JP2760833B2 - Optical pickup - Google Patents

Description

The present invention relates to an optical pickup for optically reproducing a signal (information) recorded on an optical information recording medium (hereinafter, referred to as an optical disk). In particular, the present invention relates to an optical pickup suitable for improving the frequency characteristics of a reproduction signal.

[Conventional technology]

2. Description of the Related Art A conventional optical pickup irradiates an optical disk on which a signal is optically recorded at a high density with a laser beam (light beam) after condensing it. At this time, a light spot having a diameter of about 1 μm is generated on the optical disk.
In general, an optical disk has a predetermined rotation speed (for example, 18
00 rpm. ), The light spot is relatively moving on the optical disk. On the other hand, as a recording form of a signal on an optical disk, there is a method in which a signal pit is provided in a mirror state portion (hereinafter, referred to as a mirror portion), for example. In this case, the reflectance is different between the signal pit and the mirror portion. Therefore, the amount and intensity of the reflected light (reflected light beam) of the light spot changes between the signal pit and the mirror portion. Therefore, a conventional optical pickup reproduces a signal recorded on an optical disk by detecting a change in the amount of reflected light of a light spot with a photodetector.

As a signal recording mode, there are an optical disk using a transparent portion instead of a mirror portion, and an optical disk (magneto-optical disk) using a magneto-optical effect. In the former optical pickup corresponding to the former optical disk, the change in the amount of transmitted light (transmitted light beam) of the light spot causes
The signal recorded on the optical disk is being reproduced. In the latter conventional optical pickup corresponding to the optical disk,
The signal recorded on the optical disk is reproduced by extracting the polarization of the reflected light from the light spot.

Generally, signal pits are recorded at a high density on an optical disk, and since this optical disk rotates at a relatively high rotational speed, the frequency band of a reproduction signal reproduced by an optical pickup is wide. When the density of the signal recorded on the optical disk increases and the period of the signal pit falls below about twice the diameter of the optical spot, the modulation degree (for example, the amplitude The rate of change.) As a result, in the conventional optical pickup, there is a problem that the frequency characteristic of the reproduction signal is significantly deteriorated on the high frequency side.

An example of this problem will be described with reference to FIG.

FIG. 2 is a schematic diagram conceptually showing an example of a positional relationship between a light spot and a signal pit when a laser beam is irradiated on an optical disk and a relationship between a light spot irradiation position and a detected light amount. The example shown in FIG. 2 is a case where the detected light amount decreases when a light spot is irradiated on the signal pit. FIG. 2A shows a case where the frequency of the recording signal is low and the interval between the signal pits is wide. FIG. 2B shows a case where the frequency of the recording signal is high and the interval between signal pits is narrow. An example of such a signal pit is a phase pit used in a general video disk or the like.

In FIG. 2, the light spot 21 travels in the direction of the arrow 81 (track direction), and sequentially passes through a portion of the signal pit 100 and a mirror portion (a portion other than the signal pit 100). The characteristics of the detected light amount with respect to the light spot irradiation position in this case are shown in the lower part of FIG. The horizontal axis indicates the light spot irradiation position corresponding to the position of the upper signal pit 100. The vertical axis indicates the detected light amount. The level of the detected light quantity V _M (indicated by a dashed line in the figure)
Indicates a value when the light spot 21 is stopped in the mirror section.

As shown in FIG. 2A, when the interval between the signal pits 100 is wide, when the light spot 21 is located at the center of the signal pit 100, the level of the detected light amount becomes a low value Vp. When the light spot 21 is located at an intermediate position between the signal pits 100, the level of the detected light amount becomes Vs, which is a high value. It is assumed that the interval between the signal pits 100 is reduced from the state shown in FIG. 2A, so that the period of the signal pit is shortened, and this period is smaller than about twice the diameter of the light spot 21. In this case, as shown in FIG. 2 (b), the detected light amount is changed from the level Vp. And also from level Vs To decrease. As a result, the modulation degree M = (Vs−Vp) / V _{M of} the reproduction signal reproduced by the conventional optical pickup decreases.

