JPH11125504A - Method and device for measuring minute displacement - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make measurable a displacement continuously and widely without blind zone by switching the polarization planes of measuring laser light and reference laser light, and providing a direction discrimination function when the displacement of a measuring object exceeds a frequency detection limit. SOLUTION: Measuring tunable laser 24 oscillates a measuring laser light 26, of which wave length varies following the variation of resonator length L of an external ethalon 10 and a polarization plane switcher 50 selectively switches the polarization plane of the measuring laser light 26. A polarization plane rotation mechanism 52 rotates the polarization plane of a reference laser light 32 oscillated by a reference laser 30 in accordance with the polarization plane of the switched measuring laser light 26. A polarization plane controller 54 switches and rotates the polarization plane of the measuring laser light 26 and the reference laser light 32 at every time the beat frequency detected by a beat frequency detector 40 exceeds a detection limit. Thereby, the frequency used for measurement is changed to an adjacent vertical mode frequency of the resonator so as to enable measuring displacement continuously and widely over the interval of the vertical mode frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring a minute displacement, and more particularly, to changing a wavelength of a laser beam for measurement by following a change of an external resonator length which changes according to a displacement of a measuring object. Detecting a change in the wavelength of the measurement laser light as a change in the beat frequency with respect to the wavelength of the reference laser light,
From the change in the beat frequency, determine the displacement of the measurement target,
The present invention relates to a Fabry-Perot type external resonator type minute displacement measuring method and apparatus which can be used for displacement measurement and calibration.

[0002]

2. Description of the Related Art Generally, a laser interferometer measuring a wavelength of light as one scale is used for measuring displacement with high precision and high resolution. The following method is conceivable. (Jun Ishikawa: "Prospects of subpicometer interferometers" O plus E, No. 191, 1995, pp97-102; Ishikawa, Fujimoto: "Subpicometer measurements" Materials for Research Seminar of Metrology Institute, 1994, p1-8)

Shortening the wavelength of the light source Since one scale of the interferometer is determined by the wavelength, a light source having a short wavelength is used to obtain higher accuracy and resolution. With less material and shorter wavelengths, light does not pass through the air. Therefore, the current (about 600 nm) 1
It is impractical to commercialize an interferometer using a light source with a short wavelength of about / 10.

Higher division of interference fringes When an interference image is visually read, the division of interference fringes corresponding to one graduation by wavelength is about 1/10.
Using a camera and modulating interference fringes, electrical processing,
Computer processing is performed, and in a so-called fringe scanning interferometer, two-dimensional measurement is performed with an accuracy of 1/100 wavelength or more.

Optical Heterodyne Method In the field of interferometric length measurement, in order to obtain higher resolution, the frequency of the light to be interfered is shifted to convert the light phase signal into an electric signal, and the displacement is electrically detected. The displacement is detected as a phase change of an interference signal using an optical heterodyne method. Since the limit of electrical phase detection is about 0.1 degree, resolution of 1/3600 wavelength can be obtained by dividing the interference fringe into 1/3600.

However, in any case, the accuracy and the resolution in the order of pm smaller than the atomic size are insufficient. Therefore, by changing the scale (wavelength) of the interference fringes, the change in the wavelength is detected as a change in frequency,
A wavelength-following interferometer that achieves a measurement accuracy substantially equivalent to 1/100000 division has been proposed.

As the current wavelength tracking type interferometer, a Michelson system, a semiconductor light source or an external resonator type Fabry-Perot system using a He-Ne laser and an etalon, a He-
An internal resonator Fabry-Perot system using a Ne laser, an external resonator Fabry-Perot system using a differential system, and the like have been proposed.

Among them, an external resonator type Fabry-Perot interferometer using a He—Ne laser and an etalon, which is a typical wavelength tracking type interferometer, has a displacement ΔL of a measuring object as shown in FIG. The external resonator length (the distance between the mirrors 12 and 14) L is changed by displacing one of the mirrors 12 (right side in the figure) in accordance with
The laser control unit 22 follows a change in the external resonator length according to the intensity of the external etalon 10 that is the external resonator and the intensity of the measurement laser beam 26 that has passed through the external etalon 10 and that is detected by the light receiving element 20. The wavelength was changed,
He-N, which oscillates a measuring laser beam 26 having a frequency fM
Tunable laser for measurement 24 composed of e-laser
And a reference laser beam 30 oscillating a reference laser beam 32 of frequency fR, for example, composed of an iodine-stabilized He-Ne laser, and a measurement laser beam 26 synthesized by using beam splitters (BS) 34, 36. And a beat frequency detector 40 for detecting the beat frequency fb of the reference laser beam 32.

