JP2012145362A - Method for measuring absolute distance of laser interference measuring device, and laser interference measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for measuring an absolute distance capable of favorably measuring the absolute distance to an object to be measured.SOLUTION: The method for measuring an absolute distance includes: waveform changing steps S1A, S1D and S1G for changing a resonator length with the object to be measured retained stationary and changing the wavelength of a laser beam; phase detecting steps S1B, S1E, and S1H for detecting the phases of respective interference signals at respective time points when the oscillation frequency of the laser beam is matched with the respective frequencies of a plurality of saturated absorption lines; and a first absolute distance calculating step S1J for calculating a first absolute distance to the object to be measured based on the respective frequencies of the plurality of saturated absorption lines and the phases of the respective interference signals.

Description

  The present invention relates to a method for measuring an absolute distance of a laser interferometer and a laser interferometer.

Conventionally, for example, a tracking laser interferometer is known as an apparatus for measuring a distance to an object to be measured (see, for example, Patent Document 1).
The tracking laser interferometer described in Patent Document 1 includes a retroreflector, an optical system, a biaxial rotation mechanism, and a controller described below.
The retroreflector is attached to the object to be measured and reflects incident light in a direction along the incident direction.
The interferometer emits measurement light and outputs the following signals to the controller.
That is, the interferometer outputs a first signal based on interference between return light from the object to be measured (return light reflected by the retroreflector) and reference light for reference to the controller. The interferometer outputs a second signal based on the amount of deviation of the optical axis between the measurement light and the return light from the object to be measured to the controller.
The biaxial rotating mechanism changes the emission direction of the measurement light by changing the attitude of the interferometer under the control of the controller.

Based on the second signal from the interferometer, the controller controls the operation of the biaxial rotation mechanism so that the amount of deviation of the optical axis becomes zero, and tracks the retroreflector (the measurement light emission direction is changed). Point to the retroreflector).
The tracking laser interferometer described in Patent Document 1 measures the absolute distance to the object to be measured (retroreflector) by the following method without adding a distance sensor or the like for measuring the distance. ing.
In the technique described in Patent Document 1, attention is paid to the fact that there is a predetermined relationship among the deviation amount of the optical axis, the angle adjustment amount of the biaxial rotation mechanism, and the absolute distance to the object to be measured.
Then, the biaxial rotation mechanism is operated until the deviation amount of the optical axis becomes a known value, and based on the angle adjustment amount of the biaxial rotation mechanism and the deviation amount of the optical axis that is a known value at the time of the operation. Thus, the absolute distance to the object to be measured is calculated from the above relationship.

JP 2007-309677 A

  However, in the method described in Patent Document 1, the measurement object is limited to the dynamic range of a position detector (for example, a light receiving element) that detects the amount of deviation of the optical axis. There is a problem in that it is difficult to calculate the absolute distance to.

  An object of the present invention is to provide an absolute distance measuring method and a laser interference length measuring apparatus that can favorably calculate an absolute distance to an object to be measured.

  The absolute distance measurement method of the present invention irradiates the object to be measured with laser light of a specific wavelength by changing the resonator length based on the saturated absorption line of the absorption cell, and returns light from the object to be measured and a reference for reference. An absolute distance measuring method of a laser interference length measuring apparatus for measuring a length according to an interference signal based on interference with light, wherein the laser length is changed while the measured object is stationary, and the laser is measured. A wavelength changing step for changing the wavelength of light, a phase detecting step for detecting the phase of each interference signal at each time point when the oscillation frequency of the laser light matches each frequency of the plurality of saturated absorption lines, and And a first absolute distance calculating step of calculating a first absolute distance to the object to be measured based on each frequency of a plurality of saturated absorption lines and a phase of each interference signal.

In the present invention, the phase of each interference signal at each time point when a plurality of saturated absorption lines are observed when the wavelength of the laser beam is scanned, each frequency of the plurality of saturated absorption lines, and the first to the object to be measured. It was noted that there is a predetermined relationship with the absolute distance of 1.
In the absolute distance measuring method of the present invention, the phase of each interference signal at each time point is detected by the wavelength changing step and the phase detecting step described above, and the predetermined relationship is obtained by the first absolute distance calculating step. By using this, the first absolute distance to the object to be measured is calculated.
Accordingly, the absolute distance to the object to be measured can be favorably calculated without being limited to the dynamic range of the position detector as in the prior art.

Further, as a laser interference length measuring device, the object to be measured moves only linearly so as to be closely separated from the length measuring device without moving in three dimensions, and the absolute distance to such a device to be measured is determined. In the case of a measurement structure (a structure that does not require a change in the laser beam emission direction), a two-axis rotation mechanism and a position detector such as a conventional tracking interferometer are unnecessary.
In such a laser interference length measuring device, when measuring the absolute distance to the object to be measured, if the absolute distance measuring method of the present invention is implemented, a biaxial rotating mechanism and a position detector are not added separately. The absolute distance to the object to be measured can be measured well.

In the absolute distance measuring method of the present invention, in the wavelength changing step, the resonator length is changed so that the oscillation frequency of the laser light matches each frequency of three or more saturated absorption lines, and the phase detection is performed. The step detects phases of the interference signals at three or more time points, and the first absolute distance calculating step includes the frequencies of the three or more saturated absorption lines and the three or more interference signals. Preferably, the first absolute distance is calculated on the basis of the phase.
In the present invention, since the first absolute distance is calculated based on the phase of each interference signal at three or more time points and the frequency of three or more saturated absorption lines, the first absolute distance is calculated. Can be calculated with high accuracy.

