JPH1055005A - Laser beam generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser beam generator capable of embodying wavelength conversion with high conversion efficiency by suppressing resonator loss as much as possible. SOLUTION: The optical resonator of a straight shape is consisting of two reflection mirrors 1, 2 and a nonlinear optical crystal 3. This nonlinear optical crystal 3 is formed to a trapezoidal shape in such a manner that a pair of light input and output surfaces facing each other incline with a light propagation axis. The basic wave laser beam is introduced along the light propagation axis into the optical resonator and is subjected to wavelength conversion to a second harmonic wave by the nonlinear optical crystal 3. This wave is taken outside from the reflection mirror 2. The adjustment of the beam diameter and condensing angle at the light incident surface (Y-Z surface) is made possible at the time the basic wave laser beam passes the light input and output surface of the nonlinear optical crystal 3. The conversion efficiently is improved by aligning, for example, the surface of the small receptive angle and light incident surface of the nonlinear optical crystal 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of introducing a fundamental laser beam into an optical resonator from the outside, or generating a fundamental laser beam in the optical resonator, and converting the laser beam wavelength-converted by a nonlinear optical crystal. The present invention relates to a laser light generator for generating light.

[0002]

2. Description of the Related Art Since the power density of laser light is high inside an optical resonator, efficient wavelength conversion can be expected. For example, laser light generation such as an external resonance type SHG (Second Harmonic generator) or an internal resonance type SHG is possible. Devices are known.

The internal resonance type SHG has a configuration in which a laser medium and a nonlinear optical crystal are arranged in one optical resonator, and a fundamental laser beam generated in the laser medium and a second harmonic converted by the nonlinear optical crystal. By satisfying the phase matching condition with the laser light, efficient wavelength conversion can be realized.

The external resonance type SHG has a configuration in which a nonlinear optical crystal is arranged in an optical resonator different from a laser resonator that generates a fundamental laser light, and causes the fundamental laser light to resonate in the optical resonator. This converts the wavelength to the second harmonic.

In such an external resonance type SHG, by setting a finesse value (Q value) indicating the sharpness of resonance of the optical resonator to a large value, for example, about 10 to 1000, the power density in the optical resonator can be reduced by the power of incident light. The density can be increased to several hundred times, thereby improving the conversion efficiency of the nonlinear optical crystal inside the optical resonator.

FIG. 8 shows the configuration of an external resonance type SHG laser light generator. FIG. 8 (a) shows an example using a Z-type ring resonator, and FIG. 8 (b) shows an example using a triangular ring resonator. This is an example. In FIG. 8A, the resonator optical axes intersect.
An optical resonator is formed by arranging four reflection mirrors M1 to M4 so as to form a so-called bow-tie structure, and a nonlinear optical crystal 5 is arranged between the reflection mirrors M1 and M2.
0 is arranged. The fundamental laser light F emitted from the external laser device is introduced into the optical resonator via the reflection mirror M4, and resonates inside the optical resonator. Fundamental laser light F
Is converted into a second harmonic when passing through the nonlinear optical crystal 50, and is reflected by the reflection mirrors M2 and M3.
Is extracted to the outside as the second harmonic laser light S.

In FIG. 8B, an optical resonator is formed by arranging three reflecting mirrors M1 to M3 such that the optical axis of the resonator is triangular. A non-linear optical crystal 50 is arranged between them. The fundamental laser light F emitted from the external laser device is introduced into the optical resonator via the reflection mirror M1, and resonates inside the optical resonator. When the fundamental laser light F passes through the nonlinear optical crystal 50, it is converted into a second harmonic, and the reflection mirror M
2 to the outside as the second harmonic laser light S.

As a method of increasing the finesse value of the optical resonator, it is preferable to reduce the number of reflecting mirrors in order to minimize the loss in the reflecting mirror. In addition, the reflection mirror M3 shown in FIG. 8B tends to cause a decrease in reflectance due to a large incident angle of light, which causes a decrease in the finesse value of the optical resonator. In particular, when harmonics in the ultraviolet region are generated, the nonlinear constant of the nonlinear optical crystal 50 becomes small and the conversion efficiency becomes low. Therefore, it is necessary to prepare an optical resonator having a higher finesse value. Therefore, as an optical resonator for generating ultraviolet light, FIG.
A ring resonator having four reflection mirrors shown in FIG.

On the other hand, the wavelength conversion efficiency of a nonlinear optical crystal is
The nonlinear optical crystal 50 is arranged at the beam waist position where the power density of the laser beam F increases because the power density increases in proportion to the power density of the fundamental laser beam F.

