JP2020027212A - Optical hybrid circuit - Google Patents

Abstract

To provide an optical hybrid circuit which is easy to design and also tolerant of a width error.SOLUTION: A first output part of a 4×4 coupler 40 is connected to an input part of a connection waveguide part 60, a second output part of the 4×4 coupler to a first input part of a 3×3 coupler 50, a third output part of the 4×4 coupler to a second input part of the 3×3 coupler, and a fourth output part of the 4×4 coupler to a third input part of the 3×3 coupler. The 4×4 coupler outputs interference light between light input to the first input part and light input to the third input part from a first output part and a fourth output part, and also outputs interference light between the light input to the first input part and light, obtained by rotating the phase of the light input to the third input part by π/2, from a second output part and a third output part, the 3×3 coupler outputs light input to the first input part from the third output part, light input to the second input part from the second output part, and the light input to the third input part from the first output part, respectively, and the connection waveguide part outputs the light input to the input part from an output part.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical hybrid circuit suitable for use in, for example, an apparatus for performing optical coherent detection.

  In recent years, silicon (Si) waveguides using a silicon-based material as a waveguide material have attracted attention because of their small size and excellent mass productivity. In the Si waveguide, an optical waveguide core which is substantially a light transmission path is formed using Si as a material. Then, the periphery of the optical waveguide core is covered with a clad made of, for example, silica or the like having a lower refractive index than Si. With such a configuration, the refractive index difference between the optical waveguide core and the clad becomes extremely large, so that light can be strongly confined in the optical waveguide core. As a result, it is possible to realize a small-sized curved waveguide whose bending radius is reduced to, for example, about 1 μm. Therefore, it is possible to create an optical circuit having a size similar to that of an electronic circuit, which is advantageous in reducing the size of the entire optical device.

  Further, in the case of the Si waveguide, a manufacturing process of a semiconductor device such as a CMOS (Complementary Metal Oxide Semiconductor) can be used. For this reason, the realization of photoelectric fusion (silicon photonics) in which an electronic functional circuit and an optical functional circuit are collectively formed on a chip is expected.

  A circuit useful when this Si waveguide is used for optical coherent detection is an optical hybrid circuit. A conventional example of an optical hybrid circuit will be described with reference to FIG. FIG. 8 is a schematic diagram illustrating a conventional example of an optical hybrid circuit. FIG. 8 shows only the optical waveguide core.

  The optical hybrid circuit includes first and second input branch elements 402 and 404, first to fourth optical waveguides 412, 414, 416 and 418 extending in the longitudinal direction, a phase rotation element 420, The first and second directional couplers 432 and 434 are provided. First and second optical waveguides 412 and 414 are connected to the first input branch element 402, and third and fourth optical waveguides 416 and 418 are connected to the second input branch element 404. I have. The second optical waveguide 414 and the third optical waveguide 416 cross each other, the first and third optical waveguides 412 and 416 are connected to the first directional coupler 432, and the second and fourth optical waveguides are connected. 414 and 418 are connected to the second directional coupler 434. Further, a phase rotation element 420 is provided in the fourth optical waveguide 418. The phase rotation element 420 imparts a phase rotation of 90 degrees (π / 2) to the light propagating through the fourth optical waveguide 418.

  Many improvements have been made to this optical hybrid circuit. First, in order to reduce the number of components, it has been considered to use a 2 × 4 type multi-mode interference (MMI) coupler (for example, see Patent Document 1).

  The optical hybrid circuit disclosed in Patent Document 1 is realized by one MMI coupler.

  In addition, there is no single-mode waveguide for connecting the input branch element and the directional coupler to generate a phase rotation of 90 degrees. Therefore, it is possible to avoid a phase error that tends to occur in the single mode waveguide.

  However, in order to obtain a necessary phase relationship, it is necessary to intersect three of the four optical waveguides. At the intersection of the optical waveguides, it is necessary to maintain a certain angle, and it is difficult to reduce the size of the circuit, and the design is complicated.

  In order to solve the problem of Patent Document 1, there is a technique of connecting a 2 × 2 MMI coupler to two output ports among four output ports of a 2 × 4 MMI coupler, and further using a width taper MMI coupler as the MMI coupler. (For example, see Patent Document 2).

