Patent Application
20120153190
The present invention relates to a method for manufacturing a second harmonic wave generation wavelength conversion element (hereinafter, will be referred to as a SHG wavelength conversion element or a wavelength conversion element) used for, for example, a laser light source device.
Gas laser light source devices such as argon gas laser and krypton gas laser have been conventionally known. However, the devices have low energy conversion efficiency of 0.1% and require a cooling mechanism. Thus, the devices are difficult to be reduced in size. For this reason, wavelength conversion laser devices using nonlinear optical effects which are highly efficient as video or medical laser have attracted attention. A nonlinear optical crystal having birefringence is required to obtain the nonlinear optical effects. SHG wavelength conversion elements have been used in which a ferroelectric nonlinear crystal such as a lithium niobate (LiNbO3:PPLN) crystal is periodically polarization-reversed (e.g., see Patent Literature 1).
The SHG wavelength conversion element has a narrow wavelength phase matching temperature range of ±1° C. with respect to a fundamental wave, and thus requires temperature control using a temperature control mechanism such as a Peltier element (e.g., see Patent Literature 2).
Output from wavelength conversion elements using polarization-reversed highly nonlinear optical crystals such as LiNbO3 or LiTaO3 becomes unstable due to photorefractive damage. In particular, it is known that refractive index variation occurs in about several seconds to several minutes after the incidence of a second harmonic wave such as green light.
Meanwhile, it is reported that metal additives such as magnesium, indium, scandium, and zinc are added to suppress the occurrence of optical damage. In particular, MgO-doped LN crystals have high nonlinear optical constant and favorable crystallinity most promisingly. It is reported that the occurrence of optical damage can be suppressed in a congruent PPLN crystal containing at least 5.0 mol of a metal additive (e.g., see Patent Literatures 3 and 4, and Non-Patent Literature 1).
Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2001-144354
Patent Literature 2: Japanese Patent Application Laid-Open Publication NO. 8-171106
Patent Literature 3: Japanese Patent Application Laid-Open Publication No. 5-155694
Patent Literature 4: Japanese Patent Application Laid-Open Publication No. 7-89798
Non-Patent Literature 1: “Appl. Phys. Lett.” vol. 44, p. 847, 1984, D. A. Bryan, et al.
In the configuration of the related art, however, even though metal additives are added, when the output of the second harmonic wave of the wavelength conversion element becomes 1 W or larger, the refractive index of the wavelength conversion element increases with time. Thus, the phase matching temperature varies and the output decreases. In other words, in the configuration of the related art, at least 1 W of laser light outputted using the wavelength conversion element is disadvantageously reduced with time.
An object of the present invention is to solve the problem and suppress a reduction over time in output even when high-power laser light is outputted for a long period of time.
In order to attain the object, the method for manufacturing a wavelength conversion element according to the present invention is a method for manufacturing a wavelength conversion element for converting a fundamental wave to a second harmonic wave, the method comprising the aging step of irradiating a nonlinear optical crystal with a first light beam having the same wavelength as the fundamental wave until the amount of variation per unit time in the phase matching temperature becomes a predetermined value or smaller while keeping the temperature of the nonlinear optical crystal at around the phase matching temperature after forming a periodical polarization-reversed structure in the nonlinear optical crystal.
Preferably, the output of the second harmonic wave in the aging step is not smaller than 0.5 W but smaller than 3 W.
Preferably, the integrated amount of output light of the second harmonic wave which is the product of the output of the second harmonic wave and aging time in the aging step is 600 W·hr or larger.
Preferably, the phase matching temperature is higher than 40° C. but not higher than 80° C.
The method for manufacturing a wavelength conversion element according to the present invention is a method for manufacturing a wavelength conversion element for converting a fundamental wave into a second harmonic wave, the method comprising the aging step of irradiating a nonlinear optical crystal with a first light beam having a wavelength in the vicinity of the wavelength of the fundamental wave and a second light beam having a wavelength in the vicinity of the second harmonic wave until the amount of variation per unit time in the phase matching temperature becomes a predetermined reference value or smaller after forming a periodical polarization-reversed structure in the nonlinear optical crystal.
