JPH043126A - Laser beam wavelength converting device - Google Patents

Abstract

PURPOSE:To allow wavelength conversion with high efficiency by taking the converted light formed by subjecting a laser beam to wavelength conversion by a nonlinear optical medium out of this medium to the outside under the phase match conditions set by adjusting the section of the converting luminous flux by a luminous flux converting means. CONSTITUTION:The incident condensing luminous flux B3 on the nonlinear optical medium 40 is a basic wave in wavelength conversion and is applied to the nonlinear optical medium 40 at an incident angle coinciding with the entire circumference of the annular section thereof by the size and the ratio between the major axis and minor axis of the annular ellipse given to the converting luminous flux B2 by the luminous flux converting means 20 and the condensing angle given to the condensing luminous flux B3 by a condensing means 30. A second harmonic wave is, therefore, generated with the high wavelength conversion efficiency under the exact phase matching and more particularly 90 deg. phase matching with the basic wave. This second harmonic wave is emitted as the converted light L2 in its central axis direction. The wavelength conversion efficiency is enhanced under the exact phase matching in this way.

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention utilizes the nonlinear characteristics of optical crystals such as KTiOPO (hereinafter referred to as KTP) as an optical medium.
The present invention relates to a device for projecting a laser beam oscillated by a laser beam generating means onto this and converting it into light having a wavelength different from the oscillation wavelength. [Prior art] As is well known, lasers generate coherent light with strong and sharp directivity, and their applications are not limited to material processing and measurement devices, but have recently become widespread in the fields of medicine and the chemical industry. However, with the exception of some laser beams, they oscillate only at specific wavelengths, and this is one of the biggest obstacles in terms of application. Wavelength conversion is a powerful means to solve this problem, and the principle will be briefly explained first. Generally, when two laser beams with different wavelengths are incident on a nonlinear optical crystal, a polarized wave with an intensity proportional to the optical electric field of each laser beam and a polarized wave with an intensity proportional to the product of the two electric fields is generated inside the crystal. , especially the latter polarized wave has a frequency that is the sum of the frequencies of both incident laser beams. Therefore, if this is extracted, light with a different wavelength from the original laser light can be obtained, and this is the principle of wavelength conversion called sum frequency generation. However, as is well known, what is most important in practical terms is the conversion to light with twice the original frequency. This is the above-mentioned sum frequency generation, when the frequencies of the two incident laser beams are the same, that is, a single laser beam is applied to a nonlinear optical crystal and converted into light with twice the frequency, that is, a half wavelength. Second harmonic generation or SHG (Seco
nd Harmonic Generation) method % formula % On the other hand, wavelength conversion is the opposite of the above, that is, a certain single laser beam is applied to a nonlinear optical crystal, and each Ji! It is also possible to convert into two beams whose sum of wave numbers is equal to the frequency of the original laser beam. In this case, a nonlinear optical crystal is built into the resonator and the two converted beams are converted by adjusting the resonance conditions. Make both or only one oscillate. This is called optical parametric oscillation or OPO (Optica! Paras
This is called the tric 0scillation) method. By the way, in both the SHG and OP○ systems, in order to increase the wavelength conversion efficiency to a practical level, it is necessary to make the original laser light and the converted light travel in phase within the nonlinear optical crystal. This is called phase matching. For example, in SHG, it is necessary to cause a fundamental wave, which is a polarized wave having the same frequency as that of the incident laser beam, and a second harmonic, which is a polarized wave with a double frequency thereof, to travel in the same direction under phase matching conditions. To achieve this, the speeds of both polarized waves, and therefore the refractive index of the nonlinear optical crystal, must be the same, but since the higher the frequency of light, the higher the refractive index, this phase matching condition is generally not satisfied. . Therefore, the anisotropy of the optical crystal is utilized for phase matching. For example, in a uniaxial crystal, light is divided into ordinary light and extraordinary light and travels inside the crystal, and the refractive index of ordinary light does not depend on the direction of travel, but the refractive index of extraordinary light depends on G. In SHO, by utilizing such optical anisotropy, the fundamental wave, which is the original laser light, is assigned to ordinary light, and the second harmonic, which is converted light, is assigned to extraordinary light, and then the refractive index for both is determined. The phase matching condition can be satisfied by selecting the incident angle of the laser beam with respect to the optical axis of the nonlinear optical crystal so that the angles are aligned. This method is called the angular phase matching method. The principle of this angular phase matching method will be explained with reference to FIG. This figure shows the refractive index η for ordinary and extraordinary light of a uniaxial crystal.
