JP2641258B2 - Optical head device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information processing apparatus for recording / reproducing / erasing information, and more particularly to an optical head apparatus using a holographic element. is there.

[Conventional technology]

FIG. 6 shows a conventional optical head device using a holographic element proposed earlier by the present inventor. In the figure, a light beam (2) emitted from a semiconductor laser (hereinafter referred to as LD) (1) as a light source is shown. Is emitted, and this emitted light beam (2) is condensed on an optical disc (4) as an information recording medium by a condenser lens (3). The holographic element (13) separates the reflected light beam from the optical disk (4) from the emitted light beam (2). The separated reflected light beam (6) is received by the light detector (7). (9) is an information track on the optical disk (4).

In the above configuration, the light spot (9) on the optical disk (4) is properly irradiated with the light spot to control the position of the light spot (4) in order to obtain a reproduction signal from the optical disk (4) or to write information. Has functions. The holographic element (13) previously invented by the present inventors has a beam splitter function for extracting a reflected light beam from an optical disc (4) from which a reproduced signal has been read, and astigmatism of the reflected light beam to obtain a focusing control signal. It is composed of a grating area having a function of giving an aberration and a grating area having a three-beam generating function for obtaining a tracking control signal.

 Next, description will be made in order.

Of the light beam (2) emitted from the LD (1), the 0th-order diffracted light transmitted through the holographic element (13) is a substantially aberration-free beam collected on the information track (9) on the optical disk (4). It becomes a light spot (10a). At this time, the holographic graphic element (13) has three grating regions of A, B _1, B _2, if it produced such that each of the zero-order diffraction efficiency is equal, the 0-order diffracted light emitted beam The light can be condensed by the condenser lens (3) while maintaining the intensity distribution of (2), and the discontinuity does not become the intensity distribution.

Specifically, the holographic element (13) is a phase type diffraction grating as shown in FIG.
o ₂ , Tio ₂ etc. are deposited, or plastic substrate is integrally molded with relief, etc.
It is manufactured so that the grating thickness d and the refractive index n at A, B ₁ and B ₂ are equal to each other. Assuming that the wavelength in vacuum is λ, the rays α and β in the figure are the phase differences Occurs. When the phase difference amounts are equal in the grating regions A, B ₁ , B ₂ , the respective zero-order diffracted light intensities are equal.

In FIG. 6, the condensing spot (10a) is reflected and reenters the holographic element (13). Of the three regions A, B ₁ , and B ₂ of the holographic element (13), only the light beams incident on B ₁ and B ₂ are received by the photodetector (7) for the first-order diffracted light. As shown in FIG. 8, the light detector (7)
The light is reflected from the condensing spot (10a) and divided into four central detection areas (7a) to (7f).
The light is received as a spot (6a) at d). The reproduced signal is obtained from the arithmetic output (17) of {7 (a) +7 (b) +7 (c) +7 (d)}.

In the optical disk (4), the center of rotation and the center of the disk usually do not match due to an attachment error or the like. As a result, a track shift occurs due to the rotation. Fig. 9 shows an optical disk (4)
The ideal positional relationship between the information track (9) and the condensing spot is shown, and a method of correcting the track shift will be described.

Of the light beam (2) emitted from the LD (1), the light beam incident on the grating area A of the holographic element (13) is separated into three light beams including the zero-order diffracted light, and the information track (9) is obtained. ) On the focusing spots (10a), (10e),
Irradiate (10f). Since the reading of information is performed at the spot (10a), the spot (10a) must be correctly irradiated on the information track (9). For this reason, the line connecting the spots (10e), (10a) and (10f) is arranged to be slightly inclined with the information track (9). Spot (10
e) Of the reflected light beams of (10f), the light beam incident on B ₁ and B ₂ of the holographic element (13) is the light detector (7)
8, the light is received by the light detection areas (7e) and (7f) as shown in FIG. 8, and an output proportional to the tracking error (referred to as a tracking error signal) is obtained by the differential output (11). Can be If this output is applied to a tracking actuator that moves the condenser lens (3) in a direction perpendicular to the information track (9), the light spot (10a) is constantly focused on the center of the information track (9). Can be. The method of forming three light-collecting spots on the disk (4) and detecting the track shift based on the difference in reflection intensity between the spots on both sides is called the twin spot method, which is stable to the inclination of the optical disk. This is known as a method and described in, for example, Japanese Patent Publication No. 53-13123.

