JPH056261B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical device using a semiconductor laser as a light source, in which the convergence point of oscillated light is different in a semiconductor junction plane and in a plane perpendicular to this, resulting in astigmatism. In particular, optical disc players such as audio and video players are configured to read what is recorded on the reading surface by refracting the light beam emitted from the semiconductor laser and forming an image on the reading surface. It is most suitable for use in reading devices.

For example, a gain-guiding type semiconductor laser, which is a type of double heterojunction semiconductor laser, has a noise level that is caused by the self-coupling effect caused by the return light from the reading surface, which is often seen in the same type of index-guiding type semiconductor laser. There is no increase. For this reason, their value is being reconsidered as a light source for optical reading devices, especially for video disc players, which require a high S/N ratio. This is because gain guiding type semiconductor lasers emit longitudinal multi-mode oscillations, so compared to semiconductor lasers with longitudinal single mode oscillations such as index guiding type semiconductor lasers, the oscillation is less likely to be disturbed by returned light. This is due to the nature of However, from the perspective of optical properties, the first
As shown in Figures A and B, the mode waist of the oscillated light 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). ) is different. That is, in the vertical plane (X-Z axis plane), point A coincides with the mirror surface 2, whereas in the junction plane (X-Y axis plane), it is slightly closer to the active layer 3 of the semiconductor laser, that is, the mirror plane 2. Point B enters the inner resonator. Therefore, the convergence point of the oscillated light is different between the cemented surface (X-Y axis plane) and the plane perpendicular to this (X-Z axis plane), resulting in optical astigmatism (ΔZ). It is becoming.

If this type of semiconductor laser is used as a light source for an optical reader of a video disc player,
When the light is refracted by a lens or the like and an image is formed on the reading surface of the optical disk, the reading spot is distorted due to astigmatism and becomes a flat vertically or horizontally long spot. Therefore, due to straddling adjacent tracks, the required optical system OTF (Optical Transfer
A problem arises in that the characteristics (Function) 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 used to guide the light beam to the objective lens. For this reason, if only the narrow angle part near the center is extracted as mentioned above, the efficiency of light utilization will decrease;
The disturbance of the wave front becomes smaller. Therefore, the required OTF characteristics can be obtained if the device does not require a high S/N ratio, such as an optical reading device for a digital audio disc (DAD) player.

That is, as an optical reading device for a DAD player, it does not require a high S/N ratio, and therefore does not require much light intensity. Therefore, 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 determined by the RMS value.
It is 0.056 [λ], which is well within the diffraction limit, and there is no particular problem.

However, for devices that require a high S/N ratio, such as an optical reading device for a video disc player, this method has poor utilization efficiency of the light derived from the semiconductor laser, so the laser output must be increased. This has the disadvantage of shortening the life of the semiconductor laser.

Otherwise, in order to use it as a light source for an optical reader of a video disc player, etc., which requires a high S/N ratio, considering the divergence angle of the light beam derived from current semiconductor lasers, it is necessary to This is because it is necessary to form an image even with a collimator lens having a numerical aperture. 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, the astigmatism is about 25 μm. Therefore,
When a high S/N ratio, that is, a high light intensity is required, such as in an optical reading device for a video disc player, some kind of astigmatism must be corrected.

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 to vary depending on the direction, the optical element surface, that is, the lens surface, becomes a deformed curved surface instead of a spherical surface. Therefore, it has the disadvantage 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. However, it also has the disadvantage that it is difficult to adjust the position of the optical element.

Japanese Utility Model Application Publication No. 56-45815 discloses that a plane-parallel plate is arranged in the light beam emitted from a semiconductor laser to correct astigmatism of the emitted light.

However, when astigmatism is corrected using such a plane-parallel plate, comatic aberration occurs due to the insertion of the plane-parallel plate.

In view of this problem, it is an object of the present invention to correct astigmatism of a semiconductor laser light source while suppressing the occurrence of coma aberration.

