JP3683517B2 - Method for forming domain inversion structure of ferroelectric material - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to an optical wavelength conversion element that converts a fundamental wave into a second harmonic, and more particularly, to create a predetermined pattern on a ferroelectric material having a nonlinear optical effect in order to create an optical wavelength conversion element having a periodic domain inversion structure. The present invention relates to a method for forming a domain inversion structure.
[0002]
[Prior art]
A method for converting the wavelength of a fundamental wave into a second harmonic using an optical wavelength conversion element provided with a region in which spontaneous polarization (domain) of a ferroelectric material having a nonlinear optical effect is periodically inverted has already been proposed by Bleombergen et al. (See Phys. Rev., vol. 127, No. 6, 1918 (1962)). In this method, the period Λ of the domain inversion part is
Λc = 2π / {β (2ω) -2β (ω)} (1)
Where β (2ω) is the propagation constant of the second harmonic
β (ω) is the fundamental wave propagation constant
By setting so as to be an integral multiple of the coherent length Λc given by (1), it is possible to achieve phase matching between the fundamental wave and the second harmonic. When wavelength conversion is performed using a bulk crystal of a nonlinear optical material, the phase-matching wavelength is limited to a specific wavelength unique to the crystal, but according to the above method, the period satisfying (1) for any wavelength is satisfied. By selecting Λ, it is possible to achieve phase matching efficiently.
[0003]
As a method of forming the periodic domain inversion structure as described above, conventionally,
1) LiTaO₃A method in which proton exchange is periodically performed on the −z plane of the substrate and heat treatment is performed at a temperature near the Curie point (K. Yamamoto, K. Mizuuchi, and T. Taniuchi, Optics Letters, vol. 16, No. 15, 1156 (1991). )reference)
2) Direct LiTaO electron beam at room temperature₃And LiNbO₃(See H. Ito, C. Takyu, and H. Inaba, Electrics Letters, vol. 27, No. 14, 1221 (1991))
Etc. are known.
[0004]
In the case of carrying out the method 1), it has been proposed to form a waveguide type optical wavelength conversion element by forming a third-order periodic domain inversion structure and then forming a channel waveguide by proton exchange. . In such an optical wavelength conversion element, when a cross section is observed, a semicircular periodic domain inversion structure is formed, and Ti: Al is used as a fundamental light source.₂O₃When a laser is used, a second harmonic output of 2.4 mW is obtained for a fundamental wave input of 99 mW, and a wavelength conversion efficiency close to a theoretical value in the third-order period is achieved.
[0005]
On the other hand, the optical wavelength conversion element prepared by the method 2) is LiNbO.₃Therefore, the periodic domain inversion portion penetrating from the −z plane to the + z plane is formed over the substrate thickness (for example, about 0.5 mm), so that it can be applied as a bulk type optical wavelength conversion element. In the optical wavelength conversion element in which the third-order periodic domain inversion structure is formed by this method, Ti: Al₂O₃The laser is wavelength swept to confirm phase matching in bulk.
[0006]
[Problems to be solved by the invention]
By the way, in the case of a waveguide type optical wavelength converter, in order to obtain a second harmonic of a practical level of about several mW using a single transverse mode semiconductor laser having an output of about 100 mW as a fundamental light source, It is necessary to realize a wavelength conversion efficiency that is about one digit higher than that of the third-order periodic structure.
[0007]
However, in the method 1), since the domain is inverted by the space charge electric field generated in the proton exchange region during the heat treatment, the depth of the domain inversion portion is about 1.6 μm when the primary period is 3.6 μm. Can only be obtained. For this reason, the domain inversion portion is usually much shallower than the waveguide thickness of about 2.4 μm, so that the overlap integral between the fundamental wave and the domain inversion portion cannot be increased, and the domain inversion portion is about It is only about twice as efficient. Also, in the method 1), the proton exchange region may diffuse unless it is heated rapidly to a heat treatment temperature of about 500 to 600 ° C., so that reproducibility is difficult.
[0008]
On the other hand, in the method of irradiating the electron beam of 2), it is possible to form a deep domain inversion portion that penetrates to the back of the substrate, but the problem of poor controllability of the domain inversion structure is recognized. In addition, for the convenience of the electron beam irradiation apparatus, the irradiation area at one time is limited to about 2 × 2 mm at present, so when trying to increase the wavelength conversion efficiency by increasing the interaction length of the element with respect to the fundamental wave, Although it is necessary to connect the periodic domain inversion patterns with high accuracy, there is also a problem that it is difficult to ensure the high connection accuracy. Furthermore, since the electron beam irradiation apparatus takes a long time to draw a large area, it is difficult to increase productivity by this method.
[0009]
The present invention has been made in view of the above circumstances, and a ferroelectric domain inversion structure forming method capable of forming a domain inversion structure of a predetermined period deeply, with good controllability, and easily over a large area. The purpose is to provide.
[0010]
[Means for Solving the Problems]
  One method for forming a domain inversion structure of a ferroelectric material according to the present invention is LiNbO, a ferroelectric material having a unipolarized nonlinear optical effect._ThreeSubstrate or MgO-LiNbO_ThreeAfter electrodes having a predetermined periodic pattern are formed on one main surface of the substrate, the ferroelectric is corona-charged by these electrodes and a corona wire disposed on the other main surface side of the substrate, and an electric field is applied thereto. The portion of the ferroelectric material facing the electrode is used as a local domain inversion portion, and then the substrate is heat-treated at a temperature in the range of 100 to 700 ° C.To make the refractive index of the domain inversion part uniform.It is characterized by that.
[0011]
  Another method of forming a domain inversion structure of a ferroelectric according to the present invention is LiTaO which is a ferroelectric having a unipolarized nonlinear optical effect._ThreeAfter electrodes having a predetermined periodic pattern are formed on one main surface of the substrate, the ferroelectric is corona-charged by these electrodes and a corona wire disposed on the other main surface side of the substrate, and an electric field is applied thereto. The portion of the ferroelectric material facing the electrode is a local domain inversion portion, and then the substrate is heat treated at a temperature in the range of 100 to 600 ° C.To make the refractive index of the domain inversion part uniform.It is characterized by that.
[0014]
[Operation and effect of the invention]
In the ferroelectric domain inversion structure forming method according to the present invention, when the ferroelectric is corona-charged by the electrode and the corona wire and an electric field is applied thereto, the domain inversion portion grows long along the electric field. Become. Therefore, this domain inversion part can be formed very deeply, and it is possible to sufficiently increase the wavelength conversion efficiency by ensuring a large overlap integral between the domain inversion part and the fundamental wave. Further, if the domain inversion part can be formed extremely deep in this way, the domain inversion part can be extended from one surface of the ferroelectric substrate to the other surface in the same cross-sectional shape, and the periodic accuracy can be greatly improved. . Also, this method is easier to process than electron beam irradiation and the like, and can process a large area at a time, and thus is excellent in productivity.
