JP2839502B2 - Optical information reproducing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to an optical information reproducing apparatus used for an optical disk apparatus such as an optical video disk which records and reproduces information with a laser beam.

(Prior Art) In a video disk device for reading a signal from an information disk using a light beam, an information signal is placed on the information disk with a width of 0.4 μm.
m, track pitch 1.6μ with irregularities of 0.5-1.8μm length
m is recorded spirally or concentrically. In order to read this information, a laser beam is focused on a minute spot having a diameter of about 1 μm by an objective lens, and the reflected light is guided again to a light receiving element through the objective lens, and is extracted as an electric signal.

Recently, in this optical video disk device, an attempt has been made to record much more information as the wavelength of the semiconductor laser is shortened. In this case, in addition to shortening the uneven length in the information column direction, the track pitch is narrowed to achieve high density. The problem when narrowing the track pitch is that the light beam spot from which information is read has a finite size, and the light intensity is distributed, for example, as an airy pattern. , And the signal of this track is detected at the same time. This is called crosstalk. As a method of reducing this crosstalk, Japanese Patent Laid-Open No.
The method disclosed in Japanese Patent Publication No. 731 is known. This is the third
As shown in the figure, three light beams 34, 35, and 36 are simultaneously irradiated on three adjacent tracks 31, 32, and 33, and signals from information tracks 31, 33 on both sides of the middle information track 32 are respectively added to the signals. This is a method of reducing crosstalk by performing an operation of subtracting the product of the crosstalk amount from the signal from the middle information track 32.

However, in this method, as is apparent from FIG. 3, the light beams 34, 35, and 36 irradiating the adjacent tracks 31, 32, and 33 are radiated so as to be arranged at right angles to the information track direction. Irradiated. With optical video discs,
Since the track pitch is about 1 to 1.6 μm, the beam interval is also about this value. Therefore, when the track pitch is narrow, these light beams overlap, and it is difficult to separate and detect this light beam. It becomes. Also,
In order to receive the three beams with separate photodetectors, it is common to leave a distance of several hundred μm because of the ease of making the light receiving element and the ease of adjustment, etc. The magnification of the detection optical system must be several hundred times. Such an optical system has a considerably long optical path length, is large and heavy as a whole pickup, and is not practical.

Also, the light beams 34, 36 on both sides are adjacent to the tracks 31, 32 on both sides.
When the track pitch becomes narrower, the light beams 34 and 36 also detect the information of the adjacent tracks 37 and 38. When the difference from the signal is taken from the center beam, the signal of the next adjacent track 37, 38 is picked up as crosstalk, and there is no crosstalk reduction effect.

On the other hand, conventionally, a method as shown in FIG. 10 has been used for detecting the uneven signal. That is, in (a), the laser beam from the light source 71 is condensed by the objective lens 474 through the collimation lens 472 and the beam splitter 473, and is irradiated on the information disk 475. This reflected light is collected by a condenser lens 476 to a photodetector 477.
And is detected through an amplifier 478. In this detection, the beam spot condensed by the objective lens is diffracted by the signal irregularities, which means that the amount of reflected light has been modulated. Therefore, a component obtained by Fourier transforming the product of the function of the beam spot and the irregularities is received. Is done. Therefore, only a modulation factor (MTF) uniquely determined by the numerical aperture NA of the objective lens and the light wavelength λ can be obtained as a signal. As described above, with the shortening of the wavelength of the semiconductor laser, the density of recording and reproduction is shortened by shortening the uneven wavelength and the track pitch. Is determined, there is a limit on the MTF, and therefore, in higher-density recording / reproduction, C / N degradation occurs unless NA is increased or λ is decreased.

There is also known a method in which light is received by photodetectors 479 and 480 divided into two in the direction of an uneven signal row as shown in FIG. This is obtained by emphasizing the signal diffracted at the edge of the irregularities, so high frequency detection is possible, but for low frequency irregularities (signals with long irregularities and long intervals), the detection MTF decreases. I will. Therefore, there is a problem that the reproduction of a signal including a low frequency component is inconvenient.

