JP2005274889A - Optical waveguide device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent an optical waveguide device from having a ripple in a high-frequency characteristic (S<SB>21</SB>). <P>SOLUTION: An optical waveguide 1 is equipped with a substrate main body 1 made of an electrooptic material, an optical waveguide 6, a signal electrode 4 for applying an electric signal to the optical waveguide 6, a 1st ground electrode 3A, and a 2nd ground electrode 3B. An electric connection member 7 which electrically connects the 1st ground electrode 3A and 2nd ground electrode 3B together is provided so as to pass on the optical waveguide 6. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical waveguide device such as a traveling wave optical modulator.

In the patent documents 1 and 2, the present applicant has disclosed that a thin portion is provided under the optical waveguide of the substrate of the traveling wave optical modulator, and the thickness of the thin portion is reduced to, for example, 10 μm or less. This is advantageous because high-speed optical modulation is possible without forming a buffer layer made of silicon oxide, and the product (Vπ · L) of the drive voltage Vπ and the electrode length L can be reduced.
Japanese Patent Laid-Open No. 10-133159 JP 2002-169133 A

  In the traveling waveform optical modulators described in Patent Documents 1 and 2, for example, a CPW (coplanar type) electrode and a Mach-Zehnder type optical waveguide are formed on an X plate of a lithium niobate single crystal, and each of the optical waveguides is formed. A similar electric field is applied to the branch portion and the electrode interaction length is made equal, thereby obtaining an optical modulator with zero chirp characteristics.

  However, in an actual optical transmission system, it may be advantageous to provide a predetermined chirp amount even for an optical modulator using an X plate or a Y plate as a substrate. However, it has not been studied so far that an optical modulator using an electro-optic crystal X-plate or Y-plate as a substrate has such a predetermined chirp amount.

On the other hand, the present applicant disclosed in Patent Document 3 that a plurality of ground electrodes are provided, and the width of each electrode gap between the signal electrode and each ground electrode is made different. Thus, when the width of one electrode gap and the width of the other electrode gap are made different, the electric field strength at each branch portion arranged under each electrode gap also becomes different, and a chirp characteristic is obtained.
Japanese Patent Application No. 2003-324373

  However, it has been found that a conventional optical modulator, particularly a chirped modulator having an electrode gap with an asymmetric structure as described above, has a problem that a ripple occurs in the high frequency characteristics (S21). It was found that the reason is that modes other than the CPW mode are excited because the electrode structure is asymmetric, and the modes interfere with each other.

  An object of the present invention is to prevent ripples from occurring in the high-frequency characteristics (S21) in an optical waveguide device.

  The invention according to the first aspect includes a substrate body made of an electro-optic material, an optical waveguide, and an optical waveguide including a signal electrode for applying an electrical signal to the optical waveguide, a first ground electrode, and a second ground electrode. In the waveguide device, an electrical connection member that electrically connects the first ground electrode and the second ground electrode is provided so as to pass over the optical waveguide and the signal electrode.

  The invention according to the second aspect includes a substrate body made of an electro-optic material, an optical waveguide, and a signal electrode for applying an electrical signal to the optical waveguide, a first ground electrode, and a second ground electrode. An optical waveguide device, wherein the width of the first gap between the signal electrode and the first ground electrode is different from the width of the second gap between the signal electrode and the second ground electrode. And an electrical connection member for connecting at least one of the first ground electrode and the second ground electrode to a housing for fixing the optical waveguide device.

In the invention according to the first aspect, by directly electrically connecting a pair of ground electrodes on the optical waveguide side, the ground electrodes on both sides become equipotential even at a high frequency, so that the S ₂₁ characteristic due to the unwanted mode radiation described above is obtained. I found out that I could suppress the ripple. It is considered that the CPW mode is selectively passed by the electrical connection member, and unnecessary modes other than the CPW mode are reflected by the electrical connection member. If the number of electrical connection members is increased, the resonator length of the unnecessary mode is shortened, and the resonance frequency due to the unnecessary mode is increased. For this reason, it is considered that the ripple due to the unnecessary mode can be moved to a frequency higher than the use frequency.

According to the second aspect of the invention, by connecting the ground electrode of the modulator chip and the conductive casing (package) with a wire or metal foil, a pair of ground electrodes are formed in the same manner as in the first aspect. come to be selectively excited only CPW mode to become a potential ripple S ₂₁ characteristics are reduced as a result.

Hereinafter, the present invention will be described in more detail with reference to the drawings as appropriate.
FIG. 1 is a plan view schematically showing an optical waveguide device 1 according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view of the optical waveguide device 1 of FIG.

The optical waveguide device 1 of this example includes a substrate body 2. The substrate body 2 may be bonded to a separate support substrate (not shown). In this example, the substrate body 2 has a flat plate shape, but this shape is not limited to a flat plate. On one main surface 2a of the substrate body 2, predetermined ground electrodes 3A, 3B and a signal electrode 4 are formed. In this example, the so-called coplanar type (Coplanar type)
The electrode arrangement of “waveguide: CPW electrode” is adopted, but the arrangement form of the electrodes is not particularly limited. In this example, branch optical waveguides 6a and 6b are formed in a gap 5A between the adjacent signal electrode 4 and the ground electrode 3A and a gap 5B between the signal electrode 4 and the ground electrode 3B, respectively. In contrast, a signal voltage is applied in a substantially horizontal direction. The optical waveguide 6 constitutes a so-called Mach-Zehnder type optical waveguide in plan view.

Here, in the optical waveguide device 1 of the present example, the width WA of the first gap 5A is smaller than the width WB of the second gap 5B, thereby realizing a predetermined chirp characteristic. Thus, if the width of the gap 5A and 5B are different, the ripple of the S ₂₁ characteristics are particularly likely to occur as described above.

  Here, according to the present invention, the first ground electrode 3 </ b> A and the second ground electrode 3 </ b> B are connected by the electrical connection member 7. The electrical connection member 7 passes over the optical waveguide 6. In this way, it is unparalleled to connect and electrically connect a pair of ground electrodes with an electrical connection member that passes over and crosses the optical waveguide.

  The electrical connection member 7 is not particularly limited, but a bonding wire or a metal foil is particularly preferable. The material of the bonding wire or the metal foil is not limited, but it needs to be a conductive material, and is preferably a material having flexibility or flexibility. From this viewpoint, copper with gold plating, nickel with gold plating, nickel, aluminum, gold, copper, carbon, and silver are preferable.

  The bonding method of the bonding wire or metal foil to the ground electrode is not limited, but ultrasonic bonding, spot welding, conductive adhesive, and soldering are preferable.

  Also, it is preferable to provide a plurality of electrical connection members 7, whereby the resonator length can be further shortened, and the frequency at which the ripple is generated can be shifted to the higher frequency side. From this point of view, the interval between the adjacent electrical connection members 7 is preferably less than half of the length of the interaction electrode. For example, in a 10 Gbps modulator, it is more preferably 10 mm or less. Since the necessary band varies depending on the modulation frequency, when the modulation frequency is low, the electrical connecting members 7 may be provided at longer intervals.

  Preferably, the width WA of the first gap 5A is different from the width WB of the second gap 5B. From this viewpoint, WA / WB is preferably 0.7 or less, and more preferably 0.4 or less.

  FIG. 3 is a plan view showing a state in which the optical waveguide device 11 is attached to the housing 8 according to an embodiment of the invention according to the second aspect. 4 is a cross-sectional view of the optical waveguide device 11 and the housing 8 of FIG.

The optical waveguide device 11 of this example includes a substrate body 2. The substrate body 2 may be bonded to a separate support substrate (not shown). In this example, the substrate body 2 has a flat plate shape, but this shape is not limited to a flat plate. On one main surface 2a of the substrate body 2, predetermined ground electrodes 3A, 3B and a signal electrode 4 are formed. In this example, the so-called coplanar type (Coplanar type)
The electrode arrangement of “waveguide: CPW electrode” is adopted, but the arrangement form of the electrodes is not particularly limited. In this example, branch optical waveguides 6a and 6b are formed in a gap 5A between the adjacent signal electrode 4 and the ground electrode 3A and a gap 5B between the signal electrode 4 and the ground electrode 3B, respectively. In contrast, a signal voltage is applied in a substantially horizontal direction. The optical waveguide 6 constitutes a so-called Mach-Zehnder type optical waveguide in plan view.

