JP3197820B2 - Solid-state laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to, for example, Nd: YA
The present invention relates to a solid-state laser device that generates laser light using a solid-state laser medium such as a G crystal or a Nd: YVO ₄ crystal.

[0002]

2. Description of the Related Art Heretofore, solid-state laser devices capable of efficiently generating a laser beam of a high-power diffraction-limited beam have been actively studied. FIG. 5 is a view showing an example of the structure of a solid-state laser device for generating the second harmonic of a laser beam of a high-power diffraction-limited beam (see Optical Letters, Vol. 19, p. 189).

A plurality of semiconductor laser diodes 1 are arranged.
The pumping light is generated from 00, and the pumping light propagates inside the fiber bundle 106 and is emitted from the fiber end 106a. The emitted excitation light 107 passes through a collimator lens 108 and a condenser lens 109, and is a quarter-wave plate (QWP) 101 also serving as a laser resonator mirror.
And is focused inside the Nd: YAG crystal 105. In the Nd: YAG crystal 105, a laser beam is generated by the energy of the irradiated excitation light, and the emitted laser beam is emitted from the Nd: YAG crystal 105, passes through the etalon 113 and the pinhole 112, and passes through a plane reflection mirror. Folded at 102, reflected by a concave output mirror 103 having a curvature of 40 mm, and further passed through a KTP crystal 120, which is a nonlinear optical crystal that generates a second high frequency of laser light and satisfies a type II phase matching condition, QWP1
01 reaches the resonator mirror 104 constituting the laser resonator. The laser light reflected by the resonator mirror 104 passes through the KTP 120 again and reaches the concave output mirror 103. This concave output mirror 103 is a Nd: YAG crystal 10
The spectral reflectance distribution (spectral transmittance distribution) is adjusted so that the laser light generated at 5 is reflected and the second high frequency of the laser light generated at the KTP 120 is transmitted. For this reason, the second high frequency of the light that has reached the concave output mirror 103 passes through the concave output mirror 103, is collimated by the lens 111, and is emitted outside the solid-state laser device. On the other hand, of the light reflected by the resonator mirror 104 and reaching the concave output mirror 103, laser light having a wavelength not converted by the KTP crystal 120 and generated by the Nd: YAG crystal 105 is output by the concave output mirror 103. Nd: Y
Return to the AG crystal 105. The laser light emitted from the Nd: YAG crystal 105 to the left in FIG. 4 is reflected by the QWP 101 and returns to the Nd: YAG crystal 105.

Nd: YA excited by irradiation of excitation light
In the G crystal 105, due to the temperature distribution due to heat generation, a refractive index distribution is generated in the radial direction around the optical axis of the laser beam, and an effective convex lens is formed. The laser mode in the laser resonator is determined by the effective convex lens and the concave output mirror 103. That is, in the KTP crystal 120, the laser mode is narrowed to increase the light density, thereby improving the wavelength conversion efficiency based on the nonlinear optical effect and extracting the second harmonic with high output. In addition, since the laser mode in the Nd: YAG crystal 105 expands and the overlap with the excitation mode is improved, Nd: YA
The oscillation efficiency of the laser in the G crystal 105 is improved. According to the above-mentioned document, the configuration shown in FIG. 5 yields a second high frequency of 3.5 W when irradiating 17.3 W of excitation light.

[0005]

In the solid-state laser device shown in FIG. 5, optimization of the laser mode in the Nd: YAG crystal 105 and the laser mode in the KTP crystal 120 is performed by moving the resonator mirror 104 in the optical axis direction. Then, the distance between the concave output mirror 103 and the resonator mirror 104 is adjusted. However, only the distance between the concave output mirror 103 and the resonator mirror 104 can be adjusted, and both the laser mode in the Nd: YAG crystal 105 and the laser mode in the KTP crystal 120 are optimized. And trying to optimize one will result in the other deviating from the optimal laser mode.

