JPH07117410B2 - Distance detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention uses an optical means to detect whether a target object is far or close to a specified position. The present invention relates to an automatic focus adjustment device in an optical information processing device or an optical measurement device.

[Conventional technology]

An automatic focus adjustment device in an optical information processing device or an optical measurement device incorporates a distance detection device based on the astigmatism method in order to detect whether or not it is in focus. In such a conventional distance detecting device, as shown in FIG. 6, the laser beam emitted from the laser light source 21 is reflected by the surface of the flat plate beam splitter 22 tilted with respect to the optical axis and then collected. It is adapted to be converged by the optical lens 23 and to be irradiated on the target object surface 24. Further, the reflected light flux reflected by the target object surface 24 is made incident on the condenser lens 23 to be converged light, then transmitted through the flat plate beam splitter 22 and guided to the photodetector 25.

When the converged light is incident on the flat plate beam splitter 22 tilted with respect to the optical axis, astigmatism occurs, so that the cross-sectional shape of the light flux is as shown in FIG. Immediately after passing through, it has a circular shape, but as it gets farther from this, an elliptical shape → a linear shape (spot b ₁ )
→ Oval (spot b ₂ ) → Circular (spot b ₃ ) → Oval (spot b ₄ ) → Straight (spot b ₅ ) → Oval → Circular

The photodetector 25 is arranged at a position where the cross-sectional shape of the light flux is circular (spot b ₃ ), and detects the cross-sectional shape of the light flux having astigmatism. As shown in FIG. 8, the light receiving surface of the photodetector 25 has a division line (dead zone) 25.
Four light-receiving areas 25a ・ 25b ・ 25c ・ 2 in the shape of a square at e ・ 25f
It is divided into 5d and the intersection of the division lines 25e and 25f is aligned with the optical axis, and a spot is formed in the light receiving regions 25a to 25d as described below, so that the distance can be detected from these spot shapes. Become.

When the target object surface 24 with respect to the condenser lens 23 is at a position that matches the focal length that is the prescribed position, the photodetector 25
Since the circular spot b ₃ is formed on the light receiving surface of the area, the outputs of the areas 25a to 25d are equal to each other. Next, when the target object surface 24 is closer to the condenser lens 23 than the specified position, the position where the cross-sectional shape of the light beam in which astigmatism has occurred becomes circular is far from the flat plate beam splitter 22, On the light-receiving surface of the photodetector 25, an elliptical spot b ₂ having a long axis in the X-axis direction in the figure is formed, and the area 25a · 2
The output becomes smaller in 5c, while the output becomes larger in the regions 25b and 25d. In addition, the object surface 24
Is farther than the prescribed position, the position where the cross-sectional shape of the astigmatic light beam becomes circular approaches from the flat plate beam splitter 22, so that on the light receiving surface of the photodetector 25, Y in the figure is shown. An elliptical spot b ₄ having a long axis in the axial direction is formed, and the output is reduced in the regions 25b and 25d, while
The output is larger with 25a and 25c.

Therefore, it is possible to detect whether the target object is at a position far from or close to the specified position based on the outputs from the regions 25a to 25d in the photodetector 25.

In the automatic focus adjustment device, when the distance detected by the distance detection device incorporated therein is closer than the specified distance, the condenser lens 23 is moved in a direction away from the target object surface 24, and when the distance is far, the direction is approached. Move to and make adjustments so that the distance is always the specified value.

Further, a distance detecting device that obtains astigmatism by using a cylindrical lens is also known.
The same detection principle as in the case of using the flat plate beam splitter 22 is used.

[Problems to be Solved by the Invention]

However, since the above-mentioned conventional distance detection device based on the astigmatism method detects a geometrical optical change, it detects the change in the distance of the target object plane 24 near the focus (within the depth of focus). There is a drawback that high sensitivity cannot be expected. Also, the astigmatism due to the flat plate beam splitter 22 is
Since it is proportional to the thickness of the flat plate beam splitter 22, a considerable thickness is required to obtain astigmatism comparable to that of a cylindrical lens. As a result, the flat plate beam splitter 22 becomes large in size, and there is a drawback in that the location is restricted.

[Means for Solving the Problems]

In order to solve the above-mentioned problems, a distance detection device according to the present invention is a distance detection device configured to detect a distance to a target object by receiving a light beam reflected on the target object surface, An optical system for converting the light beam reflected by the beam into a convergent light, and a phase element for changing the phase of one light of a predetermined aperture divided by two in a plane including the optical axis with respect to the light beam passing through this optical system by π And a photodetector for receiving the interference light due to the convergent light that has passed through the phase element and the convergent light that has not passed through the phase element, the photodetector having a light receiving region divided into two in a plane including the optical axis. It is characterized by that.

[Work]

According to the above configuration, by receiving the interference light due to the convergent light that has passed through the phase element and the convergent light that does not pass through the phase element, the light intensity of the two main peaks is present on the light receiving area of the photodetector. A distribution is formed. Then, the light intensity changes in the light intensity distribution of the two peaks depending on whether the target object is located far from or close to the specified position. Therefore, the distance is detected from this change in light intensity. can do.

