JP2010008574A - Wavelength conversion device using optical parametric oscillation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength conversion device using optical parametric oscillation that is available in near-infrared and infrared wavelength regions. <P>SOLUTION: The wavelength conversion device includes: a wavelength conversion element 4 wherein a periodically poled structure having positive and negative polarities alternated with a period d given by a formula (1):d=m/[(n3/3λ)-(n2/2λ)-(n1/1λ)] is formed in a nonlinear optical crystal to, by using quasi phase matching, output light beams 2 and 3 at frequencies ω1 and ω2 satisfying ω3=ω1+ω2 in response to incidence of a light beam 1 at the frequency ω3; and an optical resonator 5 wherein this element 4 is disposed. The nonlinear optical crystal includes a nitride single crystal. In the formula (1), m is the degree of phase matching, and λ1, λ2, and λ3 are wavelengths of light beams at frequencies ω1, ω2, and ω3, respectively, and n1, n2, and n3 are refractive indexes of the nitride single crystal for light beams at frequencies ω1, ω2, and ω3, respectively. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an improvement in a wavelength conversion device by optical parametric oscillation that can be used in the near-infrared and infrared wavelength regions having a wavelength of 1 μm to 14 μm.

  In recent years, semiconductor lasers with higher output, smaller size, and shorter oscillation wavelength have been developed for optical communication and high-density optical recording. In addition to these applications, there are active developments in the application of laser light in the fields of medicine and environmental measurement. It is important to further increase the output of laser light sources, make them more compact, and diversify their oscillation wavelengths. It is a difficult issue. However, it is necessary to overcome huge investment and technical problems in the development of laser light sources, and it is a fact that laser light sources with the optimum wavelength cannot always be used for medical, environmental measurement, spectroscopic analysis, etc. . Therefore, a technology for converting the wavelength using an existing laser light source as basic light is important. The wavelength conversion of laser light is realized by a combination of an existing semiconductor laser or solid state laser and a wavelength conversion element using a nonlinear optical crystal. According to this combination, continuous oscillation is possible in principle, and the repetition frequency can be increased in the case of pulse oscillation. Further, the wavelength band can be narrowed, and the quality of the spatial mode is good.

  Here, the nonlinear optical crystal is a crystal that exhibits a nonlinear optical effect. The nonlinear optical effect is an effect due to nonlinearity of the polarization response of a substance. When strong light such as laser light is incident on the substance, the polarization response to the electric field of the incident light is not proportional. This is a phenomenon in which a part of incident light is wavelength-converted. In particular, second-harmonic generation, in which light having a wavelength half that of incident light is extracted using a second-order nonlinear optical effect, is best known as a method for converting the wavelength of laser light to a short wavelength. By this method, for example, Nd: YAG laser light (wavelength: 1064 nm) can be converted to a wavelength of 532 nm, and further to 266 nm by another wavelength conversion.

  However, in this method, since the nonlinear optical crystal that performs wavelength conversion has refractive index dispersion, the wavelength of the second harmonic in the crystal is not exactly ½ of the wavelength of incident light in the crystal. A phase shift occurs between the second harmonics generated at various points in the crystal, and there is a problem that it is difficult to extract a second harmonic having a sufficient strength. For this reason, the phase is usually matched by using the crystal orientation in which the wavelength ratio between the incident light and the second harmonic is exactly ½ using the birefringence of the crystal.

  However, in phase matching using birefringence, phase matching exceeding the birefringence amount of the crystal is impossible. Therefore, as a technique for performing phase matching beyond the limit of the nonlinear optical crystal, a method called pseudo phase matching has been proposed (Patent Document 1).

  Quasi-phase matching is realized by forming a periodically poled structure in a nonlinear optical crystal. According to the quasi phase matching, even if the nonlinear optical crystal does not have an appropriate birefringence at a desired wavelength, the conversion efficiency can be improved by matching the phases of the fundamental wave and the second harmonic. In addition, wavelength conversion by quasi-phase matching does not use the birefringence of the crystal, and therefore has the advantage of avoiding deterioration in conversion efficiency and beam quality caused by the traveling direction of the fundamental wave and the second harmonic. Yes.

