JPS5837849A - Detector of focus servo signal for optical information reader - Google Patents

Abstract

PURPOSE:To reduce the effect of disturbance, by providing a double refractive crystalline plate and a polarized separating plate between a polarized beam splitter and a photodetector. CONSTITUTION:The light emitted from a light source 10 is turned into parallel beams through a lens 11 and then into a circular polarized beam by a lambda/4 plate 13 after passing through a polarized beam splitter 12 to be focused on a recording carrier 6. This focused beam is reflected by the carrier 6 and converted into a linear polarized beam by the plate 13 after passing through the lens 14 again. In this case, the polarizing direction of the returned beam is orthogonal to the incident light, and the returned beam is reflected by the splitter 12 to be made incident to a lens 15. This incident beam is equal to a linear polarized beam in the direction (y) (vertical to the surface of paper). After this, the beam passes through a double refractive crystalline plate 16 having a crystalline axis in the direction of an optical axis and is converted into an electric signal through a photodetector 18 after passing through a polarized separating plate 17.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus servo signal detection device f in an optical information reading device.

As a method for detecting focus shifts due to surface blur, etc. that may occur when reproducing recorded signals from video or audio discs using an optical information reading device, returning light becomes convergent light or diverging light due to focus shift. This can be detected by converting it into a change in the shape of a spot projected onto a detector using astigmatic convergence, or by converting it into a spot displacement in a direction perpendicular to the optical axis using a prism. Various examples have been considered.0 For example, in an optical system as shown in FIG. It is converted into circularly polarized light, focused by the focusing lens 5, reflected by the record carrier 6, passes through the lens 5 again, becomes linearly polarized light by the λ/4 plate 4, is reflected by the polarizing beam splitter 3, and enters the lens 7. At this time, the polarized light is perpendicular to the plane of the paper.The light incident on the lens T is condensed by this lens 7, but due to the properties of the cylindrical lens 8, it becomes astigmatic convergent light, which reaches the focal point on the photodetector B. Depending on the amount of shift, a circle or an ellipse is projected on the photodetector ias, which is converted into an electric signal for focus servo as shown in FIG. 2 by the photodetector 904 divided sensor.

However, the optical system shown in Fig. 1 requires many optical parts, and their positions must also be adjusted.Furthermore, the light spot reflected on the photodetector 9 is minute, so it is nice to be able to make adjustments. There was a problem that it was susceptible to disturbances such as vibration.

The present invention has been made in view of the above points,
The purpose is to provide a focus servo signal detection device that can detect focus servo signals in the far field, reduces the number of optical components, facilitates position adjustment, and is less susceptible to external disturbances. be.

Hereinafter, the present invention will be explained in detail by way of an example. FIG. After passing through, it becomes circularly polarized light by the λ/4 plate 13, and is then focused onto the record carrier 6 by the lens 14r, where it is reflected, passes through the lens 14 again, and is converted into linearly polarized light by the λ/4 plate. At this time, the polarization direction of the returned light is a direction that is direct to the incident light. Therefore, it is reflected by the polarizing beam splitter 12 and enters the lens 15. This incident light is linearly polarized light in the y direction (direction perpendicular to the plane of the paper) - it then passes through a birefringent crystal plate 16 with a coupling axis in the optical axis direction, passes through a polarization splitter plate 11, and then passes through a photodetector 11.
1F- is converted into an electrical signal. In addition, the polarization separation plate 17
As shown in FIG. 4, the polarization separation plate 17 is formed into a fan shape with a center angle α centered at an angle of 45 degrees, and each fan-shaped portion passes only the polarized light component in the direction of the arrow. A photodetector 18 that can be made from a Polaroid board, etc.
is a divided detector with a dividing line in the y direction as shown in the tss diagram, and the electric output Pm of the sensor 18a and the sensor r1
The difference (Pmu PB') between the pneumatic output P and lb is output as a focus error signal.

