JPH0927648A - Summed frequency laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a summed frequency laser of a structure, wherein the frequency conversion efficiency of the summed frequency laser is improved by bringing the mode of a solid laser into a single longitudinal mode and the fluctuations of the output of the summed frequency laser caused by a mode hopping are inhibited. SOLUTION: An optical resonator 25 is constituted of a laser medium 23 having double refraction properties, a nonlinear optical element 24 which generates a beam walk-off, and an output mirror 41 with a reflective surface which is a recess surface, and a laser oscillation beam in the resonator 25 is turned into a first fundamental wave. When a second laser beam 31 having a wavelength different from a laser oscillation wavelength is directed from the outside into the element 24 as a second fundamental wave and the first fundamental wave is mixed with the second fundamental wave in the element 24, a sum frequency beam is generate. The element 24 is an off-cut crystal and when the beams pass through this element 24, the beam walk-off is generated according to the polarization directions of the beams, whereby the optical axis of the resonator 25 is separated into two optical axes 1 and 1a of polarization and the mirror 41 is arranged so that the optical axis of the resonator 25 is constituted of the shifted optical axis 1a of polarization only.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is preferably used as a light source for optical recording, communication, measurement and the like, and by mixing two laser beams having different wavelengths to generate a sum frequency, a shorter wavelength and a higher speed are obtained. The present invention relates to a sum frequency laser device that can realize a light source that can be modulated.

[0002]

2. Description of the Related Art A solid-state laser configured to pump a laser medium made of Nd: YAG using a semiconductor laser such as a GaAlAs laser has been known. However, the wavelength of such a semiconductor laser pumped solid-state laser is still too long to be used as a light source for an optical storage device or a short wavelength coherent light source.

In order to obtain a laser beam of a shorter wavelength, a solid laser medium and a bulk crystal made of a nonlinear optical material for wavelength-converting the laser oscillation light are arranged in the resonator to make the solid laser oscillation light the second harmonic. A method of obtaining output light having a short wavelength by converting it into a wave is also known. For example, N
By combining a laser medium made of d: YAG and a non-linear optical crystal made of potassium titanyl phosphate KTiOPO ₄ (abbreviated as KTP), it is possible to wavelength-convert laser oscillation light having a wavelength of 1064 nm and output it.

Further, in order to obtain shorter wavelength blue-green light from such green light, laser oscillation light of wavelength 946 nm is converted into laser oscillation light of wavelength 473 nm by a combination of a laser medium made of Nd: YAG and a nonlinear optical crystal made of KNbO _3. An example in which the wavelength is converted to is also reported.

However, the solid-state laser device in which the solid-state laser medium and the second harmonic (SHG) generating element are arranged in such a resonator cannot directly modulate. Therefore, when it is used as a light source for optical disk recording or the like, it is necessary to separately prepare an optical modulator externally.

On the other hand, as a solid-state laser device capable of direct modulation, laser light having different wavelengths is introduced as a second fundamental wave from the outside of the cavity into a laser cavity for generating a first fundamental wave, and nonlinearity in the cavity is generated. A sum frequency laser device that generates sum frequency light by mixing with an optical element has been conventionally known.

In the process of the second harmonic generation and the sum frequency generation as described above, it is necessary to satisfy the phase matching condition in the non-linear optical element in order to improve the conversion efficiency (Japanese Patent Laid-Open No. 64-62621). ).

[0008]

In the conventional sum frequency laser device, laser oscillation of multimode (spectral width 2 to 5Å) consisting of 2 to 3 longitudinal modes is generated. Therefore, the spectrum of the output sum-frequency light reflects the spectrum of the fundamental wave, and the monochromaticity is impaired.

FIG. 13 is a graph showing an example of the spectrum of the sum frequency light output from the sum frequency laser device. Here, Nd: YVO ₄ is used as the laser medium, a-axis-KNbO ₃ is used as the nonlinear optical crystal, the laser medium is excited by pumping light with a wavelength of 809 nm and an output of 1 W using a semiconductor laser, and laser oscillation with a wavelength of 1064 nm is performed. The spectrum is being measured. From the graph, it can be seen that the longitudinal mode interval is about 2Å, and the multi-mode oscillation consists of four longitudinal modes in total. Therefore, when such multi-mode laser oscillation light is used as the fundamental wave for sum frequency generation, the sum frequency light spectrum also becomes a multi mode composed of four longitudinal modes.

In applications such as measurement and communication, a short wavelength and a single longitudinal mode are often required, and the application field is limited by the multimode sum frequency light as described above. Further, the multimode oscillation of the solid-state laser has not only the problem of monochromaticity of light but also the following problems.

1) When the optical power density for exciting the laser medium is concentrated for the purpose of increasing the output of the sum frequency light, a temperature distribution is generated in the laser medium and the laser gain distribution with respect to the wavelength is widened. . Therefore, the number of oscillation longitudinal modes also increases and the conversion efficiency decreases.
In particular, KNbO _{3 is} used as a nonlinear optical crystal for sum frequency generation.
When the crystal is used, since the crystal has a narrow wavelength tolerance, the conversion efficiency sharply decreases as the band of the oscillation wavelength broadens.

2) When the solid-state laser oscillates in the longitudinal multi-mode, some kind of disturbance causes mode hopping between the longitudinal modes, and the output of the sum frequency light fluctuates.

An object of the present invention is to provide a sum frequency laser device in which laser oscillation of a solid-state laser is set to a single longitudinal mode to improve conversion efficiency of sum frequency generation, and output fluctuation due to mode hopping can be suppressed. That is.

