JP2007272122A - Optical control element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical control element which suppresses insertion loss more than before and operates with a lower driving voltage. <P>SOLUTION: The optical control element having a crystal substrate 1 having electrooptic effect, an optical waveguide 2 formed on the substrate, and a modulating electrode for modulating light passing through the optical waveguide is characterized in that an infrared transparent conductive film 12 made of composite oxide of Ti and In is used for at least one of a signal electrode 3 and a ground electrode 4 constituting the modulating electrode. Preferably, at least one of the signal electrode 3 and ground electrode 4 has an at least two-layered structure consisting of the infrared transparent conductive film and a metal film. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a light control element, and more particularly to a light control element in which an optical waveguide and a modulation electrode are formed on a crystalline substrate having an electro-optic effect.

Conventionally, in the optical communication field and the optical measurement field, a light control element such as a waveguide type light modulator in which an optical waveguide or a modulation electrode is formed on a substrate having an electro-optic effect is widely used.
In an optical waveguide type optical switch or optical modulator using a substrate having an electro-optic effect such as LiNbO ₃ , a metal is used for an electrode to which a control signal (modulation signal) for switching or modulating an optical path is applied. It is common that

When a metal film is used as the electrode of the light control element and the light control element is operated in the TM mode, a LiNbO ₃ substrate is interposed between the optical waveguide and the metal film in order to prevent light absorption by the metal film. However, it is necessary to provide a buffer layer having a low refractive index and low light absorption. The buffer layer such as an ordinary _{SiO 2, Si 3 N 4,} SiON is used.
In addition, various methods have been proposed so far, such as making the optical waveguide into a ridge structure by using dry etching in order to realize a wide band of the optical modulation frequency and a low driving voltage.

In the following Patent Document 1 or 2, as an electrode to which a control signal is applied, a conductive film having a refractive index lower than that of the substrate is directly formed on the substrate, and in a region excluding the conductive film region on the optical waveguide. An optical device provided with a metal film has been proposed. In particular, in these documents, ITO (In ₂ O ₃ —SnO ₂ ) or an organic conductive material is exemplified as a conductive film having a refractive index lower than that of the substrate, and light that enables high-speed operation in combination with a metal film. A control element is disclosed.
Hei 1-128036 Hei 1-128038

However, in Patent Document 1 or 2 described above, there is a big problem when considering use as an actual device. That is, when an ITO material is used as a conductive material for the light control element used in the communication wavelength band, the insertion loss is large, resulting in an insertion loss level that cannot be used as an actual device. This is because ITO has excellent transmission characteristics in the visible region (transmittance of 80% or more), but in the wavelength region of 1200 nm or more, the transmittance is almost zero.
In the case of an organic conductive material, heat treatment is performed in the manufacturing process of the light control element, so that there are severe restrictions on the production such as the heat treatment temperature, and there is a concern about long-term reliability as an actual device.

Further, Patent Document 3 below discloses a material containing titanium or tungsten as an indium oxide-based oxide transparent electrode having high infrared transmittance. In Patent Document 3, as an application of the oxide transparent electrode, application to a solar cell in the infrared region and application to a light detection element are exemplified. However, like an electrode of a light control element, the refractive index of light is set. There is no disclosure of a method for use as a pattern-controlled electrode having a control function, and there is no suggestion of using it for a light control element having a novel structure to be described later.
JP 2004-207221 A

On the other hand, in Patent Document 4 or 5 below, an optical waveguide and a modulation electrode are incorporated in an extremely thin substrate (hereinafter referred to as “first substrate”) having a thickness of 30 μm or less, and the dielectric constant is lower than that of the first substrate. Other substrates are bonded to reduce the effective refractive index with respect to the microwave, thereby achieving speed matching between the microwave and the light wave.
JP-A 64-18121 JP 2003-215519 A

  In Patent Document 4 or 5, the use of a thin first substrate greatly increases the degree of freedom in designing the light control element. For example, an optical modulator having a low driving voltage can be used without using a buffer layer. It is disclosed that it can be produced. However, even in a structure that does not use a buffer layer, there is a known problem that if a metal electrode serving as a modulation electrode is disposed within a predetermined range near the waveguide, light waves are absorbed and insertion loss increases.

