JP3428067B2 - Displacement measuring method and displacement measuring device used therefor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement measuring method for an object to be measured due to optical interference and a displacement measuring apparatus used therefor.

[0002]

2. Description of the Related Art With the development of lasers, electronics, microcomputers, sensor elements and the like, a method of measuring displacement of a minute portion of a surface by optical interference using a laser beam having excellent coherence has become widespread. This displacement measurement using laser light can be performed with high accuracy without contact.
For example, it is used for measuring the surface roughness of crystal wafers, optical components, X-ray mirrors, and the like.

The above-mentioned method of measuring the displacement of a minute portion of the surface by optical interference using laser light generally has the following configuration. The laser light emitted from the laser oscillator is split into reference light and irradiation light by a half mirror, a polarization beam splitter, etc., one reference light is reflected by a reference light reflecting mirror, and the other irradiation light is applied to the DUT. It irradiates and reflects it, and these two reflected lights having a phase difference due to the difference in optical path length are caused to interfere with each other. The interference light is detected by the photodetector, and the displacement of the minute portion of the surface of the object to be measured is measured according to the state of the interference. In the method of measuring the displacement of a minute portion of the surface by optical interference using laser light, the maximum value of the discontinuous displacement that can be discriminated is half the wavelength λ of the laser light used for measurement.

However, in the conventional method of measuring the displacement of a minute portion of the surface of the object to be measured by optical interference, a light source emitting a light of a single wavelength is used, and a displacement larger than the single wavelength of this light is discontinuous. If it happens, the displacement cannot be determined. Therefore, two or more types of light sources each emitting a light of a single wavelength different from each other, for example, a light source A emitting a light of a single wavelength λ _{1 and} a light source B emitting a light of a single wavelength λ ₂ (> λ ₁ ) When light is used, the combined wavelength λ _{3 of the} two lights regulates the maximum value of the discriminable discontinuous displacement. The combined wavelength λ ₃ of both lights is given by the following equation (1). λ ₃ = 1 / (1 / λ ₁ −1 / λ ₂ ) (1) From equation (1), the smaller the difference between the wavelength λ ₁ and the wavelength λ ₂ , the larger the synthetic wavelength λ _3, so it can be determined. It can be seen that the maximum value of such discontinuous displacement can also be increased. To use two or more types of light sources that emit different wavelengths of light,
It can be obtained by a method of using two or more laser oscillators, a method of splitting white light into light of two wavelengths or more by a spectroscope or the like.

As a method for measuring the phase difference between the reflected light of the irradiation light and the reference light, heterodyne interference is generated,
Method of measuring the phase with a phase difference meter, causing heterodyne interference, measuring the beat signal, performing phase modulation by resistance heating to the optical waveguide on the glass substrate, measuring the phase from the interference change due to the phase modulation, There is a method of using a laser that emits light of two or more wavelengths at the same time.

[0006]

By the way, in the method using two or more laser oscillators, lights having different wavelengths are introduced into the same interference system, or these lights are guided to the same position on the surface of the object to be measured. This complicates the measurement system. Therefore, the configuration of the device becomes large, and optical adjustment of optical elements such as a mirror, a lens, and a prism becomes complicated, and a slight shock or vibration may cause misalignment of the optical path. Moreover, in order to detect the difference in interference between lights having slightly different wavelengths, it is necessary to use an interferometer with excellent stability. Even if white light is used in a spectroscope to split it into two lights with different wavelengths, the split light has different exit angles from the spectroscope depending on the wavelength.
Special means are required to guide the divided light to the same position on the surface of the object to be measured.