The relationship between the angle θ with respect to the z-axis, which is the optical axis of the crystal, is shown in the form of a function ηO.In the figure, the refractive index of the crystal for the ordinary ray and the extraordinary ray of the fundamental wave are each represented by a stem for convenience.
As shown in Figure 0, where the refractive index for the ordinary light and the extraordinary light of the second harmonic are respectively represented by the squares, the function of the refractive index for the fundamental wave and the second harmonic for the ordinary light is 75 (the locus of e2). are all circular shapes that are independent of the angle θ, but the refractive index function z (11) for these extraordinary rays both becomes elliptical depending on the angle θ. Refractive index for extraordinary light of the dead O and second harmonic

[If the direction of the intersection of the trajectories of O is set as ^ and the fundamental laser beam is incident on the crystal as ordinary light from the direction of this angle 9, phase matching will be achieved between it and the second harmonic generated as extraordinary light. In order to make the fundamental wave ordinary light, for example, it may be linearly polarized in a direction perpendicular to the Z-axis, and the second harmonic, which is extraordinary light, becomes linearly polarized light parallel to the Z-axis. In addition to this angular phase matching method, for example, the angle of incidence of the fundamental wave on the crystal can be determined by utilizing the fact that the refractive index for ordinary light, which is the fundamental wave, and the refractive index for extraordinary light, which is the second harmonic, have different temperature dependencies. There is a method that does not satisfy the phase matching condition by fixing θ at 90 degrees and changing the temperature, and this is usually called the temperature phase matching method. In both of these angular phase matching method and temperature phase matching method, there is a so-called type I phase matching in which the fundamental wave is the ordinary light and the second harmonic is the extraordinary light. There are two types of type D (zero phase matching) in which the phase is the extraordinary light and the second harmonic is the ordinary light. [Problem to be solved by the invention] However, none of the above conventional phase matching methods The first problem with the angular phase matching method is that the propagation directions of the ordinary light and the extraordinary light are different, which limits the range in which phase matching is satisfied, which reduces the efficiency of conversion from the fundamental wave to the second harmonic. In other words, when the phase matching condition is met at angle 9 in Figure 9 as described above, the fundamental wave, the ordinary light Lo, has a refractive index e in the figure.
The extraordinary light Le, which is the second harmonic, propagates in the direction perpendicular to the circular locus of (e), that is, in the direction of angle &, but the extraordinary light Le, which is the second harmonic, propagates in the direction perpendicular to the elliptical locus of refractive index and O, that is, in the direction forming angle 9 and angle ζ. This means that even if the second harmonic is generated from the fundamental wave, the two waves will quickly walk away from each other. Therefore, the angle ζ is called the walk-off angle and is expressed by the following equation. . tanζ-%(de)"j(5)-"(e)-J ¥5
in(2^) However, both hawk and ku in the above formula are values for the angle θ of 90 degrees. Unless the walk-off angle ζ is 0, the phase matching condition is satisfied only in a small portion of the surface of the optical crystal where the fundamental wave is incident, and the conversion efficiency decreases. The second problem with the angular phase matching method is that the tolerance limit for the angle that satisfies the phase matching condition is very narrow, only about mrad. This allowable limit also becomes narrower as the walk-off angle ζ becomes larger. On the other hand, in the temperature phase matching method, the incident angle of the fundamental wave is 9
Since it is fixed at 0 degrees, the walk-off angle becomes a part of the angle as shown in Figure 9, and the first angle in the angle phase matching method.
This problem does not occur, and the crystal temperature control
Since the circle of the refraction center O in the figure and the ellipse of the refractive index and color are made to touch each other at an angle of θ-90°, the second problem mentioned above is also alleviated, and the allowable limit of the incident angle deviation is lOmrad.