[0015]
Further, when the above heat treatment is performed, the local refractive index change caused by the electric field application is eliminated. Therefore, when light is passed through the domain inversion structure formed by this method, light scattering, diffraction, and the like are reduced, and loss is suppressed to a low level. Therefore, if the optical wavelength conversion element having this domain inversion structure is arranged in the laser resonator as described above, the internal loss of the resonator can be suppressed low and the second harmonic can be generated efficiently.
[0016]
According to each method of the present invention described above, after forming the domain inversion structure of the primary period, for example, a channel optical waveguide is formed, and the interaction length between the optical waveguide and the domain inversion region can be made sufficiently long. it can. This makes it possible to obtain a highly efficient waveguide-type optical wavelength conversion element using a laser diode or the like as a fundamental light source. Furthermore, according to this method, since the domain inversion part can be formed extremely deeply, an optical wavelength conversion element incorporated in a laser diode pumped solid-state laser or an external resonator type bulk crystal type optical wavelength conversion element can be formed. Is also possible.
[0017]
【Example】
Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings. First, a first reference example for the present invention will be described with reference to FIG. In FIG. 1, reference numeral 1 denotes MgO—LiNbO which is a ferroelectric having a nonlinear optical effect.₃It is a substrate. This substrate 1 is unipolarized to a thickness of 0.5 mm, and has the largest nonlinear optical constant d.₃₃Is optically polished on the z-plane so that it can be used effectively. A metal Ta is sputtered on the -z surface 1a of the substrate 1 to form a Ta thin film having a thickness of 50 nm, and then a periodic pattern of the Ta mask 2 as shown in FIG. Form. The period Λ of this pattern is MgO—LiNbO.₃In consideration of the chromatic dispersion of the refractive index, the thickness was set to 4 μm so as to have a primary period in the vicinity of 880 nm along the x direction of the substrate 1.
[0018]
Next, the substrate 1 is subjected to a proton exchange treatment in pyrophosphoric acid at 230 ° C. for 15 minutes to form a periodic proton exchange portion 3 having a thickness of 0.5 μm as shown in FIG. After this proton exchange, the Ta mask 2 becomes NaOH and H₂O₂It removes with the mixed etching liquid.
[0019]
Further, in order to make the potential when applying an electric field, which will be described later, uniform, metal Pt5 is EB deposited on the + z surface 1b on the back surface of the substrate. The sample thus prepared was heated to a temperature of 200 ° C. by a heater 6 connected to the ground 7 as shown in FIG. 3C, and an electric field was applied to the sample by corona charging. At this time, the distance between the corona wire 4 and the substrate 1 was set to 10 mm, and a voltage of −5 kV was applied from the high voltage power source 8 through the corona wire 4 for 10 minutes. After the above processing, the metal Pt5 deposited on the + z plane 1b is removed, the y plane is cut and polished, and then HF and HNO₃And selective etching was performed using an etchant prepared by mixing 1: 2.
[0020]
When the cross section (y plane) of this substrate 1 was observed, it penetrates from the −z plane to the + z plane at a period of Λ = 4 μm at the location where the proton exchange part 3 was formed as shown in FIG. It was confirmed that the periodic domain inversion portion 9 was formed. The arrow 10 in FIG. 1 (d) indicates the direction of polarization.
[0021]
When forming a channel waveguide after this, since the refractive index change in the proton exchanged region affects the propagation loss of the waveguide, in order to make the refractive index uniform and avoid this influence, for example at 400 ° C. Annealing is performed for 4 hours.
[0022]
Next, a second reference example for the present invention will be described. In this reference example, as the ferroelectric, for example, the same MgO—LiNbO as in the first reference example is used.₃A substrate is used. First, a mask pattern having the same period as that of the first reference example is formed on the + z plane of the substrate by photolithography. Next, after Ti metal is sputtered to form a Ti thin film having a thickness of 65 nm, a periodic pattern of Ti is formed by lift-off. Next, Ti diffusion treatment is performed at 1035 ° C. for 10 hours in a wet oxygen atmosphere to form periodic Ti diffusion portions.
[0023]
Thereafter, the sample was corona-charged while heating the sample in the same manner as in the first reference example, whereby the Ti diffusion part was used as a periodic domain inversion part. When the cross section of the y plane was observed, it was confirmed that the domain inversion part was deeply formed so as to penetrate the substrate.
[0024]
Next, a third reference example for the present invention will be described. In this reference example, the ferroelectric material is the same MgO—LiNbO as that in the first reference example.₃A substrate is used. First, on the + z plane of this substrate, SiO₂Sputtering of 100 nm thick SiO₂A thin film is formed. Thereafter, SiO 2 having the same period as that of the first reference example is obtained by photolithography and dry etching.₂A mask pattern is formed. Next, by performing heat treatment in an oxygen atmosphere at 1100 ° C., periodic Li outdiffusion portions are formed.
[0025]
Thereafter, Pt was deposited in the same manner as in the first reference example, and the sample was subjected to corona charging while heat-treating the sample, whereby the above Li diffusion region was used as a periodic domain reversal unit. When the cross section of the y plane was observed, it was confirmed that the domain inversion part was deeply formed so as to penetrate the substrate.
[0026]
In addition to the method in the first, second, and third reference examples described above, the electric field application method includes depositing metal Pt5 as an electrode on both the −z surface 1a and + z surface 1b of the substrate 1 as shown in FIG. A method of applying a DC voltage or a pulse voltage from the power supply 11 via these electrodes may be employed. In FIG. 2, the same elements as those in FIG. 1 are denoted by the same reference numerals, and redundant description thereof will be omitted (the same applies hereinafter). As the ferroelectric material, the MgO—LiNbO described above is used.₃In addition to LiTaO₃LiNbO₃Etc. can also be used.
[0027]
In the method of forming a domain inversion structure of a ferroelectric according to the present invention described above, the Curie point temperature and the electric conductivity of the bulk crystal of the ferroelectric are obtained by performing proton exchange, Ti diffusion, or Li outdiffusion. It is considered that an extremely deep domain inversion portion is selectively formed by performing heat treatment while applying an electric field.
[0028]
Next, a waveguide type optical wavelength conversion element using the primary periodic domain inversion structure created in the first reference example will be described. FIG. 3 shows a schematic configuration of the waveguide type optical wavelength conversion element. After forming the periodic domain inversion portion 9 on the substrate 1 as in the first reference example, the channel waveguide 12 is formed so that light propagates in the x direction. The channel waveguide 12 is formed as follows. A metal Ta is sputtered on the −z surface of the substrate 1 to form a 50 nm thick Ta thin film, and then a mask pattern having a width of 4 μm is formed by photolithography and dry etching. Next, the substrate 1 is subjected to proton exchange treatment in pyrophosphoric acid at 230 ° C. for 15 minutes, and the Ta mask is made of NaOH and H.₂O₂Then, the channel waveguide 12 is formed by annealing at 300 ° C. for 5 minutes. Finally, the input end 20a and the output end 20b of the channel waveguide type optical wavelength conversion element 20 thus created are edge-polished.