In the solid-state laser device shown in FIG. 5, the allowable variation range of the focal length of the equivalent convex lens formed in the Nd: YAG crystal is small because the degree of freedom of adjustment is small. Even if an attempt is made to obtain a higher output by increasing the power, the heat in the Nd: YAG crystal 105 increases due to the increase in the power of the excitation light, the refractive index distribution changes, and the equivalent focal length of the convex lens is optimal. Therefore, even if the power of the pumping light is increased, it may be difficult to obtain a higher output.

In the solid-state laser device shown in FIG. 5, since the degree of freedom of design is small, the overlap between the excitation mode and the laser mode in the Nd: YAG crystal 105 is insufficient.
A pinhole 112 for removing a higher-order laser mode and obtaining a diffraction-limited beam is arranged in the laser resonator. By arranging the pinhole 112, a higher-order laser mode can be removed and a diffraction-limited beam can be obtained. However, a loss is generated due to the removal of the higher-order laser mode, and the oscillation efficiency of the laser is reduced. I have.

Further, in the solid-state laser device shown in FIG.
Since the degree of freedom in designing the resonator is small, the KTP crystal 120
The radius of curvature of the concave output mirror 103 is set to 40 mm, and the focal length thereof is set to be extremely short so that the laser mode inside is always small. Under such conditions, oscillation does not occur unless the interval between the concave output mirror 103 and the resonator mirror 104 is extremely small, so that there is no room for adjusting the size and angle of the KTP crystal 120 for generating the second high frequency. Since a nonlinear optical crystal has a conversion efficiency proportional to the square of the length, it is impossible to arrange a KTP crystal having a length of 5 mm or more when it is desired to arrange a long KTP crystal.

In addition, since the focal length of the concave output mirror 103 is short, and hence the interval between the concave output mirror 103 and the resonator mirror 104 is narrow, the tolerance of the change of the interval is narrow. Even if the position is slightly changed, the characteristics change extremely sensitively, and there is a possibility that a problem may occur in long-term stability. The present invention has been made in view of the above circumstances, and has as its object to provide a solid-state laser device that has a large degree of freedom in design and an allowable range and is excellent in stability.

[0010]

A solid-state laser device according to the present invention for achieving the above object is as follows: (1) a solid-state laser medium which receives excitation light and is excited by the energy of the excitation light to generate laser light; A laser beam generated in the solid-state laser medium and emitted in a direction along a first optical path passing through the solid-state laser medium is reflected in a direction in which the laser light returns to the solid-state laser medium along the first optical path. 1 mirror (3) disposed at a position on the first optical path with the solid-state laser medium interposed therebetween with respect to the first mirror, transmitting the excitation light and causing the excitation light to enter the solid-state laser medium; A second mirror for reflecting the laser light emitted from the solid-state laser medium in a direction along a second optical path extending in a direction different from the direction in which the first optical path extends (4) disposed on the second optical path And the second The laser light reflected by the mirror in the direction along the second optical path,
A third mirror that reflects in a direction different from the direction in which the second optical path extends (5) laser light that is disposed on the third optical path and reflected by the third mirror in a direction along the third optical path; And a fourth mirror that reflects the laser light in a direction returning to the third mirror along the third optical path.

Here, the solid-state laser medium may be arranged on a second optical path instead of being arranged on the first optical path. According to the present invention, when adjusting the laser mode in the resonator, the first mirror is moved to the first mirror.
Can be moved in the direction along the optical path of
The fourth mirror can be moved in the direction along the third optical path, so that the degree of freedom of adjustment is high, and when the third mirror is a concave mirror, the curvature of the concave mirror may be gradual. The distance between the fourth mirror and the fourth mirror can be widened, and the rate of change in the characteristics with respect to the change in the position of the fourth mirror is small, and therefore, the long-term stability is excellent. As a result, the degree of freedom in design is greatly improved.

In the solid-state laser device according to the present invention, the third mirror is preferably a concave mirror. The thermal lens effect of the solid-state laser medium is compensated, and as described below, when a non-linear optical member is arranged between the third mirror and the above-described fourth mirror, the nonlinear optical member has This is for narrowing the laser mode in.

