JP2016156928A - Spot size converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spot size converter with a simple structure.SOLUTION: The spot size converter comprises a first waveguide core 1 and a second waveguide core 2 and has a first configuration area, a second configuration area and a third configuration area which are sequentially set. Waveguide directions (z-axis directions) of the first waveguide core and the second waveguide core are arranged in parallel with each other. The first waveguide core 1 is formed in the first configuration area and the second waveguide core 2 is formed over the whole area including the first configuration area, the second configuration area and the third configuration area. The first waveguide core 1 includes a taper structure area 1-T in which the width of the core becomes narrower toward the second configuration area and a second waveguide core 2-2 included in the second configuration area is defined as a taper structure area 2-T in which the width of the core becomes narrower as separated from the first configuration area. The first waveguide core 1 and the second waveguide core 2 are sandwiched between a lower clad layer 4 and an upper clad layer 3.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a spot size converter that outputs an enlarged or reduced mode field diameter of input light.

  In recent years, attention has been focused on technology that realizes functional elements such as optical transceivers by hybrid integration of compound semiconductors such as GaAs or InP on SOI (Silicon on Insulator) substrates on which optical waveguides that function as optical wiring layers are formed. ing. In such a functional element, an optical waveguide for guiding an optical input signal to the functional element or an optical output signal from the functional element is coupled with an external optical waveguide (or optical element) such as an optical fiber. Spot size converters are required.

  As a spot size converter for coupling an optical waveguide formed on an SOI substrate with an external optical element such as a semiconductor laser, a spot made of a tapered silicon fine wire waveguide whose waveguide width is changed in a tapered shape in the waveguide direction It is preferred to use a size converter. The higher the coupling efficiency between the optical waveguide formed on the SOI substrate and the external element, the better.

  Several suitable spot size converters used for optically coupling the input / output light between the optical waveguide and the optical element formed on the SOI substrate with high efficiency are disclosed. For example, a spot size converter including a tapered silicon fine wire waveguide and a waveguide core (polymer core) that captures light that has oozed out from the tapered portion is disclosed (see Non-Patent Document 1). In addition, in order to reduce polarization dependence, the tapered silicon wire waveguide is configured not only in the width direction but also in the longitudinal direction (thickness direction of the waveguide), and the spot size can be reduced to 10 μm. In order to enlarge, a spot size converter using a waveguide core having a rib structure with a thickness of 10 μm is disclosed (see Non-Patent Document 2). Further, a structure has been proposed in which the waveguide core is tapered and the spot size is increased to about 10 μm with a rib-type waveguide core (see Patent Document 1).

T. Shoji, et al., “Low loss mode size converter from 0.3 μm square Si wire waveguides to single mode fibers”, Electronics Letters Volume 38, Issue 25 pp. 1669-1670 (2002). M. Tokushima, et al., “Dual-Tapered 10-μm-Spot-Size Converter with Double Core for Coupling Polarization-Independent Silicon Rib Waveguides to Single-Mode Optical Fibers”, Applied Physics Express 5, pp.022202-1〜 022202-3 (2012).

JP 2013-231753 A

  However, the spot size converter described in Non-Patent Document 1 described above requires an organic material called a polymer material in addition to an inorganic material such as a silicon material. Further, in the spot size converter using the double core described in Non-Patent Document 2, the spot size is determined by the cross-sectional dimension of the waveguide core. Therefore, the cross-sectional dimension of the waveguide core is about 10 μm square, which is a silicon thin wire conductor. It is larger than the waveguide and requires additional processes such as film formation and etching process corresponding to the step processing, and the manufacturing process becomes complicated. In the spot size converter using the triple core (first to third cores) described in Patent Document 1, the second core thickness of the tapered structure is about 3 μm, and the third core thickness of the rib type is about 4 μm. Since a relatively large level difference is generated, a process corresponding to level difference processing is also required. In these additional processes, other integrated elements are damaged or a high step shape is generated. Therefore, in the integration process with other functional elements, process consistency becomes a problem.

  The inventor of the present application includes a waveguide core formed of a polymer material provided in the spot size converter disclosed in Non-Patent Document 1 and a rib provided in the spot size converter disclosed in Patent Document 1. A spot size converter with a simpler structure that does not require a step corresponding to the spot size converter disclosed in Non-Patent Document 2 without using a component corresponding to the type of waveguide core. It was found by simulation analysis that this is realized. That is, an object of the present invention is to provide a spot size converter having a simpler structure than the spot size converter disclosed in the above-mentioned prior art document.

  According to the gist of the present invention, the following features are provided.

