JPH1196557A - Optical recording medium recording signal reader - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily read a signal by irradiating at least two light beams with different light quantity distribution shapes, respectively detecting reflected light intensity of the light beams from an optical recording medium and making the signal based on a difference between detected outputs themselves, or the signals after prescribed signal processing is performed a read signal. SOLUTION: The light from a semiconductor laser 1 is made nearly parallel by a collimate lens 2 to be made incident on a polarizing beam splitter 3. The splitter 3 is arranged so that the polarization direction of the light becomes nearly 45 deg. for a polarizing beam, and the light is divided to an optical path going toward a reflection mirror 4 and the optical path going toward the polarizing beam splitter 7. The light by the reflection mirror 4 is made incident on a wave front aberration giving means 5, and is given with specified aberration or intensity distribution to be focused on photodetectors 13, 14 by an objective lens 9 and a condenser lens 11. The central part intensity of the image of the light through the wave front aberration giving means 5 is detected by the photodetector 13, and the central part intensity of the image of the light going ahead the polarizing beam splitter 3 is detected by the photodetector 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an apparatus for reading a signal recorded on an optical recording medium such as an optical disk.

[0002]

2. Description of the Related Art A known pickup for reading a recording signal from an optical recording medium includes a laser serving as a light source, a half mirror, a collimator lens, an objective lens, and a photodetector. In this configuration, the light emitted from the laser passes through a half mirror, is converted into substantially parallel light by a collimator lens, and is incident on an objective lens. The light passing through the objective lens forms a spot having a minimum spot diameter (diffraction limit) with respect to the wavelength λ and the numerical aperture NA of the objective lens. The pits or marks that can be read out by the spots, that is, the areas where the reflection characteristics of adjacent ones are different from each other, must have a size such that the spatial frequency is equal to or less than the cutoff spatial frequency of fc = 2NA / λ. This is because the light diffracted by this optical disc is divided into 0th-order light, which indicates the average reflectance, and plus and minus first-order lights, which are located at a position separated by a frequency determined by the frequency of the objective lens pupil. This is because the light component goes out of the pupil of the objective lens and the structure of the optical disk is not transmitted as information. That is, the size of the pit or mark is required to be larger than the size capable of detecting the diffracted light component on the optical disc,
Therefore, in order to store a large amount of information, for example, digital data for high-definition television for 2 hours on an optical disc having a diameter of 12 cm, a means such as using a short wavelength light source or further increasing the numerical aperture of the objective lens is required. However, these have problems that the configuration is complicated and the element lacks stability.

[0003] Further, even in a band within the cut-off frequency, a fine structure close to the cut-off frequency has a small signal modulation degree, so that a noise component contained in the signal disturbs and a good signal cannot be read. There was a problem. In addition, if the intervals between tracks are reduced to a high density, many signals of adjacent tracks are picked up, and this signal becomes noise, so that a high-quality signal cannot be read.

[0004]

SUMMARY OF THE INVENTION The present invention reduces the wavelength of a signal recorded by a short pit or mark or a signal of a closely arranged track, which cannot be read satisfactorily by such a conventional apparatus, by shortening the wavelength. It is an object of the present invention to provide a reading device capable of reading without increasing the numerical aperture or increasing the numerical aperture.

[0005]

According to the present invention, there is provided an optical recording medium recording / reading apparatus comprising: an optical recording medium on which a signal is recorded on a recording track composed of a series of areas having different reflection characteristics from each other; A beam irradiating means for irradiating at least two light beams having different light amount distribution shapes, light detecting means for respectively detecting reflected light intensity of the light beam from the optical recording medium, An optical recording medium recording signal reading device, comprising: a detection output itself of the light detection unit or a difference detection unit that sets a signal based on a difference between signals subjected to predetermined signal processing to these signals as a read signal. .

That is, in the present invention, an optical recording medium (hereinafter, simply referred to as an optical disk) is simultaneously irradiated with two types of spots having different distribution shapes, and a time difference of an electric signal corresponding to light reflected by each spot is obtained. Changes are picked up by scanning the optical disc.

