JPH0637368A - Laser and beam expander - Google Patents

Abstract

(57) [Abstract] [Purpose] A laser device in which a laser beam travels in a zigzag manner while repeating total reflection on the optically polished surface in a slab-shaped solid laser medium having opposed optically polished surfaces, and a solid-state laser by emitting light from a laser diode. As a laser device for exciting a medium, a structure of a device which is small and has a large beam spread in a laser medium for pump injection and a small aberration, and a structure of a beam expander for this purpose are provided. In a device in which a laser beam travels in a zigzag manner, a convex cylindrical lens 3 and a reflector 9 having a conical reflecting surface with an apex angle of 90 ° are provided in a cavity optical path, and a central axis of the reflecting surface is a convex cylindrical lens. In a device that is arranged so as to be parallel to the focal point of the lens 3 and is excited by a laser diode, the beam is expanded into a fan shape on the conical reflection surface and is passed through the laser medium. The beam expander is composed of a convex cylindrical lens 3 and a reflector 9 having a conical reflecting surface with an apex angle of 90 °.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state laser device and its optical element.

[0002]

2. Description of the Related Art A solid-state laser represented by a YAG laser is small and easy to use, but on the other hand, nonuniformity of optical characteristics due to heat generated in a laser medium such as a thermal lens effect causes oscillation with high beam quality. Making it difficult. As a method of solving this, a method using a slab-shaped solid-state laser medium has been attracting attention. For example, in the Nd: YAG slab laser, as shown in FIG. 3, two opposing surfaces of the Nd: YAG slab 1 are optically polished, and the laser beam 2 repeats total reflection on the polished surface (excitation surface) and proceeds in a zigzag manner. This allows the crystal thickness direction
The influence of the temperature distribution generated in the (y direction) is eliminated. On the other hand, the side surface (non-excited surface) is made adiabatic so that the temperature distribution in the width direction (x direction) becomes uniform. Therefore, slab lasers are much less susceptible to heat than conventional rod-type solid-state lasers.

Generally, when trying to reduce the beam divergence, the performance of the laser is easily affected by changes in the optical characteristics of the crystal. Therefore, when the influence of heat is large, the target performance is maintained under conditions other than the limited conditions. Difficult to do.
However, since the slab laser is less affected by heat, it is possible to adopt a resonator structure for reducing the beam divergence.

In order to reduce the beam divergence, the ratio between the resonator length and the crystal width may be increased. However, since the slab laser has a large crystal width, it is difficult to reduce the beam divergence angle in the x direction by this method. Therefore, a method of inserting a beam expander in the resonator is used. The beam expander 5 composed of the convex cylindrical lens and the concave cylindrical lens in FIG. 4 changes the beam width in the x direction. As you can see from the fact that the total reflection mirror can be seen at a distance when the beam expander, which is a kind of telescope, is viewed from the slab side, the beam expander has the same effect as increasing the cavity length. Therefore, by this method x
It is possible to reduce the beam divergence in the direction. A similar method can be performed by using the prism 8 as shown in FIG.

As a technique attracting attention as a method of solving the heat problem of the solid-state laser, there is a method of using an LD (laser diode) as an excitation source. The emission spectrum of a discharge tube conventionally used for pumping a solid-state laser is widely distributed from the ultraviolet to the infrared region, but the LD has a line spectrum and its wavelength is adjusted to a value convenient for pumping the laser medium. can do. Therefore, the LD-pumped solid-state laser has high efficiency and there are few problems caused by heat.

Another feature of using an LD as an excitation source is that the excitation light can be focused on a small spot.
By injecting the excitation light into the fundamental mode beam having the smallest diameter, fundamental mode oscillation becomes possible, and laser light with extremely small divergence can be emitted. However, the LD also has a large light emitting point when the output becomes large, and particularly when the LD has an array structure in which a large number of light emitting points are arranged in a line, it becomes difficult to focus the excitation light on a minute spot. Therefore, a method is known in which the laser beam in the resonator is expanded into a flat beam and pumping light is injected into the expanded part. Specifically, like the above slab laser,
A method using a cylindrical lens is also possible, but a method using reflection on a parabolic surface is also known (see Japanese Patent Application No. 2-41968).