[0029]
When a laser beam having a wavelength λ as a fundamental wave is incident on the waveguide type optical wavelength conversion element 20 created as described above from the input end 20a, the phase matching of the waveguide-guide mode is taken, and the output end The second harmonic wave having the wavelength λ / 2 can be efficiently emitted from 20b.
[0030]
As an example, a case where a laser diode is used as a fundamental wave light source will be described with reference to FIG. A laser beam 14 (wavelength = 880 nm) as a fundamental wave emitted from the laser diode 13 is collimated by a collimator lens 15, and then the polarization direction is aligned with the z-axis direction of the channel waveguide 12 by a λ / 2 plate 16. The light is condensed by the condenser lens 17 and converges on the end face 12a of the channel waveguide 12. As a result, the fundamental wave 14 enters the channel waveguide 12 and is guided there.
[0031]
The fundamental wave 14 traveling in the waveguide mode is phase-matched in the periodic domain inversion region in the waveguide 12 and wavelength-converted to the second harmonic 18. The second harmonic wave 18 also propagates through the channel waveguide 12 in the waveguide mode, and is efficiently emitted from the output end 20b. Since the polarization direction of the output second harmonic 18 is also the z-axis direction, MgO—LiNbO₃The largest nonlinear optical constant d₃₃Will be used. Here, when the output of the laser diode 13 was 100 mW and the interaction length with the fundamental wave of the waveguide type optical wavelength conversion element 20 was 10 mm, the second harmonic output was 12 mW.
[0032]
Since the element 20 has a periodic domain inversion structure that penetrates the substrate 1 having a thickness of 0.5 mm, the element 20 can also be used as a bulk crystal type light wavelength conversion element. Thus, for example, YAG and YVO oscillating at 1064 nm₄The optical wavelength conversion element 20 can be replaced with the optical wavelength conversion KTP conventionally disposed in the resonator of a laser diode pumped solid-state laser using a laser crystal such as LNP. For KTP nonlinear optical constant of 7 pm / V, MgO-LiNbO₃Since the effective nonlinear optical constant of is about 25 pm / V, the use of this optical wavelength conversion element 20 can greatly increase the wavelength conversion efficiency.
[0033]
In the case of KTP, since the phase matching of TYPE II is taken, there is a problem that the second harmonic output fluctuates depending on the temperature and the crystal length by changing the polarization of the fundamental wave. When TYPE is used, TYPEI phase matching is achieved, and thus such a problem can be solved.
[0034]
Furthermore, an external resonator type optical wavelength conversion element can also be produced by polishing a bulk crystal having a periodic domain inversion structure by a method of the present invention into a ring resonator. In that case, it is possible to freely obtain the second harmonic in the wavelength region of the ultraviolet or higher by arbitrarily selecting the domain inversion period.
[0035]
Next, a fourth reference example for the present invention will be described. FIG. 5 shows a process of forming a domain inversion structure according to a fourth reference example of the present invention. In FIG. 5, reference numeral 31 denotes LiNbO which is a ferroelectric having a nonlinear optical effect.₃It is a substrate. This LiNbO₃The substrate 31 is unipolarized in advance by a known method. For example, the substrate 31 is formed to a thickness of 0.5 mm, and its largest nonlinear optical material constant d₃₃Is effectively used, a z-plate optically polished on the z-plane is used. Then, as shown in FIG. 5A, a metal mask 32 arranged at a predetermined pitch is provided on the + z surface 31a of the substrate 31 by a known photolithography method. The metal mask 32 is made of, for example, Au and is formed with a thickness of 1 μm. An arrow 10 in FIG. 5 indicates the direction of polarization of the substrate 31.
[0036]
Next, this LiNbO₃Mg on the substrate 31 from the metal mask 32 side^{2 +}Ion 33 is irradiated on the entire surface. The ion acceleration voltage at this time is, for example, 100 keV, and the dose is 1 × 10.¹⁵And By this process, the substrate 31 is exposed to Mg from the portion where the metal mask 32 is not provided.^{2 +}Ions 33 are implanted. Thereafter, the metal mask 32 is removed. As a result of the above processing, the substrate 31 is formed with ion implantation portions 34 having a pattern extending in the substrate thickness direction and repeating at a predetermined period Λ, as shown in FIG. In the ion implanted part 34, the electrical conductivity is higher than that of the other parts. In the figure, reference numeral 35 denotes a portion where no ion implantation is performed.
[0037]
Next, as shown in FIG. 3C, metal electrodes 36 and 37 made of Cr are provided on the + z surface 31a and the −z surface 31b of the substrate 31, respectively, and a DC power source 38 is provided via these metal electrodes 36 and 37. An electric field is applied to the entire surface of the substrate 31. The electric field at this time is, for example, 1000 V / mm, and the application time is 10 minutes. At this time, the temperature of the substrate 31 is set to 200 ° C. lower than the Curie point.
[0038]
By applying this electric field, the direction of polarization is reversed in the ion implanted portion 34 where the electrical conductivity is increased, but the direction of polarization is not reversed in the portion 35 where the electrical conductivity is kept relatively low. The ion implantation part 34 grows along the direction of the electric field, and finally grows sufficiently deep until it penetrates the substrate 31. Therefore, the ion implantation portion 34 and the other portion 35 are formed so as to be clearly distinguished from each other over a long region in the depth (thickness) direction of the substrate 31. As a result, a domain inversion portion 39 having a pattern penetrating to the back of the substrate and repeating at a predetermined period Λ is formed on the substrate 31 with good controllability as shown in FIG. Here, the period Λ is LiNbO.₃In consideration of the wavelength dispersion of the refractive index, the thickness was set to 4 μm so as to have a primary period in the vicinity of 880 nm along the x direction of the substrate 31.
[0039]
Next, a case where an optical wavelength conversion element is formed from the substrate 31 on which the periodic domain inversion structure is formed in the reference example will be described. A bulk crystal type light wavelength conversion element 40 as shown in FIG. 6 is obtained by polishing the x-plane and the -x-plane of the substrate 31 to form the light passage surfaces 40a and 40b, respectively. The bulk crystal type optical wavelength conversion element 40 having this periodic domain inversion structure was arranged in the resonator of the laser diode pumped YAG laser shown in FIG.
[0040]
This laser diode pumped YAG laser is doped with a laser diode 44 that emits a laser beam 43 as pumping light having a wavelength of 809 nm, a condensing lens 45 that converges the laser beam 43 in a divergent light state, and Nd (neodymium). It consists of a YAG crystal 46 which is a laser medium and arranged at the convergence position of the laser beam 43, and a resonator mirror 47 which is arranged on the front side (right side in the figure) of the YAG crystal 46. The optical wavelength conversion element 40 has a crystal length of 1 mm, and is disposed between the resonator mirror 47 and the YAG crystal 46.
[0041]
The YAG crystal 46 is excited by a laser beam 43 having a wavelength of 809 nm and emits a laser beam 48 having a wavelength of 946 nm. The solid-state laser beam 48 resonates between the YAG crystal end face 46a on which a predetermined coating is applied and the mirror surface 47a of the resonator mirror 47, and enters the optical wavelength conversion element 40 to have a wavelength of 1/2, that is, 473. converted to the second harmonic 49 of nm. The solid-state laser beam 48 as the fundamental wave and the second harmonic 49 are phase matched (so-called pseudo phase matching) in the periodic domain inversion region, and only the second harmonic 49 is emitted from the resonator mirror 47.
[0042]
In this example, when the output of the laser diode 44 is 200 mW, a high output second harmonic 49 of 1 mW was obtained. Thus, it was demonstrated that the domain inversion part 39 was formed with good controllability over the entire thickness direction of the substrate 31 by obtaining extremely high wavelength conversion efficiency.
[0043]
As described above, the reference example in which the electrical conductivity of the ferroelectric is lowered by the ion implantation process has been described. However, even if ions or atoms are implanted into the ferroelectric and the electrical conductivity of the implanted portion is increased. Good. In addition to the Mg 2+ ions in the reference example, for example, H + ions may be implanted.
[0044]
The method of the above reference example is applied to create a bulk crystal type light wavelength conversion element, but this method can also be applied to create an optical waveguide type light wavelength conversion element. is there.
[0045]
Furthermore, this method is similar to the LiNbO in the above reference example.₃And LiTaO₃, KNbO₃The present invention can be similarly applied when forming a domain inversion structure in a ferroelectric such as KTP.
[0046]
The ion or atom implantation process is performed by beam irradiation using a FIB (Focusing Ion Beam) apparatus or the like in addition to the entire surface irradiation using the metal mask 32 as in the above reference example. May be. However, according to the above reference example, since the ion or atom implantation process can be performed in a short time, it can be said that this method is more preferable in terms of work efficiency.
[0047]
In order to apply an electric field to the entire surface of the substrate 31, the corona charging method applied in the first reference example may be applied in addition to applying the electric field directly using the metal electrodes 36 and 37.
[0048]
Next, a fifth reference example for the present invention will be described. FIG. 7 shows how a domain inversion structure is formed according to a fifth reference example of the present invention. In FIG. 7, 4, 5, 7 and 8 are the same corona wire, metal Pt electrode, ground, and power supply as those used in the first reference example, and 31 was used in the fourth reference example. LiNbO with the same thickness of 0.5 mm₃It is a substrate. As shown in FIG. 5A, this LiNbO₃The optically polished −z surface 31b of the substrate 31 is irradiated with laser beams 50 and 51 having a wavelength of 325 nm emitted from a He—Cd laser (not shown), and a predetermined period Λ = 4 μm by two-beam interferometry. A repeating interference pattern is formed.
[0049]
In this way, LiNbO only in the part where the laser beam intensity is high due to interference.₃A photorefractive effect is generated in the substrate 31, and the electrical conductivity of the portion is increased. In this state, LiNbO by corona charging in the same manner as in the first reference example.₃When an electric field is applied to the substrate 31, only the polarization of the portion 52 where the photorefractive effect is generated is selectively inverted to form a domain inversion portion. These domain inversion portions grow along the direction of the electric field, and finally grow sufficiently deep until they penetrate the substrate 31. Therefore, the domain inversion portion and the other portions are clearly distinguished from each other over a long region in the depth (thickness) direction of the substrate 31. As a result, a domain inversion portion 39 having a pattern penetrating to the back of the substrate and repeating at a predetermined period Λ is formed on the substrate 31 with good controllability as shown in FIG.
[0050]
In this case, LiNbO is also charged by corona charging.₃Instead of applying an electric field to the substrate 31, an electric field may be applied directly to the substrate 31 using a pair of electrodes as shown in FIG. At this time, a transparent electrode may be used on the −z surface 31 b side of the substrate 31.
[0051]
LiNbO having a periodic domain inversion structure formed as described above.₃A bulk crystal type light wavelength conversion element was formed from the substrate 31, and was used by being disposed in a resonator of a laser diode pumped YAG laser similar to that shown in FIG. Even in this case, when the output of the laser diode 44 is 200 mW, a very high output second harmonic 49 of 1 mW is obtained, and the domain inversion portion 39 is formed with good controllability over the entire thickness direction of the substrate 31. It was proved that.
[0052]
Next, examples of the present invention will be described. FIG. 8 shows how a domain inversion structure is formed according to an embodiment of the present invention. In FIG. 8, 4, 7 and 8 are the same corona wire, ground, and power source as those used in the first reference example, and 31 is the same thickness as that used in the fourth reference example. 0.5 mm LiNbO₃It is a substrate. This LiNbO₃A periodic electrode 60 that repeats with a predetermined period Λ = 4 μm is provided on the optically polished −z surface 31b of the substrate 31 by a known photolithography method.
[0053]
Then, in the same manner as in the first reference example, LiNbO is obtained by corona charging.₃When an electric field is applied to the substrate 31, the polarization of only the portion 59 facing the periodic electrode 60 of the substrate 31 is selectively inverted as shown in FIG. These domain inversion portions grow along the direction of the electric field, and finally grow sufficiently deep until they penetrate the substrate 31. Therefore, the domain inversion portion and the other portions are clearly distinguished from each other over a long region in the depth (thickness) direction of the substrate 31. In this reference example, the above electric field application is performed by applying a voltage of 5 kV for 5 minutes, and the substrate 31 is kept at 50 ° C. at that time.
[0054]
Next, the y-plane of the substrate 31 is cut and polished, and then HF (hydrofluoric acid) and HNO₃And selective etching was performed using an etching solution obtained by mixing the above and. Then, when the cross section (y plane) of the substrate 31 was observed, as shown in FIG. 8 (b), at the place where the periodic electrode 60 was formed, from the −z plane to the + z plane with a period of Λ = 4 μm. It was confirmed that the penetrating periodic domain inversion portion 39 was formed with good controllability.
[0055]
LiNbO having a periodic domain inversion structure formed as described above.₃A bulk crystal type light wavelength conversion element was formed from the substrate 31, and was used by being disposed in a resonator of a laser diode pumped YAG laser similar to that shown in FIG. Also in this case, when the output of the laser diode 44 is 200 mW, a very high output second harmonic 49 of 1 mW is obtained. From this point as well, the domain inversion unit 39 extends over the entire thickness direction of the substrate 31. It was demonstrated that it was formed with good controllability.
[0056]
Next, a sixth reference example for the present invention will be described. FIG. 9 shows how a domain inversion structure is formed according to a sixth reference example of the present invention. In FIG. 9, reference numeral 1 denotes MgO—LiNbO having a thickness of 0.5 mm similar to that used in the first reference example.₃It is a substrate. This MgO-LiNbO₃On the + z surface 1b of the substrate 1 that has been optically polished, a periodic electrode 60 that repeats at a predetermined period Λ = 4 μm is provided by a known photolithography method, as shown in FIG.
[0057]
Then, after dropping the periodic electrode 60 to the ground 7, according to the temperature program shown in FIG.₃The substrate 1 is heat treated. That is, the substrate 1 is heated to 100 ° C. and then rapidly cooled. Then, due to the pyroelectric effect, a surface charge is generated as shown in FIG. 9B, and an electric field is applied to the substrate 1 by this surface charge. In this reference example, the surface potential of the −z plane 1a of the substrate 1 was 3.5 kV, and the electric field applied between the −z plane 1a and the + z plane 1b was 70 kV / cm.
[0058]
By applying an electric field to the substrate 1 in this manner, the polarization of only the portion of the substrate 1 facing the periodic electrode 60 is selectively inverted to form a domain inversion portion. These domain inversion parts grow along the direction of the electric field, and finally grow sufficiently deep until they penetrate the substrate 1. Therefore, the domain inversion part and the other part are clearly distinguished from each other over a long region in the depth (thickness) direction of the substrate 1.
[0059]
Next, after etching the substrate 1 in the same manner as in the embodiment of the present invention, the cross section (y-plane) was observed, and as shown in FIG. 9B, the periodic electrode 60 was formed. It was confirmed that the periodic domain inversion portion 9 penetrating from the −z plane to the + z plane with a period of Λ = 4 μm was formed with good controllability at the location.
[0060]
MgO—LiNbO having a periodic domain inversion structure formed as described above.₃A bulk crystal type light wavelength conversion element was formed from the substrate 1, and was used by being disposed in a resonator of a laser diode pumped YAG laser similar to that shown in FIG. Also in this case, when the output of the laser diode 44 is 200 mW, a very high output second harmonic 49 of 1 mW is obtained. From this point as well, the domain inversion section 9 extends over the entire thickness direction of the substrate 1. It was demonstrated that it was formed with good controllability.
[0061]
Next, a seventh reference example for the present invention will be described. FIG. 11 shows how a domain inversion structure is formed according to the seventh reference example of the present invention. In FIG. 11, 1 and 60 are the same MgO—LiNbO as in the sixth reference example.₃The substrate 1 is a periodic electrode, and the substrate 1 is heat-treated with a temperature program similar to that of the sixth reference example, and receives an electric field applied by the pyroelectric effect.
[0062]
In this reference example, when the substrate 1 is rapidly cooled from 100 ° C., an electric field is additionally applied to the substrate 1 by corona charging using the corona wire 4 and the power source 8. By doing in this way, compared with the case where an electric field is applied only by the pyroelectric effect, a larger electric field can be applied to the substrate 1, and the periodic domain inversion part 9 can be formed with better controllability.
[0063]
In addition, the electric field application method by the pyroelectric effect demonstrated above can also be applied instead of each electric field application method in the method of the 1st-5th reference example and the Example of this invention. It is of course possible to apply the methods of the fifth, sixth and seventh reference examples and the method of the embodiment in order to produce an optical waveguide type optical wavelength conversion element.
[0064]
Next, an eighth reference example for the present invention will be described. FIG. 12 shows how the domain inversion structure is formed according to the eighth reference example of the present invention. In FIG. 12, reference numeral 1 denotes MgO—LiNbO which is a ferroelectric having a nonlinear optical effect.₃It is a substrate. This substrate 1 is unipolarized to a thickness of 0.5 mm, and has the largest nonlinear optical constant d.₃₃A z-plate optically polished on the z-plane is used so that can be used effectively. A metal Ta is sputtered on the -z surface 1a of the substrate 1 to form a Ta thin film having a thickness of 50 nm, and then a periodic pattern of the Ta mask 2 as shown in FIG. Form. The period Λ of this pattern is MgO—LiNbO.₃In consideration of the chromatic dispersion of the refractive index, the thickness was set to 4 μm so as to have a primary period in the vicinity of 880 nm along the x direction of the substrate 1.
[0065]
Next, the substrate 1 is subjected to a proton exchange treatment in pyrophosphoric acid at 230 ° C. for 15 minutes to form a periodic proton exchange portion 3 having a thickness of 0.5 μm as shown in FIG. After this proton exchange, the Ta mask 2 becomes NaOH and H₂O₂It removes with the mixed etching liquid.
[0066]
Further, in order to make the potential when applying an electric field, which will be described later, uniform, metal Pt5 is EB deposited on the + z surface 1b on the back surface of the substrate. The sample thus prepared was heated to a temperature of 200 ° C. by a heater 6 connected to the ground 7 as shown in FIG. 3C, and an electric field was applied to the sample by corona charging. At this time, the substrate 1 and the electric field applying means are accommodated in the sealed container 61, and the air in the sealed container 61 is reduced to a humidity of 30% or less by using the drawing passage 62 and the supply passage 63 each provided with a valve. Suppressed dry N₂This (nitrogen) gas 64 is replaced by this N₂An electric field was applied in an atmosphere of gas 64. The distance between the corona wire 4 and the substrate 1 was set to 10 mm, and a voltage of −5 kV was applied from the high voltage power source 8 through the corona wire 4 for 10 minutes.
[0067]
After the above processing, the metal Pt5 deposited on the + z plane 1b is removed, the y plane is cut and polished, and then HF and HNO₃And selective etching was performed using an etchant prepared by mixing 1: 2.
[0068]
When the cross section (y-plane) of the substrate 1 was observed, as shown in FIG. 5D, in the portion where the proton exchange part 3 was formed, every part had a period of Λ = 4 μm and any part. In addition, it was confirmed that the periodic domain inversion portion 9 penetrating from the −z plane to the + z plane was formed.
[0069]
When forming a channel waveguide after this, since the refractive index change in the proton exchanged region affects the propagation loss of the waveguide, in order to make the refractive index uniform and avoid this influence, for example at 400 ° C. Annealing is performed for 4 hours.
[0070]
Next, a ninth reference example for the present invention will be described with reference to FIG. The ninth reference example is different from the eighth reference example in the method of applying an electric field. That is, in this case, MgO—LiNbO is used in the same manner as in the eighth reference example.₃After the periodic proton exchange part 3 is formed on the substrate 1, metal Pt5 is vapor-deposited as an electrode on both the −z surface 1a and + z surface 1b of the substrate 1, and a DC voltage or pulse is supplied from the power source 11 through these electrodes. Apply voltage.