  The problem to be solved by the present invention is to solve the above-described problems, to provide an optical control element that can operate at a low drive voltage, with a lower insertion loss than in the prior art.

In order to solve the above-described problem, in the invention according to claim 1, a crystal substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating light passing through the optical waveguide are provided. In the light control element, an infrared transparent conductive film made of a composite oxide of Ti and In is used for at least one of the signal electrode and the ground electrode constituting the modulation electrode.
The “infrared transparent conductive film” in the present invention means that the transmittance is 80% or more, preferably 95% or more, and the volume resistance value is 5 × 10 ¹ Ωcm in the wavelength region of 1200 nm or more. Means something smaller.

  In the invention according to claim 2, in the light control element according to claim 1, at least one of the signal electrode and the ground electrode has a structure of two or more layers including the infrared transparent conductive film and a metal film. It is characterized by that.

  In the invention according to claim 3, in the light control element according to claim 2, the signal electrode and the ground electrode are both composed of an infrared transparent conductive film and a metal film, and the metal film of the signal electrode and the The minimum distance between the ground electrode and the metal film is 15 μm or less.

  According to a fourth aspect of the present invention, in the light control element according to any one of the first to third aspects, the optical waveguide is a ridge type optical waveguide.

  The invention according to claim 5 is the light control element according to any one of claims 1 to 4, wherein the thickness of the crystal substrate is 10 μm or less.

  In the invention according to claim 6, in the light control element according to claim 5, a ground electrode having an infrared transparent conductive film is formed on a surface different from the surface of the crystal substrate on which the signal electrode is formed. It is characterized by.

  According to a seventh aspect of the present invention, in the light control element according to any one of the first to sixth aspects, the optical waveguide has two branch waveguides that branch or multiplex, and the modulation electrode is provided in the branch waveguide. The direction of spontaneous polarization of the crystal substrate in the region to which is applied is different for each branching waveguide.

  In the invention according to claim 8, in the light control element according to any one of claims 1 to 7, a buffer layer having a thickness of 1 μm or less is formed between the infrared transparent conductive film and the crystal substrate. It is characterized by.

  According to a ninth aspect of the present invention, in the light control element according to any one of the first to eighth aspects, the infrared transparent conductive film includes an In oxide as a base and a Ti oxide of 0.3 to 1.5 wt. % Content.

  According to a tenth aspect of the present invention, in the light control element according to any one of the first to ninth aspects, the infrared transparent conductive film is formed by a sputtering method, and then adjusts a resistance value and a transmittance. The heat treatment process is performed.

  According to an eleventh aspect of the present invention, in the light control element according to any one of the first to tenth aspects, the light control element is used in a region having a longer wavelength than the optical communication wavelength band.

  According to the first aspect of the present invention, an infrared transparent conductive film made of a composite oxide of Ti and In is used for at least one of the signal electrode and the ground electrode, so that even if the wavelength used for the light control element exceeds 1200 nm, Absorption of light does not occur and a buffer layer is unnecessary or the thickness can be reduced, so that a low drive voltage device can be provided without an increase in insertion loss. Furthermore, since the infrared transparent electrode film absorbs light much less than the metal electrode, the modulation electrode can be disposed near the optical waveguide as compared with the conventional case. That is, it is possible to provide a light control element with a further low driving voltage due to a synergistic effect in which the electric field is applied to the optical waveguide more effectively.

  According to the second aspect of the present invention, since at least one of the signal electrode and the ground electrode has a structure of two or more layers including an infrared transparent conductive film and a metal film, the resistance value of the electrode itself is reduced to infrared transparent. It is possible to provide a light control element that can be made smaller than the case of using only a conductive film and that can operate at high speed even in a high-frequency region.

