Optical device and method for manufacturing optical device

Abstract

This optical device (10) comprises: a rib-type waveguide (31) that includes a core (31A) and a pair of slabs (31B) extending from both ends of the core in the width direction; a channel-type waveguide (32) that is composed of a core and positioned further forward than the rib-type waveguide (31); and a connection waveguide (35) that is disposed between the rib-type waveguide (31) and the channel-type waveguide (32) and optically connects the rib-type waveguide (31) and the channel-type waveguide (32). The connection waveguide (35) comprises: a tapered waveguide (35A) that extends forward from the front end of the core (31A) of the rib-type waveguide (31) while gradually increasing in width; and a tapered waveguide (35C) that extends rearward from the rear end of the channel-type waveguide (32) while gradually increasing in width. The connection waveguide (35) further comprises a rectangular waveguide (35B) that extends from the front end of the tapered waveguide (35A) to the rear end of the second tapered waveguide (35C) while maintaining a constant width.

Description

Optical device and method for manufacturing optical device

[0001] The present invention relates to an optical device and a method for manufacturing an optical device.

[0002] Known optical waveguides include rib-type waveguides and channel-type waveguides (Non-Patent Document 1). Rib-type waveguides and channel-type waveguides are sometimes integrated into a single optical device.

[0003] T. Tsurugaya et al., “1D Photonic-Crystal Laser with Laterally-Current-Injected Ultrasmall Buried Active Region on SiO2 / Si Substrate,” in CLEO 2023, SM2J.4 (2023).

[0004] As described above, when integrating a rib-type waveguide and a channel-type waveguide into a single optical device, alignment of the two is important. For example, when the rib-type waveguide and the channel-type waveguide are formed by separate lithography and etching processes, misalignment is likely to occur.

[0005] An object of the present invention is to increase the tolerance to misalignment between a rib-type waveguide and a channel-type waveguide.

[0006] In order to solve the above problems, the optical device of the present invention comprises a rib-type waveguide including a core and a pair of first slabs extending from both ends of the core in a width direction, a channel-type waveguide consisting of a core and located forward of the rib-type waveguide, and a connecting waveguide arranged between the rib-type waveguide and the channel-type waveguide and optically connecting the rib-type waveguide and the channel-type waveguide, wherein the connecting waveguide comprises a first tapered waveguide extending gradually wider forward from the front end of the core of the rib-type waveguide, and a second tapered waveguide extending gradually wider from the rear end of the channel-type waveguide toward the rear.

[0007] A method for manufacturing an optical device according to the present invention is a method for manufacturing an optical device including a rib-type waveguide, a channel-type waveguide, and a connection waveguide connecting the two, and includes a first step of forming a waveguide layer including a slab, a first tapered region formed so as to gradually narrow forward from at least a part of a front end of the slab, and the channel-type waveguide extending from the front end of the first tapered region, and an etching mask covering the Chanel-type waveguide, the etching mask including a linear region covering a region of the slab where a core of the rib-type waveguide is formed, and an etching mask covering the linear region. a second step of covering the waveguide layer with an etching mask having: a second tapered region that gradually widens forward from a front end of the slab and covers a portion of the slab; and a rectangular region that extends forward from the front end of the second tapered region with a width equal to that of the front end, narrower than the rear end of the first tapered region, and wider than the front end of the first tapered region, and covers the slab and at least a portion of the first tapered region; and a second step of etching a portion of the thickness direction of the waveguide layer that is not covered with the etching mask.

[0008] A method for manufacturing an optical device according to the present invention is a method for manufacturing an optical device including a rib-type waveguide, a channel-type waveguide, and a connection waveguide connecting the two, and includes a first step of forming a waveguide layer including a slab, a first tapered region formed so as to gradually narrow forward from at least a part of a front end of the slab, and the channel-type waveguide extending from the front end of the first tapered region; and a second step of covering the waveguide layer with an etching mask for covering the Chanel-type waveguide, the etching mask having a linear region covering a region of the slab where a core of the rib-type waveguide is formed, and a second tapered region extending so as to gradually widen forward from the front end of the linear region and covering a part of the first tapered region, and etching a part of the thickness direction of the region of the waveguide layer that is not covered by the etching mask.

[0009] According to the present invention, high tolerance to misalignment between the rib waveguide and the channel waveguide can be obtained.

[0010] FIG. 1 is a schematic plan view of an optical device according to a first embodiment of the present invention. FIG. 2 is a diagram illustrating a manufacturing process for the optical device of FIG. 1. FIG. 3 is a diagram illustrating a manufacturing process for the optical device of FIG. 1. FIG. 4 is a diagram illustrating a manufacturing process for the optical device of FIG. 1. FIG. 5 is a schematic plan view illustrating various parameters of the optical device according to the first embodiment of the present invention, with a photomask superimposed by a dashed-dotted line. FIG. 6 is a schematic plan view of the optical device according to the first embodiment of the present invention, with a misalignment occurring, with a photomask superimposed by a dashed-dotted line. FIG. 7 is a schematic plan view of the optical device according to the first embodiment of the present invention, with a misalignment occurring, with a photomask superimposed by a dashed-dotted line. FIG. 8 is a distribution diagram illustrating the refractive index distribution in the X-Y cross section of a rib-type waveguide. FIG. 9 is a distribution diagram illustrating the refractive index distribution in the X-Y cross section of a channel-type waveguide. FIG. 10 is a distribution diagram illustrating the optical intensity distribution of a mode calculated by a two-dimensional finite difference method in the X-Y cross section of a rib-type waveguide. FIG. 11 is a distribution showing the optical intensity distribution of modes calculated by the two-dimensional finite difference method in the XY cross section of a channel waveguide. FIG. 12 is a graph showing the calculation results of the mode conversion efficiency of the optical device of FIG. 1. FIG. 13 is a graph showing the calculation results of the mode conversion efficiency of the optical device of FIG. 1. FIG. 14 is a graph showing the calculation results of the mode conversion efficiency of the optical device of FIG. 1. FIG. 15 is a graph showing the calculation results of the mode conversion efficiency of the optical device of FIG. 1. FIG. 16 is a graph showing the calculation results of the mode conversion efficiency of the optical device of FIG. 1. FIG. 17 is a graph showing the calculation results of the mode conversion efficiency of the optical device of FIG. 1. FIG. 18 is a schematic plan view of an optical device according to a second embodiment of the present invention, in which a photomask is superimposed by a dashed-dotted line. FIG. 19 is a schematic plan view of an optical device according to the second embodiment of the present invention, in which a positional misalignment has occurred, in which a photomask is superimposed by a dashed-dotted line. FIG. 20 is a graph showing the calculation results of the mode conversion efficiency of the optical device of FIG. 18. Fig. 21 is a graph showing the calculation results of the mode conversion efficiency of the optical device in Fig. 18. Fig. 22 is a graph showing the calculation results of the mode conversion efficiency of the optical device in Fig. 18.Fig. 23 is a graph showing the calculation results of the mode conversion efficiency of the optical device in Fig. 18. Fig. 24 is a graph showing the calculation results of the mode conversion efficiency of the optical device in Fig. 18. Fig. 25 is a graph showing the calculation results of the mode conversion efficiency of the optical device in Fig. 18.

