JP5104598B2 - Mach-Zehnder optical modulator - Google Patents

Description

  The present invention relates to a Mach-Zehnder optical modulator formed on a semiconductor substrate.

A Mach-Zehnder type optical modulator using an InP-based semiconductor can be reduced in size as compared with an optical modulator using LiNbO ₃ . Therefore, a Mach-Zehnder type optical modulator using an InP-based semiconductor is a promising technique for reducing the size of a wavelength variable repeater for a high-density wavelength division multiplexing (DWDM) system. Also, chirp control is possible by push-pull driving. Therefore, it is expected as a component of a small optical repeater monolithically integrated with a semiconductor laser.

  A Mach-Zehnder modulator formed on a semiconductor substrate includes a duplexer, two modulation waveguides, and a multiplexer (Patent Document 1, Non-Patent Document 1). A traveling wave electrode to which a modulation signal is applied along the waveguide is formed on each of the two modulation waveguides. Ground electrodes are disposed on both sides of the traveling wave electrode. One ground electrode is disposed between the two modulation waveguides. The ground electrode disposed between the modulation waveguides extends to the outer region across the modulation waveguide and is connected to a terminal for applying a ground potential.

JP 2005-300570 A K. Tsuzuki et al., "A 40-Gb / s InGaAlAs-InAlAs MQW n-i-n Mach-Zehnder Modulator With a Drive Voltage of 2.3 V", IEEE Photonics Technology Letters, Vol. 17, No. 1, January 2005

  In a conventional Mach-Zehnder type optical modulator, a ground electrode disposed between two modulation waveguides and a terminal for applying a ground potential are connected. The wiring for connecting the two intersects the modulation waveguide. The modulation waveguide usually has a mesa shape formed on a substrate. In order not to adversely affect the guided light at the intersection of the wiring and the modulation waveguide, a different manufacturing procedure must be adopted for the overlapping portion of the traveling wave electrode to which the modulation signal is applied and the modulation waveguide. I must. For example, it is necessary to adopt a manufacturing procedure such as an air bridge. This complicates the manufacturing process.

An optical modulator for solving the above problems is as follows.
First and second modulation waveguides spaced apart from each other on a semiconductor substrate;
A duplexer disposed on the semiconductor substrate and connected to input ends of the first modulation waveguide and the second modulation waveguide;
A multiplexer disposed on the semiconductor substrate and connected to output ends of the first modulation waveguide and the second modulation waveguide;
A first signal electrode disposed on the first modulation waveguide along the first modulation waveguide;
A second signal electrode disposed on the second modulation waveguide and along the second modulation waveguide;
Of the substrate surfaces on both sides of the first signal electrode, a first inner ground electrode disposed on the second signal electrode side;
A first outer ground electrode disposed on the opposite side of the substrate surface on both sides of the first signal electrode from the second signal electrode;
Of the substrate surfaces on both sides of the second signal electrode, a second inner ground electrode disposed on the first signal electrode side;
A second outer ground electrode arranged on the opposite side of the substrate surface on both sides of the second signal electrode from the first signal electrode;
The first inner ground electrode and the second inner ground electrode do not overlap with any of the first modulation waveguide and the second modulation waveguide in plan view ,
And a first mesa structure and a second mesa structure formed on the surface of the semiconductor substrate and having a planar shape along the first modulation waveguide and the second modulation waveguide, respectively.
The first modulation waveguide and the second modulation waveguide are disposed in the first mesa structure and the second mesa structure, respectively;
The first signal electrode and the second signal electrode are disposed on the first mesa structure and the second mesa structure, respectively.
Furthermore, a material that is disposed on the semiconductor substrate outside the first mesa structure and the second mesa structure and has a lower dielectric constant than a material that forms the first mesa structure and the second mesa structure Having an embedding member formed of
The first and second outer ground electrodes are formed on the embedded member.

  Since the first and second inner ground electrodes do not overlap any of the first modulation waveguide and the second modulation waveguide, a special process for producing an overlapping portion is not necessary.

