JP2018005034A - Laser device and wavelength conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser device and a wavelength conversion element that make it possible to suppress ripples and side lobes in a lateral mode of an emitted laser beam and thus obtain a high-quality beam highly efficiently and easily.SOLUTION: A laser device comprises: a semiconductor laser 21 for outputting excitation light; a laser medium 13 that is disposed between a pair of mirrors 11 and 12 constituting a resonator 1, and excited by the excitation light from the semiconductor laser 21 to emit a laser beam of a fundamental wave; and a nonlinear crystal 14 that selectively transmits a low-order mode of a lateral mode of the fundamental wave in the resonator 1 and converts the selected fundamental wave of the low-order mode into a harmonic.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a laser device and a wavelength conversion element that control a mode of a beam of laser light output from a laser light source.

  A conventional laser device collects laser light from a semiconductor laser and irradiates the laser medium, and irradiates a nonlinear crystal housed in a resonator with a fundamental wave that is stimulated and emitted from the laser medium. The laser device generates a second harmonic by a nonlinear crystal, oscillates the second harmonic in a resonator, and outputs the second harmonic to the outside through an output mirror.

The quality of the transverse mode of the laser beam emitted from the laser device greatly affects the performance of the application using the laser device. A basic resonator for generating a high-quality laser beam having an M ² (M square) of approximately 1 is given priority to the lowest-order mode in a stable resonator composed of two mirrors, the so-called TEM ₀₀ mode. , A resonator that selectively oscillates.

As a basic method thereof, there is a method of increasing the ratio of diffraction loss other than in the TEM ₀₀ mode by providing a circular aperture aperture in the resonator. For example, in Patent Document 1, in order to eliminate multimode, a mode selector such as an aperture, a slit, or a knife edge is mounted in a resonator, and oscillation is suppressed by giving a loss to the multimode beam by the mode selector. ing. Patent Document 2 is known as a conventional technique.

JP-A-1990-170585 JP 2011-238792 A

  However, in Patent Document 1, ripples and side lobes in the near field and the far field are formed due to diffraction by the edge of the mode selector, and a higher-order mode may occur due to multiple reflection of diffracted light.

  Further, since the mode selector itself also receives the optical power due to the reflected light of the diffracted light, a large heat load is generated. For this reason, a cooling mechanism is provided in the aperture to radiate heat.

  Furthermore, for the multimode, when the mode selector is inserted deeply toward the center of the beam or the aperture diameter is reduced, high-precision adjustment is required. In this case, there was a case where a loss was given to the single mode.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a laser device and a wavelength conversion element that can suppress a transverse mode ripple and a side lobe of an emitted laser and can easily obtain a high-quality beam with high efficiency.

  In order to solve the above-described problems, a laser apparatus according to the present invention is arranged between a pumping light source that outputs pumping light and a pair of mirrors that form an optical resonator and is excited by pumping light from the pumping light source. A laser medium that emits a laser beam of a wave, and a nonlinear crystal that selects a low-order mode of the transverse mode of the fundamental wave in the optical resonator and converts the fundamental wave of the selected low-order mode into a harmonic. It is characterized by providing.

  According to the present invention, the nonlinear crystal selects the lower-order mode of the transverse mode of the fundamental wave in the optical resonator, and converts the selected fundamental wave of the lower-order mode into a harmonic. It is possible to provide a laser device and a wavelength conversion element that can suppress mode ripples and side lobes and can easily obtain a high-quality beam with high efficiency.

