JP2015178981A - Distance metering device and distance metering method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct a fluctuation of a phase due to a CEO on software, unlike the conventional art that corrects the fluctuation of the phase due to the CEO on hardware.SOLUTION: One embodiment according to the present invention provides a distance metering device (100) that comprises: a storage unit (131) that preliminarily stores a change direction matrix about a direction of a change of a fluctuation component of a phase due to a CEO included in interference light; and a CEO correction unit (133) that estimates the fluctuation component from a coefficient matrix adjusted so that a phase on a model matches an actual phase within a prescribed threshold on the basis of the model in which the phase on the model of the interference light is expressed by a sum of a prescribed phase and the fluctuation component and the fluctuation component is expressed by a product of the change direction matrix and the coefficient matrix of the change direction matrix, and corrects the fluctuation component by subtracting the estimated fluctuation component from the actual phase.

Description

  The present invention relates to a distance measurement technique using a laser beam, and in particular, an optical frequency comb.

  Conventionally, as a device for measuring the distance to an object, a technique for irradiating a measurement object with laser light and measuring the distance to the object based on the phase of reflected light from the object is known. . Patent Document 1 provides a mechanism that suppresses mode hopping, and a double frequency stabilization negative feedback mechanism that stabilizes one absolute frequency and repetition frequency of a longitudinal mode independently, and causes them to act simultaneously. Thus, a mode-locked pulse laser is disclosed that stabilizes the absolute frequency of all longitudinal modes. In Patent Document 2, the control unit of a device that measures the carrier envelope offset (CEO) frequency of a mode-locked laser adjusts the resonator length of the mode-locked laser and the pump light intensity to adjust the repetition frequency. And a technology for controlling the CEO frequency is disclosed.

JP 2008-251723 A JP 2014-13935 A

  As disclosed in Patent Documents 1 and 2, conventionally, the resonator length is controlled by detecting the phase fluctuation component due to carrier envelope offset (CEO) from the frequency spectrum of the laser and feeding it back. A device that makes the phase fluctuation component a constant value is used. In Patent Document 1, a mechanism that suppresses mode hop reduction in which a longitudinal mode frequency suddenly changes to another frequency, and double frequency stabilization that stabilizes an absolute frequency and a repetition frequency of a certain longitudinal mode independently. A load feedback mechanism is provided and operated simultaneously to stabilize the absolute frequency of all longitudinal modes.

  In order to correct the phase fluctuation component on hardware as in the past, a highly accurate fluctuation component detection device and a high-accuracy resonator length can be obtained under circumstances where the ambient temperature and vibration are properly controlled. Therefore, there is a problem that it is difficult to actually use it in a production line such as a factory.

  Therefore, an object of the present invention is to provide a distance measuring device and a distance measuring method for correcting the fluctuation component by software instead of correcting the fluctuation component of the phase by CEO as in the prior art. That is.

In one embodiment of the present invention, a first light generator that generates first light having a first repetition frequency mode, and a second light generation mode that has a second repetition frequency mode different from the first repetition frequency. A second light generator that generates the first light, and a first interference light between the first light and the second light emitted from the first and second light generators via the reference point. Detecting a second interference light between the first detector and the first light reflected from the measurement object and the second light reflected from the reference surface, which is irradiated through the reference point. A change direction matrix (W) indicating a change direction of a phase fluctuation component (θ ^CEO ) due to a carrier envelope offset (CEO) included in the first detector and the second interference light is stored. And the phase (θ) on the model of the first and second interference lights is a predetermined phase ((Δθ / Δf) F) and is represented by the sum of the fluctuation component (theta ^CEO), model represented by the product of the fluctuation component (theta ^CEO) changes orientation matrix (W) and its coefficient matrix (Q) (θ = Based on (Δθ / Δf) F + WQ), the fluctuation component (θ) from the coefficient matrix (Q) adjusted so that the phase (θ) on the model matches the actual phase (θ ′) within a predetermined threshold value. ^CEO) estimates the, by subtracting from the estimated fluctuation component (theta ^CEO) the actual phase (theta '), and CEO correcting unit for correcting the fluctuation component (theta ^CEO), the fluctuation component (theta ^CEO) is corrected The phase between the modes in the phase difference (Δθ ^b−a ) between the phase (θ ^b ) of the second interference light and the phase (θ ^a ) of the first interference light with the fluctuation component (θ ^CEO ) corrected. based on the phase difference ^{_{(Δθ k + 1 b-a}} -Δθ k b-a), the distance calculation unit that calculates a distance from the reference point to the object of measurement A distance measuring device is provided.

Further, according to an embodiment of the present invention, interference light having a mode with a predetermined repetition frequency is detected a plurality of times, and the phases (θ ₀ , θ ₁ ,... Θ _n ) of the interference light are distributed in a predetermined phase space. A step of calculating a distribution center of gravity (θ _G1 , θ _G2 ,...), A plurality of principal component axes passing through the center of gravity and orthogonal to each other, and a plurality of principal component axis eigenvectors (w ₁ , w ₂ , Is a step of generating a change direction matrix (W) including the change of the phase fluctuation component (θ ^CEO ) due to the carrier envelope offset (CEO) included in the interference light. And the phase (θ) on the model of the interference light is represented by the sum of a predetermined phase ((Δθ / Δf) F) and a fluctuation component (θ ^CEO ), and the fluctuation component (θ ^CEO ) Is the change direction matrix (W) and its coefficient matrix (Q) In the model (θ = (Δθ / Δf) F + WQ) represented by the product of the above, the phase (θ) on the model matches the actual phase (θ ′) of the detected interference light within a predetermined threshold value. estimating an adjusted coefficient matrix (Q) from the fluctuation component (theta ^CEO) in by subtracting estimated fluctuation component (theta ^CEO) from the actual phase (theta '), a fluctuation component (theta ^CEO) Provided is a distance measuring method including a step of correcting, and a step of calculating a distance from a reference point to a measurement object based on the corrected actual phase (θ ′).

