JP2001110068A - Optical head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an RF signal little in noise while excellently detecting a focus error signal and a tracking error signal. SOLUTION: By using a luminous flux division element 27 and luminous flux emitted from a light source 21 to an optical disk 50, the luminous flux reflected by the optical disk 50 is divided into three lines of liminous flux of convergent angles different from each other. The focus error signal and the tracking error signal are generated by a control circuit based on the luminous flux of the furthest converged position from the luminous flux division element 27 and the luminous flux of the nearest converged position. The luminous flux in the middle of convergent position from the luminous flux division element 27 is detected by a photo-detector 28 having a single light receiving surface so that RF signal is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head for writing or reading information signals on an optical recording medium.

[0002]

2. Description of the Related Art Hitherto, an optical head for writing or reading an information signal on or from an optical recording medium such as an optical disk has been proposed. The optical head includes a light source such as a semiconductor laser, and irradiates a light beam emitted from the light source to an optical recording medium. Then, an information signal is written to the optical recording medium by a light beam applied to the optical recording medium, or an information signal is read from the optical recording medium by detecting a reflected light beam of the light beam applied to the optical recording medium. .

In such an optical head as well as in an optical sensor such as a surface measuring device, for example,
Highly accurate focus control is required. In particular, in the above-described optical head, the amount of vibration of the recording surface of the optical recording medium is generally much larger than the depth of focus of the optical system. For example, while the depth of focus is several μm, the recording surface of the optical recording medium vibrates by 50 μm or more. For this reason, in an optical head, an objective lens that focuses on an optical recording medium is driven in accordance with the vibration of a recording surface so that a light beam spot for recording / reproducing a signal is accurately focused. ing. At this time, a signal indicating a relative distance (focus error) between the recording surface and the objective lens is called a focus error signal, and this signal is fed back to a drive signal of the objective lens. Such an operation is called a focus servo.

A focus error signal is generated by a so-called astigmatism method based on an output signal from an optical head. By configuring the servo loop so that the voltage value of the focus error signal becomes a negative feedback, the position of the objective lens can always be set to the position where the best focus is obtained.

In an optical head for detecting a focus error signal by the astigmatism method, as shown in FIG.
The light beam emitted from the light source 101 is collimated by the collimator lens 1.
02 is formed into a parallel beam by the beam splitter 10.
4 is reflected by the objective lens 105 and converged on the optical recording medium 106 by the objective lens 105 to form a beam spot on the optical recording medium 106. The light beam reflected by the optical recording medium 106 passes through the objective lens 105 again, passes through the beam splitter 104, and is detected by the detection lens 107 and the cylindrical lens 108 as shown in FIG.
An image is formed on the light receiving surfaces A, B, C, and D of the image No. 09. The photodetector 109 includes currents IA, IB, IC, and ID proportional to the intensity of the light beam applied to each of the light receiving surfaces A, B, C, and D.
Occurs. By performing current-voltage conversion based on these currents, voltage values VA, VB, VC, and VD can be obtained.

The image plane formed by the detection lens 107 and the cylindrical lens 108 does not form a single focal plane because of the cylindrical lens 108 but forms two focal lines separated in the optical axis direction. Since the state of the light wavefront where two focal lines exist is called astigmatism, such a method of detecting a focus error signal is called an astigmatism method. And these 2
By arranging the photodetector 109 approximately in the middle between two focal lines, a focus error signal can be detected as follows.

That is, when the image formed by the objective lens is focused on the optical recording medium, the light intensity pattern on the photodetector 8 is set so that the intensity pattern becomes substantially circular as shown in FIG. Is set. Therefore, the following calculation result (FE) of the signal strength becomes substantially zero.

FE = (VA + VC)-(VB + VD) On the other hand, if the objective lens is too far or too close to the optical recording medium, it becomes elliptical as shown in FIG. 9 or FIG. , Minus, or plus value. Therefore, the calculation result FE can be used as a focus error signal. The focus servo loop may be a feedback loop such that FE becomes zero.

