JP6118834B2 - Mode conversion element and optical functional element - Google Patents

Description

  The present invention provides, for example, a mode conversion element that converts a TM (Transverse Magnetic) polarization in a basic mode into a TE (Transverse Electric) polarization in a primary mode, and an optical functional element including the mode conversion element and an optoelectronic device About.

  In recent years, in the development of optical functional elements that are advantageous for miniaturization and mass productivity, Si fine wire waveguides that use Si (silicon) as a material for fine wire waveguides have attracted attention.

  In the Si thin wire waveguide, an optical waveguide core that substantially becomes a light transmission path is formed using Si as a material. Then, the periphery of the optical waveguide core is covered with a clad made of, for example, silica having a refractive index lower than that of Si. With such a configuration, the refractive index difference between the optical waveguide core and the clad becomes extremely large, so that light can be strongly confined in the optical waveguide core. As a result, a small curved waveguide having a bending radius reduced to, for example, about 1 μm can be realized. For this reason, it is possible to create an optical circuit having the same size as an electronic circuit, which is advantageous for downsizing the entire optical functional element.

  In addition, in the Si wire waveguide, it is possible to divert the manufacturing process of a semiconductor device such as a CMOS (Complementary Metal Oxide Semiconductor). Therefore, realization of photoelectric fusion (silicon photonics) in which electronic functional circuits and optical functional circuits are collectively formed on a chip is expected.

  Incidentally, a photodiode having a PIN structure (hereinafter also referred to as PIN-PD) is known as an optoelectronic device integrated in a Si wire waveguide (see, for example, Non-Patent Document 1). The PIN-PD is integrated in the Si fine wire waveguide as follows, for example. First, impurities are added to the Si core to impart P-type or N-type conductivity. Next, for example, Ge (germanium) is stacked on the Si core. Thereafter, by adding impurities to the surface of Ge, conductivity opposite to that of the Si core is imparted. That is, Ge is N-type when Si is P-type, and Ge is P-type when Si is N-type. As a result, a PIN-PD in which an intrinsic semiconductor region called an I-type region exists between a P-type region and an N-type region is obtained.

  Generally, a vertical PIN-PD having a structure in which a P-type region, an I-type region, and an N-type region are stacked is formed by epitaxially growing a light absorption layer on a substrate. At this time, it is known that polarization dependency occurs in the light receiving efficiency of the PD due to the strain remaining in the light absorption layer. A light absorbing layer having no strain or compressive strain has a high light receiving efficiency in the TE mode, and a light absorbing layer having a tensile strain has a high light receiving efficiency in the TM mode (see, for example, Patent Document 1).

  For this reason, in the light receiver disclosed in Patent Document 1, a strain-free or compressive-strained light absorption layer is disposed in the previous stage with respect to a PD in which InGaAs is epitaxially grown as a light absorption layer on an InP substrate, and tensile strained light is obtained. The absorption layer is arranged in the subsequent stage. The TE mode light is absorbed and the TM mode light is transmitted through the unstrained or compressive strain light absorbing layer, and the TM mode light is absorbed at the tensile strain light absorbing layer, so that the reception sensitivity is made polarization independent. .

JP-A-5-267709

OPTICS EXPRESS, Vol. 15, no. 21, 2007, pp. 13965-13971

  However, although the technique disclosed in Patent Document 1 can be applied to a PD using a ternary or quaternary compound semiconductor, the PD has a light absorption layer obtained by epitaxially growing Ge on a Si substrate. Is not applicable. The reason is as follows.

By controlling the composition of In and Ga (In _x Ga _1-x As) of the InGaAs light absorption layer epitaxially grown on the InP substrate, the lattice constant is changed from 5.653５ of GaAs (x = 0) to InAs (x = 1). ) To 6.06mm. When In _0.53 Ga _0.47 As with x = 0.53, phase matching is achieved with InP with a lattice constant of 5.8687Å. Therefore, when x is smaller than 0.53, the lattice constant is smaller than InP, and tensile strain can be applied to InGaAs. Further, when x is larger than 0.53, the lattice constant becomes larger than that of InP, and compressive strain can be applied to InGaAs.

  On the other hand, when Ge is epitaxially grown on the Si substrate, the lattice constant of the Si substrate is 5.44Å whereas the lattice constant of Ge is 5.65Å, so that only compressive strain is included in Ge. I can't.

