JPH1011773A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical head device capable of making the receiving surface of a photodetector, for detecting a RF signal, small and detecting the relevant RF signal without deteriorating the high frequency characteristic of the Rf signal. SOLUTION: The optical head device diffracts the transmitting light beam L from a laser diode 2 into three beams by a modulating diffraction grating and makes light spots b, c of ±1st order diffracted light beams, formed on the recording surface 4a of a recording medium 4, generate astigmatic aberrations. The returned three beams from the recording medium 4 are diffracted to three beams, respectively, and the RF signal is detected from the light spots a1, a2 of the returned light beams formed by ±1st order diffraction beams. A focusing error signal and a tracking error signal are formed-based on the change of lateral aberration of meridional astigmatism of light spots b1, b2 and c1, c2. Since it is not necessary to use division type for a photodetector for detecting the RF signal and the formed light spots a1, a2 are small, the high frequency characteristic of the RF signal is improved and the receiving surface of the relevant detector is made small.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical head device used for recording and / or reproducing an optical recording medium.

[0002]

2. Description of the Related Art In an optical head device, laser light emitted from a light source is condensed as a light spot on a recording surface of an optical recording medium via an objective lens, and return light reflected therefrom is detected by a photodetector. And the recorded information is reproduced. At the same time, the position of the objective lens in the focusing direction is controlled so that the laser light is focused on the recording surface without defocusing. Further, the position of the objective lens in the tracking direction is controlled so that the laser beam scans the track on the recording surface accurately.

In order to perform these tracking and focusing controls, optical information corresponding to a tracking error and a focusing error of a laser beam is detected from return light from an optical recording medium.

As a method of detecting a tracking error,
Generally, a three-beam method is employed. In the three-beam method,
The laser beam is divided into three beams in the track direction of the recording surface by a diffraction grating or the like, and a tracking error signal is formed based on the reflected light of the +/− 1 order diffracted light included in these beams.

As a method of detecting a focusing error signal, for example, an astigmatism method using a cylindrical lens, a knife edge method using a hologram element, a spot size method, and the like are known. In these methods, the returning light from the optical recording medium optically includes a focusing error component such as astigmatism, and the error component is detected by a photodetector.

An optical head device employing such a three-beam method or a knife-edge method is disclosed in, for example, Japanese Patent Application Laid-Open No. 3-225.
No. 635. In the optical head device disclosed in this publication, a tracking error signal is detected by a three-beam method, and therefore, a diffraction grating is arranged on the outward path of laser light. Further, the focusing error signal is detected by the knife edge method, and for this purpose, a hologram element is arranged on the return path of the laser beam.

[0007]

Here, in the conventional focusing error signal detection method using the astigmatism method, a four-divided photodetector having a light-receiving surface divided into four parts is used, and each light-receiving part divided into four parts is used. RF signal (recording signal) together with focusing error signal based on the amount of light received on the surface
Has been detected. Since the split type photodetector has a split dead zone on the light receiving surface, when detecting an RF signal, the high frequency characteristics of the RF signal may be degraded due to this.

In the knife edge method and the spot size method, a focusing error signal is detected by using one side of the diffracted light of the return light formed by using the hologram element, and is detected by using the diffracted light of the other side. The RF signal is being detected. In this case, the light spot formed by the diffracted light for detecting the RF signal on the light receiving surface of the photodetector has the same size as the light spot for detecting the focusing error signal. Therefore, since the degree of light collection is low, the high frequency characteristics of the RF signal may be degraded. In addition, the light receiving surface for detecting the RF signal becomes large.

An object of the present invention is to propose an optical head device capable of detecting an RF signal without deteriorating high frequency characteristics.

Another object of the present invention is to provide an optical head device capable of reducing the light receiving surface of a photodetector for detecting an RF signal.

Another object of the present invention is to provide an optical head device capable of detecting a focusing error signal and a tracking error signal by using a common photodetector.

