JP2542387B2 - 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. .

[Prior Art] FIG. 6 is a configuration diagram of an optical head device using a conventional holographic element disclosed in, for example, Japanese Patent Laid-Open No. 56-57013, in which (1) is a light source. Semiconductor laser, (2) is the luminous flux emitted from the semiconductor laser (1), (3)
Is a condenser lens for converging and irradiating the outgoing light flux (2) on the information recording medium (4), and (6) changes the traveling direction of the reflected light flux (7) reflected from the information recording medium (4) and A holographic element that gives point aberration, and (8) is a photodetector that receives the reflected light beam (7).

Next, the operation will be described. The light beam (2) emitted from the semiconductor laser (1) passes through the holographic element (6), but only the zero-order light of interference is focused on the information recording medium (4) by the focusing lens (3). . An information track (5) is formed on the information recording medium (4), and the focused spot records or reproduces information on the information track (5). The light flux (7) reflected from the information recording medium (4) passes through the condenser lens (3),
It is incident on the holographic element (6) again. The holographic element (6) is a diffraction grating in which the grating period is linearly different within the light flux aperture, and the traveling direction of the first-order diffracted light of the reflected light flux (7) is changed and astigmatism is generated. Given. Therefore, the first-order diffracted light is received by the photodetector (8) placed at a position distant from the semiconductor laser (1), whereby the information recording medium (4) is received.
You can read the information above.

In addition, the photodetector (8) is a 4-division detector section (8a to 8d)
This is arranged so that when the focused spot (10) on the information recording medium (4) is in focus, the reflected light flux (7) becomes a circle of least confusion (9) and is irradiated to the center of the detector. It is set up. On the other hand, when the focused spot (10) on the information recording medium (4) is out of focus, the spot shape on the photodetector (8) is elliptical. Therefore, for each output of the four-division detectors (8a to 8d), ((8a + 8c)-
By performing the calculation of (8b + 8d)), the focus shift amount can be detected. Then, an objective lens [condensing lens (3)] is provided by a focus actuator (not shown).
Can be displaced in the direction of the optical axis so that the information recording medium (4) can be controlled so as to always be in focus. The playback signal is (8
It is obtained by the sum output of a) + (8b) + (8c) + (8d).

In the above-mentioned conventional optical head device, since the holographic element is arranged between the semiconductor laser (1) and the condenser lens (3), the 0th-order light originally necessary on the information recording medium (4). The focused spot (11) of the first-order diffracted light is irradiated in addition to the focused spot (10). At this time, of the reflected light flux from the converging spot (11) of the first-order diffracted light, the light transmitted through the holographic element (6) in the 0th order also enters the photodetector (8). Since the focused spot (11) is focused at a position different from the focused spot (10) for reading the original signal,
The signal read by the focused spot (11) acts as noise on the original signal. Therefore, the configuration of the conventional apparatus shown in FIG. 6 has a problem that the S / N of the reproduced signal is deteriorated.

The above problem occurs because the focused spot (10) and the focused spot (11), which is unnecessary light, are irradiated to different places. Therefore, if the holographic element (6) is configured as an in-line type, the focused light is focused. The spots (10) and (11) are irradiated on the same place.

FIG. 7 shows an example in which the holographic element (6) is an in-line type, focusing on the above points. Each component is the same as the above-mentioned conventional example, and the description thereof is omitted. The holographic element (6) of this example gives astigmatism to the reflected light beam (7), but the optical axis (2A) indicated by the chain line in the figure is the light beam of the light beam (2) emitted from the semiconductor laser (1). Aligns with the axis. In other words, the holographic element (6) is an in-line type and does not have the function of changing the traveling direction of the light flux. Therefore, the holographic element (6) has a lattice shape of two symmetrical A and B as shown in FIG. It becomes a pattern with lines.

The photodetector (8) is a four-divided detector similar to the conventional example, and is arranged immediately before the emitting direction of the semiconductor laser (1). A hole is provided in the center of the photodetector (8) so that the emitted light beam (2) can be transmitted.

As for the light flux (2) emitted from the semiconductor laser (1), an aberration-free light flux that has transmitted through the holographic element (6) in the 0th order is collected by the condenser lens (3) on the information track (5) on the information medium (4). It is illuminated by light. The reflected light flux (7) on which information is written or reproduced is received by the photodetector (8) without changing the traveling direction due to the astigmatism of the light that has passed through the holographic element (6) ± 1st-order diffracted light. It The reproduction output and the focus shift detection output method are the same as those in the conventional example, and the description thereof will be omitted. Holographic element (6)
The symmetry lines A and B (Fig. 8) form a 45 ° angle with the dividing lines (8A) and (8B) of the photodetector (8). In an apparatus using such an inline-type holographic element, a focused spot (first-order diffracted light) which is unnecessary light when the emitted light (2) from the semiconductor laser (1) passes through the holographic element (6) ( 11) is radiated onto the information track (5) on the information recording medium (4) coaxially with the originally required focused spot (10) of 0th-order light. Therefore, there is an advantage that the reflected light of the focal spot (11) does not become noise because it is a signal of the in-phase component even if mixed in the photodetector (8).

