JPH1040567A - Optical head and optical information reproducing apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low price optical head which assures less amount of noise in the returning beam, easily detects defocusing signal and always supplies a high quality focus control signal, a tracking control signal of which offset is reduced and a reproducing signal for different kinds of optical discs. SOLUTION: The apparatus is driven integrally with an objective lens 7, diffracts the light flux reflected from an optical disc 9 using polarizing diffraction grating having the grating groove pattern in the shape of unequal interval curve, detects a control signal and an information signal using the diffracted light beam and non-diffracted light beam, functions as the polarizing diffraction grating for the laser beam in the wavelength λ1 and also functions to only transmit the light beam for the laser beam in the wavelength λ2, obtains good performance through the optical head loading a couple of laser sources in different wavelengths and prevents feedback of the 0-order beam to the semiconductor laser 1 by inserting a polarizing beam splitter 3 in order to use the 0-order beam for detection of defocusing and signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical head for recording or reproducing an optical information recording medium, and more particularly to an optical head suitable for improving the accuracy of tracking control and an optical information reproducing apparatus using the same.

[0002]

2. Description of the Related Art In order to correctly position a focused spot on an information signal sequence (track) of an optical information recording medium such as an optical disk, position control in an optical axis direction perpendicular to the optical disk surface (hereinafter referred to as focus control). ) And position control in the disk surface direction (hereinafter referred to as tracking control). In particular, some effective methods have been proposed as means for detecting a tracking control signal required for tracking control, but a detection method called a push-pull method is most widely used at present. This method is used when a light spot condensed on a disk is diffracted by a periodic guide groove or a series of information pits on the disk, and its 0th order light and ± 1st order diffracted light are diffracted on the pupil plane of the objective lens. If the light spot deviates from above the guide groove or directly above the information pit row, the phases of the + 1st-order diffracted light and the -1st-order diffracted light are shifted in opposite directions. It detects the imbalance between the intensity and the light intensity due to the interference between the 0th-order light and the -1st-order diffracted light. That is, the interference intensity is photoelectrically converted independently by a two-divided photodetector, and the differential output is fed back to a light spot movable means such as a two-dimensional actuator attached to an objective lens, thereby forming a control system. Position the light spot correctly on the track.

However, in the case where the tracking control of the light spot is performed by displacing the objective lens in the in-plane direction of the optical disk as described above, the light is irradiated onto the two-divided photodetector in accordance with the displacement amount of the objective lens. However, there is a problem that an offset is generated in the tracking control signal detected as a result because the light spot is also displaced. Especially in a high-density optical disk, the influence of such tracking offset cannot be ignored, and the amplitude of the reproduced signal decreases,
This causes an increase in adjacent track crosstalk, which leads to a deterioration in reproduction signal quality.

In recent years, as a conventional technique capable of improving the problem of occurrence of tracking offset due to the displacement of the objective lens and always detecting an accurate and stable tracking control signal while using a push-pull method, for example, Japanese Unexamined Patent Application Publication No.
Attention has been paid to an optical head using a polarizing diffraction grating as disclosed in Japanese Patent No. 77578. In this optical head, in addition to an objective lens, a λ / 4 plate and a polarizing diffraction grating are integrally mounted on a two-dimensional actuator. Interference regions between the order diffracted light and the zero-order light are diffracted in different directions and received on different light detection surfaces.

In this case, even if the objective lens is displaced, the boundary line of the interference area also moves in accordance with the movement of the objective lens. The tracking offset accompanying the displacement can be satisfactorily eliminated. Moreover, in this optical head, the diffraction grating mounted on the two-dimensional actuator is a polarizing diffraction grating, and a λ / 4 plate is provided between the polarizing diffraction grating and the objective lens. Diffraction efficiency becomes almost zero when transmitting through the diffractive grating, and the reflected light from the disk is diffracted at an appropriate diffraction efficiency only when transmitted again through the diffraction grating. When such a polarizing diffraction grating is used, it is possible to prevent the loss of light amount and the generation of stray light due to unnecessary diffraction occurring on the outward path, which is extremely advantageous for improving the performance of the optical head.

However, the zero-order light that is not diffracted by the polarizing diffraction grating as in this conventional example returns to the semiconductor laser as it is, and the optical head having a configuration in which only the diffracted light is detected by the photodetector has a focus control of the light spot. This is inconvenient for detecting a signal. That is, for example, a focus control signal detection method most commonly used in the conventional optical disk optical head includes a method called astigmatism method. Such focus control signal detection is performed by a reflected light spot due to defocus. Since it is designed to detect a change in the intensity distribution of the optical head, the present invention is applicable to an optical head having a configuration in which light incident on a detection system is separated from the beginning by a polarizing diffraction grating as in the above-described conventional example. Is not. For this reason, the optical head like the above-mentioned conventional example has to adopt a quite special detection method. For example, the method disclosed in the above conventional example divides a polarizing diffraction grating into a tracking signal detection area and a defocus signal detection area to detect a focus control signal,
Different diffraction directions are set so as to be incident on different light detection surfaces. However, such a configuration complicates the grating groove pattern of the polarizing diffraction grating and the configuration of the detection surface of the photodetector, thereby increasing the manufacturing cost. In addition, there is a problem that laser noise is generated when the zero-order light returns to the semiconductor laser as it is, thereby deteriorating reproduction signal quality. Further, in recent years, several types of optical disks having different substrate thicknesses and different structures are to be developed and marketed. However, the above-described conventional example employs a complicated detection system as described above, so that the structure of the optical disk is It is not possible to flexibly respond to changes in the optimal detection means due to differences in In the above-mentioned conventional example, there is no mention of such compatibility between different types of optical disks.

