JPH0731837B2 - Optical pickup device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical pickup device for detecting or recording information by irradiating the information recording surface of an information carrier with light. The present invention relates to an optical pickup device capable of reproducing / N signal.

[Prior art]

In recent years, research and development of a rewritable optical disc recording medium and an optical disc recording / reproducing apparatus using the medium have been actively conducted. One of such optical disk recording media is a magneto-optical recording medium. Signal reproduction from a magneto-optical recording medium (hereinafter simply referred to as a recording medium) is performed by utilizing a magneto-optical effect called Kerr effect or Faraday effect. That is, the polarization plane of the reflected light or the transmitted light from the recording medium is slightly rotated from the polarization plane when entering the recording medium, and the rotation component is converted into intensity modulation by a polarizing plate or the like to perform signal detection.

Since the rotation angle of this plane of polarization is approximately 1 °, the detected signal component is very small, and some contrivances have been made in the optical pickup device for signal detection.

FIG. 9 shows a schematic view of an optical pickup device used conventionally. In the figure, a light beam emitted from a semiconductor laser 1 (hereinafter simply referred to as LD) is converted into a parallel light beam by a collimator lens 2. The parallel light flux then passes through the beam splitter 3 and is focused by the objective lens 4 on the recording medium 5 into a spot of approximately φ1 μm. The light beam reflected from the recording medium 5 undergoes polarization plane modulation by the Kerr effect and Faraday effect, passes through the objective lens 4 again, and is separated from the incident light beam by the beam splitter 3. The separated light flux is partially reflected by the second beam splitter 6, passes through the lens system 7, and enters the optical sensor 8. The lens system 7 is composed of a known system, for example, an astigmatism system, a knife edge system, or a Fuco prism system.
The information on the distance between the objective lens 4 and the objective lens 4, that is, the AF error signal is obtained. In addition, a shift from the information track, that is, an AT error signal can also be obtained by the known push-pull method or the like. These error signals are fed back to a drive system (generally referred to as an actuator) for an objective lens (not shown) to perform accurate tracking at an accurate focus position and detect or record a signal.

The remaining light flux passing through the second beam splitter 6 is
It passes through the half-wave plate 9 and is split into two directions by the polarization beam splitter 10. When the optical crystal axis of the half-wave plate 9 is tilted by 22.5 ° with respect to the polarization axis of the incident light beam, the amount of light divided into two by the polarization beam splitter 10 is equal, and the polarizing plate is 45 ° for each light beam. It is equivalent to the arrangement with a transmission axis of -45 °. The two divided light beams are focused on the signal detection sensors 13 and 14 by the sensor focusing lenses 11 and 12, respectively, and the electric signals from the signal detection sensors 13 and 14 are differentiated (differential detection) to obtain a recording medium. The information signal above 5 can be detected.

The advantages of this differential detection method will be described below.

FIG. 10 schematically shows a signal amplitude component that reaches the polarization beam splitter 10 from the half-wave plate 9 and is divided here to reach the sensors 13 and 14. In the figure, when the vertical axis represents the polarization direction of the incident light beam, the light beam reflected from the recording medium 5 has a polarization plane of θ _K or −θ depending on the direction of magnetization (upward or downward) of the magnetic domain of the magneto-optical pattern.
Rotate _K. 1/2 wave plate 9 and polarization beam splitter 10
Since the combination of is equivalent to the system in which the transmission axis is inclined by 45 ° and the polarizing plate is arranged, the difference S ₁ of the projection components to the virtual transmission formula x, x ′ (the axes of the broken lines inclined by ± 45 °) is S ₁ ′ becomes the signal amplitude component.

Since the rotation angles θ _K and −θ _K change temporally depending on the magneto-optical pattern, the signal intensity changes of the signals received by the signal detection sensors 13 and 14 are as shown in FIGS. 11 (a) and 11 (b). The phases of the light beams divided into are shifted by 180 °.

Although the phase of the magneto-optical signal is inverted as described above, a normal noise component (noise from the recording medium 5, light fluctuation noise from the LD1, etc.) rides on these signals and the noise component is in phase.

Therefore, if the signals obtained from the sensors 13 and 14 are differentiated, the signal components will strengthen each other and the noise components will decrease. If the optical system is correctly arranged, S ₁ ² and S ′ ₁
^{Since 2} is equal and the noise amplitude is also equal, the signal is doubled and the noise is 0. As shown in FIG. 11 (c), the differential detection method as shown in FIG. 9 has an advantage that a signal with a good S / N can be detected.

