JPH04127487A - Laser-excited laser oscillator - Google Patents

Abstract

PURPOSE:To enhance the output of a laser-excited tie sapphire laser oscillator by composing one side in a protruding surface, the other side in a recess surface, and directing the recess surface side toward the inside of a resonator. CONSTITUTION:An exciting laser light 1 is a laser light from a CVL having relatively large thickness of about 50mm of the diameter of a beam, passed through a convex lens 2 having about 200mm of focal length, passed through a dichroic mirror 4, and condensed in a laser medium 3. The mirror 4 has a protruding surface at the outside, and a concave surface at the inside. Since the radii of curvature of both spherical surfaces are about 100mm, the light 1 is converged so that its angle is not varied, propagated between resonators, and condensed in the medium 3. An output mirror 5 has a concave surface inside, and about 100m of curvature. The mirror is disposed at an interval of about 200mm to a dichroic mirror 4 operated as a totally reflecting mirror for the oscillated laser light 6. Thus, the light 6 is not parallel beam, but formed in a shape of thick diameter of the beam on the mirror 4 and in a fine shape therebetween.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to the structure of a laser resonator in a laser-pumped laser oscillator using a laser medium capable of laser operation using pulsed light as an excitation light source.

[Conventional technology]

Generally, as an example of the structure of a resonator in a laser oscillator using a laser beam as an excitation light source, as shown in FIG. A total reflection @ 2 and an output mirror 3 are placed facing each other, and the laser beam 4 from the copper vapor laser as excitation light travels through the condensing lens 5 while being condensed. was transmitted and irradiated onto the end face of the Thai sapphire laser rod 1. As a result, the laser beam 5 oscillated as shown in the figure, and was of a vertical excitation type coaxial with the excitation laser beam 4.

Furthermore, in general optically pumped laser oscillators including laser-pumped laser oscillators, increasing the light intensity of the excitation light in the laser medium in which the laser is operated increases the excitation photon density and increases the probability that stimulated emission will occur. Laser output is improved. As a result, in order to increase the light intensity of the excitation light, especially when the excitation light is laser light, it is necessary to narrow down the excitation laser light using a condenser lens and irradiate the laser medium. As a result, an anti-reflection coating (AR
If the film is coated with co-coating, the part of the film that is irradiated with the excitation laser beam may be damaged.

In particular, as an excitation laser, a copper vapor laser (CVL),
In the case of pulsed lasers that can operate repeatedly at high speeds of approximately IKHz or higher, such as YAGL/- lasers with QXX latch or excimer lasers, the pulse width is generally about several tens of seconds plus 1 second, which is the time required for heat dissipation through heat conduction. The pulse time is orders of magnitude shorter, and the heat equivalent to the pulse is injected into the minute area of the coating surface on the end face of the laser medium, causing the coating film to melt instantly, causing evaporation and damage. Something happened. In other words, the damage threshold of the coating film is generally no more than 2 to 4 J/d.

Therefore, when using a solid-state laser lot as a laser medium, in order to avoid applying a coating film that is likely to cause damage as much as possible, it is necessary to cut both end faces of the rond so that the incident and output angles of the laser beam oscillate are Brewster's angle. As a result, reflection on this surface can be suppressed, and a laser oscillator can be constructed without using a non-reflective coating film. That is, in this case, the damage threshold at the rond end face is as high as 40 to 50 J/cd, which is more than an order of magnitude higher than the damage threshold of the coating film.

[Problem to be solved by the invention]

In the above conventional technology, no consideration is given to the coating films of the output mirror and the total reflection mirror in the laser oscillator, and the laser oscillation light generated between the resonators causes damage to the parts irradiated onto these coating films. Something happened. Especially in the case of CVL pumped Thai sapphire lasers, for example, Japan Physics This Fall Subcommittee, 6a-E5-4.198
8, page 346, the reflectance of the output mirror is as high as 80 to 98%.1For example, when the reflectance is 90%, the power of the laser beam extracted from between the resonators is higher than that of the laser beam extracted from between the resonators. , the laser power inside the resonator is about ten times higher, and as a result, the intensity of the laser light on the coating film of the total reflection mirror and the output mirror becomes extremely high, easily causing damage, and reducing the laser output. was a limiting factor.

The reason why the coating films of the output mirror and the total reflection mirror are easily damaged is that the excitation light, such as CVL laser light, needs to be focused relatively narrowly to increase the excitation photon density. In other words, in CVL, the energy of one pulse is about 20 to 20 mJ (equivalent to an average output of 10 to 100 W when the repetition rate is 5 KHz).
When a dye laser is used as the excitation light source (for example, see Optics Letters, Vol. 13, No. 5,
May issue, 1988, pages 380 to 382 (Opt
ics Letters, Vol. 13. k 5
, May 1988゜Pρ, 380-382). Since it is 11,500 times smaller than OJ, the oscillation efficiency will deteriorate unless the energy of the excitation light, which is the threshold for laser oscillation, is set to a small value of about 2 m, J or less.

