JPH0728066B2 - Rod type solid state laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention is a rod using a YAG laser or the like, which is a cylindrical rod in which a base material of a crystal or glass is doped with transition metal ions, rare earth element ions or the like as a laser medium. Type solid-state laser device.

[Conventional technology]

As is generally known, YAG (Y ₃ Al ₅ O ₁₂ ), GGG (Gd ₃ Ga ₅ O ₁₂ ), GSGG (Gd ₃ Sc
₂ Ga ₃ O ₁₂ ) and other crystals, transition ions (C
r ³⁺ ) or a rare earth element ion (Nd ³⁺ ) doped with a small amount of impurities is used. The shape is generally a cylindrical rod type, but recently, a slab type has also appeared.

Here, FIGS. 5 and 6 show a rod-type solid-state laser device, and FIGS. 7 and 8 show a configuration outline of a slab-type solid-state laser device. First, in FIGS. 5 and 6, 1 is a cylindrical laser medium rod, 2 is an optical excitation lamp, 3 is an elliptic cylindrical reflecting mirror for collecting excitation light, and 4,5 are opposed to each other on both ends of the rod 1. The total reflection mirror for the optical resonator and the output mirror, 6 is the cooling water flowing through the inside of the elliptic cylinder type reflecting mirror, and the rod 1 and the optical excitation lamp 2 are located at the respective focal points of the elliptic cylinder type reflecting mirror. There is.

By concentrating the excitation light on the laser rod 1 in such a configuration, energy is generated by the laser amplification action while the light 7 reciprocates many times in the in-system optical path between the total reflection mirror 4 and the output mirror 5 of the optical resonator. And the self-excited oscillation occurs, and the oscillated laser light is transmitted through the output mirror having the transmission characteristic and is output to the outside of the optical resonator.

On the other hand, the slab type fixed cooling device sandwiches the slab plate 8 of the laser medium whose both end faces are obliquely cut as shown in FIGS. 7 and 8 and sandwiches the slab plate 8 with the photoexcitation lamp 2 as the photoexcitation means and the excitation light condensing light. The reflector 3 is arranged. With this configuration, the light 7 incident from the obliquely cut end surface of the slab plate 8 is refracted at this end surface, and the total internal reflection is repeated on the plate side surfaces (upper and lower surfaces in the drawing) of the slab plate 7 as shown in the figure. It will start to zigzag.

[Problems to be solved by the invention]

The characteristics of the solid-state laser described above are that the density of the laser medium is high,
The point is that an essentially large power density can be obtained. However, in the rod-type solid-state laser shown in FIGS. 5 and 6, the heat generated in the laser medium causes expansion and change in the refractive index of the medium, so that high-power operation is restricted. Become. That is, the heat generated in the rod 1 is the cooling water 6 from the outside.
However, as the laser output increases, a temperature gradient occurs in the medium of the rod 1 where the temperature at the center of the rod is high and the temperature is lower toward the peripheral side surface. Moreover, when such a temperature gradient is generated inside the laser medium, the rod has the characteristics of a convex lens having a large refractive index at its center due to the so-called thermal lens effect, which adversely affects the output characteristics of the laser device. Incidentally, FIG. 9 shows this state, and the light 7 traveling inside the rod 1 does not go straight due to the thermal lens effect.

On the other hand, when the thermal lens effect cannot be ignored, the optical system is designed in advance so that the oscillation conditions of the laser device are satisfied in consideration of the thermal lens effect.
However, the temperature distribution inside the rod changes with changes in the laser output, and the focal length of the thermal lens also changes with it, which complicates the problem, and this is a major obstacle to higher output of solid-state laser devices. ing.

In this regard, in the slab type solid-state laser device shown in FIGS. 7 and 8, the slab plate 8 is arranged in the X, Y, Z coordinate system in FIG.
While the left and right side surfaces (XZ plane) of the are cooled by the cooling water 6, the laser light travels in the YZ plane in a zigzag manner.
Therefore, the influence of the temperature distribution in the Y-axis direction is offset and reduced in the process of the zigzag progression of light. As a result, it is possible to expect a laser output with high repetition rate and high average output as compared with the rod type.

However, even with such a slab type, it is difficult to eliminate the influence of the temperature distribution in the X-axis direction in FIG. That is, the cooling water is caused to flow to the side surface of the slab plate 8 so that the temperature distribution in the X-axis direction becomes as uniform as possible, but in reality, there is a temperature gradient in the X-axis direction due to uneven cooling or the like. Therefore, even in the slab type, the influence of the thermal lens effect in the X-axis direction cannot help being accepted.