  The distance measuring apparatus according to an embodiment of the present invention can more accurately measure the distance to the measurement object using the phase information in which the phase fluctuation component by the CEO is corrected. In addition, the distance measuring apparatus according to the embodiment of the present invention does not require an additional device (hardware) for correcting a phase fluctuation component caused by the CEO as in the prior art, and is used at an actual manufacturing site or the like. This leads to a reduction in manufacturing cost.

It is a block diagram of the distance measuring device concerning one embodiment. It is a conceptual diagram of the mode of an optical frequency comb. It is a conceptual diagram of the pulse of an optical frequency comb. It is a conceptual diagram of the phase spectrum of an optical frequency comb. It is a conceptual diagram of the statistical analysis method which concerns on one Embodiment of this invention. It is a conceptual diagram of the statistical analysis method which concerns on one Embodiment of this invention. It is a flowchart of the distance measurement method which concerns on one Embodiment of this invention. It is a flowchart of the distance measurement method which concerns on one Embodiment of this invention. It is an example of the phase spectrum which concerns on one Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, dimensions, materials, shapes, relative positions of components, and the like described in the following embodiments are arbitrary, and are changed according to the structure of the apparatus to which the present invention is applied or various conditions. Further, unless otherwise specified, the scope of the present invention is not limited to the form specifically described in the embodiments described below. In the drawings described below, components having the same function are denoted by the same reference numerals, and repeated description thereof may be omitted.

<Configuration of distance measuring device>
FIG. 1 is a block diagram of a distance measuring apparatus 100 according to an embodiment of the present invention.

The distance measuring device 100 includes an optical frequency comb laser device 110, a lens 120, and an arithmetic processing device 130. The distance measuring device 100 irradiates the work 140, which is a measurement object, from the optical frequency comb laser device 110 through the lens 120 with the optical frequency comb, and measures the distance to the work 140 based on the reflected light. Here, the optical frequency comb is a pulse having a plurality of discrete frequency components ("frequency mode" or simply "mode") having a predetermined frequency (oscillator modulation frequency) interval around the frequency ν _{0 of the} laser light source. Light. The distance measuring device 100 may not include the lens 120.

  The optical frequency comb laser device 110 includes a laser light source 111, beam splitters 112a and 112c, polarization beam splitters 112b and 112d, first and second optical frequency comb generators (OFCG) 113a and 113b, a frequency shifter 114, a wave plate 115, A reference surface 116, polarizers 117a and 117b, first and second detectors 118a and 118b, and a fast Fourier transform (FFT) unit 119a and 119b are provided.

The laser light source 111 emits laser light having a single frequency ν ₀ toward the beam splitter 112a. For example, the frequency ν ₀ is 2.5 THz. Laser light from the laser light source 111 is separated toward the first OFCG 113a and the frequency shifter 114 by the beam splitter 112a.

The OFCG 113a as the first light generator is composed of an electro-optic converter (EOM) and a reflecting mirror disposed so as to sandwich the EOM, and a modulation signal having a modulation frequency f _m1 from an external oscillator 113c. In response, an optical frequency comb (“measurement light Ps” as the first light) having a mode with the first repetition frequency f _m1 is generated. By adjusting the modulation frequency f _m1 of the oscillator 113c, the frequency interval f _m1 between the modes of the measurement light Ps emitted from the first OFCG 113a can be adjusted. For example, f _m1 is 25 GHz.

Similarly, the OFCG 113b as the second light generator is configured by an EOM and a reflecting mirror disposed so as to face the EOM, and receives a modulation signal having a modulation frequency f _m2 from an external oscillator 113d. Then, an optical frequency comb having a mode of the second repetition frequency _fm2 ("reference light Pr" as the second light) is generated. f _m2 is f _m2 = f _m1 + Δf, and Δf is a sufficiently smaller value than f _m1 and f _m2 , for example, 500 kHz. The oscillators 113c and 113d are phase-synchronized by a common reference oscillator (not shown), and the relative frequency between the modulation frequencies f _m1 and f _m2 is stable. Thereby, the coherence between the measurement light Ps and the reference light Ps is improved, the repetition frequency is stabilized, and a short pulse width can be generated.

The first and second OFCGs 113a and 113b include phase modulators, intensity modulators, and semiconductors using nonlinear crystals such as LiNbO ₃ , ADP (ammonium dihydrogen phosphate), and KDP (potassium dihydrogen phosphate). It may be a modulator that utilizes absorption or phase change.

Frequency shifter 114, for example, acousto-optic modulator (Acousto-Optic modulator) and is, operating in the modulation signal from the oscillator 114a, the second shifts the frequency of the laser light by the modulation frequency f _alpha from the laser light source 111 Output to the OFCG 113b. By shifting the frequency of the light incident on the second OFCG 113b by the frequency shifter 114, the beat frequency of the interference light Pi between the measurement light Ps and the reference light Pr in the detectors 118a and 118b is not a DC signal but a frequency f. It becomes an AC signal of _α , facilitating phase comparison.

The wave plate 115 is a half-wave plate and adjusts the polarization direction of the reference light Pr from the second OFCG 113b. Polarizing beam splitter 112b is emitted to the beam splitter 112c as mixed light Pi ₁ superimposed with polarized light orthogonal to the reference optical power Pr and the measurement light Ps. The beam splitter 112c separates the mixed light Pi ₁ toward the polarizer 117a and the polarization beam splitter 112d.

Polarization beam splitter 112d is a mixed light Pi ₁ from the beam splitter 112c is separated into a reference beam Pr and the measurement light Ps according to the polarization, the measurement light Ps emitted to the work 140, the reference light beam Pr to the reference surface 116 Exit. Further, the polarization beam splitter 112d returns the mixed light Pi ₂ obtained by mixing the reference beam Pr reflected from the reference surface 116 and the measurement light Ps reflected from the workpiece 140 to the beam splitter 112c. The reference surface 116 is a surface that reflects light, for example, a mirror surface.