[0011] Hereinafter, embodiments of the present invention and their modifications will be described with reference to the drawings. In the following description, the optical axis C direction (Z direction) of the waveguide is defined as the front-rear direction, the thickness direction (Y direction) of the waveguide is defined as the up-down direction, and the width direction (X direction) of the waveguide is defined as the left-right direction.

[0012] 1, an optical device 10 according to this embodiment includes an underclad 20 formed on a substrate (not shown) or functioning as a substrate, and an optical waveguide 30 formed on the underclad 20. The underclad 20 is made of silicon oxide (SiO ２ The optical waveguide 30 is made of a low refractive index material such as silicon (Si) or indium phosphide (InP). The optical device 10 also includes an overclad 90 (not shown in FIG. 1; see also FIGS. 9 and 10) of air that covers the optical waveguide 30. The overclad 90 may be made of other low refractive index materials such as silicon oxide or polymer. The vertically extending dashed dotted lines in FIG. 1 are imaginary lines that indicate the boundaries of the various parts described below.

[0013] The optical waveguide 30 includes a rib-type waveguide 31, a channel-type waveguide 32, and a connecting waveguide 35. In an ideal optical device 10 without misalignment (described below), the rib-type waveguide 31, the channel-type waveguide 32, and the connecting waveguide 35 share a common optical axis C and are formed in a shape symmetrical in the left-right direction with the optical axis as an axis of symmetry. The rib-type waveguide 31, the channel-type waveguide 32, and the connecting waveguide 35 are integrally formed from the same material. Note that FIG. 1 is a cut-out view of a main portion of the optical device 10.

[0014] The rib-type waveguide 31 includes a linear core 31A extending in the front-rear direction and a pair of plate-like slabs 31B extending from the widthwise ends of the core 31A to both the left and right sides (directions perpendicular to the optical axis C). The slabs 31B are thinner than the core 31A (see also FIG. 9).

[0015] The channel waveguide 32 is formed with only a linear core extending in the front-rear direction and does not include a slab thinner than the core. The channel waveguide 32 has the same thickness as the core 31A of the rib waveguide 31, but is narrower than the core 31A.

[0016] The connecting waveguide 35 optically connects the rib-type waveguide 31 and the channel-type waveguide 32. In particular, the connecting waveguide 35 converts the cross-sectional optical mode of the optical signal propagating through the rib-type waveguide 31 and inputs the converted optical signal to the channel-type waveguide 32. The connecting waveguide 35 may also propagate the optical signal in the reverse direction.

[0017] The connecting waveguide 35 includes a tapered waveguide 35A, a rectangular waveguide 35B, a tapered waveguide 35C, a pair of slabs 35D, and a pair of slabs 35E.

[0018] The tapered waveguide 35A extends forward from the front end of the core 31A of the rib-type waveguide 31 so as to gradually widen.

[0019] The rectangular waveguide 35B extends forward from the front end of the tapered waveguide 35A with the same width.

[0020] The tapered waveguide 35C extends forward from the front end of the rectangular waveguide 35B so as to gradually narrow. The front end of the tapered waveguide 35C is connected to the rear end of the channel waveguide 32 with the same width as the rear end of the channel waveguide 32.

[0021] The tapered waveguide 35A, the rectangular waveguide 35B, and the tapered waveguide 35C are formed to the same thickness as the core 31A and the channel waveguide 32.

[0022] The pair of slabs 35D extend to both the left and right sides from the tapered waveguide 35A and are formed thinner than the tapered waveguide 35A. The pair of slabs 35D are respectively connected to the pair of slabs 31B of the rib waveguide 31. The pair of slabs 35D constitute a rib waveguide R1 with the tapered waveguide 35A as its core.

[0023] The pair of slabs 35E extend from the rectangular waveguide 35B to both the left and right sides and are thinner than the rectangular waveguide 35B. The pair of slabs 35E are connected to the pair of slabs 35D, respectively. Each of the pair of slabs 35E includes a rectangular region 35EA extending forward from the slab 35D and a triangular region 35EB extending forward from the rectangular region 35EA and having a right-angled triangular shape with a hypotenuse approaching the rectangular waveguide 35B as it moves forward. Due to misalignment, as described below, one of the triangular regions 35EB of the pair of slabs 35E may be connected to the tapered waveguide 35C in addition to the rectangular waveguide 35B (see FIGS. 6 and 7). This is also one example of the pair of slabs 35E extending from the rectangular waveguide 35B to both the left and right sides. The pair of slabs 35E constitute a rib-type waveguide R2 having a rectangular waveguide 35B and a tapered waveguide 35C connected thereto as a core.