[0007] In view of such a problem, an object of the present invention is to provide:
Optical head and optical system capable of always detecting a good focus control signal and tracking control signal while having an inexpensive structure, reducing return light noise, flexibly responding to changes in the optimum detecting means due to a difference in optical disk structure, and To provide an information reproducing apparatus.

[0008]

In order to solve the above problems, according to the present invention, a semiconductor laser, a condensing optical system constituted by at least an objective lens, a light spot position control means, and the optical information recording medium are provided. A light branching element for branching the reflected light from the focusing optical system, and an optical head including a photodetector for receiving the reflected light branched by the light branching element and performing photoelectric conversion, the objective lens and A polarization conversion element such as a λ / 4 plate in an optical path between the light splitting elements, and a grating groove having a diffraction efficiency that changes due to a difference in polarization between incident light and reflected light and has a predetermined irregularly-spaced curved locus. And a polarizing diffraction grating having the following. The light spot position control signal, that is, the focus control signal, is obtained from the diffracted light or the undiffracted light or both the diffracted light and the undiffracted light diffracted by the polarizing diffraction grating out of the disc reflected light incident on the photodetector. And a tracking control signal. Further, a movable mechanism such as a two-dimensional actuator for integrally moving the objective lens, the polarization conversion element, and the polarizing diffraction grating based on the light spot position control signal is provided as the light spot position control means.
In this way, by using a configuration in which the light spot position control signal is detected using both the diffracted light and the undiffracted light diffracted by the polarizing diffraction grating, the astigmatic method can be used even when the polarizing diffraction grating is used. , The focus control signal can be detected, and a highly reliable optical head can be realized with a simpler configuration.

As another means of the present invention, at least two or more semiconductor laser light sources having predetermined wavelengths .lambda.1 and .lambda.2 are provided in an optical head, and a laser beam of wavelength .lambda.1 is provided as described above. Diffraction efficiency changes due to the difference in the polarization state, and for a laser beam having a wavelength of λ2, a polarizing diffraction grating having a function of achieving a diffraction efficiency of about 0% for at least two different polarization states is provided. Provided.

By the way, in the optical head configured to detect both the diffracted light and the undiffracted light by the photodetector, as will be described later in detail in an embodiment, a semiconductor laser light source passes through an objective lens onto a disk. When the condensed incident light passes through the polarizing diffraction grating, minute unnecessary light generated by being slightly diffracted becomes stray light and enters the photodetector via the disk, where each control is detected. A new problem arises that adversely affects the signal quality of the signal and the reproduced signal. In order to solve this problem, in the present invention, by making the grating groove trajectory of the deflecting diffraction grating a predetermined unequally-spaced curve, the focal point of the stray light radiated to the disk can be used for non-diffraction light used for signal detection. The position is shifted (defocused) by a predetermined interval with respect to the focused light of the light.