It is a block diagram of the laser apparatus of Example 1 of this invention. It is a figure which shows the nonlinear crystal which has a polarization inversion part of the laser apparatus of Example 1 of this invention. It is a figure which shows the nonlinear crystal which has a polarization inversion part of the laser apparatus of Example 2 of this invention. FIG. 4 is a diagram showing a state when an elliptical fundamental wave having a major axis in the lateral direction is incident on the nonlinear crystal shown in FIG. 3. It is a figure which shows the nonlinear crystal which has a polarization inversion part of the laser apparatus of Example 3 of this invention. FIG. 6 is a diagram illustrating a nonlinear crystal having a polarization inversion portion of the laser device according to the fourth embodiment of the present invention. It is a figure which shows wavelength conversion by the pseudo phase matching of the polarization inversion part of the laser apparatus of Example 4 of this invention. It is a figure which shows the conversion efficiency with respect to the polarization inversion period of the nonlinear crystal of the laser apparatus of Example 4 of this invention. It is a figure which shows the nonlinear crystal which has a polarization inversion part of the laser apparatus of Example 5 of this invention. It is a figure which shows in-plane distribution of the conversion efficiency when the polarization inversion part of the laser apparatus of Example 5 of this invention is made into high temperature and low temperature. It is a figure which shows the nonlinear crystal which has a polarization inversion part of the laser apparatus of Example 6 of this invention. It is a figure which shows the nonlinear crystal which has a polarization inversion part of the laser apparatus of Example 7 of this invention.

  Hereinafter, embodiments of a laser device and a wavelength conversion element of the present invention will be described in detail with reference to the drawings.

  The present invention uses a quasi-phase matching type polarization inversion element for a nonlinear crystal housed in a resonator and forms a polarization inversion structure in a part of the opening, thereby using the nonlinear crystal as a mode selector. It is characterized in that a low-order second harmonic (SH wave) is obtained by selectively wavelength-converting only the transverse mode. Details will be described below.

  FIG. 1 is a configuration diagram of a laser apparatus according to Embodiment 1 of the present invention. The laser device includes a resonator 1 that oscillates laser light by resonance, and an excitation optical system 2 that outputs excitation light to the resonator 1.

  The excitation optical system 2 includes a semiconductor laser 21 and a condenser lens 22. The semiconductor laser 21 corresponds to the excitation light source of the present invention, and excites the laser medium 13 to oscillate the fundamental wave.

  The semiconductor laser 21 is excited by carrier injection consisting of electrons and holes injected by current driving, and outputs excitation light 31 generated by stimulated emission that occurs when carrier pairs of the injected electrons and holes disappear. For example, a semiconductor laser 21 having an oscillation wavelength of 808 nm, an emission width of 400 μm, and a maximum output of 15 W can be used.

  The condensing lens 22 condenses the fundamental wave of the excitation light from the semiconductor laser 21 on the laser medium 13 via the mirror 11. The condensing lens 22 is subjected to a coating process with a low reflectance with respect to a wavelength of 808 nm. A dielectric film is used as a coating process.

  The resonator 1 includes a mirror 11, a laser medium 13, a nonlinear crystal 14, and a mirror 12. The entrance-side mirror 11 is a concave mirror having a curvature radius of 200 mm, has a low reflectance with respect to light with a wavelength of 808 nm, and has a high reflectance with respect to light with wavelengths of 1064 nm and 532 nm. Processing has been applied. A dielectric film is used as a coating process.

  The exit-side mirror 12 is a concave mirror having a radius of curvature of 200 mm, and has a coating process that has a high reflectance for light with a wavelength of 1064 nm and a low reflectance for light with a wavelength of 532 nm. It has been subjected. As the coating treatment, for example, a dielectric multilayer film is used.

  The laser medium 13 is a material that is a source of laser oscillation, and is disposed between a pair of mirrors 11 and 12 constituting the resonator 1 and is excited by excitation light from the semiconductor laser 21 to emit fundamental laser light. To do. The laser medium 13 is, for example, a laser crystal such as YVO4. For example, a laser crystal having an Nd concentration of 0.3 at%, an opening 4 × 4 mm, and a length of 8 mm can be used.

  Both end surfaces where the oscillation light enters are subjected to a coating process having a low reflectance with respect to wavelengths of 808 nm, 1064 nm, and 532 nm. A dielectric film is used as the coating process.