  In addition, there is a technique in which a 1 × 2 MMI coupler and a 2 × 2 MMI coupler are used as input branch elements instead of causing a 90-degree phase rotation in a single mode waveguide (for example, see Patent Document 3). In the optical hybrid circuit of Patent Document 3, the phase rotation is not caused in the single mode waveguide.

JP 2012-518202 A Patent No. 5287527 Patent No. 5243607

  However, the optical hybrid circuit of Patent Document 2 has a simple structure because no crossing of optical waveguides occurs, but it is difficult to design a width tapered MMI coupler. Further, in each of the optical hybrid circuits of Patent Documents 1 and 3, there is a drawback that a width error is weakened because a single mode waveguide is frequently used between components and the like.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an optical hybrid circuit that is easy to design and that is resistant to a width error.

  In order to achieve the above-described object, an optical hybrid circuit according to the present invention includes an optical waveguide core and a clad including the optical waveguide core.

  The optical waveguide core includes a 4 × 4 coupler, a 3 × 3 coupler, and a connection waveguide. The 4 × 4 coupler includes first to fourth input sections arranged in parallel on one end side in this order, and first to fourth output sections arranged in parallel in this order on the other end side. , The first output unit faces each other, the second input unit and the second output unit face each other, the third input unit and the third output unit face each other, and the fourth input unit and the fourth output unit. Face each other. The 3 × 3 coupler includes first to third input units arranged in parallel on one end side in this order, and first to third output units arranged in parallel in this order on the other end side. And the first output unit face each other, the second input unit and the second output unit face each other, and the third input unit and the third output unit face each other. The connection waveguide unit includes an input unit and an output unit.

  A first output of the 4 × 4 coupler is connected to an input of the connection waveguide. The second output of the 4 × 4 coupler is connected to the first input of the 3 × 3 coupler. The third output of the 4 × 4 coupler is connected to the second input of the 3 × 3 coupler. The fourth output of the 4 × 4 coupler is connected to the third input of the 3 × 3 coupler.

  In the optical hybrid circuit according to the present invention, the first light is input to the first input of the 4 × 4 coupler, and the second light is input to the third input of the 4 × 4 coupler.

  The 4 × 4 coupler is configured as a multi-mode interference coupler, and outputs the interference light between the light input to the first input unit and the light input to the third input unit to a first output unit and a fourth output unit. And the interference light between the light input to the first input unit and the light obtained by applying a phase rotation of π / 2 to the light input to the third input unit is output to the second output unit and the third output unit. Output from the output unit. The 3 × 3 coupler is configured as a multi-mode interference coupler, and outputs light input to the first input unit from the third output unit, light input to the second input unit from the second output unit, and the third input unit. Light input to the input unit is output from the first output unit. The connection waveguide section outputs light input to the input section from the output section.

  The timing at which the light input to the 3 × 3 coupler is output from the 3 × 3 coupler and the timing at which the light input to the connection waveguide is output from the connection waveguide are aligned.

  The optical hybrid circuit of the present invention has a function equivalent to that of the conventional optical hybrid circuit described with reference to FIG. Moreover, since the optical hybrid circuit of the present invention does not include the width tapered MMI coupler, the design is easy.

  Further, in the optical hybrid circuit of the present invention, even when a single mode waveguide is used, the single mode waveguide is used only in the connection waveguide section. Compared to a conventional optical hybrid circuit that uses many waveguides, it is more resistant to width errors.

FIG. 2 is a schematic plan view showing a first optical hybrid circuit. FIG. 2 is a schematic end view showing a first optical hybrid circuit. (A) and (B) are diagrams illustrating characteristics of the first optical hybrid circuit. FIG. 5 is a schematic plan view showing a second optical hybrid circuit. It is a schematic plan view showing a modification of the second optical hybrid circuit. (A) is a diagram showing characteristics of the second optical hybrid circuit, and (B) is a diagram showing characteristics of a modification of the second optical hybrid circuit. FIG. 9 is a schematic plan view showing a third optical hybrid circuit. FIG. 9 is a schematic diagram for explaining a conventional example of an optical hybrid circuit.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shapes, sizes, and arrangements of the components are only schematically shown to the extent that the present invention can be understood. Hereinafter, a preferred configuration example of the present invention will be described. However, the materials and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the present invention.

