JP2006100415A - Laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser device which is miniaturized and stabilized, and provides high laser oscillation efficiency. <P>SOLUTION: The emission light generated by a laser diode 12 passes through lenses 23 and 24, an optical fiber 25, and a lens 21. Polarization split is carried out by a polarization beam splitter 61. The light of the linear polarization of a certain direction of them is outputted to a lens 22 and the light of the linear polarization of other directions which intersect perpendicularly with this is outputted to an optical filter 62. In the light inputted into the optical filter 62 from the polarization beam splitter 61, the light of a specified wavelength which penetrates the optical filter 62 selectively is reflected by a mirror 63. It transmits the optical filter 62 again, passes through the polarization beam splitter 61 and the lens 21 etc., returns to the active region of the laser diode 12, and induces the induced emission in this active region. The light of the specified wavelength λ<SB>P</SB>inputted into the lens 22 from the polarization beam splitter 61 is radiated to a laser medium 30. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a laser apparatus.

  The laser device has a laser medium on a resonant optical path in the optical resonator, and oscillates by generating stimulated emission light in the laser medium. In addition, when the laser device includes a nonlinear optical element, the laser apparatus can generate and output light having a wavelength different from the wavelength of the stimulated emission light emitted from the laser medium.

For example, a laser device using a laser medium made of Nd ion added YVO ₄ (Nd: YVO ₄ ) is known (see Non-Patent Document 1). This laser apparatus irradiates a laser medium with excitation light having a wavelength of 808 nm capable of exciting Nd ions in the laser medium, and generates stimulated emission light having a wavelength of 1064 nm in the laser medium. To generate a second harmonic wave having a wavelength of 532 nm, and a laser beam having a wavelength of 532 nm can be output.

Such a laser apparatus having Nd: YAG or Nd: YVO ₄ as a laser medium and also having a nonlinear optical element can output green laser light having a wavelength of about 532 nm. For example, Ti sapphire which is a femtosecond oscillator It can be used as an excitation light source for supplying excitation light to a laser light source.
"High-power green laser VerdiTM", [online], Coherent Inc., [searched on September 10, 2004], Internet <URL: http://www.coherent.co.jp/products/asscw/verdi_1.html >

  By the way, in the laser apparatus as described above, when the wavelength of the excitation light irradiated to the laser medium is different, the excitation light absorption efficiency, that is, the laser oscillation efficiency in the laser medium is different. On the other hand, in order to reduce the size of the laser device, a semiconductor light emitting element such as a laser diode is used as an excitation light source that outputs the excitation light. However, the wavelength of light output from the semiconductor light emitting element varies depending on temperature and injection current, and may vary with time, and may vary depending on the individual element. Therefore, a stable laser oscillation efficiency cannot be obtained with a conventional laser device.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a laser apparatus that can stably obtain high laser oscillation efficiency and can be miniaturized.

  A laser apparatus according to the present invention includes: (1) a laser medium that emits light by the incidence of excitation light; and (2) an optical resonator that has the laser medium on a resonance optical path and causes stimulated emission in the laser medium. And (3) an active region that emits light by current injection, and a first surface and a second surface that face each other across the active region, and the light emitted from the active region is reflected by the first surface; A semiconductor light emitting device for outputting the light from the second surface to the outside; and (4) a light output from the second surface of the semiconductor light emitting device is input, and a part of the input light having a specific wavelength is emitted by the semiconductor. An optical feedback unit that feeds back the active region of the element and outputs the remainder as pumping light; and (5) an excitation optical system that makes the pumping light output from the optical feedback unit incident on the laser medium. .

  This laser device operates as follows. The first surface of the semiconductor light emitting device and the optical feedback section constitute an external resonator. The light emitted from the active region of the semiconductor light emitting device is output to the outside from the second surface of the semiconductor light emitting device and is input to the optical feedback unit. Part of the light having a specific wavelength among the light input to the light feedback section is fed back to the active region of the semiconductor light emitting device, and stimulated emission light having a specific wavelength is obtained in the active region. Accordingly, light having a specific wavelength oscillates in the external resonator composed of the first surface of the semiconductor light emitting element and the optical feedback unit, and the light having the specific wavelength is output from the optical feedback unit. Output light having a specific wavelength from the optical feedback unit is incident as excitation light on a laser medium disposed on the resonance optical path of the optical resonator by the excitation optical system. Thereby, the light emitted from the laser medium is oscillated by the optical resonator, and the oscillation light is output from the optical resonator. The wavelength of the excitation light incident on the laser medium is a specific wavelength that is selectively fed back from the optical feedback section to the active region of the semiconductor light emitting device, and is stable. For this reason, in this laser apparatus, stable laser oscillation efficiency can be obtained.

