JP2743517B2 - Laser light wavelength converter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an apparatus for projecting a laser beam onto a nonlinear optical medium such as lithium niobate and converting the laser beam into light having a different wavelength.

[Conventional technology]

As is well known, lasers generate coherent light with strong and sharp directivity, and their application fields are not limited to material processing and measurement fields, but recently have become widespread in the medical and scientific industries. However, it oscillates only at specific wavelengths except for a few, which is one of the biggest obstacles in its application. The wavelength conversion technology of laser light using a nonlinear optical crystal, which is the subject of the present invention, has attracted attention as a powerful means for solving this problem, and its principle and main conventional technology will be described below.

In general, when two laser beams having different frequencies are incident on a nonlinear optical crystal, a polarized wave having an intensity proportional to the product of the electric fields as well as a polarized wave having an intensity proportional to the optical electric field of the laser light is entered therein. Occurs. Since the latter polarized wave has a frequency equal to the sum of the frequencies of both incident laser beams, it can be converted into light having a different frequency or wavelength from the original laser beam by extracting it. This is the principle of wavelength conversion called sum frequency generation, but what is particularly important in practice is when the frequencies of two incident laser beams are the same, that is, when a single laser beam is given to a nonlinear optical crystal and the frequency is doubled. Or half-wavelength light, and second harmonic generation or
It is called SHG (Second Harmonic Generation) method.

On the other hand, it is also possible to perform a wavelength conversion opposite to the above, that is, to give a laser light of a certain frequency to a nonlinear optical crystal and convert the sum of the respective frequencies into two lights equal to the frequency of the original laser light, In this case, usually, a nonlinear optical crystal is incorporated in a resonator and its resonance condition is adjusted to oscillate and extract both or only one of the two converted lights. This is called an optical parametric oscillation or OPO (Optical Parametric Oscillation) method.

By the way, in any of the SHG and OPO systems, it is necessary to advance the laser light and the converted light in the nonlinear optical crystal with the same phase in order to increase the wavelength conversion efficiency, and this is called phase matching. For example, in the SHG, a fundamental wave which is a polarization wave having the same frequency as the incident laser light and a second harmonic which is a polarization wave having a frequency multiplied thereto advance in the same direction under the phase matching condition.
For this purpose, the velocities of the two waves and therefore the refractive index of the nonlinear optical crystal for them must be the same, but in general, the higher the frequency of the light, the higher the refractive index, so that this phase matching condition is generally not satisfied. .

There is a method of utilizing the optical anisotropy of a crystal for such phase matching. That is, in the optically anisotropic crystal, light travels by being divided into ordinary light and extraordinary light, and utilizing the fact that the refractive index of ordinary light does not depend on the traveling direction but the refractive index of extraordinary light depends,
The incident angle of the laser beam with respect to the optical axis of the nonlinear optical crystal is selected so that the refractive indices for the fundamental wave and the second harmonic are equal. This method is called an angle phase matching method.

However, this method has a problem that the phase matching condition is easily affected by the birefringence of the optical crystal. Therefore, by fixing the incident angle of the laser beam to the nonlinear optical crystal in a direction at 90 ° to the optical axis, birefringence does not occur, and by adjusting the temperature of the nonlinear optical crystal, ordinary light and abnormal There is a method of performing phase matching using the difference in temperature dependence of the refractive index of light, and this method is called a temperature phase matching method.

The actual phase matching conditions in the conventional method include the type I phase matching condition in which the fundamental wave is ordinary light and the second harmonic is extraordinary light, and conversely, the fundamental wave is extraordinary light and the second harmonic is ordinary light. There are two cases of type II phase matching conditions.

For the details of the theoretical basis of the wavelength conversion and the phase matching described in the section of [Prior Art] and the related art other than the above, refer to the description in the following (Explanation).
However, in order to understand the essential contents of the present invention, this may be skipped and proceed to the section [Problems to be Solved by the Invention].

Generally, when light is incident on a transparent crystal, a dipole is induced by polarization by an oscillating electric field, and the oscillating dipole newly generates light.

The relationship is as follows.

Are the first, second and third order polarizabilities, respectively. In general, the higher order polarizability is In ordinary light, the second-order terms and higher in equation (1) can be ignored, but with strong laser light Is large, higher-order polarization appears. Especially 2
Since the next term is larger than the third and higher terms, Is represented only by a quadratic term. Therefore , The nonlinear polarization P _NL is expressed by the following equation.

Here x, y, and z-axis shall take in the direction of the optical axis of the z-axis is taken in the direction of the crystal axis, d ₁₁ etc. are non-linear optical coefficient of the optical crystal.

Now, the electric field of two angular frequencies ω ₁ and ω _{2 in} the crystal Consider the case where As follows.

However, here Is the position vector in the optical crystal, Is a wave number vector, and φ ₁ and φ ₂ are phase angles.

And rewriting the terms in this equation (4), So that nonlinear polarization The frequency component _{0,2ω 1, ω 1 -ω 2,} ω 1 + ω 2, can be divided into 2 [omega _2. Of these, the component of frequency 0, that is, the DC component, means a rectifying action of the optical frequency electromagnetic wave in the nonlinear medium. The 2ω ₁ and 2ω ₂ components cause the second harmonic generation of ω ₁ and ω ₂ , the ω ₁ −ω ₂ component generates a difference frequency, and the ω ₁ + ω ₂ component generates a sum frequency. Polarization. However, light is not generated from all of these polarized waves of different frequencies, and the frequency of light actually generated depends on the phase matching conditions described below. One must also consider the interaction between the incident light as well as the generated light.

