JPH07302946A - Solid-state laser - Google Patents

Abstract

PURPOSE:To enable a solid-state laser to turn its oscillation mode into a single longitudinal mode by a method wherein a wavelength selection device is provided in a laser resonator, and an oscillation mode is turned to a twist mode in a laser resonator. CONSTITUTION:A laser beam 10 is emitted from a semiconductor laser 11 as pumping light and focused by a condensing lens 12. Resonator mirrors 14 and 15 are provided in the front and rear side of an Nd:YAG crystal 13 as a solid- state laser medium respectively. A KN crystal 15 is provided between the resonator mirror 14 and the Nd:YAG crystal 13, and an etalon 17 is disposed between the KN crystal 16 and the Nd:YAG crystal 13. Two quarter-wave plates 18 and 19 are provided in front and the rear of the Ncl: YAG crystal 13 between the resonator mirrors 14 and 15. An oscillation mode is turned into a twist mode between the quater-wave plates 18 and 19. By this setup, an oscillation mode can be turned into a single horizontal mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state laser, and more particularly to a solid-state laser in which an etalon or the like is arranged in a laser resonator to make an oscillation mode a single longitudinal mode.

[0002]

2. Description of the Related Art As disclosed in, for example, Japanese Patent Laid-Open No. 62-189783, there is known a solid-state laser in which a solid-state laser rod doped with a rare earth element such as neodymium is pumped by a semiconductor laser (laser diode) or the like. .

In this type of solid-state laser, for example,
Optics Letters Vol.18 (19
93) As described on p.420, in order to suppress the generation of mode-competitive noise, an etalon is placed in the resonator to select the wavelength and make the oscillation mode a single longitudinal mode. ing. Also, in order to make the oscillation mode a single longitudinal mode, for example, Optical Society of America
(Optical Society of America) Vol.
30 (1991) As described in p.2495, placing two λ / 4 plates in a solid-state laser resonator and turning the oscillation beam into a twist mode (elliptical polarization state) between them. Is also proposed.

[0004]

However, according to the experiments conducted by the present inventors, it has been found that when the above two configurations are adopted, the high output of the solid-state laser is limited. That is, in the former configuration using the etalon, when the solid-state laser is made to have a high output, the adjacent cavity mode rises, and it is difficult to realize the single longitudinal mode. FIG. 4 schematically shows this state, in which the portion indicated by a is the original longitudinal mode and the portion indicated by b is the adjacent resonator mode.

On the other hand, in the latter configuration in which the oscillation beam is made into the twist mode, when the solid laser is made to have a high output, the above-mentioned adjacent resonator mode does not stand, but the longitudinal mode is different from the original longitudinal mode. Another resonator mode stands up at a distance of several to several tens, and in this case also, it becomes difficult to realize a single longitudinal mode. FIG. 5 schematically shows this state, in which the portion indicated by a is the original longitudinal mode, and the portion indicated by c is the other resonator mode.

The present invention has been made in view of the above circumstances, and the oscillation mode can be changed to a single longitudinal mode.
Moreover, it is an object of the present invention to provide a solid-state laser capable of obtaining a high output.

[0007]

The solid-state laser according to the present invention is a solid-state laser in which a solid-state laser medium is excited by a pumping source as described above, and a wavelength selection element such as an etalon is arranged in a laser resonator,
It is characterized in that means for turning the oscillation beam into a twist mode is provided in the laser resonator.

As the wavelength selection element, in addition to an etalon made of a general parallel plate, one surface of the parallel plate is coated with a high reflectance for the solid-state laser oscillation wavelength, and the other surface is provided with It is also possible to use a material provided with an absorption thin film (ATF) that has absorption for a wavelength. On the other hand, as the means for turning the oscillation beam into the twist mode, for example, the above-mentioned two λ / 4 plates can be preferably used.

In the present invention, more preferably, F
An SR (Free Spectral Range), that is, a wavelength selection element whose mode interval substantially matches the full width at half maximum of the gain of a solid-state laser is used.