The method of measuring the phase difference between the reflected light of the irradiation light and the reference light has the following problems. That is,
When the phase is measured by the phase difference meter, it is difficult to form a small interferometer because a plurality of AOM modulators and phase modulators are required to generate the heterodyne interference, and the response speed of the phase difference meter is increased. Since it is slow, there is a drawback that the measurement takes time. When a beat signal is measured by causing heterodyne interference, a plurality of AOM modulators and phase modulators are required as in the above case, it is difficult to form an interference system in a small size, and a displacement of a half wavelength or less is difficult to detect. is there. When performing phase modulation by resistance heating on an optical waveguide on a glass substrate, the time until temperature rise is much slower than the electro-optic effect, so high-speed phase modulation cannot be performed, and there is a drawback that measurement takes time. . When a laser that emits light of two or more wavelengths is used, the beat signal between the respective wavelengths is measured, so it is difficult to detect displacements of half a wavelength or less, and displacements of a fraction of λ are practical limits. is there.

The present invention has been made in view of the above-mentioned circumstances, and can be discriminated by using a single light source capable of oscillating laser beams having a plurality of wavelengths and an integrated optical element. In addition to expanding the maximum displacement value, adjustment is easy and quick, highly accurate and highly stable displacement measurement is possible.
An object of the present invention is to provide a compact displacement measuring device using the measuring method, which is resistant to shock and vibration.

[0009]

A displacement measuring method according to the present invention divides a laser beam of the same light source into a reference beam and an irradiating beam, the reference beam is used as a reference beam reflecting mirror, and the irradiating beam is the object to be measured. In the displacement measurement method of measuring the micro displacement of the surface of the DUT from the interference light, the reflected light of the reference light and the reflected light of which the phase difference is caused by the difference of the optical path are interfered with each other, A laser diode light source that oscillates two or more types of laser light having different wavelengths depending on the value of Input side
Polarization component of either TM light or TE light depending on the polarizer
After attenuating , the laser light is split into a reference light and an irradiation light by a 3 dB coupler, and one or both of the reference light and the irradiation light is reciprocated before reciprocating in the optical waveguide on the crystal substrate. After phase modulation is performed by the phase modulator using the electro-optic effect formed on the substrate, and the reference light and the irradiation light that have been phase modulated are caused to interfere with each other by the 3 dB coupler ,
Same as attenuation by input side polarizer by output side polarizer
Flip attenuates the polarized light component in the direction that you detect the interference light, and the first feature.

In addition to the first feature, the displacement measuring method of the present invention is characterized in that the crystal substrate is LiNbO ₃ crystal substrate or LiT.
The second feature is that it is an aO ₃ crystal substrate. Further, in the displacement measuring method of the present invention, in addition to the first and second features, the interference of the reference light and the irradiation light, which are phase-modulated by the phase modulator formed on the crystal substrate, in the 3 dB coupler is a proton exchange optical waveguide. It is characterized by being done by.

The displacement measuring device of the present invention comprises a light emitting power source,
It is composed of a light source, an interference system, and a power source for phase modulation, guides light from the light source to the interference system, divides this light into reference light and irradiation light, and uses the reference light as a reference light reflecting mirror. Irradiation light is irradiated to the object to be measured, the interference light between the reflected light of the reference light and the reflected light of the irradiation light is detected, and in the displacement measuring device that measures the minute displacement of the surface of the object to be measured, the light source is A laser diode light source that oscillates two or more types of laser light having different wavelengths depending on the value of the emission current, and an interference system includes two polarizers, a 3 dB coupler coupled to these polarizers through an optical waveguide, and a reference light. The first feature is that the optical waveguide type interference system in which one or both of the optical waveguide for irradiation and the optical waveguide for irradiation light and the phase modulator provided on both are integrated on the same crystal substrate.

In addition to the first feature, the displacement measuring device of the present invention is characterized in that the crystal substrate in the optical waveguide type interference system is LiN.
The second characteristic is that it is a bO ₃ crystal substrate or a LiTaO ₃ crystal substrate. Further, the displacement measuring device of the present invention is characterized in that, in addition to the first and second characteristics, the optical waveguide type interference system is composed of a proton exchange optical waveguide.