  As described above, a technique disclosed in Patent Document 1 in which a non-strained or compression-strained light absorption layer is disposed in the previous stage and a tensile-strained light absorption layer is disposed in the subsequent stage, thereby making the reception sensitivity polarization independent. Is not applicable to PDs in which Ge is epitaxially grown on a Si substrate. According to the measurement results of the reception sensitivity performed by the inventors, the reception sensitivity is 0.74 A / W for the TE mode light, and the reception sensitivity is 0.56 A / W for the TM mode light. There is a 2 dB polarization dependency.

  As a result of earnest examination, the inventor according to this application arranges a mode conversion element including a lower slab portion on the PD input side and an upper taper portion formed on the lower slab portion, The input fundamental mode TE polarized wave is output without conversion, and the input fundamental mode TM polarized wave is converted to the primary mode TE polarized wave and output, so that the reception sensitivity depends on the polarization. The inventors have conceived that the polarization dependence of the light receiving efficiency can be reduced even in the case where there is a characteristic.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a mode conversion element suitable for an optoelectronic device having a polarization dependency in reception sensitivity, and An optical functional element including a mode conversion element and an optoelectronic device is provided.

To achieve the above object, the mode converter of the invention includes a thin-ray waveguide section, and the step slab part, and the slab portion, the optical waveguide configured by sequentially connected as an integral channel waveguide Is provided . The step slab portion includes a lower slab and an upper taper portion whose width increases from the thin waveguide portion side toward the slab portion side on the lower slab. The fundamental mode TE polarized wave input from the thin waveguide section to the step slab section is output to the slab section without being converted in polarization and mode. The fundamental mode TM polarized wave input from the thin wire waveguide section to the step slab section is converted into a primary mode TE polarized wave and output to the slab section.

  An optical functional element according to the present invention includes the above-described mode conversion element and an optoelectronic device to which light output from the wide side of the upper taper portion of the mode conversion element is input. As this optoelectronic device, for example, a photodiode or an EA modulator is used.

  According to the mode conversion element of the present invention, the input fundamental mode TE polarized wave is output as it is without being converted, and the fundamental mode TM polarized wave is converted into the primary mode TE polarized wave and output. For this reason, even when an optoelectronic device having a large polarization dependency is used in the subsequent stage, the influence of the polarization dependency can be reduced.

  In addition, when a photodiode having a polarization dependency in the reception sensitivity is used as the mode conversion element and the optoelectronic device, the polarization dependency of the light receiving efficiency can be reduced.

It is a schematic diagram of a 1st optical function element. (A) is a typical perspective view of a 1st optical function element. Further, (B) is a schematic plan view of the first optical functional element as viewed from above. It is a figure which shows the cut end surface of a 1st optical function element. It is a schematic diagram for demonstrating operation | movement of a 1st optical function element. It is process drawing (1) for demonstrating the manufacturing method of a 1st optical function element. It is process drawing (2) for demonstrating the manufacturing method of a 1st optical function element. It is process drawing (3) for demonstrating the manufacturing method of a 1st optical function element. It is a schematic diagram for demonstrating the structure and operation | movement of a 2nd optical function element. It is process drawing for demonstrating the manufacturing method of a 2nd optical function element. It is a schematic diagram for demonstrating the structure and operation | movement of a 3rd optical function element.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

(First embodiment)
The optical functional element of the first embodiment (hereinafter also referred to as the first optical functional element) will be described with reference to FIGS. 1 and 2. The first optical functional element includes a mode conversion element and a photodiode as an optoelectronic device.

  FIG. 1A is a schematic perspective view of the first optical functional element. FIG. 1B is a schematic plan view of the first optical functional element as viewed from above. The first optical functional element includes a waveguide core and a cladding around the waveguide core. In FIGS. 1A and 1B, some other components such as the cladding are illustrated. Omitted.

  2A and 2B are views showing a cut end surface of the first optical functional element. 2A shows a cut end surface taken along line II in FIG. 1B, and FIG. 2B shows a cut taken along line II-II in FIG. 1B. The end face is shown.

The first optical functional element 10 is formed on the support substrate 12, the lower cladding 14 formed on the support substrate 12, the optical waveguide 16 formed on the lower cladding 14, and the lower cladding 14 and the optical waveguide 16. And an upper clad 18. The support substrate 12 and the optical waveguide 16 are made of silicon (Si), and the lower clad 14 and the upper clad 18 are made of silicon oxide (SiO ₂ ).

