JP3758977B2 - Nonlinear optical crystal and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The invention of this application relates to a nonlinear optical crystal and a manufacturing method thereof. More specifically, the invention of this application is useful as an all-solid-state ultraviolet laser light source and the like, and a novel nonlinear optical crystal capable of wavelength conversion with high efficiency even when high-power laser light is incident and a method for producing the same, In addition, the present invention relates to a wavelength conversion element (device), and further to an all-solid-state laser light source device.
[0002]
[Prior art and its problems]
When the incident laser beam of a high intensity non-line-shaped optical crystal, light having a frequency twice or three times, etc. of the incident light by the non-linear optical effects in the strong electric field by the laser is generated from the crystal. This is the wavelength conversion using nonlinear optical effect, it is possible to perform highly efficient wavelength conversion by incident light so as to satisfy a condition called phase matching. The phase matching condition is to determine the composition and temperature of the crystal, there is a need that as the nonlinear optical crystal is uniform in composition.
[0003]
In recent years, the demand for coherent ultraviolet light sources has increased in many industrial and medical fields, such as optical lithography, high-density multilayer substrate processing, DVD disk mastering, stereolithography, laser therapy, and DNA analysis. An all-solid-state ultraviolet laser light source that combines a solid-state laser and a nonlinear optical crystal with features such as small size, easy operation, low maintenance cost, and long life has been attracting attention. More generates ultraviolet laser light having a high output, high beam quality at high power, is a fundamental wave light source of so-called high intensity, it requires high-quality non-linear optical crystal for converting an incident light with high efficiency The
[0004]
However, conventional nonlinear optical crystals absorb wavelength-converted ultraviolet light when high-intensity laser light is incident, and the crystal temperature near the ultraviolet light exit rises (referred to as self-heating), resulting in a temperature distribution in the crystal. There is a problem that the matching condition is shifted and the wavelength conversion efficiency is lowered. Measures such as crystal cooling by blowing nitrogen gas have been taken against this phenomenon, but no significant improvement in wavelength conversion efficiency has been observed.
[0005]
Therefore, the invention of this application was made in view of the circumstances as described above, solved the problems of the prior art, useful as an all-solid-state ultraviolet laser light source, etc., and made high-power laser light incident. Even in such a case, it is possible to provide a novel nonlinear optical crystal capable of wavelength conversion with high efficiency and excellent output stability, and a method for manufacturing the same, and a wavelength conversion element using the nonlinear optical crystal. -It is an object to provide an apparatus and an all-solid-state laser light source apparatus.
[0006]
[Means for Solving the Problems]
Therefore, the invention of this application provides the following invention as a solution to the above-mentioned problems.
[0007]
That is, first of all, the invention of this application is a mixed crystal nonlinear optical crystal, in which the composition of the crystal corresponds to the temperature distribution in the crystal caused by incident light absorption and satisfies the phase matching condition. Provided is a nonlinear optical crystal characterized by being changed.
[0008]
And in the second aspect of the present invention, in the second aspect of the first invention, the nonlinear optical crystal has a general composition formula:
M _X Y _1-X Ca ₄ O (BO ₃ ) ₃
(M in the formula represents a rare earth element, and 0 <X <1)
And thirdly, the rare earth element is gadolinium (Gd), lanthanum (La), or lutesium (Lu). A nonlinear optical crystal is provided.
[0009]
Furthermore, the invention of this application is fourthly the method of manufacturing a nonlinear optical crystal according to the above invention, wherein the crystal composition is changed by controlling the crystal growth conditions when growing the nonlinear optical crystal of mixed crystal. A method for manufacturing a nonlinear optical crystal characterized by the following: fifth, a method for manufacturing a nonlinear optical crystal characterized by changing the crystal composition by controlling the crystal growth rate; and sixth, a crystal growth Provided is a method for producing a nonlinear optical crystal, characterized in that the crystal composition is changed by controlling the temperature.
[0010]
The invention of this application is, in the seventh aspect, a wavelength conversion element using the above-mentioned nonlinear optical crystal, an eighth, a wavelength conversion apparatus incorporating this element, and a ninth, a solid-state laser apparatus. A combined all-solid-state laser light source device is also provided.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The invention of this application has the features as described above, and an embodiment thereof will be described below.
[0012]
First, the nonlinear optical crystal provided by the invention of this application is a mixed-crystal nonlinear optical crystal, the composition of the crystal having a composition corresponding to the temperature distribution in the crystal caused by incident light absorption, and the phase matching condition It is characterized by being changed in the direction to satisfy. In the case of a known nonlinear optical crystal, the phase matching condition is made constant by making the composition as uniform as possible. Therefore, a mixed crystal in which the composition ratio of crystal components easily changes due to a slight change in the growth condition is used as a nonlinear optical crystal. When used as a crystal, a crystal having a uniform composition is grown, for example, by increasing the growth rate as much as possible so as not to change the composition. On the other hand, in the case of the invention of this application, attention is paid to the fact that the mixed crystal can change the composition of the crystal component depending on the growth condition, and the deviation of the phase matching condition due to the self-heating of the crystal is compensated by changing the composition of the crystal. Thus, a decrease in wavelength conversion efficiency is prevented. A nonlinear optical crystal whose composition is not uniform and has been intentionally continuously changed has never been considered before, and is proposed for the first time by the invention of this application.
[0013]
As the nonlinear optical crystal that is a mixed crystal, for example, M _X Y _1-X Ca ₄ O (BO ₃ ) ₃ (wherein M represents a rare earth element and 0 <X <1) is considered. Can do. In the composition formula represented by the above-mentioned M _X Y _1-X Ca ₄ O (BO ₃ ) ₃ , M: the rare earth element is Gd _X (Gd), lanthanum (La), lutesium (Lu), etc. Y _1-X Ca ₄ O (BO ₃ ) ₃ : G d YCOB, La _X Y _1-X Ca ₄ O (BO ₃ ) ₃ : LaYCOB, Lu _X Y _1-X Ca ₄ O (BO ₃ ) ₃ : LuYCOB Etc. are considered as preferred. Among them, GdYCOB is useful, has the same degree of hardness as glass, has no deliquescence, and has little thermal expansion anisotropy. For example, it is cut and polished compared to other borate materials. Since it is extremely easy to handle as an element, it can be shown as a preferred example. These nonlinear optical crystals, for example, GdYCOB, are mixed crystals composed of GdCa ₄ O (BO ₃ ) ₃ : GdCOB and YCa ₄ O (BO ₃ ) ₃ : YCOB.
[0014]
In the composition as described above, the coefficient X depends on at least one of the wavelength and the temperature, and therefore differs depending on the applied laser beam.
For example, in the GdYCOB, when used to generate a wavelength of 360 to 420 mm / room temperature (this value also varies slightly depending on the temperature), which is the second harmonic of a Ti: sapphire laser, the coefficient X is the wavelength and temperature. Therefore, the range is 0 <X <1. On the other hand, in the case of the third harmonic generation of the Nd: YAG laser, the coefficient X depends on the temperature and is about 0.20 <X <0.30.
[0015]
In the invention of this application, the composition of the nonlinear optical crystal has a composition corresponding to the temperature distribution of the crystal generated by absorption of incident light such as a laser, and changes in a direction satisfying the phase matching condition. More specifically, FIG. 1A schematically shows the temperature distribution due to incident light absorption of the nonlinear optical crystal. In FIG. 1A, for example, when laser light or the like is incident from the end face of the nonlinear optical crystal (1), the incident light (2) is wavelength-converted to become ultraviolet light (3) and is emitted from the opposite surface. When the intensity of the incident light (2) is strong, the nonlinear optical crystal (1) absorbs the wavelength-converted ultraviolet light (3) and self-heats, so that the temperature of the incident optical path (4) in the crystal (1) is increased. Rises. In addition, the incident optical path portion (4) becomes higher in temperature toward the emission end face side where the passing amount of the ultraviolet light (3) increases. The temperature around the incident light path (4) of the crystal also rises due to heat transfer. Here, since the phase matching condition of the temperature rising portion (5) of the crystal changes in accordance with the rising temperature, it deviates from the original phase matching condition.
[0016]
FIG. 1B illustrates the temperature distribution of the incident optical path portion (4) as a temperature difference from the incident end face. The phase matching condition of the incident optical path portion (4) is deviated from the original phase matching condition in the same manner as this temperature distribution. Therefore, in the invention of this application, the composition of the nonlinear optical crystal is changed so as to have a composition corresponding to the temperature distribution by self-heating and satisfy the phase matching condition. For example, if the components constituting the mixed crystal are A and B, the composition change is relatively similar to the component A along the temperature gradient (distribution), that is, the same or almost the same as the form. It is realized by changing one concentration of B and B from “high to low” and changing the other concentration from “low to high”. This component distribution can be determined by the birefringence of the mixed crystal when components A and B are contained. As the temperature distribution becomes higher, the concentration of the component having the lower birefringence is increased from “high”. By changing the concentration of the component having the higher birefringence to “low” and “from low to high”, the phase matching condition can be satisfied.