[0013]

1 is a conceptual diagram for explaining the operation of the displacement measuring method of the present invention and the displacement measuring apparatus used therefor.
In the figure, 1 is a laser diode light source that oscillates two or more types of laser light having different wavelengths depending on the value of emission current, 2 is an optical isolator, 3 is an input side optical fiber, and 4 is a crystal substrate. An optical waveguide type interference system including the following optical elements is formed on the crystal substrate 4. That is, an input side polarizer 5, a 3 dB coupler for splitting laser light into reference light and irradiation light 6, and phase modulation electrodes 7a, 7b, 7c for applying an electric field.
And the reference light optical waveguide 8 formed between the phase modulation electrodes 7a and 7b, the irradiation light optical waveguide 10 formed between the phase modulation electrodes 7b and 7c, the output side polarizer 13, and the input end 1.
6, the output end 17 and the like constitute an optical waveguide interference system.

Reference numeral 9 is a reference light reflecting mirror provided at one end of the optical waveguide interference system, 11 is a condenser lens, 12 is an object to be measured, 14 is an output side optical fiber, and 15 is a light detecting diode.
Further, the phase modulator 18 has a structure in which electric fields in opposite directions are applied to the reference light optical waveguide 8 and the irradiation light optical waveguide 10 by the phase modulation electrodes 7a and 7c and the phase modulation electrode 7b. To do. 3 dB coupler 6 is input side polarizer 5
Both the optical waveguide 6a that connects the optical waveguide that passes below and the optical waveguide 8 for reference light, and the optical waveguide 6b that connects the optical waveguide that passes below the output side polarizer 13 and the optical waveguide 10 for irradiation light The waveguides 6a and 6b are arranged close to each other and are located in parallel with each other.

A stepwise wave-modulated light emission current as shown in FIG. 2 is passed through the laser diode light source 1 to alternately oscillate laser light of two or more kinds of wavelengths according to the stepwise wave light emission current. After passing through the optical isolator 2, this laser light propagates from the input end 16 to the optical waveguide interference system on the crystal substrate 4 via the input side optical fiber 3. In the optical waveguide interference system, the laser light passes through the input side polarizer 5 and propagates while being distributed almost equally to the reference light and the irradiation light by the 3 dB coupler 6. The reference light propagates through the reference light optical waveguide 8 between the phase modulation electrodes 7a and 7b and is reflected by the reference light reflecting mirror 9. The reflected light from the reference light reflecting mirror 9 passes through the reference light optical waveguide 8 again and returns to the 3 dB coupler 6. The staircase-shaped modulated current waveform does not necessarily need to be monotonically increasing, and the same effect can be obtained even if the predetermined current is continuous for a certain time and has an uneven waveform.

On the other hand, the irradiation light is the phase modulation electrodes 7b and 7c.
The light is propagated through the optical waveguide 10 for irradiation light in between, emitted out of the optical waveguide interference system, and passed through the condenser lens 11 to irradiate the DUT 12. The reflected light from the DUT 12 is again collected by the condenser lens 1
1, passing through the irradiation light optical waveguide 10 and returning to the 3 dB coupler 6. In the 3 dB coupler 6, the reflected light of the reference light and the reflected light of the irradiation light are caused to interfere with each other, and the optical waveguide 6 b and the output side polarizer 1
3, propagates to the output side optical fiber 14 via the output end 17, and is guided to the photodetection diode 15. The phase change of the reflected light is obtained with high accuracy by detecting the intensity change of the interference light due to the phase modulation by the light detection diode 15, and the phase change of λ / 2 on the surface of the DUT 12 is from several hundredths to several tens of thousands. A minute displacement of a minute is continuously measured.

As described above, the displacement measuring method and the displacement measuring apparatus according to the present invention measure the minute displacement of the surface of the object to be measured by irradiating the object to be measured with laser light and changing the optical path length. The light emitting power supply supplies a staircase-shaped light emitting current. According to the characteristics of the laser diode light source 1 to be used, two or more kinds of emission currents are alternately held for a certain period of time, and the waveform thereof has a stepwise waveform with respect to the time axis. This waveform may be repeated periodically. The laser diode light source 1 oscillates two or more types of laser light having different wavelengths depending on the value of the emission current, and has a Fabry-Perot resonator structure. By using such a laser diode light source 1, it is possible to obtain laser light of a plurality of wavelengths with one light source.

