JPH0536767B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is applicable to optical pickups, for example, in which the light source is a semiconductor laser, in which the convergence point of the oscillated light beam differs in the semiconductor junction plane and in the plane perpendicular thereto, causing astigmatism. Regarding optical devices such as, in particular, audio and video devices configured to refract the light beam derived from the semiconductor laser, form an image on the reading surface of an optical disk, and read information recorded on the reading surface. It is most suitable for application to the optical pickup of an optical disk player such as the above.

Background technology and its problems For example, a gain guiding type semiconductor laser, which is a type of double heterojunction semiconductor laser, has a self-coupling effect due to the return light beam from the reading surface, etc., which is often seen in the same type of index guiding type semiconductor laser. There is no increase in noise level due to For this reason, their value is being reconsidered as a light source for optical pickups, especially for video optical disc players, which require a high S/N ratio. This is because the gain guiding type semiconductor laser oscillates in longitudinal multi-mode, so the oscillation is less likely to be disturbed by the returned light beam compared to a semiconductor laser with longitudinal single mode oscillation such as the index guiding type semiconductor laser. This is due to the nature of
However, from the viewpoint of optical characteristics, as shown in Figure 1 A and B, the mode waist of the oscillated light beam of the gain guiding type semiconductor laser 1 is within the semiconductor junction plane (X-Y axis plane). and in a plane perpendicular to this (X-Z axis plane).
In other words, in the vertical plane (X-Z axis plane), the mirror surface 2
point A, which coincides with the bonding surface (X-Y
In the axial plane), point B is located inside the resonator, which is slightly deeper than the active layer 3 of the semiconductor laser, that is, the mirror surface 2.
Therefore, the convergence point of the oscillation light beam is different between the semiconductor junction surface (X-Y axis plane) and the plane perpendicular thereto (X-Z axis plane), resulting in optical astigmatism (ΔZ ).

If this type of semiconductor laser is used as a light source in the optical pickup of an optical disk player and is refracted by a lens or the like to form an image on the reading surface of the optical disk, the reading spot will be distorted and flattened due to astigmatism. It becomes a vertically long or horizontally long spot.
Therefore, due to straddling adjacent tracks, the OTF (Optical Transfer Function) of the required optical system is
A problem arises in that the characteristics cannot be obtained.

Conventionally, the following measures have been taken to solve this problem. One of them is (a) a means of extracting only a narrow angular portion near the center of the diverging light beam derived from the semiconductor laser and using it for reading to reduce wave front disturbance due to astigmatism. In practice, with this means, the influence of astigmatism differs depending on the numerical aperture of the collimator lens to guide the light beam to the objective lens. For this reason, as mentioned above, if only the narrow angle part near the center is extracted, the efficiency of light utilization will decrease;
The disturbance of the wave front becomes smaller. Therefore, the required OTF characteristics can be obtained for devices that do not require a high S/N ratio, such as the optical pickup of a digital audio optical disk (DAD) player.

That is, since a high S/N ratio is not required for the optical pickup of the DAD player, the required light intensity is not so high. For this reason, a relatively small collimator lens with a numerical aperture of, for example, 0.13 NA is sufficient, so even if a semiconductor laser with astigmatism of 25 μm is used as a light source, the wavefront disturbance will be the RMS value.
0.056 [λ], which is well within the diffraction limit, and there is no particular problem.

However, for applications that require a high S/N ratio, such as the optical pickup of a video optical disk player, this method has poor utilization efficiency of the luminous flux derived from the semiconductor laser, so it is necessary to increase the laser output. This has the disadvantage of shortening the life of the semiconductor laser.

Otherwise, considering the divergence angle of the light beam derived from current semiconductor lasers, an aperture of, for example, 0.2 NA or more is required for use as a light source for an optical pickup of a video optical disc player that requires a high S/N ratio. This is because it is necessary to form an image using several collimator lenses. However, in this case, the wavefront disturbance due to the 25 μm astigmatism has an RMS value of 0.13 [λ], which significantly deteriorates the OTF characteristics and makes it impossible to obtain the required OTF characteristics.

Incidentally, the RMS value of wavefront disturbance allowed within the diffraction limit is 0.07λ (Marechal Criterion). Conversely, the astigmatism of the laser required to satisfy this requirement is 13 μm, assuming that the numerical aperture of the collimator lens is 0.2 NA. Note that, considering the astigmatism of current gain guiding type semiconductor lasers, it is approximately 25 μm. Therefore,
When a high S/N ratio, that is, a strong light intensity is required, such as in an optical pickup of a video optical disc player, some kind of astigmatism correction is required.