Now, assuming that a third light having an angular frequency ω ₃ is generated from the above two lights, the electric field is Then, the whole electric field is Becomes Therefore, the second-order nonlinear polarization is As shown in FIG. When the generated light has the sum frequency ω ₃ = ω ₁ + ω ₂ , That is, the three modes ω ₁ , ω ₂ and ω ₃ are coupled.

Is expressed by the following Maxwell's equation.

However, here Where c is the speed of light in a vacuum, η is the refractive index of the medium, and μ is the magnetic permeability of the medium. Consider the case where three lights are traveling in the z direction, and calculate the electric field vector of each light. It expresses. here Is a unit vector, and ρ _i is a variable representing amplitude.
Substituting into equations (7), (8) and (10), Then, the following equation is obtained.

It differs depending on how to take the plane of polarization of the incident light. μ
_o is the magnetic permeability in vacuum. By solving this differential equation, the amplitudes ρ ₁ , ρ ₂ , and ρ ₃ of the _three frequency components can be obtained.

Now, the case of the second high frequency generation is considered as a special case. This corresponds to the case where ω ₁ = ω ₂ = ω, ω ₃ = ω ₁ + ω ₂ = 2ω ρω = ρ ₁ = ρ ₂ , ρ _2ω = ρ ₃ in the equation (11). can get.

When the change of the amplitude ρ _2ω of the second harmonic is obtained by solving the equation (12), the result shown in FIG. 18 is obtained. here Represents the length required for about 75% of the fundamental wave output to be converted to the second harmonic. From this result, when ΔS = 0, ρ
_It can be seen that _2ω increases uniformly with z, but when ΔS ≠ 0, ρ _2ω changes oscillatingly and wavelength conversion is not performed efficiently. Qualitatively, when ΔS = 0, the polarization wave and the second harmonic generated by the polarization wave advance in the medium in the same phase, so that the second harmonic is efficiently amplified. If it is 0, it means that the speeds of the polarization and the second harmonic are different and phase mismatch occurs.

Next, in order to efficiently perform wavelength conversion, the above-mentioned ΔS = 0
In other words, in equation (12) Is obtained. First, Is represented by Here λ _ω, λ _2ω and eta _omega, and eta _{2 [omega} is the wavelength and the refractive index of the respective fundamental wave and the second harmonic wave. Therefore Η _ω = η _2ω to _satisfy However, as can be seen from the fact that white light is split by the prism, the refractive index varies depending on the wavelength, and in general, η _2ω > η
_ω . That is, usually It is.

Various phase matching methods have been proposed to solve this problem. The first method uses a nonlinear optical crystal having optical anisotropy. In such a crystal, the incident light is divided into ordinary light and extraordinary light, and the refractive index of the former is constant irrespective of the direction of incidence, but the refractive index of the latter changes depending on the direction. However, there is a direction in which the refractive indices coincide with each other, and this direction is called an optical axis. Although there is one uniaxial crystal and two biaxial crystals with one optical axis, only uniaxial crystals will be considered now. Also, due to birefringence, the two lights are displaced in the traveling direction except for the direction at 90 ° to the optical axis. For example, when the incident angle is 0 °, that is, the angle between the crystal plane and the incident light beam is 90 °, as shown in FIG. 19, the ordinary light Lo travels linearly, but the extraordinary light Le travels refracted.
In summary, the refractive indices of the ordinary light and the extraordinary light are equal to each other in the optical axis direction, and the traveling directions of the ordinary light and the extraordinary light are equal in the direction at 90 ° to the optical axis.

FIG. 20 shows the relationship between the light direction θ and the refractive index η. Drawing, eta ^o _omega and eta ^e _omega the ordinary and extraordinary refractive index of the fundamental wave, the eta ^o _{2 [omega} and eta ^e _Zw represents the ordinary and refractive index of the extraordinary light of the second harmonic, respectively. From this, it can be seen that η ^o _ω and η ^e _2ω coincide at a certain angle θ _m with respect to the optical axis which is the z-axis in the figure. That is, incident at an angle of theta _m fundamental wave as ordinary light, so that the phase matching take between the second harmonic generated as extraordinary light. In order for the fundamental wave to be ordinary light, the fundamental wave is incident as linearly polarized light in a direction at 90 ° to the z-axis. Thereby, harmonics are emitted as linearly polarized light parallel to the z-axis. Such a type in which the fundamental wave is ordinary light and the harmonic wave is extraordinary light is generally called type I phase matching. On the other hand, the fundamental wave can be a combination of ordinary light and extraordinary light, and the harmonics can be extraordinary light.
Called II phase matching. In the following, for simplicity, type I
Let us consider only the phase matching of.

The above angle phase matching method is simple because it is only necessary to adjust the incident angle. However, as shown in FIG. 19, the traveling direction of the fundamental wave, which is ordinary light Lo, and the second harmonic, which is extraordinary light Le, are different. However, the interaction region between the two is limited to the hatched portion in the figure.
However, if θ _m can be set to 90 °, the traveling directions of both light beams coincide with each other, and efficient wavelength conversion can be performed. Generally, η ^o _ω ≠ η ^e _2ω (90 °). However, there are cases where the two coincide with each other by changing the temperature. This is because the temperature dependence of the refractive index differs between η ^o _ω and η ^e _2ω . For example, the output of a YAG laser (wavelength 1.0
6μm) as the fundamental wave, lithium niobate (L
165 ° C for iNbO ₃ ), Potassium niobate (KNbO ₃ )
By controlling the temperature to 181 ° C., this 90 ° phase matching is possible. Of course, this temperature depends on the wavelength. For example, for 0.86 μm, 90 ° C. at 25 ° C. using KNbO ₃
° Phase matching is possible.