[0014]

According to the present invention, there are provided a plurality of optical elements, one of which is a laser medium, and at least two of which are birefringent optical elements. A first light source that emits pumping light for exciting the laser medium, a second light source that supplies laser light having a wavelength different from the oscillation wavelength of the optical resonator into the optical resonator, First of the birefringent optical elements
The sum frequency laser device is characterized in that the optical axis of the optical resonator is formed by one of the two polarization optical axes separated by the beam walk-off generated in the birefringent optical element. According to the present invention, the optical resonator is a linear type, and at least one of the reflecting mirrors constituting the optical resonator is a curved mirror, and the second birefringent optical element other than the first birefringent optical element. And the curved mirror, the first birefringent optical element is arranged. Further, according to the present invention, a plurality of optical elements are interposed, one of the optical elements is a laser medium, at least two of the optical elements are birefringent optical elements, and at least one of the optical elements is An optical resonator that is a polarizing element,
A first light source that emits pumping light for exciting the laser medium; a second light source that supplies laser light having a wavelength different from the oscillation wavelength of the optical resonator into the optical resonator; and the birefringent optics One of the elements is a non-linear optical element for generating a sum frequency, and the birefringent optical element and the polarization element have a wavelength discriminating function. According to the present invention, the optical resonator is a bent type, the polarization element is a reflection mirror having reflectance dependent on polarization, and the reflection mirror is interposed between the laser medium and the nonlinear optical element. It is characterized by doing. Further, the invention is characterized in that the polarizing element is a polarizing beam splitter. Further, the invention is characterized in that the polarizing element is a Brewster plate, and a Brewster plate is interposed between the laser medium and the nonlinear optical element. Further, the present invention is characterized in that the polarizing element is formed on an end face of a nonlinear optical element cut at a Brewster angle. Further, the present invention is characterized in that the laser medium has birefringence. Further, the present invention is characterized in that the laser medium is made of Nd: YVO ₄ . Further, the present invention is characterized in that the nonlinear optical element is made of KNbO ₃ . Further, the invention is characterized in that the first and second light sources are semiconductor lasers.

[0015]

According to the present invention, in the first birefringent optical element functioning as a nonlinear optical element, beam walk-off occurs in which the direction of the wave vector of light and the direction of the pointing vector are different from each other. By forming the optical axis of the optical resonator with only one of the two polarized optical axes separated by the beam walk-off, the resonance loss with respect to the remaining polarized optical axes becomes large. Therefore, the oscillation modes can be differentiated by the polarization mode, the single longitudinal mode oscillation in the resonator can be realized, and the output fluctuation due to the longitudinal mode hopping can be suppressed.

The details will be described below. 1A and 1B show the principle of the present invention. FIG. 1A is a configuration diagram of an optical resonator 25, and FIG. 1B is a perspective view showing the arrangement of each element. The optical resonator 25 includes a laser medium 23 having birefringence, a nonlinear optical element 24 that causes beam walk-off, and an output mirror 41 having a concave reflecting surface. The oscillated light becomes the first fundamental wave.

Further, a second laser light 31 having a wavelength different from the laser oscillation wavelength of the optical resonator 25 is introduced as a second fundamental wave from the outside, and the first fundamental wave and the first fundamental wave in the nonlinear optical element 24 are introduced. When the two fundamental waves are mixed, sum frequency light is generated.

The non-linear optical element 24 is an off-cut crystal, and when a light beam passes through it, beam walk-off occurs depending on the polarization direction, whereby it is separated into two polarization optical axes 1, 1a. In FIG. 1, the output mirror 41 is arranged so that only the shifted polarization optical axis 1a constitutes the optical axis of the optical resonator 25.

Laser medium 23 and nonlinear optical element 24
1B, when the x axis and the y axis perpendicular to the optical axis are taken on the surface 24a of the nonlinear optical element 24 on the laser medium side, the laser medium 23 has birefringence. The polarization axes a and c are in a plane perpendicular to the optical axis and are arranged at angles intersecting the x axis at α = 45 °.

Consider the laser oscillation in such an arrangement. First, assuming that linearly polarized light parallel to the y-axis from the surface 24a of the nonlinear optical element 24 as a starting point travels toward the laser medium 23, it passes through the laser medium 23, is reflected by the surface 23a, and is nonlinear again. When returning to the surface 24a of the optical element 24, the polarization state of the longitudinal mode in the optical resonator is such that the phase difference δ due to the retardation of the laser medium 23 is mπ (where m is an integer and π is a circular constant). Is linearly polarized light parallel to the y-axis. on the other hand,
The longitudinal mode polarized light with δ ≠ mπ is elliptically polarized light in which the major axis of the ellipse coincides with the x axis or the y axis.

When light having such a polarization state continues to propagate and passes through the nonlinear optical element 24, beam walk-off occurs and the light is split into two polarization optical axes, and one optical axis 1 has an optical resonator. Since 25 optical axes are not formed, the resonance loss of the longitudinal mode in which the x-axis component exists becomes large. However, since the longitudinal mode having the polarization mode parallel to the y-axis propagates along the optical axis 1a of the optical resonator 25, it is not affected by the resonance loss. Therefore, oscillation in longitudinal modes other than the polarization mode parallel to the y-axis is suppressed by resonance loss, so that laser oscillation in a single longitudinal mode is possible.

In this state, sum frequency light can be generated by externally introducing a second laser light having a wavelength different from the laser oscillation wavelength into the nonlinear optical element 24. At that time, if the second laser light is also a single longitudinal mode oscillation, a single longitudinal mode sum frequency light excellent in output stability and monochromaticity can be obtained.

