JPS6334963B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a length measuring method and apparatus using coherent light.

Conventionally, there have been various techniques for using coherent (laser) light in length measurement probes. For example, as shown in FIG. 1, a parallel laser beam from a laser light source (not shown) is condensed by a lens 3, and the reflected light from the surface of an object 4 is received by the same lens 3. The reflected light that has passed through the two pinholes 5 and 6 is received by the receivers 1 and 2, and the photoelectric output is differentially amplified, and the in-focus state is detected based on the positive and negative outputs. A pinhole that vibrates in the optical axis direction is placed on the image point,
A device that detects the focused state by detecting the phase of the photoelectric output signal after passing through a pinhole, or focuses a parallel laser beam with a lens that vibrates in the optical axis direction, modulates the focal length, and then detects the reflection from the object at that time. A device that detects the focused state based on the phase difference of the photoelectric output signal of light.
Alternatively, there are methods that utilize the straightness of laser light to measure distance from geometrical relationships.

However, these techniques cannot be applied to materials such as processed metal surfaces, which have high reflectance and are on the same level as the wavelength (0.1 μm ~
When applied to the surface of an object where roughness (unevenness) of 1.0 μm) is arranged with some regularity, coherent random laser speckles are generated.
A bright and dark pattern called a pattern is formed, which acts as noise in the measurement system as described above, and is a main factor in reducing the accuracy of length measurement and positioning.

This invention actively utilizes the laser speckle pattern that is generated as a result of the interaction between coherent light and the processed metal surface, and enables stable and highly accurate length measurement that is independent of the reflectance and surface condition of the surface of the object to be measured. Another object of the present invention is to provide an optical probe system that performs shape measurement in a non-contact manner.

Generally, in the configuration shown in FIGS. 2a and 2b, when a convergent Gaussian beam made by converging laser light with a lens L is irradiated onto a diffusive semi-transparent membrane M,
The backscattered light contains a speckle pattern that depends on the surface condition of the diffusive semipermeable membrane M. Now, when the diffusive semi-permeable membrane M moves in one direction in a plane perpendicular to the optical axis as shown by arrow a in the figure, the diffusive semi-permeable membrane M and the beam waist ω (point of minimum beam radius) The direction of movement on the rear observation plane S differs depending on the positional relationship between the two. That is, in FIG. 2a, it moves in the same direction a as the diffusive semipermeable membrane M, and in FIG. 2b, it moves in the opposite direction b to the diffusive semipermeable membrane M.

If the distance between the diffusive semipermeable membrane M and the beam waist ω ₀ is not very close, the speckle pattern undergoes parallel translation while maintaining approximately the same shape (translation region). When the distance becomes very small, no translation of the speckle pattern is allowed and the speckle pattern changes shape completely;
There is a region where gushing and suction movements are dominant (boiling region). The size of this region depends on a=πω ₀ ² /λ. Here, λ is the wavelength of the laser light used, and ω ₀ is the minimum beam diameter when the Gaussian beam is focused by a lens.

Although the above is a known fact, the exact same phenomenon can be observed even when the surface to be measured is a reflective surface.

In this invention, the surface to be measured does not move;
A specially configured optical system allows convergent Gaussian
The beam is moved by a minute amount on the focal plane in a direction perpendicular to the optical axis and irradiated onto the surface to be measured. Under these irradiation conditions, the relationship between the moving direction of the speckle pattern included in the reflected light reflected from a single point on the surface to be measured and the moving direction of the beam waist is determined. Know the positional relationship between. That is, the reversal of the moving direction of the speckle pattern and the beam waist is detected, and this state is treated as a focused state.

The entire optical system is mounted on a servo mechanism and configured to be movable in the optical axis direction, and information about the movement direction is fed back to the servo mechanism to drive the optical system so that it is always in a focused state. For example, the length is measured using a linear scale or the like. In addition, by adopting a two-dimensional or three-dimensional drive device,
It is also possible to measure the shape of objects. Note that in this invention, the surface to be measured may be moved instead of moving the beam waist, and the invention can be applied even when the surface to be measured is a transmission surface.

