JPH09113216A - Relative displacement amount measuring method and device - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To accurately measure the relative displacement between two measured surfaces that are close to each other, and when it is not possible to measure two measured surfaces at the same time, such as observing the movement of a magnetic head on a hard disk. , So that the displacement amount of the first measurement surface and the surface state of the second measurement surface can be associated and observed. SOLUTION: Two coherent light beams are radiated to respective measured surfaces and a reflected light beam is caused to interfere with a reference light beam having coherence and having a frequency shift to obtain two beat signals, and each of them is obtained. The relative displacement amount between the two measured surfaces is calculated based on the difference between the phase differences obtained by comparing with the reference beat signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a relative displacement amount measuring device for measuring the displacement of a moving object in a non-contact manner, and more particularly to a substrate of an object which moves relative to the substrate on a moving substrate. The present invention relates to a relative displacement amount measuring device that measures a displacement amount.

[0002]

2. Description of the Related Art Conventionally, various devices have been developed for measuring a minute distance necessary for inspecting a precision polished surface, measuring a minute displacement of an object, or positioning. For example, methods using light such as light wave interferometry (two-beam interferometry), fringe scan method, optical heterodyne method, triangulation method, hologram interferometry method, Doppler velocity measurement method, speckle image movement measurement method are known. .

The light wave interferometry has a reference surface and obtains an in-plane distribution in the object to be measured by an interference fringe obtained by causing the reflected light from this surface and the reflected light from the object to be measured to interfere with each other, and the fluctuation of the interference fringe is obtained. The amount of displacement is measured from, which is effective for measuring strain on a wide surface. For example, there are Michelson interferometer, Fizeau interferometer, and the like. The fringe scan method calculates the amount of movement of a non-measurement object from the change in interference fringes caused by a slight movement of the measurement object in the optical axis direction. The hologram interferometry method uses a CC image to form a hologram image formed by the reflected light that reflects the unevenness of the measured surface and the light that is reflected by the reference surface.
The amount of movement and the amount of unevenness are obtained by receiving the image at D and performing signal processing on the movement of the image. The triangulation method is a method of obtaining a displacement of a surface to be measured by capturing a parallel movement of the optical axis of the reflected light due to a change in the relationship of the surface to be measured with respect to the irradiation light with a linear optical sensor such as a CCD. The Doppler velocity measuring method is to measure the frequency and obtain the amount of movement based on the fact that the frequency of reflected light slightly shifts due to the Doppler effect when the object to be measured moves. It is used for measuring objects that change at high speed. The speckle image movement measurement method is
When the surface to be measured has a certain degree of roughness, the reflected light from the surface forms speckle, and when the surface moves, the speckle also changes, so the amount of movement is measured from the amount of change in speckle. It is an effective method when the surface is not smooth.

In the heterodyne interferometry, a frequency shift is given between two interfering lights to generate a low-frequency beat signal having a frequency difference between the two light waves as a frequency, and a phase difference is detected by the optical heterodyne detection. To measure the variation of the optical path length. FIG. 4 is a drawing showing the principle of an example of the heterodyne interferometry. In the figure, 101 is a surface to be measured, 102 is a stabilized NeHe laser oscillator that generates a laser beam having an oscillation frequency f ₀ , and 103 is a laser beam emitted from the laser oscillator 102 that propagates to the next stage. Optical fiber system with lenses at the end and the exit end, 104
Is a first polarization beam splitter, 105 is a first acousto-optic modulator (AOM) that gives a frequency shift of the frequency f ₁ to the passing laser light, 106 is a first non-polarization beam splitter, and 107 is a first reflection A mirror, 108 is a second reflecting mirror, 109 is a second acousto-optic modulator that gives a frequency shift of frequency f ₂ , 110 is a second non-polarizing beam splitter, 111 is a half-wave plate, and 112 is a second 2 is a polarizing beam splitter, 113 is a quarter wavelength plate, 114 is a third reflecting mirror, 115 is a third non-polarizing beam splitter, 1
Reference numeral 16 is a first polarizing plate, 117 is a first photodetector, 118 is a second polarizing plate, and 119 is a second photodetector.

