JPH0922037A - Laser beam generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device capable of embodying a short-wavelength light source of high output by preventing the saturation of output light. SOLUTION: This laser beam generator is composed of a semiconductor laser 20 which outputs pumping light 26, a semiconductor laser 31 which outputs mixing light 27, a polarization beam splitter 37 which reflects the mixing light transmitted through the pumping light 26, a laser medium 23 consisting of Nd: YVO4, a nonlinear optical element 24 consisting of KNbO3 and an output mirror 41 consisting of a curved surface mirror. An off-cut KNbO3 crystal cut out at a crystal bearing at which a beam walk off arises is used as this nonlinear optical element 24. The blue green light which is the sum frequency of a wavelength 478nm is generated by mixing of the two basic waves; the wavelength 1064nm of the laser oscillated light and the wavelength 860nm of the mixing light.

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY The present invention enables direct modulation by generating a first laser beam from a solid-state laser by optical pumping with a semiconductor laser and mixing with a second laser beam from another semiconductor laser at a sum frequency. The present invention relates to a laser beam generator configured to obtain a short wavelength laser beam.

[0002]

2. Description of the Related Art A semiconductor laser made of GaAlAs or the like is used to produce Nd: YAG (yttrium aluminum.
A structure in which a solid-state laser made of garnet) or the like is optically pumped to generate a laser beam of another wavelength has been known. However, since the oscillation wavelength of such a solid-state laser is longer than the pumping wavelength from the semiconductor laser, it is unsuitable for applications that require an optical storage device such as an optical disk or a magneto-optical disk or a short wavelength coherent light source.

Therefore, there is a method of utilizing SHG (second harmonic generation) to obtain a laser beam of a shorter wavelength. For example, a bulk single crystal made of a solid-state laser medium and a non-linear optical material is arranged in one optical resonator to convert the laser light oscillated by the solid-state laser into a second harmonic. YAG crystal as solid-state laser medium, KTP as nonlinear optical material
By using (KTiOPO ₄ : potassium titanyl phosphate) crystal, the oscillation wavelength of 1.064 μm of the YAG laser can be converted into a green laser beam of 0.532 μm.

In order to obtain shorter wavelength blue-green light, KNbO ₃ is arranged as a non-linear optical material in the optical resonator of the YAG laser to generate the second harmonic of the oscillation wavelength of 0.946 μm of the YAG laser. Wavelength 0.47
It is also reported that it can be converted into a laser beam of 3 μm.

However, in the intracavity SHG laser in which the solid laser medium and the SHG crystal are arranged in the cavity as described above, direct modulation by an electric signal is difficult.
For application to an optical storage device such as an optical disk, it is necessary to provide an external modulator separately.

On the other hand, as a solid-state laser device capable of direct modulation, a solid-state laser medium and a nonlinear optical crystal are arranged in a laser resonator, and light having a wavelength different from the oscillation wavelength is introduced as a second fundamental wave from the outside. A configuration is known in which a first fundamental wave and a second fundamental wave oscillated from a solid laser medium are mixed in a nonlinear optical crystal to generate sum frequency light.

When the first fundamental wave propagates in a direction other than the crystal axis of the nonlinear optical crystal, the propagation direction and the propagation direction of the sum frequency do not coincide with each other, and they intersect at an angle ρ. This is called critical phase matching. The angle ρ is called the beam walk-off angle. On the other hand, when the fundamental wave propagates in the crystal axis direction, the propagation directions of the fundamental wave and the sum frequency coincide, and this is called non-critical phase matching (90 ° phase matching), and ρ = 0. Further, in the case of non-critical phase matching (ρ = 0), since the propagation directions of the fundamental wave and the harmonics are completely the same and the interaction length between the two can be long, the conversion efficiency is higher than in critical phase matching. Become.

In the case of critical phase matching, since the propagation directions of the fundamental wave and the higher harmonic wave do not coincide with each other, the interaction length between the two becomes short and the conversion efficiency decreases. Therefore, when priority is given to conversion efficiency, it is usually advantageous to use non-critical phase matching.

In the case of sum frequency generation, the degree of freedom in wavelength selection increases due to the use of two wavelengths, and even if one is fixed as the oscillation wavelength of the solid-state laser, the wavelength of the second mixing light is appropriate. By selecting, there is a possibility that non-critical phase matching will be satisfied.

In such a sum frequency generating laser, when the b-axis-KTP is used as the nonlinear optical crystal, the oscillation wavelength 809n of the semiconductor laser pumping the solid-state laser is used.
There is known a configuration capable of generating a sum frequency light having a wavelength of 459 nm under non-critical phase matching by mixing m and the oscillation wavelength of 1064 nm of a solid-state laser (Japanese Patent Laid-Open No. 64-64-
62621). However, although KTP has a high temperature tolerance and a high wavelength tolerance, the output of the sum frequency light is limited to about 1 mW because of its low conversion constant.