  The basic configuration of the spot size converter of the present invention includes a first waveguide core and a second waveguide core. Then, a first configuration region and a second configuration region are sequentially set in the waveguide direction of the first and second waveguide cores. The first waveguide core is formed in the first configuration region, and the second waveguide core is formed over the entire first configuration region and second configuration region. Further, the first waveguide core and the second waveguide core are arranged such that the symmetry center of the first waveguide core and the symmetry center of the second waveguide core overlap in the thickness direction of both the waveguide cores. The waveguide direction of the first waveguide core and the waveguide direction of the second waveguide core are arranged so as to be parallel to each other. In addition to the first and second configuration regions, a third configuration region may be further added. In this case, the second waveguide core is formed over the entire first configuration region, second configuration region, and third configuration region.

  The first waveguide core includes a tapered structure region in which the width of the core decreases toward the second configuration region, and a portion included in the second configuration region of the second waveguide core is separated from the first configuration region. Accordingly, a tapered structure region in which the width of the core is narrowed.

  The first configuration region includes a first waveguide core and a second waveguide core. In the first configuration region, propagating light propagating through the first waveguide core and propagating light propagating through the second waveguide core are mutually transmitted. Are coupled via the evanescent field. In the second configuration area, the mode field diameter of the input light is increased as the distance from the first configuration area increases. In the third configuration area, the propagation light output from the second configuration area is guided to the outside or input from the outside. Light is guided to the second component region.

  The spot size converter of the present invention can be configured by further adding a lower cladding layer and an upper cladding layer to the above basic configuration. In this case, the refractive index of the second waveguide core is smaller than the refractive index of the first waveguide core, and the refractive indexes of the lower cladding layer and the upper cladding layer are set smaller than the refractive index of the second waveguide core. The first component region is configured by laminating the lower cladding layer, the first waveguide core, the second waveguide core, and the upper cladding layer in this order, and the second component region is configured by the lower cladding layer and the second waveguide core. The third clad region is constructed by laminating the lower clad layer, the second waveguide core, and the upper clad layer in this order.

  In addition, the spot size converter of the present invention can be configured such that a first lower cladding layer, a second lower cladding layer, and an upper cladding layer are further added to the basic configuration described above. In this case, the refractive index of the second waveguide core is smaller than the refractive index of the first waveguide core, and the refractive indices of the first lower cladding layer, the second lower cladding layer, and the upper cladding layer are the same as those of the second waveguide core. Set smaller than the refractive index. The first component region is configured by laminating the first lower cladding layer, the first waveguide core, the second lower cladding layer, the second waveguide core, and the upper cladding layer in this order, and the second component region is The first lower clad layer, the second lower clad layer, the second waveguide core, and the upper clad layer are laminated in this order, and the third constituent region is composed of the first lower clad layer, the second lower clad layer, and the second waveguide. The core and the upper cladding layer are laminated in this order.

  The portion included in the third component region of the second waveguide core may be a uniform-width waveguide having a constant core width, or a segment formed by arranging periodically segmented cores in series. It can also be a waveguide.

  According to the spot size converter of the gist of the present invention, as shown in a simulation result to be described later, the optical loss can be reduced to the extent that there is no practical problem. In other words, when an optical signal is input from the outside to a functional element such as an optical transceiver formed on an SOI substrate, or an optical waveguide that guides an optical output signal from the functional element and an external optical waveguide such as an optical fiber are coupled. It is possible to provide a spot size converter that is suitable for use.

It is a figure which shows schematic structure of a 1st spot size converter, (A) is a schematic plan view which shows xz surface, (B) is cut | disconnected in the position shown with the dashed-dotted line (II) shown to (A) 2 is a schematic cross-sectional structure diagram showing a yz plane. It is a figure which shows schematic structure of the other form of a 1st spot size converter. It is a figure which shows schematic structure of a 2nd spot size converter, (A) is a top view which shows xz surface, (B) is xy plane in the position where a 1st waveguide core exists in a 1st structure area | region. (C) is a schematic cross-sectional structure diagram showing the yz plane cut at the position indicated by the dashed line (II-II) shown in (A), (D) is the first FIG. 3 is a schematic cross-sectional structure diagram showing an xy plane in three constituent regions. It is a figure which shows the mode of the beam spot of the TE wave component and TM wave component of the propagation light in each position shown by ad of a 1st spot size converter. It is a figure where it uses for description of the method of calculating | requiring mode mismatch loss. It is a figure where it uses for the thickness of the 2nd waveguide core of a 1st spot size converter as a parameter, and it uses for description of the relationship of the mode mismatch loss with respect to the width | variety of the 2nd waveguide core in a 1st structure area | region. The figure which uses for the thickness of the 2nd waveguide core of a 1st spot size converter as a parameter, and uses for the description about the influence which the length of the waveguide direction of the 2nd waveguide core of a 2nd structure area has on a mode mismatch loss It is. It is a figure which shows the mode of the beam spot of the TE wave component and TM wave component of the propagation light in each position shown by ad of the 1st spot size converter calculated | required in consideration of the radiation loss to a silicon substrate. It is a figure where it uses for description of the simulation result at the time of making the 2nd waveguide core of a 3rd structure area into a segment waveguide. It is a figure where it uses for description about the equivalent characteristic and reflection characteristic at the time of making the 2nd waveguide core of a 3rd structure area into a segment waveguide.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Each diagram showing a schematic configuration of the spot size converter of the present invention shows one configuration example according to the embodiment of the present invention, and the present invention is not limited to the illustrated example. In the following description, specific constituent materials, design conditions, and the like may be used. However, these constituent materials, design conditions, and the like are only suitable examples, and are not limited to these. Moreover, in each figure, the same component is shown with the same number, and the overlapping description may be omitted.

