JPH02195532A - Optical head device - Google Patents

Abstract

PURPOSE:To reduce the influence of the astigmatism of a condensed beam due to the astigmatic difference of a semiconductor laser and to secure an optimum focused state by specifying the angle formed between the polarizing direction of the semiconductor laser and the direction of a track of a disk. CONSTITUTION:The track direction of a recording carrier 12 is set at an axis (y), and the optical axis of a luminous flux 2 emitted from a semiconductor laser 40 is set at an axis (z). The laser 40 is set into an optical path so that an active layer 42 is turned by 45 from an axis (x) around the axis (z). Thus the best reproduction signal is obtained in a stable track follow-up servo state by securing the coincidence of optimum focus offset positions between the reproduction signal and a TE signal. Then it is possible to extremely improve the write/reproduction performance of an optical head device which applies a semiconductor laser having an astigmatic difference.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical head device, and more particularly to an optical head device for reading/writing signals from/to an optical disk, which is an optical information recording medium.

[Prior Art] As a conventional device of this type, there is one shown in FIG. 6, for example.

In the figure, (40) is a light source such as a semiconductor laser,
Emit linearly polarized light. (2) is the emitted light beam emitted from the light source (40), (3) is a diffraction grating that separates the emitted light beam (2) into three beams, and (4) is the irradiated light beam (5) and the reflected light beam (6). (7) is a collimating lens (9) that converts the irradiated light beam (5) into a parallel light beam (8).
) is a reflecting prism that reflects the parallel light beam (8) in a substantially orthogonal direction, and (10) is a reflection prism that reflects the parallel light beam (11) onto the information track (13) of the disk-shaped record carrier (12).
4) is a condensing lens that condenses light. Note that the record carrier (
12) is placed near the focal point of the condenser lens (10). Moreover, the light spot (14) actually consists of three light spots (14a), (14b), and (14c). In addition, the information track (13) includes pit (15) and land (16).
). Furthermore, the record carrier (12) is rotated by a motor (not shown). In addition, the above record carrier (1
The luminous flux reflected by 2) is again reflected by the condensing lens (1
0), passes through the collimating lens (7), is bent in an almost orthogonal direction by the half prism (4), and is reflected by the beam (6).
). (17) is a concave lens that reduces the convergence angle of the reflected light beam (6) and increases the magnification of the reflective optical system; (18) is a cylindrical concave lens that transmits through the concave lens (17) and causes astigmatism in the light beam; (19) ) is a photodetector with an element (1
9a), (19b), and (19c).

Next, the operation will be explained. The light beam (2) emitted from the semiconductor laser (40) is linearly polarized light that is polarized in a direction parallel to the PN junction surface, and is transmitted through the diffraction grating (3), beam splitter (4), and collimating lens (7). , after the traveling direction is bent by the reflecting prism (9), the condenser lens (10) separates the beams into three beams (14a), (14b),
(14C) is focused onto the record carrier (12). The light beam reflected by the carrier (12) passes through the condenser lens (10), is reflected again by the reflection prism (9), is reflected by the beam splitter (4), and then passes through the concave lens (17),
After passing through the cylindrical concave lens (18), the light detector (1
9) The three elements (19a) and (19b
), (19c) as three beams. At this time, the center detector (19a) detects the difference in the amount of reflected light when the light spot (14a) is reflected from either the pit (15) or the land (16) due to the rotation of the record carrier (12). The recorded information is converted into an electrical signal, which is then used as an audio signal, video signal, digital data, etc. by a circuit not particularly shown here.

Further, the carrier (12) shakes as it rotates, and is displaced in the optical axis direction from the focal position of the condensing lens (10) due to vibration or the like. This defocus amount can be determined using a known method (for example, Japanese Patent Application Laid-Open No.
No. 102342) detects the change in the shape of the light beam on the central photodetector (19a), 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 (12) rotates, the center beam (14a) is caused by the meandering and vibration of the track (13).
3) If the position is not correct, as is known, the amount of track deviation is detected by the photodetectors (19b) on both sides,
Track and spot (14a) based on the output difference of (19c)
A method is taken to detect and correct the amount of deviation (
For example, Japanese Patent Application Laid-Open No. 58-102342).