Further, the phase matching condition in the nonlinear optical crystal is also important, and it is important that the acceptance angle (acceptance angle) determined by the phase matching condition is large. Under non-critical phase matching conditions in which the propagation axis of the laser light coincides with the crystal axis of the nonlinear optical crystal, the acceptance angle is maximized. Therefore, it is ideal to arrange the nonlinear optical crystal so as to satisfy the non-critical phase matching condition. However, in actuality, it is quite difficult to assemble and adjust parts. Therefore, in general, a critical phase matching condition that enables phase matching within an azimuth having a certain angle with respect to the crystal axis is often adopted.
However, in the case of the critical phase matching condition, the tolerance becomes strict, and the phase matching tolerance generally differs in two directions orthogonal to the light propagation axis.

For example, when a fundamental wave having a wavelength of about 500 nm is wavelength-converted to generate a second harmonic in an ultraviolet region, a nonlinear optical crystal material satisfying a noncritical phase matching condition is not known. In general, a barium beta borate crystal (BBO) is often used as a nonlinear optical crystal for angle phase matching which is a critical type phase matching condition.

Next, the angular phase matching will be described. When the fundamental wave travels in the horizontal direction and enters the BBO crystal, B
If the c-axis of the BO crystal is in a horizontal plane, phase matching is performed. At this time, two acceptance angles, an acceptance angle on a vertical plane including the light propagation axis of the fundamental wave and an acceptance angle on a horizontal plane, are defined. For example, when a fundamental wave having a wavelength of 532 nm is converted into a second harmonic by type I phase matching, φ is converted with respect to the c-axis of the BBO crystal.
The direction tolerance (multiplied value of acceptance angle and crystal length) is 0.6
[Deg · cm], which is very large, whereas the tolerance in the orthogonal θ direction is as small as 0.016 [deg · cm]. Therefore, even if the laser beam is focused at an angle larger than the acceptance angle in the θ direction, the conversion efficiency does not increase. Therefore, the laser beam is condensed slightly in the θ direction so as to be smaller than the acceptance angle. Just do it.

FIGS. 9A and 9B are explanatory views showing a focused state of a laser beam. FIGS. 9A and 9B are cross-sectional views of a θ plane and a φ plane in a certain focused state. (D) is a sectional view of the θ plane and the φ plane in another condensing state. In FIG. 9, the solid line P indicates that the intensity of the laser beam is e from the center.
^-2 (e is natural logarithm)
The dashed line Q indicates the asymptote of the beam profile, and the solid line R indicates the range of the acceptance angle.

In the case of a Gaussian beam whose light intensity distribution shows a normal distribution, there is a certain relationship between the converging angle and the beam waist diameter. Assuming that the converging angle of the beam represented by the half angle between the center and the asymptote is α, the beam waist diameter is ω, the wavelength of the laser beam is λ, and the refractive index is n, α = λ / (π · ω · n) ) (1) is established.

In general, with respect to the c-axis of a nonlinear optical crystal, θ
And the acceptance angle δφ in the φ direction are different from each other. For example, as shown in FIGS. 9A and 9B, when the acceptance angle δφ is much larger than the acceptance angle δθ (δφ≫δ
θ), when an axially symmetric laser beam is used, φ in FIG.
Since the converging angle α2 and the receiving angle δφ almost coincide with each other on a plane, efficient wavelength conversion is performed. However, FIG.
In the θ plane of (a), the converging angle α1 is much larger than the acceptance angle δθ, and only a part of the laser light within the acceptance angle δθ contributes to the wavelength conversion. For this reason, the far-field pattern (far-field pattern) of the second harmonic generated from the nonlinear optical crystal becomes an elliptical pattern elongated in the θ direction, and the conversion efficiency is reduced.

On the other hand, as shown in FIGS.
If the laser beam is gently focused so that the focusing angle α3 in the direction becomes small and the beam waist diameter ω3 is set large,
Since the light-condensing angle α3 and the acceptance angle δθ substantially match, the conversion efficiency can be kept high in both planes.

A laser beam having a beam profile as shown in FIGS. 9 (c) and 9 (d) can be easily realized by condensing strongly in one direction and gently condensing in the vertical direction.
For example, a condensing optical system composed of a combination of a spherical convex lens and a cylindrical lens is used (Applied Physics, Vol. 61, No. 9, 93).
1 page, 1992).

FIG. 10 is a block diagram showing an example of a conventional laser light generator. This is described in the above-mentioned document, and the plasma tubes 51 and 2 in which argon ions are sealed are described.
An optical resonator of an argon ion laser is constituted by the reflection mirrors 52 and 53, and a wavelength conversion is performed by disposing a BBO crystal 54, which is a nonlinear optical crystal, in this resonator. 2 harmonic laser light S
Has been taken out.

Further, a spherical lens 57 and cylindrical lenses 55 and 56 are arranged in the resonator in order to adjust the focusing state of the laser light on the BBO crystal 54.

The spherical lens 57 has a condensing power on a plane perpendicular to the plane of FIG. 10 and a plane parallel to the plane of FIG.
Reference numerals 5 and 56 have a light condensing power only on a plane parallel to the paper surface. Therefore, in the BBO crystal 54, a focused state in which the beam waist diameter and the focused angle of the laser beam are different on the two orthogonal planes is realized.

