro-optic modulator (EOM). The acousto-optic modulator 31 can also be replaced with a semiconductor optical modulator (SOM), an electro-absorption modulator (EA), a thermo-optic modulator, or the like.

[0028] The lock-in amplifier 55 interferes with the measurement signal IS by shifting the signal phase by 90° with a reference signal RS having a specific frequency. The low-frequency components of this interference signal have a signal phase that is the phase difference between the measurement signal IS and the reference signal RS in the frequency band of the reference signal RS. By passing this interference signal through a low-pass filter, a signal is generated from which the frequency components that are out of phase with the reference signal RS are removed, allowing only the phase difference of the measurement signal IS to be detected. Creating an interference signal with a 90° phase shift is intended for absolute phase measurement. By using the driving periodic signal output from the modulation driver 32 as the reference signal RS, the frequency of the measurement signal IS, which is a beat signal, matches the frequency of the reference signal RS with high precision, enabling precise monitoring of only the phase difference caused by the optical system.

[0029] In the explanation of Figure 5, a case where a lock-in amplifier 55 is used as the phase detection device 50 is described, but the phase detection device 50 may be any device that can measure the phase difference between the measurement signal IS and the reference signal RS, and various phase meters can be used.

[0030] The optical fiber 22 constituting the multiple diffraction optical system 20 of the displacement measurement device 100 is preferably a single-mode optical fiber from the viewpoint of improving phase detection accuracy. The optical fiber 22 is preferably a diffused-core optical fiber. In a typical single-mode optical fiber, the core diameter is about 10 μm, so light from space is significantly attenuated when it is collected and introduced into the optical fiber. A diffused-core optical fiber has a larger core diameter only at the entrance, and can guide, for example, 90% or more of the light to the core. This reduces loss in the circulating optical path, making it easier to increase the number of passes or the number of turns n.

[0031] In the above explanation, the displacement measurement device 100 is described as being applied to movement in a specific axial direction or rotation around a rotation axis, but the displacement measurement device of the present invention can also be used to detect displacement along two or three orthogonal coordinate axes, for example. Specifically, by using the measurement system shown in FIG. 1 , i.e., the laser light source 10, the multi-diffraction optical system 20, the frequency modulator 30, the optical branching / coupling device 40, the phase detection device 50, etc., to detect displacement along each coordinate axis, two-dimensional or three-dimensional displacement measurement becomes possible. Furthermore, the displacement measurement device of the present invention can also be incorporated into a device that detects displacement and rotation along six axes.

[0032] For convenience of explanation, the above description has been given on the assumption that the number of revolutions of the diffracted light DL in the multiple-diffraction optical system 20 is a specific number, but the phase difference may be measured in parallel for diffracted light DL with any number of revolutions and the displacement amount may be determined based on the measured phase difference. Furthermore, the phase difference may be measured for combinations of diffracted light DL with two or more different numbers of revolutions while changing the combination depending on the situation, and the displacement amount may be determined.

[0033] The laser light source 10 does not need to have high accuracy in terms of wavelength stability of the source light SL. In other words, even if the wavelength of the source light SL fluctuates slightly, as long as the operation of the frequency modulator 30 is highly accurate, a stable beat signal can be formed based on the wavelength of the source light SL, and the phase difference of the diffracted light DL can be detected with relatively high accuracy.

[0034] The displacement measurement device 100 of the first embodiment described above includes a laser light source 10, a reusable multiple diffraction optical system 20 that causes the light source light SL emitted from the laser light source 10 to be incident on the diffraction element 2 as illumination light RL to obtain diffracted light DL, and causes the diffracted light DL from the diffraction element 2 to be incident on the diffraction element 2 again as illumination light RL to obtain diffracted light DL, a frequency modulator 30 that is incorporated on the optical path of the multiple diffraction optical system 20 and performs frequency modulation on the diffracted light DL, an optical branching device 42 that is incorporated on the optical path of the multiple diffraction optical system 20 and branches output light ML including diffracted light DLn corresponding to multiple modulated components formed by passing through the frequency modulator 30 multiple times, and a phase detection device 50 that extracts phase information related to the multiple modulated components from the output light ML branched by the optical branching device 42.