[0010]

As described above, each of the arrangement of the wavelength selection element such as the etalon in the laser resonator and the provision of the means for turning the oscillation beam into the twist mode are conventionally known. It was that.
However, since their respective purposes are the same, it has not been possible to apply both of them together to a solid-state laser.

However, according to the research conducted by the present inventors, when both a wavelength selection element such as an etalon and a means for turning the oscillation beam into a twist mode are provided together, the adjacent resonator mode by the wavelength selection element is twisted. Modes are suppressed because they are formed, while on the other hand, another resonator mode due to twist mode formation (mode in which longitudinal modes stand several to several tens of away from the original longitudinal mode) has a wavelength selection element. Since it is suppressed by the insertion, even if the output power of the solid-state laser is increased, the oscillation mode becomes a single longitudinal mode well.

As described above, according to the present invention, it is possible to obtain a high-power solid-state laser in which generation of mode competition noise is suppressed by the single longitudinal mode.

The FSR of a wavelength selection element such as an etalon
Is much larger than the full width at half maximum of the gain of the solid-state laser, the yield for producing the wavelength selection element that matches the gain peak decreases, and it becomes difficult to produce the etalon that matches the gain peak. On the contrary, when the FSR of the wavelength selection element is much smaller than the full width at half maximum of the gain of the solid-state laser, the second etalon mode stands up at a position separated by the FSR, and the longitudinal mode easily becomes multimode (FIG. 6). reference). Therefore, it is particularly preferable to use a wavelength selection element in which the FSR substantially matches the full width at half maximum of the gain of the solid-state laser, because such a problem will not occur.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the drawings. FIG. 1 shows a laser diode pumped solid state laser according to a first embodiment of the present invention. The laser diode pumped solid-state laser includes a semiconductor laser (phased array laser) 11 that emits a laser beam 10 as pumping light, and a condenser lens 12 that focuses the laser beam 10 that is divergent light, for example, a rod lens. Neodymium (N
d) YAG which is a solid-state laser medium doped
Crystal (hereinafter referred to as Nd: YAG crystal) 13 and this N
Resonator mirrors 14 and 15 arranged on the front side (right side in the figure) and the rear side of the d: YAG crystal 13, respectively, and nonlinear optics arranged between the Nd: YAG crystal 13 and the resonator mirror 14. KNbO ₃ crystal as a material (hereinafter referred to as KN crystal) 16
And an etalon (quartz plate) 17 arranged between the KN crystal 16 and the Nd: YAG crystal 13 and two λs arranged before and after the Nd: YAG crystal 13 between the resonator mirrors 14 and 15. /
It consists of four plates (sapphire plates) 18 and 19. The elements described above are mounted and integrated in a common housing (not shown). The phased array laser 11 is temperature controlled to a predetermined temperature by a Peltier element and a temperature control circuit (not shown).

As the phased array laser 11, a laser emitting a laser beam 10 having a wavelength λ ₁ = 809 nm is used. On the other hand, the Nd: YAG crystal 13 emits a laser beam 21 having a central wavelength λ ₂ = 946 nm when neodymium ions are excited by the laser beam 10. The laser beam 21 is incident on the KN crystal 16, is converted into the second harmonic wave 22 having a wavelength _{λ 3 = λ 2/2 =} 473 nm.

Here, the mirror surface 15a of the resonator mirror 15
The end face 13a of the Nd: YAG crystal 13 on the KN crystal 16 side and the mirror face 14a of the resonator mirror 14 each have a wavelength λ ₁ = 80.
The coats 30, 31, and 32 having the following characteristics are applied to 9 nm, λ ₂ = 946 nm, and λ ₃ = 473 nm. A
R is non-reflective (transmittance 99% or more), HR is highly reflective (reflectance
99.9% or more). Further, each of the end faces of the λ / 4 plates 18 and 19 is provided with a non-reflection coating for the wavelength λ ₂ = 946 nm, and both end faces of the etalon 17 are non-coated.

