JPH0728099B2 - Semiconductor laser optical collimator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor laser light collimating device used in an optical information processing device using laser light.

[Conventional technology]

Optical information processing devices such as optical disk devices, laser beam printers, and laser reading devices use parallelized laser light, but the light emitted from a semiconductor laser is
It has a radial spread angle of about 10 to 30 degrees (full width at half maximum). Therefore, the divergent light emitted from the semiconductor laser must be converted into parallel light, which requires a semiconductor laser light collimating device.

Hereinafter, a conventional semiconductor laser light collimating device will be described by taking an optical head for focusing laser light on an optical information carrier such as an optical disk as an example.

In the conventional optical head, as shown in FIG. 5, the light emitted from the semiconductor laser 1 is converted into parallel light by the collimator lens 2. At this time, the distance between the semiconductor laser 1 and the collimator lens is adjusted. , So that the divergent light from the semiconductor laser 1 becomes parallel light. Then, the light emitted from the collimator lens 2 is converted into a shaping prism 3 if necessary.
After being shaped by, the light beam is passed through the polarized beam spring 4 and the quarter-wave plate 5, and then is converged as a minute spot on the optical disk 7 by the objective lens 6. The light reflected by the optical disk 7 is separated by the 1/4 wavelength plate 5 and the polarization beam splitter 4, and for example, a photodetector (not shown)
It is converged on and used as an information signal or a servo error signal.

[Problems to be solved by the invention]

In this optical system, as described above, it is necessary to adjust the collimator to make the light emitted from the semiconductor laser 1 into parallel light by the collimator lens 2. However, this collimation adjustment is a very time-consuming operation when assembling the optical head. Particularly, in the case of recording or erasing light, a large output is required, and thus a collimating lens having a large numerical aperture NA is required. However, the depth of focus d becomes shallower as the numerical aperture NA increases. The depth of focus d is ± d = ± λ / {2 (NA) ² } where λ is the wavelength. Therefore, for example, when the numerical aperture NA is 0.5, ± d = ± 1.6
It is required to perform collimation adjustment with an accuracy of μm (λ = 780 nm) or less. Such highly accurate adjustment requires a skilled technique and is an extremely difficult task.

As a method of solving such a problem of the conventional technique, the present applicant makes the collimator lens have chromatic aberration,
It was proposed in Japanese Patent Application No. 61-22427 that the reflected light from the optical disk be returned to the semiconductor laser at a constant rate.

According to this method, it is possible to cope with a change in the distance between the semiconductor laser and the collimator lens to some extent, but when the position of the optical disc deviates from the image forming position or the reflectance of the optical disc changes significantly, the semiconductor laser is changed. Since the amount of return light of 1 changes greatly, collimation performance may change.

The present invention has been devised to solve the above-mentioned problems, and by securing a predetermined amount of light returning to the semiconductor laser, the collimation adjustment of the semiconductor laser light is greatly simplified and stable. The purpose is to obtain the collimating performance.

[Means for solving problems]

In order to achieve the above object, the semiconductor laser light collimating device of the present invention provides a collimator lens that converts light emitted from a semiconductor laser of a single longitudinal mode into parallel light with chromatic aberration, and emits light emitted from the collimator lens. Is provided with a reflecting plane for returning a part of the above to the semiconductor laser.

[Action]

In the present invention, a reflecting plane is provided in the optical path of the semiconductor laser to return a part of the light emitted from the collimator lens to the semiconductor laser. When the distance between the semiconductor laser and the collimator lens changes, the wavelength of the light emitted from the semiconductor laser changes so that the return light to the semiconductor laser becomes maximum. That is, since the collimator lens has chromatic aberration, a change in wavelength results in a change in focal length, and as a result, collimate adjustment is automatically performed. At this time, since a certain amount of light returning to the semiconductor laser is secured by the reflection plane, it is not affected by changes in the reflection state of the object irradiated with the light from the semiconductor laser, for example, an optical disk. Therefore, automation of collimation adjustment can be realized in a stable state.

