JP2000199881A - Electro-optic crystal of laser apparatus and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electro-optic crystal of a laser which is stable in transmitted beam characteristics, such as profile and transmission wave surface, of the laser beam for a long time and a process for producing the same. SOLUTION: The electro-optic crystal 13 of the laser apparatus having an incident optical system 12 and exit optical system 16 for the laser beam and voltage impressing means 18 and 19 is the electro-optic crystal of the laser apparatus consisting of a single crystal of which the essential components consist of lithium tantalate(LiTaO3) or lithium niobate(LiNbO3) and which contains magnesium oxide(MgO) as a trace component. The process for production has a stage for allowing inert gas to flow into a crucible and growing the single crystal while removing oxygen from the atmosphere in the crucible in a stage for growing the single crystal by housing raw material for the crystal into the crucible, melting the same and pulling the crystal from the crucible by using a seed crystal. The process otherwise has a stage for executing an annealing treatment at least at >=1 times while impressing voltage to the single crystal pulled up from the crucible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electro-optic crystal (EO crystal) of a laser device including a frequency modulation device of a dye laser beam and a method of manufacturing the same.

[0002]

2. Description of the Related Art For example, in a dye laser device, frequency modulation may be performed by a laser light frequency modulator provided between a laser oscillator and a dye laser amplifier.
In order to perform frequency modulation of laser light by the EO (electro-optic) effect, laser light is incident on an EO crystal exhibiting the electro-optic effect using an optical system for guiding the laser light. Simultaneously, an electric field for modulation is applied to the EO crystal by two electrodes provided with the EO crystal interposed therebetween, so that the laser light frequency modulation is applied to the incident laser light to obtain an emission laser light having an expanded spectrum width.

The EO crystal used in such a laser device is grown by using a single crystal pulling method generally called a Czochralski method. In this method, a single crystal is grown by placing a raw material for a crystal in a crucible, melting it by heating, lowering a seed crystal in the raw material, and pulling up the molten raw material.

[0004]

When a voltage is applied to such a conventionally known EO crystal and a laser beam is applied to the EO crystal, the transmitted beam of the laser beam in the EO crystal changes, Irradiation with intense laser light may disturb the beam profile or wavefront. This is presumed to be a phenomenon caused by the photorefractive effect.

[0005] For this reason, for example, a method in which an argon (Ar) gas and an oxygen (O ₂ ) gas are flown together in a crucible, and a raw material containing 10 ppm for each element.
Alternatively, a method using an impurity containing about 1 ppm or more has been proposed, but a sufficient effect has not been obtained.

Further, a method of performing an annealing treatment in the air to remove a crystal growth distortion has been proposed.
However, this also could not sufficiently suppress the phenomenon caused by the photorefractive effect.

[0007] When oxygen is not mixed in the atmosphere during the growth, the crystal is blackened. However, the above-mentioned annealing treatment cannot sufficiently make the crystal transparent, and the light is irradiated in a relatively short time. In some cases, the EO crystal had a low transmittance.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an electro-optic crystal of a laser device having a stable transmission beam characteristic such as a laser beam profile and a transmission wavefront for a long time, and a method of manufacturing the same. Aim.

[0009]

According to the present invention, a main component of an electro-optic crystal of a laser device provided with a laser beam incidence optical system, an emission optical system, and a voltage applying means is lithium tantalate or lithium niobate, It is an electro-optic crystal of a laser device made of a single crystal containing magnesium oxide as a trace component.

Further, the present invention relates to a crucible in which a raw material containing lithium tantalate or lithium niobate as a main component for an electro-optical crystal or magnesium oxide as a trace component thereof is contained in a crucible, melted, and seeded. A laser device having a step of growing a single crystal by pulling up from a crucible using a crystal, and flowing an inert gas into the crucible at the time of pulling up and growing the single crystal while removing oxygen from the atmosphere in the crucible. Is a method for producing an electro-optic crystal.