Next, the tracking servo will be described. In an optical recording medium, particularly an optical disk, a guide groove or a pit row is formed in a spiral shape on a signal recording surface, and this is generally called a recording track. The optical head can record and reproduce the information signal on the optical recording medium by tracing the light beam spot along the recording track. Since the recording track is generally eccentric about 100 μm with respect to the base material of an optical recording medium such as an optical disk, when the optical recording medium is rotated, the recording track meanders by the eccentricity. And the distance between recording tracks,
That is, the track pitch is only about 1 μm.

Therefore, it is necessary to move the light beam spot by driving an objective lens for forming a light beam spot on the optical recording medium in a direction perpendicular to the optical axis, so that the light beam spot follows the recording track. There is. Such an operation is called a tracking servo.

[0011] The photodetector 109 is, as shown in FIG.
The line dividing the light receiving surfaces A and D and the light receiving surfaces B and C is parallel to the direction of the recording track. Then, when the light beam spot traverses the recording track, the light intensity on the light receiving surface of the photodetector is modulated so that a light intensity difference appears at a position symmetrical with respect to this division line. That is, when the light beam spot deviates from the recording track in one direction, the light intensity on the light receiving surfaces A and D increases, and the light intensity on the light receiving surfaces B and C decreases. When the light beam spot deviates from the recording track in the other opposite direction, the light / dark relationship is also reversed, the light intensity on the light receiving surfaces A and D becomes weak, and the light intensity on the light receiving surfaces B and C becomes strong. When the light beam spot is on the center of the recording track, the light intensity on the light receiving surfaces A and D is equal to the light intensity on the light receiving surfaces B and C.

Therefore, the signal TE that detects the difference between the light intensities on the left and right sides of the dividing line obtained by the following calculation can be used as a tracking error signal for causing the light beam spot to follow the recording track.

TE = (VA + VD)-(VB + VC) Such a method of detecting the tracking error signal TE is called a push-pull method.

[0014]

By the way, in the above-described optical head, the total intensity RF detected by the photodetector obtained by the following calculation is converted into a so-called RF signal of the information signal from the optical recording medium. Used as a playback signal.

RF = VA + VB + VC + VD In a recording / reproducing apparatus for recording / reproducing an information signal using an optical head, a photodetector, a current-voltage converter, a voltage adder, etc. It is essential to increase the speed and bandwidth of the head amplifier system circuit. In order to increase the speed in this way, noise rises. Therefore, how to suppress noise rise is an important issue.

Here, each of the current signals IA, IB, IC, I
D after current-to-voltage conversion of D, respectively,
Rather than adding VB, VC, VD, each current signal IA,
It is preferable to perform current-voltage conversion after adding IB, IC, and ID because noise in the converted signal is reduced. The cause of this noise is thermal noise generated by a feedback resistor used for current-voltage conversion. In the former, the current-voltage conversion is performed four times, whereas in the latter, only one is necessary, so that the noise is small. For example, the current-
It is assumed that the noise of 3 dB increases each time the number of times of the voltage conversion doubles. If the current-voltage conversion is performed on each of the four light receiving surfaces, the noise increases by 6 dB.

Therefore, in order to obtain a good RF signal with less noise, it is preferable that the photodetector for detecting the RF signal should not have the light receiving section divided. However, in order to detect the focus error signal and the tracking error signal, the light receiving surface of the photodetector must be divided into a plurality of parts, and the signals from the respective light receiving surfaces must be subjected to current-voltage conversion for calculation.

Accordingly, the present invention has been proposed in view of the above situation, and is intended to obtain an RF signal with less noise while detecting a focus error signal and a tracking error signal satisfactorily. It is intended to provide an optical head that has been made.

[0019]

In order to solve the above-mentioned problems, an optical head according to the present invention comprises a light source and a light source for condensing a light beam emitted from the light source and irradiating the light beam onto an optical recording medium. Means, a light beam splitting element for splitting a light beam reflected by the optical recording medium into three light beams having different convergence angles, light detecting means for detecting the three reflected light beams, and the light detecting means And a focus error signal generating means for generating a focus error signal based on the light detection output output from the controller.

