JP2016024413A - Optical circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical circuit with which it is possible to obtain high efficiency of optical coupling between a photonic crystal resonator and an optical waveguide more easily.SOLUTION: A core 152 of an optical waveguide 151 is disposed on a photonic crystal resonator 101 and extending, with the direction parallel to the extension direction of a resonance part 121 as a waveguide direction. Unlike the resonance part 121, the optical waveguide 151 is disposed in a layer on a plane parallel to the plane of the photonic crystal resonator 101. In addition, the center line in the waveguide direction of the core 152 configuring the optical waveguide 151 is disposed between a row 131 of lattice elements adjoining the resonance part 121 and extending in the same direction as the resonance part 121 and a row 132 of lattice elements adjoining this lattice element row and extending in the same direction as the resonance part 121.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical circuit including an optical waveguide that is optically coupled to a photonic crystal resonator.

  In recent years, in order to cope with an explosive increase in traffic on the Internet, light is used for transmission between nodes, and a large capacity is realized by taking advantage of low loss. In addition, even in short-distance transmission such as between boards and between racks, replacement of electrical wiring is taking advantage of the high speed of light. Furthermore, bottlenecks in electrical wiring have been pointed out between LSIs (Large Scale Integration) and within chips, and the possibility of wiring by light is being studied. A microcavity laser is used as a light source for such an optical wiring. The microcavity laser is a micron-order size laser aimed at integration with a large-scale optical integrated circuit or LSI.

  Under such circumstances, a microcavity laser having a photonic crystal resonator has attracted attention (see Non-Patent Document 1, Non-Patent Document 2, and Non-Patent Document 3). In particular, Non-Patent Document 2 eliminates the two main causes of the device degradation shown in Non-Patent Document 1, such as device temperature rise and carrier diffusion, by a buried heterostructure (BH) structure. Means to do this have been proposed (see Non-Patent Document 2).

  Here, the above-described microcavity laser will be described with reference to FIG. FIG. 5 is a plan view (a) and a sectional view (b) showing the configuration of the microcavity laser. The cross-sectional view shows a plane perpendicular to the walk in which light is guided. In this microcavity laser, first, a line defect optical waveguide 504 is provided in a photonic crystal 503 including a base portion 501 made of an InP substrate and a plurality of columnar hollow structures 502 provided in the base portion 501. For example, the hollow structures 502 are arranged in a triangular lattice shape in a plan view. The line defect optical waveguide 504 has a structure in which the hollow structure 502 in a linearly continuous portion is eliminated from the hollow structure 502 provided at periodic intervals, and light is guided to this region.

  The line-defect optical waveguide 504 of the photonic crystal 503 having the above-described configuration includes a resonance part 521 and two mirror regions 522 sandwiching the resonance part 521. The line-defect optical waveguide 504 of the resonance part 521 includes an active medium. 505 is provided (embedded). The active medium 505 is configured such that the upper and lower sides of a core layer made of an InGaAs layer are covered with a cladding layer made of InGaAsP. Thus, the photonic crystal resonator is constituted by the resonance part 521 sandwiched between the mirror regions 522.

  Since this microcavity laser has a BH structure in which the active medium 505 is embedded in the base 501, heat generated by excitation of the active medium 505 can be efficiently released, and high carrier confinement in the resonator 521 can be achieved. Can be realized.

  Further, a structure that enables in-plane light output by coupling a laser using a BH structure with a photonic crystal line defect optical waveguide has been proposed (see Non-Patent Document 3). Microcavity lasers using photonic crystals are promising as light sources for future planar optical integrated circuits. A similar structure is also attracting attention as a device for optical information processing such as an optical memory (see Non-Patent Document 4).

  In addition, for high integration and high-density mounting of optical processing devices using optical wiring, light used in optical circuits on substrates is guided to optical circuits on other substrates where other processing is performed. It is necessary to make it wave. For example, light that resonates with a photonic crystal resonator used in a microcavity laser or the like may be extracted using an optical waveguide provided on another substrate. For example, a technique has been proposed in which an optical waveguide is disposed immediately above a photonic crystal resonator so as to be along the extending direction of the resonator (see Non-Patent Documents 5 and 6).