As described above, since the change in the diffracted light intensity distribution due to the interference of light is detected, it is possible to perform distance detection with high sensitivity and high accuracy as compared with the conventional device that detects the geometrical optical change.

Further, since the photodetector only needs to detect the light intensity change in the light intensity distribution of the two peaks, the light receiving area need only be divided into two. Therefore, the optical axis adjustment and the like become much easier than the conventional four-divided photodetector.

Further, even if a flat plate beam splitter is used in such a distance detecting device, it is not necessary to cause astigmatism in the flat plate beam splitter, so that the thickness can be reduced, and space saving and simplification of optical component arrangement can be achieved. And so on.

[Embodiment 1] The following will describe one embodiment of the present invention with reference to FIGS. 1 to 4.

As shown in FIG. 1, the distance detection device according to the present invention includes a laser light source 1, a flat plate beam splitter 2 having a transparent dielectric thin film 6, a condenser lens 3, and a photodetector 4. Is configured.

The laser light source 1 is for irradiating laser light,
The laser beam is applied to the flat plate beam splitter 2.

The flat plate beam splitter 2 is arranged at a predetermined angle with respect to the optical axis of the laser light, and reflects the laser light on the surface and guides it to the condenser lens 3, while it is reflected on the target object surface 5 and further. Converged light that has passed through the condenser lens 3 is transmitted therethrough.

The condenser lens 3 forms an optical system that converges the light flux reflected by the flat plate beam splitter 2 onto the target object surface 5 and converges the reflected light flux from the target object surface 5. Further, the pupil corresponds to a circular opening having a predetermined length. In other words, it does not transmit light except the pupil, and forms an edge portion of the circular aperture in the pupil and the mirror other than the pupil.

The dielectric thin film 6 forms a phase element, and is vapor-deposited and formed in a half region of the flat plate beam splitter 2 on the substrate back side with the center of the light flux as a boundary. The film thickness d of the dielectric thin film 6 satisfies the following first expression, where λ is the wavelength of the laser light and n is the refractive index of the dielectric. However, Θ is an angle formed by the optical axis and the flat plate beam splitter 2.

d = λ / 2nCOSΘ ... Formula 1 When light passes through a thin film that satisfies the above requirement, the phase of light is delayed by π compared with the case where it does not pass.

In this embodiment, the flat plate beam splitter 2 with the dielectric thin film 6 which also functions as a phase element is provided in the converged light traveling from the condenser lens 3 to the photodetector 4,
This is equivalent to changing the phase of half the light on the pupil of the condenser lens 3 by π, that is, providing a phase element in half of the circular aperture itself.

Here, as shown in FIG. 2, the half of the light on the pupil of the condenser lens 3 remains, but the phase of the light passing through the other half is π.
As shown in FIG. 3 (b), the diffracted light intensity distribution on the focal plane a ₁ in the case of changing only the two is symmetrical with respect to the plane including the optical axis and the Y axis. Form a mountain. On the other hand, the diffracted light intensity distributions on the front and back surfaces a ₂ and a ₃ of the focal plane a ₁ are, as shown in FIGS. 7A and 7C, two peaks asymmetrical to the plane including the optical axis and the y axis, That is, a mountain having different sizes on the left and right is formed.

As shown in FIG. 4, the photodetector 4 is divided into two light receiving regions 4a and 4b by a dividing line 4c, and the condenser lens 3
At the position where the diffracted light intensity distributions of the two peaks are equal when the distance between the target object surface 5 and the target object surface 5 is a prescribed distance (for example, focal length), the light corresponding to each peak in the light receiving regions 4a and 4b described above. The intensity can be detected.

According to the above configuration, the light intensity in the light receiving areas 4a and 4b changes as follows depending on whether the target object is located far from or close to the specified position. The distance can be detected from the intensity change.

As described above, when the target object surface 5 is located at the specified distance, the outputs based on the light reception in the light receiving regions 4a and 4b are equal to each other, which means that the target object surface 5 is located at the specified distance. I understand that. On the other hand, when the target object surface 5 is located closer than the specified distance, the light intensity of the mountain on the X-axis positive side becomes larger than the other, and there is a magnitude relationship between the outputs based on the light reception of the light receiving regions 4a and 4b. From this, it can be seen that the target object surface 5 is located closer than the specified distance. On the contrary, if the distance is far, the light intensity of the mountain on the negative side of the X axis becomes larger than the other, and the magnitude relationship opposite to the above case occurs in the output based on the light reception of the light receiving regions 4a and 4b. It can be seen that the surface 5 is located farther than the prescribed distance.

As described above, in the distance detection device according to the present invention, since the change in the diffracted light intensity distribution due to the interference of light is detected, it is possible to detect the distance with high sensitivity and high accuracy as compared with the conventional device that detects the geometrical optical change. It will be possible to do.