An optical isolator 2 provided in the middle of the optical path for propagating the laser light from the laser diode light source 1 to the optical waveguide interference system cuts off the returning light which becomes noise. Known optical fibers can be used for the input side optical fiber 3 for connecting the optical isolator 2 and the optical waveguide interference system 3, and for the output side optical fiber 14 for connecting the optical waveguide interference system and the photodetecting diode 15. The optical waveguide interference system includes an input side polarizer 5, which is formed by applying a metal film or a dielectric film on the crystal substrate 4.
Output side polarizer 13, 3 dB coupler 6 and phase modulator 18
It is formed by integrating and. The crystal substrate 4 of the optical waveguide interference system includes LiNbO ₃ (lithium niobate) or Li
Crystals of TaO ₃ (lithium tantalate) can be used.

Referring to the phase modulator, in FIG. 1, the phase modulator 18 includes a reference light optical waveguide 8 and an irradiation light optical waveguide 10, a phase modulation power supply 18a and a phase modulation electrode 7.
The electric fields in opposite directions are applied by the electrodes a, 7c and the phase modulation electrode 7b. However, it suffices if electrodes are provided on both sides of at least one of the reference light optical waveguide 8 and the irradiation light optical waveguide 10, and even if either of the phase modulation electrodes 7a and 7c is not provided, the phase modulation is performed. Bowl 1
8 works. Then, when a voltage of a required magnitude is applied between at least one of the phase modulation electrodes 7a and 7c and the phase modulation electrode 7b, the reference light optical waveguide 8 and the irradiation light optical waveguide 10 are formed. The phase difference of propagating light is modulated by the electro-optic effect. The phase modulator utilizing the electro-optic effect is made to have a response speed of several tens GHz, and high-speed modulation which cannot be obtained by other methods can be realized.

In the polarizers 5 and 13, TM light (light having an electric field component in a direction perpendicular to the substrate plane) and TE light (parallel to the substrate plane) are obtained by placing a metal on the optical waveguides 6a and 6b of the crystal substrate 4. Light having an electric field component in a direction) Either one of the polarized components is absorbed or attenuated by radiation. In addition,
If the entire crystal substrate 4 is composed of a proton exchange optical waveguide,
Since the crystal substrate 4 itself radiates TM light and propagates only TE light, it is not necessary to provide a polarizer. The polarizer is used to improve the purity of the polarized light of the reflected light from the object to be measured. This is because complete interference does not occur if the polarization remains disordered.

The 3 dB coupler 6 equally distributes the propagating laser light to the reference light and the irradiation light, and propagates them to the reference light optical waveguide 8 and the irradiation light optical waveguide 10, respectively. Then, the reflected light of the reference light from the reference light reflecting mirror 9 and the reflected light of the irradiation light from the DUT 12 are caused to interfere again by the 3 dB coupler 6. The reference light reflecting mirror 9 is provided by vapor-depositing a metal or an oxide on the end surface of the irradiation light optical waveguide 10 formed on the crystal substrate 4. By integrating the optical waveguide type interference system consisting of several kinds of optical elements on the same crystal substrate 4, it is not necessary to adjust the connection of each optical element, it is mechanically stable, and the optical coupling loss can be reduced. Further, the optical waveguide 6a, which is the reference light optical path, the optical waveguide 8 for reference light, and the optical waveguide 6b, which is the optical path for irradiation light, and the optical waveguide 10 for irradiation light, are separated by only a few hundred μm, and similarly Because of the change, the effect of thermal expansion is canceled out, and as a result, it is possible to measure the phase displacement with high accuracy and stability.