[0024] Next, a method for forming the optical device 10 will be described. First, a lithography and etching process is performed on a laminate in which a slab waveguide 50 made of a semiconductor material is laminated on the entire surface of the underclad 20, as shown in FIG. 2, to process the slab waveguide 50 into a waveguide layer 60 as shown in FIG.

[0025] The waveguide layer 60 comprises a rectangular slab region 61 located at its rear, a tapered region 62 extending forward from a part of the front end of the slab region 61 and gradually narrowing in width, and a channel waveguide 32 extending forward from the front end of the tapered region 62 with the same width as the front end.

[0026] In the lithography process, an etching mask (e.g., a photoresist or a hard mask) in the shape of the waveguide layer 60 is formed on the slab waveguide 50 by lithography. In the etching process, all portions of the slab waveguide 50 that are not covered by the etching mask are removed by etching, and the etching mask is then removed. The remaining portions of the slab waveguide 50 become the waveguide layer 60.

[0027] Thereafter, an etching mask 70 such as a photoresist or hard mask, shown by dashed lines in FIG. 4, is formed over the undercladding 20 and the waveguide layer 60 .

[0028] The etching mask 70 includes a rectangular linear region 71, a tapered region 72 that gradually widens and extends forward from the front end of the linear region 71, and a rectangular region 73 that extends forward from the front end of the tapered region 72 with the same width as the front end. The etching mask 70 also includes a rectangular region 74 that is connected to the front end of the rectangular region 73 and is located in front of the rectangular region 73 and is wider than the rectangular region 73.

[0029] The linear region 71 covers the region of the slab region 61 of the waveguide layer 60 where the core 31A of the rib-type waveguide 31 is formed. The tapered region 72 covers the region of the slab region 61 where the tapered waveguide 35A of the connecting waveguide 35 is formed. The rectangular region 73 covers the region of the slab region 61 and the tapered region 62 where the rectangular waveguide 35B and tapered waveguide 35C of the connecting waveguide 35 are formed. The rectangular region 73 is narrower than the rear end of the tapered region 62 and wider than the front end of the tapered region 62. Therefore, the outline of the rectangular region 73 intersects with the outline of the tapered region 62, and this intersection defines the rectangular waveguide 35B and tapered waveguide 35C. The shapes of the rectangular waveguide 35B and tapered waveguide 35C differ depending on the positional misalignment described below (see also FIGS. 6 and 7 ). The front of the rectangular region 73 and the rectangular region 74 cover the channel waveguide 32 .

[0030] After the waveguide layer 60 is formed, a portion of the waveguide layer 60 in the thickness direction that is not covered with the etching mask 70 is etched. As a result, the region of the waveguide layer 60 that is covered with the etching mask 70 remains as the core 31A of the rib-type waveguide 31, the connecting waveguide 35, and the channel-type waveguide 32 shown in Fig. 1, and the etched portion of the waveguide layer 60 that is not covered with the etching mask 70 becomes the slab 31B of the rib-type waveguide 31 and the slabs 35D and 35E of the connecting waveguide 35 shown in Fig. 1.

[0031] Thereafter, post-processing is performed, such as removing the etching mask 70. As a result, the optical device 10 shown in FIG.

[0032] Next, the dimensional parameters of the optical device will be described with reference to FIG. 5 . As shown in FIG. 5 , the width of the linear region 71 of the etching mask 70 is defined as Wrib. The width of the tapered region 72 at the portions located on both the left and right sides of the left and right ends of the core 31A is defined as Wtp1. The length of the tapered region 72 in the front-to-rear direction is defined as Ltp1. The width of the channel waveguide 32 is defined as Wch. The width of the tapered region 62 of the waveguide layer 60 at the portions located on both the left and right sides of the left and right ends of the channel waveguide 32 is defined as Wtp2. The length of the tapered region 62 in the front-to-rear direction is defined as Ltp2. The front-to-rear distance between the tapered region 72 and the tapered region 62 and the front-to-rear distance between the tapered region 62 and the rectangular region 74 are set to the same value, Lmg.

[0033] When the channel waveguide 32 and the rib waveguide 31 are integrated into the optical device 10 as in this embodiment, the channel waveguide 32 is formed in a first lithography / etching process, and the rib waveguide 31 is formed in a second lithography / etching process. The reason why two processes are performed in this manner is that, in the formation of the channel waveguide 32, the unnecessary portion of the slab waveguide 50, which is a semiconductor layer, is etched entirely in the thickness direction, but in the formation of the rib waveguide 31, only a part of the unnecessary portion of the slab waveguide 50 in the thickness direction is etched, and therefore both etching processes cannot be performed at the same time.

[0034] In the two lithography steps described above, due to specification limits of the lithography apparatus and / or various factors in the actual microfabrication process, a finite positional misalignment occurs between the etching mask (not shown) for forming the channel waveguide 32 and the etching mask 70 for forming the rib waveguide 31. This causes a positional misalignment between the channel waveguide 32 and the rib waveguide 31, particularly in the left-right direction. This positional misalignment makes it difficult to obtain good optical coupling between the cross-sectional optical modes (hereinafter simply referred to as modes) of the channel waveguide 32 and the rib waveguide 31. This causes problems such as increased optical loss and unintended reflection within the optical circuit.

[0035] 6 and 7 show the state in which this misalignment occurs. The misalignment amount E between the two is represented by the difference in the left-right position between the optical axis C1 of the core 31A of the rib waveguide 31 and the optical axis C2 of the channel waveguide 32. The misalignment amount E is larger in FIG. 7 than in FIG. 6. Due to the misalignment amount E, the pair of triangular regions 35EB of the slab 35E have different shapes. Compared to the case where the misalignment amount E = 0 when no misalignment occurs, one of the pair of triangular regions 35EB is larger and the other of the pair of triangular regions 35EB is smaller. The front end of the larger triangular region 35EB reaches the tapered waveguide 35C. As a result, at least a portion of the tapered waveguide 35C also serves as the core of the rib waveguide R3 having a slab on one side.