(First optical hybrid circuit)
An optical hybrid circuit according to a first embodiment of the present invention (hereinafter, also referred to as a first optical hybrid circuit) will be described with reference to FIGS. 1 and 2 are schematic diagrams for explaining the first optical hybrid circuit. FIG. 1 is a schematic plan view showing the first optical hybrid circuit. FIG. 2 is a schematic end view of the structure shown in FIG. 1 taken along line II. In FIG. 1, only the optical waveguide core is shown, and a support substrate and a clad described later are omitted.

  In the following description, the direction along the light propagation direction is the length direction for each component. The direction along the thickness of the support substrate is defined as the thickness direction. The direction perpendicular to the length direction and the thickness direction is defined as the width direction.

  The first optical hybrid circuit 100 includes a support substrate 10, a clad 20, and an optical waveguide core 30.

  The support substrate 10 is formed of, for example, a flat plate made of single crystal Si.

The clad 20 is provided on the support substrate 10. The clad 20 covers the upper surface of the support substrate 10 and includes the optical waveguide core 30. The clad 20 is formed using, for example, silicon oxide (SiO ₂ ) as a material.

  The optical waveguide core 30 is formed of, for example, silicon (Si) having a higher refractive index than the cladding 20. As a result, the optical waveguide core 30 functions as a light transmission path, and the light input to the optical waveguide core 30 propagates in a propagation direction according to the planar shape of the optical waveguide core 30.

  Here, in order to prevent light propagating through the optical waveguide core 30 from escaping to the support substrate 10, the optical waveguide core 30 is preferably formed at least 1 μm or more away from the support substrate 10.

  The thickness of the optical waveguide core 30 is desirably a thickness that can achieve a single mode condition, for example, a thickness in the range of 200 to 500 nm. For example, when the first optical hybrid circuit 100 is used in a wavelength band of 1550 nm, the thickness of the optical waveguide core 30 can be set to 200 nm.

  The optical waveguide core 30 includes a first input waveguide section 110, a second input waveguide section 120, a 4 × 4 coupler 40, a 3 × 3 coupler 50, a connection waveguide section 60, a first output waveguide section 210, and a second output waveguide section 210. An output waveguide section 220, a third output waveguide section 230, and a fourth output waveguide section 240 are provided. The first output waveguide section 210, the second output waveguide section 220, the third output waveguide section 230, and the fourth output waveguide section 240 are arranged in parallel in this order.

  Each of the 4 × 4 coupler 40 and the 3 × 3 coupler 50 is configured as a so-called multi-mode interference (MMI) coupler according to the planar shape of the optical waveguide core 30.

  The 4 × 4 coupler 40 has first to fourth input units 41a to 41d arranged in parallel in this order on one end side and first to fourth output units 42a to 42d arranged in parallel in this order on the other end side. It is comprised including. In the 4 × 4 coupler 40, the first input section 41a and the first output section 42a face each other, the second input section 41b and the second output section 42b face each other, and the third input section 41c and the third output section 41c. The unit 42c faces each other, and the fourth input unit 41d and the fourth output unit 42d face each other.

  The 4 × 4 coupler 40 mixes light input to the first input unit 41a and the third input unit 41c, and outputs the light from the first to fourth output units 42a to 42d. The 4 × 4 coupler 40 converts the interference light between the first light input to the first input unit 41a and the second light input to the third input unit 41c into a first output unit 42a and a fourth output unit. Output from 42d. The 4 × 4 coupler 40 rotates the phase of the first light input to the first input unit 41a and the second light input to the third input unit 41c by 90 degrees (π / 2). The interference light with the given light is output from the second output unit 42b and the third output unit 42c.

  The 3 × 3 coupler 50 has first to third input sections 51a to 51c arranged in parallel in this order on one end side and first to third output sections 52a to 52c arranged in parallel in this order on the other end side. It is comprised including. In the 3 × 3 coupler 50, the first input unit 51a and the first output unit 52a face each other, the second input unit 51b and the second output unit 52b face each other, and the third input unit 51c and the third output unit 51c. The part 52c faces each other.

  The 3 × 3 coupler 50 is configured as a so-called mirror image type coupler, and outputs light input to the first to third input units 51a to 51c from the first to third output units 52a to 52c. The 3 × 3 coupler 50 inputs light input to the first input unit 51a from the third output unit 52c, and inputs light input to the second input unit 51b from the second output unit 52b to the third input unit 51c. The output light is output from the first output unit 52a.