However, this example using an inline type holographic element (6) has the following problems.

Of the emitted light (2) of the semiconductor laser (1), the originally necessary light flux that passes through the holographic element (6) in the 0th order is aberration-free, and becomes a focused spot (10) of diffraction limit on the information track and is irradiated. It However, astigmatism occurs in the unnecessary light that is transmitted by the first-order diffraction. Therefore, there is a possibility that the unnecessary light that is transmitted by the first-order diffraction will be an elongated focused spot (11) on the information recording medium (4) which extends over several information tracks (5) as shown by the broken lines in FIG. is there.

At this time, since the focused spot (11) reads the information recorded on the adjacent track, the output signal of the photodetector (8) has a poor S / N as in the conventional example. .

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and the focused spot of unnecessary diffracted light on the information recording medium is essentially the 0th-order light focused spot without reading information. The object is to obtain an optical head device capable of reading information and obtaining a reproduction signal with a high S / N.

[Means for Solving Problems] Light emitted from a semiconductor laser, which is a light source, is condensed on a surface of an information recording medium by a condenser lens, and a reflected light beam from the information recording medium is subjected to astigmatism by a holographic element. In the optical head device for receiving the reflected light beam and reading the recording / reproducing information, the diffracted light when the light emitted from the semiconductor laser passes through the holographic element is the line of the information track of the information recording medium. This is a holographic element in the form of a lattice that forms an elliptical focused spot elongated in the direction.

[Operation] According to the optical head device of the present invention, of the light emitted from the semiconductor laser, the light flux diffracted and transmitted through the holographic element, that is, the light-converging spot of the light flux unnecessary for reading information on the information recording medium Are elongated spots in the line direction of the information track.

Therefore, the condensing spot of the unnecessary diffracted light does not have a resolution for reading the information written in the information track, and does not become a noise generation factor, and a signal with a high S / N can be obtained by the photodetector.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

The basic configuration is exactly the same as the conventional example, and the operation will be described with reference to FIG. Of the luminous flux emitted from the semiconductor laser (1), the luminous flux which is transmitted through the holographic element (6) in the 0th order is condensed without any aberration (shown by the solid line), and is condensed on the information track (5) as shown in FIG. The focused spot (10) is illuminated. The condensing spot (10), which is the diffraction limit, reads or records information, is reflected, and then enters the holographic element (6) again. The astigmatism is generated in the light of the reflected light flux which is first-order diffracted and transmitted through the holographic element (6), and the minimum circle of confusion is the light detection arranged between the semiconductor laser (1) and the holographic element (6). The light is received by the container (8).

Since the focus shift detector is the same as the conventional example, the description thereof is omitted.

On the other hand, among the light fluxes emitted from the semiconductor laser (1), the light flux (indicated by a broken line) that has passed through the holographic element (6) by the first-order diffraction is shown on the information recording surface (4) of the information track (5). The holographic element (6) is designed so that astigmatism is generated so that it becomes a focal line at the position (s'), which is an elongated focused spot (11) in the line direction.

Here, the astigmatic light beam is an aberration having focal lines orthogonal to each other at two points (s 'and t') as shown in the figure.

 This point will be described in more detail with reference to FIG.

FIG. 3 is an optical path diagram in which FIG. 1 is drawn in a plan view. Of the reflected light flux, the first-order diffracted light (7) given astigmatism by the holographic element (6) forms a focal line at the positions of s and t, and the photodetector is located at the middle circle of least confusion. (8)
Are arranged. The focal line position of (s) becomes an elongated focal line in the line direction (y) of the information track (5) formed on the information recording (4), and is formed on the light emitting surface of the semiconductor laser (1). The position (t) focal line is an elongated focal line in the direction perpendicular to the information track (X direction). That is, the holographic element (6) causes the astigmatic difference L in the first-order diffracted light (8).

The lattice shape of such a holographic element (6) is
When the direction of the information track (5) is the y direction, the lattice period is designed to be gradually shortened with A as a symmetry line as shown in FIG. Such a holographic element (6)
Is a kind of in-line holographic element because it does not change the optical axis.

Next, with reference to FIG. 3, it will be described that the optical head device according to the present embodiment does not cause S / N deterioration due to unnecessary diffracted light, which has been a problem in the conventional example. Here, the unnecessary diffracted light refers to the first-order diffracted light when the light flux (2) emitted from the semiconductor laser (1) passes through the holographic element (6) (indicated by a broken line in FIG. 1). This one
The second-order diffracted light has astigmatism instead of no aberration, and is equivalent to an astigmatic light flux having focal lines at positions (s) and (t).