A laser beam having an oscillation frequency f ₀ emitted from a stabilized NeHe laser oscillator 102 is focused by an optical fiber 103 having a lens at an entrance end and an exit end and enters a first polarization beam splitter 104, where It is separated into polarization components of P wave and S wave which are orthogonal to each other. First
The P-wave traveling straight through the polarization beam splitter 104 has a frequency (f ₀ + f ₁ ) while passing through the first acousto-optic modulator 105.
Becomes the polarized light of the first unpolarized beam splitter 106
And is split into a vertically refracted component and a straight component. The straight traveling component changes its direction in the first reflecting mirror 107 and is incident on the second polarization beam splitter 112, travels straight through it, and reaches the second polarizing plate 118.

On the other hand, the first polarization beam splitter 104
The S wave refracted by the second refracting mirror 108 is refracted again by the second reflecting mirror 108.
While passing through the acousto-optic modulator 108, it becomes polarized light having a frequency (f ₀ + f ₂ ) and enters the second non-polarizing beam splitter 110. The component that refracts here is the third
The light is refracted again by the reflecting mirror 114 and is incident on the third non-polarizing beam splitter 115. Here, it is combined with the component refracted by the first non-polarizing beam splitter 106 to generate an interference wave having a beat frequency (f ₁ −f ₂ ). The reference beat light is formed and projected on the first polarizing plate 116. The first photodetector 117 captures this and converts it into a reference beat signal corresponding to a change in light intensity. This reference beat signal is a waveform that is fixedly determined based on the optical path difference in the measuring device by the heterodyne interferometry and serves as a reference for comparison with the measurement signal.

Further, the first polarization beam splitter 10
The S wave component that travels straight at 4 is polarized by the ½ wavelength plate 111.
2nd polarization beam splitter 1
The light is incident on 12 and goes straight on, and is irradiated as circularly polarized light on the surface to be measured 101 by the quarter-wave plate 113, and the reflected light is 1 again.
The light passes through the quarter-wave plate 113 and enters the second polarization beam splitter 112. This incident light passes through the quarter-wave plate twice, so that the polarized light changes 90 degrees in total and becomes an S wave. Therefore, the second polarization beam splitter 112 refracts downward in the figure, interferes with the reference wave of the frequency (f ₀ + f ₁ ) incident from the reflecting mirror 107, and interferes with the second polarizing plate 11
An interference wave having a beat frequency (f ₁ -f ₂ ) is formed on the optical pickup 8, and the second photodetector 119 captures the interference wave and converts it into an electric beat signal corresponding to a change in light intensity.

The phase difference between the electric beat signal and the reference beat signal is included in the reference beat signal, which is caused by the optical path length added due to the round trip to the surface to be measured, and the optical path difference between both waves of the measurement system. However, since the latter two are peculiar to the measuring device,
The phase difference measurement result can be said to be representative of the fluctuation of the surface to be measured. In this way, the heterodyne interferometry is measured based on the phase amount, and thus is not easily affected by the fluctuation of the light amount. In addition, the heterodyne measurement method has the advantage that the signal to be handled has a low frequency and the detected phase difference is proportional to the distance. Therefore, the heterodyne measurement method is particularly suitable for measuring a very small displacement amount.

[0009]

Since these conventional displacement amount measuring methods and devices measure the surface of a sample mounted on a surface plate integrated with the device, the measuring device and the object to be measured. If the objects are placed on the same substrate, the variation amount of the surface to be measured can be directly measured. However, when the substrate on which the object to be measured is mounted moves with respect to the surface plate to which the apparatus is fixed, it is impossible to accurately measure the movement and fluctuation peculiar to the object to be measured mounted thereon. That is, in the conventional displacement amount measuring method and device, when the object to be measured is placed on another substrate placed on the surface plate of the measuring device, or the object to be measured is placed outside the surface plate of the measuring device. In such a case, the movement of the surface to be measured combined with the movement of another substrate can be measured, but the amount of fluctuation of the object to be measured itself cannot be known.