  The first input waveguide section 110 includes an input port section 111 and an input taper section 112 connected in series. The width of the input port section 111 is set to, for example, achieve a single mode condition. Therefore, the input port 111 propagates light in the fundamental mode. The input taper section 112 is connected to the first input section 41a of the 4 × 4 coupler 40. The input taper portion 112 has a continuous width from one end connected to the input port portion 111 to the other end connected to the 4 × 4 coupler 40 (ie, toward the end connected to the 4 × 4 coupler 40). Are formed in a tapered shape that expands.

  The second input waveguide unit 120 includes an input port unit 121 and an input taper unit 122 connected in series. The input port section 121 and the input tapered section 122 of the second input waveguide section 120 can be configured similarly to the input port section 111 and the input tapered section 112 of the first input waveguide section 110. The input taper section 122 of the second input waveguide section 120 is connected to the third input section 41c of the 4 × 4 coupler 40.

  The first output section 42 a of the 4 × 4 coupler 40 is connected to the input section 65 of the connection waveguide section 60. The second output section 42 b of the 4 × 4 coupler 40 is connected to the first input section 51 a of the 3 × 3 coupler 50. The third output section 42c of the 4 × 4 coupler 40 is connected to the second input section 51b of the 3 × 3 coupler 50. The fourth output section 42d of the 4 × 4 coupler 40 is connected to the third input section 51c of the 3 × 3 coupler 50.

  The output section 66 of the connection waveguide section 60 is connected to the first output waveguide section 210.

  The first output section 52 a of the 3 × 3 coupler 50 is connected to the output taper section 221 of the second output waveguide section 220. The second output section 52b of the 3 × 3 coupler 50 is connected to the output taper section 231 of the third output waveguide section 230. The third output section 52c of the 3 × 3 coupler 50 is connected to the output taper section 241 of the fourth output waveguide section 240.

  The connection waveguide section 60 includes a tapered section 61, a first arm waveguide section 62, a straight waveguide section 63, and a second arm waveguide section 64 connected in series in this order.

  The tapered portion 61 extends from one end connected to the 4 × 4 coupler 40 to the other end connected to the first arm waveguide portion 62 (ie, in a direction away from the end connected to the 4 × 4 coupler 40). Are formed in a tapered shape whose width is continuously reduced.

  The first arm waveguide section 62 is formed as a curved waveguide that is separated from the 3 × 3 coupler 50 toward the straight waveguide section 63. The linear waveguide 63 is formed as a linear waveguide connecting the first arm waveguide 62 and the second arm waveguide 64. The second arm waveguide section 64 is formed as a curved waveguide that approaches the 3 × 3 coupler 50 as it goes toward the first output waveguide section 210. The first arm waveguide section 62, the linear waveguide section 63, and the second arm waveguide section 64 are set to have widths that achieve the single mode condition. Therefore, the first arm waveguide section 62, the linear waveguide section 63, and the second arm waveguide section 64 propagate light in the fundamental mode.

  Here, one end of the tapered portion 61 connected to the 4 × 4 coupler 40 is an input portion 65. The other end of the second arm waveguide 64 opposite to the one connected to the linear waveguide 63 is defined as an output 66.

  The first output waveguide section 210 is set to a width that achieves, for example, a single mode condition. Therefore, the first output waveguide unit 210 propagates light in the fundamental mode.

  The second output waveguide section 220 includes an output taper section 221 and an output port section 222 connected in series in this order. The output taper portion 221 is continuous from one end connected to the 3 × 3 coupler 50 to the other end connected to the output port portion 222 (that is, in a direction away from the end connected to the 3 × 3 coupler 50). It is formed in a tapered shape whose width is gradually reduced.

  The third output waveguide section 230 includes an output taper section 231 and an output port section 232 connected in series in this order. The fourth output waveguide section 240 includes an output taper section 241 and an output port section 242 connected in this order in series. The output taper section 231 and the output port section 232 of the third output waveguide section 230 and the output taper section 241 and the output port section 242 of the fourth output waveguide section 240 are combined with the output taper section 221 of the second output waveguide section 220. And the output port unit 222.

  Next, the design of the above-described 4 × 4 coupler 40, 3 × 3 coupler 50, and connection waveguide unit 60 will be described.

  The length L of the MMI coupler can be expressed by the following equation (1), where W is the width, n is the equivalent refractive index, and λ is the wavelength.

