JPH0814897B2 - Optical head device - Google Patents

Description

The present invention relates to an optical head device used in an optical information recording / reproducing apparatus.

<Summary of the Invention> The present invention irradiates a recording medium with a main beam and a pair of sub-beams arranged on both sides of the main beam, and diffracts the diffracted light from the return light reflected on the recording surface by a diffraction grating means such as a hologram element. Is split as a detection beam and is incident on a photodetector consisting of a main beam photodetection element group and a pair of sub-beam photodetection element pairs. In the optical head device in which the tracking error of each of the information signals is detected by the sub-beam photodetector element pair, the sub-beam photodetector element pair has a substantially symmetrical shape and the main-beam photodetector element is By configuring the group to surround the periphery of the group, the positional shift of the spot on the photodetector due to the wavelength variation of the laser caused by the temperature variation and the extension of the spot caused by the scratch on the recording medium in the optical path are the same. Even if it occurs occasionally, noise is less likely to occur in the tracking error signal.

<Prior Art> An optical head device irradiates a recording body with a beam emitted from a light source, detects the reflected light by a light detection unit, and performs focus control /
An information signal is recorded / reproduced while performing tracking control, but a diffraction grating means such as a hologram element is used as means for branching reflected light from the irradiation light path and guiding it to a light detection means, and at the same time giving processing for focus error detection Has been developed that simplifies the structure and reduces the size and weight of the device. A conventional optical head device of this type will be described with reference to FIGS. In FIG. 2, reference numeral 1 is a laser diode, and a sub-beam generating diffraction grating 2, a hologram element 3, and an objective lens 4 are arranged on the emission optical axis thereof. The diffraction grating 2 is one element for realizing the three-beam method which is one of the methods for detecting a racking error, and the 0th-order diffracted light from the emitted beam of the laser diode 1 is used as a main beam for information recording / reproduction and ± 1st-order The diffracted light is generated as a pair of sub-beams for tracking error detection so that these three beams are arranged in a direction substantially perpendicular to the paper surface.
These three beams pass through the hologram element 3 (as 0th-order diffracted light) and are then focused by an objective lens 4 to project three spots on a recording medium 5 such as an optical disk. The return light reflected by the recording medium 5 traces the outgoing optical path in reverse, reaches the hologram element 3 via the objective lens 4, and is partially incident on the photodetector 6 due to the optical path branching effect by diffraction. However, the details will be described below.

As shown in FIG. 3, the hologram element 3 has a first region I and a second region which are different in the form of a lattice by a region dividing line 1 passing through the optical axis.
It is composed of II and. The three return lights reflected by the recording medium 5 are diffracted in each of the regions I and II, but one of the ± first-order diffracted lights transmitted through each of the regions I and II has two sets of detection beams 7- The light enters the photodetector 6 as I, 7-II. As shown in the plan view of FIG. 4, the photodetector 6 is a main beam photodetection device including elements A and B contributing to focus error detection and an element C used together with these elements for reading an information signal during reproduction. It is composed of a group and a pair of sub-beam photodetector elements which are arranged on both sides of the group and consist of elements D and E that contribute to tracking error detection. As shown in the figure, among the above-mentioned two sets of detection beams, three of 7-I are focused on the positions indicated by (I), and three of 7-II are focused on the positions indicated by (II). tie.

In this state, from each output of each element A, B, C, D, E, the focus error signal FE is based on the well-known principle, and FE = A−B, and the tracking error signal TE is obtained by the well-known 3-beam method. Based on the principle, TE = DE, and the information signal RF during reproduction is obtained by RF = A + B + C.

<Problems to be Solved by the Invention> In the hologram element 3, which is a kind of diffraction grating, when the wavelength of incident light varies, the diffraction angle also varies. That is, there is a relationship of θ = Sin ⁻¹ (λ / P) because Sin θ = λ / P between the wavelength λ of the laser light, the grating pitch P, and the diffraction angle θ. Diffraction angle θ
Also fluctuates. Generally, the wavelength λ of the light beam used is unique to the laser diode 1, but it easily fluctuates because it is affected by fluctuations in its operating temperature.
The diffraction angle of the hologram element 3 also changes depending on the temperature.
Therefore, when there is a temperature change, each spot on the photodetector 6 moves in a direction perpendicular to the lattice of the hologram element 3, that is, in the same direction as the XX 'direction in FIG.