〔Example〕

Prior to the description of the embodiments, the self-focusing phenomenon found in the optical head for an optical disk and forming the basis of the present invention will be described.

FIG. 6 is a diagram for explaining the self-focusing phenomenon. It should be noted that in the figure, the shaded lens is a lens having a predetermined chromatic aberration.

The light emitted from the semiconductor laser 1 is adjusted to parallel light by the collimator lens 2 and then converged on the optical disk 7 by the objective lens 6a. At this time, when the objective lens 6a has a predetermined amount of chromatic aberration and a predetermined amount of the light reflected by the optical disk 7 returns to the semiconductor laser 1, the range of the position of the optical disk 7 as shown in FIGS. Even if it shifts, the oscillation wavelength of the semiconductor laser 1 is automatically changed from λ ₁ to λ ₂ due to the chromatic aberration of the objective lens 6a, and the focal length of the objective lens 6a is changed from fo ₁ to fo ₂ and is automatically recorded on the optical disk 7. There is a phenomenon that it is in focus.

This is called the self-focusing phenomenon, and this phenomenon itself is the 46th Annual Meeting of the Japan Society of Applied Physics (19
Reported by Nakamura et al. In Autumn 1985 (Proceedings of the same lecture)
See 3P-X-9).

Here, the reason why the wavelength automatically changes is considered as follows. When the reflected light from the optical disk 7 returns to the semiconductor laser 1 in the single longitudinal mode, the oscillation becomes multimode, but when the optical disk 7 is focused, the amount of returned light becomes maximum and the multimode is stably performed.

On the other hand, since the objective lens 6a has chromatic aberration, the image forming position differs depending on the wavelength. As a result, it is considered that the light of the wavelength focused on the optical disk 7 is automatically selected so as to perform stable multi-mode oscillation.

The present invention applies this self-focusing phenomenon to collimator adjustment to automate the collimator adjustment.

Hereinafter, features of the present invention will be specifically described based on embodiments with reference to the drawings.

1 and 2 show a first embodiment of a semiconductor laser light collimating device according to the present invention, each showing a state in which the distance between the semiconductor laser and the collimating lens is different.

In the figure, 1 is a single longitudinal mode semiconductor laser, 2a is a collimating lens having a predetermined chromatic aberration, and 8 is a reflecting plane having a predetermined reflectance. In the figure, the shaded collimating lens has chromatic aberration, and the shaded reflecting plane has a predetermined reflectance.

In this embodiment, the collimator lens 2a is provided with a certain amount of chromatic aberration in place of the objective lens 6a in the self-focusing phenomenon shown in FIG. 6, and the reflection plane 8 is used in place of the optical disc 7 in the same phenomenon. Light is returned to the semiconductor laser 1.

With such a configuration, a self-collimating phenomenon similar to the self-focusing phenomenon is realized as follows.

The light emitted from the semiconductor laser 1 passes through the collimator lens 2a and then passes through the reflection plane 8 having a predetermined reflectance and is emitted. At this time, the collimator lens 2a has a predetermined chromatic aberration, and a certain amount of light reflected by the reflection plane 8 is reflected by the semiconductor laser 1
To return to FIG.
Even if the position of is shifted by a certain range, the oscillation wavelength of the semiconductor laser 1 automatically changes from λ ₁ to λ _2, and the focal length of the collimator lens 2a changes from fc ₁ to fc _2. The emitted light is automatically emitted as parallel light.

Here, the reason why the wavelength automatically changes is considered as follows.

When the light reflected by the reflection plane 8 returns to the semiconductor laser 1, the oscillation becomes multi-mode, but when the light emitted from the collimator lens 2a becomes a parallel light flux, the amount of light returned to the semiconductor laser 1 becomes the maximum and is the most stable. To multi-mode. on the other hand,
Since the collimator lens 2a is provided with chromatic aberration, the position of the light source when forming a parallel light flux differs depending on the wavelength. As a result, it is considered that the light of the wavelength that becomes the parallel light flux is automatically selected so as to perform stable multimode oscillation.

