JPH0721873B2 - Optical information processing device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical information processing apparatus, and more particularly to a pickup head used for reproducing and / or recording information on so-called optical disks such as optical video disks and digital audio disks. The improvement of the optical system of the part.

[Technical background of the invention and its problems]

Information is usually recorded on an optical disc by an array of fine pits. 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. The light beam from the light source is converted into a parallel beam by a collimator lens and focused on the information recording surface of the optical disc by an objective lens. Then, the light beam reflected by 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 for narrowing the light beam to a diameter of about 1 μm on the information recording surface of the optical disc.

Conventionally, a compound lens in which a plurality of single lenses are combined has been used as this objective lens. This is because it is necessary to minimize various lens aberrations such as spherical aberration, coma aberration, astigmatism, field curvature, and distortion in order to form a fine light beam spot. However, even with a compound lens, it is impossible to completely eliminate the lens aberration. Further, a high-performance compound lens has drawbacks that it is difficult to mass-produce because it is difficult to polish and assemble and adjust it, and therefore it is expensive, and the weight of a pickup head becomes large because a plurality of glass lenses are used.

In order to solve such a problem, it has been proposed to use a grating lens as the objective lens. The grating lens is a kind of diffraction grating, and is, for example, a concentric circular grating pattern formed on a glass substrate and having an unequal interval diffraction grating pattern whose pitch gradually narrows toward the periphery. Since the diffraction gratings are unequal intervals, the diffraction angles of the respective parts are slightly different, and as a result, it has a lens effect of converging the parallel light beam into one point by appropriately setting the grating pitch. Since the grating interval of the grating lens needs to be in the wavelength order of the light beam to be used, for example, a resist is applied on a glass substrate and patterned by electron beam (EB) drawing.

However, there is also a problem in such a grating lens by EB drawing. First, the diffraction efficiency is as low as 20 to 30% because the non-transmissive portions and the transmissive portions of the resist are alternately arranged and only the diffraction phenomenon is used, and the lattice spacing is on the order of wavelength. Secondly, when trying to converge the first-order diffracted light as a light beam spot, if the zero-order diffracted light illuminates the beam convergence point and its periphery, it adversely affects information reading. For this reason, the in-line type in which the first-order diffracted light beam and the incident light beam have a coaxial relationship is inconvenient, and the off-axis type in which these axes are offset must be used.
When the off-axis type grating lens is used, it becomes difficult to align the optical axis of the optical system. Third, the current EB
With the drawing technology, the beam scanning width is about 2 mm, and it is difficult to obtain a lens with the required diameter. Of course, if the scanning on the lens substrate side to be drawn is combined, a lens having a necessary diameter can be obtained, but with this, a fine diffraction grating pattern for obtaining a beam spot of about 1 μm cannot be formed with high precision. In addition, the EB drawing grating lens lacks mass productivity.

[Object of the Invention]

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

[Outline of Invention]

The present invention uses an in-line type grating lens which is entirely made of a transparent material such as glass, plastic, or a composite of both as an objective lens in an optical system of a pickup unit for reproducing and / or recording information on an optical disk or the like. In this grating lens, one surface is a smooth surface such as a flat surface or a convex surface, and the other surface is formed with concentric circular unequal-interval diffraction grating patterns having a sawtooth-shaped cross section. Mainly,
The feature is that the lens action by refraction is also used together.
The grating lens is mounted so that its smooth surface faces the optical disk.

〔The invention's effect〕

According to the present invention, the pickup unit can be made much lighter and cheaper than the conventional device using the compound lens as the objective lens. Moreover, since the objective lens in the present invention is a grating lens, lens aberration can be essentially eliminated, and a fine light beam spot can be obtained.

Further, in the present invention, unlike the grating lens using the resist pattern by EB drawing, the entire grating lens is made of a transparent material and in addition to the lens function by diffraction,
As a result of using the lens action by refraction together, it is possible to equivalently achieve a convergence efficiency of almost 100%. Since this grating lens is an in-line type, the optical axis of the optical system can be easily aligned.