Further, according to the present invention, a plurality of objective lenses having substantially no aberration are mounted as objective lenses on a plurality of types of substrates having different thicknesses of the transparent substrate, and in addition to each objective lens, a polarization conversion element and a polarizer are provided. Means in which a diffraction grating is integrated or a polarization conversion element is integrally mounted on a movable mechanism, and one of a plurality of objective lenses is selected and inserted into an optical path according to the type of an information recording medium to be reproduced. Was added.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a basic embodiment of the optical head of the present invention. The light from the semiconductor laser 1 is collimated by the collimator lens 2, passes through the polarization beam splitter 3, reflects off the rising mirror 4, and enters the polarization diffraction grating 5 provided immediately below the objective lens 7. At this time, the polarization direction of the incident light is P-polarized with respect to the polarization beam splitter 3, that is, a direction substantially parallel to the X-axis in the drawing. The polarization direction is substantially parallel to the line 100. This polarizing diffraction grating 5 is made of lithium niobate (LiNbO
It is made of a crystal plate having a refractive index anisotropy such as 3) and is attached to the movable portion of the two-dimensional actuator 8 which is the same as the objective lens 7, and is displaced integrally with the objective lens 7. Then, for example, as shown in FIG.
Lattice grooves are formed in directions opposite to each other at approximately 45 degrees with respect to the symmetry axis. In addition, for a laser beam having a polarization direction substantially parallel to the center dividing line 100 as described above, the main axis direction of the crystal is set so that the phase difference of the concave and convex portions of the lattice groove becomes substantially an integral multiple of the incident light wavelength λ. And the groove depth is designed. Therefore, when the incident light passes through the polarizing diffraction grating, almost no diffracted light is generated. Then, the light transmitted through the polarizing diffraction grating 5 is converted into circularly polarized light by the λ / 4 plate 6 bonded to the polarizing diffraction grating, incident on the objective lens 7, and collected on the optical disk 9. On the other hand, when the reflected light passes through the objective lens 7 again and passes through the λ / 4 plate 6 again, it is converted into a so-called S-polarized light in a polarization direction orthogonal to that at the time of incidence, and then enters the polarizing diffraction grating again. At this time, the unevenness of the grating groove of the polarizing diffraction grating 5 is S
Since it is set so that ± 1st-order diffracted light is generated at a predetermined diffraction efficiency with respect to polarized laser light, the disc reflected light passes through the polarizing diffraction grating as it is when transmitted again through the polarizing diffraction grating. The transmitted light is separated into the transmitted 0th-order light and ± 1st-order diffracted light that travels at an angle of approximately 45 degrees in the opposite direction to each other divided by the center dividing line. Further, these lights are set up by a mirror 4, a polarization beam splitter 3,
And a condenser lens 10 and a cylindrical lens 11
Is focused on the photodetector 12 by the The photodetector 12 has an adjacent quadrant photodetection region at the center portion and four photodetection regions around the photodetection region. The light is received at ± 4th order diffracted light in four surrounding photodetection areas. Then, by taking the differential output of the sum signal of the two detector outputs in the diagonal direction of each of the quadrant photodetectors at the center, a focus control signal in a so-called astigmatism method is obtained.
In addition, a tracking control signal by a so-called push-pull method can be obtained by taking the sum of the output signals of the two photodetectors in the diagonal direction for the peripheral portion and detecting the differential output. The detector for obtaining the data signal is not particularly limited, but may be, for example, a four-division sum signal of the central portion. However, if too many signals are calculated, the shot noise of the detector and the amplifier noise of the amplifier in the middle may increase, so it may be better to suppress the noise to an appropriate number. In addition, shot noise and amplifier noise may be dominant if the noise of the medium or light source is sufficiently small.However, if it is necessary to further reduce the noise as the density increases, it is better not to perform addition at all. Good. At this time, one of the four divisions is selected, or another beam splitter is inserted between the polarizing beam splitter 3 and the condenser lens 10, and the light is again focused on one detector by the condenser lens. What is necessary is just to make the detected output the reproduced signal.

FIG. 2 shows an embodiment similar to FIG. 1, except that the semiconductor laser 1 side and the photodetector 12 side of the polarization beam splitter 3 are replaced. This is because the polarization direction of the laser light emitted from the semiconductor laser light source and incident on the polarization beam splitter 3 is different between FIG. 1 and FIG. That is, in the embodiment of FIG. 1, P-polarized light, that is, laser light having a polarization direction in the X-axis direction in the drawing enters the separation plane of the polarization beam splitter, but in the embodiment of FIG.
Polarized light, that is, laser light having a polarization direction in the Y-axis direction in the drawing is incident. Accordingly, the polarization dependence of the diffraction efficiency of the polarizing diffraction grating 5 of FIG. 2 is opposite to that of the embodiment of FIG. 1, and is S-polarized (the polarization direction substantially perpendicular to the center dividing line 100). ), Almost no diffracted light is generated, and ± -first-order diffracted light is generated at a predetermined diffraction efficiency for P-polarized light (a polarization direction substantially parallel to the center dividing line 100). Although the optical element (for example, a beam shaping prism) for shaping the elliptical emission distribution from the semiconductor laser into a circle is omitted in FIGS. 1 and 2, the optical head equipped with such an optical element is of course omitted. In this case, the present invention can be applied without any problem.

FIG. 3 shows an example of a photodetector for detecting a focus control signal, a tracking control signal, an information signal recorded on a disc, and the like from the signal output of the photodetector 12. The photodetector 12 is a quadrant photodetection region 1 adjacent to the central portion.
There are 25, 126, 127, 128 and four photodetection areas 121, 122, 123, 124 around the area. As described above, the four divided areas 125, 126, 12
At 7 and 128, the 0th order light among the disk reflected light is received, and ± 1st order diffracted light is received at the peripheral photodetectors 121, 122, 123, and 124. At this time, the quadrant photodetector 12 at the center is
When a difference signal is detected by a differential amplifier 131 from a sum signal of two detector outputs 5, 126, 127 and 128 in diagonal directions, a focus control signal based on a so-called astigmatism method is obtained. Further, the peripheral detection areas 121 and 12
The differential signal is detected by the differential amplifier 132 from the sum signal of the two photodetector outputs in the diagonal directions 2, 123, and 124, thereby obtaining a so-called push-pull tracking control signal. The recording information signal is obtained by, for example, outputting a sum signal of the signals detected from the central four-part region by the addition amplifier 14 as in the embodiment shown in FIG. Of course, the recording information signal may be detected from the sum signal of the detection signals from the four peripheral portions.