Further, the solid-state laser device has a problem associated with the intensity distribution of the excitation light in addition to the problem due to the thermal lens effect described above. That is, in the rod-type solid-state laser device shown in FIGS. 5 and 6, the rod 1 is placed inside the elliptic cylindrical reflection mirror 3 for collecting the excitation light.
And the optical excitation lamp 2 are arranged side by side, but the amount of excitation light incident on the rod 1 is smaller than that on the surface facing the optical excitation lamp 2 from the opposite side, so that excitation in the rod is performed. The light intensity distribution becomes non-uniform. Moreover, the intensity distribution of the excitation light appears as it is as the light intensity distribution of the laser beam, and the light intensity distribution of the output laser light becomes non-uniform. On the other hand, in the slab type shown in FIGS. 7 and 8, the effects of the Y-axis direction and the light intensity distribution are canceled by the zigzag progression of light in the same manner as the thermal effects, but the effects of the excitation light intensity distribution in the X-axis direction are It will appear as it is.

The present invention has been conceived in view of the above points, and its purpose is to eliminate the drawbacks in the characteristics of the conventional solid-state laser device described above, and to improve the temperature in the laser medium and the intensity distribution of the excitation light. It is an object of the present invention to provide a rod-type solid-state laser device capable of canceling adverse effects caused by the laser light and obtaining a laser light having a uniform light intensity, high repetition, and high average output.

[Means for solving problems]

In order to achieve the above object, according to the first aspect of the present invention, a cylindrical rod using a crystal or glass as a laser medium, a total reflection mirror for an optical resonator arranged at both ends of the rod, and an output. It is composed of a mirror and an optical pumping means, and is provided with a refracting part or a reflecting part having a conical surface for the incident light and the outgoing light of the rod in the optical path in the system of the optical resonator. In a rod-type solid-state laser device configured to travel in the rod while repeating total reflection with, both end faces of the rod are formed as conical convex or concave faces having the same apex angle with the axial centers aligned as refraction portions.
The inclination angle of the conical surface with respect to the rod center axis is set so that the incident angle of the incident light parallel to the rod center axis is an angle close to the Brewster's angle.

Further, according to the second aspect of the present invention, a cylindrical rod using a crystal or glass as a laser medium, a total reflection mirror for an optical resonator, an output mirror, and optical pumping means arranged on both ends of the rod are provided. It is equipped with a refracting part or a reflecting part having a conical surface for the incident light and the outgoing light of the rod in the optical path in the system of the optical resonator, and the light incident from the end face of the rod repeats total reflection on the peripheral side face of the rod. In the rod-type solid-state laser device configured to travel in the inside, both end faces of the rod are formed as planes perpendicular to the rod axis, and a conical concave total reflection mirror is provided as a reflecting portion on one end face side of the rod. The conical lens is placed as a refraction part between the other end face side and the output mirror, and the incident angle of the conical surface with respect to the rod center axis is parallel to the rod center axis. An angle whose angle is close to Brewster's angle It shall be set to be.

[Action]

With the above-described structure, the rod is formed with a conical convex surface or a concave surface having the same apex angle at both end faces so as to form a refraction portion, or both end faces of the rod are formed into a plane perpendicular to the rod axis. In addition, a conical concave total reflection mirror is provided as a reflecting portion on one end face side of the rod, and a conical lens as a refracting portion is interposed between the other end face side and the output mirror. And the angle of the conical end surface with respect to the rod center axis is set so that the incident angle of incident light parallel to the rod center axis having the end surface as a boundary surface is an angle approximate to the Brewster's angle. Has been done.

With such a configuration, the reflection loss of the light incident on the rod is suppressed, and inside the rod, the light propagates in a zigzag manner while repeating the total reflection on the circumferential side surface of the rod in the plane in each direction including the rod central axis. Therefore, the influence due to the non-uniform distribution of temperature and pumping light intensity inside the laser medium is canceled out in all directions, and as a result, the light intensity distribution is uniform, high repetition, and high average output Laser light can be obtained.

〔Example〕

FIG. 1 is a block diagram of an embodiment of a rod-type solid-state laser device according to the present invention, and FIGS. 2 and 3 show an enlarged view of the end face of the rod in FIG. 1 and a state of light traveling inside the rod. Explanatory drawing and FIG. 4 show a constitutional view of an embodiment different from the above, in which the same members corresponding to FIGS. 5 and 6 are designated by the same reference numerals.

First, in FIG. 1, the laser medium rod 1 has both end faces.
Reference numeral 11 forms a conical convex surface with the same apex angle by aligning the axial centers. Thereby, the incident light of the light 7 reaching the end surface of the rod 1 and parallel to the rod axis is conical convex end surface 11
As a boundary surface, the light is refracted and transmitted. Further, inside the rod, zigzag progresses in each radial surface including the rod central axis while repeating total reflection on the peripheral side surface 12 of the cylindrical rod 1 as shown in the figure. The light 7 is similarly refracted on the opposite end face on the emission side to become a beam parallel to the rod axis again.