  In Non-Patent Document 5, as shown in FIG. 6, an optical waveguide 602 is combined with a photonic crystal resonator 601 provided in a line defect region of a photonic crystal in which air holes are provided in a triangular lattice shape in an InP substrate. . The optical waveguide 602 includes a lower clad layer 604 made of silicon oxide formed on a silicon base 603, a core 605 formed on the lower clad layer 604, and a benzocyclobutene (BCB) formed thereon. The upper clad layer 606 is provided.

  The photonic crystal resonator 601 is disposed on the optical waveguide 602 so that the resonating part of the photonic crystal resonator 601 and the core 605 of the optical waveguide 602 overlap each other with the same optical waveguide direction. With this arrangement, the light resonating in the photonic crystal resonator 601 is coupled to the optical waveguide 602 and extracted.

  According to the optical circuit configuration in which the optical coupling is performed as described above, the light resonating in the photonic crystal resonator is guided to the optical circuit provided on another substrate, and further optical processing is performed. High integration and high-density mounting of optical processing devices can be realized.

K. Nozaki et al., "Room temperature continuous wave operation and controlled spontaneous emission in ultrasmall photonic crystal nanolaser", Optics Express, vol.15, no.12, pp.7506-7514, 2007. S. Matsuo et al., "High-speed ultracompact buried heterostructure photonic-crystal laser with 13 fJ of energy consumed per bit transmitted", Nature Photonics, vol.4, pp.648-654, 2010. S. Matsuo et al., "20-Gbit / s directly modulated photonic crystal nanocavity laser with ultra-low power consumption", Opt. Express, vol.19, no.3, pp.2242-2250, 2011. K. Nozaki et al., "Ultralow-power all-optical RAM based on nanocavities", Nature Photonics, vol.6, pp.248-252, 2012. Y. Halioua et al., "Hybrid InP-based photonic crystal lasers on silicon on insulator wires", APPLIED PHYSICS LETTERS, vol.95, 201119, 2009. Min Qiu, "Vertically coupled photonic crystal optical filters", OPTICS LETTERS, vol.30, no.12, pp.1476-1478, 2005.

  However, in the configuration of the above-described optical circuit, there is a problem that it is not easy to obtain high optical coupling efficiency between the photonic crystal resonator and the optical waveguide. In the above-described technique, the efficiency of optical coupling between the resonating part of the photonic crystal resonator and the optical waveguide greatly affects the alignment state. For this reason, in order to obtain a high optical coupling efficiency, it is necessary to align the extending direction center line of the photonic crystal resonator and the waveguide direction center line of the optical waveguide with very high accuracy. . However, it is not easy to obtain high accuracy for this alignment.

  For example, when the lattice constant of the photonic crystal constituting the photonic crystal resonator is 422 nm, the resonance part of the photonic crystal resonator has a very narrow width of about 600 nm. In addition, in the optical waveguide using the Si core that constitutes the optical circuit as described above, the core width is very thin as several hundred nm. In order to obtain a high coupling efficiency by superposing the thin core and the resonating part in the same optical waveguide direction as described above, the alignment system needs to be at least several tens of nm or less. However, it is not easy to achieve such a high alignment accuracy, and it is not easy to obtain high efficiency of optical coupling between the photonic crystal resonator and the optical waveguide with the above-described technique. was there.

  The present invention has been made to solve the above-described problems, and is intended to make it easier to obtain high efficiency of optical coupling between a photonic crystal resonator and an optical waveguide. Objective.

  An optical circuit according to the present invention includes a base and a photonic crystal that is periodically provided at the base and includes a plurality of columnar lattice elements having different refractive indexes from the surroundings, and includes a resonance unit that extends in a predetermined direction. A photonic crystal resonator, and an optical waveguide disposed on the photonic crystal resonator and extending in a direction parallel to the extending direction of the resonant portion as a waveguide direction and disposed in a layer different from the resonant portion The center line in the waveguide direction of the optical waveguide is adjacent to the resonance portion and extends in the same direction as the resonance portion, and adjacent to the lattice element row and extends in the same direction as the resonance portion. Arranged between the grid element rows. Note that the lattice elements may be arranged in a triangular lattice or a square lattice.