809 nm 946 nm 473 nm Coat 30 AR HR-〃 31-AR HR 〃 32-HR AR Due to the coatings 30 and 32 as described above,
The laser beam 21 resonates between the mirror surface 15a and the mirror surface 14a. As described above, the laser beam 21 is incident on the KN crystal 16 in a state of being resonated, so that the laser beam 21 is sufficiently well absorbed therein, whereby the second harmonic wave 22 is efficiently generated. This second harmonic wave 22 is reflected directly or at the end face 13a of the Nd: YAG crystal 13 provided with the coating 31 in the opposite direction and is emitted from the resonator mirror 14.

Here, the etalon 17 is a laser beam.
It is arranged so as to make an angle of 30 'with the traveling direction of 21, and its FSR (Free Spectral Range) is
It is equal to the full width at half maximum of the YAG oscillation in the 946 nm band.
It is set to 8 nm. The resonance wavelength of this etalon 17 is
946.2 nm (in vacuum), which is equal to the YAG gain peak wavelength of 946.2 nm. The resonance wavelength of this etalon 17 and the gain peak wavelength of YAG are ± 0.
Even if there is a deviation of about 1 nm, there is practically no problem.

On the other hand, the two λ / 4 plates 18 and 19 are arranged so that their crystal axes are rotated by 90 °.
The arrangement of the four plates 18, 19 causes the laser beam 21 to be twisted between them. Therefore, the laser beam 21 naturally has a gain peak wavelength of 94
It oscillates at 6.2 nm. With that alone, when the output power of the solid-state laser is increased, as described above, another resonator mode stands up at a position where several longitudinal modes are separated by several to several tens, but in this case, the etalon 17 is provided. Therefore, this other resonator mode is suppressed.

On the contrary, even when the oscillation wavelength is selected only by the etalon 17, when the solid-state laser is made to have a high output, the adjacent resonator mode stands up as described above. This adjacent resonator mode is suppressed because the twist mode is formed. The oscillation state in this embodiment is shown in FIG.

As described above, according to this structure, it is possible to achieve both high output of the solid-state laser and single longitudinal mode. In this embodiment, when the output of the laser beam 10 which is the pumping light is 3 W, the second harmonic wave 22 of the single longitudinal mode of 300 mW was obtained.

Next, a second embodiment of the present invention will be described with reference to FIG. Note that, in FIG. 3, elements equivalent to those in FIG. 1 are denoted by the same reference numerals, and duplicate description thereof will be omitted (the same applies hereinafter). This second
The solid-state laser of the embodiment is also a laser diode-pumped solid-state laser. Compared with the laser-diode-pumped solid-state laser of the first embodiment, the etalon 17 and the resonator mirror 15 are omitted, and instead the wavelength selection element 40 is used.
The difference is that is used.

The wavelength selection element 40 is formed by coating a transparent parallel flat plate on the rear end face 40a with a coat 41 and on the front end face 40b with an absorbed thin film (ATF) 42. The coat 41 has the same wavelength λ ₁ = 809 n as the coat 30 applied to the resonator mirror 15 of the first embodiment device.
AR for m, HR for wavelength λ ₂ = 946 nm
Of the characteristics of. That is, in this embodiment, the wavelength selection element 40 and the resonator mirror 14 constitute a resonator for a solid-state laser. On the other hand, the absorption thin film 42 is made of a Cr film having a thickness of 5 nm.

In this structure, when the laser beam 21 is incident on the wavelength selection element 40, a standing wave may be generated in a state where the node is located on the end surface 40a on which the coat 41 is applied. In this case, The end face on which the absorption thin film 42 is formed
Even at 40b, only light having a wavelength at which the node is located produces this standing wave. In this embodiment, the central wavelength is 946
Of the multiple longitudinal modes with wavelengths of nm, only the light of the longitudinal mode with a wavelength of 946.2 nm produces this standing wave, and the node of the standing wave is the end face.
Other light not located at 40b is absorbed by the absorption thin film 42,
Oscillation is suppressed. As described above, the wavelength selection element 40
As a result, the same effect as the etalon 17 can be obtained, and in the same manner as in the first embodiment, the high output of the solid-state laser and the single longitudinal mode can be achieved at the same time.