Since the polarization of the reference beam Pr the polarization of the measuring light Ps in the mixed light Pi ₁ from the polarization beam splitter 112c are orthogonal to each other, a polarizer 117a is adjusting the direction to be oblique to both polarizations Arranged. Therefore, the detector 118a detects the interference light between the measurement light Ps and the reference light Pr.

Similarly, since the polarization of the reference beam Pr the polarization of the measuring light Ps in the mixed light Pi ₂ from the polarization beam splitter 112c are orthogonal to each other, a polarizer 117b is to be oblique to both polarizations Arrange the orientation. Therefore, the detector 118b detects interference light between the measurement light Ps reflected from the workpiece 140 and the reference light Pr reflected from the reference surface 116.

  The detector 118a receives the interference light between the measurement light Ps and the reference light Pr from the polarizer 117a, and outputs an interference signal corresponding to the interference light. The detector 118a is a detector for obtaining a reference phase in the generation process of the measurement light Ps generated by the first OFCG 113a and the reference light Pr generated by the OFCG 113b.

Here, as shown in FIG. 2B, the measurement light Ps and the reference light Pr in the detector 118a (or the detector 118b) have different repetition frequencies, so the pulse of the measurement light Ps and the reference light Pr are always somewhere. An instant (time t ₁ , t ₂ ) where the pulse overlaps appears. The overlapping moments (time t ₁ , t ₂ ) appear periodically at a repetition frequency (that is, Δf) corresponding to the difference between the repetition frequency of the measurement light Ps and the repetition frequency of the reference light Pr. Since Δf is sufficiently smaller than the light frequency, phase information can be detected by a detector (electronic circuit).

  On the other hand, the detector 118b receives the interference light of the measurement light Ps and the reference light Pr from the polarizer 117b, and outputs an interference signal corresponding thereto. The detector 118b is a detector for obtaining a delay time corresponding to the difference between the distance to the reference surface 116 and the distance to the workpiece 140 with respect to the beam splitter 112c (reference point: RP).

  The FFT units 119a and 119b input interference signals based on the interference light detected by the detectors 118a and 118b, respectively, perform predetermined signal processing such as fast Fourier transform (FFT), and the frequency of each mode of the interference light. Information, phase information, amplitude information, and the like are output to the arithmetic processing unit 130.

  The lens 120 is a lens for irradiating the workpiece 140 with the measurement light Ps from the optical frequency comb laser device 110, and returns the reflected light from the workpiece 140 to the optical frequency comb laser device 110 through the lens. The lens 120 may be attached to a moving mechanism for moving the lens to a predetermined position manually by an operator or in response to a control signal from the arithmetic processing unit 130.

  The arithmetic processing unit 130 is a computer including an input / output interface (not shown), a CPU (Central Processing Unit) (not shown), and a storage unit 131 such as a ROM (Read Only Memory) or a RAM (Random Access Memory). . The CPU of the arithmetic processing device 130 causes each function of the statistical analysis unit 132, the CEO correction unit 133, and the distance calculation unit 134 to be performed in accordance with a predetermined program stored in the storage unit 131. The description about each function part 132-134 is mentioned later.

<Relationship between phase and measurement distance>
The relationship between the phase difference (θ = φ _s −φ _r ) between the measurement light Ps (phase φ _s ) and the reference light Pr (phase φ _r ) and the distance from the reference point RP to the workpiece 140 will be described.

As shown in FIG. 2A, the measurement light Ps generated by the first OFCG 113a has a plurality of frequency modes having a frequency interval (repetition frequency f _m1 ) that matches the modulation frequency f _m1 of the oscillator 113c. Here, the frequency ν ₀ of the laser light from the laser light source 111 is set to the 0th order mode, and n is an integer of 0, ± 1, ± 2,. The reference light Pr generated by the second OFCG 113b has a plurality of frequency modes having a frequency interval (repetition frequency f _m2 ) corresponding to the frequency f _m2 (= f _m1 + Δf) of the modulation signal of the oscillator 113d. Here, the frequency ν ₀ + f _α of the light frequency-shifted by the frequency shifter 114 is set to the 0th order mode.

Each mode of the reference light Pr, for each mode of the measurement light Ps, only f _alpha differ in 0-order mode, only f _alpha + Delta] f in the primary mode differs, differ in the n-th order mode only f α + _nΔf. Therefore, the beat frequency of the kth-order mode of the interference light between the measurement light Ps and the reference light Pr is (ν ₀ + f _α + kf _m1 + kΔf) − (ν ₀ + kf _m1 ) = f _α + kΔf (k is _{0 to} n). Integer).

The amplitude E _k (t) of the electric field of the k-th mode of the interference light between the measurement light Ps and the reference light Pr in the detector 118a (or the detector 118b) is expressed by Equation 1:
It is represented by Here, A _k is the electric field of the measurement light Ps, B _k is the electric field of the reference light Pr, and θ _k is the phase φ _s of the measurement light Ps and the phase φ _{r of the} reference light Pr in the k-order mode. The phase difference between and. In other words, it is the relative phase (that is, θ _k = φ _s −φ _r ) of the _k -th mode of the measurement light Ps based on the phase φ _r of the k-th mode of the reference light Pr.

Interference signal between the following numbers of different modes, to appear on and around the modulation frequency f _m, by although the bandwidth of the detector 118a sufficiently wide as compared with f _alpha and Δf takes less than f _m, or filter etc. By using it to remove high frequency components, only the beat frequency between modes of the same order remains. Therefore, interference between modes of different orders is not considered.

The output current I _k (t) for the k-th order mode of the detector 118a (or detector 118b) is expressed by Equation 2:
It is represented by The frequency difference (f _α + kΔf) between the measurement light Ps and the reference light Pr is the beat frequency F.

