JP2017215526A - Optical waveguide structure, optical waveguide, and optical integrated circuit - Google Patents

Abstract

Kind Code: A1 A polarization conversion is suppressed in a curved waveguide region of a rib-type optical waveguide. An optical waveguide has a core (13) for propagating light in a curved light propagation direction and clads (12, 14) having a lower refractive index than the core (13). The core 13 includes a slab portion and projections 13a projecting from the slab portion. The slab portion includes a first slab portion 13c outside the curved light propagation direction with respect to the protrusion 13a and a second slab portion 13b inside the curved light propagation direction. The thickness of the first slab portion 13c is thinner than the thickness of the second slab portion 13b. [Selection drawing] Fig. 1

Description

  The present invention relates to an optical waveguide structure, and more particularly to an optical waveguide structure in a rib-type optical waveguide. The present invention also relates to an optical waveguide having such an optical waveguide structure and an optical integrated circuit having the optical waveguide.

  In recent years, digital coherent communication has been actively studied in ultra-high speed communication exceeding 100 Gb / s (bits / second). Among digital coherent communications, the dual-polarization-quadrature phase shift keying (DP-QPSK) method is expected to improve spectrum utilization efficiency and various dispersion tolerances. Energetic development is underway.

  In the DP-QPSK system, a function of polarization separation at the receiver and a 90-degree optical hybrid function for extracting phase information from the separated optical signal are required. A receiver having such a function (ICR: Integrated Coherent Receiver) is currently composed of a planar lightwave circuit (PLC) using an optical waveguide technology based on quartz. In the future, miniaturization of transceivers such as CFP2-ACO (C form-factor pluggable 2-Analog Coherent Optics) and CFP4-ACO is desired, and miniaturization for ICR is also strongly desired. Therefore, downsizing of the 90-degree hybrid mixer itself composed of PLC is also strongly desired.

In order to reduce the size of an optical integrated circuit such as a 90-degree hybrid mixer composed of PLC, a semiconductor material having a higher refractive index, particularly Si photonics using Si is promising. In silicon photonics, an optical waveguide having Si as a core and SiO ₂ as a cladding is used. Since this optical waveguide has a very high refractive index difference between the core and the clad, it is possible to confine light strongly and to realize a smaller bending radius than the quartz waveguide.

14 and 15 show examples of the configuration of an optical waveguide using Si photonics. FIG. 14 shows an optical waveguide having a general channel type structure, and FIG. 15 shows an optical waveguide having a general rib type structure. A channel-type optical waveguide 50 shown in FIG. 14 includes a substrate 21, a lower clad 22, a core 23, and an upper clad 24. The substrate 21 is a silicon substrate, and a lower layer cladding 22 is provided on the upper layer of the silicon substrate. The lower clad 22 is, for example, a SiO ₂ film, and is formed of, for example, a buried oxide film (BOX: Buried Oxide).

The core 23 is provided on the upper layer side of the lower cladding 22. The core 23 is made of, for example, a Si film having a rectangular cross section formed on the lower cladding 22 that is an insulating layer. An upper clad 24 is provided on the upper layer of the core 23 so as to cover the core 23. The upper clad 24 is, for example, a SiO ₂ film. The refractive index of the core 23 is higher than that of the lower clad 22 and the upper clad 24. The core 23 is formed along the light propagation direction, and the light incident on the core 23 is propagated along the core 23.

  A rib type optical waveguide 51 shown in FIG. 15 includes a substrate 21, a lower layer clad 22, a core 23, and an upper layer clad 24, similarly to the channel type optical waveguide 50 shown in FIG. In the channel type optical waveguide 50, the core is formed in a rectangular cross section, whereas in the rib type optical waveguide 51, the core 23 entirely covers the lower cladding 22. Moreover, the core 23 has the protrusion part (rib part) 23a and the slab part 23b. The slab part 23b is arrange | positioned at the both sides of the direction orthogonal to a light propagation direction with respect to the projection part 23a. The protruding portion 23a protrudes from the slab portion 23b to the upper clad 24 side. The protrusion 23 a is formed along the light propagation direction, and in the rib-type optical waveguide 51, the light incident on the core 23 is propagated along the protrusion 23 a.

  The channel-type optical waveguide 50 (see FIG. 14) has a configuration in which a rectangular core 23 is sandwiched between a lower clad 22 and an upper clad 24 and has a high optical confinement effect. Therefore, a steep bend radius on the order of several μm. Can be realized. However, the channel-type optical waveguide 50 has a larger propagation loss than the quartz waveguide, and is not suitable for efficiency. On the other hand, the rib-type optical waveguide 51 (see FIG. 15) has a smaller bend radius than the channel-type optical waveguide 50, although the bend radius that can be realized is larger than that of the channel-type optical waveguide 50. The channel-type optical waveguide 50 can realize a bent waveguide having a level of several hundred μm, which is one digit smaller than that of a quartz waveguide, while realizing a propagation loss similar to that of a quartz waveguide, for example.

  Here, the thing of patent document 1 is known as an optical waveguide structure in a bending part (curved part). FIG. 16 shows an optical waveguide in Patent Document 1. The optical waveguide 60 is a rib-type optical waveguide, and includes a clad 22 and a core 23 formed in an upper layer thereof. The core 23 has a protrusion 23a protruding from the slab portion. The optical waveguide 60 described in Patent Document 1 has groove portions 23c provided along the protrusion portions 23a at the curved portions, which are thinner than the normal slab portions 23b, on both sides of the protrusion portions 23a.