Next, a third embodiment of the present invention will be described with reference to FIG. The solid-state laser of this third embodiment is also a laser diode pumped solid-state laser.
Compared with the laser diode pumped solid-state laser of the embodiment, the resonator mirror 15 is omitted and the Nd: YAG crystal is used.
Nd: YLF crystal 50 was used instead of 13 and KN crystal 16
In place of, a LiNbO ₃ crystal (hereinafter referred to as LN crystal) 51 having a periodic domain inversion structure is used, and λ
/ 4 Calcite (calcite) between plate 19 and etalon 17
The difference is that the plate 52 is arranged.

The semiconductor laser 11 has a wavelength λ ₁ = 79
A device that emits a 5 nm laser beam 60 is used. The c-cut Nd: YLF crystal 50 emits a laser beam 61 having a center wavelength λ ₂ = 1313 nm when neodymium ions are excited by the laser beam 60. The laser beam 61 is incident on the LN crystal 51, is converted into the second harmonic wave 62 having a wavelength _{λ 3 = λ 2/2 =} 657 nm.

Here, the rear end surfaces 18a of the λ / 4 plate 18 and λ /
The front end face 19a of the four plate 19 and the mirror face 14a of the resonator mirror 14 have wavelengths λ ₁ = 795 nm and λ ₂ = 1313 nm, respectively.
And λ ₃ = 657 nm coat 53 with the following characteristics,
54, 55 are given. In this case, AR is also non-reflective,
HR shows high reflection.

795 nm 1313 nm 657 nm Coat 53 AR HR-〃 54-AR HR 〃 55-HR AR That is, in the present embodiment, the λ / 4 plate 18 and the resonator mirror 14 constitute a solid-state laser resonator. There is. Therefore, the solid-state laser beam 61 resonates while the LN crystal 51
And is absorbed there sufficiently, so that the second harmonic wave 62 is efficiently generated. The second harmonic wave 62 is reflected directly or at the end face 19a of the λ / 4 plate 19 provided with the coating 54 in the opposite direction, and is emitted forward from the resonator mirror 14.

In this case, the mirror surface of the resonator mirror 14
14a has a radius of curvature of 30 mm, and the solid-state laser has a cavity length of 15
mm, the calcite plate 52 has a thickness of 1.5 mm, and the LN crystal 51 has a length of 5 mm.

Also in this embodiment, since the two λ / 4 plates 18 and 19 are arranged, the laser beam 61 is turned into a twist mode between them. Therefore laser beam 61
Naturally oscillates at the gain peak wavelength. With that alone, it was found that when the output power of the solid-state laser was increased, another resonator mode stood at 0.4 nm away from the gain peak wavelength, that is, at 8 longitudinal modes apart (this phenomenon is This does not occur when the pumping power of the semiconductor laser 11 is as low as 1 W). However, also in this case, the provision of the etalon 17 suppresses this other resonator mode.

On the contrary, when the oscillation wavelength is selected only by the etalon 17, when the solid-state laser is made to have a high output, the adjacent resonator mode stands up as described above. This adjacent resonator mode is suppressed because the twist mode is formed.

Here, the etalon 17 has a thickness of 0.3 mm and its FSR (free spectral range) is Nd: Y.
2n, which is equal to the full width at half maximum of the LF oscillation in the 1313 nm band
Since it is m, the second etalon mode does not stand up. The resonance wavelength of this etalon 17 is 130.1 nm which is the gain peak wavelength of Nd: YLF and ± 0.1.
There is a deviation within nm, and the reflectance for a wavelength of 1313 nm is
20%, and the inclination angle of the laser beam 61 with respect to the traveling direction is 5 '.

Further, in this embodiment, since an anisotropic Nd: YLF crystal 50 is used as the solid laser medium,
The laser beam 61 emitted from the λ / 4 plate 19 contains two linearly polarized light components that are linearly polarized in directions orthogonal to each other. The laser beam 61 is incident on the calcite plate 52 that is cut (so-called angle cut) so that the light passing end face forms an angle with the optical axis, and only one of the linearly polarized light components is selected. The direction of linearly polarized light is LN
The light is made incident on the LN crystal 51 so as to coincide with the Z axis of the crystal 51.

In the above structure, when the output of the laser beam 60 which is the pumping light is 2 W, the red second harmonic wave 62 of the single longitudinal mode of 300 mW is obtained.