The meaning of the time delay that gives the phase θ _k (= φ _s −φ _r ) of the k-th mode in Equation 2 differs between the detectors 118a and 118b. That is, the phase θ _k detected by the detector 118a is caused by the length of the optical path length from when the light is separated by the beam splitter 112a until it is mixed by the polarization beam splitter 112b. Then, this time delay are the common to the detectors 118a and detector 118b, the detector phase that is detected by 118b theta _k ( "theta _k ^b") phase theta _k detected by the detector 118a from ( It is removed by subtracting “θ _k ^a ”). Then, the time difference of the interference signal detected by the interference signal detector 118b detected by the detector 118a, the phase difference between the modes of the reference beam Pr and the measurement light Ps in the frequency domain (θ _k ^b -θ _k ^a) The rate of change (gradient) with respect to the order k.

As can be seen from FIG. 2A, the relationship between the angular frequency ω _s of the measurement light Ps and the frequency ω _r of the reference light Pr regarding the k-th mode is
ω _r = ω _s + ω _α + kΔω
It is represented by Here, ω _s = 2πf _m1 , ω _r = 2πf _m2 , ω _α = 2πf _α , and Δω = 2πΔf.

Next, the phase phi _r ^a phase phi _s ^a and the reference light Pr of the measuring light Ps for the k-order mode towards the detector 118a from the beam splitter 112c (reference point RP) at time t, respectively,
φ _s ^a = ω _s t
φ _r ^a = ω _r t
It is.

On the other hand, the phase φ _s ^b of the measurement light Ps and the phase φ _r ^b of the reference light Pr for the k-th mode from the reference point RP to the detector 118b through the workpiece 140 and the reference surface 116 are respectively at time t.
φ _s ^b = ω _s {t−2 (L ₀ + L _s ) / V _c }
^{_{φ r b = ω r {t}} -2 (L 0 + L r) / V c}
It is. Here, V _c is the speed of light, L ₀ is the distance (optical path length) from the reference point RP to the polarization beam splitter 112d, and L _s and L _r are the distances from the reference point RP to the workpiece 140 and the reference surface 116, respectively ( Optical path length).

Then, with respect to the k-th mode, the phase θ _k ^b (= φ _s ^b −φ _r ^b ) of the interference signal output from the detector 118b and the phase θ _k ^a (= φ of the interference signal output from the detector 118a). _s ^a −φ _r ^a ), Δθ _k ^b−a ,
It becomes. In Equation 3, the first term is a phase that changes according to the distance | L _s −L _r | to the workpiece 140, and the second term is a phase offset that depends on the fixed distances L ₀ and L _r .

The distance | L _s −L _r | to be measured is obtained from the rate of change between the modes of the phase. Since the frequency interval between the k + 1-order mode and the k-th mode of the measuring light Ps as described above is _{f m1,} the difference between [Delta] [theta] _k ^b-a of the [Delta] [theta] of the k + 1-order mode _k ^{+ 1 b-a} and k-th order mode , Formula 4:
It is. Here, ω _m1 = 2πf _m1 . As can be seen from Equation 4, in order to obtain an accurate distance | L _r −L _s | from an accurate phase difference (Δθ _{k + 1} ^b−a −Δθ _k ^b−a ) between the k + 1 order mode and the kth order mode. Is preferably the value of the phase difference when Δω = 0 that is not affected by the second term of Equation 4. In order to obtain the value of the phase difference when Δω = 0, measure the phase difference by changing the value of Δω several times, and obtain the value of the phase difference by extrapolating the obtained results. The distance | L _r −L _s | may be obtained from the value. The phase difference value when Δω = 0 may be obtained by averaging the phase difference values obtained as Δω = ± (predetermined value).

In this way, the interference light detected by the detector 118b phase _{^{θ k b (= φ s b}} -φ r b) and the interference light of the phase detected by the detector _{^{118a θ k a (= φ s}} a - phase difference between the modes of the difference Δθ _k ^b-a and φ _r ^a) (i.e., based on ^{_{Δθ k + 1 b-a -Δθ}} k b-a), the distance to the workpiece 140 _| L r -L _{s |} a Can be sought.

<Phase fluctuation component by CEO>
Subsequently, regarding the influence of the carrier envelope offset (CEO) generated in the optical frequency comb, the beat frequency F (= f _α + Δf) and the phase θ (= φ _s −φ _r ) of the interference light detected by the detector are set. Think based. In the following, the interference light detected by the detector 118a is considered, but the same applies to the interference light detected by the detector 118b.

Ideally, as shown in FIG. 3A, the optical frequency comb has a frequency interval Δf and a phase difference Δθ ^a (= θ _{k + 1} ^a −θ _k ^a) between the modes of the interference light detected by the detector 118a. ) Is constant, and the phase θ ^{a of} each mode is on the ideal straight line 301. However, in reality, CEO is generated due to slight temperature changes and vibrations in the optical frequency comb generator, changes in the resonator length due to the electric field of laser confidence, and the like. Since the CEO affects the phase of each mode of the optical frequency comb as the phase fluctuation component θ ^CEO = [θ ₀ ^CEO θ ₁ ^CEO ... θ _n ^CEO ], as shown in FIG. The phase θ in each mode of the optical frequency comb deviates from the ideal straight line 301.

Further, the phase fluctuation component θ _k ^CEO by the CEO of each mode is different for each optical frequency comb so that the shape of the phase spectrum of FIG. 3B is different from that of FIG. 3C _. As the reason, for example, in the example of FIG. 2B, the interference light 203 detected by the detector at time t2 is the pulse 201 of the fourth measurement light Ps from the left and the pulse 202 of the fifth reference light Pr from the left. Consists of. Therefore, the fluctuation component phi _s ^CEO phase by CEO pulse 201 of the measuring light Ps, because different from the fluctuation component phi _r ^CEO of the pulse 202 of the reference beam Pr, also affect the interference light resulting effects are detected .

  Therefore, in order to measure the exact distance from the distance measuring device 100 to the workpiece 140, the arithmetic processing device 130 uses the software to set the phase fluctuation component by CEO on the phase information of the interference light output from the detector. It is desirable to calculate the distance using Equations 3 and 4 after the correction.