According to still another related invention, a single crystal raw material containing lithium tantalate or lithium niobate as a main component or containing magnesium oxide as a trace component is contained in a crucible, and the single crystal raw material is melted. Let
Production of an electro-optic crystal for a laser device including a step of growing a single crystal by pulling up from a crucible using a seed crystal and a step of performing annealing at least once while applying a voltage to the single crystal pulled up from the crucible Is the way.

[0012]

1 and 2 show an example of a dye laser (DL) apparatus using an EO crystal according to the present invention. FIG. 1 is a block diagram illustrating a configuration of a laser device according to the present invention. As shown in FIG. 1, this laser device includes:
Dye laser (DL) oscillator 11, EO incident optical system 12 for guiding laser light from this DL oscillator to electro-optical EO crystal, EO crystal and laser into which laser light from this EO incident optical system enters A laser light modulator 15 having an EO driver 14 for applying a voltage to the EO crystal at the time of light incidence to perform frequency modulation of the laser light, and an EO emission optical system 16 for guiding the laser light emitted from the EO crystal;
A dye laser (DL) amplifier 17 for amplifying the laser light from the EO emission optical system;

The laser light modulator 15 including the EO crystal
As shown in FIG. 2, on the ground-side lower electrode 18 which is a part of the EO driver and also serves as the EO crystal holding table,
An EO crystal 13 is provided. A silver (Ag) electrode 19 on the high voltage side is formed on the upper surface of the EO crystal 13 by sintering, and an adhesive metal tape 20 for supplying a voltage is affixed thereto. As a result, a modulation voltage is applied to the EO crystal 13 from the pair of electrodes 18 and 19 sandwiching the EO crystal 13.

Further, on the side surface 13a of the EO crystal 13,
An EO incidence optical system 12 for receiving a laser beam from a DL oscillator and an EO emission optical system 16 are optically connected. Laser light modulation was performed by such a laser device, and the emitted light was observed. The EO crystal of the present invention has good transmitted beam characteristics of laser light, and almost no deterioration of the characteristics was observed even after continuous use for a long time.

The inventor of the present invention has made intensive studies to reduce the adverse effect of the electro-optic crystal due to the photorefractive effect. It is assumed that the change in the long-term laser beam characteristics due to the photorefractive effect is correlated with, for example, the amount of undesired metal-based impurity ions contained in the EO crystal and the amount of oxygen defects and the amount of scatterers in the crystal. Is done.

EO in which the above physical quantities are not negligible
When a strong laser beam is applied to the crystal, impurity ions move and are unevenly distributed due to the electric field distribution of the laser beam. This produces a refractive index distribution in the EO crystal. When a laser beam is incident on the EO crystal having a refractive index distribution, the intensity of the laser beam locally increases or decreases, thereby accelerating the generation of the refractive index distribution. It is also considered that oxygen defects and scatterers in the crystal tend to accelerate the generation of the refractive index distribution. It is considered that these cause deterioration of the transmitted beam characteristics such as diffusion of the transmitted beam or a change in the beam profile.

In order to suppress the occurrence of these phenomena, the inventor first attempted to suppress unnecessary oxygen vacancies by mixing no or almost no oxygen in the atmosphere for growing the EO crystal. .

The EO crystal of the laser device of the present invention is a single crystal whose main component is lithium tantalate or lithium niobate, or which contains magnesium oxide as a trace component. The general formula of lithium tantalate is LiTaO
₃ , the general formula of lithium niobate is LiNbO ₃ , and the general formula of magnesium oxide is MgO. Hereinafter, description will be made mainly using such a general formula.

The EO crystal, which is the main part of the present invention, is a single crystal whose main component is LiTaO ₃ or LiNbO ₃ , or which contains MgO as a trace component. The single crystal raw material is accommodated in a crucible, the single crystal raw material is melted, and the seed crystal is pulled up from the crucible to grow a single crystal, and an inert gas is introduced into the crucible at the time of pulling. The single crystal is grown while flowing and removing oxygen from the atmosphere in the crucible. Alternatively, a step of performing annealing at least once or more while applying a voltage to the single crystal pulled from the crucible is performed.

Example 1 An electro-optic crystal according to the present invention was obtained using the following raw materials under the following conditions. Raw materials: (1) Lithium carbonate (Li ₂ CO ₃ ) Purity is 99.99% or more (impurity content specification is at most 0.2 to 2 ppm for one element). (2) Tantalum oxide (Ta ₂ O ₅ ) purity: 99.99% or more (impurity content specification is 1 at the maximum)
1 to 5 ppm or less for the element).

The molar ratio of Li ₂ CO ₃ to Ta ₂ O ₅ is about 1: 1.

Crystal growth conditions: (a) The growth atmosphere is an argon (Ar) gas flow (0% oxygen).

(B) The annealing atmosphere is an oxygen (O ₂ ) gas flow.

(C) The number of times of annealing is 1200 ° C., 30 hours × 2 times.

Thus, Li_TwoCO_Three + Ta_Two
O_Five → LiTaO_Three + CO _Two With LiT
aO_ThreeIs obtained.

The EO crystal obtained in Example 1 was applied to the laser apparatus shown in FIGS. 1 and 2, and laser light modulation was performed. First, light polarized in the electrode direction is incident on the EO crystal. At this time, the wavelength (frequency) of the incident laser light is modulated by the Pockels effect caused by applying a modulation voltage to the crystal. Then, the laser light emitted from the EO crystal is guided to the DL amplifier.

In this example, the laser light transmitted through the EO crystal was guided to the amplifier satisfactorily with respect to the beam profile and the transmitted wavefront regardless of whether or not a voltage was applied, and was output. Also,
Even after long-time continuous operation, almost no refractive index distribution or diffraction grating was generated in the EO crystal, and almost no change or deterioration in transmitted beam characteristics was recognized.

Example 2 Tantalum oxide (Ta ₂ O) as a raw material in Example 1 was used.
Using niobium oxide (Nb ₂ O ₅₎ instead of _5). Its purity is also 99.99% or more (impurity content specification is 1 to 5 ppm or less per element at maximum). In addition,
The molar ratio of Li ₂ CO ₃ to Nb ₂ O ₅ is about 1: 1. Others were manufactured under the same conditions as in Example 1.

According to the laser modulator using the EO crystal obtained in Example 2, the same good characteristics as described above were obtained.

Example 3 The raw materials were the same as in Example 1, and the conditions for growing the crystal were as follows.

(A) The growth atmosphere is an argon (Ar) gas flow (0% oxygen).

(B) The annealing atmosphere is an oxygen (O ₂ ) gas flow, during which a voltage is applied to the crystal (about 5 V / c).
m).

(C) The number of times of annealing is 30 hours × 2 times.

(D) The polling condition is that the electric field strength is 5 V /
cm, current density is 30μA / cm ^Two.

According to the laser modulator using the EO crystal obtained in the third embodiment, the laser beam transmitted through the crystal has a good beam profile and a transmitted wavefront regardless of whether a voltage is applied or not, as described above. The light was guided to the amplifier and output. Further, even if the device was operated continuously for a long period of time, no refractive index distribution or diffraction grating was generated in the crystal, and almost no change or deterioration of the transmitted beam characteristics was recognized.

Example 4 Tantalum oxide (Ta ₂ O) as a raw material in Example 3 was used.
Using niobium oxide (Nb ₂ O ₅₎ instead of _5). Its purity is also 99.99% or more (impurity content specification is 1 to 5 ppm or less per element at maximum). Others were manufactured under the same conditions as in Example 3. According to the laser modulator using the EO crystal obtained in Example 4, the same good characteristics as described above were obtained.

Example 5 The raw material in Example 1 further contained about 4 mol% of magnesium oxide (MgO). The purity of MgO is 99.
It is 99% or more (impurity content specification is 1 to 5 ppm or less for one element at the maximum). Then, an EO crystal was grown under the conditions of Example 1. According to the laser modulator using the EO crystal thus obtained, the same good characteristics as described above were obtained.