  FIG. 1 shows a plan view of a Mach-Zehnder type optical modulator according to the first embodiment. A duplexer 25 and a multiplexer 30 are disposed on the substrate 20. For example, a multimode interference (MMI) coupler is used for the duplexer 25 and the multiplexer 30. Each of the duplexer 25 and the multiplexer 30 has two input ports and two output ports.

  The first input waveguide 21 and the second input waveguide 22 are connected to the two input ports of the duplexer 25, respectively. The first modulation waveguide 28 connects one output port of the duplexer 25 and one input port of the multiplexer 30. The second modulation waveguide 29 connects the other output port of the duplexer 25 and the other input port of the multiplexer 30. The first output waveguide 31 and the second output waveguide 32 are connected to the two output ports of the multiplexer 30, respectively.

  The first modulation waveguide 28 and the second modulation waveguide 29 are arranged in parallel to each other except in the vicinity of the end portion. In the region in the vicinity of the end portion, the distance is gradually narrowed toward the end portion, and then arranged parallel to each other and connected to the duplexer 25 or the multiplexer 30. These waveguides are high-mesa optical waveguides in which a core layer and upper and lower cladding layers thereof have a mesa structure.

  A closed region 45 is defined by the duplexer 25, the first modulation waveguide 28, the multiplexer 30, and the second modulation waveguide 29.

  The first embedded member 41 is disposed outside the closed region 45 so as to be adjacent to the first modulation waveguide 28. The first embedded member 41 is formed of a low dielectric constant material having a lower dielectric constant than any of the core layer and the upper and lower cladding layers constituting the first modulation waveguide 28 and the second modulation waveguide 29. The The thickness of the first embedded member 41 is adjusted so that the upper surface thereof is substantially the same height as the upper surface of the first modulation waveguide 28.

  The first signal electrode 35 is disposed so as to overlap the portion other than the vicinity of the end portion of the first modulation waveguide 28. Outside the closed region 45, the first input terminal 35A and the first output terminal 35B are arranged. The end of the first signal electrode 35 on the demultiplexer 25 side is connected to the first input terminal 35A, and the end on the multiplexer 30 side is connected to the first output terminal 35B. The first modulation signal electrode 35, the first input terminal 35A, and the first output terminal 35B are formed of the same conductive film.

  A first outer ground electrode 37 and an inner ground electrode 39 are disposed on both sides of the first signal electrode 35. The inner ground electrode 39 is disposed inside the closed region 45, and the first outer ground electrode 37 is disposed outside the closed region 45. The inner ground electrode 39 does not overlap with either the first modulation waveguide 28 or the second modulation waveguide 29 in plan view. Further, it is not connected via a wiring on the substrate to a conductive region provided outside the closed region 45 and to which a ground potential is applied.

  Also for the second modulation waveguide 29, the second signal electrode 36, the second input terminal 36A, the second output terminal 36B, the second outer ground electrode 38, and the second embedded member 42 are arranged. Has been. The inner ground electrode 39 is shared as a ground electrode corresponding to the first signal electrode 35 and the second signal electrode 36. The inner ground electrode corresponding to the first signal electrode 35 and the inner ground electrode corresponding to the second signal electrode 36 may be separated from each other.

  The second signal electrode 36, the second input terminal 36A, the second output terminal 36B, the second outer ground electrode 38, and the second embedded member 42 are respectively formed of the first modulation waveguide 28 and the second modulation member 28. The first signal electrode 35, the first input terminal 35A, the first output terminal 35B, the first outer ground electrode 37, the first signal electrode 35, the first input terminal 35B, the first outer ground electrode 37, It has a plane shape that is symmetric with respect to one embedding member 41.

  A first signal source 51 is connected to the first input terminal 35A, and a 50Ω termination resistor 53 is connected to the first output terminal 35B. A second signal source 52 is connected to the second input terminal 36A, and a 50Ω termination resistor 54 is connected to the second output terminal 36B. The modulation signal is applied to the first input terminal 35A and the second input terminal 36A that are negatively biased in terms of DC.

