JP2002311466A - Nonlinear optical element and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonlinear optical element with low loss, high reliability, a small size and small amount of driving energy and its manufacturing method. SOLUTION: An optical anisotropic area having a locally large nonlinear optical efficient can be formed in a core area 13 by impressing an electric field from DC high voltage 18 in the surface direction of a plane substrate 12 from the outside so as to be orthogonally crossed with an optical waveguide path 11 and simultaneously irradiating short-pulse laser light in the thickness direction of the plane substrate 12. Its mechanism is not interpreted in detail, however, it is predicted that formation of an electric dipole due to various point defects and non-bridging oxygen, etc., is concerned.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element mainly used in the fields of communication, measurement, information processing, etc., and more particularly to a non-linear optical element and a method of manufacturing the same.

[0002]

2. Description of the Related Art With the development of the fields of communication, measurement, and information processing using light, nonlinear elements such as an optical element using a highly functional and highly reliable glass material, an electro-optic effect, and second harmonic generation. There is a demand for development of optical elements having characteristics. In particular, a waveguide-type optical element using a quartz-based material has received the most attention because it can form a complicated optical circuit on a flat substrate in addition to its low loss.

[0003] In these quartz-based waveguide optical devices, a light propagation region called a core region having a relatively high refractive index doped with germanium or titanium is formed on a quartz substrate.
In general, this portion is covered with a clad region having a lower refractive index.

In general, inorganic glass such as quartz glass is an optically isotropic substance, and inherently has non-linear optical characteristics such as an electro-optic effect and second harmonic generation due to its inversion symmetry. Have been considered not.

[0005] However, it has recently been clarified that even in such an optically isotropic glass material, a second-order optical nonlinearity is induced by performing the poling treatment.

Here, the poling treatment means that a DC electric field is applied to a sample at a high temperature, the electric field is kept for a certain period of time while the electric field is applied, and then the heat is released. This is a method of forming an anisotropic region. The nonlinear optical effect due to such poling treatment has been confirmed so far in various glass materials such as tellurite glass and phosphate glass in addition to quartz glass.

However, the mechanism of inducing the nonlinear optical effect by the polling process has not been elucidated in detail, and it is considered that there are some factors relating to the origin of the nonlinear optical effect.

First, due to the drift of mobile proton ions (such as Na ⁺ ) as impurities, a cation-deficient region (space charge layer) is formed near the anode during the poling process, and this cation-deficient region is oriented by the electric dipole. May result. In this case, only the surface layer of several tens μm on the anode side mainly serves as an optically anisotropic region.

On the other hand, an electric dipole generated by factors related to the presence of point defects, OH groups, non-crosslinking oxygen, etc., is a material to which an electric field is applied by a poling process if these factors are present in the entire material. Optical anisotropy is observed throughout.

The glass material which is optically isotropic as described above
New attempts to incorporate anisotropic features into the
Has gained academic and practical interest. Especially S
iO _Two-Based glass with high reliability, low loss
At the heart of optoelectronics today
It is a material that has already been made into optical fibers and planar optical waveguides.
Technology is well established. In addition, quartz glass
Wide gap, other than telecommunications field mainly in near infrared region
However, it will be a great future for devices in the ultraviolet region and far ultraviolet region
It is a promising material.

[0011]

However, the non-linear optical effects in glass materials that have been reported so far are still weak, and practical optical elements such as optical modulators, optical switches, and wavelength conversion elements are realized. Not yet. At the current level, for example, in order to perform an optical switching operation using the electro-optic effect, the element length of the nonlinear region (the length of the core region of the optical waveguide) is required to be several tens cm, and the guided light is Since the driving voltage to be controlled requires several hundred volts, it is not practical. At least it is desired to realize a nonlinear optical effect comparable to a ferroelectric material such as LiNbO ₃ already in practical use as a nonlinear optical element, and to develop an optimum element structure, material, and manufacturing method.