  In Patent Document 1, the groove 23c is formed only in the curved portion, and in the straight portion, the protruding portion 23a protrudes from the slab portion 23b to the opposite side of the clad 22. The optical waveguide 60 has a tapered portion 23d at the connection portion between the straight portion and the curved portion. The thickness of the taper portion 23d is the same as the thickness of the slab portion 23b. The width of the tapered portion 23d in the direction orthogonal to the light propagation direction gradually decreases from the straight portion to the curved portion. By reducing the width of the tapered portion 23d, the groove portion 23c approaches the protruding portion 23a from the straight portion to the curved portion. The groove 23c is formed adjacent to the protrusion 23a at the start end of the bending portion.

  As another optical waveguide structure in the bent portion (curved portion), the one described in Patent Document 2 is also known. FIG. 17 shows an optical waveguide described in Patent Document 2. The optical waveguide 70 is configured as a ridge type optical waveguide. The optical waveguide 70 includes a lower clad 73, a core 71, and an upper clad 72 in order from the substrate 74 side. The upper clad 72 includes an outer side clad 75 on the outer side of the curve and an inner side clad 76 on the inner side. In the optical waveguide 70, the guided light propagates through the core 71 along the upper clad 72.

  In Patent Document 2, the outer side cladding 75 and the inner side cladding 76 are formed by etching the upper cladding 72. In the etching of the upper cladding 72, the etching amount is different between the outer side and the inner side of the curve, and the outer side cladding 75 is etched deeper than the inner side cladding 76. Therefore, in the optical waveguide 70 described in Patent Document 2, the thickness of the side cladding formed in the upper layer of the core 71 is different between the outer side and the inner side of the curve. In the optical waveguide 70, the equivalent refractive index in the thickness direction on the outside of the curve is lower than the equivalent refractive index in the thickness direction on the inside, so that radiation loss (bending loss) is suppressed.

Japanese Patent Laid-Open No. 2-110405 Japanese Patent Application Laid-Open No. 5-288934

  Here, in an ultra-high-Δ optical waveguide such as Si photonics, polarization conversion in which the polarization of propagating light changes occurs at a bent portion where the mode shape in the optical waveguide is asymmetric. The polarization conversion itself is determined depending on the birefringence determined according to the optical waveguide structure and the symmetry of the optical mode. In general, when birefringence is small, polarization conversion is likely to occur. Even if the symmetry of the optical mode in the optical waveguide is lost, polarization conversion occurs. In Si photonics, since the refractive index difference is basically large, birefringence is large, and polarization conversion due to this is unlikely to occur. On the other hand, from the viewpoint of the symmetry of the optical mode, since the optical structure of the Si rib type optical waveguide is vertically asymmetric, the symmetry of the optical mode is easily broken.

  FIG. 18 shows the behavior of the optical mode in the rib-type optical waveguide. FIG. 18A schematically shows the optical mode shape in the optical waveguide in the straight portion (straight waveguide region), and FIG. 18B shows the optical mode shape in the optical waveguide in the curved portion (bent waveguide region). This is shown schematically. In FIGS. 2A and 2B, the optical mode shape of light propagating through the core 23 and the protrusion 23a is indicated by a broken line.

  As shown in FIG. 18A, polarization conversion does not occur in the straight waveguide region because the left and right optical modes are symmetrical in the rib-type optical waveguide (core 23). However, in the curved waveguide region of the rib-type optical waveguide, as shown in FIG. 18 (b), the light is moved toward the side wall of the optical waveguide, and a part of the optical mode penetrates the slab region of the core 23 from the protrusion 23a. put out. At this time, not only the vertical symmetry, but also the left-right symmetry is lost, so that polarization conversion occurs.

  When polarization conversion occurs in the optical waveguide, the following problem occurs in an optical device including the optical waveguide. For example, a 90-degree hybrid mixer using a silicon optical waveguide is considered as an optical device including the optical waveguide. The 90-degree hybrid mixer causes signal light and local light to interfere with each other. In such a 90 degree hybrid mixer, polarization conversion is very problematic. That is, in the 90-degree hybrid mixer, when polarization conversion occurs during wave guiding, the polarization state in each path changes. If the polarization state is different in each path, the converted light intensity cannot interfere, and the interference amplitude at the output end decreases. Since the deterioration of the interference amplitude leads to the deterioration of the SN ratio, the transmission characteristics are deteriorated. Therefore, it is desirable that polarization conversion is suppressed as much as possible in the bent waveguide region of the optical waveguide.

  Among the silicon optical waveguides, in the channel-type optical waveguide (see FIG. 14), the cross-sectional shape of the core 23 is symmetrical vertically and horizontally, and the symmetry of the optical mode is small even in the curved waveguide region. Is unlikely to occur. From the viewpoint of suppressing polarization conversion, it can be said that the channel-type optical waveguide is more advantageous than the rib-type optical waveguide. However, since there is a problem of loss as described above, there is a demand for adopting a rib-type optical waveguide. Patent Documents 1 and 2 describe suppression of radiation loss in the curved portion, but do not describe suppression of polarization conversion. Therefore, an optical waveguide structure capable of suppressing polarization conversion is desired.

  In view of the above problems, an object of the present invention is to provide an optical waveguide structure, an optical waveguide, and an optical integrated circuit capable of suppressing polarization conversion in a curved waveguide region of a rib-type optical waveguide.

  In order to achieve the above object, the present invention provides an optical waveguide structure having a core that propagates light in a curved light propagation direction, and a clad having a refractive index lower than that of the core, wherein the core includes a slab portion. And a rib portion protruding from the slab portion, wherein the slab portion is a first slab portion outside the light propagation direction with respect to the rib portion, and a second slab portion inside the light propagation direction. An optical waveguide structure including a slab portion, wherein the thickness of the first slab portion is thinner than the thickness of the second slab portion.