  Since the wavelength conversion element using such quasi-phase matching can extract light having a wavelength shorter than the incident light such as the second harmonic and the sum frequency, the application to a deep ultraviolet light source having a short wavelength has been actively studied. Yes.

  On the other hand, it is also possible to decompose the incident light into two lights having a wavelength longer than that of the incident light with such a wavelength conversion element, of which the light having a short wavelength is called signal light, and the light having a long wavelength is called idler light. When the frequency of incident light is ω3, the frequency of signal light is ω1, and the frequency of idler light is ω2, the relationship of ω3 = ω1 + ω2 is satisfied. In the case of such a long wavelength, since the gain is lower than that of the short wavelength, it is necessary to use an optical resonator together. This long wavelength conversion technique is called an optical parametric oscillation (Optical Parametric Oscillator). According to such optical parametric oscillation, for example, by using Nd: YAG laser light (wavelength 1064 nm) as incident light, it is possible to extract near infrared or infrared light in the vicinity of 2 μm with high efficiency. Furthermore, by utilizing the fact that the wavelength of these lights depends on the temperature of the wavelength conversion element, it is possible to have wide wavelength variability by controlling the temperature of the wavelength conversion element, and it can be used as a continuous light source It becomes. Such tunable laser light sources in the near-infrared to infrared range are used for spectroscopic operations of various atoms and molecules, determination of basic physical constants, and application to standards such as frequency standards, length standards and optical power standards In addition to applications in the fundamental field of isophysics and chemistry, it is practically possible to use in the environmental field such as measurement of trace amounts of gases and pollutants in the atmosphere, and is currently realized using a wide variety of laser light sources. There is great expectation that the light source for precision measurement can be replaced with a light source system based on a single principle.

As wavelength conversion elements applied to such an optical parametric oscillation wavelength converter, ferroelectric oxide crystals such as LiNbO ₃ and LiTaO ₃ are mainly known, and further doped with elements such as Mg. Thus, studies such as improving the light damage resistance have been made (Patent Document 2).

However, in crystals such as LiNbO ₃ and LiTaO ₃ used as these wavelength conversion elements, there is a problem that it is difficult to extract light with a wavelength of 5 μm or more because the absorption edge in the infrared region is around 5 μm. In addition, in a crystal such as LiNbO ₃ or LiTaO ₃ , when there is absorption specific to a material near a wavelength of 2800 nm such as OH-ion in the near infrared or infrared region, light generated in that wavelength region is absorbed. Therefore, the efficiency of the light source in the wavelength region is remarkably inferior, and in the worst case, there is a problem that the crystal is broken due to light absorption of the material itself (Patent Document 3).

In addition, since the thermal conductivity of LiNbO ₃ and LiTaO ₃ is as low as 5 to 10 W / K · m, heat dissipation is poor, and there is a problem that high output characteristics deteriorate due to thermal distortion of the crystal.
JP 2002-122898 A (Claims, paragraph 0056) JP 2002-372731 A (Claims) JP 2000-356793 A (Claims)

  The present invention was made by paying attention to such problems, and the problem is that the influence of light absorption of the material itself described in Patent Document 3 is small, excellent in high output characteristics, An object of the present invention is to provide an optical parametric oscillation wavelength converter capable of extracting light having a wavelength of 5 μm or more.

  In order to solve the above-mentioned problems, the present inventors have examined a crystal material that can function as an optical parametric oscillation wavelength conversion element in the above-described wavelength region. The target crystal for wavelength conversion element has high thermal conductivity and excellent heat dissipation, no material-specific absorption in the near infrared and infrared regions, and an absorption edge in the infrared region of 5 μm or more. In addition, it is required to have ferroelectricity for applying the quasi phase matching technique. Furthermore, in order to actually manufacture the wavelength conversion element, it is necessary to manufacture a single crystal having a certain size or more while satisfying these physical properties. The present inventors have already found that a nitride single crystal is suitable as a wavelength conversion element using quasi phase matching used in a short wavelength region (see Japanese Patent Application No. 2007-320441 specification). The inventors have discovered that this nitride single crystal is also effective as a wavelength conversion element used in an optical parametric oscillation wavelength converter, and have completed the present invention.

