Patent ID: 12242143

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained with reference to accompanying drawings. The present invention is not limited by the embodiments below. In addition, dimensions of various parts are illustrated by way of example for convenience of explanation, but the dimensions are not limited to this example and may be changed appropriately.

[a] First Embodiment

FIG.1is a block diagram illustrating an example of a configuration of an optical communication apparatus1according to a first embodiment. The optical communication apparatus1illustrated inFIG.1is connected to an optical fiber2A (2) at an output side and an optical fiber2B (2) at an input side. The optical communication apparatus1includes a digital signal processor (DSP)3, a light source4, an optical modulator5, and an optical receiver6. The DSP3is an electrical component that performs digital signal processing. The DSP3performs a process, such as encoding, on transmission data, generates an electrical signal including the transmission data, and outputs the generated electrical signal to the optical modulator5, for example. Further, the DSP3acquires an electrical signal including reception data from the optical receiver6, performs a process, such as decoding, on the acquired electrical signal, and obtains reception data.

The light source4includes, for example, a laser diode or the like, generates light at a predetermined wavelength, and supplies the light to the optical modulator5and the optical receiver6. The optical modulator5is an optical device that modulates the light supplied from the light source4by using the electrical signal output from the DSP3, and outputs the obtained optical transmission signal to the optical fiber2A. The optical modulator5includes, for example, a lead zirconate titanate (PZT) waveguide31and a signal electrode32having a micro-stripline (MSL) structure. The optical modulator5, when the light supplied from the light source4propagates through the PZT waveguide31, modulates the light by the electrical signal input to the signal electrode32, and generates an optical transmission signal.

The optical receiver6receives an optical signal from the optical fiber2B and demodulates the received optical signal by using the light supplied from the light source4. Then, the optical receiver6converts the demodulated received optical signal into an electrical signal, and outputs the converted electrical signal to the DSP3.

FIG.2is a schematic plan view illustrating an example of a configuration of the optical modulator (PZT modulator)5according to the first embodiment. The optical modulator5illustrated inFIG.2is, for example, a PZT modulator that is connected to an optical fiber4A from the light source4at an input side and is connected to the optical fiber2A for outputting a transmission signal at an output side. The optical modulator5includes a first optical input unit11, an RF modulation unit12, and a first optical output unit13. The first optical input unit11includes a first Si waveguide21and first PZT-Si waveguide bonding units22. The first Si waveguide21includes a single Si waveguide that is connected to the optical fiber4A, two Si waveguides that are branched from the single Si waveguide, four Si waveguides that are branched from the two Si waveguides, and eight Si waveguides that are branched from the four Si waveguides. The first PZT-Si waveguide bonding units22bond the eight Si waveguides in the first Si waveguide21and eight PZT waveguides in the PZT waveguide31.

The RF modulation unit12includes the PZT waveguide31, the signal electrode32, and an RF terminator33. The RF modulation unit12, when light supplied from the first Si waveguide21propagates through the PZT waveguide31, modulates the light by using an electric field applied from the signal electrode32. The PZT waveguide31is, for example, an optical waveguide that is formed by using a thin-film PZT substrate55, is repeatedly branched from the input side, and includes the eight PZT waveguides that are parallel to one another. The light that propagates through and modulated in the PZT waveguide31is output to the first optical output unit13. PZT is an inorganic material, such as a perovskite oxide, with a large electro-optic effect, such as an optical refractive index, as compared to LN.

The signal electrode32is a transmission path that is arranged at a position overlapping with the PZT waveguide31and that has the MSL structure, and applies an electric field to the PZT waveguide31in accordance with an electrical signal output from the DSP3. A terminal end of the signal electrode32is connected to the RF terminator33. The RF terminator33is connected to the terminal end of the signal electrode32and prevents unnecessary reflection of a signal that is transmitted by the signal electrode32.

The thin-film PZT substrate55is a PZT single crystal, for which a crystal direction in which an electro-optic coefficient of the thin-film PZT substrate is high corresponds to a vertical direction (X direction) with respect to an Si substrate51; therefore, the optical modulator5includes a ground electrode53between the Si substrate51and the signal electrode32, and the electric field is oriented in the vertical direction (X direction) with respect to the Si substrate51.

The first optical output unit13includes a second PZT-Si waveguide bonding unit41, a second Si waveguide42, eight child-side MZs43, four parent-side MZs44, a PR45, and a PBC46. The second PZT-Si waveguide bonding unit41bonds the PZT waveguide31in the RF modulation unit12and the second Si waveguide42. The second Si waveguide42includes eight Si waveguides that are connected to the second PZT-Si waveguide bonding unit41and four Si waveguides, among the eight Si waveguides, that merge with two Si waveguides. Further, the second Si waveguide42includes two Si waveguides, among the four Si waveguides, that merge with two Si waveguides, and a single Si waveguide that merges with the two Si waveguides. The child-side Mach-Zehnders (MZs)43are arranged for the respective eight Si waveguides in the second Si waveguide42. A set of the child-side MZs43applies bias voltage to DC electrodes on the Si waveguides to adjust bias voltage such that ON/OFF of an electrical signal corresponds to ON/OFF of an optical signal, and outputs an I signal that is an in-phase component or a Q signal that is a quadrature component. The parent-side MZs44are arranged for the respective four Si waveguides in the second Si waveguide42. A set of the parent-side MZs44applies bias voltage to DC electrodes on the Si waveguides to adjust bias voltage such that ON/OFF of an electrical signal corresponds to ON/OFF of an optical signal, and outputs an I signal or a Q signal.

The PR45rotates the I signal or the Q signal that is input from one set of the parent-side MZs44by 90 degrees, and obtains a vertically-polarized optical signal that is rotated by 90 degrees. Then, the PR45inputs the vertically-polarized optical signal to the PBC46. The PBC46couples the vertically-polarized optical signal input from the PR45and a horizontally-polarized optical signal input from the other set of the parent-side MZs44, and outputs a dual-polarized signal.