<First spot size converter>
With reference to FIGS. 1A and 1B, the configuration of the first spot size converter will be described. FIG. 1 is a diagram showing a schematic configuration of a first spot size converter. Here, for convenience of explanation, as shown in FIG. 1, the waveguide direction is defined as the z-axis direction, the width direction of the waveguide is defined as the x-axis direction, and the thickness direction of the waveguide is defined as the y-axis direction. (A) is a schematic plan view showing an xz plane, and (B) is a schematic cross-sectional structure diagram showing a yz plane cut at a position indicated by a dashed line (II) shown in (A).

  The first spot size converter is formed on a silicon substrate 10 and includes a first waveguide core 1 and a second waveguide core 2 (2-1, 2-2, 2-3). Here, as shown in FIG. 1, the first component region, the second component region, and the third component region are sequentially arranged in the waveguide direction (z-axis direction) of the first waveguide core 1 and the second waveguide core 2. Set.

  The first waveguide core 1 is formed in the first configuration region, and the second waveguide core 2 is formed throughout the first configuration region, the second configuration region, and the third configuration region. In FIG. 1, a portion included in the first configuration region of the second waveguide core 2 is indicated as a second waveguide core 2-1, and a portion included in the second configuration region is indicated as a second waveguide core 2-2. The portion included in the third configuration region is shown as a second waveguide core 2-3. The second waveguide core 2 has a flat upper surface (a surface in contact with the upper cladding layer 3 of the second waveguide core 2) over the first to third constituent regions.

  The first waveguide core 1 and the second waveguide core 2 are arranged such that the symmetry center of the first waveguide core 1 and the symmetry center of the second waveguide core 2 overlap in the thickness direction of both the waveguide cores. In addition, the waveguide direction of the first waveguide core 1 and the waveguide direction of the second waveguide core 2 are arranged so as to be parallel to each other.

  The first waveguide core 1 includes a tapered structure region 1-T in which the width of the core decreases toward the second configuration region. The second waveguide core 2-2 included in the second configuration region includes A taper structure region 2-T in which the width of the core is narrowed as the distance from the one component region is increased. The second waveguide core 2-3 included in the third configuration region is formed as a uniform waveguide having a constant waveguide width.

  The first component region includes a first waveguide core 1 and a second waveguide core 2-1. In the first configuration region, the propagation light propagating through the first waveguide core 1 and the propagation light propagating through the second waveguide core 2-1 are optically coupled via each other's evanescent field. The second component region includes the second waveguide core 2-2. In the second configuration region, the mode field diameter of the input light is enlarged by the second waveguide core 2-2 as it approaches the third configuration region. The third component region includes the second waveguide core 2-3. In the third configuration region, propagating light output from the second configuration region is guided to the outside by the second waveguide core 2-3, or light input from the outside is guided to the second configuration region.

  The first waveguide core 1 and the second waveguide core 2 (2-1, 2-2, 2-3) are sandwiched between the lower cladding layer 4 and the upper cladding layer 3. In such a configuration, the refractive index of the second waveguide core 2 is smaller than the refractive index of the first waveguide core 1, and the refractive indexes of the lower cladding layer 4 and the upper cladding layer 3 are the second waveguide core 2. Set to be smaller than the refractive index of. The first constituent region is formed by laminating the lower clad layer 4, the first waveguide core 1, the second waveguide core 2-1, and the upper clad layer 3 in this order. The clad layer 4, the second waveguide core 2-2, and the upper clad layer 3 are laminated in this order. The third constituent region is the lower clad layer 4, the second waveguide core 2-3, and the upper clad layer. The layers are stacked in the order of 3.

