JP2691773B2 - Solid-state laser device - Google Patents

Description

TECHNICAL FIELD The present invention relates to improving the quality of a laser beam generated from a solid-state laser device.

[Prior Art] FIG. 13 shows, for example, a laser handbook (Ohm,
FIG. 1) is a cross-sectional configuration diagram showing a conventional solid-state laser device shown in (1982). In the figure, (1) is a solid-state element, for example, if a YAG laser is taken as an example, Y _3-x Nd _x Al ₅ O ₁₂ (3) a flash lamp, (4) a power source for lighting the flash lamp (3), (5) a condensing reflection mirror, and (6) an exit mirror made of glass, for example. , (7) is provided on the inner surface of the exit mirror (6), for example, a partially reflective film made of TiO ₂ , and (8) is provided on the outer surface of the exit mirror and both side surfaces of the solid-state device (1), such as SiO 2. ₂ is a non-reflective film, (9) is a laser beam in a laser resonator, (10) is a laser beam extracted to the outside, (12) is an outer frame, and (20) is a total reflection mirror.

 Next, the operation will be described.

The solid state element (1) is excited by direct light from a flash lamp (3) turned on by a power supply (4) and reflected light from a converging / reflecting mirror (5) to form a laser medium. On the other hand, a laser beam (9) confined in a so-called stable resonator consisting of an exit mirror (6) having partial reflectance and a total reflection mirror (20) by a partial reflection film (7) provided on the inner surface. ) Is amplified by this laser medium every time it goes back and forth between both mirrors, and when it reaches a certain size or more, a part of it is emitted to the outside as a laser beam (10) through the exit mirror (6).

FIG. 14 shows an example of the oscillation output characteristic. Here, the solid-state element (1) is made of Y _2.4 Nd _0.6 Al ₅ O _{12 and} has a cross-sectional diameter of 8
mm, length 150 mm. The curvature of the inner surface of the exit mirror (6) and the total reflection mirror (20) are both 0.4 m, the distance between the two mirrors is 0.45 m, and the reflectance of the exit mirror is 70%. In addition, the input power is the power consumed for lighting the flash lamp, and a laser output of about 100 W is obtained with an input power of 6 kW.

[Problems to be Solved by the Invention] Since the conventional solid-state laser device uses the stable resonator as described above, the generated laser beam is in a so-called higher-order mode with a large divergence angle. The index of the degree of this higher-order mode is the ratio of the cross-sectional area of the lowest-order mode with a uniform phase that can be generated in the resonator and the cross-sectional diameter of the laser medium. In this example, the lowest order mode is a so-called Gaussian beam having a normal distribution, so the cross-sectional diameter φ ₀ defined at the point where its intensity is 1 / e ^{2 of the} center is the distance L between the two mirrors,
It is calculated that φ ₀ = 2 (λL / 2π) ^0.5 = 0.55 mm as the wavelength λ of the laser beam.

On the other hand, since the cross-sectional diameter of the laser medium is 8 mm, the ratio of the two is very large, about 15, and from this value it is empirically predicted that higher-order modes of 100th order or higher are generated. The order of the higher-order mode is experimentally measured by measuring the divergence angle of the generated laser beam.In this example, the divergence angle is grasped as 10 mrad, and the order of the corresponding higher-order mode is
It was calculated to be of the order of 100. The magnitude of the divergence angle can be said to be the laser beam focusing performance itself.

This is because the focal spot diameter φ _{s of the} condensing lens with the focal length f is roughly represented by φ _s = f · θ with respect to the divergence angle θ. This is because the size of the spot diameter is determined. 10mr here
the value ad about 10 times when compared with commercially available CO ₂ lasers, thus condensing performance of a conventional solid laser device, CO
It can be said that it is 1/10 of ₂ laser devices.

The present invention has been made to solve the above problems, and an object thereof is to obtain a solid-state laser device capable of generating a laser beam with a small divergence angle at a high output.

[Means for Solving the Problems] A solid-state laser device according to the present invention uses an unstable resonator of a negative branch and keeps a vacuum near a focus generated in the unstable resonator.

Further, as the exit mirror constituting the unstable resonator, it is preferable that the central portion has a partial reflectance and the peripheral portion has a lower reflectance than the central portion.

[Operation] The negative branch unstable resonator of the present invention generates the lowest order laser beam. Further, by maintaining a vacuum in the vicinity of the condensing point in the resonator, it is possible to prevent the instability of the resonator due to heating of the atmosphere near the condensing point in the resonator due to the high-power beam being condensed, Furthermore, it prevents airborne damage near the focal point.