The light detecting means includes a first sub-light receiving surface for detecting a reflected light beam whose light condensing position is farthest from the light beam splitting element among the three reflected light beams, and a light condensing light beam among the three reflected light beams. A second sub-light receiving surface for detecting a reflected light beam whose position is closest to the light beam splitting element, and a reflected light beam having a middle distance from the light beam splitting element at the condensing position among the three reflected light beams; It has a main light receiving surface and outputs a light detection output independently for each light receiving surface.

The focus error signal generating means is a focus error signal indicating a focus state of the light beam emitted from the light source on the optical recording medium based on the light detection outputs corresponding to the first and second sub-light receiving surfaces. Generate

[0022]

Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, a description will be given of a case where the present invention is applied to an optical head of an optical disc apparatus for recording and reproducing information signals using a rewritable optical disc as an optical recording medium. However, the present invention is not limited to this embodiment. However, the present invention can be applied to an optical head of any type of recording / reproducing apparatus using an optical recording medium in which information signals are recorded or reproduced using light, such as a read-only optical disk or a magneto-optical disk.

As shown in FIG. 1, the optical disk apparatus having the optical head according to the present invention includes a spindle motor 2 for rotating the optical disk 50. The optical head 1 according to the present invention writes a signal on the optical disc 50 that is rotated by the spindle motor 2, or
The signal written on the optical disk 50 is read. The optical disk device also includes a thread motor 4 for moving the optical head 1 to an arbitrary position on the optical disk 50, a signal to be written on the optical disk 50 by the optical head 1, or a signal from the optical disk 50 by the optical head 1. A recording / reproducing circuit 5 for generating a reproduced signal based on the read signal; an address reproducing circuit 12 for reproducing an address signal on the optical disk 50 based on a signal read from the optical disk 50 by the optical head 1; A focus servo signal and a tracking servo signal based on a signal read from the optical disk 50 by the head 1, a signal for positioning the optical head 1, a signal for controlling the rotation of the spindle motor 2, etc. Control circuit 6 for generating a control signal for controlling the operation
It has.

The spindle motor 2 is driven by a spindle driver 7 operated based on a control signal supplied from a control circuit 6, and rotates the optical disk 50 at a predetermined rotation speed.

The sled motor 4 is driven by a sled driver 8 operated based on a control signal supplied from a control circuit 6 to drive the optical head 1 to the optical disk 5.
It is moved to a predetermined position in the radial direction above zero.

As shown in FIG. 2, the optical head 1 has a light source 21 such as a semiconductor laser that emits a laser beam.
By the optical disk 50
The information signal is written on the optical disk 50 or the information signal written on the optical disk 50 is read out by irradiating the signal recording surface 51 of the optical disk. At this time,
As shown in FIG. 1, the objective lens 25 is moved toward and away from the optical disk 50 by the objective lens driver 9 operated based on the focus error signal and the tracking error signal supplied from the control circuit 6.
The focus servo and the tracking servo are performed by moving in two radial directions.

The recording / reproducing circuit 5 temporarily stores a recording signal input from an external device such as a host computer in a memory 10, sequentially reads out the recording signals from the memory 10, and performs interleaving and error correction on the recording signals. A signal to be written to the optical disk 50 is generated by adding a code or the like. Then, the recording / reproducing circuit 5 outputs a signal to be written to the optical disk 50 to the laser driver 11.

The recording / reproducing circuit 5 performs error correction processing, interleaving, and the like on the signal read from the optical disk 50 by the optical head 1 to generate a reproduced signal. The reproduced signal generated by the recording / reproducing circuit 5 is output to an external circuit such as a host computer.