  The first waveguide core 1 is preferably a silicon core formed of a silicon material. The second waveguide core 2 (2-1, 2-2, 2-3), the lower cladding layer 4, and the upper cladding layer 3 may be formed of a silicon oxide (SiOx) material having a refractive index smaller than that of silicon. Each refractive index can be appropriately set by adjusting the value of x of the oxygen content (SiOx) of the silicon oxide material. This adjustment of the value of x is realized by adjusting parameters that determine the formation conditions of the silicon oxide layer in a chemical vapor deposition (CVD) apparatus used to configure these parts. The

  The shape and refractive index of the first waveguide core 1 and the second waveguide core 2 (2-1, 2-2, 2-3), and the thickness and refractive index of the lower cladding layer 4 and the upper cladding layer 3 are: It is determined from the mode field diameter of the light propagating through the optical fiber to be coupled with the first spot size converter or the output light of the external light source. That is, it is determined in consideration of the conditions for the enlargement / reduction ratio of the spot size and the allowable value such as the light amount loss generated in the spot size converter.

  In FIG. 1, when spot-size converted light is input using the left end of the first waveguide core 1 as an input port, the propagating light propagating through the first waveguide core 1 in the basic propagation mode gradually becomes the second waveguide core. It oozes out to 2-1 and is finally coupled to the fundamental propagation mode of the second waveguide core 2-1. That is, in the first configuration region, the propagating light propagating through the first waveguide core 1 and the propagating light propagating through the second waveguide core 2-1 are optically coupled via each other's evanescent field. The propagating light propagating through the second waveguide core 2-2 has an enlarged spot size (mode field diameter) while propagating through the tapered structure region 2-T included in the second constituent region.

  The second waveguide core 2 (2-1, 2-2, 2-3) is formed on the upper surface of the lower cladding layer 4. Then, output light having a mode field diameter substantially equal to the mode field diameter in the second waveguide core 2-3 (the left end of the second waveguide core 2-3 in FIG. 1) is output from the second waveguide core 2-3. An end (the right end of the second waveguide core 2-3 in FIG. 1) is output to the outside as an output port. Thus, when input light with a small mode field diameter is input to the first waveguide core 1, the mode field diameter is enlarged and output from the output end (output port) of the second waveguide core 2-3. . This output light can be input without causing much light loss in an optical fiber or the like disposed at the subsequent stage of the output end of the second waveguide core 2-3.

  Of course, the right end of the second waveguide core 2-3 is used as an input port, the left end of the first waveguide core 1 is used as an output port, and input light having a large mode field diameter is input from the right end of the second waveguide core 2-3. It is also possible to reduce the mode field diameter and output from the left end of the first waveguide core 1.

  Further, as shown in FIGS. 2A and 2B, the second waveguide core 2-3 included in the third configuration region is formed by arranging periodically segmented cores in series. It can also be formed as a segmented waveguide.

  By using the second waveguide core 2-3 included in the third configuration region as a segment waveguide, the mode field diameter of propagating light propagating through the second waveguide core 2-3 can be easily set large. That is, there is an advantage that it is easy to increase the ratio of the mode field diameter of the input light and the output light, and the enlargement / reduction ratio of the mode field diameter can be increased. The first spot size converter shown in FIG. 2 is different only in the second waveguide core 2-3, and the other components are the same as those in FIG.

<Second spot size converter>
The configuration of the second spot size converter will be described with reference to FIGS. FIG. 3 is a diagram showing a schematic configuration of the second spot size converter, (A) is a schematic plan view showing the xz plane, and (B) is the existence of the first waveguide core in the first configuration region. (C) is a schematic cross-sectional structure diagram showing the xy plane cut at the position indicated by the dashed line (II-II) shown in (A), (D) FIG. 4 is a schematic cross-sectional structure diagram showing an xy plane in a third configuration region.

  The second spot size converter is formed on the silicon substrate 10 and includes a first waveguide core 1 and a second waveguide core 2 (2-1, 2-2, 2-3). The waveguide direction of the core 1 and the waveguide direction (z-axis direction) of the second waveguide core 2 are arranged so as to be parallel to each other. Also here, similarly to the first spot size converter, in the waveguide direction (z-axis direction) of the first waveguide core 1 and the second waveguide core 2, the first configuration region, the second configuration region, Three configuration areas are set sequentially.