  According to the invention of claim 3, since both the signal electrode and the ground electrode are composed of the infrared transparent conductive film and the metal film, the minimum distance between the metal film of the signal electrode and the metal film of the ground electrode is reduced. It becomes possible to make the thickness 15 μm or less, and it becomes possible to provide a light control element with a much lower driving voltage.

  According to the invention of claim 4, since the optical waveguide is a ridge-type optical waveguide, a waveguide with a large light confinement can be used, and the effect of reducing the waveguide interval as compared with the prior art, and an infrared transparent conductive film Can be arranged closer to the waveguide than the metal film, so that it is possible to provide a further low drive voltage type light control element. In addition, the infrared transparent conductive film can form an electrode in a region including the side surface of the ridge-type waveguide without absorbing light waves, and thus has an effect of further reducing the driving voltage.

  According to the invention of claim 5, since the thickness of the crystal substrate is 10 μm or less, the degree of freedom in design is widened. In addition to reducing the restrictions on the arrangement of the signal and ground electrodes, such as a structure that satisfies the speed matching condition and the speed matching, the substrate can be used effectively, so that a low drive voltage type light control element can be used more efficiently. It becomes possible to provide.

  According to the invention of claim 6, since the ground electrode having the infrared transparent conductive film is formed on a surface different from the surface of the crystal substrate on which the signal electrode is formed, the substrate can be effectively used and manufactured. It becomes possible to improve the reproducibility. In addition, when the thickness of the crystal substrate is 10 μm or less, an electric field can be more effectively applied to the optical waveguide by the signal electrode and the ground electrode arranged with the crystal substrate interposed therebetween. A voltage type light control element can be provided.

  According to the invention of claim 7, the optical waveguide has two branch waveguides that branch or multiplex, and the direction of the spontaneous polarization of the crystal substrate in the region where the modulation electrode applies an electric field to the branch waveguides Since each waveguide is different, differential driving can be easily performed without using an external electric device, and a light control element capable of operating at a lower driving voltage can be provided.

  According to the eighth aspect of the present invention, since the buffer layer having a thickness of 1 μm or less is formed between the infrared transparent conductive film and the crystal substrate, light wave absorption is more strictly performed without impairing the low driving voltage. And the speed matching condition can be relaxed.

  According to the invention of claim 9, since the infrared transparent conductive film is based on In oxide and Ti oxide is in the range of 0.3 to 1.5 wt%, the carrier concentration is appropriate, high transmittance and low It is possible to provide an optical control element that can form an infrared transparent conductive film having both volume resistance values, can suppress insertion loss, and can operate at a low driving voltage.

  According to the invention of claim 10, since the infrared transparent conductive film is manufactured by a sputtering method and then undergoes a heat treatment step for adjusting the resistance value and the transmittance, there is little compositional deviation between the raw material and the film formation sample, A wide process margin can be taken. In addition, since the transmittance and the volume resistance value can be adjusted in a subsequent process by heat treatment, it is possible to provide a low drive voltage type light control element with high yield.

  According to the invention of claim 11, since the light control element is used in a region having a wavelength longer than that of the optical communication wavelength band, it is possible to provide a light control element having a particularly small insertion loss as the optical communication wavelength device. Become.

Hereinafter, the present invention will be described in detail using preferred examples.
As shown in FIGS. 1 to 8, the present invention passes through a crystal substrate (1, 1 ′) having an electro-optic effect, an optical waveguide (2, 2 ′) formed on the substrate, and the optical waveguide. In a light control element having a modulation electrode for modulating light, at least one of the signal electrode 3 and the ground electrodes (4, 5) constituting the modulation electrode is red made of a composite oxide of Ti and In. The outer transparent conductive film 12 is used.