Here, the measurement tunable laser 2
The wavelength of 4 is always controlled to a wavelength at which the etalon 10 resonates. Therefore, when the mirror 12 of the etalon 10 moves, the wavelength λ (frequency f
M) also changes. This change Δλ in wavelength is used as a reference
And the displacement ΔL of the mirror 12 can be obtained from the following equation.

ΔL / L = Δλ / λ = − (Δf / fM) (1)

For example, if the mirror 12 is displaced by ΔL = 0.1 μm when the resonator length L = 100 mm, the change Δf in the beat frequency becomes 470 MHz. Therefore, if the resolution of the beat frequency is 10 KHz, the displacement resolution is about 2 pm.

Further, the displacement amount ΔL obtained by this method depends on the accuracy of the reference laser 30. For example, if an iodine-stabilized He—Ne laser is used as the reference By comparison, displacement can be absolutely measured with high accuracy of 10 ^-9 or more.

[0013]

However, in this external resonator type Fabry-Perot type wavelength tracking interferometer, the following limitations are imposed due to the limitation of the beat frequency counter and the longitudinal mode frequency interval of the laser. Had problems.

The vicinity of 0 beat where the wavelengths of the measuring laser beam and the reference laser beam become equal cannot be detected. That is, the frequency counter used for the beat frequency detector 40 is 0 Hz
Since the vicinity cannot be measured, an unmeasurable region is formed. or,
Since the beat frequency has no sign, the direction is lost when the signal fluctuates near zero.

[0015] Beat frequency detection limit (maximum several GHz
Degree) exists. That is, the detection capability of the frequency counter is generally about several GHz, and the measurement range is limited.

The oscillation frequency range of the measurement tunable laser 24 is about 1.5 GHz, and the range in which continuous measurement can be performed is limited.

Accordingly, as shown in FIG. 6, λ / 2 (≒ C /
Even when a large number of longitudinal mode frequencies are used for each 2L: C, the adjacent longitudinal modes cannot be distinguished, so that a measurement dead zone is created when switching frequencies. If a plurality of longitudinal modes exist within the detection limit of the beat frequency detector, they cannot be distinguished from each other and cannot be measured.

Therefore, the conventional wavelength tracking interferometer has a very narrow range of λ / 2, which does not need to switch the longitudinal mode frequency, at the maximum, for one moving in one direction or continuous measurement at the maximum. Only measurements could be made.

The present invention has been made to solve the above-mentioned conventional problems, and has an object to provide a direction discriminating function so that continuous measurement without a dead zone can be performed and a wide range of displacement can be measured. And

[0020]

According to the present invention, a wavelength of a measuring laser beam is changed by following a change in an external resonator length which changes according to a displacement of an object to be measured. The change is detected as a change in the beat frequency with respect to the wavelength of the reference laser beam, and the minute displacement measurement method for obtaining the displacement amount of the measurement target from the change in the beat frequency, the polarization plane of the measurement laser beam and the reference laser beam Can be selected,
When the displacement of the measurement object exceeds the detection limit of the beat frequency, the frequency used for measurement is changed to the adjacent longitudinal mode frequency of the resonator by switching the polarization plane of the measurement laser light and the reference laser light. Thus, the above problem is solved by enabling continuous displacement measurement over a wide range exceeding the interval of the longitudinal mode frequency.

In the minute displacement measuring device, an external resonator whose resonator length changes in accordance with the displacement of an object to be measured, and a measuring device whose wavelength changes following the change of the external resonator length. A tunable laser for measurement that oscillates laser light, a polarization plane switching unit for switching the polarization plane of the laser light for measurement, a reference laser that oscillates the reference laser light, and a polarization plane for the laser light for measurement. In addition, a polarization plane rotation unit for rotating the polarization plane of the reference laser beam, a beat frequency detection unit for detecting a beat frequency of the measurement laser beam and the reference laser beam, and the beat frequency is Each time the detection limit is exceeded, a polarization plane control means for switching the polarization plane of the measurement laser light and the reference laser light is provided. Reduction from and to perform a wide range of continuous displacement measurement exceeding the spacing of the longitudinal mode frequencies of the resonator is also obtained by solving the above problems.

Further, one or more of the oscillation frequencies of the tunable laser for measurement are within the oscillation frequency range of the tunable laser for measurement, excluding the unstable detection region of the beat frequency detection device and the region close to the wavelength of the reference laser beam. Different polarization plane 2
This is set so that the vertical mode frequency of the book is entered.

The polarization plane of the measuring laser light is switched by rotating the polarization plane by a polarization plane rotating means.