Further, in the solid-state laser device according to the present invention, the second high frequency of the laser light generated by the solid-state laser medium, which is disposed on the third optical path between the third mirror and the fourth mirror, is used. A non-linear optical member to be generated, a third mirror reflecting laser light generated by the solid-state laser medium and transmitting a second high frequency generated by the non-linear optical member, and a fourth mirror, It is also a preferable embodiment that the laser beam reflects both the laser light generated by the solid-state laser medium and the second high frequency generated by the nonlinear optical member.

With this configuration, it is possible to generate the second high frequency and to take out the second high frequency to the outside using the third mirror as an output mirror. Further, when the above-mentioned nonlinear optical member is arranged, an optical member having a different transmittance for a different polarization direction is provided on one of the first, second and third optical paths. Is preferred.

By doing so, the number of longitudinal modes in the oscillation spectrum is greatly reduced, the influence of the wavelength dependence of the phase matching condition in the nonlinear optical member is reduced, and the efficiency of wavelength conversion to the second high frequency is further improved. Further, in the solid-state laser device according to the present invention, the excitation light is focused in the solid-state laser medium, and the excitation light is focused in the solid-state laser medium. First
It is preferable to provide a focusing optical member that can be adjusted in the direction along the optical path.

By making the focusing position of the excitation light adjustable, it is possible to optimally adjust the overlap between the excitation mode and the laser mode in the solid-state laser medium. By adjusting the light condensing position according to the above, a laser output suitable for the power of the excitation light can be obtained.

[0017]

Embodiments of the present invention will be described below. FIG. 1 is a diagram showing a structure of a first embodiment of the solid-state laser device of the present invention. The solid-state laser device shown in FIG. 1 has a diameter of 5 m, which is one of the solid-state laser media.
An Nd: YAG crystal 5 having a length of 5 mm and a length of 5 mm is arranged, and the Nd: YAG crystal 5 is irradiated with excitation light 20. Excitation light 20 is guided to an optical fiber 21 and emitted from an end face 21 a thereof, collimated by a collimating lens 22, further passed through a condenser lens 23, and passes through the plane mirror 2, for example, in the same manner as the conventional example shown in FIG. N through
d: Light is condensed in the YAG crystal 5. Here, the condenser lens 2
Numeral 3 is fixed on a movable stage (not shown) that is movable in the directions of arrows AB shown in the figure.
Is adjusted.

Further, the plane mirror 2 is the second mirror according to the present invention.
In this example, 99.99% of the oscillation laser light having a wavelength of 1064 nm in the Nd: YAG crystal 5 is used.
It has a high reflectance of 9% or more and a wavelength of excitation light of 809 mm
95% of the light. Also Nd:
On both end surfaces 5a and 5b of the YAG crystal 5, a dielectric film having a reflectance of 0.1% or less with respect to the oscillation laser light (wavelength: 1064 nm) and a reflectance of 0.5% or less with respect to the excitation light (wavelength 809 nm). Are formed.

In the Nd: YAG crystal 5, laser oscillation occurs due to the energy of the excitation light, and the above-described wavelength 1
064 nm laser light is generated, and the generated laser light is applied to the Nd: YAG crystal 5 in a direction along the first optical path 11.
Emitted from On the first optical path 11, a plane mirror 1 (an example of a first mirror according to the present invention) having a reflectance of 95% with respect to a laser beam (wavelength: 1064 nm) is arranged. In this embodiment, the plane mirror 1 functions as an output mirror that emits laser light to the outside of the solid-state laser device, and 5% of the laser light passes through the plane mirror 1 and becomes an output laser light. 95% of the laser light emitted outside the solid-state laser device is
And returns to the Nd: YAG crystal 5 along the first optical path 11.

The laser light emitted from the Nd: YAG crystal 5 toward the plane mirror 2 is reflected in a direction along a second optical path 12 extending in a direction different from the direction in which the first optical path 11 extends. A concave mirror 3 (an example of a third mirror according to the present invention) having a radius of curvature of 70 mm and having a high reflectance of 99.9% or more with respect to light having a wavelength of 1064 nm is disposed on the second optical path 12. The laser light reflected by the plane mirror 2 and traveling along the second optical path 12 and reaching the concave mirror 3 is extended by the concave mirror 3 in a direction different from the direction in which the second optical path 12 extends. The light is reflected in a direction along the third optical path 13. On the third optical path 13, the wavelength 1064n
Resonator flat mirror 4 having a high reflectance of 99.9% or more with respect to light of m (an example of a fourth mirror according to the present invention)
The laser light reflected by the concave mirror 3 and traveling along the third optical path 13 is reflected by the resonator plane mirror 4 in a direction returning to the concave mirror 3 along the third optical path 13. Is done.