As another means for performing phase matching, there is a means for inputting a fundamental wave from a direction different from that using a waveguide.
The former utilizes the fact that the effective refractive index changes when the diameter or thickness of an optical fiber or a waveguide using a thin film is changed. That is, since the modes of light propagating through the waveguide are discrete and the dependence of the refractive index on the width of the waveguide is different, different modes are assigned to the fundamental wave and the harmonic, and the thickness of the waveguide is adjusted by adjusting the thickness and the like. Perform phase matching.

On the other hand, the latter considers phase matching as a vector quantity. In other words, although the fundamental wave and the harmonic wave have been thought to travel in the same direction in the conventional thinking, as shown in FIG. , The induced nonlinear polarization wave becomes a wave vector Propagate with Wave vector propagating in the same direction as this nonlinear polarization wave The phase matching condition for the second harmonic of Becomes Therefore, in the bulk crystal, the fundamental wave is ordinary light and the second harmonic is extraordinary light, and by adjusting α, phase matching becomes possible. In addition, it is also possible to assign a fundamental wave and a higher harmonic wave to different modes using a waveguide and adjust α instead of the width of the waveguide to perform phase matching.

[Problems to be solved by the invention]

The above-described angle phase matching method in the prior art is the simplest in principle and the most practical, but since the propagation directions of ordinary light and extraordinary light differ as shown in FIG. 19 due to the birefringence of the optical crystal,
The range in which the fundamental wave and the second harmonic can interact with each other is limited to a very narrow area with hatching in the figure, and therefore, there is a disadvantage that the wavelength conversion efficiency does not increase much.

On the other hand, the temperature phase matching method has an advantage that the propagation direction of the ordinary light and the extraordinary light can be matched to expand the interaction region of the fundamental wave and the second harmonic wave, and thus the wavelength conversion efficiency can be increased. Since the width of the change in the refractive index due to the temperature change of the crystal is usually very small, a nonlinear optical crystal to which this phase matching method can be actually applied is specified so that its optical axis and the refractive index in the 90 ° direction originally have an appropriate value. The temperature is difficult to control and the wavelength conversion efficiency fluctuates due to a slight control error because the temperature for adjusting the phase is raised to 150 ° C or higher within the wavelength range that is practically important. Have easy problems.

On the other hand, a method of adjusting the width of the waveguide is promising in principle, but it is necessary to perform the width adjustment with extremely high accuracy, so that there are many practically difficult aspects. Also, a method of injecting a laser beam into a nonlinear optical crystal from two directions is described.
When the laser beam is condensed to increase the incident energy density, a region where two incident lights overlap is reduced, and there is a problem that the wavelength conversion efficiency does not increase much.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art and to provide a practical laser light wavelength converter capable of easily performing phase matching and increasing the wavelength conversion efficiency.

It is another object of the present invention to provide a wavelength conversion device having less restrictions on the type and wavelength range of a nonlinear optical crystal. Still another object of the present invention is to provide a wavelength converter that can easily perform phase matching by means other than temperature adjustment without having to adjust the temperature with high accuracy. A more specific object of the present invention is to obtain an SHG and OPO type wavelength converter having such features.

[Means for solving the problem]

According to the present invention, an object of the present invention is to provide a wavefront converting means for converting a laser beam projected from a laser beam generating means into a convergent laser beam having a conical laser beam wavefront, and to collect the converted laser beam inside. Wavefront converting means for converting a laser beam projected from a non-linear optical medium receiving the light beam or from a laser beam generating means into a laser beam having a laser beam wavefront diverging in an annular shape, and the converted laser beam The wavelength conversion device is composed of a focusing means for focusing the light and adjusting the distance from the wavefront converting means, and a nonlinear optical medium in which the converted laser beam is focused. This is achieved by extracting wavelength-converted converted light under matching conditions.

Wavefront conversion means for converting the incident laser beam in the above configuration into a convergent laser beam having a conical laser wavefront,
Most simply, it can be constituted by a converging conical lens having a light converging property, and can also be constituted by an outwardly convex conical light receiving surface of a nonlinear optical medium.

The nonlinear optical medium which receives the laser beam converged or condensed by the wavefront converting means does not need to adjust its temperature in principle, but in practice, the phase matching on the wavelength conversion is performed by simple temperature adjustment. It is desirable to optimize the conditions.

Further, the wavefront converting means for converting the incident laser beam into a laser beam having a laser wavefront that diverges in an annular shape can be constituted by a convex conical mirror, but it is usually advantageous to constitute a concave cone lens. Advantageously, the combined focusing means comprises a convex focusing lens. At this time, it is desirable to make the light receiving surface of the condenser lens a convex conical surface so that the light flux in the lens is parallel, and make the exit surface a convex condenser surface.

When the present invention is applied to the wavelength conversion of the SHG method, it is usual to arrange the wavelength conversion device outside the resonance system of the laser light generating means, but it is also possible to incorporate the wavelength conversion device in the resonance system in some cases. When the present invention is applied to OPO-type wavelength conversion, it is advantageous to arrange the wavelength conversion device in the resonance system of the laser light generating means.