Therefore, in order to lower the threshold of laser oscillation, it is necessary to increase the reflectance of the output mirror to 80% or more, or to narrow down the CVL laser light that is the excitation light, resulting in the generation of light between the resonators. The intensity of the laser beam has increased, making it easier to damage the □ coating film on the output mirror and total reflection mirror.

In contrast, conventionally, in laser-pumped laser oscillators,
For example, by increasing the spot area of the laser beam irradiated onto the coating surface, so that the coating film applied to the output mirror and total reflection mirror is less likely to be damaged.
There are also configurations that attempt to lower the laser light intensity there.

To do this, the excitation laser beam must be shaped so that the diameter of the laser beam formed between the resonators is slightly larger on the output mirror or total reflection mirror than the diameter of the beam in the laser medium. The total reflection mirror, which is a dichroic mirror used to guide the laser medium inside the resonator, is
For example, as in the conventional example shown in FIG. 3, a concave mirror was used as the dichroic mirror 4. In that case, the cross-sectional area of the beam could be made somewhat larger than the beam cross-sectional area of the generated laser beam 6. However, conventionally, since the other surface of the dichroic mirror 4 is flat, it functions as a concave lens with respect to the excitation laser beam 1 that passes through the dichroic mirror 4. As a result, the excitation laser beam 1 first passes through the convex lens 2 and then travels while being focused.
After passing through the dichroic mirror 4, the aperture angle widens, and a considerably long distance is required to condense the light into the laser medium 3. As a result, the resonator length becomes longer, the laser output decreases, and the number of longitudinal modes of the oscillated laser light increases, making it difficult to oscillate in the -th longitudinal mode.

Further, in order to prevent the distance from the dichroic ink mirror 4 to the point at which the excitation laser beam is focused to be long, the focal length of the convex lens 2 must be sufficiently short. However, due to the short focal length of the convex lens, the beam diameter is approximately 3, especially for CVL laser beams.
If a thick laser beam of 0+nm or more is passed through, spherical aberration will increase. As a result, the focusing ability of the excitation laser beam deteriorated, making it impossible to increase the laser output.

An object of the present invention is to provide a laser-excited laser oscillator that operates at high output without increasing the resonator length and without causing damage to the output mirror or the total reflection mirror.

[Means to solve the problem]

In order to achieve the above object, in a dichroic mirror having a high transmittance for the excitation laser beam used for guiding the excitation laser beam into the laser medium in the excitation laser oscillator, one side is a convex surface. and the other surface is concave, and the concave side faces inside the resonator.

Furthermore, in order to make damage less likely to occur, the oscillator is configured as a concentric resonator.

Further, in order to omit a convex lens used for condensing the excitation laser beam, the dichroic mirror has a positive meniscus lens shape in which the radius of curvature of the convex surface is shorter than the radius of curvature of the concave surface. be.

[Effect]

It is possible to reduce the effect of a concave lens on laser light passing through a dichroic ink mirror having a convex surface on one side and a concave surface on the other surface. In particular, if the curvatures of the convex and concave surfaces are made equal, the spread angle of the laser beam passing through them does not need to change at all. As a result, the distance to the focal point of the excitation laser beam that passes through this dichroic mirror while being focused does not become long.

Further, by using a concentric resonator, a focal point can be formed in the resonator in the laser light generated between the resonators, so that the laser beam diameter can be made extremely thin near this focal point. By placing a laser medium near this focal point, the excitation laser beam can be narrowly focused here, increasing the laser output. In this case, the diameter of the laser beam at the output mirror and the total reflection mirror does not need to be reduced, so that the coating films on these mirrors are less likely to be damaged.

Furthermore, by making the dichroic mirror have the shape of a positive meniscus lens as described above, there is no change in the laser light reflected inside because it is a concave surface.
It acts as a convex lens for the excitation laser beam that passes through it. Therefore, even if the excitation laser beam travels in parallel, the excitation laser beam can be focused into the laser medium by adjusting only the curvature of the convex surface of this positive meniscus lens.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. Here, a Thai sapphire crystal is used as the laser medium 3, and both ends of the crystal are cut at Brewster's angle, thereby reducing the loss of the oscillated laser beam 6 even without AR cocoing. It looks like this. The excitation laser beam 1 is a laser beam from a CVL with a relatively thick beam diameter of approximately 50 mm, and a focal length of approximately 20 mm.
The light passes through a 0 m convex lens 2, passes through a dichroic mirror 4, and is focused into a laser medium 3. As shown in Figure 1, this dichroic mirror 4 has a convex surface on the outside and a concave surface on the inside, but since the radius of curvature of both spherical surfaces is about 1100a, the excitation laser beam 1 is focused and the angle does not change. , propagates between the resonators and can be focused into the laser medium 3.