[0021]

However, FIG.
When an optical component such as a condensing lens is disposed inside the resonator as described above, the loss of light is inevitably increased, and as a result, the finesse value of the resonator is greatly reduced, and the conversion efficiency is reduced. . On the other hand, when the acceptance angle of the nonlinear optical crystal is different in the two orthogonal directions, it is impossible to properly maintain the focused state of the laser light unless some optical component is used.

An object of the present invention is to provide a laser light generator capable of realizing wavelength conversion with high conversion efficiency while suppressing the resonator loss as much as possible.

[0023]

SUMMARY OF THE INVENTION The present invention provides at least one
An optical resonator constituted by a pair of reflecting means, and a nonlinear optical crystal disposed inside the optical resonator, wherein a fundamental laser light is introduced into the optical resonator to perform a resonance operation, and the nonlinear optical In a laser light generator that generates laser light whose wavelength has been converted by a crystal, the nonlinear optical crystal has a pair of non-parallel and opposing light input / output surfaces, and each light input / output surface is aligned with a light propagation axis. A laser light generator characterized by being inclined with respect to the laser light generator.

According to the present invention, since each light input / output surface of the nonlinear optical crystal is inclined with respect to the light propagation axis,
The focusing state of the fundamental laser light in the nonlinear optical crystal can be made different in two orthogonal directions. Therefore, the condensing state of the nonlinear optical crystal can be optimized without any extra optical components inside the optical resonator, so that the loss of the optical resonator can be suppressed from being reduced. Here, a pair of opposing light input / output surfaces of the nonlinear optical crystal may be non-parallel, but when the light input / output surface is cut along a plane including the light propagation axis, the cut lines of the light input / output surfaces are mutually reciprocal. More preferably, the angle formed is 10 ° to 160 °.

Further, the present invention comprises an optical resonator constituted by at least one pair of reflecting means, a nonlinear optical crystal and a laser medium disposed inside the optical resonator, and for exciting the laser medium. Pump light from outside is introduced into the optical resonator, a fundamental wave laser light is generated in the optical resonator by the laser medium, and a laser light wavelength-converted by the nonlinear optical crystal is generated. The laser light generator is characterized in that the crystal has a pair of non-parallel and opposing light input / output surfaces, and each light input / output surface is inclined with respect to the light propagation axis.

According to the present invention, an internal resonance type configuration in which a laser medium is disposed in an optical resonator and a fundamental laser beam is generated in the optical resonator by excitation light from an external semiconductor laser is employed. Thus, the fundamental laser light is directly subjected to wavelength conversion, so that high conversion efficiency can be realized.
As the excitation light for exciting the laser medium, output light from a semiconductor laser can be exemplified.

Further, since each light input / output surface of the nonlinear optical crystal is inclined with respect to the light propagation axis, the focusing state of the fundamental laser light in the nonlinear optical crystal can be made different in two orthogonal directions. become. Therefore, the condensing state of the nonlinear optical crystal can be optimized without any extra optical components inside the optical resonator, so that the loss of the optical resonator can be suppressed from being reduced. Here, a pair of opposing light input / output surfaces of the nonlinear optical crystal may be non-parallel, but when the light input / output surface is cut along a plane including the light propagation axis, the cut lines of the light input / output surfaces are mutually reciprocal. More preferably, the angle formed is 10 ° to 160 °.

Further, according to the present invention, the fundamental laser light incident on the nonlinear optical crystal is set so that the beam waist cross section becomes elliptical, and the incident direction of the fundamental laser light and the light input / output surface of the nonlinear optical crystal are changed. The surface including the normal direction and the direction in which the phase matching acceptance angle of the nonlinear optical crystal is small are substantially parallel to each other.

According to the present invention, the converging angle of the fundamental laser light can be set to be equal to or smaller than the acceptance angle of the nonlinear optical crystal, so that most of the fundamental laser light contributes to wavelength conversion. , High conversion efficiency can be realized.

Further, the present invention is characterized in that the light input / output surface of the nonlinear optical crystal is inclined at a Brewster angle with respect to the light propagation axis.

According to the present invention, one of the polarization components of the fundamental laser light can pass through the light input / output surface of the nonlinear optical crystal without loss, and the finesse value of the resonator can be maintained high. Further, the anti-reflection coating is not required on the light input / output surface, and the manufacturing cost can be reduced.

Further, the present invention is characterized in that the optical resonator is a folded resonator type in which the light incident direction and the light reflection direction in the reflection means coincide with each other.

According to the present invention, since the optical resonator can be configured with the number of reflecting means as small as possible, the finesse value of the resonator can be maintained high, and high conversion efficiency can be realized.

The present invention is also characterized in that the optical resonator is a ring resonator type in which the direction of light incidence and the direction of light reflection in the reflection means do not match.

