JPH0821743B2 - Solid-state laser device - Google Patents

Description

The present invention relates to a solid-state laser device that emits laser light by photoexcitation using YAG or glass doped with transition metal ions or rare earth element ions as a solid-state laser medium.

[Conventional technology]

As is well known, YAG (Y
₃ Al ₅ O ₁₂ ), GGG (Gd ₃ Ga ₅ O ₁₂ ), GSGG (Gd ₃ Ga ₅ O ₁₂ ), etc. Optical crystals and glass base materials, transition metal ions such as Cr ³⁺ and Nd ³⁺
A rare-earth element ion such as a laser-active substance is used as the laser medium, and the excitation light for that is conventionally light from an excitation light source such as a tungsten halogen lamp or a krypton discharge lamp, and in some cases sunlight. Is used. The shape of the laser medium is usually rod-shaped, but in recent years, so-called slab type has been used for high output. As is well known, FIG. 5 shows the principle configuration of a conventional solid-state laser device using a rod-shaped YAG as a laser medium.

As shown in FIG. 5A, a rod-shaped laser medium 1
The partial reflection mirror 2 and the total reflection mirror 3 are arranged so as to face the respective end faces of the laser beam, and a laser resonance system is configured with the laser medium 1, and the output of the laser light LL is extracted from the side of the partial reflection mirror 2. The light source 4 for the excitation light EL is, for example, a krypton discharge lamp, and is disposed in the closed container 5 so as to face the laser medium 1, and the laser medium is generated by the cooling medium CM that flows into the closed container 5 through the port 5a. 1 and the excitation light source 4 are strongly cooled. As shown in FIG. 2B, the closed container 5 has an elliptic cylindrical shape with both ends closed, and its inner surface is mirror-finished, and the excitation light EL from the excitation light source 4 arranged at one focus of the ellipse is The laser medium 1 is arranged at the other focus.

[Problems to be Solved by the Invention]

However, in a photo-excited solid-state laser, in general, only a part of the excitation light EL given to the laser medium contributes to the excitation of the laser-active substance or ion, and most of it is lost as heat, resulting in energy loss. There is a problem of very low efficiency. This will be described with reference to FIG.

Figure 4 (a) shows YAG with Nd ³⁺ as the laser active ion.
In the laser medium, it shows the correlation between the relative value of the efficiency η that the pumping light contributes to the pumping of Nd ³⁺ and the wavelength λ of the pumping light. As can be seen from this, the wavelength range of 720 to 820 nm shown in A in the figure is shown. And has high excitation efficiency. FIG. 2B shows the wavelength characteristic of the emission intensity I of the krypton discharge lamp as the excitation light source, and as shown in the figure, it is very convenient that the emission intensity is high in the wavelength range A described above. However, while the pumping light source has a considerable emission intensity in other ranges, especially in the short wavelength range B, the pumping efficiency of the laser medium is very low in this wavelength range B, so that pumping in the entire wavelength range Only about 5% of the energy of light contributes to the excitation of laser active ions.

In this way, the efficiency of photoexcitation is about 5%, and the remaining 95% is completely converted into heat in the laser medium. Therefore, the solid-state laser has a high density of the laser medium, that is, the density of the laser active ions. Is supposed to be able to take out a large laser output, but there is a problem in that it cannot be operated at a sufficiently high output due to thermal restrictions.

From the viewpoint of improving the photoexcitation efficiency, a solid-state laser device using a semiconductor laser as an excitation light source has recently attracted attention. As is well known in AlGaAs semiconductor lasers, the oscillation wavelength can be adjusted by changing the composition ratio of Al and Ga.For example, by adjusting this to near 810 nm where the peak of Nd ³⁺ excitation efficiency is present. In addition, the photoexcitation efficiency can be greatly improved, and the heat generation problem of the laser medium can be almost completely eliminated. However, as is well known, the output per semiconductor laser is as low as about 1 W at maximum, so even if a large number of semiconductor lasers are arranged to form an excitation light source, several W to several tens W
Only solid-state laser output can be obtained, and the solid-state laser device excited by the semiconductor laser is suitable for small output, but it is likely to reach the high output level of several hundred W or several kW currently obtained by lamp excitation. Absent.

It is an object of the present invention to solve the above problems and provide a solid-state laser device having high photoexcitation efficiency and suitable for high output.