However, in the above optical pickup device, the number of parts is large, and the position adjustment between the optical parts is required, which causes an increase in product cost and a decrease in reliability.

As an invention that partially compensates for these drawbacks, an optical pickup device is proposed in which a Wollaston prism is used instead of the polarization beam splitter 10 shown in FIG.

FIG. 12 is a schematic view of such an optical pickup device. In the figure, the light beam transmitted through the beam splitter 6 passes through the half-wave plate 9 and then enters the Wollaston prism 15 made of quartz, calcite or the like. The prism 15 is composed of a pair of prisms whose crystal axes are orthogonal to each other, and separates an incident light beam into a pair of linearly polarized lights vibrating in mutually orthogonal vibrating planes, each of which is an optical axis. It has a function of deflecting only a minute angle with respect to and emitting.

Therefore, two light spots are generated at the focal position of the sensor condenser lens 11, which are detected by the photodetectors 16 and 17, respectively.
Differential detection will be performed.

The optical pickup device shown in FIG. 12 does not require the polarization beam splitter 10 as compared with the optical pickup device using the two polarization beam splitters shown in FIG. 9, and is manufactured on a single substrate. Since a pair of photodetectors 16 and 17 are used, electrical characteristics can be made uniform, differential detection can be effectively performed, and adjustment is easy because there is only one sensor lens. Etc. There are many advantages.

However, even in this known example, the beam splitter 6, the sensor condensing lens 7, the focus error and the tracking error detection photodetector 8 are required, and there is a drawback that the configuration is complicated.

[Object of the Invention]

An object of the present invention is to provide an optical pickup capable of avoiding complication of an optical system, which is a drawback of the above-mentioned conventional example, and capable of signal detection with high S / N.

[Summary of Invention]

According to the present invention, in order to achieve the above object, a focusing optical system that irradiates light emitted from a light source onto an information recording surface and guides reflected light from the information recording surface to a light receiving portion of a photodetector. In the optical pickup device including: the focusing optical system includes a unit that separates the reflected light into a plurality of light beams having different polarization states; and the plurality of separated light beams are separated by the focusing optical system.
Focusing on one or more light spots, a light receiving unit of the photodetector being provided corresponding to the positions of the two or more light spots, and at least one of the light receiving units. It is divided into a plurality of light receiving surfaces, a set of amplifier circuits corresponding to the light receiving section is provided on the same substrate as the photodetector, and An optical pickup device is provided, in which a differential amplifier for obtaining an information signal by making a difference between outputs is built in the photodetector.

〔Example〕

Hereinafter, the optical pickup device of the present invention will be described in detail.

FIG. 1 is a schematic configuration diagram showing a first embodiment of an optical pickup device of the present invention. In the figure, the light beam emitted from the LD 20 is made into a parallel light beam by the collimator lens 21, passes through the beam splitter 22, and is condensed by the objective lens 23 on the recording medium 24 as a minute spot. The reflected light beam from the recording medium 24 passes through the objective lens 23 again, is reflected by the beam splitter 22 and passes through the half-wave plate 25, is rotated by 45 ° in its polarization plane direction, and is incident on the Wollaston prism 26. . As a result, the luminous flux is linearly polarized light vibrating in the plane of the paper,
It is divided into linearly polarized light that vibrates in a plane perpendicular to the paper surface, and is condensed by the sensor lens 27.

FIGS. 1B and 1C are diagrams showing an example of division of the light receiving section 29 of the photodetector 28. When the relationship between the objective lens 23 and the recording medium 24 is in the in-focus position, the light flux spreads in the shaded area in FIGS. 1 (b) and 1 (c). Now, the signal output from each of the light receiving surfaces A to F is I _A , I _B ,
If I _C , I _D , I _E , I _F , the focus error signal I _AF1 is
It is given by the following formula.

I _AF1 = (I _A + I _B + I _E + I _F ) − (I _C + I _D ) Now, in FIG. 1B, the recording medium 24 is the objective lens 23.
When away against the light spot is reduced as indicated by the dashed Fig. 1 (b), the result output I _C, I _D
Becomes larger and the focus error signal I _AF1 takes a negative value. Further, when the recording medium 24 approaches, the size of the light spot on the light receiving unit 29 increases, and as a result, the focus error signal I _AF1 has a positive value.

Even when the concentric circular light receiving unit 29 shown in FIG. 1 (c) is used, the focus error signal can be similarly detected by taking the difference between the outputs of the outer light receiving surface and the inner light receiving surface.

On the other hand, tracking error signal detection can also be obtained by the conventionally known push-pull method in the present invention.