[0012]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides a laser light source, an objective lens for condensing light emitted from the laser light source on a recording surface of an optical recording medium, An optical head device having light detecting means for detecting return light reflected on a recording surface, wherein a modulation diffraction grating for dividing light emitted from the laser light source into three beams including 0-order light and + -1st-order light; A light guide element for guiding the three returned beams reflected by the recording surface to the light detecting means; and a light signal detecting means for receiving the zero-order light included in the three returned lights as the light detecting means. First light detection unit for detecting the return light beam and + -1 included in the three returned beams.
A detector having a light detecting section for detecting a focusing error and a tracking error for receiving the second-order diffracted light, and detecting a focusing error based on astigmatism contained in the + -1st-order diffracted light of the three returned beams. I am trying to do it.

In the optical head device according to the present invention, since the outgoing light from the laser light source toward the recording medium is diffracted by the modulation diffraction grating, the diffracted + -1st-order diffracted light forms an image on the recording surface of the recording medium. Sometimes, if the diffraction direction is a meridional light beam, the modulation diffraction grating adds convergence and divergence effects to the + -1st order diffraction, and astigmatism occurs in the opposite direction. That is, an error component (astigmatism) for detecting a focusing error is included in the + -1st-order diffracted light on the outward path of the light. Therefore, the focusing error signal can be detected by the astigmatism method similar to the related art, using the return light of the + -1st-order diffracted light. Also, 0
The RF signal can be detected using the return light of the next light.

In this way, since the focusing error signal can be detected using the + -1st-order diffracted light, the 0th-order light can be used only for detecting the RF signal. Therefore, since it is not necessary to use a split type photodetector for the RF signal detection, there is no split dead zone on the light receiving surface, and it is possible to avoid deterioration of the high frequency characteristics of the RF signal. Further, since the RF signal is detected from the zero-order light, the degree of convergence of the light spot of the return light formed on the light receiving surface of the photodetector is smaller than that of the conventional knife edge method or spot size method. Since it is high, deterioration of the high frequency characteristics of the RF signal can be avoided, and the light receiving area thereof may be small.

Here, as the light guide element, a diffraction grating for diffracting each of the three returning beams toward the light detecting means can be employed.

In this case, the diffraction grating is arranged between the modulation diffraction grating and the objective lens, and + -1st-order diffracted light obtained by diffracting each of the returning three beams passes through the modulation diffraction grating. It is desirable to employ a device that can diffract each of the three returned beams so as to go to the light detection unit for detecting the focusing error and the tracking error without performing the above operation. According to this configuration, + −
It is possible to prevent the first-order diffracted light from passing through the modulation diffraction grating and being diffracted again to reduce the amount of light.

In order to enhance the light use efficiency, it is possible to adopt a configuration in which the diffraction grating is a polarizing diffraction grating and a quarter-wave plate is arranged between the modulation diffraction grating and the recording surface. . In this case, since the emitted light is not diffracted by the diffraction grating, it is possible to suppress a decrease in the amount of light on the recording surface.

Next, in the optical head device according to the present invention, the direction of diffraction by the modulation diffraction grating is set to be the track direction on the recording surface of the optical recording medium. If this configuration is employed, a tracking error signal can be detected based on the return light of the +/− 1st-order diffracted light in the same manner as in the conventional three-beam method. Therefore, the above-described light detecting section for detecting a focusing error can be used also as a detecting section for detecting a tracking error signal.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical head device to which the present invention is applied will be described below with reference to the drawings.

FIG. 1 shows a schematic configuration of an optical system of an optical head device according to the present invention. As shown in this figure,
The optical head device 1 has a laser diode 2 as a laser light source. The laser light L emitted from the laser diode 2 is focused by an objective lens 3 on a recording surface 4a of an optical recording medium 4 as a light spot.

On the optical path between the laser diode 2 and the objective lens 3, a modulation diffraction grating 5 and a diffraction grating 6 are arranged in this order from the laser diode 2 side. In this example, the modulation diffraction grating 5 is formed on one surface and the diffraction grating 6 is formed on the other surface with the glass substrate 7 interposed therebetween. Further, the diffraction grating 6 of the present example is a polarizing diffraction grating.
FIGS. 1B and 1C show a grating pattern of the modulation diffraction grating 5 and a grating pattern of the diffraction grating 6, respectively, and these diffraction directions are orthogonal to each other.