Other Examples Approximately 0.1 to 10 mol% of magnesium oxide (MgO) was added to the raw materials shown in Examples 2 to 4, and EO crystals were grown under the same production conditions as in each Example. . According to the laser modulator using the EO crystal obtained thereby, the same good characteristics as described above were obtained.

As described above, the EO crystal of the laser device of the present invention is contained in a crucible containing a single crystal material of LiTaO ₃ or LiNbO ₃ or containing a small amount of MgO and melting these single crystal materials. In a method for growing a single crystal having a step of growing a single crystal from a crucible using a seed crystal, an inert gas is flowed into the crucible at the time of pulling, and oxygen is removed from the atmosphere in the crucible to remove the single crystal. Develop.

It is preferable that the method further includes a step of annealing the single crystal pulled from the crucible at least once in an oxygen atmosphere. Thereby, even if oxygen vacancies occur even when growing a single crystal while removing oxygen from the atmosphere in the crucible, it can be compensated by annealing in an oxygen atmosphere.

The annealing step is preferably performed, for example, in the case of LiTaO ₃ at 1000 ° C. to 1300 ° C. for 10 to 30 minutes. Also, the annealing step is preferably 1 to
Performed three times. In the case of LiNbO ₃ , it is preferably performed at 1200 ° C. to 1250 ° C. for 10 to 30 minutes. The annealing step is preferably performed 1 to 3 times.

The inert gas has a purity of 99.9999.
% Or higher. Furthermore, it is preferable that the inert gas is an argon gas. Further, it is preferable that the inert gas is applied at a flow rate capable of sufficiently removing oxygen from at least the atmosphere in the crucible.

Further, the raw material for the single crystal composed of an oxide has a content of impurities, preferably 1 for each element.
0 ppm or less, more preferably 1 ppm or less. This makes it possible to reduce the physical quantity considered to have the above-mentioned correlation, use a high-purity raw material, and reduce the amount of metal-based impurity ions contained in the crystal.

Thus, according to the present invention, the photorefractive effect and the generation of the diffraction grating in the EO crystal are suppressed,
Short or long time transmission beam characteristics can be improved.

Further, the single crystal raw material is accommodated in a crucible, the single crystal raw material is melted, pulled up from the crucible using a seed crystal to grow a single crystal, and a voltage is applied to the obtained single crystal. The anneal process is performed at least once or more.

In this annealing step, for example, LiTaO ₃
In the case of the above, preferably 10 to 3 at 1000 to 1300C
Performed for 0 minutes. In the case of LiNbO ₃ , preferably 1
The reaction is performed at 200 ° C. to 1250 ° C. for 10 to 30 minutes. The annealing step is preferably performed 1 to 3 times.
At this time, it is desirable that the voltage applied to the single crystal has an electric field strength of preferably 5 V / cm or more. Note that this annealing step is preferably performed in an oxygen atmosphere.

After the annealing step, the electric field strength is 5 V / cm or more and the current density is 10 μA / cm ^2.
It is desirable to perform a poling process for polarizing the crystal under the above conditions.

Further, a voltage may be applied during annealing by raising the temperature to around the melting point of the single crystal, thereby smoothing the electron distribution and ion distribution in the crystal and suppressing the generation of defects. Can be. Note that oxygen defects and the like which still occur can be prevented by annealing in an oxygen atmosphere.

Further, the single crystal before poling, for example, Li
In TaO ₃ , domains in the order of wavelength are distributed in the crystal and serve as scatterers. However, by increasing the applied electric field strength and current density during poling, the
The scatterers can be further reduced by removing the minute domains.

As described above, the occurrence of the photorefractive effect can be suppressed, the transmission beam characteristics of the crystal can be improved, and continuous use for a long time can be made possible.