  The nonlinear crystal 14 is a wavelength conversion element that selects a low-order mode of the transverse mode of the fundamental wave in the resonator 1 and converts the wavelength of the selected fundamental wave of the low-order mode into a second harmonic (SH wave). A quasi phase matching type polarization inversion element is used. As the crystal of the nonlinear crystal 14, for example, MgSLT doped with Mg is used. Note that MgLN can be used instead of MgSLT.

  As shown in FIG. 2, the nonlinear crystal 14 has a polarization inversion portion 141 that is inverted in polarity at a predetermined period with respect to a part of the opening. The polarization inversion unit 141 is formed so as to satisfy a desired phase matching condition.

  As shown in FIG. 2A, the nonlinear crystal 14 is formed of, for example, a rectangular parallelepiped having an opening width size W of 1 mm, a thickness size H of 1 mm, and a light traveling direction size L of 3 mm. The polarization inversion portion 141 is formed only in the range where the width size W is 1 mm and the thickness size H is 1 mm, and the central width size w1 is 0.3 mm and the thickness size h1 is 0.3 mm. A region of the nonlinear crystal 14 other than the region of the polarization inversion portion 141 is a non-polarization inversion portion 142 that is not inverted in polarity at a predetermined period.

  FIG. 2B shows an in-plane distribution D1 of wavelength conversion efficiency in the width direction. FIG. 2C shows an in-plane distribution D2 of wavelength conversion efficiency in the thickness direction.

  In this example, a MgSLT crystal having a thickness size of 0.3 mm, a width size of 1 mm, and a MgSLT crystal whose polarization is inverted only to the central 0.3 mm is changed to a non-polarized thickness of 0.35 mm. A method such as bonding up and down by optical bonding is used.

  Here, as an example of the size of the polarization inversion portion 141, the width size w1 is 0.3 mm and the thickness size h1 is 0.3 mm. However, this size is an example and is not limited to this size. . Depending on the beam shape, it is necessary to set the size of each polarization inversion unit 141 to an appropriate size.

  Both ends of the nonlinear crystal 14 are subjected to a coating process having a low reflectance with respect to light having wavelengths of 1064 nm and 532 nm. A dielectric film is used as the coating process.

  Thus, according to the laser apparatus of Example 1, the excitation light 31 emitted from the semiconductor laser 21 is condensed in the laser medium 13 by the condenser lens 22, and the laser medium 13 is excited by the excitation light. The fundamental wave reciprocates in the resonator 1.

  Of the fundamental wave incident on the nonlinear crystal 14, the light passing through the central polarization inversion unit 141 is wavelength-converted to the SH wave only in the low-order transverse mode, and the generated SH wave becomes the low-order transverse mode. The SH wave passes through the mirror 12 and is output outside the resonator.

  For example, the laser beam passes through the polarization inversion unit 141 of 0.3 mm × 0.3 mm with respect to the beam section of the fundamental wave of the higher order mode whose beam cross section is greater than φ0.3 mm. High-order mode components are not wavelength-converted, and low-order mode components of the fundamental laser light are wavelength-converted. For this reason, it is possible to improve the transverse mode quality of the wavelength-converted SH wave.

  That is, the polarization inversion unit 141 selects the low-order mode of the transverse mode of the fundamental wave in the resonator 1 and converts the fundamental wave of the selected low-order mode into an SH wave. Ripple and side lobes can be suppressed, and a high-quality beam can be easily obtained with high efficiency.

  FIG. 3 is a diagram illustrating a nonlinear crystal having a polarization inversion portion of the laser device according to the second embodiment of the present invention. As shown in FIG. 3, the non-linear crystal 14a has a polarization inversion portion 141a that is inverted in polarity at a predetermined period with respect to a part of the opening. The polarization inversion portion 141a is formed so as to satisfy a desired phase matching condition.