In this case, since it is necessary to generate a beam walk-off in the nonlinear optical element 24, the crystal orientation satisfying the phase matching is a direction deviated from the crystal axis direction, that is, a so-called critical type phase matching (non-90 ° phase). (Matching) must be established.

On the other hand, when the non-linear optical element 24 performs so-called non-critical phase matching in which the non-linear optical element 24 is aligned with the crystal axis direction, beam walk-off does not occur in the non-linear optical element 24. Therefore, as shown in FIG.
4 and the output mirror 41, another birefringent optical element BR is interposed, and a beam walk-off is generated by this birefringent optical element BR to separate the two polarization optical axes 1, 1a.

As described above, it is sufficient that any one of the plurality of optical elements interposed in the optical resonator 25 has a birefringence property for generating a beam walk-off, and such a birefringence optical element is, for example, a laser medium. 23 or the non-linear optical element 24 may also be used, or another birefringent optical element BR may be used.

FIG. 3 is an explanatory diagram showing another principle of the present invention. The optical resonator 25 includes a laser medium 23, a non-linear optical element 24, an output mirror 41 having a concave reflecting surface, and a reflecting mirror 40 interposed between the laser medium 23 and the non-linear optical element 24. . Here, the laser medium has a birefringence and is, for example, Nd: YVO ₄ . The reflection mirror 40 has a polarization dependency in which the reflectance differs depending on the polarization direction, and forms a bent resonator structure in which the optical resonator 25 is bent at 90 °, for example.

FIG. 4 is a graph showing an example of the polarization dependence and wavelength dependence of the transmittance of the reflection mirror 40. The horizontal axis shows the wavelength and the vertical axis shows the transmittance. The incident light and the reflected light are 90 °.
This is the case when forming the angle of. In this example, when the oscillation wavelength of the optical resonator 25 is 1064 nm, the reflectance of S-polarized light (perpendicular to the plane including incident light and reflected light) is 99.9% (transmittance of 0.01%), and P-polarized light. The transmittance of (parallel to the plane) is 9
It turns out that it can be designed to 0% or more.

Returning to FIG. 3, the optical medium 25 is excited by exciting the laser medium 23 with a semiconductor laser (not shown).
A laser beam of wavelength λa is oscillated therein, and this becomes the first fundamental wave. Furthermore, another wavelength λb (≠ λ
The laser light 31 of a) is introduced into the resonator so as to pass through the reflection mirror 40, and this becomes the second fundamental wave. In the non-linear optical crystal 24, when the laser lights of the wavelength λa and the wavelength λb are mixed, the sum frequency light of the wavelength λc is generated so as to satisfy the relational expression of 1 / λc = 1 / λa + 1 / λb.

Next, the principle of forming the single longitudinal mode in FIG. 3 will be described. The optical resonator 25 is configured so that oscillation light reciprocates between the output mirror 41 and the surface 23a of the laser medium 23, which are arranged with the reflection mirror 40 interposed therebetween. Here, from the reflection mirror 40 to the laser medium 23
If the origin 40a is taken on the optical axis 2 on the side and the y axis (upper surface of the paper) and the x axis (right of paper) perpendicular to the optical axis 2 at the origin 40a are taken, the laser medium 23 has birefringence. The polarization axes a and c are in a plane perpendicular to the optical axis 2 and are arranged at angles intersecting the x axis by 45 °.

Consider the laser oscillation in such an arrangement. First, assuming that linearly polarized light parallel to the y-axis proceeds from the origin 40a on the optical axis 2 toward the laser medium 23, the light passes through the laser medium 23, is reflected on the surface 23a, and is reflected again. When I returned to 40a,
The polarization state of the longitudinal mode in the optical resonator is a linearly polarized light parallel to the y axis for the longitudinal mode in which the phase difference δ due to the retardation of the laser medium 23 is mπ (m is an integer). On the other hand, with respect to the polarized light in the longitudinal mode with δ ≠ mπ, it becomes elliptically polarized light in which the major axis of the ellipse coincides with the x axis or the y axis.

When the light having such a polarization state continues to propagate and is reflected at a right angle by the reflection mirror 40, the reflection mirror 40 exhibits a high reflectance for the y-axis polarization component which is the S polarization, while the P polarization does not. The reflection mirror 40 exhibits a low reflectance for a certain x-axis polarization component. Thus the reflection mirror 4
0 functions as a polarizing element that reflects S-polarized light and transmits P-polarized light.

Therefore, the longitudinal mode light with δ = mπ at the origin 40a, that is, the longitudinal mode light polarized in parallel with the y axis is reflected by the reflection mirror 40 in the 90 ° direction without loss, and is reflected by the nonlinear optical element 24. Incident. On the other hand, δ ≠ mπ
The longitudinal mode light having the following relationship, that is, the longitudinal mode light in which the x-axis component is present, suffers a large loss at the reflection mirror 40, and the resonator loss becomes large as a whole.

The polarization axes of the nonlinear optical element 24, that is, the b-axis and the c-axis, are arranged so as to coincide with the y-axis and the x-axis on the origin 40, as shown in FIG. In this case, the longitudinal-mode light polarized in parallel with the y-axis is reflected by the nonlinear optical element 2.
Since it coincides with the polarization axis (b axis) of 4, the light passes through the nonlinear optical element 24 along the optical axis 1 in a state where the polarization direction is preserved without being rotated by the polarization plane and is reflected by the output mirror 41. To be done. Since the light reflected by the output mirror 41 has a polarization parallel to the y-axis, it passes through the nonlinear optical element 24 again while maintaining the polarization state, and is again reflected by the reflection mirror 40.
It reflects in the 0 ° direction without loss and propagates along the optical axis 2.