In the absolute distance measuring method of the present invention, a movement amount calculating step of calculating a movement amount of the object to be measured based on the interference signal, and a second absolute distance based on the first absolute distance and the movement amount. It is preferable to provide a second absolute distance calculating step for calculating the distance.
In the present invention, the absolute distance measuring method includes the movement amount calculating step and the second absolute distance calculating step described above.
Thus, when measuring the absolute distance to the object to be measured, for example, the movement amount of the object to be measured is sequentially calculated with the movement of the object to be measured (movement amount calculating step). Then, the second absolute distance is calculated by adding or subtracting the calculated movement amount to the first absolute distance (initial position) calculated in the first absolute distance calculating step (second absolute distance calculating step). .
If the second absolute distance calculated in this way is the absolute distance to the object to be measured, the length measurement is performed in comparison with the method of sequentially performing the first absolute distance calculating step as the object to be measured moves. Can be implemented easily and quickly.

In the absolute distance measuring method of the present invention, it is preferable to include a movement amount correction step of correcting the movement amount based on the second absolute distance.
By the way, when the laser beam used for length measurement is modulated by the modulation signal, or when the air refractive index fluctuates during length measurement, etc., the movement amount calculated in the movement amount calculation step is reduced. An error corresponding to the position of the measurement object (second absolute distance) is included.
In the present invention, since the absolute distance measuring method includes the above-described movement amount correction step, an error corresponding to the position of the above-described object to be measured included in the movement amount is removed, and the movement amount is set to an appropriate value. it can.

In the absolute distance measuring method of the present invention, the laser interference length measuring device irradiates the measured object with the laser beam modulated with a modulation signal having a predetermined modulation frequency and a predetermined modulation amplitude, and the movement amount correcting step Preferably, the movement amount is corrected based on the second absolute distance, the modulation frequency, and the modulation amplitude.
In the present invention, since the movement amount correction step corrects the movement amount based on the second absolute distance, the modulation frequency, and the modulation amplitude, the error caused by the modulation of the laser beam is appropriately removed, and the movement amount is made appropriate. It can be set to any value.

In the absolute distance measuring method of the present invention, the laser interference length measuring device includes a refractive index calculating device that calculates an air refractive index, and the movement amount correcting step includes the second absolute distance and the movement of the object to be measured. It is preferable to correct the amount of movement based on the amount of change in the air refractive index before and after.
In the present invention, the movement amount correction step corrects the movement amount based on the second absolute distance and the amount of change in the air refractive index before and after the movement of the object to be measured. Appropriate removal is possible, and the amount of movement can be set to an appropriate value.

The laser interference length measuring apparatus of the present invention is configured to be capable of changing the resonator length, irradiates the laser beam to the object to be measured, a laser beam generator that generates laser light of a specific wavelength, and the object to be measured An interferometer that outputs an interference signal based on the interference between the return light from the reference light and the reference light for reference, and a control device that controls the operation of the laser light generation unit, wherein the control device includes the laser light generation unit A wavelength control unit that changes the resonator length based on a saturated absorption line of an absorption cell and changes the wavelength of the laser light, and the oscillation frequency of the laser light has a plurality of saturated absorption lines. A phase detector that detects the phase of each interference signal at each time point that matches each frequency, and each frequency of the plurality of saturated absorption lines and the phase of each interference signal, to the device under test Calculating the first absolute distance Characterized in that it comprises an absolute distance calculator.
In the present invention, since the laser interference length measuring device is a device that implements the above-described absolute distance measuring method, it can enjoy the same operations and effects as the above-described absolute distance measuring method.

The block diagram which shows the laser interference length measuring apparatus in 1st Embodiment. The block diagram which shows the laser beam production | generation part in 1st Embodiment. The block diagram which shows the control apparatus in 1st Embodiment. The figure for demonstrating the process by the wavelength control part in 1st Embodiment. The figure for demonstrating the process by the phase detection part in 1st Embodiment. The figure for demonstrating the process by the phase detection part in 1st Embodiment. The flowchart explaining the absolute distance measuring method in 1st Embodiment. The flowchart explaining the calculation process of the 1st absolute distance in 1st Embodiment. The flowchart explaining the calculation process of the 1st absolute distance in 2nd Embodiment. The figure for demonstrating the absolute distance measuring method in 2nd Embodiment.

[First embodiment]
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a first embodiment of the invention will be described with reference to the drawings.
[Configuration of laser interference length measuring device]
FIG. 1 is a block diagram showing a laser interference length measuring device 1 (hereinafter referred to as a length measuring device 1) in the first embodiment.
The length measuring device 1 is a device that measures the absolute distance from the length measuring device 1 to an object to be measured (a corner cube 20 described later) by using interference of laser light.
As shown in FIG. 1, the length measuring device 1 includes a laser light generation unit 10, a corner cube 20, an interferometer 30, a refractive index calculation device 40, a differential detection circuit 50, and a control device 60. Prepare.

[Configuration of laser beam generator]
FIG. 2 is a block diagram showing the laser light generation unit 10.
As shown in FIG. 2, the laser light generation unit 10 includes a laser generation unit 11, a laser light detection unit 12, a modulation / demodulation signal generator 13, an actuator drive circuit 14, and a lock-in amplifier 15.
The laser generator 11 includes an excitation semiconductor laser 111 that emits a laser beam L1 having a wavelength of 808 nm, and a resonance wave generator 112 that receives the laser beam L1 and outputs a laser beam L2 having a wavelength of 532 nm.
The resonant wave generating unit 112 includes an Nd: YVO4 crystal 112A that emits light having a wavelength of 1064 nm from induced radiation, a KTP crystal (nonlinear optical crystal) 112B that reflects light having a wavelength of 1064 nm and makes light having a wavelength of 532 nm, and a specific frequency of laser light. An optical element such as an etalon 112C that transmits only light and a reflecting mirror 112D that reflects light having a wavelength of 1064 nm and transmits light having a wavelength of 532 nm is housed in a resonator housing 112E.
The single-mode laser beam L2 can be obtained by disposing the etalon 112C inside the resonator casing 112E.
In addition, an actuator 112F such as a piezo element that changes the position of the reflecting mirror 112D (changes the resonator length) by applying a voltage is disposed inside the resonator housing 112E.