  The first waveguide core 1 is formed in the first configuration region, and the second waveguide core 2 is formed throughout the first configuration region, the second configuration region, and the third configuration region. Here, similarly to the above-mentioned first spot size converter, the second waveguide core 2 is formed on the upper surface of the waveguide (the upper cladding layer 3 of the second waveguide core 2 and the second waveguide core 2 over the first to third constituent regions). The contact surface is flat.

  The first waveguide core 1 includes a tapered structure region 1-T in which the width of the core decreases toward the second configuration region. The second waveguide core 2-2 included in the second configuration region includes A taper structure region 2-T in which the width of the core is narrowed as the distance from the one component region is increased. The second waveguide core 2-3 included in the third configuration region is formed as a uniform waveguide having a constant waveguide width. Further, as in the case of the first spot size converter described above, the second waveguide core 2-3 included in the third constituent region is formed by arranging periodically segmented cores in series. It can also be formed as a segmented waveguide.

  The first configuration region includes the first waveguide core 1 and the second waveguide core 2-1. In the first configuration region, the propagating light propagating through the first waveguide core 1 and the second waveguide core 2-1 are included. And the propagating light propagating through are coupled to each other via the mutual evanescent field. The second configuration region includes a second waveguide core 2-2, and the second waveguide core 2-2 expands as the mode field diameter of the input light approaches the third configuration region. Including two waveguide cores 2-3, propagating light output from the second configuration region is guided to the outside by the second waveguide core 2-3, or light input from the outside is guided to the second configuration region . These points are common to the first spot size converter described above.

  The refractive index of the second waveguide core 2 is smaller than the refractive index of the first waveguide core 1, and the refractive indexes of the first lower cladding layer 4-1, the second lower cladding layer 4-2, and the upper cladding layer 3 are It is set smaller than the refractive index of the second waveguide core 2. Then, the first component region is laminated in the order of the first lower cladding layer 4-1, the first waveguide core 1, the second lower cladding layer 4-2, the second waveguide core 2-1, and the upper cladding layer 3. The second component region is formed by laminating the first lower cladding layer 4-1, the second lower cladding layer 4-2, the second waveguide core 2-2, and the upper cladding layer 3 in this order, The three constituent regions are formed by laminating the first lower cladding layer 4-1, the second lower cladding layer 4-2, the second waveguide core 2-3, and the upper cladding layer 3 in this order.

  According to the second spot size converter, since the second lower cladding layer 4-2 functions as the lower cladding layer for the second waveguide core 2 together with the first lower cladding layer 4-1, the mode field diameter is increased. There is an effect that the absorption loss of the photoelectric field of the propagating light caused by the silicon substrate 10 caused by the expansion of the above can be reduced.

<Operation simulation of spot size converter>
The results of numerical simulation will be described. The numerical simulation was performed by combining a three-dimensional beam propagation method (3D-Beam Propagation Method: 3D-BPM) and a three-dimensional FDTD method (3D-Finite-difference time-domain method: 3D-FDTD). 3D-BPM was applied to the region where the refractive index change in the propagation direction was gentle, and 3D-FDTD was applied to the region where the refractive index change in the propagation direction was steep. In carrying out the numerical simulation, the wavelength of light targeted for spot size conversion was set to 1.55 μm.

≪Simulation about spot size≫
An operation simulation result performed on the first spot size converter will be described. In the simulation, the first waveguide core 1 was formed of a silicon material and the refractive index was 3.48. Also, the second waveguide core 2, the lower cladding layer 4 (and the first lower cladding layer 4-1, the second lower cladding layer 4-2), and the upper cladding layer 3 are formed of a silicon oxide material, and the second conductor is formed. Assuming that the refractive index of the waveguide core 2 is 1.52, the lower cladding layer 4 (and the first lower cladding layer 4-1 and the second lower cladding layer 4-2), and the upper cladding layer 3 have a refractive index of 1.44, the first spot The mode field diameter at various places of the size converter was simulated. In this simulation, the thickness (the thickness of the core in the y-axis direction) and the width (the width of the core in the x-axis direction) of the first waveguide core 1 are both set to 300 nm. The detailed dimensions of the first and second waveguide cores, the thicknesses of the lower cladding layer 4 and the upper cladding layer 3, and the like were appropriately set in consideration of the convenience of simulation. Here, the refractive index of the first lower cladding layer 4-1 and the refractive index of the second lower cladding layer 4-2 are set equal, but the refractive indexes of both need not necessarily be set equal.