On the other hand, since the polarization component in the x-axis direction coincides with the polarization axis (c-axis) of the non-linear optical element 24, it also passes through the non-linear optical element 24 while maintaining the polarization state and is reflected by the output mirror 41, The P-polarized light enters the reflection mirror 40, and similarly suffers a large loss.
In this way, the longitudinal mode having the x-axis component on the origin 40 suffers two round trip losses in the resonator.

As described above, since the longitudinal mode having a polarization component in the x-axis direction has a large resonance loss, laser oscillation is suppressed, and only the longitudinal mode having a polarization component in the y-axis direction has an influence of the resonance loss. Laser oscillation is possible without receiving it. In this way, it is possible to realize laser oscillation in a single longitudinal mode with little level fluctuation due to longitudinal mode hopping.

In this state, the second laser light 31 having a wavelength λb (≠ λa) different from the laser oscillation wavelength λa is introduced into the optical resonator 25 so as to pass through the reflection mirror 40, and the laser light is emitted. When the polarization direction of 31 coincides with the x-axis or the y-axis, the sum frequency light of wavelength λc is efficiently generated in the nonlinear optical element 24 and is emitted from the output mirror 41. At that time, if the second laser light is also a single longitudinal mode oscillation, a single longitudinal mode sum frequency light excellent in output stability and monochromaticity can be obtained.

In the configuration of FIG. 3, since the reflection mirror 40 is responsible for the longitudinal mode discrimination, it is not necessary to generate the beam walk-off in the nonlinear optical element 24. Therefore, the non-linear optical element 24 has a non-critical phase matching (90 ° phase matching) in which the crystal axis is aligned with the azimuth capable of phase matching.
It is also possible, that beam walk-off does not occur. Further, the nonlinear optical element 24 may be set to the critical type phase matching and used in combination with the longitudinal mode discrimination by the beam walk-off.

FIG. 5 shows another structural example of mode discrimination using a polarizing element, FIG. 5 (a) is a structural diagram of the optical resonator 25, and FIG. 5 (b) is a perspective view showing the arrangement of each element. is there. The optical resonator 25 includes a laser medium 23, a non-linear optical element 24, and an output mirror 41 and a reflection mirror 42 each having a concave reflecting surface.
And the Brewster plate 5 interposed between the laser medium 23 and the nonlinear optical element 24. The Brewster plate 5 has a constant reflectance for S-polarized light which is a linearly polarized light perpendicular to the paper surface of FIG. 5A, is parallel to the paper surface, and is perpendicular to the optical axis of the optical resonator 25. Is the linearly polarized light of P
It functions as a polarizing element that transmits all polarized light. That is, the Brewster plate 5 is used in place of the reflection mirror 40 shown in FIG. 3 for longitudinal mode discrimination.

Next, the principle of forming the single longitudinal mode in FIG. 5 will be described. Here, on the surface 23b of the laser medium 23 on the Brewster plate 5 side, the y-axis (upper surface of the paper) and the x-axis (front of the paper vertical) perpendicular to the optical axis are taken. Surface 23b
Assuming that linearly-polarized light parallel to the y-axis has traveled through the laser medium 23 with the origin as the origin, when the light passes through the laser medium 23, is reflected by the reflection mirror 4, and returns to the surface 23b again, The polarization state of the longitudinal mode in the resonator is linearly polarized light parallel to the y axis for the longitudinal mode in which the phase difference δ due to the retardation of the laser medium 23 is mπ (m is an integer). On the other hand, with respect to the polarized light in the longitudinal mode with δ ≠ mπ, it becomes elliptically polarized light in which the major axis of the ellipse coincides with the x axis or the y axis.

When light having such a polarization state continues to propagate and pass through Brewster plate 5, y, which is P-polarized light, is generated.
The axial polarization component passes without loss, and the x-axis polarization component that is S-polarized light undergoes a certain loss.

Therefore, the longitudinal mode light in which δ = mπ on the surface 23b, that is, the longitudinal mode light polarized in parallel with the y-axis enters the nonlinear optical element 24 without being lost by the Brewster plate 5. On the other hand, the longitudinal mode light in which δ ≠ mπ, that is, the longitudinal mode light in which the x-axis component exists, will be lost by the Brewster plate 5.

The polarization axes b and c of the nonlinear optical element 24 are arranged so as to coincide with the x and y axes, as shown in FIG. 5 (b). In this case, since the longitudinal mode light polarized in parallel with the y-axis matches the polarization axis (c-axis) of the nonlinear optical element 24, it passes through the nonlinear optical element 24 while preserving the polarization direction. The light is reflected by the output mirror 41, passes through the nonlinear optical element 24 again while maintaining the polarization state, and passes through the Brewster plate 5 again without loss.

On the other hand, the polarization component in the x-axis direction coincides with the polarization axis (b-axis) of the non-linear optical element 24, and thus similarly passes through the non-linear optical element 24 while maintaining the polarization state and is reflected by the output mirror 41, The S-polarized light enters the Brewster plate 5 and is subject to a certain loss. Thus on the surface 23b x
A longitudinal mode having an axial component will suffer two round trip losses in the resonator.

As described above, since the longitudinal mode having a polarization component in the x-axis direction has a large resonance loss, the laser oscillation is suppressed, and the longitudinal mode having a polarization component only in the y-axis direction is affected by the resonance loss. Laser oscillation is possible without receiving it. In this way, it is possible to realize laser oscillation in a single longitudinal mode with little level fluctuation due to longitudinal mode hopping.

