JPH065581B2 - Optical head - Google Patents

Description

The present invention relates to an optical reproducing device for optically reading information recorded on a disc such as a video disc, or an optical reproducing device for optically recording and reproducing information on the disc. And an optical system for recording / reproducing and servo.

Configuration of Conventional Example and Problems Thereof In recent years, an optical information processing apparatus using a semiconductor laser has been actively developed in place of the gas laser. The optical disk device is one example. An optical disk device is a device that narrows the light of a semiconductor laser into a minute spot light of φ1 μm or less on a disk which is an information recording medium, and reproduces a signal recorded on the disk or records and reproduces information on the disk at high density. Is.

A semiconductor laser used in a recordable / reproducible optical disk device is required to be capable of outputting a high output optical power.
In general, a semiconductor laser capable of high-power CW (steady-state) oscillation has an anisotropic beam divergence angle because its light emitting region has different aspect ratios.

For example, FIG. 1 shows an example of the emitted light distribution in the parallel and vertical directions in the far-field image of a semiconductor laser capable of high-power CW oscillation. The half-value angle in each direction where the light intensity is half the peak is shown. _Where θ ₁₁ and θ _ι , θ ₁₁ = 5 °, θ _ι = 12.5 °, θ _ι / θ ₁₁ = 2.5 (1). In FIG. 1, the vertical axis represents the light intensity and the horizontal axis represents the spread angle. Thus high-power semiconductor laser beam拡Ri angle ratio _θ ι / θ ₁₁ is about 2.5 times.

An example of an optical disk device for narrowing the light of a semiconductor laser having such an anisotropic divergence angle to a minute isotropic spot light that is round on the disk is disclosed in JP-A-55-108.
Proposed in Japanese Patent No. 612. The outline is shown in FIG. That is, the light beam emitted from the semiconductor laser 1 is collected by the condenser lens 2 to be converted into parallel light, which has a lens action only in the direction parallel to the cemented surface of the semiconductor laser 1 and has a concave and convex cylindrical arrangement. Lens 3, 4
With this configuration, a parallel beam is expanded only in the parallel direction to the bonding surface of the semiconductor laser, and the beam diameter is made almost equal to that in the vertical direction. Light is obtained. But,
This method has the drawback that (1) the optical system becomes large.
Also, at least two servo technologies are required in such a device. One is that the disk causes surface wobbling in a direction perpendicular to the rotating direction of the disk as it rotates, and an optical system is provided so that a minute spot light of φ1 μm or less with respect to the surface wobbling can always be irradiated onto the disk. This servo is made to follow, and this servo is called focus servo. On the other hand, the track moves in the radial direction of the disk due to eccentricity or the like with the rotation of the disk, but in response to this, the optical system always follows so that the minute spot light irradiates the track. It is called a tracking servo.

The servo signal (error signal) for performing the focus servo and the tracking servo is obtained from the reflected light of the disk, and a concrete conventional optical system is also shown in FIG.

In FIG. 2, a polarizing beam splitter 7 transmits or reflects light depending on the polarization direction of the laser. 8 is a λ / 4 plate, and the reflected light from the disk 6 passes through the λ / 4 plate 8 again,
The polarization direction is changed and reflected by the polarization beam splitter 7. Reference numeral 9 is a convex lens, and 10 is a splitting mirror. The splitting mirror splits the light beam into two parts, and the direction of the light beam is changed to be guided to photodetectors 11 and 12, respectively. The photodetector 11 is divided into two as shown in 11a and 11b when viewed from the light incident direction.
The focus error signal for the focus servo is obtained from the difference between the outputs of a and 11b. Also, the photodetector 12 has a 2
The photodetectors 12a and 12 are divided into two parts.
The tracking error signal for the tracking servo is obtained from the difference in the output of b, and the reproduction signal for reading the information recorded on the disk is obtained from the relaxation of the outputs of the four photodetectors.

The conventional optical system shown in FIG. 2 has the following drawbacks in addition to the above-mentioned two drawbacks.

(2) An optical element consisting of a split mirror is required to split the reflected light into two, and changes in the mirror over time, such as dust adhering to the surface or temperature fluctuations, displacement of the split mirror due to vibration, etc. Disturbs signals and impairs control performance.

(4) The optical components such as the polarization beam splitter, λ / 4 plate, convex lens, and mirror are all spatially separated and scattered, so that the optical system as a whole is large and the light reflection at each optical component is large. The loss or the light transmission loss becomes large, and the laser light cannot be effectively used.