Furthermore, the grating lens in the present invention is manufactured by a recent ultra-precision lathe process using a diamond bite having a tip shape of 100 to 200Å, and mass-produces a large diameter lens which cannot be obtained by EB drawing with high precision, injection molding, compression. It can be mass-produced by replica technology such as photopolymer.

Further, in the present invention, one surface of the grating lens is a smooth surface such as a flat surface or a convex surface and is opposed to the optical disk, so that deterioration of the objective lens performance due to adhesion of dust or the like to the grating surface can be prevented.

Further, since the in-line type grating lens is arranged with the smooth surface positioned on the optical disk side and the surface on which the unequal-interval diffraction grating pattern is formed positioned on the light source side, for example, parallel light coming from the light source side, The light beam is diffracted by the diffraction grating to change its direction, and then refracted on the smooth surface. Therefore, the degree of refraction of light can be increased and the focal length can be shortened as compared with the case where the in-line type grating lens is arranged in a relationship opposite to the above relationship. That is, the required focal length can be obtained with a small objective lens. In this way, the objective lens can be miniaturized, so that the device can be miniaturized, and as a result, high-speed driving can be realized and the access time can be shortened.

Further, since the in-line type grating lens is arranged in the above relationship, even if a part of the light beam coming from the light source side is reflected by the surface of the diffraction grating, the surface of the diffraction grating is inclined with respect to the optical axis. Therefore, the reflected light beam does not follow the reverse optical path and directly enter the photodetector.
Therefore, it is possible to prevent the occurrence of beam interference that tends to occur when the light beam reflected by the objective lens and the light beam reflected from the information recording surface are incident on the photodetector.

Example of Invention

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an optical configuration of a pickup section in an optical information reading apparatus according to an embodiment. Reference numeral 1 denotes a semiconductor laser as a light source, which emits a light beam having an output of 3 mW and a wavelength of 780 nm, for example. The light beam from the semiconductor laser 1 is divided into three beams of ± first-order diffracted light and zero-order diffracted light by the linear grating 2. One beam is for reading information,
The other two are used for tracking. The diffracted light beam obtained from the grating 2 passes through the polarizing beam splitter 3 and is converted into a parallel light beam by the collimator lens 4. Then, this light beam is bent at a right angle by the prism mirror 5, passes through the 1/4 wavelength plate 6, becomes circularly polarized light, and is incident on the objective lens 7, and the information recording surface on which the pits of the optical disk 8 are arranged as beam spots. Is irradiated.

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 90 degrees from the beginning by the 1/4 wavelength plate 6 and enters the polarizing beam splitter 3,
It is separated here and is guided to a photodiode array 10 as a photodetector through a beam conversion system 9 in which a concave lens, a cylindrical lens and the like are combined, and an RF signal such as an image, a sound signal or a code / data signal, as well as a focus error. A signal and a tracking error signal are obtained.

In the above-mentioned configuration, the objective lens 7 has the second
To as an enlarged cross-sectional view diagram, using an in-line type grating lens _71. The grating lens ₇₁ is made of a transparent material such that the whole is adhered to a plastic film grating transparent acrylic or single spherical glass, the surface facing the optical disc 8 is a smooth surface such as a plane or spherical, cylindrical body 7 _A concentric diffraction grating pattern having a sawtooth-shaped cross section similar to that of the Fresnel lens is formed on the other surface which is held by ₂ and is not exposed to the outside air. This diffraction grating pitch has a focal length of 4 mm for a wavelength of 780 nm, considering the thickness and refractive index of the substrate portion of the grating lens 7 ₁ and the cover glass 8 ₂ of the optical disc 8.
Light beam to be converged on the information recording surface _81, is precisely set at unequal intervals as. This grating lens
The point that ₁ is not a simple grating is not a combination of a light transmitting part and a blocking part as in a so-called normal grating, but the thickness of the transparent material is continuously changed by a taper with a constant curved surface between the grooves. That is, the lens function is provided by refraction. The shape of the serrated portion is precisely processed so that the focal length due to this refraction is also 4 mm.