  2 is a cross-sectional view taken along one-dot chain line 2A-2A in FIG. A first modulation waveguide 28 and a second modulation waveguide 29 having a semi-insulating buried hetero (SI-BH) structure are formed on a substrate 20 made of n-type InP having a (001) plane as a main surface. Has been. Each of the first modulation waveguide 28 and the second modulation waveguide 29 has a laminated structure in which a lower cladding layer 61, a core layer 62, and an upper cladding layer 63 are formed in this order. A contact layer 64 is formed on the upper cladding layer 63. The lower cladding layer 61 is disposed on the entire surface of the substrate 20, and the upper surface of the lower cladding layer 61 in a region other than the first and second modulation waveguides 28 and 29 is the first and second modulation waveguides 28. , 29 lower than the upper surface. The width (mesa portion width) of each of the first modulation waveguide 28 and the second modulation waveguide 29 is, for example, 1.4 to 1.8 μm.

The lower cladding layer 61 is made of n-type InP, and its impurity concentration is 5 × 10 ¹⁷ cm ⁻³ , and the thickness of the portion where the modulation waveguides 28 and 29 are formed is 1.4 μm. The upper cladding layer 63 is made of p-type InP and has a thickness of 1.3 μm. The impurity concentration of the 0.5 μm thick portion on the core layer 62 side is 6.5 × 10 ¹⁷ cm ⁻³ , and the impurity concentration of the 0.8 μm thick portion on the contact layer 64 side is 1.6 × 10 ^18. cm- ³ . The contact layer 64 has a p-type InGaAsP layer having a thickness of 0.1 μm and an impurity concentration of 2 × 10 ¹⁸ cm ^{−3, and} a p-type layer having a thickness of 0.5 μm and an impurity concentration of 1.5 × 10 ¹⁹ cm ⁻³ thereon. And an InGaAs layer.

  The core layer 62 has a stacked structure in which an i-type InP layer having a thickness of 50 nm, a multiple quantum well layer, and an i-type InP layer having a thickness of 100 nm are formed in this order. For example, a multiple quantum well layer having an absorption edge of 1.4 μm has a stacked structure in which i-type InP barrier layers having a thickness of 5 nm and i-type InGaAsP well layers having a thickness of 10 nm are alternately stacked. For example, the number of barrier layers is 34, and the number of well layers is 33. As a result, a 1.55 μm band optical waveguide is formed.

  A semi-insulating high resistance film 65 made of semi-insulating InP is formed on the side surfaces of the first modulation waveguide 28 and the second modulation waveguide 29. A protective film 67 made of silicon oxide covers the surface of the semi-insulating high resistance film 65 and the surface of the lower cladding layer 61. Note that an opening is formed in the protective film 67 so that the lower cladding layer 61 between the first modulation waveguide 28 and the second modulation waveguide 29 is exposed.

  The first embedded member 41 is disposed on the substrate outside the first modulation waveguide 28, and the second embedded member 42 is disposed on the substrate outside the second modulation waveguide 29. Has been. The embedded member 43 is disposed on the side surface of the protective film 67 disposed on the side surface on the inner side (second modulation waveguide side) of the first modulation waveguide 28, and the inner side (second surface) of the second modulation waveguide 29. The embedding member 44 is disposed on the side surface of the protective film 67 disposed on the side surface on the first modulation waveguide side). For example, benzocyclobutene (BCB) is used for the first embedding member 41, the second embedding member 42, and the embedding members 43 and 44. In place of benzocyclobutene, other low dielectric constant materials such as polyimide organic compounds, epoxy organic compounds, acrylic organic compounds, and the like may be used.

  A first signal electrode 35 is formed on the contact layer 64 on the first modulation waveguide 28. A second signal electrode 36 is formed on the contact layer 64 on the second modulation waveguide 29. A first outer ground electrode 37 is formed on the first embedded member 41, and a second outer ground electrode 38 is formed on the second embedded member 42. An inner ground electrode 39 is formed on the lower cladding layer 61 between the first modulation waveguide 28 and the second modulation waveguide 29. The inner ground electrode 39 is disposed in an opening provided in the protective film 67 and is in contact with the lower cladding layer 61. A back electrode 70 is formed on the back surface of the substrate 20. A ground potential is applied to the back electrode 70.