  As shown in FIG. 3A, the width size W of the opening is 1 mm, the thickness size H is 1 mm, and the light traveling direction size L is 3 mm. The polarization inversion portion 141a is formed only in the range where the width size W is 1 mm and the thickness size H is 1 mm, and the central width size w2 is 0.3 mm and the thickness size H is 1 mm. A region other than the region of the polarization inversion portion 141a in the nonlinear crystal 14a is a non-polarization inversion portion 142a that is not inverted in polarity at a predetermined period.

  FIG. 3B shows an in-plane distribution D3 of wavelength conversion efficiency in the width direction. FIG. 3C shows an in-plane distribution D4 of wavelength conversion efficiency in the thickness direction.

  According to the thus configured nonlinear crystal 14a, the domain-inverted structure is not limited in the thickness direction of the domain-inverted portion 141a, and the domain-inverted structure is limited only in the width direction. Therefore, as shown in FIG. 4, when an elliptical fundamental wave BM1 having a major axis in the width direction (lateral direction) is transmitted through the polarization inversion unit 141a, higher-order mode components RE2, RE3 in the major axis direction are transmitted. Are not wavelength-converted, and the low-order mode component RE1 is wavelength-converted to output a low-order transverse mode SH wave.

  Thus, according to the laser device of Example 2, when the fundamental wave has an elliptical shape with a long axis in the width direction, the width direction of the domain-inverted region is not subjected to wavelength conversion of the higher-order mode of the long axis. Since the size of the wave is limited, the transverse mode quality of the SH wave can be improved.

  FIG. 5 is a diagram illustrating a nonlinear crystal having a polarization inversion portion of the laser device according to the third embodiment of the present invention.

  As shown in FIG. 5, the nonlinear crystal 14b is formed with a polarization inversion portion 141b in which the polarization is inverted at a predetermined period with respect to a part of the opening. The polarization inversion portion 141b is formed so as to satisfy a desired phase matching condition.

  As shown in FIG. 5A, the width size W of the opening is 1 mm, the thickness size H is 1 mm, and the light traveling direction size L is 3 mm. The polarization inversion unit 141b changes the size w3 in the width direction of the polarization inversion region along the light traveling direction according to the beam shape of the low-order mode of the fundamental wave. In this example, the size w3 in the width direction at both ends in the light traveling direction of the polarization inversion portion 141b is the largest, and the size w3 in the width direction in the middle of the light traveling direction is the smallest.

  A region of the nonlinear crystal 14b excluding the region of the polarization inversion portion 141b is a non-polarization inversion portion 142b that has not undergone polarization inversion at a predetermined period.

  FIG. 5B shows an in-plane distribution D5 of wavelength conversion efficiency in the width direction. FIG. 5C shows an in-plane distribution D6 of wavelength conversion efficiency in the thickness direction. The in-plane distribution D5 of wavelength conversion in the width direction has a single peak characteristic composed of left and right inclined portions D51 and a flat portion D52.

  As described above, according to the laser device of Example 3, the polarization inversion unit 141b changes the width w3 of the polarization inversion region along the light traveling direction according to the beam shape of the low-order mode of the fundamental wave. Therefore, the conversion efficiency from the fundamental wave to the SH wave can be smoothed, and the low-order mode SH wave can be output from the polarization inversion unit 141b, thereby improving the transverse mode quality of the SH wave. it can.

  In addition, the polarization inversion part 141b may smooth the conversion efficiency by simply changing the size w3 in the width direction of the polarization inversion region along the light traveling direction.

  FIG. 6 is a diagram illustrating a nonlinear crystal having a polarization inversion portion of the laser device according to the fourth embodiment of the present invention.

  As shown in FIG. 6A, the nonlinear crystal 14c is inverted in polarity at the first polarization inversion period, the first polarization inversion unit 141c having the first polarization inversion period equal to the coherent length, Polarization inversion is performed at the polarization inversion period, and two second polarization inversion portions 143c having a second polarization inversion period different from the coherent length are formed. The two second polarization inverting parts 143c are arranged so as to sandwich the first polarization inverting part 141c.

