JPS58137141A - Focus detecting method - Google Patents

Abstract

PURPOSE:To perform high-precision focus detection without increasing the size of a device, by guiding reflected light to be used for the detection without guiding it directly to a reflecting member, allowing diffracted light to enter a diffraction grating to strike the reflecting member. CONSTITUTION:The transmission type diffraction grating 10 is arranged in an end surface of a polarization beam splitter 3. Reflected light from an irradiated body 6 is made into parallel luminous flux by an objective lens 5 before entering the diffraction grating 10 and primary diffracted light is also made into parallel luminous flux. Then the diffracted light from the diffraction grating 10 is allowed to enter a prism 7 as the reflecting member. For this purpose, variation of the distribution of the quantity of reflected light from a reflecting surface is utilized to perform the focus detection to improve the precision of the focus detection without increasing the size of the device.

Description

[Detailed description of the invention] The present invention relates to a focus detection method.

In recent years, it has become practical to record and read out information signals from sources such as disk memories and video disks. In order to perform the above recording and reading, generally, parallel light beams are converted into parallel beams using an objective lens. record information signals through
Alternatively, it is focused onto a recording medium to be read out. In order to perform recording or reading properly, it is necessary for the objective lens to accurately focus the focused light onto the recording medium. Here, it is necessary to accurately detect and adjust the positional relationship between the objective lens and the recording medium. In such cases, the method of detecting the positional relationship between the objective lens and the object to be irradiated with the focused light from the objective lens is known as a focus detection method.
Conventionally, the following methods are known.

That is, the light from the light source is made into a parallel beam, focused on an object to be irradiated by an objective lens, and at least a part of the light reflected from the object to be irradiated is guided to a transparent reflective member via the objective lens. The reflective surface of this reflective member is set so as to form a nearly critical angle with respect to one of the reflected light rays, and changes in the light intensity distribution of the light reflected by the reflective surface or the light transmitted through the reflective surface,
Or, by detecting the change in the amount of reflected light and transmitted light, a focus error signal of the objective lens with respect to the irradiated object is obtained α.Hereinafter, I! This focus detection method will be briefly explained with reference to FIG.

In FIG. 1 (1), 1 is a light source, 2 is a collimating lens, 3 is a polarizing beam splitter, 4 is a quarter wavelength plate, 5 is an objective lens, 6 is an object to be irradiated, Reference numeral 7 indicates a prism as a reflecting member, and reference numeral 8 indicates a photodetector. As the light source 1, a semiconductor laser is suitable.

The light emitted from the light source 1 is collimated by the collimating lens 2 and enters the polarizing beam splitter 3. A part of the light is reflected in a direction parallel to the optical axis of the objective lens 5, and is converted into a parallel beam by the collimating lens 2. enters the objective lens 5 via the wave plate 4,
The irradiated object 6 is, for example, a video disc or the like, and is converged by the lens 5 and is incident on the irradiated object 6.

Now, if the positional relationship between the irradiated object 6 and the objective lens 5 is appropriate, FIG. 1 (1) shows this state.
In this case - the reflected light from the irradiated object 6 is reflected by the objective lens 5
After passing through the lens 5, the light becomes a parallel light beam parallel to the optical axis, passes through the quarter-wave plate 4, and enters the polarizing beam splitter 3 from the left side in the drawing.

Since this light has passed through the 1/4 wavelength plate twice, the polarization plane 7 has been rotated 90 degrees from the state where it was separated by the tin ritter 3. 3 and enters the prism 7, which is a reflecting member.

The reflective surface 9 of the prism 7 is such that the angle of incidence θ with respect to the reflective surface 9 is a critical angle for the reflected light from the irradiated object 6 that is incident on the prism and coincides with the optical axis. I'm in a hurry to make it happen. In the state shown in FIG. 1, since the light beam incident on the prism 7 is a parallel light beam, the angle of incidence on the reflecting surface 9 is the same in all parts, and therefore the reflectance is also the same.

The light detection unit 8 includes light receiving elements 81 and 82, and receives the light beam reflected by the reflecting surface 9. As mentioned above, Figure 1 (1)
In the state shown in , the angle of incidence of the light beam incident on the reflecting surface 9 is constant everywhere, and the reflectance is the same everywhere, so in this state, the intensity of the light received by the light receiving elements 81 and 82 is the same. Yes, light receiving elements 81, 82
The difference between the outputs is 0.

That is, when the output difference between the light receiving elements 81 and 82 is 0, the irradiated object 6 is at the focal point of the objective lens 5,
It is known that the relative positional relationship between the objective lens 5 and the irradiated object 6 is correct.

However, as shown in FIG.
When the object 6 moves away from the object 6, the reflected light from the object 6 passes through the objective lens 5, becomes a convergent light beam, and enters the prism 7. In addition, Figure 1 (
II), 0III, the friend who avoids complications,
Light source 11 collimating lens 2, polarizing beam splitter
3.''/4 wavelength plate 4 (1 description is omitted.