  Next, with reference to FIGS. 3A to 3K, a method of manufacturing the optical modulator according to the first embodiment will be described.

  As shown in FIG. 3A, a lower clad layer 61, a core layer 62, an upper clad layer 63, and a contact layer 64 are sequentially formed on the substrate 20. These films are formed by epitaxially growing a compound semiconductor by molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD).

  A mask pattern 100 is formed on the contact layer 64. The mask pattern 100 includes the first input waveguide 21, the second input waveguide 22, the duplexer 25, the first modulation waveguide 28, the second modulation waveguide 29, and the multiplexer shown in FIG. 30, corresponding to the planar shapes of the first output waveguide 31 and the second output waveguide 32. For the mask pattern 100, silicon oxide, silicon nitride, resist material, or the like is used.

As shown in FIG. 3B, etching is performed from the contact layer 64 to the surface layer portion of the lower cladding layer 61 using the mask pattern 100 as an etching mask. For this etching, dry etching or wet etching can be applied. In dry etching, inductively coupled plasma of chlorine-based gas such as SiCl ₄ or Cl ₂ can be used. For wet etching, hydrochloric acid or the like can be used. By this etching, a mesa structure including the first modulation waveguide 28 and the second modulation waveguide 29 is formed.

  As shown in FIG. 3C, a buried layer 65a made of semi-insulating InP is selectively grown on the lower cladding layer 61 under the condition that it does not grow on the mask pattern 100. For example, MOCVD is applied to the growth of the buried layer 65a. After the formation of the buried layer 65a, the mask pattern 100 is removed.

  As shown in FIG. 3D, a mask pattern 101 is formed on the first modulation waveguide 28 and the second modulation waveguide 29. The width of the mask pattern 101 is slightly wider than the core layer 62. For the mask pattern 101, silicon oxide, silicon nitride, resist material, or the like is used.

  As shown in FIG. 3E, the upper portions of the buried layer 65a and the lower cladding layer 61 are etched using the mask pattern 101 as an etching mask. A part (semi-insulating high-resistance film) 65 of the buried layer 65a remains on the side surface of the mesa structure constituting the first modulation waveguide 28 and the second modulation waveguide 29. After etching the buried layer 65a, the mask pattern 101 is removed.

  An SI-BH structure is obtained in which an upper clad layer 64 and a lower clad layer 61 are respectively disposed above and below the core layer 62, and buried layers 65a are disposed laterally.

  As shown in FIG. 3F, the entire surface of the substrate is covered with a protective film 67. For the protective film 67, for example, silicon oxide, silicon nitride, or the like is used. For forming the protective film 67, electron beam evaporation or the like is used.

  As shown in FIG. 3G, a low dielectric constant material 46 is formed on the surface of the protective film 67 by applying a low dielectric constant material by spin coating or the like and performing curing. For example, benzocyclobutene (BCB) is used as the low dielectric constant material. The low dielectric constant layer 46 embeds low regions on both sides of the first modulation waveguide 28 and the second modulation waveguide 29 and is also deposited on these waveguides.

  As shown in FIG. 3H, the low dielectric constant layer 46 is etched back until the protective film 67 disposed on the first modulation waveguide 28 and the second modulation waveguide 29 is exposed.

  As shown in FIG. 3I, a mask pattern 102 is formed on the substrate surface. The mask pattern 102 corresponds to the planar shape of the first embedding member 41 and the second embedding member 42 shown in FIG. The mask pattern 102 extends from a region outside the first modulation waveguide 28 and the second modulation waveguide 29 to a slightly inner side than the inner side surface of these waveguides.