As described above, conventionally, a method in which the poling treatment for forming an optically anisotropic region by applying a high voltage is performed under a high-temperature superheated state has been mainly studied. In the optical waveguide used in the present invention as well, the poling process can be performed by heating the entire or local area of the optical waveguide while applying a high voltage. In the case of overall heating, the element may be placed in a high-temperature electric furnace, and a high voltage may be applied therein. In the case of local heating, it is not possible to locally overheat the optical waveguide while applying a high voltage by irradiating a CO ₂ laser oscillating in an infrared wavelength band that is well absorbed by a general glass material such as quartz. Easy. However, in this thermal poling, at the same time as the orientation of the electric dipole by the high-voltage electric field, the irradiated portion is brought into a high temperature state by absorption of the high-energy laser light, and the electric dipole is relaxed by molecular vibration. As a result, a large nonlinear effect cannot be obtained. Further, such a high-temperature heat treatment has a problem that the dopant in the core region is dissipated or the shape of the core region is softened and deformed by heat.

An object of the present invention is to solve the above-mentioned problems and to provide a small-sized nonlinear optical element having low loss, high reliability, small driving energy, and a method of manufacturing the same.

[0014]

In order to solve the above-mentioned problems, it is necessary to find optimal conditions from all viewpoints, such as the composition of the glass material, the production method, the poling method, and the structure of the optical element.

In view of the above, the present inventors have paid attention to glass materials to which various dopants have been added and laser irradiation used as a means for improving the orientation of electric dipoles.
Glass materials have many excellent properties such as low loss, high reliability, and workability for forming optical circuits as described above,
Some passive elements have already been put into practical use for communication. If a nonlinear optical effect can be exhibited with high efficiency by using such a material, it can be considered that it can be applied to more optical functional elements.

Further, by using a laser having a short pulse high peak power intensity for the poling process, despite the irradiation of a laser beam having an extremely high laser power intensity, the irradiated portion hardly generates heat and has sufficient energy. Can be given. As a result, relaxation of the orientation of the electric dipole due to molecular vibration can be suppressed, and a large nonlinear optical effect can be obtained even with a quartz glass material.

In order to achieve the above object, a nonlinear optical element according to the present invention comprises a planar substrate, a core region formed on the planar substrate, and a cladding region which has a lower refractive index than the core region and covers the core region. And an optically anisotropic region having a large nonlinear optical coefficient locally in the core region.

According to the method of manufacturing a nonlinear optical element of the present invention, a core region is formed on a flat substrate, and the core region is covered with a cladding region having a lower refractive index than the core region to form an optical waveguide. In the manufacturing method of the optical element, an electric field of DC high voltage is applied from the outside in the plane direction of the plane substrate so as to be orthogonal to the optical waveguide, and at the same time, a short pulse is applied in the thickness direction of the plane substrate so as to be orthogonal to the electric field and the optical waveguide. By irradiating a laser beam, an optically anisotropic region is formed in the core region.

In addition to the above configuration, in the method of manufacturing a nonlinear element according to the present invention, it is preferable that the polarization direction of the electric field of the short pulse laser beam and the direction of the electric field are substantially parallel.

In addition to the above structure, the method of manufacturing a nonlinear element according to the present invention is characterized in that the wavelength of the short pulse laser beam is 1.8 μm or less, the repetition frequency is 100 kHz or more, and the short pulse laser beam It is preferable that the peak power intensity be 10 ⁵ W / cm ² or more.

In addition to the above-described structure, the method for manufacturing a nonlinear element according to the present invention is characterized in that the optical waveguide has quartz glass as a main component and any one of germanium, titanium, tin, phosphorus, boron, terbium and erbium in the core region. It is preferable to use the one added.

In addition to the above configuration, in the method of manufacturing a nonlinear element of the present invention, it is preferable that at least a part of the electrode for applying a high voltage is left as an electrode for controlling signal light in the core region.

In addition to the above configuration, in the method of manufacturing a nonlinear element of the present invention, it is preferable that at least a part of the electrode for applying a high voltage is covered with a glass material.

In addition to the above structure, the method of manufacturing a nonlinear element according to the present invention comprises forming a cladding region around a core region into a substantially trapezoidal cross-sectional shape, and positioning the upper bottom of the cladding region from the position of an electrode for applying a high voltage. Is also preferably high.