A quarter of the diffraction grating 6 is located on the side of the objective lens 3.
A wave plate 8 is provided. In this example, the laser beam L is diffracted by the modulation diffraction grating 5, but the diffraction grating 6
In some cases, the polarization direction of the laser light L and the crystal axis direction of the diffraction grating 6 are set so as not to be affected by diffraction. The laser beam L passes through the quarter-wave plate 8 and becomes circularly polarized light. After being reflected on the recording surface 4a of the recording medium, it again passes through the quarter-wave plate 8 and returns to linearly polarized light. The directions are orthogonal. For this reason, the return light from the recording medium 4 is subjected to a diffraction effect by the diffraction grating 6.

The return light from the recording medium 4 is received by the photodetector 10 after being diffracted by the diffraction grating 6. Based on the amount of light received by the photodetector 10, an RF signal is detected, and a focusing error signal and a tracking error signal are detected.

As shown in FIG. 1D, the photodetector 10 of this embodiment has two photodetector groups 20 and 30 arranged symmetrically with respect to the laser diode 2. The photodetector group 20 includes a photodetector 21 located at the center for detecting an RF signal, and photodetectors 22 and 23 located on both sides thereof for detecting a focusing error and a tracking error. Similarly, the other photodetector group 30 includes a photodetector 31 located at the center for detecting an RF signal, and photodetectors 32 and 33 located on both sides for detecting a focusing error and a tracking error. .

Here, the photodetectors 22, 23 and the photodetectors 32, 33 are three-division type photodetectors. That is, the photodetector 22 includes the light receiving surfaces 22a, 22b, and 22c.
3a, 23b and 23c. The photodetectors 32 and 33 also have light receiving surfaces 32a, 32b, and 3 respectively.
2c and 33a, 33b, 33c.

FIG. 2 shows the diffraction effect of the output light by the modulation diffraction grating 5, the diffraction effect of the return light by the diffraction grating 6, and the state of the light spot formed on the recording surface and the light receiving surface of the detector. is there. FIG. 3 shows the optical path of the first-order diffracted light diffracted by the modulation diffraction grating 5.

The operation of detecting an optical signal in the optical head device 1 according to the present embodiment will be described with reference to these drawings. The laser emission light L from the laser diode 2 first passes through the modulation diffraction grating 5 and undergoes a diffraction action, and is mainly a zero-order light L.
It is split into three beams of (0) and + -1st order diffracted light L (+1), L (-1). The diffraction direction of the modulation diffraction grating 5 is set to be the direction of the recording track 4b of the recording medium.

The three beams thus formed pass through the diffraction grating 6, pass through a quarter-wave plate 8, are converted into circularly polarized light, and then enter the objective lens 3. Through the objective lens 3, the three beams are focused as light spots a, b, and c on a recording track 4 b formed on a recording surface 4 a of the recording medium 4.

Return three beams Lr reflected by the recording medium 4
(0), Lr (+1), and Lr (-1) correspond to the objective lens 3
Then, the light returns to the 板 wavelength plate 8, passes therethrough, and is converted into linearly polarized light again. However, the polarization direction is switched to a direction orthogonal to the laser emission light L. As a result,
Return three beams Lr (0), Lr (+1), Lr (-1)
Undergoes a diffraction effect when passing through the diffraction grating 6 and is diffracted into three beams. This diffraction direction is set to be a direction orthogonal to the diffraction direction of the modulation diffraction grating 5.

As a result, the zero-order light Lr (0) included in the three returned beams is the zero-order light Lr (0) o, and the + -1st-order diffracted lights Lr (0) +1 and Lr (0) -1. Diffracted into a beam. Similarly, the + 1st-order diffracted light Lr (+1) also includes the 0th-order light Lr (+1) o and the + 1st-order diffracted light Lr (+1) +
The light is diffracted into three beams of 1, Lr (+1) -1, and the -1st-order diffracted light Lr (-1) is also 0th-order light Lr (-1) o and +
-1st order diffracted light Lr (-1) +1, 3 of Lr (-1) -1
Diffracted into a beam.

Of these diffracted lights, + 1st-order diffracted light Lr
(0) +1, Lr (+1) +1, Lr (-1) +1
The light beams are condensed as light spots a1, b1, and c1, respectively, on the light receiving surfaces of the light detectors 21, 22, and 23 that constitute the light detector group 20 of the light detection device 10. On the other hand,-
First order diffracted light Lr (0) -1, Lr (+1) -1, Lr
(-1) -1 collects light spots a2, b2, and c2 on the light receiving surfaces of the photodetectors 31, 32, and 33 that constitute the photodetector group 30 of the photodetector 10, respectively.