L = 4nW ² / (Nλ) (1)
In the above equation (1), N is a parameter for image formation. In the MMI coupler, when the input light is divided into four equal parts, N = 4. The width of the 4 × 4 coupler 40 as _{W 1,} from the above equation (1), the length _{L 1} of the 4 × 4 coupler 40 _{becomes ^{L 1 = nW 1 2 / λ}} .

The width W ₂ of the 3 × 3 coupler 50 is approximately ／ of the width W ₁ of the 4 × 4 coupler 40. In a so-called mirror image type coupler, N = 1. From the above equation (1), the length _{L 2} of the 3 × 3 coupler ^{_{50, L 2 = 4nW 2 2 /}} λ = 4n (3W 1/4) 2 / λ = a (9/4) _{L 1.}

Accordingly, the length _{L 2} of the 3 × 3 coupler 50 becomes 2.25 times the length _{L 1} of the 4 × 4 coupler 40.

  The connection waveguide section 60 is formed as a single mode waveguide as described above. For this reason, a difference occurs in the group refractive index between the connection waveguide unit 60 and the 3 × 3 coupler 50 as the MMI coupler, and a difference also occurs in the speed of light propagating through each. Thus, by adjusting the optical path length of the connection waveguide section 60, the timing at which the light input to the 3 × 3 coupler 50 is output from the 3 × 3 coupler 50 and the light input to the connection waveguide section 60 are reduced. The timing output from the connection waveguide unit 60 can be made to correspond.

  When the first optical hybrid circuit 100 is used for coherent detection, signal light is input to the first input waveguide unit 110 as first light. The signal light is sent to the first input unit 41a of the 4 × 4 coupler 40 through the input port unit 111 and the input taper unit 112 sequentially. Local light is input to the second input waveguide unit 120 as the second light. The local light is sent to the third input unit 41 c of the 4 × 4 coupler 40 through the input port unit 121 and the input taper unit 122 in order.

  The signal light and the local light sent to the 4 × 4 coupler 40 are mixed in the 4 × 4 coupler 40. Then, the signal light and the interference light of the local light are output from the first to fourth output units 42a to 42d, respectively. The second output unit 42b and the third output unit 42c output interference light between the signal light and the local light having a phase rotation of 90 degrees (π / 2). From the first output unit 42a and the fourth output unit 42d, interference light between the signal light and the local light that is not given a phase rotation is output (JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.28, NO.9, MAY 1 , 2010 pp.1323-1331).

The phase difference φ (x, y) of the light output from each of the output units 42a to 42d with respect to the light input to each of the input units 41a to 41d of the 4 × 4 coupler 40 is lower than the phase difference φ (x, y) except for the common phase. It is represented by equations (2) and (3). Here, N = 4. Also, [psi ₀ indicates the phase of the input light. In addition, x indicates the number of the input units 41a to 41d, and y indicates the number of the output units 42a to 42d. Therefore, here, x is an integer of 1 to 4, and y is an integer of 1 to 4. The phase difference between the light input from the x-th input unit 41 and the light output from the y-th output unit 42 is expressed by the following equation (2) when x + y is an even number, and is expressed by the following equation (2) when x + y is an odd number. It is represented by the following equation (3).

φ (x, y) = π + φ _{N− (y−x) / 2} = { ₀ + π + {π (y−x) (2N−y + x)} / 4N (2)
φ (x, y) = π + φ N- (y + x-1) / 2 = ψ 0 + {π (y + x-1) (2N-y-x + 1)} / 4N ··· (3)
Further, the output power P (y) from the y-th output unit when the light input to the first input unit 41a and the light input to the third input unit 41c interfere is represented by the following equation (4).

P (y) = [P ₁ ^1/2 exp {iφ (1, y)} + P ₃ ^1/2 exp {iφ (3, y) + iφ ₀ }] [P ₁ ^1/2 exp} −iφ (1, ^{_{y)} + P 3 1/2 exp}} {-iφ (3, y) -iφ 0}] / 4 = [P 1 + P 3 +2 (P 1 P 3) 1/2 cos {iφ (1, y) -iφ (3, y) −iφ ₀ }] / 4 (4)
Here, φ ₀ is the phase difference between the light input to the first input unit 41a and the light input to the third input unit 41c, and P ₁ is the input of the light input to the first input unit 41a. the power, P ₃ is the input power of light input to the third input unit 41c, respectively.