  As a crystalline substrate having an electro-optic effect, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used. In particular, lithium niobate (LN) or lithium tantalate (LT) crystals having a high electro-optic effect are preferably used. In addition, when an X-cut substrate is used as the crystalline substrate, the modulation electrode is disposed so as to sandwich the optical waveguide in a direction parallel to the surface of the crystal substrate. In general, a modulation electrode is disposed directly on an optical waveguide via a layer. However, in the case of a Z-cut type structure using polarization inversion, for example, a push-pull operation can be performed by arranging a signal electrode in the center of a Mach-Zehnder type optical waveguide. In this case, the buffer layer is not necessarily required.

  The modulation electrode includes various electrodes formed on the crystalline substrate in order to drive a light control element such as a signal electrode, a ground electrode, or a DC electrode. In order to form an infrared transparent conductive film as a modulation electrode, an electrode pattern is formed by a photolithography method and formed by a lift-off method, a mask material is formed so that a predetermined electrode pattern remains, dry etching, or wet It is possible to form by etching

A composite oxide film of In and Ti is used as the infrared transparent conductive film. This is because high transmittance is exhibited even in a wavelength region of 1200 nm or more. In the case of a light control element for a communication wavelength band, for example, a light control element used at 1550 nm, the transmittance of the infrared transparent conductive film at 1550 nm is preferably 80% or more, and more preferably 95% or more. The rate is more preferred. The smaller the volume resistance value, the better. In the case of a composite oxide film of In and Ti, the value can be expected to be about 1 × 10 ⁻⁴ Ωcm. However, when these light control elements are used in a low frequency region (generally 1 GHz or less), or when an infrared transparent conductive film is used as a ground electrode, the volume resistance value is less limited. Even about 10 ¹ Ωcm is sufficient.

Infrared transparent conductive films vary greatly in physical properties depending on the production conditions. Table 1 shows the transmittance and the volume resistance value depending on the difference in oxygen partial pressure supplied at the time of production. The common production conditions for the infrared transparent conductive film in Table 1 were DC power of 300 W, sputtering gas Ar + O ₂ , and the film thickness was set to 200 nm.
The transmittance was measured using a spectrophotometer U4000 (manufactured by Hitachi, Ltd.). In the table, the values when the test light is 1550 nm are shown.
The volume resistance value was measured using a four-probe method using a resistance measuring instrument (manufactured by Dia Instruments, MCP-T610 type).

  Table 2 shows the transmittance and volume resistance value due to the difference in heat treatment conditions after preparation with an oxygen partial pressure of 1%. The heat treatment was performed in the atmosphere for 1 hour.

  The infrared transparent conductive film has different physical property values depending on the thickness to be formed. This is considered to be due to an unintended influence of the substrate temperature rise during film formation. However, even in this case, desired physical property values can be obtained by optimizing the heat treatment conditions in the subsequent steps.

(Specimen 1)
An optical waveguide was formed by a Ti diffusion method using a Z-cut LN substrate having a thickness of 500 μm as a crystalline substrate. Next, after forming 0.7 μm of an infrared transparent conductive film using a DC magnetron sputtering apparatus, the waveguide loss at each polarization at a wavelength of 1550 nm was determined by a cut-back method. The transmittance at 1550 nm of the prepared infrared transparent conductive film was measured using a spectrophotometer U4000 (manufactured by Hitachi, Ltd.). The volume resistance value was measured by a four-probe method using a resistance measuring instrument (manufactured by Dia Instruments, MCP-T610 type).

(Specimen 2)
A sample prepared in the same manner as in Evaluation Example 1 was evaluated in the same manner except that it was heated in the atmosphere at 200 ° C. for 1 h.
(Specimen 3)
A sample prepared in the same manner as in Evaluation Example 1 was evaluated in the same manner except that it was heated in the atmosphere at 400 ° C. for 1 h.
The evaluation results of the above test bodies 1 to 3 are shown in Table 3.