In this state, when the second laser light having a wavelength λb (≠ λa) different from the laser oscillation wavelength λa is introduced into the optical resonator 25, the sum frequency light of the wavelength λc is generated in the nonlinear optical element 24. The output mirror 41 is generated efficiently.
Radiated from At that time, if the second laser light is also a single longitudinal mode oscillation, a single longitudinal mode sum frequency light excellent in output stability and monochromaticity can be obtained.

In the configuration of FIG. 5, since the Brewster plate 5 is responsible for the longitudinal mode discrimination, the nonlinear optical element 24
May be either non-critical phase matching or critical phase matching.

FIG. 6 shows another structural example of mode discrimination using a polarizing element, and FIG. 6 (a) is a structural diagram of an optical resonator 25.
FIG. 6B is a perspective view showing the arrangement of each element. The optical resonator 25 includes a laser medium 23, a nonlinear optical element 24,
The output mirror 41 and the reflection mirror 4 whose reflection surfaces are concave surfaces
2 and the surfaces 24a and 24b, which are the light input / output end faces of the nonlinear optical element 24, are inclined so as to form the Brewster angle with respect to the optical axis. When the incident angle of light reaches the Brewster angle, it has a constant reflectance with respect to S-polarized light which is linearly polarized light in a direction perpendicular to the paper surface of FIG.
Since the P-polarized light, which is a linearly polarized light parallel to the paper surface and perpendicular to the optical axis of the optical resonator 25, is totally transmitted, the surfaces 24a and 24b of the nonlinear optical element 24 function as a polarizing element. It can thus be used as an alternative to the Brewster plate 5 of FIG. 5 for longitudinal mode discrimination. Further, since the principle of the single vertical mode in FIG. 6 is the same as that in FIG. 5, duplicated description will be omitted.

FIG. 7 is a diagram showing another configuration example of mode discrimination using a polarizing element. The optical resonator 25 includes a laser medium 23, a non-linear optical element 24, and a reflecting mirror 42 having a concave reflecting surface, and a surface 24a, which is a light input / output end surface of the non-linear optical element 24, with respect to the optical axis. The surface 24b is inclined to have the Brewster angle, and the surface 24b is formed to have a high reflectance so as to function as an output mirror. Here again, when the incident angle of light reaches the Brewster angle, it has a constant reflectance with respect to S-polarized light which is linearly polarized light in a direction perpendicular to the paper surface of FIG. Since the P-polarized light, which is a linearly polarized light perpendicular to the optical axis of the optical resonator 25, is totally transmitted, the surface 24a of the nonlinear optical element 24 functions as a polarizing element. Further, the principle of the single vertical mode is the same as that of FIG.

Since the mode discrimination can be realized by the above-mentioned structure, the longitudinal mode of the solid-state laser oscillation can be unified, and the single-mode sum frequency light can be obtained.

Examples of the birefringent optical element that can be used in the present invention include materials classified into the following (1) and (2). It should be noted that which of the materials corresponds to the first birefringent optical element and the second and subsequent birefringent optical elements is
It can be appropriately combined depending on the purpose of use of the solid-state laser device.

(1) Laser medium M: YVO ₄ , M: LiYF ₄ , M: LaF ₃ , M: Ca
GdAlO ₄ , M: La ₂ O ₂ S, M: LaMgAl ₁₁ O
₁₉ , M: La ₂ Be ₂ O ₅ , M: YAlO ₃ , M _x La _1-x
P ₅ O ₁₄ , LiM _x Gd _1-x P ₄ O ₁₂ , KM _x Gd _1-x P ₄ O
_{12, M x Gd 1-x} Al 3 (BO 3) 4, M x Y 1-x A l3 (B
Materials typified by O ₃ ) ₄ , K ₅ Bi _1-x M _x (MoO ₄ ) ₄ and the like can be listed. Here, M is Nd, Er, Ho,
A rare earth element such as Tm or Yb, and a mixture of two or more of these elements may be used. Further, in addition to this, Cr or the like may be included. Besides this, Cr: BeAl ₂ O ₄ , Ti: Al ₂
All birefringent laser media such as O ₃ can be used.

(2) Non-linear optical medium KTiOPO ₄ , KNbO ₃ , LiNbO ₃ , β-BaB
₂ O ₄ , LiB ₃ O ₅ , Ba ₂ NaNb ₅ O ₁₅ , LiI
It is possible to use all birefringent non-linear optical materials having the second harmonic wave, the third harmonic wave, and higher harmonic conversion action, such as O ₃ , KDP, and ADP.

The present invention basically provides means for suppressing mode hopping and reducing output fluctuations.
It is needless to say that the present invention is not limited to the solid-state laser device for generating an in-resonator sum frequency, and is also effective for generating a difference frequency, for example.

[0054]

【Example】

(Embodiment 1) FIG. 8 is a block diagram showing an embodiment of the present invention. Here, a laser beam having a wavelength of 1064 nm is oscillated by a solid-state laser, and a wavelength of 8 nm is emitted from a semiconductor laser outside the resonator.
An example in which a 60-nm laser beam is introduced to generate sum frequency light having a wavelength of 475 nm will be described.

The sum frequency laser device includes an optical resonator 25 that constitutes a solid-state laser, a semiconductor laser 20 that outputs pumping light 26 that excites the solid-state laser, and a semiconductor laser 31 that outputs laser light 27 of the second fundamental wave. Etc.

