JPS60182526A - Optical information processor - Google Patents

Abstract

PURPOSE:To attain the high performance and the light weight for an optical information processor by using an in-line type grating lens made of a transparent material to an objective lens of an optical system at a pickup part. CONSTITUTION:An in-line type grating lens 71 is totally made of a transparent material, and the surface of the lens 71 opposite to an optical disk 8 is smoothed. While concentric diffraction grating patterns having sawtooth-shaped sections like a Fresnel lens are formed on the surface at the other side which is held by a barrel 72 and not exposed to open air. The pitches of the diffraction gratings are set precisely at unequal intervals in consideration of the thickness and diffractive index of the substrate part of the lens 71 and the cover glass 82 of a disk 8 and so that the light beams are focused on an information recording surface 81 with 4mm. focal distance set to a waveform of 780nm. Furthermore, the lens 71 is not equal to a mere grating lens since the lens function is provided between grooves due to the refraction caused by varying consecutively the thickness of said transparent material by a taper having a fixed curved surface.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an optical information processing device, and particularly to an optical information processing device, particularly for information reproduction and/or recording of so-called original discs such as optical video discs and digital audio discs. This invention relates to improvements in the optical system of the pickup head used.

[Technical background of the invention and its problems]

Optical discs typically record information using a minute array of bits. A pickup head that reads information from an optical disc is equipped with a light source such as a semiconductor laser and a photodetector such as a photodiode, and converts the light beam from the light source into a collimated beam through a collimator lens, which then uses an objective lens to record information on the optical disc. The information recording surface is converged and scanned, and the reflected light beam from the information recording surface is detected by a photodetector. The most important component in the optical system of this pickup head is an objective lens that focuses the light beam onto the information recording surface of the optical disk to a diameter of about 1 μm.

Conventionally, a compound lens, which is a combination of a plurality of single lenses, has been used as this objective lens. This is because in order to form a fine light beam spot, it is necessary to eliminate various lens aberrations such as spherical aberration, coma aberration, astigmatism, field curvature, and distortion as much as possible. However, even with a compound lens, it is impossible to completely eliminate lens aberration. In addition, high-performance compound lenses are difficult to mass-produce because they are difficult to polish and assemble and adjust, making them expensive.Additionally, the use of multiple glass lenses increases the weight of the pickup head. .

In order to solve such problems, it has been proposed to use a grating lens as an objective lens. A grating lens is a type of diffraction grating, for example, in which a concentric diffraction grating pattern is formed on a glass substrate, with irregularly spaced diffraction grating patterns whose pitch gradually narrows toward the periphery. Since it is an unequally spaced diffraction grating, the diffraction angles of each part are slightly different, so that by setting the grating pitch appropriately, it has a lens effect that converges the p-parallel light beam to one point. Since the lattice spacing of the grating lens must be on the order of the wavelength of the light beam used, for example, a resist is coated on a glass substrate and then turned by electron beam (EB) writing.

However, there are also problems with such a grating lens based on EB drawing. First, since only the diffraction phenomenon is utilized by alternately arranging opaque and transparent parts of the resist, the diffraction efficiency is as low as 20 to 30 inches. i2, -
When trying to converge the pointing light as a light beam spot next time, if the zero-order diffracted light illuminates the beam convergence point and its surroundings, it will adversely affect information reading. For this reason, an in-line type in which the next pointing light beam and the incident light beam are coaxial is inconvenient, and an off-axis type in which these axes are shifted is required. If an off-axis grating lens is used, it becomes difficult to align the optical axis of the optical system. Third, with the current EB lithography technology, the beam scanning width is about 2 mm, making it difficult to obtain a lens with the required diameter. Of course, by combining the scanning of the lens substrate side to be drawn, a lens of the required diameter can be obtained, but this method cannot produce a fine diffraction grating pattern with high precision to obtain a beam spot of 1 μm and 8 degrees. Additionally, grating lenses based on EB drawing lack mass productivity.