From the above characteristics of the infrared transparent conductive film, it is easy to adjust the transmittance and volume resistance value by adjusting the oxygen partial pressure of the sputtering gas and adjusting the heat treatment conditions after forming the conductive film. Understandable. Furthermore, the adjustment can be similarly made for the excess loss in the TM mode and the TE mode.
Note that in a composite oxide film of In and Ti, if the In oxide is a base material and the Ti oxide is in the range of 0.3 to 1.5 wt%, the transmittance is in a wavelength region of 1200 nm or more. It has also been confirmed that a material having a volume resistance value of 80% or more and smaller than 5 × 10 ¹ Ωcm can be realized.

Next, an example of a light control element having a novel structure using an infrared transparent conductive film will be described.
1 to 8 all show sectional views of an optical modulator having a light control element, particularly a Mach-Zehnder type waveguide.
FIG. 1 shows an example in which a Z-cut crystal substrate 1 is used. When the infrared transparent conductive film 12 is used for the ground electrode 4 constituting the modulation electrode (see FIG. 1A), the modulation electrode is used. The case where the infrared transparent conductive film 12 is used for the signal electrode 3 to configure (see FIG. 1B) is shown.
Reference numeral 2 denotes a branching waveguide of the Mach-Zehnder type waveguide, 11 denotes a buffer layer, and 10 denotes a metal film.

  With the configuration shown in FIG. 1, when the infrared transparent conductive film 12 is used, the buffer layer 11 can be omitted, and an electric field can be efficiently applied to the optical waveguide. An element can be realized.

FIG. 2 shows an example in which an X-cut crystal substrate 1 ′ is used. When the infrared transparent conductive film 12 is used for the ground electrode 4 constituting the modulation electrode (see FIG. 2A), the modulation electrode The case where the infrared transparent conductive film 12 is used for the signal electrode 3 that constitutes (see FIG. 2B) is shown.
With the configuration shown in FIG. 2, when the infrared transparent conductive film 12 is used, the infrared transparent conductive film can be disposed closer to the optical waveguide 2 than the metal film, and an electric field can be efficiently applied to the optical waveguide. Thus, it is possible to realize a light control element with a low driving voltage.

FIG. 3 shows an example in which the Z-cut crystal substrate 1 is used. When the ground electrode 4 constituting the modulation electrode is formed by a combination of the infrared transparent conductive film 12 and the metal film 10 (FIG. 3A). )) And the case where the signal electrode 3 constituting the modulation electrode is formed of a combination of the infrared transparent conductive film 12 and the metal film 10 (see FIG. 3B).
In the case of FIG. 3 as well, in the case of using the infrared transparent conductive film 12, the buffer layer 11 can be omitted when the infrared transparent conductive film 12 is used, whereby an electric field is efficiently applied to the optical waveguide. Therefore, a light control element with a low driving voltage can be realized. In addition, since the metal film is disposed in contact with the infrared transparent conductive film, the electric resistance value of the ground electrode (in the case of FIG. 3A) or the signal electrode (in the case of FIG. 3B) is lowered, and the high frequency It is possible to provide a light control element that enables higher-speed operation in the region.

FIG. 4 shows an example in which an X-cut crystal substrate 1 ′ is used. When the ground electrode 4 constituting the modulation electrode is formed of a combination of the infrared transparent conductive film 12 and the metal film 10 (FIG. 4 ( a) and a case where the signal electrode 3 constituting the modulation electrode is formed of a combination of the infrared transparent conductive film 12 and the metal film 10 (see FIG. 4B).
In the case of FIG. 4 as well, as in the configuration of FIG. 2, the infrared transparent conductive film can be disposed closer to the optical waveguide 2 than the metal film, an electric field can be efficiently applied to the optical waveguide, and low drive is achieved. A voltage light control element can be realized. In addition, since the metal film is disposed in contact with the infrared transparent conductive film, the electrical resistance value of the ground electrode (in the case of FIG. 4A) or the signal electrode (in the case of FIG. 4B) is lowered, and the high frequency It is possible to provide a light control element that enables higher-speed operation in the region.