Further, since the photodetector 4 only needs to detect the light intensity change in the light intensity distribution of the two peaks, it is only required to be divided into two light receiving regions 4a and 4b. The dividing line 4c that divides the two light receiving regions 4a and 4b into two is, as shown in FIG. 1, that the aperture is divided into two parts by a phase element that changes the phase of the light by π and a part that does not change the phase of the light flux. It is set on a plane including the optical axis 7 (a plane including the optical axis and the y axis). As is clear from FIGS. 3 (a) and 3 (b), the light intensity in the vicinity of the dividing line 4c is substantially zero (corresponding to between two peaks of the diffracted light intensity distribution), and Even if the position of the photodetector 4 is slightly shifted in the X-axis direction, the light intensity incident on the light receiving regions 4a and 4b hardly changes. On the other hand, in the conventional photodetector, since the light receiving area is divided into four and the light intensity on the dividing line is the highest, it is sensitive to the positional deviation of the photodetector. Therefore, the optical axis adjustment and the like become much easier than the conventional four-divided photodetector.

In addition, the flat plate beam splitter 2 used in this embodiment
With respect to the above, since it is not necessary to generate astigmatism, it is possible to reduce the thickness thereof, and it is possible to achieve space saving and simplification of optical component arrangement. Further, since the dielectric thin film 6 is vapor-deposited on the flat plate beam splitter 2 so as to have the function of the phase element, the number of optical components does not increase, and there is no adverse effect such as complication of the structure.

Although the pupil of the condenser lens is a circular aperture in this embodiment, a condenser lens having a relatively large pupil may be used and an optical member having a predetermined aperture may be provided in the optical path. is there.

[Second Embodiment] The following will describe another embodiment of the present invention with reference to FIG. It should be noted that members having the same functions as those used in the above-described embodiment are designated by the same reference numerals and the description thereof is omitted.

In the distance detection device according to the present embodiment, as an optical system for making the light beam reflected by the target object surface as convergent light,
A condenser lens 11 that collimates the laser light emitted from the laser light source 1, and a parallel light flux that is reflected by the flat plate beam splitter 2 through the condenser lens 11 is converted into a target object surface 5
A condenser lens 12 for converging the flat beam splitter 2 and a condenser lens 13 for condensing the parallel light flux transmitted through the flat plate beam splitter 2 on the photodetector 4 are provided. It is arranged in the parallel light flux between 13.

According to the above configuration, the power efficiency of the light reaching the target object surface 5 from the laser light source 1 is increased in addition to the above-described excellent effect, and as a result, the intensity of the light reaching the photodetector 4 is increased. Since it also becomes stronger, it becomes possible to further improve the accuracy of distance detection.

〔The invention's effect〕

As described above, the distance detection device according to the present invention is a distance detection device that receives the light flux reflected on the target object surface and detects the distance to the target object, and is reflected by the target object surface. An optical system for converting the light flux into a convergent light, a phase element for changing the phase of one light of a predetermined aperture divided by two in a plane including the optical axis with respect to the light flux passing through the optical system by π, It is a configuration including a photodetector having a light receiving region divided into two in a plane including the optical axis for receiving the interference light by the convergent light having passed through the phase element and the convergent light not passing through the phase element.

As a result, it is possible to perform highly sensitive and highly accurate distance detection, and at the same time, it is possible to achieve various excellent effects such that the optical axis of the photodetector is remarkably easily adjusted.

[Brief description of drawings]

1 to 4 show an embodiment of the present invention. FIG. 1 is a schematic configuration diagram of a distance detecting device. FIG. 2 is an explanatory diagram showing the focal plane of the converged light that has been transmitted through the case where the half of the phase on the pupil of the condenser lens is changed by π, and the front and back surfaces thereof. FIG. 3 (a) is an explanatory view showing the light intensity distribution on the front surface a ₂ of the focal plane, FIG. 3 (b) is an explanatory view showing the light intensity distribution on the focal plane a ₁ , and FIG. 3 (c) is the focus. FIG. 7 is an explanatory diagram showing a light intensity distribution on a rear surface a ₃ of the surface. FIG. 4 is an explanatory view showing the light receiving surface of the photodetector. FIG. 5 shows another embodiment and is a schematic configuration diagram of a distance detecting device. 6 to 8 show a conventional example. FIG. 6 is a schematic configuration diagram of the distance detecting device. FIG. 7 is an explanatory diagram of astigmatism caused by the flat plate beam splitter. FIG. 8 is an explanatory view showing a light receiving surface form and a beam spot shape of the photodetector in the distance detecting device. 1 is a laser light source, 2 is a flat plate beam splitter, 3 is a condenser lens (optical system), 4 is a photodetector, 4a and 4b are light receiving regions,
5 is the surface of the target object, 6 is the dielectric thin film (phase element), 11.
12 and 13 are condenser lenses (optical systems).

Claims (1)

[Claims] 1. A distance detecting device for detecting a distance to a target object by receiving a light beam reflected by the target object surface, for converting the light beam reflected by the target object surface into convergent light. An optical system, a phase element for changing the phase of one light of a predetermined aperture divided into two parts by a plane including the optical axis with respect to a light flux passing through the optical system, a convergent light transmitted through this phase element, and a phase element Receive the interference light due to the convergent light that does not pass through,
And a photodetector whose light-receiving region is divided into two in a plane including the optical axis.