The address reproducing circuit 12 reproduces an address signal on the optical disk 50 based on the signal read from the optical disk 50 by the optical head 1 and sends it to the control circuit 6. The guide grooves provided along the recording tracks on the optical disk 50 adopt a so-called wobble method meandering with a small amplitude. This meandering method is obtained by frequency-modulating an address uniquely determined on the optical disk. This meandering appears as a wobble signal on a so-called push-pull signal. Therefore, address information on the optical disk can be obtained by frequency-demodulating the wobble signal.

The control circuit 6 generates a focus servo signal and a tracking servo signal from the signal read from the optical disk 50 by the optical head 1 and supplies the focus servo signal and the tracking servo signal to the objective lens driver 9. . The control circuit 6 generates a signal for positioning the optical head 1 from the signal read from the optical disk 50 by the optical head 1 and supplies the signal to the thread driver 8. A signal for performing the rotation control is generated, and this signal is supplied to the spindle driver 7.

When an information signal is recorded on the optical disk 50 by the optical disk device having the above-described configuration, first, the optical disk 50 is rotated by the spindle motor 2 and the recording / reproducing circuit 5 is connected to the host computer or the like. A recording signal is input from an external device.

When a recording signal is input to the recording / reproducing circuit 5, the recording / reproducing circuit 5 outputs the signal to the optical disk 5 based on the recording signal.
A signal to be written to 0 is generated, and this signal is output to the laser driver 11. The laser driver 11 modulates the intensity of the laser beam emitted from the light source of the optical head 1 according to this signal.

The optical head 1 is moved by the thread motor 4 to a predetermined position on the optical disk 50 in the radial direction of the optical disk 50. Then, the optical head 1 condenses the laser light flux intensity-modulated by the laser driver 11 by the objective lens 25 and irradiates the condensed light spot onto the signal recording surface 51 of the optical disc 50. Thus, a predetermined information signal is written at a predetermined position on the signal recording surface 51 of the optical disk 50.

The optical head 1 receives the return light reflected by the signal recording surface 51 of the optical disk 50 by the photodetector 28, as shown in FIG. The return light received by the photodetector 28 is converted into an electric signal and supplied to the control circuit 6. The control circuit 6 generates a focus error signal and a tracking error signal based on the signal supplied from the photodetector of the optical head 1 and supplies the generated signal to the objective lens driver 9. Then, based on the focus error signal and the tracking error signal supplied from the control circuit 6, the objective lens driver 9 moves the objective lens 25 of the optical head 1 toward and away from the optical disc 50 and in the radial direction of the optical disc 50. By performing the movement operation in the axial direction, focus servo and tracking servo are performed.

When an information signal is reproduced from the optical disk 50 by the optical disk device, first, the optical disk 50 is rotated by the spindle motor 2 and the optical head 1 is moved by the thread motor 4 to a predetermined position on the optical disk 50. Is moved to the position.

When the optical head 1 is moved to a predetermined position on the optical disk 50, a laser beam having a predetermined intensity is emitted from the light source 21 of the optical head 1. The laser beam emitted from the light source 21 of the optical head 1 is
To irradiate a predetermined position on the signal recording surface 51 of the optical disk 50.

The laser beam radiated to a predetermined position on the signal recording surface 51 of the optical disk 50 is modulated according to the information signal written at this position, and contains a signal component. The light is reflected by the recording surface 51. The return light reflected by the signal recording surface 51 of the optical disk 50 is received by the photodetector 28 of the optical head 1, converted into an electric signal, and supplied to the recording / reproducing circuit 5.

The recording / reproducing circuit 5 generates a reproduced signal based on the signal supplied from the photodetector of the optical head 1.
Then, the reproduced signal generated by the recording / reproducing circuit 5 is
It is output to an external circuit such as a host computer.

The signal received by the photodetector 28 of the optical head 1 and converted into an electric signal is supplied to the control circuit 6. The control circuit 6 generates a focus servo signal and a tracking servo signal based on the signal supplied from the photodetector 28 of the optical head 1 and supplies the generated signal to the objective lens driver 9. Then, based on the focus error signal and the tracking error signal supplied from the control circuit 6, the objective lens driver 9 moves the objective lens 25 of the optical head 1 toward and away from the optical disc 50 and in the radial direction of the optical disc 50. By performing the movement operation in the axial direction, focus servo and tracking servo are performed.