Furthermore, as an exit mirror that constitutes an unstable resonator,
By providing the transmittance distribution in the central portion and the peripheral portion, it is possible to generate a highly focused laser beam having a medium clogging.

[Embodiment] An embodiment of the present invention will be described below with reference to FIG.
In FIG. 1, (1) is a solid-state element, for example, if a YAG laser is taken as an example, a rod-shaped crystal made of Y _3-x Nd _x Al ₅ O ₁₂ , (2) is provided on the end face of this rod. It is a total reflection mirror and forms a magnifying mirror. (3) is an excitation light source,
For example, an arc lamp, a flash lamp, (4) a power source for turning on the excitation light source (3), (5) a converging reflection mirror, and (6) an exit mirror made of glass, for example.
It forms a collimating mirror. (7) is a partial reflection film made of, for example, TiO ₂ provided in the central portion of the inner surface of the exit mirror (6), (8) is a peripheral portion of the partial reflection film (7) on the inner surface of the exit mirror, and the exit mirror (6) Outer surface of a solid element (1)
A non-reflective film made of, for example, SiO ₂ provided on each side surface of the laser beam, (9) and (11) are laser beams generated in a laser resonator including an exit mirror (6) and a total reflection mirror (2), 10) is a laser beam extracted to the outside of the laser resonator, (12) is an outer frame, (30) is a vacuum container, and (31) is a window.

 Next, the operation will be described.

The solid-state element (1) is excited by the direct light from the excitation light source (3) turned on by the power source (4) and the reflected light from the condensing reflection mirror (5) to form a laser medium. On the other hand, the laser beams (9) and (11) generated in the laser resonator composed of the exit mirror (6) and the total reflection mirror (2) are amplified by this laser medium every time the laser beams reciprocate between the two mirrors. When the size exceeds a certain level, a part of the size is emitted to the outside as a laser beam (10).

The laser resonator will be described in more detail. The exit mirror (6) and the total reflection mirror (2) constitute a so-called negative branch unstable resonator. The laser beam (11) is amplified by the solid-state element (1), then is condensed by the total reflection mirror (2) and is amplified by the solid-state element (1) to become a laser beam (9). In the peripheral part of 9), most of it is emitted to the outside due to the action of the non-reflective film on the inner peripheral part of the exit mirror (6), and in the central part, a part of it is emitted to the exit mirror (6).
The partial reflection film (7) is taken out to the outside, and the remaining portion reciprocates in the laser resonator as a laser beam (11). External laser beam (10)
Is a solid block. In addition, in such an unstable resonator, the difference in loss between the lowest-order mode and the higher-order mode in which the phases are aligned is larger than that in the stable resonator. Will only be. Also, the radius of curvature of the outer surface of the exit mirror is smaller than the radius of curvature of the inner surface, and the exit mirror has a meniscus shape, and the outgoing beam is made to be substantially parallel light. From these facts, the most ideal laser beam in which the phase is clogged is obtained.

FIG. 2 shows an example of the oscillation output characteristic.
Here, the solid-state device (1) is Y _2.4 Nd excited by a lamp.
_0.6 Al ₅ O ₁₂ with a cross-sectional diameter of 8 mm and length of 150 m
m, the curvature of the inner surface of the exit mirror (6), and the total reflection mirror (2) are 0.48 m and 0.8 m, respectively, and the distance between the two mirrors is
The reflectance of the partially reflective film (7) at the center of the inner surface of the exit mirror is 0.44 m, and is 80%. The input power is the power consumed to light the lamp, and with a power input of 6 kW, a laser output of about 100 W is obtained.

FIG. 3 shows a pattern obtained by focusing a laser beam by a lens at a laser output of 100 W by the solid-state laser device according to one embodiment of the present invention (FIG. 3 (a)) and the conventional solid-state laser device (FIG. 3 (b)) is shown in comparison. According to the present invention, not only is the width of the pattern reduced to about 1/40 of the conventional one, but the central axial strength is 10%.
You can see that it is more than 00 times.

Prior to this experiment, the inventors performed the experiment without using the vacuum container (30). However, in this case, the laser output becomes unstable as the oscillation output increases, and the maximum is 10
There were times when I was staggered. As a result of detailed investigation of the cause, the laser output shows that the light-collecting density at the light-collecting point inside the resonator
We have reached 20kW / cm ² for 100W, and found that this highly focused laser beam is absorbed by the atmosphere near the focal point and raises its temperature, making the atmosphere unstable. Based on this consideration, when the vicinity of the converging point is held in a vacuum container as shown in FIG. 1 to prevent temperature rise near the converging point, the laser output is stabilized. It was realized.