As another means, (b) means is used to correct astigmatism using an optical element such as a cylindrical lens, which has different power (coupling force) depending on the direction. However, in this case, since the power of the optical element is made different depending on the direction, the optical element surface, that is, the lens surface, becomes a deformed curved surface instead of a spherical surface. Therefore, the disadvantage is that it is extremely difficult to form the surface. In addition, since there is power and the power differs depending on the direction, it is necessary to adjust the angular position of the optical element with respect to the optical axis, the position in the optical axis direction, and the directional position of the power with respect to astigmatism. Moreover, there is a drawback that it is difficult to adjust the position of the optical element.

Purpose of the Invention The present invention was invented in view of the above-mentioned circumstances, and its purpose is to use a semiconductor laser with astigmatism, even if it has a high S/N.
Even when a high ratio is required and a high light intensity is required, an optical element with easy surface formation and easy position adjustment can be easily incorporated into a device, and the required OTF characteristics can increase the laser output. The objective is to provide an optical device that can be obtained without any problems.

Summary of the Invention An optical device according to the present invention uses a semiconductor laser as a light source in which the convergence point of an oscillated light beam differs in a semiconductor junction plane and in a plane perpendicular thereto, resulting in astigmatism, and a cap covers the semiconductor laser. In the optical device covered with the cap, the window of the cap through which the light beam derived from the semiconductor laser passes has a normal vector that is within the semiconductor bonding surface of the semiconductor laser in order to correct astigmatism of the derived light beam. The astigmatism to be corrected is A _S , and the thickness of the parallel plane plate is the amount of astigmatism to be corrected. t′, the refractive index of the parallel plane plate is N,
When the inclination angle of the normal vector of the parallel plane plate with respect to the optical axis is U _P ′, the formula: A _S = t′/√N ² −sin ² U _P ′ [N ² cos ² U _P ′/N ² −sin ² U _P ′
-1], the desired amount of astigmatism correction is obtained, and when the amount of spherical missing coma generated in the parallel plane plate is coma _S and 1/2 of the aperture angle of the derived light beam is U, the formula: coms _S = t′・U ²・U _P ′・(N ² −1)/2N ³ In order to obtain a sufficiently small amount of missing pieces,
The invention is characterized in that the plate thickness t' and the inclination angle U _P ' of the parallel plane plate are set respectively.

As a result, the plane-parallel plate is an optical element whose surface is easy to form and whose position is easily adjusted.Moreover, the plane-parallel plate is provided in the window of the cap of the semiconductor laser, so that it can be easily and accurately incorporated into the device.
The required OTF characteristics can be obtained without increasing the laser power.

Embodiment Hereinafter, an embodiment in which the present invention is applied to an optical pickup will be explained with reference to the drawings, in which only the aperture optical system thereof is taken out.

FIG. 2 shows a cross section of the semiconductor laser taken along the optical axis within the semiconductor junction surface, and FIG. 3 shows a cross section taken along the optical axis within the plane perpendicular to the semiconductor junction surface of the semiconductor laser. Note that the coordinate axes shown in the figure are the first
It matches the diagram.

The semiconductor laser 1, which is a light source, is a gain guiding type, which is a type of double heterojunction semiconductor laser. As mentioned above, in this semiconductor laser 1, the convergence point of the oscillation light beam enters the resonator slightly deeper than the mirror surface 2 within the semiconductor junction surface (X-Y axis plane).
In a plane perpendicular to this (X-Z axis plane), it coincides with the mirror surface 2, resulting in astigmatism.

On the optical axis of the optical path of the light flux emitted from the semiconductor laser 1, the focal point of the light flux is set on the semiconductor laser 1 in order to refract the emitted light flux into a parallel light flux.
A collimator lens 4 having a required numerical aperture is provided to be positioned on the mirror surface 2 of the lens. Therefore, in the optical path of the divergent light beam between the semiconductor laser 1 and the collimator lens 4, the window W of the cap (CAP) that covers the semiconductor laser 1 through which the light beam led out from the semiconductor laser 1 passes. A parallel plane glass 5 having a predetermined thickness t and capable of transmitting a light beam is provided. As shown in the figure, the parallel plane glass 5 is provided so that its normal vector is inclined at a predetermined angle _UP within the semiconductor bonding surface (X-Y axis plane) of the semiconductor laser 1 with respect to the optical axis. ing. Thereby, astigmatism of the light beam derived from the semiconductor laser 1 is corrected. (The plate thickness t, angle U _P, etc. will be described later.) In addition, in order to receive the parallel light beam from the collimator lens 4 and refract it so as to form an image on the reading surface 6 of a video optical disk, etc., the reading surface An objective lens 7 whose focal point is located at 6 is provided on the optical axis. This image formation allows what is recorded on the reading surface 6 to be read.