  In the laser apparatus according to the present invention, it is preferable that the wavelength of the light returned by the feedback unit is variable. In this case, the wavelength of the light oscillated in the external resonator composed of the first surface of the semiconductor light emitting element and the optical feedback section is variable, and therefore, the wavelength <specific wavelength> of the excitation light irradiated to the laser medium is variable. It becomes.

  In the laser device according to the present invention, the optical feedback unit includes an absorption or interference optical filter that selectively transmits light of a specific wavelength and a mirror, and outputs the light output from the second surface of the semiconductor light emitting element. Among them, it is preferable that light of a specific wavelength is transmitted through an optical filter and reflected by a mirror, so that the light of a specific wavelength is returned to the active region of the semiconductor light emitting device. In this case, the wavelength (specific wavelength) of the light oscillated in the external resonator becomes the wavelength of the light that is transmitted through the optical filter, reflected by the mirror, and fed back to the active region of the semiconductor light emitting device. The specific wavelength can be changed by changing the inclination of the optical filter.

  In the laser apparatus according to the present invention, the optical feedback section includes a diffraction grating element, diffracts the light output from the second surface of the semiconductor light emitting element by the diffraction grating element, and emits light having a specific wavelength among the diffracted light. It is preferable to feed back to the active region of the semiconductor light emitting device. In this case, the wavelength (specific wavelength) of the light oscillated in the external resonator is the wavelength of light that is fed back to the active region of the semiconductor light emitting element among the diffracted light diffracted by the diffraction grating element. And the said specific wavelength can be changed by changing the inclination of a diffraction grating element.

The laser apparatus according to the present invention further includes a nonlinear optical element that is provided on the resonance optical path of the optical resonator and that receives light emitted from the laser medium and generates light having a wavelength different from the wavelength of the input light. Is preferred. In this case, light having a wavelength different from that of the light emitted from the laser medium is output from the nonlinear optical element. Here, the nonlinear optical element is, for example, LBO (LiB ₃ O ₅ ), type I pseudo phase matching type PPLN (Periodically Poled LiNbO ₃ ), KTP (KTiOPO ₄ ), or the like. Among these, LBO and type I pseudo phase matching type PPLN are particularly preferable because they are strong and stable against optical damage. The wavelength conversion in the nonlinear optical element depends on processes such as harmonic generation, sum frequency generation, difference frequency generation, parametric oscillation, and the like.

In the laser apparatus according to the present invention, the laser medium is preferably Nd ion-added vanadate, and more specifically, Nd ion-added GdVO ₄ or Nd ion-added YVO ₄ is preferable. At this time, the excitation light wavelength (specific wavelength) is preferably in the wavelength range of 808 ± 5 nm, or preferably in the wavelength range of 880 ± 5 nm. In this case, the wavelength of light emitted from the laser medium is around 1063 nm, and the wavelength of the second harmonic generated by the nonlinear optical element is around 531 nm.

  The laser apparatus according to the present invention can stably obtain high laser oscillation efficiency and can be downsized.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  (First embodiment)

  First, a first embodiment of a laser apparatus according to the present invention will be described. FIG. 1 is a configuration diagram of a laser apparatus 1 according to the first embodiment. The laser device 1 shown in this figure includes a drive circuit 11, a laser diode 12, lenses 21 to 24, an optical fiber 25, a laser medium 30, a nonlinear optical element 40, mirrors 51 to 54, a polarization beam splitter 61, an optical filter 62, and A mirror 63 is provided.

The laser medium 30 is made of Nd ion-added vanadate, and more specifically, Nd: GdVO ₄ or Nd: YVO ₄ is preferable. When a pumping light having a wavelength λ _P (808 nm band or 880 nm band) that can excite Nd ions, which are active elements, is incident on the laser medium 30, the pumping light λ _P is absorbed and Nd ions are excited and excited. When the Nd ions transition from the upper laser level to the lower laser level, light of wavelength λ ₁ (1063 nm) is emitted.

