JP2001109027A - Method of preventing decrease in transmittance due to coloring of nonlinear optical crystal for wavelength conversion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily prevent such a phenomenon that a nonlinear optical crystal used for wavelength conversion of a solid laser light is colored by the laser light and its transmittance decreases with time. SOLUTION: The nonlinear optical crystal used for wavelength conversion of a solid laser light is annealed in an environment containing water to permeate the water content into the crystal. Thus, decrease in the transmittance due to coloring of the crystal by repeated use can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser device used in the field of material processing and the like, and it is intended to prevent a decrease in transmittance due to coloring of a nonlinear optical crystal used for wavelength conversion of solid-state laser light. Accordingly, the present invention relates to a technique for annealing a nonlinear optical crystal for wavelength conversion, which enables a longer life of the nonlinear optical crystal.

[0002]

2. Description of the Related Art In recent years, with the increase in the output of semiconductor lasers, semiconductor laser pumped solid-state lasers (Diodes)
Pumped Solid State Laser,
A laser device using DPSSL) can be made more efficient and smaller, and a solid-state laser has been widely used in place of a conventional gas laser in various laser application fields such as material processing. However, most solid-state lasers have an oscillation wavelength in the near infrared region. Therefore, in order to replace a conventional excimer laser or the like with a solid-state laser, the output wavelength of the solid-state laser is shortened by wavelength conversion to increase the absorptance to the workpiece, or to increase the brightness by reducing the focusing size. There is a need.

Heretofore, most of the above-mentioned wavelength conversion of solid-state laser light has been performed using a so-called non-linear optical crystal. Specifically, one side of the solid-state laser light beam has approximately one side. A cubic nonlinear optical element (crystal) of about 5 to 10 mm is installed, and a harmonic of the input laser light is generated by the so-called nonlinear susceptibility of the nonlinear optical crystal, which is reduced to 1/2 to 1/5 of the fundamental wavelength. It outputs a harmonic laser light.

The nonlinear optical crystal used for the wavelength conversion has a large nonlinear optical constant and high conversion efficiency of laser light, and has a wide phase matching angle and a wide range of phase matching temperature, so that the conversion efficiency of laser light is stable. It is necessary to have various requirements such as that it can withstand long-term use, and that it has high chemical stability, high mechanical strength, little absorption of fundamental and harmonics, Requirements such as high damage resistance to the laser are required.

However, it is almost impossible at the present time to obtain a nonlinear optical crystal having all of these requirements. Therefore, in actual wavelength conversion using a nonlinear optical crystal, the crystal is Various inconveniences have occurred due to exposure to intense laser light. Among them,
In particular, a problem that needs to be urgently solved is a problem of a decrease in the transmittance of laser light due to coloring of crystals.

That is, the decrease in the transmittance is caused by the fact that the crystal itself is exposed to intense laser light and is colored according to the use time, and a color center (color center) existing inside the crystal is used. Is said to be caused by absorbing laser light. Also,
This phenomenon is conspicuous when using short-wavelength laser light or harmonic light, but generally used Y light is used.
KTP crystal (KTiOPO ₄ ) used for generation of second harmonic of AG laser (Yttrium Aluminum Garnet Laser)
Crystal), which is called gray tracking.

By the way, in order to avoid a decrease in the transmittance due to the above-mentioned coloring, the position of the crystal is moved with the use time so that the influence of the defect does not directly appear on the beam or the temperature of the crystal during use is reduced. Measures have been taken to increase the temperature up to 80 ° to 100 ° C. and use the crystal while removing defects in the crystal by an annealing effect. In some cases,
A measure is taken to prevent a decrease in transmittance by using an expensive crystal with extremely few defects manufactured by a special manufacturing method called gray tracking resistance.