FIG. 1 shows a second embodiment, which is the same as the first embodiment described above until the returned light is reflected by the polarizing beam splitter 12 and enters the birefringent crystal plate 19. However, since there is no lens 15 in the first embodiment, the light is not converged by the lens 15 and enters the birefringent crystal 19. This birefringent crystal plate 1s has its crystal axis 22 aligned with the first
As shown in the enlarged view in Figure 1, the return after passing through the zero-birefringent crystal plate 19, which is located in the ξ-2 plane, that is, at an angle of 45 degrees to the plane of the paper, and which has an inclination of θ with respect to the optical axis 2. The light passes through a polarization separation plate 20 and is converted into an electrical signal by a photodetector 21. Note that the polarization separation plate 20 is divided into four parts as shown in Figure 8, and each divided part allows only the polarized light component in the direction of the arrow to pass through. Further, as shown in FIG. 9, the photodetector 21 consists of four divided sensors 218 to 21d, and the electrical signals detected by each sensor 21a to 21d are transmitted from Q to Q.
D, then (QA −QB ) −(Q:o −
Qc) is output as a focus error signal.

Next, the effect will be explained. In both the first and second embodiments described above, the returned light incident on the birefringent crystal plates 16 and 19 is polarized light in the y direction perpendicular to the plane of the paper. If the coordinates are determined as shown in FIG. 10 when the polarizing beam splitter 12 is viewed from the photodetector 21 side, the polarization in the y direction is E = (0 + a cos ωt)
It is expressed as This polarized light is birefringent crystal plate 1'6.19
For example, in the case of an axial crystal, there is polarized light in the plane that includes the traveling direction and convergence axis, called ordinary light, and polarized light that oscillates perpendicularly to this (extraordinary light). The difference in refractive index causes a phase difference after passing through the crystal. Now, the incident polarized light P (PJ * Py) becomes polarized light p I (p, I, p, I) by passing through the crystal.
In the present invention, the polarization splitting plate t7%2
Using G and photodetector 18.21, birefringent crystal plate 11
1.1! IIc S (=<P's>t-<
In the first embodiment, as shown in the sixth factor, the incident angle i of the ray passing through the point P(g, y) and entering the birefringent crystal plate 16r is , mi =
It is expressed as (r)v)/L (where L is the distance from the reference coordinate O to the focal point). If the angle between OP and the X-axis is φ, the ordinary light Po and the extraordinary light Pe are PG"aQIIωtIIthφ pm=aasωt"amφ, and the two lights Po after passing through one birefringent crystal plate 16.

The phase difference δ of Pe is 1 δe= (NX-No”) ・tm (2r/λ)・
1/(2Ne No). However, t: crystal thickness, λ: wavelength of incident light, No=
Refractive index of ordinary light 1. Ne: Refractive index of extraordinary light.

Therefore, the ordinary light Po' and the extraordinary light Pe' after passing through the birefringent crystal plate 16 can be expressed as: Po'=aselfφ(ωt+δ) Pe'MaaII$−ωt. Therefore, the polarized light Pg', Py' in the X direction and the y direction is the island Pg' m Po' amφ + Pe'cm (φ
) Rot a drawing φ・tassel φ11 width (ωt+δ) − a blood φ − (2)
φ・plane ωt Py' m Po'dnφ+Pe'm(φ10/2>
.

madnφ#bi(ωt+δ)+aaIIφ11co! ω
t, and Δ5- (P
Mu-PB) becomes . However, W: radius of the fan shape of the polarization splitting plate 1T, C=
C(Ne-No)/(2Ne No): ]-(
2fJ'/λ], and if the amount of defocus is D, then the distance can be found as a function of D.

Now, for example, the focal length of the lens 14 is 4.3s+m.

The focal length of lens 15 is 7! IIL both lenses 14 and 15
Between 5+i+ and lens 140 aperture 43s+m.

The distance from the lens 15 to the birefringent crystal plate 16 is 041
1. The thickness of the crystal plate 16 is 5tg, and the photodetector 18
If the diameter of the fin is four fins, a focus error signal as shown in FIG. 12 will be obtained.

