JP2009181108A - Optical waveguide element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical waveguide element such that increase in drive voltage is suppressed, the propagation loss of light waves is reduced and the optical response characteristic at a high-frequency band is not deteriorated by a propagation loss reduction means. <P>SOLUTION: The optical waveguide element includes: a substrate having an electrooptical effect; a Mach-Zehnder type optical waveguide formed on the main face of the substrate; a control electrode for controlling the light wave which propagates in the optical waveguide, wherein the Mach-Zehnder type optical waveguide is divided into: an action part which is an optical waveguide part which controls the light wave with the control electrode; and input/output parts (2, 21 and 22) which are optical waveguide parts which input the light wave to the action part or output the light wave from the action part, wherein the control electrodes are constituted of a signal electrode 3 and grounding electrodes (321 to 323), and an optical loss reduction means is provided at least at one side of the grounding electrodes on the input/output parts, when the grounding electrodes are arranged striding over the input/output parts. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device including a Mach-Zehnder optical waveguide and a control electrode for controlling a light wave guided in the optical waveguide.

In recent years, in the fields of optical communication and optical measurement, a waveguide type optical modulator in which an optical waveguide is formed on a substrate having an electro-optic effect and a control electrode for controlling a light wave guided in the optical waveguide is formed. Such optical waveguide elements are often used.
The optical waveguide device shown in FIGS. 1 and 2 has a Mach-Zehnder type optical waveguide formed on a substrate. FIG. 1 shows an example in which an X plate is used as a substrate, and FIG. 2 shows an example in which a Z plate is used as a substrate. In addition, (a) of each figure is a top view of an optical waveguide element, FIG.1 (b) or (c) is each sectional drawing in AA 'or BB' of FIG.1 (a), Furthermore, FIG. 2B or FIG. 2C is a cross-sectional view taken along the line CC ′ or DD ′ in FIG.

  In the optical waveguide device of FIG. 1, a so-called coplanar type control electrode is formed in which the signal electrode 3 is disposed so that the ground electrodes 31 and 32 are sandwiched therebetween. For this reason, it is necessary to arrange the ground electrode 32 so as to straddle the optical waveguide 2 locally, and the ground electrode 32 comes into contact with the optical waveguide 2 as shown in FIG. The light wave propagating through the optical waveguide 2 is absorbed or scattered by the ground electrode 32, and the propagation loss of the light wave increases.

On the other hand, in the optical waveguide device shown in FIG. 2, the ground electrode 35 is not only locally disposed so as to straddle the optical waveguide, as in the optical waveguide device shown in FIG. , 22 is necessary. For this reason, in order to reduce the loss of the light wave propagating through the optical waveguide, a buffer layer 4 such as SiO ₂ is formed as shown in FIGS. In FIG. 2A, the buffer layer 4 is omitted to show the positional relationship between the optical waveguide (2, 23, etc.) and the control electrodes (signal electrodes 33, 36, ground electrodes 34, 35, 37). ing. In addition, as an example using a buffer layer, the thing of patent document 1 etc. are known.
JP-A-9-185025

  As shown in FIG. 2, the buffer layer 4 can be expected to contribute to reducing loss when an electrode is disposed on the optical waveguide. However, since the gap between the electrode and the optical waveguide is increased, the buffer layer 4 is applied to the optical waveguide. In order to maintain the strength of the electric field at a predetermined level or higher, it is necessary to increase the driving voltage applied to the electrodes. For this reason, it becomes difficult to reduce the power consumption and cost of the driving device.

  The problem to be solved by the present invention is to solve the above-mentioned problems and reduce the propagation loss of the light wave while suppressing the increase of the driving voltage, so that the optical response characteristic in the high frequency band is not deteriorated by the propagation loss reducing means. It is to provide an optical waveguide device maintained in the above.

  In the invention according to claim 1, a substrate having an electro-optic effect, a Mach-Zehnder type optical waveguide formed on the main surface of the substrate, and a control electrode for controlling a light wave guided in the optical waveguide. In the optical waveguide element provided, the Mach-Zehnder type optical waveguide includes an action portion that is an optical waveguide portion that controls the light wave by the control electrode, and an optical waveguide that introduces a light wave into the action portion or derives a light wave from the action portion. When the control electrode is composed of a signal electrode and a ground electrode, and the ground electrode is disposed across the input / output unit, at least on the input / output unit. One of the ground electrodes is provided with light loss reducing means.