As described above, the optical disk apparatus records and reproduces a signal on and from the optical disk 50 while performing the focus servo and the tracking servo. Is always connected to the signal recording surface 51 of the optical disk 50, and the light spot is made to appropriately follow the recording track of the optical disk 50 so that the signal can be recorded and reproduced appropriately.

In the part of the optical disk device which performs focus servo, that is, in the focus servo system circuit, first, in the focus error signal generation circuit of the control circuit 6, based on the signal supplied from the photodetector of the optical head 1, A focus error signal is generated. Then, this focus error signal is supplied to the arithmetic unit,
In this arithmetic unit, the amount of deviation from the target value supplied from the target value control circuit is calculated. A signal indicating the amount of deviation between the focus error signal and the target value calculated by the calculator is supplied to the objective lens driver 9 via a phase compensation circuit.

Then, based on the signal indicating the amount of deviation between the focus error signal and the target value, the objective lens driver 9 moves the focus error signal generated by the focus error signal generation circuit in a direction approaching the target value. The objective lens of the head 1 is moved.

As described above, the focus servo system circuit of the optical disk device is controlled so that the focus error signal generated based on the signal received by the photodetector 28 of the optical head 1 becomes equal to the target value. It is a closed loop. That is, this focus servo system circuit feeds back a signal indicating the amount of deviation between the focus error signal and the target value to the optical head 1 and
5 is controlled so that the focus error signal becomes equal to the target value.

In this focus servo system circuit, the target value supplied from the target value control circuit is normally set to zero. Therefore, in this case, the focus error signal generated by the focus error signal generation circuit is supplied to the objective lens driver 9 as it is via the phase compensation circuit.

However, when the optical disk 50 is a so-called land / groove type optical disk which is configured to record a signal on both a land portion and a groove portion, a signal is recorded on a land. Different target values may be set depending on whether a signal is recorded in a groove. In this case, a signal indicating the amount of deviation between the focus error signal and each target value is supplied to the objective lens driver 9 via the phase compensation circuit.

The optical head 1 provided in the optical disk device
As shown in FIG. 2, a light source 21, a collimator lens 22 for converting a laser beam emitted from the light source 21 into parallel light, and a ± 1 order laser beam converted to parallel light by the collimator lens 22. An irradiation optical system having a diffraction grating 23 for splitting the light into three light beams of light and zero-order light is provided. Further, the irradiation optical system converges the beam splitter 24 that bends each optical path of the laser beam transmitted through the diffraction grating 23, and condenses each laser beam whose optical path is bent by the beam splitter 24. An objective lens 25 for irradiating the optical spot on the signal recording surface 51 of the optical disk 50;

The diffraction grating 23 does not need to be used in the case where a simple push-pull method is used in detecting a tracking signal without using a differential push-pull method described later.

The optical head 1 also includes, as an optical system for receiving light, a light beam splitting element 27 for splitting the light beam reflected by the optical disk 50 into three light beams having different convergence angles. That is, the reflected light beam reflected by the signal recording surface 51 of the optical disk 50 and transmitted through the objective lens 25 and the beam splitter 24 is split by the light beam splitting element 27 into three reflected light beams having different convergence angles. The light receiving optical system includes a condenser lens 26 for converging the reflected light beam passing through the light beam splitting element 27.
And a photodetector 28 that receives the return light transmitted through the light beam splitting element 27 and the condenser lens 26.

The optical head 1 allows the optical disk 50
When reading the information signal recorded in the
A laser beam having a predetermined intensity is emitted. The laser beam emitted from the light source 21 is collimated by the collimator lens 22 and then enters the diffraction grating 23. The three light beams split by the diffraction grating 23 form, by the objective lens 25, light spots arranged on the signal recording surface 51 of the optical disk 50 in a direction substantially along the tangent to the recording track. These luminous fluxes are reflected on the signal recording surface 51 after being modulated according to the information signal recorded on the signal recording surface 51, and are incident on the luminous flux splitting element 27 via the objective lens 25 and the beam splitter 24. Is done. At this time, each reflected light beam is substantially a parallel light beam.