On the other hand, in the second embodiment, only the incident angle and the phase difference δ of the birefringent crystal plate 19 are expressed differently.
This is the same as the 14th example. Assuming that the crystal axis 22 of the birefringent crystal plate 1S is in the direction of the medical fiber of the crystal, the inclination of its normal line with respect to the light -zi is θ, so that the light ray passing through the locus (η, ξ) is The angle of incidence l' is C1)l l' = (ξ+θ+L'rxsl)
/ %'e'' + '7'' + L''. However, L/ is the distance from the coordinate center to the focal point.

Also, the normal vector n of the crystal plane is n5=(0,gtIIθ,-(2)θ)(η-ξ-2 coordinate)2 coordinate), and the x-y-z coordinate point (2e y * The plane of polarization of the ordinary light incident on point (0, O, L') is inclined with respect to the x-x plane, and if this angle is β, then it is expressed as follows. After passing through the birefringent crystal plate 19, the polarization intensity difference △S in the X direction and the y direction is △S =<Px'>
1-<Py'>t a! =-(ax2β+5In2β-(2)β). The electrical signal S detected by the photodetector 21 is 5=(Qmu〇B)
-(QD-QC) kyu (f public s-4, ps) is obtained as follows.

Now, for example, the focal length of the lens 14 is 43 m.

The distance from the lens 14 to the birefringent crystal plate 19 is T,
It is assumed that the thickness of the crystal plate 19 is 311, that the crystal @22 is aligned with the normal line of the crystal plate 190, that the angle between it and the optical axis 2 is θ-0.48 rad, and that the diameter of the photodetector 21 is 4 fl. In this case, a focus error signal as shown in FIG. 13 is obtained.

As explained above, according to the invention, there is no need to place a photodetector near the imaging point, so its position can be easily adjusted, it is less susceptible to disturbances, and the optical path length can be shortened. Furthermore, since the crystal plate, polarization splitting plate, and photodetector can be integrally made and incorporated, the number of parts can be reduced and mass productivity can be improved.

[Brief explanation of the drawing]

8g1 is an explanatory diagram of a conventional optical system for detecting focus servo signals, FIG. 2 is a characteristic diagram of the focus servo signal detected in FIG. 1, and FIG. 3 is a focus servo according to the first embodiment of the present invention. An explanatory diagram of the optical system for signal detection.
The figure is a plan view of the senna portion of the photodetector in Fig. 3, Fig. 6 is an explanatory diagram of light incident on the birefringent crystal plate in Fig. 3C, and Fig. 7 is a focus of the second embodiment of the present invention. An explanatory diagram of the optical system for detecting servo signals, Fig. 8 is a plan view of the polarization separation plate in Fig. 1, Fig. 9 is a plan view of the sensor portion of the photodetector in Fig. 1, and Fig. 10 is a coordinate diagram. Figure 11 is an enlarged perspective view of the relationship between the polarizing beam splitter and the birefringent crystal plate in Figure 1. is a characteristic diagram of the focus servo signal in the first embodiment, and FIG. 13 is a characteristic diagram of the focus servo signal in the second embodiment. Lens, 12...Polarizing beam splitter, 13...λ/4
Plate, 14... Lens, 15... Lens, 16...
Torsional refractive crystal plate, 17... Polarization separation plate, 1... Photodetector, 18... Birefringence crystal plate, 20... Polarization separation plate, 21... Photodetector. Patent applicant: Pioneer Corporation Figure 7 Figure 9 Figure 12 Figure 13

Claims (1)

[Claims] Light from a light source is guided to a record carrier through a polarizing beam splitter, a quarter plate, and a focusing lens in this order, and a return light from the record carrier is guided in order through the focusing lens, the λ/4 plate, and the polarizing beam splitter. In the optical information reading device, a birefringent crystal plate and a polarization separation plate are interposed between the polarization beam splitter and the photodetector in order from the polarization beam splitter to the photodetector. The returned light is incident non-parallel to the crystal axis of the birefringent crystal plate, and the polarized light component in a specific direction of the light that has passed through the birefringent crystal plate is separated and detected by the photodetector. 1. A focus servo signal detection device in an optical information reading device, characterized in that the device is configured to generate a focus servo signal.