[0007]

FIG. 1 is a diagram showing a configuration of a first embodiment of the present invention. Light emitted from the semiconductor laser 1 is made substantially parallel by the collimator lens 2 and enters the polarization beam splitter 3. Here, the polarization direction of the light is approximately 4 with respect to the polarization axis direction of the polarization beam splitter 3.
It is arranged so as to be at an angle of 5 degrees, and is thus split by the polarizing beam splitter 3 into an optical path toward the reflection mirror 4 and an optical path toward the polarizing beam splitter 7 as it is. The light reflected by the reflection mirror 4 enters the wavefront aberration applying means 5. Here, the wavefront aberration applying means 5 uses a hologram plate or a glass plate having a partially different thickness, refractive index, or transmittance, and gives a specific aberration or intensity distribution to light transmitted therethrough. This light is reflected again by the reflection mirror 6, and is combined by the polarization beam splitter 7 with light that has traveled straight through the polarization beam splitter 3. At this time, since the two lights are separated by the polarization beam splitter 3, the polarization directions are different. This light is controlled by the objective lens 9 so as to be focused on the information recording surface of the optical disk 10, and the light modulated by the information pits or marks is collected again by the objective lens 9, and then is focused by the beam splitter 8 to the focusing lens 11. Be guided.
Here, the polarization direction of this light is divided by the polarization beam splitter 12, and the light is focused on the photodetectors 13 and 14 by the condenser lens 11, respectively. Wavefront aberration imparting means 5
Is detected by the photodetector 13 at the center of the image, and the light at the center of the image is detected by the photodetector 14 on the light that has traveled straight through the polarization beam splitter 3. Here, the spot on the optical disk is shown in FIG.
(A). FIG. 2 (a)
, 21, 22, and 23 denote pit trains serving as information tracks, and a beam spot 24 formed by a polarization component that has passed through the wavefront aberration imparting means 5 travels straight through the polarization beam splitter 3 and does not receive a specific aberration. The intensity distribution is such that the intensity at the central portion in the direction along the track is lower than that of the component beam spot 25. FIG. 3 shows FIG.
And the beam distribution shape 31 (corresponding to the beam spot 25) due to the light in the polarization direction that has traveled straight through the polarization beam splitter 3 is substantially the diffraction limit, which is not affected by aberration or the like. Beam. Further, the beam distribution shape 32 (beam spot 24) in the polarization direction via the wavefront aberration imparting means 5
) Has a shape whose head is suppressed by the aberration and intensity distribution given by the wavefront aberration applying means 5. That is, assuming that the full width at half maximum with respect to the maximum value is the beam diameter, the beam diameter is expanded. Alternatively, the shape is determined by the wavefront aberration applying means 5 such that the head portion is missing when compared with the actual intensity. Here, the beam distribution shapes 31 and 32 are the results of calculations and confirmation that both are shapes that can be realized using this objective lens. Here, the size of each photodetector is set to be a region of 33 smaller than the beam diameter (for example, the diameter of the first dark ring) on the detection surface. The central portion of the detected signal is detected by the photodetector 14, and the central portion of the signal detected by the beam having the beam distribution shape 32 is detected by the photodetector 13.

If at least one of the light detecting means 13 and 14 is located at an image forming position on the optical disk surface in the tangential direction of the recording track, the length of the light detector is The length of the upper two beam spots should be shorter than any of the tangential lengths of the tracks. Further, at least one of the light detection means 13 and 14 is
When the optical detector is placed at an image forming position in the direction perpendicular to the tangent to the recording track on the optical disc surface, the length of the photodetector is perpendicular to the tangent to the track of the two beam spots on the photodetection surface. It is shorter than any of the lengths in the direction.

