JP2000164959A - Solid state laser with monolithic pumping cavity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid state laser having an active element with a laser- rod form, at least a diode laser that is aligned in parallel with an axis of the laser rod, and a so-called pumping cavity. SOLUTION: A laser includes a rod-shaped active element 1, at least a diode laser line 7 for pumping the active element 1, and the pumping cavity that surrounds the active element 1. In this case, the active element 1 is a monolithic cylindrical block 2 with a groove in the axial direction of the cylindrical shape for fixing the active element 1 with an adhesive 4, and the block 2 has a reflective layer on its surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an active element (AE) in the form of a laser rod, at least one diode laser aligned parallel to the axis of the laser rod,
A so-called pumping cavity (reflector device that surrounds the AE, emits a diode laser train, and retroreflects the pumping radiation onto the AE that is not directly directed onto the AE and / or is not completely absorbed by the first traverse of the AE. )). Those skilled in the art will further appreciate that further means, such as resonator devices or cooling devices, belong to solid-state lasers, but these are not important in describing the nature of the solid-state laser according to the present invention, so these This device will not be considered.

[0002]

2. Description of the Related Art Emission in spectral range safe for eyes 3
Solid state lasers with AEs fabricated from level media are promising light sources with potentially high potential applications. Examples of such applications are: laser radar equipment, laser distance and laser velocity measurement equipment, air pollution recording equipment, and medical laser equipment.

[0003] Unlike other optically pumped lasers, lasers with AEs made from three-level media are relatively sensitive to pumping inhomogeneities due to the generally high laser threshold. Yb: Er: Glass, Tm: Ho: YAG, Yb:
Co-active three-level media, such as Ho: YAG, are particularly sensitive due to the absence of absorption saturation reactions.

[0004] In the case of three-level media, slight inhomogeneities in the pumping energy distribution that do not affect the absorption efficiency in the case of four-level media can cause a significant decrease in the overall efficiency of the laser. The parameters that are critical to efficient and homogeneous absorption of pumping radiation are, in essence, the shape and dimensions of the pumping cavity, the reflectivity of the surface of the pumping cavity, the optical properties of the medium between the reflective surface and the AE and the heat transfer The rate, the number and location of gaps that direct pumping radiation into the pumping cavity, the amount of heat input to the AE, and the pumping wavelength.

[0005] Various solutions are known from the prior art, using a cylindrically oriented reflecting surface in which the pumping radiation is launched into the pumping cavity through one or more gaps.

US Pat. Nos. 3,821,663, 4,969,155,
Nos. 5,033,058 and 5,307,365, and H. Ajer, S.
Optics Letter by Landro, G. Rustad and K. Stenersen
s The publication on page 17, 1785 (1992) should be mentioned in this context.

The use of a pumping cavity having a direct reflecting surface and a cylindrical shape with a circular cross section is disclosed in US Pat. No. 5,033,05.
As described in Nos. 8 and 5,307,365, necessarily causes a non-uniform angular distribution of the pumping radiation,
As a result, the spatial pumping power density in the AE becomes inhomogeneous. These solutions also provide, for example, an optical element placed between the laser light source and the pumping cavity and / or directly on the surface of the pumping cavity, so that the angular distribution of the pumping radiation is geometrically given by the AE. Must match the conditioned solid angle. As a result, the overall device is complicated in structure and thus more expensive. Of course, adding an optical element also causes a loss.

As described in US Pat. No. 3,821,663, limiting the pumping cavity to a reflective layer that is in direct contact with the surface of the AE causes the formation of a so-called "hot zone" of excessive pumping power density.

[0009] Increasing the size of the pumping cavity makes convection cooling of the AE difficult. Therefore, at high pulse repetition rates with high average power, the heat generated in the AE is dissipated by liquid cooling. In this case, too, the construction costs are relatively high and the efficiency is reduced due to the shading of the surface area by the necessary holding elements.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to achieve a high degree of homogeneity in the pumping light distribution around the AE and a substantially higher absorption efficiency compared to known solid state lasers. To manufacture a solid-state laser having a simple structure.

