JPH0150975B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an apparatus for reading information by focusing a reading light spot onto an information track recorded spirally or concentrically on a recording medium through an objective lens. The present invention relates to a focus detection method for detecting misalignment.

The above-mentioned information reading device is known from the prior art, and examples of recording media having information tracks include those called video discs and audio discs. On these video disks and audio disks, video signals and audio signals encoded into information tracks are recorded as optical information such as optical transmission characteristics, reflection characteristics, and phase characteristics. The information recorded on the disk is recorded by rotating the disk at high speed, focusing the laser light emitted from the laser light source onto the information track through an objective lens, and detecting the transmitted light or reflected light modulated by the information track. I'm reading it. One of the characteristics of such recording media is that the information recording density is very high, so that the width of each information track is very narrow and the spacing between successive information tracks is also very narrow. In order to accurately read the original information from such a narrow information track, it is necessary to keep the objective lens always in focus on the disk surface and to reduce the diameter of the light spot on the disk surface. It needs to be made smaller. However, since the distance between the disk and the objective lens varies, it is not possible to maintain a focused state at all times. For this reason, in such an optical reading device, focusing control is performed by a servo mechanism to detect the defocus of the objective lens with respect to the disk surface, and to displace the objective lens in the direction of its optical axis based on this defocus signal. ing.

FIG. 1 is a diagram for explaining a focus detection method in a conventional optical reading device. The light (P-polarized light) emitted from the laser light source 1 is made into parallel light by a collimator lens 2, and passes through a polarizing prism 3 having a polarizing film, a quarter-wave plate 4, and an objective lens 5 to a disk including an information track 6a. It is focused on 6. This light beam is reflected by the information track 6a having an uneven pit shape, and is reflected by the objective lens 5.
The light then passes through a quarter-wave plate 4 and enters the polarizing prism 3. The reflected light incident on the polarizing prism 3 is 1/4
Since the light becomes S-polarized by the action of the wave plate 4, this light is reflected by the polarizing prism 3. The light beam reflected by the polarizing prism 3 is focused by a condenser lens 7 and a cylindrical lens 8. Here, cylindrical lens 8
Since the beam has a focusing effect only in one axis direction, the shape of the focused beam formed by the focusing lens 7 and the cylindrical lens 8 will change when the position of the disk 6 shifts up or down, causing the beam to be correctly focused on the information track (focus position). It deforms in the direction orthogonal to the boundary. Conventionally, this shape change was divided into four parts, for example, into a photodetector (not shown).
A focus error signal is obtained by detection, and focusing control is performed using this signal.

However, the above-mentioned conventional focus detection method requires a long optical path length to focus the reflected light from the video disk 6 after being reflected by the polarizing prism 3, so it has the disadvantage that the optical system becomes large. be. Furthermore, since the photodetector that obtains the focus error signal needs to be accurately placed in two axes: the optical axis direction and the plane orthogonal to the optical axis direction, it is difficult to adjust its position. Furthermore, since the area in which an error signal can be obtained due to changes in the shape of the focused beam is narrow, there is a drawback that no signal can be obtained if the beam is too far away from the focused state.

On the other hand, the applicant has already proposed a focus detection method and device that allows for a compact optical system, easy arrangement of a photodetector for obtaining a focusing error signal, and always detects the focus state with high precision. are doing. FIG. 2 is a diagram showing the configuration of essential parts of an example of an optical information reading device using a focus detection device proposed earlier by the applicant. 1
2 to make parallel light, a polarizing prism 13 having a polarizing film, a quarter wavelength plate 14 and an objective lens 15.
The information is converged on a disk 16 having an information track. This light beam is reflected by the information track 16a having an uneven pit shape, passes through the objective lens 15 and the quarter-wave plate 14, and enters the polarizing prism 13. Since the reflected light beam incident on the polarizing prism 13 becomes S-polarized light due to the action of the quarter-wave plate 14, this light is reflected by the polarizing prism 13. The parallel light beam reflected by this polarizing prism 13 is made incident on the detection prism 17, and its reflecting surface 18
A photodetector 19 receives the light beam reflected by the photodetector 19 . The reflective surface 18 is set at a critical angle or slightly smaller than a critical angle with respect to an incident light beam (parallel light beam) in a focused state. In this way, in the focused state, all the light reflected by the polarizing prism 13 is totally reflected by the reflecting surface 18 (actually, since the condition of the reflecting surface is not perfect, some light is transmitted in the n direction shown in the figure). ), when the disc 16 deviates from the focused state in the direction a, the light beam reflected by the polarizing prism 13 becomes a light beam having an inclination component as shown by the outermost rays a _i1 to a _i2 with respect to the reflecting surface 18. Become. Also disk 1
6 deviates from the focused state in the b direction, the reflective surface 18
The incident light beam has a tilt component as shown by the outermost rays b _i1 to b _i2 . That is, when the disk 16 is out of focus, the light beams incident on the reflective surface 18 change continuously around the critical angle, except for the central ray (dotted chain line) on the optical axis. Therefore,
When the disk 16 is displaced in directions a and b and is out of focus, the intensity of reflection at the reflective surface 18 changes rapidly with a slight change in the angle of incidence near the critical angle, so that the light flux reflected from the reflective surface 18 The light and dark states are reversed at the plane of the paper containing the central ray (output optical axis), that is, the plane perpendicular to the plane of incidence on the reflective surface 18. On the other hand, in the focused state,
Since the light is totally reflected uniformly, such brightness and darkness do not appear. The photodetector 19 has such a reflective surface 18
This detects the distribution of the amount of light reflected from the second
As shown in the figure, there are two light-receiving areas 1 divided into two by a boundary plane perpendicular to the incident plane.
It consists of 9A and 19B.