  The low dielectric constant layer 46 is etched using the mask pattern 102 as an etching mask. For the etching of the low dielectric constant layer 46, for example, reactive ion etching using oxygen as an etching gas is used. As a result, the first embedded member 41 remains outside the first modulation waveguide 28 and the second embedded member 42 remains outside the second modulation waveguide 29. An embedded member 43 and an embedded member 44 made of a low dielectric constant material remain on the inner side surfaces of the first modulation waveguide 28 and the second modulation waveguide 29, respectively. The protective film 67 is exposed inside the two embedded members 43 and 44. After the low dielectric constant layer 46 is etched, the mask pattern 102 is removed. When the mask pattern 102 is removed, the protective film 67 formed on the first modulation waveguide 28 and the second modulation waveguide 29 is exposed.

  As shown in FIG. 3J, the exposed protective film 67 is removed using, for example, hydrofluoric acid. The contact layer 64 on the first modulation waveguide 28 and the second modulation waveguide 29 is exposed, and the lower cladding layer 61 is interposed between the first modulation waveguide 28 and the second modulation waveguide 29. Exposed.

  As shown in FIG. 3K, a first signal electrode 35, a second signal electrode 36, a first outer ground electrode 37, a second outer ground electrode 38, and an inner ground electrode 39 are formed. These electrodes have a laminated structure in which an Au layer, a Zn layer, and an Au layer are sequentially formed from the substrate side. Vapor deposition and a lift-off method can be used for forming these electrodes. Note that Au plating may be performed on the Au layer formed by vapor deposition.

  Finally, as shown in FIG. 2, the back electrode 70 is formed on the back surface of the substrate 70. The back electrode 70 has a laminated structure in which an AuGe layer and an Au layer are formed in order from the substrate side.

  Next, the operation principle of the optical modulator according to the first embodiment will be described. An optical signal is input from one of the first input waveguide 21 and the second input waveguide 22 shown in FIG. The demultiplexer 25 demultiplexes the input optical signal into the first modulation waveguide 28 and the second modulation waveguide 29 for the time being.

  Modulation signals are input from the first signal source 51 and the second signal source 52 to the first signal electrode 35 and the second signal electrode 36, respectively. The input modulation signal propagates through the first signal electrode 35 and the second signal electrode 36. In order to perform impedance matching with the terminating resistance, the characteristic impedance of the first signal electrode 35 and the second signal electrode 36 is designed to be about 50Ω. The characteristic impedance depends on the geometric shapes of the first signal electrode 35, the second signal electrode 36, the first outer ground electrode 37, the second outer ground electrode 38, and the inner ground electrode 39.

  In FIG. 2, when a reverse bias voltage is applied between the lower cladding layer 61 and the upper cladding layer 63, the absorption edge of the multiple quantum well layer in the core layer 62 changes. When the absorption edge changes, the refractive index of the waveguide changes due to the Kramers-Kronig relationship. As a result, the phase of the optical signal propagating through the first modulation waveguide 28 and the second modulation waveguide 29 changes.

  When a modulation signal is not applied to either the first signal electrode 35 or the second signal electrode 36, the optical signal propagating through the first modulation waveguide 28 and the second modulation waveguide 29 is phase-shifted. The signal is not modulated and input to the multiplexer 30 in phase. When modulated signals having opposite phases are applied to the first signal electrode 35 and the second signal electrode 36, the optical signals propagating through the first modulation waveguide 28 and the second modulation waveguide 29 are mutually transmitted. Subject to different phase modulation. As a result, the signal is input to the multiplexer 30 with an opposite phase.

  When an optical signal having the same phase is input from the first modulation waveguide 28 and the second modulation waveguide 29 to the multiplexer 30, the optical signal is output to the second output waveguide 32. When an optical signal is input to the multiplexer 30 with an opposite phase, the optical signal is output to the first output waveguide 31. As described above, the optical signal can be extracted from one of the first output waveguide 31 and the second output waveguide 32 by switching between application and non-application of the modulation signal.

  In the first embodiment, the inner ground electrode 38 is disposed in the closed region 45 and does not overlap any of the first modulation waveguide 28 and the second modulation waveguide 29. Further, the inner ground electrode 39 is not connected via a wiring on the substrate to a conductive region to which a ground potential outside the closed region is applied in a plan view. For this reason, a special process such as formation of an air bridge straddling the first modulation waveguide 28 and the second modulation waveguide 29 is not necessary, and a decrease in yield can be suppressed.