[0036] The misalignment amount E in Figure 7 is the maximum misalignment amount Emax allowed in the optical device 10. With a misalignment at the maximum misalignment amount Emax, the left and right sides of the channel waveguide 32 overlap with the left and right sides of the rectangular region 73 of the etching mask 70. If a misalignment exceeding the maximum misalignment amount Emax occurs, the channel waveguide 32 and the rearward extension from the channel waveguide 32 in the tapered region 62 will no longer be covered by the etching mask 70 and will be etched away. As a result, the transmission efficiency of the optical signal between the channel waveguide 32 and the rib waveguide 31 will decrease. Conversely, if the misalignment is equal to or less than the maximum misalignment amount Emax, the erosion will be prevented, the transmission efficiency of the waveguide optical signal will be ensured, and good optical coupling will be obtained between the cross-sectional optical modes (hereinafter simply referred to as modes) of the channel waveguide 32 and the rib waveguide 31. This is because the connecting waveguide 35 is provided with tapered waveguides 35A and 35C that gradually become wider in opposite directions, so that no step shape that would hinder the propagation of light is provided in the optical waveguide 30, even in the optical device 10 with the positional deviation amount E=Emax shown in Figure 7.

[0037] In this embodiment, for example, an optical signal input to the core 31A of the rib waveguide 31 is amplified without mode conversion by the tapered waveguide 35A constituting the rib waveguide R1. This optical signal is then input to the rectangular waveguide 35B, whose width does not change. In the rib waveguide R2 having the rectangular waveguide 35B as its core (or the rib waveguides R2 and R3 having the rectangular waveguide 35B and the tapered waveguide 35C as their cores when misalignment occurs), the triangular region 35EB of the slab 35E causes the slab to gradually become smaller toward the front. Therefore, the rib waveguide R2 (R2 and R3) gradually becomes closer to a channel waveguide toward the front, and the mode of the optical signal propagating through the rib waveguide R2 is gradually converted to the mode of the channel waveguide. The optical signal is then input to the channel waveguide 32. As a result, mode conversion between the channel waveguide 32 and the rib waveguide 31 occurs gradually, resulting in low-loss optical coupling. In this embodiment, the triangular region 35EB is always formed even if misalignment occurs, so the above-mentioned good optical coupling is obtained. This is achieved by overlapping the rectangular region 73 of the etching mask 70 with the tapered region 62 of the waveguide layer 60, defining the rectangular waveguide 35B and tapered waveguide 35C of the connecting waveguide 35 with the outline of the tapered region 62 and the outline of the rectangular region 73, and providing the slab 35E (particularly the triangular region 35EB) on the left and right outer sides thereof.

[0038] As shown in FIG. 7, if the left-right distance between the left-right ends of the tapered region 62 and the rectangular region 73 of the etching mask 70 when the misalignment amount E=Emax is defined as Wmin (the maximum width of the triangular region 35EB), the widths Wtp1 and Wtp2 are defined by the following equations (1) and (2).

[0039] From the above formula, by setting Emax, Wmin, Wrib, and Wch to appropriate values, and particularly by determining Emax to a desired value taking into consideration misalignment of the etching mask, etc., appropriate Wtp1 and Wtp2 can be obtained. Wmin may be set to an appropriate value according to the degree of light seeping out in the left and right directions of the mode formed in the rib-type waveguide 31, for example.

[0040] The lengths Ltp1 and Ltp2 can be set to any length as long as the desired thermal insulation (light propagation efficiency) is achieved. Furthermore, Lmg defines a margin area for absorbing misalignment in the front-to-rear direction, and therefore Lmg can be set appropriately according to the value of the maximum expected misalignment in the front-to-rear direction. While the front-to-rear lengths Ltp1, Ltp2, and Lmg are all optional parameters, if the area available for this embodiment is limited, constraints are imposed on the total length Ltot = Ltp1 + Lmg + Ltp2. In this case, effectively allocating lengths to each of Ltp1 and Ltp2 is particularly important for achieving excellent connection characteristics. Therefore, Ltp1 and Lt2 can be set, for example, according to the following equations (3) and (4).

[0041] The above formula is derived from the condition that the divergence angles of the tapered regions 72 and 62 are equal (Wtp1 / Ltp1=Wtp2 / Ltp2), which allows the same degree of heat insulation to be obtained in both tapered regions 72 and 62, and under the condition that the total length Ltot is a given, it is possible to obtain excellent rib-to-channel connection characteristics (high optical transmission efficiency) between the rib-type waveguide 31 and the channel-type waveguide 32. As a modified example, the slab region 61 may be formed in a shape in which Lmg=0 and the rectangular region 35EA is not formed.

[0042] To confirm the effects of this embodiment, we will show the results of calculating the rib-channel connection characteristics using the two-dimensional finite difference method (2D-FDM) and the three-dimensional finite difference time domain (3D-FDTD) method for a specific assumed structure. The various parameters assumed in the calculations are shown in the table below.

[0043] InP as the core material and SiO as the undercladding material ２, and air was assumed as the overcladding material. The width Wrib and thickness Tcore of the core 31A of the rib-type waveguide 31 were set to 600 nm and 250 nm, respectively. The thickness Tside of the slab 31B was set to 120 nm. The width Wch and thickness Tcore of the channel-type waveguide 32 (core) were set to 500 nm and 250 nm, respectively. These shape parameters satisfy the single-mode condition for propagating an optical signal with a wavelength of 1550 nm through the optical waveguide 30, and are typical shape parameters for an optical waveguide made of InP. Wmin was set to 200 nm (i.e., a positional misalignment occurred), and Lmg was set to 1 μm.

[0044] 8 and 9 show the refractive index distributions in the XY cross section of the rib-type waveguide 31 and the channel-type waveguide 32, respectively. FIGS. 10 and 11 show the optical intensity distributions of the modes calculated by the two-dimensional finite difference method in the XY cross section of the rib-type waveguide 31 and the channel-type waveguide 32, respectively. As can be seen from FIGS. 8 to 11, a fundamental mode with an intensity distribution that reflects the core shape is formed in each of the rib-type and channel-type waveguides. In both of these fundamental modes, the electric field component in the left-right direction (X direction) is dominant, and the TE ００ can be considered as a mode.