In this example, the + 1st-order diffracted light Lr
(0) +1, Lr (+1) +1, Lr (-1) +1, and -1st order diffracted light Lr (0) -1, Lr (+1) -1,
The direction of diffraction by the diffraction grating 6 and the optical path length between the diffraction grating 6 and the modulation diffraction grating 5 are set so that Lr (-1) -1 does not pass through the modulation diffraction grating 5.

Here, in the optical head device 1 of this embodiment, the laser output light L is diffracted by using the modulation diffraction grating 5 to form three beams. As a result, as can be seen from FIG. 3, the position of the meridional image plane when considered except for the recording surface 4a is the position indicated by the broken line 4a1, and the meridional image surface after reflection by the recording surface 4a is the position indicated by the broken line 4a2. Become. For this reason, among the light spots of the respective beams formed on the recording surface of the recording medium 4, astigmatism occurs in the light spots b and c located on both sides. The meridional image plane on the light receiving surface side of the detector 10 due to the return light of these light spots is at the position indicated by the broken line 10a. Therefore, the light spots b1 and b2 formed on the light receiving surface of the photodetector when the return light of these light spots forms an image.
And the width of the transverse aberration of meridional astigmatism in the light spots c1 and c2 changes in the opposite direction according to the direction of defocus.

That is, as shown in FIG. 2 (D), when the focus is on, the light spots b1, b2 and the light spots c1, c2 have an elliptical shape having the same dimensions. On the other hand, at the front focus position, the widths of the light spots b1 and b2 are narrow, whereas the widths of the light spots c1 and c2 are wide. Conversely, at the rear focal position, the light spots b1, b2
Becomes wider, whereas the widths of the light spots c1 and c2 become narrower.

Therefore, the photodetector 10 detects a change in the width of these light spots as a change in the amount of light received by each detector, and generates a focus error signal FE. For example, as shown in FIG. 4A, the light receiving surfaces 22b, 22c, and 3 on both sides of the three-segmented photodetectors 22, 32.
The sum S1 of the amount of light received at 2b and 32c and the amount of light received at the central light receiving surfaces 23a and 33a of the three-divided photodetectors 23 and 33 on the opposite side is calculated. In addition, the sum S2 of the amount of light received by the central light receiving surfaces 22a and 32a of the three-divided photodetectors 22 and 32 and the amount of light received by the three divided detectors 23b and 23c and 33b and 33c on the opposite side is calculated. Then, these sum signals S1 and S2
, A focus error signal FE is formed.

Next, the tracking error signal TE is formed by a generally employed three-beam method.
That is, as shown in FIG. 4B, the difference between the amount of received light S3 of the three-divided photodetectors 22 and 32 in the photodetector 10 and the amount of received light S4 of the three-divided photodetectors 23 and 33 on the opposite side. , A tracking error signal TE is formed.

On the other hand, the RF signal is detected based on the amount of light received by the photodetectors 21 and 31 arranged at the center in the photodetector 10.

In this embodiment, the photodetector 10 includes two photodetector groups 20, 30. Basically, it is sufficient if one of the photodetector groups 20 or 30 is provided. Alternatively, both the photodetector groups 20 and 30 may be provided so that one side detects a tracking error signal and the other side detects a focusing error signal.

(Other Embodiments) In the above example, a quarter-wave plate and a diffraction grating 6 are used as light guide elements for guiding return light to each photodetector of the photodetector. . Instead, only the diffraction grating may be used, or the return light may be separated using a half mirror.

[0040]

As described above, in the optical head device according to the present invention, the laser emission light is diffracted into three beams by the modulation diffraction grating, and then condensed on the recording surface as a light spot on the recording medium. Is caused to generate astigmatism. Therefore, according to the present invention, a focusing error signal can be detected based on the return light of the +/− 1st-order diffracted light on both sides where astigmatism has occurred. Also, the return light of the zero-order light can be used exclusively for detecting the RF signal.