FIG. 5 shows an example using an X-cut crystal substrate 1 ′, in which a ground electrode 4 and a signal electrode 3 constituting a modulation electrode are formed by a combination of an infrared transparent conductive film 12 and a metal film 10. Shows the case.
In the case of FIG. 5, the infrared transparent conductive film can be disposed closer to the optical waveguide 2 than the metal film, an electric field can be efficiently applied to the optical waveguide, and a light control element with a low driving voltage is realized. It becomes possible. In particular, the interval between the metal films can be reduced until the interval W between the metal films of each electrode is 15 μm or less, and further 10 μm or less. When a conventional infrared transparent conductive film is not used (usually, a metal film) The interval is about 20 μm, which is the limit.) The interval is about half, and the light control element can be operated with a lower driving voltage.
In addition, since the metal film is arrange | positioned in contact with an infrared transparent conductive film, it cannot be overemphasized that the electrical resistance value of each electrode also falls.

FIG. 6 shows an example of a light control element having a ridge-type optical waveguide 2 ′.
FIG. 6A is an example using a Z-cut type crystal substrate 1, and a combination of an infrared transparent conductive film 12 and a metal film 10 together with a ground electrode 4 and a signal electrode 3 constituting a modulation electrode. The case where it formed in is shown. FIG. 6B similarly shows an example using an X-cut crystal substrate 1 ′.

When the ridge type optical waveguide is used, a waveguide with a large light confinement can be used, and the effect of reducing the waveguide interval compared to the conventional case, and the infrared transparent conductive film 12 is more conductive than the metal film 10. Due to the synergistic effect that can be disposed close to the waveguide 2 ', it is possible to provide a further low drive voltage type light control element. In addition, in the case of an infrared transparent conductive film, an electrode can be formed in a region including the side surface of the ridge-type optical waveguide 2 ′ without absorbing light waves (not shown). is there.
The right ground electrode 4 (infrared transparent conductive film 12) in FIG. 6A is configured to straddle the right groove forming the right ridge-type optical waveguide 2 ′. It is also possible to arrange the infrared transparent conductive film 12 along. Further, the infrared transparent conductive film 12 in FIG. 6B can be disposed so as to be in contact with the side surface of the ridge-type optical waveguide 2 ′.

FIG. 7 shows an example where the thickness of the crystal substrate is 10 μm or less. An example in which a ridge type optical waveguide 2 ′ is used as the optical waveguide is shown.
FIG. 7A shows an example in which a Z-cut crystal substrate 1 is used. The modulation electrode disposed on the upper side of the substrate 1 is composed of a combination of an infrared transparent conductive film 12 and a metal film 10. Yes. Similarly, FIG. 7B is an example using an X-cut crystal substrate 1 ′.

When the crystal substrate is thin, it is possible to form a modulation electrode such as a ground electrode on the lower side of the substrate by using an infrared transparent conductive film. In FIG. 7 (a), since it is a Z-cut type crystal substrate, a back electrode 5 serving as a ground electrode is provided on the lower side of the optical waveguide 2 ′ so as to generate an electric field in the vertical direction of the drawing. The conductive film 12 is used.
In FIG. 7B, since it is an X-cut type crystal substrate, a back electrode 5 serving as a ground electrode is disposed on the lateral side of the optical waveguide 2 ′ so as to generate a horizontal electric field in the drawing. The transparent conductive film 12 is used.

  When the back electrode 5 serving as the ground electrode as shown in FIG. 7 is used, for example, the ground electrode on the upper side of the substrate can be eliminated, and the light control element can be configured more compactly. Effective use is possible. Furthermore, since the electrode arrangement is three-dimensional, the shape and arrangement of the electrode for more efficiently applying an electric field to the optical waveguide can be achieved, and the production reproducibility of the light control element can be improved.

FIG. 8 shows an example using polarization reversal.
FIG. 8 uses a Z-cut type crystal substrate 1, and the modulation electrode disposed on the upper side of the substrate 1 is composed of a combination of an infrared transparent conductive film 12 and a metal film 10. Inside the crystal substrate 1, as indicated by arrows A and B, the direction of spontaneous polarization in the crystal is different on the left and right sides of the drawing with a boundary line C as a boundary.

When 2 is a branching waveguide such as a Mach-Zehnder type optical waveguide, the direction of spontaneous polarization in the crystal substrate 1 including each branching waveguide 2 is different for each branching waveguide. As described above, when an electric field is applied to the two branch waveguides using a common electrode (signal electrode 3), the electro-optic effects generated in the branch waveguides are opposite to each other. This is the same as preparing an independent electrode for each branching waveguide and performing differential driving using an external electric device. Therefore, with the configuration of FIG. 8, it is possible to provide a light control element that can easily realize differential driving and can operate at a lower driving voltage.
FIG. 8B shows an example in which the thickness of the crystal substrate is 10 μm or less, and has the same effect as the example of FIG. 7 in addition to the above effect.

Example 1
An optical waveguide was formed by a Ti diffusion method using an X-cut LN substrate having a thickness of 500 μm as a crystalline substrate. Next, a photoresist mask having an opening in the ground electrode pattern is formed on the substrate through a photolithography process, and an infrared transparent conductive film is formed with a DC magnetron sputtering apparatus, and then immersed in an organic solvent. The pattern was formed by the lift-off method.
The infrared transparent conductive film was performed under the conditions of DC power of 300 W and oxygen partial pressure of 1%.
After forming the infrared transparent film, heat treatment was performed at 180 ° C. for 1 hour in the air. Thereafter, an Au electrode having a height of 20 μm was formed as a signal electrode by electrolytic plating using the resist pattern as a guide. Although the distance between the signal electrode and the ground electrode was narrowed by 5 μm compared to the conventional one, the increase in insertion loss of the manufactured light control element was 0.3 dB or less, and the drive voltage was reduced by 20% compared to the conventional one.

(Example 2)
A Z-cut LN substrate having a thickness of 500 μm was used as the crystalline substrate, and an infrared transparent conductive film serving as a lower ground electrode was formed on one surface of the substrate with a thickness of about 1 μm. The infrared transparent conductive film was formed by a reactive sputtering method with a DC power of 300 W and an oxygen partial pressure of 1%, and heat treatment was performed in the atmosphere at 180 ° C. for 1 hour.
Next, the substrate with infrared transparent conductive film was applied to a new Z-cut LN substrate with an adhesive applied to the surface on which the infrared transparent conductive film was formed, and then bonded together. After the substrate is pasted and fixed on a polishing jig, one surface of the substrate with the infrared transparent conductive film is fixed on a lapping machine polishing machine (carrier: epoxy resin containing glass fiber, lapping agent: GC # 1200 20 wt% aq). Then, rough polishing is performed until the thickness of the substrate becomes approximately 50 μm under conditions of a speed of 35 min ⁻¹ and a lap pressure of 12.75 to 9.81 kPa. Thereafter, precision mirror polishing was performed to a set thickness by mechanochemical polishing (CMP) using a nonwoven fabric as the pad material and colloidal silica as the processing liquid. The set thickness of the substrate was 4 μm.

  A ridge type optical waveguide having a Mach-Zehnder type interference system was formed on the substrate. For the formation, a dry etching method using a photoresist as a mask material was used. The etching depth was 2 μm. An infrared transparent conductive film of 0.5 μm was formed on the ridge-type waveguide, and a modulation electrode was formed by electroplating.

(Comparative Example 1)
Instead of the infrared transparent conductive film formed as the lower ground electrode, an SiO ₂ buffer layer of 0.5 μm and an Au electrode of 1 μm are formed, and instead of the infrared transparent conductive film formed on the ridge-type waveguide, SiO _{2 A} light control element was produced in the same manner as in Example 2 except that 0.5 buffer layer was formed.