The condenser lens 11 is provided for adjusting the luminous flux of the irradiation light for irradiating the DUT 12 and the reflected light flux thereof. The photodetection diode 15 converts the interference light into an electric signal, and for example, a pn junction photodiode, a pin photodiode, an avalanche photodiode (APD) or the like can be used.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Based on FIG. 1, which is a conceptual diagram for explaining the operation of a displacement measuring device used in the displacement measuring method of the present invention,
After actually manufacturing the displacement measuring device, the displacement was measured.
X-cut LiNbO with a thickness of 1 mm and a diameter of 75 mm
_An exchange mask of SiO ₂ was applied to the _3- wafer crystal substrate 4. Next, proton exchange was performed using benzoic acid, and a proton exchange waveguide propagating in the Y-axis direction and a 3 dB coupler 6 were provided. Further, metal was vapor-deposited on the crystal substrate 4 to form the phase modulation electrodes 7a, 7b and 7c of the phase modulator 18.
Then, the crystal substrate 4 was diced into dimensions of 3 mm (width) × 30 mm (length) × 1 mm (thickness). Since the extinction ratio of the light propagating through the proton exchange waveguide was 50 dB or more, it was shown that this proton exchange waveguide sufficiently functions as a polarizer.

The half-wave voltage of the phase modulator 18 was 2.8V. Here, the half-wave voltage means a voltage required to give a phase change of πrad to the light propagating through the phase modulator 18. Since the phase modulator 18 was created for both the reference light and the irradiation light to perform the push-pull operation, the voltage required to give a phase change of πrad while the reference light and the irradiation light reciprocate respectively. Was 2.8V / 4 = 0.7V.

In order to reduce reflection loss, an antireflection film was coated between the input side optical fiber 3, the output side optical fiber 14 and the crystal substrate 4. Reference light reflection mirror 9
Was formed by evaporating aluminum on the end surface of the reference light optical waveguide 8. On the other hand, the end surface of the irradiation light optical waveguide 10 was also coated with an antireflection film in order to reduce reflection loss. Then, the irradiation light emitted from the end surface of the irradiation light optical waveguide 10 is concentrated by the condenser lens 11 to be measured 1.
Then, the reflected light was guided to the irradiation light optical waveguide 10 again.

The laser diode light source 1 used was a Fobry-Perot resonator type with a laser oscillation wavelength of 830 nm. The optical isolator 2 has a laser oscillation wavelength of 83
The one for 0 nm was used. In addition, the light detection diode 1
For 5, a Si-pin type photodiode was used.
In addition, a polarization maintaining optical fiber was used as the input side optical fiber 3, and a multimode optical fiber with a core diameter of 50 μm was used as the output side optical fiber 14. Phase modulation power supply 18a
Is a 12-bit DA driven by a microcomputer
It was composed of a converter and adjusted so that the output voltage range was ± 2.8V.

The laser light oscillated from the laser diode light source 1 was passed through the optical isolator 2 and then guided through the input side optical fiber 3 to the optical waveguide type interference system. The interference light is guided to the photodetection diode 15 through the output side optical fiber 14 which is a multimode optical fiber. The laser oscillation wavelength of the laser diode light source 1 is 830.1 nm and 83
By switching to 1.8 nm, the phase of the reflected light from the DUT 12 was measured at each wavelength. The phase measurement error at this time was 0.006 rad or less. 0.0
The phase difference of 06 rad is λ × 0.006 / (2π) =
This corresponds to an optical path length change of 0.8 nm. Therefore, the displacement resolution of this displacement measuring device is 0.4 nm or less. The wavelength λ _{3 of the} synthetic wave is 0.4 mm from the equation (1), and theoretically, the optical path difference of 0.4 mm, that is, the discontinuous displacement of 0.2 mm or less can be measured. As the object to be measured 12, an aluminum plate provided with overhanging step scratches was used. As a result, a step difference of 0.13 mm could be measured, and the measurement result shown in FIG. 3 was obtained.