The present invention uses a semiconductor laser as a light source, in which the convergence point of the oscillated light differs in the semiconductor junction plane and in the plane perpendicular thereto and causes astigmatism. At least one transparent or semi-transparent parallel flat plate is provided, and its normal vector is tilted at a predetermined angle within the semiconductor junction surface of the semiconductor laser with respect to the optical axis to correct astigmatism of the derived light beam. An optical device using a semiconductor laser, in which the amount of astigmatism to be corrected is A _s , the thickness of the parallel plate is t′, the refractive index of the parallel plate is N, and light with a normal vector of the parallel plate When the inclination angle with respect to the axis is U _p ′, the formula When the desired amount of astigmatism correction is obtained, and when U is 1/2 of the aperture angle of the derived light beam, the amount of spherical missing coma generated in the parallel plate coma _s = t′・U ²・U An optical system using a semiconductor laser, characterized in that the thickness t' and the inclination angle U _p ' of the parallel plate are respectively set so that _p '·(N ² -1)/2N ³ becomes a sufficiently small value. It is related to the device.

This configuration suppresses coma aberration to a sufficiently low level and corrects astigmatism.

Hereinafter, an embodiment in which the present invention is applied to an optical reading device using a semiconductor laser will be explained with reference to the drawings, with only the aperture optical system thereof being taken out.

FIG. 2 shows a cross section of the semiconductor laser along the optical axis within the bonded surface, and FIG. 3 shows a cross section of the semiconductor laser along the optical axis within a plane perpendicular to the bonded surface. Note that the coordinate axes shown in the figure are aligned with those in FIG.

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 enters the resonator slightly deeper than the mirror surface 2 in the junction plane (X-Y axis plane), and the convergence point in the plane perpendicular to this (X-Z axis plane) 2) coincides with the mirror surface 2, resulting in astigmatism. Then, in order to bend the emitted light onto the optical axis of the optical path of the light flux derived from this semiconductor laser 1 so that it becomes a flat light flux,
A collimator lens 4 having a required numerical aperture is provided to locate its focal point on the mirror surface 2 of the semiconductor laser 1. Furthermore, in the optical path of the light beam in a diverging state between the semiconductor laser 1 and the collimator lens 4, the normal vector thereof is set within the junction plane (X-Y axis plane) of the semiconductor laser 1 with respect to the optical axis. A piece of _parallel plane glass 5 having a predetermined thickness t is provided which can be tilted at an angle Up to transmit a light beam. 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 6 An objective lens 7 whose focal point is located on the optical axis 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.

As shown in the figure, for example, a parallel plane glass 5' (thickness t', refractive index N) is positioned at an angle U _p ' with respect to the optical axis in the optical path of a converged light beam (numerical aperture NA=sinU). ing. The amount of astigmatism (astigmatism) that occurs at this time is, for example, WJSmith; Modern
Optical Engineering (MCGraw-Hill N.
According to Y.1966), it is as follows.

Note that lt' is the distance to the convergence point in the plane (meridian plane) including the normal line and optical axis, and ls' 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・U _p ′・(N ² −1)/2N ³ (2).

Based on equations (1) and (2) above, at the required numerical aperture, for example NA=sinU=0.2, the plate thickness t′ and the inclination angle U _p ′
It can be seen that by selecting , it is possible to generate astigmatism of the same amount and opposite sign as the astigmatism generated in the semiconductor laser 1, and to suppress coma aberration that occurs at the same time 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 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) = - It becomes 25μm. As mentioned above, the astigmatism of the current gain guiding type semiconductor laser 1 is about 25 μm, so it is clear that the astigmatism can be sufficiently corrected. 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 astigmatism is approximately squared and coma is proportional to the inclination angle U _p '.
In order to generate a certain level of astigmatism, it is better to reduce the plate thickness t' of the plane-parallel glass 5' and to greatly correct the inclination angle U _p ', thereby simultaneously suppressing the coma aberration that occurs.

