Patent Application
20210026222
Materials used for optical devices that utilize a second-order nonlinear optical phenomenon primarily include ferroelectric optical crystals such as a LiNbO3 (LN) crystal, a KTiOPO4 (KTP) crystal, a LiB3O5 (LBO) crystal, or a β-BaB2O4 (BBO) crystal. Optical devices utilizing these crystals have been developed in a wide range of application fields with wavelength conversion as a primary application. In the field of laser processing, for example, optical devices utilizing these crystals are shortened in wavelength using a second harmonic generation (SHG) of an optical fiber laser. The optical device using these crystals enables fine processing because it can perform selective processing using absorption wavelength dependence of the material and can reduce a beam spot diameter. In the field of optical communication, optical devices utilizing these crystals are used as optical wavelength conversion devices that perform simultaneous wavelength conversion from C-band WDM signals to L-band signals in order for effective utilization of wavelength resources in wavelength division multiplexing (WDM) optical communication. In the field of measurement, attention is paid to terahertz spectroscopy, which allows observation of intermolecular vibrations caused by hydrogen bonding and the like, and optical devices utilizing these crystals are used as light sources generating terahertz light.
Recently, compound semiconductor crystals such as GaAs, GaP, GaN, CdTe, ZnSe, or ZnO have also been used as materials for optical device utilizing the second-order nonlinear optical phenomenon. These materials have attracted attention as materials for a second-order nonlinear device due to a remarkable progress in techniques of fabricating a periodically spatially-poled structure, which is essential for the second-order nonlinear optical device in addition to having a large second-order nonlinear optical constant.
Wavelength conversion manners can be classified into angular phase matching and quasi phase matching (QPM) by periodically-poling. Among these, the quasi phase matching enables generation of various phase matching wavelengths and wavelength conversion in all transparent regions of a material by properly designing a poling pitch. In addition, in the quasi phase matching, there is no walk-off angle due to the angular phase matching. Therefore, a beam quality is good and an interaction length can be lengthened. The quasi phase matching is suitable for efficiency improvement and suppression of coupling loss, and is an effective manner in processing and measurement.
Patent Literature 1: PCT International Application Publication No. 2017/110792
A wavelength conversion optical device according to one embodiment includes: a substrate having a virtual plane including a virtual axis and first and second regions facing each other with the virtual plane interposed therebetween and including multiple first crystal regions and multiple second crystal regions alternately arranged on the virtual axis, in which each of the multiple first crystal regions includes a pair of portions arranged in a direction intersecting a first plane with the first plane interposed therebetween, the first plane being located in the first region and being parallel to the virtual plane, and directions of spontaneous polarizations of each of the pair of portions being directions away from the first plane, and each of the multiple second crystal regions includes a pair of portions arranged in a direction intersecting a second plane with the second plane interposed therebetween, the second plane being located in the second region and being parallel to the virtual plane, and directions of spontaneous polarizations of each of the pair of portions being directions away from the second plane.
In addition, a method for manufacturing a wavelength conversion optical device according to one embodiment includes: a first step of forming multiple first crystal regions by irradiating a substrate with laser light, the substrate having a virtual plane including a virtual axis and first and second regions facing each other with the virtual plane interposed therebetween, the laser light having a wavelength included in an absorption wavelength band of the substrate and having a light intensity distribution that gradually decreases as a distance from a plane including a central axial line of the laser light increases, the multiple first crystal regions each including a pair of portions arranged in a direction intersecting a first plane with the first plane interposed therebetween, the first plane being in the first region and being parallel to the virtual plane, and directions of spontaneous polarizations of each of the pair of portions being directions away from the first plane; and a second step of forming multiple second crystal regions by irradiating the substrate with the laser light, the multiple second crystal regions each including a pair of portions arranged in a direction intersecting a second plane with the second plane interposed therebetween, the second plane being in the second region and being parallel to the virtual plane, and directions of spontaneous polarizations of each of the pair of portions being directions away from the second plane, in which the multiple first crystal regions and the multiple second crystal regions are alternately formed on the virtual axis.
As a wavelength conversion optical device that performs quasi phase matching, a wavelength conversion optical device obtained by a combination of molding-in-place of glass and wavelength conversion has been proposed (see, for example, Patent Literature 1). Advantages of such a wavelength conversion optical device are a point that it is possible to process the glass into various shapes such as a fiber form and a thin film form since a substrate material is the glass and a point that a wavelength conversion function can be imparted to the shape. Patent Literature 1 discloses a method for forming a polarization-ordered structure defined in a polarization direction by irradiating a laser in a state of applying an electric field.