A configuration of the optical modulator5according to the first embodiment will be described in detail below.FIG.3is a schematic cross-sectional view of an exemplary cross section of the optical modulator5taken along a line A-A inFIG.2, andFIG.4is a schematic cross-sectional view of an exemplary cross section of the optical modulator5taken along a line B-B inFIG.2. The A-A cross section illustrated inFIG.3and the B-B cross section illustrated inFIG.4are cross-sections of the RF modulation unit12. The RF modulation unit12includes the Si substrate51, a Box layer52that is laminated on the Si substrate51and that is made of silicon dioxide (SiO2) or the like, and the ground electrode53that is laminated on the Box layer52and that has the MSL structure. The RF modulation unit12includes the first Si waveguide21that is formed on the Box layer52and a first cladding layer54that is laminated on the ground electrode53. Further, the RF modulation unit12includes the thin-film PZT substrate55that is laminated on the first cladding layer54, a second cladding layer56that is laminated on the thin-film PZT substrate55, and the signal electrode32that is laminated on the second cladding layer56and that has the MSL structure.

The Si substrate51is a Si substrate with a thickness of about several hundred μm, for example. The Box layer52is a substrate made of SiO2, titanium dioxide (TiO2), or the like, for example. The ground electrode53is an electrode that is made of metal, such as aluminum, that has a thickness of 0.1 μm or more, and that has a ground potential. The ground electrode53is able to reduce an influence of the electric field from the signal electrode32on the Si substrate51and reduce a high-frequency loss. The first cladding layer54is a layer that is made of SiO2, TiO2, or the like and that has a thickness of 0.3 to 0.5 nm, for example. Similarly, the second cladding layer56is a layer that is made of SiO2, TiO2, or the like and that has a thickness of 0.2 to 3 μm, for example. The second cladding layer56is able to prevent an optical loss caused by the signal electrode32that is arranged above the thin-film PZT substrate55, for example.

The thin-film PZT substrate55with a thickness of 0.5 to 3 μm is sandwiched between the first cladding layer54and the second cladding layer56, and the PZT waveguide31that protrudes upward is formed in the center of the thin-film PZT substrate55, for example. A width of the protrusion that serves as the PZT waveguide31is, for example, about 1 to 8 μm. The thin-film PZT substrate55and the PZT waveguide31are covered by the second cladding layer56, and the signal electrode32is arranged on a surface of the second cladding layer56. In other words, the signal electrode32faces the ground electrode53across the PZT waveguide31and constitutes a transmission path of the MSL structure.

It is desirable to form a film of the ground electrode53of the MSL structure through Si wafer processing. Further, it is desirable to select a material by taking into account adhesiveness of the ground electrode53and the first cladding layer54. Furthermore, it is desirable that the signal electrode32is made of a material for which a high-frequency loss is small and which is different from the material of the ground electrode53.

The signal electrode32is an electrode that is made of a metal material, such as gold or copper, has a width of 2 to 10 μm, and has a thickness of 1 to 20 μm, for example. The ground electrode53is an electrode that is made of a metal material, such as aluminum, and has a thickness of 0.1 μm or more, for example. A high-frequency signal corresponding to an electrical signal output from the DSP3is transmitted by the signal electrode32, so that an electric field in a direction from the signal electrode32to the ground electrode53is generated and the electric field is applied to the PZT waveguide31. As a result, a refractive index of the PZT waveguide31is changed in accordance with the electric field applied to the PZT waveguide31, so that it is possible to modulate light that propagates through the PZT waveguide31. The thin-film PZT substrate55that forms the PZT waveguide31is a PZT single crystal, and therefore, a crystal direction (crystal orientation) is the vertical direction (X direction) with respect to the Si substrate51, which is the same as a direction of the electric field.

The optical modulator5includes a Si optical integrated circuit wafer500and a thin-film PTZ substrate wafer550. The Si optical integrated circuit wafer500is a wafer that forms the first optical input unit11, the RF modulation unit12, and the first optical output unit13in the optical modulator5. The thin-film PTZ substrate wafer550is a wafer for forming the thin-film PZT substrate55that is a PZT single crystal and that forms the RF modulation unit12.

FIG.5is a schematic cross-sectional view of an exemplary cross section of the optical modulator5taken along a line C-C inFIG.2. The C-C cross-section illustrated inFIG.5is a cross-section of the second PZT-Si waveguide bonding unit41. The second PZT-Si waveguide bonding unit41includes the Si substrate51, the Box layer52that is laminated on the Si substrate51, the first Si waveguide21that is laminated on the Box layer52, and the first cladding layer54that covers the first Si waveguide21. Further, the second PZT-Si waveguide bonding unit41includes the thin-film PZT substrate55that includes the PZT waveguide31and that is laminated on the first cladding layer54, and the second cladding layer56that is laminated on the thin-film PZT substrate55.

FIG.6is a schematic cross-sectional view of an exemplary cross section of the optical modulator5(the Si optical integrated circuit wafer500) taken along a line D-D. The Si optical integrated circuit wafer500illustrated inFIG.6is constructed with a silicon on insulator (SOI) wafer. The Si optical integrated circuit wafer500includes the Si substrate51, the Box layer52that is laminated on the Si substrate51, the first Si waveguide21that is laminated on the Box layer52, and the first cladding layer54that is laminated on the first Si waveguide21. The first cladding layer54is a dielectric, such as a SiO2film, with a low refractive index, for example. A surface of the first cladding layer54is smoothed by chemical mechanical polishing (CMP) to reduce unevenness. Meanwhile, resin with a low refractive index may be adopted as a dielectric that covers the entire wafer surface, for example.