  As described above, according to the present invention, it is possible to obtain an excellent effect that the high efficiency of optical coupling between the photonic crystal resonator and the optical waveguide can be obtained more easily.

FIG. 1 is a plan view showing a configuration of an optical circuit according to an embodiment of the present invention. FIG. 2 is a perspective view showing the configuration of the optical circuit in the embodiment of the present invention. FIG. 3 is an explanatory diagram for explaining the principle regarding the optical coupling structure between the photonic crystal resonator 101 and the optical waveguide 151. FIG. 4 is a characteristic diagram showing the result of deriving the optical coupling efficiency between the resonance part and the core by computer simulation when the optical waveguide is displaced in the x-axis direction. FIG. 5 is a plan view (a) and a sectional view (b) showing the configuration of the microcavity laser. FIG. 6 is a perspective view showing a configuration of an optical circuit in which an optical waveguide is disposed on a photonic crystal resonator and optically coupled.

  Hereinafter, embodiments of the present invention will be described with reference to FIGS. FIG. 1 is a plan view showing a configuration of an optical circuit according to an embodiment of the present invention. FIG. 2 is a perspective view showing the configuration of the optical circuit in the embodiment of the present invention. This optical circuit includes a photonic crystal resonator 101 and an optical waveguide 151.

  The photonic crystal resonator 101 is composed of a photonic crystal including a base 102 and a plurality of lattice elements 103 provided on the base 102. The lattice elements 103 are, for example, hollow cylinders, and are arranged in a triangular lattice shape in plan view. Each lattice element 103 has the same shape. Such a photonic crystal includes a line defect optical waveguide 104 formed by a portion (line defect portion) where the lattice element 103 is not formed.

  The line defect optical waveguide 104 includes a resonating part 121 and two mirror regions 122 sandwiching the resonating part 121. A resonator is provided by providing the resonance unit 121. The line defect optical waveguide 104 of the resonance part 121 is provided (embedded) with a medium part 105 made of a material having a refractive index different from that of the base part 102. Here, the line defect optical waveguide 104 extends in the first direction, and the resonance part 121 also extends in the first direction. 1 and 2, the z-axis direction is the first direction. Note that the photonic crystal resonator 101 can be a laser by forming the medium unit 105 from an active medium.

An optical waveguide 151 is disposed on the photonic crystal resonator 101 having the above-described configuration. In FIG. 1 and FIG. 2, the optical waveguide 151 is disposed on the y-axis direction that is the normal direction of the plane of the photonic crystal resonator 101. 1 and 2, the core 152 constituting the optical waveguide 151 is shown, and other configurations such as cladding are omitted. The optical waveguide 151 may be, for example, a Si fine wire waveguide having a core 152 made of silicon (Si). The Si wire waveguide may be formed, for example, using an insulating film (for example, SiO ₂ ) provided on a Si substrate as a lower clad. Alternatively, the Si substrate may be formed with a hollow structure, and air may be used as the cladding.

  Here, in the present invention, the core 152 of the optical waveguide 151 is disposed on the photonic crystal resonator 101 and extends in the direction parallel to the extending direction of the resonance part 121 as the waveguide direction. Unlike the resonance part 121, the optical waveguide 151 is arranged in a layer on a plane parallel to the plane of the photonic crystal resonator 101. In addition, the center line in the waveguide direction of the core 152 constituting the optical waveguide 151 is adjacent to the resonance element 121 and the lattice element array 131 extending in the same direction as the resonance element 121 and the lattice element array. And the resonance element 121 and the row 132 of lattice elements extending in the same direction.

  1 illustrates the case where the center line of the core 152 is disposed between the row 131 and the row 132 on the right side of the plane of the resonance unit 121, but the present invention is not limited to this. The same applies to the case where the center line of the core 152 is arranged between the lattice element row 133 and the lattice element row 134 on the right side of the resonance portion 121 in the drawing.