Also, the disk surface is not usually flat, and the rotation causes surface runout. Next, the correction method will be described. In FIG. 6, the lattice regions B ₁ and B ₂ of the holographic element (13) are shown.
Has a stripe shape in which the grating period gradually varies in the x direction within the aperture, and gives astigmatism to the first-order diffracted light. At this time, in the quadrant detection area of the photodetector (7), depending on the direction of deviation of the optical disc (4) from the focal position of the condenser lens (3),
As shown in FIG. 10, from the reference state of FIG.
The direction of the ellipse changes by 90 ° as shown in FIG. The change in the shape of the light spot on the light detector (7) is detected by a calculation output (12) (focus error signal) shown in FIG. The focus error signal (12) is given by [{(7a) + (7c)} − {(7b) + (7d)}]. When the focusing spot (10a) is in focus, (7a), (7
The outputs of b), (7c) and (7d) are all equal, and the focus error signal (12) becomes zero. Therefore, by operating the focusing actuator that moves the condenser lens (3) in the optical axis direction at the output (12), it is possible to correct the defocus on the optical disk (4).

A method of giving astigmatism to the reflected light beam (6) from the optical disk (4) and detecting a focus shift from a change in the shape of the light beam (astigmatism method) is disclosed in, for example, Japanese Patent Publication No. 53-39123. Have been described.

Among the reflected light beams of the condensing spots (10a), (10e) and (10f), the light beam incident on the grating area A of the holographic element (13) is not received by the photodetector (7), and is not shown in FIG. (14-1) to (14-5). Spots (14-1) to (14-5) are unnecessary diffracted light for the reproduced signal, and the unnecessary diffracted light does not enter the photodetector (7), so that a high-quality reproduced signal can be obtained.

[Problems to be solved by the invention]

Since the conventional optical head device is configured as described above, it has the following problems.

As shown in FIG. 6, the grating area A of the holographic element (13) has a substantially rectangular shape that is long in the information track direction (Y). Therefore, the three spots on the information track (9) do not become circular condensing spots as shown in FIG. 9, but as shown in FIG. 11, the condensing spots (10e) and (10e)
f) becomes an ellipse elongated in the x direction. This will be described in detail below.

Generally, when a light beam transmitted through an aperture as shown in FIG. 12A is collected, the numerical aperture of the collected light beam is set to NA (NA = sin
α), the light intensity distribution of the condensing spot is as shown in FIG. 7B, and when the spot diameter D is a circular aperture, In the case of a rectangular aperture, it is λ / NA. Since d, which determines the effective aperture diameter necessary for actually converging the converged light flux of NA, is given by d = 2 · f · NA, where f is the focal length of the converging lens, the larger the d, the larger the NA Large, resulting in a smaller spot diameter D.

Normally, in order to read information pits of 1 μm or less on the optical disk (4), the NA of the condenser lens (3) adopts a value of 0.45 to 0.55. Holographic element of FIG. 6 (13)
Shows only an opening portion (referred to as an effective opening diameter) of the emitted light beam (2) corresponding to the above NA. Assuming that the effective aperture diameter of the illustrated holographic element (13) is d, the condensing spot (10a) of the zero-order light beam transmitted through this aperture d has a spot diameter of about 1 μm, and information can be read. However, the converging spots (10e) and (10f) of the diffracted light from the grating area A have an opening diameter of d in the holographic element (13) in the y-direction and an opening diameter of d in the x-direction in the holographic element (13).
Less than. If the opening diameter in the x direction is, for example, d / 5, the first
As shown in FIG. 1, the spot diameter in the x direction is an elliptical spot that is five times the spot diameter in the y direction. That is, the spot diameter in the y direction is about 1 μm, which is almost the same as that of the condensing spot (10a), but the spot diameter in the x direction is about 5 μm.
Assuming that the distance between the information tracks (9) in the x direction is about 1.6 μm, the condensing spots (10e) and (10f) straddle two or more information tracks (9). Since the tracking error signal uses the signal component in the direction (x direction) crossing the information track (9), the light spots (10e) and (10f)
It is also desired that the spot diameter in the x direction is as small as about 1 μm.

Therefore, a good tracking error signal cannot be obtained with the diffracted light by the conventional grating region A.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide an optical head device using a holographic element capable of obtaining a high-performance tracking error signal.

[Means for solving the problem]

In the optical head device according to the present invention, the grating region having the three-beam generating function of the holographic element is disposed substantially at the center of the effective light beam emitted from the LD, and is substantially rectangular or long in the direction orthogonal to the information track. Elliptical area,
The major axis was set as the effective aperture diameter in the holographic element.

[Action]

According to the present invention, the ± primary converging spot on the information track becomes an ellipse long in the direction of the information track, and when the converging spot crosses in the direction perpendicular to the information track, a traversing signal is given to the reflected light beam, and a tracking error is generated. A signal can be obtained.

〔Example〕

FIG. 1 shows an embodiment of the present invention, in which a grating area A of a holographic element (13) has a substantially rectangular opening which is long in the x direction, and the opening diameter in the x direction is an outgoing light beam (2). Is equal to the effective aperture diameter of the holographic element (13).