[0035] In the displacement measurement device 100, the phase detection device 50 extracts phase information regarding the diffracted light DL corresponding to the multiple modulation components from the output light ML branched by the optical branching device 42. This phase information reflects the phase changes that are sequentially added up, i.e., accumulated, due to multiple diffractions by the diffractive element 2. In other words, the relative displacement of the diffractive element 2 can be detected with high accuracy based on phase information with sensitivity multiplied by the number of diffractions, i.e., phase information in which the number of diffractions is the number of light divisions. Here, the multiple diffraction optical system 20 does not need to change the optical path depending on the number of diffractions to selectively extract the final diffracted light DL, and the structure can be easily simplified.

[0036] Furthermore, in the displacement measurement device 100, the multiplied multi-diffracted light can be acquired as a group of output light having a time difference. Therefore, the number of revolutions and frequency can be freely set and changed without changing the basic structure of the optical system, by simply changing, adjusting, selecting, etc. the phase detection device 50 (or its preceding stage) according to the desired resolution, displacement speed, sampling rate, performance of the sampling device, etc.

[0037] In the above description, a single laser light source 10 introduces light source light SL of a substantially single wavelength into the multiple diffraction optical system 20, but if the wavelength difference is slight, for example, about 10 nm, light source light SL of multiple wavelengths may be introduced into the multiple diffraction optical system 20. In this case, displacement measurement can be performed in parallel for multiple wavelengths.

[0038] [Verification Experiment] Verification experiments conducted on the measurement principle and resolution of the displacement measuring device 100 according to the embodiment will be described below.

[0039] An experiment was carried out using the displacement measurement device 100 shown in Fig. 5. However, the displacement measurement device 100 used in the experiment did not incorporate an optical amplifier 60. A wavelength-tunable laser was used as the laser light source 10, and the wavelength was 1550 nm and the frequency was 2.0 x 10 １４Hz. In the optical branching / coupling device 40, which is a two-branching coupler, 75% of the light amount is branched to the diffraction grating side. A holographic diffraction grating with a pitch of 2.2 μm was used as the diffraction element 2, which is a diffraction grating, so that the ±1st-order diffracted light intensity is uniform. The amount of optical frequency shift in the AOM 31 was set to 80 MHz. The beat signal phase was measured by a lock-in amplifier 55 with a frequency range of DC-600 MHz. In this case, it is expected that a signal train with frequencies of 80 MHz, 160 MHz, 240 MHz, etc. will be observed as the beat signal output.

[0040] 6 is a diagram illustrating the driving waveform used in the experiment for the piezo stage 81. The stage 81 was reciprocated in the horizontal direction DR1 with a displacement of ±100 nm using a 1 Hz square wave.

[0041] Figure 7 shows the time change in beat signal phase at measured frequencies of 80 MHz, 160 MHz, and 240 MHz. Each beat signal changes over time in a rectangular wave pattern, and it is believed that the displacement of the diffraction element 2 (a diffraction grating) caused by the stage 81 is detected as the beat signal phase. Furthermore, the signal phase is doubled or tripled for signals corresponding to 80 MHz, 160 MHz, and 240 MHz, confirming that the phase change is amplified according to the beat signal frequency. Furthermore, the height gap of the signal phase due to the rectangular wave is approximately 30° for the 80 MHz signal and approximately 100° for the 240 MHz signal, so the measurement results are generally consistent with the displacement amplitude of 200 nm of the stage 81. From the above, it can be concluded that the measurement principle of the displacement measurement device 100 has been verified.

[0042] Fig. 8 is a diagram for explaining an experiment on the number of light circuits or the number of light divisions, and is a partial modification of the displacement measurement device 100 shown in Fig. 5. In this case, measurements were performed using a spectrum analyzer 56 instead of the lock-in amplifier 55.

[0043] 9A shows the frequency spectrum from 0 to 1 GHz, measured by the spectrum analyzer 56, of the frequency distribution of the beat signal intensity, which is the measurement signal IS from the optical detector 51, without amplification by the optical amplifier 60. It was confirmed that a large number of beat signals were output at intervals of 80 MHz, which matches the amount of frequency shift of the AOM 31, and the spectrum analyzer even confirmed a beat signal of 560 MHz, which had phase information corresponding to the number of optical revolutions or divisions of 7.