FIG. 4 is a schematic diagram showing the problems of the conventional push-pull tracking control signal detecting means in order to clarify the effects of the present invention. Generally, since the optical disk 9 is a replaceable medium, if the center of a helical or concentric track is not necessarily aligned with the rotation center of the spindle motor and is mounted in an eccentric state, a stationary light spot will Since the recording track appears to fluctuate, the objective lens is appropriately displaced using, for example, an actuator or the like in order to keep the light spot always following the track. Therefore, as shown on the left side of the figure,
In addition to the case where the center of the objective lens coincides with the optical axis, the case where the objective lens is displaced from the optical axis by δ to move the light spot by a predetermined displacement amount δ as shown on the right side of the drawing also moved. Of course, it can happen. In such a field, the center of the contour of the reflected light beam is shifted by δ with respect to the optical axis of the incident light beam (the center axis of the light beam contour), and the point having the maximum intensity is 2δ.
It shifts. On the other hand, in the conventional push-pull method, since the dividing boundary of the two light detection areas for detecting the difference signal does not move while being fixed, if such a shift of the reflected light flux occurs, two light detection areas are inevitably generated. Unbalance occurs in the output of the light detection area, and as a result, an offset occurs in the tracking control signal. On the other hand, when the polarizing diffraction grating is mounted on the same actuator as the objective lens and driven integrally, as in the present invention, no matter how much the objective lens is displaced, the displacement of the division boundary line with respect to the center of the contour of the reflected light flux will be improved. Does not occur. Therefore, the offset component of the tracking control signal due to the output imbalance as described above can be significantly reduced.

FIG. 5 is a schematic view showing a specific embodiment of the polarizing diffraction grating. Here, an example using a lithium niobate (LiNbO3) substrate is shown as an example. The lithium niobate substrate 51 has, for example, a main axis of refractive index anisotropy in the in-plane direction 58 of the drawing. Therefore, a proton exchange region 52 and a dielectric film 53 are formed in and on the substrate according to a predetermined lattice groove pattern. At this time, a phase difference φo between an ordinary ray 54 (a ray having a polarization direction substantially perpendicular to the plane of the paper) transmitted on the lattice groove pattern and an ordinary ray 55 transmitted between the lattice groove patterns, and an extraordinary ray (substantially parallel to the plane of the paper) The phase difference φe between the light beams 56 and 57 having different polarization directions is expressed by the following equation.

[0018]

(Equation 1)

Furthermore, in the diffraction grating of a rectangular groove in which the width of the concave part and the convex part of the grating groove are equal, given the above-mentioned phase differences φo and φe, the diffraction efficiency of ± 1st-order diffracted light with respect to ordinary ray and extraordinary ray respectively ( ± 1st order diffracted light I ± with respect to 0th order light I0
(Intensity ratio of 1) I ± 1 / I0 is determined by the following equation.

[0020]

(Equation 2)

Here, nd, Δno, and Δne are values uniquely determined as physical constants. If the above equation is solved as a simultaneous equation in which the thickness Td of the dielectric film and the depth Tp of the proton exchange region are unknown values, the polarization that provides a desired diffraction efficiency with ordinary light and extraordinary light with respect to laser light of a predetermined wavelength is obtained. Diffraction gratings can be designed. For example, the refractive index of the dielectric film nd = 2.2, Tp = 1.36 μm, Td = 0.045
When it is set to μm, the diffraction efficiency of ordinary light is 0%, that is, no diffracted light is generated with respect to the semiconductor laser light having a wavelength of 680 nm, while the ± 1st-order diffracted light is separated and generated with the diffraction efficiency of about 30% for the extraordinary light. it can.