As an improvement measure for such deterioration of the frequency characteristic,
Generally, a correction circuit is provided in a signal reproduction circuit. Japanese Patent Application Laid-Open No. Sho 62-2443 discloses a correction circuit of this type.
No. 7 publication.

[Problems to be solved by the invention]

The above prior art has a problem that the circuit scale becomes large in order to add a correction circuit for improving the frequency characteristic, and the waveform of the reproduced signal is distorted due to a change in the phase of the high frequency component of the reproduced signal. .

An object of the present invention is to provide a simple optical means or to configure the photodetectors so that their detection sensitivities are partially different, so that the modulation degree of the reproduced signal can be changed without changing the phase of the high frequency component. It is an object of the present invention to provide an optical pickup capable of suppressing a reduction in the optical pickup.

[Means for solving the problem]

 First, the principle of the present invention will be described with reference to the drawings.

FIG. 3 is a diagram showing the intensity distribution of a light beam (detected light beam) whose light amount is detected by a photodetector when a light spot is on a signal pit. FIG. 3A shows a case where the frequency of the recording signal is low and the interval between signal pits is wide. FIG. 3B shows a case where the frequency of the recording signal is high and the interval between signal pits is narrow.

In FIG. 3, the horizontal axis indicates the distance between each point measured from the optical axis of the detected light beam (the central axis of the light beam) in a predetermined direction corresponding to the track direction of the optical disk. The vertical axis indicates the relative value of the intensity of each detection light (reflected light beam or transmitted light beam) with reference to the intensity of the optical axis of the detection light obtained when no signal pit exists on the optical disk. ing. In addition, the range of the diameter of the detection light beam is described using a dashed line. As is clear from FIG.
When the interval between the signal pits is narrowed, the intensity of the detection light beam having a light spot on the signal pit increases at the optical axis. As a result, as described in FIG. 2, the light amount Vp of the entire light flux increases. On the other hand, the intensity of the outer edge portion of the detection light beam does not change even if the interval between the signal pits changes.

FIG. 4 is a diagram showing an intensity distribution of a detected light beam when a light spot is between signal pits. 4th
FIG. 7A shows a case where the interval between signal pits is wide. Also,
FIG. 4B shows a case where the interval between signal pits is narrow.

In FIG. 4, the horizontal axis represents the distance from the optical axis in the track direction, and the vertical axis represents the relative value of the intensity of the detection light. Note that the range of the diameter of the detection light beam is described by a dashed line. In the detection light beam when there is a light spot between the signal pits, the intensity of the optical axis decreases when the interval between the signal pits is reduced. As a result, as described in FIG. 2, the light amount Vs of the entire light flux decreases. on the other hand,
The intensity of the outer edge portion of the detection light beam hardly changes even if the interval between signal pits changes.

That is, as can be seen from FIGS. 3 and 4, the fluctuation in the light amount due to the change in the interval between the signal pits is mainly influenced by the light amount in the optical axis portion of the detection light beam. Therefore, if the absolute amount of light amount of the optical axis portion of the detection light beam is reduced,
That is, the rate of change in the detected light amount is also reduced. Alternatively, in the photodetector, if the detection sensitivity of the optical axis portion of the detection light beam is reduced, the rate of fluctuation of the output signal for which the light amount is detected is also reduced.

Here, for simplicity of explanation, a case where the amount of light detected decreases when a light spot is generated on a signal pit has been described as an example. However, conversely, even in the case of a signal pit having an increased light amount, for example, in the case of using a reflected light beam, in the case of a phase change type amplitude pit in which the reflectivity of the pit portion is increased, the above-described explanation is applied. Exactly the same principle holds. In addition, even if the signal recording principle is different, such as a magneto-optical disk, at the time of reproduction, the signal is reproduced from the change in the amount of the detected light beam by the combination of the analyzer and the photodetector. The principle holds.

