JPS63241305A - Fringe scanning method - Google Patents

Abstract

PURPOSE:To perform uniform fringe scanning without disturbing a wave front over the entire range of beam, by performing fringe scanning while the phase scanning between reference beam and signal beam is changed by the voltage applied to a member having electrooptical effect. CONSTITUTION:When the voltage from an oscillator 15 is applied to the electrooptical crystal 18 provided on the common beam path 2 of reference beam and signal beam as crystal apply voltage 17 through an amplifier 16, the phase difference between the reference beam and the signal beam receives relative modulation in proportion to said voltage to perform fringe scanning. When the applied voltage data at this time is taken in a computer and the image processing of the interference fringe pattern subjected to modulation is performed, the interpolation between fringes can be performed and the profile of an object to be measured can be measured with measuring resolving power higher than lambda/4.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a fringe scanning method in a laser interferometer.

In particular, since high-precision and high-speed fringe scanning can be achieved, improvements in the measurement accuracy of laser interferometers based on this principle can be expected.

[Conventional technology]

Regarding the conventional fringe scanning method in a laser interferometer, see "Optics" Vol. 15, No. 1, February 1986.

This is done with PZT as discussed in the paper entitled "Sinusoidal Phase Modulation Interferometry" on pages 25-30.

This technique will be explained with reference to FIG. A laser beam 2 from a laser oscillator 1 is expanded by a beam expander 3, split by a beam splitter 4, and irradiated on a reference reflecting surface 24 and a surface to be measured 10, respectively.The reflected light from both is then sent back to the beam splitter 4. They are combined to cause interference, and this interference fringe pattern is photoelectrically converted in the camera 12, and the image information is roughly taken into the computer 14 via the A/D converter 13 for image processing.

Here, in order to improve the measurement resolution, the voltage from the oscillator 15 is used as the PZT applied voltage 23 through the amplifier 16 to drive the PZT 22, and by sweeping the reflective reference plane 7 bonded to the PZT 22, the reference optical path is directly set. Fringe scanning was realized by changing the length 5.

[Problem that the invention seeks to solve]

In the above-mentioned conventional technology, since the reflecting reference plane is displaced by mechanical displacement using PZT, the reference reflecting plane does not maintain a completely perpendicular attitude to the beam due to the characteristics of the PZT used, and as a result, The problem was that the reference optical path length became non-uniform within the range of a reference beam with a certain spread.

This was directly reflected in the interference fringe pattern and caused measurement errors.

In addition, PZT has a very large problem with hysteresis in its dynamic characteristics, which is a bottleneck in achieving high precision in fringe scanning.The purpose of the present invention is to solve the above problems all at once. The purpose of this invention is to realize high precision and high speed fringe scanning.

[Means for solving problems]

The above purpose is to fix the reference plane and keep the reference optical path length physically constant, instead of mechanically changing the optical path length of the reference beam using PZT, and to set the reference plane at a position before the beam is expanded by the beam expander and This is achieved by providing an electro-optic crystal on the common optical path of the light and signal light and applying a voltage to it to statically change the relative phase difference.

[Effect]

An electro-optic crystal is an optically anisotropic crystal that is used by passing light so that the polarization planes of the reference light and signal light, whose polarization planes are orthogonal to each other, coincide with the two axis directions of the crystal. For example, it is an optical element that changes the phase of two linearly polarized lights at different rates in proportion to the voltage applied to the crystal (the electric field generated thereby), and as a result, the phase between the two lights changes in proportion to the applied voltage. Since the phase difference changes relatively accurately,
The optical path difference of the apparent reference beam can be freely controlled by applying voltage to the crystal. Therefore, the electro-optic crystal installed in front of the beam expander can change the phase difference without disturbing the wavefront of the laser beam, resulting in uniform fringe scanning throughout the wide measurement beam range. can be realized. Furthermore, compared to PZT, electro-optic crystals have almost no hysteresis and have much better linearity, making it possible to achieve highly accurate fringe scanning, and because of its good responsiveness, it is possible to increase the frequency of fringe scanning.

〔Example〕

Embodiments of the present invention will be described below with reference to FIG.

This embodiment is a flatness measuring device using laser interference. Laser light 2 from a laser oscillator 1 that oscillates linearly polarized light
passes through the electro-optic crystal 18 and reaches the beam expander 3, where the expanded beam is sent to the polarizing beam splitter 4 (
Hereinafter abbreviated as P, B, 8.4), a beam (hereinafter referred to as the S-polarized component beam) 5 has a vibration plane perpendicular to the plane of the paper and a beam (hereinafter referred to as the P-polarized component beam) has a vibration plane parallel to the plane of the paper. It is divided into 8 parts. The S-polarized component beam 5 is
P, B, S.

4 and passes through the one-wavelength plate 6, it is reflected at a reference reflecting surface 7 whose flatness is known and passes through the one-wavelength plate 6 again, and then P, B, 8.
However, since the plane of vibration of this light that has made one round trip through the -wave plate 6 has been rotated by 90 degrees, it now passes through P, B, and 8.4, and reaches the polarizing plate 11.

On the other hand, the P-polarized component beam 8, which first passes through P, B, 5.4, passes through the one-wavelength plate 9, is reflected at the measurement target surface 10, passes through the one-wavelength plate 9 again, and becomes P, B, and S.