Next, the result of measuring how much the accuracy of the collimation adjustment is relaxed when using such a collimator is shown as an example.

8% of the light emitted from the semiconductor laser 1 using a collimating lens 2a with a numerical aperture NA of 0.5 and an amount of chromatic aberration of an object point of 1 μm / nm
Was returned to the semiconductor laser 1 from the reflection plane 8. as a result,
Even if the distance between the semiconductor laser 1 and the collimator lens 2a is shifted ± 10 μm from the optimum position, the light emitted from the collimator can be regarded as parallel light. On the other hand, in the conventional collimator, only parallel light of ± 1.6 μm can be regarded as parallel light as described above.

As is clear from this comparison, according to the present embodiment, the positioning accuracy required for the collimator adjustment was greatly relaxed.

As described above, in order to realize the self-collimating phenomenon, it is necessary that the collimator lens 2a have chromatic aberration. However, if the amount of chromatic aberration is too large,
On the contrary, due to chromatic aberration, the light emitted from the collimator lens 2a undergoes color separation, and parallel light cannot be obtained as a whole. Here, the light corresponding to the shift within the depth of focus of 2d can be regarded as parallel light, and therefore the chromatic aberration may be 2d / Δλ or less. However, Δλ is the half-width of the envelope of the oscillation spectrum of the laser light multimoded by the return light.

Further, in this example, the accuracy required for collimation adjustment was measured while changing the amount of light returning from the reflecting plane 8. As a result, it was found that in order to obtain a sufficient effect, 0.1% or more of the emitted light amount of the semiconductor laser 1 needs to be returned from the reflecting plane 8 to the semiconductor laser 1.
However, if the amount of reflected light is excessively large, the original utilization efficiency of the laser light is reduced, so it is practical to set the upper limit to about 30%.

FIG. 3 shows a second embodiment in which the semiconductor laser light collimating device of the present invention is applied to an optical head. In the figure, parts corresponding to the parts shown in FIG. 5 are designated by the same reference numerals.

In FIG. 3, the light emitted from the semiconductor laser 1 passes through a collimating lens 2a having a predetermined chromatic aberration and a shaping prism 3 and is incident on a reflecting flat plate 8a having a predetermined reflectance.
Part of the light returns to the semiconductor laser 1 by the reflecting flat plate 8a,
The rest passes through the polarization beam splitter 4, the quarter-wave plate 5 and the objective lens 6, and then converges on the optical disc 7. The reflection plate 8a can be formed, for example, by coating the surface of glass with a reflection film.

With the above-described configuration, the self-collimating phenomenon described above can be realized by the semiconductor laser 1, the collimating lens 2a having chromatic aberration, and the reflecting flat plate 8a as in the embodiment shown in FIGS. it can.

Further, the light beam splitting means composed of the polarization beam splitter 4 and the quarter-wave plate 5 prevents the reflected light from the optical disk 7 from affecting the self-collimating phenomenon as a disturbance. That is, since the reflected light from the optical disk 7 does not return to the semiconductor laser 1, even if the position of the optical disk 7 deviates from the focus position or the reflectance of the optical disk 7 largely changes, there is no effect.

Therefore, even if the distance between the semiconductor laser 1 and the collimator lens 2a is different within a certain range as shown in FIGS. 3A and 3B, the focal length of the collimator lens 2a is from fc ₁ to fc ₂
Therefore, the light emitted from the collimator lens 2a automatically becomes parallel light. Further, this action is not affected by the optical disc 7.

Next, the result of measuring how much the accuracy of the collimation adjustment is relaxed when using such a collimator is shown as an example.