  By disposing the inner ground electrode 39 at a position lower than the outer first outer ground electrode 37 and the second outer ground electrode 38, the characteristic impedance of the first signal electrode 35 and the second signal electrode 36 can be reduced. It can be close to 50Ω.

  In the first embodiment, the first embedded member 41 and the second embedded member 42 are disposed outside the first modulation waveguide 28 and the second modulation waveguide 29, respectively. The first input terminal 35 </ b> A and the first output terminal 35 </ b> B that are continuous with the first signal electrode 35 are disposed on the first embedded member 41. As shown in FIG. 2, the step between the upper surface of the mesa portion where the first modulation waveguide 28 is formed and the upper surface of the first embedded member 41 is different from the upper surface of the mesa portion and the lower cladding. It is lower than the step between the upper surface of the protective film 67 covering the layer 61. For this reason, compared with the case where the 1st embedding member 41 is not arrange | positioned, formation of the 1st signal electrode 35, 1st input terminal 35A, and 2nd output terminal 35B is easy. Similarly, it is easy to form the second signal electrode 36, the second input terminal 36A, and the second output terminal 35B.

  FIG. 4 shows a simulation result of the electric field in the space above the substrate. The arrow in FIG. 4 represents the direction of the electric field, and the length of the arrow represents the strength of the electric field. FIG. 4 shows an electric field when a positive voltage is applied to the second signal electrode 36. Although the ground voltage is not directly supplied to the inner ground electrode 39 from the ground terminal on the substrate surface, an electric field directed to the inner ground electrode 39 is generated, and the inner ground electrode 39 functions sufficiently as a ground electrode. I understand.

  FIG. 5 shows the optical response characteristics obtained by the electromagnetic field distribution simulator and the small optical signal calculation. The horizontal axis represents the length of the modulation waveguide in the unit “mm”, and the vertical axis represents the maximum modulation bandwidth in the unit “GHz”. Here, the “maximum modulation bandwidth” means a frequency when the intensity of the optical signal output to the output waveguide decreases by 3 dB.

  It can be seen that when the length of the modulation waveguide is 1 mm and 1.5 mm, maximum modulation bandwidths of 13 GHz and 9 GHz are obtained, respectively. Thus, sufficient high-speed modulation is possible without directly applying a ground voltage to the inner ground electrode 39.

  Next, the second to fifth embodiments will be described with reference to FIGS. Hereinafter, description will be made focusing on differences from the optical modulator according to the first embodiment, and description of common parts will be omitted.

  FIG. 6 is a sectional view of an optical modulator according to the second embodiment. In the first embodiment, as shown in FIG. 2, a semi-insulating high layer made of semi-insulating InP is formed on the side surfaces of the mesa portions constituting the first modulation waveguide 28 and the second modulation waveguide 29. Although the resistance film 65 is disposed, the semi-insulating high resistance film is not disposed in the second embodiment. The protective film 67 is in contact with the side wall of the mesa portion.

  In the method of manufacturing the optical modulator according to the second embodiment, after removing the mask pattern 100 shown in FIG. 3B, the protective film 67 shown in FIG. 3F is formed without forming the buried layer 65a shown in FIG. 3C. Is done.

  Since the low dielectric constant embedded members 41 to 44 are disposed on both sides of the mesa portion via the protective film 67, the capacity can be reduced. In addition, since the optical confinement effect in the lateral direction becomes strong, effective phase modulation can be performed. For this reason, the maximum modulation bandwidth is expected to be wider than that of the optical modulator of the first embodiment.

  FIG. 7 shows a cross-sectional view of an optical modulator according to the third embodiment. In the third embodiment, the protective film 67, the first embedded member 41, the second embedded member 42, and the embedded members 43 and 44 of the optical modulator of the first embodiment shown in FIG. Not formed. A first outer ground electrode 37 and a second outer ground electrode 38 are formed on the surface of the lower cladding layer 61.