The movement of the spot on the photodetector 6 due to this temperature fluctuation does not directly affect the function of the optical head. However, when a spot is elongated due to a scratch on the recording body 5 which will be described in detail below, a synergistic effect with the spot may cause an abnormality in tracking error detection. Hereinafter, this will be described mainly with reference to FIG.

The recording body 5 generally has a structure in which a recording surface on which information pits are formed is covered with a protective layer formed of a transparent resin. This protective layer directly protects the recording surface from scratches and stains, and even if scratches or the like occur on the surface of the protective layer itself, the position is the defocus position of the beam. It also has the effect of reducing the risk of obstructing the beam. But,
The scratches on the protective layer may cause the following problems. In general, the scratches on the protective layer are often formed as a set of one groove or a plurality of parallel grooves. When the beam passes through such scratches, it is diffracted and rarely receives refraction and reflection,
A scattered light beam inclined from the original optical axis is generated in the direction perpendicular to the groove. The delicate form such as the emission angle and the light amount distribution of the scattered light flux is based on the size of the scratches, the number of the flaws, the cross section, the form of the groove inner surface, and the like and cannot be unconditionally predicted. As shown by the broken line in the figure, the original spot shape is stretched and deformed on the photodetector 6 in a direction perpendicular to the flaw. When the spot is at the original reference position, that is, at each position shown by (I) and (II) in FIG. Since the positional relationship is symmetrical with respect to the center of the container 6, there is no great adverse effect on the detection of each signal. But,
If the positional deviation of the spots due to the wavelength variation occurs at the same time, the effect cannot be ignored.

The positions of the spots indicated by (I) ′ and (II) ′ in FIG. 5 are, as shown in FIG. 4, distances m ₁ and m ₂ from the original positions (I) and (II) due to wavelength fluctuations. The figure shows the state of movement in the arrow X direction. In such a state, when the spot extension due to the scratch occurs at an inclination (eg, θ≈45 °) with respect to the array of the elements as shown in FIG. 5, the spot in (I) 'and the spot in (II)' There will be a difference in the irradiation area occupied by each element from the spot in FIG. Especially, the spots 8 and 9 of the main beam, which is the 0th-order diffracted light of the diffraction grating 2,
Each of the spots 10, 11, 12, and 13 of the sub-beam that is the first-order diffracted light has a light amount several times as large as that of the first-order diffracted light, and the influence thereof is great. Fifth
In the case of the state shown in the figure, the main beam spots 8 and 9 irradiate not only the elements A, B and C to be originally irradiated, but also the elements D and E which are a pair of tracking error detecting elements. Letting e and d be the amounts of light that the main beam spots 8 and 9 leak into the elements E and D, the tracking error signal TE is TE = (D + d) − (E + e) = D−E + ( d-e), and the noise of (d-e) is added to the original signal. However, as shown in FIG. 5, due to the positional shift of the spot between the light amounts d and e, d >> There is a light amount difference of e, and this noise (d−e) has a very large value.

As a result, when the positional shift of the spot on the photodetector due to the wavelength variation and the extension of the spot due to the scratch on the recording medium occur in the same way, the tracking control may not be normally performed depending on the scratch direction. This caused a serious hindrance to the function of the optical head.

The present invention focuses on the above problems, and an object thereof is to provide an optical head device capable of normally performing tracking control even when the above-described state occurs.

<Means for Solving the Problems> The present invention irradiates a recording medium with a main beam and at least a pair of sub-beams arranged on both sides thereof, and makes return light reflected on a recording surface enter a diffraction grating means, The diffraction beam is used to split the detection beam, and the detection beam is guided to a photodetection device including a main beam photodetection element group and a pair of sub-beam photodetection element pairs. In an optical head device in which a focus error or an information signal is detected by a group and a tracking error is detected by the sub-beam photodetection element pair, the sub-beam photodetection element pair has a substantially symmetrical shape and the main beam is detected. The configuration is such that the periphery of the photodetection element group for beams is surrounded.

<Operation> According to the above configuration, since the sub-beam photodetection element pair is in a state of surrounding the main beam photodetection element group, movement of the spot on the photodetector due to wavelength fluctuation When the spots on the recording medium are simultaneously elongated by the scratches, the leakage of the main beam spot to each of the sub-beam photodetector element pairs, even if the directions of extension are in any direction, are such that the light amounts are almost symmetrical to each other. It becomes a distribution and is canceled by the calculation of the tracking error signal, so that it does not occur as noise.