Examples embodying this invention will be described below. FIG. 3 is a block diagram showing a length measuring device using semiconductor laser light based on the above principle.

A semiconductor laser 11, a condenser lens 12, and an objective lens 13 are provided on a movable stage 10 that can be moved in the optical axis direction, and the beam waist position is set on the optical axis between the lenses 12 and 13 in a plane perpendicular to the optical axis. An acousto-optic deflection element 14 for moving the beam within the beam splitter 14 and a beam splitter 17 for reflecting part of the reflected light in a direction perpendicular to the optical axis are provided. and a one-dimensional solid-state image sensor 19 are provided, and these are placed on the movable table 10.

Coherent light emitted from a semiconductor laser 11 with an oscillation wavelength in the 0.8 μm band and a fundamental mode converges and diverges through a condenser lens 12 and is narrowed down again by an objective lens 13 to have a beam waist in front of the objective lens 13. It becomes a convergent Gaussian beam. The signal generated by the signal generator 15 is amplified and modulated by the amplifier 16 to drive the acousto-optic deflection element 14.

The convergent Gaussian beam exiting the objective lens 13 is
The reflected light including the speckle pattern is reflected by the surface to be measured (not shown) and is focused again by the lens 13, and then sent to the beam splitter 17 and the imaging lens 18.
The image is formed on a one-dimensional solid-state image sensor 19 at .

The element arrangement direction of the one-dimensional solid-state image sensor 19 is adjusted in advance so that it is parallel to the moving direction of the speckle pattern.
The direction of movement of the pattern is discriminated by signal processing electronics 20 as a phase lead or lag of the video signal output from the one-dimensional solid-state image sensor 19 to determine the positional relationship between the surface to be measured and the beam waist.
Then, the output of the electronic circuit 20 is input to the servo motor drive circuit 21, and the servo motor 22 for driving the moving table 10 in the optical axis direction is driven according to the determination result of the signal processing electronic circuit 20. 10
move forward or backward. 23 is semiconductor laser 1
This is a constant voltage power supply applied to 1.

The relationship between the moving direction of the speckle pattern and the phase lead and lag of the video signal from the one-dimensional solid-state image sensor 19 is shown in Fig. 4 a, b, Fig. 5 a, b, Fig. 6 a, b, and Fig. 6. It is shown in Fig. 7a, b and Fig. 8 a, b.

When the coherent light from the semiconductor laser 11 is irradiated onto the surface A to be measured as a converged Gaussian beam by the lenses 12 and 13, the reflected light interferes with each other, resulting in a bright and dark pattern called a random speckle pattern. As described above, the light that forms an image on the one-dimensional solid-state image sensor 19 includes dark areas S ₁ , S _{2 .} . . as shown in FIGS. 5a and 5b.

Assuming that the positional relationship between the beam waist ω and the surface to be measured A is near the focused state as shown in Figure 4a, the position of the beam waist ω is aligned with the optical axis using the acousto-optic deflection element (Figure 3). When moved to the position ω' in a vertical plane, the image forming plane on the one-dimensional solid-state image sensor 19 on which the reflected light RL forms an image,
The speckle pattern moves in the direction of arrow a as shown in FIGS. 5a-b. That is, a phase delay occurs in the video signal from the one-dimensional solid-state image sensor 19 as shown in FIGS.

On the other hand, the moving table 10 (third
If the beam waist ω is moved from the ω position to the ω' position, the speckle pattern will be as shown in Figures 7a to 7b. Move in the direction of arrow b on the image plane. That is, the video signal from the one-dimensional solid-state image sensor 19 has a phase advance as shown in FIGS. 8a and 8b.