The optical resonator 25 comprises a laser medium 23 made of Nd: YVO ₄ doped with 1% of Nd and KNbO _3.
And the output mirror 41 having a concave reflecting surface. The surface 23b of the laser medium 23 and the surface 24a of the nonlinear optical element 24 are in contact with each other. Further, on the surface 23a of the laser medium 23, a minute spherical surface (not shown) forming a curved mirror is formed.

The surface 23a of the laser medium 23 has a reflectance of 99.9% at a wavelength of 1064 nm, which is the oscillation wavelength of the laser medium 23, and a transmittance of 95 at a wavelength of 809 nm of the pumping light 26. % Coating is applied. The surface 23b of the laser medium 23 on the side of the non-linear optical element 24 is provided with a coating having a transmittance of 99.9% or more for a wavelength of 1064 nm. The reflection surface of the output mirror 41 has a wavelength of 106
A coating having a reflectance of 99.9% with respect to 4 nm is applied. In addition, each surface 2 of the nonlinear optical element 24
4a and 24b have a transmittance of 9 at a wavelength of 1064 nm.
It has a 9.9% coating.

The semiconductor laser 20 which outputs the pumping light 26 is controlled to a predetermined temperature by a Peltier temperature adjusting circuit (not shown). Semiconductor laser 20 and optical resonator 2
5, a collimator lens 21a, a polarization beam splitter 37 for introducing the second fundamental wave laser beam 27, and a condenser lens 21b are arranged. A collimator lens 33 is arranged between the semiconductor laser 31 that outputs the laser light 27 and the polarization beam splitter 37.

The pumping light 26 emitted from the semiconductor laser 20 has a lens 21a, a polarization beam splitter 37, and
When passing through the lens 21b and focused on the laser medium 23, an inversion distribution is formed in the laser medium 23, and Nd: Y
In the case of VO ₄ crystal, laser oscillation with a wavelength of 1064 nm occurs. This laser oscillation light is emitted from the surface 2 of the laser medium 23.
It reciprocates between the small spherical surface on 3a and the output mirror 41.

On the other hand, the laser light 27 having a wavelength of 860 nm and an output of 100 mW emitted from the semiconductor laser 31 passes through the lens 33 and is reflected by the polarization beam splitter 37.
The surface 24a of the nonlinear optical element 24 is formed by the lens 21b.
The light is focused so as to form a beam waist on the upper side and is coaxial with the laser oscillation light in the optical resonator 25.

FIG. 9 shows KNb which is the nonlinear optical element 24.
O ₃ is a diagram showing a cut-out direction of the crystal. The KNbO ₃ crystal is a biaxial crystal, and the polarization axis with the largest refractive index is b
Axis, the smallest polarization axis is the c-axis, and the rest is the a-axis (refractive index of each axis is nb>na> nc), the apex angle θ with respect to the c-axis
= 90 ° and a so-called ab axis crystal cut out in the direction of φ = 62 ° from the a axis in the ab plane is used. Further, the b ′ axis in the ab plane and oriented in the direction φ = 62 ° from the b axis is the polarization axis. The orientation of the non-linear optical element 24 is arranged so that these b ′ axis and c axis are perpendicular to the optical axis 1. The thickness of the KNbO ₃ crystal is 5 mm.

In such a KNbO ₃ crystal, the first fundamental wave having a wavelength of 1064 nm linearly polarized in the b′-axis direction,
Similarly, a second linearly polarized light with a wavelength of 860 nm in the b'axis direction.
By mixing with the fundamental wave, the sum frequency light having a wavelength of 475 nm is linearly polarized in the c-axis direction and generated.

In the optical resonator 25 of FIG. 8, the c-axis polarization component of the laser oscillation light propagates along the optical axis 1,
The b′-axis polarization component causes a beam walk-off due to the birefringence of the nonlinear optical element 24, and propagates along the optical axis 1a shifted corresponding to the beam walk-off angle ρ. K
When NbO ₃ is used, a beam walk-off of ρ = 0.9 ° occurs. Therefore, when the crystal length is 5 mm, a beam shift of about 80 μm occurs at the crystal end face. Therefore, the output mirror 41 forms a resonator optical axis only for the optical axis 1a of the two optical axes 1, 1a separated by the beam walk-off, and does not constitute a resonator for the optical axis 1. It is located in. Therefore, since the reflecting surface of the output mirror 41 has a concave shape, light incident at a position deviated by about 80 μm from the axis of symmetry is reflected off-axis. In addition, 2
If the two optical axes 1 and 1a are not sufficiently separated, an aperture that passes only the beam along the optical axis 1a may be interposed between the nonlinear optical element 24 and the output mirror 41.

FIG. 10 is a layout diagram of the polarization axes of the laser medium 23 and the nonlinear optical element 24. N of the laser medium 23
The d: YVO ₄ crystal is a negative uniaxial crystal and has one a
The axis is cut out parallel to the optical axis 1, and the remaining a-axis and c-axis are arranged so as to be perpendicular to the optical axis 1. The c-axis, which is the polarization axis of the laser medium 23, is arranged so as to intersect the c-axis, which is the polarization axis of the nonlinear optical element 24, at 45 °. Such a laser medium 23 corresponds to a second birefringent optical element.

