JP6279983B2 - Optical circuit - Google Patents

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. It means that that have been proposed.

Here, the above-described microcavity laser will be described with reference to FIG. FIG. 8 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 direction in which light is guided. In this microcavity laser, first, a line defect optical waveguide 804 is provided on a photonic crystal 803 including a base 801 made of an InP substrate and a plurality of columnar hollow structures 802 provided on the base 801. The hollow structures 802 are arranged in a triangular lattice shape in plan view, for example. The line defect optical waveguide 804 has a structure in which the hollow structure 802 in a linearly continuous portion is eliminated from the hollow structure 802 provided at periodic intervals, and light is guided to this region.

The line-defect optical waveguide 804 of the photonic crystal 803 having the above-described configuration includes a resonance part 821 and two mirror regions 822 sandwiching the resonance part 821, and the line-defect optical waveguide 804 of the resonance part 821 has an active medium. 805 is provided (embedded). Active medium 805, the upper and lower core layers ing from InGaAs layer has the structure covered with a clad layer made of InGaAsP. Thus, the photonic crystal resonator is constituted by the resonance part 821 sandwiched between the mirror regions 822.

  In this microcavity laser, since the active medium 805 has a BH structure embedded in the base 801, heat generated by excitation of the active medium 805 can be efficiently released, and high carrier confinement in the resonator 821 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. 9, an optical waveguide 902 is combined with a photonic crystal resonator 901 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 902 includes a lower clad layer 904 made of silicon oxide formed on a silicon base 903, a core 905 formed on the lower clad layer 904, and a benzocyclobutene (BCB) formed thereon. The upper clad layer 906 is formed.

  The photonic crystal resonator 901 is disposed on the optical waveguide 902 so that the resonating part of the photonic crystal resonator 901 and the core 905 of the optical waveguide 902 are overlapped with each other in the same optical waveguide direction. With this arrangement, the light resonating in the photonic crystal resonator 901 is coupled to the optical waveguide 902 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 high optical coupling efficiency, it is necessary to align the resonating part of the photonic crystal resonator and 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 is composed of a photonic crystal including a base and a plurality of columnar lattice elements that are periodically provided at the base and have different refractive indexes from the surroundings, and the resonance part extends in the first direction. The provided photonic crystal resonator and the photonic crystal resonator disposed on the photonic crystal resonator extend in a second direction different from the first direction and intersect with a layer different from the resonance unit, and the resonance unit at the intersection And an optical waveguide that is optically coupled.

In the above optical circuit, the intersection angle between the first direction and the second direction is an angle formed by the sum vector of two lattice vectors in the array of lattice elements and one of the two lattice vectors. The 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 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 an enlarged plan view of the line defect optical waveguide 104 in the photonic crystal resonator 101 in which the lattice elements 103 are arranged in a triangular lattice. FIG. 5 is a characteristic diagram showing a result of deriving the optical coupling efficiency between the resonance unit 121 and the core 152 by computer simulation when the optical waveguide 151 is displaced in the x-axis direction . FIG. 6 is an explanatory diagram for explaining the tolerance for the intersection angle between the first direction and the second direction. FIG. 7 is a characteristic diagram showing the relationship of the coupling efficiency with respect to the distance between the resonating unit 121 and the core 152. FIG. 8 is a plan view (a) and a sectional view (b) showing the configuration of the microcavity laser. FIG. 9 is a perspective view showing a configuration of an optical circuit in which an optical waveguide is arranged 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.

  Here, in the present invention, the core 152 of the optical waveguide 151 extends in the second direction different from the first direction described above, intersects with the resonator 121 in a different layer, and the resonator 121 and the light at the intersection. Are connected. In FIG. 2, the crossing angle between the first direction and the second direction is φ. Here, the optical waveguide 151 is formed in a layer on a plane parallel to the plane of the photonic crystal resonator 101.