That is, the invention according to claim 1
A periodic polarization inversion structure in which positive and negative polarities alternate with a period d represented by the following formula (1) is formed on the nonlinear optical crystal, and light having a frequency ω3 is made incident by using pseudo-phase matching. A wavelength conversion element that outputs light having a frequency ω1 and a frequency ω2 that satisfies the relationship ω1 + ω2 and an optical resonator that includes an input-side mirror and an output-side mirror and in which the wavelength conversion element is disposed between the mirrors. In the optical parametric oscillation wavelength converter,
The nonlinear optical crystal is composed of a nitride single crystal.

d = m / [(n3 / λ3) − (n2 / λ2) − (n1 / λ1)] (1)
[In the above formula (1), m is the phase matching order, λ1, λ2, and λ3 are the wavelengths of the light of the frequencies ω1, ω2, and ω3, respectively, and n1, n2, and n3 are the nitridings for the light of the frequencies ω1, ω2, and ω3, respectively. It is the refractive index of a single crystal]
The invention according to claim 2
In the optical parametric oscillation wavelength converter according to the invention of claim 1,
The nitride single crystal is a nitride represented by Al _1-x Ga _x N (where 0 ≦ x ≦ 1),
The invention according to claim 3
In the optical parametric oscillation wavelength converter according to the invention of claim 1,
The nitride single crystal is AlN.

Next, the invention according to claim 4 is:
In the optical parametric oscillation wavelength converter according to any one of claims 1 to 3,
The nitride single crystal is composed of a bulk crystal,
The invention according to claim 5
In the optical parametric oscillation wavelength converter according to any one of claims 1 to 3,
The nitride single crystal is formed of a thin film formed on a substrate.

The invention according to claim 6
In the optical parametric oscillation wavelength converter according to the invention of claim 5,
The substrate on which the thin film is formed is any one of Si, GaAs, AlN, InP, AlGaN, Al ₂ O ₃ , β-Ga ₂ O ₃ ,
The invention according to claim 7 provides:
In the optical parametric oscillation wavelength converter according to any one of claims 1 to 6,
The nitride single crystal is manufactured by a vapor phase growth method.

Next, the invention according to claim 8 is:
In the optical parametric oscillation wavelength converter according to any one of claims 1 to 6,
The nitride single crystal is manufactured by a liquid phase growth method or a solution growth method,
The invention according to claim 9 is:
In the optical parametric oscillation wavelength conversion element according to the invention of claim 1,
At least one of the lights output after wavelength conversion has a wavelength of 1 μm to 14 μm.

According to the optical parametric oscillation wavelength converter according to the present invention,
Effect of wavelength conversion to long wavelength light with wavelength of 5μm or more by quasi phase matching and continuous light source with wide wavelength variability by temperature control of wavelength conversion element, and solidification and miniaturization of laser device have.

  Hereinafter, embodiments of the present invention will be described in detail.

  An optical parametric oscillation wavelength converter according to the present invention includes a wavelength conversion element in which a periodic polarization inversion structure is formed in a nonlinear optical crystal made of a nitride single crystal to realize quasi phase matching, and has a high reflectance. It is placed in an optical resonator composed of a pair of mirrors (input side mirror and output side mirror) installed facing each other, and pump laser light is incident from the input side mirror of the optical resonator to output on the opposite side. Two lights (signal light and idler light) that have been wavelength-converted are extracted from the side mirror.

  FIG. 1 is a schematic configuration diagram of an optical parametric oscillation wavelength converter according to the present invention.

  The wavelength conversion element applied to this optical parametric oscillation wavelength converter is composed of a substantially rectangular parallelepiped nitride single crystal (nonlinear optical crystal), and the nitride single crystal is represented by the following formula (1). A periodic domain-inverted structure is formed in which the positive and negative polarities alternate as shown in FIG. 2 in the period d (indicated by reference numeral 6 in FIG. 1).