[0011]

This object is achieved according to the invention with a solid-state laser by means of a singularity representing the features of claim 1. Advantageous structures are set out in the dependent claims.

[0012]

DETAILED DESCRIPTION OF THE INVENTION It is important to the invention that the pumping cavity be made up of a monolithic block. The material of this block is a material that is transparent to the pumping radiation, such as optical glass, molten glass or crystalline glass, sapphire and undoped yttrium aluminum garnet (YAG). This block material must have a high degree of thermal conductivity and be able to efficiently dissipate the heat generated within the AE.

The specific choice of material depends on the cross-sectional shape of the pumping cavity and the required average power of the laser.

Although the block is cylindrical, various cross-sectional shapes are possible, and such shapes are described in detail below using specific examples. A tubular groove running in the axial direction of the cylinder is common to all structures. In this case, the axis of the groove can coincide with the axis of the cylinder or be parallel to the axis of the cylinder. The AE is mounted in this groove using an adhesive.

A reflective layer is formed on the outer surface, and depending on the cross-sectional shape, both a reflective coating in a scattering manner and a coating in a directed manner are conceivable. The surface layer has at least one gap running parallel to the axis of the cylinder, through which pump radiation from a diode laser train arranged upstream is emitted into the block.

As with other radiation components that radiate through AK (as is), radiation components that are not absorbed after colliding directly with the AE are reflected one or more times by the reflective surface of the block and then re-emitted. Collide with

Since the radiation can traverse the AE multiple times, the requirement for pump wavelength stability is reduced.
This situation is important because when the diode laser heats up, the wavelength of the diode laser fluctuates. Further, laser diodes exhibit wavelength errors for each column, and similarly for each diode in the column.

A particularly high degree of absorption is achieved in the case of solid-state lasers with an AE made from a three-level medium (k: absorption coefficient of the three-level medium at the pumping wavelength, d: diameter of the AE).

When kd <0.1, the loss in the reflective layer is
It increases very sharply assuming an actual reflectance value.

When kd> 5, the homogeneity of the absorbed energy distribution becomes significant assuming ideal and homogeneous illumination. A high degree of homogeneity and a high absorption efficiency of the pumping light distribution is achieved under the following conditions.

A. Block with diffusely reflective coating: In this case, the larger components of the inner surface reflect diffusely.

A-1. The coating on the block surface is applied by a diffuse reflection method. This coating can be composed of a metal oxide powder such as, for example, MgO, ZrO ₂ , ZnO. The reflectivity can have a value of 98% or less.

A-2. Before depositing the reflective coating, the surface of the block is smooth and there are no special requirements for the exact geometry. In special cases, the cross-sectional shape of the block can be circular. The absorption efficiency of a block with a diffuse reflection surface depends very slightly on its cross-sectional shape. Optimal ratio of the maximum cross section of the block (diameter for a circle
The optimal ratio between D) and the diameter of the AE essentially depends on the value kd. When kd = 0.1-5.0, the optimum ratio D / d is within the limit 1-4.

A-3. If the block is too large to allow heat accumulated in the AE to dissipate on its surface or to incorporate the block into special equipment, one dimension of the block The diameter can be reduced. Increasing the other dimensions of the cross section allows the diode radiation to be launched into blocks on this axis.

A-4. In order not to significantly reduce the efficiency due to the loss of pumping radiation in the gap, the width of the gap t
_sl must satisfy the following condition: t _sl <2 · P · (1-
K _refl ) / N _si , (P: outer circumference of block cross section, K _refl : reflectance coefficient of reflection layer, N _si : number of gaps). Coefficient 2 on right side of inequality sign
Considers the reflection due to the gap inside the block in the case of diffuse illumination.