Of the two opposing surfaces of the laser medium 30, the first film 31 is formed on one surface, and the second film 32 is formed on the other surface. The first film 31 has a high transmittance at the excitation light wavelength λ _P and a high reflectance at the wavelength λ ₁ . The second film 32 has a high reflectance at the excitation light wavelength λ _P and a high transmittance at the wavelength λ ₁ .

The nonlinear optical element 40 receives light emitted from the laser medium 30 and generates a second harmonic wave having a wavelength λ ₂ (531 nm) different from the wavelength λ _{1 of the} input light. The nonlinear optical element 40 is, for example, LBO (LiB ₃ O ₅ ), type I pseudo phase matching type PPLN (Periodically Poled LiNbO ₃ ), KTP (KTiOPO ₄ ), or the like. The wavelength conversion in the nonlinear optical element 40 may be based on processes such as sum frequency generation, difference frequency generation, parametric oscillation, etc. in addition to harmonic generation. On both surfaces of the nonlinear optical element 40, the wavelength lambda ₁ and wavelength lambda ₂ transmittance is high in each film.

  A laser diode 12 that is a semiconductor light emitting element has an active region that emits light when a drive current is supplied from a drive circuit 11, and a first surface 12a and a second surface 12b that face each other across the active region. The light emitted from the active region is reflected by the first surface 12a, and the light is output to the outside from the second surface 12b. A high reflection film is formed on the first surface 12a, and a low reflection film is formed on the second surface 12b. The laser diode 12 outputs light in a wavelength band including a wavelength that can excite the active element Nd ions in the laser medium 30.

The lens 23 converts the excitation light λ _P diverged and output from the second surface 12 b of the laser diode 12 into parallel light, and the lens 24 converts the parallel light by the lens 23 into the first end surface 25 a of the optical fiber 25. Condensed to The optical fiber 25 guides the excitation light λ _P incident on the first end face 25a and outputs the light from the second end face 25b to the outside. The lens 21 converts the excitation light λ _P diverged and output from the second end face 25 b of the optical fiber 25 into parallel light, and the lens 22 transmits light that has been converted into parallel light by the lens 21 through the polarization beam splitter 61. And the excitation light λ _P is incident on the laser medium 30 from the side where the first film 31 is formed. These optical systems constitute an excitation optical system that makes the excitation light λ _P incident on the laser medium 30.

Each of the mirrors 51 to 53 has a high reflectance at the wavelength λ ₁ of the light generated in the laser medium 30. The mirror 52 has a high transmittance at the wavelength λ ₂ of the second harmonic generated by the nonlinear optical element 40. Each of the mirrors 51 and 52 has a concave reflecting surface, and can collect light when incident light is reflected. The first film 31 and the mirror 53 of the laser medium 30 constitute a Fabry-Perot optical resonator, and the laser medium 30, the mirror 51, the mirror 52, and the nonlinear optical element are sequentially arranged on the resonance optical path of the optical resonator. 40 is arranged. The mirror 54 reflects the second harmonic λ ₂ transmitted through the mirror 52 and output to the outside of the optical resonator.

  The polarization beam splitter 61 is provided between the lens 21 and the lens 22, and polarization-separates the light output as parallel light by the lens 21 and outputs linearly polarized light in a certain direction to the lens 22. Then, linearly polarized light in another direction orthogonal to this is output to the optical filter 62. The optical filter 62 is of an absorption type or an interference type that selectively transmits light of a specific wavelength, transmits light of a specific wavelength out of light that has arrived from the polarization beam splitter 61, and outputs the light to the mirror 63. The light having a specific wavelength that has arrived from the mirror 63 is transmitted and output to the polarization beam splitter 61. The optical filter 62 is, for example, a dielectric multilayer filter or an etalon filter. The optical filter 62 and the mirror 63 may be integrated.