  First, the structure of the mode conversion element 20 will be described. The optical waveguide 16 of the mode conversion element 20 is configured by sequentially connecting a thin-line waveguide portion 22, a step slab portion 24, and a slab portion 28.

  The thickness of the optical waveguide 16 of the mode conversion element 20 is, for example, 300 nm. The width of the thin-line waveguide portion 22 is set to 300 nm so as to be a single mode waveguide for S-band signal light having a wavelength of 1460 nm to 1530 nm. The slab part 28 is a part connected to the photodiode, and the width of the slab part 28 is about 2 μm. Here, the direction perpendicular to the upper surface of the support substrate is the thickness direction, and the direction perpendicular to both the light propagation direction and the thickness direction is the width direction.

  The step slab portion 24 includes a lower slab portion 25 and an upper taper portion 27 formed on the lower slab portion 25. The lower slab portion 25 is formed in a flat plate shape having a rectangular planar shape, and the thickness of the lower slab portion 25 is 150 nm, which is half that of the thin wire waveguide portion 22.

  The upper taper portion 27 is formed in a taper shape whose width increases from one side of the lower slab portion 25 toward the opposite side. Here, the width of the upper tapered portion 27 increases from the thin-line waveguide portion 22 side toward the slab portion 28 side. That is, the upper taper portion 27 is formed in a reverse taper shape whose width increases along the waveguide direction from the thin wire waveguide portion 22 side toward the slab portion 28 side. The thickness of the upper taper portion 27 is 150 nm, which is half of the thin-line waveguide portion 22, similarly to the lower slab portion 25. The thicknesses of the upper taper portion 27 and the lower slab portion 25 are not necessarily equal, but it is advantageous because the mode conversion efficiency is increased.

  In the configuration example shown in FIG. 1, the width of the end portion of the upper taper portion 27 on the thin wire waveguide portion 22 side is equal to the width of the thin wire waveguide portion 22 and is the minimum. Further, the width of the end portion of the upper taper portion 27 on the slab portion 28 side is equal to the width of the slab portion 28 and is the maximum.

  In consideration of a decrease in mode conversion efficiency and loss of an optical signal, the upper taper portion 27 of the step slab portion 24 does not have an equal width region, and the thin waveguide portion 22 and the slab portion 28 are tapered. It is preferable to have a configuration without an area. However, although there may be some degradation in performance, the region where the lower slab portion 25 is provided may be made larger or smaller than the region corresponding to the portion where the width of the upper tapered portion 27 changes.

  Further, the width of the lower slab part 25 may be at least equal to the maximum width of the upper taper part 27. However, in consideration of manufacturing errors, the width of the lower slab portion 25 is preferably equal to or greater than the maximum width of the upper tapered portion 27. Since the length is larger than the width, manufacturing errors can be ignored in the length direction.

  The step slab portion 24 mainly functions as a mode conversion element. Therefore, depending on other elements connected to the narrow side of the upper taper portion 27, the structure of the optoelectronic device connected to the wide side of the upper taper portion 27, and the like, the thin wire waveguide portion 22 and the slab portion 28 are used. It may be configured not to include

  Next, the photodiode 30 will be described. The photodiode 30 is disposed on the slab portion 28 (the width of the upper taper portion 27 is large) of the mode conversion element 20. Light output from the wide side of the upper taper portion 27 of the mode conversion element 20 is input to the photodiode 30.

  In the optical waveguide 16 of the photodiode 30, a low concentration p-Si region 32 to which a low concentration impurity is added is formed. A Ge layer 42 is formed on the low concentration p-Si region 32. A high concentration p-Si contact region 34 to which a high concentration impurity is added is formed in a part of the region where the Ge layer 42 of the low concentration p-Si region 32 is not formed. Further, a high concentration n-Ge contact region 44 is formed in the region above the Ge layer 42. A contact electrode 50 is formed on each of the high-concentration p-Si contact region 34 and the high-concentration n-Ge contact region 44, and a pad electrode 52 is formed on the contact electrode 50. The optical waveguide 16 and the Ge layer 42 are covered with the upper clad 18, but the pad electrode 52 on the contact electrode 50 is exposed.