When it is necessary to know the relative displacement amount between the surface of the object to be measured and the substrate on which the object to be measured is actually placed,
For example, in order to analyze the vibration of the outer wall on the surface of the transformer, it is necessary to measure the vibration phase difference between the parts at different positions, and to measure that the plate thickness of the work in the NC machine tool changes due to the temperature change during processing. However, in developing a hard disk, there is an urgent need to accurately measure the flying height of a pickup on a rotating hard disk, that is, the so-called flying height. The flying height on the hard disk has become extremely small in order to support high-density recording, and in order to measure this, the measurement sensitivity must be extremely high. Moreover, since the flying height changes depending on the rotation speed of the disk, it must be measured under the same conditions when actually operating. Moreover, although the surface of the substrate of the hard disk changes greatly due to the unevenness of the substrate and the surface treatment, the state of the substrate surface immediately below the part where the pickup is sliding cannot be measured simultaneously because it is behind the pickup. It is important to measure the flying height for the performance evaluation of the hard disk, but the conventional displacement amount measuring method and apparatus could not perform the appropriate measurement due to the above restrictions.

The problem to be solved by the present invention is to measure the relative amount of displacement between two adjacent points with high accuracy by utilizing the advantage of the heterodyne interferometry, and in particular, the surface of the substrate on which the sample is mounted fluctuates. Even in such a case, it is an object of the present invention to provide a relative displacement amount measuring method and device capable of canceling the movement and measuring the fluctuation of only the surface to be measured. It is another object of the present invention to provide a relative displacement amount measuring method and device capable of measuring a relative displacement amount between two sample surfaces even if they cannot be simultaneously measured.

[0012]

In order to solve the above-mentioned problems, the relative displacement amount measuring method of the present invention irradiates two measured surfaces with a light beam and reflects the reflected light from the measured surfaces. An optical beat signal is generated for each light, and the relative movement between both measured surfaces is measured by the phase difference between the beat signal and the reference signal. In addition, the second relative displacement amount measuring method of the present invention includes the first coherent light beam that irradiates the first surface to be measured and reflects a light beam that has coherence with the first light beam and has a different frequency. The first beat signal is obtained by interfering with the reference light, this phase is compared with the phase of the reference beat signal to obtain the first phase difference, and the second coherent light is irradiated onto the second measured surface and reflected. Light beam having a coherency with the second reference light beam having a different frequency to obtain a second beat signal, and this phase is compared with the phase of the reference beat signal to obtain a second phase difference. It is characterized in that the relative displacement amount between the first measured surface and the second measured surface is calculated from the difference between the first phase difference and the second phase difference. In the relative displacement amount measuring method of the present invention, the first coherent light and the second coherent light are P-wave and S-wave, respectively, in which one laser light is separated and relatively frequency-shifted. The second coherent light may be used as the reference light and the first coherent light may be used as the second reference light. Further, the reference beat signal may be an electric signal obtained by causing the first coherent light and the second coherent light to interfere with each other.

In order to solve the above-mentioned problems, the relative displacement amount measuring apparatus of the present invention is such that an irradiation device for irradiating two measured surfaces with a light beam and a reflected light reflected from the measured surface are received. And a signal processing circuit for measuring the relative movement between the measured surfaces based on the phase difference between the beat signal and the reference signal. Further, a reference signal generation device may be provided that causes the light beams before being irradiated onto the surface to be measured to interfere with each other to acquire an optical beat signal and use it as a reference signal.

A second relative displacement amount measuring device of the present invention is a laser oscillator and a first relative displacement amount measuring device which receives a laser beam and is orthogonal to each other.
A light splitting device that splits the polarized light into a polarized light and a second polarized light, relatively shifts the polarized light in frequency, and supplies the polarized light as a first coherent light and a second coherent light, respectively. And a second probe that receives and supplies a light beam that irradiates and is reflected by the surface to be measured of the second probe and a second coherent light beam.
Of the second probe and the second probe, which receives and supplies the light beam that irradiates and is reflected on the surface to be measured, and generates the interference light between the light from the first probe and the second coherent light, and inputs the interference light to generate the first beat. A first beat signal generator for obtaining a signal, and a second beat signal generator for producing an interference light of the light from the second probe and the first coherent light and inputting the interference light to obtain a second beat signal , A signal processing circuit for obtaining a phase difference between the first beat signal and the second beat signal with reference to a reference beat signal and displaying a signal based on the phase difference. Further, a reference beat signal generator for generating interference light of the first coherent light and the second coherent light and inputting the interference light to obtain a third beat signal is provided, and the signal processing circuit inputs the third beat signal. It may be a reference signal. In addition, it is preferable that the first probe and the second probe be mounted on one stage so that the interval of the irradiation light from both can be adjusted.