  In the invention according to claim 2, in the optical waveguide element according to claim 1, the light loss reducing means is configured such that the width of the ground electrode straddling the input / output portion is compared with the width of the ground electrode before and after straddling. The ground electrode that is narrow and straddles the input / output section is disposed in the vicinity of the signal electrode.

  According to a third aspect of the present invention, in the optical waveguide device according to the first aspect, the light loss reducing means is a wire or ribbon bonding in which a ground electrode straddling the input / output unit is disposed in the vicinity of the signal electrode. It is characterized by being.

  According to a fourth aspect of the present invention, in the optical waveguide device according to the first aspect, the optical loss reducing means includes a ground electrode that straddles the input / output unit and is disposed in the vicinity of the signal electrode. It is a bridge-like electrode connecting electrodes.

  According to a fifth aspect of the present invention, in the optical waveguide device according to the first or second aspect, the optical loss reduction means includes a buffer between the input / output unit and a ground electrode disposed across the input / output unit. A layer is disposed.

  The invention according to claim 6 is the optical waveguide device according to any one of claims 1 to 5, wherein the substrate is an X plate of lithium niobate or tantalite niobate.

  The invention according to claim 7 is the optical waveguide element according to any one of claims 1 to 6, wherein the thickness of the substrate is 50 μm or less.

  According to the first aspect of the present invention, the Mach-Zehnder type optical waveguide includes an action portion that is an optical waveguide portion that controls the light wave by the control electrode, and an optical waveguide that introduces the light wave into the action portion or derives the light wave from the action portion. The control electrode is composed of a signal electrode and a ground electrode, and when the ground electrode is disposed across the input / output unit, at least one of the input / output unit Since the optical loss reducing means is provided on the ground electrode, the propagation loss of the light wave propagating through the optical waveguide can be suppressed, and the optical loss reducing means is implemented so that the electrode arrangement in the vicinity of the optical waveguide can be maintained in the coplanar type. Therefore, it is possible not to deteriorate the optical response characteristics in the high frequency band.

  According to the invention according to claim 2, the light loss reducing means is such that the width of the ground electrode straddling the input / output part is narrower than the width of the ground electrode before and after straddling, and the ground electrode straddling the input / output part is Since it is arranged in the vicinity of the signal electrode, the overlapping portion between the optical waveguide and the ground electrode can be reduced to reduce the optical loss, and the straddling ground electrode is arranged in the vicinity of the signal electrode. In addition, the optical response characteristics in the high frequency band do not deteriorate.

  According to the invention of claim 3, the light loss reducing means requires that a ground electrode straddling the input / output section is a wire or ribbon bonding disposed in the vicinity of the signal electrode, and therefore a ground electrode that contacts the optical waveguide needs to be disposed. Therefore, the optical loss can be suppressed to the maximum. In addition, since wire bonding or the like is arranged in the vicinity of the signal electrode, the optical response characteristics in the high frequency band do not deteriorate as described above.

  According to the invention of claim 4, the optical loss reducing means is in contact with the optical waveguide because the ground electrode straddling the input / output section is a bridge-shaped electrode that is disposed in the vicinity of the signal electrode and connects the ground electrodes before and after the straddle. Therefore, it is not necessary to arrange a grounding electrode, and light loss can be suppressed to the maximum. In addition, since the bridge electrode is disposed in the vicinity of the signal electrode, the optical response characteristic in the high frequency band does not deteriorate as described above.

  According to the invention of claim 5, since the optical loss reducing means arranges the buffer layer between the input / output unit and the ground electrode arranged across the input / output unit, the propagation loss of the light wave propagating through the optical waveguide Can be reduced.

  According to the invention of claim 6, since the substrate is an X plate of lithium niobate or tantalate niobate, even when an electrode is arranged directly on the substrate, grounding is performed via the light loss reducing means of claims 1 to 5. Since the electrodes can be disposed so as to straddle the optical waveguide, it is possible to suppress the propagation loss of the light wave propagating through the optical waveguide while suppressing an increase in the driving voltage of the optical waveguide element.

  With the invention according to claim 7, since the thickness of the substrate is 50 μm or less, it becomes easy to achieve speed matching between the light wave propagating through the optical waveguide and the drive signal, and an optical waveguide element having a good optical response up to a higher frequency region is realized. It becomes possible to do. In addition, since the substrate is thin, light waves propagating through the optical waveguide are generally easily absorbed by the ground electrode straddling the optical waveguide. However, the light loss reducing means according to claims 1 to 5 can suppress these problems. It becomes possible.