[Purpose of the invention]

In view of the above points, it is an object of the present invention to provide an optical information processing device having a high-performance, lightweight, and inexpensive pickup head.

[Summary of the invention]

The present invention uses an in-line grating lens made entirely of a transparent material as an objective lens in an optical system of a pickup unit that reproduces and/or records information on an optical disc or the like.

This grating lens has one smooth surface.
, which has a concentric non-uniformly spaced diffraction grating with a serrated cross section formed on the other surface, and is characterized by the fact that it mainly uses a lens effect due to diffraction phenomenon, but also uses a lens effect due to refraction. be. Further, this grating lens is mounted so that its smooth surface faces the optical disk.

〔Effect of the invention〕

According to the present invention, the pickup section can be made much lighter and cheaper than conventional devices that use a compound lens as an objective lens. Moreover, since the objective lens in the present invention is a grating lens, essentially lens aberration f
: can be eliminated and a fine light beam spot can be obtained.

In addition, in the present invention, unlike a grating lens using a resist pattern drawn by EB, the entire grating lens is made of a transparent material, and as a result of using a lens effect due to refraction as well as a lens effect due to diffraction, it converges isometrically. Efficiency of nearly 100% can be achieved. By using this grating lens as an in-line type, the optical axis of the optical system can be easily aligned.

Furthermore, the grating lens of the present invention can be mass-produced with high precision using the latest ultra-precision lathe machining using a diamond cutting tool with a tip shape of 100 to 200X, which cannot be obtained with EB drawing, or mass-produced using replica technology. can do.

Furthermore, in the present invention, one surface of the grating lens is made smooth and faces the optical disk.
Deterioration of objective lens performance due to dust adhering to the grating surface can be prevented.

[Embodiments of the invention]

The present invention will be described in detail below with reference to the drawings. FIG. 1 shows the optical configuration of a pickup section in an optical information reading device according to an embodiment. Reference numeral 1 denotes a semiconductor laser as a light source, which emits, for example, an original beam with an output of 3 mW and a wavelength of 780 nm. A light beam from a semiconductor laser 1 is split by a linear grating 2 into three beams: a tenth-order pointing light and a zero-order diffracted light. One beam is used for information reading υ, and the other two beams are used for tracking.

The diffracted light beam obtained from this grating 2 passes through a polarizing beam splitter 3 and is converted into a collimated light beam by a collimator lens 4. And this light beam is
It is bent at a right angle by a prism mirror 5, passes through a 1/4 wavelength plate 6, becomes circularly polarized light, enters an objective lens 7, and is irradiated onto the information recording surface of an optical disk 8 where pits are arranged as a non-pot beam. .

The reflected light beam containing information from the optical disk 8 returns through the same optical system. At this time, the plane of polarization is rotated by 90 degrees from the initial position by the quarter-wave plate 6 and enters the polarizing beam splitter 3, where it is separated and transmitted through a beam conversion system 9 that combines a concave lens, a cylindrical lens, etc. The signal is guided to a photodiode array 10 as a photodetector, and in addition to an image or audio signal, a focus error signal and a tracking error signal are obtained.

In the above configuration, the second objective lens 7 is
As shown in the enlarged cross-sectional view in the figure, an in-line grating lens 71 is used. The entire grating lens 7, TD-1 is formed of a transparent material such as transparent acrylic, and the surface facing the optical disk 8 is a smooth surface, and the other surface that is held by the cylinder body 72 and is not exposed to the outside air. , a concentric diffraction grating pattern with a sawtooth cross section similar to that of a Fresnel lens is formed. This diffraction grating pitch measure takes into consideration the thickness and refractive index of the substrate portion of the grating lens 71 and the cover glass 8□ of the optical disk 8, and the focal length is 4 for a wavelength of 780 nm.
The holes are precisely set at irregular intervals so that the light beam is focused on the information recording surface 81. The grating lens 71 is not just a grating, but rather than a combination of a light transmitting part and a blocking part as in a so-called normal grating, the thickness of the transparent material is made continuous between the grooves by the grooves having a certain curved surface. This means that it has a lens effect due to refraction. The shape of the serrated portion is precisely machined so that the focal length due to this refraction is also 4 points.