Further, the length of the external resonator is equal to the length of the resonator of the tunable laser for measurement.

[0025]

Embodiments of the present invention will be described below in detail with reference to the drawings.

In this embodiment, as shown in FIG. 1, an external etalon 1 having a pair of mirrors 12, 14 similar to the conventional example is used.
0 and the external etalon 1 by a heater or a piezoelectric element controlled by outputs of the light receiving element 20 and the laser control unit 22.
The measurement tunable laser 24 oscillates the measurement laser light 26 whose wavelength changes following the change of the cavity length L of 0.
A reference laser 30 that oscillates a reference laser beam 32;
An external resonator type wavelength tracking interferometer including a measurement laser beam 26 combined by the beam splitters 34 and 36 and a beat frequency detector 40 for detecting a beat frequency of the reference laser beam 32, A polarization plane switch 50 for selecting and switching the polarization plane of the measurement laser light 26, and the reference laser light 3 according to the polarization plane of the measurement laser light 26 switched by the polarization plane switch 50.
A polarization plane rotating mechanism 52 for rotating the polarization plane 2 and the polarization of the measurement laser beam 26 and the reference laser beam 32 each time the beat frequency detected by the beat frequency detector 40 exceeds the detection limit. A polarization plane controller 54 for switching and rotating the plane is provided.

The polarization plane switching device 50 and the polarization plane rotation mechanism 52 provided on the exit side of the measurement tunable laser 24 and the reference laser 30 are constituted by, for example, a combination of a wave plate and a polarization plate. The plane of polarization can be switched or rotated.

The resonator of the tunable laser 24 for measurement and the resonator length of the external etalon 10 are the same, so that the longitudinal mode frequency is the same.

The longitudinal mode frequency interval between the measurement laser beam 26 and the external etalon 10 and the frequency of the reference laser beam 32 satisfy the conditions shown in FIG. Specifically, it is the oscillation frequency range R (about 1.5 GHz) of the tunable laser 24 for measurement, and the reference laser frequency fR
One or two etalons 10 (one for each polarization plane) in a region other than the nearby a region and the b region where the detection is unstable due to a decrease in gain, as indicated by the gain profile G Of the vertical mode frequencies fM and fM1.
The reason why the difference between the longitudinal mode frequency fM and the reference laser frequency fR is H or more is to avoid a detection unstable region near 0 beat. The width H of the hysteresis is limited by the detection lower limit of the beat frequency counter, and is generally 100
MHz. Also, the k region needs to cover at least the region where the beat frequency is unstable.

In order to satisfy such conditions, an iodine stabilized laser can be used as the reference laser 30. Such an element-stabilized laser can select a frequency by selecting an absorption line. Further, the tunable laser 24 for measurement can change the center frequency Gc of the gain profile G by changing the composition ratio of the isotope of the enclosed Ne gas.

In a measurement system satisfying the above conditions, when the beat frequency is detected near the unmeasurable region (0 beat) while detecting the beat frequency, the polarization plane is switched and the beat frequency is shifted by the vertical mode. To enable continuous measurement. When the polarization plane is switched, since the resonator lengths of the measurement tunable laser 24 and the external etalon 10 are the same, the longitudinal modes of the two are equal, and the resonance state is maintained even if the wavelength changes by switching the polarization plane. Is done. Further, since the relationship with the frequency (wavelength) of the reference laser can be always grasped, it has a direction discriminating function and the degree of freedom of measurement is expanded.

For example, if the resonator length L is 215 mm, the longitudinal mode is 700 MHz, and the center frequency GC of the gain profile G is set 250 MHz higher than the frequency fR of the reference laser, the relationship shown in FIG. Can be measured.

The method for calculating the switching timing of the polarization plane and the beat frequency will be specifically described with reference to FIG. 4 showing the measurement procedure.

As the tube (resonator) of the initial measurement tunable laser 24 is extended, when the laser frequency matches the resonance frequency of the resonator of the external etalon 10, the amount of light transmitted through the etalon 10 becomes maximum. Become. The laser control unit 22 controls the resonator of the tunable laser 24 for measurement by the output of the light receiving element 20 so that this light amount always becomes maximum. That is, the measurement tunable laser 24 is connected to the external etalon 10.
Lock at the resonance frequency of At this time, locking is performed in a direction in which the frequency becomes lower (the wavelength becomes longer).
The relationship with the reference laser frequency is determined, and the resonance frequency is determined (the same holds true for the higher frequency direction).