Hereinafter, the optical waveguide device according to the present invention will be described in detail.
An optical waveguide device according to the present invention includes a substrate having an electro-optic effect, a Mach-Zehnder optical waveguide formed on a main surface of the substrate, and a control electrode for controlling a light wave guided in the optical waveguide. In the optical waveguide device comprising: When the control electrode is composed of a signal electrode and a ground electrode, and the ground electrode is disposed across the input / output unit, the control electrode is divided into an input / output unit that is an optical waveguide portion. An optical loss reducing means is provided on at least one of the ground electrodes (light wave introduction side or light emission side).

  FIG. 3 is a diagram for explaining the above-mentioned “acting portion”. The signal electrode 3 constituting the control electrode cooperates with a ground electrode (not shown) to apply an electric field to the optical waveguide and propagate in the optical waveguide. The region for controlling the light wave is the region b in the figure, and the optical waveguides (21, 22) in this region are called action parts. The optical waveguide disposed in the region a between the side end where the light wave is input to the optical waveguide element and the action part is referred to as “input part”, and the side between the side end where the light wave is output from the optical waveguide element to the action part. The optical waveguide disposed in the region c is called an “output unit”, and both are collectively called an “input / output unit”.

As the substrate 1, for example, lithium niobate, tantalate niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), a quartz-based material, and a combination thereof can be used. In particular, lithium niobate (LN) or niobate tantalate having a high electro-optic effect in the X plate is preferably used.
The thickness of the substrate to be used is preferably 150 μm or less, particularly 50 μm or less, as shown in Patent Document 2 below. By using such a thin plate, it becomes easy to achieve speed matching between the light wave propagating through the optical waveguide and the drive signal, and it becomes possible to realize an optical waveguide element having a good optical response up to a higher frequency region. However, when the thickness of the substrate is 50 μm or less, if there is a ground electrode on the optical waveguide, the light wave propagating through the optical waveguide is absorbed by the electrode and the optical loss is increased, and the scattered light wave is propagated to the optical waveguide output section. It has been experimentally found that the characteristics such as the on / off extinction ratio are significantly deteriorated by coupling into the optical fiber connected to the optical waveguide output section. It has been found that application of the present invention can avoid such characteristic deterioration that is likely to occur when a thin plate is used.
JP 2006-284963 A

As a method for forming the optical waveguide, it can be formed by diffusing Ti or the like on the substrate surface by a thermal diffusion method or a proton exchange method.
The control electrode can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like.

Next, the optical loss reducing means that is a feature of the present invention will be described.
FIG. 4 shows a first example in which light loss reducing means is arranged at the input portion of the optical waveguide. As an optical loss reducing means, the width of the ground electrode 322 straddling the input portion of the optical waveguide (the optical waveguide 2 in FIG. 4) is narrower than the width of the ground electrodes (321, 323) before and after the straddle. The ground electrode 322 straddling the input section is disposed in the vicinity of the signal electrode 3.
The “neighborhood” used in the present invention may be a range in which the optical response characteristic does not deteriorate. In FIG. 4, it is arranged at the position closest to the signal electrode, but is not limited thereto.

  The width of the ground electrode 322 straddling the optical waveguide is preferably as narrow as possible from the viewpoint of reducing optical loss, and is preferably 75 μm or less. On the other hand, if the width of the ground electrode 322 is too narrow, the optical response characteristic with respect to the drive signal propagating through the signal electrode deteriorates.

The position of the ground electrode 322 across the optical waveguide (the distance n ₂ between the ground electrode 322 and the signal electrode) is the distance n ₁ between the ground electrode 321 and the signal electrode 3 or the distance n between the ground electrode 323 and the signal electrode 3. ₃ , the coplanar control electrode can be continuously formed, and the electric field strength between the signal electrode and the ground electrode generated in response to the drive signal propagating through the signal electrode. It is possible to stabilize the optical response characteristic in the high frequency region.

FIG. 4 shows a configuration in which the ground electrode straddles the optical waveguide before the Y-branch of the Mach-Zehnder type optical waveguide. However, the present invention is not limited to this, and the ground electrode straddles the branch waveguides 21 and 22. It is also applicable to cases. In that case, every time the branching waveguides 21 and 22 are straddled, the width of the ground electrode straddling the optical waveguide is set narrow as shown in FIG. 4, and the position of the ground electrode is arranged in the vicinity of the signal electrode. It is preferable to do.
Moreover, although FIG. 4 demonstrated the input part of the optical waveguide, it cannot be overemphasized that the optical loss reduction means of this invention is applicable similarly to an output part.
The same applies to the following optical loss reducing means.

FIG. 5 shows a second example of the optical loss reducing means.
As the optical loss reduction means, a ground electrode straddling the input section is a wire 324 or ribbon bonding arranged in the vicinity of a signal electrode (not shown but assumed to be arranged on the right side of the figure). .

The number of wires, the width of the ribbon, and the like are not particularly limited, but can be set as appropriate as long as the coplanar control electrode is maintained and the propagation of the drive signal is not impaired.
The distance between the ground electrodes 321 and 323 is 200 μm or less, preferably 50 μm or less, although it depends on the width of the optical waveguide 2 disposed therebetween. The same applies to the other optical loss reducing means. In addition, the distance between the wire or ribbon straddling the optical waveguide and the signal electrode is set to be approximately the same as the distance between the ground electrode 321 and the signal electrode or the distance between the ground electrode 323 and the signal electrode, as in the first example. It is preferable to do.

FIG. 6 shows a third example of the optical loss reducing means.
As an optical loss reducing means, a ground electrode straddling the input part is disposed in the vicinity of the signal electrode (not shown but assumed to be disposed on the right side of FIG. 6A), and before and after the straddle. This is a bridge electrode 325 that connects the ground electrodes 321 and 323. Note that the distance between the bridge electrode and the signal electrode straddling the optical waveguide is the same as the distance between the ground electrode 321 and the signal electrode or the distance between the ground electrode 323 and the signal electrode, as in the first or second example. It is preferable to set the degree.
The bridge-like electrode can be formed by bonding a metal plate (eg, gold, SUS, etc.) with solder or conductive paste. Further, a high frequency capacitor (for example, a multilayer ceramic capacitor, a single plate capacitor, or a thin film capacitor) may be used instead of the metal plate.

FIG. 7 shows a fourth example of the optical loss reducing means.
As the optical loss reducing means, the buffer layer 5 is disposed between the input unit 2 and the ground electrode 326 disposed across the input unit.
As the buffer layer, SiO ₂ can be used. However, when SiO ₂ is formed on the entire substrate like an optical waveguide device using a Z-plate, the distance between the control electrode and the optical waveguide is increased, and the drive voltage is increased. I will let you. Therefore, in the present invention, the buffer layer is preferably provided in a place excluding the region b in FIG. 3, and more preferably provided only in a place where the ground electrode straddles the optical waveguide.

  In order to evaluate the influence of the ground electrode straddling the optical waveguide on the optical loss, an optical waveguide element having a Mach-Zehnder type optical waveguide using an LN X plate was prepared. The chip length of the element was about 60 mm, and a ground electrode straddling the optical waveguide as shown in FIG. Next, the optical loss of the optical waveguide device was measured when the width of the ground electrode 322 was changed in the range of 0 to 75 μm.

  As a measurement test method, as shown in FIG. 8, a DFB laser 40 (wavelength 1550 nm, manufactured by NTT Electronics, model number NLK1L5GAAA) is used as a light source, and light from the light source is polarized by a polarization controller 41 (applied photoelectric laboratory). The polarization plane was adjusted with a product of model number MPCA-1550) and made incident on the optical waveguide element 42 through a polarization maintaining fiber (PMF). The optical waveguide device applies a DC voltage from a DC power source 43 (manufactured by Kikusui Electronics Co., Ltd., model number PMM18-2.5DU) through a DC application probe to the top of the modulation curve that maximizes light transmission. The light wave emitted from the optical waveguide element was incident on an optical power meter 44 (manufactured by Agilent Technologies, model number 81533B) via a single mode fiber, and optical loss was measured.

  FIG. 9 is a graph showing changes in optical loss when the width of the ground electrode is changed to 0 μm, 37.5 μm, and 75 μm. The light loss is improved as the width of the ground electrode is reduced. Is easily understood.