FIG.7is an exemplary schematic cross-sectional view of the thin-film PTZ substrate wafer550. The thin-film PTZ substrate wafer550illustrated inFIG.7includes a Si substrate551, a ZrO2film552, a Pt layer553, an SRO film554, and a PZT layer555. The ZrO2film552is a film that includes a film portion of 10 nm or more and a protruding portion of 3 to 8 nm, and that is formed such that a first layer including ZrO2and having elasticity due to inclusion of crystal defects of up to 8% is formed on an Si single crystal by epitaxial growth, for example. The Pt layer553is a layer that is formed such that a second layer including Pt and having a film thickness of 20 nm or more is formed on the ZrO2film552by epitaxial growth, for example. The SRO film554is a film that is formed such that a third layer including strontium oxide (SRO) and having a film thickness of 20 nm or more is formed on the Pt layer553by epitaxial growth, for example. The PZT layer555is a layer that is formed such that a fourth layer including a thin film, such as PZT, with a piezoelectric effect and an electro-optic effect and having a film thickness of about 2 um is formed on the SRO film554by epitaxial growth, for example. The first layer, the second layer, the third layer, and the fourth layer are sequentially subjected to epitaxial growth, and the thin-film PTZ substrate wafer550that is a multi-layer film including the PZT layer555as a PZT single crystal is formed (reference: International Publication Pamphlet NO. WO2020/179210). As epitaxial growth, a vapor deposition method may be used for the ZrO2film552, and a physical vapor deposition (PVD) method, such as sputtering, may be used for the Pt layer553and the SRO film554. Further, it may be possible to heat the Si substrate551to 450 to 600° C. to promote epitaxial growth.

Here, the ZrO2crystal as the first layer is a tetragonal crystal, but has crystal defects of up to 8%, and, it is presumed that, if crystal defects are present, atoms located adjacent to defective vacancies have elasticity in a direction in which lattice distortion is reduced. With use of the elasticity of the ZrO2crystal, it is possible to implement a function that allows a change in a crystal structure. Further, formation the protruding portion made of the ZrO2crystal indicates that when a material density is supersaturated in a film formation process, crystal growth may occur such that a certain axis of the crystal grows in an anisotropic manner along a certain ridge while forming a pyramid structure.

FIG.8AtoFIG.8Eare diagrams for explaining an exemplary process of manufacturing the RF modulation unit12of the optical modulator5.FIG.8Ais a diagram for explaining an exemplary process of manufacturing the RF modulation unit12, andFIG.8Bis a diagram for explaining an exemplary process of manufacturing the RF modulation unit12(adhesive layer). The PZT layer555of the thin-film PTZ substrate wafer550illustrated inFIG.8Bis bonded, by wafer bonding, on the first cladding layer54in the Si optical integrated circuit wafer500via an adhesive layer556.

FIG.8Cis a diagram for explaining an exemplary process of manufacturing the RF modulation unit12(adjustment of a dimension between a bottom surface of the PZT layer555and an upper surface of the first Si waveguide21). Meanwhile, for convenience of explanation, it is assumed that the first Si waveguide21has a thickness of, for example, 220 nm, and the PZT layer555has a thickness of, for example, 1 μm. The thin-film PTZ substrate wafer550is bonded, by wafer bonding, on the Si optical integrated circuit wafer500. Further, thicknesses of the adhesive layer556and the first cladding layer54are adjusted such that a thickness dimension between the bottom surface of the PZT layer555illustrated inFIG.8Cand the upper surface of the first Si waveguide21falls within a range of 100 to 300 nm, for example. As a result, it is possible to achieve optical coupling from the first Si waveguide21to the PZT waveguide31.

FIG.8Dis a diagram for explaining an exemplary process of manufacturing the RF modulation unit12(polishing process). After the PZT layer555of the thin-film PTZ substrate wafer550is bonded on the Si optical integrated circuit wafer500, the Si substrate551, the ZrO2film552, the Pt layer553, and the SRO film554are removed by using the polishing process while remaining the PZT layer555in the thin-film PTZ substrate wafer550.

FIG.8Eis a diagram for explaining an exemplary process of manufacturing the RF modulation unit12(formation of the thin-film PZT substrate). As illustrated inFIG.8E, the PZT waveguides31in protruding shapes and a PZT slab558are further formed on a surface of the PZT layer555by using photolithography, on the PZT layer555of the thin-film PTZ substrate wafer550subjected to the polishing process. As a result, the thin-film PZT substrate55is formed on the first cladding layer54. Meanwhile, the PZT slab558of the thin-film PZT substrate55is able to increase a bonding force between the first cladding layer54on the Si optical integrated circuit wafer500and the thin-film PZT substrate55.

Then, the second cladding layer56is formed on the thin-film PZT substrate55(seeFIG.4). Further, the signal electrode32having the MSL structure is formed on the second cladding layer56. As a result, the RF modulation unit12as illustrated inFIG.4is completed.

The optical modulator5of the first embodiment includes the Si substrate51, the ground electrode53that is laminated on the Si substrate51and that has a ground potential, and the PZT waveguide31that is formed of the thin-film PZT substrate55laminated on the ground electrode53. Further, the optical modulator5includes the signal electrode32that is arranged at a position facing the ground electrode53across the PZT waveguide31in the vertical direction of the Si substrate51and that applies a high-frequency signal to the PZT waveguide31. As a result, with use of the PZT waveguide31that has a large electro-optic effect as compared to an LN waveguide, it is possible to improve modulation efficiency and it is possible to reduce a device size and driving voltage. With use of the thin-film PZT substrate55, which is a PZT single crystal having a large electro-optic effect as compared to LN, as a material of the optical modulator5, the PZT single crystal is able to achieve an electro-optic coefficient that is three times larger than that of an LN single crystal.