Furthermore, the first light beam and the second light beam may enter in parallel to each other from a propagation direction thereof.
Furthermore, the first light beam and the second light beam may enter so as to cross each other in the nonlinear optical crystal.
Preferably, the method for manufacturing a wavelength conversion element further includes the heating step of retaining the nonlinear optical crystal at a predetermined heating temperature for predetermined heating time after forming the periodical polarization-reversed structure in the nonlinear optical crystal but before the aging step.
Preferably, the heating temperature is 85° C., and the heating time is 125 hours or longer.
Preferably, the wavelength conversion element is stored at 80° C. or lower after the aging step.
As described above, the wavelength conversion element is irradiated with the first light beam having the same wavelength as the fundamental wave after the formation of the periodical polarization-reversed structure in the nonlinear optical crystal, so that the variation of the phase matching temperature can be saturated beforehand. Thus, it is possible to suppress a reduction over time in output even when high-power laser light is outputted for a long period of time.
Background of the Invention
First, the background of the present invention will be described.
The inventors revealed by experiment that a reduction in output during high-power wavelength conversion, which is the problem to be solved by the present invention, was caused by a change in the phase matching temperature of a wavelength conversion element. The wavelength conversion element used in the experiment was a Mg-doped LiNbO3 crystal having a periodically polarization-reversed structure with a period of about seven microns and a phase matching temperature of about 50° C. The phase matching temperature indicates a temperature at which the conversion efficiency from a fundamental wave to a second harmonic wave peaks, and the temperature varies depending on the wavelength of the fundamental wave and the period of polarization reversal. In the experiment, such a wavelength conversion element was used, and light with a fundamental wave of 7 W (having a wavelength of 1064 nm) was collected in the wavelength conversion element, to perform wavelength conversion for obtaining a second harmonic wave having a wavelength of 532 nm (about 2 W). At this point, when time variation in output was observed, the output was reduced to not higher than half of the initial output in several hours. Concurrently, the phase matching temperature of the wavelength conversion element became higher than the set temperature. The change of the phase matching temperature is thought to have been induced by refractive-index variation caused by the high-power fundamental wave and the second harmonic wave. This is conceived for the following reasons. First, it is reported that the refractive-index variation of radiated light is caused by optical damage. However, optical damage does not occur on light having a wavelength of 532 nm in Mg-doped LiNbO3. Further, the refractive-index variation due to optical damage is a reversible phenomenon in which the refractive index returns to the original state when light radiation is stopped. In contrast, the variation of the phase matching temperature observed in the experiment was an irreversible phenomenon in which the refractive-index variation was kept even when the wavelength conversion element had been left at 50° C. for several months. Moreover, the refractive-index variation with temperature observed in the experiment occurred not when light having a wavelength of 532 nm or 1064 nm was singly radiated but when the fundamental wave and the second harmonic wave were concurrently radiated. It is considered from these factors that the reduction in output in the experiment, which had not been observed, was caused not by optical damage but by the refractive-index variation due to the concurrent radiation of the fundamental wave and the second harmonic wave. Furthermore, the phase matching temperature has been specific to a wavelength conversion element, and it has not been known that the phase matching temperature varies when the output of the fundamental wave is increased. Even though the phase matching temperature varied, wavelength conversion at another phase matching temperature did not cause a reduction in conversion efficiency. However, the variation of the phase matching temperature caused a difference between the set temperature and the phase matching temperature, thereby having reduced the output. As has been discussed, it is found that when a high-power second harmonic wave is outputted, it is important to avoid the variation of the phase matching temperature. The present invention is characterized in that the phase matching temperature is prevented from varying in the case where a high-power second harmonic wave is outputted.
