KR910008500B1 - Optical apparatus - Google Patents

Abstract

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Description

Optics

1A and 1B illustrate astigmatism of a gain guiding semiconductor laser.

2 is a cross-sectional view along an in-plane optical axis perpendicular to the bonding surface of the semiconductor laser in the optical system of the optical reading device of one embodiment to which the present invention is applied.

3 is an enlarged view of a happy prism.

4 shows a change in refractive index ratio of a sensitivity coefficient of astigmatism.

* Explanation of symbols for main parts of the drawings

1 semiconductor laser 4 collimating lens

5: composite prism 8: photodetector

5d: parallel plane plate 5f: translucent reflective film

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an optical device using a semiconductor laser having a semiconductor laser as a light source in which the focusing point of oscillating light is different and astigmatism in the plane of the semiconductor assembly and in a plane perpendicular thereto, in particular, by refraction of the luminous flux derived from the semiconductor laser. The present invention relates to an optical device suitable for application to an optical reading device of a disc player, such as audio and video, which is configured to read what is recorded on this reading surface by forming an image.

For example, a kind of gain guiding semiconductor laser of a double heterojunction semiconductor laser is an increase in the noise level due to the self-coupling effect of the return light from the reading surface or the like, which often occurs in the same kind of index guiding semiconductor laser. There is no. For this reason, it is valuable as a light source of the optical reading device of a video disk player which requires a high S / N ratio.

This is due to the intrinsic property that the gain guiding semiconductor laser is less likely to be scattered by the return light than the semiconductor laser of the longitudinal single mode oscillation such as the index guiding semiconductor laser due to the longitudinal multimode oscillation. However, from the viewpoint of optical characteristics, as shown in Figs. 1A and 1B, the mode west of the oscillating light of the gain guiding semiconductor laser 1 is in the plane (XY axis plane) of the semiconductor and perpendicular to it (XZ). Axial plane). In other words, the point A coincides with the mirror surface 2 in the vertical plane (XZ axis plane), whereas the junction point (XY axis plane) slightly points in the active layer 3 of the semiconductor laser, and eventually the point B entered into the resonator inside the mirror plane 2. Becomes Therefore, in the joint surface (X-Y axis surface) and the surface perpendicular | vertical to this (X-Z axis surface), the connection point of oscillation light differs and it is set as astigmatism ((DELTA) 2) which is referred to as an optical image.

If this kind of semiconductor laser is used as a light source such as a video disk full layer and refracted by a lens or the like to form an image on the reading surface of the disk, the reading spot is distorted due to astigmatism, resulting in a flat longitudinal or horizontal spot. As a result, the position where the reproduction information signal and the tracking error signal are optimally matched, the external scattering margin for the servo such as defocus and disk cue decreases, and the desired optical system such as spanning adjacent tracks is reduced. There arises a problem that the OTF characteristic (optical transmission characteristic) cannot be obtained.

Conventionally, the following means have been taken to solve this problem. One of them is (a) a means for extracting only a narrow angle portion of the center root from the divergent light beams derived from the semiconductor laser for use in reading out and reducing the scattering of the wavefront due to astigmatism. As a means, the effect of astigmatism differs depending on how much numerical collimation lens guides the light beam to the objective lens. For this reason, when only a narrow angle portion near the center is drawn out as described above, light utilization efficiency decreases, but wavefront scattering becomes small. Therefore, if the high S / N ratio is not required, such as a digital audio disc (DAD) player, the required OTF characteristic is obtained.

That is, since the high S / N ratio is not required for the DAD player, the required light intensity is not necessary to that extent. For this reason, a relatively small collimation lens of 0.13NA, for example, is sufficient, so even if a semiconductor laser with astigmatism of 25 µm is used as a light source, wavefront scattering is within a diffraction limit of 0.056 (λ) with an RMS value. There is no.

However, in the case of requiring a high S / N ratio such as a video disc player or the like, according to this means, the use efficiency of the light derived from the semiconductor laser is poor, so that the laser output must be increased and the lifetime of the semiconductor laser is shortened. Occurs.

Otherwise, a light source such as a video disk player that requires a high S / N ratio may be formed as a collimating lens having a numerical aperture of 0.2 NA or more, taking into account the divergence angle of the light beam from which the current semiconductor laser is derived. Because there is. In this case, however, the wavefront scattering due to astigmatism of 25 µm becomes 0.13 (λ) in terms of RMS, which significantly deteriorates the OTF characteristic and thus fails to obtain the required OTF characteristic.

In addition, the RMS value of wavefront scattering that is allowed within the diffraction limit is 0.07 (λ). Conversely, the astigmatism of the laser necessary to satisfy this becomes 13 μm when the numerical aperture of the collimating lens is 0.2 NA. In addition, considering the astigmatism of the current gain guiding semiconductor laser, it is about 20 to 25 µm. Therefore, when a high S / N ratio, such as a video disc player or the like, has a strong light intensity, certain astigmatism correction is required.