[0044] FIG. 9B shows the frequency distribution of the beat signal intensity measured by the spectrum analyzer 56 while amplification was being performed by the optical amplifier 60, and it was confirmed that the beat signal was output at up to 960 MHz, which corresponds to 12 optical circuits or 12 optical divisions.

[0045] Second Embodiment A displacement measurement device according to a second embodiment will be described below with reference to Fig. 10. The displacement measurement device according to the second embodiment is a partial modification of the displacement measurement device according to the first embodiment, and a description of the same parts as those of the displacement measurement device according to the first embodiment will be omitted.

[0046] The displacement measurement device 100 of the second embodiment includes two multiple-diffraction optical systems 20 and 220, each equipped with a frequency modulator 30 and 230. One multiple-diffraction optical system 20 forms an optical path for circulating +1-order diffracted light DL, and the other multiple-diffraction optical system 220 forms an optical path for circulating −1-order diffracted light DL. The +1-order diffracted light DL, which has been frequency-modulated by Δf by the frequency modulator 30 while circulating through the multiple-diffraction optical system 20, enters the phase detection device 50 via couplers 7a and 7b. The −1-order diffracted light DL, which has been frequency-modulated by Δf by the frequency modulator 30 while circulating through the multiple-diffraction optical system 220, enters the phase detection device 50 via couplers 7a and 7b. In other words, the phase detection device 50 acquires both a +1-order diffracted light signal and a −1-order diffracted light signal.

[0047] Figure 11 shows a frequency spectrum from 0 to 1.5 GHz, obtained by measuring the frequency distribution of the beat signal intensity from the photodetector 51 using the spectrum analyzer 56 instead of the phase detector 50. It can be seen that numerous beat signals are output at 80 MHz intervals, which corresponds to the frequency shift of the AOM 31. The spectrum analyzer also confirmed a 960 MHz beat signal with phase information corresponding to 12 optical cycles or optical divisions. By performing measurements using ±1st-order diffracted optical signals, a beam division number of 24 can be achieved. This results in a spatial period of 21 nm for a grating width of 500 nm, which can be said to meet the requirements for picoscale displacement measurement.

[0048] [Third Embodiment] A displacement measurement device according to a third embodiment will be described below with reference to Fig. 12. The displacement measurement device according to the third embodiment is a partial modification of the displacement measurement device according to the first embodiment, and a description of the same parts as those of the displacement measurement device according to the first embodiment will be omitted.

[0049] In the displacement measurement device 100 of the third embodiment, the multiple diffraction optical system 320 is of a partially reciprocating type. Therefore, the multiple diffraction optical system 320 incorporates a circulator 28, and a single collimating lens 323 is arranged at an angle with respect to the diffraction element 2. Light travels back and forth in an optical fiber 322 extending between the circulator 28 and the collimating lens 323. In other words, illumination light RL propagates from the circulator 28 toward the collimating lens 323, and diffracted light DL propagates from the collimating lens 323 toward the circulator 28. In this system, the angle of incidence of the illumination light RL emitted from the collimator lens 323 with respect to the diffraction element 2 is θ, the diffraction angle is α, the grating interval or pitch is d, and the wavelength of the illumination light RL is λ, and the equation for +1st order diffraction is d(sin θ+sin α)=λ. If the diffracted light DL returns to the direction of incidence, then θ=α, and since 2d sin θ=λ, then θ=sin-1(λ / 2d). In this case, the angle of incidence θ is the Littrow angle. When the illumination light RL is incident on the diffraction element 2 at the Littrow angle, the multiple diffraction optical system 320 can be configured simply.

[0050] [Fourth embodiment] A displacement measurement device of the fourth embodiment will be described below. The displacement measurement device of the fourth embodiment has a structure similar to that of the displacement measurement device 100 of the first embodiment shown in Fig. 1 etc., but performs calibration to compensate for the obtained displacement amount by utilizing an error of the diffraction element 2 obtained by a scale pitch measurement device described later.