[Action]

The present inventor has arrived at the present invention by analyzing in detail the manner in which light passing through the conical lens is collected. First, in order to facilitate understanding, a laser beam as a fundamental wave is applied to the optical crystal from two directions. The case of incidence will be described. FIG. 11 shows a state in which light L1 and L2 of wavelength λ _ω are incident on the optical medium and intersect from two directions in which the angle between the direction Q and α is α, and a thin line perpendicular to the light traveling direction is shown. Wavefront of light. The wavelength λ _Q when the electric field created by these lights L1 and L2 in the crossing portion is viewed in the Q direction is expressed by the following equation as is apparent from the figure.

λ _Q = λ _ω / cos α (16) By changing the angle α in this way, the wavelength λ _Q of the fundamental wave in the Q direction can be manipulated, and the harmonic wave that is wavelength-converted in the optical medium from the fundamental wave Since the wave is generated in the Q direction, phase matching between the harmonic and the fundamental wave becomes possible. However, if the fundamental wave is collected in order to perform the wavelength conversion efficiently, the interaction range between the two beams is narrowed.

Therefore, the present inventor paid attention to the fact that wavefront conversion means such as a conical lens can be used. FIG. 12 illustrates the principle of the present invention.

In FIG. 12 (a), for example, a fundamental wave which is a parallel laser beam 22 is given to a convex conical lens as a wavefront converting means 40 to be converted into a convergent beam, and the intersection of two lights in FIG. 11 is performed. A hatched area is created in the optical medium 70 in a view similar to the section. In FIG. 12 (b), after the fundamental wave which is the laser beam 21 is given to the concave conical lens as the wavefront converting means 50 and is once converted into the divergent light beam 23, the convex light is condensed as the light collecting means 60. The lens also creates a hatched area in the optical medium 70 similar to the light crossover in FIG. When FIG. 12 is compared with FIG. 11, they are similar, but this is apparent because they are represented as a two-dimensional diagram. In the case of the present invention, hatching is different from the intersection of light in FIG. The difference is that the parts are made of convergent or convergent light flux.

That is, in the case of the present invention, the following Kirchhoff showing the state of propagation of light through the aperture A as shown in FIG.
It is necessary to use the following equation.

However, where u _a is the amplitude distribution of the electric field on the opening A, u _p is an amplitude distribution when light propagating from the opening A has reached the other side P, R is on the point on the opening A and the plane P Is the length of the vector connecting the point A, the angle θ _B is the angle between the vector and the normal to A, and ds is the area element on the opening A. In order to discuss the light-condensing process by the conical lens, it is sufficient to consider that when the plane wave reaches the aperture portion, the phase is delayed by the lens.
Now, phase shift If, for example, as apparent from FIG. 14 corresponding to the conical lens 40, Becomes here Is the wave number of light in air, θ _L is the angle formed by the conical surface of the lens with the bottom surface of the lens, η _L is the refractive index of the lens, d is the diameter of the lens, and r is the distance from the center of the lens. Assuming that the absolute value of the electric field strength at the opening surface A is 1, By substituting this into equation (17) and performing numerical integration, the light distribution is obtained. u _p is the energy density I because it is complex amplitude representing the field strength I = | u _p | ² .................. obtained as (21).

FIG. 15 shows the energy density on the center line of the beam after passing through the lens in this manner, that is, the energy density on the Q axis in FIG.

Note that the distance l on the horizontal axis is the distance from the conical lens 40 on the Q axis, and the energy density I on the vertical axis is the energy density of the incident light.
Is a relative value when As apparent from FIG. 15, I increases in proportion to the distance l, but suddenly decreases at about 2.6 cm. This corresponds to the distance at which the overlapped light is separated again, that is, the end of the hatched area in FIG. This calculation shows that the energy density on the center line is about 4 × 10 ⁴ times the input light. Note that the vibration superimposed on the increase curve is caused by diffraction at the end of the opening A.

Next, FIG. 16 shows the result of obtaining the energy density I in the direction perpendicular to the Q axis at the 15th l = 2.5 cm position, and the horizontal axis is the radius r in the direction of 90 ° from the Q axis. It can be seen that the energy density I has a large peak near the center with a radius of about 2 μm.

It may be obtained the real part of u _p to know the field intensity rather than energy density. Fig. 17 shows l on the Q axis = 2.499 to 2.5 cm
The electric field E in the section of 10 μm is enlarged and shown. The wavelength on the Q axis is λ _Q = 1.082 μm. On the other hand, when the above lens is used, the angle α formed by the refracted light beam with the z axis
Is 11.026 °, so from equation (16), λ _Q = 1.083 μm
It can be seen that the wavelength of the oscillating electric field on the Q axis substantially matches the predicted value obtained by the equation (16).

As can be seen from the above, the wavefront conversion means shown in FIG.
By condensing the fundamental wave into the nonlinear optical medium 70 by using the wavefront converting means 50 and the condensing means 60 shown in FIG. The concentration of the fundamental wave into the optical medium by the wavefront converting means and the focusing means is appropriately selected while the concentration of the fundamental wave is focused on the density, so that the width of the high energy density region and the oscillating electric field of the fundamental wave inside the high energy density area are increased. Can be adjusted to a desired value in the direction of the center line.

The present invention utilizes such functions of the wavefront converting means and the condensing means. The center of the above-mentioned fundamental wave which can be phase-matched with the converted light such as the second harmonic from the type of the nonlinear optical medium and the wavelength of the fundamental wave. First, the wavelength in the linear direction is determined, and the converging angle of the fundamental wave into the optical medium by the wavefront converting means and the condensing means is adjusted so as to satisfy the phase matching condition. This substantially satisfies the phase matching condition, but when fine adjustment is required, the temperature of the nonlinear optical crystal or the distance between the wavefront converting means and the light condensing means can be adjusted.