Therefore, the influences of the internal temperature of the rod 1 and the excitation light intensity distribution can be canceled out as in the case of the slab type solid-state laser device of FIG. Moreover, the rod type is different from the slab type in that the optical path of the light traveling in the laser medium is not limited to a constant direction, but travels along each radial surface including the rod center axis, so that By canceling the influence, it becomes possible to obtain an output laser light having a uniform light intensity distribution.

Next, FIG. 2 shows an enlarged end view of the rod 1. 9 in the figure
Is a rod fixing frame, and 10 is an O-ring for sealing. Here, the inclination angle θ of the end face 11 with respect to the rod center axis is made equal to the angle γ formed between the transmitted light (refracted light) of the end face 11 and the rod center axis, and the light transmitted through the end face 11 is parallel to the conical end face. It is set to proceed. That is, when the incident angle of the light 7 on the rod end face 11 is α and the refraction angle is β, sin α / sin β = n (1) is established according to Snell's law. Note that n is the relative refractive index of the laser medium with respect to air. On the other hand, the incident angle α of light is expressed as α = (π / 2) −θ (2), and θ = γ, the refraction angle β
However, it is necessary that β = α−θ = (π / 2) −2θ (3). Here, substituting the equations (2) and (3) into the equation (1), cos θ / cos 2θ = n (4), and by solving this, the inclined surface θ of the end face 11 is obtained. If YAG (refractive index n = 1.823) is used as the laser medium, θ = 31 degrees, which is an approximate value to the Brewster angle α _B = 61.25 degrees determined by tan α _B = n. Therefore, the p-polarized light component of which the direction of the vibration electric field of light is within the incident surface is transmitted through the laser medium without being reflected by the end surface 11. That is, as described above, the inclination angle θ of the conical end faces 11 formed at both ends of the rod is set to the end face 11 of the rod 1.
On the other hand, by setting the incident angle α of the light incident parallel to the rod axis to be close to the Brewster's angle as described above, the reflection loss at the end face 11 can be suppressed to be low. Note that this condition is the same on the exit side of the rod 1.

When glass (n = 1.5) is used as the laser medium, θ = 27 degrees from the above equation (4). Even in this case, the incident angle α = 90−27 = 63 degrees is Brewster's angle α _B =
It is a value close to 56.3 degrees. Similarly, GGG as a laser medium
Even when (n = 1.95), GSGG (n = 1.942) or the like is used, the above Brewster angle condition is almost satisfied.

Next, regarding the optical path of the light traveling through the laser medium inside the rod, first, YAG is used as the laser medium, and the inclination angle of the conical end surface 11 of the rod 1 is set to θ = 31 degrees as described above. Then, the incident angle δ (FIG. 2) of the light traveling inside the rod 1 with respect to the rod peripheral side surface 12 is δ = 90−γ = 59
It becomes degree. Moreover, since the outer peripheral side of the peripheral side surface 12 of the rod 1 is the medium of the cooling water, the relative refractive index n'at the interface between the laser medium and the cooling water medium is n '= 1.823 / 1.333 = 1.37 .... (6) Thus, the critical angle [delta] _C becomes 47 degrees from _{sinδ C = 1 / 1.37 ...... (} 7). Therefore, the above-mentioned incident angle δ becomes larger than the critical angle δ _C and satisfies the condition of total reflection, whereby the light travels in zigzag while repeating total reflection inside the rod. Most of the solid materials described above as the laser medium satisfy this total reflection condition. Exceptionally, in the case of glass, δ ≈ δ _C , but by coating the peripheral surface of the rod 1 with an appropriate material, or by using gas instead of water as a cooling medium to flow on the peripheral surface of the rod 1, Can meet the reflection condition. Also, in the peripheral surface portion locally covered with the fixed frame 9 or the O-ring 10 at both ends of the rod in FIG. 2, by applying an appropriate coating treatment to this portion, total reflection on the peripheral surface of the rod is performed in the same manner as described above. You can meet the conditions.

Next, the length dimension of the rod 1 will be described. That is, in order to cancel the influence of temperature and excitation light intensity distribution inside the rod in the process of light traveling inside the rod, it is necessary that all light rays passing through the rod travel the same distance inside the rod. . FIG. 3 shows the rod 1 shown in FIGS. 1 and 2.
Ray a of light incident on the inside of the cone from its conical end face 11,
The optical path is drawn for B and C, where the diameter of the rod 1 is d, and the direction of the central axis that the light ray travels from the point at which the light is totally reflected on the rod peripheral side surface 12 to the time at which it is totally reflected. The distance l is l = d / tan (α−β) (8) Therefore, if the total length L of the cylindrical portion of the rod 1 is selected so that L = md / tan (α−β) (9) where m is an integer of 1 or more, zigzag is performed while repeating total reflection in the rod. All the light rays traveling along the optical path having the same length can satisfy the above-mentioned condition.