  According to the above-described embodiment, as will be described later, since tolerance for misalignment between the photonic crystal resonator 101 and the optical waveguide 151 is increased, the optical coupling between the two can be performed without requiring high alignment accuracy. Be able to get.

  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. Hereinafter, a case where the base 102 and the core 152 are made of InP will be described as an example.

  3A-1 is a plot of the mode shape of the photonic crystal resonator 101 in the “wave number space”, and FIG. 3A-2 shows the direction in which the core 152 extends in the z direction. The mode shape of the optical waveguide 151 having a core width of 400 nm aligned with the axis is also plotted in the “wave number space”.

As shown in (a-1) in FIG. 3, full photonic crystal resonator 101 is found to have a linear mode in the direction parallel to the kx axis near the normalized wave number kz = 0.5. On the other hand, as shown in FIG. 3A-2, it can be seen that the optical waveguide 151 (core 152) also has a linear mode in the direction parallel to the kx axis in the vicinity of kz = 0.5. When both positions are arranged so as to coincide with each other in the wave number space, both modes can be combined with the maximum coupling efficiency. This condition means that in the real space, the distance between the nodes in the z-axis direction field is the same for both the photonic crystal resonator 101 and the core 152.

  In the mode of the photonic crystal resonator 101 shown in FIG. 3A-3, by arranging the core 152 (optical waveguide 151) at the position of the frame 301, the overlapping state between both modes becomes the best, and both modes The maximum coupling efficiency is achieved. Note that the width of the frame 301 approximately matches the width of the medium part 105 (resonant part 121).

  By the way, as shown in (a-1) of FIG. 3, the mode of the photonic crystal resonator 101 existing in the vicinity of kz = 0.5 is kx = 0 of the K vector shown in (a-1) of FIG. It can be seen that there are two peaks in the vicinity and in the vicinity of kx = 0.5 of the K ′ vector shown in FIG. In particular, the peak near kx = 0.5 is stronger.

  From the above, by arranging both as described below, both modes can be coupled with high coupling efficiency. First, in order to match the length of the K vector in (a-2) of FIG. 3 with the direction of the K ′ vector, the mode of the optical waveguide 151 is tilted as shown in (b-2) of FIG. In order to increase the length, the width and thickness of the optical waveguide 151 (core 152) on the x-axis are increased. Thus, if the mode peak of the optical waveguide 151 is arranged so as to coincide with the peak in the vicinity of kx = 0.5 among the modes of the photonic crystal resonator 101, both modes can be coupled with high coupling efficiency. become.

  In order to obtain the above-described relationship, in the real space, as shown in FIG. 3B-3, the core 152 (optical waveguide 151) is disposed at the position of the frame 302, and the resonance part 121 extends. The optical waveguide 151 may be extended in a second direction different from the first direction, and the optical waveguide 151 may be crossed with the resonance unit 121 in a different layer. The resonating unit 121 and the optical waveguide 151 are optically coupled at the intersection.

  Here, the intersection angle φ between the first direction and the second direction in the real space corresponds to the inclination from the kz axis in the wave number space shown in (b-1) of FIG. The peak in the vicinity of kx = 0.5 of the mode of the photonic crystal resonator 101 exists at a position of φ = 30 ° in the wave number space as shown in (b-1) of FIG. Therefore, as shown in FIG. 3B-2, the mode of the optical waveguide 151 is tilted by φ = 30 ° in the wave number space, that is, the extending direction of the optical waveguide 151 is changed in the real space to the resonance portion 121. By tilting 30 ° from the extending direction (z-axis direction), the maximum coupling efficiency can be obtained between the two.

