JPH0463556B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to improvement of beam quality in a laser device, particularly a high-power laser device.

[Conventional technology]

Figure 9 shows, for example, a laser handbook (Laser handbook).
Handbook1979.North−Holland Publishing
1 is a cross-sectional side view showing a conventional laser device having an unstable resonator described in J. Campany. In the figure, 1 is a collimating mirror made of a concave mirror, 2 is a magnifying mirror made of a convex mirror placed opposite to this collimating mirror, and both mirrors 1,
2 constitutes a total reflection mirror. 3 is the laser medium, which is CO ₂
For example, a gas laser such as a laser is a gas medium excited by an electric discharge, and a solid laser such as a YAG laser is a glass medium excited by a flash lamp, etc. 4 is a wind mirror, and 5 is a window. A non-reflection coating film applied on the mirror surface, 6 a box covering the surrounding area, 7 a laser beam generated within an optical resonator formed by mirrors 1 and 2, and 8 taken out from the periphery of the magnifying mirror. This is a laser beam.

Next, the operation will be explained.

The mirrors 1 and 2 constitute a so-called unstable resonator, and the laser beam reflected and expanded by the magnifying mirror 2 is amplified by the laser medium 3 and collimated into a parallel beam by the collimating mirror 1. It is reflected onto the periphery of the mirror and taken out from the wind mirror 4 as a ring-shaped beam. Since the extracted ring-shaped laser beam 8 is obtained with almost the same phase, it becomes a medium-height beam by condensing it with a lens or the like, making it possible to efficiently cut, weld, etc. iron plates.

In addition, the degree of condensation is determined by the ratio of the inner diameter to the outer diameter (M value (Magnification factor)) of the ring-shaped beam extracted, and the larger the M value, that is, the more the beam is extracted from the center, the better the beam is focused. be illuminated. However, if the M value is increased, the oscillation efficiency deteriorates significantly, so the upper limit of the M value that is actually used industrially is about 2.

[Problem that the invention seeks to solve]

Conventional laser devices are configured as described above, so increasing the M value in order to improve light focusing characteristics will deteriorate the oscillation efficiency.
There was a problem in that the value could not be raised to near infinity where the best light collection performance could be obtained. In addition, since the wind mirror 4 is heated unevenly by the ring-shaped laser beam, uneven internal stress is generated.
There were problems such as disrupting the phase distribution of the passing laser beam and deteriorating the focusing performance.

This invention was made to solve the above-mentioned problems, and it is possible to increase M without reducing the oscillation efficiency.
The object of the present invention is to easily obtain a laser device that can extract a high-quality laser beam with a value close to infinity.

[Means for solving problems]

In the laser device according to the present invention, the magnifying mirror has partial transparency, and the laser beam passing through the magnifying mirror and the laser beam passing through the peripheral portion of the mirror are separated into the resonator or outside the laser beam output side of the resonator. A phase compensation mirror is provided to cancel the phase difference.

[Effect]

The phase compensation mirror according to the present invention eliminates the phase difference between the laser beam passing through the magnifying mirror section and the mirror peripheral section, and produces a laser beam with a uniform phase.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a cross-sectional side view showing a laser device according to an embodiment of the present invention. In the figure, 4 is a convex mirror that also serves as a wind mirror, and has a partial reflectance in the center of the surface facing the collimating mirror 1. A partially reflective film 20 (for example a ZnSe thin film) is provided and acts as a magnifying mirror. In addition, the peripheral part of the reflective film 20 and the other surface are coated with a non-reflective coating film 5.
(e.g. PbF ₂ thin film). Furthermore, there is a step 200 in the center between the convex mirror 4 and the collimating mirror 1, and a non-reflection coating film 5 is provided on both sides.
A phase compensation mirror 9 is provided.

The following operation will be explained.

Reflection film 20 of collimating mirror 1 and convex mirror 4
The part constitutes a so-called unstable resonator, and the laser beam 7 reflected and expanded by the reflection film 20 of the convex mirror 4 is amplified by the laser medium 3, and
The laser beam is collimated into a parallel beam by a collimating mirror 1, and extracted as a laser beam 8 from a convex mirror 4 to the outside.

This laser beam 8 consists of a portion 70 that passes through the partially reflective film 20 and a portion 71 that passes through the non-reflection coating film 5, both of which are separated by a phase compensation mirror 9.
The phase of the laser beam 8 is corrected and the laser beam 8 is outputted to the outside as a laser beam 8 having an equal phase. Since this laser beam is centered, the M value defined in the conventional unstable resonator corresponds to infinity.

FIGS. 2a and 2b are characteristic diagrams schematically showing pattern shapes when a laser beam generated in an unstable resonator according to a conventional method and an embodiment of the present invention are focused by a lens, respectively, and the horizontal axis is distance from the optical axis,
The vertical axis is the beam intensity.

In this experiment, in order to make the oscillation characteristics of the two almost the same, the reflectance of the reflective film 20 was 50%, and the reflectance of the reflective film 20 was 50%.
The ratio between the zero diameter and the beam outer diameter was set to 1.5. (i.e.,
Magnifying mirror 2 of conventional unstable resonator with M=1.5
The unstable resonator of the present invention was made by giving 50% partial transparency to the resonator. ) Furthermore, the curvatures of both surfaces of the convex mirror 4 were made the same (the thickness was made constant), so that the laser beam 8 remained a parallel beam even after passing through the convex mirror 4. Figure 2 a, b
Comparing the respective light focusing performances shown in , it can be seen that the one according to the present invention (FIG. 2b) has a high center intensity and can obtain a laser beam concentrated on the optical axis.