Therefore, in order to achieve the above-mentioned object, the present invention provides a light path at a central portion and a light amount at a peripheral portion of a reflected light beam in a track direction of an optical information recording medium in an optical path from an objective lens to a photodetector. Are provided, and the photodetector is configured to detect the amount of light in the central part and the amount of light in the peripheral part of the reflected light beam passing through the means. This means includes, for example, an optical filter in which the transmittance (or reflectance) of the portion where the central portion of the reflected light beam enters is lower than the transmittance (or reflectance) of the portion where the peripheral portion of the reflected light beam enters. I do.

In order to achieve the above object, according to the present invention, a photodetector is provided with a photodetection sensitivity and a peripheral portion of a photodetection surface of a portion where a central portion of a reflected light beam with respect to a track direction of an information recording medium enters. It is configured so as to differ from the light detection sensitivity of the light detection surface of the portion.

[Action]

The above-described optical filter having the first filter region and the second filter region partially reduces the amount of light in the optical axis portion of the detection light beam. For this reason, although the amount of light detected by the photodetector is reduced as a whole, the fluctuation when the interval between the signal pits is reduced is reduced. This allows
Since the decrease in the degree of modulation in the reproduced signal at the high frequency range is suppressed, the frequency characteristics are improved.

Further, the photodetector provided with the first detection region and the second detection region described above, has a detection sensitivity of the first region where the optical axis portion of the detection light beam and the vicinity thereof enter, with respect to the second detection region. Has been reduced. For this reason, the output signal of the photodetector is prevented from lowering in the modulation factor at a high frequency, so that the frequency characteristic is improved.

Note that the optical filter or the photodetector does not cause a waveform distortion due to a phase change in the reproduced signal, unlike the case where the correction circuit is provided.

〔Example〕

Hereinafter, an optical pickup of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view showing a first embodiment of the optical pickup of the present invention.

In FIG. 1, a light beam 20 emitted from a semiconductor laser device 1 passes through a diffraction grating 2, a polarizing beam splitter 3, a collimating lens 4, a rising mirror 5, a quarter wave plate 6, and an objective lens 7, and then to an optical disk 8 And a light spot 21 is generated.

The optical disk 8 reflects the condensed light beam 20. The tracks of the optical disk 8 are provided with signal pits 100 corresponding to recording signals, and the reflected light is reflected by the signal pits 100 and other portions. Change the amount of light. The laser light (light beam) reflected from the optical disk 8 is again
After passing through an objective lens 7, a quarter wave plate 6, a rising mirror 5, and a collimating lens 4, the light is reflected by the polarization beam splitter 3 and enters the optical filter 50. And optical filters
The light beam 22 transmitted through 50 reaches the photodetector 11 via the cylindrical lens 9 and the concave lens 10. Then, the light amount of the light beam 22 is detected by the photodetector 11 whose detection surface is divided into six. Then, the photodetector 11 outputs a detection signal 11 'according to the detected light amount. Based on this light amount detection signal 11 ', the signal reproduction circuit 12 outputs a reproduction signal,
Actuator for moving the position of the objective lens 7
A position control signal (a signal for controlling the position of the light spot 21, that is, a focus control signal and a tracking control signal) is output to 13.

Note that the specific configuration of the signal reproducing circuit 12 and the method of creating the focus control signal and the tracking control signal for controlling the position of the light spot 21 are well-known techniques, and are not directly related to the present invention. Is omitted.

The optical filter 50 is, as shown in FIG. 5, a filter 50 corresponding to the circumferential direction (track direction) of the optical disk 8.
In the upward direction (the y direction in FIGS. 1 and 6), the light transmittance of the band-like region 50a including the part where the central part (the optical axis and the vicinity of the optical axis) of the light beam of the light beam 22 enters is shown. The transmittance has a distribution in which the transmittance changes stepwise so as to be lower than the region 50b outside the region 50a. Such an optical filter
By passing through the light beam 50, the light beam 22 enters the photodetector 11 in a state where the light amount at the central portion (optical axis portion) of the light beam is restricted.