  The polarization beam splitter 61, the optical filter 62, and the mirror 63 function as an optical feedback unit. That is, the optical feedback unit inputs the light output from the second surface 12b of the laser diode 12, feeds back a part of the input light having a specific wavelength to the active region of the laser diode 12, and the remaining part. Output as excitation light. Further, the first surface 12a of the laser diode 12 and the mirror 63 constitute an external resonator, and oscillate at the wavelength of light (specific wavelength) that the optical feedback unit selectively feeds back to the active region of the laser diode 12. Then, the oscillation light having the specific wavelength is output as excitation light.

The laser device 1 operates as follows. When a drive current is supplied from the drive circuit 11 to the laser diode 12, light is emitted in the active region of the laser diode 12. The emitted light is output to the outside from the second surface 12 b of the laser diode 12, condensed by the lenses 23 and 24 onto the first end surface 25 a of the optical fiber 25, and incident on the optical fiber 25 from the first end surface 25 a. The light is guided through the fiber 25 and output from the second end face 25b of the optical fiber 25 to the outside. The excitation light λ _P output from the second end face 25 b of the optical fiber 25 is converted into parallel light by the lens 21 and input to the polarization beam splitter 61. The light input from the lens 21 to the polarization beam splitter 61 is polarized and separated by the polarization beam splitter 61, and the linearly polarized light in a certain direction is output to the lens 22, and the linearly polarized light in another direction orthogonal to this. Is output to the optical filter 62.

  Of the light input from the polarizing beam splitter 61 to the optical filter 62, the light having a specific wavelength selectively transmitted through the optical filter 62 is reflected by the mirror 63, and is transmitted through the optical filter 62 again. 21, returns to the active region of the laser diode 12 through the optical fiber 25 and the lenses 24 and 23, and induces stimulated emission in the active region. Thus, the external resonator composed of the first surface 12a of the laser diode 12 and the mirror 63 oscillates at the wavelength (specific wavelength) of light that selectively passes through the optical filter 62.

The light λ _P having a specific wavelength input from the polarization beam splitter 61 to the lens 22 is collected by the lens 22, passes through the first film 31, and is irradiated onto the laser medium 30. This excitation light λ _P is reflected by the second film 32 and reciprocates in the laser medium 30.

When the laser medium 30 is irradiated with the excitation light λ _P , light having a wavelength λ ₁ is emitted from the laser medium 30. The first film 31 and the mirror 53 of the laser medium 30 constitute an optical resonator, and the laser medium 30 is disposed on the resonance optical path of the optical resonator. λ ₁ is obtained. The stimulated emission light λ ₁ is incident on the nonlinear optical element 40 disposed on the resonance optical path of the optical resonator, thereby generating a second harmonic having a wavelength λ ₂ different from the wavelength λ ₁ of the stimulated emission light. To do. The second harmonic λ ₂ generated by the nonlinear optical element 40 passes through the mirror 52 and is output to the outside of the optical resonator.

As described above, since the laser device 1 according to the present embodiment generates the excitation light using the small laser diode 12, the overall size can be reduced. The wavelength λ _P of the excitation light incident on the laser medium 30 is a specific wavelength that is selectively fed back from the optical feedback section to the active region of the laser diode 12, and is excellent in monochromaticity and stable. Therefore, the laser device 1 according to the present embodiment can stably obtain high laser oscillation efficiency because the excitation light wavelength is stable.

Further, if the inclination of the optical filter 62 is changed, the center transmission wavelength in the optical filter 62 changes, so that the specific wavelength λ _P that is selectively fed back from the optical feedback unit to the active region of the laser diode 12 can be changed. And the wavelength λ _P of the excitation light incident on the laser medium 30 can be adjusted. Therefore, in the laser apparatus 1 according to the present embodiment, even if the pumping light absorption spectrum differs depending on the individual laser medium 30, the pumping light wavelength λ _P is matched to the peak value of the spectrum of the laser medium 30 being used. Therefore, it is possible to obtain a higher laser oscillation efficiency more stably.

In the laser device 1, the excitation light is guided by the optical fiber 25 and enters the laser medium 30. Since the main body including the laser diode 12 and the like and the head portion including the laser medium 30 and the nonlinear optical element 40 and the like can be connected via a flexible optical fiber 25, only a small head portion is provided. , The irradiation position of the laser beam λ ₂ can be easily adjusted.