  The first domain inversion unit 141c gradually increases the width w4 of the domain inversion region in a taper shape along the light traveling direction. The two second domain-inverted portions 143c gradually decrease the size in the width direction of the domain-inverted regions in a tapered shape along the light traveling direction.

  FIG. 6B shows an in-plane distribution D7 of wavelength conversion efficiency in the width direction. FIG. 6C shows an in-plane distribution D8 of wavelength conversion efficiency in the thickness direction. The in-plane distribution D7 of wavelength conversion in the width direction has a characteristic composed of left and right vertical portions D71, left and right inclined portions D72, and a flat portion D73.

  Here, the polarization inversion period and the coherent length will be described with reference to FIG. FIG. 7 is a diagram illustrating wavelength conversion by quasi phase matching of the polarization inversion unit of the laser device according to the fourth embodiment of the present invention. In FIG. 7, the up and down arrows of each polarization inversion region indicate that the polarization is inverted. Moreover, the SHG output with respect to the propagation distance when the laser light (arrow to the right shown by a thick line) permeate | transmits the polarization inversion part is shown.

  In the case where phase matching is not achieved, if there is a difference in phase velocity between the fundamental wave and the SH wave, as the fundamental wave propagates through the crystal, the SH waves that are generated one after another gradually phase. Is generated. The generated SH waves gradually increase in intensity as they are added, but when the phase difference of the generated SH waves becomes π at two points apart from each other (ie, the coherent length Lc), they cancel each other. On the contrary, the strength decreases. For this reason, the intensity of the SH wave periodically repeats strength as shown by the dotted line in FIG.

  In order to stably increase the SH output intensity, by reversing the polarization of the crystals and reversing the phase of the SH wave generated at this phase in the mutually canceling phases, the SH output intensity is always increased as shown by the solid line. increase.

  As shown in FIG. 7, the polarization inversion period Λ is the sum of the length polarized in the upward direction and the length polarized in the downward direction. The coherent length Lc is a length at which the phase difference of the SH wave is π. In the example shown in FIG. 7, the polarization inversion period Λ is equal to the coherent length Lc, and the phases are matched in a pseudo manner. As shown in FIG. 7, the first polarization inversion unit 141c has a first polarization inversion period equal to the coherent length Lc, and is phase-matched in a pseudo manner.

  On the other hand, the two second domain inversion units 143c have a second domain inversion period different from the coherent length Lc. That is, the second polarization inversion period is shifted from the coherent length Lc.

  FIG. 8 is a diagram showing the conversion efficiency with respect to the polarization inversion period of the nonlinear crystal 14c of the laser device according to the fourth embodiment of the present invention. As shown in FIG. 8, when the polarization inversion period is the coherent length Lc, the conversion efficiency from the fundamental wave to the SH wave is the maximum value Po. For this reason, the first polarization inversion unit 141c can obtain the conversion efficiency of the maximum value Po.

Further, the two second polarization inversion 143c, since the second polarization inversion period is different from the coherence length Lc, a second polarization inversion period is, for example, when a polarization inversion period Λ1 is the conversion efficiency P ₁ becomes , when the second polarization inversion period is, for example, a polarization inversion period Λ2 becomes conversion efficiency P _2.

  As described above, according to the laser device of the fourth embodiment, the second polarization inversion unit 143c shifts the second polarization inversion period from the coherent length Lc, as shown in FIG. It can have a smooth distribution like an aperture. Thereby, the SH wave of a low-order mode can be output and the transverse mode quality of SH wave can be improved.

  FIG. 9 is a diagram illustrating a nonlinear crystal having a polarization inversion portion of the laser device according to the fifth embodiment of the present invention. FIG. 9B shows an in-plane distribution D9 of wavelength conversion efficiency in the width direction. FIG. 9C shows an in-plane distribution D10 of wavelength conversion efficiency in the thickness direction.