The larger the tilt angle of the etalon 17, the more improved the wavelength selectivity. But etalon 17
Needs to suppress another resonator mode that may stand relatively far from the gain peak wavelength. Since the gain of such a mode is lower than the original one, the etalon tilt angle is set to 5 ' Even if it is set relatively small like
The other resonator mode is suppressed. If the tilt angle of the etalon 17 can be set small in this way, the loss due to it can be suppressed low, and high output can be realized.

However, when the output of the laser beam 60 which is the pumping light is increased to, for example, 3 W and 10 W, it becomes difficult to suppress the other resonator mode, and therefore the etalon tilt angle is increased to about 10 'and 30', respectively. Therefore, it is desirable to increase the wavelength selectivity.

Next, a fourth embodiment of the present invention will be described with reference to FIG. The solid-state laser of the fourth embodiment is also a laser diode pumped solid-state laser,
It differs from the laser diode pumped solid-state laser of the embodiment only in that a Brewster plate 70 is used instead of the calcite plate 52 as a polarization control element.

When the Brewster plate 70 and the LN crystal 51 having the periodic domain inversion structure are combined, the directions of linear polarization of the laser beam 61 as the fundamental wave and the second harmonic wave 62 coincide with each other. As is well known, the Brewster plate 70 causes almost no loss for P-polarized light.
Therefore, when the directions of the linearly polarized light of the laser beam 61 and the second harmonic wave 62 coincide with each other, both of them can be incident on the Brewster plate 70 in the P-polarized state to suppress the loss. . If so, there is no need to apply an AR (antireflection) coating to the second harmonic wave 62 on the Brewster plate 70.

The present invention can be similarly applied to solid-state lasers other than the laser diode pumped solid-state laser described above. Further, needless to say, the solid laser medium, its pumping source, the nonlinear optical material for wavelength conversion, and the like other than those in each of the above embodiments can be appropriately used.

[Brief description of drawings]

FIG. 1 is a schematic side view showing a solid-state laser according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an oscillation state of the solid-state laser of the first embodiment.

FIG. 3 is a schematic side view showing a solid-state laser according to a second embodiment of the present invention.

FIG. 4 is an explanatory diagram showing an oscillation state of a conventional solid-state laser.

FIG. 5 is an explanatory diagram showing another oscillation state of the conventional solid-state laser.

FIG. 6 is an explanatory view showing another oscillation state of the solid-state laser of the present invention.

FIG. 7 is a schematic side view showing a solid-state laser according to a third embodiment of the present invention.

FIG. 8 is a schematic side view showing a solid-state laser according to a fourth embodiment of the present invention.

[Explanation of symbols]

 10, 60 Laser beam (pumping light) 11 Semiconductor laser 12 Condenser lens 13 Nd: YAG crystal 14, 15 Resonator mirror 16 KN crystal 17 Etalon 18, 19 λ / 4 plate 21, 61 Laser beam (solid-state laser beam) 22 , 62 Second harmonic 30, 31, 32, 41, 53, 54, 55 Coat 40 Wavelength selection element 42 Absorption thin film 50 Nd: YLF crystal 51 LN crystal with periodic domain inversion structure 52 Calcite plate 70 Brewster plate

Claims (4)

[Claims] 1. A solid-state laser in which a solid-state laser medium is excited by a pumping source, wherein a wavelength selection element is arranged in a laser resonator, and means for turning an oscillation beam into a twist mode in the laser resonator is provided. A solid-state laser characterized in that. 2. The solid-state laser according to claim 1, wherein the FSR (free spectral range) of the wavelength selection element is substantially equal to the full width at half maximum of the gain of the solid-state laser. 3. A nonlinear optical crystal having a periodic domain inversion structure in the laser resonator for wavelength-converting a solid-state laser beam, and a predetermined direction from the solid-state laser beam with respect to an optical axis of the nonlinear optical crystal. 3. A solid-state laser according to claim 1, further comprising: a polarization control element that selects one linearly polarized light component to be incident on the crystal and makes it enter the crystal. 4. The solid-state laser according to claim 3, wherein the polarization control element is a Brewster plate.