The output mirror 5 has a concave inner surface, and its curvature is approximately 1.
00m+. This output mirror includes a dichroic mirror 4 that acts as a total reflection mirror for the oscillated laser beam 6;
They are placed at intervals of approximately 200mm. Therefore, the resonators form a concentric system, and the laser beam 6 formed between the resonators
The beam is not a parallel beam as shown in the figure, but has a large beam diameter at the output lR5 and the surface of the dichroic mirror 4, and a narrow beam between them. Therefore, these outputs #! 5 and the coating film on the dichroic mirror 4 are less likely to be damaged. Furthermore, the beam diameter of the light 6 becomes small in the laser medium, and the excitation light is also sufficiently focused here, so that the gain becomes high and, as a result, a high-output laser light 6' is extracted.

FIG. 2 shows another embodiment of the invention. The excitation laser beam 1 is a second harmonic green laser beam from a YAG laser operated at about 20 KHz by an acousto-optic (A○) Q switch.

This excitation laser beam 1 passes through a dichroic mirror 7 and enters the resonator from a dichroic mirror 4 functioning as an output mirror. This dichroic mirror 4 has a convex surface with a curvature of 34 m on the outside and a curvature of about 1100 on the inside.
It has an n concave surface.

As a result, the excitation laser beam 1 that has been traveling from the outside as a nearly parallel beam passes through the dichroic mirror 4 and then travels into the laser medium 3 while being focused so as to be focused at a point that has traveled approximately 100 mm. incident on . A convex lens 8 with a focal length of about 100 mm is placed about 200 degrees from the dichroic ink mirror 4, and a diffraction grating 9 is placed immediately next to it, forming a resonator. As a result, the generated laser beam 6 forms a concentric system in which the laser beam 6 is focused between the resonators as shown in the figure. As a result, as in the embodiment shown in FIG. The coating film on these surfaces is less likely to be damaged.

Furthermore, in this embodiment, as in the embodiment shown in FIG. 1, a Thai sapphire crystal is used as the laser medium 3, and it has wavelength tunability. Therefore, a diffraction grating 9 is used as a wavelength selection element. Furthermore, a pinhole plate 10 is used to roughly select the wavelength to be oscillated.

That is, the focal length of the convex lens 8 is approximately 100 I, but it depends on the wavelength of the laser beam 6 to be precise. Therefore,
As shown in the figure, laser light with a wavelength that focuses on the distance between the pinhole plate 10 placed near the focal point of the convex lens 8 and the convex lens 8 is easily oscillated. In other words, laser beams of other wavelengths do not focus exactly on the pinhole plate 10, so this hole (here approximately 0.1 m
The size is about m. ), the percentage of laser light that can pass through becomes smaller, making it difficult to oscillate at that wavelength.

Further, since the wavelength can be selected in this way, a plane reflecting mirror may be used in place of the diffraction grating 9 depending on the case.

This embodiment is different from the embodiment shown in FIG. 1 in that the excitation laser beam 1 is input into the resonator from a dichroic mirror 4 used as an output mirror. This makes it possible to use the diffraction grating 9 on the total reflection side.

[Effects of the Invention] According to the present invention, the coating film on the output mirror and the total reflection mirror is less likely to be damaged.
A laser-excited Thai-sapphire laser oscillator that uses the second harmonic of a laser as an excitation light source and uses a Thai-sapphire crystal cut at a B-12 Euster angle as a laser medium can have a high output.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of an apparatus as an embodiment of the present invention, and FIG.
The figure shows an explanatory diagram of an apparatus as another embodiment of the present invention, and FIG. 3 shows an explanatory diagram of an apparatus as a conventional example. 1... Laser light for excitation, 2... Convex lens, 3...
Laser medium, 4... Dichroic mirror, 5...
Output mirror, 6.6'... Laser light, 7... Dichroic mirror 8... Convex lens, 9... Diffraction grating, 10
...Pinhole board. Diagram

Claims (1)

[Claims] 1. Regarding the excitation laser beam used for guiding the excitation laser beam of a laser-pumped laser oscillator using a laser medium capable of laser operation using a laser beam as an excitation light source into the laser medium. A dichroic mirror having a high transmittance and having a convex surface on one side and a concave surface on the other side, the concave side facing inside a resonator. oscillator. 2. The laser-pumped laser oscillator according to claim 1, wherein the resonant optical system in the laser-pumped laser oscillator is a concentric system. 3. The laser-excited laser oscillator according to claim 1, wherein the dichroic mirror has a positive meniscus lens shape in which the radius of curvature of the convex surface is shorter than the radius of curvature of the concave surface. 4. A laser-pumped laser oscillator according to claim 1, wherein the laser medium is a Thai sapphire crystal, and the pumping laser beam is a laser beam from a copper vapor laser or a second harmonic of a YAG laser operated by a Q switch.