As described above, according to the present invention, the energy of the fundamental wave is concentrated at a high density in the wavelength conversion action region in the nonlinear optical medium while easily and accurately performing the phase matching between the fundamental wave and the harmonic.
In addition, the above-described problem can be solved by sufficiently increasing the area of this region to increase the wavelength conversion efficiency.

〔Example〕

Hereinafter, some embodiments of the present invention will be described with reference to FIGS. 1 to 10.

FIG. 1 is a configuration diagram of a laser light wavelength conversion device according to the present invention using a convex conical lens as the wavefront conversion means 40.
In this embodiment, the laser light is converted into the second harmonic light by SHG wavelength conversion. The laser light generating means 10 shown in the figure
As usual, laser medium 11, such as YAG rod, total reflection mirror
12, a partially reflective output mirror 13, a polarizer 14, etc., and the excitation light source and the like are omitted from the figure for simplification. The laser light generating means 10 oscillates a laser light 20 having a wavelength of, for example, 1.063 μm, which is linearly polarized by the polarizer.

In this example, a beam expander 15 including a concave lens 15a and a convex lens 15b is provided in association with the laser light generating means 10, whereby a narrow laser light flux 21 from the laser light generating means 10 is a parallel laser in this example. The light beam 22 is once spread. When the beam diameter of the original laser beam 20 is large, the beam expander 15 is of course unnecessary.

The convex conical lens as the wavefront converting means 40 receives the laser beam 22 spread by the beam expander 15 on a flat light receiving surface, converts it into a convergent laser beam, and is disposed close to the emission side thereof. The light is condensed inside the nonlinear optical medium 70.

As the nonlinear optical medium 70, for example, MgO-doped lithium niobate (LiNbO ₃ ) crystal is used. In the SHG type wavelength conversion as in this example, the above-mentioned type I phase matching is advantageous, so the polarization direction is the optical axis. At right angles to The nonlinear optical medium 70 and the wavefront converting means 40 are housed in a container 80, and the temperature is kept at a constant value by means of a simple temperature controller 90 or the like.

In the above-described type I phase matching in this embodiment,
The inclination angle of the conical lens as the wavefront conversion means 40 is set to, for example, 12
° or 14.5 °, the laser beam 20 is projected into the nonlinear optical medium 70 at an angle of 2.8 ° or 3.43 ° with respect to the center line,
The phase matching condition is achieved by controlling the temperature of the nonlinear optical medium 70 to 59 ° C. or 36 ° C. Under this condition, the converted light 30, which is the second harmonic light converted from the laser light 20, which is the fundamental wave, is of course generated as a thin light beam near the center line of the nonlinear optical medium 70, and as shown in the drawing from the exit end face thereof. Is taken out of the container 80. The laser beam 20, which is a fundamental wave radially emitted from the end face, is absorbed by appropriate means.

2 is different from the previous embodiment in that the incident side end face of the nonlinear optical medium 70 is polished into a conical optical surface and is used as the wavefront converting means 41. The same and the same reference numerals are given. As can be easily understood, the operation principle of this embodiment is the same as that of the previous embodiment, and has the advantage that the overall configuration can be simplified.

On the other hand, the embodiment of FIG. 3 is significantly different from the above-mentioned two embodiments in that the wavelength converter is incorporated in the lens resonance system. In this embodiment, the nonlinear optical medium 70 is used instead of the output mirror 13 incorporated in the laser light generating means 10.
An output mirror 16 having a concave spherical reflecting surface is arranged opposite to the right end face of the laser beam, and a laser beam 20 as a fundamental wave radially emitted from the nonlinear optical medium 70 is reflected by the It is sent back to the medium 70.

Therefore, the laser light 20 is confined in the laser resonance system between the output mirror 16 and the total reflection mirror 12 without being emitted to the outside, and the wavefront converting means 40 and the nonlinear optical medium 70 are incorporated in the resonance system. . On the left end face of the nonlinear optical medium 70 facing the wavefront conversion means 40, a wavelength selective coating having high transmittance for the laser light 20 but high reflectance for the converted light 30 is applied, The converted light 30 generated in the medium 70 is extracted to the outside from a right end face thereof through a small exit port opened at the center of the output mirror 16.

In this embodiment, a high-intensity laser beam in the resonance system is supplied to the wavelength converter as a fundamental wave, and the second-order nonlinear polarization of the nonlinear optical medium is proportional to the square of the intensity of the fundamental wave. The wavelength conversion efficiency can be significantly increased as compared with the example, and converted light with high intensity can be extracted. As can be easily understood, in this embodiment, similarly to the embodiment of FIG. 2, the end face of the nonlinear optical medium can be formed as a conical surface to serve as a wavefront converting means.

In the embodiment shown in FIG. 4, the wavelength conversion device is placed outside the laser resonance system, but the end face of the nonlinear optical medium 72 opposite to the light receiving side is formed as a conical surface, and a highly reflective coating is applied to perform wavefront conversion. Used as means 42. Nonlinear optical medium
When the laser beam 22 which is a fundamental wave enters the 72 from a flat light-receiving surface, the laser beam 22 is reflected by the wavefront converting means 42 and converted into a conical convergent laser beam, and the converted light 30 which is a harmonic wave from the condensing portion. It is generated and in this example is taken out at right angles from the plane parallel plate 17 arranged on the input side of the fundamental wave. The parallel plane plate 17 is provided with a coating that is highly transmissive to the fundamental wave and highly reflective to the harmonics.