By the way, even if the actual polarization diffraction grating is designed so that the diffraction efficiency is completely 0% with respect to the laser beam having a predetermined polarization direction, as described above, the width of the dielectric film is small. Due to error factors such as a shift in the width of the proton exchange region and the like, it is not possible to achieve 0% completely, and a slight amount of diffracted light is actually generated. This diffracted light becomes a stray light component that does not contribute to actual signal reproduction at all. The present inventors have analyzed how this stray light affects various signal reproduction performances of the optical head. As a result, the outgoing laser light condensed on the disk via the objective lens is diffracted by such a polarizing diffraction grating. When stray light was generated due to poor efficiency polarization dependence, it was found that this stray light reflected the disk along an optical path substantially similar to that of the signal reproduction light (zero-order light) and was incident on the photodetector. Moreover, it has been clarified that the stray light interferes with the signal reproduction light on the light detection surface and significantly deteriorates the quality of the detection signal, such as a waveform distortion of the detection signal and an increase in noise components. Therefore, in order to minimize the influence of the stray light component caused by the poor diffraction efficiency polarization dependence of the polarizing diffraction grating, in the present invention, the trajectory (grating pattern) of the diffraction grating groove is formed into an irregularly spaced curve. That is, by using a so-called holographic diffraction grating manufacturing technique, by forming the grating pattern into a substantially circular arc shape or a substantially elliptical arc shape instead of a normal straight line, a wavefront aberration corresponding to a predetermined defocus on the wavefront of the ± 1st-order diffracted light. To defocus the condensed spot of the stray light component from the disk surface. For example, FIG. 6 shows an example of a grating groove pattern of a polarizing diffraction grating, in which trajectories of the grating grooves are schematically represented by a line every several tens. Boundary line 10 in the center of the figure
Lattice grooves that are inclined by about 45 degrees in opposite directions to each other with 0 as a boundary are not straight lines but are substantially arc-shaped. In addition, the groove pitch is not equal, but gradually widens from the upper part to the lower part of the figure at a predetermined change rate. By using a polarizing diffraction grating having such irregularly-spaced curved grating groove patterns, even if diffraction light is generated in the outward light incident on the disk, the generated ± 1st-order diffraction light, that is, the stray light component, is generated by the objective lens. 7, the signal reproduction light (zero-order light) spot 300 properly focused on the recording surface of the disk as shown in FIG. 7 has a negative direction (far side from the disk) and a positive direction. Condensed spots 301 and 302 are formed at positions separated by a predetermined distance ε (on the side closer to the disk). Therefore, these stray light components are incident on the disk in a defocused state. For this reason, the disk reflected light of these stray light components reaches the light detection surface with almost no modulation by the information signal or the like recorded on the disk.
Moreover, since these stray light components are defocused with respect to the signal light spot, the spot shape on the detection surface is different from that of the signal reproduction light, and the portion of the signal reproduction light that interferes with the spot on the detection surface is greatly reduced. Decrease. Therefore, the deterioration of the detection signal quality caused by the interference between the stray light component and the signal reproduction light is almost eliminated, and a good signal can be obtained.

FIG. 8 shows an example of a configuration for obtaining a focus control signal, two types of tracking control signals, and a recorded information reproduction signal from the photodetector 12, as another embodiment of the present invention. That is, in the present embodiment, as the tracking control signal detecting means, similar to the embodiment of FIG.
Push-pull method using 21, 122, 123, 124, and four divided areas 125, 126, 127,
This is an example that has both of the so-called phase difference detection method using the detection signal obtained from the control signal 28. The optical head of the present embodiment is exactly the same as the embodiment of FIG. 1 or FIG. 2 except for the signal detection system shown in the figure. The phase difference detection method is a detection method suitable for a read-only disc in which an uneven signal pit array is provided in advance. Tracking control is performed by sampling the signal output of each of the four divided detection areas at the edge of the signal pit and performing arithmetic processing. Although a signal is obtained, its detection principle is already known and is not directly related to the present invention. In general, this phase difference detection method has a feature that an offset of a tracking control signal due to a displacement of an objective lens is very small, and furthermore, the same method as a focus control signal detection method such as an astigmatism method detects light from a four-division light detection area. Since the detection can be performed using the output signal, there is an advantage that the detection optical system can be simplified. Therefore, by adopting the configuration as in the present embodiment, the first tracking control signal is detected from the ± 1st-order diffracted light by the polarization diffraction grating using the push-pull method, and the astigmatism method is used from the undiffracted light. Thus, a focus control signal can be detected by using the non-diffracted light, and a second tracking control signal based on a so-called phase difference detection method can also be detected from the undiffracted light. Generally, the push-pull method is a recording / reproducing type (R) in which a spiral or concentric guide groove is provided in advance.
AM) This is a tracking control signal detection method suitable for a disk, and the phase difference detection method is a detection method suitable for a read-only disk in which signal pit rows having irregularities are provided in advance as described above. Therefore, when an optical head as in the present embodiment is used, a single optical head can handle a plurality of types of disks by appropriately switching the optimal tracking control signal detection method according to the type of the disk. Usually, the tracking signal is a signal in a higher frequency range than the focus control signal. Therefore, in the embodiment of FIG. 8, the focus control signal (AF signal) detection system includes a tracking control signal (TR signal) based on a phase difference detection method. A low-pass filter 15 is inserted to prevent leakage. On the other hand, in the tracking control signal detection system using the phase difference detection method, the sampling circuit 16 is inserted because only the output at the edge of the pit needs to be sampled.

FIG. 9 shows an embodiment of an optical head for accommodating two types of disks having different substrate thicknesses. This embodiment is different from the embodiment of FIG. 1 in that two objective lenses (71, 7) are used.
2) Mounted. These two objective lenses 71 and 72
Are integrally mounted on the lens variable two-dimensional actuator 81, and detect discs having different substrate thicknesses.
It is used by switching appropriately. Examples of two types of discs with different substrate thicknesses are 1.2 mm thick CDs or CDs.
-Compatible playback of a ROM disk and a 0.6 mm thick DVD-ROM or DVD-RAM disk may be performed. Here, the objective lens 71 is a DVD-ROM or DVD-RAM disk 91, and the objective lens 72 is a CD.
Alternatively, a CD-ROM disk is assumed, and a polarizing diffraction grating 5 and a quarter-wave plate 6 are mounted on the objective lens 71 side as in the embodiment of FIG. 1 or FIG. Has only a quarter-wave plate 6 and no polarizing diffraction grating. Since the DVD-RAM disk has a structure in which a spiral or concentric guide groove is provided, a push-pull method using ± 1st-order diffracted light by a polarization diffraction grating is an optimal tracking control signal detection method. Whereas CD and CD-RO
This is because the M disk has a configuration in which a signal pit row having concavities and convexities is arranged, and the tracking control signal detection method is switched to the phase difference detection method described above. However, of course, also in CD and CD-ROM discs, signal pits having irregularities are present intermittently over one round of the disc, so that the tracking control signal can be detected by a push-pull detection method in a low frequency band. is there.