That is, the tracking error signal can be detected by calculating I _AT1 = (I _A + I _C + I _E ) − (I _B + I _D + I _F ).

Next, the reason why the information signal by magneto-optical can be differentially detected by the optical pickup device of the present invention will be described.

In FIG. 1, the reflected light from the recording medium 24 has its polarization plane rotated by θ _K or −θ _K by the magnetic pattern (the direction of magnetization is upward or downward) of the recording medium 24 due to the magneto-optical effect. It is incident on the half-wave plate 25. The half-wave plate 25 is arranged with its crystal axis at an angle of 22.5 ° with respect to the crystal axis of the Wollaston prism 26.
Since the Wollaston prism 26 splits the incident light flux into two linearly polarized lights having vibrating surfaces orthogonal to each other as described above, when these light fluxes are separated and detected by the light receiving section 29,
As shown in the figure, the signals can be detected as signals with inverted phases.

In FIG. 2, the X axis is the crystal axis of the first prism of the Wollaston prism 26, and the Y axis is the axis showing the component orthogonal to this. The light flux that has been rotated by θ _K or −θ _K from the plane of polarization of the incident light due to the magneto-optical effect (the broken line shows the plane of polarization in FIG. 2) is rotated by 45 ° by the half wave plate 25. receive. After this, 2 by Wollaston prism 26
Since it is divided, its fluctuation amplitudes (S ₁ and S ₁ ′) are equal,
The phase becomes two light fluxes that are inverted. Therefore, if these two light fluxes are detected by the light receiving portion 29, the magneto-optical signal of the recording medium 24 can be obtained. That is, in the photodetector 28 of FIG. 1 (b), the magneto-optical information signal I _S is I _S = (I _A + I _B + I _C + I _D + I _E + I _F ) − (I _G + I _H + I _I + I _J + I _K + I _L ) In the photodetector 28 of FIG. 1 (c), I _S = (I _A + I _B + I _C + I _D ) − (I _E + I _F + I _G + I _H ).

As described above, by implementing the present invention, only one beam splitter element was required, which was required in the conventional optical head, which was three, and it was possible to reduce the size and weight of the device and increase the reliability. Further, since the photodetector 28 is formed with a pair of light receiving portions 29 for performing differential detection on the same substrate, it is possible to perform differential detection that is strong against external fluctuations such as changes in operating temperature. It was

Furthermore, by forming a set of amplifier circuits on the same substrate as the photodetector, the frequency characteristics and temperature characteristics between these circuits can be matched well, and the signal quality can be further improved. Further, by incorporating a differential amplifier for performing differential detection as described above in the photodetector, it is possible to perform strong detection against noise.

In the above description, the half wave plate 25 is arranged to facilitate the arrangement and fine adjustment of the optical system. However, the crystal axis direction of the prism is arranged at 45 ° to detect light. The same effect can be obtained by arranging the container 28 at a position corresponding to this.

Next, a second embodiment of the present invention is shown in FIG. In the figure, the light beam that has passed through the beam splitter 22 and the half-wave plate 25 (the optical elements before the beam splitter 22 are omitted in FIG. 3) passes through a focusing lens 27 to become a focused light beam, and then the focused light beam is focused. Light has a crystal axis oriented at 2
In addition to the two prisms 30a and 30b (constituting a Wollaston prism), a polarizing plate 30c having the same crystal axis direction as that of the prism 30b is provided to enter the polarization separation element 30. The light flux transmitted through the polarization separation element 30 is emitted in different directions between the ordinary ray and the extraordinary ray, and the equivalent optical thickness changes due to the difference between the refractive index No and Ne for the ordinary ray and the extraordinary ray. The converging positions of ordinary rays and extraordinary rays are different. For this reason, the light receiving section 29 of the photodetector 28 is placed at a position before convergence with respect to a light beam in one polarization direction and in a divergent light beam after convergence with respect to the other light beam.

4 (a) shows the focus error signal obtained in the embodiment of FIG. FIGS. 4 (b) and 4 (c) are focus error signals obtained by a single concentric circular photodetector, respectively, but these show undesired side peaks with respect to the original S-shaped curve. Have On the other hand, in the embodiment shown in FIG. 3, since the photodetectors in the convergent light state and the divergent light state are used in combination, unnecessary peaks are eliminated as shown in FIG. The focus error signal of the character can be obtained. In Fig. 3, by moving the Wollaston prism (referred to as 30a, 30b) back and forth,
The distance between the two light spots on the photodetector 28 can be finely adjusted.