It will be eased to a certain extent. This method of setting the incident angle θ to 90 degrees is advantageous in solving the phase matching problem caused by birefringence of the optical crystal, and is called the 90 degree phase matching method. However, in order for this 90 degree phase matching to be possible, the values of the refractive index C for the ordinary light of the fundamental wave and the refractive index C for the extraordinary light of the second harmonic at the 90 degree angle of the optical crystal are originally close to each other.
Moreover, there must be speed dependence on temperature, and in reality, there are only a limited number of crystals that meet this condition. moreover,
If the temperature to be controlled is high, the constant temperature device becomes very large-scale, and even a very small temperature control error causes the phase matching condition to shift easily. In order to solve this gap, the inventor of this case filed a patent application in 1983.
Domestic priority ratio li with No. 3-256595 as the earlier application!
(Japanese Patent Application No. 1-260568) proposed a wavelength conversion device that can satisfy the above-mentioned 90 degree phase matching condition without controlling the crystal temperature. The outline will be explained below with reference to FIG. In FIG. 10, the laser light emitting means 1 shown in the dark path
The laser beam B1 from 0 is converted into an annularly diverging laser beam B5 by a wavefront converting means 50 such as a concave conical lens, and then converted into a condensed beam B6 by a condensing means 60, and then into a nonlinear optical medium 70. Projected. The nonlinear optical medium 70 is a uniaxial crystal such as lithium niobate, and its input and output end surfaces are finished parallel to the optical axis, and the center line of the condensed light beam B6 is perpendicular to the incident surface. In other words, it is projected so that the angle with the optical axis is 90 degrees. The 90 degree phase matching condition in this case is expressed by the following equation, where α is the intersection angle between the condensed light beam B6 and the center line inside the crystal, and the refractive index η is the value relative to 90 degrees. cos cr=/ As can be easily seen from the diagram, changing the distance d between the wavefront conversion means 50 and the condensing means 60 changes the diameter of the ring of the laser beam B5 incident on the condensing means 60. The intersection angle α is set to U81 so that the above 90 degree phase matching condition is not satisfied by the Al1 node with the interval X! I can. The second harmonic L2, which is the converted light, is extracted in the direction of the central axis of the nonlinear optical medium 70. According to this method, as long as there is a crossing angle α that satisfies the above equation, 90 degree phase matching at room temperature is possible by adjusting the distance d without controlling the temperature of the crystal. However, this method has limitations regarding the optical crystal and 90 degree phase matching conditions. In other words, it is necessary to use a uniaxial optical crystal as a nonlinear optical medium, and 2
This principle cannot be applied to biaxial crystals in which the propagation speed of each polarized light depends on its direction.Furthermore, since the condensed light beam B6 is conical, the refractive index of the fundamental wave changes depending on the propagation direction within the crystal. It must not be incident as an ordinary light, and the fundamental wave must be incident as an ordinary light. The principle can only be applied to the aforementioned type I phase matching in which the second harmonic is assigned to extraordinary light. An object of the present invention is to obtain a laser beam wavelength conversion device that can utilize uniaxial and biaxial optical crystals as a nonlinear optical medium and is suitable for both type (2) and type (2) phase matching. C! ! ! [Means for Solving the Problem] According to the present invention, this object is to receive a laser beam projected from a laser beam generating means, convert it into an elliptic annular beam, and to calculate the diameter of the ellipse in the cross section of this converted beam. A laser beam wavelength conversion device is constituted by a light flux converting means capable of 1111 clauses, a light condensing means for condensing the light flux converted by the light flux converting means, and a nonlinear optical medium that receives the condensed light flux by the light condensing means. This is achieved by converting the wavelength of laser light using a nonlinear optical medium and extracting the converted light to the outside under phase matching conditions that are adjusted by adjusting the cross section of the converted light beam by the converting means. It is preferable that the above-mentioned light beam converting means is constituted by an annularizing optical means that converts the laser beam from the laser beam ordering means into an annular cross section, and an ovalizing optical means that converts this annular cross section into an elliptical annular cross section. The annular optical means in this case is composed of a pair of conical lenses having the same apex angle, and the interval between these conical lenses is adjustable, so that the diameter of the annular light beam produced by this optical means is determined by the tA node of the lens interval. It is advantageous to make it more adjustable. In addition, a wedge prism or an lindrical lens can be used as the ovalization optical means, and especially for the former, if the inclination with respect to the annular luminous flux is set to M node, the major axis of the elliptical annular shape of the luminous flux converted by the luminous flux conversion means can be used. The ratio of the minor axis to 3, that is, the shape of the ellipse @
It is particularly advantageous in that it is adjustable. An ordinary convex lens can be used as the condensing means in the above structure, and in this case, it is advantageous to set the converted light beam given thereto from the 1 m light beam means as a parallel light beam. Further, as the nonlinear optical medium, either a uniaxial optical crystal such as lithium niobate or a biaxial optical crystal such as KTP can be used. Furthermore, it is of course advantageous to set the phase matching between the laser beam and the converted light in such a nonlinear optical medium under a 90 degree phase matching condition. [Function] Hereinafter, the function of the configuration of the present invention described in the previous section will be explained as shown in Figures 3 to 8.