  Therefore, the inventor of the present invention considers that there is a tendency in the phase of each mode of the optical frequency comb affected by the phase fluctuation component due to the CEO, and examines the phase information of the actually detected interference light multiple times. As a result, it was found that there is a tendency in the phase change caused by the CEO. Therefore, the tendency of phase change caused by CEO is investigated using statistical analysis techniques, and knowledge gained empirically is accumulated, and based on that knowledge when measuring the actual distance in the field such as factories. It was found that by correcting the phase change caused by CEO on the software, it can be used for accurate distance measurement. The method will be described below.

<Correction method of phase fluctuation component by CEO>
In the present embodiment, principal component analysis (PCA) is used as a statistical analysis method.

As a model using PCA, interference light detected by a detector has _{0 to n-} order modes, and a matrix θ = [θ ₀ θ ₁ ... Θ _n ] of each phase θ is expressed by Equation 5:
Consider the model represented by The parameter P to be adjusted in this model is P = [Δθ / Δf, Q].

Here, Δθ / Δf is the inclination of the ideal straight line 301 and is a constant that is ideally determined according to the configuration of the optical frequency comb laser device 110 (such as the modulation frequency of the OFCG). Thus, it is determined by performing PCA processing. Further, F is a matrix of frequency of _{0th to n-th} mode F = [F ₀ F ₁ ... F _n ], and θ ^CEO is a matrix of phase fluctuation components due to ^CEO of each mode θ ^CEO = [θ ₀ ^CEO θ ₁ ^CEO ... θ _n ^CEO ]. W is a change direction matrix W = [w ₁ w ₂ ...] In which directions in which the phase θ of each mode can change due to phase fluctuations caused by the CEO (that is, the phase change direction caused by the CEO) are arranged. Q is a coefficient matrix Q = [q ₁ q ₂ ...] Representing the amount of change of each element w ₁ , w ₂ ,. Note that θ ^CEO = WQ, and the dimensions of the matrices W and Q may coincide with each other and may coincide with the dimensions of the matrices F and θ (ie, n + 1 dimensions) or may have fewer dimensions. .

  As described above, it has been found that there is a tendency in the change of the phase fluctuation component due to the CEO of the optical frequency comb. Therefore, the following method is used as the correction method of the phase fluctuation component by the CEO of the present embodiment.

First, a plurality of optical frequency combs (that is, interference light beams of the measurement light Ps and the reference light Pr) are detected by the detectors 118a and 118b under the configuration of the distance measuring device 100 used in the actual site such as a factory and the use conditions thereof. The information of a plurality of phase spectra is acquired by performing detection once, and the change direction matrix W is generated in advance for each of the detectors 118a and 118b based on the information. Thereafter, until the phase (θ) on the model of Formula 5 matches (or matches or substantially matches) within a predetermined threshold with the actual phase (θ ′) of the interference light actually detected at the site. The parameter P = [Δθ / Δf, Q] is adjusted using a regression analysis such as a least square method. Estimating a phase fluctuation component θ ^CEO (= WQ) due to CEO of detected interference light from the value of the parameter P = [Δθ / Δf, Q] when they match within the predetermined threshold value Can do. Then, the phase fluctuation component θ ^CEO due to the CEO can be corrected by subtracting the estimated fluctuation component θ ^CEO from the actual phase (θ ′) of the actually measured phase spectrum. Further explanation will be given below with a simple example. The predetermined threshold value is a value empirically determined according to the measurement accuracy. For example, when the measurement accuracy is increased, the predetermined threshold value is set so that the phase on the model and the actual phase coincide with each other 98 to 100%.

First, how to obtain the change direction matrix W will be described. Note that a specific calculation method of PCA is generally well known, and thus the description thereof is omitted. In addition, although an actual optical frequency comb includes several hundred or more modes, in order to simplify the explanation, here, the interference light detected by the detector 118a (or the detector 118b) is the frequency. assumed to have only second order mode of F ₁ of the first-order mode and the frequency _F 2 (> _{F 1).}

When the detector 118a detects the interference light between the measurement light Ps and the reference light Pr a plurality of times (for example, 1000 times), the phases θ ₁ and θ ₂ as shown in Table 1 for the modes Mode1 and Mode2 of the interference light. Is obtained. As described previously, the phase theta, the difference between the phase (phi _r) of the reference beam Pr and phase (phi _s) of the measuring light Ps, is θ = φ _s -φ _r.

As shown in FIG. 4 (a), the obtained phase theta _1, phase point taking _{θ 2 (θ 1, θ 2} ) to be distributed on a two-dimensional theta ₁ - [theta] ₂ phase space. Next, as shown in FIG. 4B, the centroid G (θ _G1 , θ _G2 ) of the distribution set 401 is obtained, and the first principal component axis 402 passing through the centroid G and the centroid G in descending order of eigenvalues. And a second principal component axis 403 that is orthogonal to the first principal component axis 402 is formed. Then, as shown in FIG. 4 (c), first and obtains eigenvectors _w 1, _{w 2} extending from the center of gravity G of the second principal component axis 402, 403, changes direction matrix W consisting of eigenvectors _w 1, _{w 2} = [W ₁ w ₂ ] is obtained.

Here, the position of the center of gravity G (θ _G1 , θ _G2 ) is the center of the distribution of the phase points, and is considered to represent the phase value that is least affected by the phase fluctuation component θ ^CEO caused by the CEO. . Therefore, at this time, the Δθ of the parameter (Δθ / Δf) may be determined as Δθ = θ _G2 −θ _G1 from the position (θ _G1 , θ _G2 ) of the center of gravity G. Note that the value of Δf is a constant determined in advance by subtracting the modulation frequency f _m1 of the first OFCG 113a from the modulation frequency f _m2 of the second OFCG 113b.

After obtaining the change direction matrix W = [w ₁ w ₂ ] in this way, as shown in FIG. 4C, the detector 118a generates 2 interference lights between the measurement light Ps and the reference light Pr. It is assumed that the actual phase 404 (θ ₁₁ , θ ₁₂ ) and the actual phase 405 (θ ₂₁ , θ ₂₂ ) are obtained by performing the measurement once.