(Problems to be Solved by the Invention) As described above, in order to eliminate crosstalk, in the crosstalk reduction method of irradiating three light beam spots arranged vertically in three information tracks to the center of each track, If the track pitch is reduced, it becomes difficult to separate and detect the three beams, the optical system becomes too large, and crosstalk corresponding to the signal of the next adjacent track appears. .

Furthermore, in the conventional detection method, the signal modulation degree is determined by the MTF determined by the NA of the objective lens and the wavelength of light, and when the size of the unevenness is reduced, the level of the detection signal is reduced and C Since / N deteriorates, it is impossible to record and reproduce information at higher density.

Therefore, according to the present invention, first, the tracking operation can be easily performed without increasing the size of the optical pickup.
It is another object of the present invention to provide a high-density optical information reproducing apparatus in which crosstalk on both sides is reduced and crosstalk of a track outside the crosstalk is not affected. The use of different wavelengths can provide a greater degree of modulation, and the noise component does not increase at the same rate, thus improving the C / N of the reproduced signal and enabling higher density recording and reproduction It is an object of the present invention to provide a high-resolution optical information recording / reproducing apparatus that can perform the above-described operations.

[Configuration of the invention]

(Means for Solving the Problems) The present invention comprises a light source for emitting a light beam, an optical means for guiding the light beam emitted from the light source to an information medium and condensing it on a minute spot, and a reflection from the information medium. A photodetector for receiving light or transmitted light and converting it into an electric signal, divided into two regions in the information column direction, and a first adding means for summing outputs of the two photodetectors by the photodetector; ,
Subtraction means for taking the difference between the outputs of the two photodetectors; phase adjustment means for adjusting the phases of the outputs of the addition means and the subtraction means; and a summation of the outputs of the addition means and the subtraction means. The present invention relates to an optical information reproducing apparatus provided with two adding means.

(Operation) In the present invention, an MTF that receives reflected light from an information disk with a photodetector divided into two, and gradually reduces from a low frequency to a high frequency determined by the NA of the objective lens and the wavelength λ of the light, which is obtained from the sum thereof, By taking the difference between the signal component
By adding a signal component having an MTF that has a peak before reaching a cutoff frequency determined by NA and λ after adjusting the phase, a modulation factor larger than that of the MTF with a single photodetector is obtained. The amplitude can be increased. Further, in the difference output, the noise component can be reduced and the noise can be reduced with respect to the signal amplitude, so that the C / N can be improved. Therefore, the C / N ratio can be improved even when the objective lens having the same NA and the light wavelength are used, and higher-density recording and reproduction can be realized.

(Example) Hereinafter, an example according to the first invention of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing the configuration of one embodiment of the present invention. A laser beam 12 emitted from a semiconductor laser 11 is converted into a parallel beam by a collimation lens 13 and split into three beams through a diffraction grating 14. Polarizing beam splitter 14,
The beam that has passed through the quarter-wave plate 16 is
The light is condensed on two beam spots and irradiated on the information disk 18. The light reflected by the information disk 18 returns to the original optical path, is reflected by the polarization beam splitter 15, passes through the focusing lens system 19, and is received by the three photodetectors 110, 111, 112. Each photodetector output is amplified by preamplifiers 113, 114, 115,
The first beam output 113 passes through a delay circuit 116 having a delay time of 2τ (τ is determined by τ = l / v from the rotational linear velocity v of the information disk and the three beam intervals l), and is passed to the gain adjustment circuit 117. Reach. The second beam output 114 passes through a delay circuit 118 having a delay time of τ. The third beam output 115 passes through the gain adjustment circuit 119 as it is. The first beam output and the third beam output are then added in an adder circuit 120. This output is subtracted by the subtraction circuit 121 from the second beam output. This output is an information signal.