  The present invention also includes a core for propagating light and a clad having a refractive index lower than that of the core, and the core is disposed on both sides of the rib portion and a direction perpendicular to the light propagation direction with respect to the rib portion. The rib portion is an optical waveguide protruding from the slab portion, and is a straight portion that propagates light in a straight line, and a direction perpendicular to the light propagation direction of the rib portion. The slab portions disposed on both sides of the rib portions are straight portions having the same thickness and curved portions that propagate light along the curved direction, and are disposed on both sides of the rib portion in a direction perpendicular to the light propagation direction. An optical waveguide having a curved portion in which the thickness of the slab portion outside the curve is thinner than the thickness of the slab portion inside the curve is provided.

  The present invention has a core for propagating light and a clad having a refractive index lower than that of the core on the substrate, and the core has a rib portion and a direction perpendicular to the light propagation direction with respect to the rib portion. The rib portion is an optical integrated circuit in which an optical waveguide protruding from the slab portion is formed, and the optical waveguide has one or more curved portions, Provided is an optical integrated circuit in which at least one of the one or more curved portions has a slab portion disposed outside the curved portion, the thickness of which is smaller than the thickness of the slab portion disposed inside the curved portion.

  The optical waveguide structure, optical waveguide, and optical integrated circuit of the present invention can suppress polarization conversion in the curved waveguide region of the rib-type optical waveguide.

Sectional drawing which shows the optical waveguide structure in the counterclockwise waveguide area | region which concerns on one Embodiment of this invention. Sectional drawing which shows the optical waveguide structure in the clockwise waveguide area | region which concerns on one Embodiment of this invention. (A) is a figure which shows typically the optical mode shape in the optical waveguide in a linear waveguide area | region, (b) is a figure which shows typically the optical mode shape in the optical waveguide in a curved waveguide area | region. The graph which shows the relationship between the bending amount of an optical waveguide, and a polarization conversion rate. Sectional drawing which shows the optical waveguide parameter of an asymmetrical optical waveguide. The graph which shows the relationship between the bending amount of an optical waveguide, and a polarization conversion suppression rate. The graph which shows the relationship between the minimum value of polarization conversion suppression rate, and the amount of overetching ROH. The top view which shows the connection part provided between a linear part and a bending part. The top view which shows the connection of the linear part and bending part via the connection part in an optical waveguide. The graph which shows the relationship between half taper length TL and polarization conversion suppression rate. The graph which shows the relationship between TW / TL and a polarization conversion suppression rate. The graph which expands and shows a part of graph shown by FIG. The graph which shows the relationship between TW / TL and the excess loss of a connection part. Sectional drawing which shows the structural example of the channel-type optical waveguide using Si photonics. Sectional drawing which shows the structural example of the rib-type optical waveguide using Si photonics. The perspective view which shows the optical waveguide described in patent document 1. FIG. The perspective view which shows the optical waveguide described in patent document 2. FIG. (A) And (b) is a figure which shows typically the behavior of the optical mode in a rib type | mold optical waveguide.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 1 and 2 each show an optical waveguide structure according to an embodiment of the present invention. The optical waveguide 10 includes a substrate 11, a lower layer cladding 12, a core 13, and an upper layer cladding 14, similarly to a general rib-type optical waveguide (see FIG. 15). Note that the optical waveguide having the structure shown in FIGS. 1 and 2 is used in a curved portion where the light propagation direction is curved in an optical integrated circuit. The optical waveguide structure in the straight line portion that propagates light in the linear direction may be the same as the structure of a general rib-type optical waveguide shown in FIG.

The substrate 11 is, for example, a silicon substrate, and a lower layer cladding (first cladding) 12 is provided on the upper layer of the silicon substrate. The lower cladding 12 is, for example, a SiO ₂ film, and is formed of, for example, a buried oxide film (BOX: Buried Oxide). The core 13 is provided on the upper layer side of the lower cladding 12. The core 13 is made of, for example, a Si film.

An upper clad (second clad) 14 is provided on the upper layer of the core 13 so as to cover the core 13. The upper layer cladding 14 is, for example, a SiO ₂ film. Instead of using the SiO ₂ film as the upper clad 14, an air layer may be used as the upper clad. The refractive index of the core 13 is higher than that of the lower clad 12 and the upper clad 14, and light incident on the core 13 is confined in the core 13.

  Here, FIG. 1 shows an optical waveguide structure in a curved portion in which the light propagation direction curves to the left side toward the paper surface, and FIG. 2 shows an optical waveguide structure in a curved portion in which the light propagation direction curves to the right side in the paper surface. Is shown. In the following description, the inside of the curved light propagation direction refers to the side on which the center of the circle is placed when the curved light propagation direction is approximated by a circle and the degree of curvature is represented by a radius of curvature. Further, the outside of the curved light propagation direction refers to the side opposite to the side where the center of the circle is placed when the curved light propagation direction is approximated by a circle and the degree of curvature is represented by a radius of curvature.

  The core 13 has a protruding portion (rib portion) 13a and slab portions 13b and 13c. The protrusion 13a protrudes from the slab portions 13b and 13c to the upper clad 14 side. The protrusion 13a is formed along the light propagation direction. The slab part (first slab part) 13c is located outside the light propagation direction curved with respect to the projection part 13a, and the slab part (second slab part) 13b is curved with respect to the projection part 13a. It is located outside the light propagation direction.

  As shown in FIGS. 1 and 2, in the present embodiment, the thickness of the first slab portion 13 c outside the light propagation direction curved with respect to the protrusion 13 a is the second thickness inside the light propagation direction. It is thinner than the thickness of the slab part 13b. In other words, the protruding amount of the protruding portion 13a with respect to the first slab portion 13c is larger than the protruding amount of the protruding portion 13a with respect to the second slab portion 13b. Alternatively, the height from the upper surface of the first slab portion 13c to the outer end of the curved light propagation direction of the protrusion 13a is the height of the curved light propagation direction of the protrusion 13a from the upper surface of the second slab portion 13b. It is higher than the height to the inner edge. The first slab part 13c and the second slab part 13b are formed, for example, by etching a flat silicon film constituting the core 13 with different etching amounts on both sides of the region to be the protrusion 13a.