[0045] Next, a mode emission source and a mode receiver that emit and receive light with mode distributions specified by these fundamental modes were set for the rib waveguide 31 and the channel waveguide 32, respectively, and these were connected by an optical waveguide 30. The optical power transmittance from the mode emission source connected to the rib waveguide 31 to the mode receiver connected to the channel waveguide 32 was calculated using the three-dimensional finite-difference time-domain method. This optical power transmittance represents the mode conversion efficiency from the fundamental mode of the rib waveguide 31 to the fundamental mode of the channel waveguide 32, and is therefore appropriate as a figure of merit for the optical waveguide 30 of the optical device 10 according to this embodiment.

[0046] Here, to confirm the dependence of the mode conversion efficiency on the misalignment amount E, the maximum misalignment amount Emax, and the total length Ltot, calculations were performed by sweeping each parameter as follows. Ltp1 and Ltp2 were derived using the above equations (3) and (4). E (nm) = {0, 100, 200, 300, 400, 500, 600, 700} Emax (nm) = {300, 600} Ltot (μm) = {30, 60, 90}

[0047] The calculation results of the mode conversion efficiency for light with a wavelength of 1550 nm when Emax = 300 nm are shown in Figures 12 and 13. In Figure 12, the axial range of the mode conversion efficiency is 0-100%, and in Figure 13, the axial range of the mode conversion efficiency is 97-100%. The calculation results of the mode conversion efficiency for light with a wavelength of 1550 nm when Emax = 600 nm are shown in Figures 14 and 15. In Figure 14, the axial range of the mode conversion efficiency is 0-100%, and in Figure 15, the axial range of the mode conversion efficiency is 97-100%.

[0048] 12 and 13 show that when the maximum misalignment amount Emax = 300 nm, by setting Ltot to 30 μm or more, very good rib-to-channel connection characteristics are obtained, with the mode conversion efficiency exceeding 99.3% over the entire range of E satisfying E≦Emax (= 300 nm). Figures 12 and 13 also show that the mode conversion efficiency rapidly deteriorates when the misalignment amount E exceeds Emax (= 300 nm). This is because, as described above, when E > Emax, the region not covered by the etching mask 70 invades the channel waveguide 32, impairing the thermal insulation of the mode conversion. This is a reasonable result within the expectations of this embodiment.

[0049] 14 and 15 show that when the maximum misalignment amount Emax is 600 nm, tolerance to a larger misalignment amount E than in FIGS. 12 and 13 is achieved, reflecting the larger Emax setting. For example, a mode conversion efficiency of 99.2% or more is achieved when Ltot = 60 μm, and 99.4% or more is achieved when Ltot = 90 μm over the entire range of E satisfying E≦Emax (= 600 nm). When E exceeds Emax at 700 nm, the mode conversion efficiency rapidly deteriorates. Therefore, the results in FIGS. 14 and 15 again confirm the effect of this embodiment, that very high mode conversion efficiency can be achieved at any misalignment amount E less than the maximum misalignment amount Emax that can be arbitrarily set. However, when Ltot = 30 μm, when Emax = 600 nm, in contrast to when Emax = 300 nm, very high mode conversion efficiency is not necessarily achieved; for example, the mode conversion efficiency at E = 200 nm is below 98%. This reflects the fact that as Emax is increased from 300 nm to 600 nm, Wtp1 and Wtp2 increase in accordance with the above formulas (1) and (2), and the taper length required to obtain sufficient thermal insulation in these tapered waveguides 35A and tapered regions 62 (i.e., to keep the taper divergence angle sufficiently small) is increased.

[0050] 12 to 15, it can be seen that the total length Ltot in this embodiment can be set to a length that is necessary and sufficient for the desired maximum misalignment amount Emax (allowable misalignment amount) and the mode conversion efficiency. In this calculation example, for example, if the lower limit of the mode conversion efficiency is set to 99.5%, by setting Ltot = 60 μm for Emax = 300 nm and Ltot = 90 μm for Emax = 600 nm, it is possible to obtain a highly efficient optical device 10 (rib-channel connection structure) that is necessary and sufficient for each maximum misalignment amount Emax.

[0051] 16 shows the wavelength dependence of the mode conversion efficiency when Emax = 300 nm and Ltot = 60 μm, and FIG. 17 shows the wavelength dependence of the mode conversion efficiency when Emax = 600 nm and Ltot = 90 μm. As shown in FIGS. 16 and 17, as long as E≦Emax (= 600 nm) is satisfied, a very high mode conversion efficiency comparable to that at 1550 nm is obtained over a wide wavelength range. Therefore, the effect of this embodiment does not show significant wavelength dependence and can be stably obtained over a wide wavelength range.

[0052] 18 and 19 , an optical device 110 according to a second embodiment has a connection waveguide 135 instead of the connection waveguide 35. The following description will focus on differences from the first embodiment. In the following description, elements similar to those in the first embodiment are denoted by the same reference numerals, and detailed descriptions will be based on those in the first embodiment.

[0053] The connection waveguide 135 has the same function as the connection waveguide 35, but has a different shape from the connection waveguide 35. The connection waveguide 135 includes a tapered waveguide 135A, a tapered waveguide 35C, and a pair of slabs 35E. The connection waveguide 135 does not have a configuration corresponding to the rectangular waveguide 35B.

[0054] The tapered waveguide 135A extends forward from the front end of the core 31A of the rib-type waveguide 31 so as to gradually widen.

[0055] The tapered waveguide 135C extends forward from the front end of the tapered waveguide 135A so as to gradually narrow. The front end of the tapered waveguide 135C is connected to the rear end of the channel waveguide 32 with the same width as the rear end of the channel waveguide 32.

[0056] The tapered waveguide 135A and the tapered waveguide 135C are formed to have the same thickness as the core 31A and the channel waveguide 32.