  FIG. 4 shows a state of a spot (beam spot) of propagating light at various points (positions a to d shown in FIG. 4A) of the first spot size converter. FIG. 4B shows the propagation light spot for the TE wave component and the TM wave component with black and white shading depending on the intensity of the photoelectric field. The photoelectric intensity of the beam spot is maximum at the center, and decreases exponentially as it goes to the periphery. In FIG. 4 (A), the beam spots corresponding to the respective locations indicated by a to d are shown in correspondence with (a) to (d) in FIG. 4 (B).

  The mode field diameter of propagating light at the position indicated by a in the first waveguide region 1 in the first component region, and the position indicated by d in the second waveguide core 2-3 at the boundary between the second component region and the third component region. When the mode field diameter of the propagating light at is compared, it is clearly larger at the latter position. In addition, it can be seen that the mode field diameter is similarly expanded or reduced for the TE wave component and the TM wave component at both positions.

  The intensity of the output light output with the mode field diameter enlarged or reduced by the first spot size converter is the light intensity of the input light which is the mode-target field converted light input to the first spot size converter. Is less than. This is due to an absorption loss that occurs during propagation through the first and second waveguide cores and a light amount loss due to mode mismatch. The amount of light loss due to mode mismatch (hereinafter referred to as mode mismatch loss) is a first configuration region in which propagating light propagating through the first waveguide core 1 and propagating light propagating through the second waveguide core 2-1 are optically coupled to each other. Mainly occurs.

≪Mode mismatch loss≫
With reference to FIG. 5, a method for obtaining the mode mismatch loss by simulation of the mode field diameter will be described. The mode mismatch loss generated in the first constituent region is caused by the photoelectric field distribution of the propagation light near the end, which is the narrowest portion of the waveguide width of the tapered structure region 1-T of the first waveguide core 1, and the second waveguide. It is calculated as being proportional to the value of the overlap integral with the photoelectric field distribution of propagating light propagating through the core 2-1.

  5A is a cross-sectional view taken along a plane perpendicular to the waveguide direction in the vicinity of the narrowest end of the waveguide width of the tapered structure region 1-T of the first waveguide core 1 in FIG. (A-2) is a cross-sectional view taken along a plane perpendicular to the waveguide direction, where the first waveguide core 1 in the vicinity of the narrowest end of the waveguide width of the tapered structure region 1-T does not exist. is there. (B-1) shows the photoelectric field distribution of propagating light superimposed on (A-1), and (B-2) shows the photoelectric field distribution of propagating light superimposed on (A-2). is there. The larger the difference in the shape of the photoelectric field distributions shown in (B-1) and (B-2), the greater the mode mismatch loss.

  With reference to FIG. 6, the relationship of the mode mismatch loss to the width W of the second waveguide core 2-1 in the first structure region will be described using the thickness H of the second waveguide core 2 as a parameter. Refer to FIGS. 1A and 1B for thickness H and width W. In the simulation shown here, for convenience of calculation, the silicon substrate 10 in the first configuration region is not considered, and the width W of the second waveguide core 2-1 is set to 1 to 3 μm. FIG. 6A shows the result for the TE wave component, and FIG. 6B shows the result for the TM wave component. The horizontal axis in FIG. 6 indicates the width W of the second waveguide core 2-1 in units of μm, and the vertical axis indicates the mode mismatch loss in dB scale. In FIG. 6, the thickness H of the second waveguide core 2 is determined by assuming that (a) is H = 0.5 μm, (b) is H = 1 μm, (c) is H = 2 μm, (d) is H = 3 μm. It is a result. It can be seen that if the width W of the second waveguide core 2-1 is about 3 μm, the mode mismatch loss can be sufficiently reduced even if the thickness H of the second waveguide core 2 is thin.

  Next, referring to FIG. 7, the length in the waveguide direction of the tapered structure region 2-T of the second waveguide core 2-2 in the second constituent region, with the thickness H of the second waveguide core 2 as a parameter. The result of simulating how the mode mismatch loss is affected when L is changed will be described. Here, the width of the end at which the width of the tapered structure region 2-T of the second waveguide core 2 is minimized is set to 0.5 μm. FIG. 7A shows the result for the TE wave component, and FIG. 7B shows the result for the TM wave component. The horizontal axis of FIG. 7 indicates the length L in the waveguide direction of the tapered structure region 2-T of the second waveguide core 2-2 in units of μm, and the vertical axis indicates the tapered structure region 2-T. The generated mode mismatch loss (optical loss) is shown in dB scale. 7A and 7B, (a) shows the results obtained with H = 1 μm, (b) with H = 2 μm, and (c) with H = 3 μm.