The optical modulator5includes the first cladding layer54that is laminated between the ground electrode53and the thin-film PZT substrate55, and the second cladding layer56that is laminated on the thin-film PZT substrate55and that covers the PZT waveguide31. The signal electrode32is arranged at a position overlapping with the PZT waveguides31on the surface of the second cladding layer56. The signal electrode32generates an electric field inside the PZT waveguide31in the vertical direction (vertical direction (X direction) inFIG.4) with respect to the Si substrate51. The crystal direction of the PZT waveguide31is also the vertical direction (X direction) with respect to the Si substrate51. In other words, the crystal direction of the PZT waveguide31is the same as the direction of the electric field, so that it is possible to improve efficiency in electric field application, reduce the driving voltage, and largely improves the modulation efficiency. Further, with use of PZT, it is possible to improve the modulation efficiency (voltage×electrode length). As a result, it is possible to reduce voltage and a device size. Furthermore, it is possible to achieve high modulation efficiency as compared to LN even if the electrode length is reduced, so that it is possible to further reduce the size of the optical modulator5in accordance with a reduced electrode length.

The optical modulator5includes the first cladding layer54that covers the first Si waveguide21, and the second cladding layer56that covers the thin-film PZT substrate55. A thickness of the first cladding layer54between the upper surface of the first Si waveguide21and the bottom surface of the thin-film PZT substrate55is set to 100 nm to 300 nm to achieve optical coupling between the first Si waveguide21and the PZT waveguide31. As a result, it is possible to achieve optical coupling between the first Si waveguide21and the PZT waveguide31.

The first cladding layer54is made of a dielectric or resin with a low refractive index. As a result, it is possible to bond the first cladding layer54and the thin-film PZT substrate55.

The optical modulator5includes a dielectric57that is a SiO2film and that is a dielectric formed on a back surface of the Si substrate51. As a result, it is possible to reduce an influence of warpage due to an influence of thermal history or the like in the process of manufacturing the optical modulator5.

Meanwhile, the case has been described in which the RF modulation unit12of the optical modulator5of the first embodiment is configured such that the PZT layer555is bonded on the Si optical integrated circuit wafer500via the adhesive layer556as illustrated inFIG.8D. However, in the process of manufacturing the optical modulator5, if warpage of the Si optical integrated circuit wafer500increases due to an influence of thermal history or the like, it may become difficult to adsorb the Si optical integrated circuit wafer500on a wafer stage by an exposure device or the like that is used to form a pattern of the optical modulator5. Therefore, to cope with the situation as described above, as illustrated inFIG.9, the dielectric57, such as an SiO2film, is formed on the back surface of the Si substrate51of the Si optical integrated circuit wafer500to prevent warpage of the Si optical integrated circuit wafer500. As a result, it is possible to avoid a situation in which the Si optical integrated circuit wafer500is not adsorbed on the wafer stage by the exposure device or the like that is used to form a pattern of the optical modulator5.

Further, the case has been described in which the RF modulation unit12is configured such that the PZT layer555of the thin-film PTZ substrate wafer550is bonded on the first cladding layer54of the Si optical integrated circuit wafer500via an adhesive layer556A. However, an appropriate change is applicable, and it may be possible to form a bonding portion between the first cladding layer54and the PZT layer555without using the adhesive layer556A.

FIG.10is a schematic cross-sectional view illustrating a modification of the cross section of the optical modulator5taken along the line B-B inFIG.2. In the RF modulation unit12illustrated inFIG.10, opening portions561are formed, through an etching process, in portions of the second cladding layer56that covers both sides of the PZT waveguides31of the thin-film PZT substrate55. Further, in the RF modulation unit12, the signal electrodes32are arranged on the second cladding layer56on the PZT waveguides31. By opening the portions of the second cladding layer56that covers the both sides of the PZT waveguides31, an influence of the second cladding layer56is reduced and electric fields that are generated in the vertical direction from the signal electrodes32to the ground electrode53is applied to the PZT waveguides31. As a result, it is possible to improve the modulation efficiency.

FIG.11Ais a schematic cross-sectional view illustrating a modification of the cross section of the optical modulator5taken along the line B-B inFIG.2. In the RF modulation unit12illustrated inFIG.11A, an opening portion562that exposes a part of a surface of the ground electrode53covered by the first cladding layer54is formed by etching the second cladding layer56, the thin-film PZT substrate55, and the first cladding layer54. Then, a metal film that is made of the same material as the ground electrode53is formed in the opening portion562, and a ground electrode531for exposure is formed. As a result, the ground electrode53and the ground electrode531for exposure are electrically bonded, and it becomes possible to easily establish a ground connection by exposing the ground electrode531on the second cladding layer56.

Meanwhile, for convenience of explanation, the case has been described in which, in the optical modulator5of the first embodiment, the first Si waveguide21and the PZT waveguide31are directionally coupled. However, an appropriate change is applicable, and the first Si waveguide21and the PZT waveguide may be butt-coupled.

It is necessary to provide the first cladding layer54between the thin-film PZT substrate55and the ground electrode53, and increase a thickness of the first cladding layer54to laminate the ground electrode53. Therefore, a distance between the PZT waveguide31and the first Si waveguide21is increased with an increase in the thickness of the first cladding layer54, so that a bond length between the PZT waveguide31and the first Si waveguide21is increased. Therefore, to cope with the situation as described above, a Si-PZT waveguide may be provided between the PZT waveguide31and the first Si waveguide21to achieve optical coupling.