[0017]
More specifically, in the MYCOB crystal (M: rare earth element), for example, the case where the rare earth element (M) is La, Gd, or Lu is considered. The relationship between the birefringence of the MCOB crystal containing these rare earth elements (M): La, Gd, and Lu and the YCOB crystal containing Y is higher in the order of La <Gd <Y <Lu depending on the contained elements. Become. Therefore, when producing a mixed crystal of LaYCOB or GdYCOB, the La concentration or Gd concentration is changed from “high to low” as the crystal temperature distribution increases, corresponding to FIGS. 1A and 1B. Can be changed from “low to high”. On the other hand, when producing a mixed crystal of LuYCOB, it is only necessary to change the Y concentration from “high to low” and the Lu concentration from “low to high” as the temperature distribution of the crystal increases.
[0018]
The range in which the composition is changed is a range including the incident optical path portion in the crystal. For example, when considering only the Gd concentration distribution in the GdYCOB crystal, the Gd concentration distribution and the temperature distribution of the entire crystal may be the same distribution as shown in FIG. 2A, or as shown in FIG. The Gd concentration distribution and the temperature distribution of the incident optical path in the crystal may be the same distribution. However, in the case of (b), it is necessary to provide a concentration distribution so that the equal refractive index surface formed by connecting the same refractive index in the mixed crystal and the phase matching direction (incident optical path) are orthogonal to each other. It is. If the equal refractive index surface is inclined with respect to the phase matching direction (incident optical path), the laser is bent. Incidentally, a means for changing the Gd concentration distribution of the entire crystal as shown in FIG. 2A is appropriate.
[0019]
In determining the coefficient X in the composition change as described above, first, the temperature distribution generated with respect to the output can be estimated, and then the composition corresponding to the temperature gradient (distribution), that is, the range of the coefficient X can be determined. . Then, the growth conditions are adjusted so that the crystal has a desired composition distribution. In that case, the type and size of the nonlinear optical crystal, the type and intensity of the laser used, and the like are taken into consideration. By changing the composition of the nonlinear optical crystal in this way, for example, the birefringence of the GdYCOB crystal can be changed between the birefringence of GdCOB 0.033 and the birefringence 0.041 of YCOB at a wavelength of 1064 nm. The phase matching condition can also be controlled.
[0020]
Thereby, even if the nonlinear optical crystal is self-heated, a nonlinear optical crystal capable of wavelength conversion with high efficiency is realized without shifting the phase matching condition in the crystal.
The nonlinear optical crystal of the invention of this application can be manufactured by controlling the growth conditions and changing the crystal composition during the crystal growth. Non-linear optical crystal growth methods include various known crystal production methods such as the Cz method (Czochralski method), the flux method, and the top seeded solution growth (TSSG) method. The method can be used. Examples of the conditions to be controlled include a growth speed (pulling speed), a growth temperature, a pulling rotation speed, and the like. These conditions may be controlled in combination of two or more.
[0021]
For example, when a GdYCOB crystal is grown from a melt by the Cz method, the composition ratio of Gd and Y changes even if the growth rate is constant, and the rate of change varies depending on the growth rate. Of course, changing the growth rate during crystal growth also changes the crystal composition ratio. This depends on the difference in segregation coefficient between Gd ³⁺ ions (ion radius 0.938Å) and Y ³⁺ ions (ion radius 0.900Å) due to the difference in ion radius. Also, regarding the growth temperature during crystal growth, the growth rate and the segregation coefficient depend on the temperature, so that the composition ratio of Gd and Y of the GdYCOB crystal also changes by changing this. This depends on the difference in growth temperature between GdCOB and YCOB. Therefore, by appropriately controlling the growth conditions such as the growth rate (pulling rate) and the growth temperature, it is possible to obtain a GdYCOB crystal in which the composition ratio of Gd and Y is changed with a desired gradient.
[0022]
Many physical properties of the mixed crystal type nonlinear optical crystal continuously change depending on the composition. Therefore, by controlling various physical property parameter values that affect the phase matching conditions, desired phase matching conditions are maintained.
[0023]
Then, an Example is shown below and embodiment of this invention is described in more detail.
[0024]
【Example】
<A> GdYCOB crystal was grown from the melt by induction heating type Cz method. There are three growth conditions: (a) 1 mm / h, 10 rpm, (b) 2 mm / h, 15 rpm, (c) 4 mm / h, 10 rpm. a) (b) (c) could be obtained.
[0025]
The compositions in these crystals were analyzed, and the results are shown in FIG. FIG. 3 shows the relationship between the growth length from the seed crystal and the Gd concentration. The crystal (a) was found to have a large change in Gd concentration from approximately 25.9% to 28.5% in the length direction of 120 mm. The Gd concentration of the crystal (c) was about 26%, and the variation in composition was almost uniform at 0.5% or less. The composition variation of the crystal (b) was between the crystal (a) and the crystal (c), but the Gd concentration at the initial stage of growth was lower than that of the crystal (a) (c). Thus, it was shown that the composition of the crystal can be changed variously by controlling the crystal pulling speed and the rotation speed.