As described above, the coloring phenomenon called gray tracking that occurs in the nonlinear optical crystal can be prevented to some extent by raising the temperature of the crystal during use to about 80 ° C. to 100 ° C. . However, in order to keep the temperature of the crystal constant,
It is necessary to separately provide a heating device such as a small oven,
Not only does the structure of the wavelength conversion mechanism become complicated and large, but it also causes a problem that the startup time of the laser device is prolonged. In addition, when a crystal having few defects in the crystal lattice formed by a special manufacturing method is used as the nonlinear optical crystal, the decrease in transmittance can be slightly reduced, but it is difficult to reduce the transmittance to almost zero. There is a problem that the manufacturing cost of the conversion mechanism rises significantly.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to reduce the crystal temperature in order to prevent the above-described problem in the conventional nonlinear optical crystal for wavelength conversion of solid-state laser light, that is, to reduce the transmittance due to the coloring of the crystal. The method of maintaining the temperature at or above a predetermined temperature during use causes an increase in the size of the wavelength conversion mechanism and an increase in the rise time of the laser device, and a decrease in transmittance even when using a crystal manufactured by a special method with fewer defects. Is not able to be significantly improved, and the problem that the production cost of the wavelength conversion mechanism rises is to be solved.
Annealing the nonlinear optical crystal under the specified conditions makes it possible to almost completely prevent the decrease in transmittance due to the coloring of the crystal even when used for a long time in a strong laser beam. It is an object of the present invention to provide a method for preventing a decrease in transmittance due to coloring of crystals.

[0010]

When a laser beam is incident on a KTP crystal, electrons are transferred by the photon energy of the laser. Fe ³⁺ + h ⁺ → Fe ⁴⁺ (1) Ti ⁴⁺ + e ⁻ → Ti ³⁺ (2) Titanium Ti is a main component of the KTP crystal, while iron Fe is an impurity mixed therein. h and e represent holes (positrons) and electrons. In other words, when irradiating the laser,
Iron, which is an impurity, emits electrons (forms positrons) and
Value. On the other hand, titanium receives floating (free) electrons and changes to trivalent. What is confirmed as the deterioration of the transmittance is the change of titanium to trivalent. The changed valences (tetravalent, trivalent) that are unstable in both iron and titanium return to the original stable trivalent, tetravalent with the laser irradiation time (the reverse process of the above equation). The time required to return to the original state varies depending on the crystal, but it is known that the state returns to the original state relatively quickly when heated to about 100 ° C.

As described above, if the crystal returns to the original state by heating the crystal after the laser irradiation, if the crystal is in a heated state even during the laser irradiation,
It is expected that even if deterioration occurs, it will return to its original state immediately and the deterioration will be suppressed. About 100 crystals during conventional laser irradiation
The degradation prevention method of heating to ° C. is based on the above formula (
The deterioration is suppressed by causing the valence changes of 1) and (2) to occur on the left and right sides.

Therefore, the inventors of the present invention applied a simple annealing treatment (heat treatment) to the crystal in advance, instead of preventing the transmittance from being lowered by heating during the use of the crystal, thereby coloring the crystal after the treatment. I thought that it would not occur. In order to realize this idea, the present inventors first use KTiOPO ₄ (KT) as a nonlinear optical crystal.
P) Select multiple crystals and use multiple KTPs with different growth methods
For crystals, Nd: YAG with a fundamental wavelength of 1064 μm
Second harmonic of laser light (wavelength 532 μm, pulse width 3
The decrease in the transmittance due to the coloring of the crystal under irradiation of (μs, repetition frequency: 100 Hz) was measured, and the crystal that easily caused coloring and the crystal that hardly caused coloring were classified. Next, elemental analysis was performed on each of the classified crystals to clarify the differences, and the effects of these differences on the occurrence of coloring were examined from various angles.

As a result, the water (OH) existing in the crystal
Group) has an important relationship with the coloring of the crystals, and it is known that the crystals having more OH groups are less likely to be colored. In addition, based on the knowledge, the inventors have conceived that, on the contrary, a crystal having a small number of OH groups and being easily colored is subjected to a heat treatment (annealing) to give water to thereby convert the crystal into a crystal which is less likely to be colored. That is, impurities such as Fe contained in the crystal are unlikely to be present as a single element, and are easily associated with oxygen or OH. Fe + H ₂ O → FeOH + H (3) Also, once in the form of FeOH, it is considered that the formation of holes does not occur unless a stronger laser beam is used. On the other hand, it is considered that the role of filling oxygen deficiency or the transfer of electrons with water to titanium, which is the main component, is easier than in the case of impurities such as Fe. It can be suppressed for a long time.