As the light beam splitting element 27, a hologram can be used. This hologram is formed by using a plane wave as a reference light and using convergent light by an astigmatic convex lens or divergent light by an astigmatic concave lens as object light. Therefore, when a plane wave reflected by the signal recording surface 51 of the optical disk 50 and passing through the objective lens 25 is incident on this hologram, each of these incident lights is the same plane wave as the incident wave.
Next-order light, convergence light by the convex lens used to form the hologram, or + 1st-order light having the same wavefront as the divergence light by the concave lens, and divergence light or convergence light having a complex conjugate relationship with the + 1st-order light It is split into three light beams of a certain -1 order light.

When the reflected light beam split by the hologram enters the condenser lens 26, the photodetector 2
On the light receiving surface 8, three beam spots are formed for each of the three light beams divided by the diffraction grating 23.

Since the three reflected light beams split by the light beam splitting element 27 have different convergence angles before entering the condenser lens 26, the + 1st order light and the -1st order light of the light beam splitting element 27 are , And zero-order light are defocused in opposite directions. That is, of these three light beams, the + 1st-order light and the −1st-order light are the reflected light beam whose light-converging position is farthest from the light beam splitting element 27 and the reflected light beam whose light-converging position is closest to the light beam splitting element 27.

The optical detector 28 of the optical head
As shown in FIG. 3, a first sub-light-receiving surface 30 for detecting a reflected light beam having a condensing position farthest from the light beam splitting element 27 among the three light beams split by the light beam splitting element 27, The second sub-light receiving surface 31 for detecting a reflected light beam whose light condensing position is closest to the light beam splitting element 27 of the three light beams, and the distance from the light beam splitting element 27 to the light condensing position of the three light beams is intermediate And a main light receiving surface 32 for detecting the reflected light flux.

The main light receiving surface 32 has a single light receiving surface composed of a single light receiving element. The first and second sub-light receiving surfaces 30, 31 are respectively provided with the respective sub-light receiving surfaces 30, 3,
1 are formed by at least two light receiving elements, that is, light receiving surfaces B and D including light receiving elements including a central portion and light receiving surfaces A and C including light receiving elements including outer peripheral portions of the respective sub light receiving surfaces 30 and 31. ing. In this embodiment, each sub light receiving surface 3
There are two light receiving elements each including the central part of 0 and 31, and four light receiving elements each including the outer peripheral part of each sub light receiving surface 30 and 31.

Each of the light receiving surfaces 30, 31, 32 generates a current proportional to the intensity of the light beam applied to the light receiving surface corresponding to the light receiving element constituting each light receiving surface. By performing current-voltage conversion based on these currents, a voltage value can be obtained.

Since the main light receiving surface 32 has a single light receiving surface composed of a single light receiving element, the current generated by receiving the zero-order light is also a single current. A voltage value obtained by current-voltage conversion of the current generated by the main light receiving surface 32 is a so-called RF signal.

When the light beam emitted from the light source is focused on the signal recording surface of the optical disk, the light intensity pattern of the ± primary light of the light beam splitting element 27 on each of the sub-light receiving surfaces 30 and 31 is Are substantially equal to each other. Accordingly, the light receiving surfaces A, B, C,
The voltage values obtained by current-to-voltage conversion of the current generated from D are VA30, VB30, VC30, and VD30, respectively.
In the second sub-light receiving surface 31, the light receiving surfaces A, B, C,
The voltage values obtained by current-voltage conversion of the current generated from D are VA31, VB31, VC31, and VD31, respectively.
Then, when the light beam emitted from the light source is in focus on the optical disc, the following calculation result (FE) of the signal intensity becomes substantially zero.