According to the present invention, an electric field from a DC high-voltage power supply is externally applied in the plane direction of the plane substrate so as to be orthogonal to the optical waveguide, and at the same time, the thickness of the plane substrate is orthogonal to the electric field and the optical waveguide. By irradiating the short pulse laser beam in the direction, an optically anisotropic region having a large nonlinear optical coefficient can be formed in the core region. Although the mechanism has not been elucidated in detail, it is presumed that formation of an electric dipole due to various point defects, non-bridging oxygen and the like is involved.

[0026]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a conceptual diagram showing one embodiment of a method for manufacturing a nonlinear optical element according to the present invention.

An optical waveguide 11 as a nonlinear optical element is formed on a quartz substrate 12 and has a core region 13 made of silica-based glass to which various dopants are added, and has a relatively lower refractive index than the core region 13. Cladding region surrounding the core region 13
It consists of 14 and.

First, the core region 13 is made of a material such as germanium.
-Using a quartz plate containing punt as the target material,
Core film on quartz substrate 12 by microwave sputtering
And then pattern the waveguide by photolithography.
And CHF _ThreeReactive ion using gas
Formed into a rectangular cross-sectional shape by etching
is there.

The cladding region 14 is formed by a flame deposition method. After the core region 13 is formed in the clad region 14, glass fine particles obtained by adding P (phosphorus) and B (boron) to SiO ₂ are deposited on the entire upper surface of the quartz substrate 12 by a flame hydrolysis reaction. The glass particles are vitrified by sintering the entire substrate 12 at a high temperature of 1300 ° C.

Here, when phosphorus is added to quartz, the refractive index increases, and when boron is added, the refractive index decreases. Therefore, by adjusting the amount of addition of phosphorus and boron, the refractive index of the cladding region 14 becomes substantially equal to the refractive index of the quartz substrate 12. In the present embodiment, the cross-sectional dimension of the core region 13 is about 6 × 6 μm, and the relative refractive index difference between the core region 13 and the cladding region 14 (or the quartz substrate 12) is 0.8%.
And

A pair of electrodes 15 and 16 are provided on both sides of the clad region 14 thus formed so as to sandwich the core region 13.
Was formed. An electrode protection layer 17 is formed on the upper portions of these two electrodes 15 and 16 to suppress dielectric breakdown due to application of a high voltage. The electrodes 15 and 16 are connected to a DC high-voltage power supply 18 via through holes formed in the electrode protection layer 17.
An electric field is generated by the DC high-voltage power supply 18 in the plane direction (lateral direction in the figure) of the quartz substrate with respect to the core region 13 of the optical waveguide 11. By irradiating a short pulse high peak power intensity laser beam in the direction perpendicular to the core region 13 and the electric field (from above in the figure) in the direction of arrow A simultaneously with the generation of the electric field,
An optically anisotropic region is formed in the core region 13 of the optical waveguide 11.

The laser light having a short pulse and high peak power intensity is condensed in the vicinity of the core region 13 by a condensing lens (not shown), and while a high voltage is applied between the electrodes 15 and 16 in the longitudinal direction of the core region 13. Irradiation while scanning. In the present embodiment, the intensity of the electric field applied to the core region 13 by the electrodes 15 and 16 is set to 10 ⁶ to 10 ⁷ V / cm.

In addition, simultaneously with the application of a high DC voltage, the core region
The laser beam of short pulse high peak power intensity applied to 13 has a repetition pulse frequency of 200 kHz and a pulse width of 200 kHz.
A so-called femtosecond laser having an fs of 800 nm was used. This laser light is hardly absorbed by the quartz glass and the average power is at most about 200 mW, so that the irradiation area in the core area 13 hardly generates heat. However, since the pulse width is very narrow, the peak power becomes very large. Therefore, when a laser beam is irradiated simultaneously with application of a high voltage, the electric dipole is oriented by the energy of the laser beam. It is important that the wavelength of the laser beam is transparent to a glass material such as quartz that constitutes the optical waveguide. In general, for infrared light having a wavelength of 1.8 μm or more, laser light is absorbed by many glass materials including quartz glass. Therefore, most of the energy of the laser light is converted into heat. In addition to causing thermal deformation of the waveguide shape, relaxation of the electric dipole due to molecular vibration occurs simultaneously, so that a large nonlinear effect cannot be expected.