[0057] The pair of slabs 35E extend from the tapered waveguide 135A to both the left and right sides and are thinner than the tapered waveguide 135A. The pair of slabs 35E are connected to the pair of slabs 31B, respectively. Each of the pair of slabs 35E has a triangular shape that gradually narrows toward the front. Due to a positional misalignment, as described below, one of the pair of slabs 35E may be connected to the tapered waveguide 135C in addition to the tapered waveguide 135A (see FIG. 19). This is also one example of the pair of slabs 35E extending from the tapered waveguide 135A to both the left and right sides. The pair of slabs 35E constitute a rib-type waveguide with the tapered waveguides 135A and 135C connected to it as its core.

[0058] Next, a description will be given of a method for forming the optical device 110. The method for forming the optical device 110 is almost the same as that of the first embodiment, except that an etching mask 170 is used instead of the etching mask 70.

[0059] The etching mask 170 includes a rectangular linear region 171, a tapered region 172 that gradually widens and extends forward from the front end of the linear region 171, and a rectangular region 173 that extends forward from the front end of the tapered region 172 with the same width as the front end. The etching mask 170 also includes a rectangular region 174 that is connected to the front end of the rectangular region 173 and is positioned forward of the rectangular region 173 and is wider than the rectangular region 173.

[0060] The linear region 171 covers the region of the slab region 61 of the waveguide layer 60 where the core 31A of the rib-type waveguide 31 is formed. The tapered region 172 covers the region of the tapered region 62 where the tapered waveguide 135C of the connecting waveguide 135 is formed. The tapered region 172 and the tapered region 62 have the same taper length Ltot but taper in opposite directions. Therefore, the outlines of the tapered region 172 and the tapered region 62 intersect, and this intersection defines the tapered waveguides 135A and 135C. The shapes of the tapered waveguides 135A and 135C differ depending on the positional misalignment described below. The rectangular region 173 and the rectangular region 174 cover the channel-type waveguide 32.

[0061] The portions of the thickness that are not covered by the etching mask 170 are etched to form slabs that are thinner than the core, and the etching mask 170 is removed to form the optical device 110 shown in Figures 18 and 19.

[0062] 19 shows the optical device 110 when the misalignment amount E is the maximum value Emax. As in the first embodiment, if a misalignment exceeding the maximum misalignment amount Emax occurs, the channel waveguide 32 and the rearward extension portion of the channel waveguide 132 in the tapered region 62 will no longer be covered by the etching mask 70 and will be etched away. As a result, the transmission efficiency of the optical signal between the channel waveguide 32 and the rib waveguide 31 will decrease. Conversely, if the misalignment is equal to or less than the maximum misalignment amount Emax, the erosion will be prevented, the transmission efficiency of the waveguide optical signal will be ensured, and good optical coupling will be obtained between the cross-sectional optical modes (hereinafter simply referred to as modes) of the channel waveguide 32 and the rib waveguide 31. This is because the connecting waveguide 35 has tapered waveguides 135A and 135C that gradually become wider in opposite directions, so that even in the optical device 10 with a positional misalignment amount E = Emax, no step shape that would hinder the progression of light is formed in the optical waveguide 30.

[0063] In the second embodiment, the total length Ltot of the connecting waveguide 135 is shortened by the amount corresponding to the absence of the rectangular waveguide 35B, thereby reducing the size of the optical device 110. In the second embodiment, the tapered regions 62 and 172 for forming the tapered waveguides 135A and 135C are set to the same length Ltot, preventing the total length Ltot of the connecting waveguide 135 from becoming too long.

[0064] The width of the linear region 171 of the etching mask 170 is represented by Wrib, and the width of the tapered region 172 at the portions located on both the left and right sides of the left and right ends of the linear region 171 is represented by Wtp1. The width of the channel waveguide 32 is represented by Wch. The width of the tapered region 62 of the waveguide layer 60 at the portions located on both the left and right sides of the left and right ends of the channel waveguide 32 is represented by Wtp2.

[0065] Wtp1 and Wtp2 are given by the following formulas (5) and (6) based on Emax and Wmin. In particular, by determining Emax to a desired value in consideration of the misalignment of the etching mask, etc., appropriate Wtp1 and Wtp2 can be obtained.

[0066] In order to confirm the effect of this embodiment, similarly to the first embodiment, the mode conversion efficiency of the structure of this embodiment at a wavelength of 1550 nm when Emax = 300 nm and Emax = 600 nm was calculated based on the parameter settings specified in Table 1. The results are shown in Figures 20 and 21 (Emax = 300 nm) and Figures 22 and 23 (Emax = 600 nm). Note that, in this case, in order to see the effect of shortening the total length Ltot, the calculations were performed by adding 10 μm and 20 μm to the value of Ltot.

[0067] First, the calculation results in Figures 20 to 23, focusing on the dependence on the misalignment amount E, reveal that, compared to the first embodiment, the decrease in mode conversion efficiency with increasing E is somewhat significant, even within the range where E≦Emax is satisfied. As a result, the mode conversion efficiency is significantly lower than that of the first embodiment, especially for misalignment amounts E near Emax. On the other hand, focusing on the dependence on the total length Ltot, Figures 20 and 21 show that even with a very short total length of Ltot = 10 μm, characteristics are obtained that are not significantly different from, for example, Ltot = 90 μm. This indicates that, although the lower limit of the mode conversion efficiency is lower than that of the first embodiment, a rib-channel connection structure can be realized with a very short total length of, for example, Ltot = 10 μm. Similarly, in Figures 22 and 23, when Ltot = 30 μm is set, characteristics comparable to those obtained with longer Ltots are obtained, and it can be seen that the necessary and sufficient total length is shortened to approximately 30 μm. For example, when Ltot = 10 μm and 30 μm in Figures 20 to 23, the lower limit of the mode conversion efficiency within the range satisfying E≦Emax is approximately 99.0 to 99.1%. Although this does not reach the lower limit when Ltot is set sufficiently long as in the first embodiment, it significantly exceeds the lower limit when Ltot = 30 μm in Figure 12, for example. Therefore, although this structure does not achieve a mode conversion efficiency as high as that of the first embodiment, it has the characteristic of being able to significantly shorten the necessary and sufficient total length Ltot, and is particularly effective in cases where it is desired to keep the total length as short as possible.

