JPS61271633A - Optical head device - Google Patents

Abstract

PURPOSE:To improve the read characteristic and the yield at mass-production by arranging a parallel plane component to a non-parallel part of a luminous optical flux in a conveyed optical system with a tilt so as to eliminate the astigmatism specific to the condenser system and obtaining easily the condenser characteristic of the diffraction limit. CONSTITUTION:The astigmatism corresponding to an angle theta is generated by arranging a parallel plane glass plate 50 with a tilt by the angle theta with respect to the optical axis of a collimate lens 7. Thus, the astigmatism of the entire condenser system is eliminated by setting a rotational position in the light axis direction of the plate 50 to correct the astigmatism of the optical system other than the plate 50. Thus, the condensing characteristic of the diffraction limit is obtained easily even when the arrangement error of the component and the astigmatism of a semiconductor exist so as to improve the read characteristic of an optical head and the yield at mass-production.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to the structure of an optical head device, and more specifically to an optical head that outputs/writes all signals from an optical disk, which is an optical information recording medium. This method is most suitable for use as an aberration correction method when astigmatism occurs in the light beam focused on the medium due to deviations in the design of purchased optical components or defects in the light source itself.

[Conventional technology]

Examples of this type of prior art include ■ Japanese Patent Application Laid-Open No. 58-102
No. 342, ■ “Modern Opt, 1cal En
gineeringJ (M, C6Gray-Hill
N, Y, 1966), ■ “Lens Design Techniques P,
35-36” (Engineering and Industrial Technology Association), ■[0ptics
P, 156 J (vilsy N, Y,).

In addition to this technique, there has been a conventional device of this type, for example, the one shown in FIG. In the figure, fi+ is a light source such as a semiconductor laser, (2) is the emitted light beam emitted from the light source (11), (3) is a diffraction grating that separates the emitted light beam (2) into three light beams, and (4) is the irradiated light beam. (5) and reflected luminous flux (6)
(7) is a collimating lens that converts the irradiated light beam (5) into a parallel light beam (8); (9) is a reflection prism that reflects the parallel light beam (8) in a direction orthogonal to the beam;
α1 is an objective lens that focuses the parallel light beam aυ onto the information track α of the disc-shaped record carrier σ2 as a light spot α4. Note that the record carrier α3 is placed near the focal point of the objective lens a1. Moreover, the light spot α4 is actually three light spora) (14a).

(14b) and (14c). Also, the information track α consists of bit α9 and land H. Furthermore, the record carrier α is rotated by a motor (not shown). Further, the light beam reflected by the recording carrier a2 passes through the objective lens Ql and the collimating lens (7) again, and is bent by the half prism (4) in a direction orthogonal to the beam, becoming a reflected light beam (6). (Iη is a concave lens that reduces the convergence angle of the reflected light beam (6) and increases the magnification of the reflective optical system;
On the side is a cylindrical concave lens that produces astigmatism in the light beam transmitted through the concave lens αD (11 is a photodetector element (
19a), (19b).

(19c).

Next, the operation will be explained. The light beam (2) emitted from the semiconductor laser passes through a diffraction grating (3), a half prism (4),
Transmitted through collimating lens (7) 1 reflection prism 191
The beam whose traveling direction is bent at , is focused onto the record carrier a3 by the objective lens α1 as three beams (14a), (14b), and (t4c). The light beam reflected by the carrier (13) passes through the objective lens, is reflected again by the reflecting prism (2), is reflected by the half prism (4), and then passes through the concave lens αη and the cylindrical concave lens before passing through the photodetector a. The three elements that make up the value (19a), (19b),
(19c) as three beams. On this occasion,
The central detector (19a) detects pit α, land (light spora from either Ie) by the rotation of the record carrier (1
The information recorded on the carrier is converted into an electrical signal based on the amount of reflected light when 4a) is reflected, and then used as an audio signal, video signal, digital data, etc. by a circuit not particularly shown here. be done. Also, carrier t
L2 is the objective lens (
11 in the optical axis direction. This defocus amount is determined by a known method (Reference ■) using the central photodetector σI (
a) It is detected from a change in the shape of the light beam on the disk, and is corrected by a servo circuit (not shown) so that the light beam on the disk is always kept in focus.

Furthermore, when the carrier α2 rotates, the track αj meanders.