For example, if two beam distribution shapes in a tangential direction (or a direction orthogonal to the tangential direction) of a recording track are as shown in FIG. 3, the length of the photodetector is two beam spots. The beam spots 31 and 32 have a length 33 shorter than the length 34. FIG. 4 shows the result of calculating the state of signal detection at this time.
In FIG. 4A, in a recording signal 41 which is a signal recorded on the optical disc, a mark having a low reflectance is recorded where the output is low. The shortest mark length is only about 60% of the detection resolution determined by the beam diameter, and this mark cannot be decomposed and detected by a normal detection method. In FIG. 4B, detection signals 42 and 43 are signals detected by the beam distributions 31 and 32 when the optical disk is rotated, and are irradiated at overlapping positions, but have different polarization directions. It can be detected separately. Here, since the average reflectance in the range where the spot falls upon scanning the optical disk changes, the optical disk is modulated by the rotation of the optical disk, but does not directly reflect the recording signal 41. However, this 2
The two spots are illuminated at overlapping positions, and by obtaining the difference between the detection signals 42 and 43 by the subtraction circuit 15, a change in the amount of light due to the difference between the distributions of the beam distributions 31 and 32 can be taken out. Scanning makes it possible to detect the recorded recording signal 41 as a temporal change. The subtraction circuit 15 outputs the level difference between the detection signals 42 and 43 as an analog value. At this time, the signal modulation degree 44 is smaller than that in the case where a large pit is detected by the conventional signal detection method. However, in this method, two signals are extracted from the light emitted from the same light source, and the difference between the two signals is obtained. , Most of the noise components are cancelled, and a good signal-to-noise ratio can be obtained. Therefore, according to the present invention, it is possible to detect a small signal conventionally buried in noise. Further, only the central part of the beam is taken out by the photodetector, and by calculating this difference, only a portion where there is a difference in intensity between the two beams is detected as a signal. It can be considered as a temporal change in intensity.

Further, as a second embodiment, if the intensity distribution difference is also given by the polarization direction in the direction orthogonal to the track, that is, in the direction across the adjacent track, a signal in which the influence of the adjacent track is reduced can be obtained. Can be taken out. The beam on the optical disk at this time is shown in FIG.
(B). In FIG. 2B, 26, 2
Reference numerals 7 and 28 denote pit trains serving as information tracks. A beam spot 29 based on the polarized light component passing through the wavefront aberration providing means 5 designed to also provide an effect in the radial direction is specified by traveling straight through the polarizing beam splitter 3. The intensity distribution is such that the intensity at the central portion in the direction along the track is lower than that of the beam spot 30 of the polarization component to which no aberration is given.

FIG. 5 shows a configuration for realizing the second embodiment. In FIG. 5, the optical system includes a semiconductor laser 51 as a light source, a collimator lens 52, a polarization hologram element 53, a reflection prism 54, and a beam splitter 5.
5, objective lens 56, optical disk 57, condenser lens 5
8, polarization beam splitter 59, photodetectors 60 and 61
It is composed of Light having linearly polarized light emitted from the semiconductor laser 51 is converted into parallel light by a collimator lens 52 and enters a polarization hologram element 53. Here, the polarization hologram element 53 is a hologram element having a structure as shown in FIG. 6, and is composed of a hologram 62 made of, for example, lithium niobate crystal having birefringence and an ultraviolet curable material having no birefringence. It is composed of a portion 63 using a transparent resin. Here, the refractive index of the portion 63 having no birefringence is selected to be equal to the refractive index of the portion 62 having birefringence for ordinary light, and this element acts as a hologram only for extraordinary light. It is configured as follows. Here, the semiconductor laser 51 changes its polarization direction,
45 with respect to the anisotropic axis of the hologram 62 having birefringence.
The light transmitted through the polarization hologram element 53 is different depending on the polarization direction.
The reflected light from the optical disk is collected again by the objective lens 56, passes through the beam splitter 55,
Are collected on photodetectors 60 and 61. Here, the light is split into two by the polarization beam splitter 59 according to the polarization direction.

The hologram element 53 may be modified from FIG. 6 to be as shown in FIG. As a third embodiment, FIG.
Shows a method of irradiating a pit with a beam having a polarization direction of 45 degrees with respect to the track direction. In FIG.
The reflected light of the beam applied to the pits has the same length of the pits in its polarization direction (d ₁ = d ₂ ), so that the pits are equally affected. As shown in the first and second embodiments, since the present invention separates the reflected light from the optical disk by the beam splitter, it is better that the pits have the same effect on both beams. In other words, this method is differently affected by both beams because the lengths of the pits seen from the polarization directions of the two irradiated beams are different,
Both beams are prevented from having the same polarization direction.