From the above equation (4), the above equation in cos (4), first to fourth output unit output power phase of P (y) of 42a~42d are each _{0 + φ 00, π / 2} + φ 00, 3π / 2 + φ ₀₀ and π + φ ₀₀ to become. Here, φ ₀ = −3π / 4−φ ₀₀ .

  As a result, the phase of the output light from the first output unit 42a is + π / 2 for the output light from the second output unit 42b, + 3π / 2 for the output light from the third output unit 42c, and the fourth output. The output light from the section 42d has a phase difference of + π (Appl. Opt 33, pp. 3905).

  Therefore, a phase difference of π occurs between the output light from the first output unit 42a and the output light from the fourth output unit 42d. A phase difference of π occurs between the output light from the second output unit 42b and the output light from the third output unit 42c. Then, a phase difference of π / 2 occurs between the output light from the first output unit 42a and the output light from the second output unit 42b. Further, a phase difference of π / 2 occurs between the output light from the third output unit 42c and the output light from the fourth output unit 42d.

  The light output from the first output section 42 a of the 4 × 4 coupler 40 is sent to the input section 65 of the connection waveguide section 60. The light transmitted to the input section 65 of the connection waveguide section 60 passes through the taper section 61, the first arm waveguide section 62, the linear waveguide section 63, and the second arm waveguide section 64 in this order, and is output from the output section 66. Is output.

  The light output from the second output section 42 b of the 4 × 4 coupler 40 is sent to the first input section 51 a of the 3 × 3 coupler 50. The light input to the 3 × 3 coupler 50 from the first input unit 51a is output from the third output unit 52c. The light output from the third output section 42c of the 4 × 4 coupler 40 is sent to the second input section 51b of the 3 × 3 coupler 50. The light input to the 3 × 3 coupler 50 from the second input unit 51b is output from the second output unit 52b. The light output from the fourth output section 42 d of the 4 × 4 coupler 40 is sent to the third input section 51 c of the 3 × 3 coupler 50. The light input to the 3 × 3 coupler 50 from the third input unit 51c is output from the first output unit 52a.

  The light output from the output unit 66 of the connection waveguide unit 60 is sent to the first output waveguide unit 210. The light output from the first output section 52a to the third output section 52c of the 3 × 3 coupler 50 is sent to the second output waveguide section 220 to the fourth output waveguide section 240, respectively.

  The light sent to the first output waveguide section 210 to the fourth output waveguide section 240 is sent to the first balanced light receiver 80 and the second balanced light receiver 90. The first balance light receiver 80 and the second balance light receiver 90 are each provided with a pair of photodiodes (PD: Photodiode). Here, the first balanced light receiver 80 includes PDs 81 and 82, and the second balanced light receiver 90 includes PDs 91 and 92. The PDs 81 and 82 included in the first balanced light receiver 80 and the PDs 91 and 92 included in the second balanced light receiver 90 are arranged in parallel. The first balanced light receiver 80 and the second balanced light receiver 90 receive light when they are input to the PDs 81 and 82 and the PDs 91 and 92. The light sent to the first output waveguide unit 210 is input to one PD 81 of the first balanced light receiver 80 via the first output waveguide unit 210. The light sent to the second output waveguide unit 220 is input to the other PD 82 of the first balanced light receiver 80 via the output taper unit 221 and the output port unit 222. Further, the light sent to the third output waveguide section 230 is input to one PD 91 of the second balanced light receiver 90 via the output taper section 231 and the output port section 232. Further, the light sent to the fourth output waveguide section 240 is input to the other PD 92 of the second balanced light receiver 90 via the output taper section 241 and the output port section 242.

  By providing the connecting waveguide section 60 and the 3 × 3 coupler 50 between the 4 × 4 coupler 40 and the first to fourth output waveguide sections 210 to 240, the PDs 81 and 82 of the first balanced light receiver 80 are provided. The phase difference of each input light is π. Further, the phase difference of each light input to the PDs 91 and 92 of the second balance light receiver 90 is π. Then, the phase difference between the light input to the PDs 81 and 82 of the first balanced light receiver 80 and the light input to the PDs 91 and 92 of the second balanced light receiver 90 is π / 2.

  Therefore, the first optical hybrid circuit 100 has a function equivalent to the conventional optical hybrid circuit described with reference to FIG.