In the case of the solid-state laser device having the structure shown in FIG. 1, the plane mirror 1 and the plane mirror 2 are moved by moving the plane mirror 1 in the direction in which the first optical path 11 extends.
Can be adjusted, and the distance between the concave mirror 3 and the resonator flat mirror 4 can be adjusted by moving the resonator flat mirror 4 in the direction in which the third optical path 13 extends. It is possible to make Nd: YA
The laser resonator is kept stable by compensating for the thermal lens effect inside the G crystal 5, and the overlap between the excitation mode and the laser mode inside the Nd: YAG crystal 5 is optimally adjusted to increase the efficiency of laser oscillation. be able to. Therefore, by employing the structure shown in FIG. 1, it is possible to obtain a laser output with the highest efficiency in any excited state in any excited state, and it is not necessary to dispose a pinhole or the like for adjusting a laser mode. A diffraction limited beam can be obtained.

Also, as described above, the condenser lens 23
Is movable in the directions of arrows AB, and the position of the condenser lens 23 is adjusted so that the power of the output laser light emitted from the output mirror 1 is maximized. This condenser lens 2
Adjusting the position of 3 adjusts the focal length and the excitation mode of the thermal lens inside the Nd: YAG crystal 5, and thus the output characteristics are adjusted to be optimal according to the laser resonator.

In the embodiment shown in FIG. 1, an Nd: YAG crystal is used as an example of the solid-state laser medium according to the present invention. The solid-state laser medium used here is Nd: YA.
It need not be a G crystal, but may be a Nd: YVO ₄ crystal, a Nd: YLF crystal, or any other solid state laser medium. As a pumping light source used for pumping, for example, a semiconductor laser diode, a pumping light source that generates pumping light of a suitable wavelength corresponding to a solid-state laser medium to be used is selected.

FIG. 2 is a view showing the structure of a second embodiment of the solid-state laser device of the present invention. The difference from the embodiment shown in FIG. 1 is that the Nd: YAG crystal 5 is arranged in the second optical path 12. Thus, the solid-state laser medium according to the present invention may be arranged on the first optical path as shown in FIG. 1 or may be arranged on the second optical path as shown in FIG.

FIG. 3 is a diagram showing the structure of a third embodiment of the solid-state laser device of the present invention. The differences from the first embodiment shown in FIG. 1 will be described. In the embodiment shown in FIG. 1, the plane mirror 1 is used as an output mirror.
In this embodiment, the plane mirror 1 has a wavelength of 1064 nm.
And has a high reflectivity of 99.9% or more with respect to the laser light, and reflects all the laser light. In this embodiment, the solid-state laser medium has a diameter of 3 mm × a length of 3 mm.
Nd: YVO ₄ crystal 51 is used. Further, the concave mirror 3 is similar to the case of the first embodiment shown in FIG.
The concave mirror 3 has a radius of curvature of 70 mm.
Is 99.9% of the laser beam generated by the Nd: YVO ₄ crystal 51 with a wavelength of 1064 nm in the present embodiment.
While having a high reflectivity as described above, L
It has a transmittance of 95% with respect to the light having a wavelength of 532 nm, which is the second harmonic of the laser light having a wavelength of 1064 nm, generated by the BO crystal 6, and functions as an output mirror of the second harmonic. Further, the resonator plane mirror 4 has a wavelength of 1064n.
m has a high reflectivity of 99.9% or more with respect to the light of m, and 9% with respect to the light of the wavelength 532 nm of the second harmonic.
It has a high reflectance of 9.5% or more.