d = m / [(n3 / λ3) − (n2 / λ2) − (n1 / λ1)] (1)
[In the above formula (1), m is the phase matching order, λ1, λ2, and λ3 are the wavelengths of the light of the frequencies ω1, ω2, and ω3, respectively, and n1, n2, and n3 are the nitridings for the light of the frequencies ω1, ω2, and ω3, respectively. It is the refractive index of a single crystal]
Then, as shown in FIG. 1, a nitride single unit in which a periodic polarization inversion structure is formed in an optical resonator 5 composed of an input side mirror 51 and an output side mirror 52 which are installed facing each other. A wavelength conversion element 4 made of a crystal (nonlinear optical crystal) is arranged, and from one end face of the wavelength conversion element 4 to the boundary surface of the periodic polarization inversion structure via the input side mirror 51 as shown in FIGS. When incident light 1 having a predetermined frequency ω 3 is inputted vertically, a signal wave 2 having a frequency ω 1 and an idler wave 3 having a frequency ω 2 are output from the other end face of the wavelength conversion element 4. The input side mirror 51 and the output side mirror 52 oscillate and function as a wavelength conversion device based on optical parametric oscillation. At this time, if the nitride single crystal end face which becomes the light incident surface or the light exit surface of the wavelength conversion element 4 is subjected to optical polishing, and an antireflection film corresponding to the wavelength of the transmitted light is further formed, the wavelength conversion element 4 Efficiency can be increased. There is no particular limitation on the method of forming such a periodically poled structure, and a periodic domain-inverted structure can be easily formed even on a nitride single crystal by using the usual high voltage application method. Here, as the nitride single crystal (nonlinear optical crystal) constituting the wavelength conversion element of the optical parametric oscillation wavelength converter according to the present invention, Al _1-x Ga _x N (where 0 ≦ x ≦ 1) A mixed crystal of AlN and GaN expressed by the following formula is suitable, and in particular, it is effective to use AlN for extracting long wavelength light having a wavelength of 5 μm or more. This is because long-wavelength light (14 μm) having a wavelength of 5 μm or more can be extracted because nitride single crystal AlN has an absorption edge of 14.8 μm in the infrared region.

Furthermore, the thermal conductivity of AlN is 250 W / K · m, which is 25 times higher than that of LiNbO ₃ or LiTaO _3, and the heat dissipation of the crystal is excellent.

Moreover, as a form of the nitride single crystal, a bulk crystal or a thin film formed on a substrate can be used, and as the substrate on which the thin film is formed, Si, GaAs, AlN, InP, AlGaN, A substrate composed of either Al ₂ O ₃ or β-Ga ₂ O ₃ can be used. Furthermore, as a method for producing the nitride single crystal used in the present invention, a vapor phase growth method (sublimation method, metalorganic vapor phase growth method, hydride vapor phase growth method, molecular beam epitaxy method), liquid phase growth method, solution Although a growth method can be used, it is not limited by the manufacturing method, growth conditions, or the like.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, the technical content of this invention is not limited at all by an Example.

  In this example, a wavelength conversion element was manufactured using an AlN single crystal grown by a sublimation method. In addition, the following description is only an illustration of this invention and is not limited to this.

  Here, as shown in FIG. 3, the sublimation method provides a temperature distribution having a high temperature portion 12 and a low temperature portion 13 in the growth crucible 7 by the heating device 8, and is arranged on the high temperature portion 12 side. In this method, the grown crystal 10 is produced by sublimating the deposited raw material 11 and depositing it on the seed crystal 9 arranged on the low temperature part 13 side.

  In the sublimation method in this example, a graphite crucible with a thickness of 10 mm having a space of an inner diameter of 50 mmφ and a height of 80 mm is placed in a quartz container that can be evacuated and supplied with high-purity nitrogen gas using high-frequency induction heating. I set it. On the upper low temperature side of the graphite crucible, an AlN single crystal substrate (seed crystal) having a thickness of 1 mm and a diameter of 25 mm with a principal surface orientation of c-plane and a mirror-finished surface by chemical polishing was set.

  AlN polycrystalline powder was used as a raw material, and was placed on the high temperature side below the graphite crucible. The atmosphere is high-purity nitrogen 101 kPa, the portion where the seed crystal at the top of the graphite crucible is placed by high-frequency induction heating is 2200 ° C. as the low temperature side, the portion where the raw material at the bottom of the graphite crucible is placed at 2250 ° C. as the high temperature side, 80 hours An AlN crystal was grown. And after completion | finish of growth, it cooled to room temperature and obtained the AlN crystal | crystallization.