  FIG. 3A schematically shows the optical mode shape in the optical waveguide in the straight portion (straight waveguide region), and FIG. 3B shows the optical mode shape in the optical waveguide in the curved portion (bent waveguide region). This is shown schematically. As described above, in the linear waveguide region, the optical waveguide structure is the same as the general rib-type optical waveguide shown in FIG. 15, and the core 13 is formed symmetrically. As shown in FIG. 3A, in the linear waveguide region, the left-right symmetry of the optical mode shape is obtained in the rib-type optical waveguide (core 13), and polarization conversion hardly occurs.

  In the present embodiment, an optical waveguide structure in which the outer slab region is thinner than the inner side in the curved waveguide region of the rib-type optical waveguide is employed (hereinafter, an optical waveguide having such a structure is also referred to as an asymmetric optical waveguide). Call). By introducing an asymmetric structure in the rib-type optical waveguide, as shown in FIG. 3B, light confinement in the optical waveguide can be strengthened in the bent waveguide region, and light oozes out into the slab region. Can be suppressed. By suppressing the bleeding of the optical mode, the symmetry of the optical mode can be improved in the bent waveguide region. As a result, polarization conversion can be suppressed in the bent waveguide region as compared with a general rib type optical waveguide structure.

  In Patent Document 2, the outer side cladding 75 of the upper cladding 72 formed on the upper portion of the core 71 (see FIG. 17) is formed thinner than the inner side cladding 76 in the bent portion of the optical waveguide. On the other hand, in the present embodiment, the first slab portion 13c on the outer side in the bending direction of the core 13 is formed thinner than the inner second slab portion 13b. Thus, the configuration of the optical waveguide differs between the optical waveguide 10 according to the present embodiment and the optical waveguide 70 described in Patent Document 2.

  Here, polarization conversion changes depending on how much light passes through the waveguide region in the optical waveguide. Hereinafter, the polarization conversion efficiency with respect to the bending amount of the optical waveguide will be described. The polarization conversion efficiency is defined by a ratio indicating how much incident light is converted into TM mode (Transverse Magnetic mode) when TE mode (Transverse Electro mode) light is incident on the optical waveguide. Specifically, the polarization conversion rate is defined by the ratio (TM / TE) between the TM mode and the TE mode. When the polarization conversion does not occur, the polarization conversion rate is 0, and when all the light is converted to the TM mode, the polarization conversion rate is infinite.

  FIG. 4 shows the relationship between the bending amount of the optical waveguide and the polarization conversion rate. In the graph shown in FIG. 4, the horizontal axis represents the amount of bending (bending angle) in the optical waveguide, and the vertical axis represents the polarization conversion rate. Here, it is assumed that light is circulating around an optical waveguide having a constant bending radius (curvature radius) in a pseudo manner. With respect to two optical waveguides having different design parameters, the ratio of the TE mode to the TM mode at each bending angle was obtained with reference to the position where the TE mode light was incident (bending angle 0). ) And (b) were obtained.

  In the polarization conversion in the optical waveguide, the amount of transition from one mode to the other mode is determined according to the bending angle of the optical waveguide. Also, the amount of mode change varies periodically. That is, the light that has once shifted from the TE mode to the TM mode at a certain bending angle is converted to the TE mode again while proceeding through the curved waveguide region, and this is repeated. In the example of the graph (a) shown in FIG. 4, the polarization conversion rate is periodically converted with a period of about 180 degrees in the bending angle, and in the example of the graph (b), the polarization conversion rate with a period of about 300 degrees in the bending angle. Changes periodically.

  For example, in the graph (a), after reaching the peak of the polarization conversion rate near the bending angle of 90 degrees, the polarization conversion ratio becomes 0 at the bending angle of 180 degrees. This indicates that the light converted from the TE mode to the TM mode by the bending angle of 90 degrees returns to the original TE mode in the vicinity of 180 degrees. In this case, if light is extracted near a bending angle of 180 degrees, it appears as if polarization conversion does not occur in the optical waveguide. However, in reality, polarization conversion occurs at an intermediate bending angle.

  For example, consider an optical waveguide that guides light to an optical coupler that interferes with light in a 90-degree hybrid mixer. If the polarization state of the output light at the point where light is output from the optical waveguide to the optical coupler is the same as that upon incidence on the optical waveguide, it can be said to be synonymous with an optical waveguide in which no polarization conversion occurs. However, actually, when polarization conversion occurs in the middle of the optical waveguide, a phase difference is caused due to birefringence in the optical waveguide, and a delay occurs in the same optical waveguide. This is synonymous with the occurrence of a phase error. Therefore, it is desired that polarization conversion is suppressed at any bending angle. That is, in the graph shown in FIG. 4, it is desired to suppress the peak value (worst value) of the polarization conversion rate.

  The present inventor uses simulations to determine the influence of the difference between the thickness of the first slab portion 13c and the thickness of the second slab portion 13b on polarization conversion in the asymmetric optical waveguide shown in FIGS. Examined. FIG. 5 shows optical waveguide parameters of the asymmetric optical waveguide. In the simulation, the height SiH from the lower clad 12 to the protrusion 13a is 1.5 μm, the width (rib width) RibW in the horizontal direction of the protrusion 13a is 2.0 μm, and the protrusion 13a is the second slab portion. The amount protruding from 13b (rib height) RibH was set to 0.85 μm. The bend radius of the asymmetric optical waveguide was 300 μm. The inventor defines the difference between the thickness of the second slab portion 13b and the thickness of the first slab portion 13c as the overetching amount ROH, and changes the full vector for the asymmetric optical waveguide while changing the overetching amount ROH. Using a three-dimensional beam propagation method, we calculated how much polarization conversion occurred.

