JP3044667B2 - Optical reader - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for reading information from an information medium such as a compact disk, a magneto-optical disk, a phase change disk, and a bar code, and The present invention relates to an optical spot forming apparatus capable of reading information and reducing the numerical aperture of an objective lens, and an optical reading apparatus using the same.

[Conventional technology]

FIG. 8 shows the configuration of a conventional optical reader for an optical disk.

Laser light, which is detection light emitted from the semiconductor laser 1, is converted into a parallel light beam by the collimating lens 2. This parallel light beam is sent to the objective lens 6 by the beam splitter 4 and is focused on the recording surface of the disk 7 by the objective lens 6. The return light reflected from the recording surface of the disk 7 is
The light is split by the beam splitter 4 in a right angle direction, collected by the light receiving lens 8, and detected by the pin photodiode 10. The modulation of light by the information pits P formed on the recording surface of the disk 7 is detected by the pin photodiode 10, whereby the signal is reproduced.

[Problems to be solved by the invention]

In this type of reader, the cutoff frequency of the optical system
f _c (Maximum spatial frequency that can be read by the optical system)
Is determined by f _c = 2 × NA / λ (1) NA is the numerical aperture of the objective lens 6, and λ is the wavelength of the detection light. The output wavelength λ of the current semiconductor laser is 0.67 to 0.
Since it is 83 μm, in order to read data having a length of about 1 μm, it is necessary to use a large objective lens 6 having a numerical aperture NA of about 0.4 to 0.6.

In the optical reader, a method for reading higher-density data is as follows:
It is necessary to use an objective lens having a short numerical aperture or an objective lens having a large numerical aperture NA. However,
First, it is difficult to efficiently use light having a wavelength shorter than the output wavelength λ of the current semiconductor laser. Although the numerical aperture of the objective lens is NA, if an objective lens having a numerical aperture NA larger than that of the current optical pickup is used, performance degradation due to disturbance such as surface deflection of the disk 7 becomes remarkable. The numerical aperture NA of the objective lens 6 is determined as follows: NA = a / F (2) where F is the focal length and a is the radius of the light beam. In addition, since the objective lens 6 moves in the optical axis direction by, for example, about ± 0.5 mm by the focus correction operation, the focal length F needs to be about 3 mm in order to prevent the objective lens 6 from hitting the disk 7 by the correction operation. is there. Therefore, in order to obtain a large numerical aperture NA, it is necessary to use a light beam having a large diameter. The use of a large-diameter light beam increases the size of the optical device in the reader and makes the reader larger.

The present invention has been made to solve the above-mentioned conventional problems, and does not shorten the wavelength of detection light to be used and does not increase the numerical aperture of an objective lens. It is an object of the present invention to provide a light spot forming apparatus which enables the above and an optical reader using the same.

[Means for solving the problem]

The light spot forming apparatus according to the present invention includes a light emitting element, an optical system that separates light emitted from the light emitting element into two light beams and provides a phase difference of 180 degrees between the two light beams, such that the two light beams overlap each other. Light condensing means for converging light to form two small diameter spots due to light interference.

For example, the optical system includes a separating unit that separates the light from the light emitting element into two light beams, and a polarizing unit that provides a phase difference of 180 degrees to the two separated light beams. The one formed by a diffraction grating having an inter-grating pitch that determines the pitch of the spots so that the spots overlap when the two luminous fluxes are collected, and a grating depth that gives a phase difference of 180 degrees between the two luminous fluxes. Become.

Further, an optical reader according to the present invention includes a light spot forming apparatus according to any one of the above, a recording medium irradiated with a small diameter spot formed by the light spot forming apparatus, and a recording medium having the small diameter. And a light receiving element for detecting at least one of the return light from the spot.

In this case, the light emitted from the light emitting element is
A main beam including two light beams different from each other by 0 degrees, and a separating unit for separating the main beam into two sub beams are provided. In the light receiving element, return light of the main beam from a recording medium is provided.
It is possible that both the return light of the sub-beam from the recording medium is detected. An optical system that forms two minute spots generated by the interference into two interfering light beams on a recording medium, and a light receiving element that detects at least one of the return light of the two minute spots from the recording medium includes: It is characterized by being provided.