For this reason, although the amount of light detected by the photodetector 11 is reduced as a whole, the fluctuation when the interval between the signal pits 100 is narrowed is suppressed to be low. This state is shown in FIG.
FIG. 6 is a schematic view similar to FIG. 2, and FIG.
6 shows a case where the optical filter 50 is not provided, and FIG. 6B shows a case where the optical filter 50 is provided. The sixth
As can be seen from FIG. And in (b) It is. Therefore, the frequency characteristics of the reproduced signal are improved.

FIG. 7 shows the results of actually measuring the frequency characteristics of the case where the optical filter shown in FIG. 5 is used and the case where there is no conventional optical filter.

In FIG. 7, the horizontal axis represents the spatial frequency of the signal pit, and the vertical axis represents the relative value of the signal modulation in decibels.

As is clear from the figure, when the optical filter 50 is used, the modulation degree of a high-frequency signal in which the spatial frequency of the signal pit 100 is about 400 pit / mm or more increases, and the frequency characteristics are improved. The signal pit 100 has a spatial frequency of 400 pits /
mm is the frequency of the recording signal, which is about 2.3 MHz, assuming, for example, that the rotation speed of the optical disk 8 is 1800 rpm and the position of the recording track is a radius of 30 mm.

Also, as shown in FIG. 7, the effect of improving the frequency characteristics changes depending on the width W of the low transmittance region of the optical filter 50 and the transmittance of that portion.

FIG. 8 shows the low transmittance region 50 of the optical filter 50 on the horizontal axis.
The ratio of the width W of a to the diameter d of the light beam 22 of the light beam 22 incident on the optical filter 50 is taken, and the vertical axis represents the relative value T of the transmittance of the low transmittance region 50a to the high transmittance region 50b. FIG. 9 is a diagram illustrating contour lines of the improvement amount of the signal modulation obtained by providing the optical filter 50 when the spatial frequency of the signal pit 100 is 700 pit / mm.

The dashed line in FIG. 8 is a contour line in which the total light amount detected when the optical filter 50 is provided is 60% of the total light amount detected when the optical filter 50 is not provided. That is, when the optical filter 50 is provided, in order to secure a light amount of 60% or more of the total detected light amount when the filter is not provided, the width W of the low transmittance region 50a of the optical filter 50 is required.
And the relative transmittance T must be determined so as to be positioned at the upper left of the broken line in FIG. As is clear from FIG. 8, the spatial frequency of the signal pit 100 is 700 pit / mm.
In order to ensure an improvement of 2 dB or more in signal modulation degree and at least 60% of the total amount of light detected when there is no optical filter 50, the low transmittance area of the optical filter 50
It is necessary to combine the width W of 50a and the relative transmittance T so as to be positioned within the hatched portion in the figure.

Therefore, at least the width W of the low transmittance area 50a is set to 0.
3 to 0.8 times, low transmittance area 5 for high transmittance area 50b
By setting the relative value T of the transmittance of 0a to 0.3 to 0.8, the spatial frequency of the signal pit can be reduced in a state where the detected total light amount is 60% or more of the value when the optical filter 50 is not provided. At 700 pit / mm (corresponding to about 4 MHz under the same conditions as the above-described optical disk rotation speed and recording track position), the modulation degree is improved by about 2 dB or more, and it can be seen that good frequency characteristics can be obtained.

In the above embodiment, the optical filter 50 whose transmittance changes stepwise is used. However, at least in the track direction of the optical disc 8, if the transmittance of the region where the central portion of the light beam enters is set lower than the transmittance of the region where the peripheral portion of the light beam enters,
An optical filter having any transmittance distribution has a similar effect of improving frequency characteristics. For example, an optical filter having a trapezoidal transmittance distribution as shown in FIG.
As shown in the figure, an optical filter having a transmittance distribution approximating a curve represented by a predetermined function having a smooth change such as a Gaussian function may be used.