The following will specifically describe embodiments of a method for manufacturing a wavelength conversion element according to the present invention with reference to the accompanying drawings.
First, a method for manufacturing a wavelength conversion element according to a first embodiment of the present invention will be described with reference to
The respective steps of
First, a nonlinear optical crystal substrate to be the material of a wavelength conversion element is prepared.
In the first embodiment, a wafer used for manufacturing a nonlinear optical crystal substrate 1 is a LiNbO3 crystal which has a thickness of 1 mm and a diameter of 76.2 mm, contains 5.0 mol % of magnesium oxide, and has crystal orientation along the z axis.
a) is the cross-sectional view of the nonlinear optical crystal substrate 1 used in the first embodiment. The nonlinear optical crystal substrate 1 is a rectangular parallelepiped with a thickness of about 1 mm, a width of about 10 mm, and a length of about 25 mm, which is obtained by cutting out the wafer with a thickness of 1 mm and a diameter of 76.2 mm.
Next, polarization-reversed portions 2 are periodically formed inside the nonlinear optical crystal substrate 1 (in other words, a periodically polarization-reversed structure is formed).
In this step, first, an electrode pattern (not shown) is formed in portions of the nonlinear optical crystal substrate 1 where the polarization-reversed portions 2 are formed. In the first embodiment, the period of the polarization-reversed portions 2 (corresponding to A in
In the formation of the electrode pattern, a sputtering device is used to form tantalum (Ta) thin films on surfaces 1a of the nonlinear optical crystal substrate 1, and a coater/developer is used to apply photoresists over the tantalum thin films. Next, a mask with a repeated pattern to be an electrode and the substrate with the photoresists applied thereon are made to contact each other and are exposed by an exposure unit. Thereafter, the photoresists with the pattern on the mask printed thereon are developed by the coater/developer and are etched to form the electrode pattern.
A pulsed electric field is applied to the electrode pattern to form the periodical polarization-reversed portions 2. Atom migration in the crystal due to the application of the pulsed electric field reverses the polarization orientation of the electrode pattern portion in the crystal orientation, so that the periodical polarization-reversed portions 2 are formed.
The electrode pattern is then removed. In the case where the electrode pattern is formed of tantalum, a fluoro-nitric acid solution is used.
As described above, the periodical polarization-reversed portions 2 are formed in the nonlinear optical crystal substrate 1 (in other words, the periodical polarization-reversed structure is formed) in this step as shown in
Next, two ends 1b of the nonlinear optical crystal substrate 1 are optically polished, and then anti reflective films are formed on the optically polished surfaces by the sputtering device.
This allows light such as a laser beam to be inputted to or outputted from the nonlinear optical crystal substrate 1.
As shown in
As described above, the fundamental wave is an optical wave which is inputted to the wavelength conversion element 3 by the laser light source device for which the nonlinear optical crystal substrate 1 (that is, the wavelength conversion element 3 after the aging step) is used. In the first embodiment, as described above, the light having a wavelength of 1064 nm as the fundamental wave is inputted to the wavelength conversion element 3, and the second harmonic wave having a wavelength of 532 nm is outputted from the wavelength conversion element 3. Thus, the wavelength of the first light beam 4 is 1064 nm.
As shown in
The nonlinear optical crystal substrate 1 is placed on a temperature controller 6 such that the temperature of the nonlinear optical crystal substrate 1 is electronically variable. With this configuration, the temperature of the nonlinear optical crystal substrate 1 is controlled to around the phase matching temperature by the temperature controller 6.
As described above, the periodical polarization-reversed structure including the periodical polarization-reversed portions 2 is formed in the nonlinear optical crystal substrate 1. The collected first light beam 4 is converted to a second harmonic wave 7 in the nonlinear optical crystal substrate 1.
Further, an area where the first light beam 4 passes through the nonlinear optical crystal substrate 1 is set as a first light beam propagation area 8, and an area where the second harmonic wave 7 passes through the nonlinear optical crystal substrate 1 is set as a second harmonic beam propagation area 9.