Such a grating lens 71 can be mass-produced using a mold produced by ultra-high precision lathe processing.

Next, the design criteria for the above grating lens 7□ will be explained in detail with reference to FIGS. 3 and 4. The two axes in FIG. 3 are optical axes, where the refractive index of the grating lens 71 is ns, the thickness is h8, and the cover glass 82 of the optical disk.
The refractive index is nc1, the thickness is h, and the working distance is d. The pitch of the unequal recessed diffraction grating, expressed as the distance on the X-axis in the figure, is determined as follows. First, on the Z-axis, the optical path length from the grating lens surface to point P on the information recording surface 81 of the optical disc is expressed as L(o)=d+nBb4+ncb(4--.-(1). The optical path length from a point at a distance of X from the center on the lens surface to the beam convergence point PA on the information recording surface is expressed as follows.The distance X is as follows.

Now, when the m-th grating position from the center of the grating lens surface is defined as Xm, the condition that the first-order diffracted light from this position overlaps the transmitted light that has gone straight from the center in the same phase at point P on the information recording surface is as follows: When the wavelength is λ, t(Xm) = t(o) + mλ ・・・・・・・・・
...It is expressed as (4). Therefore, by substituting equation (4)'k into equation (2), finding ψ□ when x = xm, and substituting this ψ□ into equation (3), xm is calculated. On the other hand, in order for the first-order diffracted light in the vicinity of this xm position to strengthen each other at point P, the grating pitch in this vicinity is determined by the following equation.

A specific numerical example will be given. The grating lens 71 is made of transparent acrylic, and 18=l, 5, hs=2.5
[rmn], the cover glass 8□ of the disk is nc=1
．． 55, hc = 1.2 [If n + ml and working distance is d23 [a] a, then the number of gratings N = 884C] and the radius of the outermost groove is XN =
It becomes 2.8 [m]. Further, the grating pitch may be approximately 300 [μm] near the center, gradually decreasing toward the periphery, and approximately 1.5 [μm] near the outermost periphery.

Next, the sawtooth cross-sectional shape of the grating surface will be explained with reference to FIG. Rare1 <'x ('If the height of the grating at xm is Th fm(X), then in Fig.
) sin θ −n4 sin θ, , , , , , , ,
(6) Self ψ = n, 5i11ψ8 ・・・・・・・・・
(7) ψ8 = θ-03 (8). Using these equations, fm(x) at which the refracted light beam converges on the above-mentioned point P on the information recording surface is:
It is the solution of the following differential equation (9) to 0◇.

However, the boundary conditions are x=xm and fm(X)=O.

By creating a sawtooth cross section with a curved surface based on the above equation, it has a lens effect due to refraction in addition to the diffraction effect, and has a focal length of 4M and a numerical aperture of approximately 0.000 nm for a light beam with a wavelength of 780 nm. 47, the effective convergence efficiency is about 10
A 0% grating lens is obtained.

Although the details of the objective lens 7 have been explained above, an in-line grating lens as shown in FIG. 5 can also be used for the collimator lens 4 in the optical system shown in FIG. In this case as well, it is assumed that the entire structure is made of a transparent material, and that an unevenly spaced diffraction grating i4 turns having a sawtooth cross section is formed on one surface. However, the diffraction grating pattern differs from that of the objective lens in that it has an elliptical shape, as is clear from FIG. This is because the light beam emitted from a normal striped semiconductor laser has a radiation angle in the vertical direction that is wider than the radiation angle in the direction parallel to the substrate, resulting in a beam with an elliptical cross section. This is to convert it into a parallel light beam. This grating lens as a collimator lens can also be formed by ultra-high precision lathe processing, similar to the objective bins.