  The obtained AlN crystal has a columnar shape with a diameter of about 30 mm and a thickness of about 10 mm, and there are parts that are partly polycrystallized on the outer periphery of the crystal, but other parts are single crystals, The crystal was grown taking over the c-plane which is the crystal orientation.

  An AlN single crystal plate having a necessary size was cut out from the AlN single crystal thus obtained to produce a wavelength conversion element.

  The optical parametric oscillation wavelength converter in the present embodiment is a periodic polarization in which positive and negative polarities alternate with a period d represented by the following formula (1) as shown in FIG. 2 on an AlN single crystal plate which is a wavelength conversion element. An inversion structure is formed, and light having a wavelength of 1064 nm is used as incident light (fundamental wave) by using pseudo phase matching, and light having a wavelength of 1 μm to 14 μm is output.

d = m / [(n3 / λ3) − (n2 / λ2) − (n1 / λ1)] (1)
[In the above formula (1), m is the phase matching order, λ1, λ2, and λ3 are the wavelengths of the light of the frequencies ω1, ω2, and ω3, respectively, and n1, n2, and n3 are the nitridings for the light of the frequencies ω1, ω2, and ω3, respectively. It is the refractive index of a single crystal]
In this example, a periodic domain-inverted structure was formed using the ferroelectricity of AlN. Specifically, the polarity of the spontaneous polarization was reversed by applying a high electric field from the outside in the c-axis direction, which is the spontaneous polarization axis of AlN, in the direction opposite to the direction of the spontaneous polarization to form a polarization inversion structure.

  First, the obtained AlN single crystal was sliced perpendicularly to the c-axis and processed into a 10 mm × 10 mm × 0.5 mm thin plate-like AlN single crystal plate in which the thickness direction was the c-axis. With regard to this AlN single crystal plate, the thinner one can increase the electric field strength applied to the inside of the crystal when processing the domain-inverted structure, but it is desirable to make it thicker than the beam diameter of the light incident as the fundamental wave. . Considering these conditions, the thickness of the AlN single crystal plate is suitably about 0.5 mm to 1.0 mm.

  Next, an electrode having a periodic structure corresponding to the periodic domain-inverted structure to be formed was formed on a plane perpendicular to the c-axis ([0001] plane) of the thin AlN single crystal plate. The electrode having a periodic structure may be formed on at least one of the opposing [0001] planes (upper surface and lower surface), and the electrode formed on the other surface may be uniform over the entire surface. Of course, electrodes having the same periodic structure may be formed on both sides. In this embodiment, as shown in FIG. 4, a uniform lower electrode film 16 is formed on the lower surface of the AlN single crystal plate 15, and an upper electrode film 17 having a periodic structure is formed on the upper surface by sputtering. .

  Although platinum was used as the material of the electrode material, it is possible to use other metal materials such as aluminum and nickel-chromium alloy as the electrode film in addition to platinum. As a method for forming the electrode film, a conventional thin film forming method such as a vacuum deposition method or an ion plating method can be used in addition to the sputtering method. If an appropriate method is selected depending on the size of the element, the material of the electrode film, and the like. good.

  As a method for forming a periodic pattern on the electrode film, a photolithography technique generally used in the manufacture of semiconductor devices was applied.

  From the above steps, after forming an electrode having a predetermined periodic structure on the AlN single crystal plate, a high voltage was applied to the electrode to invert the spontaneous polarization of the corresponding part. The applied voltage was adjusted in accordance with the coercive electric field and the element thickness of the AlN single crystal plate within a range of several kV to 10 kV, and applied in a pulse form. The duration of one pulse was set to about several tens of μs to 200 μs. The applied voltage and application time per pulse are optimal by observing the formed polarization inversion structure by monitoring the total amount of charge flowing when applying an electric field, in-situ observation of polarization inversion, etching after device creation, etc. Turned into.

  Since the electric field applied at the time of polarization inversion has a certain extent in the crystal, the length of the domain-inverted part after processing does not completely match the length of the electrode, and the length of the electrode is usually set to the desired polarization. It is set to be shorter than the length of the reversing part. In this embodiment, the optimum electrode length is experimentally obtained. However, the optimum electrode length may be obtained in advance by simulating the electric field strength distribution in the crystal.