In FIG. 2, when the disk 16 is displaced in the direction a, among the light incident on the reflective surface 18, the light beam below the center ray in the figure is composed of all the incident light beams starting from the outermost incident ray a _i1 . The angle of incidence of the ray will be less than the critical angle. Therefore, there is transmitted light in this part, and the outermost transmitted light ray a _t1
A bundle of rays including from n to n is transmitted. The intensity of the reflected ray bundle including the outermost reflected ray a _r1 to the center ray is weakened by the amount of light transmitted. Furthermore, among the light beams incident on the reflective surface 18, the angle of incidence of all the light beams above the central light beam in the figure, starting with the outermost incident light beam a _i2 , is larger than the critical angle. Therefore, there is no transmitted light in this part, and all the incident light rays are reflected as being included in the light flux including from the outermost reflected light ray a _r2 to the center ray. Therefore, in this case, the light amount distribution on the photodetector 19 is such that the light receiving area 19A becomes dark and the light receiving area 19B remains bright.

On the other hand, when the disk 16 is displaced in the b direction, the relationship of the inclination of the incident light beam to the reflective surface 18 is opposite to that in the a direction described above, and therefore The relationship between light and dark is reversed. The reflected light and transmitted light on the reflecting surface 18 in this case are indicated by symbols b _r1 , b _r2 and b _t2 , respectively. Note that in the focused state, the amounts of light incident on the light receiving areas 19A and 19B of the photodetector 19 are equal. Therefore, by detecting the difference between the outputs of the light receiving areas 19A and 19B, a focusing error signal representing the amount and direction of deviation can be obtained from the amount and polarity, and based on this signal, the objective lens It is possible to perform focusing control to control the movement of 15 in the optical axis direction, and
The information signal recorded on the disk 16 can be detected from the sum of the outputs of the light receiving areas 19A and 19B. Moreover, in the focused state, there is almost no transmitted component on the reflective surface 18, so the loss of light quantity is extremely small, and when the focus is out of focus, the light beam on either side of the center ray is completely absorbed. Since the reflected light beam is reflected and the reflected intensity of the light beam on the other side is extremely reduced, the difference in light amount between the light receiving areas 19A and 19B becomes significant. Therefore, focus detection can be performed more accurately than in the case shown in FIG.

In the example shown in FIG. 2, the light reflected by the reflective surface 18 is received by the photodetector 19 having two divided light receiving areas 19A and 19B. The focus error signal can also be obtained by receiving the light with one photodetector, or by receiving the reflected light and the transmitted light with two photodetectors.

Further, as shown in FIG. 3, the applicant makes a polarized beam enter a polarizing prism 13 from a laser light source 11, and reflects this S-polarized beam by the polarizing prism 13 to form a quarter wavelength plate 14 and an objective lens 15. The reflected light passes through the objective lens 15 and the 1/4 wavelength plate 14, becomes P-polarized light, enters the polarizing prism 13, and is transmitted through the polarizing prism 13.
The light receiving area 19A is divided into four parts in a direction parallel to the plane of incidence on the reflection surface 18 of the detection prism 17 and in a direction perpendicular to the incident plane on the reflection surface 18 of the detection prism 17.
An optical information reading device in which light is received by a photodetector 19 having 19D has also been proposed. According to this device, the light receiving areas 19A to 19D of the photodetector 19
An information signal is generated from the sum of the outputs of the light receiving areas 19A and 19B and the sum of the outputs of the light receiving areas 19C and 19B.
Tracking that expresses a focus error signal from the difference between the sum of the outputs of the light receiving areas 19A and 19D and the positional deviation of the beam spot with respect to the information track 16a from the difference between the sum of the outputs of the light receiving areas 19A and 19D and the sum of the outputs of the light receiving areas 19B and 19C. Error signals can be detected simultaneously.