In short, by moving the movable stage 10 by the basic feed unit step l, the phase of the video signal due to the speckle pattern is inverted. In the figure, SP indicates video sweep start and EP indicates video sweep end.

In this way, when the phase of the speckle pattern is reversed by moving the moving table 10 by the basic feed unit step l, it is assumed that the in-focus state is included within the range of this basic feed unit step l. Therefore, the feed amount of the basic feed unit step 1 may be determined depending on the required measurement accuracy. Depending on the required measurement accuracy, point _a (= 1/2 l), point a ₂ , or point a ₃ may be used as the focal point position, and thereby the distances L ₁ , L ₂ , and L ₃ with respect to the reference position P can be calculated. can be measured.

Furthermore, by moving the movable table 10 not only in the optical axis direction but also in two-dimensional and three-dimensional directions, it is possible to measure not only the length but also the shape of the object.

As described above, the present invention enables highly accurate and stable measurement on the micron order with a point-like non-contact optical probe in measuring the length or shape of a processed metal surface that is a reflective surface. Moreover, it has the feature of being easy to operate.

[Brief explanation of drawings]

Figure 1 shows an example of the conventional method, Figure 2a,
b is an explanatory diagram of the characteristics depending on the positional relationship between the beam waist and the diffusing semi-transparent membrane when the laser beam is a convergent Gaussian beam, FIG. 3 is a block diagram showing an example of the device of the present invention, and FIG. Figures a and b are explanatory diagrams showing the positional relationship between the beam waist and the surface to be measured;
Figures 5a, b and 7a, b are explanatory diagrams showing the movement of speckle patterns, respectively; Figures 6a, b
and FIGS. 8a and 8b are explanatory diagrams showing the phase delay and lead of the video signal, respectively. 10...Moving table, 11...Semiconductor laser, 1
2...Condenser lens, 13...Objective lens,
14... Acousto-optic deflection element, 15... Signal generator, 17... Beam splitter, 18... Imaging lens, 19... One-dimensional solid-state image sensor, 20... Signal processing electronic circuit, 21... Servo Motor drive circuit, 22...servo motor,
ω, ω′...beam waist, A...surface to be measured,
RL...Reflected light, S ₁ , S ₂ , S ₃ , S ₃ '... Dark part of speckle pattern.

Claims (1)

[Claims] 1. Length measurement, shape measurement, etc. of a surface to be measured where a speckle pattern is formed in the reflected light or transmitted light of a reflective surface, a transmitting surface, etc. that is irradiated with coherent (laser) light. In the length measurement method, the beam waist of a focused laser beam or the surface to be measured is moved in a plane perpendicular to the optical axis, and the beam waist and the surface to be measured are determined by the direction of movement and the direction of movement of the speckle pattern. Determine the positional relationship with the surface, assume that the positional relationship is within a predetermined range as a focused state, and turn the optical system so that the measured surface and the beam waist are in a focused state. A length measurement method characterized by measuring length and shape by driving in the axial direction and measuring the distance traveled. 2. An optical system equipped with means for irradiating a surface to be measured with a convergent laser beam of coherent light and moving the beam waist in a plane perpendicular to the optical axis, and combining the reflected light from the surface to be measured. a light-receiving optical system that forms an image on an image plane; a means for detecting a speckle pattern of reflected light that forms an image on the image plane and converts it into a video signal; and detecting a phase lead or lag of the video signal of the means. a signal processing electronic circuit; a servo mechanism that drives the entire optical system in the optical axis direction using a signal from the signal processing electronic circuit so that the positional relationship between the beam waist and the surface to be measured is always in a focused state; and the servo mechanism. A length measuring device comprising a moving distance measuring means for measuring the length and shape of a surface to be measured based on the amount of movement of an optical system in the optical axis direction. 3. The length measuring device according to claim 2, wherein the means for moving the beam waist in a plane perpendicular to the optical axis comprises a signal generator and an acousto-optic deflection element driven by the signal.