As another means, (b) means for correcting astigmatism with optical elements having different powers (coupling forces) in different directions, such as cylindrical lenses or the like, are used. However, in this case, since the power of the optical element is changed depending on the direction, the optical element surface, that is, the lens surface, becomes a spherical surface rather than a spherical surface. Therefore, it has the drawback that the formation of the surface is extremely difficult. In addition, there is power, and the power differs depending on the direction. In addition to the angular position with respect to the optical axis of the optical element, the position in the coaxial direction and the directional position of the power with respect to astigmatism must be adjusted. It also has the drawback of being difficult.

In order to solve such a conventional drawback, the present inventor has already inserted at least one transparent or translucent parallel plane plate having a predetermined thickness in an optical path of a light beam derived from a semiconductor in Japanese Patent Application No. 57-25522, It has been proposed that the normal vector is inclined at a predetermined angle with respect to the optical axis to correct astigmatism of the derived light beam.

According to this means, for example, even if a high S / N ratio is required and a strong light intensity is required, the OTF characteristic is required as an optical element that does not increase the laser output, and is easy to form a surface and has a simple position adjustment. To achieve this.

Where No, do and Upo are the indices of refraction, thickness and optical axis of the parallel plane plate.

Therefore, if Upo = 45 ° No = 1.5, the thickness required to produce astigmatism of 20 μm is d ≒ 0.07mm.

On the other hand, for the purpose of correcting astigmatism only, when the aberration of the transmitted light wavefront is a problem, it is not necessary to make the surface precision of the parallel plane plate very high.

However, in order to make use of the above-mentioned source without increasing the component score of the apparatus, if a polarizing reflecting film is deposited by a translucent reflecting film on a thin parallel plane plate, the plane of the stacked parallel plane plate may be As a result, a large astigmatism coma aberration occurs on the reflected wave surface. As a result, the OTF characteristics deteriorate, and when considered as an optical reading device, there is a problem in the reproduction of faithful information signals.

The present invention has been invented in view of the above circumstances, and an object of the present invention is to provide an optical device that can make use of the source without increasing the number of parts of the device.

In the present invention, the optical device provides a composite prism composed of at least two prisms having substantially the same refractive index for correcting astigmatism of the derived light beam in an optical path of the light beam derived from the semiconductor laser 1, and the composite prism is (a) a film of a parallel plane layer having a refractive index different from the refractive index is deposited on a predetermined surface of one prism; (b) a translucent reflective film or a polarizing reflective film is deposited on a predetermined surface of the other prism; (c The predetermined surface of the one prism and the predetermined surface of the other prism are coupled to each other.

As a result, the reflective surface of the deposited semi-transparent reflective film or polarizing reflective film is maintained by a prism, and thus high surface accuracy can be maintained, which can also function as a beam splitter, and has desired characteristics without increasing the component score.

In the embodiment in which the optical device of the present invention is applied to an optical reading device of a disc player such as audio and video, the optical system will be extracted and described with reference to the drawings.

FIG. 2 shows a cross section along an optical axis in a plane perpendicular to the bonding surface of the semiconductor laser, and the coordinate axes shown in the drawing are aligned with FIG.

The semiconductor laser 1 which is a light source is a gain guiding type which is a kind of double heterojunction semiconductor laser. As described above, the semiconductor laser 1 has a connection point of the oscillation light in the resonator slightly smaller than the mirror surface 2 in the joint surface (XZ axis surface), and coincides with the mirror surface 2 in the surface perpendicular to the mirror surface (XZ axis surface). Astigmatism.

And on the optical axis of the optical path of the luminous flux derived from this semiconductor laser 1, the collimating lens 4 of the required numerical aperture which positions an exiting light so that it may become a parallel luminous flux is provided.

The composite prism 5 is provided in the optical path of the light beam in the divergent state between the semiconductor laser 1 and the collimating lens 4. As shown in FIG. 3, this composite prism 5 is composed of two right-angle triangular prisms 5a and 5b having the same refractive index N _H and the right angle Up _θ . Then, a film 5d, which is a parallel plane layer having a thickness d having a refractive index N _L smaller than the refractive index N _H , is deposited on the slope 5b of one prism 5a, and the slope 5e of the other prism 5b. A semi-transparent reflective film 5f is deposited on the surface, and the slopes 5c and 5e sides of both prisms 5a and 5b are joined by known adhesive means. Therefore, the composite prism 5 has a function of adjusting astigmatism described later of the semiconductor laser 1 by the film 5d and a beam splitter function by the semi-transparent reflective film 5f.

In addition, an objective lens 7 for focusing the reading surface 6 on the optical axis in order to receive parallel light beams from the collimating lens 4 and to be refracted so as to form an image on the reading surface 6 such as a video disc. It is arranged.

On the other hand, the reflected light beam from the reading surface 6 resulting from this phase, i.e., the return light, traverses the optical path through the objective lens 7 and the collimating lens 4 in reverse, and thus the translucent reflective film of the composite prism 15 ( The photodetector 8 is provided in the position reflected by 5f). The photodetector 8 reads the light recorded on the reading surface 6 and the like by the reflection of the light beam from the reading surface 6 on the light receiving surface. That is, the reproduction information signal, the tracking error signal, and the like are detected and obtained.