  According to the above-described embodiment, the optical waveguide 151 may be shifted between the interval between the row 131 (row 133) and the row 132 (row 134), so that the photonic crystal resonator 101 and the optical waveguide 151 The tolerance for misregistration increases, and optical coupling between the two can be obtained without requiring high alignment accuracy.

  This will be described in more detail below. First, the principle regarding the optical coupling structure between the photonic crystal resonator 101 and the optical waveguide 151 will be described with reference to FIG. In the following, first, the base portion 102 is made of InP, and the medium portion 105 is made of InGaAsP. Moreover, the lattice element 103 is assumed to be composed of air as a hollow cylinder. Therefore, the refractive index is medium part 105> base part 102> lattice element 103. The case where the core 152 of the optical waveguide 151 is made of silicon and the clad is made of air will be described as an example.

  First, FIG. 3A is a plot of the mode shape of the photonic crystal resonator 101 in the “wave number space”. 3B-1 is a plot of the mode shape of the optical waveguide 151 with the core width of 300 nm aligned with the z-axis in the direction in which the core 152 extends in the “wave number space”. Also, (b-2) of FIG. 3 is a plot of the mode shape of the optical waveguide 151 with the core width of 400 nm aligned in the direction in which the core 152 extends along the “wave number space”. 3B-3 is a plot of the mode shape of the optical waveguide 151 with the core width of 500 nm aligned in the z-axis with the direction in which the core 152 extends in the “wave number space”. In any case, the thickness of the core is 245 nm.

  As shown in FIG. 3A, when the extending direction is aligned with the z axis, the mode of the photonic crystal resonator 101 is in a direction parallel to the x axis near the normalized wave number Kz = 0.5. It has a linear mode. As shown in (b-1), (b-2), and (b-3) of FIG. 3, the mode of the optical waveguide 151 is also parallel to the x-axis when the waveguide direction is aligned with the z-axis. It has a linear mode in any direction. The position on the kz axis in this mode is adjusted by the width and thickness of the core 152. When the center line in the waveguide direction of the core 152 coincides with the center line in the extending direction of the photonic crystal resonator 101, the mode coupling coefficient of both is maximized.

  Next, (c) of FIG. 3 shows (a) of FIG. 3 in real space. The wave number matching condition between the resonance unit 121 and the core 152 in the wave number space corresponds to a condition in which the phase in the z-axis direction of the photonic crystal resonator mode and the phase of the optical waveguide mode match in real space. However, since the mode shape of the photonic crystal resonator in the real space is extremely different from the mode shape of the optical waveguide, it is necessary to consider the matching of the field distribution in the real space.

  As shown in FIG. 3C, the mode of the photonic crystal resonator has a mode shape just above the z-axis and about ± 0.5 × a√3 (a is a lattice constant of the photonic crystal) from here. The phase of the mode shape at a position distant in the x-axis direction (near the hole array closest to the line defect waveguide) is shifted by π. For this reason, there is a node 301 where the field intensity is zero between the hole array closest to the resonance part 121 and the z-axis.

  On the other hand, since the optical waveguide 151 does not have a node region as described above, when the center of the core 152 in the waveguide direction is disposed on the node 301 as shown in FIG. The coupling efficiency between the mode and the optical waveguide mode is extremely lowered. In order to increase the coupling efficiency, it is important to arrange the core 152 away from the node 301 in the waveguide direction center. Conventionally, the center line in the waveguide direction of the core 152 is strictly aligned with the center line (z-axis) in the extending direction of the resonance part 121, thereby avoiding the node 301 and increasing the coupling efficiency.

  On the other hand, in the present invention, the coupling efficiency is increased by giving a predetermined offset to the positional relationship between the extending direction center line of the resonance portion 121 and the center line of the core 152 in the waveguide direction. FIG. 4A shows the amount of deviation Δx between the center line in the extending direction of the resonance part 121 and the center line in the waveguide direction of the core 152 in the direction perpendicular to the waveguide direction (x-axis direction). Shows the relationship with the coupling efficiency (η). The coupling efficiency (η) is converted from the reflectance (R) when input from the optical waveguide to the resonance part.