FE = ｛(VA30 + VC30)-(VB
30 + VD30)｝-｛(VA31 + VC31)-(V
B31 + VD31)｝ = (VA30−VA31) + (V
C30-VC31)-(VB30-VB31)-(VD
30-VD31) When the light beam emitted from the light source is defocused, the calculation result FE changes with a polarity corresponding to the direction of the defocus. That is, the calculation result FE is a focus error signal for the light beam irradiated on the optical disk. Thus, in this optical head, of the light beams split by the light beam splitting element 27, ±
Using the primary light, it is possible to detect the amount of defocus for the light beam irradiated on the optical disk.

Further, in this optical head, a control circuit serving as a tracking error signal generating means controls a light beam irradiated on the optical disk based on light detection outputs corresponding to the first and second sub-light receiving surfaces 30, 31. A tracking error signal indicating a state of following a recording track on the optical disk is generated. In generating the tracking error signal, the first or second sub light receiving surface 30,
In each of 31, the so-called push-pull method can be used. In this case, the tracking error signal TE30, TE31, or TE can be obtained by the following calculation.

TE30 = (VA30 + VB30)-(V
C30 + VD30) TE31 = (VA31 + VB31)-(VC31 + VD
31) TE = TE30 + TE31 (Either TE30, TE31 or TE may be used.) In this optical head, the 0th-order light and ± 1st-order light divided into three by the diffraction grating 23 are used. The tracking error signal can be detected by a so-called differential push-pull method. In this case, as shown in FIG. 4, the sub-light receiving surfaces 33 and 35 for receiving ± first-order light by the light beam splitting element 27 of the + 1st-order light reflected by the diffraction grating 23 and the −1st-order light Sub light-receiving surfaces 34 and 36 for receiving ± first-order light by the light beam splitting element 27 of the reflected light beam are further provided. Each of the sub light receiving surfaces 33, 35, 34, 36 is composed of two light receiving elements. The two light receiving elements forming the sub light receiving surfaces 33, 34, 35, 36 constitute a light receiving surface divided by a line in a direction corresponding to a tangential direction of a recording track of the optical disk.

In this case, the first and second sub-light receiving surfaces 3
Sub light receiving surfaces 33 and 35 for receiving ± 1 order light by the light beam splitting element 27 of ± 1 order light by the diffraction grating 23 and 0, 31;
Based on the light detection signals at 34 and 36, the influence of the movement of the light beam spot on the light receiving surface due to the offset of the objective lens can be offset, and the tracking error signal can be detected by the so-called differential push-pull method.

In this optical head, as the light beam splitting element 27, instead of the hologram as described above, as shown in FIG. 5, an off-axis Fresnel zone plate whose center is shifted from the optical axis is used. (F
ZP) can be used.

The Fresnel zone plate has a plurality of regions, as shown in FIG. 5, and, like a grating (diffraction grating), has a constant optical phase difference between transmitted lights in adjacent regions. I have. In the Fresnel zone plate, the divided shape of each region is concentric. The Fresnel zone plate has the same function as a lens, and outputs conjugate positive and negative diffracted light, like a grating. An off-axis Fresnel zone plate is obtained by cutting a part of the Fresnel zone plate off the center.

Further, in this optical head, a liquid crystal plate can be used as the light beam splitting element 27.
That is, if a voltage is applied to a liquid crystal plate having electrodes concentrically divided similarly to the above-described Fresnel zone plate so that transmitted light has the same phase difference as transmitted light of the Fresnel zone plate, Has the same effect as the Fresnel zone plate. Assuming that the phase difference of the transmitted light is approximately linear with the voltage, the voltage waveform may be similar to the desired phase difference.

[0065]

As described above, in the optical head according to the present invention, the light beam splitting element is used to split the light beam emitted from the light source and reflected by the optical recording medium into three light beams having different convergence angles. I do. Then, the focus error signal generation means generates a focus error signal based on the reflected light beam and the closest reflected light beam of which the condensing position is farthest from the light beam splitting element among the three reflected light beams. Further, the tracking error signal generation means generates a tracking error signal based on the reflected light beam whose light collection position is farthest from the light beam splitting element and the closest reflected light beam among the three reflected light beams.