[Means for solving the problem]

According to the present invention, according to the present invention, in a solid-state laser device that optically excites a laser active substance in a solid laser medium to emit laser light, from a semiconductor that absorbs irradiation light and emits fluorescence having a wavelength suitable for photoexcitation of the laser active substance. It is achieved by providing the following light conversion means, and by irradiating the light conversion means with light and using the fluorescence emitted from the light conversion means as excitation light to optically excite the laser active substance in the laser medium.

The light conversion means referred to in the above configuration is for converting the irradiation light it receives into fluorescence, and the semiconductor used for this is one that can easily match the wavelength or wavelength range of fluorescence to the wavelength range in which the photoexcitation efficiency of the laser active substance is high. In this sense, Al _x Ga _1-x As is preferably used in this sense, and the wavelength of fluorescence can be adjusted by the content x of Al. Further, in order to make the wavelength range of the fluorescence as narrow as possible so that it can be accurately matched to the peak of the photoexcitation efficiency of the laser active material, it is desirable that the semiconductor has a direct transition type energy band structure. Also in this respect, it is suitable as a semiconductor for the light converting means in the present invention. This AlGaAs
In the case of using, the light converting means may of course have a single layer structure, but in some cases a composite layer structure having a so-called double heterojunction structure can be used.

As a practical form of the light converting means, it is preferable that the semiconductor is formed in a thin layer or a film shape, and the light converting means is formed in the form of a plate-like member sandwiched between a pair of transparent holding plates, for example, glass plates. It is suitable for construction and incorporation in a solid state laser device.

[Action]

As is well known, a semiconductor has an energy band structure consisting of a valence band with a low energy level and a conduction band above it. When receiving irradiation light, electrons in the valence band are excited into the conduction band. , When this excited electron transits to the valence band again or falls, it emits fluorescence with a wavelength λ determined by the energy gap Eg between the conduction band and the valence band. The wavelength of this fluorescence is represented by λ = Eg / hc (1). However, h is a blank constant and c is the speed of light. Further, this semiconductor hardly absorbs a wavelength component longer than the above wavelength λ in the irradiation light received by the semiconductor, but absorbs a wavelength component below that very well.

The present invention utilizes this principle. For example, the emission intensity distribution of the light source shown in FIG. 4 (b) will be described. The wavelength λ of the fluorescence emitted by the light conversion means using the above-mentioned semiconductor is determined by the laser active material. By arranging it in the wavelength range A where the photoexcitation efficiency is good, not only the wavelength component in this range A in the irradiation light received by the light converting means from this light source but also the wavelength component in the range B having a shorter wavelength than that are absorbed. The wavelength component in the wavelength range B, which has not been used until now, is effectively used for the optical excitation of the laser active substance by converting the fluorescence into the fluorescence in the wavelength range A and giving it as excitation light to the laser medium.

When the energy band structure of the above-mentioned semiconductor is viewed in more detail with respect to the electron momentum or wave number vector,
The valence band has an upwardly convex band shape and the conduction band has a downwardly convex band shape. However, a so-called direct transition type semiconductor has a band structure in which the top of the valence band and the bottom of the conduction band substantially coincide with each other. In this case, the electrons excited from the valence band to the conduction band by the irradiation light relax their momentum and fall to the bottom of the conduction band within a very short relaxation time, from which they transit to the top of the valence band and emit fluorescence. Generate. The direct transition type semiconductor has almost no change in electron momentum during this transition, so that the quantum efficiency of fluorescence generation is higher than that of the indirect transition type and is almost the same.
Since it is 100% and the wavelength range of fluorescence is narrow, it has a very advantageous advantage for the present invention.

By selecting the component ratio of Al and Ga, AlGaAs described above can generate fluorescence in the wavelength range in which the photoexcitation efficiency of the laser active material is high, and within this component range, a direct transition type energy band structure. I have

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 exemplifies a schematic structure of a solid-state laser device according to the present invention corresponding to FIG. 5 described above, and corresponding parts are designated by the same reference numerals. In the following, the description of the same parts as those described above will be omitted.

In the embodiment shown in FIG. 1, the laser medium 1 is a so-called slab type suitable for a high-power solid-state laser device,
As can be seen from the cross section of FIG. 2B, it is formed into a plate-like body having a slightly flat rectangular cross section, and its material is, for example, Nd ^3+.
Doped YAG is used. As described above, this laser medium has the optical excitation efficiency η of the wavelength characteristic shown in FIG.

In the solid-state laser device using the slab type laser medium 1, as shown in FIG.
The laser light LL entering and exiting from the pair of oblique end faces 1a travels in a zigzag shape while being totally reflected by the pair of upper and lower plate surfaces in the figure, thereby reducing the so-called thermal lens effect of the laser medium 1. It Further, as shown in FIG. 3B, thermal insulation 1b is applied to the left and right side surfaces of the laser medium 1 in the figure to eliminate the temperature gradient in the left and right direction of the cross section and further reduce the thermal lens effect. Has become.

In order to apply the pumping light EL to the slab-shaped laser medium 1 from the pair of plate surfaces, the light converting means 10 according to the present invention are provided above and below the slab-shaped laser medium 1 as shown in FIG. The conversion means 10 is formed in a plate-like body in which a thin layer or film of AlGaAs is sandwiched between a pair of glass plates, and is fixed to the left and right side walls of the closed container 5 in FIG. Irradiation light IR is given to each of the pair of light conversion means 10, and accordingly, in this example, the irradiation light source 6 is provided.
Are arranged in the upper and lower parts of the closed container 5, and are cooled by the cooling medium CM flowing in the closed container 5. As the irradiation light source 6, in addition to various high-power discharge lamps and incandescent lamps, sunlight can be used as described above, but in this embodiment, the emission intensity I of the wavelength characteristic of FIG. 4 (b) is used. A krypton discharge lamp with a distribution of shall be used.

Now, for the laser medium 1 having the photoexcitation efficiency η shown in FIG. 4 (a), the wavelength within the range A in the figure, particularly the photoexcitation efficiency η
Since the light conversion means 10 that emits fluorescence having a wavelength of 810 nm having the maximum peak of is advantageous, the composition ratio of Al and Ga in AlGaAs as a semiconductor for the light conversion means 10 is selected to match this. . Assuming that the Al component ratio is x, Al _x Ga
The energy gap Eg of a _1-x As semiconductor is expressed by the following equation.

Eg = 1.424 + 1.247x (eV) (2) Since the energy gap Eg of the above formula (1) where wavelength λ is 810 nm is 1.53eV, if this is put into the above formula (2), x = 0.086. By providing this composition to the AlGaAs semiconductor, the fluorescence of the wavelength suitable for the maximum photoexcitation efficiency of the laser medium 1 using Nd ³⁺ doped YAG is generated.
AlGaAs in the composition region that can be generated in 10 and has a low Al component ratio is the above-mentioned direct transition type semiconductor. However, the wavelength λ of the fluorescence emitted from this has a slight temperature dependence in practice, but since the degree is usually 0.3 nm / ° C, it is 100 nm, which is the width of the wavelength range in FIG. 4 (a). In comparison, the variation width of the fluorescence wavelength is sufficiently small and there is no problem in practical use.

In the solid-state laser device according to the present invention configured as described above, the semiconductor of the light conversion means 10 absorbs the wavelength components in the range A and B of FIG. 4 (b) in the irradiation light IR from the irradiation light source 6. Then, the fluorescence can be converted into fluorescence having a wavelength corresponding to the highest peak of the photoexcitation efficiency η in FIG. The light conversion means 10 cannot be used in the range B
Since the wavelength component of is also effectively used and is converted into fluorescence having a wavelength corresponding to the maximum photoexcitation efficiency of the laser active material, the photoexcitation efficiency of the solid-state laser device can be remarkably improved. Moreover, unlike the conventional semiconductor laser, the entire surface of the light conversion means 10 serves to convert the irradiation light IR into the fluorescence as the excitation light EL, so that excitation light of large energy can be given to the laser medium I, Therefore, the present invention can provide a high-power solid-state laser device.

FIG. 2 shows an example of the structure of the light conversion means 10 using a single-layer semiconductor. A state during manufacture is shown in FIG. 2 (a) and a state at the time of completion is partially enlarged in FIG. 2 (b). Shown in cross section. As shown in FIG. 3A, first, the AlGaAs layer 14 having the above-described composition is grown on the GaAs substrate 11 to have a thickness of, for example, 0.3 mm by the liquid phase epitaxial method, and the portion of the GaAs substrate 11 is polished or etched. After removing it, the shape is adjusted to a predetermined size, and a transparent plate such as glass as shown in FIG.
The light conversion means 10 is formed by arranging and sandwiching between 10a.

FIG. 3 shows an optical conversion means 10 using a semiconductor of a composite layer having a double heterojunction structure in the same manner as in FIG. First, as shown in FIG.
= 0.2 thick AlAs layer 12 was grown thick by liquid phase epitaxial method, then AlGaAs layer 13 with x = 0.45 and AlGaAs layer with x = 0.08 by vapor phase epitaxial method.
The layer 14 and the AlGaAs layer 15 having a composition of x = 0.45 are successively grown, the total thickness of the AlGaAs layers 12 to 15 is set to about 0.3 mm which is the same as before, and the GaAs substrate 12 is removed as before. This composite semiconductor layer is sandwiched between a pair of transparent plates 10a and 10b to form the light conversion means 10, as in the case of FIG.

The composite semiconductor layer configured as described above has a double heterojunction structure in which a heterojunction is formed between the AlGaAs layer 14 hatched in the figure and the AlGaAs layers 13 and 15 having different Al component ratios above and below the AlGaAs layer 14. Excited electrons or carriers generated in the composite semiconductor layer due to absorption of irradiated light from the transparent plate 10a side are collected in the AlGaAs layer 14 having a small energy gap and transition to the valence band thereof. The fluorescence emitted at this time is given to the laser medium 1 as excitation light from the transparent plate 10b side, for example. This excited electron Al
The concentration on the GaAs layer 14 can be promoted by appropriately providing a potential difference between the layers in the composite semiconductor layer.

The quantum efficiency of the composite semiconductor layer in the embodiment of FIG. 3 in terms of fluorescence generation is not necessarily as good as in the case of FIG. 2, but in practice, the Al component of the AlGaAs layer 14 is easier to manage, so This has an advantageous advantage in manufacturing a semiconductor for a light conversion means.

As can be inferred from the embodiments described above, the present invention is not limited to such examples and can be implemented in various modes.

〔The invention's effect〕

As described above, according to the present invention, in a solid-state laser device that optically excites a laser active substance in a solid laser medium to emit laser light, a semiconductor that absorbs irradiation light and emits fluorescence having a wavelength suitable for photoexcitation of the laser active substance. Since the light conversion means is composed of the laser light and the irradiation light is applied to the light conversion means to excite the laser-active substance in the laser medium by using the fluorescence emitted from the light conversion means as excitation light, the laser light of the conventional laser medium in the irradiation light is emitted. The semiconductor of the light conversion means is made to absorb the wavelength component in the range that could hardly be absorbed due to the low photoexcitation efficiency of the active substance, and the type and composition of this semiconductor are adapted to the laser active substance, whereby the absorbed irradiation light is absorbed. It can be converted into fluorescence having a wavelength with good photoexcitation efficiency and given to the laser medium as excitation light, and the wavelength component of this irradiation light can be effectively used and read. By the improvement of excitation efficiency The active substance, the excitation efficiency of the solid-state laser apparatus can be significantly improved than before.

Further, since the entire surface of the light converting means in the present invention serves to convert the irradiation light into fluorescence for photoexcitation, it is possible to give excitation light having a large energy to the laser medium from the light converting means, and Is efficiently used for photoexcitation of the laser active material, heat generation due to the excitation light in the laser medium can be significantly reduced as compared with the prior art, and it is possible to provide a solid-state laser device with a large output by the combined effect of the present invention. It becomes possible by carrying out.

Further, by using a direct transition type semiconductor for the light conversion means, the above-mentioned effect can be further enhanced.

[Brief description of drawings]

1 to 4 relate to the present invention, FIG. 1 is a longitudinal and transverse sectional view of an embodiment of a solid-state laser device according to the present invention, and FIGS. 2 and 3 are of different embodiments of a light converting means. Partially enlarged cross-sectional view, FIG. 4 is a diagram illustrating the photoexcitation efficiency of the laser active substance and the wavelength characteristic of the light intensity distribution of the irradiation light. FIG. 5 is a vertical and horizontal sectional view of a typical conventional solid-state laser device. In the figure, 1: laser medium, 2: partial reflection mirror, 3: total reflection mirror, 5: closed container of solid-state laser device, 6: irradiation light source, 10: light conversion means, 10a, 1
0b: Transparent plate for light conversion means, 11: GaAs substrate, 12, 13, 15: AlGaA
s layer, 14: AlGaAs layer as a semiconductor for light conversion means, EL: excitation light, I: light intensity of irradiation light, IR: irradiation light, LL: laser light,
λ: wavelength, η: photoexcitation efficiency of laser active substance.

Claims (1)

[Claims] 1. A solid-state laser device for optically exciting a laser active substance in a solid laser medium to emit laser light, wherein the light conversion means is composed of a semiconductor which absorbs irradiation light and emits fluorescence having a wavelength suitable for photoexcitation of the laser active substance. The solid-state laser device is characterized in that irradiation light is provided to the light conversion means, and fluorescence emitted from the light conversion means is used as excitation light to optically excite the laser active substance in the laser medium.