OBJECT OF THE INVENTION The present invention has been made mainly in view of the above-mentioned drawbacks, and provides a novel optical head using an optical prism having a smaller recording / reproducing optical system and having less optical loss and aging. The purpose is to

According to the present invention, a condensing lens that collects light emitted from a semiconductor laser, which is a light source for irradiating a minute spot on a disc, and converts the light into parallel light, and an enlarging means for enlarging one of the luminous flux widths of the parallel light. And a polarizing beam splitter which is placed in the expanded parallel light path and splits an incident light beam into reflected light or transmitted light according to the polarization direction, and placed in the optical paths of the reflected light and the transmitted light. Λ / 4 plate and the λ
A part of the reflected light from the disc that has passed through the aperture lens, the λ / 4 plate and the polarization beam splitter, and an aperture lens that narrows the reflected light that has passed through a / 4 plate onto the disc. With a total reflection surface that reflects the light at a substantially right angle, a transmission surface that transmits the other part of the reflected light from the disk, and a semi-convex lens for narrowing the reflected light reflected at a substantially right angle on the total reflection surface. Divided prism, a first photodetector which is placed at approximately the image forming position of the reflected light narrowed down by the semi-convex lens and is divided into two, and two divided light which receives the light transmitted through the transmission surface of the divided prism. By including the second photodetector, an optical system that is smaller than the conventional optical system and has less change over time and light loss is realized.

Description of Embodiments The present invention will be described in detail below with reference to the drawings. FIG. 3 is a diagram showing an embodiment of the present invention. The same components as those in FIG. 2 are designated by the same reference numerals. In FIG. 3, a collimated beam P having a luminous flux width I ₁ collimated by a condenser lens enters from the a surface of the expanding prism 13, is expanded into parallel light having a luminous flux width I ₂ , and is totally reflected on the b surface, c A parallel beam Q having a luminous flux width I ₂ is output from the surface. At this time, the optical axis X of the incident light P
-X 'and the optical axis YY' of the emitted light Q are substantially orthogonal to each other.

Hereinafter, the magnifying prism 13 will be described in detail.

Incident light P (parallel light) enters the end face a of the magnifying prism 13, is refracted, and enters the prism. The condition at this time is given by Equation (2) from Snell's law. n ₀ sin θ ₁ = n ₁ sin θ
₂ ………… (2) where n ₀ (≈1): Refractive index of air n ₁ : Refractive index of prism θ ₁ : Incident angle θ ₂ : Refraction angle Parallel light flux width I ₁ at the end face a is I ₂ But the ratio m
= I ₂ / I ₁ is given by equation (3).

m = I ₂ / I ₁ = cos θ ₂ / cos θ ₁ (3) Next, the optical axes XX ′ and YY ′ of the incident light P and the outgoing light Q.
To be substantially orthogonal to each other, the light is totally reflected by the end surface b of the magnifying prism 13. The incident angle θ ₃ at this time is given by the equation (4).

θ ₃ ≈ (90 + θ ₁ −θ ₂ ) / 2 (4) A prism for totally reflecting the light at the incident angle θ ₃ on the end face b and for causing the reflected light to be substantially vertically incident on the end face c. The respective vertex angles of θ ₄ ≈θ ₂ + θ ₃ (5) θ ₅ ≈θ ₃ (6)

If a prism satisfying the above conditions is manufactured, the incident light flux
I ₁ becomes the m-fold output light flux I ₂ , and the optical axis XX ′ of the incident light P
And the optical axis Y-Y 'of the emitted light Q are substantially orthogonal to each other.

Specifically, assuming that the divergence angle ratio of the semiconductor laser is 2.5, which is the condition of the expression (1), and m = 2.5 and the refractive indices are n ₀ = 1 and n ₁ = 1.51, the respective apex angles are obtained as follows.

θ ₁ = 72 °, θ ₂ = 39 °, θ ₅ ≈61.5 °, θ ₄ ≈100.5 ° The polarization direction of the light beam entering the end face a is preferably P-polarized with respect to the end face a. This is because the semiconductor laser is generally deflected in the direction parallel to the bonding surface, and θ ₁₁ <θ _{ι as} shown in FIG.
This is because it is necessary to expand the beam diameter in the parallel direction.
Further, the incident angle of light is as large as θ ₁ = 72 °, and the P deflection is more likely to pass through the end face a. An antireflection film is vapor-deposited on the end face a so that the reflection loss at that time is minimized under the conditions of a specific laser wavelength to be used and a specific incident angle θ ₁ . In this way, the beak diameter is expanded by the magnifying prism 13 only in the direction parallel to the junction surface of the semiconductor laser, and the semiconductor laser is converted into substantially circular parallel light and then totally reflected. Then, after being reflected by the polarization beam splitter 7 and passing through the λ / 4 plate 8, a substantially circular isotropic minute spot light is obtained on the disk 6 by the diaphragm lens 5.

On the other hand, the reflected light from the disk 6 passes through the polarization beam splitter 7 and is guided to a split prism 14 for reproducing the signal recorded on the disk 7 and for performing focus tracking control.

Hereinafter, this split prism will be described in detail.

FIG. 4 shows an oblique view of the split prism 14. That is,
In the reflected light path, spatially about half of the light reflected from the disk is totally reflected by the total reflection surface d, and is guided to the half-convex lens 15 which is formed by dividing the convex lens into about half parallel to the optical axis direction. The optical axis center of the convex lens and the laser center are almost coincident with each other.

Therefore, about half of the reflected light is focused by the semi-convex lens 15 and is applied to the photodetector placed at the image forming position. Photo detector 1
Seen from the light incident surface, 1 has a two-divided structure shown by 11a and 11b, and a focus error signal can be obtained from the output difference of each photodetector of 11a and 11b.