In such an optical head device, a record carrier (12
) In order to maximize the information density stored on the disk and use it as a large-capacity information medium, the pit length and track spacing are set so that the optical head's condensing system, which extends from the semiconductor laser to the condensing lens, is ideally diffraction-limited. The dimensions are small enough to be marginally readable under normal conditions. That is,
Typically, when the laser wavelength is λ-780 nm and the numerical aperture on the disc side of the condensing lens is about NA-0.5, the spot diameter focused at the diffraction limit is about λ/NA-1.6 μm. However, in line with this, the track spacing is 1.6 μm.
One minimum pit length is 0.8 μm, that is, approximately one hand of the minimum spot diameter. As mentioned above, in order for the condensing system of the optical head device to have diffraction-limited condensing characteristics, the light emitted from the semiconductor laser (40) must pass through the condensing lens (10) and become a spot (14). The optical path must be guided with almost no aberration throughout the entire optical path.

■ The semiconductor laser itself has no aberrations.

Two conditions are necessary.

[Problem A to be Solved by the Invention] However, semiconductor lasers generally have a certain degree of astigmatism. First, astigmatism of a semiconductor laser will be explained.

The area of the light emitting point of a semiconductor laser is usually 2 μm x 0°1 μm.
The light is emitted from a very small area that can be regarded as a point light source.

FIG. 7 shows an example of the structure of a double heterojunction semiconductor laser. In such a semiconductor laser, as shown in Fig. 7(a),
, (b) and (c), the laser chip (40)
The beam waist of the emitted light flux is the semiconductor junction surface (X-Y
It is known that there is a difference between the plane (axis plane) and the plane perpendicular to this plane (X-Z axis plane), and this difference is particularly large in gain guiding type semiconductor lasers like the one shown in this figure. It is being In the vertical plane (x-y axis plane), point A, which coincides with the mirror surface (41), is the mode waist, whereas in the junction plane (X-Y axis plane), the activity of the semiconductor laser chip (40) is slightly lower. Point B, which is inside the layer (42), that is, inside the resonator deeper than the mirror surface (41), is the mode waist. This difference results in optical astigmatism, and this difference is particularly large in the gain-guiding type lens, and some exist as much as about 25 μm.

The difference between the points A and B is called an astigmatism difference, and the relationship between this astigmatism difference and wavefront aberration will be explained below with reference to the no-tease region in FIG. 8.

In the figure, E indicates the exit pupil with radius a, and the pupil coordinates (also,
y). The X-direction lateral aberration X' at the spherical image plane Ps is expressed by equation (1) using the wavefront aberration W.

X −=R/n −・aW/anu ---(,1)
Also, from the figure, when R)Δ, that is, the astigmatism difference is very small, equation (2) holds true.

X'-nu/R・△...(2)(1)
, If we eliminate X′ from equation (2) and find the wavefront aberration, we get (3
) formula is obtained. However, n'-1 is assumed to be in air.

-1/2 (also/R)2△... (3)
Equation (3) represents the astigmatism coefficient in the Seidel format, and when this is converted into the standard deviation Wrms of the wavefront aberration at the point of the circle of least confusion, which is the best focal point, the equation (4) is obtained.

Wrms - (1/4f) Na2.DELTA (4) However, in the formula 4), Na-a/R, and a is the radius of the exit pupil as shown in FIG.

For example, △-20μm, collimating lens NA-0,1
2. If λ-0.78μm, then from equation (4), Wrms
Mar
In some cases, it becomes 50% or more of the standard Wrms <0.07λ of chal, and it can be seen that this is a factor that deteriorates the good writing/reading characteristics of the optical head.

Furthermore, when astigmatism exists in the laser beam, it also affects the characteristics of each signal with respect to defocusing. The characteristics for this out-of-focus will be explained below.

In the focused state, the spot on the disk is in a circle of least confusion as shown in FIG. 9(b). Although only the central beam spot (14a) is shown in FIG. 9, the spots (14a) on both sides sandwiching the central beam spot (14a) are shown.
b) (14c) also has the same shape.

If there is no astigmatism, the spot (14a) will be in the state shown in FIG. 9(b), but when the focus is shifted, its diameter will increase and the photodetector (19b) will be obtained as a reproduced signal and track deviation amount. The amplitude of the output difference signal (hereinafter referred to as the TE multiplied signal) of (19c) is attenuated with the maximum in the state shown in FIG. 9(b). Therefore, by adjusting the overall offset including the control circuit so that the reproduction state becomes as shown in FIG. A state that is resistant to misalignment can be obtained.