Then, from Equation 5, the parameter P = [Δθ / Δf, so that the value of θ = (Δθ / Δf) F + WQ matches the values of the phase components θ ₁₁ and θ ₁₂ of the phase 404 within a predetermined threshold value. Q] is adjusted. From the coefficient matrix Q = [q ₁ q ₂ ] when the values match within a predetermined threshold, the phase fluctuation component θ ^CEO = WQ caused by the CEO can be estimated. By subtracting the estimated phase fluctuation component θ ^{CEO due} to ^CEO from the actual phase 404 (θ ₁₁ , θ ₁₂ ), a phase spectrum in which the phase fluctuation component due to CEO is corrected can be obtained. The same applies to the phase 405.

  This will be described again from another point of view with reference to FIG. In FIG. 5, the phase spectrum is drawn as a continuous curve or straight line for the sake of simplicity.

First, the interference light of the measurement light Ps and the reference light Pr is transmitted a plurality of times (for example, several thousand times) by the detectors 118a and 118b under the configuration of the distance measuring device 100 used in the actual site such as a factory and the use conditions thereof. ) The detected phase spectrum of the interference light is detected and stored in the storage unit 131. Then, based on the information of the phase spectrum stored in the storage unit 131 by the statistical analysis unit 132 of the arithmetic processing unit 130, the change direction matrix indicating the change direction of the phase fluctuation component by CEO for each of the detectors 118a and 118b. W ₁ and W ₂ are generated in advance and stored in the storage unit 131 (S1).

As described above, the value of Δθ / Δf is determined in advance based on the center of gravity G (θ _G1 θ _G2 θ _G3 ...) Used when obtaining the deformation direction matrix W = [w ₁ w ₂ w ₃ . You may keep it. For example, the value of Δθ / Δf may be determined in advance by setting the average value of phase differences (θ _{Gk + 1} −θ _Gk ) between adjacent modes of the center of gravity G as Δθ of Δθ / Δf.

  Thereafter, when the actual distance measuring apparatus 100 is in operation, the interference light is detected by the detector 118a (or the detector 118b), and the actual phase spectrum 501 of the detected interference light is acquired (S2).

Then, the change direction matrix W stored in the storage unit 131, the model of Expression 5, and the initial parameter P = [(Δθ / Δf) ₀ , Q ₀ ] arbitrarily set in advance by the CEO correction unit 133 are stored. Based on the above, the phase spectrum 502 on the model is constructed, and the phase spectrum 502 on the model is compared with the actual phase spectrum 501 (S3).

When the two values do not match within the predetermined threshold value, the value of the parameter P is adjusted by the CEO correction unit 133 (P ₀ to P ₁ ), and the phase spectrum 503 on the model is reconstructed. The phase spectrum 503 is compared with the actual phase spectrum 501 (S4).

The CEO correction unit 133 repeats these operations, and determines the parameter P when the phase spectrum 504 on the model matches the actual phase spectrum 501 within a predetermined threshold (S5). That is, the difference (θ′−θ) between the phase (θ ′) of the actual phase spectrum 501 and the phase (θ) of the phase spectrum on the model constructed by adjusting the parameter P takes the minimum value for each mode. The value of P = [Δθ / Δf, Q] is determined (argmin | θ′−θ | ² process).

Thereafter, the CEO correction unit 133 estimates the phase spectrum 505 of the phase fluctuation component θ ^CEO (= WQ) due to the CEO based on the determined coefficient matrix Q (S6). Thereafter, the CEO correction unit 133 can correct the fluctuation component θ ^CEO by subtracting the estimated phase spectrum 505 of the fluctuation component θ ^CEO from the actual phase spectrum 501. As a result, a phase spectrum 506 in which the fluctuation component θ ^CEO is corrected can be obtained.

In this way, by correcting the phase fluctuation component due to the CEO of the phase spectrum of the interference light detected by the detectors 118a and 118b, the distance calculation unit 133 can obtain a more accurate phase difference Δθ _{k + 1} ^b from Equation 4. ^−a− Δθ _k ^b−a can be obtained. As a result, the distance measuring device 100 can measure a more accurate distance to the workpiece 140.

  6A and 6B are flowcharts for measuring the distance to the workpiece 140 using the distance measuring apparatus 100. FIG.

In step S601, the distance measuring device 100 is arranged at a place such as a factory where the distance to the measurement object is actually measured, and operating conditions of various devices of the distance measuring device 100 are set. For example, adjustment of the output of the laser light source 111, adjustment of the modulation frequencies f _m1 and f _m2 of the first and second OFCGs 113a and 113b, adjustment of the modulation frequency f _α of the frequency shifter 114, and the like.

  In step S602, a reference body is placed at a position where the workpiece 140 is to be placed. The reference body is an object used to generate the change direction matrix W, and is, for example, a mirror surface. Note that a workpiece 140 used in actual distance measurement may be arranged instead of the reference body.

  In step S603, the optical frequency comb laser device 110 is operated, and the interference light between the measurement light Ps and the reference light Pr is detected by the detectors 118a and 118b. Detectors 118a and 118b output interference signals based on the detected interference light to FFTs 119a and 119b, respectively.

In step S604, the FFTs 119a and 119b perform FFT processing on the input interference signal, and calculate the beat frequency (mode) information of the detected interference light, the phase θ information (θ-F phase spectrum) for each mode, and the like. The data is output to the storage unit 131 of the device 130. The storage unit 131 stores and accumulates the detected phase spectrum of the interference light. Θ is the difference between the phase φ _s of the measurement light Ps and the phase φ _r of the reference light Pr.

  In step S605, the arithmetic processing unit 130 determines whether or not the phase spectrum has been accumulated a predetermined number of times for each of the detectors 118a and 118b. Here, the predetermined number of times may be several hundred to several thousand. If the predetermined number of times has not been performed (No in S605), Steps S603 to S605 are repeated.