Next, the operation of the phase compensation mirror 9 will be described.

The laser beams 70 and 71 pass through the portions of the wind mirror 4 coated with two types of coating films 20 and 5, respectively, and are extracted to the outside. Since the phase change imparted to the laser beam passing through it differs depending on the type of coating film, if there is no phase compensation mirror, the equal phase laser beam 7 generated inside would be extracted to the outside as a laser beam with a spatial phase distribution. Ru.

FIG. 3 shows the focusing characteristics for the difference between the phase given to the laser beam passing through the partial reflection film 20 on the inner surface of the wind mirror 4 and the phase given to the laser beam passing through the non-reflection coating film 5. An example calculation is shown. As for the focusing characteristics, the beam intensity when the laser beam is focused by a lens is 1/1 of the center.
It shows the diameter of a condensing spot defined by a diameter of e ² and the power concentration, which indicates what percentage of the total power is contained within that diameter.

The smaller the former, the narrower the laser beam becomes.
In addition, the larger the latter, the more power can be concentrated within a narrowed area, and therefore the more efficient processing can be performed, so it can be judged that the light collection characteristics are better. Third
The figure shows that in order to keep the focal spot diameter small and the power concentration high, the phase difference needs to be kept within about 45°. But for example
Taking mirrors for CO ₂ lasers as an example, it is often observed that this phase difference is 60° or more.

The phase compensation mirror is configured to cancel this phase difference. In the example shown in FIG.
A step 200 is provided between the portions where 0 and 71 pass to vary the thickness of each portion.

For example, the partial reflection film 2 on the inner surface of the wind mirror 4
The phase given to the laser beam passing through 0 is
It is assumed that the phase is δ (degree) ahead of the phase given to the laser beam passing through the non-reflection coating film 5. The depth d of the step 200 at the center of the phase compensation mirror 9 for correcting this is determined by (n-1)d/λ=δ. However, n is the refractive index of the material of the phase compensation mirror 9, and λ is the wavelength of the laser beam.

In the above embodiment, a transmission type mirror is used as the phase compensation mirror 9, but a reflection type mirror may be used as shown in FIG. In this case, the laser beam can be generated as linearly polarized light, and is ideal for high-precision machining, where linearly polarized light is converted into circularly polarized light outside the laser device. Further, the installation location of the phase compensation mirror 9 is not limited to the inside of the resonator, but may be installed outside the resonator as shown in FIGS. 5 and 6. Further, although an example has been shown in which the phase change on the phase compensation mirror surface is realized by a step difference, anti-reflection films 5 and 55 having different configurations may be used as shown in FIG. 7. For example, if we take a mirror for a CO ₂ laser, the substrate (ZnSe) is PbF ₂ 1
A non-reflective coating film can be achieved by using two layers of ZnSe and ThF ₄ , but in this case, the phase difference due to passing through both films is 20° or more.
In short, it is sufficient if the configuration is devised to create a phase difference between the central portion and its surroundings.

Further, as shown in FIG. 8, a concave mirror may be used as the magnifying mirror.

〔Effect of the invention〕

As described above, according to the present invention, since the magnifying mirror of the unstable resonator is configured to have partial transparency, it is possible to obtain a laser beam with good focusing characteristics without sacrificing oscillation efficiency. By using the laser beam as a lever, it is possible to perform high-speed, efficient, and highly accurate laser processing. Furthermore, since the laser beam heats the entire wind mirror, thermal stress is less likely to occur and the beam can be stably extracted for a long period of time.

Furthermore, since the phase compensation mirror is provided separately from the resonator mirror, each can be designed optimally, and the same phase beam can be easily extracted to the outside.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional side view showing a laser device according to an embodiment of the present invention, FIGS. 2a and 2b are characteristic diagrams showing the focusing characteristics of a conventional laser device and a laser device according to an embodiment of the present invention, respectively. FIG. is a curve diagram explaining the necessity of phase compensation which is the basis of this invention, FIGS. 4 to 8 are cross-sectional side views showing laser devices according to other embodiments of this invention, and FIG. 9 is a diagram showing a conventional laser device. It is a cross-sectional side view showing a laser device. In the figure, 1 is a collimating mirror, 3 is a laser medium, 4 is a convex mirror, 5 is a non-reflection coating film, 7, 70, 71, 8 are laser beams, 9 is a phase compensation mirror, 20 is a partial reflection film, 200 is a step , 41 is a concave mirror. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1. A magnifying mirror having partial transmittance and consisting of a concave or convex mirror, disposed opposite to this magnifying mirror,
an unstable resonator comprising a collimating mirror that reflects a laser beam reflected and expanded by the magnifying mirror onto the magnifying mirror and a peripheral portion of the mirror; A laser device provided with an external phase compensation mirror for canceling a phase difference between a laser beam passing through the magnifying mirror and a laser beam passing around the mirror. 2. The laser device according to claim 1, wherein the magnifying mirror is formed by providing a reflective film having partial reflectance in the center of a concave or convex mirror. 3. The laser device according to claim 2, wherein the concave or convex mirror has the same curvature on both sides. 4. The laser device according to any one of claims 1 to 3, wherein the phase compensation mirror is a transmission mirror. 5. The laser device according to any one of claims 1 to 4, wherein the phase compensation mirror is a reflection mirror. 6. The laser device according to any one of claims 1 to 5, wherein a step is provided on the surface of the phase compensation mirror to correspond to the magnifying mirror portion and the peripheral portion of the mirror. 7. The laser device according to any one of claims 1 to 6, wherein thin films having different configurations are provided on the surface of the phase compensation mirror, corresponding to the magnifying mirror portion and the peripheral portion of the mirror.