In addition, the same reference numerals as those in FIG. 6 indicate the same parts.

With the above configuration, ± 1 of the luminous flux incident on the grating area A
The second-order diffracted transmitted light is applied to the optical disk (4) as condensing spots (10e) and (10f). FIG. 2 shows the state of the three spots on the information track (9). As in the conventional example, the condensing spot (10a) is small enough to read out the information written in the information track (9), and the NA of the condensing lens (3) is set so as to converge, for example, about 1 μm. Have been.
The spot diameters in the x direction of the condensing spots (10a), (10e) and (10f) are substantially equal. Since the spot diameter in the x direction is smaller than the interval (about 1.6 μm) in the x direction of the information track (9), the condensing spots (10e) and (10f) transmit the signal traversing the information track (9) in the x direction. Can be read.

Therefore, a tracking error signal can be obtained from the operation output (11) in FIG. 8 by making the differential of the reflected light beams of the condensing spots (10e) and (10f) similar to the conventional example.

Further, in FIG. 2, the focusing spots (10e), (1
0f) has a large spot diameter in the information track direction. Assuming that the aperture diameter in the y direction of the grating region A of the holographic element (13) is about 1/5 of the x direction, the light spot (10e),
The spot diameter in the information track direction of (10f) reaches about 5 μm. Therefore, it is impossible to read the signal in the y direction, which is the original reproduction signal.

As described above, the tracking error signal is a signal in the x direction, and the signal in the y direction becomes noise with respect to the tracking error signal. Therefore, since the signal in the y direction is not mixed into the tracking error signal in this embodiment, a tracking error signal having a high S / N can be obtained.

The holographic element (13) of the above embodiment is
Although the grid area A is divided into the grid area B by a straight line in the x direction and has a substantially rectangular opening, an elliptical opening may be used as shown in FIG. 3, and the opening in the direction orthogonal to the information track is sufficiently provided. Take large.

The grating regions B ₁ and B ₂ impart astigmatism to the diffracted light (6) having a focal line in the ± 45 ° direction with respect to the x direction. As another embodiment, as shown in FIG. The grating regions B ₁ , B _{2 of the} holographic element (13) may be designed to give astigmatism having focal lines in the x, y directions to the diffracted light (6). FIG. 5 shows the shape of the light detector at this time. The reproduction signal, tracking error signal, and focus error signal are obtained from operation outputs (17), (11), and (12), respectively.

〔The invention's effect〕

As described above, according to this finding, the grating area having the three-beam generating function of the holographic element is formed by opening the opening in the direction orthogonal to the information track direction with a substantially rectangular or substantially rectangular beam substantially equal to the effective light beam diameter of the light beam emitted from the LD. Because of the elliptical aperture shape, a high-performance tracking error signal is obtained, and there is an effect that high-performance reproduction and writing can be performed.

[Brief description of the drawings]

1 is an optical path diagram of one embodiment of the present invention, FIG. 2 is a schematic diagram for explaining the operation of FIG. 1, FIG. 3 is a plan view of a modification of the holographic element, and FIG. FIG. 5 is a circuit diagram for explaining the operation of the embodiment shown in FIG. 6 to 12 show a conventional optical head device, FIG. 6 is an optical path diagram, FIG. 7 is an explanatory diagram of a diffraction grating, FIG. 8 is a circuit diagram for explaining operation, and FIG. Schematic diagram for explanation, No.
FIG. 10 is a schematic diagram for explaining the astigmatism method, FIG. 11 is a schematic diagram for explaining the problem, and FIG. 12 is an explanatory diagram of the aperture and the diameter of the condensing spot. (1) ... semiconductor laser (light source), (3) ... condensing lens (condensing means), (4) ... optical disk (information recording medium), (7) ... photodetector, (9) …… information track,
(13) holographic element, (A) first grid region, (B ₁ ), (B ₂ ) second grid region. In the drawings, the same reference numerals indicate the same or corresponding parts.

Continuation of the front page (56) References JP-A-62-139146 (JP, A) JP-A-63-56819 (JP, A) JP-A-64-53353 (JP, A) JP-A-1-149235 (JP) JP-A-1-151022 (JP, A) JP-A-1-253842 (JP, A)

Claims (1)

(57) [Claims] 1. A light source, a condensing means for converging and irradiating a light beam emitted from the light source onto an information track on an information recording medium, a photodetector for receiving a reflected light beam from the information recording medium,
A first grating region for separating the emitted light beam into at least three light beams and directed to the information recording medium, and a second grating region for separating the reflected light beam from the information recording medium from the emitted light beam are in the same plane. And the first grating region of the holographic element has an opening that is arranged substantially at the center of the effective opening of the emitted light beam and that is long in a direction orthogonal to the information track direction. An optical head device comprising: