JP2001100163A - Optical waveguide element and phase control method for optical waveguide element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new optical waveguide type element which permits ON/OFF control over an external signal at a high quenching ratio by compensating effective refractive index variation of a substrate and an optical waveguide due to defects and stress generated at the time of the production and a phase control method for the optical waveguide element. SOLUTION: This invented optical waveguide element 10 has a substrate 1 having electrooptic effect, a Mach-Zehnder type optical waveguide 2 for guiding a light wave, and a signal electrode 4 and ground electrodes 5-1 and 5-2 for controlling the light wave guided in the optical waveguide 2. At least part of 1st and 2nd branch optical waveguides 3-1 and 3-2 constituting the optical waveguide 2 on the substrate, a film body 6 of a material having a higher effective refractive index than the optical waveguide is formed to thickness less than a quarter as large as the wavelength of the light wave. Here, the film body 6 is formed by forming a specific thin film and then trimming the thin film so that phase control is placed in the best state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide device and a phase control method for the optical waveguide device, and more particularly, to a waveguide type optical intensity modulator, a phase modulator, and a high speed / large capacity optical fiber communication system. The present invention relates to an optical waveguide element and a phase control method of the optical waveguide element that can be suitably used for a polarization scrambler or the like.

[0002]

2. Description of the Related Art With the recent progress in high-speed, large-capacity optical fiber communication systems, niobium acid has been used instead of the conventional direct modulation of a laser diode because of wide band characteristics, low chirp characteristics, and small propagation loss. Practical use of a waveguide type external modulator using lithium (LiNbO _3; hereinafter, sometimes abbreviated as LN) is in progress.

In such a waveguide type optical modulator, a predetermined high frequency external electric signal is applied to a modulation electrode provided in the vicinity of a Mach-Zehnder type optical waveguide constituting the optical modulator. The on / off of the external signal is controlled by changing the phases of the light waves guided in the two branched optical waveguides and utilizing the extinction of the light waves when these light waves are combined. However, due to process errors, formation of various layers, and installation of the modulation electrode, various defects, stresses, and the like are present in the optical modulator. For this reason, even though the optical modulators are manufactured under exactly the same conditions, the effective effective refractive indexes of the substrates and the optical waveguides constituting each optical modulator may be different from each other.

Therefore, the operating points of the optical modulators may be different from each other, and even if a high-frequency external electric signal is applied at the initially set voltage and frequency, the light propagates through the two branched optical waveguides. In some cases, the phase of the light wave cannot be changed as initially set. For this reason, the quenching of the light waves when these light waves are combined becomes insufficient, and as a result, it may not be possible to sufficiently turn on / off the external signal.

In order to cope with such a problem, conventionally, a DC voltage bias is applied to the optical modulator to control the operating point of the optical modulator, thereby guiding the light through the two branched optical waveguides. The external signal is turned on / off by setting the phase change of the light wave to an optimum state and obtaining sufficient extinction.

Further, Japanese Patent Application Laid-Open No. Hei 4-337707 discloses that a Ti—Au vapor-deposited film or an Au plated film is formed on at least one of the two branch optical waveguides as described above. A method of controlling the operating point by using a change in the effective effective refractive index resulting from this distortion, and applying an independent DC voltage different from the external modulation voltage to the modulation electrode. A method is disclosed in which a temperature change is applied to an optical waveguide and the operating point is controlled using an effective refractive index change caused by the temperature change.

Japanese Patent Application Laid-Open No. 4-258918 describes that a phase adjusting region is provided near at least one of two branch optical waveguides constituting a Mach-Zehnder type optical waveguide. According to the above publication, the phase adjustment region is formed, for example, by a Si substrate having an opening at a portion of the substrate constituting the optical modulator where the branch optical waveguide is formed.
After the O ₂ film is formed, the opening is formed by applying a photoresist or the like. Further, Japanese Patent Application Laid-Open No. 7-28006 discloses that a light transmitting film such as a SiO ₂ film is partially provided on at least one of the branch optical waveguides to give a distortion to the branch optical waveguide. A method similar to that disclosed in Japanese Patent Application Laid-Open No. 4-337707, in which an operating point is adjusted by using a change in effective effective refractive index, is disclosed.