However, the polarization-ordered structure for realizing quasi phase matching is fine, and an interval between adjacent polarization-ordered structures is extremely short. Therefore, an interval between a positive electrode and a negative electrode configured to apply the electric field becomes narrow, and thus, manufacturing steps may be complicated in order to avoid dielectric breakdown when a high voltage is applied.
An object of the present disclosure is to provide a wavelength conversion optical device and method for manufacturing the wavelength conversion optical device capable of forming a polarization-ordered structure for realizing quasi phase matching by a simple method.
According to the wavelength conversion optical device and the method for manufacturing the wavelength conversion optical device according to the present disclosure, it is possible to form the polarization-ordered structure for realizing the quasi phase matching by a simple method.
First, contents of embodiments of the present disclosure will be listed and described. A wavelength conversion optical device according to one embodiment includes: a substrate having a virtual plane including a virtual axis and first and second regions facing each other with the virtual plane interposed therebetween and including multiple first crystal regions and multiple second crystal regions alternately arranged on the virtual axis, in which each of the multiple first crystal regions includes a pair of portions arranged in a direction intersecting a first plane with the first plane interposed therebetween, the first plane being located in the first region and being parallel to the virtual plane, and directions of spontaneous polarizations of each of the pair of portions being directions away from the first plane, and each of the multiple second crystal regions includes a pair of portions arranged in a direction intersecting a second plane with the second plane interposed therebetween, the second plane being located in the second region and being parallel to the virtual plane, and directions of spontaneous polarizations of each of the pair of portions being directions away from the second plane.
In the wavelength conversion optical device, one portions of the multiple first crystal regions and one portions of the multiple second crystal regions are alternately arranged on the virtual axis. A direction of spontaneous polarizations in the one portion of the first crystal region is a direction away from the first plane located in one of the first region and the second region (hereinafter, collectively referred to as “a pair of regions”). In addition, a direction of spontaneous polarizations in the one portion of the second crystal region is a direction away from the second plane located in the other of the pair of regions. Further, the first and second planes extend along the virtual plane. Accordingly, polarization orientations that intersect the virtual axis and are opposite to each other appear alternately on the virtual axis. Therefore, quasi phase matching by periodically-poling can be performed on light propagating on the virtual axis. In addition, each of the crystal regions of the wavelength conversion optical device can be easily formed by irradiating the substrate with laser light having a wavelength included in an absorption wavelength of the substrate. That is, according to the wavelength conversion optical device, a polarization-ordered structure for realizing quasi phase matching can be formed by a simple method.
In addition, a method for manufacturing a wavelength conversion optical device according to one embodiment includes: a first step of forming multiple first crystal regions by irradiating a substrate with laser light, the substrate having a virtual plane including a virtual axis and first and second regions facing each other with the virtual plane interposed therebetween, the laser light having a wavelength included in an absorption wavelength band of the substrate and having a light intensity distribution that gradually decreases as a distance from a plane including a central axial line of the laser light increases, the multiple first crystal regions each including a pair of portions arranged in a direction intersecting a first plane with the first plane interposed therebetween, the first plane being in the first region and being parallel to the virtual plane, and directions of spontaneous polarizations of each of the pair of portions being directions away from the first plane; and a second step of forming multiple second crystal regions by irradiating the substrate with the laser light, the multiple second crystal regions each including a pair of portions arranged in a direction intersecting a second plane with the second plane interposed therebetween, the second plane being in the second region and being parallel to the virtual plane, and directions of spontaneous polarizations of each of the pair of portions being directions away from the second plane, in which the multiple first crystal regions and the multiple second crystal regions are alternately formed on the virtual axis.
In the method for manufacturing a wavelength conversion optical device, one portions of the multiple first crystal regions and one portions of the multiple second crystal regions are alternately arranged on the virtual axis. A direction of spontaneous polarizations in the one portion of the first crystal region is a direction away from the first plane located in one of the pair of regions. In addition, a direction of spontaneous polarizations in the one portion of the second crystal region is a direction away from the second plane located in the other of the pair of regions. Further, the first and second planes extend along the virtual plane. Accordingly, polarization orientations that intersect the virtual axis and are opposite to each other appear alternately on the virtual axis. Therefore, quasi phase matching by periodically-poling can be performed on light propagating on the virtual axis. In addition, in the method for manufacturing a wavelength conversion optical device, each of the crystal regions is easily formed by irradiating the substrate with laser light having a wavelength included in an absorption wavelength of the substrate. At this time, the substrate is irradiated with the laser light having the light intensity distribution that gradually decreases as the distance from the plane along the central axial line of the laser light increases. According to the method for manufacturing a wavelength conversion optical device, the directions of the spontaneous polarizations of each crystal region can be easily set to the directions away from the first and second planes, respectively. That is, according to the method for manufacturing wavelength conversion optical device, a polarization-ordered structure for realizing quasi phase matching can be formed by a simple method.