Furthermore, even when a reflection type optical filter such as a case where a light beam is reflected by an optical filter and incident on a photodetector is used in addition to the transmission type filter, the reflectance of the central portion is changed to that of the peripheral portion. By making it lower than that, the same effect as in the above-described embodiment of the transmission filter can be obtained.

In addition to the above embodiments, the optical filter 50 and other optical elements can be formed by forming a filter film on or in an appropriate surface of each optical element in the optical pickup through which a light beam reflected or transmitted by the optical disk is transmitted. It is possible to improve the frequency characteristics without increasing the number of components by combining them.

For example, FIG. 11 is a view showing a second embodiment of the optical pickup of the present invention, in which the polarization beam splitter 3 in the optical pickup is provided on the surface on the photodetector 11 side. This is an example in which a dielectric film or a metal film is provided so as to have a transmittance distribution similar to that of the optical filter shown in FIG.

FIG. 12 is a view showing a third embodiment of the optical pickup of the present invention, in which an optical beam incident on the optical disk 8 is formed by using a half mirror 40 inclined at 45 ° to the optical axis. This is an example in which an optical pickup configured to separate reflected light from the optical disk 8 and an optical filter 50 is formed on a surface of the half mirror 40 on the photodetector 11 side.

FIG. 13 is a perspective view showing a fourth embodiment of the optical pickup of the present invention.

In FIG. 13, the same components as those of the embodiment described in FIG. 1 are denoted by the same reference numerals.

In this embodiment, instead of providing the optical filter 50,
The photodetector 30 whose detection sensitivity is changed according to the position on the detector surface is arranged. That is, for example, in the example of FIG. 14A, of the six divided detection surfaces in the photodetector 30,
The sensitivity distribution is set so that the detection sensitivity is higher at the peripheral position, at least in the direction corresponding to the track direction of the optical disk 8 (the y direction in the figure) at the center of the quadrant detection surface 31 for detecting the reproduction signal. ing.

In the example of FIG. 14 (b), the four-divided detection surface 31 is further divided into three regions of a band-shaped region 31a at the center and outer edges 31b and 31c as shown in the figure. When reproducing the recorded signal, the detection signals from the detection surface central region 31a and the detection signals from the outer edge portions 31b, 31c are independently amplified by separate preamplifiers 33a, 33b, and then added by an addition circuit 34 to add the signals. Input to the reproduction circuit. (Note that, in the figure, a circuit for creating a focus control signal and a tracking control signal is omitted for simplicity.) In addition, the amplification factor Mb of the preamplifier 33b is set to
By making it larger than the amplification factor Ma of 3a, the apparent detection sensitivity differs between the center and the outer edge.

By using such a photodetector 30, the same effect of improving the frequency characteristics as in the case of using the optical filter 50 can be obtained.

Moreover, in this embodiment, it is not necessary to separately provide the optical filter 50, so that the number of optical components can be made equal to that of the conventional optical pickup.

Further, unlike the case where the optical filter 50 is used, there is an advantage that the recorded signal can be reproduced efficiently because the detected total light amount does not decrease.

In the above-described embodiments, the light amount fluctuation of the light beam reflected or transmitted through an optical disk such as an optical disk having an uneven phase type signal pit, an amplitude pit type optical disk represented by a phase change method, and a hole type optical disk. The same effect can be obtained with a disc based on any recording principle as long as the disc can reproduce a recording signal from the disc. Therefore, like a magneto-optical disk,
Even in the case of a disk in which the polarization direction of reflected or transmitted light from an optical disk changes according to a recording signal, only a light beam having a predetermined polarization direction is selectively reflected or transmitted, as in an optical pickup for a conventional magneto-optical disk. An optical pickup configured to convert a change in the polarization direction of detection light into a change in the amount of light and detect the change in the amount of light using a light detection element and a photodetector to reproduce a recording signal is the same as in each of the above-described embodiments. The frequency characteristics can be improved by using an optical filter or a photodetector.

FIG. 15 is a view showing a fifth embodiment of the optical filter of the present invention, which shows an embodiment in which the optical filter is applied to an optical pickup for a magneto-optical disk.