The embodiment shown in FIG. 5 uses a nonlinear optical medium 70 having flat end faces, and is a conical mirror which is concave on the end face opposite to the light receiving side, and is functionally equivalent to the wavefront conversion means 42 shown in FIG. A simple wavefront converting means 43 is provided. In this embodiment, a gap is formed between the nonlinear optical medium 70 and the wavefront converting means 43 so that all fundamental waves cannot be collected in the nonlinear optical medium 70, but there is an advantage that the processing of the optical crystal becomes easy.

FIG. 6 shows an embodiment of the present invention in which the distance between the two means is adjustable by combining the wavefront converting means and the condensing means.
FIG. 11A shows a state where the interval is large, and FIG. 10B shows a state where the interval is small. In this embodiment, a concave conical lens is used for the wavefront converting means 50, the light receiving surface of which is formed flat, and the thin laser beam 21 generated by the laser light generating means 10 is beam-formed as in the previous embodiments. The laser beam is received without passing through the dilator, and is converted into the laser beam 23 which diverges in an annular shape as shown in FIG.

Focusing means for receiving this annularly divergent laser beam 23
Since the light receiving surface 60 is formed as a convex conical surface having the same angle as the conical surface of the wavefront converting means 50, the laser beam becomes an annular parallel light beam inside thereof, and in this example, a convex spherical surface is formed. The light exiting surface or the light condensing surface formed in the above-mentioned manner converges on the focal point F of the light converging surface in the nonlinear optical medium 70 as shown by the annular laser beam 24 in the figure. The exit surface of the light condensing means 60 is not limited to such a spherical shape, and may be formed, for example, in a convex conical surface as shown in FIG. 12 (b).

In this embodiment, fine adjustment of the angle α with respect to the center line of the laser light as the fundamental wave focused in the nonlinear optical medium 70 by adjusting the distance x between the wavefront converting means 50 and the focusing means 60. Accordingly, phase matching between the fundamental wave and the harmonic can be easily achieved. That is, when the interval x is increased as shown in FIG. 6 (a), the annular diameter of the laser beam 23 incident on the condensing means 60 is large, and therefore the angle α is large. When the distance x is small as in b), the annular diameter of the laser beam 23 is small, and therefore the angle α is small.

Thus, the embodiment of FIG. 4 has an advantage over the embodiment of FIGS. 1 to 5 in that the angle is fixed in that the projection angle α of the fundamental wave onto the nonlinear optical medium can be easily adjusted. Having.

Of course, even if the angle α is fixed, the phase matching condition can be satisfied if the angle of the conical surface of the wavefront conversion means is accurately formed and this angle α is adjusted to a predetermined value, and the temperature of the nonlinear optical medium can be controlled. Then, this condition can be finely adjusted. However, the optical characteristics of the nonlinear optical medium tend to fluctuate depending on the crystal formation conditions, making it difficult to predict the angle to be given to the conical surface. Depending on the type of crystal, the temperature dependence of the refractive index for ordinary light and extraordinary light In some cases, it may be difficult to obtain accurate phase matching.

As shown in FIG. 6, a wavelength converter in which the distance between the two means can be adjusted by combining the wavefront converting means and the condensing means is highly effective when applied to such a case, and always satisfies the phase matching condition and ensures non-linear optics. The need to adjust the temperature of the medium can be eliminated.

In the embodiment shown in FIG. 6, the laser beam 24 is
When the annular angle width δ in the figure of the annular laser beam 24 is too large, the projection angle α of the laser beam has a distribution, and the To eliminate this inconvenience, the diameter of the laser beam 21 applied to the wavefront conversion means 50 must be sufficiently reduced.

The embodiment of FIG. 7 eliminates this restriction, and uses a wavefront converting means 51 which is substantially the same as the conical lens of FIG. 4, but has a slightly concave light receiving surface. The converted annular laser light beam 23 has a higher divergence than the case of FIG. 4, and the laser light beam in the condensing means 60 receiving the light beam has a slightly divergent light beam deviating from the parallel light beam. The converging laser light beam 24 projected on the optical medium 70 becomes a light beam having a uniform projection angle α equivalent to the case of FIG. It should be noted that also in this embodiment, the exit surface of the wavefront converting means 51 and the light collecting means
By making the cone angles of the light receiving surfaces 60 uniform, the uniformity of the projection angle α can be maintained even if the distance between the wavefront converting means 51 and the light collecting means 60 changes.

The embodiment of FIG. 8 has a function equivalent to that of FIG. In this embodiment, the wavefront converting means 52 has a concave light-receiving surface as in the case of FIG. 7, but its emission surface is formed as a convex conical surface. Except that the laser beam 23 converted by the wavefront converting means 52 becomes a divergent light beam once collected, the projected light beam 24 from the light collecting means 60 is the same as in the case of FIG.
It is also the same that it is desirable to make the cone angles of the output surface of the wavefront conversion means 51 and the light receiving surface of the light collection means 60 uniform.

The embodiments described above are all suitable for the wavelength conversion of the SHG system, but the embodiments after FIG. 9 are suitable for the wavelength conversion of the OPO system. The principle of the OPO method has been explained before using equations (7) to (1).
1) As described by the equation and the like, a light having a frequency of ω ₃ is projected as a converted wave and a conversion having both or one of ω ₁ and ω _{2 in} a relationship of ω ₃ = ω ₁ + ω ₂ is performed. It extracts light. At this time, the phase matching is performed at frequencies ω ₁ and ω ₂
Is the sum of the two wavenumber vectors corresponding to the frequency ω
It is satisfied under the condition that it becomes equal to the wave number vector corresponding to ₃ .