Meanwhile, while the PZT modulator is illustrated in the optical modulator5of the first embodiment, it may be possible to use a barium titanate (BiTiO3: hereinafter, referred to as BTO) modulator in which BTO is adopted instead of PZT.FIG.11Bis an exemplary schematic cross-sectional view of a thin-film BTO substrate wafer550B. The thin-film BTO substrate wafer550B illustrated inFIG.11Bincludes the Si substrate551, the ZrO2film552, the Pt layer553, the SRO film554, and a BTO film555B. As described above, the ZrO2crystal as a tetragonal crystal has crystal defects and atoms located adjacent to defective vacancies have elasticity to implement a function to allow a change in a crystal structure, so that when a material density is supersaturated in a film formation process, crystal growth occurs such that a certain axis of the crystal grows in an anisotropic manner along a certain ridge while forming a pyramid structure: thus, epitaxial growth is possible even in BTO.

Further, in BTO, phase transition may occur at around 0° C. to 5° C. in an operation temperature range (for example, −5° C. to 75° C.) of the modulator: therefore, to stabilize properties in the operation temperature range, it may be possible to reduce the Curie temperature by doping BTO with Sr, Zr, La, KF, or the like to reduce phase transition temperature to −5° C. or less from around 0° C. to 5° C., or it may be possible to control the temperature at 25° C. to 45° C. by a Peltier device, for use of the BTO. Meanwhile, the Peltier device is mounted on a surface opposite to a surface on which the Si substrate551is laminated inFIG.11B. Further, by controlling the temperature of the Peltier device at constant temperature, it is possible to stabilize the properties in the operation temperature range of the modulator.

Furthermore, the BTO film555B of the thin-film BTO substrate wafer550is bonded, by wafer bonding, on the first cladding layer54in the Si optical integrated circuit wafer500via the adhesive layer556. As a result, through the manufacturing process as illustrated inFIG.8AtoFIG.8E, the thin-film BTO substrate instead of the thin-film PZT substrate55is formed on the first cladding layer54. Meanwhile, a process of forming the thin-film BTO substrate is different in that the thin-film BTO substrate wafer550using the BTO film instead of the PZT film is used, but the other formation processes are substantially the same as the processes of forming the thin-film PZT substrate55, and therefore, detailed explanation thereof will be omitted. Then, the second cladding layer56is formed on the thin-film BTO substrate (seeFIG.4). Moreover, the signal electrode32having the MSL structure is formed on the second cladding layer56. As a result, the RF modulation unit with the BTO modulator is completed. A BTO waveguide formed in the thin-film BTO substrate has a certain shape that is similar to the PZT waveguides31formed on the thin-film PZT substrate55as illustrated inFIG.8E.

Further, while the PZT modulator is described by way of example in the optical modulator5of the first embodiment, it may be possible to adopt a lanthanum-doped lead zirconate-lead titanate (PLZT) modulator in which PLZT is used instead of PZT, and an embodiment for this will be described below as a second embodiment. Meanwhile, the same components as those of the first embodiment are denoted by the same reference symbols, and repeated explanation of the configuration and the operation thereof will be omitted.

[b] Second Embodiment

FIG.12is a schematic plan view illustrating an example of a configuration of an optical modulator (PLZT modulator)5A according to the second embodiment. The optical modulator5A illustrated inFIG.12is a PLZT modulator. The optical modulator5A includes the first optical input unit11, an RF modulation unit12A, and the first optical output unit13. The first optical input unit11includes the first Si waveguide21and a first PLZT-Si waveguide bonding unit22A. The first Si waveguide21includes a single Si waveguide that is connected to the optical fiber4A, two Si waveguides that are branched from the single Si waveguide, four Si waveguides that are branched from the two Si waveguides, and eight Si waveguides that are branched from the four Si waveguides. The first PLZT-Si waveguide bonding unit22A bonds the eight Si waveguides in the first Si waveguide21and eight PLZT waveguides in a PLZT waveguide31A.

The RF modulation unit12A includes the PLZT waveguide31A, a signal electrode32A, and the RF terminator33. The RF modulation unit12A, when light supplied from the first Si waveguide21propagates through the PLZT waveguide31A, modulates the light by using an electric field applied from the signal electrode32A. The PLZT waveguide31A is, for example, an optical waveguide that is formed by using a thin-film PLZT substrate55A, is repeated branched from the input side, and includes eight PLZT waveguides that are parallel to one another. The light that propagates through and modulated in the PLZT waveguide31A is output to the first optical output unit13. PLZT is an inorganic material, such as a perovskite oxide, with a large electro-optic effect, such as an optical refractive index, as compared to LN.

The signal electrode32A and a ground electrode53A have coplanar waveguide (CPW) structures. The signal electrode32A and a pair of ground electrodes53A sandwiching the signal electrode32A are arranged above the PLZT waveguide31A. The signal electrode32A applies an electric field to the PLZT waveguide31A in accordance with an electrical signal output from the DSP3. A terminal end of the signal electrode32A is connected to the RF terminator33.

The optical modulator5A includes the signal electrode32A and the pair of ground electrodes53A having the CPW structures above the PLZT waveguide31A, and a direction of the electric field is a width direction with respect to the Si substrate51(left to right (Z direction) inFIG.13). The thin-film PLZT substrate55A is a PLZT single crystal, and a crystal direction of the thin-film PLZT substrate55A is the width direction (Z direction) with respect to the Si substrate51, which is the same as the direction of the electric field.

The first optical output unit13includes a second PLZT-Si waveguide bonding unit41A, the second Si waveguide42, the eight child-side MZs43, the four parent-side MZs44, the PR45, and the PBC46. The second PLZT-Si waveguide bonding unit41A bonds the PLZT waveguide31A in the RF modulation unit12A and the second Si waveguide42. The second Si waveguide42includes eight Si waveguides that are connected to the second PLZT-Si waveguide bonding unit41A and four Si waveguides, among the eight Si waveguides, that merge with two Si waveguides.