The relative displacement amount measuring method of the present invention irradiates two measured surfaces with a light beam and generates optical beat signals for the respective reflected lights reflected from the measured surfaces. By using a separately prepared reference signal, it becomes possible to calculate the phase difference between them, so the same applies to the case where the measured surfaces move independently and in relation to each other. The relative movement between the measured surfaces can be measured. In addition, the second relative displacement amount measuring method of the present invention causes the first coherent light that irradiates and reflects the first measured surface to interfere with the first reference light that has coherence and has a different frequency. Since the first beat signal is obtained and this phase is compared with the phase of the reference beat signal, the first phase difference can be obtained, and the second coherent light that irradiates and reflects the second measured surface is converted into a light beam. Since the second beat signal is obtained by interfering with the second reference light having the coherence with and different frequency, and this phase is compared with the phase of the reference beat signal, the second phase difference can be obtained.
The relative displacement amount of the first measured surface and the second measured surface can be obtained from the difference between the first phase difference and the second phase difference. The first coherent light and the second coherent light respectively separate the one laser light and relatively shift the frequency of the P wave and the S wave.
When the second coherent light is used as the first reference light and the first coherent light is used as the second reference light, the reference light serving as the respective reference is easily obtained and the beat Since the frequencies are easily the same, the structure of the measuring device is simplified. When the reference beat signal is an electric signal obtained by causing the first coherent light and the second coherent light to interfere with each other, the reference lights are used to obtain the reference signal having the same frequency without special adjustment. be able to.

Therefore, if one of the surfaces to be measured is a plane fixed to the magnetic head to be measured and the other is the surface of the hard disk, the flying height can be directly measured by a single relative displacement amount measuring device. It is also possible to observe irregularities on the hard disk surface corresponding to changes in flying height.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The relative displacement amount measuring method of the present invention comprises:
By measuring two independent measured surfaces independently by the optical heterodyne method, the principle is to measure the relative movement between the two measured surfaces. An optical beat signal is generated for each of the reflected lights reflected from the surface to be measured, and the relative motion between the surfaces to be measured is measured by the phase difference between the beat signal and the reference beat signal. If the beat signal generated by the reflected light from the surface to be measured interferes with the reference light causes a deviation ΔΨ from the phase difference from the reference beat signal, and this is caused by the fluctuation ΔL of the surface to be measured, ΔΨ = 4πΔL /
The relationship of λ holds. Therefore, the phase difference ΔΨ may be known in order to obtain the fluctuation of the surface to be measured. By configuring an optical circuit that obtains two ΔΨ on the two measured surfaces with respect to the reference beat signal, it becomes possible to obtain the relative displacement amount between the two measured surfaces. When the surface to be measured is located at different positions within one surface, the vibration of one surface to be measured is measured, and it is used to observe the resonance vibration of the iron plate during the self-excited vibration of the transformer, for example. You can In addition, the substrate surface separated from the measuring device is
When measuring the change in the work plate thickness that changes due to the temperature rise during machining in the NC machine tool, in real time as the measured surface of No. 2 and the surface of the plate placed on the substrate as the second measured surface. There is. The reference beat signal can be obtained by a reference interferometer by causing known light to interfere with each other, but can also be electrically generated by an electronic circuit. Also,
According to the relative displacement amount measuring method of the present invention, the measured surface fixed to the magnetic head and the second measured surface located below the magnetic head on the hard disk surface are measured and corresponded to each other. It is possible to observe how the flying height of the magnetic head changes depending on the surface condition and rotating condition of the rotating hard disk magnetic disk.