FIG. 4A shows that the frequency fM of the tunable laser 24 for measurement (for example, vertical polarization with a deflection angle of 90 °) is higher than the frequency fR of the reference laser 30 by the beat frequency fb. ing. From the change in the beat frequency fb, the change .DELTA.L in the resonator length L of the etalon 10, that is, the displacement of the measurement object is obtained. In this initial stage, the beat frequencies fb of fR and fM become the final beat frequency fs as they are.

Switching of Polarization Plane (First Time) When the beat frequency is detected and it is recognized that the frequency FM of the tunable laser 24 for measurement has entered the region a or b in FIG. Is switched from, for example, vertically polarized light to horizontally polarized light having a deflection angle of 0 °.
This recognition can be determined from the beat frequency fb constantly monitored. Even in the initial stage, when the frequency of the tunable laser for measurement 24 is swept (extending the resonator), the frequency fM of the tunable laser for measurement 24 is increased.
When the (wavelength) enters the region a (when the tube is extended), the polarization plane is similarly switched.

FIG. 4B shows a case where the frequency fM of the tunable laser 24 for measurement gradually decreases and enters the region a. This recognition can be understood from the fact that the detected beat frequency fb has become H or less. At this time, by switching the polarization plane of the light incident on the etalon 10 by the polarization plane switch 50, the light incident on the etalon 12 can be switched to the adjacent light fM1 (for example, horizontal polarization) having a different frequency for the longitudinal mode. it can. This prevents the beat frequency fb from becoming zero or losing sight of the relationship with the reference frequency fR.

By this operation, the reference frequency fR and the measurement frequency taking the beat are changed from the vertical polarization fM to the horizontal polarization fM.
M1 and the detected beat frequency changes from fb to fb1
Changes to However, the beat frequency fb with respect to the actual reference frequency is obtained by subtracting the number of waves for longitudinal mode (C / 2L) from fb1 as shown in the following equation.

Fb = fb1- (C / 2L) (2)

Accordingly, the final beat frequency fs before switching the polarization plane next time is as shown in the following equation.

Fs = fb1- (C / 2L) (3)

When the polarization plane is switched, the frequency locked to the resonance frequency of the etalon is forcibly changed, so that it is considered that the frequency shifts from the resonance frequency and the lock is released. Since the resonator lengths of the tunable laser 24 and the etalon 10 are the same,
Since the longitudinal mode is the same and the new frequency fM1 becomes the resonance frequency of the etalon 10 like the previous frequency fM, the locked state is maintained.

Switching of polarization plane (second time) Thereafter, when the measurement frequency gradually increases and the tunable laser frequency fM1 for measurement enters the region b, the polarization plane is switched from horizontal deflection to vertical deflection again, and the etalon is switched. The frequency incident on 10 is switched from fM1 to fM2. At the same time, the detected beat frequency changes from fb1 to fb2. Where fb1
The following relationship is established between fb2 and fb2.

Fb1 = fb2 + (C / 2L) (4)

Accordingly, the actual beat frequency fs from the reference frequency is as shown in the following equation.

Fs = fb2 + (C / 2L)-(C / 2L) (5)

Thus, the resonator length of the etalon is λ / 2
Even if the frequency changes, the frequency of incidence on the etalon is switched together with the plane of polarization, so that the amount of change in the resonance frequency can be continuously measured as the beat frequency fs.
Therefore, the change amount of the etalon from this beat frequency fs,
That is, the displacement of the measurement object can be obtained.

Switching of Polarization Plane (Nth Time) The switching of the longitudinal mode frequency and the polarization plane as described above is repeated, and the accumulated value is used as the displacement amount. The accumulated final beat frequency fs is obtained by subtracting the longitudinal mode frequency (C / 2L) from the detected beat frequency when the polarization plane is switched in the area a, and the detected beat frequency when the polarization plane is switched in the area b. To the vertical mode frequency. Therefore, the final beat frequency fs when the polarization plane is switched n times is expressed by the following equation.

Fs = fbn- (C / 2L) * l + (C / 2L) * m (6) where l is the number of times the polarization plane is switched in the region a, and m
Is the number of times the polarization plane is switched in the region b.

Therefore, from this beat frequency fs (Δf), the change amount ΔL of the etalon is obtained by the equation (1).

By performing such a measurement, a continuous and wide measurement can be performed using the tunable range of the measurement tunable laser 24 as a measurement range, and also a reciprocating measurement can be performed.

For example, assuming that the resonator length is 215 mm and the beat measurement resolution (accuracy) is 1 KHz, the calculation is 0.5
Measurement can be performed with a resolution (accuracy) of pm. Moreover, as long as the tunable laser for measurement can follow the change of the etalon, the measurement can be performed continuously, and the measurement range is very wide.
In addition, since this measurement can be performed with reference to the length standard laser, it is an absolute measurement that can be directly compared with the length standard.