As shown in FIG. 2, the laser light detector 12 includes a λ / 2 plate 121, polarizing beam splitters 122 and 123, a λ / 4 plate 124, an iodine cell (absorption cell) 125, a reflecting mirror 126, A photodetector 127.
Then, after passing through the λ / 2 plate 121, the laser light L2 is separated by the polarization beam splitter 122 into laser light L3 used for length measurement and the like and laser light L4 used for frequency stabilization control described later. Is done.
Further, the laser beam L4 passes through the polarization beam splitter 123, the λ / 4 plate 124, and the iodine cell 125, and then is reflected by the reflecting mirror 126 toward the iodine cell 125.
The laser beam L4 passes through the iodine cell 125 and the λ / 4 plate 124 again, is reflected by the polarization beam splitter 123 toward the photodetector 127, is photoelectrically converted by the photodetector 127, and is emitted. Output as an output signal S1.

The modulation / demodulation signal generator 13 outputs a modulation signal Sm1 having a modulation frequency of 1 fHz to the actuator drive circuit 14, and outputs modulation signals Sm2 and Sm3 having a frequency of 2f and 3fHz to the lock-in amplifier 15.
The actuator drive circuit 14 drives the actuator 112F under the control of the control device 60 (applies a voltage V to the actuator 112F), and modulates the laser light L2 with the modulation signal Sm1 from the modulation / demodulation signal generator 13.
The lock-in amplifier 15 demodulates the optical output signal S1 obtained by exciting the laser light L2 modulated based on the modulation signal Sm1 by the actuator driving circuit 14 with the modulation signals Sm2 and Sm3 having the frequencies 2f and 3fHz, respectively. Next, third differential signals S2 and S3 are output, respectively.

[Composition of corner cube]
The corner cube 20 reflects incident light along the incident direction, and is attached to an object to be measured (not shown).
In the present embodiment, the object to be measured moves so as to be linearly close to and separated from the interferometer 30. That is, the same applies to the corner cube 20.

[Configuration of interferometer]
The interferometer 30 emits the laser beam L3 generated by the laser beam generator 10 to the object to be measured (corner cube 20), and returns light from the object to be measured (return light reflected by the corner cube 20). ) And interference signals SiA to SiD based on the interference with the reference light for reference are output.
In the present embodiment, the interferometer 30 is a known Michelson interferometer.
As shown in FIG. 1, the interferometer 30 includes λ / 2 plates 31, 36, polarizing beam splitters 32, 39A, 39B, λ / 4 plates 33, 35, 38, a corner cube 34, and a beam splitter 37. And PD (Photo Detector) 30A-30D.
After the laser light L3 generated by the laser light generation unit 10 and input to the interferometer 30 passes through the λ / 2 plate 31, the polarization beam splitter 32 and the reference light Lr for reference which is an S-polarized component, Separated into measurement light Lm for measurement.
Further, after passing through the λ / 4 plate 33, the reference light Lr is reflected by the corner cube 34 toward the λ / 4 plate 33 and passes again through the λ / 4 plate 33 (through the λ / 4 plate 33). Passing twice) becomes P-polarized light and passes through the polarization beam splitter 32.

On the other hand, the measurement light Lm for measurement passes through the λ / 4 plate 35 and is then emitted to the object to be measured.
Further, the measurement light Lm is reflected by the corner cube 20 toward the interferometer 30 (λ / 4 plate 35) and passes again through the λ / 4 plate 35 (passes through the λ / 4 plate 35 twice). Thus, it becomes S-polarized light and is reflected by the polarization beam splitter 32.

Then, the reference light Lr that has passed through the polarization beam splitter 32 and the measurement light Lm reflected by the polarization beam splitter 32 become interference light Li.
After the interference light Li passes through the λ / 2 plate 36, it is separated into two interference lights Li1 and Li2 by the beam splitter 37.
After the interference light Li1 passes through the λ / 4 plate 38, it is separated into an S-polarized component and a P-polarized component by the polarization beam splitter 39A.
The S-polarized component and the P-polarized component are photoelectrically converted by the PDs 30A and 30B, respectively, and output as interference signals SiA and SiB, respectively.

On the other hand, the interference light Li2 is separated into an S-polarized component and a P-polarized component by the polarization beam splitter 39B.
The S-polarized component and the P-polarized component are photoelectrically converted by the PDs 30C and 30D, respectively, and output as interference signals SiC and SiD, respectively.
Note that, since the above-described optical path is traced, the phases of the interference signals SiA to SiD are shifted by 90 °.

  For example, when the plane waves Er and Em of the reference light Lr and the measurement light Lm are represented by the following formula (1), the intensity I of light irradiated on any of the four PDs 30A to 30D is represented by the following formula (2 ).

Here, in Equation (2), λ ₀ , f, and c are the vacuum wavelength, the oscillation frequency of the laser light L2, and the light velocity, respectively.
Further, d is the optical path length until the reference light Lr reaches the corner cube 34. d ′ is the optical path length until the measurement light Lm reaches the corner cube 20.
In the present embodiment, the origin is a location where d′−d = 0.