In step S606, the statistical analysis unit 132 distributes the phase information (θ ₀ , θ ₁ ,... Θ _n ) of the accumulated phase spectrum in a predetermined (n + 1 dimension) phase space. The statistical analysis unit 132 obtains the center of gravity G (θ _G0 , θ _G1 ,... Θ _Gn ) of the _distribution , forms first to m-th principal component axes that pass through the center of gravity G and are orthogonal to each other, and the eigenvector w for each of them. ₁ , w ₂ ,... W _m are obtained. Here, the phase spectrum has 0 to n-order modes, and m is an integer of n + 1 or less.

In step S607, the statistical analysis unit 132, the eigenvectors _{w 1, w 2, ... w} m Sorting changing direction matrix _{W = [w 1, w 2} , ... w m] is determined and be stored in the storage unit 131 deep. The average of the phase differences (θ _{Gk + 1} −θ _Gk ) between adjacent modes of the center of gravity G (θ _G0 , θ _G1 ,... Θ _Gn ) is set as the value of Δθ of Δθ / Δf, and the value of Δθ / Δf is determined in advance. However, it may be stored in the storage unit 131. In this case, the parameter P described later is only the coefficient matrix Q. Note that the statistical analysis unit 132 generates the change direction matrix W for each of the detectors 118a and 118b, and the change direction matrix for the detector 118a is W ^a = [w ₁ ^a , w ₂ ^a _,. ^a ] and the change direction matrix for the detector 118b is W ^b = [w ₁ ^b , w ₂ ^b ,... w _m ^b ].

  Subsequently, as shown in FIG. 6B, in step S608, the work 140 is placed at a predetermined position manually by the operator or by an automatic transfer device (not shown). At this time, the lens focus of the lens 120 is adjusted. In step S609, the detectors 118a and 118b detect the interference light between the measurement light Ps and the reference light Pr from the optical frequency comb laser device 110.

  In step S610, detectors 118a and 118b output interference signals based on the detected interference light to FFTs 119a and 119b, respectively. The FFTs 119a and 119b perform FFT processing on the input interference signal, and the detected interference light beat frequency F (mode) information, phase θ information for each mode (θ-F phase spectrum), etc. The data is output to the storage unit 131 and stored in the storage unit 131. Here, an actual phase spectrum related to the detector 118a is a phase spectrum sa, and an actual phase spectrum related to the detector 118b is a phase spectrum sb.

In step S611, the CEO correction unit 133 sets the initial value P ₀ = [(Δθ / Δf) ₀ , Q ₀ ] of the parameter P stored in advance in the storage unit 131 and the deformation direction matrix generated in advance. Based on W ^a and W ^b , a phase spectrum on the model is constructed using Equation 5. Here, the phase spectrum on the model relating to the detector 118a is referred to as a phase spectrum ma, and the phase spectrum on the model relating to the detector 118b is referred to as a phase spectrum mb.

  In step S612, the CEO correction unit 133 determines whether or not the actual phase spectrum sa matches the phase spectrum ma on the model within a predetermined threshold value. Similarly, the CEO correction unit 133 determines whether or not the actual phase spectrum sb and the phase spectrum mb on the model match within a predetermined threshold value.

If the values do not match within the predetermined threshold (No in step S612), in step S613, the CEO correction unit 133 adjusts the parameter P by argmin | θ′−θ | ² processing or the like. The parameter P may be adjusted separately for each of the phase spectrum ma on the model and the phase spectrum mb on the model. The phase spectrum on the model is reconstructed using the adjusted parameter P (step S611), and steps S611 to S613 are repeated until the phase spectrum on the model matches the actual phase spectrum within a predetermined threshold. .

When the phase spectrum on the model and the actual phase spectrum match within the predetermined threshold (Yes in step S612), the CEO correction unit 133 determines in step S614 that the phase spectrum matches the predetermined threshold. From the parameter P, the values of the coefficient matrix Q and Δθ / Δf are determined. Here, the coefficient matrix for the detector 118a and ^{Q a,} the coefficient matrix for the detector 118b to ^{Q b.} Note that if the value of Δθ / Δf has been previously determined for each of the detectors 118a and 118b in step S607, the value of Δθ / Δf is not determined.

In step S615, the CEO correcting unit 133 estimates the phase fluctuation component θ ^CEO (= WQ) due to CEO from the determined coefficient matrices Q ^a and Q ^b, and the phase fluctuation component θ due to CEO estimated from the actual phase spectrum. ^The fluctuation component θ ^CEO is corrected by subtracting ^CEO . Actual subtracted theta ^CEOA is ⁽⁼ W ^{a Q} a) for the phase spectrum sa, for the actual phase spectrum sb subtracting ^{θ CEOb (= W b Q b} ). As a result, the phase fluctuation component due to the CEO can be corrected from the actual phase spectrum.

In step S616, the distance calculation unit 134, a phase theta _k ^a of the corrected k-order mode of the interference light detected by the detector 118a, the interference light detected by the detector 118b corrected k-order mode phase calculating the difference Δθ _k ^b-a of the theta _k ^b (equation 3), calculates the phase difference ^{_{Δθ k + 1 b-a -Δθ}} k b-a between the modes (equation 4).

  In step S617, the distance calculation unit 134 calculates the distance to the workpiece 140 based on the calculation result in step S616, stores the distance in the storage unit 131, and outputs it to an output device (not shown) such as a display. If there is another workpiece 140 to be measured in step S618, steps S608 to S618 are repeated, and if there is no workpiece 140 to be measured, the flow ends.

  As described above, the distance measuring apparatus 100 can more accurately measure the distance to the workpiece 140 using the phase information in which the phase fluctuation component by the CEO is corrected. Further, the distance measuring apparatus 100 can correct the phase fluctuation component caused by the CEO on the software, and the distance measuring apparatus 100 can add an additional device (hardware) for correcting the phase fluctuation component caused by the CEO as in the prior art. Wear) is not necessary. Therefore, the distance measuring device 100 can be used at an actual manufacturing site or the like, which leads to cost reduction.