The above-described aging step is performed while the amount of variation in the phase matching temperature of the nonlinear optical crystal substrate 1 with respect to time is determined. Specifically, the aging step is performed until the amount of variation per unit time in the phase matching temperature of the nonlinear optical crystal substrate 1 becomes the reference value or smaller.
At an initial stage when the first light beam 4 starts entering, the temperature of the nonlinear optical crystal substrate 1 is controlled with a target temperature set at the phase matching temperature before the aging step continuation determination step. Thereafter, the temperature of the nonlinear optical crystal substrate 1 is regularly varied by the temperature controller 6 (every ten hours in the first embodiment) to measure output at measured temperatures, and the temperature at which the output peaks is calculated as the phase matching temperature at that point. The calculated temperature is determined to be the phase matching temperature, the target temperature is changed, and the first light beam 4 continues entering while the nonlinear optical crystal substrate 1 is kept at the changed target temperature which is the phase matching temperature at that stage. At this point, a difference between the phase matching temperature the previous time (ten hours before) and the phase matching temperature this time is determined, and the time variation is calculated. When the variation (that is, the amount of variation per unit time in the phase matching temperature) is larger than the predetermined reference value, the first light beam 4 continues entering. When the variation (that is, the amount of variation per unit time in the phase matching temperature) is not larger than the predetermined reference value, the first light beam 4 stops entering.
As described above, after the completion of this step, the wavelength conversion element 3 (
In the first embodiment, the reference value of the amount of variation per unit time in the phase matching temperature of the nonlinear optical crystal substrate 1 is 0.0025° C./hr. The continuation of the aging step is determined such that the aging step (that is, the incidence of the first light beam 4) continues until the amount of variation per unit time in the phase matching temperature of the nonlinear optical crystal substrate 1 becomes 0.0025° C./hr or smaller.
The following will describe the reason that the reference value of the amount of variation per unit time in the phase matching temperature of the nonlinear optical crystal substrate 1 is 0.0025° C./hr.
In the case where the amount of variation per unit time in the phase matching temperature of the nonlinear optical crystal substrate 1 is larger than 0.0025° C./hr, since the variation with time of the phase matching temperature of the nonlinear optical crystal substrate 1 is extremely large, the variation with time of the phase matching temperature of the nonlinear optical crystal substrate 1 cannot be complemented by Auto Power Control (APC) which is generally used for the control of light outputted from a laser light source. However, in the case where the amount of variation per unit time in the phase matching temperature of the nonlinear optical crystal substrate 1 is not larger than 0.0025° C./hr, the variation with time of the phase matching temperature can be complemented. Conversely, in the case where the output is not complemented according to the variation of the phase matching temperature by APC, the reference value may be reduced, and the wavelength conversion element 3 may be subjected to the aging step such that a reduction in output according to the variation of the phase matching temperature during an operation can be tolerable to the laser light source device.
The above description is about the method for manufacturing a wavelength conversion element according to the first embodiment of the present invention. The wavelength conversion element manufactured thus is then mounted on a wavelength conversion unit and is used for the laser light source device.
Further,
As shown in
As shown in
Moreover, as shown in
Auto Power Control (APC) has been conventionally used to suppress a reduction in output light. The common APC can complement a reduction in the output of the second harmonic wave substantially equivalent to 0.4° C. which is the amount of variation in the phase matching temperature. Thus, the above-described aging step and the APC can be combined.
Specifically, as shown in
It is possible to carry out the above-described method for manufacturing a wavelength conversion element with other condition settings. The following will describe the details of the other conditions.
In the case where the first light beam was radiated such that the output of the second harmonic wave was below 0.5 W, a reduction over time in the output of the second harmonic wave was not suppressed. Further, a stable reduction over time in the output of the second harmonic wave could not be suppressed in the case where the output of the second harmonic wave was 3 W or larger. Thus, when radiating the first light beam 4, the output of the second harmonic wave has to be not smaller than 0.5 W but smaller than 3 W.