Next, a third embodiment of the present invention is shown in FIG. In the third embodiment, a light beam from a beam splitter (not shown in FIG. 5) is separated into an ordinary ray and an extraordinary ray by a prism 31 made of crystal, and then a focusing lens 27 is used to detect the light.
Focused on 28. At this time, the photodetector 28 can be arranged at an appropriate angle with respect to the luminous flux.

Not only the prism but also the condenser lens 27 can be made of crystal. FIG. 6 shows such an embodiment. The optical element indicated by 32 in the figure has a prism on one surface and a lens on the other surface, and as a result, the convergent and divergent light beams separated according to the polarization state of the incident light on the photodetector 28. Is obtained.

Further, FIG. 7 shows an embodiment in which, in addition to the prism 31, the focusing lens 27 is a teletype telephoto lens 33. With such a configuration, the separation of the luminous flux required for differential detection can be performed more satisfactorily.

The present invention can be carried out by using an optical medium which constitutes the beam splitter 22 'as shown in FIG. 8 in addition to the above-mentioned embodiments.

In FIG. 8, the light beam from the laser 20 is converted into a parallel light beam by the collimator lens 21, and the beam splitter
It is reflected by the reflecting surface 22 '. This incident light flux is a completely linearly polarized light, and there is no difference in effect regardless of whether the optical medium forming the beam splitter 22 'is isotropic such as glass or birefringent such as quartz. The light beam reflected from the magneto-optical medium 24 again passes through the beam splitter 22 '. At this time, if the second prism is made of a uniaxial crystal having a crystal axis in an appropriate direction, this light beam will be reflected. Sensor separated by linearly polarized light by sensor lens 27 '
It is incident on 28 and differential detection is performed. At this time, the crystal axis may form an angle of 45 ° with the vertical axis in the figure, and may be rotated by about 45 ° with respect to the plane of the paper.

The Wollaston prism or an example using a single prism has been shown above as a means for separating the light flux by the difference in the polarization state of the incident light flux, but the separation means is not limited to this, and the Lotion prism, Various types such as a Senarmont prism can be used. Further, an example using a crystal parallel plate or a convex lens has been shown as a means for producing a light flux that converges or diverges according to the difference in the polarization state. However, it is also possible to use various means such as a combination of convex and concave teletype lenses. Is obviously usable.

〔The invention's effect〕

As described above, according to the configuration of the present invention,
It is possible to provide a differential detection type optical pickup device that is low in cost and has a good S / N.

According to this optical pickup device, in addition to the above advantages,
Advantages include the fact that the number of parts can be reduced, that the parts can be integrated, axis alignment is easy, and it is resistant to axis misalignment during use, and that the differential amplifier circuit can be built into the optical sensor to resist noise. There is.

Furthermore, since one set of amplifier circuits is formed on the same substrate, the frequency characteristics and temperature characteristics between the circuits can be well matched, and the signal quality can be further improved.

[Brief description of drawings]

FIG. 1 (a) is a schematic view showing a first embodiment of the optical pickup device of the present invention, and FIGS. 1 (b) and 1 (c) are enlarged views of a photodetector used in the optical pickup device. Is. FIG. 2 is a schematic diagram showing the signal amplitude component of the light flux. FIG. 3 is a schematic view showing a second embodiment of the present invention,
The figure shows the focus error signal. 5, FIG. 6, FIG. 7, and FIG.
It is a schematic diagram showing a 3rd, 4th, 5th, and 6th example. FIG. 9 is a schematic view of a conventional optical pickup device.
10 and 11 are diagrams for explaining the differential detection method. FIG. 12 is a schematic diagram showing an optical pickup device using a Wollaston prism. 22: Polarizing beam splitter, 28: Photodetector, 23: Objective lens, 29: Light receiving part, 24: Recording medium surface, 30: Polarization separating element, 2
5: 1/2 wave plate, 26: Wollaston prism, 27: Sensor lens

Claims (1)

[Claims] 1. An optical pickup device having a focusing optical system for irradiating an information recording surface with light emitted from a light source and guiding reflected light from the information recording surface to a light receiving portion of a photodetector. Has a means for separating the reflected light into a plurality of light beams having different polarization states, and the plurality of separated light beams are separated by the focusing optical system.
Focusing on one or more light spots, a light receiving unit of the photodetector being provided corresponding to the positions of the two or more light spots, and at least one of the light receiving units. It is divided into a plurality of light receiving surfaces, a set of amplifier circuits corresponding to the light receiving section is provided on the same substrate as the photodetector, and An optical pickup device, wherein a differential amplifier for obtaining an information signal by making a difference between outputs is built in the photodetector.