[0018]

[Embodiment 1] FIG. 1 is a block diagram showing a first embodiment of a relative displacement amount measuring apparatus of the present invention. In the figure, 1 is a first measured surface to be measured, and 2 is a second measured surface in the vicinity of the first measured surface. 3 is a stabilized NeHe laser oscillator, 4 is an optical circuit, 5 is a first probe, 6 is a second probe, 7 is an amplifier, 8 is an indicator, 9 is a signal processing circuit, and 10 is a display. The laser light of the oscillation frequency f ₀ emitted from the stabilized NeHe laser oscillator 3 is
The light is focused by the lens 11 and is incident on the core of the optical fiber 12, propagates in the optical fiber 12 and is diffused on the end face on the exit side, and is made into substantially parallel light rays by the lens 13 and is incident on the optical circuit 4.

The optical circuit 4 includes a first polarization beam splitter 14, a first acousto-optic modulator (AOM) 15, a first AOM controller 16 for controlling the first acousto-optic modulator, and a first AOM controller 16.
Non-polarizing beam splitter 17, first reflecting mirror 18, second
Acousto-optic modulator (AOM) 19, second AOM controller 20 for controlling second acousto-optic modulator, second non-polarizing beam splitter 21, third non-polarizing beam splitter 2
2, a first photodetector 23, a fourth non-polarizing beam splitter 24, and a fourth photodetector 25.

The laser light incident on the optical circuit 4 is separated by the first polarization beam splitter 14 into two polarization components of P wave and S wave which are orthogonal to each other. The P wave traveling straight through the first polarization beam splitter 14 is the first AOM controller 16
As a result, polarized light of frequency (f ₀ + f ₁ ) is obtained while passing through the first acousto-optic modulator 15, which is controlled to perform frequency modulation of a minute modulation frequency f ₁ . This polarized light is split by the first non-polarizing beam splitter 17 into a component refracted in the vertical direction and a component going straight. The rectilinear component enters the first polarization maintaining optical fiber 26 and is propagated to the first probe 5. Also,
The S wave refracted by the first polarization beam splitter 14 is the first
The component reflected by the reflecting mirror 18 passes through a second acousto-optic modulator (AOM) 19 arranged in parallel with the first acousto-optic modulator 15 and goes straight to a component in the vertical direction by a second non-polarizing beam splitter 21. Separate into the components to be used. Second acousto-optic modulator 1
9 is controlled by the second AOM controller 20 to perform frequency modulation of a minute modulation frequency f ₂ ,
The S-wave polarization component is polarized at the frequency (f ₀ + f ₂ ). The component that has proceeded straight through the second non-polarizing beam splitter 21 is propagated to the second probe 6 via the second polarization maintaining optical fiber 27.

The first probe 5 includes a third unpolarized beam splitter 30, a first quarter-wave plate 31, and a first lens 3.
2 and the second reflecting mirror 33, and P of the frequency (f ₀ + f ₁ ) propagated in the first polarization maintaining optical fiber 26.
A light beam which is made to travel straight on the wave polarization component and is projected on the first measurement surface 1 and reflected is refracted by the third non-polarization beam splitter 30 and the reflection mirror 33, and the first multimode fiber 28
Incident on The P-wave polarization component that has entered the first probe 5 passes through the first quarter-wave plate 31 twice within the probe until it enters the first multi-mode fiber 28, and the polarization changes by 90 degrees. In addition, there is a phase difference that advances while propagating in the optical path that reciprocates between the first probe 5 and the first measured surface 1. The second probe 6 includes a fourth non-polarizing beam splitter 34, a second quarter-wave plate 35, a second
It is composed of the lens 36 and the third reflecting mirror 37, and has a frequency (f ₀ +) propagated through the second polarization maintaining optical fiber 27.
The S-polarized light component of f ₁ ) which is projected onto the second surface to be measured 2 and reflected is refracted by the fourth non-polarizing beam splitter 34 and the third reflecting mirror 37, and the second multimode fiber 29 Incident on. The S-wave polarization component that has entered the second probe 6 has its polarization changed by 90 degrees by the second quarter-wave plate 35 before it enters the second multi-mode fiber 29. It has a phase difference that advances while propagating in the reciprocal optical path between the two measured surfaces 2.