<Example> Hereinafter, an example of the present invention will be described in detail with reference to FIG.

The present invention is one in which the structure of the photodetector is devised, and the other configurations are the same as those of the above-described conventional technique shown in FIGS.

FIG. 1 shows a plan view of a photodetector 6'of an optical head device to which the present invention is applied. Similarly to the prior art, the photodetector 6'is a main beam photodetection element group consisting of elements A'and B'that contribute to focus error detection and together with these elements C'used for reading an information signal during reproduction. , A pair of sub-beam photodetector elements D ', E'consisting of tracking error detection elements D', E'are symmetrical to each other and have main beam photodetector elements A ', B', C. It is shaped so as to surround the ‘

In this apparatus, when the spot movement on the photodetector 6'due to the wavelength variation in the X direction and the spot extension due to a scratch on the recording medium occur at the same time as in the prior art example shown in FIG. think of. As shown in FIG. 1, if the spots are elongated in the direction inclined with respect to the arrangement of the elements as in FIG. 5, the main beam spots 8'and 9'are leaked to the elements E'and D ', respectively. It will be crowded. Assuming that the amounts of light leaking at this time are e'and d ', the tracking error signal TE is TE = (D' + d ')-(E' + e ') = D'-E, as in the prior art, with respect to the original equation. ′ + (D′−e ′), and (d′−e ′) is added to the original signal. As can be seen from FIG. 1, the elements D and E are both main beam photodetection elements. Since they are shaped so as to surround the groups A ', B', and C ', the leak amounts are substantially equal to each other, and the relationship of d'.apprxeq.e' is established. Therefore, from the above formula, TE = D'-E '+ (d'-e') = D'-E '+ 0 = D'-E' holds, and the tracking error signal TE is obtained with almost no noise. can do. This relationship is always established regardless of the direction of the spot extension, that is, regardless of the direction of the scratch on the recording medium. Therefore, in this case, almost no noise is mixed into the tracking error signal. You can

In the above-described embodiment, the detection beam is divided into two by the dividing line 1 passing through the optical axis when the return light is branched by the hologram element.
Although an example in which the focus error signal is obtained by dividing the focus error signal by the principle of the knife edge method is applied, the present invention is not limited to this, and the detection beam is not divided, and astigmatism is generated by the hologram element or the like. Can be applied to a method that does not perform beam splitting, such as an astigmatism method in which a focus error signal is detected by a four-quadrant split photodetector element group.

<Effects of the Invention> As described above, according to the present invention, even when the movement of the spot on the photodetector due to wavelength fluctuation and the extension of the spot due to scratches on the recording medium occur at the same time, the direction of extension However, it is possible to prevent almost no noise from being mixed into the tracking error signal, and it is possible to configure an optical head device that is resistant to temperature changes and scratches on the recording medium.

[Brief description of drawings]

FIG. 1 is a plan view showing an embodiment of a photodetector of an optical head device to which the present invention is applied, FIGS. 2 and 3 are optical path diagrams of a hologram element utilizing optical head device and plan views of a hologram element, and FIG. FIG. 5 and FIG. 5 show plan views of photodetectors in the prior art. 1 ... Laser diode, 2 ... Diffraction grating (for generating three beams), 3 ... Hologram element (diffraction grating means), 5 ...
... Recording body, 6 '... Photodetector, 7-I, 7-II ... Detection beam, A', B ', C' ... Main beam photodetection element group,
D ', E' ... A pair of sub-beam photodetector elements.

Claims (1)

[Claims] 1. A recording medium is irradiated with a main beam and at least a pair of sub-beams arranged on both sides of the recording medium, and the return light reflected by the recording surface is incident on a diffraction grating means for detection using the diffraction action. The beam is branched, and the detection beam is guided to a photodetector composed of a main beam photodetection element group and a pair of return beam photodetection element pairs, and a focus error or information signal is generated in the main beam photodetection element group. In the optical head device configured to detect the tracking error by the sub-beam photodetector element pair, the sub-beam photodetector element pair has a substantially symmetrical shape, and
An optical head device characterized in that it is configured to surround the periphery of the main beam photodetection element group.