In the above publication, an opening is provided in a part of the buffer layer on or in the vicinity of the branch optical waveguide, and the effective effective area is changed by changing the area, shape, and position of the opening. A method for controlling an operating point using a change in refractive index is disclosed. Furthermore, the above-mentioned publication discloses that a cyanoacrylate-based polymer adhesive is applied to the inside of the opening to cause expansion / contraction distortion in the branch optical waveguide, and an operating point is obtained by utilizing a change in effective effective refractive index caused by the distortion. JP-A-4-2589
A method similar to Japanese Patent Publication No. 18 is disclosed. According to these methods, a sufficiently high extinction ratio can be obtained by supplementing the effective effective refractive index fluctuation of the optical waveguide or the like in the optical modulator by operating point control, thereby enabling external signals to be turned on / off. .

[0009]

However, in the method of biasing a DC voltage as described above, a so-called D
A phenomenon called C drift occurs. For this reason, a control circuit for separately monitoring the operating point shift caused by the DC drift is required, which causes a problem that the system is complicated and the cost is high.

Further, Japanese Patent Application Laid-Open Nos. Hei 4-337707, Hei 4-258918 and Hei 7-28006
In the method described in Japanese Patent Application Laid-Open Publication No. H10-163, operating point control is limited to a certain range. For this reason, in the case where the change of the effective refractive index with time is extremely large as described above, the phase change of the light wave guided in the branch optical waveguide is controlled by these methods to obtain a sufficiently high extinction ratio. It was difficult. For this reason, such an optical modulator cannot be put to practical use, and this has been a cause of deteriorating the manufacturing yield of the optical modulator.

[0011] Even if it is possible to control the phase change of the light wave guided in the branching optical waveguide in these methods, for example, in the method described in JP-A-4-258918, the light wave It is necessary to provide a relatively large phase adjustment region in the interaction region between the electrode and the modulation electrode. For this reason, there has been a problem that the size of the optical waveguide element is increased as a whole.

The present invention provides a novel optical waveguide type which compensates for the effective effective refractive index fluctuation of a substrate or an optical waveguide due to a defect or stress generated at the time of fabrication and enables on / off control of an external signal with a high extinction ratio. It is an object of the present invention to provide a phase control method for a device and an optical waveguide device.

[0013]

In order to achieve the above object, an optical waveguide device according to the present invention comprises a substrate having an electro-optic effect, a Mach-Zehnder optical waveguide for guiding a light wave, A modulation electrode for controlling a light wave guided through the optical waveguide. And at least a part of the portion on the substrate where the first and second branch optical waveguides constituting the Mach-Zehnder type optical waveguide are located is made of a material having a higher effective refractive index than the optical waveguide. To form a film body. Further, the thickness of the film body is set to be not more than の of the wavelength of the light wave guided through the optical waveguide.

Further, the phase control method for an optical waveguide device according to the present invention provides a substrate having an electro-optic effect, a Mach-Zehnder type optical waveguide for guiding an optical wave, and a light wave guided through the optical waveguide. In the optical waveguide device having a modulation electrode for controlling, at least a part of a portion on the substrate where the first and second branch optical waveguides constituting the Mach-Zehnder type optical waveguide are located. And a film body made of a material having a higher effective refractive index than the optical waveguide and having a thickness of １ ／ or less of the wavelength of the light wave guided through the optical waveguide. Then, by trimming the film while monitoring the optical output from the optical waveguide, the first waveguide guided in the first branch optical waveguide is trimmed.
The phase between the branched guided light and the second branched guided light guided in the second branched optical waveguide is adjusted.