Further, in the above means, a diffraction grating is used as an optical element for dividing the detection light emitted from the light emitting element into two light beams that interfere with each other.

Further, in the above means, the diffraction grating is formed with a grating groove having a pitch and a depth for dividing the detection light into two light beams that interfere with each other, and a grating for dividing the detection light emitted from the light emitting element into three beams. Is what it is.

[Action]

The light spot forming apparatus of the present invention converts the detection light into two light beams that interfere with each other. The interference of the two light beams makes it possible to form two very small spots on the recording medium. By detecting one of the returning light of the minute spot from the recording medium, high-density data can be read. High-density data can also be read by processing such as reading the return light from the two minute spots together and correcting the read phase difference.

Preferably, since a diffraction grating is used as an element for dividing the detection light into two light beams, the optical system can be manufactured at low cost.

In addition, the diffraction grating enables the detection light to be separated into two light beams and the formation of a three-beam spot, so that a high-density reading device can be configured without increasing the number of components of a conventional optical device such as a compact disk player. become able to.

〔Example〕

 Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a side view showing an optical element configuration of an optical reader according to a first embodiment of the present invention.

Reference numeral 1 denotes a semiconductor laser, 2 denotes a collimating lens, 4 denotes a beam splitter, 6 denotes an objective lens, and 7 denotes a disk as a recording medium. In the present embodiment, the Wollaston prism 3 is used as an element for separating the laser light output from the semiconductor laser 1 into two light beams. Further, a polarizer 5 for obtaining an interference component of two light beams is provided in front of the beam splitter 4. A light receiving lens 8 is provided in the direction of reflection by the beam splitter 4.
The return light from the disk 7 is focused by the light receiving lens 8. This converged light is condensed to one light beam by a slit 9a formed in the slit plate 9, and a pin photodiode
Received by 10

 Next, a reading operation according to the above embodiment will be described.

Laser light emitted from the semiconductor laser 1 is converted into a parallel light beam by the collimator lens 2. The light is further divided into ordinary light and extraordinary light by the Wollaston prism 3. The ordinary light and the extraordinary light are output from the Wollaston prism 3 as two light beams at a slight angle. The two light beams are converged by the objective lens 6 and formed on the recording surface of the disk 7 such that two spots overlap each other. However, the ordinary light and the extraordinary light do not interfere with each other because they have polarization planes orthogonal to each other. The polarizer 5 is configured to transmit only a 45-degree light component with respect to each polarization plane of the two light beams. The two light beams pass through the polarizer 5 and are 180 degrees apart from each other.
Light having a phase difference of FIG. 7 shows the light intensity of the spot, in which the horizontal axis shows the distance and the vertical axis shows the light intensity. In this figure, S ₁ and S ₂ denotes a minute spot by the interference of two light beams. The spot overlaps with two light beams having a phase difference of 180 deg, the dark portion A occurs where the light intensity becomes zero at the center portion by the interference of each other, thus very small as indicated by said S ₁ and S ₂ on both sides System spots are formed in close proximity.

Return light from two light fluxes formed on the disk 7 is reflected by the beam splitter 4 and collected by the light receiving lens 8, and the collected return light is converted into two small spots S ₁ formed on the disk. Similarly the spot diameter and S ₂ is what is suppressed by interference with. Therefore, the same light modulation component as that obtained by diffracting each of the small-diameter spots S ₁ and S ₂ by the pits P of the disk 7 can be extracted. In the embodiment of FIG. 1, by using the slit 9a, only the focused light corresponding to one minute spot is received by the pin photodiode 10, and the focused light corresponding to the other minute spot is blocked by the slit plate 9. You.

Here, using the same numerical aperture of the objective lens and the detection light of the same wavelength, alone light and the diameter of the spot S ₀ formed by bundles, two light spot diameter of the minute spot S ₁ or S ₂ formed by the interference of beams Comparing with the above, it is sufficiently possible to make the latter spot diameter about 75% larger than the former spot diameter. In this case, the maximum reading frequency can be increased 1.3 times. Therefore, when an objective lens having the same numerical aperture as the detection light having the same wavelength as the conventional one is used, it is possible to read data at a higher density than the conventional one by using the two-beam interference.

 FIG. 2 shows a second embodiment of the present invention.