[0068] 24 and 25 show the wavelength dependence of the mode conversion efficiency of this structure calculated when Emax = 600 nm and Ltot = 30 μm, assuming Emax = 300 nm and Ltot = 10 μm, and it can be seen that, as with the first embodiment, as long as E≦Emax is satisfied, a high mode conversion efficiency equivalent to that at 1550 nm is obtained over a wide wavelength range. In other words, as with the first embodiment, this structure also does not show significant wavelength dependence, and the above-mentioned effect can be obtained stably over a wide wavelength range.

[0069] (Scope of the present invention) The present invention is not limited to the above-described embodiments and modifications. For example, the present invention includes various modifications to the above-described embodiments and modifications that can be understood by a person skilled in the art within the scope of the technical concept of the present invention. The configurations listed in the above-described embodiments and modifications can be combined as appropriate within a range that does not contradict. In addition, any of the above-described configurations can be deleted.

[0070] (Additional Notes) The following are exemplary configurations of the above-described embodiments and modifications disclosed in this specification. The above-described embodiments and modifications may be modified as appropriate to include the following additional notes.

[0071] (Supplementary Note 1) An optical device comprising: a rib-type waveguide including a core and a pair of first slabs extending from both ends of the core in a width direction; a channel-type waveguide consisting of a core and located forward of the rib-type waveguide; and a connection waveguide disposed between the rib-type waveguide and the channel-type waveguide and optically connecting the rib-type waveguide and the channel-type waveguide, wherein the connection waveguide comprises: a first tapered waveguide extending from a front end of the core of the rib-type waveguide toward the front and gradually increasing in width; and a second tapered waveguide extending from a rear end of the channel-type waveguide toward the rear.

[0072] According to the above configuration, the first tapered waveguide and the second tapered waveguide can absorb slight misalignment between the rib waveguide and the channel waveguide, thereby achieving high tolerance to misalignment between the rib waveguide and the channel waveguide (increasing the amount of allowable misalignment).

[0073] (Supplementary Note 2) The optical device according to Supplementary Note 1, wherein the connecting waveguide further includes a rectangular waveguide extending with the same width from the front end of the first tapered waveguide to the rear end of the second tapered waveguide.

[0074] According to Supplementary Note 2, the first tapered waveguide and the second tapered waveguide can absorb slight misalignment between the rib waveguide and the channel waveguide, thereby achieving high tolerance to misalignment between the rib waveguide and the channel waveguide.

[0075] (Supplementary Note 3) The optical device according to Supplementary Note 2, wherein the connecting waveguide further comprises a second slab extending from both widthwise ends of at least the first tapered waveguide and the rectangular waveguide among the first tapered waveguide, the rectangular waveguide, and the second tapered waveguide, and the second slab comprises: a pair of first regions extending from both widthwise ends of the first tapered waveguide, each thinner than the first tapered waveguide; and a pair of second regions extending from both widthwise ends of at least the rectangular waveguide among the rectangular waveguide and the second tapered waveguide, each thinner than the rectangular waveguide, and each of the pair of second regions is formed in a right-angled triangular shape having a hypotenuse that approaches the rectangular waveguide and the second tapered waveguide as it extends forward.

[0076] According to the above configuration, a second rib-type waveguide consisting of a first tapered waveguide and a first region is formed downstream of the rib-type waveguide, and an optical signal input from the upstream rib-type waveguide can be broadened by the second rib-type waveguide without mode conversion. Furthermore, a pair of second regions, each thinner than the rectangular waveguide, is formed extending from both ends of at least the rectangular waveguide of the rectangular waveguide and the second tapered waveguide in the width direction, and each of the pair of second regions is formed in a right-angled triangular shape with a hypotenuse that approaches the rectangular waveguide and the second tapered waveguide as it extends forward. Therefore, a waveguide in which mode conversion from rib type to channel type gradually occurs downstream of the second rib-type waveguide is formed. This allows smooth mode conversion, particularly by the rectangular waveguide, and achieves high mode conversion efficiency.

[0077] (Supplementary Note 4) The optical device according to Supplementary Note 1, wherein a front end of the first tapered waveguide is directly connected to a rear end of the second tapered waveguide with the same width.

[0078] According to the above configuration, since a rectangular waveguide or the like is not provided, the overall length of the connecting waveguide is shortened, and the optical device is made smaller.

[0079] (Supplementary Note 5) A method for manufacturing an optical device (particularly, any of the optical devices of Supplementary Note 1 to 4) including a rib-type waveguide, a channel-type waveguide, and a connection waveguide connecting the two, comprising: a first step of forming a waveguide layer including a slab, a first tapered region formed so as to gradually narrow forward from at least a part of the front end of the slab (in the first embodiment and the like, the tapered region 62 extends from a part of the side of the front end of the slab region 61, but the tapered region 62 may extend from the entire side; the same applies hereinafter), and the channel-type waveguide extending from the front end of the first tapered region; a second step of covering the waveguide layer with an etching mask that covers the Chanel-type waveguide, the etching mask having: a linear region that covers a region of the slab in which a core of the rib-type waveguide is formed; a second tapered region that extends forward from a front end of the linear region and gradually becomes wider to cover a part of the slab; and a rectangular region that extends forward from the front end of the second tapered region with the same width as the front end, a width that is narrower than a rear end of the first tapered region and a width that is wider than the front end of the first tapered region, the rectangular region covering the slab and at least a part of the first tapered region, and etching a part of a thickness direction of the waveguide layer that is not covered with the etching mask.