Using a collimator lens 2a having a numerical aperture NA of 0.5 and an amount of chromatic aberration of an object point of 1 μm / nm, 7% of the light emitted from the semiconductor laser 1 was returned from the reflecting plate 8a to the semiconductor laser 1. The reflectance of the reflecting flat plate 8a is 10%. As a result, the semiconductor laser 1
Distance from the collimating lens 2a to ± 10 μm from the optimum position
Even if it was shifted, the light emitted from the collimator could be regarded as parallel light.

Also in this example, it was found that the adjustment accuracy becomes gentle as the amount of chromatic aberration increases and that the amount of chromatic aberration needs to be 0.5 μm / nm or more in order to obtain a sufficient effect. Further, it was found that 0.1% or more of the emitted light amount of the semiconductor laser 1 should be returned from the reflecting flat plate 8a to the semiconductor laser 1 in order to obtain a sufficient effect.

FIG. 4 shows a third embodiment of the present invention.

This embodiment is different from the second embodiment in that it is an independent reflecting flat plate.
8a is not provided, but the incident end face of the polarization beam splitter 4a is the reflecting plane 8. This can be easily realized by applying the same coating as that of the half mirror to the incident end surface of the polarization beam splitter 4a.

According to this embodiment, the effect of self-collimating can be obtained without increasing the number of parts as compared with the conventional optical head shown in FIG.

The means for forming the reflection plane is not limited to the above-mentioned embodiment, and for example, the emission plane of the beam shaping prism 3 may be used to form the reflection plane.

Further, a beam splitter, a half mirror, or the like may be used as the light beam separating means.

As described above, according to the present invention, a part of the light from the collimator lens is reflected by the reflecting plane, passes through the collimator lens having chromatic aberration, and is returned to the semiconductor laser. The amount of light is stably secured by the reflection plane. Therefore, an optical information carrier irradiated with laser light, for example,
The wavelength of the semiconductor laser is controlled so that the laser light from the semiconductor laser is always parallel light without being affected by the reflected light from the optical disc. As a result, the accuracy required for the collimating adjustment between the semiconductor laser and the collimating lens is greatly relaxed, and the collimating adjustment based on the mechanical accuracy becomes possible. In addition, the range in which the collimator is automatically adjusted is widened, and highly reliable collimated light can be obtained even if the position of the component is slightly changed due to thermal expansion or vibration during use. Further, since the collimate adjustment is automatically performed, highly reliable collimated light can be obtained as compared with the conventional collimate adjustment that relies on manual labor.

[Brief description of drawings]

FIG. 1 is a block diagram of a semiconductor laser light collimating device according to a first embodiment of the present invention, FIG. 2 shows a case where the distance between a semiconductor laser and a collimating lens is changed, and FIG. 3 shows the present invention as an optical head. FIG. 4 is a configuration diagram of a second embodiment applied to the present invention, FIG. 4 is a configuration diagram of a third embodiment where the present invention is applied to an optical head,
FIG. 5 is a configuration diagram showing a conventional optical head, and FIG. 6 is a diagram for explaining the principle of the self-focusing phenomenon which is the basis of the present invention. 1: Semiconductor laser, 2, 2a: Collimating lens 3: Shaping prism, 4, 4a: Polarization beam splitter 5: 1/4 wave plate, 6, 6a: Objective lens 7: Optical disk, 8: Reflection plane 8a: Reflection plate

Claims (3)

[Claims] 1. A collimating lens for converting light emitted from a semiconductor laser in a single longitudinal mode into parallel light has chromatic aberration, and a part of light emitted from the collimating lens is reflected back to the semiconductor laser. A semiconductor laser light collimating device having a flat surface. 2. The semiconductor laser light collimating device according to claim 1, wherein the collimating lens has an amount of chromatic aberration of an object point of 2d / Δλ or less. Where d: depth of focus Δλ: half-width of envelope of oscillation spectrum of laser light multimode by return light 3. The amount of light returning from the reflecting plane to the semiconductor laser is 0.1% with respect to the emitted light amount of the semiconductor laser.
The semiconductor laser light collimating device according to claim 1, wherein the above is the case.