  Here, the meaning of φ = 30 ° will be described with reference to FIG. FIG. 4 is an enlarged plan view of the line defect optical waveguide 104 in the photonic crystal resonator 101 in which the lattice elements 103 are arranged in a triangular lattice. In FIG. 4, the state of the triangular lattice is indicated by a dotted line. The angle formed by the two lattice vectors 401 and 402 of this triangular lattice is 60 °. The lattice vector 401 is the first direction, which is the z-axis direction in FIG. A sum vector 403 of the lattice vector 401 and the lattice vector 402 is a vector inclined by 30 ° from the z-axis direction. Therefore, the maximum coupling efficiency can be obtained by extending the optical waveguide 151 in the second direction different from the first direction by the “angle formed by the sum vector of two lattice vectors in the arrangement of the lattice elements 103”. become.

  Next, the tolerance of the coupling efficiency with respect to the positional deviation between the photonic crystal resonator 101 and the optical waveguide 151 will be described. In the following, the strength of misalignment tolerance according to the present invention is proved by estimating the coupling efficiency with respect to misalignment between the photonic crystal resonator 101 and the optical waveguide 151. First, the tolerance when the optical waveguide 151 is displaced with respect to the x direction perpendicular to the first direction is evaluated. The x direction is the width direction of the line defect optical waveguide 104.

  Below, the case where the base 102 is comprised from InP and the core 152 is comprised from Si is made into an example. Since Si has a higher refractive index than InP, for the matching described with reference to FIG. 3, the width of the core 152 is required to be narrower than that of InP. For this reason, when optical coupling is performed in a state in which the extending direction of the optical waveguide and the extending direction of the resonance portion coincide with each other, InP is used as the tolerance of optical coupling with respect to an error in the positional deviation of the optical waveguide (core). Note that the situation is even more severe than in the case.

  FIG. 5 shows the optical coupling efficiency between the photonic crystal resonator 101 (resonant unit 121) and the optical waveguide 151 (core 152) when the optical waveguide 151 is offset (displaced) in the x-axis direction by computer simulation. It is a characteristic view which shows the derived | led-out result. In FIG. 5, the horizontal axis indicates the offset amount in the x-axis direction of the optical waveguide, and the vertical axis indicates the coupling efficiency. The size of the medium part 105 in which light is strongly confined in the photonic crystal resonator 101 is set to (width) 350 nm in the x-axis direction and (length) about 3.8 μm in the z-axis direction. The core 152 has a width of 420 nm, and the distance in the y-axis direction between the resonating unit 121 and the core 152 is constant at 300 nm. Further, the crossing angle between the first direction and the second direction was 30 ° at which the maximum coupling efficiency was obtained.

  A gray area in the graph of FIG. 5 indicates an area having an offset amount that is twice the width of the medium portion 105 in which light is strongly confined in the photonic crystal resonator 101. As shown in FIG. 5, it can be seen that a high coupling efficiency of 60% or more can be obtained even in a region exceeding the width of the medium portion 105. When optical coupling is performed in a state where the extending direction of the optical waveguide and the extending direction of the resonance portion coincide with each other, considering that the positional deviation accuracy requires several tens of nm or more, the positional deviation tolerance is about one digit higher. You can see that

  Note that the broken line in the graph of FIG. 5 indicates the boundary position of the resonance part 121 in the z-axis direction. In this example, if the offset in the x-axis direction is greater than about 1000 nm, the core 152 resonance part 121 cannot be covered, and optical coupling between them cannot be obtained. As described above, the reduction in the coupling efficiency due to the positional deviation depends on the length of the resonance part 121 in the extending direction, and if the resonance part 121 having a longer length in the extending direction is used, even higher positional deviation is allowed. You can get a degree.

  Subsequently, the tolerance for the intersection angle between the first direction and the second direction is evaluated. FIG. 6A is a distribution diagram showing the resonance mode of the photonic crystal resonator 101 in the wave number space, and FIG. 6B shows the crossing angle φ with respect to the first direction and the second direction. The result of having derived | led-out the Q value of the photonic crystal resonator 101 by computer simulation is shown. Here, as the Q value of the photonic crystal resonator 101 decreases, the mode that resonates in the resonance unit 121 is efficiently coupled to the optical waveguide 151 (that is, the coupling efficiency between the two is high).