  FIG. 6 shows the simulation results. In the graph shown in FIG. 6, the horizontal axis represents the bending angle in the optical waveguide, and the vertical axis represents the degree of polarization conversion (index). For example, a polarization conversion suppression rate is used as an index of polarization conversion. The polarization conversion inhibition rate is defined by −10 log (TM / TE). The polarization conversion suppression rate corresponds to the reciprocal of the polarization conversion efficiency (see FIG. 5). In FIG. 6, graph (a) shows the polarization conversion inhibition rate when ROH is 0 μm, and graph (b) shows the polarization conversion inhibition rate when ROH is 0.1 μm. Graph (c) shows the polarization conversion inhibition rate when ROH is 0.15 μm, graph (d) shows the polarization conversion inhibition rate when ROH is 0.2 μm, and graph (e) shows that ROH is 0. The graph shows the polarization conversion inhibition rate when ROH is 0.3 μm, and the graph (g) shows the polarization conversion inhibition rate when ROH is 0.35 μm. The graph (h) shows the polarization conversion inhibition rate when the ROH is 0.4 μm.

  Referring to graphs (a) to (h), the polarization conversion suppression rate decreases as the bending angle increases, and after the polarization conversion suppression rate reaches a minimum value (worst value) at a certain bending angle, You can see how the conversion inhibition rate increases. The minimum value of the polarization conversion suppression rate changes depending on the overetching amount ROH. Specifically, the polarization conversion suppression rate is the lowest when the overetching amount ROH is 0 μm (graph (a)), and the polarization conversion suppression rate is when the overetching amount ROH is 0.2 μm (graph (d)). It is the highest.

  FIG. 7 shows the relationship between the minimum value of the polarization conversion suppression rate and the overetching amount ROH. In the graph shown in FIG. 7, the horizontal axis represents the overetching amount ROH, and the vertical axis represents the polarization conversion suppression rate. When the polarization conversion inhibition rate at each bending angle was calculated while changing the overetching amount ROH, and the minimum value of the polarization conversion inhibition rate was plotted, the graph shown in FIG. 7 was obtained. Referring to FIG. 7, when the overetching amount ROH is 0 μm, that is, when the optical waveguide has a symmetric structure, polarization conversion occurs most, and the thickness of the outer slab part is slightly reduced to suppress polarization conversion. I understand that I can do it. In the example of FIG. 7, the polarization conversion suppression efficiency takes the maximum value (best value) when the overetching amount ROH is 0.22 μm. At this time, it can be seen that the polarization conversion suppression rate can be improved by 27 dB or more compared to the case where the overetching amount ROH is set to 0 μm.

  Here, the optical waveguide structure according to the present embodiment is applied to, for example, an optical integrated circuit. The optical integrated circuit includes, for example, an interference system device including a Mach-Zehnder interferometer and / or a variable optical attenuator (VOA) as its constituent elements. May be a 90-degree optical hybrid mixer, an optical switch, etc. An optical waveguide formed in an optical integrated circuit generally includes a straight portion and a bent portion (curved portion).

  As described above, the optical waveguide structure in the straight portion is the same as the structure of the general rib-type optical waveguide shown in FIG. In the present embodiment, the asymmetric optical waveguide is used in the bent waveguide region of the optical waveguide. When the asymmetric optical waveguide is applied to the linear portion, polarization conversion occurs in the linear portion because the asymmetry is stronger than that of a normal rib-type optical waveguide having a symmetric structure. Therefore, it is desirable to use a normal rib optical waveguide having a symmetric structure for the straight portion.

  In the present embodiment, in the rib-type optical waveguide used for the straight portion, the thicknesses of the slab portions arranged on both sides in the direction orthogonal to the light propagation direction of the protrusions are equal to each other. On the other hand, in the asymmetrical optical waveguide (asymmetrical rib-type optical waveguide) used for the bent portion, the slab portion (the first slab portion) on the outer side of the curve among the slab portions arranged on both sides in the direction orthogonal to the light propagation direction of the protrusion 13a The thickness of the first slab portion 13c) is thinner than the thickness of the slab portion (second slab portion 13b) inside the curve. In this case, if an optical waveguide having a symmetric structure and an asymmetric optical waveguide are directly connected, a mode mismatch may occur, resulting in excessive loss.

  In order to suppress the occurrence of the excess loss, the optical integrated circuit according to the present embodiment preferably has a connection portion between the straight portion and the bent portion of the optical waveguide. FIG. 8 shows a connecting portion provided between the straight portion and the bent portion. The connecting portion shown in FIG. 8 is provided between a curved portion and a straight portion that are curved to the right as viewed in the drawing. In FIG. 8, position B indicates the boundary between the straight line portion and the connecting portion, and position D indicates the boundary between the bent portion and the connecting portion. The length of the connection portion in the light traveling direction is assumed to be TL.

  In the optical waveguide, in the straight portion (position A), the thicknesses of the slab portions on both sides of the protrusion 13a are equal to each other, and the thickness of the slab portion in the straight portion is an asymmetric optical waveguide (see FIG. 1 or FIG. 2). Is equal to the thickness of the second slab portion 13b. Hereinafter, for convenience, the slab portion in the straight portion and the second slab portion 13b are regarded as the same.

  Further, the straight portion of the optical waveguide is not limited to a portion where the optical waveguide is formed in a completely linear shape, and includes a portion that extends substantially linearly. For example, when moving 1 mm in a certain direction, a displacement of about 10 μm in a direction orthogonal thereto can be regarded as a straight line. Further, when the radius of curvature (its absolute value) is sufficiently large, for example, when the radius of curvature is about 1 cm or 10 cm, it can be regarded as a straight line. Furthermore, when it has a sufficiently large radius of curvature with respect to the radius of curvature of the bent portion, for example, when it has a radius of curvature of about 30 times the radius of curvature of the bent portion, it can be regarded as a straight line.