A configuration of the optical modulator5A according to the second embodiment will be described in detail below.FIG.13is a schematic cross-sectional view of an exemplary cross section of the optical modulator5A taken along a line E-E inFIG.12. The E-E cross-section illustrated inFIG.13is a cross-section of the RF modulation unit12A. The RF modulation unit12A includes the Si substrate51, the Box layer52that is laminated on the Si substrate51and that is made of SiO2, the first cladding layer54that is laminated on the Box layer52. The RF modulation unit12A includes the first cladding layer54that is laminated on the thin-film PLZT substrate55A, the second cladding layer56A that is laminated on the thin-film PLZT substrate55A, and the signal electrode32A and the pair of ground electrodes53A that are laminated on a surface of a second cladding layer56A and that have the CPW structures.

The Si substrate51is a Si substrate with a thickness of about several hundred μm, for example. The Box layer52is a substrate made of SiO2, TiO2, or the like, for example. The ground electrodes53A are electrodes that are made of metal, such as copper, that have thicknesses of 1 μm or more, and that have ground potentials, for example. The first cladding layer54is a layer that is made of SiO2, TiO2, or the like, that has a high refractive index, and that has a thickness of 0.3 to 0.5 μm, for example. Similarly, the second cladding layer56A is a layer that is made of SiO2, TiO2, or the like and that has a thickness of 0.2 to 3 μm, for example. The second cladding layer56A is able to prevent an optical loss caused by the signal electrode32A that is arranged above the thin-film PLZT substrate55A, for example.

The thin-film PLZT substrate55A with a thickness of 0.5 to 3 μm is sandwiched between the first cladding layer54and the second cladding layer56A, and the PLZT waveguide31A that protrudes upward is formed in the center of the thin-film PLZT substrate55A, for example. A width of the protrusion that serves as the PLZT waveguide31A is, for example, about 1 to 8 μm. The thin-film PLZT substrate55A and the PLZT waveguide31A are covered by the second cladding layer56A, and the signal electrode32A and the ground electrodes53A are arranged on the surface of the second cladding layer56A. In other words, the pair of ground electrodes53A are arranged on the PLZT waveguides31A, and the signal electrode32A constitutes a transmission path of the CPW structure.

The signal electrode32A is an electrode that is made of a metal material, such as gold or copper, has a width of 2 to 10 μm, and has a thickness of 1 to 20 μm, for example. The ground electrodes53A are electrodes that are made of, for example, a metal material, such as gold or copper, and have thicknesses of 1 μm or more, for example. A high-frequency signal corresponding to an electrical signal output from the DSP3is transmitted by the signal electrode32A, so that an electric field in a direction from the signal electrode32A to the ground electrodes53A is generated and the electric field is applied to the PLZT waveguide31A. As a result, a refractive index of the PLZT waveguide31A is changed in accordance with the electric field applied to the PLZT waveguide31A, so that it is possible to modulate light that propagates through the PLZT waveguide31A. Further, the thin-film PLZT substrate55A that forms the PLZT waveguide31A is a PLZT single crystal, and therefore, a crystal direction is also the width direction (Z direction). In other words, the crystal direction of the PLZT waveguide31A is the same as the direction of the electric field, so that it is possible to improve efficiency in electric field application and reduce driving voltage. Further, it is possible to largely improve the modulation efficiency.

The optical modulator5A includes the Si optical integrated circuit wafer500and a thin-film PLZT substrate wafer550A. The thin-film PLZT substrate wafer550A is a wafer for forming the thin-film PLZT substrate55A that is a PLZT single crystal and that forms the RF modulation unit12A.

FIG.14is a schematic cross-sectional view of an exemplary cross section of the optical modulator5A taken along a line F-F inFIG.12. The F-F cross-section illustrated inFIG.14is a cross-section of the second PLZT-Si waveguide bonding unit41A. The second PLZT-Si waveguide bonding unit41A includes the Si substrate51, the Box layer52that is laminated on the Si substrate51, the first Si waveguide21that is laminated on the Box layer52, and the first cladding layer54that covers the first Si waveguide21. Further, the second PLZT-Si waveguide bonding unit41A includes the thin-film PLZT substrate55A that includes the PLZT waveguide31A and that is laminated on the first cladding layer54, and the second cladding layer56A that is laminated on the thin-film PLZT substrate55A.

FIG.15is an exemplary schematic cross-sectional view of the thin-film PLZT substrate wafer550A. The thin-film PLZT substrate wafer550A illustrated inFIG.15includes a sapphire substrate551A and a PLZT layer552A. The sapphire substrate551A is a material with a certain lattice constant that is similar to that of PLZT.

An organometallic compound that is a reaction product of Pb, La, Zr, Ti and an organic compound is coated on the sapphire substrate551A by a spin coating method or the like without hydrolysis (coating process). The sapphire substrate551A that has been subjected to the coating process is thermally decomposed in an atmosphere containing oxygen, at temperature, such as 200° C. to 400° C., at which crystallization does not occur, and at a temperature rise rate of, for example, 1 to 100° C. per seconds, so that an amorphous thin film with a film thickness of, for example, 200 nm or less is formed (thermal decomposition process).

Subsequently, the temperature of the sapphire substrate551A subjected to the thermal decomposition process is increased to crystal growth temperature of, for example, 600° C. to 800° C. in an atmosphere containing dry oxygen, and the sapphire substrate551A is heated for, for example, 10 seconds to 12 hours. Then, a PLZT single crystal thin film is formed, by solid-phase epitaxy, on the sapphire substrate551A (crystallization process). After the crystal growth, the temperature of, for example, 100° C. to 600° C. is maintained, and cooling is performed at a speed of, for example, 0.01° C. to 100° C. per second. By repeating a series of processes from the coating process to the crystallization process as described above for multiple times, it is possible to obtain the PLZT layer552A with a desired film thickness of about 2 um, for example.