  As shown in FIG. 9A, the nonlinear crystal has a Fan-out-shaped domain-inverted portion in which the domain-inverted period gradually changes in the width direction. The polarization inversion unit selects a low-order mode of the transverse mode of the fundamental wave in the resonator, and converts the selected fundamental wave of the low-order mode into a harmonic.

  In this polarization inversion portion, the conversion efficiency is maximized at a location where the polarization inversion period matches the coherent length Lc with respect to the width direction. Further, the in-plane distribution is such that the conversion efficiency decreases as the polarization inversion period deviates from the coherence length Lc. By selecting the polarization inversion period Λ1 and the polarization inversion period Λ2 so that the size w5 in the width direction is an appropriate width for the beam shape, it is possible to output low-order mode SH waves and improve transverse mode quality Can be made.

  Further, since the coherent length Lc changes according to the temperature of the polarization inversion element, the position where the conversion efficiency is maximized is shifted in the width direction by changing the temperature of the wavelength conversion element. For this reason, it is not necessary to arrange the wavelength conversion element at the center of the beam, and even when the installation position of the wavelength conversion element is deviated from the center of the beam, by adjusting the temperature of the wavelength conversion element, low-order SH Waves can be output and the transverse mode quality can be improved.

  FIG. 10 shows the in-plane distribution of conversion efficiency when the polarization inversion part is at a high temperature and a low temperature. D11 shown in FIG. 10B is an in-plane distribution of conversion efficiency when the temperature of the wavelength conversion element is set to a temperature T that maximizes the conversion efficiency at the center of the element. D12 is an in-plane distribution of conversion efficiency when the temperature is slightly higher than the temperature T. D13 is an in-plane distribution of conversion efficiency when the temperature is slightly lower than the temperature T.

  FIG. 11 is a diagram illustrating a nonlinear crystal having a polarization inversion portion of the laser apparatus according to Example 6 of the present invention. The nonlinear crystal 14d shown in FIG. 11 is formed of a rectangular parallelepiped and is composed of a quasi phase matching type polarization inverting element. The nonlinear crystal 14d includes a first coating portion 151 disposed in the center and having a cylindrical shape, and a second coating portion 152 disposed so as to surround the first coating portion 151, and is used as a mode selector.

  The first coating unit 151 is provided with an AR coating (Anti-Reflection Coating) that provides low reflection with respect to the fundamental wave and the SH wave. The AR coat is an antireflection film that reduces reflection, has a reflectance of 0.5% or less, and a dielectric film is used.

  The second coating portion 152 is provided with an AR coating that provides low reflection with respect to the fundamental wave, and with an HR coating that provides high reflection with respect to the SH wave. The HR coat (High Reflection Coating) has a reflectance of, for example, 99.5% or more, and a dielectric film is used.

  According to the laser apparatus of Example 6 configured as described above, since the AR coating is applied to the first coating unit 151 and the second coating unit 152, the fundamental wave is generated from the first coating unit 151 and the second coating unit. The SH wave is transmitted through the part 152 and only the first coating part 151 is transmitted. That is, since it is transparent to the fundamental wave, ripples and side lobes can be suppressed, and low-order SH waves can be obtained.

  FIG. 12 is a diagram illustrating a nonlinear crystal having a polarization inversion portion of the laser apparatus according to Example 7 of the present invention. The nonlinear crystal 14e shown in FIG. 12 is made of a rectangular parallelepiped and made of a quasi-phase matching type polarization inversion element. The nonlinear crystal 14e includes a first coating portion 151 disposed in the center and having a cylindrical shape, and a third coating portion 153 disposed so as to surround the first coating portion 151, and is used as a mode selector.

  Since the structure of the 1st coating part 151 is the same as what was shown in FIG. 11, the description is abbreviate | omitted. The third coating portion 153 is provided with an HR coating that is highly reflective to the fundamental wave and the SH wave.