(Measurement example)
FIG. 7 is an exemplary graph showing the relationship between the actual phase spectrum of this embodiment and the phase spectrum on the model. A black dotted line 701 is a phase spectrum that is simulated by a model that does not consider the phase fluctuation component θ ^{CEO due} to ^CEO . This model is a model that does not have the second term of Equation 5, and is a model in which θ = (Δθ / Δf) F, and is a model that adjusts only (Δθ / Δf) as a parameter.

On the other hand, the black solid line 702 is a phase spectrum that is simulated using a model (that is, Equation 5) that takes into account the phase fluctuation component θ ^CEO due to the CEO according to the present embodiment. A gray solid line 703 is a phase spectrum actually measured under the same conditions as in the simulation.

As can be seen from FIG. 7, the phase spectrum 702 using the model according to the present embodiment is closer to the actual phase spectrum 703 than the phase spectrum 701 of the model that does not consider the phase fluctuation component θ ^{CEO due} to ^CEO. I understood it. Therefore, it can be seen that the model according to this embodiment is useful for estimating the actual phase spectrum of the interference light detected by the detector more accurately.

(Other embodiments)
The statistical analysis method according to the present invention is not limited to principal component analysis (PCA), and methods such as multiple regression analysis, cluster analysis, factor analysis, discriminant analysis, and regression analysis may be used. Further, the present invention is not limited to the configuration of the optical frequency comb laser apparatus shown in FIG. 1, and can be applied to any application in which a phase fluctuation component due to phase CEO is to be corrected under a situation where the phase is to be accurately obtained.

100: Distance measuring device,
113a: first optical frequency comb generator (OFCG) (first optical generator)
113b: second optical frequency comb generator (OFCG) (second optical generator)
116: Reference plane 118a: First detector 118b: Second detector 130: Arithmetic processing device 131: Storage unit 132: Statistical analysis unit 133: CEO correction unit 134: Distance calculation unit 140: Workpiece (measurement object)

Claims (4)

A first light generator for generating first light having a mode of a first repetition frequency;
A second light generator for generating a second light having a second repetition frequency mode different from the first repetition frequency;
A first detector for detecting a first interference light between the first light and the second light emitted from the first and second light generators via a reference point;
A second detector for detecting a second interference light between the first light reflected from the measurement object and the second light reflected from the reference surface via the reference point;
A storage unit storing a change direction matrix (W) indicating a change direction of a phase fluctuation component (θ ^CEO ) due to a carrier envelope offset (CEO) included in the first and second interference lights;
It said first and second interference light model on the phase of the (theta) is represented by the sum of the predetermined phase ((Δθ / Δf) F) and the fluctuation component (theta ^CEO), the fluctuation component (theta ^CEO ) Based on a model (θ = (Δθ / Δf) F + WQ) represented by the product of the change direction matrix (W) and its coefficient matrix (Q), the phase (θ) on the model is the actual phase. The fluctuation component (θ ^CEO ) is estimated from the coefficient matrix (Q) adjusted to match (θ ′) within a predetermined threshold, and the estimated fluctuation component (θ ^CEO ) is calculated as the actual fluctuation component (θ ^CEO ). A CEO correction unit that corrects the fluctuation component (θ ^CEO ) by subtracting from the phase (θ ′);
The fluctuation component (theta ^CEO) wherein is corrected second interference light phase (theta ^b) and the fluctuation component (theta ^CEO) wherein is corrected first interference light phase (theta ^a) and position of the based on the phase difference a phase difference between the modes in ^{(Δθ b-a) (Δθ} k + 1 b-a -Δθ k b-a), and a distance calculation unit that calculates a distance to the measurement object from the reference point A distance measuring device. The phases (θ ₀ , θ ₁ ,... Θ _n ) of the first and second interference lights detected a plurality of times by the first and second detectors are distributed on a predetermined phase space, respectively, A center of gravity (θ _G1 , θ _G2 ,...) Of the distribution is obtained, a plurality of principal component axes passing through the center of gravity and orthogonal to each other are formed, and eigenvectors (w ₁ , w ₂ ,. The distance measuring device according to claim 1, further comprising a statistical analysis unit that generates the change direction matrix (W) including the eigenvectors (w ₁ , w ₂ ,...) By obtaining. The distance measuring device according to claim 2, wherein the predetermined phase ((Δθ / Δf) F) is determined based on a difference (θ _{Gk + 1} −θ _Gk ) between adjacent elements of the center of gravity. Detecting interference light having a mode of a predetermined repetition frequency a plurality of times, and distributing the phase (θ ₀ , θ ₁ ,... Θ _n ) of the interference light in a predetermined phase space;
Obtaining a center of gravity (θ _G1 , θ _G2 ,...) Of the distribution;
Forming a plurality of principal component axes passing through the center of gravity and orthogonal to each other, and generating a change direction matrix (W) composed of eigenvectors (w ₁ , w ₂ ,...) Of the plurality of principal component axes, A change direction matrix (W) indicates a direction of change of a phase fluctuation component (θ ^CEO ) due to a carrier envelope offset (CEO) included in the interference light, and
The phase (θ) on the model of the interference light is represented by the sum of a predetermined phase ((Δθ / Δf) F) and the fluctuation component (θ ^CEO ), and the fluctuation component (θ ^CEO ) is the change direction matrix. In the model (θ = (Δθ / Δf) F + WQ) represented by the product of (W) and its coefficient matrix (Q), the actual phase (θ of the interference light detected by the phase (θ) on the model is displayed. ') Estimating the fluctuation component (θ ^CEO ) from the coefficient matrix (Q) adjusted to match within a predetermined threshold;
Correcting the fluctuation component (θ ^CEO ) by subtracting the estimated fluctuation component (θ ^CEO ) from the actual phase (θ ′);
Calculating a distance from a reference point to a measurement object based on the corrected actual phase (θ ′).