FIG. 10 shows an arrangement of an optical system in the case of reproducing a CD or CD-ROM disc 92 in the optical head of FIG. Here, since no polarizing diffraction grating is provided, no diffracted light is generated, and the disc reflected light does not enter the four detection regions around the detector 12. That is, the focus control signal based on the astigmatism method and the tracking control signal based on the phase difference detection method are simultaneously detected only from the output signals of the four divided areas at the center.

FIG. 11 shows another embodiment of the present invention. The same parts as those in the embodiment of FIGS. 1 and 2 are denoted by the same reference numerals. The optical head shown in this embodiment has basically the same configuration as that of the embodiment shown in FIG. 1 or FIG. 2, except that two semiconductor laser light sources having different wavelengths are mounted in one optical head. Are different. Laser light emitted from the semiconductor laser light source 1 having the wavelength λ1 passes through the polarization beam splitter 3 and enters the wavelength-selective mirror 30.
On the other hand, a laser beam emitted from a so-called semiconductor laser module 21 in which a semiconductor laser light source having a wavelength λ2 and a photodetector are stored in the same package is used as a hologram optical element 22, an aberration correction lens 23 (function of this aberration correction lens). Will be described later), and also enters the wavelength-selective mirror 30. Since the wavelength-selective mirror 30 has a function of transmitting light of the wavelength λ1 and reflecting light of the wavelength λ2, these lights are transmitted or reflected by the wavelength-selective mirror 30 and have substantially the same optical path. And the light is condensed on the optical disk 9 by the objective lens 7 through the collimator lens 2, the rising mirror 4, the polarizing diffraction grating 5, and the λ / 4 plate 6. Each light spot condensed on the optical disk 9 returns along the same optical path as the outward path after the disk reflection, and the optical path is separated again by the wavelength selective mirror 30 for each wavelength. The light of the wavelength λ1 of the separated light reflected from the disk follows the same optical path as in the embodiment of FIG. However, in the present embodiment, the function of the condenser lens 10 in the embodiment of FIG. 1 is also used for the collimator lens 2, so that the condenser lens 10 is omitted.

On the other hand, the light of wavelength λ2 is
3, the light reaches the hologram optical element, is diffracted by the hologram optical element, and
Each signal is detected by being incident on a photodetector (not shown) provided in 1. Since the optical head using the semiconductor laser module and the hologram optical element is a known technique, a detailed description thereof will be omitted. Further, the present embodiment is not limited to the configuration in which the semiconductor laser module and the hologram optical element are mounted, but is a so-called bulk type configuration in which a semiconductor laser light source and a photodetector are separately provided as in an ordinary optical head. But of course you can.

In recent years, there are many disks on the market where the wavelength of usable laser light is severely restricted. For example, a disc generally called a CD-R is a recordable disc using an organic dye as a recording medium, and is a disc of a type that performs recording and reproduction with a laser beam having a wavelength of 780 nm. However, when a short-wavelength laser beam having a wavelength of about 630 to 680 nm is incident on the CD-R, the energy is almost absorbed by the medium and does not reflect off the disk.
In fact, reproduction in this wavelength band is impossible. On the other hand, DV
For high-density disks such as D disks, wavelengths of 630-6
Short-wavelength laser light of about 80 nm is indispensable as signal reproduction light. Therefore, in the embodiment of FIG. 11, in order to realize a compatible device capable of reproducing both high-density disks such as a CD disk, a CD-R disk, and a DVD disk, for example, the wavelength λ1 = 650 nm and the wavelength λ2 = 780 nm.
The two types of semiconductor laser light sources described above are mounted, the type of the disk is determined by a disk determiner (not shown), and the optimum light source is appropriately switched based on the result of the determination. That is, when reproducing a high-density disk such as a DVD disk, the semiconductor laser 1 having a wavelength of λ1 = 650 nm is turned on, and when reproducing a CD disk or a CD-R disk, the semiconductor laser 21 having a wavelength of λ2 = 780 nm is turned on. It does. In this embodiment, unlike the embodiments of FIGS. 9 and 10, the same objective lens 7
High-density disks such as disks and CD disks and CD-
The objective lens 7 is designed to reproduce an R disk, but this objective lens 7 focuses the laser beam having a wavelength of λ1 = 650 nm on an optical disk such as a DVD disk. Designed. Therefore, when the objective lens 7 is used to focus a laser beam having a wavelength of λ2 = 780 nm on a CD-R disc or a CD disc, aberrations caused by the difference in the thickness of the disc substrate and the difference in the wavelength of the laser light are reduced. Occurs, and the focusing state of the light spot is significantly deteriorated. In the present embodiment, the aberration correction lens 23 is provided in the optical path of the wavelength λ2 = 780 nm in order to suppress the deterioration of the focused state of the spot due to the occurrence of the aberration. That is, the wavelength λ2 = 780 nm
Is focused by the objective lens 7 through the aberration correcting lens 23, the aberration correcting lens 23 is irradiated with a good light spot with sufficiently reduced aberration on the recording surface of the CD-R disc. Is designed. Therefore, for example, as the objective lens 7, the wavelength λ1 =
For a laser beam of 650 nm, a light spot is correctly focused on a high-density disc such as a DVD disc.
When an objective lens or a collimating lens or an optical element having a special function of correctly forming a light spot on a D or CD-R disc is used, or as shown in FIGS. When a lens is provided to switch to an objective lens suitable for each disc, the aberration correction lens 23 can be omitted as a matter of course.