On this phase matching, the OPO system differs from the SHG system only in the point that the wavelength of the converted wave is shorter than the wavelength of the converted wave, and as can be seen from the above-described embodiment suitable for SHG, the same applies according to the present invention. By any means, this phase matching condition can usually be satisfied without utilizing the birefringence of the optical crystal. However,
In the case of the OPO method, the nonlinear optical medium is placed in a resonance system tuned to the frequency of the converted light in order to increase the wavelength conversion efficiency,
It is common to amplify the converted light generated in the medium and then extract it outside.

FIG. 9 shows an embodiment of an OPO type wavelength converter corresponding to FIG. 1, in which a conical lens is used for the wavefront converting means 40 as in FIG. The laser beam 20 to be converted is transparent to the end face 70a, but the converted light is coated with a wavelength-selective coating having high reflectivity, and the output mirror 18 is partially reflective to face the emission side end face 70b. Is arranged. The converted light 31 generated in the medium is amplified in a resonance system between the incident side end face 70a and the output mirror 18 and then extracted outside.

In such OPO wavelength conversion, when the wavelength of the laser light 20 to be converted is 1.06 μm, for example, 3.00 μm.
Both or one converted light 31 at a wavelength of 1.64 μm is generated. Of course, the setting of the wavelength of the converted light 31 is performed by finely adjusting the distance of the output mirror 73 from the nonlinear optical medium 70 and selecting the resonance frequency of the resonance system. In addition,
As described above, it is advantageous to perform the SHG wavelength conversion under the 90 ° phase matching condition, but the OPO wavelength conversion is advantageous in both the optical axis direction and the 90 ° direction of the optical crystal. Available to

FIG. 10 shows an embodiment of an OPO type wavelength converter corresponding to the previous FIG. 7, in which the wavefront converting means 51 and the condensing means 60 are used in combination as in FIG. A resonance system is configured by using the incident side end face 73a and the emission side end face 73b of the optical medium 73 for the laser light 20 as the light to be converted as a reflection mirror for the converted light 31, and the above-described two types of generated converted light can be generated. The resonance system is tuned to the shorter wavelength of 1.64 μm, for example.

To configure such a resonance system, both end surfaces 73a and 73b of the nonlinear optical medium 73 are highly transmissive to the laser light 20, but the former is highly reflective to the converted light 31. A wavelength-selective coating is applied so that the latter is, for example, 90% reflective. Of course, instead of using the output side end face 73b as a reflecting mirror, a partially reflecting output mirror 18 is provided opposite to the reflecting mirror as in the case of FIG. 9 to facilitate fine adjustment of the resonance system or selection of the wavelength of the converted light. You may be able to. It is desirable that the emission-side end face 73b is formed in a convex spherical shape as shown in the figure to stabilize the resonance operation of the resonance system. In the embodiment of FIG. 10 configured as described above, the converted light
31 is the emission side end face 73b coated with 90% reflective coating
As shown in the drawing.

In the above-wavelength conversion in FIG. 9 and OPO method of FIG. 10 described, or converted light formula omega ₃ sum omega ₁ + omega ₂ to satisfy omega ₁ and omega ₂ throat frequencies above occurs, resonance Of course, the resonance state of the system is related, but mainly depends on the state of phase matching.

Therefore, in the embodiment shown in FIG. 9, the wavelength of the converted light to be generated can be continuously controlled by changing the phase matching condition by adjusting the temperature of the nonlinear optical medium 70. Similarly, in the embodiment of FIG. 10, the projection angle of the laser beam 20 to the nonlinear optical medium 73 is adjusted by changing the distance between the wavefront conversion unit 51 and the light collection unit 60, and the frequency satisfying the phase matching condition is changed. The wavelength of the converted light can be controlled continuously. In particular, the embodiment shown in FIG. 10 has the advantage that the wavelength control of the converted light can be performed freely or frequently and over a wide range as compared with the case of the temperature adjustment shown in FIG.

As described above, the present invention has been described with reference to some examples. However, the present invention is not limited to such examples, and the present invention can be implemented in various modes within the gist of the present invention. For example, the condensing means is terminologically distinguished from the wavefront converting means in order to facilitate understanding, but this is also a kind of wavefront converting means, and not only a lens but also an optical means such as a mirror can be appropriately used. However, it is easily understood that the light receiving surface may be formed flat, and the light condensing surface is not limited to the spherical shape but may be appropriately formed to be a conical surface or the like.

〔The invention's effect〕

As described above, in the present invention, wavefront converting means for converting a laser beam projected from a laser beam generating means into a convergent laser beam having a conical laser wavefront, and converting the converted laser beam inside Wavefront converting means for converting the laser beam projected from the laser light generating means by a non-linear optical medium received so as to be focused or a laser beam having a laser light wavefront diverging in an annular shape,
A laser light wavelength converter is formed by a light collecting means that collects the converted laser light flux and adjusts an interval with the wavefront converting means, and a nonlinear optical medium in which the converted laser light flux is collected. By converting the wavelength-converted converted light from the laser light received by the nonlinear optical medium under the phase matching condition from the nonlinear optical medium, the converted light between the fundamental wave and the harmonics in the nonlinear optical medium or the converted light can be converted. The wavelength conversion efficiency can be increased while the phase matching condition with light is much more easily and reliably satisfied as compared with the related art.