  (Second Embodiment)

  Next, a second embodiment of the laser apparatus according to the present invention will be described. FIG. 2 is a configuration diagram of the laser apparatus 2 according to the second embodiment. Compared with the laser apparatus 1 (FIG. 1) according to the first embodiment described above, the laser apparatus 2 according to the second embodiment shown in FIG. 2 has a diffraction grating element 64 instead of the optical filter 62 and the mirror 63. The difference is that it is provided and that the excitation optical system does not include the lenses 23 and 24 and the optical fiber 25.

  The laser apparatus 2 according to the second embodiment includes a polarization beam splitter 61 and a diffraction grating element 64 as an optical feedback unit. The polarization beam splitter 61 is provided between the lens 21 and the lens 22, and polarization-separates the light output as parallel light by the lens 21 and outputs linearly polarized light in a certain direction to the lens 22. Then, linearly polarized light in another direction orthogonal to this is output to the diffraction grating element 64. The diffraction grating element 64 has a reflection type diffraction grating formed on the surface facing the polarization beam splitter 61, diffracts the light reaching from the polarization beam splitter 61 with a diffraction angle corresponding to the wavelength, and identifies the diffracted light. The light having the wavelength is fed back to the active region of the laser diode 12. The first surface 12a of the laser diode 12 and the diffraction grating element 64 form an external resonator, and the wavelength of the light (specific wavelength) that the diffraction grating element 64 selectively feeds back to the active region of the laser diode 12. It oscillates and outputs the oscillation light of this specific wavelength as excitation light.

The laser device 2 according to the second embodiment includes the lens 22 as an excitation optical system. The lens 22 condenses the electromotive light λ _P transmitted through the polarization beam splitter 61 out of the parallel light by the lens 21 and from the side on which the first film 31 is formed without passing through the optical fiber. The excitation light λ _P is incident on 30.

  The laser device 2 operates as follows. When a drive current is supplied from the drive circuit 11 to the laser diode 12, light is emitted in the active region of the laser diode 12. The emitted light is output to the outside from the second surface 12 b of the laser diode 12, converted into parallel light by the lens 21, and input to the polarization beam splitter 61. Light input from the lens 21 to the polarization beam splitter 61 is polarized and separated by the polarization beam splitter 61, and linearly polarized light in a certain direction among them is output to the lens 22, and linearly polarized light in another direction orthogonal thereto. Is output to the diffraction grating element 64.

  The light input from the polarization beam splitter 61 to the diffraction grating element 64 is diffracted by the diffraction grating element 64 at an angle corresponding to the wavelength. The light having a specific wavelength diffracted in a specific direction returns to the active region of the laser diode 12 through the polarization beam splitter 61 and the lens 21 and induces stimulated emission in the active region. As described above, the external resonator composed of the first surface 12 a of the laser diode 12 and the diffraction grating element 64 oscillates at the wavelength (specific wavelength) of the light fed back by the diffraction grating element 64.

The excitation light λ _P having a specific wavelength input from the polarization beam splitter 61 to the lens 22 is collected by the lens 22, passes through the first film 31, and is irradiated onto the laser medium 30. This excitation light λ _P is reflected by the second film 32 and reciprocates in the laser medium 30. When the laser medium 30 is irradiated with the excitation light λ _P , light having a wavelength λ ₁ is emitted from the laser medium 30. The first film 31 and the mirror 53 of the laser medium 30 constitute an optical resonator, and the laser medium 30 is disposed on the resonance optical path of the optical resonator. λ ₁ is obtained. The stimulated emission light λ ₁ is incident on the nonlinear optical element 40 disposed on the resonance optical path of the optical resonator, thereby generating a second harmonic having a wavelength λ ₂ different from the wavelength λ ₁ of the stimulated emission light. To do. The second harmonic λ ₂ generated by the nonlinear optical element 40 passes through the mirror 52 and is output to the outside of the optical resonator.

Thus, since the laser apparatus 2 according to the present embodiment generates the excitation light using the small laser diode 12, the overall size can be reduced. Further, the wavelength λ _P of the excitation light incident on the laser medium 30 is a specific wavelength that is selectively fed back from the optical feedback section to the active region of the laser diode 12, and is excellent in monochromaticity and stable. Therefore, the laser apparatus 2 according to the present embodiment can stably obtain high laser oscillation efficiency because the excitation light wavelength is stable.