The present inventors have conducted intensive studies to achieve the above-mentioned object. As a result, by forming a film made of a material having a higher effective refractive index than the optical waveguide formed on the optical waveguide element on the substrate, it is possible to greatly change the effective refractive index of the optical waveguide in such a portion. I found that it is possible.

However, on the other hand, if the optical waveguide is formed on the substrate and then the film is formed on the portion of the substrate where the optical waveguide is formed, the film is higher than the optical waveguide. Due to the refractive index, a light wave guided in the optical waveguide leaks to the film body, which makes it impossible to use the optical waveguide element substantially. Therefore, the present inventors have further conducted extensive research in order to prevent the leak. As a result, it has been found that the leak can be prevented by setting the thickness of the film having the high effective refractive index within a certain range with respect to the wavelength of the light wave guided through the optical waveguide. The present invention has been made as a result of extensive research and search as described above by the present inventors.

According to the present invention, as in the method of applying a DC bias described above, the system becomes complicated,
There is no problem such as high cost. Further, since the film body of the present invention is made of a material having a high effective refractive index, the effective refractive index of the substrate and the optical waveguide formed on the substrate can be largely changed. Therefore, even when the effective refractive index fluctuation of the manufactured optical waveguide element is large and the operating point is largely deviated from the set value,
A sufficiently high extinction ratio can be obtained by compensating for such effective refractive index fluctuation. Further, even when the size of the film body is relatively small, a high extinction ratio can be obtained by compensating for the above-described fluctuation of the effective refractive index.

Further, the film body according to the present invention and Japanese Patent Application Laid-Open No.
The phase adjustment region and the like described in US Pat. No. 5,918,918 are generally formed outside a region where a light wave guided in an optical waveguide interacts with a high-frequency external electric signal applied to a modulation electrode. In forming the film body and the phase adjustment region as described above, it is necessary to positively form a space for forming these in a certain size outside the region where the interaction occurs. . For this reason, the optical waveguide element needs to be formed larger than originally required in consideration of this space.

In the present invention, even when the size of the film body is made relatively small, the effective refractive index fluctuation can be sufficiently compensated, so that the above-mentioned space can be made relatively small. Therefore, the size of the optical waveguide element can be approximated to the original size, and compared with the conventional method of forming the phase adjustment region and the like as described above,
The optical waveguide element can be reduced in size.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings, according to embodiments of the present invention. FIG.
FIG. 2 is a plan view showing an example of the optical waveguide device of the present invention.
In FIG. 1, in order to clarify the features of the present invention,
The configuration, size, and the like are drawn differently from the actual ones.

An optical waveguide device 10 shown in FIG. 1 has a substrate 1 having an electro-optic effect, a Mach-Zehnder type optical waveguide 2 for guiding light waves formed in the substrate 1, and And a ground electrode 5-1, 5-2, which is a modulating electrode for modulating a light wave propagating through the waveguide. The Mach-Zehnder type optical waveguide 2 has a first branch optical waveguide 3-1 and a second branch optical waveguide 3-2.

Further, the optical waveguide element 10 has a film body 6 in a portion on the substrate 1 where the first branch optical waveguide 3-1 is located. The film body 6 has a length L
The region where the light wave guided in the optical waveguide 2 and the high-frequency external electric signal applied to the signal electrode 4 and the ground electrodes 5-1 and 5-2 interact (hereinafter referred to as "interaction region L")
In some cases). Note that the length L of the interaction region is generally defined by the length in which the signal electrode and the ground electrode exist substantially in parallel.

The value of the effective refractive index of the film body 6 is not particularly limited as long as it is larger than the effective refractive index of the optical waveguide 2. However, the substrate 1 needs to have an electro-optic effect, and generally, lithium niobate (LiNb
O ₃ ), lithium tantalate (LiTaO ₃ ), and lead lanthanum zirconate titanate (PLZT). These materials have an effective refractive index of 2.0 to 2.3. Therefore, the film body 6 is preferably made of a material having an effective refractive index of 3 or more. Examples of such a material include silicon, germanium, and gallium arsenide. Among these, silicon can be preferably used from the viewpoint of film forming control and trimming control.