As explained above, in this example, an in-line grating lens obtained by machining is used as an objective lens, the pickup part is lighter compared to a device using a conventional compound lens, and replica creation technology is used. It can be made cheaper. Moreover, since this grating lens essentially utilizes diffraction, it can almost eliminate lens aberrations, and also combines the lens action of refraction in addition to the diffraction phenomenon, effectively achieving a convergence efficiency of about 100.
This makes it possible to improve the performance of the pickup section.

Furthermore, since it can be obtained by machining, there is no need for polishing or assembly processes, so it is excellent in mass production. In addition, one smooth surface of the grating lens faces the optical disk,
By placing the grating surface inside the optical barrel,
It is possible to prevent deterioration of lens performance due to the adhesion of dirt and dust.

In the embodiment, only information reproduction has been described, but the present invention can of course be applied to an apparatus that optically records information using the same optical system. Further, the apparatus of the present invention can be applied not only to video discs and audio discs but also to information processing of optical memories based on the same principle.

Further, the smooth surface of the grating lens does not have to be a flat surface, but may be a spherical surface with a gentle constant curvature, for example, as shown in FIG. Further, in the embodiment, the grating surface having a serrated cross section is a curved surface similar to that of a Fresnel lens, but even if it is approximated by a flat surface as shown in Fig. 7, a certain level of performance can be obtained.

The present invention is also effective when an optical system is used in which a normal beam splitter is used in place of the polarizing beam splitter and the 174 wavelength plate is omitted.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the optical configuration of a pickup section in an optical information reading device according to an embodiment of the present invention, FIG. 2 is an enlarged sectional view of the objective lens section, and FIGS. 3 and 4 are objective lenses. FIG. 5 is a diagram showing a configuration example of the collimator lens of the pickup section, and FIGS. 6 and 7 are diagrams illustrating the grating lens used in the objective lens and/or collimator lens. It is a figure showing a modification of . DESCRIPTION OF SYMBOLS 1... Semiconductor radar (light source), 2... Grating (equally spaced straight lines), 3... Polarizing beam sinter, 4... Collimator lens, 5... Prism mirror, 6... 1/4 wavelength plate, 7...Objective lens, 7□・
... In-line grating lens, 72 ... Cylindrical body, 8 ... Optical disk, 8□ ... Information recording surface, 8□
...Cover glass, 9...Beam conversion system, 10...
・Photodiode array (photodetector). Applicant's agent Patent attorney Takehiko Suzue Figure 1 Figure 2 Figure 1

Claims (4)

[Claims] (1) A light source that sends out a light beam for recording and a light beam from this light source that is used to reproduce and/or reproduce information on a recording medium on which information read by light beam scanning is recorded. An optical information system comprising an optical system including a collimator lens and an objective lens for converging the information on the information recording surface, and a yC detector for detecting a light beam containing information obtained from the information recording surface. In the processing device f1, as the objective lens of the optical system No. 111, the surface facing the recording medium is a smooth surface, and a concentric irregularly spaced diffraction grating pattern with a sawtooth cross section is formed on the other surface. NoL is an optical information processing device characterized by using an inline grating lens made entirely of transparent material. (2) Claim 1 in which an in-line grating lens is used as the collimator lens of the optical system.
The optical information processing device described in Section 1. (3) In addition to the collimator lens and the objective lens, the optical system includes a polarizing beam splitter between the collimator lens and the light source, and a polarizing beam splitter between the collimator lens and the objective lens.
The optical system according to claim 1, comprising a four-wavelength plate, wherein a reflected light beam containing information from the information recording surface of the recording medium is separated by the polarizing beam splitter and guided to the photodetector. Expression information processing device. (4) The optical system has a beam splitter between the collimator lens and the light source, and the reflected light beam containing information from the information recording surface of the recording medium is separated by the beam splitter to An optical information processing device according to claim 1, which is guided by a photodetector.