On the other hand, the other half of the reflected light passes through the transmission surface e and is guided to the photodetector 12. When the light detector 12 is viewed from the light incident surface 12a, it has a two-piece structure shown in 12b, a tracking error signal from the output difference of each photodetector of the 12 _a, 12 _b is obtained. The tracking error signal is obtained from the movement of the so-called far-field image.

The reproduction signal from the disk is obtained, for example, by relaxing the outputs of both the photodetectors 11 and 12.

The principle of obtaining the focus error signal with the configuration of FIG. 3 will be described in detail.

FIG. 5 is a simplified diagram of FIG. 3 for explaining only the method of obtaining the focus error signal.
The same components as those in the figure are designated by the same reference numerals. In FIG. 5, a is a distance between the aperture lens 5 and the disc 6 that is too close to the desired distance, b is a desired distance, that is, c when the incident light is focused on the disc surface, and
Indicates the case where the distance is longer than the desired distance.

First, as shown in FIG. 5A, when the diaphragm lens 5 and the disk 6 are too close to each other than the desired distance, the image forming position A ₁ of the reflected light focused by the semi-convex lens 11 is set to the photodetector 1.
Farther than 1. Therefore, in this case, the amount of light received by the photodetector 11a is larger than the amount of light received by the photodetector 11b. On the contrary, as shown in FIG.
If the disk 6 and the disk 6 are further apart than the desired distance, the amount of light received by the photodetector 11b becomes larger than the amount of light received by the photodetector 11a (the image forming position A _{3 of the} reflected light is on the convex lens side). Approach).

Further, when the incident light is focused on the disk 6 as shown in FIG. 5B, the reflected light A ₂ imaged by the semi-convex lens is irradiated onto the photodetector 11, so that the photodetector 11
The light receiving amounts of a and 11b are equal.

Therefore, focus servo can be realized by performing servo so that the light receiving amounts of the two photodetectors 11a and 11b become equal.

As shown in FIG. 3, the magnifying prism and the split prism are bonded together with the λ / 4 plate to the polarization beam splitter to form a one-piece structure.

However, since it is necessary to finely adjust the rotation of the λ / 4 plate, it is possible to configure only the λ / 4 plate separately.

EFFECTS OF THE INVENTION As described above, according to the configuration of the present invention, since the reflected light is divided into two by the split prism without using the split mirror, there is no change in the characteristics of the split mirror with time and positional deviation.

In addition, since each prism and polarization beam splitter have a one-piece structure in the entire optical system, there are few light entrance and exit surfaces and reflection loss at each end surface of the optical component is reduced, so that laser light can be effectively used. Moreover, there is an effect that the size of the optical system can be significantly reduced as compared with the conventional one.

[Brief description of drawings]

FIG. 1 is a light intensity distribution characteristic diagram of a far-field image of a semiconductor laser,
FIG. 2 is a diagram showing a configuration of a conventional optical disk device, FIG. 3a is a front view of an optical head in one embodiment of the present invention, and b.
Is a side view, c is a plan view, FIG. 4 is a perspective view of a split prism in the embodiment, and FIGS. 5a, 5b, and 5c are views for explaining a method of obtaining a focus error signal. 1 ... Semiconductor laser, 2 ... Focusing lens, 5 ... Aperture lens, 6 ... Disk, 7 ... Polarizing beam splitter,
8 ... λ / 4 plate, 11 ... First photodetector, 12 ... Second photodetector, 13 ... Enlarging prism, 14 ... Dividing prism, 15 ... Semi-convex lens, I ₁ ... Shorter parallel light width, a: light incident surface of the expanding prism, b: total reflection surface of the expanding prism, d: total reflection surface of the split prism, e ......
Transmission surface of the split prism.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kenji Koishi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References

Claims (2)

[Claims] 1. A condenser lens for collecting emitted light from a semiconductor laser, which is a light source for irradiating a minute spot on a disc, and converting it into parallel light, and enlarging means for enlarging one of the luminous flux widths of the parallel light. A polarization beam splitter placed in the expanded parallel light path and splitting an incident light beam into reflected light or transmitted light according to a polarization direction, and λ placed in the optical paths of the reflected light and the transmitted light. / 4 plate and the λ
In an optical head including a diaphragm lens that narrows the reflected light that has passed through a / 4 plate onto the disk, one of reflected light from the disk that has passed through the diaphragm lens, the λ / 4 plate, and the polarization beam splitter. A total reflection surface that reflects a portion substantially at a right angle, a transmission surface that transmits the other portion of the reflected light from the disk, and a semi-convex lens for narrowing the reflected light reflected at a substantially right angle on the total reflection surface. And a first photodetector, which is placed at approximately the image forming position of the reflected light focused by the semi-convex lens and is split into two, and two splits that receive the light transmitted through the transmission surface of the split prism. Optical head provided with a second photodetector. 2. A magnifying means, a polarization beam splitter, and λ / 4.
The optical head according to claim 1, wherein the plate and the split prism are integrally structured.