In such an arrangement, if the position of the output mirror 41 is adjusted so that the resonator optical axis is formed only with respect to the polarization component of the b'axis that causes beam walk-off,
The b'-axis polarized component oscillates predominantly. When the b′-axis polarization component passes through the laser medium 23, its polarization direction is inclined by 45 ° with respect to the polarization axis of the laser medium 23, and the laser medium 23 functions as a phase plate. For the longitudinal mode in which the phase difference δ when the laser medium 23 travels back and forth is δ = mπ, the nonlinear optical element 24 is used after the laser medium 23 travels back and forth.
When it returns to the surface 24a of the above, it becomes a linearly polarized light. Therefore, when entering the nonlinear optical element 24 again, there is no component parallel to the c-axis to which resonance loss is imparted, so that the longitudinal mode with δ = mπ predominantly oscillates.

On the other hand, in the longitudinal mode having the b′-axis polarization component on the surface 24a of the nonlinear optical element 24, the phase difference δ when reciprocating the laser medium 23 is δ ≠ mπ,
After returning to and from the laser medium 23, when it returns to the surface 24a, it becomes elliptically polarized light. Therefore, when the light is incident on the non-linear optical element 24 again, there is a component parallel to the c-axis to which resonance loss is imparted, so that laser oscillation is suppressed.

FIG. 11 is a graph showing an example of the laser oscillation spectrum in FIG. In the arrangement described above, the longitudinal mode interval of the optical resonator 25 is 0.51Å, but the overall width of the oscillation spectrum is smaller than that, and single longitudinal mode oscillation is confirmed, and the output of the pumping light 26 is 1 W.
Even single vertical mode was retained.

Next, from the semiconductor laser 31 to the wavelength 860n
When a laser beam 27 of m and an output of 100 mW is introduced into the optical resonator 25, a sum frequency light of an output of 15 mW is output from the output mirror 41 along the optical axis 1a, which is about the same as that in the conventional multimode oscillation. We were able to achieve 1.5 times higher output. Furthermore, the single longitudinal mode suppresses the mode hopping, and the output fluctuation of the sum frequency light is greatly reduced.

(Embodiment 2) FIG. 12 is a block diagram showing another embodiment of the present invention. Here, a solid-state laser has a wavelength of 10
An example will be described in which a 64 nm laser beam is oscillated and a laser beam having a wavelength of 690 nm is introduced from a semiconductor laser outside the resonator to generate a sum frequency light having a wavelength of 418 nm.

The sum frequency laser device includes an optical resonator 25 that constitutes a solid-state laser, a semiconductor laser 20 that outputs pumping light 26 that excites the solid-state laser, and a semiconductor laser 31 that outputs laser light 27 of the second fundamental wave. Etc.

The optical resonator 25 comprises a laser medium 23 made of Nd: YVO ₄ doped with 1% of Nd, a reflection mirror 40 having polarization dependence and wavelength dependence, and KNbO.
_The nonlinear optical element 24 is composed of ₃ and the output mirror 41 having a concave reflecting surface, and the surface 23b of the laser medium 23 and the surface 24a of the nonlinear optical element 24 are in contact with each other.

The surface 23a of the laser medium 23 has a reflectance of 99.9% at a wavelength of 1064 nm, which is the oscillation wavelength of the laser medium 23, and a transmittance of 95 at a wavelength of 809 nm of the pumping light 26. % Coating is applied. The surface 23b of the laser medium 23 on the reflection mirror 40 side is coated with a transmittance of 99.9% or more for a wavelength of 1064 nm. The reflection surface of the output mirror 41 has a wavelength of 1064n.
A coating having a reflectance of 99.9% with respect to m is applied. In addition, each surface 24 of the nonlinear optical element 24
a and 24b have a transmittance of 9 at a wavelength of 1064 nm.
It has a 9.9% coating.

The reflection mirror 40 has an S-polarized light reflectance of 99.9% and a P-polarized light transmittance of 90 for a wavelength of 1064 nm.
%, And the multilayer coating is applied so that the transmittance of S-polarized light is 90% with respect to the wavelength of the second fundamental wave of 690 nm.

The semiconductor laser 20 which outputs the pumping light 26 is controlled to a predetermined temperature by a Peltier temperature adjusting circuit (not shown). Semiconductor laser 20 and optical resonator 2
A lens 21 a for collimator and a lens 21 b for condensing are arranged between the lens 5 and the lens 5. Also, laser light 2
A lens 28 for a collimator is arranged between the semiconductor laser 31 that outputs 7 and the reflection mirror 40.

The pumping light 26 emitted from the semiconductor laser 20 is linearly polarized in the direction perpendicular to the plane of the drawing, passes through the lenses 21a and 21b, and is focused on the laser medium 23. An inversion distribution is formed,
In the case of Nd: YVO ₄ crystal, laser oscillation with a wavelength of 1064 nm occurs. The laser oscillation light is emitted from the laser medium 23.
It reciprocates between the surface 23a of the and the output mirror 41.

On the other hand, a semiconductor laser (TOLD-9 manufactured by Toshiba
The laser light 27 having a wavelength of 690 nm and an output of 30 mW emitted from the 151 MD) 31 passes through the lens 28 and the reflection mirror 40 and is condensed so as to form a beam waist on the surface 24 a of the nonlinear optical element 24. It becomes coaxial with the laser oscillation light in the resonator 25.

As shown in FIG. 3, N of the laser medium 23 is
The d: YVO ₄ crystal is a negative uniaxial crystal and has one a
The axis is cut out parallel to the optical axis 1, and the remaining a-axis and c-axis are arranged so as to be perpendicular to the optical axis 1. The c axis, which is the polarization axis of the laser medium 23, is arranged so as to intersect the b axis, which is the polarization axis of the nonlinear optical element 24, at 45 °. Such a laser medium 23 corresponds to a second birefringent optical element. The KNbO ₃ crystal that is the nonlinear optical element 24 is an a-axis crystal cut out so that the b-axis is perpendicular to the plane including the optical axis 1 and the optical axis 2.