  The bottom surface 2c of the substrate body 2 is placed in the inside 9 of the housing 8, and placed on the bottom 8b of the housing 8 and fixed. In this state, the side surface 2 b of the substrate body 2 of the optical waveguide 11 faces the side wall 8 a of the housing 8. Then, the first ground electrode 3A is electrically connected to the side wall 8a of the housing 8 by the electrical connecting member 10A, and the second ground electrode 3B is connected to the side wall 8a of the housing 8 by the electrical connecting member 10B. Connect electrically.

  Specific materials for the electrical connection members 10A and 10B are not particularly limited, but bonding wires and metal foils are particularly preferable. The material of the bonding wire or the metal foil is not limited, but it needs to be a conductive material, and is preferably a material having flexibility or flexibility. From this viewpoint, copper with gold plating, nickel with gold plating, nickel, aluminum, gold, copper, carbon, and silver are preferable.

  The bonding method of the bonding wire or metal foil to the ground electrode and the housing 8 is not limited, but ultrasonic bonding, spot welding, silver paste, conductive adhesive, and soldering are preferable.

  In addition, it is preferable to provide a plurality of electrical connection members 10A and 10B, respectively, whereby the resonator length can be further shortened, and the frequency at which the ripple is generated can be shifted to the higher frequency side. From this point of view, the distance between the adjacent electrical connection members 10A and 10B is preferably less than or equal to half the interaction electrode length, and more preferably 10 mm or less in the case of 10 Gbps. Since the necessary band varies depending on the modulation frequency, when the modulation frequency is low, the electrical connection members 7 may be provided at longer intervals.

  In the invention according to the second aspect, the width WA of the first gap 5A is different from the width WB of the second gap 5B. From this viewpoint, WA / WB is preferably 0.7 or less, and more preferably 0.4 or less.

Hereinafter, preferred embodiments of the invention according to the first and second aspects will be further described.
The substrate body constituting the optical waveguide substrate is made of a ferroelectric electro-optic material, preferably a single crystal. Such a crystal is not particularly limited as long as it can modulate light. Examples thereof include lithium niobate, lithium tantalate, lithium niobate-lithium tantalate solid solution, potassium lithium niobate, KTP, GaAs, and quartz. it can.

  The electrode is not particularly limited as long as it is a material having low resistance and excellent impedance characteristics, and can be composed of a material such as gold, silver, or copper.

  A buffer layer can be provided between the surface of the substrate body and the signal electrode and the ground electrode. A known material such as silicon oxide, magnesium fluoride, silicon nitride, and alumina can be used for the buffer layer.

  In a preferred embodiment, the optical waveguide is formed on one main surface side of the substrate body. The optical waveguide may be an optical waveguide formed inside the substrate body by an internal diffusion method or an ion exchange method, such as a titanium diffusion optical waveguide or a proton exchange optical waveguide. The electrode is provided on one main surface side of the substrate body, but may be directly formed on one main surface of the substrate body or may be formed on the buffer layer.

  In a preferred embodiment, the thickness of the optical waveguide substrate is 200 μm or less, more preferably 100 μm or less. Moreover, when providing a recessed part in an optical waveguide board | substrate, it is preferable that the thickness of the area | region of a recessed part shall be 100 micrometers or less, 50 micrometers or less, Furthermore, it is preferable to set it as 30 micrometers or less.

(Experiment A)
An embodiment of the invention according to the first aspect will be described.
The optical modulator 1 shown in FIGS. 1 and 2 was manufactured (example of the present invention). Or the thing which remove | eliminated the electrical connection member 7 from the optical modulator 1 shown in FIG. 1, FIG. 2 was produced (comparative example).