  According to the laser apparatus of the seventh embodiment configured as described above, the AR coating is applied to the fundamental wave and the SH wave on the first coating part 151, and the fundamental wave and the SH wave are applied to the third coating part 153. Since the HR coating is applied, the first coating unit 151 transmits the fundamental wave and the SH wave, and the third coating unit 153 reflects the fundamental wave and the SH wave SH wave.

  That is, since it acts as a mode selector for the fundamental wave, the effect of suppressing ripples and side lobes cannot be obtained. However, it is not necessary to prepare a mode selector separately, and since it is highly reflective to the fundamental wave, the effect of not having heat can be obtained even if the optical power is received.

  Note that the coatings shown in FIGS. 11 and 12 are examples, and various modifications may be mentioned, such as coating reflectance, shape, and coating on both sides or only one side of the crystal. The AR coating and HR coating described in the sixth and seventh embodiments are also applied to the domain-inverted portion in which the domain-inverted structure is formed only in a part of the opening described in the first to fifth embodiments. You can also

  The present invention is applicable to a semiconductor laser device.

DESCRIPTION OF SYMBOLS 1 Resonator 2 Excitation optical system 11, 12 Mirror 13 Laser medium 14, 14a-14e Nonlinear crystal 21 Semiconductor laser 22 Condensing lens 141, 141a-141c Polarization inversion part 142, 142a-142c Non-polarization inversion part 151 1st coating part 152 2nd coating part 153 3rd coating part BM1 Fundamental wave RE1-RE3 Mode component

Claims (8)

An excitation light source that outputs excitation light;
A laser medium disposed between a pair of mirrors constituting an optical resonator and emitting a fundamental laser beam when excited by excitation light from the excitation light source;
Selecting a low-order mode of the transverse mode of the fundamental wave in the resonator, and converting the fundamental wave of the selected low-order mode into a harmonic; and
A laser device comprising: The nonlinear crystal is formed with a polarization inversion portion that is inverted in polarity at a predetermined period with respect to a part of the opening,
The said polarization inversion part selects the low-order mode of the transverse mode of the said fundamental wave in the said resonator, and converts the fundamental wave of the selected low-order mode into a harmonic. Laser device.   When the fundamental wave has an elliptical shape with a major axis in the width direction, the polarization inversion unit does not convert the wavelength of the higher-order mode of the major axis of the fundamental wave in the width direction. The laser device according to claim 2, wherein:   The laser device according to claim 2, wherein the polarization inversion unit changes a size in a width direction of the polarization inversion region along a light traveling direction. The nonlinear crystal includes a first polarization inversion portion whose polarity is inverted at a first polarization inversion period, the first polarization inversion portion having a coherent length equal to the coherent length, and a polarization inversion at a second polarization inversion period. A second domain-inverted portion having a domain-inverted period different from the coherent length is formed,
Each of the first polarization inversion unit and the second polarization inversion unit selects a low-order mode of the transverse mode of the fundamental wave in the resonator, and harmonics the selected fundamental wave of the low-order mode. The laser device according to claim 1, wherein the laser device is converted into: The non-linear crystal is formed with a domain-inverted portion that is domain-inverted so that the domain-inverted period gradually changes in the width direction,
The said polarization inversion part selects the low-order mode of the transverse mode of the said fundamental wave in the said resonator, and converts the fundamental wave of the selected low-order mode into a harmonic. Laser device.   2. The laser device according to claim 1, wherein the nonlinear crystal is subjected to a coating process having a high reflectivity with respect to the wavelength of the harmonics at a part of the opening. A wavelength conversion element that converts a fundamental wave into a harmonic,
It consists of a nonlinear crystal in which a domain-inverted part that is domain-inverted at a predetermined period is formed on a part of the opening,
The polarization inversion unit selects a low-order mode of a transverse mode of the fundamental wave, and converts the fundamental wave of the selected low-order mode into a harmonic wave.

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