[Configuration of refractive index calculation device]
The refractive index calculating device 40 includes a temperature sensor, an atmospheric pressure sensor, a humidity sensor, and an arithmetic device, although not specifically shown.
The temperature sensor, the atmospheric pressure sensor, and the humidity sensor respectively measure the temperature, atmospheric pressure, and humidity in the vicinity of the optical path of the measurement light Lm output from the interferometer 30 to the corner cube 20.
The computing device calculates the air refractive index based on the temperature, atmospheric pressure, and humidity measured by each sensor.
For example, the arithmetic device calculates the air refractive index by substituting the values of temperature, atmospheric pressure, and humidity measured by the above sensors into Edren's empirical formula.
Then, the refractive index calculating device 40 outputs a signal Sn corresponding to the calculated air refractive index.

[Differential detection circuit]
The differential detection circuit 50 inputs interference signals SiA to SiD from the interferometer 30. The differential detection circuit 50 differentially detects the interference signals SiA and SiB, differentially detects the interference signals SiC and SiD, and outputs a two-phase interference signal Si that is 90 ° out of phase.

[Configuration of control device]
FIG. 3 is a block diagram showing the control device 60.
The control device 60 includes a CPU (Central Processing Unit), a memory, and the like, and executes various processes according to programs stored in the memory.
Specifically, the control device 60 controls the operations of the laser light generation unit 10 and the refractive index calculation device 40, and also includes the two-phase interference signal Si from the differential detection circuit 50 and the signal from the refractive index calculation device 40. Based on Sn, the absolute distance to the object to be measured is measured.
As shown in FIG. 3, the control device 60 includes a wavelength control unit 61, a phase detection unit 62, a first absolute distance calculation unit 63, a movement amount calculation unit 64, a second absolute distance calculation unit 65, A movement amount correction unit 66 and a memory 67 are provided.

FIG. 4 is a diagram for explaining processing by the wavelength control unit 61. Specifically, FIG. 4 is a diagram showing saturated absorption lines in the wavelength band of 532.245 nm.
4A and 4B show the case where the output value of the secondary and tertiary differential signals S2 and S3 is the vertical axis, the frequency is the horizontal axis, and the resonator length is changed (laser beam). It is a figure which each shows the waveform of the secondary and tertiary differential signal S2, S3 in the case of changing the wavelength (oscillation frequency) of L2. FIG. 4C is an enlarged view of the region Ar1 in FIG. FIG. 4D is an enlarged view of the region Ar2 in FIG.
As shown in FIG. 4B, saturated absorption lines a1 to a15 are observed in order from the lower frequency side in the secondary differential signal S2. Further, as shown in FIG. 4A, saturated absorption lines having substantially the same characteristics as the secondary differential signal S2 are also observed in the tertiary differential signal S3.

The wavelength control unit 61 controls the operation of the actuator drive circuit 14 based on the secondary and tertiary differential signals S2 and S3 from the interferometer 30 (adjusts the voltage V applied to the actuator 112F). Implement frequency stabilization control.
That is, the wavelength controller 61 changes the voltage V applied to the actuator 112F to a voltage V that matches the frequency at which the oscillation frequency of the laser light L2 is the center of a specific saturated absorption line, and then performs second and third order differentiation. The signals S2 and S3 are always observed.
Then, the wavelength control unit 61 makes the output value of the secondary differential signal S2 equal to or higher than a predetermined voltage value (a voltage value that can be recognized as a saturated absorption line), and the output value of the tertiary differential signal S3 is close to 0V. The voltage V is adjusted.
By such frequency stabilization control, the oscillation frequency of the laser light L2 is stabilized at a frequency that becomes the center of a specific saturated absorption line.
In other words, the wavelength of the laser beam L2 is stabilized to a wavelength corresponding to the frequency that is the center of the specific saturated absorption line.

5 and 6 are diagrams for explaining processing by the phase detection unit 62. FIG. Specifically, FIG. 5 is a diagram showing a change in intensity (amplitude) of the interference signal Si with time. FIG. 6 is a diagram showing a Lissajous waveform due to the two-phase interference signal Si.
5 and 6, in the two-phase interference signal Si, an interference signal in which the interference signals SiA and SiB are differentially detected is defined as an interference signal SiAB, and an interference signal in which the interference signals SiC and SiD are differentially detected is illustrated. The interference signal SiCD is used.
The phase detector 62 detects the phase φ of the interference signal Si based on the two-phase interference signal Si from the differential detection circuit 50, and calculates the variation Δφ of the phase φ of the interference signal Si.
As described above, since the two-phase interference signal Si is 90 degrees out of phase, it becomes a substantially circular Lissajous signal as shown in FIGS.
Then, the phase detector 62 detects the change in the phase L of the interference signal Si by detecting the change in the Lissajous signal.

Specifically, the phase detection unit 62 increments the count value N by 1 each time the Lissajous signal changes by one turn in a predetermined direction. The phase detector 62 counts down the count value N by 1 each time the Lissajous signal changes by one turn in the direction opposite to the predetermined direction.
The count value N corresponds to a value obtained by normalizing the variation Δφ of the phase φ of the interference signal Si by 2π.
Further, the phase detector 62 equally divides the Lissajous signal into several hundred to several thousand within one round of the Lissajous signal, and changes the count value N according to the change of the Lissajous signal.
For example, when the Lissajous signal is divided into 100 equal parts, the phase detection unit 62 changes the count value N by 0.01 according to the change of the Lissajous signal.