This will be explained with reference to the figures. Figure 3 shows the refractive index plane of KTP, which is a biaxial crystal, and the thick solid line is, for example, a wavelength of 1.06 oscillated by a YAG laser.
n laser beam, the thick broken line represents the refractive index for the second harmonic of half wavelength 0.53n converted by a biaxial crystal, the z-axis is the so-called optical axis of this crystal, and the upper The solid line and the broken line are the lines of intersection of the refractive index plane with the X)'+3'Z+zx plane. The length of a line segment from the origin to one point on the refractive index surface represents the refractive index of light in which the direction of this line segment is the wavefront propagation direction, that is, the normal hector direction perpendicular to the wavefront. However, since the actual change in the refractive index is very small, this figure shows the refractive index range of 1.7 to 1.9 enlarged as shown in Figure 1. The rate is 1
It is a double curved surface consisting of two planes that intersect only on two planes, and there are two refractive indices for one normal vector,
These correspond to two lights whose polarization directions are orthogonal to each other. Now, of the two refractive indices for one normal vector, the larger one is represented by the superscript g, and the smaller one is represented by the superscript S, and the two refractive indices for the fundamental wave, which is laser light, are expressed as follows: η: is expressed separately, and the two refractive indices for the second harmonic, which is the converted light, are expressed with edge skipping. Among these, for example, if we explain the refractive index η and v2 of the fundamental wave corresponding to a certain normal vector on the yz field surface, the smaller one is the refractive index for light vibrating in the X-axis direction, and the propagation direction is As long as it is within this yz plane, the relationship between the vibration direction and the Z axis does not change, so its value is constant, and therefore the y
z This 72 curve on the field surface becomes a circle. However, since the larger value is a refractive index corresponding to light whose vibration direction is in the yz plane, its value changes depending on the propagation direction, and the curve corresponding to it becomes an ellipse. Now, the above-mentioned type D that passes through such a biaxial optical crystal.
In phase matching, a fundamental wave is usually divided into two components whose vibration directions are orthogonal to each other within a crystal, and a second harmonic is generated from these two fundamental wave components as converted light. Now, it is assumed that converted light corresponding to the smaller refractive index C of the two refractive indices for the second harmonic is generated, and that the refractive index for the second harmonic and the two fundamental waves is the third one.
Assuming that it lies on a line passing through the origin of the figure, the phase matching condition becomes l ni siro (+7 2 + η:). In Fig. 3, the curved surface of the refractive index, which corresponds to the right side of this equation, is shown by a thin line, and the upper The condition that the equation is not satisfied is the intersection line between this curved surface and a surface corresponding to a small refraction mass for the second harmonic, and this is shown by a thick dashed-dotted line in the figure. Now, if such phase matching is further changed to 90 degree phase matching, t
Since the angle θ between the line connecting the point on the dashed line and the origin with the Z axis needs to be 90°, the line B on the XY screen in the figure is in the direction of this 90° phase matching, and the The angle φ with the y-axis is 23.5° in the case of KTP, however,
The curved surface of the thin line in the figure and the curved surface of the refractive index °C are this line B on the XY screen.