The third non-polarization beam splitter 22 of the optical circuit 4 has a polarization component of the frequency (f ₀ + f ₁ ) returned from the first probe 5 via the first multi-mode fiber 28 and a second non-polarization component. The polarization beam splitter 21 receives and interferes with the polarization component of the frequency (f ₀ + f ₂ ) refracted, and sends it to the first photodetector 23 as beat light of the frequency (f ₁ −f ₂ ). The polarization component returned from the first probe 5 is almost 90.
The second non-polarizing beam splitter 2
It can interfere with the polarized component refracted at 1. The first photodetector 23 receives the beat light generated by this interference and outputs a first beat signal having a frequency (f ₁ -f ₂ ) corresponding to the beat. Similarly, the fourth non-polarizing beam splitter 24 also returns the frequency (f ₀ + f ₂ ) returned from the second probe 6 via the second multi-mode fiber 29.
And the polarized component of the frequency (f ₀ + f ₁ ) refracted by the first non-polarizing beam splitter 17 to cause interference and the second optical detection as beat light of the frequency (f ₁ −f ₂ ). Send to container 25. The second photodetector 25 outputs a second beat signal having a frequency (f ₁ −f ₂ ) corresponding to the intensity change of the beat light.

The first beat signal has a phase that reflects the optical path difference that two polarizations have followed since being split by the first polarization beam splitter 14, and the first beat signal has a phase other than the fixed optical path in the measuring device. Information about a round-trip optical path between the probe 5 and the first measured surface 1 is included. However, even if the first beat signal itself is observed, the reference phase cannot be determined, so that this information cannot be extracted. Therefore, it is necessary to know the change in the distance between the first probe 5 and the first measured surface 1 by obtaining the phase difference from the first beat signal using the reference beat signal having the same frequency as the phase reference. It is possible. Similarly, for the second beat signal, the reference beat signal and the second beat signal
By taking the phase difference from the beat signal, it is possible to know the change in the distance between the second probe 6 and the second measured surface 2.
Therefore, by comparing the phase differences between the acquired beat signal and the reference beat signal, the distance between the first measured surface 1 and the second measured surface 2, that is, the relative displacement amount can be obtained.

A displacement amount of the first beat signal and the second beat signal is obtained by a signal processing circuit 9 via an amplifier 7, and the result is displayed on a liquid crystal display 10 and in a signal form that an external device can input. Output. The difference signal amplified by the amplifier 7 is displayed on the indicator 8 so that the operator can monitor the measurement state. The signal processing circuit 9 includes a phase detection unit and an arithmetic processing unit, detects the phase difference between the first beat signal and the second beat signal based on the internally generated reference beat signal, and detects the difference between the two phase differences. Is calculated to obtain the relative displacement amount between the first measured surface and the second measured surface. The positions and emission directions of the first probe 5 and the second probe 6 may be provided separately so that they can be changed according to the measurement purpose and the object to be measured, but depending on the measurement purpose, they should be fixed in a predetermined state. Is desirable. Therefore, the two probes can be integrally formed, or the probe portions that are separately formed can be fixed to a member having high rigidity. When measuring the flying height of a magnetic head on a rotating hard disk, it is necessary to observe the movement of the magnetic head and the state of the hard disk surface that causes the movement. However, since the surface of the hard disk directly below the magnetic head is hidden by the magnetic head, both cannot be measured at the same time. In such a case, the first probe 5 aims at a part of the magnetic head to obtain a first beat signal, and the second probe 6 aims at a part of the hard disk fed immediately below the magnetic head to cause a second beat signal. Then, the signal processing circuit 9 corrects the hard disk portion based on the time it takes to move from the aiming position to just below the magnetic head.

FIG. 2 is a block diagram showing one embodiment of the probe portion. As shown in FIG. 2, the first probe 5 and the second probe 5
The probe 6 can be mounted on the base 38 and the distance between the two can be adjusted using the movable stage 39. It should be noted that, for example, if the optical circuit 4 uses an appropriate wave plate to convert the polarized light into an appropriate polarized light, the same result is obtained.
In the above, the structure that emphasizes the symmetry of the parts functions is used.
In the second probe 6 in FIG. 2, the input / output relationship of the beam is expressed opposite to that in FIG. Unless the polarization state is changed in the optical circuit 4, the S-polarization component propagates in the second polarization maintaining optical fiber 27, and the P-wave polarization component returns from the second surface to be measured. This is because the fourth non-polarizing beam splitter 34 considers the relationship in which the S-wave polarization component is refracted and the P-wave polarization component goes straight.