On the other hand, a first photodetector output 110 and a third photodetector output 112
That is, when a difference is obtained by the differential amplifier 122, a tracking signal is obtained. When the tracking signal is added to the objective lens driving device 124 through the driving circuit 123, the light beam can follow the track.

In addition, as the optical pickup, a focus servo mechanism is required to keep the distance between the information disk and the objective lens constant. A well-known method such as an astigmatism method may be used.

FIG. 2 is a diagram showing the relationship between an information track and a beam spot according to the present invention. As shown in (a), the center beam 22 is positioned at the center of the track 25 from which information is to be read,
The beams 21 and 23 on both sides are arranged so as to cover the boundary near the center between the tracks 24 and 26 on both sides. This can be done by rotating the diffraction grating 14. The distribution of the light beam intensity at this time is as shown in FIG. In other words, the center beam 22 is located on the center line of the track 25, but due to the distribution of light intensity, its skirt and side lobes are applied to the adjacent tracks 24 and 26, and the detection thereof causes crosstalk. Become. Further, since the first beam 21 and the third beam 23 are located at the boundary between the adjacent tracks 24 and 26, signals of the first track 24 and the third track 26 can be detected, respectively. But,
Since the side lobes cover the second track 25, the signal of this center track is also detected to some extent. The gain of the signal components on both sides of the first track 24 and the third track 26 detected from the second beam 22 is made equal to the signal component of the track obtained from the first beam 21 and the third beam 23. Is adjusted and subtracted from the signal from the center beam 22 to cancel the crosstalk component. At this time, the first beam signal,
Since the center beam component included in the third beam signal becomes extremely small, the center signal component is almost kept as it is.

Further, as is apparent from (b), since the first beam 21 and the third beam 23 are respectively shifted toward the center by half of the track width from the track center, the outer tracks 27 and 2
8 has less foot and side lobes.
Therefore, the crosstalk component from this track is very small.

By the way, as shown in (a), the positions of the three light beams are moved back and forth in the information track direction. Assuming that the linear velocity of the information disk is v (m / s) and the beam interval is 1 (μm), the signals obtained from the three beams have a time shift of τ = 1 / v (μs). Therefore, when calculating the sum or difference of the three beams, the output of the preceding first beam 21 is delayed by 2τ and the output of the second beam 22 is delayed by τ to execute the calculation, thereby eliminating the time lag.

In the case of rotation at a constant rotation speed, the linear velocity v changes according to the reproduction radial position of the information disk, so that the delay time τ also changes. In this case, however, the radial position information is detected to change the delay time. A delay line may be used.

As a method of generating three beams, not only a diffraction grating but also three semiconductor lasers may be used.

As described above, according to the present invention, even if the track pitch becomes narrower than the light beam diameter, crosstalk from an adjacent track is canceled out, so that a good reproduction signal without crosstalk can be obtained. Further, the amount of crosstalk from the outer adjacent track can be kept small, so that higher density recording / reproducing is possible.

An embodiment according to the second invention of the present invention will be described below with reference to the drawings.

FIG. 4 is a diagram showing the configuration of one embodiment of the present invention. The laser beam emitted from the semiconductor laser 401 is converted into parallel light by the collimation lens 402, and the beam that has passed through the beam splitter 403 is condensed into a minute spot by the objective lens 404, and is irradiated on the information disk 405. The reflected light diffracted by the signal irregularities recorded on the information disk 405 returns to the original optical path, is reflected by the beam splitter 403, and is received by the two-divided photodetectors 407 and 408 through the focusing lens diameter 406. The two divisions are made in a direction that matches the concave / convex row (information row). The outputs of the respective photodetectors are added by an addition circuit (first addition means) 409, and at the same time, a difference is obtained by a subtraction circuit (subtraction means) 410. The difference signal passes through a delay circuit (phase adjusting means) 411 and is adjusted to have the same phase as the sum signal, and is added by a second adding circuit (second adding means) 412 to be an information signal.