As is clear from
As described above, the first light beam 4 having the same wavelength as the fundamental wave is radiated on the wavelength conversion element 3 after the periodical polarization-reversed structure is formed on the nonlinear optical crystal, so that the variation of the phase matching temperature can be saturated beforehand. Thus, a reduction over time in output can be suppressed even when high-power laser light is outputted for a long period of time.
Moreover, the period of the polarization-reversed portions 2 of the nonlinear optical crystal substrate 1 was changed to change the phase matching temperature of the wavelength conversion element 3, and effects on the phase matching temperature due to the radiation of the first light beam 4 were examined. Consequently, even though the aging step was performed such that the integrated amount was 1000 W·hr or larger, the amount of variation in the phase matching temperature was not saturated when the phase matching temperature was 40° C. or lower. Further, when the phase matching temperature exceeded 80° C., the effects caused by the radiation of the first light beam 4 could not be stably produced. According to the results, the period of the polarization-reversed portions 2 of the nonlinear optical crystal substrate 1 has to be designed such that the phase matching temperature is higher than 40° C. but not higher than 80° C.
Evaluations were performed on the storage temperature of the wavelength conversion element 3 subjected to the aging step. The wavelength conversion element 3 irradiated with the first light beam 4 such that the integrated amount was 600 W·hr was stored in high-temperature environment, and then the amount of variation in the phase matching temperature was evaluated. The phase matching temperature of the wavelength conversion element 3 shifted to the high temperature side by about 1° C. from the initial phase matching temperature with the irradiation of the first light beam 4.
As shown in
Thereafter, when the wavelength conversion element restored to the initial phase matching temperature was continuously operated again, the phase matching temperature shifted from the initial phase matching temperature to the high temperature side again. Thus, when the wavelength conversion element is restored to the initial phase matching temperature in the high-temperature environment after the aging step, the effects of the aging step are lost, thereby causing the variation of the phase matching temperature again. According to the result, the wavelength conversion element 3 has to be stored at a temperature of 80° C. or lower after the aging step.
In the first embodiment, the element is composed of LiNbO3 having a congruent composition with a magnesium oxide content of 5.0 mol-%. However, the variation of the phase matching temperature can be saturated by the aging step under certain conditions, even when the element is composed of LiTaO3 having a congruent composition with a magnesium oxide content of 5.0 mol %, or LiNbO3, LiTaO3, or KTiOPO4 having a stoichiometric composition with a magnesium oxide content of at least 1 mol.
In the first embodiment, the wavelength conversion using the nonlinear optical effect of the optical element is explained by way of example. However, an optical element having a polarization-reversed structure for matching the phases of light using the period of the polarization reversal or matching the velocities of light and a microwave may be applied. Further, in the first embodiment, the conversion (generation of the second harmonic wave) from infrared light (1064 nm) into visible light (532 nm) is explained by way of example. However, a system for matching the phases of light with sum frequency generation or difference frequency generation using the period of the polarization reversal or parametric oscillation may be applied.
In the first embodiment, the wavelength of the first light beam 4 is 1064 nm but may be 900 nm to 1200 nm in the vicinity of 1064 nm.
The following will describe a method for manufacturing a wavelength conversion element according to a second embodiment of the present invention.
The second embodiment is different from the first embodiment in that, in the aging step of the method for manufacturing the wavelength conversion element 3, a first light beam 4 having the same wavelength as a fundamental wave and a second light beam 10 having the same wavelength as a second harmonic wave are radiated on a nonlinear optical crystal substrate 1, so as to enter parallel to a direction in which the first light beam 4 and the second light beam 10 propagate, and the radiation continues until the amount of variation per unit time in the phasing matching temperature of the nonlinear optical crystal substrate 1 becomes a predetermined reference value or smaller. The steps explained in the first embodiment can be performed other than the method of light radiation in the aging step, and an explanation thereof is omitted.