Next, the reason why astigmatism is corrected even with the plane-parallel glass 5 will be explained based on FIG. 4.

The same applies to the case of a divergent state, but for example, as shown in the figure, the optical path of a light beam in a convergent state (numerical aperture NA
Assume that a parallel plane glass 5' (thickness t', refractive index N) is positioned at an angle U _P ' with respect to the optical axis. The amount of astigmatism that occurs at this time (astigmatism)
For example, WJSmith; Modern Optical
According to MCGraw-Hill NY1966, it is as follows.

A _s =l _s ′−l _t ′=t′/√N ² −sin ² U _P ′ [N ² cos ² U _P ′/(N ² −sin ² U _P ′)−1] ……(1) Note that l _t ′ is the distance to the convergence point in the plane that includes the normal and optical axis (meridian plane), and l _s ′ is the distance to the convergence point in the plane perpendicular to this (the spherical plane). .

On the other hand, the amount of spherical coma (spherical aberration) is coma _s = t′・U ²・_UP ′・(N ² −1)/2N ³ (2).

According to equations (1) and (2) above, the required numerical aperture, e.g.
At NA=sinU=0.2, by selecting the plate thickness t' and the tilt angle U _P ', it is possible to generate astigmatism of the same amount and opposite sign as the astigmatism generated in the semiconductor laser 1, and at the same time. It can be seen that it is possible to suppress the generated coma aberration to a sufficiently low level. That is, for any U _P ′≠0, the amount of astigmatism (astigmatism) is A _s =l _s ′−l _t ′<0.

Therefore, if the meridian plane is taken as the semiconductor junction surface of the semiconductor laser 1, the astigmatism of the semiconductor laser 1 can be corrected.

For example, if t' = 0.1 mm, U _P ' = 45°, and N = 1.5, the amount of astigmatism (astigmatism) is A _s = l _s ' - l _t ' = -0.025 (mm) = -25μm. Since the astigmatism of the current gain guiding type semiconductor laser 1 is about 25 μm as described above, it is clear that this can sufficiently correct the astigmatism. Moreover, the coma aberration that occurs at this time is approximately 0.02 [λ] in RMS value of wavefront aberration, and can be completely ignored.

Note that both astigmatism and coma are proportional to the thickness t' of the plane-parallel plate glass 5', and for the tilt angle U _P ', the astigmatism is approximately squared and proportional to the coma aberration, so they are constant. In order to generate the astigmatism, it is better to correct the plate thickness t' of the parallel plane glass 5' by making it smaller and increasing the inclination angle _UP ', so that the coma aberration that occurs at the same time can be kept low.

As described above, even if the parallel plane glass 5 of a predetermined plate thickness t and provided with the normal vector inclined at a predetermined angle _UP within the semiconductor bonding surface (X-Y axis plane) of the semiconductor laser 1 with respect to the optical axis, the semiconductor It can be seen that the astigmatism of the laser 1 is corrected.

Therefore, the reading spot imaged on the reading surface 6 by the objective lens 7 becomes approximately circular due to the correction of astigmatism by the plane-parallel glass 5, and the required OTF characteristics can be obtained without straddling adjacent tracks. .

In this embodiment, astigmatism was corrected using the plane-parallel glass 5, but a plane-parallel plate made of sapphire or the like may also be used.In short, it is a plane-parallel plate through which the light beam from the semiconductor laser 1 can pass. All you have to do is

Further, in this embodiment, only one parallel plane glass 5 is used, but astigmatism can be corrected if there is a plate thickness t required to correct a certain astigmatism. Therefore, this plate thickness t may be divided to provide two, three, etc. parallel plane glasses 5.

Furthermore, in this embodiment, the semiconductor laser 1 is a gain guiding type double heterojunction semiconductor laser, but the convergence point of the oscillation light beam differs between the semiconductor junction plane and the plane perpendicular to this, resulting in astigmatism. It goes without saying that the present invention can be applied to any semiconductor laser.

In addition, the normal vector of the parallel plane glass 5 is tilted by 45 degrees (= U _P ) with respect to the optical axis, and the surface opposite to the semiconductor laser 1 is coated with a vapor-deposited film to be used as a semiconducting transparent mirror and a beam splitter. It is also possible to use both.

Application Example The present invention provides a distance measuring device, an object movement measuring device,
It can be applied to information recording devices (optical audio, video disk master manufacturing devices, etc.), information transmission devices, and the like.