[0071]
In this case as well, the electric field applied to the substrate 1 is dry N with the humidity kept at 30% or less in the sealed container 61.₂Perform in an atmosphere of (nitrogen) gas 64. Thereby, in this case as well, as in the eighth reference example, the periodic domain inversion portions 9 having a uniform pitch and a uniform depth are formed.
[0072]
Note that when an electric field is applied to a ferroelectric substance by corona charging as in the eighth reference example, an electric field cannot be applied in a vacuum, but the ferroelectric substance as in the ninth reference example or a tenth reference example described later. When applying an electric field by directly arranging electrodes on the electrode, the electric field may be applied not only in a dry atmosphere but also in a vacuum.
[0073]
Next, a tenth reference example for the present invention will be described with reference to FIG. In FIG. 14, reference numeral 65 denotes LiTaO which is a ferroelectric having a nonlinear optical effect.₃It is a substrate. This substrate 65 is unipolarized to a thickness of 0.5 mm, and has the largest nonlinear optical constant d.₃₃A z-plate optically polished on the z-plane is used so that can be used effectively. After depositing metal Pt on the + z surface 65a of the substrate 65, a desired periodic pattern is formed by photolithography and dry etching to form a metal Pt electrode (periodic electrode) 66 as shown in FIG. . On the other hand, by depositing Pt on the −z surface 65b of the substrate 65, a ground electrode 67 made of metal Pt that adheres entirely to the −z surface 65b is formed.
[0074]
Next, the substrate 65 is accommodated in a sealed container 61 and dried N with a humidity of 30% or less.₂The substrate 65 is heated in an atmosphere of gas 64 (for example, in the range of room temperature to 300 ° C.), and a DC voltage is applied by a DC power source 68 so that an electric field of several tens of V / mm to several thousand V / mm is formed on the + z side. As a result, in this case as well, as shown in FIG. 5B, the periodic domain inversion portion 9 having a uniform pitch and a uniform depth is formed in the portion where the electrode 66 and the electrode 67 face each other.
[0075]
As a comparative example, an electric field is applied in an air atmosphere with a humidity of 70%, and the others are the same as in the above 10th reference example.₃A periodic domain inversion portion was formed on the substrate 65. In that case, although the periodic domain inversion part was formed, the depth and period (width) were not uniform. This is presumably because the surface resistance of the substrate 65 decreased due to the humidity in the air atmosphere, and the resistance became non-uniform within the surface, resulting in an increase in leakage current.
[0076]
Next, a waveguide-type optical wavelength conversion element using the primary periodic domain inversion structure created in the tenth reference example will be described. FIG. 15 shows a schematic configuration of the waveguide type optical wavelength conversion element 70. After the periodic domain inversion unit 9 is formed on the substrate 65 as in the tenth reference example, a proton exchange channel waveguide 72 is formed so that light propagates in the x direction. Thereafter, the input end 70a and the output end 70b of the optical wavelength conversion element 70 were edge-polished. In this case, dry N when an electric field is applied₂The humidity of the gas 64 was particularly 10% or less. The period Λ of the periodic domain inversion unit 9 is 4 μm.
[0077]
When a laser beam having a wavelength λ as a fundamental wave is incident on the waveguide type optical wavelength conversion element 70 created as described above from the input end 70a, the phase matching of the waveguide-guide mode is taken, and the output end The second harmonic wave having the wavelength λ / 2 can be efficiently emitted from 70b.
[0078]
As an example, Ti: Al is used as the fundamental light source.₂O₃A case where a laser is used will be described. Ti: Al₂O₃The laser beam 74 as a fundamental wave emitted from the laser 73 is aligned with the polarization direction in the z-axis direction of the substrate 65 by the λ / 2 plate 16, condensed by the condenser lens 17, and converged on the end face 72 a of the channel waveguide 72. To do. As a result, the fundamental wave 74 enters the channel waveguide 72 and is guided there.
[0079]
The fundamental wave 74 traveling in the waveguide mode is phase-matched in the periodic domain inversion region in the waveguide 72 and wavelength-converted to the second harmonic wave 78. The second harmonic wave 78 also propagates through the channel waveguide 72 in the waveguide mode, and is efficiently emitted from the output end 70b. Here, when the output of the laser 73 was 40 mW and the interaction length with the fundamental wave of the waveguide type optical wavelength conversion element 70 was 5 mm, the second harmonic output was 3 mW.
[0080]
On the other hand, when the waveguide type optical wavelength conversion element in which the periodic domain inversion structure is formed by the method of the comparative example described above is used, the output of the laser 73 is 40 mW, and the interaction length with the fundamental wave of the optical wavelength conversion element Was 5 mm, the second harmonic output was 0.3 mW. As described above, according to the present invention, it has been confirmed that a second harmonic output that is one digit higher than that in the case of producing an optical wavelength conversion element by a conventional method can be obtained.
[0081]
The methods of the eighth to tenth reference examples described above are the above-described MgO—LiNbO as the ferroelectric material.₃And LiTaO₃LiNbO is not limited to using LiNbO₃Of course, the present invention can be applied when other ferroelectric materials such as the above are used.
[0082]
Next, an eleventh reference example for the present invention will be described. MgO-LiNbO as shown in FIG.₃A periodic domain inversion portion 9 was formed on the substrate 1 in the same manner as in the first reference example. Note that the period at this time was set to 4.6 μm so as to be the primary end in the vicinity of 946 nm of YAG. Next, this substrate 1 is made of MgO—LiNbO.₃For 3 hours at 600 ° C., lower than the Curie point (1210 ° C.). If heat treatment is performed at a temperature lower than the Curie point in this way, the polarization direction set in a predetermined direction by applying an electric field will not be changed by this heat treatment.
[0083]
Before and after the above heat treatment, the substrate 1 was observed with a polarizing microscope from the polished x-plane. Under the polarizing microscope, if the refractive index of the substrate 1 does not change locally and is uniform, the entire surface is quenched and darkened at the extinction position. When the substrate 1 before the heat treatment was observed, it was found that only the part where the proton exchange part 3 (see FIG. 1) was formed was not quenched at the extinction position, and the refractive index was clearly changed by applying an electric field. . In contrast, when the substrate 1 after the heat treatment was observed, the entire surface was quenched at the extinction position, and it was confirmed that the local refractive index change of the proton exchange part 3 disappeared and the refractive index was made uniform. In this way, the effect of the heat treatment can be confirmed by observation with a polarizing microscope.
[0084]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 1 on which the periodic domain inversion structure is formed by the method of the eleventh reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. The second harmonic 49 has a clean profile free from scattered light and stray light. On the other hand, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as described above in the resonator of the laser diode pumped YAG laser without performing the heat treatment described above. When the output of the laser diode 44 was 200 mW, it was 1 mW. As described above, when the optical wavelength conversion element is disposed in the resonator of the solid-state laser, it has been demonstrated that the internal loss of the resonator is reduced due to the above-described heat treatment, and the wavelength conversion efficiency is improved.
[0085]
Next, a twelfth reference example for the present invention will be described. MgO-LiNbO₃A periodic domain inversion portion 9 was formed on the substrate 1 in the same manner as in the second reference example. The substrate 1 was then subjected to the same heat treatment as in the eleventh reference example.
[0086]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 1 on which the periodic domain inversion structure is formed by the method of the twelfth reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0087]
Next, a thirteenth reference example for the present invention will be described. MgO-LiNbO₃A periodic domain inversion portion 9 was formed on the substrate 1 in the same manner as in the third reference example. The substrate 1 was then subjected to the same heat treatment as in the eleventh reference example.
[0088]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 1 on which the periodic domain inversion structure is formed by the method of the thirteenth reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0089]
Next, a fourteenth reference example for the present invention will be described. LiNbO as shown in FIG.₃A periodic domain inversion portion 39 was formed on the substrate 31 in the same manner as in the fourth reference example. Next, this substrate 31 is coated with LiNbO.₃For 3 hours at 600 ° C., lower than the Curie point (1130 ° C.).
[0090]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 31 on which the periodic domain inversion structure is formed by the method of the fourteenth reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0091]
Next, a fifteenth reference example for the present invention will be described. LiNbO as shown in FIG.₃A periodic domain inversion portion 39 was formed on the substrate 31 in the same manner as in the fifth reference example. Next, this substrate 31 was subjected to the same heat treatment as in the 14th reference example.
[0092]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 31 on which the periodic domain inversion structure is formed by the method of the fifteenth reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0093]
Next, a sixteenth reference example for the present invention will be described. LiNbO as shown in FIG.₃A periodic domain inversion portion 39 was formed on the substrate 31 in the same manner as in the previous example. Next, this substrate 31 was subjected to the same heat treatment as in the 14th reference example.
[0094]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 31 on which the periodic domain inversion structure is formed by the method of the sixteenth reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0095]
Next, a seventeenth reference example for the present invention will be described. MgO-LiNbO as shown in FIG.₃A periodic domain inversion portion 9 was formed on the substrate 1 in the same manner as in the sixth reference example. Next, this substrate 1 is made of MgO—LiNbO.₃For 3 hours at 600 ° C., lower than the Curie point (1210 ° C.).
[0096]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 1 on which the periodic domain inversion structure is formed by the method of the seventeenth reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0097]
Next, an eighteenth reference example for the present invention will be described. MgO-LiNbO as shown in FIG.₃A periodic domain inversion portion 9 was formed on the substrate 1 in the same manner as in the seventh reference example. Next, the substrate 1 was subjected to the same heat treatment as in the seventeenth reference example.
[0098]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 1 on which the periodic domain inversion structure is formed by the method of the eighteenth reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0099]
Next, a nineteenth reference example for the present invention will be described. MgO-LiNbO as shown in FIG.₃A periodic domain inversion portion 9 was formed on the substrate 1 in the same manner as in the eighth reference example. Next, the substrate 1 was subjected to the same heat treatment as in the seventeenth reference example.
[0100]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 1 on which the periodic domain inversion structure is formed by the method of the nineteenth reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0101]
Next, a twentieth reference example for the present invention will be described. MgO-LiNbO as shown in FIG.₃A periodic domain inversion portion 9 was formed on the substrate 1 in the same manner as in the ninth reference example. Next, the substrate 1 was subjected to the same heat treatment as in the seventeenth reference example.
[0102]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is prepared from the substrate 1 on which the periodic domain inversion structure is formed by the method of the twentieth reference example, and this is converted into a laser diode pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0103]
Next, a twenty-first reference example for the present invention will be described. LiTaO as shown in FIG.₃The periodic domain inversion part 9 was formed on the substrate 65 in the same manner as in the tenth reference example. Next, the substrate 65 is made of LiTaO.₃For 3 hours at 560 ° C. lower than the Curie point (610 ° C.).
[0104]
A bulk crystal type optical wavelength conversion element having a crystal length of 1 mm is formed from the substrate 65 on which the periodic domain inversion structure is formed by the method of the twenty-first reference example, and this is converted into a laser diode-pumped YAG laser as shown in FIG. Placed in the resonator. In this case, when the output of the laser diode 44 was 200 mW, the second harmonic 49 having an output of 10 mW was obtained. On the other hand, when the above-mentioned heat treatment was not performed, the second harmonic output was measured by placing the optical wavelength conversion element prepared in the same manner as above in the resonator of the laser diode-pumped YAG laser. When the output of the laser diode 44 was 200 mW, it was 1 mW.
[0105]
Next, a preferable temperature range for the heat treatment will be described. LiNbO as sample₃A plurality of crystals were prepared, and a domain inversion structure was formed on each of these crystals by the same method as in the 14th reference example. Next, these LiNbO₃The crystal was subjected to the above-mentioned heat treatment by changing the temperature appropriately within a range of 100 to 1000 ° C. lower than its Curie point 1130 ° C. At this time, the rate of temperature rise was 30 ° C./min, the temperature holding time was 2 hours, and the temperature was naturally cooled.
[0106]
As described above, when each sample heat-treated at different temperatures was observed with a polarizing microscope, it was confirmed that the local refractive index change of the domain inversion portion disappeared and the refractive index was made uniform.
[0107]
On the other hand, bulk crystal type light wavelength conversion elements were formed from these samples in the same manner as described above. Each of these optical wavelength conversion elements was placed in the resonator of a laser diode pumped YAG laser in the same manner as described above. For the sample in which the heat treatment temperature T is set within the range of 100 ° C. ≦ T ≦ 700 ° C., the generation of the second harmonic that is blue light is confirmed, and the second output of 10 mW when the output of the laser diode is 200 mW. Harmonics were obtained. However, the generation of the second harmonic was not confirmed for the sample in which the heat treatment temperature T was set in the range of 700 ° C. <T ≦ 1000 ° C. As for the sample in which the generation of the second harmonic was not confirmed, when the shape of the domain inversion portion was observed by etching, the periodic domain inversion structure was not confirmed at all, and the polarization direction of the domain inversion portion was changed by the heat treatment. It became clear.
[0108]
When heat treatment was performed at a temperature of less than 100 ° C., it was confirmed by the above polarizing microscope observation that the local refractive index change (refractive index step) of the domain inversion portion was not lost. Then, when the sample was placed in the resonator of the laser diode pumped YAG laser as described above, the generation of the second harmonic as blue light was confirmed, but 1.0 mW with respect to the output 200 mW of the laser diode. Only the second harmonic of the output was obtained. This is because the internal loss increased due to scattering of the fundamental wave due to the refractive index step. From these results, it is clear that there is no heat treatment effect below 100 ° C.
[0109]
From the above, LiNbO₃On the other hand, when heat treatment at a temperature lower than the Curie point is applied, it can be said that the heat treatment temperature T should be set within the range of 100 ° C. ≦ T ≦ 700 ° C.