According to the present invention, since the optical resonator can be configured with the number of reflecting means as small as possible, the finesse value of the resonator can be kept high, and high conversion efficiency can be realized.

Also, according to the present invention, the optical resonator comprises a first reflector passing through the nonlinear optical crystal between the first reflector and the second reflector.
It is a ring resonator type including a light propagation axis and a second light propagation axis that does not pass through the nonlinear optical crystal.

According to the present invention, since a ring-type optical resonator can be constituted by only two reflecting means, the finesse value of the resonator can be kept high, and high conversion efficiency can be realized.

Further, according to the present invention, the optical resonator is a ring resonator type, and the fundamental laser light is obliquely incident on the light input / output surface of the nonlinear optical crystal so that the astigmatism is corrected. It is characterized by.

According to the present invention, since the fundamental laser light is obliquely incident on the light input / output surface of the nonlinear optical crystal, astigmatism of the laser light, that is, astigmatism, can be corrected. The focusing state can be optimized.

Further, according to the present invention, the nonlinear optical crystal is CsL
It is characterized by being formed of iB ₆ O ₁₀ (cesium lithium baud rate).

According to the present invention, wavelength conversion can be realized with high conversion efficiency.

Further, according to the present invention, the nonlinear optical crystal is a BBO
(Beta barium borate crystal).

According to the present invention, wavelength conversion can be realized with high conversion efficiency.

Hereinafter, the principle of the present invention will be described in detail.

FIG. 1 is a configuration diagram showing an example of a folded resonator. The optical resonator is of a linear type composed of two reflecting mirrors 1 and 2, and a nonlinear optical crystal 3 is arranged inside the optical resonator. The nonlinear optical crystal 3 is formed in a trapezoid so that a pair of opposing light input / output surfaces are inclined with respect to the light propagation axis. Therefore, the light propagation axis of the optical resonator is bent due to refraction at each light input / output surface, and a folded resonator type is used so that the light incident direction on the reflection mirrors 1 and 2 and the light reflection direction match. Make up. The fundamental laser light is
An external laser light source is introduced into the optical resonator along the light propagation axis.

With such a configuration, when the laser light traveling along the light propagation axis passes through the light input / output surface of the nonlinear optical crystal 3, the beam diameter on the light incident surface (the YZ plane parallel to the plane of FIG. 1). In addition, it is possible to adjust the light converging angle, and the conversion efficiency can be improved by, for example, making the surface of the nonlinear optical crystal 3 having a small acceptance angle coincide with the light incident surface.

Further, since the light input / output surface of the nonlinear optical crystal 3 is inclined with respect to the light propagation axis, the light reflected on the light input / output surface deviates from the light propagation axis, and returns to the external laser light source as light. It is possible to reliably prevent the laser beam from returning, and this contributes to stabilization of the external laser light source.

FIG. 2 is a configuration diagram showing an example of the ring resonator. The optical resonator is of a ring resonator type including two reflection mirrors 1 and 2, and a nonlinear optical crystal 3 is disposed inside the optical resonator, and a nonlinear optical crystal is disposed between the reflection mirror 1 and the reflection mirror 2. 3 and a second light propagation axis that does not pass through the nonlinear optical crystal 3.

The nonlinear optical crystal 3 is formed in a trapezoid so that a pair of opposing light input / output surfaces are inclined with respect to the light propagation axis. Therefore, the first light propagation axis of the optical resonator is bent by refraction at each light input / output surface. The fundamental laser light is introduced from an external laser light source into the optical resonator along the first light propagation axis or the second light propagation axis.

With such a configuration, when the laser light traveling along the first light propagation axis and the second light propagation axis passes through the light input / output surface of the nonlinear optical crystal 3, the light incident surface (parallel to the paper of FIG. 2) (The YZ plane) and the light collection angle can be adjusted. For example, the conversion efficiency can be improved by matching the plane of the nonlinear optical crystal 3 having a small acceptance angle with the light incident plane. .

Further, since the light input / output surface of the nonlinear optical crystal 3 is inclined with respect to the first light propagation axis, the reflected light on the light input / output surface deviates from the first light propagation axis, and the external laser It can be reliably prevented from returning to the light source as return light, which contributes to stabilization of the external laser light source.

As described above, at least two reflecting mirrors 1 and 2 and a nonlinear optical crystal 3 are used with a small number of parts.
Since an optical resonator having a high finesse value can be realized, compact and highly efficient wavelength conversion can be performed.

FIG. 3 is an explanatory diagram showing a state in which a laser beam is refracted at a crystal interface. When the fundamental laser beam F travels at an incident angle θ1 with respect to the normal direction N of the surface 3a of the nonlinear optical crystal 3, it is refracted at a refraction angle θ2 with respect to the normal direction N according to Snell's law.