In another sum frequency generating laser, when a-axis -KNbO ₃ is used as the nonlinear optical crystal,
When laser light having a wavelength of 1064 nm is used as the first fundamental wave, there are two cases where non-critical (90 degrees) phase matching is possible.
1) In the a-axis-KNbO ₃ cut out in the b-axis direction,
By mixing the first fundamental wave 1064 nm and the second fundamental wave 910 nm, sum frequency light with a wavelength of 490 nm is generated. 2) In the b-axis-KNbO ₃ cut out in the a-axis direction, the first fundamental wave 1064 nm and the second fundamental wave 695 nm
By mixing and, sum frequency light with a wavelength of 420 nm is generated (Japanese Patent Application No. 5-247089).

In such a non-critical phase matching, even if the pump light intensity to the laser medium is relatively weak, that is, the light intensity of the second fundamental wave in the resonator is relatively weak, the sum is particularly efficient. It is possible to perform frequency generation.

On the other hand, an example of obtaining a sum frequency laser by critical phase matching without forming a resonator is OPTICS LETTERS Vol.16, No.7.
P449-P451 (1991). According to this, Ar
A laser and a semiconductor laser are mixed in a β-BaB ₂ O ₄ crystal to obtain a sum frequency output of 1.4 μW at a wavelength of 369 nm.

[0014]

The sum frequency conversion efficiency of β-BaB ₂ O ₄ is low, and in order to obtain a high output, use KNbO ₃ whose sum frequency conversion efficiency is two orders of magnitude higher than that of β-BaB ₂ O _4. Is desirable.

However, the nonlinear optical material KN
The bO ₃ crystal has a high conversion efficiency but an extremely narrow temperature tolerance (for example, 0.3 ° C./c on the a-axis of KNbO ₃ ).
m), the conversion efficiency decreases due to a slight non-uniform temperature distribution.

The presence of blue light in the KNbO ₃ crystal increases the absorption at the wavelength of the incident fundamental wave, BEIRA (Blu
e Enhanced IR Absorption) is known to occur (H. Mabuchi, ES Polzik, and HJ Kimbl
e; J.Opt.Soc.Am.B, 11, 2023 (1994)). When the intensity of the input fundamental wave with a wavelength of 860 nm increases in the second harmonic generation using a-axis -KNbO ₃ , the output of blue light with a wavelength of 430 nm is saturated due to the BEIRA phenomenon and narrow temperature tolerance. It is known.

The cause is considered as follows. When the fundamental wave is input from the outside, the intensity distribution of the blue light generated by the second harmonic generation in the propagation axis direction is not uniform in the nonlinear optical crystal and increases with the square of the distance from the incident end face. become. On the other hand, since the absorption of the fundamental wave is proportional to the blue light intensity, the absorption coefficient of the fundamental wave also has a distribution that increases with the square of the distance from the incident end face. For this reason, the heat generation due to the absorption of the fundamental wave in the nonlinear optical crystal does not have a uniform distribution, and a temperature distribution is generated in the propagation axis direction (LE Busse, L. Goldb
erg, and MRSurette and G.Mizell, J.Appl.Phys.75
(2), 1102 (1994)). This temperature distribution is usually about several degrees centigrade, which is not a problem in a nonlinear optical crystal having a relatively wide temperature tolerance, but since the KNbO ₃ crystal has a narrow temperature tolerance, the conversion efficiency may be lowered.

In the case of the sum frequency laser, the effect of such a phenomenon has not been clarified hitherto. In the sum frequency laser using KNbO ₃ crystal, blue light can be efficiently generated even when the excitation light intensity is relatively weak.
However, when the intensity of the pumping light to the pumping laser medium is increased in order to obtain a higher output blue light, it has been found that the conversion efficiency is lowered.

FIG. 11 shows BEIRA in sum frequency generation.
It is a graph which shows the example of the efficiency fall by a phenomenon. Here, an Nd: YVO ₄ crystal as a laser medium and an a-axis-KNbO ₃ crystal as a nonlinear optical material are arranged in an optical resonator, and the laser medium is excited by a first semiconductor laser to generate a first fundamental wave (wavelength 1064 nm). ), And the laser light from the second semiconductor laser is emitted to the second fundamental wave (wavelength 690n
As m), blue light having a wavelength of 418 nm is generated by being introduced into the resonator and mixed. At that time, the temperature of the KNbO ₃ crystal was controlled by the Peltier device. In this graph, with the intensity of the second fundamental wave constant (maximum 17 mW obtained from a 690 nm semiconductor laser), the horizontal axis represents the output of the pumping light that excites the laser medium, and the vertical axis represents the output of the sum frequency light. Show.

As shown by the broken line in the graph, the output of blue light should change linearly with the output of pumping light. However, as shown by the solid line, in reality, the pumping light output saturates near 400 mW, and the output decreases conversely even if the pumping light is further increased. The reason for this is that as the intensity of the first fundamental wave (wavelength 1064 nm) increases inside the resonator, the amount of absorption increases and the statistical amount of light absorption due to the BEIRA phenomenon due to the increase in blue light intensity significantly increases. Therefore, it is considered that the influence of heat generation is significant. Therefore, it can be seen that there is a certain limit in increasing the sum frequency output even if the pumping light intensity is increased.

As another configuration example, Nd: YVO ₄ crystal is used as a laser medium and b is used as a nonlinear optical material in the optical resonator.
The axis-KNbO ₃ crystal is arranged, and the first fundamental wave (wavelength 10
64 nm) is laser-oscillated, and the second fundamental wave (wavelength 910 nm) is externally introduced into the resonator for mixing to generate blue-green light with a wavelength of 490 nm. Saturation of frequency output was observed.

Thus, in the sum frequency laser using KNbO ₃ , the saturation of the optical output due to the enhancement of the pumping light intensity is
It is thought to be due to the following two reasons. 1) In a sum frequency laser, a fundamental wave (wavelength 1064 nm) having a high light intensity that exceeds 100 W of internal power is confined and used in the resonator. Therefore, even if the light absorption coefficient is slightly increased, the heat generation amount is remarkably high. Will increase. 2) Since the output blue light is emitted in one direction, a large temperature distribution is likely to occur along the propagation axis.

An object of the present invention is to provide a laser beam generator capable of preventing a saturation of output light and realizing a high output short wavelength light source.

[0024]

According to the present invention, a resonator for a first fundamental wave, a laser medium arranged in the resonator for performing laser oscillation of the first fundamental wave, and the resonance are provided. A non-linear optical crystal that is arranged in a container and that generates a sum frequency light by mixing the first fundamental wave and the second fundamental wave, and light introduction for introducing the second fundamental wave into the non-linear optical crystal The nonlinear optical crystal has a crystallographic axis direction and an incident direction of the first fundamental wave so that a beam walk-off of the first fundamental wave and the sum frequency light propagating in the nonlinear optical crystal occurs. A laser beam generator characterized in that a relationship is defined. Further, the present invention is characterized in that the nonlinear optical crystal is formed of KNbO ₃ . Further, according to the present invention, in the three crystal axes of KNbO ₃ , when the a-axis refractive index na, the b-axis refractive index nb, and the c-axis refractive index nc satisfy the relationship of nb>na> nc, from the c-axis Where θ is the apex angle and φ is the angle from the a-axis in the ab plane, the propagation direction is set to a direction satisfying the relationships of 80 ° ≦ θ ≦ 110 ° and 15 ° ≦ φ ≦ 75 °. To KNb
The feature is that O ₃ is cut out. The present invention also provides the first wavelength of substantially 1064 nm.
It is characterized in that the fundamental wave and the second fundamental wave having a wavelength of substantially 780 nm are mixed to generate a coherent light beam having a wavelength of substantially 450 nm. The present invention also provides the first wavelength of substantially 1064 nm.
It is characterized in that the fundamental wave and the second fundamental wave having a wavelength of substantially 860 nm are mixed to generate a coherent light beam having a wavelength of substantially 475 nm. The present invention also provides the first wavelength of substantially 1064 nm.
It is characterized in that the fundamental wave and the second fundamental wave having a wavelength of substantially 830 nm are mixed to generate a coherent light beam having a wavelength of substantially 466 nm.

[0025]

According to the present invention, the nonlinear optical crystal is cut out in the crystal orientation causing the beam walk-off to reduce the spatial overlap between the light beams of the first and second fundamental waves and the sum frequency light beam. The temperature rise due to the BEIRA phenomenon can be suppressed. Therefore, even if the pumping light of the laser medium becomes high, the sum frequency output is not saturated, so that high conversion efficiency and high output can be realized. This will be described in detail below.

FIG. 1 is an explanatory diagram showing the difference between the temperature distribution and the beam propagation direction in a nonlinear optical crystal. FIG.
(A) shows the case where there is no beam walk-off, the propagation direction of the fundamental wave and the propagation direction of the sum frequency match, and the crossing angle ρ is 0. FIG. 1B shows the case where there is beam walk-off, and the propagation direction of the fundamental wave and the propagation direction of the sum frequency intersect at a constant angle ρ. Here, z is the position along the optical axis from the incident end face of the crystal, and x is the position in the direction perpendicular to the optical axis.

As shown in FIG. 1A, the temperature inside the crystal rises exponentially as the light travel distance increases, and the temperature becomes considerably high near the emission end face of the crystal. On the other hand, as shown in the temperature distribution of FIG. 1 (b), it can be seen that the temperature rise is significantly suppressed as compared with FIG. 1 (a).

The following three conditions are necessary in order to prevent a decrease in efficiency due to the BEIRA phenomenon while maintaining high conversion efficiency in sum frequency generation.

(A) The crystal orientation satisfies the phase matching condition in the sum frequency generation.

(B) A crystalline material showing a large value of the effective non-linear constant for generating the sum frequency in this direction.

(C) To have an appropriate beam walk-off angle ρ.

Here, KNbO _{3 is used} as the nonlinear optical crystal.
Will be specifically described by taking as an example the case where the first fundamental wave has a wavelength of 1064 nm.

First, regarding the condition (A), if the refractive index which is a function of the light traveling direction and the wavelength is n, the crystal angle for phase matching can be calculated by the following equation (1).

[0034]

[Equation 1]

Here, as shown in FIG. 2, the angles θ and φ indicating the crystal orientations are, as shown in FIG. 2, the a-axis refractive index na, the b-axis refractive index nb and the c-axis of the three crystal axes in the KNbO ₃ crystal. The refractive index nc has a relationship of nb>na> nc, and the apex angle from the c-axis is defined as θ, and the angle from the a-axis in the ab plane is defined as φ. In addition, λ1, λ2, and λ3 are respectively the first
It is the wavelength of the fundamental wave, the second fundamental wave, and the sum frequency.

FIG. 3 is a graph showing the direction of phase matching with respect to the wavelength change of the second fundamental wave. Here, the apex angle θ =
It is fixed at 90 °, the horizontal axis shows the mixing wavelength λ 2 which is the second fundamental wave, and the vertical axis shows the phase matching angle φ. For example, when the mixing wavelength is 780 nm, the cutout angle of the crystal satisfying the phase matching is θ = 90 ° and φ = 42 °. Further, when the mixing wavelength is 860 nm, the crystal cutting angle is θ = 90 ° and φ = 63 °.

As described above, it is understood from the equation (1) that the crystal orientation for phase matching is uniquely determined when the mixing wavelength is determined.

Next, the condition (B) will be examined. When the crystal orientation capable of phase matching is determined as described above, it is required that the effective nonlinear constant along the orientation is large. The effective nonlinear constant deff in the ab plane of the crystal is obtained by the following equation (2). Here, d32 and d31 are nonlinear optical constants, and na and nb are refractive indices of the a-axis and the b-axis.

[0039]

[Equation 2]

For example, φ = 43 °, d32 = 15 pm /
Substituting V and d31 = 18 pm/V, deff = 16pm
/ V, which is a sufficiently large value.

Next, the condition (C) will be examined. a-b
The beam walk-off angle ρ in the plane is given by the following equation (3).
Is determined by

[0042]

(Equation 3)

FIG. 4 is a graph showing changes in the beam walk-off angle ρ with respect to the crystal angle φ. Here, the horizontal axis represents the crystal angle φ and the vertical axis represents the beam walk-off angle ρ. Looking at this graph, the beam walk-off angle ρ becomes approximately 0.96 ° which is the maximum at the crystal angle φ = 43 °. As will be described later, this numerical value is less affected by the BEIRA phenomenon and satisfies the above three conditions A to C.

Thus, the case of the apex angle θ = 90 ° was considered. However, since KNbO ₃ is a biaxial crystal, phase matching may occur even in an orientation shifted from θ = 90 °, but it is effective. The non-linear constant deff tends to decrease, and the beam walk-off angle ρ has a large value. If the beam walk-off angle ρ becomes larger than necessary, the conversion efficiency will decrease. Therefore, 80 ° ≦ θ ≦ 110 °
It is preferable to achieve phase matching in the range of
90 ° is more preferable.

Next, the above three conditions A to C and BEIRA
The causal relationship with the efficiency decrease due to the phenomenon will be described. BE
The amount of absorption of the fundamental wave due to the IRA phenomenon is proportional to the degree of spatial overlap between the sum frequency light and the fundamental wave light. In the case of non-critical phase matching (90 degree phase matching), which enables phase matching in the direction that matches the crystal axis of the nonlinear optical crystal, the two fundamental wave lights and the sum frequency light propagate coaxially along the crystal axis. Therefore, the spatial overlap becomes large. The intensity distribution of the sum frequency light on the propagation axis rises as the square of the propagation distance in the crystal. Therefore, the temperature distribution in the crystal rises with the square of the propagation distance, and reaches the maximum temperature near the emission end face.

On the other hand, when phase matching is possible in a direction deviated from the crystal axis, the generated sum frequency light propagates in a direction deviated from the propagation direction of the fundamental wave by the beam walk-off angle ρ. Therefore, the spatial overlap between the two is reduced, and the temperature distribution along the propagation axis of the fundamental wave is reduced.

Next, the BEIRA phenomenon in the presence of beam walk-off will be considered quantitatively. Here, for simplification, the sum frequency generation when the two fundamental waves have the same wavelength will be described.

Letting the spatial electric field distributions of the fundamental wave and the sum frequency be E1 (x, y) and E2 (x, y, z), they can be expressed by the following equations (4) and (5). Here, W ₀ is the mode radius of the fundamental wave, L is the crystal length, and ρ is the beam walk-off angle.

[0049]

(Equation 4)

FIG. 5 shows the intensity distribution of the sum frequency light at the position z in the crystal from the end face. FIG. 5 (a) shows the beam walk-off state and FIG. 5 (b) shows the beam walk-off state. The vertical axis represents the intensity of the sum frequency light, and the horizontal axis represents the propagation axis of the fundamental wave (z
The position is in a plane (xy plane) perpendicular to the axis. Here, the mode radius W ₀ of the fundamental wave E ₁ is 40 μm,
When the crystal incident end face is z = 0, crystal positions z = 1, 2,
The intensity distributions at 3, 4, and 5 mm are shown. Further, the intensity of the output light is a value obtained by integrating the intensity distribution in the xy plane.

FIG. 5A shows the beam walk-off angle ρ.
It shows a case of non-critical phase matching at = 0 °, and it is found that the intensity of the sum frequency light sharply increases as the crystal position z increases from 1 mm. Further, the center of the intensity distribution of the fundamental wave and the center of the intensity distribution of the sum frequency coincide with each other.

FIG. 5B shows the beam walk-off angle ρ.
= 0.9 °, the case of critical phase matching is shown in FIG.
Similarly, it can be seen that the intensity of the sum frequency light sharply increases as the crystal position z increases from 1 mm. Also, because of beam walk-off, the center of the intensity distribution of the sum frequency is gradually shifted from the center of the intensity distribution of the fundamental wave. Further, in the case of the critical phase matching, it is unavoidable that the beam walk-off substantially reduces the conversion efficiency as compared with the non-critical phase matching, but comparing each graph at z = 5 mm, It can be seen that the peak value slightly decreases, but the overall integrated value hardly decreases.

Further, the absorption amount of the fundamental wave which increases due to the presence of the sum frequency is proportional to the overlap integral of the intensity distribution of the sum frequency and the intensity distribution of the fundamental wave. In the case of no beam walk-off, the intensity distributions of the fundamental wave and the sum frequency always overlap with each other, so that the absorption amount increases in proportion to the square of the distance z along the propagation axis. On the other hand, in the case of the beam walk-off, the intensity distribution of the sum frequency gradually shifts with respect to the propagation axis of the fundamental wave, so that the overlap integral of both is saturated rapidly.

FIG. 6 is a graph showing changes in the infrared absorption coefficient along the propagation axis z. When ρ = 0 °, the crystal position z
, The absorption coefficient increased sharply, but ρ = 0.9
In the case of °, it can be seen that the increase of the absorption coefficient is significantly suppressed and the occurrence of the temperature distribution in the propagation axis direction is suppressed to about 1/4 or less.

When the beam walk-off angle becomes too large, the temperature rise due to BEIRA is suppressed, but this time the sum frequency generation efficiency is extremely reduced, which is disadvantageous for higher output. For example, the beam walk-off angle ρ =
When the case of 4 ° is calculated, the output becomes extremely low. Therefore, in KNbO ₃ , the angle φ from the a-axis in the ab plane satisfies the relationship of 15 ° <φ <75 °, so that an appropriate beam walk-off angle is set from the viewpoint of temperature distribution and conversion efficiency. it can.

Thus, the sum frequency output does not decrease so much,
Moreover, it can be seen that a method of setting an appropriate beam walk-off angle according to the type of the nonlinear optical crystal is extremely effective in suppressing the temperature rise due to the absorption of the fundamental wave due to the BEIRA phenomenon.

Specifically, a semiconductor laser, a laser medium,
Considering the availability of nonlinear optical crystals and the like, the configuration examples in the following table are preferable, and a light source with a high output and a short wavelength can be easily realized.

[0058]

[Table 1]

[0059]

【Example】

(Embodiment 1) FIG. 7 is a block diagram showing a first embodiment of the present invention. In this embodiment, the wavelength is 1064 nm and the wavelength is 860.
A wavelength of 4 is obtained by mixing the two fundamental waves of nm.
An example of generating a 75 nm sum frequency blue-green light will be described. This laser beam generator includes a semiconductor laser 20 (output 1 W) that outputs pumping light 26 having a wavelength of 809 nm that excites a laser medium 23, a collimator lens 21a that collects the pumping light 26, and a mixing light 27 that has a wavelength of 860 nm. Output semiconductor laser 31 (output 20
0 mW), a collimator lens 33 that collects the mixing light 27, a polarization beam splitter 37 that transmits the pumping light 26 and reflects the mixing light 27, and a condensing lens 21b that focuses the pumping light 26 and the mixing light 27. , Nd doped with about 1% Nd: YVO ₄
Laser medium 23, a nonlinear optical element 24 made of KNbO _3, and an output mirror 41 made of a curved mirror.

The surface 23a on the light incident side of the laser medium 23 and the output mirror 41 constitute an optical resonator 25. The surface 23b of the laser medium 23 and the nonlinear optical element 24
And the surface 24a thereof are in contact with each other. Further, the semiconductor laser 20 for pumping is controlled to a predetermined temperature by a Peltier temperature adjusting circuit (not shown).

The surface 23a of the laser medium 23 has a reflectance of 99.9% at a wavelength of 1064 nm, which is the oscillation wavelength of the laser medium 23, and a transmittance of 95 at a wavelength of 809 nm of the pumping light 26. % Coating is applied. The surface 23b of the laser medium 23 is coated with a transmittance of 99.9% or more for a wavelength of 1064 nm.

Since the polarization direction of the pumping light 26 emitted from the semiconductor laser 20 coincides with the vertical upper portion 30 of the optical axis 29, it passes through the polarization beam splitter 37 as it is. When the pumping light 26 is condensed by the condenser lens 21b and is incident on the laser medium 23, a population inversion is formed in the laser medium 23. The laser medium 23 is N
It is formed of d: YVO ₄ , and laser oscillation with a wavelength of 1064 nm occurs in the optical resonator 25.

The non-linear optical element 24 is made of KNbO ₃ crystal, and its crystal orientation is, as shown in FIG. 8, a direction of an angle θ = 90 ° with respect to the c-axis and φ = 62 ° with respect to the a-axis. And a so-called ab axis cut crystal cut out along this crystal orientation is used, and the crystal thickness is 7 mm. The three crystal axes are the refractive indices n of the a-axis.
The refractive index nb of the a and b axes and the refractive index nc of the c axis are nb>
The relationship is defined as na> nc. On the surface 24a of the nonlinear optical element 24 on the laser medium 23 side, the wavelength 1
A transmittance of 99.9% for 064 nm and a wavelength of 86
An optical coating having a transmittance of 95% for 0 nm is applied.

On the other hand, the polarization direction of the mixing light 27 emitted from the semiconductor laser 31 is the same as the vertical direction of the paper surface of FIG. 7, so that it is reflected to the right by the polarization beam splitter 37. This mixing light 27 is used by the condenser lens 2
The light is focused by 1b, passes through the laser medium 23, and is incident on the surface 24a of the nonlinear optical element 24 so as to form a beam waist. First fundamental wave (wavelength 1064nm)
Laser oscillation light and the second fundamental wave (wavelength 860n
The respective polarization directions of the mixing light 27 of m) are arranged so as to be parallel to the ab plane of the KNbO ₃ crystal which is the nonlinear optical element 24. Then, the sum of the first fundamental wave and the second fundamental wave (wavelength 478 nm) is generated by the non-linear optical effect of the non-linear optical element 24, and the coherent output beam 29 having the polarization direction parallel to the c-axis is output from the output mirror. Four
Emitted from 1.

FIG. 9 is a graph showing the sum frequency output with respect to the output change of the pumping light 26 with the mixing light intensity of 860 nm being kept constant at 120 mW. As shown by the solid line, the sum frequency output changes linearly with the pumping light intensity, and it can be seen that when the pumping light is increased to 1 W, the sum frequency output of about 30 mW is stably obtained. In addition, the non-critical phase matching shown in FIG.
In comparison with the case where the sum frequency of 418 nm is generated by the combination with 90 nm, the saturation or decrease of the sum frequency output with respect to the change of the pumping light intensity is not observed in FIG. 9. Although the mixing light intensities of the two do not match, the sum frequency output increases as the mixing light output increases, which is considered to be strongly influenced by the BEIRA phenomenon. However, the mixing light output of this embodiment is 12
Despite the large value of 0 mW, the sum frequency light output is remarkably increased. From this point as well, the effect of improving the BEIRA phenomenon is clear.

As described above, by using the off-cut KNbO ₃ crystal to perform the critical phase matching, a large output of the sum frequency can be realized.

(Second Embodiment) FIG. 10 is a block diagram showing a second embodiment of the present invention. In this embodiment, the wavelength is 1064n.
An example of generating sum frequency blue light having a wavelength of 450 nm by mixing two fundamental waves of m and a wavelength of 780 nm will be described. This laser beam generator is similar to the configuration shown in FIG. 7, except that a dichroic mirror 40 for introducing mixing light into the resonator 25 is interposed between the laser medium 23 and the nonlinear optical element 24. Points,
The difference is that a minute spherical surface 23c is formed on the light incident surface of the laser medium 23.

The laser beam generator comprises a laser medium 23.
A semiconductor laser 20 that outputs a pumping light 26 having a wavelength of 809 nm that excites the laser light, collimator lenses 21a and 21b that collect the pumping light 26, and a laser medium 23 made of Nd: YVO ₄ doped with Nd of about 1%, A semiconductor laser 31 (model number SDL-5401) that outputs the mixing light 27 having a wavelength of 780 nm, and the mixing light 27.
Collimator lens 33a for condensing light and mixing light 2
The anamorphic prism pair 32 for shaping the beam shape of No. 7, the focusing lens 33b for focusing the beam-shaped mixing light 27, and the laser oscillation light of wavelength 1064 nm amplified by the laser medium 23 Reflected,
Further, it is composed of a dichroic mirror 40 that transmits the mixing light 27 having a wavelength of 780 nm, a nonlinear optical element 24 made of KNbO ₃ , an output mirror 41 made of a curved mirror, and the like.

A fine spherical surface 23c is formed on the light incident surface 23a of the laser medium 23 by the photolithography technique. The fine spherical surface 23c and the output mirror 41 are formed.
And constitute an optical resonator 25. The surface 23a of the laser medium 23 has a wavelength of 1 which is the oscillation wavelength of the laser medium 23.
A coating having a reflectance of 99.9% is applied to 064 nm. The surface 23b of the laser medium 23 is coated with a transmittance of 99.9% or more for a wavelength of 1064 nm.

When the pumping light 26 emitted from the semiconductor laser 20 is condensed by the condenser lens 21b and enters the laser medium 23, a population inversion is formed in the laser medium 23. The laser medium 23 is made of Nd: YVO ₄ , and laser oscillation with a wavelength of 1064 nm occurs in the optical resonator 25. Also, a semiconductor laser 2 for pumping
Zero is controlled to a predetermined temperature by a Peltier temperature adjusting circuit (not shown).

The non-linear optical element 24 is formed of KNbO ₃ crystal, and the crystal orientation is as shown in FIG. 8 with a direction of an angle θ = 90 ° with respect to the c-axis and φ = 42 ° with respect to the a-axis. The so-called ab axis cut crystal cut out along this crystal orientation is used, and the crystal thickness is 5
mm. The surface 24a of the nonlinear optical element 24 on the laser medium 23 side has a transmittance of 9 for a wavelength of 1064 nm.
9.9% and a transmittance of 95% for a wavelength of 450 nm
Optical coating is applied. The non-linear optical element 24 is controlled to a predetermined temperature by a Peltier temperature adjusting circuit (not shown), and achieves phase matching by temperature tuning.

On the other hand, the mixing light 27 emitted from the semiconductor laser 31 is condensed by the condenser lens 33b,
It passes through the dichroic mirror 40 and becomes coaxial with the laser oscillation light. The mixing light 27 is incident on the surface 24a of the nonlinear optical element 24 so as to form a beam waist. The respective polarization directions of the laser oscillation light which becomes the first fundamental wave (wavelength 1064 nm) and the mixing light 27 which becomes the second fundamental wave (wavelength 780 nm) are K of the nonlinear optical element 24.
It is arranged so as to be parallel to the ab plane of the NbO ₃ crystal. Then, due to the nonlinear optical effect of the nonlinear optical element 24, the sum frequency of the first fundamental wave and the second fundamental wave (wavelength 450
nm) and a coherent output beam 35 having a polarization direction parallel to the c-axis is emitted from the output mirror 41.

In this embodiment, when the output of the semiconductor laser 20 for pumping is set to 1 W and the output of the semiconductor laser 31 for mixing is set to 70 mW, the output is 12 m.
With W, it was possible to obtain a sum frequency blue light having a wavelength of 450 nm.