However, if there is astigmatism in the laser beam,
Even on the carrier, as shown in Figure 8 above, the beam waist is different in the junction plane of the semiconductor laser and in the plane perpendicular to it, and the direction of the junction plane and the information track are almost perpendicular or parallel. Due to out-of-focus, a state like that shown in FIG. 9(a) or (C) exists. The reproduced signal and TE multiplied signal utilizes the difference in the amount of reflected light between the bit (15) and the land (16), and the smaller the beam diameter in the direction across the bit (15) and land (16), the larger the signal amplitude. It has been known. Therefore, in FIG. 9(a), the direction along the track (13), and in FIG. 9(c), the direction along the track (13).
The signal amplitude in the direction across is maximum. Since the reproduced signal is a signal in the direction along the track and a signal in the direction across the TE double signal track, for the above reason, the reproduced signal and the TE
There are states in which the focus offset position at which the double signal is maximized is different. FIG. 10 shows the results of measuring the reproduced signal and the TE times signal amplitude when defocus is actually caused in an optical head equipped with a semiconductor laser having astigmatism. The horizontal axis is the amount of defocus. It is clearly seen that the positions of the maximum point of the reproduced signal and the maximum point of the TE multiplied signal are shifted.

By the way, in general, it is not possible to obtain a large reproduced signal, so by reproducing the signal in its largest state, the S/N can be increased and good characteristics can be obtained. In other words, the operation will be performed in the state shown in m10 diagram (a),
In this case, as shown in the figure, the TE multiplied signal is not at its maximum, but in the case of the relationship between the reproduced signal and the TE multiplied signal as shown in Fig. 6, when defocusing to the negative side, the TE multiplied signal attenuation is large, and the track following becomes difficult. This had a significant impact on the stability of the servo, resulting in an unstable state in some cases. On the other hand, Fig. 10 (e
), the stability of the track following servo can be ensured, but a large reproduced signal cannot be obtained, and furthermore, there is a problem that the reproduced signal deteriorates significantly when defocused to the positive side. As described above, in an optical head device equipped with a semiconductor laser having an astigmatism difference, when the track direction and the polarization direction of the semiconductor laser or the direction of the active layer are approximately perpendicular or parallel, the reproduced signal and the TE Since the focus offset position where the signal is maximum is different, the S/N of the reproduced signal is
It was difficult to make the tracking servo large and to stabilize the track following servo, and also to equalize the effect of signal attenuation when defocusing to the plus or minus side.

This invention was made to solve the above-mentioned problems, and in an optical head device equipped with a semiconductor laser having an astigmatic difference, it is possible to solve the astigmatism of a focused beam due to the astigmatism difference of the semiconductor laser. The object of the present invention is to provide an optical head device that can reduce the influence and have an optimal focusing state.

[Means for Solving the Problems] The optical head device of the present invention sets the polarization direction of the semiconductor laser and the direction of the track on the disk at an angle of 45 degrees, thereby improving the defocus characteristics of the reproduced signal and the TE multiplied signal. This is to reduce the influence of astigmatism.

C effect] In the present invention, by setting the polarization direction of the semiconductor laser and the direction of the track on the disk to 45 degrees, the influence of astigmatism due to the semiconductor laser is reduced, and the playback signal and TE
This makes the double signal defocus the same and satisfies both reproduction performance and track following servo characteristics.

[Example] An example of the present invention will be described below with reference to FIGS. 1 to 5.

FIG. 1 shows one embodiment of the present invention, and in this FIG.
For convenience, the optical path reflected by the beam splitter (4) and directed toward the photodetector (19), which is unnecessary for the explanation of the present invention, is omitted from the configuration of the conventional example shown in the figure, but the semiconductor laser (40) which is a laser light source device , diffraction grating (3), beam splitter (4), collimating lens (7), reflective prism (9), condensing lens (10), concave lens (17),
All components such as the cylindrical concave lens (18) are included.

The feature of this embodiment is that the direction of the active layer (42) of the semiconductor laser (40), that is, the polarization direction, is tilted by 45 degrees as shown by E in the figure (compared to the conventional example). In other words,
As shown in FIG. 1, the track direction on the record carrier (12) is the y-axis, and the optical axis of the emitted light beam (2) from the semiconductor laser (40) is the two-axis, and the direction of the active layer (42) is the two-axis. A semiconductor laser (40) is installed in the optical path so as to rotate by 45 degrees around the y-axis.

The operation of this embodiment will be explained below. Of course, in this optical system as well, the method of detecting the reproduced signal and the TE multiplied signal is exactly the same as in the conventional example.

Now, as mentioned above, since the direction of the active layer is rotated by 45 degrees from the y-axis, the focused spot (14) on the carrier (12)
The shape when out of focus is different from the conventional one. This situation will be explained with reference to FIG.