Further, if the inclination of the diffraction grating element 64 is changed, the specific wavelength λ _P that is selectively fed back from the diffraction grating element 64 to the active region of the laser diode 12 can be changed, and the excitation light incident on the laser medium 30 can be changed. it is possible to adjust the wavelength λ _P. Therefore, in the laser apparatus 2 according to the present embodiment, even if the pumping light absorption spectrum differs depending on the individual laser medium 30, the pumping light wavelength λ _P matches the peak value of the spectrum of the laser medium 30 being used. Therefore, it is possible to obtain a higher laser oscillation efficiency more stably.

  (Modification)

  The present invention is not limited to the above embodiment, and various modifications can be made. For example, an optical feedback unit that returns a part of light having a specific wavelength to the active region of the semiconductor light emitting element includes the optical filter 62 and the mirror 63 in the first embodiment, and the reflective diffraction in the second embodiment. Although the lattice element 64 is included, other configurations may be used. For example, in the first embodiment, a transmission type diffraction grating element may be used instead of the optical filter. In this case, the wavelength of light to be returned is changed by changing the inclination of both the diffraction grating element and the mirror. Can be changed.

Further, the direction when the excitation light λ _P is incident on the laser medium 30 is the same as the resonance optical path of the optical resonator in the above embodiment, but may be inclined with respect to the resonance optical path of the optical resonator, It may be perpendicular to the resonant optical path of the optical resonator.

  Moreover, you may further provide the Q switch means to change the Q value of an optical resonator. In this case, pulse laser oscillation with a large peak power can be obtained. Here, the Q switch means is, for example, a saturable absorber, an acousto-optic modulator, an electro-optic modulator, or the like.

1 is a configuration diagram of a laser apparatus 1 according to a first embodiment. It is a block diagram of the laser apparatus 2 which concerns on 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 ... Laser apparatus, 11 ... Drive circuit, 12 ... Laser diode, 21-24 ... Lens, 25 ... Optical fiber, 30 ... Laser medium, 31 ... High reflection film, 32 ... Low reflection film, 40 ... Nonlinear optical element Reference numerals 51 to 54: mirrors, 61: polarizing beam splitters, 62: optical filters, 63: mirrors, 64: diffraction grating elements.

Claims (8)

A laser medium that emits light upon incidence of excitation light; and
An optical resonator having the laser medium on a resonant optical path and causing stimulated emission in the laser medium;
An active region that emits light by current injection; and a first surface and a second surface that are opposed to each other with the active region interposed therebetween. The light emitted from the active region is reflected by the first surface, and the light is reflected A semiconductor light emitting element that outputs to the outside from the second surface;
The light output from the second surface of the semiconductor light emitting device is input, a part of the input light having a specific wavelength is fed back to the active region of the semiconductor light emitting device, and the remainder is output as the excitation light. An optical feedback section to
An excitation optical system that causes the excitation light output from the optical feedback section to enter the laser medium;
A laser device comprising:   2. The laser device according to claim 1, wherein the wavelength of the light returned by the feedback unit is variable.   The optical feedback unit includes an absorption type or interference type optical filter that selectively transmits the light of the specific wavelength and a mirror, and has a specific wavelength of light output from the second surface of the semiconductor light emitting device. 2. The laser device according to claim 1, wherein light is transmitted through the optical filter and reflected by the mirror to return the light having the specific wavelength to the active region of the semiconductor light emitting element.   The optical feedback unit includes a diffraction grating element, diffracts light output from the second surface of the semiconductor light emitting element by the diffraction grating element, and emits light of the specific wavelength out of the diffracted light to the semiconductor light emitting device. 2. The laser device according to claim 1, wherein the laser device is fed back to the active region of the element.   A non-linear optical element that is provided on a resonance optical path of the optical resonator and that receives light emitted from the laser medium and generates light having a wavelength different from the wavelength of the input light; The laser device according to claim 1.   The laser device according to claim 1, wherein the laser medium is Nd ion-added vanadate. The laser device according to claim 6, wherein the laser medium is Nd ion added GdVO ₄ . The laser device according to claim 6, wherein the laser medium is Nd ion-doped YVO ₄ .

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