<B> From the temperature-phase matching characteristics and composition-phase matching characteristics of the GdYCOB crystal, in the case of the third harmonic generation of the Nd: YAG laser, the coefficient X of Gd _X Y _1-X Ca ₄ O (BO ₃ ) ₃ Depends on the temperature distribution and was estimated in the range of about 0.20 <X <0.28. From this, the GdYCOB crystal (a) grown in the above <A> is cut to 15 mm and polished, the fundamental wave (Nd: YAG laser ) is incident from the polished surface, and the maximum average output of the generated ultraviolet laser is obtained. It was measured. The output of the fundamental wave was changed from 3.5 to 10 W, and the output of the ultraviolet laser was 1/3 wavelength (355 nm).
[0026]
FIG. 4 shows the measurement result of the maximum average output of the ultraviolet laser . As shown in FIG. 2, when the composition gradient and the temperature distribution tendency are made coincident and the light is incident from the polishing surface having the higher Gd concentration, the output is significantly higher than the input as shown in FIG. It was confirmed to rise.
[0027]
On the other hand, when the incident / exit direction was reversed and the light was incident from the polished surface having the lower Gd concentration, the output characteristics as shown in FIG. However, because of the deviation from the phase matching condition due to self-heating, the output decreased as shown in (a ′). Then, since the degree of self-heating is reduced, the characteristic of (b) is obtained, but after that, it moved between (b) and (a ′).
[0028]
Thus, if the composition gradient and the direction of temperature distribution due to self-heating of the crystal are matched, ultraviolet light can be generated efficiently, and if this correspondence is reversed, not only the efficiency decreases, but also the laser It was confirmed that the output became unstable.
[0029]
From the above, it is possible to convert the wavelength of the crystal with high efficiency by using a nonlinear optical crystal whose composition is changed in a direction that satisfies the phase matching condition corresponding to the temperature distribution in the crystal caused by absorption of incident light. It has been shown. Also, the output is stable.
[0030]
Of course, the present invention is not limited to the above examples, and it goes without saying that various aspects are possible in detail.
[0031]
【The invention's effect】
As described above in detail, the present invention is useful as an all-solid-state ultraviolet laser light source and the like, and is capable of wavelength conversion with high efficiency even when high-power laser light is incident, and can provide a stable output. A nonlinear optical crystal and a method for manufacturing the same are provided. The nonlinear optical crystal also provides an all-solid-state ultraviolet laser light source device by combining a wavelength conversion element / device with excellent characteristics and a solid-state laser.
[Brief description of the drawings]
FIG. 1A is a schematic diagram illustrating a state of self-heating by absorption of incident light of a nonlinear optical crystal, and FIG. 1B is a diagram illustrating a temperature distribution of the incident optical path portion.
FIGS. 2A and 2B are conceptual diagrams illustrating Gd concentration distribution in a GdYCOB crystal in the invention of this application.
FIG. 3 is a diagram illustrating the relationship between the growth length of GdYCOB crystals (a), (b), and (c) obtained in Examples and the Gd concentration.
FIG. 4 is a diagram illustrating measurement results of the maximum average output of an ultraviolet laser generated when an Nd: YAG laser is incident on a GdYCOB crystal.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Nonlinear optical crystal 2 Incident light 3 Ultraviolet light 4 Incident optical path 5 Temperature rise part

Claims (9)

A nonlinear optical crystal which is a mixed crystal nonlinear optical crystal, wherein the composition of the crystal is changed in a direction corresponding to a temperature distribution in the crystal caused by incident light absorption and satisfying a phase matching condition. Nonlinear optical crystals are expressed as a general composition formula:
M _X Y _1-X Ca ₄ O (BO ₃ ) ₃
(In the formula, M represents a rare earth element, and 0 <X <1.)
The nonlinear optical crystal according to claim 1, wherein 3. The nonlinear optical crystal according to claim 2, wherein the rare earth element is gadolinium (Gd), lanthanum (La), or lutesium (Lu). 4. The method for producing a nonlinear optical crystal according to claim 1, wherein the crystal composition is changed by controlling crystal growth conditions when growing a nonlinear optical crystal of mixed crystal. Manufacturing method. The method for producing a nonlinear optical crystal according to claim 4, wherein the crystal composition is changed by controlling the crystal growth rate. 5. The method for producing a nonlinear optical crystal according to claim 4, wherein the crystal composition is changed by controlling the crystal growth temperature. 4. A wavelength conversion element, wherein the nonlinear optical crystal according to claim 1 is a laser light wavelength conversion element. A wavelength conversion device comprising the wavelength conversion element according to claim 7 incorporated therein. An all-solid-state laser light source device, wherein the nonlinear optical crystal according to any one of claims 1 to 3 or the wavelength conversion element according to claim 7 is combined with a solid-state laser device or a solid-state laser device.

Images