By the way, in order to reproduce an information disk with such an optical pickup, the light beam spot must follow the fluctuation of the information disk. For this purpose, the focusing servo and tracking servo are performed by an astigmatism method or a three-beam method. A well-known method such as that described above may be used, and the description is omitted because it is not related to the object of the present invention.

FIG. 5 is a diagram showing the reproduction principle of the present invention. As shown in (a), the light diffraction phenomenon due to the information unevenness is considered separately for the incident beam side and the reflected beam side. When a light beam having a wavelength λ is condensed by an objective lens 404 having a numerical aperture NA and is irradiated on the information unevenness 421, the amplitude distribution of the light beam spot is defined as w (x) (handled in one dimension for easy description of the principle). Assuming that the phase change of the distribution is (x), the distribution P (x) of the light diffracted by the unevenness is a Fourier transform of w (x) (x). Is represented by The light intensity Q collected by the same objective lens 14 (NA = a / f where a is the lens aperture radius and f is the focal length) is It is. When the modulation degree of the signal obtained in this way, that is, the MTF is represented on the spatial frequency axis, the curve (1) in FIG. The cut-off spatial frequency is 2NA / λ.

The light intensity when the difference between the two photodetectors 407 and 408 is obtained is Becomes The MTF at this time is a curve (2) in FIG.

(C) shows a correspondence between the two signals on the time axis. (4) shows the shape of the unevenness. Since the light is greatly diffracted at the concave portion of the unevenness, the amount of light received by the two photodetectors 407 and 408 is reduced, so that the sum signal is reduced and a waveform as shown in (5) is obtained. In addition, since the diffraction in the direction of the uneven row becomes large at the edge of the unevenness, a large amount of light is received by one of the two photodetectors 407 and 408, so that the amplitude becomes large at the edge where the difference signal is obtained. The signal of (6) is obtained. When comparing (5) and (6), there is a phase shift of about 90 °. Therefore, if the difference signal is delayed so that the phases of the difference signal and the sum signal are matched, and the maximum amplitude is added, the degree of modulation can be increased in a high band as shown by the curve (3) in FIG.

Let's focus on this noise. FIG. 6 is a diagram showing the relationship between the irradiated light beam 432, the unevenness 433, and the photodetectors 407 and 408. Since the light beam 432 reproduces a signal while tracing on the unevenness 433, the detection beam on the photodetectors 407 and 408 is divided into two light detectors 407 and 40 only at the edge portions of the unevenness.
Move perpendicular to boundary 8 431. Considering the noise on the information disk, irregularities on the surface and fluctuations in the depth, width, and length of the irregularities are the causes of the noise, but this is similar to the signal, and is perpendicular to the boundary 431 at the edge of the noise. If the difference is taken, the noise will be superimposed on the signal in the same MTF form. However, the component spreading in the direction of the boundary 431 becomes large except at the edge, and this change appears as the same change in both the two-segment photodetectors. Therefore, if the difference is taken, the noise component is very small. The noise at the edge described above can be removed by passing the reproduced signal through a limiter or the like. For the sum signal, the noise also overlaps with the MTF applied. In addition, since the laser noise is considered to be almost in phase with the photodetectors 407 and 408, it can be reduced by taking a difference. Therefore, if the sum signal and the difference signal are considered in total, the C / N can be improved.

FIG. 7 shows another embodiment. As apparent from FIG. 5 (c), the phase of the sum signal and the difference signal is shifted by about 90 °, and the difference signal is a form obtained by differentiating the sum signal. be able to.

Also, the phases may be matched by delaying the sum signal.