  In the method of manufacturing the optical modulator according to the third embodiment, after removing the mask pattern 101 shown in FIG. 3E, the first signal electrode 35, the second signal electrode 36, and the first outer side shown in FIG. 3K. A ground electrode 37, a second outer ground electrode 38, and an inner ground electrode 39 are formed. The first input terminal 35A, the first output terminal 35B, the second input terminal 36A, and the second output terminal shown in FIG. 1 are formed on the semi-insulating high resistance film 65.

  Also in the third embodiment, it is not necessary to form a wiring that intersects the first modulation waveguide 28 and the second modulation waveguide 29.

  FIG. 8 is a sectional view of an optical modulator according to the fourth embodiment. In the fourth embodiment, the semi-insulating high resistance film 65 of the optical modulator of the first embodiment shown in FIG. 2 is not disposed. Further, in the region where the first outer ground electrode 37 is disposed, the protective film 67 and the first embedded member 41 are removed, and in the region where the second outer ground electrode 37 is disposed, the protective film 67. And the 2nd embedding member 42 is removed. Therefore, the first outer ground electrode 37 and the second outer ground electrode 38 are in contact with the surface of the lower cladding layer 61.

  In the method of manufacturing the optical modulator according to the fourth embodiment, after removing the mask pattern 100 shown in FIG. 3B, the buried layer 65a shown in FIG. 3C is not formed, and the protective film 67 shown in FIG. 3F is formed. Is formed. Further, an opening is formed in a region where the first outer ground electrode 37 and the second outer ground electrode 38 are arranged in the mask pattern 102 shown in FIG. 3I. As a result, the first embedded member 41 and the second embedded member 42 in the region where the first outer ground electrode 37 and the second outer ground electrode 38 are disposed are removed. In the etching process of the protective film 67 shown in FIG. 3J, the protective film 67 in the region where the first outer ground electrode 37 and the second outer ground electrode 38 are disposed is removed.

  Also in the fourth embodiment, it is not necessary to form a wiring that intersects the first modulation waveguide 28 and the second modulation waveguide 29.

  FIG. 9 is a plan view of an optical modulator according to the fifth embodiment. A wavelength tunable semiconductor laser element 80 is formed on the same substrate 20 on which the optical modulator is formed. Laser light emitted from the wavelength tunable semiconductor laser element 80 is input to the first input waveguide 21. Note that the second input waveguide 22 may be input. Further, instead of the wavelength tunable semiconductor laser element, a fixed wavelength distributed feedback (DFB) laser element may be used.

  In the first to fifth embodiments, n-type InP is used for the substrate 20. However, other compound semiconductors may be used, and p-type InP or semi-insulating InP may be used. In that case, it is desirable to form the lower cladding layer 61 only in the vicinity of the mesa structure. When p-type InP is used, the lower cladding layer 61 is p-type and the upper cladding layer 63 is n-type.

  When semi-insulating InP is used for the substrate 20, the back electrode 70 is not formed. In the configuration of the third and fourth embodiments shown in FIGS. 7 and 8, the first outer ground electrode 37 and the second outer ground electrode 38 are connected to the inner ground electrode 39 via the lower cladding layer 61. A ground potential is applied.

  Moreover, in the said 1st-5th Example, although the core layer 62 was made into the multiple quantum well structure, it is good also as a quantum wire structure, a quantum dot structure, or a bulk structure.

  In the first to fifth embodiments, an InP-based compound semiconductor layer is grown on an InP substrate to form an optical waveguide. However, an InGaAsP-based or AlGaInAs-based compound semiconductor layer is grown on the InP substrate. An optical waveguide may be formed.