In this embodiment, the basic structure of the optical system is the same as that of the embodiment shown in FIG. 1, but the slit plate 9 shown in FIG. The light is received by each of the two light receiving portions 11a and 11b of the photodiode 11. The same pit P in two and minute spots S ₁ and S ₂ wherein
When a signal is read, a phase difference of the signal corresponding to the spot pitch is generated. By compensating the phase difference of the signal converted into an electric signal in each of the light receiving units 11a and 11b on a circuit, a signal waveform is obtained. Can be compensated, and highly accurate detection becomes possible.

More developmental, since the two micro spots S ₁ and S ₂ scans the same recording unit writes the information by one of the spot in an optical memory device, it is also possible to perform the read-after-write the other spots.

 FIG. 3 shows a third embodiment of the present invention.

In this embodiment, the diffraction grating 12 is used as means for forming two light beams having a phase difference of 180 degrees. In this case, the pitch δ of the spot formed when each of the two light beams is converged on the disk is determined by the pitch δ ₁ between the gratings of the diffraction grating 12. This interstitial pitch δ ₁ is 600
About 700 μm is appropriate. Since it is necessary to form a minute spot S ₁ and S ₂ due to interference in the 180deg together two light fluxes of the phase, the phase difference between the two light beams is determined by the grating depth d, the grating depth By setting d, two light beams having a phase difference of 180 degrees can be formed.

Also in this embodiment, since the two beams having a phase difference of 180deg is superimposed on the recording surface of the disk 7, so that the spot S ₁ and S ₂ of the small diameter adjacent as shown in FIG. 7 is formed . In this embodiment, since there is no need to use the expensive Wollaston prism 3 as shown in the above two embodiments, a low-cost optical device is obtained.

 FIG. 4 shows a fourth embodiment of the present invention.

In this embodiment, a reader such as a compact disk or a laser disk is constructed by applying the third embodiment shown in FIG.

Reference numeral 15 shown in FIG. 4 is a diffraction grating. This diffraction grating
Reference numeral 15 designates a laser beam emitted from the semiconductor laser 1 which is divided into two light beams having a phase difference of 180 deg, and can form three beams. 5 and 6 show the above diffraction grating
15 structures are shown for each example. In the diffraction grating 15 shown in FIG. 5, a grating groove 15a for splitting two light beams is formed on the left surface of the drawing. Like the diffraction grating 12 shown in FIG. 3, the pitch [delta] ₁ of the grating grooves 15a is about 600～700Myuemu, also the phase difference of 180deg by the grating depth d is set. On the right surface of the diffraction grating 15, a grating groove 15b for forming three beams is formed. Pitch [delta] _a of the grating grooves 15b is about 30 to 40 .mu.m. In the diffraction grating 15 shown in FIG. 6, the grating grooves 15a and 3
The structure is such that the beam grating groove 15b is overlapped.

The laser beam from the semiconductor laser 1 passes through the grating groove 15a of the diffraction grating 15 to form a main beam separated into two light beams having a phase difference of 180 deg. Small spots S ₁ and S ₂
Is formed. Further, two sub-beams sandwiching the main beam are formed by transmitting through the grating groove 15b, thereby forming two sub-spots on the disk 7.
The minute spot S ₁ and S ₂ which is formed by the main beam
The return light diffracted by the pit is reflected by the beam splitter 16
Through the concave lens 17 and received by the pin photodiode 18. In the pin photodiode 18, the return light that is based on two small spots S ₁ and S ₂ are both received. Then, the phase difference of the signal due to the light reception is corrected, and the recording signal of the disk is read. The return light from the two sub-spots is also received by the pin photodiode 18, and a tracking error signal is obtained from the phase difference between the EF outputs.

When the interference of two light beams is used in the above-described reading apparatus, when the same objective lens 6 is used as in the related art, it is possible to form a smaller spot on the disk than in the conventional apparatus. Conversely, if an attempt is made to form a spot having the same diameter as that of a conventional apparatus on a disk using two-beam interference, the numerical aperture NA of the objective lens 6 can be smaller than in the conventional case. The NA of the objective lens used in conventional readers is about 0.45 for compact discs and about 0.4 for laser discs.
It was about 5 to 0.53. On the other hand, if the same spot diameter is formed by using two-beam interference,
Can be reduced to about 0.34 for compact discs and about 0.38 to 0.40 for laser discs.