In the above two techniques, an optical element that expands or contracts the laser beam in only one direction plays an important role. This type of beam expander also plays an important role when the laser beam is focused linearly in order to use the laser for welding.

[0008]

When the beam expander is used for a slab laser, the larger the expansion ratio M, the smaller the beam expansion. If f _P is the focal length of the convex lens and f _N is the focal length of the concave lens, then M = f
_Since it can be expressed as _P / f _N , in order to increase M, f _P
Should be increased or f _N should be decreased. However, f
_{When P} is increased, it is necessary to increase the distance between the convex lens and the concave lens, which increases the size of the device. On the other hand, if f _N is made small, there arises a problem that the aberration of the lens becomes large. Even in the case of using prisms, the enlargement ratio of one prism is small. Therefore, in order to increase M, a large number of prisms are required and a large device is required.

These problems also apply to the LD pumped solid-state laser. In this case, the beam diameter in the fundamental mode is 20
Since it is about 0 μm, this is the width of the LD array (about 10 m
In order to extend to m), it is necessary to set M = 50,
This is difficult to achieve with normal lens pairs. The above method using a parabolic reflecting surface is convenient for increasing M, but it is actually difficult to perform parabolic processing with optically sufficient surface roughness.

The above problem is the same when the beam expander is used for other purposes, and if M is set to be large, the distance between the lenses becomes large and the apparatus becomes large. An object of the first invention is to provide a slab type solid-state laser device with a small size and an M size.
It is to provide a beam expander with a large aperture and a small aberration, and enable oscillation of laser light with a small beam divergence angle.

A second object of the present invention is to provide a small-sized beam expander with a large M and a small aberration in an LD pumped solid-state laser device, and to enable oscillation of laser light with a small beam divergence angle. . A third object of the present invention is to provide a beam expander which is widely applicable to laser processing and the like, is small, has a large M, and has a small aberration.

[0012]

SUMMARY OF THE INVENTION An object of the first invention is to allow a laser beam to pass in a zigzag manner in a slab-shaped solid laser medium having opposed optical polishing surfaces while repeating total reflection on the optical polishing surface. In the slab laser described above, a convex cylindrical lens for converging laser light only in the width direction of the slab, or an optical element having a function equivalent to that is provided in the optical path in the cavity, and the apex angle is 90 °.
This is achieved by arranging the conical reflecting surface of (1) in the intracavity optical path so that its central axis is parallel to the focal line of the optical element.

A second object of the present invention is to provide a laser device which oscillates a laser beam by exciting a solid-state laser medium in a laser resonator with light emitted from an LD, in which the apex angle is 90 ° and the central axis is the optical axis. A conical reflecting surface that is parallel to the conical reflecting surface is provided in the resonator, and the laser light in the resonator is reflected by the conical reflecting surface in the direction of 90 ° with respect to the central axis in a fan shape, and the flat laser beam is reflected by the solid state. This is achieved by injecting the excitation light into the flat laser beam in the solid laser medium so as to pass through the laser medium.

A third object of the present invention is to provide a convex cylindrical lens, or an optical element having a function equivalent to that of the convex cylindrical lens, and an apex angle of 90 °.
It is achieved by arranging both of them so that the central axis of the conical reflecting surface is parallel to the focal line of the optical element.

[0015]

The operation of the present invention will be described with reference to FIG. Now, a lens having a conical depression with an apex angle of 90 ° as shown in the figure.
(Concave axicon) Consider a case where the laser light 2 traveling parallel to the central axis C of the cone is reflected by this surface with respect to the conical reflection surface. When the reflection state is viewed from the direction B perpendicular to the central axis C, the light appears to be reflected in the 90 ° direction. On the other hand, when viewed from the direction A parallel to the central axis C, the light appears to spread out in an arc shape. That is, it can be seen that the conical surface has the same function as the concave cylindrical lens. Strictly speaking, the light spreads as an arc having the central axis C of the cone as the center of curvature, so no aberration occurs. Further, since the light can be greatly spread by making the incident light incident near the apex of the cone, it is possible to manufacture a beam expander having a small size and a large expansion ratio M by using this principle.

Of course, the convex axicon 1 as shown in FIG.
The same effect can be obtained by using 0.