  In the connecting portion, among the slab portions arranged on both sides in the direction orthogonal to the light propagation direction of the protruding portion 13a, the thickness of the slab portion on the side corresponding to the outside of the curved portion is a predetermined distance from the protruding portion 13a. It is equal to the thickness of the slab part 13b in the straight part up to a position far away. Moreover, the thickness of the slab part corresponding to the outer side is thinner than the thickness of the slab part 13b at a position farther from the position away from the protrusion 13a by a predetermined distance. The thickness of the slab part at a position farther from the position away from the protrusion 13a by a predetermined distance is equal to the thickness of the first slab part 13c in the asymmetric optical waveguide, for example. Hereinafter, for the sake of convenience, the slab part formed up to a position away from the protrusion 13a by a predetermined distance is regarded as the same as the second slab part 13b, and the slab part far from the predetermined distance is the first. The slab portion 13c is regarded as the same.

  Specifically, at the position B that is the boundary between the connecting portion and the straight portion, the second slab portion 13b is formed to a position that is a predetermined distance TW away from the end of the protruding portion 13a, and a position farther than that. Then, the 1st slab part 13c is formed. The thickness of the second slab portion 13b is, for example, 0.85 μm, and the thickness of the first slab portion 13c is 0.22 μm thinner than the thickness of the second slab portion 13b. That is, the thickness of the first slab portion 13c is 0.63 μm.

  In the connecting portion, the boundary between the first slab portion 13c and the second slab portion 13b approaches the protruding portion 13a from the connecting portion with the straight portion to the connecting portion with the bent portion. In other words, in the connecting portion, the maximum value of the predetermined distance is TW, and the predetermined distance decreases from the connecting portion with the straight portion to the connecting portion with the bent portion. For example, in the position C shown in FIG. 8, the length of the second slab portion 13b is shorter than that of the position B, and the first slab portion 13c is approaching the protruding portion 13a. At the position D that is the boundary between the connecting portion and the bent portion, the predetermined distance is 0, and the first slab portion 13c is formed adjacent to the protruding portion 13a.

  In the connecting portion, the second slab portion 13b is formed in a single taper shape only on the side corresponding to the outside of the bent portion. In other words, in the connecting portion, the second slab portion 13b has an asymmetrical taper structure (inclined structure) with the protrusion 13a as the center. A first slab portion 13c that is further dug down by an overetching amount ROH is formed in a region outside the asymmetrical single taper structure. In the connecting portion, the width of the second slab portion 13b formed in a one-taper shape is gradually narrowed from the straight portion to the bent portion, thereby converting the optical waveguide having a symmetric structure into an asymmetric optical waveguide. The By using such a connection portion, it is possible to avoid a sudden structural change from a linear rib-type optical waveguide having a symmetric structure to an asymmetric optical waveguide having a bent portion.

  FIG. 9 shows the connection between the straight portion and the bent portion via the connecting portion. In this example, the bent portion of the optical waveguide bends the light traveling direction by 90 degrees between the vertical direction and the horizontal direction on the paper surface. For example, light traveling in a symmetrical optical waveguide (straight line portion) having second slab portions 13b on both sides of the protrusion 13a from the lower side to the upper side of the paper surface is only one taper outward in the bending direction. Is incident on a connecting portion (first connecting portion) in which a second slab portion 13b is formed. As the optical waveguide heads toward the bent portion, the asymmetry is gradually strengthened in the one-tapered structure of the first connecting portion, and the first is formed on the outer side in the bending direction at the boundary between the first connecting portion and the bent portion. It is converted into an asymmetric optical waveguide having a slab portion 13c.

  The light incident on the bent portion is bent 90 degrees in the traveling direction at the bent portion. In the bent portion, the light symmetries are maintained because the thickness of the first slab portion 13c on the outer side in the bending direction of the protruding portion 13a is smaller than the thickness of the inner second slab portion 13b (FIG. 3B). ), And polarization conversion in the bent portion is suppressed. The light whose traveling direction is bent by 90 degrees is a straight line extending in the left-right direction toward the paper surface through the connection portion in which the second slab portion 13b is formed in a single taper shape (reverse taper shape) only outside the bending direction. Incident on the part. As the optical waveguide is directed from the bent portion toward the straight portion, the asymmetry is gradually weakened in the reverse piece taper structure of the second connection portion, and the protrusion 13a is formed at the boundary between the second connection portion and the straight portion. Is converted into an optical waveguide having a symmetric structure having second slab portions 13b on both sides.

  It should be noted that the slab portion outside the bending direction in the bent waveguide region only needs to be thinly formed to a certain distance from the protrusion 13a, and after that distance, it has the same thickness as the slab portion in the straight portion. It may be. For example, in the bent waveguide region, the first slab portion 13c only needs to be formed to a position about 1 μm to 2 μm away from the end of the protrusion 13a, and the second slab portion 13b is located farther than that. It may be formed. The same applies to the connecting portion.

  In general, in an optical integrated circuit such as a 90-degree hybrid mixer, a large number of linear optical waveguides and curved optical waveguides are mixed. In the case where the connecting portion is provided in all of the connecting portions of the straight optical waveguide and the bent optical waveguide, a large number of connecting portions are provided, so that the excess loss in the connecting portion alone and the polarization conversion suppression rate are very important. Become. The excess loss and the polarization conversion suppression rate at the connecting portion are the length TL of the connecting portion (the second slab portion 13b formed in a single taper shape) and the piece of the second slab portion 13b formed in a single taper shape. It is considered that it changes depending on the taper width (TW in FIG. 8). The inventor conducted a simulation and examined the influence of the length (one taper length) TL and the one taper width TW of the connection portion on the polarization conversion suppression rate and excess loss.