Meanwhile, a PLZT film as a first layer that is formed first on the sapphire (Al2O3) substrate551A has a film thickness of 1 to 40 nm such that crystal grains of a PLZT film having a composition in which 0<x<0.30 and 0<y<0.20 when Pb(1−x)La(x) (Zr(y)Ti(1−y)) (1−x/4)O3 are decomposed in an island-like manner. As a result, it is possible to easily perform epitaxial growth with a single-phase perovskite while avoiding a pyroclore layer. The second and subsequent layers on the first layer are formed by causing epitaxial growth to occur on a single-phase perovskite having a composition in which 0<x<0.20 and 0.20<y<1.0 when Pb(1−x)La(x) (Zr(y)Ti(1−y)) (1−x/4)O3. As a result, even if the PLZT film of the first layer is decomposed in an island-like manner, it is possible to fill gaps without forming spaces between the islands, so that it is possible to smooth the surfaces of the PLZT films of the second and subsequent layers and prevent scattering. Meanwhile, from the necessity of preventing scattering at interfaces, refractive indices of the PLZT film of the first layer and the PLZT films of the second and subsequent layers are set such that a refractive index difference is, for example, 0.01 or less by appropriately adjusting amounts x and y.

FIG.16AtoFIG.16Eare diagrams for explaining an exemplary process of manufacturing the RF modulation unit12A of the optical modulator5A.FIG.16Ais a diagram for explaining an exemplary process of manufacturing the RF modulation unit, andFIG.16Bis a diagram for explaining an exemplary process of manufacturing the RF modulation unit (adhesive layer). InFIG.16AandFIG.16B, the adhesive layer556A is laminated on the surface of the first cladding layer54of the Si optical integrated circuit wafer500, and the PLZT layer552A of the thin-film PLZT substrate wafer550A is bonded on the adhesive layer556A by wafer bonding.

FIG.16Cis a diagram for explaining an exemplary process of manufacturing the RF modulation unit12A (adjustment of a dimension between a bottom surface of the PLZT layer552A and an upper surface of the first Si waveguide21). Meanwhile, for convenience of explanation, it is assumed that the first Si waveguide21has a thickness of, for example, 220 nm, and the PLZT layer552A has a thickness of, for example, 1 μm. A thickness dimension between the upper surface of the first Si waveguide21and the bottom surface of the PLZT layer552A is, for example, 100 nm to 300 nm. Then, a thickness between the upper surface of the first Si waveguide21and the bottom surface of the PLZT layer552A is set to, for example, 100 nm to 300 nm, so that it is possible to achieve optical coupling from the first Si waveguide21to the PLZT waveguide31A.

FIG.16Dis a diagram for explaining an exemplary process of manufacturing the RF modulation unit12A (polishing process). After the thin-film PLZT substrate wafer550A is bonded on the first cladding layer54of the Si optical integrated circuit wafer500via the adhesive layer556A, the sapphire substrate551A is removed by using the polishing process while remaining the PLZT layer552A in the thin-film PLZT substrate wafer550A.

FIG.16Eis a diagram for explaining an exemplary process of manufacturing the RF modulation unit12A (formation of the thin-film PLZT substrate). The PLZT waveguides31A in protruding shapes and a PLZT slab558A are further formed on a surface of the PLZT layer552A by using photolithography, on the PLZT layer552A that forms the thin-film PLZT substrate55A and that is subjected to the polishing process. Then, the thin-film PLZT substrate55A is formed on the first cladding layer54of the Si optical integrated circuit wafer500. Meanwhile, the PLZT slab558A of the thin-film PLZT substrate55A is able to increase a bonding force between the first cladding layer54on the Si optical integrated circuit wafer500and the thin-film PLZT substrate55A.

Then, the second cladding layer56A is laminated on the thin-film PLZT substrate55A and the PLZT waveguides31A. The RF modulation unit12A forms the second cladding layer56A that is laminated on the thin-film PLZT substrate55A and forms the signal electrode32and the pair of ground electrodes53A that are laminated on the second cladding layer56A and that have the CPW structures. As a result, the RF modulation unit12A is completed.

FIG.17is a diagram for explaining a result of comparison between the LN waveguide121and the PLZT waveguide31A in relation to driving voltage with respect to the electrode length. InFIG.17, a vertical axis represents the driving voltage and a horizontal axis represents the electrode length of the signal electrode. The electrode length is a length of the signal electrode. The LN waveguide121is a waveguide of the RF modulation unit120illustrated inFIG.20. The PLZT waveguide31A is a waveguide of the RF modulation unit12A illustrated inFIG.12. To ensure the same refractive index, if the electrode length of the signal electrode32A of the PLZT waveguide31A is 12 mm, a driving voltage of “1” is needed, but is the electrode length of the signal electrode122of the LN waveguide121is 12 mm, a driving voltage of “3” is needed. Therefore, the PLZT waveguide31A has an electro-optic effect that is three times larger than that of the LN waveguide121.

The optical modulator5A according to the second embodiment includes the Si substrate51and the PLZT waveguide31A that is formed by the thin-film PLZT substrate55A laminated on the Si substrate51. Further, the optical modulator5A includes the second cladding layer56A that is laminated on the PLZT waveguide31A, and includes the signal electrode32A and the ground electrodes53A that are formed on the second cladding layer56A and that have the CWP structures. As a result, with use of the PLZT waveguide31A that has a large electro-optic effect as compared to an LN waveguide, it is possible to improve modulation efficiency and it is possible to reduce a device size and driving voltage. With use of the thin-film PLZT substrate55A, which is a PLZT single crystal having a large electro-optic effect as compared to LN, as a material of the optical modulator5A, the PLZT single crystal is able to achieve an electro-optic coefficient that is three times larger than that of an LN single crystal.