When a recorded CD-R disc is reproduced, the tracking control signal detecting means most preferably uses a detection method called a three-spot method (a description is omitted because it is a known technique). General. Also, when playing back a CD disk, it is common to detect the tracking control signal using the three-spot method or the above-described phase difference detection method. Therefore, FIG.
When reproducing a CD or CD-R disc in the first embodiment, light diffracted by the polarizing diffraction grating 5 out of the disc reflected light is unnecessary. However, as described above, in the embodiment of FIG. 11, the same objective lens is used for both reproducing a DVD disk and reproducing a CD or CD-R disk.
Light of wavelength λ1 = 650 nm for use with CD and CD-R
The laser light having a wavelength of λ2 = 780 nm passes through the polarizing diffraction grating 5. In such a case, for example, as shown in FIG. 12, with respect to the laser light of λ1 = 650 nm for DVD, the forward light beam incident on the polarizing diffraction grating 5 in the P-polarized state as described above is While only the return light flux that is hardly diffracted (that is, diffraction efficiency ≒ 0) and is reflected by the disk and enters the polarizing diffraction grating 5 in the S-polarized state is diffracted at the predetermined diffraction efficiency k, C
Regarding the laser beam of λ2 = 780 nm for D and CD-R, almost no diffracted light is generated regardless of the forward and backward light fluxes (that is, diffraction efficiency ≒ 0), and the laser beam is simply transmitted at a predetermined transmittance. What is necessary is just to provide the polarizing diffraction grating 5 with the function just to make it. In order to realize a polarizing diffraction grating having such a function, in addition to the formulas (1) and (2), the optical path length difference between the grating groove and the grating groove for S-polarized light (abnormal light) is a wavelength. The depth Tp of the proton exchange region and the dielectric film thickness Td may be designed to be equal to an integral multiple of λ2 = 780 nm. That is, the phase difference Φe relating to the S-polarized light (abnormal light) is expressed by the following equation

[0030]

(Equation 3)

It can be seen that Tp and Td may be determined so as to satisfy the condition.

(Note that nd, Δn
o and Δne are assumed to be constant independently of the wavelength. However, these physical constants actually have wavelength dependence of incident light. Therefore, the value obtained by solving the above equation is an approximate solution. For example, under the above-mentioned condition, that is, nd = 2.2, Δno = −0.04, Δne = 0.
12 and K = 0, L = 1, Td =
0.16 μm, Tp = 4.88 μm, wavelength λ2 = 78
For a laser beam of 0 nm, the diffraction efficiency can be made almost zero for both ordinary and extraordinary rays. At this time, the wavelength λ1 = 650
For a laser beam of nm, the diffraction efficiency is almost zero for ordinary rays and about 14% for extraordinary rays. By using such a polarizing diffraction grating, generation of light unnecessary for signal detection can be suppressed, and there is a great effect on securing signal reproduction performance of an optical head for disc compatibility.

[0033]

According to the present invention, return light noise is reduced,
An optical head that can easily detect a defocus signal and can always obtain a high-quality focus control signal, tracking control signal, and reproduction signal for different types of optical disks can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a basic embodiment of the present invention.

FIG. 2 is a perspective view showing another embodiment of the present invention.

FIG. 3 is a schematic plan view showing a detector arrangement and a signal calculation method used in a basic embodiment of the present invention.

FIG. 4 is a schematic diagram for explaining a cause of occurrence of a tracking control signal offset in the push-pull method.

FIG. 5 is a schematic sectional view for explaining the structure and operation of a polarizing diffraction grating;

FIG. 6 is a schematic plan view showing an example of a grating groove pattern of the polarizing diffraction grating of the present invention.

FIG. 7 is a schematic front view illustrating a main part of an optical head for explaining the operation of the polarizing diffraction grating of the present invention.

FIG. 8 is a schematic plan view showing a detector arrangement and a signal calculation method according to another embodiment of the present invention.

FIG. 9 is a schematic front view of a main part showing another embodiment of the present invention.