In this way, when the beam enters the prism 7 as a convergent beam, the angle of incidence on the reflecting surface 9 becomes smaller than the critical angle in the portion of the beam above the optical axis, with the optical axis as the boundary in the figure. The part of the beam below the axis becomes larger than the critical angle. Then, in the portion of the light beam with an incident angle smaller than the critical angle, the reflectance on the reflecting surface decreases from the state shown in FIG. 1(1), and therefore the light intensity received by the light receiving element 82
It decreases from the state shown in FIG. That is, 100% of the light is transmitted to the light receiving element 81.
K is incident. Therefore, the intensity of the light received by the light receiving element 81 is greater than the state shown in FIG. 1(I). Therefore,
If the outputs of the light-receiving elements 81 and 82 are respectively fiIl and I2, since the differences I and -I2 are positive,
The relative positional relationship between the objective lens 5 and the irradiated object 6 as shown in FIG. 1 can be known.

On the other hand, as shown in FIG.
Since the reflected light from the irradiated object 6 becomes a diverging source and enters the reflecting member 7, the receiving source intensity of the light receiving element 82 becomes larger than that of the light receiving element 81. , the output difference ll-l2U is negative.

In the end, the output difference between the light receiving elements 81 and 82 can be used as a focus error signal of the objective lens with respect to the irradiated object, and by displacing the objective lens 5 with respect to the irradiated object 6 according to this focus error signal, , the irradiated object 6 and the focus position of the objective lens 5 can be made appropriate. Note that when there is an information signal to be read on the irradiated object 6, the output sum of the light receiving elements 81 and 82 corresponds to this information signal, so from the state shown in FIG. 1 (1),
The system excluding the irradiated object 6 can be used, for example, as a video disc read-out device.

By the way, in reality, in order to properly adjust the relative position between the irradiated object and the objective lens, in the focus detection described above, a change in the angle of incidence of the light incident on the reflective surface on the order of 0.01 degrees is detected. I must be able to do it. For this reason, extremely high accuracy is required for focus detection. As a way to meet this demand for high precision, we have traditionally used prisms as reflective materials. Instead, an IC with a vertical cross-sectional shape of a parallelogram is used, and its two mutually opposing parallel surfaces are used as reflecting surfaces, and the reflected light from the irradiated object is reflected by the two reflecting surfaces. A method is known in which the light intensity of the reflected light component that is not totally reflected is gradually attenuated by repeating the reflection more than once, thereby amplifying the difference in light intensity between the reflected light component and the totally reflected component. However, there is a problem with implementing this method, as the size of the device increases as the reflecting member becomes larger.
Furthermore, extremely high parallelism is required for the two surfaces used as reflective surfaces in the reflective member.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a focus detection method that can perform focus detection with high accuracy and without increasing the size of the device in the focus detection method described above.

The present invention will be explained below.

A feature of the focus detection method according to the present invention is that in the above focus detection method, the reflected light used for detection is not guided directly to a reflecting member, but is first guided to a diffraction grating, and the diffracted light by this diffraction grating is The point is to make the light incident on the reflective member.

The invention will now be described in detail with reference to FIG.

g2ffio (in IJ, the symbol 10i diffraction grating is shown. This diffraction grating 10 is a transmission type, and has a refractive index n
1 medium, and the number of lattice feet is D.

As shown in the figure, if the diffraction angle of the diffracted light is θm1, and the refractive index of the medium through which the diffraction source propagates is 02, the incident angle is 0 disk, and the wavelength is human, the following relationship holds true as is well known.

nX8tnθi+n2sinθm=m (1) mi
This is the order of diffraction. In implementing the present invention, diffracted light of any order can be used in principle, but in reality, it is most practical to use a first-order diffraction source with a high light intensity, so this will be discussed below. will be explained in one dimension only.

(If we take the differential on both sides of the υ equation, we get -n1cosθidθ4-n2cosθmdθm=0ko'rL. Therefore, H, cosθ1)n2cOsθm
If so, 1dθrr4 > jdθ1$ holds true. That is, in this case, the variation dO for zero incident angles
i can be increased with a variation dOm of the diffraction angle θm.

FIG. 2 (Ill) shows a reflection type diffraction grating with reference numeral 11. In this case, K Its , ('-O3θi>co
Soll can satisfy \ya (1 JdOml>1d01) The present invention utilizes the principle that the variation in the angle of incidence of light incident on the diffraction grating can be increased in the diffracted light.

This will be explained below using a specific example.

FIG. 3 shows one embodiment of the present invention. To avoid clutter, the same reference numerals as in FIG. 1 have been used for things that can avoid confusion.