FIG. 9 shows a fourth embodiment in which a composite hologram is used in place of the polarization hologram. In FIG. 9, a composite hologram 93 is formed by bonding diffraction gratings as shown in FIG. 10A so as to be at 90 degrees to each other (102, 103), and further, as shown in FIG. Are attached to each other, which has a structure as shown in FIGS. 10B and 10C. Here, FIGS. 10B and 10C show the structure viewed from the lateral direction with respect to each other. The beam emitted from the semiconductor laser 95 has its polarization direction changed by the hologram layer 101, and is split into the same beam in a traveling direction having a different intensity distribution. Subsequently, the light goes straight to the objective lens 92 and is focused on the optical disk 91. The beam reflected from the optical disk and returned returns to the hologram layer 103 in a direction different from that of the light emitted from the semiconductor laser 95, and is further transmitted to the two photodetectors 96 and 97 by the hologram layer 102. And divided
The difference between the detection signals of the photodetectors 96 and 97 is obtained by a subtraction circuit 90 similar to the subtraction circuit 15. As described above, by arranging the composite hologram in the common portion between the optical path from the light source to the optical disk and the optical path from the optical disk to the photodetector, it is possible to reduce the size of the entire pickup 94.

Although the composite hologram is used in the above embodiment, it is also possible to use a liquid crystal panel to partially change the transmittance or the wavefront depending on the polarization direction so as to correspond to various media. is there. FIG. 11 shows a fifth embodiment in which light is separated by wavelength regardless of the polarization direction. FIG. 11 shows that the wavelength is 630 nm, for example.
, A semiconductor laser light source 113 having a wavelength of 780 nm, and a collimator lens 11
2 and 114, dichroic prism 11 used to combine light from semiconductor lasers 111 and 113
5. Reflecting prism 116, beam splitter 117, objective lens 118, optical disk 119, condensing lens 12
0, a system composed of a dichroic prism 121 and photodetectors 122 and 123 is shown. Light emitted from the semiconductor lasers 111 and 113 is collimated by a collimator lens 112.
And 114 are combined so as to be substantially parallel, and the dichroic prism 115 is combined so that the optical axes are substantially the same. This light is reflected by the reflection prism 116, passes through the beam splitter 117, and is focused on the optical disk 119 by the objective lens 118. Here, since the two spots have different wavelengths and distribution shapes on the pupil plane,
Form spots of different shapes. For example, when the semiconductor laser 111 is slightly defocused and condensed, the two beam diameters are equal and the center intensities are different from each other.
31 and 32 can be obtained. The reflected light obtained at the two wavelengths passes through the objective lens 118 and the beam splitter 117, and the light of 630 nm is transmitted to the photodetector 123 by the dichroic prism 121.
The light of 780 nm is focused on the photodetector 122. In this way, it is possible to obtain signals at the respective central portions by overlapping lights having different distribution shapes, and by obtaining difference signals of the photodetectors 122 and 123 by a subtraction circuit 124 similar to the subtraction circuit 15, As in the above-described embodiment, a fine structure can be detected. In such a configuration using two wavelengths, the noise margin can be reduced because the noise of the light source cannot be canceled as compared with the configuration using polarized light. In particular, by using light sources of two wavelengths, a configuration suitable for a pickup device that reads shared recording information with a low-density optical disk is obtained.

In this example, the wavelengths are 630 nm and 7
Although the description has been made with 80 nm as a matter of course, the wavelength is not limited to this. For example, a laser that oscillates in a multimode mode can be used to separate its spectrum or use a laser beam with a short wavelength by using optical nonlinearity. Various configurations are possible such as using the fundamental wave and the second harmonic in the SHG element that obtains the following.

As a sixth embodiment, the shapes of two spots on an optical disk are superimposed with different beam diameters at different center positions as shown in FIG.
As shown in FIG. 2B, one of the beams may be configured to have a bimodal property. Various configurations are possible as long as such overlapping portions exist and different signals can be obtained by separation.

In the above embodiment, the distribution in the radial direction of the optical disk is not mentioned, but it is sufficient if the two beams are substantially equal, and the distribution is detected so that a pit image is formed on the photodetector. In this case, it is also possible to use a spot having a shape extending over multiple tracks. At this time, the distribution in the disk radial direction does not necessarily have to be equal, and the same effect can be exerted if the distribution in the scanning direction has the above-described relationship.

In the above embodiment, a confocal or semi-confocal configuration in which a fine structure exceeding the cutoff frequency can be read out such that only the central portion of each of the imaged light spots is taken out by a photodetector. However, if it is not necessary to read such a fine structure, the beam is not imaged on the photodetector, but the overall intensity of the signal light is detected using a large photodetector, and the difference is calculated. It may be configured as follows. In this case, only the bandwidth expansion effect due to noise reduction among the effects described above is obtained,
There is an advantage that the adjustment of the pickup becomes easy.