[0026]

Second Embodiment FIG. 3 is a block diagram showing a second embodiment of the relative displacement amount measuring device of the present invention. The second embodiment is the first
The major difference from the embodiment is that instead of electrically generating the reference beat signal, it is directly optically created from the polarization components of the P wave and S wave separated from the laser light and frequency-modulated by an acousto-optic modulator. The beat light thus generated is converted into an electric signal by a photodetector and used as a reference beat signal. Therefore, the optical circuit 40 substituting for the optical circuit 4 of the first embodiment has the same configuration as the optical circuit 4 except that the fifth non-polarizing beam splitter 41, the sixth non-polarizing beam splitter 42, the fourth reflecting mirror 43, and the seventh non-polarizing beam are used. A beam splitter 44 and a third photodetector 45 are added, and the amplifier 7 receives and processes the output of the third photodetector 45 in addition to the first photodetector 23 and the second photodetector 25. Has become.

In FIG. 3, the parts having the same functions as those in FIG. 1 are designated by the same reference numerals and the description thereof is omitted. Referring to FIG. 3, the P wave subjected to the frequency modulation of the modulation frequency f ₁ by the first polarization beam splitter 14 goes straight on by the fifth non-polarization beam splitter 41 and is reflected by the component which proceeds to the first non-polarization beam splitter 17. Then, it is divided into components which proceed to the seventh non-polarizing beam splitter 44. The S wave that has been frequency-modulated at the modulation frequency f ₂ by the second polarization beam splitter 19 is reflected by the sixth non-polarization beam splitter 42 to go straight to the second non-polarization beam splitter 21 and is reflected by the fourth component. It is divided into components that travel to the seventh non-polarizing beam splitter 44 via the reflecting mirror 43. If an appropriate phase shifter is interposed in the optical path of either or both of the P wave and the S wave so that both waves interfere with each other, they are combined by the seventh non-polarizing beam splitter 44 and the third photodetector is used. The light incident on the light beam 45 has a phase (f
It becomes a beat light of _1- f ₂ ). The third photodetector 45 converts this beat light into an electric signal and supplies it to the amplifier 7. In the signal processing circuit 9, the beat signal provided from the third photodetector 45 is used as a reference beat signal to perform the processing described in the first embodiment, and the relative distance between the first measured surface 1 and the second measured surface 2 is increased. Calculate the amount of displacement.

In the above two embodiments, the coherent light beam for irradiating the surface to be measured is obtained by dividing it from the output of one stabilizing laser oscillator. In signal processing, an appropriate reference beat signal is mediated. Therefore, coherent light beams having coherence with frequencies close to each other may be used, and the light beams are not limited to light from the same oscillator. In the configuration of the optical circuit, the optical elements are specifically arranged for ease of explanation, but it goes without saying that various modifications satisfying the configuration of the present invention are possible.

[0029]

As described above, the relative displacement amount measuring method and apparatus of the present invention irradiate two objects with two coherent light rays to generate beat light with reference light. Since the relative displacement between two objects is measured based on the phase difference between the two beat signals compared with the reference beat signal, the relative displacement of the object on the fluctuating substrate with respect to the substrate is measured. In particular, the flying height of the magnetic head on the hard disk can be observed while making it correspond to the disk surface state,
High industrial availability.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a block diagram showing an example of an embodiment of the probe of the present invention.

FIG. 3 is a block diagram showing a second embodiment of the present invention.

FIG. 4 is a drawing showing the principle of an example of heterodyne interferometry.