Specifically, an X-cut 3 inch wafer (LiNbO ₃ single crystal) was used, and a Mach-Zehnder type optical waveguide 6 was formed on the surface of the wafer by a titanium diffusion process and a photolithography method. The size of the optical waveguide can be, for example, 10 μm at 1 / e ² . Next, the signal electrode 4 and the ground electrodes 3A and 3B were formed by a plating process. The width WA of the first gap 5A was 20 μm, the width WB of the second gap 5B was 95 μm, the electrode thickness was 25 μm, and the electrode length was 40 mm.

  Next, the polishing dummy substrate was fixed to the polishing surface plate, and the substrate body for the modulator was attached thereon with the electrode surface facing downward. Next, the substrate body 2 for the modulator was thinned to a thickness of 8.5 μm by horizontal polishing, lapping and polishing (CMP). Next, the substrate body 2 was fixed on a flat support substrate. The resin for adhesion and fixing had a resin thickness of 50 μm. The end face of the optical waveguide (connecting portion to the optical fiber) was polished, and the wafer was cut by dicing to obtain each chip. The chip width was 2 mm and the total thickness of device 1 was 0.5 mm.

  In the example of the present invention, the ground electrodes 3A and 3B were connected by a wire 7 made of aluminum at intervals of 1.25 mm. In the comparative example, the electrical connection member 7 was not provided.

  A polarization-maintaining optical fiber for 1.55 μm or a single-core fiber array holding a 1.3 μm single-mode fiber is manufactured, and the former is connected to the optical modulator chip 1A with the latter as the input side and the latter as the output side. The waveguide was aligned and bonded with an ultraviolet curable resin.

It was then measured S ₂₁ characteristics vector network analyzer. The probe used was a CPW probe “ACP 50-250” manufactured by Cascade. The results are shown in FIG. 5 (comparative example) and FIG. 6 (invention example).

  In FIG. 5, some ripples were observed in the region of 3 GHz or more and 10 GHz or less. For this reason, there arises a problem that the signal wave is distorted by the modulation. In FIG. 6, the position of the ripple moved to the high frequency side, and no ripple was observed below 15 GHz. If wires are provided at even shorter intervals, ripple-free operation can be achieved up to a higher frequency.

(Experiment B)
An embodiment of the invention according to the second aspect will be described.
The optical modulator 11 shown in FIGS. 3 and 4 was manufactured and attached to the housing 8 (example of the present invention).
Specifically, an X-cut 3 inch wafer (LiNbO ₃ single crystal) was used, and a Mach-Zehnder type optical waveguide 6 was formed on the surface of the wafer by a titanium diffusion process and a photolithography method. The size of the optical waveguide can be, for example, 10 μm at 1 / e ² . Next, the signal electrode 4 and the ground electrodes 3A and 3B were formed by a plating process. The width WA of the first gap 5A was 20 μm, the width WB of the second gap 5B was 95 μm, the electrode thickness was 25 μm, and the electrode length was 40 mm.

  Next, the polishing dummy substrate was fixed to the polishing surface plate, and the substrate body for the modulator was attached thereon with the electrode surface facing downward. Next, the substrate body 2 for the modulator was thinned to a thickness of 8.5 μm by horizontal polishing, lapping and polishing (CMP). Next, the substrate body 2 was fixed on a flat support substrate. The resin for adhesion and fixing had a resin thickness of 50 μm. The end face of the optical waveguide (connecting portion to the optical fiber) was polished, and the wafer was cut by dicing to obtain each chip. The chip width was 2 mm and the total thickness of device 1 was 0.5 mm.

  In the example of the present invention, the ground electrode 3A and the casing 8 and the ground electrode 3B and the casing 8 were connected at intervals of 5 mm by 250 μm thick gold ribbons 10A and 10B.

  A polarization-maintaining optical fiber for 1.55 μm or a single-core fiber array holding a 1.3 μm single-mode fiber is manufactured, and the former is connected to the optical modulator chip 1A with the latter as the input side and the latter as the output side. The waveguide was aligned and bonded with an ultraviolet curable resin.