  The operation of the first optical functional element will be described with reference to FIG. FIG. 3 is a schematic diagram for explaining the operation of the first optical functional element. FIG. 3A shows a case where a TE mode polarized wave (hereinafter also referred to as TE0) is input, and FIG. 3B shows a fundamental mode TM polarized wave (hereinafter referred to as TM0). It may be referred to as “)”. 3A and 3B, like FIG. 1B, only the optical waveguide is shown, and other components such as the cladding are not shown.

  The TE0 component and the TM0 component of the signal light input to the mode conversion element 20 of the optical functional element 10 propagate through the thin wire waveguide 22, the step slab part 24, and the slab part 28, and are output from the mode conversion element 20.

  At this time, the TE0 component of the light input in the waveguide direction is output from the mode conversion element 20 as the TE0 component without being converted in polarization and mode in the step slab portion 24. The TE0 component output from the mode conversion element 20 is input to the photodiode 30.

  On the other hand, the TM0 component of the light input in the waveguide direction is converted into a first-order mode TE polarized wave (hereinafter also referred to as TE1) in the step slab portion 24, and the mode conversion element is used as the TE1 component. 20 is output. The TE1 component output from the mode conversion element 20 is input to the photodiode 30.

  As described above, in the first optical functional element 10, the mode conversion element 20 converts the TM polarization of the fundamental mode of the signal light into the TE polarization of the primary mode. For this reason, even if there is a significant polarization dependency in the reception sensitivity of the photodiode 30, it is possible to reduce the polarization dependency of the light receiving efficiency as the entire optical functional element.

  According to the study by the inventors, by appropriately designing the shapes of the step slab part 24 and the slab part 28, the conversion loss from the TM0 component to the TE1 component is reduced with respect to the signal light of the S band, particularly the wavelength of 1520 nm. It has been found to be about 0.2 dB. As a result, the polarization dependency of the reception sensitivity as the optical functional element can be reduced from about 1.2 dB to about 0.2 dB.

  As described above, when the mode conversion element 20 is used, even if a photodiode having a polarization dependency in the reception sensitivity is used as the optoelectronic device, the polarization dependency of the light receiving efficiency is reduced as a whole of the optical functional element. can do.

  With reference to FIGS. 4-6, the manufacturing method of a 1st optical function element is demonstrated. 4-6 is process drawing for demonstrating the manufacturing method of a 1st optical function element.

First, an SOI substrate in which a SiO ₂ layer and a Si layer are stacked on a Si substrate is prepared. A Si substrate, a SiO ₂ layer, and a Si layer are used as a support substrate, a lower cladding layer, and an optical waveguide layer, respectively.

  Next, the Si layer is patterned by photolithography and dry etching to form a half-etch pattern 60 (see FIGS. 4A, 4B, and 4C). FIG. 4A is a perspective view of the half-etch pattern 60. FIG. 4B shows a cut end surface of the SOI substrate on which the half-etched pattern 60 is formed, taken along the line I-I in FIG. FIG. 4C shows a cut end surface of the SOI substrate on which the half-etched pattern 60 is formed, taken along the line II-II in FIG.

  Next, the half-etch pattern 60 is patterned by photolithography and dry etching to form the optical waveguide 16 (see FIGS. 5A, 5B, and 5C). FIG. 5A is a perspective view of the optical waveguide 16. FIG. 5B shows a cut end surface of the SOI substrate on which the optical waveguide 16 is formed, taken along the line II in FIG. FIG. 5C shows a cut end surface of the SOI substrate on which the optical waveguide 16 is formed, taken along the line II-II in FIG.

  Next, a process for forming a photodiode will be described with reference to FIGS. 6 (A) to 6 (D) show end faces corresponding to the cut end faces taken along the line II-II in FIGS. 4 (A) and 5 (A).

First, a resist pattern 80 is formed on the lower clad 14 and the optical waveguide 16 by photolithography. The resist pattern 80 is formed with an opening 81 that substantially exposes the optical waveguide 16 in the photodiode formation region. The area of the opening 81 is about several μm × several tens of μm. Thereafter, for example, boron (B) as a p-type impurity is added to the optical waveguide 16 from the opening 81 to form a low concentration p-Si region 32. The impurity concentration of the low-concentration p-Si region is, for example, about 1 × 10 ¹⁹ cm ⁻³ (FIG. ^6A ).