As described above, the normal vector is set at a predetermined angle within the junction plane (X-Y axis plane) of the semiconductor laser 1 with respect to the optical axis.
It can be seen that the astigmatism of the semiconductor laser 1 can be corrected even with the parallel plane glass 5 having a predetermined thickness t and provided at an angle U _p .

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

Furthermore, although the parallel plane glass 5 is provided in the optical path of the divergent light beam, as is clear from the above-mentioned reason for astigmatism correction, the parallel plane glass 5 is provided in the optical path of the convergent light beam, for example between the objective lens 7 and the reading surface 6. It may be provided in between.

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 oscillated light is different in the junction plane and in the plane perpendicular to the junction plane, which causes astigmatism. It goes without saying that the present invention can be applied to any laser.

Incidentally, 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, so that it can be used as a semi-transparent mirror and used as a beam splitter. It is also possible to use both. It can also be used as a semiconductor laser cap window.

The present invention also provides a distance measuring device using a semiconductor laser, an object movement measuring device, an information recording device (optical audio, video disk master manufacturing device, etc.),
It can be applied to information transmission devices, etc.

As described above, in order to correct the astigmatism of a semiconductor laser light source, when a parallel plate is placed in the laser beam, the amount of correction of the astigmatism is equal to the thickness of the parallel plate and the inclination angle. The thickness and inclination angle of the parallel plate are set so that the comatic aberration generated in the parallel plate is sufficiently small. In other words, both astigmatism and coma are proportional to the thickness of the parallel plate, and since astigmatism is approximately squared and coma is proportional to the tilt angle, the desired astigmatism is generated in the parallel plate. for,
By setting the plate thickness to be small and the tilt angle to be large, coma aberration occurring in the parallel plate can be suppressed to a low level.

Therefore, according to the present invention, when a parallel plate is placed in a light beam to correct astigmatism of a semiconductor laser light source, there is little deterioration of coma aberration caused by the parallel plate, and therefore the light source is relatively inexpensive and simple. A highly accurate optical device can be constructed using the optical element and the optical element.

[Brief explanation of the drawing]

Figures 1A and 1B are diagrams explaining astigmatism of a gain guiding type semiconductor laser, and Figure 2 shows a semiconductor laser in an aperture optical system of an optical reading device using a semiconductor laser as an example of the present invention. Figure 3 is a cross-sectional view taken along the optical axis within the junction plane of the semiconductor laser in Figure 2, Figure 4 is a cross-sectional view taken along the optical axis within a plane perpendicular to the junction plane of the semiconductor laser in Figure 2, and Figure 4 is an astigmatic cross-sectional view of the semiconductor laser in Figure 2 taken along the optical axis. It is a figure explaining correction of aberration. In addition, in the symbols used in the drawings, 1...
. . . semiconductor laser, 5 . . . parallel plane glass.

Claims (1)

[Scope of Claims] 1. A light source is a semiconductor laser in which the convergence point of the oscillated light differs in the semiconductor junction plane and in the plane perpendicular thereto and causes astigmatism. At least one thick transparent or semi-transparent parallel flat plate is provided, and its normal vector is tilted at a predetermined angle within the semiconductor bonded surface of the semiconductor laser with respect to the optical axis to correct astigmatism of the derived light beam. In this optical device using a semiconductor laser, the amount of astigmatism to be corrected is A _s , the thickness of the parallel plate is t′, the refractive index of the parallel plate is N, and the normal line of the parallel plate is The angle of inclination of the vector with respect to the optical axis is
When U _p ′, the expression When the desired amount of astigmatism correction is obtained, and when U is 1/2 of the aperture angle of the derived light beam, the amount of spherical missing coma generated in the parallel plate coma _s = t′・U ²・U An optical system using a semiconductor laser, characterized in that the thickness t' and the inclination angle U _p ' of the parallel plate are respectively set so that _p '·(N ² -1)/2N ³ becomes a sufficiently small value. Device.