Further, as shown in FIG. 4, for example, when a Q-switch element (40) such as a Pockel element is inserted into the resonator to obtain a laser output having a high steeple value, and when the vacuum container (30) is not present, Is a breakdown in the air due to the high electric field strength generated by the focusing of the laser beam near the focusing point in the resonator,
Although the laser oscillation sometimes stopped, this air breakdown could be completely suppressed by keeping the vicinity of the focusing point in the vacuum container as shown in FIG.

In the above embodiment, the central part of the inner surface of the exit mirror is a partially reflective film, and the peripheral part is formed of a non-reflective film. However, the reflectivity of the exit mirror gradually decreases as it goes from the central part to the peripheral part. May be something.

Further, the exit mirror may be one in which the central portion has a total reflection property. In this case, a ring-shaped laser beam is generated, and the diffracted light due to the ring-shaped deteriorates the condensing performance. In comparison with this, a sufficiently high quality laser beam can be generated.

In the above embodiment, the phase difference between the laser beams passing through the central portion and the peripheral portion of the inner surface of the exit mirror was small and did not cause any problem. However, depending on the configuration of the partial reflection film (7), this may cause a problem. It is also conceivable, in that case, means for canceling the phase difference between both laser beams may be provided,
For example, as shown in FIG. 5, a step (7
0) can be realized.

Further, although the exit mirror is formed in a meniscus shape and the emitted beam is made to be substantially parallel light in the above-mentioned embodiment, it may be made to be parallel light by a lens provided outside the exit mirror.

Further, although the total reflection mirror (2) is formed on the side surface of the solid-state element in the above-mentioned embodiment, both may be separated as shown in FIG. The solid state element is also shown in FIGS.
It may be located near the exit mirror as shown.

Further, in the above-mentioned embodiment, an example in which the condensed laser beam is reflected from the total reflection mirror (2) has been shown, but a parallel laser beam may be reflected.
FIG. 9 corresponds to FIG. 6, FIG. 7, FIG.
Modifications as shown in FIGS. 10, 11 and 12 are possible.

In the above embodiment, the vacuum container (30) is arranged near the converging point to maintain the vacuum in the vicinity of the focal point in the resonator.
It goes without saying that the inside of the outer casing (12) may be evacuated to a vacuum to serve as a vacuum container.

[Advantages of the Invention] As described above, according to the present invention, since the negative branch unstable resonator is used and the means for keeping the vicinity of the condensing point in the inside of the resonator in vacuum is provided, high quality with a small divergence angle is provided. There is an effect that the laser beam can be stably generated even in a high output region.

Furthermore, if an exit mirror that constitutes an unstable resonator is one that has a partial reflectance in the central portion and a lower reflectance in the peripheral portion compared to the central portion, it is possible to form a clogged shape with good light-collecting properties. It is possible to obtain a high quality beam.

[Brief description of the drawings]

FIG. 1 is a sectional configuration diagram showing a solid-state laser device according to an embodiment of the present invention, FIG. 2 is a characteristic diagram showing its output characteristics, and FIG.
FIGS. 4A and 4B are characteristic diagrams showing focusing patterns in an embodiment of the present invention and a conventional solid-state laser device, and FIGS. 4 to 12 are solid-state diagrams according to other embodiments of the present invention. FIG. 13 is a sectional configuration diagram showing a laser device, FIG. 13 is a sectional configuration diagram showing a conventional solid-state laser device, and FIG. 14 is a characteristic diagram showing laser output in the conventional solid-state laser device. (1) ... solid-state element, (2), (20) ... total reflection mirror, (3) ... light source, (6) ... exit mirror, (7) ...
… Partially reflective film, (8) …… Non-reflective film, (9), (10),
(11) …… Laser beam, (30) …… Vacuum container, (31)
...... Window In the figures, the same reference numerals indicate the same or corresponding parts.

Claims (2)

(57) [Claims] 1. A solid-state element forming a laser medium, a light source for exciting the solid-state element, a negative launch unstable resonator composed of a plurality of mirrors, and a vicinity of a condensing point generated in the unstable resonator. A solid-state laser device equipped with a means for maintaining a vacuum. 2. The solid-state laser device according to claim 1, wherein the exit mirror of the unstable resonator has a partial reflectance in the central portion and a lower reflectance in the peripheral portion as compared with the central portion.