Therefore, the first-order diffracted light condensed by the condenser lens (3) is elongated in the line direction (y direction) of the information track (5) on the information recording medium (4), and the condensed spot (11) shown in FIG. ) Is irradiated as an elliptic spot. This is an astigmatic light flux equivalent to the reflected light flux (7) because the light source position (s) and the light focus position (s ′) have a conjugate object image plane relationship via the condenser lens (3). The unnecessary diffracted light is
As a focal point in the line direction (y direction) of the information track at the position (s '), as a focal line in a direction (x direction) perpendicular to the line direction of the information track at a position (t') having a conjugate relationship with the position (t). This is because the image is formed. At this time, the ratio of the astigmatic difference l and L is almost equal to the longitudinal magnification of the condenser lens (3).

As described above, the unnecessary first-order diffracted light of the condensing optical system in this embodiment becomes the condensing spot (11) which is elongated in the line direction of the information track (5), and has the following advantages.

Since it is desired to increase the recording density on the information recording medium (4), small pits are recorded so that only the diffraction limited focal light spot (10) can read information. Therefore, the resolution in the line direction of the information track (5), which is the direction in which information is read, is about the diameter of the diffraction-limited converging spot (10). Since the unnecessary diffracted light is elongated in the line direction of the information track (6), the resolution in this direction is particularly low, and therefore information cannot be read. Further, since the spots are not illuminated on the adjacent tracks, the information cannot be read out. Therefore,
Information can be read only by the originally required 0th-order light condensing spot (10), and in FIG. 1, for example, the photodetector (8) does not need the reflection / anti-bundle from the information recording medium of the diffraction-proof. However, only the DC component is added, and the signal read by the 0th-order light does not become noise. Therefore, a high quality signal with a high S / N ratio can be obtained.

Although only the first-order diffracted light has been described as the unnecessary diffracted light in this embodiment, the same applies to the -1st-order diffracted light.
If the focal line at the position (t ') is generated only on the opposite side of the position (s'), the focal spot at the position (s') also becomes an elongated focused spot in the line direction of the information track (5), and the same effect as the above is obtained. Play.

Another advantage of this embodiment is the ease of making the holographic element (6). In other words, it is easy to mass-produce because it has only a substantially linear grid as shown in FIG. 4 and has a simple grid shape in which the grid spacing gradually changes.

In this embodiment, the optical head device using the inline holographic element was described, but the optical head device using the off-axis holographic element as in the above-mentioned conventional example is also shown in FIG. As shown, the holographic element (6) is designed so that the focused spot (11) of the unnecessary diffracted light (shown by the broken line in the figure) has a slender spot shape in the line direction (y direction) of the information track (5). All that is required is to achieve the same effect as above.

The purpose of the present invention is to prevent the condensing spot (11) of unnecessary diffracted light from reading the information on the information recording medium (4), and only to make the spot elongated in the line direction of the information track (5). Not only the resolution is low enough,
Even if the spot is sufficiently larger than the diffraction limit, the effect can be obtained.

[Effects of the Invention] As described above, according to the present invention, the focused spot of unnecessary diffracted light with which the surface of the information recording medium is irradiated is sufficiently larger than the diffraction limit, for example, a spot elongated in the line direction of the information track. Therefore, the unnecessary diffracted light cannot read information, and even if it is received by the photodetector, it does not become noise to the original signal due to the emitted light flux that has transmitted through the holographic element in the 0th order. This makes it possible to obtain a high-quality reproduced signal with a high S / N ratio.

[Brief description of drawings]

FIG. 1 is a block diagram of the optical head device of this embodiment, FIG. 2 is an explanatory view showing the shape of a focused spot, FIG. 3 is an optical path diagram in which the embodiment is drawn in plan, and FIG. 4 is an embodiment. FIG. 5 is an optical path diagram showing another application example of the present invention, FIG. 6 is a configuration diagram of a conventional example, and FIG. 7 is an apparatus using an inline holographic element. FIG. 8 is an explanatory diagram showing a grid shape of an inline holographic element, and FIG. 9 is an explanatory view showing a shape of a focused spot. (1) is a semiconductor laser, (3) is a condenser lens, (4) is an information recording medium, (5) is an information track, (6) is a holographic element, (8) is a photodetector, (10), (11) is a focused spot. The same reference numerals in the drawings indicate the same or corresponding parts.

Claims (2)

(57) [Claims] 1. A semiconductor laser as a light source, a condenser lens for condensing emitted light of the semiconductor laser on a surface of an information recording medium, and a semiconductor laser arranged between the semiconductor laser and the condenser lens. A holographic element that gives astigmatism to the reflected light flux reflected by the holographic element, and receives / records the reflected light flux.
In an optical head device having a photodetector for reading reproduction information and detecting a condensed state on the information medium, in the holographic element, the diffracted light when the emitted light from the semiconductor laser is transmitted is transmitted. An optical head device, which has a lattice shape so as to form an elongated elliptical focused spot in a line direction of an information track of the information recording medium. 2. The optical head device according to claim 1, wherein the holographic element is an in-line holographic element.