Here, the light incident surface is defined by the YZ plane including the incident direction of the fundamental laser beam F and the normal direction N of the surface 3a of the nonlinear optical crystal 3, and the refractive index of the surface incident side medium is n1, Assuming that the refractive index of the nonlinear optical crystal 3 is n2, the beam radius of the fundamental wave laser light F before incidence is W1, and the beam radius of the fundamental wave laser light F after refraction is W2, the beam shapes along the surface 3a match. Thus, the following equation is established. n1 · sin θ1 = n2 · sin θ2 (2) 2 · W2 = 2 · W1 · (cos θ2 / cos θ1) (3)

On the other hand, the beam diameter in a plane perpendicular to the light incident surface does not change before and after refraction. Therefore, the beam diameter on the light incident surface can be converted by the oblique incidence of the laser light.

The above description is applied when the laser beam behaves as perfect parallel light. However, since the actual laser beam becomes a Gaussian beam having a beam waist due to diffraction, the beam contour shape is a ray matrix (ray matrix). ).

FIG. 4 is a graph showing the result of calculating the beam diameter at the nonlinear optical crystal in the ring resonator of FIG. Here, a BBO crystal having a crystal length of 5 mm is used as the nonlinear optical crystal 3, the light incident angle θ1 to the nonlinear optical crystal 3 is 70 degrees, and the radius of curvature of the reflection mirrors 1 and 2 is 10 m.
m, the inclination angles of the reflection mirrors 1 and 2 are set to 10 degrees, the horizontal axis represents the linear distance between the reflection mirrors 1 and 2, and the vertical axis represents the beam radius at the crystal center. The solid line indicates the light incident surface (Y
The beam diameter on the Z plane) and the broken line are perpendicular to the light incidence plane (ZX).
Plane).

The inclination of the reflection mirrors 1 and 2 gives
The effective mirror radius of curvature differs between the YZ plane and the ZX plane,
Since an astigmatism phenomenon occurs in which the resonator conditions and beam diameters differ on the YZ plane and the ZX plane, the calculation was performed in consideration of these effects.

Referring to the graph, the beam diameter on the YZ plane is calculated to be about 60 μm, and the beam diameter on the ZX plane is calculated to be about 30 μm. It can be seen that the former can be enlarged to about twice the latter. Accordingly, the converging angle on the YZ plane is also ZX according to equation (1).
It can be set to about 1/2 smaller than the surface.

By arranging such that the light incident surface and the direction in which the phase matching acceptance angle of the nonlinear optical crystal is small are substantially parallel, most of the fundamental laser light F contributes to wavelength conversion. Conversion efficiency is significantly improved.

Further, by applying a non-reflective coating to the light input / output surface of the nonlinear optical crystal, it is possible to suppress optical loss. Also, by inclining the light input / output surface at a Brewster angle with respect to the light propagation axis, a low-loss optical interface can be realized without coating. In particular, when the acceptance angle of the nonlinear optical crystal is different in the two orthogonal directions, it is preferable to arrange the nonlinear optical crystal such that the polarization direction in which the non-reflective refraction by the Brewster angle is established coincides with the direction in which the acceptance angle decreases. In the BBO crystal, since the polarization direction of the fundamental wave laser light does not coincide with the direction in which the acceptance angle decreases, it is preferable to apply an anti-reflection coating.

[0062]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) FIG. 5 is a configuration diagram showing a first embodiment of the present invention. Here, an example will be described in which green laser light having a wavelength of 532 nm is introduced into a ring-type external resonator SHG and converted into laser light in the ultraviolet region having a wavelength of 266 nm.

The laser light generator comprises a ring-shaped optical resonator comprising two reflecting mirrors 1 and 2 and a nonlinear optical crystal 3 disposed inside the optical resonator. A nonlinear optical crystal 3 is provided between the reflection mirror 1 and the reflection mirror 2.
And a second light propagation axis not passing through the nonlinear optical crystal 3 are formed.

The nonlinear optical crystal 3 is formed in a trapezoid so that a pair of opposing light input / output surfaces 3a and 3b are inclined with respect to the first light propagation axis. B as nonlinear optical crystal 3
When a BO crystal is used, it is cut out so that the direction of θ = 47.43 ° and φ = 90 ° in the polar coordinate format coincides with the light propagation axis (first light propagation axis) based on the crystal c axis. Are arranged such that the θ direction is within the paper plane (YZ plane) of FIG.

The bottom surface and the top surface of the nonlinear optical crystal 3 are parallel to the first light propagation axis, and the bottom surface and the light input / output surface 3
a and 3b are cut and optically polished so that the left and right base angles intersecting with each other are β = 58 °.
The entire resonator is arranged so that the angles of incidence on a and 3b are γ = 70 °. The light input / output surfaces 3a and 3b are provided with a non-reflective coating so that the light input / output surfaces 3a and 3b are totally transmitted under the conditions of an incident angle γ = 70 ° and a wavelength of 532 nm. The optical path length of the optical resonator can be arbitrarily adjusted by moving the nonlinear optical crystal 3 in the vertical direction (parallel to the Z direction).