As an example, the optimal diameter D = 4-5 mm is given by d
= 2 mm, K _refl = 0.98 and kd = _0.5-2.0 . If there are two gaps, their width must not exceed 0.25 mm. In the case of the embodiment of kd> 2.0, even if a wider gap is applied, the efficiency is not significantly impaired. A gap of this size can be simply formed, and arranging the openings of the diode laser array with respect to the gap is similarly simple.

B. Block with specular reflection coating. It is assumed that a substantial component of the inner surface of this block is specularly reflected.

B-1. The reflectivity inside this block is achieved by a metal layer (gold, silver, aluminum, etc.) deposited on the outside of the block, or by a dielectric multilayer system with high reflectivity to the pumping wavelength. can do. Total internal reflection may be used while leaving a part of the surface uncoated. The choice of the reflection coefficient depends on the number of times the reflection traverses the block, this number being essentially given by the optical density of the AE. Optical density is high enough (kd> = 0.2
~ 0.3), near infrared (IR) if the coating can be done by welding aluminum chemically or in vacuum
K _refl = 0.92 to _{0.95 in the} spectral domain.

B-2. Pumping radiation is generated on one or two opposing sides, especially when using a plurality of diode laser trains, one or two opposing gaps or one or two opposing gap groups. Enter the block from.

B-3. The cross sections of the blocks are
And two flat or substantially flat surfaces arranged parallel or substantially parallel to the plane of symmetry of the divergence angle of the diode radiation entering the block from one or more gaps. These surfaces may be flat or out of flat geometry. The surface area in the “AE-gap” direction is ΔW, and in this case, it is necessary to satisfy ΔW> 0.3 · D. Here, D: the maximum dimension of the cross section. Acceptable error δh from flat geometry with respect to mean plane
Must be δh <0.2 · D. Some possible variations of such an arrangement are shown in FIGS. 4, 5 and 6. The dimensions are determined by the optical density of the AE. AE optical density is kd =
If varying within the limits of 0.2-5.0, the index of the largest section for the AE diameter is D / d = 1.5-6.0 and the index of the smallest section for the AE diameter is 1.0-2.5.

B-4. To avoid a perceptible decrease in efficiency due to radiation loss due to radiation leakage from the gap, the gap width t _sl or the total gap width of closely adjacent gap groups must satisfy the following conditions: one when pumping at the end _{t sl << d · exp (2} · kd) when pumping at both _{t sl << d · exp (kd} )

For example, if the diameter of the AE is 2 mm and the absorption coefficient is 5c
For m ^−1, the overall gap width acceptable without significant reduction in efficiency is <0.5-1.5 mm.

B-5. In the case of a pumping intake at both ends, there is an axis of symmetry, and the AE is arranged coaxially with this axis. In the case of a pumping intake at one end, the AE moves away from the gap or gaps onto the opposing wall and the distance of the AE from the wall is not allowed to exceed the diameter of the AE.

The present invention will be described below with reference to the drawings. In the present description, the individual figures show the parts of the solid-state laser which are the essence of the invention, the pumping cavities differing in cross section.

The solid-state laser part which is the essence of the present invention is illustrated.
Shown in 1. The pumping cavity according to the invention is a cylindrical monolithic block 2, which is elliptical in cross section. On the surface of the block 2, a layer 5 that reflects in a scattering manner is applied. This layer 5 has a gap 6 running parallel to the axis of the cylinder passing through the apex of the ellipse. This gap 6 is arranged downstream of the diode laser train 7, and the radiation of the diode laser train 7 is directed into the block 2 via the gap 6. Inside the block 2, a tubular groove 3 is arranged parallel to the axis of the cylinder, and the rod-shaped AE 1 is fixed with an adhesive 4 so that the axis X of the AE is parallel to the axis of the cylinder.