[0051] Magnesium oxide is preferably contained in the single crystal in an amount of 0.1 to 10 mol%. The magnesium oxide (MgO) used has a purity of 99.9999.
% Or higher. by this,
MgO doping of the crystal has the effect of increasing the photoconductivity in the crystal and relaxing or compensating for the internal electric field generated locally by light irradiation. Therefore, the occurrence of the photorefractive effect can be suppressed, and the short-time and long-time transmitted beam characteristics can be improved. In addition, it is also possible to manufacture by combining various kinds of annealing treatment and poling treatment.

The electro-optical crystal (EO crystal) of such an embodiment is made of a material for a SAW device, a high-speed modulator,
The present invention can also be applied to an apparatus such as an optical switch or a waveguide type SHG device of a blue laser.

[0053]

According to the present invention, there is provided a laser device which has good transmitted beam characteristics such as a laser beam profile and a transmitted wavefront, and hardly deteriorates these characteristics even when used continuously for a long time. can get.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an example of a laser system according to the present invention.

FIG. 2 is a perspective view showing a part of a laser light modulation device according to the present invention.

[Explanation of symbols]

 13: EO crystal, 12: incidence optical system, 16: emission optical system, 19: electrode for voltage application.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H079 AA02 AA12 DA03 EA11 EB04 HA25 4G077 AA02 AB01 BC32 BC37 CF10 EA06 EB01 FE03 FH02 HA02 PA06

Claims (11)

[Claims] 1. An electro-optic crystal of a laser device comprising a laser beam incident optical system, an output optical system, and a voltage applying means, wherein the electro-optic crystal is mainly composed of lithium tantalate or lithium niobate, and a trace amount thereof. An electro-optic crystal for a laser device, comprising a single crystal containing magnesium oxide as a component. 2. The lithium tantalate or lithium niobate has a purity of 99.99% or more, and the magnesium oxide has a purity of 99.9999% or more.
An electro-optic crystal for the laser device according to claim 1. 3. The electro-optic crystal according to claim 1, wherein the magnesium oxide is contained in the single crystal in an amount of 0.1 to 10 mol%. 4. A method for manufacturing an electro-optic crystal of a laser device comprising a laser light incident optical system, an output optical system, and a voltage applying means, wherein lithium tantalate or lithium niobate is used as a main component for the electro-optic crystal. Or a step of accommodating a raw material containing a small amount of magnesium oxide in a crucible, melting the raw material, pulling the raw material from the crucible using a seed crystal to grow a single crystal, and inactive into the crucible at the time of pulling. A method for producing an electro-optic crystal for a laser device, comprising growing a single crystal while removing oxygen from the atmosphere in the crucible by flowing a gas. 5. The inert gas has a purity of 99.999.
The method for producing an electro-optic crystal of a laser device according to claim 4, wherein the content is 9% or more. 6. The method according to claim 4, further comprising the step of annealing the single crystal pulled from the crucible at least once in an oxygen atmosphere. 7. The method of manufacturing an electro-optic crystal for a laser device according to claim 4, wherein said raw material has an impurity content of 10 ppm or less for each element. 8. The method of manufacturing an electro-optic crystal for a laser device according to claim 4, wherein said raw material has an impurity content of 1 ppm or less for each element. 9. A method for manufacturing an electro-optic crystal of a laser device comprising a laser beam incident optical system, a laser beam emitting optical system, and a voltage applying means, wherein lithium tantalate or lithium niobate is used as a main component for the electro-optic crystal. Alternatively, a raw material containing a small amount of magnesium oxide is stored in a crucible, the raw material is melted, pulled up from the crucible using a seed crystal to grow a single crystal, and a voltage is applied to the single crystal pulled up from the crucible. A method for performing an annealing process at least once or more while producing an electro-optic crystal for a laser device. 10. The method according to claim 9, wherein the annealing is performed in an oxygen atmosphere. 11. After the step of performing the annealing treatment,
Electric field strength of 5 V / cm or more and current density of 10 μA / cm
10. The method of manufacturing an electro-optic crystal for a laser device according to claim 9, further comprising a step of performing a poling process for polarizing the crystal under ^two or more conditions.