[0080] According to the above configuration, an optical device having the configuration of, for example, the first embodiment can be obtained, and high tolerance to misalignment between the rib-type waveguide and the channel-type waveguide can be obtained.

[0081] (Supplementary Note 6) A method for manufacturing an optical device including a rib-type waveguide, a channel-type waveguide, and a connection waveguide connecting the two, comprising: a first step of forming a waveguide layer including a slab, a first tapered region formed to gradually narrow forward from at least a part of a front end of the slab, and the channel-type waveguide extending from the front end of the first tapered region; and a second step of covering the waveguide layer with an etching mask that covers the Chanel-type waveguide, the etching mask having a linear region covering a region in the slab where a core of the rib-type waveguide is formed, and a second tapered region that gradually widens forward from the front end of the linear region and covers a part of the first tapered region, and etching a part of a thickness direction of the region of the waveguide layer that is not covered by the etching mask.

[0082] According to the above configuration, an optical device having the configuration of, for example, the second embodiment can be obtained, and high tolerance to misalignment between the rib type waveguide and the channel type waveguide can be obtained.

[0083] (Supplementary Note 7) The method for manufacturing an optical device according to Supplementary Note 5 or 6, wherein a width of the first tapered region and a width of the second tapered region are determined based on an allowable amount of misalignment between the core of the rib-type waveguide and the channel-type waveguide, and the first step and the second step are performed with the determined widths.

[0084] With the above configuration, the user can set the amount of misalignment and determine the width of the tapered region (Wtp2, Wtp1, etc.) based on that amount, thereby improving the design freedom of optical devices while maintaining high tolerance to misalignment between the rib-type waveguide and the channel-type waveguide.

[0085] 10...optical device, 20...underclad, 30...optical waveguide, 31...rib-type waveguide, 31A...core, 31B...slab, 32...channel-type waveguide, 35...connecting waveguide, 35A...tapered waveguide, 35B...rectangular waveguide, 35C...tapered waveguide, 35D...slab, 35E...slab, 35EA...rectangular region, 35EB...triangular region, 50...slab waveguide, 60...waveguide layer, 61...slab region, 62...tapered region, 70...etching mask, 71...line shaped region, 72...tapered region, 73...rectangular region, 74...rectangular region, 90...overclad, 110...optical device, 132...channel type waveguide, 135...connecting waveguide, 135A...tapered waveguide, 135C...tapered waveguide, 170...etching mask, 171...linear region, 172...tapered region, 173...rectangular region, 174...rectangular region, C...optical axis, C1...optical axis, C2...optical axis, R1...rib type waveguide, R2...rib type waveguide, R3...rib type waveguide.

Claims

1. An optical device comprising: a rib-type waveguide including a core and a pair of first slabs extending from both ends of the core in the width direction; a channel-type waveguide consisting of a core and located forward of the rib-type waveguide; and a connecting waveguide located between the rib-type waveguide and the channel-type waveguide and optically connecting the rib-type waveguide and the channel-type waveguide, wherein the connecting waveguide comprises: a first tapered waveguide that gradually widens and extends forward from the front end of the core of the rib-type waveguide; and a second tapered waveguide that gradually widens and extends backward from the rear end of the channel-type waveguide.

2. The optical device according to claim 1, wherein the connecting waveguide further comprises a rectangular waveguide extending with the same width from the front end of the first tapered waveguide to the rear end of the second tapered waveguide.

3. The optical device according to claim 2, wherein the connecting waveguide further comprises a second slab extending from both ends in the width direction of at least the first tapered waveguide and the rectangular waveguide out of the first tapered waveguide, the rectangular waveguide and the second tapered waveguide, and the second slab comprises: a pair of first regions extending from both ends in the width direction of the first tapered waveguide and thinner than the first tapered waveguide; and a pair of second regions extending from both ends in the width direction of at least the rectangular waveguide out of the rectangular waveguide and the second tapered waveguide and thinner than the rectangular waveguide, and each of the pair of second regions is formed in the shape of a right triangle having a hypotenuse that approaches the rectangular waveguide and the second tapered waveguide as it extends forward.

4. The optical device according to claim 1, wherein the front end of the first tapered waveguide is directly connected to the rear end of the second tapered waveguide with the same width.

5. A method for manufacturing an optical device comprising a rib-type waveguide, a channel-type waveguide, and a connecting waveguide connecting the two, comprising: a first step of forming a waveguide layer including a slab, a first tapered region formed so as to gradually narrow forward from at least a part of the front end of the slab, and the channel-type waveguide extending from the front end of the first tapered region; a second step of covering the Chanel-type waveguide with an etching mask having: a linear region covering a region of the slab where the core of the rib-type waveguide is formed; a second tapered region extending so as to gradually widen forward from the front end of the linear region and covering a part of the slab; and a rectangular region extending forward from the front end of the second tapered region with the same width as the front end, narrower than the rear end of the first tapered region, but wider than the front end of the first tapered region, and covering the slab and at least a part of the first tapered region; and a second step of etching a part of the thickness direction of the region of the waveguide layer not covered by the etching mask. A method for manufacturing an optical device comprising:

6. A method for manufacturing an optical device having a rib-type waveguide, a channel-type waveguide, and a connecting waveguide connecting the two, comprising: a first step of forming a waveguide layer including a slab, a first tapered region formed so as to gradually narrow forward from at least a part of the front end of the slab, and the channel-type waveguide extending from the front end of the first tapered region; and a second step of covering the waveguide layer with an etching mask that covers the Chanel-type waveguide, the etching mask having a linear region covering a region of the slab where the core of the rib-type waveguide is formed, and a second tapered region that gradually widens forward from the front end of the linear region and covers part of the first tapered region, and etching a part of the thickness direction of the region of the waveguide layer that is not covered by the etching mask.

7. A method for manufacturing an optical device according to claim 5 or 6, wherein the width of the first tapered region and the width of the second tapered region are determined based on the amount of misalignment allowed between the core of the rib-type waveguide and the channel-type waveguide, and the first step and the second step are performed using the determined widths.

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