In the embodiment shown in FIG. 1, an example in which both end faces 11 of the rod are formed as conical convex faces as a refraction portion for incident light and emitted light of the rod 1 is shown. It is possible to achieve various effects. With this concave structure, the processing of the rod end surface is slightly more difficult than the convex surface shown in FIG.

Yet another embodiment is shown in FIG. In this embodiment, a normal laser rod 1 whose both end faces form a plane perpendicular to the center axis of the rod is used as a laser medium rod, and light which has a mirror-like conical concave surface as a reflecting portion on one side facing the end face. A conical concave total reflection mirror 13 for a resonator is provided as a reflection portion, and a conical lens 14 is interposed as a refraction portion between the end surface of the rod and the output mirror 5 on the other side. The conical concave total reflection mirror 13 and the conical surface of the conical lens 14 substantially satisfy the Brewster's angle condition as in the previous embodiment, and inside the rod 1, the light is totally reflected on the peripheral side surface. The angle is selected so as to proceed zigzag while repeating.

According to this structure, as in the embodiment of FIG. 1 described above, the influence of the temperature and the excitation light intensity distribution in the rod is affected in the process of zigzag while the light repeatedly repeats total reflection on the peripheral side surface of the rod. Can be offset or reduced. Moreover, the fourth
In the illustrated embodiment, there is no restriction on the length dimension of the rod 1, and even if the rod length is arbitrarily selected, the optical path lengths of all light rays passing through the rod are equal.

〔The invention's effect〕

As described above, according to the first invention and the second invention of the present invention, as a result of adopting the above configuration, the temperature distribution in the rod and the excitation light intensity, which have been problems in the conventional solid-state laser device, are obtained. It is possible to obtain an excellent output laser beam having a uniform light intensity distribution, high repeatability, and high average output by canceling out and eliminating the influence due to the nonuniformity of the distribution.

[Brief description of drawings]

1 is an overall configuration diagram of an embodiment of the present invention, FIG. 2 is an enlarged view of a rod end surface portion in FIG. 1, FIG. 3 is an explanatory diagram showing an optical path of a light ray in the rod, and FIG. Is a schematic view of another embodiment, FIGS. 5 and 6 are schematic side views of a conventional rod-type solid-state laser device, and end views thereof, and FIGS. 7 and 8 are schematic diagrams of a slab-type solid-state laser device. FIG. 9 is a side view and an end view thereof, FIG. 9 is an explanatory view of a thermal lens effect for the laser medium rod of FIG. 7, and 10 is an explanatory view showing an optical path passing through the slab plate of FIG. 7 in three dimensions. Is. In each figure, 1: laser medium rod, 2: optical pumping lamp, 3: pumping light focusing reflector, 4: total reflection mirror for optical resonator, 5: output mirror, 6: cooling water, 7: light, 11: conical end surface of rod, 12: peripheral side surface of rod, 13: conical concave total reflection mirror, 14: conical lens, θ: inclination angle of conical end surface, α: incident angle of light, β: refraction angle.

Claims (2)

[Claims] 1. A system of an optical resonator comprising a cylindrical rod using a crystal or glass as a laser medium, a total reflection mirror for an optical resonator arranged at both ends of the rod, an output mirror, and an optical pumping means. The inner optical path is provided with a refracting portion or a reflecting portion having a conical surface for the incident light and the outgoing light of the rod, and the light incident from the end face of the rod travels inside the rod while repeating total reflection on the peripheral side surface of the rod. In the rod-type solid-state laser device, both end surfaces of the rod are formed as conical convex or concave surfaces having the same apex angle with the axial centers aligned as the refraction portion, and the inclination angle of the conical surface with respect to the rod central axis is A rod-type solid-state laser device characterized in that an incident angle of incident light parallel to is set to an angle close to Brewster's angle. 2. A system of an optical resonator comprising a cylindrical rod having a crystal or glass as a laser medium, a total reflection mirror for an optical resonator arranged on both ends of the rod, an output mirror, and an optical pumping means. The inner optical path is provided with a refracting portion or a reflecting portion having a conical surface for the incident light and the outgoing light of the rod, and the light incident from the end face of the rod travels inside the rod while repeating total reflection on the peripheral side surface of the rod. In this rod-type solid-state laser device, both end faces of the rod are not parallel to the rod axis, and a conical concave total reflection mirror is arranged as a reflection part on one end face side of this rod, and the other end face side A conical lens is interposed between the output mirror and the output mirror as a refraction part, and the inclination angle of the conical surface with respect to the rod center axis is an angle at which the incident angle of the incident light parallel to the rod center axis is close to the Brewster angle. So that Rod type solid state laser apparatus characterized by being.