Here, the depth of focus of the optical reading device for an optical disk is obtained by λ / (NA) ² , the tolerance for the skew of the disk is proportional to λ / (NA) ³ , and the tolerance for unevenness in the thickness of the disk is λ. / (NA) is proportional to ⁴ . As described above, when two-beam interference is used, the NA can be reduced to about 75% of the conventional value. Therefore, the depth of focus is about 1.8 times the conventional value, the skew tolerance is about 2.4 times the conventional value, and the thickness tolerance is about 3.2 times the conventional value.

For example, the allowable value for the skew of a disk is ± 1 ° in the case of a compact disk, but is ± 2.4 ° in the case of two-beam interference. By increasing the tolerance as described above, the tilt adjustment of the optical axis of the objective lens and the tilt adjustment of the disk, which are required in the conventional compact disk device and laser disk device, are almost unnecessary.

The present invention is not limited to the case where the recording medium is an optical disk, but can be used for reading a bar code, for example.

〔effect〕

As described above, the light spot forming apparatus of the present invention can irradiate a target with a spot smaller than the spot diameter determined by the numerical aperture of the lens and the wavelength of light. Therefore, in the optical reader using this light spot forming apparatus, high-density data can be read, and it is not necessary to increase the numerical aperture of the objective lens. Conversely, even if the numerical aperture of the objective lens is reduced, the same reading accuracy as the conventional one can be maintained, and downsizing and simplification of assembly adjustment can be achieved.

Preferably, a diffraction grating is used as an element for obtaining two-beam interference, so that it can be manufactured at low cost.

Further, since two-beam interference and three beams can be set by a common diffraction grating, a highly accurate disc player or the like can be manufactured with a minimum number of parts.

[Brief description of the drawings]

FIG. 1 is a side view showing a configuration of an optical reader according to a first embodiment of the present invention, FIG. 2 is a side view showing a configuration of an optical reader according to a second embodiment of the present invention, and FIG. FIG. 4 is a side view showing a configuration of an optical reader according to a third embodiment of the present invention. FIG. 4 is a side view showing a configuration of an optical disk pickup as a specific example of the optical reader of the present invention.
FIG. 6 and FIG. 6 are side views showing the structure of the diffraction grating for each embodiment.
FIG. 7 is a diagram showing the difference in beam spot diameter between a single light beam and two-beam interference, and FIG. 8 is a side view showing the structure of a conventional optical reader. 1 ... Semiconductor laser, 2 ... Collimate lens, 3 ...
Wollaston prism, 4 …… Beam splitter, 5…
... Polarizer, 6 ... Objective lens, 7 ... Disc, 8 ...
Receiving lens, 9 ... Slit plate, 10 ... Pin photodiode, 11 ... Binded pin photodiode, 12,15 ... Diffraction grating.

Claims (5)

(57) [Claims] 1. A light emitting device, an optical system for separating light emitted from the light emitting device into two light beams and providing a phase difference of 180 degrees between the two light beams, and condensing the two light beams so as to overlap each other. A light spot forming apparatus provided with a light condensing means for forming two small diameter spots due to light interference. 2. The optical system according to claim 1, wherein the optical system includes: a separating unit that separates the light from the light emitting element into two light beams; and a polarizing unit that provides a phase difference of 180 degrees to the two separated light beams. Light spot forming device. 3. The optical system has an inter-grating pitch that determines the pitch of the spots so that the spots overlap when the two light fluxes are collected, and a grating depth that provides a phase difference of 180 degrees between the two light fluxes. 2. The light spot forming device according to claim 1, wherein the light spot forming device is formed by a diffraction grating. 4. A light spot forming apparatus according to claim 1, a recording medium irradiated with a minute diameter spot formed by the light spot forming apparatus, and a light spot formed on the recording medium. And a light-receiving element for detecting at least one of the return lights of the above. 5. The light emitted from the light emitting element has a phase of 180.
Separating means for separating a main beam including two light beams having different degrees and two sub-beams is provided, and the light receiving element includes a main beam returning light from the recording medium and a sub-beam returning light from the recording medium. 5. The optical reader according to claim 4, wherein both are detected.