In the second embodiment, for example, a light beam having a wavelength of 1064 nm can be used as the first light beam 4, and a light beam having a wavelength of 532 nm can be used as the second light beam 10.
The first light beam 4 and the second light beam 10 are radiated, so that the inside of the nonlinear optical crystal substrate 1 comes closer to a state in which the temperature is regulated and the second harmonic wave (532 nm) is being generated from the light having the wavelength of 1064 nm. Thus, the same state as in the aging step of the first embodiment can be obtained and the phase matching temperature can be saturated beforehand without keeping the temperature of the nonlinear optical crystal substrate 1 at around the phase matching temperature during the aging step, so that high output can be maintained in wavelength conversion. Hence, a temperature control system is not necessary for the nonlinear optical crystal substrate 1. As a result, the manufacturing cost for the aging step of the wavelength conversion element 3 can be reduced and the wavelength conversion element 3 can be easily manufactured.
In the second embodiment, the light having the same wavelength of 1064 nm as the fundamental wave is used as the first light beam 4 but may be light having a wavelength (900 nm to 1200 nm) in the vicinity of the wavelength of the fundamental wave.
In the second embodiment, the light having the wavelength of 532 nm is used as the second light beam 10 but may be light having a wavelength (350 nm to 600 nm) in the vicinity of the wavelength of the second harmonic wave.
The following will describe a method for manufacturing a wavelength conversion element according to a third embodiment of the present invention.
The third embodiment is different from the second embodiment in that, in the aging step of the method for manufacturing the wavelength conversion element 3, a first light beam 4 having the same wavelength as a fundamental wave and a second light beam 10 having the same wavelength as a second harmonic wave are radiated on a nonlinear optical crystal substrate 1, so as to cross each other in the nonlinear optical crystal substrate 1, and the radiation continues until the amount of variation per unit time in the phase matching temperature of the nonlinear optical crystal substrate 1 becomes a predetermined reference value or smaller. In the third embodiment, the wavelength of the first light beam 4 is 1064 nm and the wavelength of the second light beam 10 is 532 nm.
This configuration eliminates the need to coaxially arrange the optical axes of the first light beam 4 and the second light beam 10 during the light incidence on the nonlinear optical crystal substrate 1. Further, the phase matching temperature can be saturated beforehand without keeping the temperature of the nonlinear optical crystal substrate 1 at around the phase matching temperature during the aging step, so that high output can be maintained in wavelength conversion, similarly to the second embodiment. Hence, the optical system of the first light beam 4 and the optical system of the second light beam 10 can be relatively easily designed, so that the manufacturing cost of the wavelength conversion element 3 can be reduced further than that in the second embodiment.
In the third embodiment, the first light beam 4 has the same wavelength of 1064 nm as the fundamental wave but may have a wavelength (900 nm to 1200 nm) in the vicinity of the wavelength of the fundamental wave.
In the third embodiment, the second light beam 10 has the wavelength of 532 nm but may have a wavelength (350 nm to 600 nm) in the vicinity of the second harmonic wave.
The following will describe a method for manufacturing a wavelength conversion element according to a fourth embodiment of the present invention.
The fourth embodiment is different from the first embodiment in that, in the method for manufacturing a wavelength conversion element 3, a temperature controller mounting step (step A in
In the temperature controller mounting step (step A), the nonlinear optical crystal substrate 1 having a periodical polarization-reversed structure formed on a nonlinear optical crystal is mounted on a temperature controller 12. The aging step is performed with the nonlinear optical crystal substrate 1 put on the temperature controller 6 to evaluate the element characteristics in
As shown in
Such a manufacturing method enables the temperature controller 12 of the wave conversion unit 11 to control the temperature of the nonlinear optical crystal substrate 1 in the aging step 4, as compared to the first embodiment. Thus, it is possible to eliminate the step of incorporating the nonlinear optical crystal substrate 1 into the wavelength conversion unit 11 at the stage of manufacturing a final product. As a result, the wavelength conversion unit 11 can be easily manufactured.