The invention of the present application has been made based on the above findings and ideas of the present inventors. The invention of claim 1 is to provide a nonlinear optical crystal used for wavelength conversion of solid-state laser light by using an environment containing moisture. An object of the present invention is to prevent a decrease in transmittance due to coloring of a crystal due to repeated use by annealing under conditions.

According to a second aspect of the present invention, in the first aspect of the present invention, the nonlinear optical crystal is annealed for at least 2 hours or more in an atmosphere whose visibility is controlled at a temperature of 80 ° C. or more. Things.

According to a third aspect of the present invention, in the first aspect of the present invention, the nonlinear optical crystal is made of KTP, KDP, LBO, BBO.
Or CLBO.

According to a fourth aspect of the present invention, in the first aspect of the present invention, the nonlinear optical crystal converts the wavelength of the solid-state laser light into a wavelength of 1/2 to 1/5 of the fundamental wavelength. .

[0018]

When a crystal having a low water concentration in a crystal and easily causing defects, that is, coloring, is annealed in a water-containing atmosphere,
Moisture in the atmosphere penetrates into the inside of the crystal, and the permeated water impedes the generation of crystal defects that occur when intense laser light is applied, thereby preventing a decrease in transmittance. It is to be noted that the mechanism of preventing the water (OH group) penetrating into the crystal from discoloring the crystal is not yet clear, but the mechanism for generating the lattice defect as described above or the process of relaxing the generated lattice defect is not clear. It is presumed that the permeated OH groups may make some contribution. In addition, the same effect as that of the present invention has been observed in quartz glass irradiated with ultraviolet laser light, and quartz glass containing an appropriate concentration of OH group is unlikely to be colored by strong laser light. I know.

According to the present invention, the OH group concentration in the nonlinear optical crystal for wavelength conversion can be easily controlled by a simple heat treatment. It turned out to be an important function. By the way, it has been confirmed that, in the reverse process, when a crystal that is hardly colored is dehydrated by annealing in an atmosphere free of moisture (in vacuum), the crystal is converted into a material that is easily colored. The coloring property of the crystal can be freely controlled by the heat treatment. It should be noted that the fact that moisture is taken into the crystal by the heat treatment can be inferred from the fact that when the temperature of the glass or the like is increased, various gases are easily transmitted, but the measurement of the OH group concentration in the crystal before and after the heat treatment is not possible. Has not done so far.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic explanatory view of an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an annealing furnace (heat treatment furnace);
Is a heater, 3 is a support, 4 is an object to be processed (non-linear optical crystal), 5 is an air communication hole, 6 is a steam supply hole, and the annealing furnace 1 has a thermometer, a pressure gauge, and a control. A board and the like are provided. At the time of annealing the object to be processed (non-linear optical crystal) 4, first, the crystal 4 is placed on a support, the heater 2 is operated with the valve 5 a opened, and the temperature in the annealing furnace 1 is set to 80 ° C. to 600 ° C. After the temperature is raised to ° C., this temperature state is maintained for about 2 to 24 hours. afterwards,
The heater 2 is shut off, the inside of the annealing furnace 1 is opened, and the furnace temperature is lowered by natural cooling. This completes the annealing process of the crystal 4, that is, the process of preventing the transmittance from being lowered by coloring the nonlinear optical crystal 4 for wavelength conversion.

In the embodiment of FIG. 1, the heater 2
Although an electric heater is used as the heater, any kind of heater 2 and annealing furnace 1 may be used.
In the embodiment of FIG. 1, the atmosphere in the annealing furnace 1 is set to the atmosphere, and the crystal 4 is annealed in the presence of moisture contained in the atmosphere (air) A. However, the valve 6a is opened. Then, steam S is supplied from the steam supply hole 6 into the furnace, and the crystals 4 are grown in an atmosphere having a higher or lower moisture content.
May be annealed. In other words, it is sufficient that the atmosphere for annealing the crystal 4 contains an appropriate amount of moisture, and the moisture content can be optimized. Further, the atmosphere in the annealing furnace 1 may be an inert gas such as N ₂ , Ar, He, CO ₂ other than the air A.

Annealing temperature of the crystal (heat treatment temperature)
The optimal temperature is 80 ° C. or higher. If the temperature is lower than about 80 ° C., moisture in the atmosphere is only adsorbed and desorbed on the outer surface of the crystal, and a sufficient coloring prevention effect cannot be obtained.
In addition, the upper limit temperature of the annealing process varies depending on the type of the crystal to be annealed. In the case of a KTP crystal, if the temperature exceeds about 600 ° C., the crystal structure is modified and the function as a nonlinear optical crystal is hindered. , Annealing temperature 6
It cannot be higher than 00 ° C.

The annealing time of the crystal 4 depends on the annealing temperature, but is optimally about 2 to 24 hours. If the annealing time is less than 2 hours, water (OH
This is because the base may not sufficiently penetrate, and if the annealing time exceeds 24 hours, the concentration of the water permeated into the crystal is substantially saturated.

The object to be processed (nonlinear optical crystal) 4 may be any crystal as long as it is used for wavelength conversion of solid-state laser light, and may be either a water-soluble crystal or a water-insoluble crystal. The present invention can be applied. In the present embodiment, a so-called KTiOPO ₄ crystal (called a KTP crystal) is used as the object to be processed (non-linear optical crystal) 4, but a KH ₂ PO ₄ crystal (called a KDP crystal) or a LiB ₃ crystal is used.
O ₅ (referred to as LBO crystal), β-BaB ₂ O ₄ (BBO
Crystals), CsLiB ₆ O ₁₀ (referred to as CLBO crystals), and the like may be used as the object 4.

As will be described later, in the present embodiment, the transmittance of the non-linear optical crystal subjected to the annealing is measured only when the second harmonic of a so-called solid laser beam is irradiated. However, it is needless to say that the non-linear optical crystal subjected to the annealing treatment according to the present invention has an excellent coloring prevention effect even when irradiated with a harmonic other than the second harmonic.

[0026]

FIG. 2 shows a method for measuring the transmittance of a nonlinear optical crystal 4 after annealing according to the present invention. In FIG. 2, 7 is a test laser light generator,
8a and 8b are reflection plates, 9 is a collimating lens, and 10a and
10b is a beam slitter, 11a and 11b are photodiodes, 12 is a damper, 13 is a laser beam generator for adjusting the optical axis, and 14a and 14b are reflectors.

The nonlinear optical crystal 4 for measuring the transmittance includes:
Preliminary humidity of about 70-80% in the atmosphere, about 40
KT annealed at 0 ° C for 12 hours
A P crystal (about 5 mm × 5 mm × 5 mm) was used.

The test laser light generator 7 includes Nd:
A laser equipped with a YAG laser is used, and the second harmonic (wavelength 532 μm) of the laser light from the laser light generator 7 is used.
m, pulse width 3 μs, output 0.2 J, repetition frequency 1
00 Hz) is reduced to a laser beam having a diameter of 3 mm by the collimating lens 9 through the reflection plates 8a and 8b.
The crystal 4 was irradiated (irradiation intensity: 200 Mw / cm ² ).
The maximum irradiation time of the test laser beam is about 1.5 × 10 ⁴ seconds as shown in FIG. 3 described later. Also, N
d: The wavelength of the fundamental wave of the YAG laser beam is 1,064 μm.
m.

The laser light generator 13 for adjusting the optical axis has H
An e-Ne laser is used, and the reflectors 14a and 1
4b, the collimator lens 9, and the beam splitter 10a, and then enter the crystal 4. In the measurement of the transmittance, a part of the laser light being irradiated before and after the crystal 4 as the measurement data is extracted by the beam slitters 10a and 10b, and the energy of each extracted laser light is determined by PIN.
The measurement was performed using the photodiodes 11a and 11b, respectively. Then, the transmittance was determined from the ratio of the measured energy on the input and output sides, and the rate of crystal deterioration was determined from the transmittance with respect to the irradiation time.

FIG. 3 shows the state of the change in the transmittance measured by the measuring method shown in FIG. 2. The irradiation intensity of the laser beam (second harmonic) on the crystal 4 is 200 Mw.
/ Cm ² . In FIG. 3, the curve U is
This shows the change in the transmittance of the nonlinear optical crystal 4 subjected to the annealing treatment as described in the above embodiment, and the transmittance of about 0.8 at the initial stage of the start of laser beam irradiation (T = 0) is a value that is substantially unchanged. It can be seen that almost no decrease in transmittance due to the so-called coloring of crystals has occurred.