As described above, by using the off-cut KNbO ₃ crystal to perform the critical phase matching, the high output of the sum frequency can be realized.

[0075]

As described in detail above, according to the present invention, by using a material cut out in a crystal orientation causing beam walk-off, for example, an off-cut KNbO ₃ crystal, as the non-linear optical crystal, The spatial overlap between the wave light beam and the sum frequency light beam is reduced, and the temperature rise and instability due to the BEIRA phenomenon can be suppressed. Therefore, since the sum frequency output is not saturated even if the pumping light of the laser medium becomes high, it is possible to easily realize a short wavelength light source with high conversion efficiency and high output.

By modulating the second fundamental wave,
It is possible to modulate sum frequency light without using an external modulator.

[Brief description of drawings]

1A and 1B are explanatory views showing a difference between a temperature distribution and a beam propagation direction in a nonlinear optical crystal. FIG. 1A shows a beam walk-off state and FIG. 1B shows a beam walk-off state.

FIG. 2 is a diagram defining angles θ and φ indicating crystal orientation in a KNbO ₃ crystal.

FIG. 3 is a graph showing a direction of phase matching with respect to a wavelength change of a second fundamental wave.

FIG. 4 is a graph showing changes in beam walk-off angle ρ with respect to crystal angle φ.

5A and 5B show the intensity distribution of the sum frequency light at the crystal position z from the end face, where FIG. 5A shows beam walk-off and FIG. 5B shows beam walk-off.

FIG. 6 is a graph showing changes in infrared absorption coefficient along the propagation axis z.

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

FIG. 8 is a view showing a cutout orientation of a KNbO ₃ crystal.

9 is a graph showing the output of the sum frequency with respect to the output change of the pumping light 26. FIG.

FIG. 10 is a configuration diagram showing a second embodiment of the present invention.

FIG. 11 is a graph showing an example of efficiency reduction due to the BEIRA phenomenon in sum frequency generation.

[Explanation of symbols]

 20 Semiconductor lasers for pumping 21a, 33, 33a Collimator lenses 21b, 33b Condensing lens 23 Laser medium 24 Nonlinear optical element 25 Optical resonator 26 Pumping light 27 Mixing light 29, 35 Output beam 32 Anamorphic prism pair 40 Dichroic mirror 41 output mirror

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[Procedure amendment]

[Submission date] November 7, 1995

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

FIG.

[Fig. 2]

[Figure 8]

[Figure 3]

FIG. 4

[Figure 5]

FIG. 6

FIG. 10

FIG. 7

[Figure 9]

FIG. 11

Claims (6)

[Claims] 1. A resonator for a first fundamental wave, a laser medium arranged in the resonator for lasing the first fundamental wave, and a laser medium arranged in the resonator for the first fundamental wave. A nonlinear optical crystal that generates a sum frequency light by mixing the first fundamental wave and the second fundamental wave; and a light introducing unit that introduces the second fundamental wave into the nonlinear optical crystal. The crystal is characterized in that the relationship between the direction of the crystal axis and the incident direction of the first fundamental wave is determined so that beam walk-off of the first fundamental wave and the sum frequency light propagating in the crystal occurs. And laser beam generator. 2. The laser beam generator according to claim 1, wherein the nonlinear optical crystal is made of KNbO ₃ . 3. When the a-axis refractive index na, the b-axis refractive index nb, and the c-axis refractive index nc among the three crystal axes in KNbO ₃ satisfy the relationship of nb>na> nc, Where θ is the apex angle and φ is the angle from the a-axis in the ab plane, the propagation direction is set to a direction satisfying the relationships of 80 ° ≦ θ ≦ 110 ° and 15 ° ≦ φ ≦ 75 °. To KNb
The laser beam generator according to claim 2, wherein O ₃ is cut out. 4. A first having a wavelength of substantially 1064 nm
3. The laser beam generation according to claim 1, wherein the fundamental wave and the second fundamental wave having a wavelength of substantially 780 nm are mixed to generate a coherent light beam having a wavelength of substantially 450 nm. apparatus. 5. The first having a wavelength of substantially 1064 nm
3. The laser beam generation according to claim 1, wherein the fundamental wave and the second fundamental wave having a wavelength of substantially 860 nm are mixed to generate a coherent light beam having a wavelength of substantially 475 nm. apparatus. 6. A first having a wavelength of substantially 1064 nm
The laser beam generation according to claim 1 or 2, wherein the fundamental wave and the second fundamental wave having a wavelength of substantially 830 nm are mixed to generate a coherent light beam having a wavelength of substantially 466 nm. apparatus.