  FIG. 10 shows the relationship between the single taper length TL and the polarization conversion suppression rate. In the graph shown in FIG. 10, the horizontal axis represents the single taper length TL, and the vertical axis represents the polarization conversion suppression rate. In the simulation, polarization conversion suppression when each taper length is changed while changing the single taper length TL for each of the case where the single taper width TW is 0.5 μm, 1.0 μm, and 2 μm. When the rate was determined, the result shown in FIG. 10 was obtained.

  Referring to FIG. 10, it can be seen that the polarization conversion suppression rate gradually decreases as the one taper length TL becomes longer. The reason why the polarization conversion suppression rate decreases is that although the optical waveguide is smoothly changed from a symmetric structure to an asymmetric structure at the connection portion, the connection portion itself has an asymmetric structure, so that the light propagation distance becomes long. This is considered to be because polarization conversion occurs in the connection portion. On the other hand, when the single taper width TW is changed, when compared with the same single taper length TL, the polarization conversion suppression rate improves as the single taper width TW increases. From these facts, it is considered that the polarization conversion suppression rate can be increased by shortening the length of the connecting portion having an asymmetric structure as much as possible and making it a steep single taper structure.

  The gradient in the length direction of the connecting portion of the second slab portion 13b formed in a single taper shape at the connecting portion is given by TW / TL. FIG. 11 shows the result of normalizing the polarization conversion inhibition rate by TW / TL. In the graph shown in FIG. 11, the horizontal axis represents TW / TL, and the vertical axis represents the polarization conversion suppression rate. Referring to FIG. 11, it can be seen that the polarization conversion suppression rate increases as the value of TW / TL increases, that is, as the gradient of the one-taper structure increases. Therefore, from the viewpoint of suppressing polarization conversion, it is preferable that the value of TW / TL is large.

FIG. 12 is an enlarged view of a part of the graph shown in FIG. When TW / TL is in the range of 0.005 or less and TW / TL is x and the polarization conversion suppression rate is y, an approximate expression representing the relationship between x and y is obtained. Y = 3.548 × 10 ⁸ x ⁵ -1.130 An approximate expression of × 10 ⁸ x ⁴ + 7.912 × 10 ⁶ x ³ -2.318 × 10 ⁵ x ² + 3.630 × 10 ³ x + 2.225 × 10 ¹ was obtained. Considering 30 dB as one index of the polarization conversion suppression rate, in order to obtain a polarization conversion suppression rate of 30 dB or more, TW / TL needs to be 0.0025 or more.

  FIG. 13 shows the relationship between TW / TL and excess loss at the connection. In the graph shown in FIG. 13, the horizontal axis represents TW / TL, and the vertical axis represents the excess loss of the connection portion. Referring to FIG. 13, it can be seen that the excess loss of the connection itself is lower as TW / TL is smaller. Therefore, from the viewpoint of excess loss, the value of TW / TL is preferably small.

  Here, the excess loss of the connection part itself is lower than 0.01 dB, and the loss generated in each connection part is very low. However, in an optical integrated circuit, there may be 100 or more portions where the straight portion and the bent portion are connected. When the number of connection portions increases, even if the excess loss per location is low, the excess loss of the connection portions cannot be ignored when viewed as a whole.

  As described above, the value of TW / TL is preferably large from the viewpoint of suppressing polarization conversion, and is preferably small from the viewpoint of excess loss. If the value of TW / TL is small, excess loss can be reduced, but the effect of suppressing polarization conversion is diminished. Conversely, if the value of TW / TL is large, polarization conversion can be suppressed, but excess loss increases. In the optical integrated circuit, it is preferable to select the single taper length TL and the single taper width TW in the connection portion in accordance with the balance between the excess loss and the polarization conversion suppression rate.

  In the present embodiment, in the curved waveguide region of the rib-type optical waveguide, the thickness of the first slab portion 13c on the outer side in the bending direction of the protruding portion 13a is made thinner than the thickness of the inner second slab portion 13b. In this way, by introducing an asymmetric structure in the bent waveguide region, light confinement in the optical waveguide can be strengthened in the bent waveguide region, and the leakage of light into the slab region can be suppressed. . As a result, the symmetry of the optical mode can be improved in the curved waveguide region, and polarization conversion can be suppressed as compared with a general rib-type optical waveguide structure.

  In the present embodiment, the second slab portion 13b is formed in a one-taper shape only at the side corresponding to the outside of the bent portion at the connection portion between the optical waveguide having a symmetric structure and the asymmetric optical waveguide. Is provided. In the connection portion, the width of the second slab portion 13b gradually narrows from the straight portion toward the bent portion only on the side corresponding to the outside in the bending direction. By using such a connecting portion, the structure can be gently converted from a symmetric structure in the straight portion to an asymmetric structure in the bent portion, or from an asymmetric structure in the bent portion to a symmetric structure in the straight portion. As a result, it is possible to suppress mode mismatch and suppress occurrence of excess loss due to the mode mismatch.

  In the above embodiment, an example in which the optical waveguide is a silicon waveguide has been described. However, the present invention is not limited to this. The optical waveguide may be a semiconductor waveguide such as an InP compound. As described above, in the above embodiment, polarization conversion can be suppressed by making the outer slab portion slightly thinner than the inner slab portion in the curved waveguide region. The numerical values such as the Si film thickness, the rib width, and the rib height described above are examples, and are not particularly limited to the above-described values.

  In the above-described embodiment, the example in which the width of the slab portion corresponding to the outside in the bending direction is reduced linearly from the straight portion toward the bending portion in the connection portion is not limited thereto. The width | variety of the slab part formed in the outer side of a bending direction in a connection part should just be decreased monotonously toward a bending part from a linear part. For example, the width of the slab portion formed on the outer side in the bending direction at the connecting portion may be reduced according to a quadratic function or a higher order function from the straight line portion toward the bending portion.