When the central beam (14a) is not positioned correctly on the track 13 due to vibration, it is known that the track misalignment can be corrected by detecting the track and spot ( 14a) is taken to detect and correct the entire amount of deviation. (Refer to Document ■) In such an optical head device, in order to increase the information density stored on the record carrier t17J as much as possible and use it as a large-capacity dispersive information medium, the pit length and track spacing are set to be shorter than that of a semiconductor laser. The dimensions are small enough to be marginally readable when the condensing system of the optical head leading to the lens is in an ideal diffraction-limited state.

That is, typically when the laser wavelength is λ-780 nm and the numerical aperture of the objective lens is about NA-0.5, the spot diameter focused at the diffraction limit λ is about /NA-1.6 μm I.
K, but in line with this, the track spacing is 1.6 μm,
The minimum three-dimensional bit length is 0.5 μm, that is, approximately one half of the minimum spot diameter. As mentioned above, in order for the condensing system of the optical head device to have a diffraction-limited condensing characteristic.

Two conditions are required: ■ The light emitted from the semiconductor laser passes through the objective lens α1 and is guided with almost no aberration on the entire optical path leading to the spot lJ4.■ The semiconductor laser itself must have no aberrations. be.

[Problem that the invention seeks to solve]

There are three reasons why the conventional optical head device suffers from the above-mentioned condensing system causing astigmatism and deviating from the aberration-free system. The first is astigmatism, which occurs when the light beam transmitting surfaces of parallel plane optical components such as the diffraction grating (3) and half mirror (4) are tilted and not perpendicular to the optical axis of the output light beam (2). In this case, the second problem is that the light emitting point of the semiconductor laser is decentered from the optical axis of the collimating lens, resulting in an image height, causing astigmatism in the collimated light (8), and at the same time, the collimated light is deflected by the polarization of the semiconductor laser. The third case is when the beam enters the objective lens (1) obliquely due to the center, resulting in an image height and astigmatism.The third case is when the emitted light beam of the semiconductor laser itself has astigmatism.

The causes of the above three astigmatisms will be explained in more detail below.

(1) Inclination of diffraction diffraction or half prism The light beam (2) emitted from the semiconductor laser is a diverging light beam, which is the same in this case, but 2 For example, as shown in Fig. 4, it is in a convergent shape. Assume that a parallel plane glass (7) is arranged in the optical path of the light beam (numerical aperture NA-*U) at an angle U with respect to the optical axis Gυ. According to literature (2), the astigmatism (astigmatism) that occurs at this time is expressed as equation 111.

A ri 1 -1 88 t Note that t is the distance to the convergence point in the plane that includes the normal and the optical axis (the meridional plane), and l is the distance to the convergence point in the plane perpendicular to this (the sagittal plane). It is.

Now, in FIG. 3 showing a conventional example, when the diffraction grating (3) or Nof prism (4), which are parallel plane components, are tilted, astigmatism occurs according to the IL1 formula. As a specific example of this, when a half prism with a refractive index of N-1.5 and a diffraction grating length of t-5 as is tilted by 1.0@, the astigmatism difference generated is 0.17 μm and 056 μm, respectively. becomes.

(II) Image height due to eccentricity of semiconductor laser Generally, in a lens system, the object point (in this example, the emission point of the semiconductor laser) is shifted from the optical axis and has an image height.

Astigmatism occurs. FIG. 5 shows an example of calculation of astigmatism generation according to document ◎. As shown in the graph on the right side of the figure, it can be seen that the imaging positions of the meridian ray and the spherical ray become separated with respect to the incident angle, that is, the image height, and the astigmatism increases. As a lens for an optical head device, for an image height corresponding to an incident angle of 1°, a collimating lens has an image height of, for example, 10 μm.
Some objective lenses produce astigmatism of 5 μm.

(iii) Astigmatism of a semiconductor laser The area of the light emitting point of a semiconductor laser is usually 2 μm×0. It is about IA1, and is regarded as a zo point light source, and light is emitted from a much smaller area.

FIG. 6 shows an example of the structure of a double heterojunction semiconductor laser. In such a semiconductor laser.

As shown in FIGS. 1(A) and 1(B), the mode waist of the emitted light beam of the semiconductor laser chip (40) is within the semiconductor junction surface (X-Y axis plane) and in a plane perpendicular to this (X-Z axis plane). It is known that this difference is particularly large in a gain guiding type semiconductor laser as shown in this figure.
On the other hand, in the junction plane (X-Y axis plane), the active layer (42) of the semiconductor laser chip (40), that is, the mirror surface, is slightly (41) Point B, which is further inside the resonator, is the mode waist.