Next, the gap distance between the ground electrodes 321 and 523 in FIG. 4 is set to 200 μm, and the width of the ground electrode across the optical waveguide is set to 0 μm (that is, cut on the optical waveguide), 37.5 μm, and 75 μm. When set, the optical response characteristics with respect to the driving frequency were evaluated.
In the measurement test, the optical waveguide device was mounted on a modulator package and evaluated using an optical component analyzer (manufactured by Agilent Technologies, model number 86030A).

Further, in order to evaluate the influence of the change in the width of the ground electrode on the optical response characteristics, the same measurement was performed when a normal electrode having a narrow width as shown in FIG. 4 was used.
Further, the same measurement was performed when the gap distance was set to 50 μm, the width of the ground electrode was set to 0 μm, and five wires (Au, φ25 μm) as shown in FIG. 5 were connected.
These results are shown in FIG.

  As shown in FIG. 10, when the width is 75 μm and the width is 37.5 μm, the optical response characteristics are the same as the normal case, and when the width of the ground electrode is 0 μm (when cut on the optical waveguide). Although the optical response characteristic is deteriorated, it shows the same optical response characteristic by connecting with a wire, and the optical waveguide element of the present invention can easily prevent the optical response characteristic in the high frequency region from being deteriorated. To be understood.

  As described above, according to the present invention, there is provided an optical waveguide device that reduces propagation loss of light waves while suppressing an increase in driving voltage, and maintains optical response characteristics in a high frequency band by a propagation loss reducing unit. It becomes possible to provide.

It is the schematic of the optical waveguide element using the board | substrate of the conventional X board. It is the schematic of the optical waveguide element using the board | substrate of the conventional Z board. It is a figure explaining the action part of the optical waveguide which concerns on this invention. It is a figure which shows the 1st example of the optical loss reduction means which concerns on this invention. It is a figure which shows the 1st example of the optical loss reduction means which concerns on this invention. It is a figure which shows the 1st example of the optical loss reduction means which concerns on this invention. It is a figure which shows the 1st example of the optical loss reduction means which concerns on this invention. It is a figure which shows arrangement | positioning of each apparatus at the time of measuring and testing an optical loss. It is a graph which shows the relationship of the optical loss with respect to the width | variety of a ground electrode. It is a graph which shows the optical response characteristic with respect to the drive frequency at the time of using the ground electrode which straddles an optical waveguide.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2,21,22,23 Optical waveguide 3,33,36 Signal electrode 31,32,34,35,37,321,322,323,326 Ground electrode 4,5 Buffer layer 324 Wire 325 Bridge electrode 40 Laser Light source 41 Polarization controller 42 Optical waveguide element (test object)
43 DC power supply 44 Optical power meter

Claims (7)

A substrate having an electro-optic effect;
A Mach-Zehnder type optical waveguide formed on the main surface of the substrate;
In an optical waveguide device comprising a control electrode for controlling a light wave guided in the optical waveguide,
The Mach-Zehnder type optical waveguide includes an action portion that is an optical waveguide portion that controls light waves by the control electrode, and an input / output portion that is an optical waveguide portion that introduces light waves into the action portion or derives light waves from the action portion. Divided into
When the control electrode is composed of a signal electrode and a ground electrode, and the ground electrode is disposed across the input / output unit, at least one of the ground electrodes on the input / output unit is provided with light loss reduction means. An optical waveguide device characterized by the above.   2. The optical waveguide element according to claim 1, wherein the light loss reducing means has a width of the ground electrode straddling the input / output portion that is narrower than a width of the ground electrode before and after the straddle, and on the input / output portion. An optical waveguide device characterized in that a ground electrode straddling the wire is disposed in the vicinity of the signal electrode.   2. The optical waveguide device according to claim 1, wherein the light loss reducing means is a wire or ribbon bonding in which the ground electrode straddling the input / output portion is disposed in the vicinity of the signal electrode. element.   2. The optical waveguide element according to claim 1, wherein the light loss reducing means is a bridge-like electrode in which a ground electrode straddling the input / output unit is disposed in the vicinity of the signal electrode and connects the ground electrodes before and after the straddle. An optical waveguide device characterized by the above.   3. The optical waveguide device according to claim 1, wherein the light loss reducing means is configured to dispose a buffer layer between the input / output unit and a ground electrode disposed across the input / output. A characteristic optical waveguide device.   6. The optical waveguide device according to claim 1, wherein the substrate is an X plate of lithium niobate or tantalate niobate.   7. The optical waveguide device according to claim 1, wherein the substrate has a thickness of 50 [mu] m or less.

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