As can be seen from the fact that they intersect at points corresponding to , the aforementioned walk-off occurs between the fundamental wave and the second harmonic. In order to eliminate such a walk-off, the present invention is directed to, for example, θ
The direction A in the figure, which coincides with the y-axis where =900 and ψ-90', is the 90 degree phase matching direction. but,
As can be seen from the figure, the above-mentioned phase matching conditional expression does not hold in this y-axis direction, and the difference becomes arsenic〈'A (η+dead), although the difference is small. Therefore, in the present invention, in order to compensate for this difference, a condensed light beam is created by the condensing means in the previous type structure and made to enter the nonlinear optical crystal, and this condensed light beam is changed to the intersection angle explained in FIG. By using it as α, the phase matching condition is satisfied. However, if the fundamental wave is made obliquely incident on the center line of a nonlinear optical crystal and split into two polarized lights inside it, the values of refraction for these two fundamental waves will differ depending on the refractive index. , of course their propagation speeds will be different,
Furthermore, if the incident angle of the fundamental wave is changed around the Muni y-axis direction, the above-mentioned refractive index value will also change, albeit very slightly, so the incident angle or intersection angle must be set with these considerations in mind. . In Fig. 4, the center line is oriented in the y-axis direction (J trilinear optical medium 4
The direction of the fundamental wave incident on the end face 41 of 0 and the two directions in which it propagates within the crystal are shown in the form of arrow wave number vectors. 0 As usual, the wave number vector is the wave front of the light mentioned above. A vector whose direction is the same as the normal vector perpendicular to , and whose length is 2χ/λ−ηω/C. However, λ is the wavelength of light, and ω is the angular frequency 10 is the speed of light. Now, the fundamental wave includes the y-axis and is at an angle ρ with the Z-axis as shown in the figure.
After entering the crystal from the direction of angle γ with respect to the y-axis along the plane PL, it is assumed that the two polarized lights propagate within the crystal. , and let the refractive index for each be the above-mentioned death and death. For simplicity, let us assume that both the g-wave and the g-wave are within the plane PL, and let the angles that their propagation directions make with the y-axis be α and α, respectively, as shown in the figure. The following equation holds true. sin α 3 = sin γ / η 2 (1) sin α, = sin r / death (2) Furthermore, as shown in Fig. 5 in terms of hectors, the g wave of the fundamental wave incident from the direction of angle T and the direction of angle - d g of the fundamental wave incident from
g of the second harmonic corresponding to the small refractive index μ mentioned above from the wave.
Assume that a wave is generated, and the wave number vectors corresponding to the fundamental g-wave, the fundamental g-wave, and the second harmonic g-wave are respectively nail, f:, and sha as shown in the figure, then the phase matching condition is L=Nail+1! Then, further expressing this in terms of refractive index, 2 η:, = η2 cos αS ten η! CoSα
, (3)08η25rnas-y725lna,
(4) is obtained. Now, the general relationship between the propagation direction of light and the refractive index is F
The rcsnclO formula is known, and can be expressed, for example, as follows for the fundamental g wave. S・Z +Sy t − (η9−” +(y5)−”Dl2)−”+(
? ri-”S z ”=. (η2)−”+(da, −z
(51 However, η2, E, and H2 are the refractive indices when the vibration directions of the fundamental S wave are in the x, y, and z directions, respectively, and SX,
Sy and Sz are the x, y, and z components of the normal vector S, as seen from the angle relationship diagram in Figure 6, and 5x
=sines 51np, 5y=cosα5Sz=
sin αs cosp (6), and of course the fundamental g wave is also expressed by the following Frens
The elO formula etc. are obtained. (e)-"+ <v2)-'(ri-"ten (