This difference results in optical astigmatism, and this difference is particularly large in the above-mentioned gain guiding type lens, and there are some that exist as much as about 25 μm.

Now, the norm of Mare and chal has conventionally been used as the allowable optical system as a nine-diffraction limited optical system. According to this, the RMS value of wavefront aberration (W ) m
S must be w (rLorλ) (λ: ms-wavelength of light) The relationship between the astigmatism step and wavefront aberration, which are classified and discussed in three cases, will be explained below in the no-chance section of Fig. 7. In the figure, E indicates the exit pupil with radius a, and the pupil coordinates are expressed as (X, y).The unidirectional transverse aberration I at the spherical image plane P is calculated using the wavefront aberration W,8. ) is expressed by the formula.

Also, from FIG. 2, when R)Δ, that is, the astigmatism difference is extremely small, (3) holds true.

By eliminating I from R +21, +3) and finding the wavefront aberration, equation (4) is obtained. However, nwl is assumed to be in the air.

Now, assuming that the observation surface of the transverse aberration is the circle of least confusion, the wavefront aberration W in the y direction at the circle of least confusion is similarly expressed by equation (6).

Therefore, the two-dimensional wavefront aberration W based on the y direction is (7)
It can be found as shown in the formula.

1 ・3. . (7) X 74 where -(-), and r is the polar coordinate radius of (x, y).

At this time, the value of is 0 to Ha (Na numerical aperture) within the pupil.
Takes the value within. Using (7), the rma wavefront aberration Wr m
Therefore, if the function number Na of the system is given, then from equation (8), rm
The astigmatism difference Δ that satisfies s<”72 can be found.

For example, in an optical head for an optical video disc, Na of the collimating lens is +12. Semiconductor laser wavelength α8
A value of approximately μm is used, and in this case, Wrm.

The tolerance for astigmatic difference with <0.07λ is Δ(117μ
m. Therefore, with respect to this astigmatism difference tolerance, the astigmatism produced by a combination of the factors (1), (Ii), and Qii) above inhibits the function of the total focusing system as a diffraction-limited optical system.

It can be seen that the deterioration of the OFF state is a factor in deteriorating the read signal.

In addition, in FIG. 7, there is a relationship of R>>Δ.

[Means for solving problems]

(Purpose) The optical head device according to the present invention uses optical components to correct astigmatism caused by defective components, which was revealed in the explanation of the above problems, and to enable the condensing system to function as a diffraction-limited optical system. It provides a means to improve.

(Means) In the optical head device of the present invention, the astigmatism caused by tilting the parallel plane component placed in the emitted light beam part in the condensing optical system, that is, any part where the light beam is not parallel, A method is used to correct the astigmatism of the condensing system.

[For production]

In the optical system head according to the present invention, astigmatism is corrected by utilizing the fact that the tilted parallel flat plate component generates an astigmatism difference according to the above-mentioned formula 111.

[Embodiments of the invention]

FIG. 1 shows only the condensing system part of the optical system of an optical head device according to an embodiment of the present invention.

In the figure, (50) is a parallel plane glass plate with thickness t and refractive index N, and collimating lens (7) shown as a dotted line.
is arranged at an angle θ with respect to the optical axis of the lens.

Note that the other parts of the optical system are the same as those shown in FIG. 3 showing the conventional example, so their explanation will be omitted here.

Next, a first embodiment of the invention will be described below.

As explained above, a parallel plane plate placed in a diverging (or converging) beam produces an astigmatism difference of the amount expressed by equation 111. Therefore, the parallel plane plate (50) in FIG. 1 intentionally generates an astigmatism difference equal to the amount obtained by replacing U in equation (1) with θ. Therefore, the problem (g) is (1).

If the total astigmatism difference resulting from the sum of the astigmatism differences caused by each factor discussed in the section of Qil) is Δ, then
Glass plate (5
The inclination θ, thickness t, and refractive index N of the glass plate (50) that generate an astigmatism difference equivalent to that of an optical system other than 0) are given. For example, N-ml, 5. tm (assuming L5m, the astigmatism difference equivalent to the astigmatism difference of 30 μm is the inclination angle θ-2
It is given by 2.5°. Therefore, the glass plate (50)
The glass plate (50) is arranged so that the astigmatism difference generated by the glass plate (50) corrects the astigmatism difference of the optical system other than the glass plate (50).
The astigmatism of the total focusing system shown in FIG. 1 can be removed by setting the optical axis direction rotational position {circle around (2)} (as shown). In addition, in the above rotational adjustment (2), for example, if the rotation position is changed by 90 degrees, the relationship between the meridian image point and the spherical defect point will be reversed, and in short, the front-back relationship between the focal point pt in the y direction and the focal point P in one direction in Fig. 7 will be reversed. It is based on the principle of becoming.