In FIG. 15, a linearly polarized laser beam 20 emitted from the semiconductor laser device 1 passes through a collimating lens 4, a beam shaping prism 15, a polarizing beam splitter 3a, a rising mirror 5, and an objective lens 7, and is then on a magneto-optical disk 8 '. Is irradiated as a light spot 21. The light beam reflected by the magneto-optical disk 8 ′ passes through the objective lens 7 and the rising mirror 5 again,
The light is reflected by the polarization beam splitter 3a, and further divided by the polarization beam splitter 3b into a light flux 22 'for detecting a focus control signal and a tracking control signal and a light flux 22 for detecting a magneto-optical signal. Then, the light enters the light detectors 62a and 62b through the detection lens 60 and the light detection element 61. The signals detected by the photodetectors 62a and 62b are amplified by preamplifiers 63a and 63b, and then supplied to a subtraction circuit 64.
These signals are subtracted by a subtraction circuit 64, taken out as a differential signal, and supplied to a magneto-optical signal reproducing circuit. Such a configuration is a general configuration of an optical pickup for a magneto-optical disk except for the optical filter 50.

Note that the technique of reproducing the magneto-optical signal is a known technique, and thus the description thereof is omitted.

The optical filter 50 used in this embodiment has the same frequency characteristic improvement effect as that of each embodiment by using a filter having partially different light transmittance or reflectance as described in each embodiment described above. Is obtained.

Further, utilizing the principle of magneto-optical signal detection, as shown in FIG.
A similar frequency improving effect can be obtained by providing an optical filter 50 'in which the optical element 51 having an optical rotation such as a wave plate is provided in a strip shape in the light beam 22 of the optical pickup. This is because, when the polarization direction of the light beam 22 is rotated from the reference direction, the difference of the transmittance or the reflectance used in each of the above-described embodiments is utilized by utilizing the property that the differential signal of the magneto-optical output is reduced. It has the same function as the optical filter 50 used.

FIG. 17 is a schematic diagram for explaining the function of the optical filter 50 'of FIG. Now, as shown in Fig. 17 (a),
When the reflection axis 202 and the transmission axis 203 of the analyzer 61 are inclined by 45 ° with respect to the polarization direction 201 of the incident laser light, the polarization direction of the light beam 22 rotated by ± θk by the magneto-optical signal
Photodetector 62a with respect to 204 and 205, the magneto-optical signal 210a detected by 62b, the amplitude of the differential output _{^{211a | P 1 2 -S 1 2}} | becomes the maximum value. In such a state, when the polarization direction of the light beam 22 is rotated by α by the optical rotation optical element 51 as shown in FIG. 17 (b), the magneto-optical signals 210b and 210b detected by the photodetectors 62a and 62b
The amplitude of the differential output _{^{211b | P 2 2 -S 2 2}} | _{^{is, | P 1 2 -S 1 2}} | decreased from, the signal amplitude ratio is expressed by the following equation.

| P ₂ ² −S ₂ ² | / | P ₁ ² −S ₁ ² | = COS 2α− (1) Accordingly, by appropriately setting the optical rotation angle of the optical rotation optical element, the light beam is obtained according to the equation (1). The magneto-optical signal obtained from the central portion of 22 can be reduced to an arbitrary ratio, and can function similarly to the optical filter shown in each of the above-described embodiments.