v=)−”S・j −0 (de)−’+(death) bow (7
)Sx=s:nrr, sinρ, 5y-cosα
,. 5z-sincr, cosp
(81 Therefore, specifying the angle ρ of the plane PL in Fig. 4, the phase matching condition (3), (related to equation 41)
After solving equations 5) to (8) with the angles α, and α, and the refractive index and death as unknowns, the results are expressed as (11, (2) f
C, it is possible to calculate the incident angle T of the fundamental wave that satisfies the phase matching condition for each angle ρ. FIG. 8 shows, as small circles, the results of such calculations while changing the angle ρ on the horizontal axis in 5° increments, and the vertical axis represents the incident angle T that should be given to the fundamental wave in order to satisfy the phase matching condition. Therefore, if the incident angle T of the fundamental wave is set in the illustrated relationship for each angle ρ formed by the plane PL in FIG. 4 with the two axes, the phase matching condition can be accurately satisfied. However, in the actual laser beam wavelength conversion device X, it is not possible to set the incident angle γ strictly in this way, so some kind of practical means that can replace ζ is required. Therefore, the inventor of the present invention attempted to approximate the relationship between the angle ρ and T using an ellipse. As shown in Figure 7, consider an ellipse whose minor axis is the value of the angle T for ρ-0° and the major axis is the value for ρ-90°, and calculate the value of the angle T for each angle ρ from the origin to this ellipse. This is the distance to the upper point. As can be seen from Figure 6, which shows the results of the ellipse approximation in Figure 8 with a solid line, this ellipse proximal matches the exact one very well, and numerically speaking, both have an error within 5 x 10-'%. Match. The present invention has succeeded in accurately satisfying the stone phase matching condition by utilizing this elliptical proximity. That is, in the present invention, as described in the above configuration, the laser beam received from the laser beam generating means is first converted into an elliptic annular converted beam by the beam converting means, and then condensed into a nonlinear optical medium by the condenser, and the converted beam is converted into a converted beam. By making it possible to adjust the diameter 1 of the annular ellipse, for example, the size of the major axis and minor axis, or the ratio of both diameters, within the light flux converting means, it is possible to satisfy the phase matching condition with the same precision as before, and this This enables highly efficient wavelength conversion even under type n phase matching using a biaxial optical crystal. [Examples] Hereinafter, some embodiments of the present invention will be described with reference to the drawings. In the following examples, a biaxial KTP crystal is used as the nonlinear optical medium, and the original laser light is wavelength-converted to the second harmonic by SHO under the 90-degree phase matching condition of type (■). do. FIG. 1 is a block diagram of a first embodiment of a laser beam wavelength conversion device according to the present invention. The laser light generating means 10 shown on the left side of the figure is a solid-state laser device that oscillates laser light to be wavelength-converted, and uses a laser medium 11. Slightly concave total reflection mirror 121 Partially reflective output mirror 13. It consists of a polarizer 14 and the like, and a light source for optical excitation and the like are omitted from the figure for simplification. The laser beam generating means 10 oscillates a laser beam having a wavelength of, for example, 1.06 nm, which is linearly polarized by a polarizer 14, and outputs a laser beam B1 having a circular cross section in this example. In this example, the beam converting means 2o receiving the laser beam B1 includes a pair of conical lenses 21, 22 and a wedge prism 23, each having an equal apex angle and facing convex surfaces. The conical lenses 21 and 22 are annular optical means for converting the light beam into an annular shape, and the laser beam B1 is transmitted through the small diameter conical lens 2.