  Next, S21 characteristics were measured with a vector network analyzer. The probe used was a CPW probe “ACP 50-250” manufactured by Cascade. The results are shown in FIG. 7 (comparative example) and FIG. 8 (invention example).

The S ₂₁ characteristics of the optical waveguide device 11, prior to attachment to the housing (in the state of bare chips) was determined (Comparative Example). As a result, a number of ripples were observed at 3 to 22 GHz. In contrast, implementing the optical waveguide device 11 in the metal package, Figure 8 shows the S ₂₁ characteristics after connecting the ground electrode to the metal housing 8 is as described above. In FIG. 8, it can be seen that the ripple is reduced particularly at 22 GHz or less.

It is a top view which shows roughly the optical waveguide device 1 which concerns on invention of 1st aspect. It is a cross-sectional view schematically showing the optical waveguide device 1 of FIG. It is a top view which shows roughly the optical waveguide device 11 and the housing | casing 8 which concern on invention of 2nd aspect. FIG. 4 is a cross-sectional view schematically showing the optical waveguide device 11 and the housing 8 of FIG. 3. In Comparative Examples experiment A, it is a graph showing the relationship between the S ₂₁ characteristics and frequency. 1, employing an optical waveguide device 1 of FIG. 2, in the present invention example of experiment A, is a graph showing the relationship between the S ₂₁ characteristics and frequency. In Comparative Examples experiment B, and a graph showing the relationship between the S ₂₁ characteristics and frequency. Figure 3, employing an optical waveguide device 11 and the housing 8 in FIG. 4, in the present invention example of experiment B, and a graph showing the relationship between the S ₂₁ characteristics and frequency.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 11 Optical waveguide device 2 Substrate main body 3A 1st ground electrode 3B 2nd ground electrode 4 Signal electrode 5A 1st gap 5B 2nd gap 6 Optical waveguide 7 Electrical connection member which passes on optical waveguide 6 8 Case Body 10A Electrical connection member for connecting first ground electrode 3A and housing 8 10B Electrical connection member for connecting second ground electrode 3B and housing 8 WA Width of first gap 5A WB Second gap 5B width

Claims (10)

An optical waveguide device comprising a substrate body made of an electro-optic material, an optical waveguide, and a signal electrode for applying an electrical signal to the optical waveguide, a first ground electrode and a second ground electrode,
An optical waveguide device, wherein an electrical connection member for electrically connecting the first ground electrode and the second ground electrode is provided so as to pass over the optical waveguide.   The width of the first gap between the signal electrode and the first ground electrode is different from the width of the second gap between the signal electrode and the second ground electrode. The optical waveguide device according to claim 1.   The optical waveguide device according to claim 1, wherein the electrical connection member is made of a bonding wire.   The optical waveguide device according to any one of claims 1 to 3, wherein a thickness of the substrate body is 200 µm or less.   The optical waveguide device according to any one of claims 1 to 4, wherein the optical waveguide device is a traveling wave type optical modulator. An optical waveguide device comprising a substrate body made of an electro-optic material, an optical waveguide, and a signal electrode for applying an electrical signal to the optical waveguide, a first ground electrode and a second ground electrode,
A width of a first gap between the signal electrode and the first ground electrode is different from a width of a second gap between the signal electrode and the second ground electrode; It comprises an electrical connection member for electrically connecting at least one of the first ground electrode and the second ground electrode to a housing for fixing a waveguide device, Optical waveguide device.   The electrical connection member includes a first electrical connection member for connecting the first ground electrode to the housing, and a second electrical connection member for connecting the second ground electrode to the housing. The optical waveguide device according to claim 6, comprising:   The optical waveguide device according to claim 6 or 7, wherein the electrical connection member is made of a metal foil or a bonding wire.   The optical waveguide device according to any one of claims 6 to 8, wherein a thickness of the substrate body is 200 µm or less.   The optical waveguide device according to claim 6, wherein the optical waveguide device is a traveling wave optical modulator.

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