[0110]
On the other hand, LiTaO having a Curie point of 610 ° C₃Also, a plurality of samples were prepared, and a domain inversion structure was formed on each of these crystals by the same method as in the twenty-first reference example. The heat treatment temperature range was 100 to 600 ° C. lower than the Curie point of 610 ° C. At this time, the temperature raising rate was 30 ° C./min, the temperature holding time was 2 hours, and the temperature was naturally cooled.
[0111]
As described above, when each sample heat-treated at different temperatures was observed with a polarizing microscope, it was confirmed that the local refractive index change of the domain inversion portion disappeared and the refractive index was made uniform.
[0112]
On the other hand, bulk crystal type light wavelength conversion elements were formed from these samples in the same manner as described above. Each of these optical wavelength conversion elements was placed in the resonator of a laser diode pumped YAG laser in the same manner as described above. For the sample in which the heat treatment temperature T is set within the range of 100 ° C. ≦ T ≦ 600 ° C., the generation of the second harmonic which is blue light is confirmed, and the second output of 10 mW when the output of the laser diode is 200 mW. Harmonics were obtained. However, generation of the second harmonic was not confirmed for the sample in which the heat treatment temperature T was set in the range of 600 ° C. <T ≦ 610 ° C.
[0113]
When heat treatment was performed at a temperature of less than 100 ° C., it was confirmed by the above polarizing microscope observation that the local refractive index change (refractive index step) of the domain inversion portion was not lost. Then, when the sample was placed in the resonator of the laser diode pumped YAG laser as described above, the generation of the second harmonic as blue light was confirmed, but 1.0 mW with respect to the output 200 mW of the laser diode. Only the second harmonic of the output was obtained. This is because the internal loss increased due to scattering of the fundamental wave due to the refractive index step. From these results, it is clear that there is no heat treatment effect below 100 ° C.
[0114]
From the above, LiTaO₃On the other hand, when heat treatment at a temperature lower than the Curie point is applied, it can be said that the heat treatment temperature T should be set within a range of 100 ° C. ≦ T ≦ 600 ° C.
[0115]
In order to investigate the effect of the heat treatment described above and the preferable temperature range, the periodic domain inversion portion was formed on each substrate by a method different from the method of the present invention, but the heat treatment was performed after the periodic domain inversion portion was formed. Therefore, even when the periodic domain inversion portion is formed by the method of the present invention, it is considered that the effect of improving the wavelength conversion efficiency by the heat treatment and the preferable temperature range of the heat treatment are the same as described above.
[0116]
In addition, the ferroelectric in which the domain inversion structure is formed according to the present invention can be applied as an optical wavelength conversion element of an external resonator type laser by applying an appropriate polishing and coating to form an element of a ring resonator. . In such a case, the same operation and effect as when applied to a laser diode pumped solid-state laser can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing how a periodic domain inversion structure is formed by the method of the first reference example for the present invention.
FIG. 2 is a schematic diagram showing an example of electric field applying means.
FIG. 3 is a schematic perspective view of a waveguide type optical wavelength conversion element having a periodic domain inversion structure.
4 is a schematic side view showing a usage state of the optical wavelength conversion element of FIG. 3;
FIG. 5 is a schematic view showing how a periodic domain inversion structure is formed by the method of the fourth reference example according to the present invention.
FIG. 6 is a side view of a solid-state laser including an optical wavelength conversion element in which a domain inversion structure is formed according to the present invention.
FIG. 7 is a schematic view showing how a periodic domain inversion structure is formed by the method of the fifth reference example according to the present invention.
FIG. 8 is a schematic diagram showing how a periodic domain inversion structure is formed by the method of one embodiment of the present invention.
FIG. 9 is a schematic view showing how a periodic domain inversion structure is formed by the method of the sixth reference example according to the present invention.
FIG. 10 is a graph showing a temperature program for heat treatment in the method of the sixth reference example.
FIG. 11 is a schematic view showing how a periodic domain inversion structure is formed by the method of the seventh reference example according to the present invention.
FIG. 12 is a schematic view showing how a periodic domain inversion structure is formed by the method of the eighth reference example according to the present invention.
FIG. 13 is a schematic view showing how a periodic domain inversion structure is formed by the method of the ninth reference example according to the present invention.
FIG. 14 is a schematic diagram showing how a periodic domain inversion structure is formed by the method of the tenth reference example for the present invention.
FIG. 15 is a schematic side view showing a usage state of an optical wavelength conversion element formed by the method of the tenth reference example.
[Explanation of symbols]
1 MgO-LiNbO₃Unipolarized substrate (z plate)
2 Ta mask pattern
3 Proton exchange
4 Corona wire
5, 67 Metal Pt electrode
6 Heater
7 Earth
8, 11, 68 power supply
9, 39 Domain reversal part
12, 72 channel waveguide
13 Laser diode
14, 74 Laser beam (fundamental wave)
15 Collimating lens
16 λ / 2 plate
17 Condensing lens
18, 78 Second harmonic
20, 70 Waveguide type wavelength converter
31 LiNbO₃substrate
32 metal mask
33 Mg 2+ ions
34 Ion implantation part
35 Parts that are not ion-implanted
36, 37 Metal electrodes
38 DC power supply
40 Bulk crystal type wavelength converter
43 Laser beam (pumping light)
44 Laser diode
45 condenser lens
46 YAG crystal
47 Cavity mirror
48 Solid-state laser beam (fundamental wave)
49 Second harmonic
50, 51 Laser beam producing photorefractive effect
52 Areas where photorefractive effects occur
60, 66 periodic electrodes
61 Airtight container
64 Dry N₂gas
65 LiTaO₃substrate
73 laser

Claims (2)

After electrodes having a predetermined periodic pattern are formed on one main surface of a LiNbO ₃ substrate or MgO-LiNbO ₃ substrate which is a unipolarized ferroelectric material having a nonlinear optical effect, these electrodes and the other of the substrates The ferroelectric material is corona-charged by a corona wire disposed on the main surface side of the substrate , and an electric field is applied thereto, and a portion of the substrate facing the electrode is a local domain inversion portion. ferroelectric domain inversion structure formation method according to claim Rukoto to equalize the refractive index of the domain reversals was heat-treated at a temperature in the range of to 700 ° C.. After electrodes having a predetermined periodic pattern are formed on one main surface of a LiTaO ₃ substrate, which is a unipolarized ferroelectric substance having a nonlinear optical effect, these electrodes are arranged on the other main surface side of the substrate. was the ferroelectric by a corona wire to apply an electric field thereto by corona charging, the opposing portions to the electrode of the substrate and local domain reversals, the scope of the subsequent substrate 100 to 600 ° C. ferroelectric domain inversion structure formation method which was heat-treated at a temperature of the inner and said Rukoto to equalize the refractive index of the domain reversals.

Images