1 into an enlarged light beam Be that spreads out in a conical ring shape, the large diameter conical lens 22 converts the light into an annular light beam Br which is a parallel light beam in the form of a ring in this example. The mutual spacing d allows ti1 nodes. The wedge prism 23 is an ovalization optical means that converts the annular light beam Br into an elliptic annular converted light beam B2, and has a wedge angle θW and an inclination angle θ! For example, the wedge prism 23 has the function of reducing the diameter of the annular beam Br only in the vertical direction in the figure and converting it into a parallel beam with an elliptical annular cross section. Place it with a wedge angle θW suitable for giving the cross section,
This ratio can be finely adjusted by the inclination angle θ1. Therefore, in the light beam converting means 20 of FIG. 1, the size of the elliptic annular cross section of the converted light beam B2 is
and 22, and the ratio of major axis to minor axis of the ellipse can be set and adjusted by the wedge angle θW and the inclination angle θI of the wedge prism 23. The condensing means 30 may be an ordinary convex lens, and converts the converted light beam B2, which is a parallel light beam with an elliptical annular cross section, into an elliptic cone-shaped condensed light beam B3, and projects it onto the nonlinear optical medium 40 from the end surface 4. Focuses light on an internal focal point. Note that the nonlinear optical medium 40 is prepared so that its center line coincides with, for example, the y-axis in FIG. 4. In this way, the condensed light beam B entering the nonlinear optical section W40
3 is, of course, the aforementioned fundamental wave in terms of wavelength conversion, and is based on the size and major axis: minor axis ratio of the annular ellipse given to the converted beam B2 by the beam converting means 20, and the condensed beam 83 by the condensing means 30. Due to the given condensing angle, an incident angle corresponding to angle T in FIG. 8 is applied to the nonlinear optical section R40 over the entire circumference of the annular cross section. Therefore, in the nonlinear optical section f40, the second harmonic is generated with high wavelength conversion efficiency under precise phase matching 9 with the fundamental wave, especially under 90 degree phase matching, and this second harmonic is generated as converted light L2 as shown in the figure. is emitted in the direction of its center line. FIG. 2 shows a second embodiment of a laser beam wavelength conversion device according to the present invention, and parts corresponding to those in FIG. 1 are given the same reference numerals. This embodiment differs from the previous embodiment in that a cylindrical lens 24 is used as the ovalization means in the light flux conversion means 20. This cylindrical lens 24 has a curvature only in the direction of the plane of the drawing, and in this example is formed so that the entrance surface is concave, the exit surface is convex, and the focal points of both surfaces coincide. Therefore, the diameter of the annular light beam Br having an annular cross section received from the conical lens 22 is expanded only in the vertical direction in the figure by this lens 24, and is converted into a converted light beam B2 which is a parallel light beam even in this example of an elliptic annular shape. . As before, the size of the elliptical annular cross section of this converted light beam B2 can be adjusted by the distance d between the pair of conical lenses 2] and 22, but in this example, the ratio of the major axis to the minor axis of the ellipse is determined by the cylindrical lens. It is set by the curvature of the exit surface and the lens thickness of 24, and as in the case of the previous example, the fine adjustment is determined by the inclination angle. 1if
B can't do it. However, once the major axis: minor axis ratio is set accurately, the converted luminous flux B2 by the interval d does not need to be finely adjusted.
In many cases, the phase matching condition can be satisfied simply by adjusting the size of the cross section of the nonlinear optical medium 40 and, therefore, the converging angle of the condensed beam B3.On the other hand, in this embodiment, as can be seen from the figure, the center of the nonlinear optical medium 40 Since it is only necessary to match the line direction with the direction of the laser beam B1, alignment of the optical axis of the entire wavelength conversion device is easier than in the previous embodiment, and there is an advantage that the converted light L2 is also emitted in the same direction as the laser beam B1. can get. In the embodiments described above, type H is used for biaxial optical crystals.