It is necessary that the thickness of the film body 6 is not more than 1/4 of the wavelength of the light wave guided in the optical path disruption 2, more preferably not more than 1/10, especially not more than 1/15. The following is preferred. Thus, it is possible to effectively prevent the light wave guided in the optical waveguide 2 from leaking to the film body 6.

In the optical waveguide device 10 shown in FIG. 1, the film body 6 is provided only on the portion of the substrate 1 where the first branch optical waveguide 3-1 is located. However, the formation of the film is not limited to the portion where the first branch optical waveguide is located, but may be provided at the portion where the second branch optical waveguide is located. Furthermore, a film body can be formed in each of the portions where the first and second branch optical waveguides are located. Further, as shown in FIG.
For example, the first branch optical waveguide 3-
A plurality may be provided in the portion where 1 is located.

In the present invention, the phase of the light wave guided in the branched optical waveguide is controlled by forming the film body as follows. Hereinafter, a method for forming the film body 6 will be described with reference to FIG. First, the first and second branched optical waveguides 3 are placed on a substrate 1 made of lithium niobate or the like.
The Mach-Zehnder type optical waveguide 2 having -1 and 3-2 is formed by, for example, a thermal titanium diffusion method or a proton exchange method. Next, by using the vapor deposition method and the plating method together, the signal electrode 4 and the ground electrodes 5-1 and 5--5 are sandwiched between the branch optical waveguides 3-1 and 3-2.
Form 2

Next, a thin film made of a material having a high effective refractive index such as silicon is placed on a branch optical waveguide on the substrate 1 such that its thickness is 1/4 or less of the wavelength of a light wave guided through the optical waveguide 2. It is formed in a predetermined size in a portion where 3-1 is located. Here, the thin film can be formed by an evaporation method, a sputtering method, or the like. Next, optical fibers are connected to the entrance 2A and the exit 2B of the optical waveguide 2, respectively. Then, the same light wave as that used when completed as an actual optical waveguide element is guided in the optical waveguide 2. This light wave passes through the optical fiber and enters the entrance 2A.
From the optical waveguide 2. Then, the light wave passes through the first and second branch optical waveguides 3-1 and 3-2, and then passes through the optical fiber from the exit 2B to the outside.

When a light wave is guided in the optical waveguide 2 as described above, the signal electrode 4 and the ground electrodes 5-1 and 5
-2, the same high-frequency external electric signal as used for the actual modulation is applied. And the emission port 2 at this time
The intensity of the output light from B is monitored by a photodetector or the like. Next, the thin film is trimmed by a YAG laser or the like while monitoring the output light intensity. When the output light intensity becomes substantially zero, the trimming is stopped and the film body 6 is completed. Exit 2
The fact that the output light intensity at B becomes 0 means that the effective refractive index fluctuation of the optical waveguide 2 and the like is compensated, and the first branched lightwave and the second branched lightwave guided through the first branched optical waveguide 3-1. This indicates that the phase with the second branched light wave guided through the optical waveguide 3-2 has been adjusted to an optimal state (a state in which the phase is shifted by about 180 degrees).

According to the present invention, the film is formed from a high effective refractive index material having a high effect of compensating for the effective refractive index fluctuation of the optical waveguide element, and the size of the film is adjusted by trimming. Can be. Therefore,
The effective refractive index fluctuation in a sufficiently wide range can be compensated almost completely almost completely, and the yield of the optical waveguide element can be improved.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. (Example) In this example, an optical waveguide device 10 as shown in FIG. 1 was manufactured. X of lithium niobate as substrate 1
Using a cut plate, a photoresist was formed to a thickness of 0.5 μm on the X-cut plate using a spin coater, and then exposed and developed to form an optical waveguide pattern having a development width of 7 μm. Next, a titanium layer having a thickness of 800 .ANG.
The titanium was diffused into the substrate 1 by performing a heat treatment at 1000 ° C. for 10 hours in an electric furnace to form an optical waveguide 2 (effective refractive index 2.10) having a width of 9 μm. Note that the first branch optical waveguide 3-1 and the second
Also have a width of 9 μm.
It was formed to become.

Next, after forming a titanium layer on the entire surface of the substrate 1 to a thickness of 0.05 μm by a vapor deposition method, an Au layer having a thickness of 0.1 μm is formed on the titanium layer by a sputtering method.
It was formed to 2 μm. Next, after a chromium mask was provided on the Au layer, a photoresist was formed to a thickness of 25 μm by a spin coater. Thereafter, an exposure and development treatment are performed to form an electrode pattern, and electroplating is performed using the electrode pattern as a mask to obtain a thickness of 15 μm and a width of 5 μm.
A μm signal electrode 4 and ground electrodes 5-1 and 5-2 were formed.

At this point, a light wave having a wavelength of 1.55 μm was introduced into the optical waveguide 2 and a modulation curve of the optical waveguide element before the film body 6 was formed was obtained. As shown in FIG. A curve was obtained.

Next, a thin film made of silicon (effective refractive index: 3.5) was formed to a thickness of 0.05 μm, a width of 10 μm, and a length of 150 μm outside the interaction region L by vapor deposition. Then, the light wave is introduced into the optical waveguide 2,
In a state where a high-frequency external electric signal having a frequency of 10 kHz was applied to the signal electrode 4, the thin film was trimmed while monitoring the output light intensity of the light wave to form a film body 6. As a result, when the film body 6 was formed to have a length of 120 μm, the output light intensity became almost zero. When the modulation curve at this time was obtained, a curve as shown in FIG. 2B was obtained. As is clear from FIGS. 2A and 2B, in the optical waveguide device of the present embodiment and the method of controlling the phase thereof, the modulation curve is shifted by about 4/5 wavelength.

COMPARATIVE EXAMPLE A thin film made of a photoresist (effective refractive index: 1.7) instead of silicon was formed to a thickness of 0.5 μm.
An optical waveguide device was manufactured in the same manner as in the example except that the optical waveguide device was formed to have a length of m, a width of 10 μm and a length of 18 mm. At this time,
When the modulation curve was measured in the same manner as in the example, a curve as shown in FIG. 2C was obtained. Thereafter, the thin film was trimmed in the same manner as in the example to form a film having the same size as the film 6 in the example. When the modulation curve at this time was measured in the same manner, a curve as shown in FIG. 2D was obtained.

As is apparent from FIGS. 2A, 2C and 2D, the modulation curve at the time when the thin film made of the photoresist shown in FIG. 2C is formed is shown in FIG. 2A. Approximately 3/5 from the modulation curve without the thin film shown
It can be seen that the wavelength is shifted by the wavelength. The trimming of the thin film shifts the modulation curve to the left. When the thin film made of photoresist is formed to the same size as the film body 6, a modulation curve as shown in FIG. 2D is obtained. I understood that. FIG. 2 (d) shifts from FIG. 2 (a) by only about 4 × 10 ⁻³ wavelengths, and it can be seen that these are almost superimposed.

That is, according to the above embodiment, the optical waveguide device and the phase control method of the optical waveguide device according to the present invention provide:
It can be seen that the phase shift of about 4/5 wavelength can be simply and accurately performed by the trimming. On the other hand, in the comparative example using a photoresist having an effective refractive index lower than that of the optical waveguide, even when a thin film larger than the film body 6 is formed, the phase shift is larger than that in the case where the film body 6 is formed. It turns out to be small. It can be seen that when a film body having the same size as the film body 6 is formed by trimming, only a phase shift of about 1/200 of the above embodiment can be obtained.