2 and 3, the cross section of the block 2 is
It has a shape similar to a rectangle whose flank is a convex arc. In these two examples, the surface is coated with a reflective surface in a directed manner. In the case of FIG. 2, as in FIG. 1, there is only one gap 6, but in FIG. 3, the pumping radiation enters the block 2 from the two gaps 6.

FIGS. 4 to 6 further show a cross section of the block 2 coated with the reflective layer 5 in a directed manner.

The long side parallel to the plane shown in FIG. 2 and FIG. 3 of the block cross section has a different configuration in these figures. These long sides have in common that they do not exceed the maximum error Δh (less than 0.2 · D) over the entire length ΔW of 0.3 · D (D = maximum dimension of the block cross section) (as is in the original text).

In the case of FIGS. 1 to 6, the AE 1 is arranged at the center in the case of the block 2 having two gaps 6, and may be arranged further away from the gap 6 when there is only one gap 6. I understand.

Instead of one gap 6 or two gaps 6 arranged on both sides, it is also possible to arrange gap groups. Therefore, the pumping power can be increased by using a plurality of diode laser arrays 7.

[0041]

According to the present invention, the pumping cavity is formed of a monolithic block, so that a high degree of homogeneity is obtained in the pumping light distribution around the active element, and a solid-state laser having a simple structure with high absorption efficiency is formed.

[Brief description of the drawings]

FIG. 1 shows an elliptical cross section.

FIG. 2 shows a cross section similar to a rectangle whose side is a convex arc, and AEs are eccentrically arranged.

FIG. 3 shows a cross section similar to a rectangle whose flank is a convex arc, and AE is arranged at the center.

FIG. 4 shows a cross section similar to a rectangle whose side is a convex arc and whose long side is a concave arc.

FIG. 5 shows a section similar to a rectangle having a convex arc on the side and a cut at the center of the long side.

FIG. 6 shows a cross section similar to a rectangle having a convex arc on the side and a V-shaped long side.

[Explanation of symbols]

1 active element, 2 cylindrical block, 5 layers, 6 gaps,
7 Diode laser train

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Igor Kuzema Russian Federation 197 198 St. Petersburg Burg Szczeskaya Street 14-17 (72) Inventor Alexandra Levoskin Russia 190008 St. Petersburg Burg Pskovskaya Street 22-57 (72) Inventor Artua Mak Russian Federation 199155 St. Petersburg Burg Kola Prestelio Territory Street 40-1-224 (72) Inventor Vyatslav Boutsenkov Russia 195257 St. Petersburg Grazdansky Avenue 79-1-127 (72) Alexei Petrov Russian Federation 196233 St. Petersburg Vitebsky Avenue 67 123

Claims (6)

[Claims] At least one diode laser train (7) for generating pumping radiation and a rod-shaped active element (1)
A solid-state laser having a cylindrical pumping cavity disposed around the active element (1) such that its cylindrical axis runs parallel to the axis of the active element (1) and the diode laser array (7). Wherein the pumping cavity is
The active element (1) is a monolithic block (2) having a continuous tubular groove (3) fixed by an adhesive (4), and the surface of the block (2) has at least one gap (6). A solid-state laser having a reflective layer, wherein pumping radiation emitted by the diode array (7) is emitted into the block (2) through the gap (6). 2. The block (2) according to claim 1, wherein the block (2) has a cross-sectional shape having a maximum range (D) in a connecting line direction between the gap (6) and a cylindrical axis. Solid state laser. 3. The solid-state laser according to claim 2, wherein the cross-sectional shape is elliptical. 4. The method according to claim 2, wherein the cross-sectional shape is similar to a rectangle, and the flanks have a convex arc shape, and a long side thereof has a length of 0.3 · D or more. Solid state laser. 5. The solid-state laser according to claim 4, wherein the long sides are surfaces whose planes are parallel to each other. 6. The solid-state laser according to claim 4, wherein the long side has an error of 0.2 · D or less from a surface parallel to the plane in the middle.