[0051] Figure 13 is a diagram illustrating a scale pitch measurement device 1100 obtained by partially rearranging the displacement measurement device 100 shown in Figure 1 etc. The scale pitch measurement device 1100 includes a laser light source 1010, a multiple diffraction optical system 1020, an optical branching / coupling device 1040, a wavelength detection device 1050, and a processing unit 90. In the scale pitch measurement device 1100, the exit collimator lens 23, the input collimator lens 24, the stage 81, and the processing unit 90 are elements common to the displacement measurement device 100, and the displacement measurement device 100 can be rearranged into the scale pitch measurement device 1100 by assembling the laser light source 1010, part of the multiple diffraction optical system 1020, the optical branching / coupling device 1040, the wavelength detection device 1050, etc., instead of the laser light source 10 etc. shown in Figure 1.

[0052] In the scale pitch measurement apparatus 1100, the laser light source 1010 emits source light SL2 over a predetermined wavelength band including a plurality of wavelengths. Specifically, the laser light source 1010 is a pulsed light source such as a fiber laser or solid-state laser, and emits source light SL2 that is an optical frequency comb pulse in the predetermined wavelength band.

[0053] The multiple-diffraction optical system 1020 includes a multicore fiber 1022, an exit collimator lens 23, an entrance collimator lens 24, a fiber connector 26, etc. The multiple-diffraction optical system 1020 makes the light source light SL2 from the laser light source 1010 incident on the diffraction element 2 as illumination light RL2, and causes the diffracted light DL2 emitted from the diffraction element 2 at a plurality of diffraction angles corresponding to a plurality of wavelengths to be incident again on the diffraction element 2 as illumination light RL2, thereby obtaining diffracted light DL2 emitted at a plurality of diffraction angles. The multicore fiber 1022 is specifically a single-mode fiber having three cores C1, C2, and C3, and the cores C1, C2, and C3 exposed at the end face EF are spaced apart at equal intervals in the vertical direction, i.e., the Z direction, and arranged at different height positions. The fiber connector 26 couples the diffracted components D1, D2, and D3 that have propagated through the multiple cores C1, C2, and C3 of the multicore fiber 1022 to multiple optical fibers F1, F2, and F3 of equal length, respectively. Here, the inclination angles of the diffracted components D1, D2, and D3 incident on the cores C1, C2, and C3 correspond to the diffraction angles α1, α2, and α3 of the diffracted components D1, D2, and D3. Note that α1 = α2 + δα, and α3 = α2 - δα.

[0054] The optical branching / coupling device 1040 is incorporated in the optical path of the multiple diffraction optical system 20, and also functions as the multiple diffraction optical system 20, combining the light source light SL2 from the laser light source 1010 with the diffracted light DL2 (including multiple diffracted components D1, D2, and D3) from the fiber connector 26, and coupling the combined light to the optical fiber 22. The optical branching / coupling device 1040 also functions as an optical branching device, and branches the output light ML2, which includes the diffracted light DL2 that has sufficiently circulated, out of the multiple diffraction optical system 20.

[0055] The wavelength detector 1050 measures wavelength information of the diffracted components D1, D2, and D3, which are multiple diffracted components, from the output light ML2 branched by the optical branching / coupling device 1040. The wavelength detector 1050 includes an acousto-optic shutter 57 and a spectrometer 58. The acousto-optic shutter 57 is a high-speed shutter that operates in synchronization with the laser light source 1010 and extracts a desired pulse from the output light ML2. The spectrometer 58 measures the wavelength components of the output light ML2; specifically, it measures the wavelengths λ1, λ2, and λ3 of the diffracted components D1, D2, and D3, and outputs the wavelength information resulting from the measurement to the arithmetic processing device 90.

[0056] The arithmetic processing device 90 calculates the error or deviation ΔΛ, which is the deviation of the grating pitch Λ of the diffraction element 2 from the reference value Λ0, based on premise information such as wavelength information, which is the measured values ​​of the wavelengths λ1, λ2, λ3 of the diffracted components D1, D2, D3 obtained by the spectrometer 58, diffraction angle information α2, δα of the diffracted components D1, D2, D3, and the angle of incidence θ. The arithmetic processing device 90 measures the error of the diffracting element 2 for each position in that direction while referring to the position information of the stage 81, and stores the deviation ΔΛ(X) in the storage device 93. When operating as the displacement measurement device 100, the arithmetic processing device 90 calculates a corrected displacement amount by calibrating the obtained original displacement amount with the integrated value of the deviation ΔΛ(X).