However, when performing type II phase matching, only the direction in which the fundamental wave is incident with respect to the optical axis of the crystal changes; The present invention can also be applied to any of these cases, since it is sufficient to specialize the embodiment. Furthermore, although 90-degree phase matching as shown in the embodiment is usually the most desirable, various actual phase matching conditions can be selected depending on the type of optical crystal used for wavelength conversion. If the laser beam is generated on the side of the laser beam generating means, the annularization means as in the embodiment is not necessarily necessary, and even if necessary, various other known optical means can be used instead of the conical lens as in the embodiment. These can be configured as appropriate by combining them. Optical means such as those exemplified in the embodiment can be appropriately selected to ellipse the luminous flux within this luminous flux conversion means, and it goes without saying that the luminous flux converted by this does not necessarily have to be a parallel luminous flux as in the embodiment. be. [Effects of the Invention] As explained above, in the present invention, the laser beam generated by the laser beam generating means is converted into a beam having an elliptic annular cross section by the beam converting means, and the size of the cross section of the converted beam and the major axis of the ellipse are : By adjusting the minor axis ratio, this converted light beam is made incident on the nonlinear optical medium for wavelength conversion in the form of an annular condensed light beam that is focused inside the nonlinear optical medium for wavelength conversion by a focusing means, and this incident laser beam is converted. The following effects can be achieved by converting the wavelength into light. (1) The condensed light beam from the condensing means is incident on the nonlinear optical medium in an annular manner with a predetermined intersection angle with respect to its central axis, and by adjusting this intersection angle, the incident laser beam is converted into converted light. Since the phase matching conditions necessary for the wavelength conversion gap can be set, even when using a biaxial crystal as a nonlinear optical medium, there is no need to control the temperature of the optical medium as in the conventional temperature phase matching method. Phase Matching at Room Temperature 1 Particularly under 90 degree phase matching conditions, wavelength conversion can be performed with higher efficiency than before. (2) Even when wavelength conversion is performed using a biaxial optical crystal as a nonlinear optical medium under phase matching conditions of type (■), the converted light beam is converted into an elliptic annular shape with a desired size and major axis: minor axis ratio by the beam converting means. It is possible to give a cross section and use a converging means to convert it into an annular condensed beam having a desired convergence angle and input it into the nonlinear optical medium. The wavelength of the incident laser light is converted into converted light under strict phase matching conditions, given by an incident angle of substantially the same elliptical approximation.9(3)2
In addition to type ■ phase matching that uses an axial optical crystal, type ■ phase matching that uses axial optical crystals, and type ■ that uses uniaxial optical crystals.
Since the principles of the present invention can be applied to phase matching of various types of optical crystals, the present invention allows highly efficient wavelength conversion to be performed under phase matching conditions that best suit the characteristics of various optical crystals. As described above, the present invention has the advantage that various crystals can be used for wavelength conversion, wavelength conversion can be performed at room temperature without temperature control, and wavelength conversion efficiency can be increased under accurate phase matching.

[Brief explanation of the drawing]

1 to 9 relate to the present invention, where FIG. 1 is a block diagram of a first embodiment of a laser beam wavelength conversion device according to the present invention, FIG. 2 is a block diagram of the second embodiment, and FIG. is a perspective view showing the refractive index plane of a KTP crystal as a biaxial optical crystal, FIG. Figure 6 is a diagram of the wave number vector of the fundamental wave. Figure 6 is a perspective view showing the normal vector and its angular relationship regarding the propagation of light. Figure 7 is a diagram showing the key points of ellipse approximation for the incident angle of the fundamental wave. The figure is a diagram showing the theoretical result of the incident angle of the fundamental wave satisfying the phase matching condition and its elliptical approximation, and FIG. 9 is a diagram showing the refractive index of a uniaxial optical crystal. FIG. 10 is a configuration diagram showing the main parts of a laser light wavelength conversion device according to the applicant's prior application. In these figures, 10: laser light generating means or solid-state laser device, 20:
Luminous flux converting means, 21. 22: Conical lens for layering the luminous flux, 23: Wedge prism for ovalizing the luminous flux, 24: Cylindrical lens for ovalizing the luminous flux, 30: Condensing means or convex lens, 40: Nonlinear optical medium, B1: Laser beam generated by the laser beam generation means, B2: Elliptic annular converted beam, B3: Condensed beam incident on the nonlinear optical medium, d: Distance between the pair of conical lenses for adjusting the diameter of the ellipse, L: Laser light, L2: converted light or second harmonic, θ1: inclination angle of the wedge prism for adjusting the major axis: minor axis ratio of the ellipse, θW: wedge angle of the wedge prism for adjusting the major axis: minor axis ratio of the ellipse,
2: Optical axis of optical crystal.

Claims (1)

[Claims] A beam converting means that receives a laser beam projected from a laser beam generating means and converts it into an elliptical ring-shaped beam, and is capable of adjusting the diameter of an ellipse in a cross section of the converted beam, and condenses the converted beam by the beam converting means. The nonlinear optical medium is equipped with a condensing means and a nonlinear optical medium that receives the condensed light beam by the condensing means, and the nonlinear optical medium converts the laser beam under phase matching conditions adjusted by adjusting the cross section of the converted light beam by the light beam converting means. A laser light wavelength conversion device that outputs the converted light to the outside.