  The optical loss generated in the tapered structure region 2-T includes the photoelectric field distribution of the propagation light at the narrowest end of the tapered structure region 2-T, the second constituent region of the second waveguide core 2-3, It was calculated as being proportional to the value of the overlap integral with the photoelectric field distribution of propagating light propagating in the basic propagation mode in the vicinity of the boundary with the three constituent regions. The longer the length L in the waveguide direction of the taper structure region 2-T of the second waveguide core 2-2, the lower the loss. The light loss generated in the taper structure region 2-T with a length of about 500 μm is sufficient. It turns out that it becomes small.

≪Radiation loss to silicon substrate≫
The simulation results for the first spot size converter shown in FIGS. 4, 6, and 7 show that part of the electric field component of the light propagating through the first and second waveguide cores is silicon via the lower cladding layer 4. This is obtained by ignoring the radiation loss of the photoelectric field due to reaching the substrate 10 to the silicon substrate. However, the absorption loss of the photoelectric field of the propagating light by the silicon substrate 10 accompanying the expansion of the mode field diameter in the region including the tapered structure region 2-T of the second waveguide core 2 cannot be ignored. Therefore, the thickness of the lower cladding layer 4 is set to 4 μm, and the same result as that shown in FIG. 4 will be described with reference to FIG.

  The state of the spot (beam spot) of the propagation light at the positions a to d in FIG. FIG. 8B shows the spot of the propagating light for the TE wave component and the TM wave component, with black and white shading depending on the intensity of the photoelectric field. As in FIG. 4, the mode field diameter of the propagation light at the position indicated by a of the first waveguide core 1 in the first configuration region, and the second waveguide core 2- at the boundary between the second configuration region and the third configuration region. When the mode field diameter of the propagating light at the position indicated by d in 3 is compared, it is clearly increased at the latter position. In addition, it can be seen that the mode field diameter is expanded or reduced in the same way for the TE wave component and the TM wave component at both positions.

  Although the mode mismatch loss and the absorption loss due to the silicon substrate also occur with the change in the shape of the beam spot, the inventors of the present application have confirmed that the optical loss amount is 1.5 dB or less. That is, the thickness of the lower clad layer 4 is set to 4 μm or more, or the structure is provided with the first and second lower clad layers, so that the optical loss amount can be suppressed to a level that hardly causes a problem in practice. I confirmed.

<< Effects of segmented waveguide >>
With reference to FIG. 9, the simulation result when the second waveguide core 2-3 is a segmented waveguide will be described. FIG. 9 shows the state of the spot of propagating light at various positions (positions a to e) when the second waveguide core 2-3 is a segmented waveguide in the first spot size converter. This is shown in the same format as in FIG.

  Even if a segment waveguide is employed as the second waveguide core 2-3, the same result as that obtained when a uniform-width waveguide having a constant core width is employed as shown in FIG.

  When the second waveguide core 2-3 is a segment waveguide, wavelength selection by Bragg reflection is performed at this portion. Therefore, a simulation was conducted to examine the effect of wavelength selection. The shape of each segment in the cross section (xy plane) orthogonal to the waveguide direction was formed as a square having a side of 1 μm, and the segment arrangement period was set to 0.5 μm. The refractive index of the core constituting the segment of the second waveguide core 2-3 was set to 1.52.

  With reference to FIG. 10, the equivalent characteristic and reflection characteristic of the second waveguide core 2-3 will be described. The horizontal axis of FIG. 10 shows the wavelength scaled in units of μm, and the vertical axis shows the transmitted light intensity and the reflected light intensity on an arbitrary scale. (a) shows the transmitted light spectrum, and (b) shows the reflected light spectrum. The effect of Bragg reflection appears around the wavelength of 1.45 μm. The effect of Bragg reflection can be neglected by separating the wavelength of light subject to spot size conversion from 1.45 μm. In this case, the effect of Bragg reflection can be ignored if spot size conversion is performed for beams having a wavelength of 1.3 μm or less or 1.5 μm or more. If spot size conversion of a beam having a wavelength in the vicinity of 1.45 μm is performed, the segment arrangement period may be adjusted so that the peak position of the Bragg reflection is separated from 1.45 μm.

  By considering the wavelength of the beam to be subjected to spot size conversion so as not to be affected by Bragg reflection, the beam spot diameter can be converted at any wavelength. Further, by making the second waveguide core 2-3 a segmented waveguide, the equivalent refractive index with respect to the propagating light propagating through this portion can be reduced, so the mode of the propagating light propagating through the second waveguide core 2-3. Easy to set large field diameter. As a result, it becomes easy to increase the ratio of the mode field diameter between the input light and the output light, so that it is possible to enjoy the advantage that the enlargement / reduction ratio of the mode field diameter can be increased.