[0057] In the above-described scale pitch measurement device 1100, the scale pitch measurement method includes the steps of: making light source light SL2 spanning a predetermined wavelength band including a plurality of wavelengths incident on the diffraction element 2 as illumination light RL2; using a reusable multiple diffraction optical system 1020, making diffracted light DL2 emitted from the diffraction element 2 at a plurality of diffraction angles corresponding to the plurality of wavelengths incident again on the diffraction element 2 as illumination light RL2 to obtain diffracted light DL2 emitted at a plurality of diffraction angles; branching output light ML2 including diffraction components D1, D2, and D3, which are multiple diffraction components formed by passing through the multiple diffraction optical system 1020 multiple times, from the optical path of the multiple diffraction optical system 1020; measuring wavelength information of the multiple diffraction components D1, D2, and D3 from the branched output light ML2; and calculating the deviation of the scale or scale pitch of the diffraction element 2 from the obtained wavelength information.

[0058] The following describes in detail a pitch calibration method for the diffraction element 2, which is a linear scale. The diffraction element 2 provides a relationship between the angle of incidence and the angle of diffraction depending on the grating pitch at each position and the wavelength of light incident thereon. In practical devices, it is basically impossible to know the values ​​of anything other than the optical wavelength with high accuracy, so it is necessary to measure the optical wavelength under several conditions and estimate the value of the grating pitch. Therefore, we propose an unprecedented method of measuring the grating pitch with ultra-high accuracy by incorporating a multicore fiber 1022 into the basic structure of the displacement measurement device 100. When using a laser light source 1010 that generates pulses with a wide wavelength range, such as an optical frequency comb, the diffraction angle at the diffraction element 2 varies depending on the wavelength. The end face EF of the multicore fiber 1022 is located at the point where the diffracted light DL2 is coupled. If the cores C1, C2, and C3 in the multicore fiber 1022 are arranged vertically, the angular arrangement of the cores C1, C2, and C3 with respect to the linear scale surface parallel to the XY plane on which the diffraction element 2 is located will differ, resulting in slightly different wavelengths of the coupled diffracted light DL2. In a specific embodiment, the wavelength difference is on the order of several nanometers. In the multicore fiber 1022, the cores C1, C2, and C3 have small sizes, specifically, approximately φ10 μm, so the wavelengths that can be coupled are limited, and the arrangement of the cores C1, C2, and C3 can have a spatial wavelength filter effect. However, the sizes of the cores C1, C2, and C3 are finite, and each diffracted component D1, D2, and D3 still has a wavelength width. However, since the diffracted components D1, D2, and D3 are filtered each time they circulate through the multiple diffraction optical system 1020, after a sufficient number of circumnavigations, for example, 10 circumnavigations, the wavelength width becomes very narrow, making it possible to measure the wavelength of the diffracted light with high resolution. To acquire light for each circumnavigation, an optical frequency comb pulse light source (repetition rate of approximately 30 MHz) is used as the laser light source 1010, and only pulses with a desired number of circumnavigations are acquired using the acousto-optic shutter 57. The acquired light is then dispersed to determine the wavelengths λ1, λ2, and λ3. The operation timing of the acousto-optical shutter 57 is set by using the light emission of the laser light source 1010 as a trigger and adding a time delay according to the optical path length of the multi-diffraction optical system 20, and specifically, a high-speed shutter operation is performed at, for example, about 70 MHz.Fig. 14A shows the frequency characteristics of the source light SL emitted from the laser light source 1010, Fig. 14B shows the diffracted light DL2 at a stage where the number of revolutions around the multiple diffraction optical system 1020 is small, Fig. 14C shows the diffracted light DL2 at a stage where the number of revolutions around the multiple diffraction optical system 1020 is relatively increased, and Fig. 14D shows the diffracted light DL2 at a stage where the number of revolutions around the multiple diffraction optical system 1020 is sufficient, for example, 10. The wavelength width of the diffracted light DL2 at the stage where it has sufficiently circulated is extremely narrow.