The first absolute distance calculation unit 63 is based on the signal Sn from the refractive index calculation device 40, the frequency of the saturated absorption line stored in the memory 67, and the count value N counted by the phase detection unit 62. A first absolute distance (geometric distance) to the object to be measured is calculated.
The movement amount calculation unit 64 is based on the signal Sn from the refractive index calculation device 40, the frequency of the saturated absorption line stored in the memory 67, and the count value N counted by the phase detection unit 62. The amount of movement (geometric distance) of the object is calculated.

The second absolute distance calculation unit 65 adds or subtracts the movement amount calculated by the movement amount calculation unit 64 to the first absolute distance calculated by the first absolute distance calculation unit 63, thereby measuring the measurement target. A second absolute distance to the object is calculated.
The movement amount correction unit 66 corrects the movement amount calculated by the movement amount calculation unit 64 based on the second absolute distance calculated by the second absolute distance calculation unit 65.
In addition, the specific process of each structure 63-65 is demonstrated in detail when demonstrating the following absolute distance measuring methods.
The memory 67 stores a control program and information used in each of the configurations 61 to 66 described above.

[Absolute distance measurement method]
FIG. 7 is a flowchart for explaining the absolute distance measuring method.
Next, an absolute distance measuring method using the length measuring apparatus 1 described above will be described.
As shown in FIG. 7, the absolute distance measurement method includes a first absolute distance calculation step (S1), a movement amount calculation step (S2), a second absolute distance calculation step (S3), and a movement amount correction step. (S4) is performed in order.

[First Absolute Distance Calculation Step (S1)]
FIG. 8 is a flowchart for explaining the first absolute distance calculation step S1.
The user operates the length measuring device 1 in a state where the object to be measured is stationary at the initial position, and causes the control device 60 to calculate the first absolute distance to the object to be measured (initial position) (step S1). .
The first absolute distance calculation step S1 is performed when the length measuring device 1 is started.
First, wavelength control unit 61, stabilizes the first wavelength lambda ₁ corresponding to the frequency which is the center of the wavelength of the laser beam L2 by the frequency stabilization control described above first saturated absorption line (step S1A).
Here, the first saturated absorption line is, for example, a saturated absorption line a10 (FIG. 4).
Next, the phase detector 62 counts the change in the Lissajous signal and stores the count value N in the memory 67 (step S1B).
Next, the control device 60 causes the refractive index calculation device 40 to calculate the air refractive index. Then, the controller 60 receives the signal Sn from the refractive index calculator 40, and stores the calculated air refractive index n of the _{(n 1)} in the memory 67 (step S1C).

Then, the wavelength control unit 61, stabilizes the second wavelength lambda ₂ of the wavelength corresponding to the frequency which is the center of the second saturated absorption line of the laser beam L2 by the frequency stabilization control described above (Step S1D) .
Here, the second saturated absorption line is, for example, a saturated absorption line a1 (FIG. 4).
Next, the phase detector 62 counts the change of the Lissajous signal and stores the count value N in the memory 67 (step S1E), as in step S1B.
Next, similarly to step S1C, the control device 60 stores the air refractive index n (n ₂ ) calculated by the refractive index calculation device 40 in the memory 67 (step S1F).

Then, the wavelength control unit 61, the wavelength of the laser beam L2 by the frequency stabilization control described above the first wavelength lambda ₁ again, stabilize (step S1G).
Next, the phase detection unit 62 counts the change of the Lissajous signal as in steps S1B and S1E, and stores the count value N in the memory 67 (step S1H).
Next, similarly to steps S1C and S1F, the control device 60 stores the air refractive index n (n ₁ ) calculated by the refractive index calculation device 40 in the memory 67 (step S1I).
Steps S1A, S1D, and S1G described above correspond to the wavelength changing step according to the present invention. Steps S1B, S1E, and S1H correspond to the phase detection step according to the present invention.

Next, the first absolute distance calculation unit 63 calculates a first absolute distance as described below (step S1J: first absolute distance calculation step).
Here, the first absolute distance L is given by the following equation (3).

In equation (3), n is the air refractive index and λ is the wavelength of the laser beam L2. In the expression (3), M is φ / (2π).
Here, when the wavelength of the laser beam L2 is changed by δλ while the object to be measured is stationary (with the first absolute distance L being constant), the phase of the interference signal Si is changed and M is only δM. Suppose it changes.
In this case, the first absolute distance L is given by the following equation (4).

  Then, by removing M from the equations (3) and (4), the following equation (5) is obtained.

In step S1A～S1F, since the wavelength of the laser light L2 from the first wavelength lambda ₁ is changed to the second wavelength lambda _2, in the formula (5), the lambda and lambda _1, the lambda + [delta] [lambda] lambda ₂ Then, the equation (5) can be transformed into the following equation (6).

The first absolute distance calculator 63 uses the equation (6), the process of the wavelength of the laser beam L2 is changed from the first wavelength lambda ₁ to the second wavelength lambda ₂ (step S1A～S1F) 1, from the information stored in the memory 67 (two count values N, air refractive indexes n ₁ , n ₂ , and frequencies that are the centers of the first and second saturated absorption lines), the first first The absolute distance of is calculated.
Incidentally, .DELTA.M, because the wavelength of the laser beam L2 is equivalent to the phase change of the interference signal Si at the time of being changed from the first wavelength lambda ₁ to the second wavelength lambda _2, the memory 67 at step S1B It can be obtained from the stored count value N and the count value N stored in the memory 67 in step S1E.
Further, the wavelengths λ ₁₀ and λ _{20 in} vacuum of the first and second wavelengths λ ₁ and λ ₂ are derived from the frequencies that are the centers of the first and second saturated absorption lines stored in the memory 67 in advance. Can be sought.