In the present embodiment, the polarization plane switch 50
And the polarization plane rotation mechanism 52 is configured by a combination of a wave plate and a polarization plate. However, the configuration for changing the polarization plane is not limited to this, and electric fields such as a liquid crystal, an electro-optical element, and a Faraday rotation element are used. An element that modulates light passing through the crystal by a magnetic field can be used to change the polarization angle.

In the above embodiment, the polarization plane switching device 50 is used to switch the adjacent longitudinal mode frequencies of the measurement laser light. However, the polarization plane rotation mechanism 5 for the reference laser light is used.
A rotation mechanism similar to that of the second embodiment may be used.

In the above embodiment, the He-Ne laser was used as the tunable laser for measurement, the iodine stabilized laser was used as the reference laser, and the etalon was used as the external resonator. The configuration of the vessel is not limited to this.

[0056]

According to the present invention, it is possible to continuously and minutely measure a minute displacement on the order of pm over a wide range. Furthermore, measurement based on a length standard becomes possible, and measurement in which the direction of displacement changes is also possible. Therefore, the present invention has excellent effects such as less restriction on measurement and expansion of the application range of measurement and calibration.

[Brief description of the drawings]

FIG. 1 is an optical path diagram showing a configuration of an embodiment of a minute displacement measuring device according to the present invention.

FIG. 2 is a diagram illustrating an example of a relationship between a gain profile and an oscillation wavelength for explaining the wavelength condition of the embodiment.

FIG. 3 is a diagram showing a specific example of an oscillation wavelength of the measurement laser light source in the embodiment.

FIG. 4 is a diagram showing a measurement procedure in the embodiment.

FIG. 5 is an optical path diagram showing a configuration of a conventional external cavity type Fabry-Perot type wavelength tracking interferometer.

FIG. 6 is a diagram illustrating an example of a longitudinal mode frequency.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... External etalon 12, 14 ... Mirror 20 ... Light-receiving element 22 ... Laser control part 24 ... Measurement tunable laser 26 ... Measurement laser beam 30 ... Reference laser 32 ... Reference laser beam 40 ... Beat frequency detector 50 ... Polarization Surface switch 52: Polarization plane rotation mechanism 54: Polarization plane control unit

Claims (5)

[Claims] 1. The method according to claim 1, further comprising: changing a wavelength of the measuring laser light so that the external resonator maintains a resonance state with respect to a change in the external resonator length that changes according to a displacement of the measurement object. A change in the wavelength of the reference laser beam as a change in the beat frequency with respect to the wavelength of the reference laser beam, and the displacement amount of the object to be measured is determined from the change in the beat frequency. When the displacement of the measurement object exceeds the detection limit of the beat frequency, the polarization plane of the measurement laser light is switched and the polarization plane of the reference laser light is rotated. In this way, the frequency used for measurement was changed to the longitudinal mode frequency adjacent to the resonator, enabling continuous displacement measurement over a wide range beyond the longitudinal mode frequency interval. Minute displacement measurement method to. 2. An external resonator whose resonator length changes according to a displacement of a measurement object, and a wavelength so that the external resonator maintains a resonance state with respect to the change of the external resonator length. A tunable laser for measurement that oscillates a measurement laser light having a change, a polarization plane switching unit for switching a polarization plane of the measurement laser light, a reference laser that oscillates a reference laser light, and the measurement laser. A polarization plane rotating unit for rotating the polarization plane of the reference laser beam in accordance with the polarization plane of the light, and a beat frequency detection unit for detecting the beat frequency of the measurement laser beam and the reference laser beam. A polarization plane control means for switching the polarization plane of the measurement laser light and the reference laser light every time the beat frequency exceeds the detection limit, and A minute displacement measuring apparatus characterized by performing continuous displacement measurement over a wide range exceeding the interval of the longitudinal mode frequency of the resonator from a change in the port frequency. 3. An oscillation frequency range of said tunable laser for measurement, excluding an unstable detection region of said beat frequency detection means and a region close to a wavelength of a reference laser beam. A minute displacement measuring device characterized in that two vertical mode frequencies having different surfaces are set. 4. A minute displacement measuring apparatus according to claim 2, wherein the polarization plane of the measurement laser light is switched by rotating a polarization plane by a polarization plane rotating means. 5. The minute displacement measuring apparatus according to claim 2, wherein the length of the external resonator is equal to the length of the resonator of the tunable laser for measurement.