This will be described taking the case of SHG as an example. The projection angle of the fundamental wave onto the nonlinear optical medium is α, the refractive index of the fundamental wave for ordinary light is n _1o , and the optical axis and the 90 ° direction refractive index for the fundamental wave and the second harmonic extraordinary light are n _1e respectively. and When n _2e, in the present invention, the phase matching condition of type I phase matching conditions cosα = n _2e / n _1o type II, and _{cosα = 2n 2e / (n 1o} + n 1e). In conventional wavelength conversion, of course, α = 0, so
Since cosα = 1, it is necessary to equalize the numerator and denominator in the above equation for phase matching, select a nonlinear optical medium that almost satisfies the condition for the refractive index, and raise the temperature to 150 ° C. as described above. It is necessary to precisely control the above. In the present invention, the projection angle α can be easily selected by setting the convergence or converging angle of the wavefront converting means with respect to the fundamental wave, or adjusting the distance between the wavefront converting means and the converging means. .

As can be seen from the above, according to the present invention, (a) various optical crystals can be used for wavelength conversion, (b) wavelength conversion can be performed at room temperature or a temperature close thereto, and (c) temperature control can be simplified or required. And (d) the wavelength conversion efficiency can be increased by appropriately selecting the angle of projection onto the nonlinear optical medium.

When the present invention is applied to wavelength conversion of the OPO method in particular, phase matching can be performed without using birefringence of the optical crystal. Therefore, high wavelength conversion efficiency can be achieved by utilizing characteristics of various optical crystals.

When performing wavelength conversion using the birefringence of an optical crystal,
In order to convert the fundamental wave into ordinary light and the converted wave into extraordinary light, polarization must be generated in a direction 90 ° from the polarization direction of the fundamental wave. Accordingly, as can be seen from the above equation (2), diagonal coefficients d _ii such as d ₁₁ and d _{22 in} the matrix of the second-order nonlinear optical coefficient of the optical crystal could not be used for wavelength conversion.

However, usually, the diagonal coefficient _dii is much larger than other coefficients, and for example, MNA (2-methyl-4-nititium) which has recently been noticed in that the nonlinear optical coefficient is particularly large.
Roaniline) A value of d ₁₁ is also 8.6 times the d _12. Since it is not necessary to use birefringence in the present invention, it is possible to increase the wavelength conversion efficiency by using a large diagonal coefficient having a nonlinear optical coefficient which has not been effectively used conventionally. Further, optical crystals such as tellurium and potassium arsenic, which have a large nonlinear optical coefficient but do not have birefringence, which could not be used until now, can be used.

[Brief description of the drawings]

FIGS. 1 to 5 are diagrams each showing an apparatus configuration of a different embodiment of a laser beam wavelength conversion apparatus using a wavefront conversion means alone, and FIGS. FIG. 9 and FIG. 10 are device configuration diagrams of an embodiment in which the present invention is applied to OPO wavelength conversion, and FIG. 9 and FIG.
FIG. 11 is a schematic diagram showing a wavefront of crossed light projected from two directions, FIG. 12 is a schematic diagram for explaining the principle of the laser light wavelength converter according to the present invention, and FIG. 13 is a diagram showing Kirchhoff's variables. FIG. 14 is an explanatory diagram showing a phase shift of light by a conical lens as a wavefront converting means, and FIG. 15 is a diagram showing a relationship between an energy density of a light beam condensed by the conical lens and a position on a center line. FIG. 16 is a diagram showing a change in the energy density of the condensed light flux in a direction perpendicular to the center line, FIG. 17 is a diagram showing a distribution of electric field intensity of light on the center line, and FIG. Is
Diagram showing an increase in the power of the second harmonic during wavelength conversion of the SHG method, FIG. 19 is a schematic diagram showing the separation of ordinary and extraordinary light by birefringence, and FIG. 20 is a diagram of phase matching using birefringence. FIG. 21 is a diagram of the refractive index showing the principle, and FIG. 21 is a vector diagram showing the relationship between the wave vector of the fundamental wave incident from two directions and the wave vector of the nonlinearly polarized wave. In these figures, 10: laser light generating means, 11: lens medium, 12: total reflection mirror, 13: output mirror, 14: polarizer, 15: beam expander, 16:
Output mirror, 17: parallel plane plate, 18: output mirror, 20: laser beam, 21 to 24: laser beam, 30, 31, converted light, 40 to 43, 50 to
52: wavefront converting means, 60: condensing means, 70 to 73: nonlinear optical medium.

Claims (2)

(57) [Claims] 1. A wavefront converting means for converting a laser beam projected from a laser beam generating means into a convergent laser beam having a conical laser wavefront, and the converted laser beam is focused inside. A laser light wavelength conversion device, comprising: a nonlinear optical medium receiving the nonlinear optical medium so as to extract converted light whose wavelength has been converted from the laser light received by the nonlinear optical medium under the phase matching condition from the nonlinear optical medium. 2. A wavefront converting means for converting a laser beam projected from a laser beam generating means into a laser beam having a laser wavefront which diverges in an annular shape, and a beamfront converting means for condensing the converted laser beam. A non-linear optical medium in which the converted laser beam is condensed, and converts the converted light wavelength-converted from the laser light received by the non-linear optical medium under phase matching conditions. A laser light wavelength conversion device designed to extract light from a nonlinear optical medium.