Effects of the Invention The present invention provides a parallel plane plate whose normal vector is tilted at a predetermined angle with respect to the optical axis within the semiconductor junction surface of the semiconductor laser in order to correct astigmatism of a light beam derived from a semiconductor laser. has been established. Therefore,
When a semiconductor laser is used as a light source, which causes astigmatism because the oscillation light flux differs between the semiconductor junction plane and the plane perpendicular to it, and a high S/N ratio is required and a large light intensity is required. However, desired OTF characteristics can be obtained without increasing laser power.

Furthermore, astigmatism and coma aberration of a plane-parallel plate are proportional to the thickness of the plane-parallel plate, and astigmatism and coma aberration are proportional to the square of the inclination angle of the plane-parallel plate. Focusing on this, the thickness and inclination angle of the parallel plane plate were respectively set so as to obtain a desired amount of astigmatism correction that reduces astigmatism and to obtain a sufficiently small amount of missing coma. Therefore, while astigmatism due to the semiconductor laser can be suppressed to a desired low value, coma aberration due to the parallel plane plate can be suppressed to a low value.

In addition, since the optical element that corrects astigmatism is a parallel plane plate, it is extremely easy to form the surface.
In addition, it is only necessary to adjust the angular position of the normal vector of the parallel plane plate within the semiconductor junction surface of the semiconductor laser with respect to the optical axis, and for this purpose, the position adjustment of the optical element for astigmatism correction is required. It's extremely simple.

Further, the parallel plane plate constituting the astigmatism correcting optical element constitutes the window of the cap that covers the semiconductor laser. Therefore, since there is no need to separately provide a support member for incorporating the plane parallel plate into the optical device, the structure can be simple and compact, and the optical device can be accurately mounted on the basis of the stem of a semiconductor laser. can be incorporated into

Furthermore, the window of the cap that covers the semiconductor laser is constructed from a parallel plane plate. Therefore, in an optical device configured to incorporate a conventional index guiding type semiconductor laser, it is easy to replace the laser chip with a gain guiding type semiconductor laser having astigmatism and change the shape of the cap. Since the present invention can be applied through simple operations, two types of semiconductor lasers can be used in an optical device of the same design, without creating a design burden and without requiring changes to the assembly line. As a result, no cost increase occurs. In addition, the inside of the cap is normally kept in an inert gas atmosphere to protect the semiconductor laser, but by simply covering the semiconductor laser with the cap and then introducing an inert gas into the cap, the inert gas atmosphere protects the semiconductor. The laser can be protected and astigmatism of the semiconductor laser can be corrected, thereby simplifying the manufacturing process.

[Brief explanation of drawings]

Figures 1A and 2B are diagrams explaining astigmatism in a gain guiding type semiconductor laser, and Figure 2 shows light passing through the semiconductor junction surface of the semiconductor laser in an aperture optical system of an optical pickup to which the present invention is applied. Figure 3 is a cross-sectional view taken along the optical axis in a plane perpendicular to the semiconductor junction surface of the semiconductor laser in Figure 2. Figure 4 explains the correction of astigmatism using parallel plane glass. This is a diagram. In addition, in the symbols used in the drawings, 1...
. . . semiconductor laser, 5 . . . parallel plane glass.

Claims (1)

[Scope of Claims] 1. The light source is a semiconductor laser in which the convergence point of the oscillated light beam is different in the semiconductor junction plane and in the plane perpendicular thereto, resulting in astigmatism, and the semiconductor laser is covered with a cap. In the optical device, the window of the cap through which the light beam derived from the semiconductor laser passes has a normal vector aligned with the optical axis within the semiconductor junction surface of the semiconductor laser in order to correct astigmatism of the derived light beam. It is composed of at least one transparent or translucent parallel plane plate of a predetermined thickness tilted at a predetermined angle with respect to the astigmatism amount A _S , the thickness of the parallel plane plate t′, and When the refractive index of the plane-parallel plate is N, and the angle of inclination of the normal vector of the plane-parallel plate with respect to the optical axis is U _P ′, the formula: A _S = t′/√N ² −sin ² U _P ′ [N ² cos ² U _P ′/N ² −sin ² U _P ′
-1], the desired amount of astigmatism correction is obtained, and when the amount of spherical missing coma generated in the parallel plane plate is coma _S and 1/2 of the aperture angle of the derived light beam is U, the formula: coms _S = t′・U ²・U _P ′・(N ² −1)/2N ³ In order to obtain a sufficiently small amount of missing pieces,
An optical device characterized in that a thickness t' and an inclination angle U _P ' of the parallel plane plate are set respectively.