  When the drive voltage of the light control element in Example 2 and Comparative Example 1 was compared, VπL = 6.1 Vcm in Comparative Example 1, whereas VπL = 3.7 Vcm in Example 2 was significantly low. Voltageization was achieved.

  According to the light control element of the present invention, the use of the infrared transparent conductive film made of a composite oxide film of In and Ti can significantly reduce the drive voltage without increasing the insertion loss of light waves. This makes it possible to propose a new light control element structure. Furthermore, according to the present invention, it is possible to significantly reduce the power consumption of the light control element, which has been increasing with the improvement of the information network.

It is a figure which shows the example of the light control element which used the infrared transparent conductive film for a part of modulation electrode with a Z cut type | mold board | substrate. It is a figure which shows the example of the light control element which used the infrared transparent conductive film for a part of modulation electrode by X cut type | mold board | substrates. It is a figure which shows the example of the light control element which used the combination of an infrared transparent conductive film and a metal film for a part of modulation electrode with a Z cut type | mold board | substrate. It is a figure which shows the example of the light-control element which used the combination of the infrared transparent conductive film and the metal film for a part of modulation electrode by X cut type | mold board | substrates. It is a figure which shows the example of the light control element which used the combination of the infrared transparent conductive film and the metal film for all the modulation | alteration electrodes by X cut type | mold board | substrates. It is a figure which shows the example of the light control element using a ridge type | mold optical waveguide. It is a figure which shows the example of the light control element which used the thin plate for the crystal substrate. It is a figure which shows the example of the light control element which used polarization inversion for the crystal substrate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Z cut type crystal substrate 1 'X cut type crystal substrate 2 Optical waveguide 2' Ridge type optical waveguide 3 Signal electrode 4 Ground electrode 5 Back surface electrode 10 Metal film 11 Buffer layer 12 Infrared transparent conductive film

Claims (11)

In a light control element having a crystal substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating light passing through the optical waveguide,
An optical control element, wherein an infrared transparent conductive film made of a composite oxide of Ti and In is used for at least one of a signal electrode and a ground electrode constituting the modulation electrode.   2. The light control element according to claim 1, wherein at least one of the signal electrode and the ground electrode has a structure of two or more layers including the infrared transparent conductive film and a metal film. Light control element.   3. The light control element according to claim 2, wherein the signal electrode and the ground electrode are both composed of an infrared transparent conductive film and a metal film, and a minimum of the metal film of the signal electrode and the metal film of the ground electrode. A light control element having an interval of 15 μm or less.   4. The light control element according to claim 1, wherein the optical waveguide is a ridge type optical waveguide.   5. The light control element according to claim 1, wherein the crystal substrate has a thickness of 10 [mu] m or less.   6. The light control element according to claim 5, wherein a ground electrode having an infrared transparent conductive film is formed on a surface different from the surface of the crystal substrate on which the signal electrode is formed.   7. The light control element according to claim 1, wherein the optical waveguide has two branch waveguides to be branched or combined, and the crystal substrate in a region where the modulation electrode applies an electric field to the branch waveguides. The direction of the spontaneous polarization of the light control element is different for each branching waveguide.   8. The light control element according to claim 1, wherein a buffer layer having a thickness of 1 μm or less is formed between the infrared transparent conductive film and the crystal substrate. Control element.   9. The light control element according to claim 1, wherein the infrared transparent conductive film includes an In oxide as a base material and a Ti oxide in a range of 0.3 to 1.5 wt%. A light control element characterized by that.   10. The light control element according to claim 1, wherein the infrared transparent conductive film is formed by a sputtering method and then undergoes a heat treatment step for adjusting a resistance value and a transmittance. A light control element. 11. The light control element according to claim 1, wherein the light control element is used in a region having a wavelength longer than an optical communication wavelength band.

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