In Figs. 2 and 3, the detection prism 1
Although the S-polarized light and the P-polarized light are incident on the reflective surface 18 of the detector prism 17, the respective reflection intensities of the S-polarized light and the P-polarized light at the detection prism 17 are
Regarding R _S and R _P , as shown in FIG. 4, the change in reflection intensity near the critical angle is steeper for P-polarized light than for S-polarized light. Therefore, if the P-polarized light is made incident on the detection prism 17 as shown in FIG. 3, the focus error can be detected with higher sensitivity. However, as is clear from FIGS. 2 and 3, the arrangement of the laser light source 11, the detection prism 17, and the photodetector 19 is different when the S-polarized light is incident on the detection prism 17 and when the P-polarized light is incident on the detection prism 17. Due to the different locations, the third
It may not be possible to arrange it as shown in the figure. In addition,
The reflection intensity shown in FIG. 4 is when the refractive index of the detection prism 17 is 1.50.

Furthermore, in a focus detection device using a detection prism set to a substantially critical angle as described above, the applicant uses a long detection prism 17' as the detection prism as shown in FIG.
It has also been proposed that the reflected light beam from the disk 16 that has been reflected or transmitted by the detector is totally reflected multiple times between parallel opposing reflecting surfaces 18' of the detection prism 17', and is incident on the photodetector 19. When total reflection is performed multiple times in this way, when the reflection ratio for one total reflection is 1/T, for N total reflections it becomes 1/T ^N , and the sensitivity increases exponentially. Therefore, the incident light on the detection prism 17' (disk 1
Highly sensitive focus detection is possible even if the reflected light beam from 6 is S-polarized light, and detection sensitivity can be further increased when the incident light is P-polarized light. but,
In such a method, the dimensions of the detection prism 17' become large, making it impossible to construct a compact optical system, and the parallelism of both reflecting surfaces 18' is required, making it difficult to create the detection prism 17'. , there are defects that make it expensive.

The purpose of the present invention is to detect focus using total reflection at the critical angle mentioned above, and to detect the focus without performing multiple reflections.
The present invention aims to provide a focus detection method that can improve detection sensitivity in polarized light.

The focus detection method of the present invention focuses light emitted from a light source onto an object using an objective lens,
At least a part of the reflected light beam is directed to a reflective surface of a prism coated with an anti-reflection thin film so that one of the light beams is at a critical angle or slightly smaller than this, and is S-polarized with respect to the reflective surface. A focus error signal of the objective lens with respect to the object to be irradiated is obtained by detecting a change in the amount of reflected light from the reflecting surface.

The present invention will be described in detail below with reference to the drawings.
FIG. 6 shows the configuration of an example of an apparatus for carrying out the focus detection method according to the present invention.The configuration other than the detection prism is the same as that of the apparatus shown in FIG. They are shown with the same reference numerals as , and their explanation will be omitted. In the present invention, an antireflection thin film 20 is deposited on the reflective surface 18 of the detection prism 17 by vapor deposition, sputtering, etc., and one of the S-polarized incident light beams (reflected light beam from the disk 16) is directed to the reflective surface 18. The two light beams are incident on the detection prism 17 such that the angle of incidence at the interface between the antireflection thin film 20 and the air is at or slightly smaller than the critical angle, and changes in the amount of light reflected from the interface are detected. Possibly the objective lens 15
Focus detection for the disk 16 is performed. In this example, the antireflection thin film 20 is formed of a medium having a lower refractive index than the detection prism 17, for example, _MgF2 . That is, the refractive index of the glass of the detection prism 17 is n ₁
When the refractive index of the antireflection thin film 20 is n ₂ , n ₂ <n ₁ is satisfied. Further, the film thickness d of the antireflection thin film 20 is set to d=λ/4n ₂ cosθ, where λ is the wavelength of the incident light and θ is the incident angle near the critical angle. In other words, for incident light near the critical angle, the antireflection thin film 2
The phase difference between the reflected light from the top and bottom surfaces of 0 is set to 1/2 wavelength.

Here, as the detection prism 17, the refractive index n ₁ =
1.51 as the antireflection thin film 20 with a refractive index n ₂
= 1.4, and this anti-reflection thin film 20 has a λ/
The change in reflectance when the anti-reflection thin film 20 is coated to a thickness of 4 is the change in reflectance R S for S-polarized light and the change in reflectance R _S for P-polarized light when the anti-reflection thin film 20 is not coated, as shown by the solid line in FIG. It is intermediate between R _P and can improve detection sensitivity in S-polarized light. Note that this reflectance change can be made more steep as the difference between n ₁ and n ₂ is increased.