Next, after removing the resist pattern 80, a resist pattern 82 is formed on the lower clad 14 and the optical waveguide 16 by photolithography. The resist pattern 82 is provided with two rectangular openings 83 that expose a part of the low-concentration p-Si region 32 of the optical waveguide 16 in parallel to the light propagation direction. Thereafter, for example, boron (B) is added as a p-type impurity from the opening 83 to the low-concentration p-Si region 32 of the optical waveguide to form the high-concentration p-Si contact region 34. The impurity concentration of the high-concentration p-Si contact region 34 is, for example, about 1 × 10 ²⁰ cm ⁻³ (FIG. 6B).

  Next, after removing the resist pattern 82, a Ge layer 42 having a thickness of about 1 μm is selectively grown on the low concentration p-Si region 32 (FIG. 6C).

Next, a resist pattern 84 is formed on the lower cladding 14, the optical waveguide 16, and the Ge layer 42 by photolithography. In the resist pattern 84, an opening 85 exposing a part of the Ge layer 42 is formed. Thereafter, for example, phosphorus (P) is added as an n-type impurity from the opening 85 to the Ge layer 42 to form a high concentration n-Ge contact region 44. The impurity concentration of the high concentration n-Ge contact region 44 is, for example, about 1 × 10 ²⁰ cm ⁻³ (FIG. 6D).

Next, after removing the resist pattern 84, an SiO ₂ film that covers the optical waveguide 16 and the Ge layer 18 is formed on the lower clad 14. This SiO ₂ film becomes the upper clad 18. Thereafter, contact holes are formed in the upper clad 18 by photolithography and dry etching. Next, the contact electrode 50 is formed by vacuum depositing, for example, aluminum (Al) as an electrode material and filling the contact hole. Finally, for example, aluminum (Al) is vacuum-deposited as an electrode material on the upper clad 18, and then an Al film is patterned by photolithography and dry etching to form a pad electrode 52 on the contact electrode 50. FIG. And the 1st optical function element demonstrated with reference to FIG. 2 is obtained.

(Second Embodiment)
With reference to FIG. 7 (A) and (B), the optical function element (henceforth a 2nd optical function element) of 2nd Embodiment is demonstrated. In addition, the description which overlaps with a 1st optical function element may be abbreviate | omitted.

  The second optical functional element 70 includes a mode conversion element 20 that is sequentially connected, an electro-absorption (EA) modulator 72 as an optoelectronic device, and an output-side waveguide 74.

  FIGS. 7A and 7B are schematic views for explaining the configuration and operation of the second optical functional element. FIG. 7A shows a case where the TE0 component is input, and FIG. 7B shows a case where the TM0 component is input. 7A and 7B, like FIG. 1, FIG. 3A and FIG. 3B, only the optical waveguide is shown, and other components such as the cladding are not shown.

  Since the mode conversion element of the second optical functional element is configured in the same manner as the mode conversion element of the first optical functional element, redundant description is omitted.

  The EA modulator 72 is disposed on the slab portion 28 side of the mode conversion element 20. Light output from the wide side of the upper tapered portion 27 of the mode conversion element 20 is input to the EA modulator 72. As the EA modulator 72, any suitable conventionally known one can be used.

  The output side waveguide 74 is provided on the output side of the EA modulator 72. The output-side waveguide 74 includes a tapered portion 75 and a uniform width waveguide 77. A tapered portion 75 is disposed on the EA modulator 72 side.

  The taper portion 75 decreases in width from the EA modulator 72 side toward the equal width waveguide 77 side. The equal width waveguide 77 is configured such that the TE1 component propagates.

  The TE0 component and TM0 component of the signal light input to the mode conversion element 20 of the optical functional element 70 propagate through the thin wire waveguide portion 22, the step slab portion 24, and the slab portion 28, and are output from the mode conversion device 20. .

  At this time, the TE0 component is output from the mode conversion element 20 as the TE0 component without being converted in polarization and mode in the step slab portion 24. The TE0 component output from the mode conversion element 20 is input to the EA modulator 72. The TE0 component that has undergone predetermined modulation by the EA modulator 72 is output from the second optical functional element 70 via the taper portion 75 and the equal-width waveguide 76.