As described above, according to the present invention, since the zero-order light included in the three beams can be used only for detecting the RF signal, the photodetector that receives the zero-order light can be divided into photodetectors. There is no need to use a container. For this reason, the photodetector has no divided dead zone, and the high frequency characteristics of RF signal detection are improved. Further, since the diameter of the light spot for detecting the RF signal can be made sufficiently small, the light receiving surface of the photodetector for detecting the RF signal can be made small, which also improves the high frequency characteristics of the RF signal detection. .

If the diffraction direction by the modulation diffraction grating is matched with the tracking direction of the recording medium, a tracking error signal can be detected using return light of the +/− 1 order diffracted light in the same manner as in the conventional three-beam method. In this case, the focusing error signal and the tracking error signal can be detected by the common photodetector.

Further, in the optical head device of the present invention, when a quarter-wave plate and a polarizing diffraction grating are used, the loss of the amount of laser light can be suppressed, and an RF signal, a tracking error signal, a focus error signal Detection accuracy can be improved.

[Brief description of the drawings]

FIG. 1A is a schematic configuration diagram of an optical system of an optical head device to which the present invention is applied, FIG. 1B is an explanatory diagram illustrating a grating pattern of a modulation diffraction grating, and FIG. 1C is a diffraction grating that diffracts return light. FIG. 3D is an explanatory view showing a lattice pattern, and FIG. 4D is an explanatory view showing a configuration of a photodetector.

2A is a conceptual diagram for explaining the operation of the optical system of FIG. 1, FIG. 2B is an explanatory diagram showing a light spot formed on a recording surface, and FIG. FIG. 4D is an explanatory view showing an example of a formed light spot, and FIG. 4D is an explanatory view showing a change in the shape of the light spot formed on the side of the photodetector.

FIG. 3 is a light beam explanatory diagram showing an optical path of a light beam of a first-order diffracted light diffracted by a modulation diffraction grating.

FIG. 4A is an explanatory diagram showing a circuit configuration for forming a focus error signal, and FIG. 4B is an explanatory diagram showing a circuit configuration for forming a tracking error signal.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Optical head device 2 Laser diode 3 Objective lens 4 Recording medium 4a Recording surface 4b Recording track 5 Modulation diffraction grating 6 Diffraction grating 8 1/4 wavelength plate 10 Photodetector 21, 31 Photodetector for RF signal detection 22, 23 , 32, 33 Light detectors L (0), L (+1), L (-1) for detecting a focusing error signal and a tracking error signal Diffracted light Lr (0) +1, Lr (+1) +1 of the outgoing light L Lr (-1) +1
Diffraction light of return light Lr (0) -1, Lr (+1) -1, Lr (-1) -1
Diffracted light of return light a, b, c Light spot on recording track a1, a2, b1, b2, c1, c2 Light spot on photodetector

Claims (5)

[Claims] 1. An optical system comprising: a laser light source; an objective lens for condensing light emitted from the laser light source on a recording surface of an optical recording medium; and light detecting means for detecting return light reflected on the recording surface. In the head device, a modulation diffraction grating that divides light emitted from the laser light source into three beams including 0-order light and + -1st-order light, and guides the returning three beams reflected on the recording surface to the light detection unit. An optical element, wherein the light detection means includes: a first light detection unit for detecting a recording signal that receives the zero-order light included in the three returned beams; A light detecting section for detecting a focusing error that receives the folded light, wherein a focusing error is detected based on astigmatism contained in the +/− 1st-order diffracted light of the three returned beams. To be Optical head device. 2. The optical head device according to claim 1, wherein the light guide element is a diffraction grating that diffracts each of the three returning beams toward the light detecting unit. 3. The diffraction grating according to claim 2, wherein the diffraction grating is arranged between the modulation diffraction grating and the objective lens, and the + -1st-order diffracted light obtained by diffracting each of the returning three beams is the modulation diffraction light. The return 3 is directed to the focusing error detecting light detecting section without passing through the grating.
An optical head device for diffracting each of the beams. 4. The diffraction grating according to claim 2, wherein the diffraction grating is a polarizing diffraction grating, and further includes a quarter-wave plate disposed between the modulation diffraction grating and the recording surface. An optical head device characterized by the above-mentioned. 5. The optical detector according to claim 1, wherein the direction of diffraction by the modulation diffraction grating is a track direction on a recording surface of the optical recording medium, and the light detection for detecting the focusing error is performed. An optical head device, wherein the unit also serves as a detection unit for detecting a tracking error signal.