  In addition to the drive electrodes, phase modulation electrodes may be provided in the optical modulators employed in the first to fifth embodiments.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

It is a top view of the optical modulator by a 1st Example. It is sectional drawing of the optical modulator by a 1st Example. It is sectional drawing in the manufacture middle stage of the optical modulator by a 1st Example. It is sectional drawing in the manufacture middle stage of the optical modulator by a 1st Example. It is sectional drawing in the manufacture middle stage of the optical modulator by a 1st Example. It is sectional drawing in the manufacture middle stage of the optical modulator by a 1st Example. It is a diagram which shows the simulation result of the electric field of the space on the board | substrate of the optical modulator by a 1st Example. It is a graph which shows the relationship between the maximum modulation bandwidth of the optical modulator by a 1st Example, and the length of a modulation waveguide. It is sectional drawing of the optical modulator by a 2nd Example. It is sectional drawing of the optical modulator by a 3rd Example. It is sectional drawing of the optical modulator by a 4th Example. It is a top view of the optical modulator by a 5th Example.

Explanation of symbols

20 substrate 21 first input waveguide 22 second input waveguide 25 duplexer 28 first modulation waveguide 29 second modulation waveguide 30 multiplexer 35 first signal electrode 36 second signal electrode 37 first outer ground electrode 38 second outer ground electrode 39 inner ground electrode 41 first embedded member 42 second embedded member 43, 44 embedded member 45 closed region 46 low dielectric constant layer 51 first Signal source 52 second signal source 53 first termination resistor 54 second termination resistor 61 lower cladding layer 62 core layer 63 upper cladding layer 64 contact layer 65 semi-insulating high resistance film 65a buried layer 67 protective film 70 Back ground electrode 80 Wavelength variable semiconductor laser element 100, 101, 102 Mask pattern

Claims (5)

First and second modulation waveguides spaced apart from each other on a semiconductor substrate;
A duplexer disposed on the semiconductor substrate and connected to input ends of the first modulation waveguide and the second modulation waveguide;
A multiplexer disposed on the semiconductor substrate and connected to output ends of the first modulation waveguide and the second modulation waveguide;
A first signal electrode disposed on the first modulation waveguide along the first modulation waveguide;
A second signal electrode disposed on the second modulation waveguide and along the second modulation waveguide;
Of the substrate surfaces on both sides of the first signal electrode, a first inner ground electrode disposed on the second signal electrode side;
A first outer ground electrode disposed on the opposite side of the substrate surface on both sides of the first signal electrode from the second signal electrode;
Of the substrate surfaces on both sides of the second signal electrode, a second inner ground electrode disposed on the first signal electrode side;
A second outer ground electrode arranged on the opposite side of the substrate surface on both sides of the second signal electrode from the first signal electrode;
The first inner ground electrode and the second inner ground electrode do not overlap with any of the first modulation waveguide and the second modulation waveguide in plan view ,
And a first mesa structure and a second mesa structure formed on the surface of the semiconductor substrate and having a planar shape along the first modulation waveguide and the second modulation waveguide, respectively.
The first modulation waveguide and the second modulation waveguide are disposed in the first mesa structure and the second mesa structure, respectively;
The first signal electrode and the second signal electrode are disposed on the first mesa structure and the second mesa structure, respectively.
Furthermore, a material that is disposed on the semiconductor substrate outside the first mesa structure and the second mesa structure and has a lower dielectric constant than a material that forms the first mesa structure and the second mesa structure Having an embedding member formed of
An optical modulator in which the first and second outer ground electrodes are formed on the embedded member .   The optical modulator according to claim 1, wherein the first inner ground electrode and the second inner ground electrode share one conductive pattern. It said semiconductor substrate comprises a conductive layer made of a conductive semiconductor material formed on the surface portion, the first and second inner ground electrode, according to claim 1 or 2 in contact with the conductive layer Light modulator. The semiconductor substrate is formed of a conductive semiconductor material;
Further, the optical modulator according to any one of claims 1 to 3 having a back ground electrode formed on the back surface of the semiconductor substrate. The first inner ground electrode and the second inner ground electrode are surrounded by the first modulation waveguide, the second modulation waveguide, the duplexer, and the multiplexer in a plan view. It closed to the conductive regions ground potential is applied outside the region, the optical modulator according to any one of claims 1 to 4 are not connected through the wiring on the board.

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