In this embodiment, the polarization direction of the electric field of the laser light is substantially the same as that of the electric field lines of the DC high-voltage electric field in order to enhance the orientation of the electric dipole by irradiating the laser light with the short pulse high peak power intensity. It was made parallel.

In the present embodiment, the core region 13 is formed by a high frequency sputtering method using a quartz plate containing a dopant such as germanium as a target material as described above. Germanium is added to increase the refractive index more than pure quartz. However, to increase the refractive index, not only germanium but also titanium or phosphorus may be added. To further enhance the nonlinear optical effect, it is effective to add an element such as tin, phosphorus, boron, terbium, and erbium.

The method of forming the core region 13 is not limited to the sputtering method, but may be an electron beam evaporation method or a flame deposition method.

The electrodes 15 and 16 shown in FIG. 1 are electrodes for applying a high voltage used for forming an optically anisotropic region in the core region 13, and these electrodes 15 and 16 are directly applied to a DC high voltage. If the voltage power supply 18 is replaced by an appropriate signal light control power supply, it can be used as a drive electrode for controlling the signal light propagating through the core region 13 of the optical waveguide 11 at high speed.

FIG. 2 is a conceptual diagram showing another embodiment of the method for manufacturing a nonlinear optical element according to the present invention.

The difference from the embodiment shown in FIG. 1 is that the surrounding cladding region 24 surrounding the core region 23 on the quartz substrate 22 has a trapezoidal cross-sectional shape. The point is that the position is higher than the positions of the high-voltage application electrodes 25 and 26 used for forming the optically anisotropic region in the core region 23.

The optical waveguide 21 as this nonlinear optical element
Since the position of the upper bottom of the cladding region 24 is higher than the positions of the electrodes 25 and 26, the DC high voltage electric field is efficiently applied into the core region 23, and at the same time, the electrode 25 is applied by applying a high voltage from the DC high voltage power supply 28. , 26 can be further suppressed. In the structure shown in FIG. 2, if the electrodes 25 and 26 used for applying a high voltage during the poling process are used as driving electrodes for controlling the signal light propagating through the optical waveguide, the structure shown in FIG. The signal light can be controlled. In the figure, reference numeral 27 denotes an electrode protection layer.

The cladding region 24 having such a trapezoidal cross section can be easily formed by a plasma CVD method or a high frequency bias sputtering method.

FIG. 3 is an external perspective view showing another embodiment of the nonlinear optical element of the present invention.
1 shows a Mach-Zehnder type optical modulator 31.

The optical modulator 31 has a core region 33 on a quartz substrate 32.
Is formed, and a cladding region 34 is formed so as to surround the core region 33. Y branches 35 and 36 are formed on the input side and output side of the signal light, respectively. The signal light incident in the direction of arrow C is divided into two equal parts by the Y branch 35 on the input side and
The light propagates through the waveguide arms 37 and 38. This waveguide arm is provided on both sides of the cladding region 34 of one waveguide arm 37.
Two electrodes 39 and 40 are formed with 37 interposed therebetween. Both electrodes
39 and 40 are connected to an AC power supply 41. Waveguide arm 37
The signal light propagating inside undergoes a phase change due to the electric field of the AC voltage. By combining this signal light with the signal light propagating through the other waveguide arm 38 at the Y branch 36 on the output side, the output intensity changes in accordance with the phase difference between the two signal lights, and Emit.

Here, the polarization direction of the signal light incident on the optical modulator 31 is set to be parallel to the direction of the AC electric field.
The electrodes 39 and 40 for causing a phase change are both electrodes.
The electrode used during the poling process for inducing optical anisotropy in the core of the waveguide arm 37 sandwiched between 39 and 40 can be used as it is. Electrode during polling process
DC high voltage power supply is connected between 39 and 40, several hundred to several kV
Is applied. On the other hand, in order to control the propagation of signal light and operate as a modulator, it is sufficient to connect an AC power supply of at most several volts.