Next, correction of astigmatism by the film 5d on the parallel plane side of the composite prism 15 will be described.

The boiling point gap amount (astigmatism) generated in the light flux in the diffusion state passing through the blocking film 5d passing through the composite prism 15 is expressed as follows.

here

to be. This Nr is a refractive index ratio obtained by converting the refractive index N _L of the film 5d into the refractive index N _L of the prisms 5a and 5b.

Observing the above formulas (1) and (2), the following is readily known.

① When N _H = 1, the form is consistent.

(N ₀ = N _r , Upo = Up, do = d)

(2) When the refractive index ratio Nr is Nr> 1, the polarity of the boiling point gap amount (astigmatism) becomes the same as in the formula (1), but when Nr <1, the polarity is reversed.

(3) When Nr <1, the denominator is close to zero, which is larger than the absolute value of the astigmatism difference (astigmatism) when Ne> 1 given the same inclination angle Up thickness d. Sensitivity factor at tilt angle Up = 45 ° now

The change in the refractive index ratio Nr is expressed as shown in FIG.

Therefore, it is preferable to use Nr <1, but in this case, in order to correct the astigmatism of the semiconductor laser, it is necessary to use it as S-polarized incidence as shown in FIG. In other words, it is necessary to orthogonal to the incidence surface formed by the slope normal of the film 5d and the light beam optical axis. In addition, when used as Nr> 1, the composite prism 5 is arrange | positioned so that P-polarized light may be incident.

However, for the deposition of the refractive index N _L = 1.52 as the film 5d by using the SF 11 of the refractive index N _H = 1.766 (λ = 780 nm) with the right angle Up _θ (= tilt angle) = 45 ° on the prisms 5a and 5b. When the glass is deposited with an electron beam such that the thickness is d = 0.03mm (including the thickness of the adhesive having a refractive index of 1.52), the astigmatism amount (astigmatism) Aso that can be corrected from the above formula (2) becomes 18.6 μm. .

As a result, the read spot formed by the objective lens 7 on the read surface 6 is almost circular in shape as astigmatism is corrected, and the reflecting film is formed by depositing the translucent reflecting film 5f on the slope 5e of the prism. The surface precision of the reflecting surface of (5f) is also kept high, so that the required OTF characteristic of the whole is obtained.

That is, since the surface accuracy of the reflective surface of the reflective film 5f is poor due to curvature or the like, the detection of the focus error signal is not disturbed.

In the present embodiment, the composite prism 5 is inserted into the optical path of the ore in the divergent state, but may be inserted between the objective lens 7 and the reading surface 6 in the optical path of the luminous flux in the connected state, for example.

The present invention has the following effects. First, by depositing a semi-transparent reflective film or a polarizing reflective film on the surface of the prism, the reflective surface of the reflective film can maintain high surface precision with the prism, so that the OTF characteristics can be maintained without deterioration, thus increasing the parts score of the stationary part. Don't let that happen. Second, since it was deposited as a film forming a parallel plane layer on the surface of one prism, unlike the parallel plane plate, there is almost no need for polishing and cleaning, thereby reducing the cost. Third, since the number of parts does not increase, work such as component attachment is reduced.

Moreover, when the configuration described in the aspect of the embodiment is taken, the sensitivity of the astigmatism increases, so that the thickness of the film serving as the planar layer can be reduced for the astigmatism deposited on the surface of one prism, thereby reducing the deposition cost.

Claims (5)

A semiconductor laser (1) which generates astigmatism because the connection points of the oscillating light are different in the bonding surface of the semiconductor laser (1) and in the surface perpendicular to the semiconductor laser (1); An optical device comprising at least one pair of prisms 5a and 5b disposed in an optical path of a luminous flux derived by the semiconductor laser 1 and composed of at least two prisms having the same refractive index to correct astigmatism. And a film 5d as a parallel plane plate having a predetermined thickness different from that of the prism is deposited on one of the prisms 5a and 5b, and a translucent reflective film on the other one of the prisms 5a and 5b. Or a polarizing reflective film (5f) is deposited, and predetermined surfaces (5c, 5e) of the prism are bonded to each other such that the parallel plane plate (5d) and the translucent reflective film or polarizing reflective film (5f) are sandwiched therebetween. The optical device of claim 1, wherein a refractive index of the parallel plane plate is smaller than a prism refractive index of the pair of prisms. The optical device according to claim 2, wherein the pair prism is arranged such that the normal vector of the parallel plane plate layer is perpendicular to the bonding surface of the semiconductor laser and inclined at a predetermined angle with respect to the optical axis in the plane extending along the optical axis. The optical device according to claim 1, wherein the refractive index of the parallel plane plate is larger than the prism refractive index of the twin prism. The optical device according to claim 4, wherein the pair prism is arranged such that the normal of the layer of the parallel plane plate is inclined at a predetermined angle with respect to the optical axis in the bonding surface of the semiconductor laser.