Similarly, the first absolute distance calculator 63 uses the equation (6), the wavelength of the laser beam L2 is the second wavelength lambda ₂ from the first step was changed to a wavelength lambda ₁ (Step S1D In S1I), the second first absolute distance is calculated from the information stored in the memory 67.
Note that δM can be obtained from the count value N stored in the memory 67 in step S1E and the count value N stored in the memory 67 in step S1H.
Next, the first absolute distance calculation unit 63 averages the two calculated first absolute distances (step S1K).

[Movement amount calculation step (S2)]
In the subsequent steps S2 to S4, after the length measuring device 1 is started (after step S1), the wavelength of the laser light L2 is set according to the frequency at the center of the specific saturated absorption line by the above-described frequency stabilization control. This is a step of calculating a movement amount or the like when the object to be measured actually moves in a state where the wavelength is stabilized.
Here, the moving amount D of the object to be measured is given by the following equation (7).

Here, in Formula (7), n is an air refractive index after the to-be-measured object moves. Further, λ ₀ is a wavelength in a vacuum having a wavelength corresponding to the frequency that is the center of the specific saturated absorption line. Further, N is a value (Δφ / 2π) obtained by normalizing the phase variation Δφ of the interference signal Si before and after the movement of the object to be measured by 2π. If the count value N of the phase detection unit 62 is reset to 0 before the movement of the object to be measured, the count value N counted by the phase detection unit 62 after the movement of the object to be measured is expressed by the equation (7). Equivalent to N in the middle.

The movement amount calculation unit 64 is stored in the memory 67 with the air refractive index n calculated by the refractive index calculation device 40 after the measurement object is moved and the count value N counted by the phase detection unit 62. The displacement D is calculated by substituting the wavelength λ ₀ in vacuum obtained from the frequency that becomes the center of the specific saturated absorption line into the equation (7).

[Second Absolute Distance Calculation Step (S3)]
The second absolute distance calculation unit 65 adds the movement amount D calculated in step S2 to the first absolute distance to the object before measurement (positioned at the initial position) averaged in step S1K. Alternatively, the second absolute distance to the object to be measured after movement is calculated by subtraction (step S3: second absolute distance calculation step).

[Movement Correction Step (S4)]
Incidentally, the laser light L2 is modulated based on the modulation signal Sm1. When the laser beam L2 is thus modulated, assuming that the modulation frequency (1 fHz) of the modulation signal Sm1 is Ω and the amplitude is λ ′, the original phase φ ′ of the interference signal Si is expressed by the following equation (8): Given in.

  Here, assuming that λ >> λ ′, the equation (8) can be changed to the following equation (9).

That is, in step S2, since the movement amount D is calculated not using the original phase φ ′ but using the phase φ (corresponding to the count value N), according to the second term on the rightmost side of the equation (9). Error is included.
Here, (d′−d) in the second term on the rightmost side of Equation (9) corresponds to the second absolute distance to the measured object after movement calculated in step S3.
Therefore, the movement amount correction unit 66 calculates the second absolute distance calculated in step S3, the modulation frequency Ω and the amplitude λ ′ stored in the memory 67, and the specific saturated absorption line stored in the memory 67. Based on the vacuum wavelength λ ₀ obtained from the center frequency, an error corresponding to the second term on the rightmost side of Equation (9) is calculated.
Further, the movement amount correction unit 66 corrects the movement amount D by removing the calculated error from the movement amount D calculated in step S2.

And the control apparatus 60 outputs the 2nd absolute distance calculated in step S3, and the movement amount D correct | amended in step S4 as a measurement result.
The second absolute distance to be output may be a value obtained by adding the movement amount D corrected in step S4 to the first absolute distance averaged in step S1K.

The first embodiment described above has the following effects.
In the present embodiment, the phase of each interference signal Si (corresponding to δM) at each time point when a plurality of saturated absorption lines are observed when the wavelength of the laser beam L2 is scanned, and each frequency ( It was noted that there is a relationship of equation (6) between the wavelengths λ ₁₀ and λ ₂₀ in vacuum) and the first absolute distance L to the object to be measured.
In the absolute distance measuring method of the present embodiment, the phase of each interference signal at each time point is detected by the wavelength changing steps S1A, S1D, S1G and the phase detecting steps S1B, S1E, S1H, and the first absolute distance calculating step The first absolute distance L is calculated by using Equation (6) from S1J.
Accordingly, the absolute distance to the object to be measured can be favorably calculated without being limited to the dynamic range of the position detector as in the prior art.

Further, as the length measuring device 1, as in the present embodiment, the object to be measured moves only linearly so as to be closely separated from the length measuring device 1, and the absolute distance to the object to be measured is measured. In the case of the configuration, a two-axis rotation mechanism and a position detector such as a conventional tracking interferometer are not necessary.
And, when measuring the absolute distance to the object to be measured in the length measuring device 1 as in the present application, if the absolute distance measuring method of the present application is performed, a biaxial rotation mechanism and a position detector are added separately. The absolute distance to the object to be measured can be measured well.

  Further, in the first absolute distance calculating step S1, two first absolute distances are calculated as the first absolute distance calculated in the step S1, and the two first absolute distances are averaged. The thing is adopted. For this reason, the first absolute distance calculated in step S1 can be made highly accurate.

The absolute distance measuring method includes a movement amount calculating step S2 and a second absolute distance calculating step S3.
Thus, when measuring the absolute distance to the object to be measured, the movement amount D of the object to be measured is sequentially calculated along with the movement of the object to be measured (movement amount calculation step S2). Then, the second absolute distance is calculated by adding or subtracting the calculated movement amount D to the first absolute distance L calculated in the first absolute distance calculating step S1 (second absolute distance calculating step S3). ).
If the second absolute distance calculated in this way is the absolute distance to the object to be measured, it is sequentially compared with the method of performing the first absolute distance calculating step S1 as the object to be measured moves. Measure length easily and quickly.