As described above, according to the present invention, the following excellent effects are exhibited.

Conventionally, by applying a DC high-voltage electric field from the outside so as to be orthogonal to the optical waveguide and simultaneously irradiating a laser beam with a short pulse and high peak power intensity so as to be orthogonal to the electric field and the optical waveguide, In an optical waveguide using a glass material in which only a nonlinear optical effect is realized, a very large optically anisotropic region can be efficiently induced. Thus, switching, modulation, and the like of light utilizing the non-linear optical effect such as the electro-optical effect and the second harmonic generation
It is possible to realize a small-sized non-linear optical element having low loss, high reliability, small driving energy, such as wavelength conversion.

[0048]

In summary, according to the present invention, the following excellent effects are exhibited.

It is possible to provide a small-sized nonlinear optical element having low loss, high reliability, small driving energy, and a method of manufacturing the same.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing one embodiment of a method for manufacturing a nonlinear optical element of the present invention.

FIG. 2 is a conceptual diagram showing another embodiment of the method for manufacturing a nonlinear optical element of the present invention.

FIG. 3 is an external perspective view showing another embodiment of the nonlinear optical element of the present invention.

[Explanation of symbols]

 11, 21 Optical waveguide 12, 22, 32 Quartz substrate 13, 23, 33 Core region 14, 24, 34 Cladding region 15, 16, 25, 26, 39, 40 Electrode 17, 27 Electrode protective layer 18, 28 DC high voltage Power supply 31 Optical modulator 35, 36 Y branch 37, 38 Waveguide arm 41 AC power supply

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H047 KA04 KB09 LA12 NA02 NA08 PA00 PA03 PA04 PA05 PA21 PA24 QA04 RA08 TA01 TA31 2H079 AA02 BA01 DA05 EA05 EB05 HA12 2K002 CA02 FA30 GA07

Claims (8)

[Claims] 1. A nonlinear optical element comprising: a planar substrate; a core region formed on the planar substrate; and a cladding region having a lower refractive index than the core region and covering the core region. A nonlinear optical element wherein an optically anisotropic region having a large nonlinear optical coefficient is locally formed in a core region. 2. A method for manufacturing a nonlinear optical element, comprising: forming a core region on a planar substrate; and covering the core region with a cladding region having a lower refractive index than the core region to form an optical waveguide. At the same time as applying an electric field of DC high voltage from the outside in the plane direction of the flat substrate so as to be orthogonal to the optical waveguide, a short pulse laser beam is applied in the thickness direction of the flat substrate so as to be orthogonal to the electric field and the optical waveguide. A method for manufacturing a nonlinear optical element, comprising forming an optically anisotropic region in the core region by irradiation. 3. The method for manufacturing a nonlinear optical element according to claim 2, wherein the direction of polarization of the electric field of the short-pulse laser light is substantially parallel to the direction of the electric field. 4. The wavelength of the short pulse laser beam is 1.8 μm.
The nonlinear optical element according to claim 2 or 3, wherein the repetition frequency is 100 kHz or more, and the peak power intensity in the optical waveguide irradiated with the short pulse laser light is 10 ⁵ W / cm ² or more. Manufacturing method. 5. The optical waveguide according to claim 2, wherein said optical waveguide is composed mainly of quartz glass and said core region is doped with any of germanium, titanium, tin, phosphorus, boron, terbium and erbium. 5. The method for manufacturing a nonlinear optical element according to any one of items 1 to 4. 6. The method for manufacturing a nonlinear optical element according to claim 2, wherein at least a part of the electrode for applying a high voltage is left as an electrode for controlling signal light in the core region. 7. The method for manufacturing a nonlinear optical element according to claim 2, wherein at least a part of the high voltage application electrode is covered with a glass material. 8. The cladding region around the core region is formed in a substantially trapezoidal cross-sectional shape, and the position of the upper bottom of the cladding region is higher than the position of the electrode for applying high voltage.
8. The method for manufacturing a nonlinear optical element according to any one of items 1 to 7.