The nonlinear optical crystal 3 is mounted on a temperature adjusting element (not shown) such as a Peltier element to achieve phase matching by temperature adjustment.

The reflection mirrors 1 and 2 are constituted by concave mirrors having a radius of curvature of 10 mm, and both surfaces of the reflection mirror 1 have a reflectance of 99.0% with respect to the wavelength of 532 nm of the fundamental laser light F.
The reflection mirror 2 has a reflectivity of 99.99% with respect to a wavelength of 532 nm on both surfaces and a wavelength of 2
A coating that provides a transmittance of 85% for 66 nm is applied. The reason why the reflectivity of the reflecting mirror 1 for introducing the fundamental laser light F is lower than that of the reflecting mirror 2 is to realize the maximum conversion efficiency by performing optical impedance matching in the resonator. .

On the other hand, the external laser device 20 comprises a Nd: YVO ₄ crystal as a laser medium and KT as a nonlinear optical element.
It is composed of an internal resonance type SHG laser device or the like in which P (potassium titanyl phosphate KTiOPO ₄ ) is arranged in an optical resonator, and generates a fundamental wave laser beam F having a wavelength of 532 nm and an output of 100 mW.

The fundamental laser light F passes through the half-wave plate 10 and is adjusted so that the polarization direction coincides with the direction perpendicular to the plane of the drawing (X direction). In other words, the light is adjusted to the focusing state as described in detail with reference to FIG. 2 and the like, passes through the reflection mirror 1, and is introduced into the optical resonator along the first light propagation axis.

The fundamental laser light introduced into the optical resonator repeats reflection between the reflection mirror 1 and the reflection mirror 2 and resonates, and the wavelength is 266 nm by the nonlinear optical crystal 3.
Is converted to a second harmonic. An actuator 30, such as a piezo element, is installed on the back surface of the reflection mirror 2, and the position of the reflection mirror 2 is finely adjusted so that the resonance frequency of the optical resonator matches the longitudinal mode frequency of the fundamental laser light. As a result, sharp optical resonance is realized.

When the fundamental laser light is obliquely incident on the light input / output surfaces 3a and 3b of the nonlinear optical crystal 3, the condensing angle in the light incident surface of the nonlinear optical crystal 3 is reduced, and the fundamental light is By arranging them so that the direction in which the acceptance angle is small coincides, most of the fundamental laser light contributes to wavelength conversion.

The second harmonic laser light S generated by the nonlinear optical crystal 3 passes through the output side reflection mirror 2 and is extracted to the outside.

As described above, the far-field pattern of the second harmonic laser light S becomes closer to a circle and an output about twice as large as that of the conventional art.

(Second Embodiment) FIG. 6 is a configuration diagram showing a second embodiment of the present invention. Here, the wavelength 532n
An example in which m green laser light is introduced into the linear external resonator SHG and converted into a laser light in the ultraviolet region with a wavelength of 266 nm will be described.

The laser light generating device is composed of a folded optical resonator composed of two reflecting mirrors 1 and 2 and a nonlinear optical crystal 3 disposed inside the optical resonator. A single light propagation axis is formed between the reflecting mirror 2. Fundamental laser light F in nonlinear optical crystal 3
Is adjusted according to the details described in FIG. 1 and the like. Also, as a characteristic of the folded optical resonator, since there is a possibility that return light from the optical resonator returns to the external laser device 20 coaxially, the optical isolator 12 for cutting the return light is used.
Are arranged to prevent a mode jump or the like of the external laser device 20. The other configuration is the same as that of FIG. 5, and a duplicate description will be omitted.

With this configuration, the fundamental laser light F
Is converted by the nonlinear optical crystal 3 into the second harmonic laser light S, and the far-field pattern of the second harmonic laser light S becomes closer to a circle as compared with the related art, and the output is improved about twice.

(Third Embodiment) FIG. 7 is a configuration diagram showing a third embodiment of the present invention. Here, the wavelength 809n
An example in which m excitation light is introduced into a ring-type internal resonator SHG to generate a fundamental laser light having a wavelength of 1064 nm and further convert the laser light into a green laser light having a wavelength of 532 nm will be described. As a crystal that generates green laser light, generally K
Although a TP crystal is used, it is known that an increase in output causes damage called gray tracking. Therefore, there is a BBO crystal as a material that hardly causes gray tracking and has excellent damage resistance. However, when the BBO crystal has a large beam walk-off angle and forms an optical input / output surface perpendicular to the optical axis as in the related art, Since the output light has an elliptical shape, it has hardly been put to practical use. The present invention aims at practical use of a crystal having high damage resistance and used for angle phase matching, such as a BBO crystal.