FIG. 8 shows another embodiment. When a large number of low-frequency signals with a long unevenness and a long unevenness wavelength are included, the differential output (b) has a pulse-like waveform having a peak only at the edge of the unevenness as shown in FIG. So this cannot be added late. In particular, since the MTF of the sum signal (5) is large in the low frequency region, the difference signal may be added only in the high frequency region. The circuit configuration in this case is shown in FIG.
The high-frequency component of the sum output 409 of the two photodetectors 407 and 408 (the area where the detection signal becomes a substantially sine wave) is separated by the high-pass filter 51, and the state of the high-frequency signal and the low-frequency signal is detected by the comparator 52. Based on this signal, the switch circuit 453 is turned on only when the high-frequency signal is reproduced, and is added to the sum signal of the delayed difference signal. By doing so, the C / N of the high frequency component can be improved. The same is true even if the high-pass filter 51 detects a low-pass signal with a low-pass filter.

FIG. 9 shows another embodiment capable of improving the C / N of the low frequency component. When the length of the unevenness is long, the difference signal (6) has a positive or negative pulse at the edge portion as shown in FIG. If a negative state is maintained in the case of a pulse in the direction, a signal equivalent to the uneven shape as shown in (7) can be obtained.
An example of a circuit for realizing this state is shown in FIG. Difference output 410
The state is detected by the positive comparator 461 and the negative comparator 462, and the waveform equal to the irregular period is obtained by the state holding circuit 463 (7).
Get. Since the comparators 461 and 462 compare at the rising or falling of the pulse, there is a slight time deviation from the unevenness, that is, the sum signal. When the time is adjusted by the delay circuit 464 and the sum is added to the sum signal, the signal addition of the low frequency component can be performed. In the uneven row, the edge position of the unevenness is effective as a signal, so that the edge position may be detected.

As described above, according to the present invention, the signal amplitude can be increased up to the high frequency range, and the noise component can be kept relatively low, so that the C / N of the reproduced signal can be improved. This means that information recording / reproducing with higher density or higher C / N can be realized even if the same NA objective lens and laser light wavelength are used as before.

Further, since the method described in the present invention can obtain the same effect also in the tracking direction in the direction perpendicular to the concave and convex rows,
With the same configuration, the track pitch can be narrowed without reducing the C / N of the tracking signal.

〔The invention's effect〕

According to the present invention, it is possible to provide an optical information reproducing apparatus having a good influence of crosstalk without increasing the size of an optical pickup. Furthermore, since the C / N of the reproduction signal can be improved, it is possible to provide an optical information reproducing apparatus with higher density and higher resolution.

[Brief description of the drawings]

FIG. 1 is a view showing an embodiment of the present invention, FIG. 2 is a view for explaining the effect of the present invention, FIG. 3 is a view showing a conventional example, and FIG. 4 is a view showing an embodiment of the present invention. , FIG. 5 is a view showing the principle of reproduction of the present invention, FIG. 6 is a view showing the relationship between the light beam and the photodetector of the present invention, FIG. 7, FIG. 8 and FIG. FIG. 10 is a view showing an embodiment, and FIG. 10 is a view showing a conventional example.

Claims (3)

(57) [Claims] A light source for emitting a light beam; an optical unit for guiding the light beam emitted from the light source to an information medium and condensing the light beam on a minute spot; and receiving reflected light or transmitted light from the information medium to receive the light beam. A photodetector for converting into an electric signal, divided into two regions in the information column direction, a first adding means for summing two photodetector outputs by the photodetector, and the two photodetector outputs Subtraction means for taking the difference between the two, a phase adjustment means for adjusting the phases of the outputs of the addition means and the subtraction means, and a second addition means for taking the sum of the outputs of the addition means and the subtraction means. An optical information reproducing apparatus, characterized in that: 2. The optical information reproducing apparatus according to claim 1, wherein said phase adjusting means delays an output of said subtracting means. 3. An optical information reproducing apparatus according to claim 1, wherein said phase adjusting means is an integrating circuit for passing an output of said subtracting means.