[0017]

The present invention will be described in detail based on the following examples. FIG. 1 shows an embodiment of the first invention. In the figure, 1 is a YAG slab, 9 is a concave axicon, 7 is a total reflection mirror, 3 is a convex cylindrical lens, and 6 is an output mirror. In this embodiment, the YAG slab is excited by the light from an excitation lamp (not shown) to oscillate the laser. In the YAG slab, the width of the laser light is almost equal to the width of the slab. In the slab, light follows the zigzag path, which is not shown in this figure.
The light emitted from the YAG slab to the output side is condensed by the convex cylindrical lens 3, collimated into parallel light by the concave axicon 9, and then output from the output mirror 6. In this embodiment, the beam expander composed of the convex cylindrical lens 3 and the concave axicon 9 plays two roles of removing higher-order modes and reducing the output laser beam diameter.

FIG. 2 shows an embodiment of the second invention. In the figure, the same numbers as in FIG. 1 represent the same names. In this embodiment, the laser light is spread by the concave axicon 9 in only one direction, enters the YAG slab, is reflected by the total reflection surface 12, and is then reflected by the concave cylindrical mirror 11. Excitation light 13 is emitted from an LD array not shown in the figure, and as shown in the figure, is incident on a linearly visible laser light portion from the total reflection surface in the YAG slab. Since the width of this linear portion becomes narrower as the mode becomes lower, the fundamental mode oscillation becomes possible by condensing and injecting the pumping light into a line width not larger than the width of the fundamental mode. 1 and 2 are also embodiments of the third invention.
Each of them is provided with a slab-shaped laser medium as a constituent element of the beam expander, here a reflector (concave axicon 9) having a conical reflection surface with an apex angle of 90 ° on the output mirror 6 side of the Nd: YAG slab 1. . In particular, the embodiment of FIG. 1 is provided with a convex cylindrical lens 3 in addition to the concave axicon 9 to allow laser light to be linearly focused and applied to welding, thereby enabling a wide range of applications.

[0019]

According to the first aspect of the present invention, it is possible to provide a small slab solid state laser device having a small beam divergence. According to the second aspect of the invention, L is small and has a small beam divergence.
A D-pumped solid-state laser device can be provided. According to the third invention, it is possible to provide a small-sized beam expander having a large expansion rate.

[Brief description of drawings]

FIG. 1 is a laser device configuration diagram showing an embodiment of a first invention.

FIG. 2 is a laser device configuration diagram showing an embodiment of a second invention.

[Fig. 3] Structural principle diagram of slab-shaped solid-state laser medium

FIG. 4 is a configuration diagram of a slab type laser device with a built-in beam expander according to a first conventional example.

FIG. 5 is a configuration diagram of a slab type laser device with a built-in beam expander according to a second conventional example.

FIG. 6 is an explanatory diagram of a reflection state of a laser beam on a conical surface.

FIG. 7 is an explanatory diagram of a reflection state of a laser beam on a conical surface.

[Explanation of symbols]

 1 Nd: YAG Slab 3 Convex Cylindrical Lens 9 Concave Axicon 11 Concave Cylindrical Mirror

Claims (3)

[Claims] 1. A laser device in which a laser beam passes in a zigzag manner while repeating total reflection on the optical polishing surface in a slab-shaped solid laser medium having opposed optical polishing surfaces, in an optical path in a resonator. Is provided with a convex cylindrical lens for condensing laser light only in the width direction of the slab, or an optical element having a function equivalent to that, and a conical reflecting surface having an apex angle of 90 ° has a central axis of the optical element. The laser device is arranged in the optical path in the resonator so as to be parallel to the focal line of the laser device. 2. A laser device that oscillates laser light by exciting a solid-state laser medium in a laser resonator with light emitted from a laser diode so that the apex angle is 90 ° and its central axis is parallel to the optical axis. A conical reflecting surface is provided in the resonator, and the laser light in the resonator is fan-reflected by the conical reflecting surface in the direction of 90 ° to the central axis, and the flat laser beam passes through the solid laser medium. Thus, the excitation light is injected into the flat laser beam in the solid-state laser medium. 3. A convex cylindrical lens, or an optical element having a function equivalent to that, and a conical reflecting surface having an apex angle of 90 °, and the central axis of the conical reflecting surface is parallel to the focal line of the optical element. The beam expander is characterized in that both are arranged so that