[0019]

As described above, according to the present invention, an optical disk is irradiated with two beams having different distribution shapes in the scanning direction so as to overlap each other, and separated and detected, and a signal is detected. , It is possible to detect a small signal due to a short pit conventionally buried in noise. Further, since only a portion where the intensity difference between the two beams is detected is detected as a signal, a pit smaller than the cutoff spatial frequency can be regarded as a time change of the zero-order light intensity, and crosstalk can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 shows a beam spot according to the first and second embodiments of the present invention.

FIG. 3 shows a beam distribution shape according to the first embodiment of the present invention.

FIG. 4 shows a calculation result of signal detection in the first embodiment of the present invention.

FIG. 5 is a configuration diagram showing a second embodiment of the present invention.

FIG. 6 shows one hologram element in a second embodiment of the present invention.

FIG. 7 shows another hologram element according to the second embodiment of the present invention.

FIG. 8 is a diagram illustrating a third embodiment of the present invention.

FIG. 9 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 10 shows a hologram element according to a fourth embodiment of the present invention.

FIG. 11 is a configuration diagram showing a fifth embodiment of the present invention.

FIG. 12 is a diagram illustrating a sixth embodiment of the present invention.

[Description of Signs] 1, 51, 95, 111, 113 Semiconductor laser 2, 52, 112, 114 Collimator lens 3, 7, 12, 59 Polarizing beam splitter 8, 55, 117 Beam splitter 4, 6 Reflecting mirror 5 Wavefront aberration Application means 9, 56, 92, 118 Objective lens 10, 57, 91, 119 Optical disk 11, 58, 120 Condensing lens 13, 14, 60, 61, 96, 97, 122, 123
Photodetectors 15, 90, 124 Subtraction circuits 21-23, 26-28 Pit trains 24, 25, 29, 30 serving as information tracks Beam spots 31, 32 Beam distribution shapes 41 Recording signals 42, 43 Detection signals 44 Signal modulation Degree 53 Polarization hologram element 54, 116 Reflection prism 93 Composite hologram element 94 Pickup 115, 121 Dichroic prism

Claims (10)

[Claims] 1. A reading device for reading a signal from an optical recording medium on which a signal is recorded on a recording track composed of a series of areas having different reflection characteristics between adjacent ones, wherein at least the light quantity distribution shapes are different. Beam irradiating means for irradiating two light beams; light detecting means for respectively detecting the reflected light intensity of the light beam from the optical recording medium; detecting output itself of the light detecting means or a predetermined signal processing on these An optical recording medium recording signal reading device, comprising: a difference detection unit that sets a signal based on a difference between the applied signals as a read signal. 2. The optical recording according to claim 1, wherein the two beams are irradiated so as to overlap each other on a recording surface of the recording medium, and have different intensity distributions in a tangential direction of the recording track. Medium recording signal reader. 3. The optical system according to claim 1, wherein the two beams are irradiated so as to overlap each other on the optical recording medium, and are irradiated with different intensity distributions in a direction orthogonal to a tangent to the recording track. An optical recording medium recording signal reading device according to claim 1. 4. An optical recording medium recording signal reading apparatus according to claim 1, wherein said two beams have different polarization directions. 5. An apparatus according to claim 1, wherein said two beams have a polarization direction of 45 degrees with respect to a tangential direction of a track. 6. An apparatus according to claim 1, wherein said beam irradiation means includes a hologram element. 7. The apparatus according to claim 1, wherein said beam irradiating means and said light detecting means are one.
2. The hologram element of claim 1 is shared.
An optical recording medium recording signal reading device according to claim 1. 8. An optical recording medium recording signal reading apparatus according to claim 1, wherein said plurality of beams have different wavelengths. 9. The light detecting means is located at an image forming position on the medium surface in a tangential direction of the recording track, and the length of the detecting means is longer than any one of the lengths of the two light beam spots on the detecting surface. 2. An optical recording medium recording signal reading apparatus according to claim 1, wherein the length of the signal is also shortened. 10. The light detecting means is located at an image forming position on the medium surface in a direction orthogonal to a tangent to the recording track, and the length of the detecting means is the length of two light beam spots on the detecting surface. 2. An optical recording medium recording signal reading apparatus according to claim 1, wherein the length is shorter than any one of the above.