  In the above-described embodiment, the example in which the connection portion is provided between the straight portion and the bent portion in the optical integrated circuit has been described. However, the present invention is not limited to this. It is good also as connecting a straight part and a bending part directly, without providing a connection part. Alternatively, in the case where the connection portion is provided, the connection portion does not need to be provided between all the bent portions of the optical waveguide in the optical integrated circuit and the straight portion, and without the connection portion, There may be a location where the bent portion is connected.

  As mentioned above, although embodiment of this invention was described in detail, this invention is not limited to above-described embodiment, A change and correction are added with respect to the said embodiment in the range which does not deviate from the meaning of this invention. Also included in the present invention.

(Appendix)
In the optical waveguide structure of the present invention, the first slab portion and the second slab portion of the core may be formed in a flat plate shape, and the rib portion may have a rectangular cross-sectional shape. Here, the “flat plate shape” means, for example, that the cross-sectional shape of the core has a flat aspect ratio (the length (height) in the protruding direction of the rib portion is greater than the length (width) in the direction perpendicular to the protruding direction) Is also short).

10: Optical waveguide 11: Substrate 12: Lower clad (first clad)
12: Lower clad 13: Core 13a: Projection (rib)
13b: Slab part (second slab part)
13c: Slab part (first slab part)
14: Upper clad (second clad)
21: Substrate 22: Clad 22: Lower layer clad 23: Core 23a: Projection (rib)
23b: Slab part 23c: Groove part 23d: Tapered part 24: Upper layer clad 50: Channel type optical waveguide 51: Rib type optical waveguide 60, 70: Optical waveguide 71: Core 72: Upper clad 73: Lower clad 74: Substrate 75: Outer side cladding 76: inner side cladding

Claims (13)

An optical waveguide structure having a core that propagates light in a curved light propagation direction, and a clad having a lower refractive index than the core,
The core includes a slab part and a rib part protruding from the slab part,
The slab part includes a first slab part located outside the light propagation direction with respect to the rib part, and a second slab part located inside the light propagation direction,
An optical waveguide structure in which the thickness of the first slab part is thinner than the thickness of the second slab part.   The optical waveguide structure according to claim 1, wherein a protruding amount of the rib portion with respect to the first slab portion is larger than a protruding amount of the rib portion with respect to the second slab portion.   The height from the upper surface of the first slab portion to the outer end of the rib portion in the light propagation direction is from the upper surface of the second slab portion to the inner side of the rib portion in the light propagation direction. The optical waveguide structure according to claim 1, wherein the optical waveguide structure is higher than a height to the end.   The optical waveguide structure according to any one of claims 1 to 3, wherein the core is made of silicon.   5. The optical waveguide structure according to claim 1, wherein the cladding includes a first cladding in which the rib portion is disposed on a side opposite to a side protruding from the slab portion.   6. The optical waveguide structure according to claim 5, wherein the clad further includes a second clad disposed on a side where the rib portion protrudes from the slab portion.   The optical waveguide structure according to claim 6, wherein the first clad and the second clad are made of silicon oxide. A core for propagating light and a clad having a refractive index lower than that of the core, and the core includes a rib portion and slab portions disposed on both sides of the rib portion in a direction perpendicular to the light propagation direction; And the rib portion is an optical waveguide protruding from the slab portion,
A straight line portion that propagates light in a straight line, and a straight line portion in which the thicknesses of the slab portions arranged on both sides in the direction perpendicular to the light propagation direction of the rib portion are equal to each other;
Among the slab portions that are disposed on both sides in the direction orthogonal to the light propagation direction of the rib portion, the thickness of the slab portion outside the curve is a curved portion that propagates light along the curved direction. An optical waveguide having a curved portion thinner than a thickness of an inner slab portion. Among the slab portions arranged on both sides in the direction orthogonal to the light propagation direction of the rib portion between the straight portion and the curved portion, the slab portion on the side corresponding to the outside of the curved portion in the curved portion is: It has a thickness equal to the thickness of the slab part in the linear part up to a position away from the rib part by a predetermined distance, and at a position farther than a position away from the rib part by a predetermined distance from the rib part. It further has a connection part having a thickness thinner than the thickness of the slab part up to a position separated by a predetermined distance,
The optical waveguide according to claim 8, wherein the predetermined distance decreases from the connection portion with the straight portion to the connection portion with the curved portion.   10. The optical waveguide according to claim 9, wherein the thickness of the slab portion on the side corresponding to the outer side of the bending is equal to the thickness of the outer slab portion of the bending portion in the connection portion of the connection portion with the bending portion.   The thickness of the slab part arranged on both sides of the straight part in the direction orthogonal to the light propagation direction of the rib part, the thickness of the slab part inside the curve in the curved part, and the inside of the curve in the connecting part The optical waveguide according to claim 9 or 10, wherein the thicknesses of the corresponding slab portions are equal to each other. The substrate has a core for propagating light and a clad having a refractive index lower than that of the core, and the core is disposed on both sides of the rib portion and the direction perpendicular to the light propagation direction with respect to the rib portion. Slab part, and the rib part is an optical integrated circuit in which an optical waveguide protruding from the slab part is formed,
The optical waveguide has one or more curved portions;
An optical integrated circuit in which at least one of the one or more curved portions has a slab portion disposed outside the curved portion, wherein the thickness of the slab portion is smaller than the thickness of the slab portion disposed inside the curved portion. The optical waveguide further includes a linear portion that propagates light linearly,
13. The optical integrated circuit according to claim 12, wherein in the straight portion, the thicknesses of the slab portions arranged on both sides in the direction orthogonal to the light propagation direction are equal to each other.

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