  In addition, the structure factor used for calculation of the result shown to (a) of FIG. 4 is shown next. The base material was InP, the lattice constant a = 422 nm, the lattice element was a hollow cylinder with a radius of 100 nm, and the base plate thickness was 245.4 nm. The medium part is made of InGaAsP and has a width in the x-axis direction of 350 nm × a length in the z-axis direction of 3.8 μm × a thickness in the y-axis direction of 145.4 nm. The core of the optical waveguide is made of silicon and has a width in the x-axis direction of 420 nm × a thickness in the y-axis direction of 245 nm.

  As described above, a photonic crystal resonator mode node exists in the vicinity of Δx = 200 nm, and the coupling efficiency is drastically reduced in the vicinity of the node. Therefore, in the conventional coupling structure, the cores are arranged so that the center position of Δx = 0. For this reason, the coupling efficiency is drastically reduced when the center position of the core is slightly shifted in the x-axis direction. On the other hand, according to the arrangement of the core according to the present invention, the center position of the core is arranged in the region 401 near Δx = 500 nm.

  As shown in FIG. 4A, it can be seen that a coupling efficiency of 90% or more is obtained when the positional deviation amount Δx is 360 nm to 720 nm. This range is the amount of positional deviation allowed in the configuration of the present invention. Since the conventional technique has a thickness of about several tens of nanometers, a positional shift of one digit or more can be allowed. When the region 401 shown in FIG. 4A is shown in real space, as shown in the region 402 and the region 403 in FIG. 4B, the first row counting from the nearest hole row to the resonance portion. By arranging the center of the core between the second hole row and the second hole row, it is possible to realize a coupling structure that allows an alignment error of one digit or more than the conventional one.

  As described above, in the present invention, the center line in the waveguide direction of the optical waveguide is aligned with the lattice element row (first hole row) adjacent to the resonance portion and extending in the same direction as the resonance portion. It was arranged between a lattice element row (second hole row) adjacent to the lattice element row and extending in the same direction as the resonance part. As a result, it is possible to realize a coupling structure that relaxes the alignment accuracy in the substrate plane direction required for high-efficiency coupling between the resonance part and the optical waveguide, and allows an alignment error of one digit or more than before. High efficiency of optical coupling between the nick crystal resonator and the optical waveguide can be obtained more easily.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, the resonance unit 121 is not limited to the configuration in which the medium unit 105 is provided, and may have a configuration in which the width of the line defect optical waveguide 104 is partially modulated. The line defect optical waveguide 104 may be terminated with the lattice element 103, and various photonic crystal resonators using photonic crystal line defects can be used.

  In the above description, the case where the lattice elements 103 are arranged in a triangular lattice shape in plan view has been described. However, the present invention is not limited to this, and the lattice elements 103 may be arranged in a square lattice. The lattice element 103 may be a part having a refractive index different from that of the base 102, and is not limited to being hollow, and may be a cylindrical shape made of a material having a refractive index different from that of the base 102.

  DESCRIPTION OF SYMBOLS 101 ... Photonic crystal resonator, 102 ... Base part, 103 ... Lattice element, 104 ... Line defect optical waveguide, 105 ... Medium part, 121 ... Resonant part, 122 ... Mirror area | region, 151 ... Optical waveguide, 152 ... Core.

Claims (2)

A photonic crystal resonator including a base and a photonic crystal that is periodically provided on the base and includes a plurality of columnar lattice elements having a refractive index different from that of the surroundings, and includes a resonator that extends in a predetermined direction;
An optical waveguide disposed on the photonic crystal resonator and extending in a direction parallel to the extending direction of the resonance unit as a waveguide direction and disposed in a layer different from the resonance unit;
A center line in the waveguide direction of the optical waveguide is adjacent to the resonance portion and extends in the same direction as the resonance portion, and adjacent to the lattice element row and extends in the same direction as the resonance portion. An optical circuit characterized in that the optical circuit is arranged between existing rows of lattice elements. The optical circuit according to claim 1.
The optical element, wherein the lattice elements are arranged in a triangular lattice or a square lattice.