  On the other hand, the TM0 component is converted into the TE1 component in the step slab portion 24, and is output from the mode conversion element 20 as the TE1 component. The TE1 component output from the mode conversion element 20 is input to the EA modulator 72. The TE1 component that has undergone predetermined modulation by the EA modulator 72 is output from the second optical functional element 70 via the tapered portion 75 and the equal-width waveguide 77.

  As described above, in the second optical functional element 70, the mode conversion element 20 converts the TM polarization of the fundamental mode of the signal light into the TE polarization of the primary mode and then inputs it to the EA modulator 72. For this reason, even if there is a significant polarization dependency in the reception sensitivity of the EA modulator 72, the polarization dependency of the light receiving efficiency can be reduced as the entire optical functional element.

  With reference to FIG. 8, the manufacturing method of a 2nd optical function element is demonstrated. FIG. 8 is a process diagram for explaining a method of manufacturing the second optical functional element.

First, an SOI substrate in which a SiO ₂ layer and a Si layer are stacked on a Si substrate is prepared.

  Next, the Si layer is patterned by photolithography and dry etching to form a half-etch pattern 62 (see FIG. 8A). FIG. 8A is a perspective view of the half-etch pattern 62.

  Next, the half-etch pattern 62 is patterned by photolithography and dry etching to form the optical waveguide 17 (see FIGS. 8B and 8C). FIG. 8B is a perspective view of the optical waveguide. FIG. 8C shows a cut end surface of the SOI substrate on which the optical waveguide is formed, taken along the line II in FIG. 8B. In this step, the optical waveguide 17 including the mode conversion element 20 and the output side waveguide 74 is formed. Thereafter, an electrode for supplying current to the EA modulator 72 is formed in the terrace region 78 between the mode conversion element 20 and the output-side waveguide 74 as necessary.

  Next, the EA modulator 72 prepared in advance in the terrace region 78 is mounted and die-bonded to obtain the second optical functional element 70 described with reference to FIG.

(Third embodiment)
With reference to FIG. 9 (A) and (B), the optical functional element (henceforth a 3rd optical functional element) of 3rd Embodiment is demonstrated. In addition, the description which overlaps with a 2nd optical function element may be abbreviate | omitted.

  The third optical functional element 90 is different from the second optical functional element in that the mode converting element 21 is provided instead of the output-side waveguide, and the other configuration is the same as that of the second optical functional element.

  The mode conversion element 21 provided on the output side of the EA modulator 72 has the same structure as the mode conversion element 20 provided on the input side of the EA modulator 72, and is arranged so as to be symmetric with respect to the EA modulator 72. Has been. That is, the mode conversion element 21 on the output side is provided with a slab part 98 on the output side of the EA modulator 72, a thin-line waveguide part 92 is provided on the output side of the third optical functional element 90, and the slab part 98 A step slab portion 94 is provided between the thin wire waveguide portions 92.

  The TE0 component and the TM0 component of the signal light input to the mode conversion element 20 on the input side of the third optical functional element 90 propagate through the thin wire waveguide 22, the step slab portion 24, and the slab portion 28, and enter the mode on the input side. Output from the conversion element 20.

  At this time, the TE0 component is output from the input-side mode conversion element 20 as the TE0 component without being converted in polarization and mode in the step slab portion 24 of the input-side mode conversion element 20. The TE0 component output from the mode conversion element 20 on the input side is input to the EA modulator 72. The TE0 component that has undergone predetermined modulation by the EA modulator 72 is output from the third optical functional element 90 via the mode conversion element 21 on the output side.

  On the other hand, the TM0 component is converted into a TE1 component in the step slab portion 24 of the input-side mode conversion element 20, and is output from the input-side mode conversion element 20 as a TE1 component. The TE1 component output from the mode conversion element 20 on the input side is input to the EA modulator 72. The TE1 component that has undergone predetermined modulation by the EA modulator 72 is converted to a TM0 component in the step slab portion 94 of the output-side mode conversion element 21, and is output from the third optical functional element 90 as the TM0 component.

  As described above, in the third optical functional element 90, the TM0 component of the signal light is converted into the TE1 component in the mode conversion element 20 on the input side. For this reason, even if there is a significant polarization dependency in the reception sensitivity of the EA modulator 72, the polarization dependency of the light receiving efficiency can be reduced as the entire optical functional element. Further, in the mode conversion element 21 on the output side, the TE1 component of the signal light is converted into the TM0 component. Therefore, the signal light modulated by the EA modulator 72 can be output while maintaining the polarization state of the signal light input to the third optical functional element 90.