Similarly to FIG. 9, the state of the central beam spot (14a) in the state of the circle of least confusion (b) and in the cases of defocusing on the minus side and the plus side (a) and (C) are shown. In the state of FIG. 2(b), the spot (14a) is the circle of least confusion, so the beam shape is approximately circular, which is the same as the state of the conventional example shown in FIG. 9(b). Next, in the state shown in Figure 2 (a) or (C), the semiconductor laser (40) is rotated by 45 degrees around the optical axis, so the direction of the focal line due to astigmatism is rotated by 45 degrees with respect to the track direction. are doing.

Therefore, in the state of FIG. 2(a) or (C), the beam diameter in both the track direction and the track cross direction increases, so that the reproduced signal and the TE multiplied signal are also attenuated compared to the state of FIG. 2(b). Figure 3 shows the defocus characteristics. The maximum point of the reproduced signal and T
A result is obtained in which the maximum points of the E times signal are almost the same. Furthermore, when the attenuation rate is compared with that of the conventional example, it has been experimentally revealed that the attenuation rate is small in the apparatus of the present invention. Furthermore, as is clear from a comparison of FIG. 10 and FIG. 3, the device of the present invention has a smaller attenuation with respect to defocus, so it can obtain more stable performance with respect to defocus than a device using a conventional optical system. .

Although the type of condensing lens was not specifically mentioned in the above experimental example, if an aspherical lens made of a plastic lens or a press-molded glass lens is used as the condensing lens, the aspherical lens produced by the semiconductor laser can be By setting the direction of this aspherical lens so as to cancel the point aberration, the astigmatism of the reflected light beam itself can be minimized (Japanese Patent Application Laid-Open No. 6O-149941).

Therefore, in the present invention, for example, if a plastic lens is used, astigmatism can be further reduced by rotating the gate direction of the lens by 45 degrees to match the polarization direction of the semiconductor laser, and even better light focusing characteristics can be obtained. It will be done.

In addition, although the above experimental example explained the case where an infinite conjugate type (parallel incidence type) lens was used as the condensing lens (10), the optical system of this experimental example was changed to the collimating lens (
7) is removed and the diffused emitted light of the semiconductor laser (40) is focused on the condenser lens (10) as shown in FIGS. 4(a) and (b).
), or an optical system using a flat plate (4″) as the beam splitter (4) as shown in Figures 5(a) and (b). For optical head devices using various optical systems such as
Similar results can be obtained by rotating by 5°.

[Effects of the Invention] As described above, according to the present invention, in an optical head device equipped with a semiconductor laser having an astigmatic difference, the polarization direction of the semiconductor laser is made to form an angle of 45° with respect to the track. Since the optimum focus offset position of the signal and the TE multiplied signal are made to match, the best reproduction signal can be obtained in a stable track following servo state. /This has the effect of significantly improving playback performance compared to conventional head manufacturing methods.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an optical head device according to an embodiment of the present invention, FIG. 2 is a diagram showing a state of defocusing of a spot on a record carrier in the optical head device of the present invention, and FIG. A diagram showing reproduction and TE multiplied signal defocus characteristics in the optical head device of the invention, FIG. 4 is an explanatory diagram of the optical system of another embodiment of the invention,
FIG. 5 is an explanatory diagram of the optical system of still another embodiment of the present invention, and FIG.
Figure 7 is a diagram showing the configuration of a conventional optical head device, Figure 7 is an explanatory diagram showing astigmatism of a semiconductor laser, Figure 8 is an explanatory diagram showing the correspondence between astigmatism and astigmatism difference, and Figure 9 is a conventional diagram. Figure 10 shows the defocusing of a spot on a record carrier in a conventional optical head device.
It is a figure which shows the E times signal defocus. In the figure, (4) is a beam splitter, (10) is a condensing lens, (12) is a record carrier, and (40) is a semiconductor laser. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] a laser light source device that emits linearly polarized light; a condensing lens that irradiates the light beam emitted from the laser light source device onto an information recording track of a record carrier; In an optical head device comprising an optical system comprising a beam splitter means for separating a beam from an emitted light beam of the device and directing the beam to a photoelectric converter side, an optical head device for controlling a reproduction signal of an information recording track obtained from the photoelectric converter and track following. The optical head device is characterized in that the polarization direction of the laser light source device is set at approximately 45 degrees or approximately -45 degrees with respect to the information recording track so that the optimum focus positions of the error signals coincide with each other.