  The domain-inverted structure finally formed on the AlN single crystal plate is as shown in FIG. The arrow on the crystal end face indicates the polarization direction. In the actual polarization reversal step, a margin portion where no electrode is formed is provided in the peripheral portion of the AlN single crystal plate in order to ensure insulation between the upper and lower electrodes. Therefore, there is an incomplete part of the domain-inverted structure in the peripheral part of the AlN single crystal plate, but if the domain-inverted structure is formed at a predetermined period in the part along the optical axis, the wavelength conversion element As a matter of course it will function as well.

  According to the optical parametric oscillation wavelength conversion device of the present invention, a continuous light source having a wide wavelength variability by wavelength conversion to near infrared or infrared light by quasi phase matching and temperature control of the wavelength conversion element is obtained. It is realized and has industrial applicability that the laser device can be solidified and miniaturized.

1 is a schematic configuration diagram of an optical parametric oscillation wavelength converter according to the present invention. Explanatory drawing which shows the polarization inversion structure of the wavelength conversion element in the optical parametric oscillation wavelength converter which concerns on this invention. BRIEF DESCRIPTION OF THE DRAWINGS Schematic explanatory drawing which shows the sublimation method of Example 1. 1 is a schematic diagram of an electrode pattern formed in Example 1. FIG.

Explanation of symbols

1 Incident light ω3
2 Signal light ω1
3 idler light ω2
4 Pseudo phase matching wavelength conversion element 5 Optical resonator 6 Period d
7 Growing crucible 8 Heating device 9 Seed crystal 10 Grown crystal 11 Raw material 12 High temperature part 13 Low temperature part 14 Growth orientation 15 Single crystal plate 16 Lower electrode film 17 Upper electrode film 51 Input side mirror 52 Output side mirror

Claims (9)

A periodic polarization reversal structure in which positive and negative polarities alternate with a period d represented by the following formula (1) is formed in the nonlinear optical crystal, and light having a frequency ω3 is made incident by using pseudo phase matching. A wavelength conversion element that outputs light having a frequency ω1 and a frequency ω2 that satisfy the relationship of ω1 + ω2 and an optical resonator that includes an input-side mirror and an output-side mirror and in which the wavelength conversion element is disposed between the mirrors. In an optical parametric oscillation wavelength converter,
An optical parametric oscillation wavelength converter, wherein the nonlinear optical crystal is composed of a nitride single crystal.
d = m / [(n3 / λ3) − (n2 / λ2) − (n1 / λ1)] (1)
[In the above formula (1), m is the phase matching order, λ1, λ2, and λ3 are the wavelengths of the light of the frequencies ω1, ω2, and ω3, respectively, and n1, n2, and n3 are the nitridings for the light of the frequencies ω1, ω2, and ω3, respectively. It is the refractive index of a single crystal] 2. The optical parametric oscillation wavelength converter according to claim 1, wherein the nitride single crystal is a nitride represented by Al _1-x Ga _x N (where 0 ≦ x ≦ 1).   2. The optical parametric oscillation wavelength converter according to claim 1, wherein the nitride single crystal is AlN.   The optical parametric oscillation wavelength converter according to any one of claims 1 to 3, wherein the nitride single crystal is composed of a bulk crystal.   The optical parametric oscillation wavelength converter according to any one of claims 1 to 3, wherein the nitride single crystal is formed of a thin film formed on a substrate. 6. The optical parametric oscillation wavelength according to claim 5, wherein the substrate on which the thin film is formed is any one of Si, GaAs, AlN, InP, AlGaN, Al ₂ O ₃ , and β-Ga ₂ O _3. Conversion device.   7. The optical parametric oscillation wavelength converter according to claim 1, wherein the nitride single crystal is manufactured by a vapor phase growth method.   7. The optical parametric oscillation wavelength converter according to claim 1, wherein the nitride single crystal is manufactured by a liquid phase growth method or a solution growth method.   2. The optical parametric oscillation wavelength converter according to claim 1, wherein at least one of the lights output after wavelength conversion has a wavelength of 1 [mu] m to 14 [mu] m.

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