4, the vibration plane is rotated 90 degrees as before, so it is reflected by P, B, and 8.4, and reaches the polarizing plate 11. At the time of incidence on the polarizing plate 11, the vibration planes of both lights are orthogonal to each other, so they do not interfere, but if the transmission axis of the polarizing plate 11 is set at 45 degrees with respect to the vibration plane of both lights, the transmission axis will change. The directional components are transmitted through each other and an interference fringe pattern is generated. This interference fringe pattern is photoelectrically converted in the camera 12, and further A/
The image information is sent to the computer 4 via the D converter 13.
and process the image. Then, the profile of the surface to be measured can be measured using this image processing data. However, the measurement resolution in this case is λ/4, where λ is the wavelength of the laser, and a fringe scanning method is known as a means for obtaining even higher measurement resolution.

A feature of the present invention is that this fringe scanning is realized by an electro-optic crystal 18 provided before the beam expander. When the voltage from the 1-shot device 15 is applied as the crystal applied voltage 17 via the amplifier 16 to the electro-optic crystal 18 provided on the common beam path 2 for the reference light and the signal light, the reference light and the signal light are The phase difference between the two is relatively modulated, and fringe scanning can be performed. By importing the applied voltage information into a computer and performing image processing on the modulated interference fringe pattern, interpolation between the fringes can be performed, and the profile of the measured object can be determined with a measurement resolution higher than λ/4. Can be measured.

Next, another embodiment will be explained with reference to FIG.

This embodiment is a displacement measuring device using laser interference.

Laser light 2 from laser oscillation H#1 that oscillates linearly polarized light
passes through the electro-optic crystal 18 and reaches the polarizing beam splitter 4. The S-polarized component beam 5, which has a plane of vibration perpendicular to the plane of the paper, is reflected. 8 is transmitted and is called signal light. The S-polarized light component 5 reflected at P, B, 3°4 passes through the one-wavelength plate 6, is reflected by the reflective film 7, passes through the one-wavelength plate 6 again, and becomes P,
B, 3°4, but this time the vibration plane is rotated 90 degrees, so it passes through P, B, 8.4 and reaches the polarizing plate 11.

On the other hand, the P-polarized component beam 8 passes through the one-wavelength plate 9, is reflected by the reflecting mirror 10 attached to the displacement measurement object, passes through the one-wavelength plate 9 again, and reaches P, B, 8.4, but the vibration plane is 9
Since it is rotated 0 degrees, P, B, S.

4 and reaches the polarizing plate 11. At the time of incidence on the polarizing plate 11, the vibration planes of the two lights are perpendicular to each other, so they do not interfere, but the transmission axis of the polarizing plate 11 is set at 45
If it is set to 100 degrees, the components in the transmission axis direction will pass through each other and cause interference. Assuming that the laser wavelength is λ, the intensity of this interference light changes from bright to dark or from dark to bright every time the object to be measured λ10 is displaced by -. ), and the number of brightness changes is determined by the control circuit 2.
By counting λ with the counter 1, the displacement of the WA constant object 10 can be measured with resolution. Here, in order to further improve the measurement resolution, a fringe scanning method is applied, which is also realized by the electro-optic crystal 18 in this embodiment. Electro-optic crystal 18 amplifier 16 provided on the common beam bus 2 for reference light and signal light
When a high frequency crystal applied voltage 17 is applied through the crystal, the phase difference between the reference light and the signal light is relatively modulated in proportion to this.

As a result, the interference signal 20 is modulated. At this moment, at the moment when the interference signal 20 becomes the brightest, the phase difference between the reference light and the signal light generated by the displacement of the measurement object 1o and the phase difference between the two lights generated in the electro-optic crystal 18 are just balanced. As a result, the phase difference becomes zero, so if the crystal applied voltage 17 at this moment is sampled by the sampling circuit in the control circuit 21, the crystal applied voltage which has been investigated in advance for the electro-optic crystal to be used can be obtained. The phase difference caused by the displacement is determined from the proportional relationship between the sensitivity and the phase difference generated.By further converting this into the amount of displacement of the object to be measured 10, high-precision displacement measurement exceeding the resolution is possible. It becomes possible.

In addition, in the case of this example, unlike the conventional PZT method, the optical path length can be changed without any mechanical moving parts, so high-speed and high-frequency fringe scanning can be achieved, and the time resolution of the measurement system can be greatly improved. This also has the effect of improving measurement resolution.

〔Effect of the invention〕

According to the present invention, uniform fringe scanning can be performed over the entire range of the beam without disturbing the wavefront, and furthermore, electro-optic crystal has extremely small hysteresis compared to PZT, and the amount of phase change that occurs with respect to applied voltage is Since the linearity of is also very good, the accuracy of fringe scanning is dramatically improved.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a device according to an embodiment of the present invention, FIG. 2 is a configuration diagram of a device according to another embodiment, and FIG. 3 is a device configuration diagram of a conventional example.
It is the cause. DESCRIPTION OF SYMBOLS 1... Laser oscillator, 4... Polarizing beam splitter, 18... Electro-optic crystal, 22... Piezo element.

Claims (1)

[Claims] 1. A member having an electro-optic effect is provided in the common optical path of the two linearly polarized lights in a laser interference measurement system in which two linearly polarized lights whose polarization planes are perpendicular to each other are used as reference light and signal light, respectively, and a voltage is applied to this member. A reference light beam is provided by applying a voltage to the member having the electro-optic effect.
A fringe scanning method characterized by performing fringe scanning by changing the phase difference between signal lights.