In such a KNbO ₃ crystal, the first fundamental wave linearly polarized in the b-axis direction and having a wavelength of 1064 nm and the second fundamental wave linearly polarized in the b-axis direction and having a wavelength of 690 nm are mixed to give a wavelength of 418 nm. The sum frequency light is linearly polarized in the c-axis direction and generated.

With such a configuration, the reflection mirror 40
The longitudinal mode is differentiated by the resonance loss due to, the single longitudinal mode oscillation is realized, the output fluctuation due to the mode hopping is suppressed, and the sum frequency light with the output of 10 mW and the wavelength of 418 nm can be obtained.

[0080]

As described in detail above, according to the present invention, a polarization mode is provided by giving a resonance loss to a mode other than a predetermined longitudinal mode by using an optical element having beam walk-off and polarization dependence. Can be differentiated, and a single longitudinal mode laser oscillation can be realized in a solid-state laser. Therefore, mode hopping to another longitudinal mode is suppressed, the output level fluctuation of the sum frequency light can be significantly reduced, and the conversion efficiency of sum frequency generation is also improved.

[Brief description of drawings]

FIG. 1 shows the principle of the present invention.
1A is a configuration diagram of the optical resonator 25, and FIG. 1B is a perspective view showing the arrangement of each element.

FIG. 2 is a configuration diagram showing another example of the optical resonator 25.

FIG. 3 is an explanatory diagram showing another principle of the present invention.

FIG. 4 is a graph showing an example of polarization dependence and wavelength dependence of a reflection mirror 40.

5A and 5B show another configuration example of mode discrimination using a polarization element, FIG. 5A is a configuration diagram of an optical resonator 25, and FIG. 5B is a perspective view showing an arrangement of each element.

6A and 6B show another configuration example of mode discrimination using a polarization element, FIG. 6A is a configuration diagram of an optical resonator 25, and FIG. 6B is a perspective view showing an arrangement of each element.

FIG. 7 is a diagram showing another configuration example of mode discrimination using a polarization element.

FIG. 8 is a configuration diagram showing an embodiment of the present invention.

FIG. 9 is a view showing a cutting direction of a KNbO ₃ crystal which is a nonlinear optical element 24.

10 is a layout diagram of polarization axes of a laser medium 23 and a nonlinear optical element 24. FIG.

FIG. 11 is a graph showing an example of the laser oscillation spectrum in FIG.

FIG. 12 is a configuration diagram showing another embodiment of the present invention.

FIG. 13 is a graph showing an example of a spectrum of sum frequency light output from the sum frequency laser device.

[Explanation of symbols]

 1, 1a, 2 optical axis 5 Brewster plate 20, 31 Semiconductor lasers 21a, 21b, 28, 33 Lens 23 Laser medium 24 Non-linear optical element 25 Optical resonator 26 Pumping light 27 Laser light 31 Laser light 37 Polarizing beam splitter 40, 42 reflective mirror 41 output mirror BR birefringent optical element

Claims (11)

[Claims] 1. An optical resonator in which a plurality of optical elements are interposed, one of the optical elements is a laser medium, and at least two of the optical elements are birefringent optical elements, and the laser medium is excited. A first light source that emits pumping light for operating the optical resonator, a second light source that supplies laser light having a wavelength different from the oscillation wavelength of the optical resonator into the optical resonator, and a second light source of the birefringent optical element. 1. A sum frequency laser device in which the optical axis of the optical resonator is formed by one of the two polarized optical axes separated by the beam walk-off generated in the birefringent optical element. 2. The optical resonator is a linear type, and at least one of the reflecting mirrors forming the optical resonator is a curved mirror, and a second birefringent optical element other than the first birefringent optical element. The sum frequency laser device according to claim 1, wherein a first birefringent optical element is arranged between the curved mirror and the curved mirror. 3. A plurality of optical elements are interposed, one of the optical elements is a laser medium, at least two of the optical elements are birefringent optical elements, and at least one of the optical elements is An optical resonator that is a polarizing element, a first light source that emits pumping light for exciting the laser medium, and a laser beam having a wavelength different from an oscillation wavelength of the optical resonator are supplied into the optical resonator. One of the second light source and the birefringent optical element is a nonlinear optical element for generating a sum frequency, and the birefringent optical element and the polarizing element have a wavelength discriminating function. apparatus. 4. The optical resonator is of a bent type, the polarization element is a reflection mirror whose reflectance has polarization dependency, and the reflection mirror is interposed between the laser medium and the nonlinear optical element. The sum frequency laser device according to claim 3, wherein 5. The sum frequency laser device according to claim 3, wherein the polarization element is a polarization beam splitter. 6. The sum frequency laser device according to claim 3, wherein the polarizing element is a Brewster plate, and a Brewster plate is interposed between the laser medium and the nonlinear optical element. . 7. The sum frequency laser device according to claim 3, wherein the polarization element is formed on an end face of a nonlinear optical element cut at a Brewster angle. 8. The sum frequency laser device according to claim 1, wherein the laser medium has birefringence. 9. The sum frequency laser device according to claim 1, wherein the laser medium is made of Nd: YVO ₄ . 10. The sum frequency laser device according to claim 1, wherein the nonlinear optical element is formed of KNbO ₃ . 11. The sum frequency laser device according to claim 1, wherein the first and second light sources are semiconductor lasers.