As described above, the present invention has been described in detail based on the embodiments of the present invention with specific examples. However, the present invention is not limited to the above-described contents, and may be implemented in any form without departing from the scope of the present invention. Deformation and modification are possible.

[0038]

As described above, according to the optical waveguide device and the method of controlling the phase of the optical waveguide device of the present invention, a film made of a material having an effective refractive index higher than that of the optical waveguide is formed on the substrate. At least a part of the two branch optical waveguides constituting the optical waveguide is formed by trimming. For this reason,
The variation in the effective refractive index of the optical waveguide element can be compensated within a wide range. Therefore, it is possible to improve the yield of the optical waveguide device. Furthermore, even when the size of the film body is sufficiently reduced, a relatively large effective refractive index fluctuation can be compensated, so that the ratio of the film body to the whole element is reduced, and the element itself is downsized. You can also.

[Brief description of the drawings]

FIG. 1 is a plan view showing an example of an optical waveguide device of the present invention.

FIG. 2 is a modulation curve of an optical waveguide element in an example and a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Optical waveguide 3-1 1st branch optical waveguide 3-2 2nd branch optical waveguide 4 Signal electrode 5-1, 5-2 Ground electrode 6 Film body 10 Optical waveguide element

Continued on the front page (72) Inventor Hirotoshi Nagata 585 Tomicho, Funabashi-shi, Chiba Sumitomo Osaka Cement Co., Ltd. (72) Inventor Kenichi Kuboji 585 Tomicho, Funabashi-shi, Chiba Sumitomo Osaka Cement Co., Ltd. F-term in the new technology research laboratory (reference) 2H047 KA12 LA12 NA02 PA12 QA03 RA08 TA42 2H079 AA02 AA12 CA04 DA03 EA05 HA11 JA05

Claims (6)

[Claims] An optical waveguide including a substrate having an electro-optic effect, a Mach-Zehnder type optical waveguide for guiding a light wave, and a modulation electrode for controlling the light wave guided through the optical waveguide. A waveguide element, wherein at least a part of the substrate on which the first and second branch optical waveguides constituting the Mach-Zehnder type optical waveguide are located, has a higher effective refractive index than the optical waveguide. An optical waveguide device, comprising: forming a film body made of the above material; and setting the thickness of the film body to 以下 or less of the wavelength of the light wave guided through the optical waveguide. 2. The film according to claim 1, wherein the film is made of a high refractive material having an effective refractive index of 3 or more.
3. The optical waveguide device according to claim 1. 3. The optical waveguide device according to claim 1, wherein the thickness of the film body is set to 1/10 or less of a wavelength of the light wave guided through the optical waveguide. 4. An optical waveguide comprising a substrate having an electro-optic effect, a Mach-Zehnder optical waveguide for guiding an optical wave, and a modulation electrode for controlling the optical wave guided through the optical waveguide. A phase control method of a waveguide element, wherein at least a part of a portion on the substrate where the first and second branch optical waveguides configuring the Mach-Zehnder type optical waveguide are located, is more than the optical waveguide. A thin film having a thickness of 1/4 or less of the wavelength of the light wave guided through the optical waveguide is formed, while being made of a material having a high effective refractive index;
While monitoring an optical output from the optical waveguide while an external modulation signal is applied to the modulation electrode, the thin film is trimmed to form a predetermined film body, and the first film is formed.
A first branch lightwave guided in the branch optical waveguide of
A phase control method for an optical waveguide device, wherein a phase between the second optical waveguide and the second optical waveguide guided in the optical waveguide is adjusted. 5. The film according to claim 4, wherein the film is made of a high refractive material having an effective refractive index of 3 or more.
3. The phase control method for an optical waveguide device according to item 1. 6. The phase of the optical waveguide device according to claim 4, wherein the thickness of the film body is set to 1/10 or less of the wavelength of the light wave guided through the optical waveguide. Control method.