Accordingly, an RF signal can be obtained by detecting a reflected light beam having a middle distance from the light beam splitting element at the condensing position among the three reflected light beams by a light detecting element having a single light receiving surface. Can be.

That is, the present invention can provide an optical head which can detect a focus error signal and a tracking error signal satisfactorily and can obtain an RF signal with less noise.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an optical disk device configured using an optical head according to the present invention.

FIG. 2 is a side view showing a configuration of the optical head.

FIG. 3 is a front view showing a shape of a light receiving surface of a photodetector constituting the optical head.

FIG. 4 is a front view showing another example of the shape of the light receiving surface of the photodetector.

FIG. 5 is a front view showing a configuration of a Fresnel zone plate which is a light beam splitting element.

FIG. 6 is a side view showing a configuration of a conventional optical head.

FIG. 7 is a front view showing a shape of a light receiving surface of a photodetector constituting the conventional optical head.

FIG. 8 is a front view showing a shape of a light beam spot received on a light receiving surface of a photodetector in a focused state in the conventional optical head.

FIG. 9 is a front view showing a shape of a light beam spot received on a light receiving surface of a photodetector when the optical disk moves away from the conventional optical head and becomes out of focus.

FIG. 10 is a front view showing a shape of a light beam spot received on a light receiving surface of a photodetector when the optical disk approaches and becomes out of focus in the conventional optical head.

FIG. 11 is a front view showing a shape of a light beam spot received on a light receiving surface of a photodetector when a light beam spot on an optical disk crosses a recording track in the conventional optical head.

[Explanation of symbols]

Reference Signs List 1 optical head, 6 control circuit, 21 light source, 25 objective lens, 27 light beam splitting element, 28 light detector, 30
1st sub light receiving surface, 31 2nd sub light receiving surface, 32 main light receiving surface, 33, 34, 35, 36 sub light receiving surface, 50 optical disk, 51 signal recording surface

Claims (6)

[Claims] A light source; a light condensing means for condensing a light beam emitted from the light source and irradiating the light beam onto an optical recording medium; and a light beam reflected by the optical recording medium having different convergence angles. A light beam splitting element for splitting the light beam into three light beams, light detecting means for respectively detecting the three reflected light beams, and a light detection output output from the light detecting means.
A focus error signal generating means for generating a focus error signal, wherein the light detecting means detects a reflected light beam whose focus position is the farthest from the light beam splitting element among the three reflected light beams. Surface, a second sub-light-receiving surface for detecting a reflected light beam whose light condensing position is closest to the light beam splitting element among the three reflected light beams, and the light beam splitting at a light condensing position of the three reflected light beams A main light-receiving surface for detecting a reflected light beam at an intermediate distance from the element, and independently outputting a light detection output for each light-receiving surface; An optical head for generating a focus error signal indicating a focused state of the light beam emitted from the light source on the optical recording medium, based on a light detection output corresponding to the sub light receiving surface of the optical head. 2. The optical head according to claim 1, wherein the light beam splitting element is a Fresnel zone plate whose center is deviated from the optical axis. 3. The optical head according to claim 1, wherein the light beam splitting element is a hologram. 4. The optical head according to claim 1, wherein the main light receiving surface is formed by a single light receiving element. 5. The first and second sub-light receiving surfaces, respectively,
2. The optical head according to claim 1, wherein the optical head is formed by at least two light receiving elements including a light receiving element including a central portion of the sub light receiving surface and a light receiving element including an outer peripheral portion of the sub light receiving surface. . 6. A tracking error for generating a tracking error signal indicating a tracking state of a light beam emitted from a light source on a recording track on an optical recording medium, based on a light detection output corresponding to the first and second auxiliary light receiving surfaces. 2. The optical head according to claim 1, further comprising a signal generator.