In the third optical path 13 between the concave mirror 3 and the resonator plane mirror 4, a type I phase matching crystal orientation that generates a second harmonic of wavelength 532 when light of wavelength 1064 nm is incident. An LBO crystal 6, which is a nonlinear optical crystal, is provided. On both end surfaces of the LBO crystal 6, a dielectric film having a reflectance of 0.2% or less for light having a wavelength of 1064 nm and a reflectance of 0.4% or less for light having a wavelength of 532 nm is formed. The axial direction of the LBO crystal 6 is Nd: YV
By arranging the O ₄ crystal 51 in the same direction as the c-axis direction, the conversion efficiency to the second harmonic is maximized. Of the second harmonics having a wavelength of 532 nm emitted bidirectionally from the LBO crystal 6, the light directed toward the resonator mirror 4 is reflected by the resonator mirror 14, is collected in the same direction, and goes out of the resonator from the concave mirror 3. Taken out. The lens 7 is a lens for collimating 532 nm light.

Here, by changing the distance between the plane mirror 1 and the plane mirror 2 and the distance between the concave mirror 3 and the resonator plane mirror 4, the thermal lens effect in the Nd: YVO ₄ crystal 51 is compensated for and the laser is used. Keep the resonator stable,
In addition, the overlap between the excitation mode and the laser mode in the Nd: YVO ₄ crystal 51 and the laser mode in the LBO crystal 6 can be independently and optimally adjusted, so that the second harmonic output can be performed with the highest efficiency. Can be obtained.
Also, the degree of freedom in designing the laser mode in the resonator is high, and even if the radius of curvature of the concave mirror 3 is increased to 70 mm, the laser mode in the LBO crystal 6 arranged in the third optical path 13 can be reduced to a small value. Therefore, the distance between the concave mirror 3 and the cavity plane mirror 4 can be increased to about 40 mm, a long nonlinear optical crystal can be arranged with a margin, and the second harmonic Generation efficiency can be improved. In addition, the allowable range of the change in the distance between the concave mirror 3 and the resonator flat mirror 4 is wide, the adjustment is easy, and the influence of the output fluctuation due to disturbance is small.

In the third embodiment, the LBO crystal is used as the nonlinear optical material for generating the second harmonic. However, it is not always necessary to use the LBO crystal. For example, the LBO crystal may be a BBO crystal or a KTP crystal. And other non-linear optical materials. FIG. 4 is a diagram showing the structure of a fourth embodiment of the solid-state laser device of the present invention. The differences from the embodiment shown in FIG. 3 will be described.

In the fourth embodiment shown in FIG. 4, an Nd: YAG crystal 5 is used as a solid-state laser medium for obtaining an oscillating laser beam, as in the first embodiment shown in FIG. As the nonlinear optical material arranged in the third optical path 13, unlike the LBO crystal 6 in the third embodiment shown in FIG. 3, a KTP crystal 61 satisfying the type II phase matching condition is used.

In the fourth embodiment shown in FIG. 4, a Brewster plate 8 is further arranged on the first optical path 11. In the Brewster plate 8, the polarization direction having the highest transmittance is one of the crystal axes of the KTP crystal 61.
It is fixed at 45 ° to the axis direction.
Under these conditions, the generation efficiency of the second harmonic in the KTP crystal 61 is maximized, and at the same time, the number of longitudinal modes of the oscillation spectrum is greatly reduced in order to perform the wavelength selection action most effectively as a birefringent filter. The influence of the wavelength dependence of the matching condition is reduced, and the conversion efficiency to the second harmonic is further improved.

[0031]

Embodiments of the present invention will be described below. When a solid-state laser device having the structure of the first embodiment shown in FIG. 1 was assembled and 20 W 809 nm excitation light was input, 8 W 1
A laser output having a favorable transverse mode distribution with a diffraction limit of 064 nm was obtained. Further, even when the solid-state laser device was left standing for a long time with the laser output continued, the laser output intensity did not change at all and was stable.

A solid-state laser device having the structure of the third embodiment shown in FIG. 3 was assembled, and a 532-nm second harmonic light of a diffraction-limited beam of 4 W was obtained with respect to an excitation light input of 20 W and 809 nm. In addition, the long-term stability in this case was also very good as described above. Further, the fourth type shown in FIG.
The solid-state laser device having the structure of the embodiment is assembled, and 20W8
For the excitation light input of 09 nm, a 532 nm second harmonic of a 6 W diffraction limited beam was obtained. The long-term stability was also very good.