[0059] Even if the grating pitch Λ, incident angle θ, and central diffraction angle α (for example, α2) of the diffraction element 2 are unknown, the grating pitch Λ can be estimated by solving the following diffraction condition equations for these three unknowns: Λ(sin θ+sin(α2+δα))=λ1 ... (diffraction condition equation for core C1) Λ(sin θ+sin α2)=λ2 ... (diffraction condition equation for core C2) Λ(sin θ+sin(α2-δα))=λ3 ... (diffraction condition equation for core C3) and using the wavelengths λ1, λ2, and λ3 of the three measured diffraction components D1, D2, and D3. Here, the diffraction angle α2 is the central diffraction angle, the diffraction angle α1 above it is given by α1=α2+δα, and the diffraction angle α3 below the center is given by α3=α2-δα. The deviation amount δα of the diffraction angle can be determined from the distance from the incident position of the illumination light RL on the diffraction element 2 to the end face EF of the multicore fiber 1022 and the relative spacing between the cores C1, C2, and C3 of the multicore fiber 1022. Such estimation or calculation of the grating pitch Λ becomes possible when the arithmetic processing device 90 executes a program for finding solutions to the above simultaneous equations stored in the storage device 93.

[0060] The arithmetic processing device 90 calculates an error or deviation ΔΛ, which is the deviation of the grating pitch Λ of the diffraction element 2 from a reference value Λ0, based on information such as the wavelengths λ1, λ2, λ3 of the diffracted components D1, D2, D3 obtained by the spectrometer 58 and the deviation δα of the diffraction angles of the diffracted components D1, D2, D3. The arithmetic processing device 90 measures the error of the diffracting element 2 for each position in that direction while referring to the position information of the stage 81, and stores the deviation ΔΛ(X) in the storage device 93. When operating as the displacement measuring device 100, the arithmetic processing device 90 calculates a corrected displacement amount by calibrating the obtained original displacement amount using the deviation ΔΛ(X) or its integrated value stored in advance in the storage device 93.

[0061] In this method, the deviation of the grating pitch Λ can be measured at high speed without scanning or repeated measurements, and therefore it is easy to measure the entire scale of the diffraction element 2. If the diffracted light wavelength can be measured with an accuracy of several pm, －６ It becomes possible to measure the grating pitch Λ with a relative accuracy of

[0062] In the above, the multicore fiber 1022 may have four or more cores and may be arranged two-dimensionally. The cores C1, C2, and C3 do not need to be aligned horizontally or in the X direction, as long as they are equally spaced vertically. Furthermore, the spacing between the cores C1, C2, and C3 does not need to be equal, as long as it is measurable. In principle, the diffracted components D1, D2, and D3 can be separated and propagated by methods other than the multicore fiber 1022. However, the multicore fiber 1022 is preferable from the viewpoint of distinguishing the minute deviation in the diffraction angle δα. The fiber connector 26 and the optical branching / coupling device 1040 can be modified, such as multiplexing in multiple stages or collectively multiplexing / demultiplexing, as long as their functions are achieved. The measurement of the grating pitch Λ using the scale pitch measurement device 1100 may be performed after the displacement measurement using the displacement measurement device 100. In this case, the displacement can be calibrated after the measurement.

[0063] [Others] The displacement measuring device, scale pitch measuring device, displacement measuring method, scale pitch measuring method, etc. according to the present invention have been described above as embodiments and examples, but the displacement measuring device, scale pitch measuring device, displacement measuring method, scale pitch measuring method, etc. according to the present invention are not limited to those described above.

[0064] For example, the diffracted light DL that circulates through the multiple diffraction optical system 20 may be, for example, +2nd order or higher diffracted light DL.

[0065] The multi-diffraction optical system 20 is not limited to one that uses optical fibers, but may be one that spatially guides light using elements such as mirrors.

[0066] In the above explanation, the diffraction element 2 is described as being of a reflective type, but the diffraction element 2 may also be of a transmissive type, in which case a mirror can be placed on the opposite side of the diffraction element 2 to form a circular or round-trip optical path.

[0067] This application claims priority based on Japanese Patent Application No. 2024-027297, filed on February 27, 2024, the entire disclosure of which is incorporated herein by reference.