〔The invention's effect〕

The present invention provides a certain degree of detection by providing an optical filter having a simple configuration in the detection optical path as described above, or by making the detection sensitivity of the photodetector different between the central part and the peripheral part of the incident light beam. This has the effect of suppressing the decrease in the modulation factor of the high frequency band of the reproduction signal while securing the total light amount, and improving the frequency characteristics. The improvement effect of such frequency characteristics is as follows.
Unlike an improvement measure using a conventional electronic circuit, there is an advantage that waveform distortion due to a phase change of a high-frequency component is eliminated.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of the present invention, FIG. 2 is a schematic diagram showing an example of the characteristic of the detected light amount in a conventional optical pickup, and FIGS. 3 and 4 are for explaining the principle of the present invention. 5, 5, 9, 10 and 16 are front views showing an embodiment of the optical filter used in the present invention.
FIG. 7, FIG. 7 and FIG. 8 are characteristic diagrams showing the effect of the optical filter described in FIG. 5, FIG. 11 to FIG.
15 is a perspective view showing another embodiment of the present invention, FIG. 14 is a front view showing the photodetector described in FIG. 13, and FIG.
FIG. 17 is a schematic diagram for explaining the principle of the optical filter shown in FIG. 1 ... Semiconductor laser element, 7 ... Objective lens, 8 ... Optical disk, 11 ... Photodetector, 50 ... Optical filter.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Ritsuo Imada 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside the Home Appliances Research Laboratory, Hitachi, Ltd. (72) Inventor Tohru Sasaki 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa (56) References JP-A-61-233443 (JP, A) JP-A-60-191445 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G11B 7 / 135

Claims (9)

(57) [Claims] 1. A semiconductor laser device, an objective lens for condensing a light beam emitted from the semiconductor laser device on an optical information recording medium, and a light for detecting a light amount of a reflected light beam from the optical information recording medium In an optical pickup including a detector, in the optical path from the objective lens to the photodetector, the amount of light at the center and the amount of light at the periphery of the reflected light beam with respect to the track direction of the optical information recording medium. An optical pickup, wherein the photodetector detects a light amount at a central portion and a light amount at a peripheral portion of the reflected light beam passing through the device. 2. The method according to claim 1, wherein a transmittance (or reflectance) of a portion where the central portion of the reflected light beam is incident is higher than a transmittance (or reflectance) of a portion where the peripheral portion of the reflected light beam is incident. The optical pickup according to claim 1, wherein the optical pickup is a low optical filter. 3. The transmittance (or reflectance) of a portion of the optical filter where the central portion of the reflected light beam enters.
3. The optical pickup according to claim 2, wherein the light transmittance is 0.3 to 0.8 times a transmittance (or a reflectance) of a portion where a peripheral portion of the reflected light beam is incident. 4. 4. The transmittance (or reflectance) at each position in the optical filter from a portion where the central portion of the reflected light beam is incident to a portion where the peripheral portion of the reflected light beam is incident is as follows: The optical pickup according to claim 1, wherein the optical pickup changes monotonously according to a distance from a predetermined position in the optical filter. 5. The optical filter according to claim 1, wherein the optical filter includes an optical rotation element for rotating a polarization direction of the light beam by a predetermined angle at a portion where an optical axis of the reflected light beam (or the transmitted light beam) is incident, and a predetermined polarization. The optical pickup according to claim 1, further comprising an analyzer that selectively transmits (or reflects) a light beam having a direction in a predetermined direction. 6. A semiconductor laser device, an objective lens for condensing a light beam emitted from the semiconductor laser device on an optical information recording medium, and a light for detecting the amount of reflected light beam from the optical information recording medium. An optical pickup including a detector, wherein the photodetector has a photodetection sensitivity of a photodetection surface of a portion where a central portion of the reflected light beam in the track direction of the information recording medium is incident and a portion where a peripheral portion is incident. An optical pickup characterized in that the light detection sensitivity of the light detection surface is different. 7. The photodetector according to claim 1, wherein a light detection sensitivity of a part where a central part of the reflected light beam is incident is 0.3 to 0.8 times a light detection sensitivity of a part where a peripheral part of the reflected light beam is incident. 7. The optical pickup according to claim 6, wherein: 8. The photodetector is divided into at least a first region where a central portion of the reflected light beam is incident and a second region where at least a peripheral portion of the reflected light beam is incident. Amplifying the signal detected from the area of
7. The optical pickup according to claim 6, wherein a gain of the amplifier is different from a gain of a second amplifier for amplifying a signal detected from the second region. 9. The optical pickup according to claim 8, wherein the gain of the first amplifier is 0.3 to 0.8 times the gain of the second amplifier.