  In addition, in the first optical hybrid circuit 100, the intersection of the optical waveguides does not occur, so that the manufacturing is simple, and further, since the width tapered MMI coupler is not included, the design is easy. In the first optical hybrid circuit 100, the connection waveguide section 60 is provided between the first input waveguide section 110 and the second input waveguide section 120 and the first to fourth output waveguide sections 210 to 240. Only a single mode waveguide is used. For this reason, it is more resistant to a width error than a conventional optical hybrid circuit that frequently uses a single mode waveguide.

  In the first optical hybrid circuit 100, the first input waveguide unit 110 and the second input waveguide unit 120 are connected to the 4 × 4 coupler 40 using the input taper unit 112 and the input taper unit 122. Further, in the first optical hybrid circuit 100, the 4 × 4 coupler 40 and the connection waveguide unit 60 are connected by using the tapered portion 61. Further, in the first optical hybrid circuit 100, the 3 × 3 coupler 50, the second output waveguide section 220, and the third output waveguide section 230 are formed by using the output taper section 221, the output taper section 231 and the output taper section 241. And the fourth output waveguide section 240. Thereby, in the first optical hybrid circuit 100, it is possible to suppress the variation in the input intensity and the output intensity between the constituent elements and to reduce the propagation loss.

  Further, in the first optical hybrid circuit 100, the connection waveguide section 60 includes the first arm waveguide section 62 and the second arm waveguide section 64, so that the connection waveguide section 60 and the 3 × 3 coupler 50 A large gap can be secured. Thereby, undesired coupling between the light propagating through the connection waveguide section 60 and the light propagating through the 3 × 3 coupler 50 can be prevented.

(Characteristic evaluation)
The inventor performed a simulation for evaluating the characteristics of the first optical hybrid circuit 100 by using a three-dimensional beam propagation method (BPM: Beam Propagation Method).

In this simulation, light was input from the first input waveguide unit 110 and the second input waveguide unit 120, and the intensity of light output from each of the first to fourth output waveguide units 210 to 240 was obtained. In this simulation, the optical waveguide core 30 was made of Si and the clad 20 was made of SiO ₂ . The optical waveguide core 30 had a thickness of 200 nm as a whole. Light input to the first input waveguide unit 110 and the second input waveguide unit 120 was assumed to be TE polarized light having a wavelength of 1550 nm.

  FIG. 3 shows the result of the simulation. FIGS. 3A and 3B are diagrams showing the relationship between the output from each output waveguide and the phase difference in two first optical hybrid circuits 100 having different component sizes. 3A and 3B, the horizontal axis represents the phase difference (degree) and the vertical axis represents the output light intensity. 3A, a curve 301 indicates the output intensity from the first output waveguide unit 210, a curve 302 indicates the output intensity from the second output waveguide unit 220, and a curve 303 indicates the output intensity from the third output waveguide unit 230. , And the curve 304 shows the output intensity from the fourth output waveguide section 240. In FIG. 3B, a curve 311 represents the output intensity from the first output waveguide unit 210, a curve 312 represents the output intensity from the second output waveguide unit 220, and a curve 313 represents the third output waveguide unit. The output intensity from 230 and the curve 314 represent the output intensity from the fourth output waveguide section 240, respectively.

  In the first optical hybrid circuit 100 according to FIG. 3A, the width of the 4 × 4 coupler 40 is 5.6 μm and the length is 49.6 μm. The width of the 3 × 3 coupler 50 was set to 4.2 μm and the length was set to 112.6 μm. The width of the input taper portion 112, the input taper portion 122, and the taper portion 61 on the side connected to the 4 × 4 coupler 40 was 1 μm. The width of the output taper portion 221, the output taper portion 231 and the output taper portion 241 on the side connected to the 3 × 3 coupler 50 was 1 μm.

  On the other hand, in the first optical hybrid circuit 100 according to FIG. 3B, the width of the 4 × 4 coupler 40 is 6.787 μm and the length is 75 μm. The width of the 3 × 3 coupler 50 was 5.009025 μm and the length was 168.75 μm. The width of the input taper portion 112, the input taper portion 122, and the taper portion 61 on the side connected to the 4 × 4 coupler 40 was 1.2 μm. The width of the output taper portion 221, the output taper portion 231 and the output taper portion 241 on the side connected to the 3 × 3 coupler 50 was 1.2 μm.