In FIG. 7, the laser light generating device includes a ring-shaped optical resonator composed of two reflecting mirrors 1 and 2 and a nonlinear optical crystal 3 and a laser medium 4 disposed inside the optical resonator. Be composed. A first light propagation axis passing through the nonlinear optical crystal 3 and a second light propagation axis passing through the laser medium 4 are formed between the reflection mirror 1 and the reflection mirror 2.

The nonlinear optical crystal 3 is formed in a trapezoid so that a pair of opposing light input / output surfaces 3a and 3b are inclined with respect to the first light propagation axis. B as nonlinear optical crystal 3
When a BO crystal is used, it is cut out so that the direction of θ = 28.8 ° and φ = 90 ° in the polar coordinate format coincides with the light propagation axis (first light propagation axis) based on the crystal c axis. Are arranged such that the θ direction is within the paper plane (YZ plane) of FIG.

The bottom surface and the top surface of the nonlinear optical crystal 3 are parallel to the first light propagation axis, and the bottom surface and the light input / output surface 3
a and 3b are cut and optically polished so that the left and right base angles intersecting with each other are β = 58 °.
The entire resonator is arranged so that the angles of incidence on a and 3b are γ = 70 °. The light input / output surfaces 3a and 3b are provided with a non-reflective coating so that the light input / output surfaces 3a and 3b are totally transmitted under the conditions of an incident angle γ = 70 ° and a wavelength of 1064 nm. The optical path length of the optical resonator can be arbitrarily adjusted by moving the nonlinear optical crystal 3 in the vertical direction (parallel to the Z direction).

The nonlinear optical crystal 3 is mounted on a temperature adjusting element (not shown) such as a Peltier element to achieve phase matching by temperature adjustment.

The laser medium 4 is made of Nd doped with 1% of Nd.
d: It is formed of a YAG crystal, and its light input / output surface is formed perpendicular to the optical axis. Further, a magnetic field generator (not shown) for applying a magnetic field H parallel to the second light propagation axis to the laser medium 4, for example, a permanent magnet is provided. Has been realized.

The reflection mirrors 1 and 2 are constituted by concave mirrors having a radius of curvature of 20 mm. Both sides of the reflection mirrors 1 and 2 have a transmittance of 95% for the wavelength of 810 nm of the excitation light and a reflectance of 9 for the wavelength of 1064 nm of the fundamental laser light F.
9.9%, and the wavelength of the second harmonic laser beam S is 532 nm.
With a coating having a transmittance of 95%.

On the other hand, outside the optical resonator, the laser medium 4
A semiconductor laser 40 for generating pump light E having a wavelength of 810 nm for pumping light is provided, and the pump light E is introduced into the optical resonator so that the optical axis of the pump light E coincides with the second light propagation axis. You. The semiconductor laser 40 may be provided on only one side of the extension of the second light propagation axis, or may be provided on both sides of the second light propagation axis to increase the intensity of the excitation light E incident on the laser medium 4. It is. At the subsequent stage of the semiconductor laser 40, a condenser lens 11 for efficiently condensing the excitation light E on the laser medium 4 is provided.

Next, the operation will be described. Excitation light E emitted from the semiconductor laser 40 passes through the reflection mirror 1, is introduced into the optical resonator, and enters the laser medium 4. The laser medium 4 is excited by the excitation light E to generate a laser gain, and the fundamental laser light F oscillates in the ring-shaped optical resonator including the reflection mirrors 1 and 2. At this time, by applying a magnetic field to the laser medium 4, only oscillation in one direction (clockwise in FIG. 7) is allowed.

When the output of the fundamental laser light F is sufficiently high in the optical resonator, the wavelength is converted to the second harmonic laser light S by the nonlinear optical crystal 3 and is extracted from the reflection mirror 2 to the outside.

With such a configuration, the far-field pattern of the green laser light is closer to a circle, compared to a conventional structure in which the light input / output surface is perpendicular to the light propagation axis.
Its output has been greatly improved. Further, since a crystal having excellent damage resistance such as a BBO crystal can be used, a short-wavelength laser light source having high damage resistance and high reliability and a long life can be realized.

In the above description, an example is shown in which a BBO crystal having a diagonal cut on both sides is used as the nonlinear optical crystal 3, but other nonlinear optical crystals, for example, CsLiB ₆ O
₁₀ (cesium lithium baud rate, abbreviated as CLBO) or the like can be used.

[0089]

As described above in detail, according to the present invention, since each light input / output surface of the nonlinear optical crystal is inclined with respect to the light propagation axis, the fundamental laser light is focused on the nonlinear optical crystal. The state can be made different in the two orthogonal directions. Therefore, the condensing state of the nonlinear optical crystal can be optimized without any extra optical components inside the optical resonator, so that the loss of the optical resonator can be suppressed from being reduced.