Furthermore, since the absolute distance measurement method includes the movement amount correction step S4, an error corresponding to the position of the object to be measured included in the movement amount D can be removed, and the movement amount D can be set to an appropriate value.
In particular, since the movement amount D is corrected based on the second absolute distance, the modulation frequency Ω, and the modulation amplitude λ ′, an error caused by the modulation of the laser light L2 is appropriately removed, and the movement amount D is set to an appropriate value. It can be.

[Second Embodiment]
Next, a second embodiment of the present invention will be described.
In the following, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.
In the present embodiment, the first absolute distance calculation step S1 is different from the first embodiment. Other steps and the configuration of the length measuring device 1 are the same as those in the first embodiment.

FIG. 9 is a flowchart for explaining the first absolute distance calculating step S1 in the second embodiment.
The user operates the length measuring device 1 with the measurement object stationary, and causes the control device 60 to execute the following processing.
First, the wavelength control unit 61 stabilizes the wavelength of the laser light L2 to the wavelength corresponding to the frequency that is the center of a predetermined saturated absorption line by the above-described frequency stabilization control (step S10A: wavelength changing step).

Next, the phase detector 62 counts the change in the Lissajous signal and stores the count value N in the memory 67 (step S10B: phase detection step).
Next, the control device 60 causes the refractive index calculation device 40 to calculate the air refractive index. And the control apparatus 60 inputs the signal Sn from the refractive index calculation apparatus 40, and memorize | stores the calculated air refractive index n in the memory 67 (step S10C).
And the control apparatus 60 repeatedly implements step S10A-S10C until it stabilizes the wavelength of the laser beam L2 to each wavelength according to each frequency used as the center of all the saturated absorption lines a1-a15 in step S10A. (Step S10D).

After step S10A is completed (after the scanning of the wavelength of the laser beam L2 is completed), the first absolute distance calculation unit 63 calculates the first absolute distance as described below (step S10E: first absolute) Distance calculation step).
Here, the first absolute distance L is given by the following equation (10) when the oscillation frequency of the laser beam L2 is f and the light velocity is c.

FIG. 10 is a diagram for explaining a first absolute distance calculation method.
Therefore, as shown in FIG. 10, the horizontal axis is the change amount δ (2 nf / c) of 2 nf / c, and the vertical axis is the change amount δM of M, which is stored in the memory 67 by repeatedly performing steps S10A to S10C. The amount of change δ (2 nf / c) and the amount of change δM are obtained from the obtained information, etc., and the obtained point group (P1 to P4 in FIG. 10) is linearly approximated by the least square method, and the inclination is obtained. Thus, the first absolute distance L is obtained.
In FIG. 10, only four sets (P1 to P4) of the change amount δ (2 nf / c) and the change amount δM are illustrated for convenience of explanation.

For example, when the wavelength of the laser beam L2 is stabilized to a wavelength corresponding to the frequency that becomes the center of the saturated absorption line a1 (hereinafter, referred to as the first time point), the wavelength corresponds to the frequency that becomes the center of the saturated absorption line a2. The amount of change δ (2 nf / c) and the amount of change δM between the stabilized time points (hereinafter referred to as the second time point) can be obtained as follows.
That is, the amount of change δ is based on the air refractive index n stored in the memory 67 at the first and second time points and the frequencies at the centers of the saturated absorption lines a1 and a2 stored in the memory 67 in advance. (2nf / c) can be obtained.
Further, the change amount δM can be obtained based on the respective count values N stored in the memory 67 at the first and second time points.
As described above, the first absolute distance calculation unit 63 calculates the first absolute distance L.

According to the second embodiment described above, there are the following effects in addition to the same effects as in the first embodiment.
In the present embodiment, the wavelength of the laser light L2 is stabilized to each wavelength corresponding to each frequency that is the center of all the saturated absorption lines a1 to a15, and the phase of each interference signal Si at the time of stabilization and the saturation. Since the first absolute distance L is calculated based on the frequencies that are the centers of the absorption lines a1 to a15, it is possible to calculate the first absolute distance L with higher accuracy than in the first embodiment. .

[Third embodiment]
Next, a third embodiment of the present invention will be described.
In the following, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.
In the first embodiment, the movement amount correction unit 66 corrects the movement amount D so as to remove an error included in the movement amount D due to the laser light L2 being modulated by the modulation signal Sm1.
On the other hand, in the third embodiment, the movement amount correction unit 66 includes the second absolute distance calculated by the second absolute distance calculation unit 65 and the air refractive index calculated by the refractive index calculation device 40. Based on the above, the movement amount D is corrected.

Specifically, the movement amount correction unit 66 in the third embodiment corrects the movement amount D as described below.
That is, the movement amount correction unit 66 sets the second absolute distance calculated by the second absolute distance calculation unit 65 as _DL, and the air refraction before and after the movement of the measurement object calculated by the refractive index calculation device 40. the variation rate in the case of the [Delta] n, the [Delta] n · D _L is the change in optical path length added to or subtracted to the amount of movement D, and corrects the amount of movement D.

According to the third embodiment described above, there are the following effects in addition to the same effects as in the first embodiment.
In the present embodiment, the movement amount correction step S4 corrects the movement amount D based on the second absolute distance and the amount of change in the air refractive index n before and after the movement of the object to be measured. The error caused by the fluctuation can be appropriately removed, and the movement amount D can be set to an appropriate value.