[0033]

As described above, according to the present invention,
It is possible to realize a solid-state laser device which has a large degree of freedom in design and an allowable width and is excellent in stability.

[Brief description of the drawings]

FIG. 1 is a diagram showing a structure of a first embodiment of a solid-state laser device of the present invention.

FIG. 2 is a diagram illustrating a structure of a solid-state laser device according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a structure of a third embodiment of the solid-state laser device of the present invention.

FIG. 4 is a diagram showing the structure of a fourth embodiment of the solid-state laser device of the present invention.

FIG. 5 is a diagram showing a structural example of a conventionally proposed solid-state laser device that generates a second harmonic of a laser beam of a high-power diffraction-limited beam.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 plane mirror 2 plane mirror 3 concave mirror 4 resonator plane mirror 5 Nd: YAG crystal 6 LBO crystal 7 lens 8 Brewster plate 11 first optical path 12 second optical path 13 third optical path 20 excitation light 21 optical fiber 22 Collimating lens 23 Condensing lens 51 Nd: YVO ₄ crystal 61 KTP crystal

Continuation of the front page (56) References JP-A-1-128484 (JP, A) JP-A-7-193309 (JP, A) JP-A 8-95105 (JP, A) JP-A-4-361580 (JP U.S. Pat. No. 5,130,997 (US, A) U.S. Pat. No. 5,467,749 (US, A) Optics Letters, Vol. 18 No. 24 (December 15, 1993) pp. 2111-2113 V.I. See Magni et al. , (58) Fields surveyed (Int. Cl. ⁷ , DB name) H01S 3/05-3/139

Claims (5)

(57) [Claims] We claim: 1. excitation light is excited by the energy of the incident excitation light via the solid-state laser medium for generating a laser beam, generated in the solid-state laser medium a solid laser medium
A first mirror for reflecting the laser light emitted in a direction along a first optical path extending linearly in a direction along which the laser light returns to the solid-state laser medium along the first optical path; A first optical path between the first mirror and the first mirror, the excitation light being transmitted through the solid-state laser medium, the excitation light being incident on the solid-state laser medium; And travels on the first optical path.
A second mirror that reflects the generated laser light in a direction along a second optical path that extends linearly in a direction different from the direction in which the first optical path extends, and is disposed on the second optical path; The light is reflected by the second mirror in a direction along the second optical path and travels on the first optical path.
A third mirror that reflects the laser light that has traveled in a direction along a third optical path that extends linearly in a direction different from the direction in which the second optical path extends, and is disposed on the third optical path; A fourth mirror that reflects the laser light reflected by the third mirror in a direction along the third optical path in a direction in which the laser light returns to the third mirror along the third optical path; wherein the first, second, and fourth mirrors der plane mirror
The third mirror is a concave mirror, and the third mirror is
The lens effect due to the heat generated by the solid-state laser medium and
The only optical element with optical power to compensate
Solid-state laser apparatus, characterized in that that. 2. The solid-state laser device according to claim 1, wherein the solid-state laser medium is arranged on the second optical path instead of being arranged on the first optical path. . 3. The third mirror and the fourth mirror
The solid state laser disposed on the third optical path between
Non-linear generation of second high frequency of laser light generated in medium
An optical member, wherein the third reflection mirror is generated in the solid-state laser medium.
Reflected by the nonlinear optical member
The fourth mirror transmits the generated second high frequency wave, and the fourth mirror is a laser generated by the solid-state laser medium.
Laser light and a second high frequency generated by the nonlinear optical member.
2. A light source which reflects both of
Or the solid-state laser device according to 2. 4. The optical path according to claim 1, wherein
Different transmission directions for different polarization directions
An optical member having an excess ratio is provided.
3. The solid-state laser device according to 3 . 5. The solid-state laser medium according to claim 2, wherein
The excitation light, which is located at a position with the
The light-collecting position of the excitation light to be focused in the laser medium
3. The solid-state laser device according to claim 1, further comprising a focusing optical member that is adjustable in a direction along the optical path .