The optical modulator5A includes the thin-film PLZT substrate55A, the second cladding layer56A that is laminated on the thin-film PLZT substrate55A and that covers the PLZT waveguide31A, and the signal electrode32A and the ground electrode53A that are formed on the second cladding layer56A and that have the CWP structures. The signal electrode32A generates an electric field in the PLZT waveguide31A in the width direction (Z direction) of the Si substrate51. The crystal direction of the PLZT waveguide31A is the width direction (Z direction). In other words, the crystal direction of the PLZT waveguide31A is the same as the direction of the electric field, so that it is possible to improve efficiency in electric field application, reduce the driving voltage, and largely improves the modulation efficiency. Further, with use of PLZT, it is possible to improve the modulation efficiency (voltage×electrode length). As a result, it is possible to reduce voltage and a device size. Furthermore, it is possible to achieve high modulation efficiency as compared to LN even if the electrode length is reduced, so that it is possible to further reduce the size of the optical modulator5A in accordance with a reduced electrode length.

The optical modulator5A includes the first cladding layer54that covers the first Si waveguide21and the second cladding layer56A that covers the thin-film PLZT substrate55A. A thickness of the first cladding layer54between the upper surface of the first Si waveguide21and the bottom surface of the thin-film PLZT substrate55A is set to 100 nm to 300 nm to achieve optical coupling between the first Si waveguide21and the PLZT waveguide31A. As a result, it is possible to achieve optical coupling between the first Si waveguide21and the PLZT waveguide31A.

The first cladding layer54is made of a dielectric or resin with a low refractive index. As a result, it is possible to bond the first cladding layer54and the thin-film PLZT substrate55A.

The optical modulator5A includes a dielectric57A that is a SiO2layer and that is a dielectric formed on the back surface of the Si substrate51. As a result, it is possible to reduce an influence of warpage due to an influence of thermal history or the like in the process of manufacturing the optical modulator5A.

Meanwhile, the case has been described in which the RF modulation unit12A of the optical modulator5A of the second embodiment is configured such that the PLZT layer552A is bonded on the Si optical integrated circuit wafer500via the adhesive layer556A as illustrated inFIG.16D. However, in the process of manufacturing the optical modulator5A, if warpage of the Si optical integrated circuit wafer500increases due to an influence of thermal history or the like, it may become difficult to adsorb the Si optical integrated circuit wafer500on a wafer stage by an exposure device or the like that is used to form a pattern of the optical modulator5A. Therefore, to cope with the situation as described above, as illustrated inFIG.18, the dielectric57A, such as an SiO2film, is formed on the back surface of the Si substrate51of the Si optical integrated circuit wafer500to prevent warpage of the Si optical integrated circuit wafer500. As a result, it is possible to avoid a situation in which the Si optical integrated circuit wafer500is not adsorbed on the wafer stage by the exposure device or the like that is used to form a pattern of the optical modulator5A.

Further, the case has been described in which the RF modulation unit12A is configured such that the PLZT layer552A of the thin-film PLZT substrate wafer550A is bonded on the first cladding layer54of the Si optical integrated circuit wafer500via the adhesive layer556A. However, an appropriate change is applicable, and it may be possible to form a bonding portion between the first cladding layer54and the PLZT layer552A without using the adhesive layer556A.

The case has been described in which the RF modulation unit12A illustrated inFIG.13has a CPW electrode structure in which the signal electrode32A is arranged between the pair of ground electrodes53A on the first cladding layer54, but the structure is not limited to the electrode structure as illustrated inFIG.13, and an appropriate change is applicable.

FIG.19is a schematic cross-sectional view illustrating a modification of the cross section of the optical modulator5A taken along the line E-E inFIG.2. In the RF modulation unit12A illustrated inFIG.19, the second cladding layer56A is partly etched on both sides of the PLZT waveguides31A of the thin-film PLZT substrate55A are etched. Further, an opening portion563A and an opening portion564A for exposing parts of a surface of the PLZT slab558A of the thin-film PLZT substrate55A are formed on the second cladding layer56A. Then, ground electrodes53B are formed in the opening portion563A so as to protrude from the surface of the second cladding layer56A. Further, a signal electrode32B is formed in the opening portion564A so as to protrude from the surface of the second cladding layer56A. The signal electrode32B and the ground electrode53B are arranged on lateral sides of the PLZT waveguides31A. As a result, the signal electrode32B and the ground electrodes53B are arranged on both sides of the PLZT waveguides31A and an electric field is applied in the width direction (left to right (Z direction)), so that it is possible to further improve the modulation efficiency.

Meanwhile, for convenience of explanation, the case has been described in which, in the optical modulator5A of the second embodiment, the first Si waveguide21and the PLZT waveguide31A are directionally coupled. However, an appropriate change is applicable, and the first Si waveguide21and the PLZT waveguide31A may be butt-coupled.

InFIG.17, the relationship between the driving voltage and the electrode length with which the PLZT waveguide31A has the electro-optic effect that is three times larger than that of the LN waveguide121is illustrated. However, even the PZT waveguide31is able to achieve the same effect as the PLZT waveguide31A, and has the electro-optic effect that is three times larger than that of the LN waveguide121.

Further, the thin-film substrates (50and55A) as single crystal perovskite oxides that have large electro-optic effects as compared to lithium niobate are illustrated. However, an appropriate change is applicable, and it may be possible to adopt a thin-film substrate made of a polycrystal perovskite oxide that has a large electro-optic effect as compared to lithium niobate. Furthermore, an appropriate change is applicable to the thin-film substrate, and the thin-film substrate may be a thin film without a slab.

According to one embodiment of the optical device and the like disclosed in the present application, it is possible to improve modulation efficiency and it is possible to reduce a device size and driving voltage.

All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.