The following will describe a method for manufacturing a wavelength conversion element according to a fifth embodiment of the present invention.
The fifth embodiment is different from the first embodiment in that, in the method for manufacturing a wavelength conversion element 3, a heating step (step B) is provided after the formation of a periodical polarization-reversed structure on a nonlinear optical crystal but before the aging step. The following will describe the heating step. Other steps are the same as the steps and conditions in the first embodiment, and an explanation thereof is omitted. Further, a nonlinear optical crystal substrate can be irradiated with a second light beam 10 as in the second and third embodiments, and can be mounted on a temperature controller as in the fourth embodiment.
In the heating step (step B), a nonlinear optical crystal substrate 1 having a periodical polarization-reversed structure formed on a nonlinear optical crystal is placed on a temperature controller 6 as shown in
Effects of the fifth embodiment will be described with reference to
As shown in
The following will describe a comparison between when heating is performed and when heating is not performed (the first embodiment) in the amount of variation per unit time in the phase matching temperature of the nonlinear optical crystal substrate 1 relative to the irradiation time of a first light beam 4.
The results shown in
Generally, a periodical polarization-reversed structure is formed by an external electric field, so that areas having a spontaneous polarization reversed with a micron-order short-period structure are adjacent to each other to form LiNbO3 and LiTaO3 crystals. The boundary between the areas having the reversed spontaneous polarization is called a domain wall. Further, the spontaneous polarization of the crystal is reversed, so that the crystal has a distortion therein. The distortion includes charge localization caused by the movement of lithium ions and a structural distortion occurring on the domain wall due to a change of the crystal structure. The charge localization forms charge distribution in the direction of the spontaneous polarization and generates an electric field facing the spontaneous polarization. The electric field reduces the refractive index of the crystal due to electro-optical effects. The charge localization is trapped in a shallow impurity level and is gradually discharged with time, so that electric localization is reduced. This is considered to be a cause for the variation with time in which the phase matching temperature of the wavelength conversion element gradually increases over a long period of time. The movement of charge trapped in the impurity level is effectively accelerated by increasing the temperature to accelerate a reduction in the charge localization. This is the reason that the heating step of the present invention is effective. Heating is performed at 85° C. or lower, so that the reduction in the charge localization caused by the polarization reversal or the heating step can be accelerated and the variation with time of the phase matching temperature can be suppressed. In contrast, the heating temperature was increased to higher than 90° C., so that the refractive index of the crystal was reduced again and the variation with time was reset to the original state (a state before the variation with time). This is because free charge due to the crystal defects is rapidly increased when the temperatures of the LiNbO3 and LiTaO3 crystals are increased to 90° C. or higher. The temperature increase to 90° C. or higher is known as a cause for the reduction of optical damage. The increased free charge constitutes the state of charge localization in the crystal again with the internal electric field of the spontaneous polarization. Thus, the variation with time is considered to be reset to the start condition.
As described above, in the method for manufacturing a wavelength conversion element, the periodical polarization-reversed structure is formed in the nonlinear optical crystal and the heating step is provided before the aging step, so that the reduction of the charge localization caused by the polarization reversal or the heating step can be accelerated. Thus, time for the aging step can be shortened.
In the fifth embodiment, heating is performed by the temperature controller 6 but may be performed by, for example, a thermostatic bath.
The present invention is useful for, for example, a method for manufacturing a second harmonic wave generation wavelength conversion element which can suppress a reduction over time in output and output a stable second harmonic wave in the long term and is used for, for example, a laser light source device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-197506 | Aug 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/004948 | 8/6/2010 | WO | 00 | 2/27/2012 |