In the above-described wavelength conversion optical device, the substrate may have the channel optical waveguide structure having the virtual axis as an optical axis. Similarly, the method for manufacturing a wavelength conversion optical device may further include a step of forming a channel optical waveguide structure having the virtual axis as an optical axis on the substrate, before or after the first step and the second step. As a result, the light propagation efficiency on the virtual axis can be enhanced. In this case, the channel optical waveguide structure may be formed by a dicing saw or dry etching. As a result, it is possible to easily form the channel optical waveguide structure on the substrate made of a crystalline material or an amorphous material.
The substrate may include at least one of a fresnoite-type crystal (specific examples; Sr2TiSi2O8 and Ba2TiGe2O8), a BaO—TiO2—GeO2—SiO2-based glass, and a SrO—TiO2—SiO2-based glass. For example, the above-described polarization-ordered structure can be easily formed by the irradiation of the laser light in these substrates. In this case, the substrate may include at least one of the BaO—TiO2—GeO2—SiO2-based glass and the SrO—TiO2—SiO2-based glass, and may contain, as an additive, metal included in any group of lanthanoids, actinides, and Groups 4 to 12. As a result, the absorption of the laser light in the substrate can be enhanced, and the above-described polarization-ordered structure can be formed more efficiently.
In the method for manufacturing a wavelength conversion optical device described above, a CO2 laser, a Yb-doped fiber laser, or a Ti:S laser may be used as a light source of the laser light. Alternatively, light obtained by converting a wavelength of light output from the CO2 laser, the Yb-doped fiber laser, or the Ti:S laser may be used as the laser light. With any one of these, the substrate can be irradiated with laser light in an infrared region included in absorption wavelengths of many substrates with a relatively high light intensity.
In the method for manufacturing a wavelength conversion optical device described above, a light-absorbing material may be arranged on a surface of the substrate and the substrate may be irradiated with the laser light. As a result, the absorption of the laser light in the substrate can be enhanced, and the above-described polarization-ordered structure can be formed more efficiently. In this case, the light-absorbing material may be a carbon paste. As a result, the light-absorbing material that efficiently absorbs the laser light can be easily arranged on the substrate.
In the method for manufacturing a wavelength conversion optical device described above, the substrate may be irradiated with the laser light through an optical component converting a light intensity distribution of the laser light into the light intensity distribution that gradually decreases as the distance from the plane including the central axial line of the laser light increases. As a result, the above-mentioned light intensity distribution can be realized with a simple configuration. The optical component is, for example, a diffractive optical element or an aspheric lens.
Specific examples of the wavelength conversion optical device and the method for manufacturing the wavelength conversion optical device according to embodiments of the present disclosure will be described below with reference to the drawings. Incidentally, the present invention is not limited to these examples, but is illustrated by the claims, and equivalence of and all modifications within the scope of the claims are intended to be included therein. In the following description, the same elements in the description of the drawings will be denoted by the same reference signs, and redundant descriptions will be omitted.
The substrate 2 includes multiple crystal regions 10A (first crystal regions) having a substantially rectangular planar shape and multiple crystal regions 10B (second crystal regions) having a substantially rectangular planar shape.
As illustrated in
As illustrated in
As described later, the above-described polarization-ordered structure in the crystal regions 10A and 10B is formed by irradiating the substrate 2 with laser light in an infrared region. When the substrate 2 includes at least one of the BaO—TiO2—GeO2—SiO2-based glass and the SrO—TiO2—SiO2-based glass, the substrate 2 may contain metal included in any group of lanthanoids, actinoids, and Groups 4 to 12 as an additive in order to enhance absorption of laser light having a specific wavelength in the infrared region. Examples of the lanthanoid-based or actinoid-based metal include Yb, Tm, and Er. In addition, examples of metal belonging to Group 4 to Group 12 include Ti, Cr, and Zn.