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

Next, the above-mentioned design criteria for the grating lens 7 ₁
A detailed description will be given with reference to FIGS. The Z axis in FIG. 3 is the optical axis, and the refractive index of the grating lens 7 ₁ is n _S ,
The thickness h _S, the refractive index of the cover glass ₈₂ of the optical disk n _C,
The thickness is h _C and the working distance is d. The pitch of the non-equidistant diffraction grating represented by the distance on the x-axis in the figure is obtained as follows. First, on the Z-axis, the optical path length from the grating lens surface to the point P on the information recording surface ₈₁ of the optical disc is represented by l (o) = d + n S h S + n C h C ...... (1) . On the other hand, the optical path length from the point of distance x from the center on the grating lens surface to the point P of beam convergence on the information recording surface is Is represented. The distance x is Is.

Now, assuming that the m-th grating position from the center of the grating lens surface is xm, the condition that the first-order diffracted light from this position overlaps the transmitted light traveling straight through the center and the point P on the information recording surface in phase is used. Letting the wavelength be λ, it is expressed as follows: l (xm) = 1 (o) + mλ (4) Then, substituting equation (4) into equation (2), x
X is obtained by finding m when == xm and substituting this m into the equation (3). On the other hand, in order for the first-order diffracted lights in the vicinity of the xm position to reinforce each other at the point P, the grating pitch p in this vicinity can be obtained by psinm = λ (5).

Here are specific numerical examples. The grating lens 7 ₁ is transparent acrylic, n _S = 1.5, h _S = 2.5 [mm], the cover glass 8 _{2 of the} disk is n _C = 1.55, h _C = 1.2 [mm],
If the working distance is d = 3 [mm], the number of gratings is N = 884 [lines] and the radius of the outermost groove is x _N =
It will be 2.8 [mm]. The lattice pitch is 300 near the center.
It is about [μm], and becomes gradually smaller toward the periphery, and it may be about 1.5 [μm] near the outermost periphery.

Next, the sawtooth cross-sectional shape of the grating surface will be described with reference to FIG. Assuming that the height of the grating at xm _-1 <x <xm is fm (x), in Fig. 4, f '(x) = -tan θ ... (5) sin θ = n _S sin θ _S ... (6) sin = n _S sin _S (7) _S = θ−θ _S (8) Using these equations, fm (x) for the refracted light beam to converge on point P on the information recording surface is obtained as the solution of the differential equations (9) to (11) below.

However, the boundary conditions are x = xm and fm (x) = 0.

By having a sawtooth cross section with a curved surface obtained from the above formula, in addition to the diffraction effect, it also has a lens effect by refraction, the focal length is 4 mm for a light beam with a wavelength of 780 nm,
With a numerical aperture of about 0.47, a grating lens with a focusing efficiency of 100% can be obtained effectively.

Although the details of the objective lens 7 have been described above, an in-line type grating lens as shown in FIG. 5 can be used as the collimator lens 4 in the optical system of FIG. Also in this case, it is assumed that the whole is made of a transparent material, and one surface is formed with a non-equidistant diffraction grating pattern having a sawtooth cross section. However, the diffraction grating pattern
As is apparent from FIG. 5, it is different from the case of the objective lens in that it has an elliptical shape. This is because the light beam emitted from a normal semiconductor laser with a stripe structure has a larger emission angle in the vertical direction than the emission angle in the direction parallel to the substrate and is an elliptical beam with a uniform diameter. This is for converting into a parallel light beam. The grating lens as the collimator lens can also be formed by ultra-high precision lathe processing similarly to the objective lens.