In the present invention, the antireflection thin film 20 is coated on the detection prism 17 as described above, but this thin film does not necessarily have to be a single layer, but can also be a multilayer film. FIG _{. 8 shows a thin film layer 20-1 of low refractive index (n 2 =} 1.4) made of MgF ₂ and _a high refractive index (n ₃ = 2.5) thin film layer 20-2 and
A thin film layer 20 of low refractive index (n ₄ = 1.4) made of MgF ₂
3 are sequentially laminated, and the thickness d ₁ of each thin film layer,
d ₂ and d ₃ are respectively d ₁ = λ/4n ₂ cosθ ₂ and d ₂ = λ/4n ₃ cos
θ ₃ , d ₃ =λ/4n ₄ cosθ ₄ (where θ is the incident angle near the critical angle). In this case, the change in reflectance is the 9th
As shown by curve A in the figure, it is possible to obtain a change in reflectance greater than that obtained when P-polarized light is reflected three times as shown in FIG. Although various other configurations are possible for the multilayer film, it is preferable to alternately laminate thin film layers with a high refractive index and a low refractive index relative to the refractive index of the detection prism 17, with the top layer facing the air. The layer is configured to be a thin film layer with a low refractive index. Curve B in FIG. 9 shows a thin film layer with a high refractive index of 2.5 (TiO ₂ ) and a thin film layer with a low refractive index of 1.4 (MgF ₂ ) on the reflective surface 18 of the detection prism 17 with a refractive index of 1.51. The curve C shows the change in reflectance when the layers are stacked in sequence, and the curve C shows the change in reflectance when thin film layers having the same refractive index as above are stacked in the order of high-low-high-low.

The present invention is not limited only to the embodiments described above, and numerous modifications and changes are possible. For example, various modifications can be made to the configuration of the optical system other than the prism and the thin film. Further, the object to be irradiated is not limited to a disk, and the present invention can be applied to various objects on which the light beam should be focused.

According to the present invention, as shown in FIG. 2, the change in reflectance due to the change in the incident angle of S-polarized light is less This makes it possible to obtain a focus error signal with extremely high sensitivity. Also,
Since there is no need to perform multiple total reflections, the prism can be made small and lightweight, and parallelism of the reflecting surfaces is not required as in a multi-reflection prism. In addition, especially in the case of a multilayer film structure, the change in reflectance becomes even steeper due to the antireflection effect, and even higher detection sensitivity can be obtained. Furthermore, if a hard material such as TiO ₂ or SiO ₂ is used as the medium for the antireflection thin film, the reflective surface can be effectively protected.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an apparatus for implementing an example of a conventional focus detection method, and FIG. 2 is a diagram showing the configuration of an example of an apparatus for implementing the focus detection method previously proposed by the applicant. Figure 3 is a perspective view showing the configuration of another example, Figure 4 is a graph showing changes in reflectance in a total reflection prism for S-polarized light and P-polarized light, and Figure 5 is a focus detection method previously proposed by the applicant. Diagram 6 showing a modification of the main part of still another example of the apparatus for carrying out the method
The figure is a diagram showing the configuration of an example of an apparatus for carrying out the focus detection method according to the present invention, and FIG. 7 shows how reflectance changes in the focus detection method according to the present invention and the focus detection method shown in FIG. FIG. 8 is a graph showing the multilayer thin film structure of the antireflection thin film, and FIG. 9 is a graph showing the change in reflectance in the case of the multilayer thin film structure. 11... Laser light source, 13... Polarizing prism,
14...1/4 wavelength plate, 15...Objective lens, 16
... Disc, 17 ... Prism, 19 ... Photodetector, 20, 21, -1 to 20-3 ... Thin film.

Claims (1)

[Claims] 1. Light emitted from a light source is focused onto an object to be irradiated by an objective lens, and at least a part of the reflected light is directed toward a reflective surface of a prism coated with an antireflection thin film. One light beam is made to have a critical angle or slightly smaller than this, and is incident on the reflective surface as S-polarized light, and a change in the amount of light reflected from the reflective surface is detected. A focus detection method characterized by obtaining a focus error signal for an irradiated object. 2 The refractive index of the anti-reflection thin film is made lower than that of the prism, and the film thickness is set such that the phase difference between the reflected light from the upper and lower surfaces of the thin film is 1/2 wavelength for incident light near the critical angle. A focus detection method according to claim 1, characterized in that: 3. The antireflection thin film is a multilayer film in which thin film layers having a lower refractive index and a higher refractive index than the prism are alternately laminated such that the thin film layer facing the air layer is the thin film layer having a low refractive index, and Claim 1, characterized in that the thickness of each thin film layer is set such that the phase difference between the reflected light from the upper and lower surfaces of the thin film layer is 1/2 wavelength with respect to the incident light near the critical angle. Focus detection method.