A description will be provided with reference to
In addition, the central planes P1 of the multiple crystal regions 10A and the central planes P2 of the multiple crystal regions 10B are alternately arranged in an extending direction of the virtual axis AX (that is, the optical waveguide direction D1). In other words, the central planes P1 and the central planes P2 are alternately arranged when viewed from the direction D2. However, respective end portions of the central planes P1 and P2 adjacent to each other when viewed in the direction D2 may overlap each other or may be separated from each other. In an example illustrated in
Further, the portion F2 (see
In the wavelength conversion optical device 1A having the above-described structure, a wavelength conversion region B1 is formed inside the substrate 2. The wavelength conversion region B1 is an optical waveguide that extends along the optical waveguide direction D1 with the virtual axis AX as the optical axis. One end B1a of the wavelength conversion region B1 reaches the end plane 2a of the substrate 2, and the other end B1b of the wavelength conversion region B1 reaches the end face 2b of the substrate 2. Light of a predetermined wavelength incident from the one end B1a propagates inside the wavelength conversion region B1 and is emitted from the other end B1b.
A method for manufacturing the wavelength conversion optical device 1A of the present embodiment having the structure described above will be described.
Subsequently, the multiple crystal regions 10A and the multiple crystal regions 10B are formed in the substrate 2 by irradiating a plate face of the substrate 2 with laser light having a wavelength included in an absorption wavelength of the substrate 2. Specifically, as illustrated in
In addition, when it is desired to shorten a period (A in
As illustrated in
In this Step S5, an optical component for converting the light intensity distribution of the laser light output from the light source described above into the light intensity distribution that gradually decreases as the distance from the reference plane PB increases may be used. By irradiating the substrate 2 with the laser light via such an optical component, the light intensity distributions illustrated in
By irradiating the substrate 2 with the laser light having the light intensity distribution as described above, the crystal regions 10A (see
When the substrate 2 is irradiated with such laser light, as illustrated in graphs G1 to G3 of
When a formation process of the crystal region described above is examined, it can be said that the orientation of the spontaneous polarizations generated in the crystal region follows an inclination direction of the light intensity distribution of the laser light. The laser light with which the substrate 2 is irradiated in Step S5 has a light intensity distribution that gradually decreases as the distance from the reference plane PB increases, as illustrated in
From the above, in
In addition, a peak intensity of the laser light with which the substrate 2 is irradiated in Step S5 is set to be higher than the crystal growth threshold T2 and be lower than the damage threshold T3. In this case, formation of perforations in the centers of the crystal regions 10A and 10B is suppressed, and it is thus possible to suppress deterioration of device performance due to cracks or the like caused by the perforations.
Finally, a heat treatment is performed on the substrate 2 to remove distortion of the substrate 2 again (Step S6). At this time, a heat treatment temperature is, for example, 760° C., and a heat treatment time is, for example, one hour. The wavelength conversion optical device 1A of the present embodiment is manufactured through the above steps.
Effects obtained by the wavelength conversion optical device 1A and the method for manufacturing a wavelength conversion optical device 1A according to the present embodiment described above will be described. As described above, in the present embodiment, the portions F2 of the multiple crystal regions 10A and the portions F3 of the multiple crystal regions 10B are alternately arranged on the virtual axis AX. The direction of the spontaneous polarizations A1 in the portion F2 of the crystal region 10A is the direction away from the central plane P1 located in the region 2c. The direction of the spontaneous polarizations A2 in the portion F3 of the crystal region 10B is the direction away from the central plane P2 located in the region 2d. In addition, the central planes P1 and P2 extend along the virtual plane PA including the virtual axis AX. Therefore, polarization orientations that intersect the virtual axis AX and are opposite to each other appear alternately on the virtual axis AX. Therefore, quasi phase matching by periodically-poling can be performed on light propagating on the virtual axis AX. Incidentally, the conversion wavelength is controlled by the period A (see
In addition, each of the crystal regions 10A and 10B described above can be easily formed by irradiating the substrate 2 with the laser light having the wavelength included in the absorption wavelength of the substrate 2. That is, according to the wavelength conversion optical device 1A, the polarization-ordered structure for realizing quasi phase matching can be formed by a simple method. In addition, in the method for manufacturing a wavelength conversion optical device 1A described above, each of the crystal regions 10A and 10B is formed by irradiating the substrate 2 with the laser light having the wavelength included in the absorption wavelength of the substrate 2 (Step S5). At this time, the substrate 2 is irradiated with the laser light having the light intensity distribution that gradually decreases as the distance from the reference plane PB along the central axial line O of the laser light increases. In this case, as described with reference to
In addition, the direction of the spontaneous polarizations A1 in the portion F2 of each crystal region 10A described above is aligned with the direction away from the central plane P1, and similarly, the direction of the spontaneous polarizations A2 in the portion F3 of each crystal region 10B is aligned with the direction away from the central plane P2. Therefore, it is easy to align the directions of the spontaneous polarizations A1 and A2 with each other in a direction orthogonal to the light waveguide direction (virtual axis AX). For example, as compared with a case where the crystal regions 100 having radial spontaneous polarizations A illustrated in
In addition, the substrate 2 may include at least one of the fresnoite-type crystal, the BaO—TiO2—GeO2—SiO2-based glass, and the SrO—TiO2—SiO2-based glass as in the present embodiment. For example, the above-described polarization-ordered structure can be easily formed by the irradiation of the laser light in these substrates 2. Further, when the substrate 2 includes at least one of the BaO—TiO2—GeO2—SiO2-based glass and the SrO—TiO2—SiO2-based glass, the substrate 2 may contain, as an additive, metal included in any group of lanthanoids, actinides, and Groups 4 to 12. As a result, the absorption of the laser light in the substrate 2 can be enhanced, and the above-described polarization-ordered structure can be formed more efficiently.