As described above, in the present embodiment, the in-line type grating lens obtained by machining is used as the objective lens, the pickup unit is lighter than the device using the conventional compound lens, and the cost is low due to the replica making technique. Can be Moreover, since this grating lens essentially uses diffraction, lens aberration can be almost eliminated, and in addition to the diffraction phenomenon, the lens function by refraction is also used, and the focusing efficiency is approximately 100%.
It is possible to improve the performance of the pickup section. In addition, since it is obtained by machining, it does not require a polishing process or an assembling process, so it is excellent in mass productivity. Further, since one convex surface or flat surface of the grating lens is opposed to the optical disk, and the objective lens 7 is provided with the grating surface positioned on the side of the light source that can be housed in the optical lens barrel, dust or dust adheres. It is possible to prevent the deterioration of the lens performance due to. Further, since the objective lens 7 is arranged in the above-described relationship, the parallel light beam coming from the light source side is diffracted by the diffraction grating to change its direction, and then refracted by the smooth surface. Therefore, the degree of refraction of light can be increased and the focal length can be shortened as compared with the case where the objective lens is arranged in a relationship opposite to the above relationship. That is, the required focal length can be realized with a small objective lens. in this way,
Since the objective lens 7 can be downsized, the device can be downsized, and as a result, high-speed driving can be realized and the access time can be shortened.

Furthermore, since the objective lens 7 is arranged in the above relationship, even if a part of the light beam coming from the light source side is reflected by the surface of the diffraction grating, the surface of the diffraction grating is inclined with respect to the optical axis. Therefore, the reflected light beam does not follow the reverse optical path and directly enter the photodetector 10. Therefore,
Can be a reflected light beam from the light beam and the information recording surface ₈₁ is reflected by the objective lens 7 is also prevented occurrence of possible easily beam interference when incident on the light detector 10.

In the embodiments, 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. The device of the present invention can be applied not only to video discs and audio discs, but also to information processing of optical memories according to the same principle.

Further, the smooth surface of the grating lens does not have to be a flat surface, and may be, for example, a spherical surface having a gentle constant curvature as shown in FIG. Further, in the embodiment, the surface where the grating surface has a saw-tooth cross section is the same curved surface as in the Fresnel lens, but a certain performance can be obtained even if it is approximated by a straight surface as shown in FIG.

The present invention is also effective when an ordinary beam splitter is used as the optical system instead of the polarizing beam splitter and a quarter wave plate is omitted.

[Brief description of drawings]

FIG. 1 is a diagram showing an optical configuration of a pickup section in an optical information reading apparatus according to an embodiment of the present invention, FIG. 2 is an enlarged sectional view of an objective lens section thereof, and FIGS. 3 and 4 are objective lenses. For explaining the design criteria of the grating lens as an example, FIG. 5 is a diagram showing a configuration example of the collimator lens of the pickup section, and FIGS. 6 and 7 are grating lenses used for the objective lens and / or the collimator lens. It is a figure which shows the modification of. 1 ... Semiconductor laser (light source), 2 ... Grating (equal-spaced linear shape), 3 ... Polarizing beam splitter, 4
…… Collimator lens, 5 …… Prism mirror, 6 ……
1/4 wavelength plate, 7 ... Objective lens, 7 ₁ ... In-line type grating lens, 7 ₂ ... Cylindrical body, 8 ... Optical disc,
8 ₁ …… Information recording surface, 8 ₂ …… Cover glass, 9 …… Beam conversion system, 10 …… Photodiode array (photodetector).

Claims (4)

[Claims] 1. An optical information processing apparatus comprising an objective lens for converging a light beam emitted from a light source onto an information recording surface of a recording medium, wherein the objective lens has a concentric circular shape having a sawtooth cross section on one surface. Of an in-line type grating lens having a non-equidistant diffraction grating pattern and the other surface formed into a smooth surface, and is arranged so that the light beam is incident on the one surface. A characteristic optical information processing device. 2. The optical information processing apparatus according to claim 1, wherein the other surface of the objective lens is formed as a non-planar surface. 3. The optical information processing apparatus according to claim 2, wherein the other surface of the objective lens is formed into a spherical surface. 4. Each of the diffraction gratings of the non-equidistant diffraction grating pattern formed on the one surface of the objective lens is formed so that the surface on which the light beam is incident is a surface including a curved surface. The optical information processing device according to claim 1.