In the figure, reference numeral 10 denotes a transmission type diffraction grating as shown in FIG. In the state shown in FIG. 3, the object 6 to be irradiated is exactly at the focus of the objective lens 5. Therefore, the reflection ft, c from the irradiated object 6 is converted into a parallel beam by the objective lens 5, and enters the diffraction grating lO, and the first-order diffraction source is also a parallel beam. The attitude of the reflecting surface 9 is adjusted so that this parallel light beam is incident at a critical angle θ. In the state shown in Fig. 3, the light receiving elements 81 and 82
It is easy to understand that the output difference II I2 is 0.
Good morning.

When the object 6 to be irradiated moves away from the objective lens 5 (see FIG. 1+II)), the light beam incident on the diffraction grating 10 becomes a convergent light beam, as shown in FIG.
The first-order diffracted light also becomes convergent, the amount of light incident on the light receiving element 81 increases, and the output difference IIIzU becomes positive.

Conversely, as shown in FIG. 1 (2), the irradiated object 6
However, when the objective lens 5 is near the lens, as shown in FIG. 5, the light incident on the diffraction grating 10 becomes divergent, the amount of light incident on the light receiving element 82 increases, and the output difference l1- I2 becomes negative.

In this way, the output difference is 1. By using -12 as a focus error signal, it becomes possible to adjust the objective lens 5 to the correct position with respect to the irradiated object 6.

Applying the explanation given in Figure 2 to this example, we find that H2==lXnl, generally nl>1, and θI
is θIzO in the normal detection state, and 0m is generally a finite value, so condition (2) is very effectively satisfied, and the variation in the angle of incidence on the diffraction grating 10 is effectively reflected in the diffracted light. can be amplified. As the direction of the diffracted light increases, the angle of incidence on the reflecting surface 9 also increases. Therefore, in the present invention (ri,
The accuracy of focus detection is the same as above. Let's use an example to evaluate how much accuracy can be improved. Assuming that the diffraction grating 10 is a glass plate (refractive index 1.5) l/r,
If we consider the case where a grid formed by marking with a ruling engine is used under the conditions of 2 stations - 0,985 and 0.996, in the example shown in Fig. 3, , if the angle of incidence θ1 on the diffraction grating 10 changes from the level θi = 0 in the range Δθl2±03 degrees, the change in the angle of incidence on the reflecting surface will be as shown in Figure 6. becomes.

For the song flilJ6-1, when λ/D = 0.996,
When curve 6-2n”/n=0.985, curve 6-3 [
, shows the case shown in FIG. Compared to the conventional example, λ
/D=0.985, the angle of incidence on the reflecting surface can be increased by about 5 times, and in the case of λ 0996, it can be increased by about 10 times.
This greatly improves detection accuracy.

FIG. 7 shows an example using the reflective diffraction grating 11 shown in FIG. 2 ([I). Here, too, the same ones as in Prisoner 1 are used to avoid any risk of confusion. Reference numeral 3' indicates a polarizing beam splitter, and this polarizing beam splitter 3' also serves as a reflecting member. Since the method for performing focus detection is considered to be clear based on the explanation so far, the explanation will be omitted here.

A feature of the present invention, as mentioned above, is the use of a diffraction grating. In this case, in order to make the reflected light from the irradiated object effectively contribute to the detection, it is necessary to use a diffraction grating with as high a diffraction efficiency as possible.

Therefore, in implementing the present invention, it is preferable to use a blazed grating or a volume phase grating that has a diffraction efficiency close to 100%.

In the example used in the above description, focus detection is carried out using detection of a change in the light amount distribution of reflected light by a reflecting surface. However, the present invention is not limited to such an example, and may be implemented by detecting the distribution of the original amount of light transmitted through the reflective surface, or by detecting a change in the amount of light between the reflected light and the transmitted source. It is also possible to do so.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining a conventionally proposed focus detection method, FIG. 2 is a diagram for explaining the present invention in detail, and FIGS. FIG. 6 is a diagram for explaining a specific example of the present invention, and FIG. 7 is a diagram showing another specific example of focus detection according to the present invention. be. DESCRIPTION OF SYMBOLS 1... Light source, 2... Collimating lens, 3... Polarizing beam splitter-14... l/4 wavelength plate, 5...
・Objective lens, 6... Irradiated object, 7... Film as a reflecting member, 8... Light detection section, 9... Reflective surface, 10... Transmission type diffraction grating, 11...・Reflection type diffraction grating, 81.82... Light receiving element, 3'... Polarizing beam splinter that also serves as a reflecting member.

Claims (1)

[Claims] Light from a light source is collimated and focused onto an object to be irradiated by an objective lens, and at least a part of the reflected light from the object to be irradiated is transmitted through the objective lens to a transparent beam. changing the light quantity distribution of the reflected light or transmitted light by the reflecting surface, by setting the attitude of the reflecting surface of the reflecting member to be approximately at a critical angle with respect to one of the reflected rays; Alternatively, in a focus detection method that obtains a focus error signal of the objective lens with respect to the irradiated object by detecting a change in the amount of reflected light and transmitted light, at least a part of the reflected light is guided to a diffraction grating. . A focus detection method, characterized in that the diffracted light by the diffraction grating is made incident on a reflecting member.