  The manufacturing method of the third optical functional element 90 is the same as that of the second optical functional element except for the shape of the half-etched pattern for forming the output-side mode conversion element 21, and thus the description thereof is omitted. .

(Other configuration examples)
In the first optical functional element described above, an example in which a vertical PD with a PIN structure is used as an optoelectronic device has been described. However, the present invention is not limited to this. A PD having a horizontal PIN structure may be used. In the first optical functional element, the coupling of light from the Si waveguide to the Ge layer is evanescent coupling, but may be butt coupling coupling. Further, instead of PIN-PD, an avalanche photodiode may be used as the optoelectronic device.

  In the second optical functional element and the third optical functional element, a EA modulator prepared in advance is mounted on the terrace. However, as in the first optical functional element, an optical waveguide and an EA modulator are formed. May be. Moreover, the optoelectronic device to be mounted is not limited to the EA modulator. A semiconductor optical amplifier or the like can be mounted to reduce the polarization dependence of gain. A PD may be mounted on the terrace.

Although an example in which the optical waveguide is made of Si has been described here, the optical waveguide may be made of a material having a higher refractive index than the material forming the cladding. For example, when SiO ₂ is used for the cladding, SiO _x (0 <x <2), SiON, or SiN may be used as a material for forming the mode conversion element. When the optical waveguide is made of Si, SiO _x (0 <x <2), SiON, or SiN may be used as a material for forming the cladding.

DESCRIPTION OF SYMBOLS 10 1st optical function element 12 Support substrate 14 Lower clad 16, 17 Optical waveguide 18 Upper clad 20, 21 Mode conversion element 22, 92 Thin wire waveguide part 24, 94 Step slab part 25 Lower slab part 27 Upper taper part 28, 98 Slab part 30 Photodiode 32 Low concentration p-Si region 34 High concentration p-Si contact region 42 Ge layer
44 High-concentration n-Ge contact region 50 Contact electrode 52 Pad electrode 60, 62 Half-etch pattern 70 Second optical functional element 72 EA modulator 74 Output side waveguide 75 Tapered portion 77 Equal width waveguide 80, 82, 84 Resist pattern 90 Third optical functional element

Claims (10)

A thin wire waveguide section;
Step slab part,
With the slab
Are sequentially connected as an integral channel waveguide,
The step slab part is
The lower slab part,
On the lower slab part, an upper taper part whose width increases from the thin wire waveguide part side toward the slab part side;
With
The fundamental mode TE polarization input to the step slab part from the thin wire waveguide part is output to the slab part without polarization and mode conversion,
Wherein the wire waveguide section is inputted to the stepped slab section, TM polarization of the fundamental mode, features and to makes the chromophore at the distal end o de be output are converted to TE-polarized first-order mode in the slab portion Conversion element. The width of the lower slab portion, mode converter according to claim 1, wherein at the maximum width or more upper tapered portion. Mode conversion device according to claim 1 or 2, characterized in that equal the thickness of the upper tapered portion and the lower slab section. The mode conversion element according to any one of claims 1 to 3 , wherein the material is silicon. The mode conversion element is formed using an SOI substrate in which a silicon oxide layer and a silicon layer are stacked on a silicon substrate,
The silicon substrate is used as a support substrate,
The silicon oxide layer is used as a lower cladding layer;
Mode conversion device according to any one of claims 1 to 3, characterized in that said silicon layer is used as an optical waveguide layer. The mode conversion element according to any one of claims 1 to 5 ,
An optical functional element, comprising: an optoelectronic device to which light output from a wide side of the upper tapered portion is input. The input and output sides of the optoelectronic device are each provided with the mode conversion element according to any one of claims 1 to 5 ,
Light output from the wide side of the upper taper portion of the mode conversion element on the input side is input to the optoelectronic device,
The optical functional element, wherein the light output from the optoelectronic device is input to the wide side of the upper taper portion of the output mode conversion element. The optical functional device according to claim 6 , wherein the optoelectronic device is a photodiode. 9. The optical functional element according to claim 8 , wherein a material of the light absorption layer of the photodiode is germanium. 8. The optical functional element according to claim 6, wherein the optoelectronic device is an EA modulator.

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