Further, since the converging angle of the fundamental laser light can be set to be equal to or less than the acceptance angle of the nonlinear optical crystal, most of the fundamental laser light contributes to wavelength conversion,
High conversion efficiency can be realized.

Further, it is preferable that the light input / output surface of the nonlinear optical crystal is inclined at a Brewster angle with respect to the light propagation axis, so that one of the polarization components of the fundamental laser light is converted into the light input / output of the nonlinear optical crystal. The surface can pass without loss and the finesse value of the resonator can be kept high. In addition, the anti-reflection coating is not required on the light input / output surface, and the manufacturing cost can be reduced.

Further, by forming the optical resonator of the folded resonator type or the ring resonator type, the number of the reflection means can be reduced as much as possible to form the optical resonator, so that the finesse value of the resonator is maintained high. And high conversion efficiency can be realized.

In this way, a laser light generator capable of realizing wavelength conversion with high conversion efficiency while suppressing the resonator loss as much as possible can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating an example of a folded resonator.

FIG. 2 is a configuration diagram illustrating an example of a ring resonator.

FIG. 3 is an explanatory diagram showing a state where laser light is refracted at a crystal interface.

FIG. 4 is a graph showing a result of calculating a beam diameter at a nonlinear optical crystal in the ring resonator of FIG. 2;

FIG. 5 is a configuration diagram showing a first embodiment of the present invention.

FIG. 6 is a configuration diagram showing a second embodiment of the present invention.

FIG. 7 is a configuration diagram showing a third embodiment of the present invention.

8A and 8B show the configuration of an external resonance type SHG laser light generator. FIG. 8A shows a Z-shaped ring resonator, and FIG. 8B shows a triangular ring resonator.

FIG. 9 is an explanatory diagram showing a focused state of laser light, and FIG.
9A and 9B are cross-sectional views of a θ plane and a φ plane in a certain light condensing state, and FIGS. 9C and 9D are θ sectional views in another light condensing state.
It is sectional drawing of a plane and (phi) plane.

FIG. 10 is a configuration diagram illustrating an example of a conventional laser light generator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 2 Reflection mirror 3 Nonlinear optical crystal 4 Laser medium 10 1/2 wavelength plate 11 Condensing lens 12 Optical isolator 20 External laser device 30 Actuator 40 Semiconductor laser

Claims (10)

[Claims] 1. An optical resonator comprising at least one pair of reflection means, and a nonlinear optical crystal disposed inside the optical resonator, wherein a fundamental laser light is introduced into the optical resonator. Resonant operation,
In a laser light generator that generates laser light whose wavelength has been converted by the nonlinear optical crystal, the nonlinear optical crystal has a pair of optical input / output surfaces that are non-parallel and opposed to each other. A laser light generator characterized by being inclined with respect to a propagation axis. 2. An optical resonator comprising at least one pair of reflection means, a nonlinear optical crystal and a laser medium disposed inside the optical resonator, and an excitation light for exciting the laser medium. Is introduced into the optical resonator from the outside, the fundamental wave laser light is generated in the optical resonator by the laser medium, and the laser light whose wavelength is converted by the nonlinear optical crystal is generated. A laser light generator comprising: a pair of non-parallel and opposing light input / output surfaces, wherein each light input / output surface is inclined with respect to a light propagation axis. 3. The fundamental laser beam incident on the nonlinear optical crystal is set so that the beam waist cross section is elliptical, and the incident direction of the fundamental laser beam and the normal direction of the light input / output surface of the nonlinear optical crystal. 3. The laser light generating apparatus according to claim 1, wherein a plane including the following is substantially parallel to a direction in which the phase matching acceptance angle of the nonlinear optical crystal is small. 4. The laser light generator according to claim 1, wherein the light input / output surface of the nonlinear optical crystal is inclined at a Brewster angle with respect to the light propagation axis. . 5. The laser light generating device according to claim 1, wherein the optical resonator is of a folded resonator type in which a light incident direction and a light reflecting direction of the reflecting means coincide with each other. . 6. The laser light generating device according to claim 1, wherein the optical resonator is of a ring resonator type in which the direction of light incidence and the direction of light reflection in the reflecting means do not match. . 7. A ring including a first light propagation axis passing through the nonlinear optical crystal and a second light propagation axis not passing through the nonlinear optical crystal between the first reflection means and the second reflection means. The laser light generator according to claim 1, wherein the laser light generator is of a resonator type. 8. An optical resonator of a ring resonator type, wherein a fundamental laser beam is obliquely incident on an optical input / output surface of a nonlinear optical crystal so that astigmatism is corrected. The laser light generator according to claim 4, 6 or 7. 9. The nonlinear optical crystal is CsLiB ₆ O ₁₀
The laser light generator according to claim 1, wherein the laser light generator is formed of (cesium lithium baud rate). 10. The laser beam generator according to claim 1, wherein the nonlinear optical crystal is formed of BBO (beta barium borate crystal).