In the first embodiment, a correction example was shown in which a parallel plane glass plate (50) was newly inserted into the nine diverging beam parts of the LD output beam, but there are other correction examples as well. The diffraction grating (3) is tilted as in the embodiment. Figures 2(b) to 3
It is also possible to generate an astigmatism difference for correction by tilting the half prism (4) as in the embodiment.
An astigmatism correcting condensing system can be constructed based on exactly the same principle as the first embodiment described above. Further, the parallel plane glass plate in the first embodiment has two diffraction gratings (3) and a semiconductor laser (1).
) is shown in Fig. 1, but in addition to this, as shown in Figs. Even if it is between the half prism (4) and the diffraction grating (3),
Alternatively, even if it is between the information storage carrier (l) and the objective lens H, astigmatism correction is possible at a location other than the parallel rays in the condensing optical system. (
50) In other words, as long as it is a parallel plane plate through which the semiconductor laser #4 can pass.

Correction by adding a tilted parallel plane glass plate (first example)
It goes without saying that a similar correction is possible.

〔Effect of the invention〕

As described in detail above, according to the present invention, the non-parallel portion of the light beam in the condensing optical system of the optical head device.

By arranging the parallel plane components at an angle, it is possible to remove the astigmatism that the condensing system originally had, so the wavefront aberration of the light beam condensed onto the information medium becomes extremely small.

Even when there are component placement errors and astigmatism of the semiconductor laser, it becomes easier to obtain diffraction-limited focusing characteristics, improves the readout characteristics of the optical head, and greatly contributes to improving yields in the event of wasted production. .

[Brief explanation of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention, and FIG. 2 is a diagram showing a second embodiment of the present invention. Third. A diagram showing the fourth embodiment, FIG. 3 is a diagram showing a conventional optical head device, FIG. 4 is an explanatory diagram of aberrations generated when a convergent light beam is incident on a parallel plate, and FIG. 5 is a diagram of incident light on a lens. Fig. 6 is an explanatory diagram of the occurrence of astigmatism when the image height (incidence angle) is
The figure is a no-chage diagram that calculates the astigmatism RMS value from the astigmatism difference. In the figure: 1j... Laser light source device, (7) and αt.
- Condensing lens system, a2... Record carrier, (4)... Beam splitter, (5). a... Photoelectric converter

Claims (8)

[Claims] (1) A laser light source device, a condensing lens system that irradiates the information recording track of the record carrier with the emitted light beam of the laser light source device, and a condensing lens system that irradiates the light beam emitted from the laser light source device onto the information recording track of the record carrier, and emits the light beam that has been reflected after being focused on the record carrier and is emitted from the laser light source device. In an apparatus comprising an optical system comprising a beam splitter means for separating a beam of light and directing it toward a photoelectric converter, the beam of light between a semiconductor laser in the optical system and a record carrier becomes a parallel beam, and a parallel beam arranged in a portion An optical head device comprising a flat optical element arranged at an angle with respect to the optical axis of the condenser lens system. (2) The optical head device according to claim 1, wherein the light beam irradiated onto the information recording track is divided into a plurality of light spots by a diffraction grating. (3) Claim 1, wherein the parallel plate optical element is disposed between the beam splitter and the condensing lens system.
The optical head device described in Section 1. (4) The optical head device according to claim 1, wherein the parallel plate optical element is disposed between the semiconductor laser and the beam splitter. (5) The optical head device according to claim 2, wherein the parallel plate optical element is arranged between the laser light source device and the diffraction grating. (6) Claim 2, wherein the parallel plate optical element is disposed between the diffraction grating and the beam splitter means.
The optical head device described in Section 1. (7) The optical head device according to claim 2, wherein the parallel plate optical element is the diffraction grating. (8) The optical head device according to claim 1, wherein the beam splitter is composed of a cemented right-angle prism, and the parallel plate optical element is the beam splitter.