[Explanation of symbols]

1, 2, 101 Surface to be measured 3, 102 Stabilized NeHe laser oscillator 4, 40 Optical circuit 5, 6 Probe 7 Amplifier 8 Indicator 9 Signal processing circuit 10 Display 11, 13 Lens 12 Optical fiber 14, 104, 112 Polarizing beam splitter 15 , 19, 105, 109 Acousto-optic modulator (AO
M) 16, 20 AOM controller 17, 21, 22, 24, 30, 34, 41, 42, 4
4, 106, 110, 115 Non-polarizing beam splitter 18, 43 Reflector 23, 25, 45, 45, 117, 119 Photodetector 26, 27 Polarization-maintaining optical fiber 31, 35, 113 Quarter wave plate 32, 36 Lens 33, 37, 107, 108, 114 Reflecting mirror 28, 29 Multimode fiber 103 Optical fiber system 111 1/2 wavelength plate 116, 118 Polarizing plate

Claims (9)

[Claims] 1. A light beam is applied to each of the two measured surfaces to generate an optical beat signal for each of the reflected lights reflected from the measured surface, and both are generated by the phase difference between the beat signal and the reference signal. A relative displacement amount measuring method for measuring a relative motion between measured surfaces. 2. A first beat by irradiating a first coherent light beam onto a first surface to be measured and causing a reflected light beam to interfere with a first reference light beam having a different coherence and a different frequency. The signal is obtained and this phase is compared with the phase of the reference beat signal.
Of the second coherent light is radiated to the second surface to be measured and the light reflected by the second coherent light is caused to interfere with the second reference light having a different coherency and a different frequency. A beat signal is obtained, this phase is compared with the phase of the reference beat signal to obtain a second phase difference, and the difference between the first phase difference and the second phase difference is used to calculate the first measured surface and the second measured surface. A relative displacement measuring method for obtaining a relative displacement. 3. The relative displacement amount measuring method according to claim 2, wherein each of the first coherent light and the second coherent light separates one laser light and relatively shifts the frequency. A relative displacement amount measuring method comprising a P wave and an S wave, wherein the second coherent light is used as the first reference light, and the first coherent light is used as the second reference light. 4. The relative displacement amount measuring method according to claim 3, wherein the reference beat signal is an electric signal obtained by causing the first coherent light and the second coherent light to interfere with each other. Displacement measurement method. 5. An apparatus for measuring a small amount of displacement by using an optical heterodyne detection method, wherein an irradiation apparatus for irradiating two measured surfaces with a light beam and a reflected light reflected from the measured surface are received. And a relative displacement amount measuring device comprising a photodetector device for respectively generating an optical beat signal, and a signal processing circuit for measuring a relative motion between both measured surfaces by a phase difference between the beat signal and the reference signal. 6. The relative displacement amount measuring device according to claim 5, further comprising: interfering a light beam before irradiating the surface to be measured to obtain an optical beat signal and using it as the reference signal. A relative displacement amount measuring device comprising: 7. A laser oscillator, which receives a laser beam, divides it into a first polarized wave and a second polarized wave which are orthogonal to each other, and relatively shifts the frequencies of both polarized waves to obtain a first coherent light. A light splitting device that supplies as second coherent light, a first probe that receives and supplies a light beam that irradiates the first measured surface with the first coherent light and receives and reflects the light beam, and the second coherent light A second probe that receives and supplies a light beam that irradiates and reflects the second surface to be measured, and generates interference light between the light from the first probe and the second coherent light, and inputs the interference light. A first beat signal generating device for obtaining a first beat signal; and a second beat signal for generating an interference light between the light from the second probe and the first coherent light and inputting the interference light Beat signal generator, and the first B A relative displacement amount measuring device including a signal processing circuit that obtains a phase difference between the first beat signal and the second beat signal with a reference beat signal as a reference and displays a signal based on the phase difference. 8. The relative displacement amount measuring device according to claim 7, further comprising generating interference light between the first coherent light and the second coherent light and inputting the interference light to obtain a third beat signal. A relative displacement amount measuring device comprising a reference beat signal generator, wherein the signal processing circuit inputs the third beat signal and uses it as the reference signal. 9. The relative displacement amount measuring device according to claim 7 or 8, wherein the first probe and the second probe are mounted on a single stage so that an interval of irradiation light from both can be adjusted. A relative displacement amount measuring device comprising: