JP2007266120A - Solid-state laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid-state laser device capable of achieving high-efficiency oscillation with a small excitation power. <P>SOLUTION: The solid-state laser device is provided with a semiconductor laser that emits excitation light, a lens that collects the excitation light emitted from the semiconductor laser, and a solid-state laser medium that emits infrared light while receiving the excitation light. A thickness of the solid-state laser medium in a direction orthogonal to the optical axis is less than three times of a beam diameter of a TEM00 mode. Heat-dissipation means are respectively provided vertically in the thickness direction of the solid-state laser medium. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a solid-state laser device, and more particularly to a solid-state laser device that excites a solid-state laser medium with laser light from a semiconductor laser.

  Semiconductor laser (LD) excitation type solid-state laser devices are highly efficient, small and robust, and are used for marking and thin film processing. In a solid crystal to which Nd (neodymium) is added, a 0.9 μm band and a 1.3 μm band oscillate with a wavelength of 1 μm band. By generating second-harmonic generation (SHG) of those wavelengths, visible light of green 532 nm, blue 450-470 nm, and around 670 nm can be obtained. It can be used for processing according to the characteristics of the object to be processed, and it is highly necessary in the fields of measurement, medical care, entertainment, projection display devices and the like.

  Green light is already highly efficient and has been put to practical use in thin film processing and silicon wafer markers. However, in blue, there is no highly efficient laser light source in the 900 nm band, which is the fundamental wave of blue light. This is because a gas laser such as a low-efficiency Ar ion laser or a semiconductor laser whose output power is only several tens of mW is not suitable for a watt class output such as processing.

As an example of a solid-state laser medium that oscillates light in the 900 nm band, for example, Nd: YAG (Y ₂ Al ₅ O ₁₂ ) can be given. When this medium is used, a fundamental wave having a wavelength of 946 nm is generated, and the second harmonic becomes blue light having a wavelength of 473 nm.

Patent Document 1 describes that a fundamental wave of 946 nm is generated by a semiconductor laser (LD) pumped Nd: YAG laser, and blue light having a wavelength of 473 nm is obtained at 1.1 W (watts) by nonlinear wavelength conversion. Moreover, although the thickness (optical axis direction) of the YAG crystal is 2 to 5 mm, there is no description about the size in the direction perpendicular to the optical axis. Moreover, the LD power for excitation is as large as 20W.
JP 2000-101170 A

  The present invention provides a solid-state laser device capable of high-efficiency oscillation with a small excitation power.

  According to one aspect of the present invention, a semiconductor laser that emits excitation light, a lens that collects the excitation light emitted from the semiconductor laser, and a solid-state laser medium that emits infrared light upon receiving the excitation light The solid-state laser medium has a thickness in a direction perpendicular to the optical axis that is not more than three times the beam diameter of the TEM00 mode, and heat dissipating means are provided above and below the solid-state laser medium in the thickness direction. A solid-state laser device is provided.

  According to the present invention, a solid-state laser device capable of high-efficiency oscillation with a small excitation power is provided.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic view illustrating the main configuration of a solid-state laser apparatus according to an embodiment of the invention.
Here, a specific example using an Nd: YAG laser with a wavelength of 946 nm will be described as an example of the solid-state laser medium. However, the present invention is not limited to this, and other solid-state laser media include, for example, Nd: YVO _4. Or Nd: GdVO ₄ may be used.

  In this specific example, the excitation light source is a single stripe broad area type semiconductor laser 1 having an output of about several watts. In such a W (watt) class semiconductor laser, the direction perpendicular to the pn junction (the x direction shown in FIG. 1B and also referred to as the “fast axis”) is parallel to the pn junction. The beam characteristics are different in the horizontal direction (the y direction in FIG. 1A and also referred to as “slow axis”). That is, the size of the light source at the end face of the semiconductor laser 1 is, for example, about 1.5 μm in the vertical direction (X direction) and about 200 μm in the horizontal direction (Y direction). Further, the full angle of the beam spread is, for example, about 40 ° in the vertical direction (X direction) and about 9 ° in the horizontal direction (Y direction).

  Laser light emitted from such a semiconductor laser 1 is collimated in the fast axis direction by the cylindrical lens 2, and then collimated in the slow axis direction by another cylindrical lens 3. Thereafter, the light is condensed and irradiated into the solid-state laser medium 6 by the condenser lens 4 through the high reflection side mirror 5 of the resonator. The high reflection side mirror 5 is coated with an optical film so as to be highly reflective only at an oscillation wavelength of 946 nm. The focal lengths of the cylindrical lenses 2 and 3 and the condenser lens 4 are such that the laser light from the semiconductor laser 1 is condensed to the same extent as the beam waist in the oscillation mode (diameter to 0.1 mm) when viewed in the horizontal direction. Selected as Further, by making the oscillation wavelength of the semiconductor laser 1 coincide with the Nd: YAG absorption wavelength of about 808 nm at the operating point, the absorption efficiency can be increased.

  The solid-state laser medium 6 is disposed with its upper and lower surfaces and left and right side surfaces in close contact with a metal heat sink 7. The laser light incident / exit surface of the solid-state laser medium 6 is optically polished, and an anti-reflection coating is formed with an excitation wavelength of 808 nm and oscillation wavelengths of 946 nm and 1064 nm. The laser resonator is composed of the high reflection side mirror 5 and the output mirror 8, and the interval is set to a length obtained by adding 2 to 3 mm to the length of the solid-state laser medium 6.

The output mirror 8 is made non-reflective at a wavelength of 1064 nm that easily oscillates, and is set to a transmittance of 4 to 5% at an oscillation wavelength of 946 nm. By forming the characteristics of these resonator mirrors in accordance with desired wavelengths, the respective oscillation wavelengths can be obtained in the same manner.
In this specific example, the resonator mirror is separately provided outside the solid-state laser medium 6, but it is also possible to form a resonator by directly forming a reflection film on the end surface of the solid-state laser medium 6. .

On the other hand, as the material of the solid-state laser medium 6, for example, Nd: YVO ₄ , Nd: GdVO _4, etc. can be used in addition to Nd: YAG. In these cases also, oscillation can be performed at 914 nm and 912 nm, respectively, by using the same shape as Nd: YAG. For example, the reflectance of the resonator mirror may correspond to 100% reflection on the high reflection side and 97% reflection on the output side, corresponding to each wavelength. These crystals have birefringence, and if the C-axis of the crystal is in the horizontal direction, it may coincide with the polarization plane of the semiconductor laser 1 for excitation.

FIG. 2 is a schematic perspective view illustrating the form of the solid-state laser medium 6 in the present embodiment.
The solid laser medium 6 is formed with a small thickness in a direction perpendicular to the optical axis L. That is, in the case of the specific example shown in FIG. 2, the thickness t in the vertical direction of the solid-state laser medium 6 is reduced. The thickness t of the solid-state laser medium 6 is preferably slightly larger than the beam diameter of the excited TEM00 mode. More specifically, the thickness t of the solid-state laser medium 6 is preferably not less than the beam diameter of the TEM00 mode and not more than three times the beam diameter. When the thickness t of the solid-state laser medium 6 is made smaller than the beam diameter of the TEM00 mode, excitation becomes insufficient. On the other hand, when the thickness of the solid laser medium 6 is more than three times the beam diameter of the TEM00 mode, the next mode TEM01 mode is likely to occur. This is because the heat dissipation becomes insufficient.

  The wide upper and lower surfaces and left and right side surfaces of the solid-state laser medium 6 are disposed in close contact with the metal heat sink 7. As a material of the heat sink 7, it is desirable to use a metal having high thermal conductivity such as copper or aluminum. Furthermore, heat generated in the solid-state laser medium 6 can be quickly removed via the heat sink 7 by heat conduction by a thermoelectric element (not shown) or water cooling. As a result, almost the entire solid-state laser medium 6 can be maintained at about room temperature.

  As an example, the length of the solid-state laser medium 6 in the optical axis direction (the direction of the arrow L in FIG. 2) is about 5 mm, the width w is about 5 mm, and the thickness t is about 0.3 mm. . Here, when the thickness t is set to about 0.3 mm, the diffraction loss is small in the TEM00 mode and the loss is increased with respect to the TEM10 mode, which is the next mode, so that stable TEM00 mode oscillation can be obtained.

  By reducing the thickness t in this way, it becomes possible to remove heat uniformly (one-dimensionally) from the oscillation region in the solid-state laser medium 6 toward the upper and lower surfaces of the crystal. Further, by reducing the thickness t, the heat generation area by the excitation light can be brought close to the cooling heat sink 7 and the thermal resistance can be lowered to about 1/10 of the conventional one. As a result, the temperature of the solid-state laser medium can be lowered by 30 to 50 degrees compared to the conventional (thickness t and width W are 3 to 4 mm, respectively), and oscillation in a quasi-three level energy system is easier than before. Became.

  FIG. 3 is a schematic view illustrating the temperature distribution in the solid-state laser medium 6 of this example. 3A is a schematic view of the solid-state laser medium 6 and the heat sink 7 as seen from the laser light emission surface, and FIG. 3B is a graph showing the temperature distribution in the width W direction of the solid-state laser medium 6. FIG.

  In the case of the solid-state laser medium 6 of this specific example, as shown by arrows in FIG. For this reason, the temperature in the crystal reaches the maximum temperature at the center of the excitation spot 6E, and decreases rapidly away from the center, resulting in an approximately uniform temperature distribution in the width direction. Since the axial direction of the refractive index due to the stress caused by such temperature distribution is the width W direction of the solid-state laser medium 6, it is easy to control so as to efficiently oscillate polarized light in the width W direction, i. State Laser Engineering "Springer 1999, p449).

  YAG crystals are optically isotropic. That is, the refractive index of the YAG crystal is constant regardless of the polarization direction. However, it is not optically isotropic because it causes distortion when stressed. That is, birefringence having a different refractive index depending on the polarization direction occurs. In this case, in the first-order approximation, the maximum refractive index change occurs in the stress direction. In the solid-state laser medium 6 of the present embodiment, stress is generated in the direction of the width W due to the formation of a temperature distribution as shown in FIG. 3B in the solid-state laser medium having a plate shape. The direction of the main axis of birefringence. As a result, the TEM00 mode is likely to occur in the width W direction, and stable oscillation of the TEM00 mode is promoted. This effect tends to become more prominent as the width W of the solid-state laser medium 6 is larger than the thickness t, and is particularly noticeable when the width W is 5 times or more the thickness t.

On the other hand, when YVO ₄ or GdVO ₄ is used as the material of the solid-state laser medium 6, since these crystals originally have birefringence, it is desirable to make the principal axis direction coincide with the width W.

With the above configuration, a highly useful TEM00 mode can be efficiently obtained with a low threshold. As will be described in detail later, infrared light in the TEM00 mode is required to obtain visible light of the second harmonic using a nonlinear crystal such as LiB ₂ O ₃ or LiNbO ₃ . On the other hand, according to the present embodiment, in order to make the heat dissipation of the solid-state laser medium 6 one-dimensional, a high-efficiency TEM00 mode light source can be obtained by forming a thin plate shape with a large cross-sectional aspect ratio. realizable.

  In the specification of the present application, “infrared light” refers to light having a wavelength band with a lower limit of 800 nm at the long wavelength end of visible light and an upper limit of 1 μm.

FIG. 4 is a graph illustrating the oscillation characteristics of the solid-state laser device of this embodiment. The horizontal axis represents LD excitation power, and the vertical axis represents output power at a wavelength of 946 nm.
According to this embodiment, the threshold input power is lowered to 2 to 3 W, which is half of the conventional value (thickness t and width W are 3 to 4 mm, respectively), and a TEM00 mode output of about 1 W is obtained at input 4 to 5 W. It was. In addition, the slope efficiency was also improved by promoting the heat radiation from the solid-state laser medium 6.

Conventionally, a crystal such as Nd: YAG or Nd: YVO _{4 serving} as a solid-state laser medium has a cross-sectional shape of about 3 to 4 mm in length and width (that is, thickness t and width W in FIG. 2) viewed in the optical axis direction. It was. Among them, the spot diameter that excites and contributes to oscillation is about 0.1 to 0.3 mm. The generated heat is transmitted through the solid crystal and is conducted to the external cooling medium. The heat conductivity of the crystal forming the solid-state laser medium is 0.1 to 0.01 W / cmK, which is 1/10 to 1/100 compared to a metal such as copper, so that heat is not easily transmitted. In particular, in the three-level system, the disadvantage is that the lower-level heat distribution increases due to temperature rise, the oscillation threshold is further higher, and excessive pumping power is required.

  In the 900 nm band oscillation of Nd: YAG or the like, there is a distribution due to heat because the lower energy level of the laser active ions contributing to the oscillation is the energy level above the ground state. Therefore, there is reabsorption, and excitation power is required to saturate it. For this reason, the oscillation threshold is increased, and an extra pumping power is required. This is a so-called quasi-three-level oscillation scheme. In the 1064 nm band oscillation, since it is a four-level system, the lower level distribution is almost zero at room temperature, and the threshold is low.

On the other hand, according to the present embodiment, infrared light such as Nd-added YAG and YVO ₄ , particularly 3, is formed by thinly forming a solid laser medium and forming a thin plate shape with a large cross-sectional aspect ratio. High-efficiency oscillation can be achieved in the 900 nm band, which is a level system oscillation.

  On the other hand, crystals used as the solid-state laser medium 6 tend to have improved thermal conductivity at low temperatures. For example, the thermal conductivity of a YAG crystal at a temperature of 70 K increases to nearly 10 times the room temperature. That is, according to this embodiment, by reducing the thickness t of the solid-state laser medium 6 and promoting heat dissipation, the thermal conductivity is also increased at the same time, and a cooling effect can be obtained in a superimposed manner.

FIG. 5 is a schematic diagram showing a solid-state laser device of a second specific example of the present embodiment.
In this specific example, laser light emitted from the solid-state laser medium 12 can be incident on the nonlinear crystal 13 and only the second harmonic can be extracted.
Also in this specific example, a semiconductor laser 9 is provided as an excitation light source. The temperature of the semiconductor laser 9 is appropriately controlled by the cooling system 22 and its oscillation characteristics are controlled. The laser light emitted from the semiconductor laser 9 is collimated and condensed in the excitation optical system 10 and enters the solid-state laser medium 12 through the high reflection mirror 14. The configuration of the excitation optical system 10 can be the same as that described above with reference to FIG. 1. For example, the cylindrical lenses 2 and 3 and the condenser lens 4 can be provided as appropriate.

The solid laser medium 12 and the heat sink 11A may be the same as those described above with reference to FIGS. A non-linear crystal 13 is disposed downstream of the solid-state laser medium 12. As the non-linear crystal 13, for example, a crystal that has high transparency up to the visible light region, such as β-BaB ₂ O ₄ , KNbO ₃ , LiB ₃ O ₅ , and BiBO and can achieve phase matching near room temperature can be used. The temperature control of the nonlinear crystal 13 needs to be more accurate than the solid-state laser medium 12.

  For this reason, in this specific example, as in the specific example shown in FIG. 2, a heat sink 11 </ b> B made of copper or the like is provided so as to surround the periphery of the optical axis of the nonlinear crystal 13. And these heat sinks 11A and 11B are joined to the heat sink 20. The heat radiating plate 20 is formed integrally by adhering a thermal insulator 21 having a low thermal conductivity such as a plastic material around the middle of a material having a good thermal conductivity such as copper or aluminum. By this insulator 21, the solid-state laser medium 12 and the nonlinear crystal 13 are thermally separated.

  Then, Peltier elements 17A and 17B are provided below each of these so that the temperature can be controlled. In this way, the temperature of the solid-state laser medium 12 and the nonlinear crystal 13 can be independently adjusted by the Peltier elements 17A and 17B, and the maximum efficiency can be obtained. Even if the Peltier elements 17A and 17B contract due to temperature changes, the optical axes of the solid-state laser medium 12 and the nonlinear crystal 13 are kept constant, and stable wavelength conversion can be maintained.

The mirror constituting the resonator is 100% reflected with respect to the fundamental wave, and the fundamental wave is confined in the resonator. Then, the mirror is formed so that only the output-side mirror 15 is transparent to the blue light that is the second harmonic. Accordingly, blue light of 473 nm, 457 nm, and 456 nm can be efficiently generated for Nd: YAG, Nd: YVO ₄ , and Nd: GdVO ₄ that are materials of the solid-state laser medium 12, respectively. An output power of ˜1 W is obtained.

  As described above, according to this embodiment, the threshold of the 900 nm band semiconductor laser pumped solid-state laser device that is difficult to oscillate can be lowered and oscillated with high efficiency. In addition, the volume of the material used is about 1/10 as compared with the conventional one, so that weight reduction and compactness are realized, and the material can be effectively used.

Further, using such a semiconductor laser excitation type solid-state laser device as a light source of fundamental light, a nonlinear crystal such as β-BaB ₂ O ₄ , KNbO ₃ , LiNbO ₃ , LiB ₃ O ₅ is installed in the laser resonator. Thus, light in the visible region that is the second harmonic can be efficiently generated. Further, the second harmonic can also be generated by installing the nonlinear crystal outside the resonator and condensing and irradiating with a lens.

  According to this embodiment, a small and efficient solid-state laser device can be provided. Examples of application destinations include microfabrication, medical treatment, measurement, entertainment, and a projection display device. With the same configuration, it is possible to oscillate at a wavelength of 1064 nm or 1340 nm, and it can be easily applied to a light source of green light or red light which is the second harmonic.

  The embodiments of the present invention have been described above with reference to specific examples. However, the present invention is not limited to these specific examples. For example, the structure, material, size, formation method, arrangement relationship, and the like of each element constituting the solid-state laser device are appropriately selected by those skilled in the art as long as they include the gist of the present invention. Is done.

It is a schematic diagram which illustrates the principal part structure of the solid-state laser apparatus which concerns on embodiment of this invention. It is a perspective schematic diagram which illustrates the form of the solid-state laser medium 6 in this embodiment. It is a schematic diagram which illustrates temperature distribution in the solid-state laser medium 6 of the specific example of this invention. It is a graph which illustrates the oscillation characteristic of the solid-state laser apparatus of this embodiment. It is a schematic diagram showing the solid-state laser apparatus of the 2nd example of this embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Semiconductor laser, 2, 3 Cylindrical lens, 4 Condensing lens, 5 High reflection side mirror, 6 Solid laser medium, 6E Excitation spot, 7 Heat sink, 8 Output mirror, 9 Semiconductor laser, 10 Excitation optical system, 11A, 11B Heat sink , 12 solid-state laser medium, 13 nonlinear crystal, 14 highly reflective mirror, 15 output mirror, 17A, 17B Peltier element, 18 case, 20 heat sink, 21 thermal insulator, 22 semiconductor laser cooling system

Claims (9)

A semiconductor laser that emits excitation light; and
A lens for collecting the excitation light emitted from the semiconductor laser;
A solid-state laser medium that receives the excitation light and emits infrared light; and
With
The solid-state laser medium has a thickness in a direction perpendicular to the optical axis that is three times or less the beam diameter of the TEM00 mode,
A solid-state laser device, wherein heat dissipating means is provided above and below the solid-state laser medium in the thickness direction.   2. The solid-state laser device according to claim 1, wherein the solid-state laser medium has a width in a direction perpendicular to the optical axis of 5 times or more of the thickness.   The solid-state laser device according to claim 1, wherein the heat radiating means is a heat sink made of metal in contact with the solid-state laser medium.   The solid-state laser device according to claim 3, wherein the heat sink is provided so as to surround the optical axis of the solid-state laser medium.   5. The solid-state laser device according to claim 1, further comprising a nonlinear crystal that receives the infrared light emitted from the solid-state laser medium and emits a second harmonic thereof. 6. . Further comprising a radiator plate connecting the second and third members having high thermal conductivity on both sides of the first member having low thermal conductivity,
6. The solid state laser device according to claim 5, wherein the solid state laser medium is supported on the second member, and the nonlinear crystal is supported on the third member.   The solid-state laser device according to any one of claims 1 to 6, wherein the solid-state laser medium is made of a YAG crystal to which Nd is added, and emits the infrared light having a wavelength of 946 nanometers. The solid-state laser device according to claim 1, wherein the solid-state laser medium is made of a YVO ₄ crystal doped with Nd and emits the infrared light having a wavelength of 914 nanometers. The solid-state laser device according to claim 1, wherein the solid-state laser medium is made of a GdVO ₄ crystal doped with Nd and emits the infrared light having a wavelength of 912 nanometers.

Images