1: First waveguide core
1-T, 2-T: Tapered structure region
2: Second waveguide core
2-1: Second waveguide core in the first component region
2-2: Second waveguide core in the second component region
2-3: Second waveguide core in the third region
3: Upper cladding layer
4: Lower cladding layer
4-1: First lower cladding layer
4-2: Second lower cladding layer
10: Silicon substrate

Claims (8)

A spot size converter comprising a first waveguide core and a second waveguide core,
In the first configuration region and the second configuration region, which are sequentially set in the waveguide direction of the first waveguide core and the second waveguide core,
The first waveguide core is formed in the first constituent region;
The second waveguide core is formed over the entire area of the first configuration region and the second configuration region,
The first waveguide core and the second waveguide core are arranged so that the symmetry center of the first waveguide core and the symmetry center of the second waveguide core overlap in the thickness direction of the two waveguide cores. And the waveguide direction of the first waveguide core and the waveguide direction of the second waveguide core are arranged so as to be parallel to each other,
The first waveguide core includes a tapered structure region in which the width of the core decreases toward the second configuration region;
The portion included in the second configuration region of the second waveguide core is a tapered structure region in which the width of the core decreases as the distance from the first configuration region increases.
In the first configuration region, propagating light propagating through the first waveguide core and propagating light propagating through the second waveguide core are optically coupled via each other's evanescent field,
The spot size converter characterized in that, in the second configuration region, the mode field diameter of the input light is enlarged as the distance from the first configuration region increases. Furthermore, a third component area is provided,
In the first configuration region, the second configuration region, and the third configuration region, which are sequentially set in the waveguide direction of the first waveguide core and the second waveguide core,
The second waveguide core is formed over the entire first configuration region, the second configuration region, and the third configuration region,
The propagating light output from the second configuration area is guided to the outside in the third configuration area, or light input from the outside is guided to the second configuration area. Spot size converter. Furthermore, a lower clad layer and an upper clad layer are provided,
The refractive index of the second waveguide core is smaller than the refractive index of the first waveguide core, and the refractive indexes of the lower cladding layer and the upper cladding layer are smaller than the refractive index of the second waveguide core,
The first component region is laminated in the order of the lower cladding layer, the first waveguide core, the second waveguide core, and the upper cladding layer,
2. The spot size converter according to claim 1, wherein the second constituent region is laminated in the order of the lower cladding layer, the second waveguide core, and the upper cladding layer. Furthermore, a lower clad layer and an upper clad layer are provided,
The refractive index of the second waveguide core is smaller than the refractive index of the first waveguide core, and the refractive indexes of the lower cladding layer and the upper cladding layer are smaller than the refractive index of the second waveguide core,
The first component region is laminated in the order of the lower cladding layer, the first waveguide core, the second waveguide core, and the upper cladding layer,
The spot size converter according to claim 2, wherein the second configuration region and the third configuration region are laminated in the order of the lower cladding layer, the second waveguide core, and the upper cladding layer. . Furthermore, a first lower cladding layer, a second lower cladding layer, and an upper cladding layer,
The refractive index of the second waveguide core is smaller than the refractive index of the first waveguide core, and the refractive indexes of the first lower cladding layer, the second lower cladding layer, and the upper cladding layer are Smaller than the refractive index of the waveguide core,
The first component region is laminated in the order of the first lower cladding layer, the first waveguide core, the second lower cladding layer, the second waveguide core, and the upper cladding layer,
2. The spot according to claim 1, wherein the second constituent region is laminated in the order of the first lower cladding layer, the second lower cladding layer, the second waveguide core, and the upper cladding layer. Size converter. Furthermore, a first lower cladding layer, a second lower cladding layer, and an upper cladding layer,
The refractive index of the second waveguide core is smaller than the refractive index of the first waveguide core, and the refractive indexes of the first lower cladding layer, the second lower cladding layer, and the upper cladding layer are Smaller than the refractive index of the waveguide core,
The first component region is laminated in the order of the first lower cladding layer, the first waveguide core, the second lower cladding layer, the second waveguide core, and the upper cladding layer,
The second configuration region and the third configuration region are laminated in the order of the first lower cladding layer, the second lower cladding layer, the second waveguide core, and the upper cladding layer. Item 3. The spot size converter according to Item 2.   The portion included in the third configuration region of the second waveguide core is a uniform-width waveguide having a constant core width, according to any one of claims 2, 4, and 6. Spot size converter. The portion of the second waveguide core included in the third constituent region is a segmented waveguide formed by arranging periodically segmented cores in series. The spot size converter according to any one of 6 and 6.

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