Claims

1. A displacement measurement device comprising: a laser light source; a reusable multiple diffraction optical system that causes light source light emitted from the laser light source to be incident as illumination light on a diffraction element to obtain diffracted light, and causes the diffracted light from the diffraction element to be incident again as illumination light on the diffraction element to obtain diffracted light; a frequency modulator that is incorporated in the optical path of the multiple diffraction optical system and performs frequency modulation on the diffracted light; an optical branching device that is incorporated in the optical path of the multiple diffraction optical system and branches output light including multiple modulated components formed by passing through the frequency modulator multiple times; and a phase detection device that extracts phase information related to the multiple modulated components from the output light branched by the optical branching device.

2. The displacement measurement device according to claim 1, wherein the phase detection device extracts the phase information from a beat signal obtained by interfering between the light source light and the diffracted light as the output light from the optical splitter.

3. A displacement measurement device according to claim 2, wherein the phase detection device comprises a frequency filter that extracts a beat signal component having a frequency n times the frequency difference between the source light and the nth diffracted light.

4. The displacement measuring device according to claim 3, wherein the phase detection device is a lock-in amplifier that also functions as the frequency filter, and the drive signal of the frequency modulator is used as a reference signal.

5. The displacement measuring device according to claim 1, wherein the frequency modulator includes either an acousto-optic modulator or an electro-optic modulator.

6. The displacement measuring device according to claim 1, wherein the multiple diffraction optical system is a circular optical system, and couples the diffracted light from the diffraction element onto the optical path from the laser light source to the diffraction element.

7. The displacement measuring device according to claim 1, wherein the optical splitter also serves as an optical coupler / splitter that couples the diffracted light from the diffraction element onto the optical path from the laser light source to the diffraction element.

8. The displacement measurement device according to claim 1, wherein the optical path of the multiple diffraction optical system from an inlet for acquiring the diffracted light to an outlet for emitting the illumination light is connected by an optical fiber.

9. The displacement measuring device according to claim 1, further comprising an optical amplifier incorporated in the optical path of said multiple diffraction optical system for amplifying said diffracted light.

10. The displacement measuring device according to claim 1, further comprising a processor that determines the amount of displacement of said diffraction element based on the change in said phase information.

11. The displacement measuring device according to claim 10, wherein the arithmetic processing device acquires a deviation in the scale pitch of the diffraction element, and calibrates the displacement amount of the diffraction element based on the deviation.

12. A displacement measurement method comprising the steps of: using a reusable multiple diffraction optical system, irradiating light source light emitted from a laser light source onto a diffraction element as illumination light to obtain diffracted light, and irradiating the diffracted light from the diffraction element again as illumination light to obtain diffracted light; frequency modulating the diffracted light using a frequency modulator incorporated in the optical path of the multiple diffraction optical system; branching output light containing multiple modulated components formed by passing through the frequency modulator multiple times from the optical path of the multiple diffraction optical system; and extracting phase information related to the multiple modulated components from the branched output light.

13. A scale pitch measurement device comprising: a laser light source that emits light source light over a predetermined wavelength band including a plurality of wavelengths; a reusable multiple diffraction optical system that causes the light source light from the laser light source to be incident on a diffraction element as illumination light, and that causes the diffracted light emitted from the diffraction element at a plurality of diffraction angles corresponding to the plurality of wavelengths to be incident on the diffraction element again as illumination light to obtain diffracted light emitted at the plurality of diffraction angles; an optical branching device that branches output light including multiple diffraction components formed by passing through the multiple diffraction optical system a plurality of times; a wavelength detection device that measures wavelength information of the multiple diffraction components from the output light branched by the optical branching device; and a processing device that calculates the deviation of the scale pitch of the diffraction element from the wavelength information obtained by the wavelength detection device.

14. The scale pitch measurement device according to claim 13, wherein the multiple diffraction optical system includes a multi-core fiber, and the diffracted light beams having different diffraction angles from the diffraction element are incident individually on multiple cores of the multi-core fiber.

15. The scale pitch measurement device according to claim 14, wherein the optical branching device combines the multiple diffraction components including a plurality of diffraction components corresponding to the plurality of wavelengths propagating through the plurality of cores of the multicore fiber.

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