In addition, a CO2 laser, a Yb-doped fiber laser, or a Ti:S laser may be used as the light source of the laser light as in the present embodiment. Alternatively, light obtained by converting a wavelength of light output from the CO2 laser, the Yb-doped fiber laser, or the Ti:S laser may be used as the laser light. With any one of these, the substrate can be irradiated with laser light in an infrared region included in absorption wavelengths of many materials used as a substrate material with a relatively high light intensity.
As in the present modification, in the wavelength conversion optical device, the substrate 2 may have the channel optical waveguide structure 21 having the virtual axis AX as the optical axis. In addition, a method for manufacturing the wavelength conversion optical device may further include a step of forming the channel optical waveguide structure 21 in the substrate 2. As a result, the light propagation efficiency on the virtual axis AX (wavelength conversion region B1) can be enhanced.
Incidentally, as a method for forming the channel optical waveguide structure in the substrate 2, various methods other than the above method are conceivable. Examples thereof include a method of cutting the substrate 2 with a dicing saw while leaving a portion which is to serve as a channel optical waveguide structure, a method of partially changing a refractive index by diffusing an additive such as Ge and T1 into the substrate 2, and the like.
According to the method of the present modification, the absorption of the laser light La in the substrate 2 can be enhanced, and the above-described polarization-ordered structure can be formed more efficiently. In addition, a carbon paste may be used as the light-absorbing material 31 in this case. As a result, the light-absorbing material 31 that efficiently absorbs the laser light can be easily arranged on the substrate 2. In addition, the carbon paste has a wide absorption band, and thus, can absorb light in a wavelength band oscillated by a fiber laser, a solid-state laser, or a semiconductor laser other than the CO2 laser. Further, the carbon paste can be easily removed by washing or the like after the laser light irradiation.
Incidentally, various methods other than the above method are conceivable as a method of enhancing the absorption efficiency of the laser light. For example, there is a method of increasing a light absorption rate of the substrate 2 in advance by a reduction reaction before laser light irradiation, and restoring the light absorption rate by an oxidation reaction after the laser light irradiation.
The wavelength conversion optical device according to the present disclosure is not limited to the above-described embodiments, and various other modifications can be made. For example, the embodiments and the respective modifications described above may be combined with each other in accordance with necessary purposes and effects. In addition, the fresnoite-type crystal, the BaO—TiO2—GeO2—SiO2-based glass, and the SrO—TiO2—SiO2-based glass have been exemplified as the substrate material in the above-described embodiment. However, the substrate used in the present disclosure can be made of various materials that are crystalline or amorphous and transparent to a desired wavelength. In addition, the crystal regions 10A and 10B illustrated in the above-described embodiment do not have perforations formed by melting the substrate 2. However, the first and second crystal regions of the present disclosure may also have perforations. In addition, the central planes P1 and P2 of the crystal regions 10A and 10B have been described as examples of the first and second planes, respectively, in the above-described embodiment, but the position of each of the first and second planes of the present disclosure may be deviated from the center of each of the first and second crystal regions in the direction D2.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-085055 | Apr 2018 | JP | national |
This application is a continuation application of PCT/JP2019/017277 claiming the benefit of priority of the Japanese Patent Application No. 2018-085055 filed on Apr. 26, 2018, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2019/017277 | Apr 2019 | US |
| Child | 17039223 | US |