On the other hand, the curve V shows the change in the transmittance of the nonlinear optical crystal (KTP crystal, 5 mm × 5 mm × 5 mm) 4 which has not been subjected to the annealing treatment according to the present invention. After use for 10 ⁴ (sec), the transmittance drops to about 0.68.

The curve W indicates that the nonlinear optical crystal (KTP crystal, 5 mm × 5 mm × 5 mm) 4 that has been annealed (400 ° C., 12 hours) in the above embodiment is again annealed (400 mm) in a vacuum. C. for about 72 hours) to dehydrate the water in the crystal and show the change in transmittance. From this curve W, it can be seen that by performing the annealing treatment in a vacuum, the concentration of water (OH group) in the crystal is reduced, thereby reducing the effect of preventing coloring.

[0033]

According to the present invention, it is possible to almost completely prevent a decrease in transmittance due to coloring of the crystal by a very simple processing operation of annealing the nonlinear optical crystal in a moisture-containing atmosphere. . As a result, there is no need to use an expensive crystal manufactured by a special manufacturing method as in the past, or to use an oven device for continuously heating the crystal in use to a predetermined temperature. The life of the crystal for use can be greatly extended at low cost. As described above, the present invention has found a method for controlling gray tracking of a so-called nonlinear optical crystal extremely easily. According to the present invention, one of the major problems in practical use of solid-state lasers has been eliminated. This shows an epoch-making path for general application.
The present invention has such excellent practical utility.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of one embodiment of the present invention.

FIG. 2 is an explanatory diagram of a method for measuring the transmittance of a nonlinear optical crystal subjected to an annealing process according to the present invention.

FIG. 3 is a diagram showing a change in transmittance of a nonlinear optical crystal or the like annealed according to the present invention.

[Explanation of symbols]

1 is an annealing furnace, 2 is a heater, 3 is a support, 4 is an object to be processed (non-linear optical crystal) 5 is an air communication hole, 6 is a vapor supply hole, 7 is a test laser light generator, 8 is a reflector, 9 is a collimating lens, 10
Is a beam slitter, 11 is a photodiode, 12 is a damper, 13 is a laser beam generator for adjusting the optical axis, and 14 is a reflector.

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[Procedure amendment]

[Submission date] October 7, 1999 (1999.10.
7)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0015

[Correction method] Change

[Correction contents]

According to a second aspect of the present invention, in the first aspect of the present invention, the nonlinear optical crystal is annealed for at least two hours in an atmosphere where the humidity is controlled at a temperature of 80 ° C. or more. It is.

 ──────────────────────────────────────────────────続 き Continuing from the front page (71) Applicant 500085552 Shinji Motokoshi 1-8-4 Utsuhoncho, Nishi-ku, Osaka City Osaka Science and Technology Center Building Laser Technology Research Institute Foundation (72) Inventor Shinji Motokoshi Osaka City 1-8-4 Utsumotocho, Nishi-ku Osaka Science and Technology Center Building Laser Technology Research Institute (72) Inventor Takahisa Mineno 2-6-6 Yamadaoka, Suita City, Osaka Pref. Osaka University Laser Fusion Research Center F Terms (reference) 2K002 AA04 AB12 CA02 CA04 DA01 FA30 GA05 HA20

Claims (4)

[Claims] 1. A non-linear optical crystal used for wavelength conversion of solid-state laser light is annealed under an environmental condition containing moisture to prevent a decrease in transmittance due to coloring of the crystal due to repeated use. A method for preventing a decrease in transmittance due to coloring of a nonlinear optical crystal for wavelength conversion. 2. The coloring of the nonlinear optical crystal for wavelength conversion according to claim 1, wherein the nonlinear optical crystal is annealed for at least 2 hours in an atmosphere in which the visibility is controlled at a temperature of 80 ° C. or more. To prevent the transmittance from decreasing. 3. The nonlinear optical crystal is made of KTP, KDP, LB.
The method for preventing a decrease in transmittance due to coloring of a nonlinear optical crystal for wavelength conversion according to claim 1, wherein the method is any of O, BBO, and CLBO. 4. The transmittance by coloring the nonlinear optical crystal for wavelength conversion according to claim 1, wherein the nonlinear optical crystal converts the wavelength of the solid-state laser light into a wavelength of 1/2 to 1/5 of the fundamental wavelength. Drop prevention method.