FIG. 10 is a schematic front view of a main part showing another embodiment of the present invention.

FIG. 11 is a schematic front view of a main part showing another embodiment of the present invention.

FIG. 12 is a characteristic diagram for explaining one characteristic of the polarizing diffraction grating of the present invention.

[Explanation of symbols]

1 ‥‥ semiconductor laser, 5 ‥‥ polarizing grating, 6
{1/4 wavelength plate, 7} objective lens, 8 ‥‥ two-dimensional actuator, 81 ‥‥ two-lens interchangeable two-dimensional actuator, 9 ‥‥ optical disc, 91 ‥‥ SD optical disc, 92 ‥‥ CD optical disc, 10 ‥‥ Condensing lens, 1
1 ‥‥ cylindrical lens, 12 ‥‥ photodetector

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masachi Fukui 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture In the Visual Information Media Division of Hitachi, Ltd.

Claims (6)

[Claims] 1. A condensing optic comprising at least a semiconductor laser light source and an objective lens for converging a laser beam emitted from the semiconductor laser light source onto an optical information recording medium loaded on a transparent substrate through the substrate. System, a light spot position control means for positioning a light spot focused on the optical information recording medium on an information signal sequence of the optical information recording medium, and a light reflected from the optical information recording medium. An optical head comprising a light branching element for branching from a condensing optical system, and a photodetector for receiving the reflected light branched by the light branching element and photoelectrically converting the reflected light, between the objective lens and the light branching element; A polarization conversion element that changes the polarization state of incident light condensed on the optical information recording medium through the objective lens in the optical path and the polarization state of light reflected from the optical information recording medium, Diffraction efficiency changes due to the difference in the polarization state between the incident light and the reflected light, and a polarizing diffraction grating having a grating groove of a predetermined irregularly spaced curved locus is provided. It is necessary to operate the light spot position control means from at least the diffracted light or the undiffracted light or both the diffracted light and the undiffracted light diffracted by the polarization diffraction grating out of the reflected light reaching the photodetector. A movable mechanism is provided for detecting a light spot position control signal and moving the objective lens, the polarization conversion element, and the polarizing diffraction grating integrally based on the light spot position control signal as the light spot position control means. An optical head, characterized in that: 2. A first semiconductor laser light source for emitting laser light of a predetermined wavelength λ1, a second semiconductor laser light source for emitting laser light of a wavelength λ2 different from λ1, and a laser light emitted from each of the semiconductor lasers. A condensing optical system constituted by at least an objective lens for converging the optical information recording medium loaded on a transparent substrate through the substrate, and each light spot condensed on the optical information recording medium is Light spot position control means for positioning on the information signal sequence of the optical information recording medium, an optical branching element for branching the reflected light from the optical information recording medium from the condensing optical system, and branching by the optical branching element An optical head having a photodetector for receiving the reflected light and performing photoelectric conversion on the optical information recording medium via the objective lens in an optical path between the objective lens and the light splitting element. A polarization conversion element that changes the polarization state of the incident light and the polarization state of the reflected light from the optical information recording medium, and the diffraction efficiency of the laser light having the wavelength λ1 is different due to the difference in the polarization state. A polarizing diffraction grating having a function of changing the diffraction efficiency of the laser light having the wavelength λ2 to at least 0% for at least two different polarization states. And the light spot position control means from at least the diffracted light or the undiffracted light or both the diffracted light and the undiffracted light diffracted by the polarization diffraction grating out of the reflected light reaching the photodetector via the light branching element. A light spot position control signal necessary for operation is detected, and the object laser is used as the light spot position control means based on the light spot position control signal. The optical head is characterized in that a movable mechanism for moving integrally's and said polarization conversion element and said polarization diffraction grating. 3. The optical head according to claim 1, wherein a plurality of objective lenses having substantially no aberration on a plurality of types of substrates having different thicknesses of the transparent substrate are mounted as objective lenses. A lens is mounted on a movable mechanism integrally with the polarization conversion element and the polarization diffraction grating, or integrally with the polarization conversion element, and selects one of a plurality of objective lenses according to the optical information recording medium to be recorded and reproduced, thereby selecting an optical path. An optical head to which means for inserting the optical head is added. 4. A focus control signal used for controlling a position of the light spot in an optical axis direction among the light spot position control signals in the optical head according to claim 1, 2 or 3. An optical system comprising: astigmatism detection means; and at least push-pull detection means for detecting a tracking control signal used for position control of the light spot in an in-plane direction of the optical information recording medium. head. 5. The optical head according to claim 1, wherein the light passes through the polarizing diffraction grating, passes through the objective lens, and then passes through the objective lens. Among the light spots of the incident light condensed on the information recording medium, the light spot of the diffracted light diffracted by the polarizing diffraction grating is shifted from the undiffracted light spot by at least a predetermined amount in the optical axis direction. An optical head having a function of adding a predetermined phase distribution to a wavefront of diffracted light so that the wavefront is focused at a different position. 6. An optical information reproducing apparatus equipped with the optical head according to claim 1.