Note that the present invention is not limited to the above-described embodiment, and modifications, improvements, and the like within a scope in which the object of the present invention can be achieved are included in the present invention.
In each of the above embodiments, the second absolute distance is calculated by adding or subtracting the movement amount D to or from the first absolute distance L, and the second absolute distance corresponding to the current position of the object to be measured is calculated as the second absolute distance. Although the absolute distance of was output, it is not restricted to this. That is, the first absolute distance L may be calculated sequentially according to the movement of the object to be measured, and the first absolute distance L may be output as the absolute distance corresponding to the current position of the object to be measured. .

In the first embodiment, steps S1G to S1I and S1K shown in FIG. 8 are omitted, and the first absolute distance calculated in step S1 is the first first absolute value described in the first embodiment. The distance may be adopted.
In the second embodiment, the wavelength of the laser beam L2 is stabilized to each wavelength corresponding to each frequency that is the center of all the saturated absorption lines a1 to a15. However, the present invention is not limited to this. Of the saturated absorption lines a1 to a15, it is preferable to stabilize the wavelength of the laser light L2 to each wavelength corresponding to each frequency that is the center of three or more saturated absorption lines.

  In each of the above embodiments, the length measuring device 1 is configured to measure the absolute distance to the object to be measured that linearly moves so as to be close to and separated from the length measuring device 1, but is not limited thereto. Absent. That is, as a length measuring device, a configuration in which a two-axis rotation mechanism and a position detector are provided as in a conventional tracking laser interferometer, and the absolute distance to an object to be measured that moves in three dimensions may be measured. .

  The present invention is based on the interference between the return light from the object to be measured and the reference light for reference by irradiating the object to be measured with laser light having a specific wavelength by changing the resonator length based on the saturated absorption line of the absorption cell. The present invention can be used in a laser interference length measuring device that measures length according to an interference signal.

DESCRIPTION OF SYMBOLS 1 ... Laser interferometer 10 ... Laser beam production | generation part 30 ... Interferometer 40 ... Refractive index calculation apparatus 60 ... Control apparatus 61 ... Wavelength control part 62 ... Phase detection Unit 63 ... First absolute distance calculation unit 68 ... Frequency detection unit S1A, S1D, S1G, S10A ... Wavelength changing step S1B, S1E, S1H, S10B ... Phase detection step S1J, S10E ... 1st absolute distance calculation process S2 ... Movement amount calculation process S3 ... 2nd absolute distance calculation process S4 ... Movement amount correction process

Claims (7)

Based on the saturated absorption line of the absorption cell, the length of the resonator is changed to irradiate the object to be measured with laser light of a specific wavelength, and the interference signal is based on the interference between the return light from the object to be measured and the reference light for reference. An absolute distance measurement method of a laser interference length measuring device that performs length measurement according to
A wavelength changing step for changing the wavelength of the laser beam by changing the resonator length while the object to be measured is stationary,
A phase detection step of detecting the phase of each interference signal at each point in time when the oscillation frequency of the laser light matches each frequency of the plurality of saturated absorption lines;
A first absolute distance calculating step of calculating a first absolute distance to the object to be measured based on each frequency of the plurality of saturated absorption lines and a phase of each interference signal. Distance measurement method. The absolute distance measuring method according to claim 1,
The wavelength changing step includes
Changing the resonator length so that the oscillation frequency of the laser beam matches each of the three or more saturated absorption lines,
The phase detection step includes
Detecting the phase of each interference signal at three or more time points;
The first absolute distance calculating step includes:
The absolute distance measuring method, wherein the first absolute distance is calculated based on each frequency of the three or more saturated absorption lines and a phase of the three or more interference signals. In the absolute distance measuring method according to claim 1 or 2,
A moving amount calculating step of calculating a moving amount of the object to be measured based on the interference signal;
An absolute distance measuring method comprising: a second absolute distance calculating step of calculating a second absolute distance based on the first absolute distance and the amount of movement. In the absolute distance measuring method according to claim 3,
An absolute distance measurement method comprising: a movement amount correction step of correcting the movement amount based on the second absolute distance. The absolute distance measuring method according to claim 4,
The laser interference length measuring apparatus irradiates the object to be measured with the laser beam modulated with a modulation signal having a predetermined modulation frequency and a predetermined modulation amplitude,
The movement amount correction step includes
The absolute distance measuring method, wherein the movement amount is corrected based on the second absolute distance, the modulation frequency, and the modulation amplitude. In the absolute distance measuring method according to claim 4 or 5,
The laser interference length measuring device is
A refractive index calculating device for calculating the air refractive index;
The movement amount correction step includes
The absolute distance measurement method, wherein the movement amount is corrected based on the second absolute distance and a change amount of the air refractive index before and after the movement of the object to be measured. A laser beam generator configured to change the resonator length and generate a laser beam having a specific wavelength; and
An interferometer that irradiates the object to be measured with the laser light and outputs an interference signal based on interference between return light from the object to be measured and reference light for reference;
A control device for controlling the operation of the laser light generation unit,
The controller is
A wavelength control unit that controls the operation of the laser light generation unit and changes the wavelength of the laser light by changing the resonator length based on a saturated absorption line of an absorption cell;
A phase detection unit for detecting the phase of each interference signal at each time point when the oscillation frequency of the laser light matches each frequency of the plurality of saturated absorption lines;
A laser comprising: a first absolute distance calculating unit that calculates a first absolute distance to the object to be measured based on each frequency of the plurality of saturated absorption lines and a phase of each interference signal; Interferometric length measuring device.

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