[0028]

As described above, the displacement measuring method for a minute portion on the surface of an object and the displacement measuring apparatus used therefor according to the present invention are a single light source capable of oscillating laser beams of a plurality of wavelengths and an integrated optical component. By using, it is possible to increase the maximum value of the discontinuous displacement that can be discriminated, and it is possible to perform easy, quick, highly accurate and highly stable displacement measurement.
In addition, miniaturization of the device is promoted, and a displacement measuring device that is resistant to shock and vibration can be obtained.

[Brief description of drawings]

FIG. 1 is a conceptual diagram for explaining an operation of a displacement measuring method of the present invention and a displacement measuring device used for the method.

FIG. 2 is a waveform diagram of an emission current of a laser diode in the displacement measuring method of the present invention and the displacement measuring apparatus used therefor.

FIG. 3 is a diagram showing measurement results obtained in an example of the displacement measuring device of the present invention.

[Explanation of symbols]

1 Laser diode light source 3 Input side optical fiber 4 Crystal substrate 5 Input side polarizer 6 3dB coupler 7a Phase modulation electrode 7b Phase modulation electrode 7c Phase modulation electrode 8 Optical waveguide for reference light 9 Reference light reflection mirror 10 Optical waveguide for irradiation light 12 DUT 13 Output side polarizer 14 Output side optical fiber 15 Photodetection diode 18 Phase modulator 18a Phase modulation power supply

Claims (6)

(57) [Claims] 1. A laser beam of the same light source is divided into reference light and irradiation light, and the reference light is irradiated to a reference light reflecting mirror and the irradiation light is irradiated to an object to be measured. In a displacement measuring method for measuring a minute displacement of the surface of the object to be measured from the interference light by interfering each reflected light of the reference light and the irradiation light having a phase difference, the wavelength varies depending on the value of the emission current. A laser diode light source that oscillates more than one type of laser light is caused to flow the emission current that has been modulated in a staircase shape to alternately oscillate the laser beams of two or more types of wavelengths, and the input side polarizer configured on the crystal substrate TM light, TE
After attenuating either polarization component of the light, the laser light is split into reference light and irradiation light by a 3 dB coupler, and one or both of the reference light and the irradiation light is
Before going back and forth through the optical waveguide on the crystal substrate, phase modulation is performed by a phase modulator using the electro-optic effect formed on the crystal substrate, and the phase-modulated reference light and irradiation light are the 3 dB coupler. After interfering with
Attenuated at the input side polarizer by the output side polarizer
Attenuates the same direction of the polarization component with a displacement measuring method characterized that you detect the interference light. 2. A light source for light emission, a light source, an interference system, and a power source for phase modulation, which guides light from the light source to the interference system and splits the light into reference light and irradiation light. The reference light is reflected by the reference light reflecting mirror, and the irradiation light is applied to the object to be measured, and the interference light between the reflected light of the reference light and the reflected light of the irradiation light is detected, and the measured object is measured. In a displacement measuring device for measuring a minute displacement of a surface of an object, the light source is a laser diode light source that oscillates two or more types of laser light having different wavelengths depending on a value of a light emission current, and the interference system includes two polarizers. And a 3 dB coupler coupled to the polarizer via an optical waveguide, and a phase modulator provided on one or both of the reference light optical waveguide and the irradiation light optical waveguide, and a phase modulator integrated on the same crystal substrate. Displacement characterized by being a waveguide type interference system Constant apparatus. 3. The crystal substrate is a LiNbO ₃ crystal substrate or L
The displacement measuring method according to claim 1, which is an iTaO ₃ crystal substrate. 4. A crystal substrate in an optical waveguide type interference system is L
The displacement measuring device according to claim 2, which is an iNbO ₃ crystal substrate or a LiTaO ₃ crystal substrate. 5. The displacement measurement according to claim 1 or 3, wherein the interference of the reference light and the irradiation light phase-modulated by the phase modulator formed on the crystal substrate in the 3 dB coupler is caused by the proton exchange optical waveguide. Method. 6. The displacement measuring device according to claim 2, wherein the optical waveguide type interference system is composed of a proton exchange optical waveguide.