  The medium portion 105 is 350 nm in the x-axis direction and about 3.8 μm in the z-axis direction. The core 152 is made of Si and has a width of 420 nm. Further, the distance in the y-axis direction between the resonating unit 121 and the core 152 is constant at 300 nm. Further, the offset in the width direction of the resonance part 121 in the core 152 was set to zero.

  As shown in FIG. 6B, the minimum Q value (that is, the maximum coupling efficiency) is obtained at an angle φ = 30 °. It can also be seen that the coupling efficiency hardly changes if the angular deviation is about ± 10 °. As described above, the coupling structure using the optical circuit of the present invention can obtain high coupling efficiency even if the crossing angle between the first direction and the second direction is shifted to some extent from the angle at which the maximum coupling efficiency is obtained. In other words, high coupling efficiency can be obtained without requiring a high alignment system for the crossing angle. From another viewpoint, it can be said that the Q value of the photonic crystal resonator 101 can be finely adjusted by adjusting the crossing angle between the first direction and the second direction.

  Next, the relationship of the coupling efficiency with respect to the space between the resonating unit 121 and the core 152 will be described with reference to FIG. FIG. 7 also shows the relationship between the Q value of the photonic crystal resonator 101 with respect to the interval Δy between the resonance unit 121 and the core 152 in the y-axis direction when the offset in the width direction of the resonance unit 121 in the core 152 is zero. FIG. FIG. 7A shows a case where the crossing angle φ = 0 ° between the first direction and the second direction, and FIG. 7B shows a case where φ = 30 °. Comparing FIG. 7A and FIG. 7B, it can be seen that both have substantially the same characteristics. This means that even in the configuration of the present invention, the controllability in the Δy direction is comparable to the configuration in which the optical waveguide is disposed along the extending direction of the resonance portion.

  As described above, in the present invention, the resonance unit extends in the second direction different from the first direction on the photonic crystal resonator provided with the resonance unit extending in the first direction. Optical waveguides that intersect with different layers are arranged. As a result, according to the present invention, the tolerance of the optical coupling efficiency with respect to the positional deviation between the photonic crystal resonator and the optical waveguide, particularly the tolerance in any direction in the plane parallel to the photonic crystal substrate is increased. This makes it possible to obtain optical coupling between the two without requiring high alignment accuracy, and to easily obtain high efficiency of optical coupling between the photonic crystal resonator and the optical waveguide. Be able to.

  In particular, the highest coupling efficiency can be obtained by setting the crossing angle between the first direction and the second direction to an angle formed by the sum vector of two lattice vectors in the arrangement of lattice elements of the photonic crystal. Furthermore, when the optical waveguide is arranged along the extending direction of the resonance part, the Q value of the photonic crystal resonator can be adjusted by adjusting the distance between the photonic crystal resonator and the optical waveguide. In addition to this, in addition to this, adjustment can be made by changing the intersection angle between the first direction and the second direction.

  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 lattice elements 103 are arranged in a triangular lattice shape in plan view. However, the present invention is not limited to this, and the lattice elements 103 may be arranged in a square lattice (square lattice). In this case, the angle formed by the sum vector of two lattice vectors in the array of lattice elements is 45 °, and the highest coupling efficiency can be obtained if the crossing angle between the first direction and the second direction is 45 °. become. 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 resonance comprising 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 periphery, and a resonance portion is provided extending in the first direction And
An optical waveguide that is disposed on the photonic crystal resonator, extends in a second direction different from the first direction, intersects with a layer different from the resonance unit, and is optically coupled to the resonance unit at the intersection It equipped with a door,
Crossing angle between the first direction and the second direction, the sum vector of two lattice vectors in the sequence of the grid element, that is the angle between one of the lattice vector of the two lattice vectors An optical circuit characterized by that. The optical circuit of claim 1 Symbol placement,
The optical element, wherein the lattice elements are arranged in a triangular lattice or a square lattice.