  According to FIGS. 3A and 3B, the output light from the first to fourth output waveguide sections 210 to 240 has the desired phase relationship described above. Also, comparing FIGS. 3A and 3B, in the first optical hybrid circuit 100 in which the width of the 4 × 4 coupler 40 and the 3 × 3 coupler 50 is small, the peak of the output intensity from each output waveguide unit is obtained. However, in the first optical hybrid circuit 100 in which the widths of the 4 × 4 coupler 40 and the 3 × 3 coupler 50 are large, the peaks of the output intensity from the output waveguide unit are aligned. Therefore, it is understood that the peak of the output intensity can be made uniform by setting the width of the 4 × 4 coupler 40 and the 3 × 3 coupler 50 to be large.

  In addition, even when the same simulation was performed only for the 4 × 4 coupler 40, it was found that the peak of the output intensity could be made uniform by setting the width to be large (not shown). Also, it was found that when the input taper portion 112, the input taper portion 122, the taper portion 61, the output taper portion 221, the output taper portion 231 and the output taper portion 241 were not provided, the peak of the output intensity varied (see FIG. Zu).

(Second optical hybrid circuit)
An optical hybrid circuit according to a second embodiment of the present invention (hereinafter, also referred to as a second optical hybrid circuit) will be described with reference to FIG. FIG. 4 is a schematic plan view showing the second optical hybrid circuit. In FIG. 4, only the optical waveguide core is shown, and the support substrate and the clad are omitted. The second optical hybrid circuit differs from the above-described first optical hybrid circuit in that the connection waveguide section is a 1 × 1 coupler. The same components as those of the first optical hybrid circuit are denoted by the same reference numerals, and description thereof will be omitted.

  In the second optical hybrid circuit 200, the connection waveguide section 260 is a 1 × 1 coupler configured as an MMI coupler. In the following description, the connection waveguide 260 may be referred to as a 1 × 1 coupler 260.

  In the 1 × 1 coupler 260, the input unit 261 is arranged on one end side, and the output unit 262 is arranged on the other end side. The input unit 261 is arranged on one end of the 1 × 1 coupler 260 on the side close to the 3 × 3 coupler 50, and the output unit 262 is arranged on the other end of the 1 × 1 coupler 260 on the side close to the 3 × 3 coupler 50. , The input unit 261 and the output unit 262 are arranged to face each other.

  The input section 261 of the 1 × 1 coupler 260 is connected to the first output section 42 a of the 4 × 4 coupler 40. The output section 262 of the 1 × 1 coupler 260 is connected to the first output waveguide section 210. The 1 × 1 coupler 260 sends the light sent from the first output section 42 a of the 4 × 4 coupler 40 to the first output waveguide section 210.

Here, in the 1 × 1 coupler 260, when the light input from the input unit 261 is output from the output unit 262 facing thereto, the parameter N for image formation in the above equation (1) is １ ／. Accordingly, as the width of the 1 × 1 coupler 260 _{W 3,} 1 × 1 length _{L 3} of the coupler 260 _{becomes ^{L 3 = 8nW 3 2 / λ}} . As described above, the length _{L 2} of the 3 × 3 coupler 50 _{is ^{L 2 = 4nW 2 2 / λ}} . Therefore, when the length L ₃ of the 1 × 1 coupler 260 is equal to the length L ₂ of the 3 × 3 coupler 50, the width of the 1 × 1 coupler 260 is 1 / √2 of the width of the 3 × 3 coupler 50. Becomes

  Further, in the second optical hybrid circuit 200, the first output waveguide section 210 is formed by the output taper section 211 and the output port connected in series in this order, similarly to the second to third output waveguide sections 220 to 230. It comprises a part 212. The output taper section 211 is connected to the output section 262 of the 1 × 1 coupler 260. The output taper section 211 is continuous from one end connected to the 1 × 1 coupler 260 to the other end connected to the output port section 212 (ie, in a direction away from the end connected to the 1 × 1 coupler 260). It is formed in a tapered shape whose width is gradually reduced.

  In the second optical hybrid circuit 200, the connection waveguide section 260 is configured by a 1 × 1 coupler. For this reason, both the connection waveguide section 260 and the 3 × 3 coupler 50 are configured by MMI couplers. As a result, the group refractive index difference between the connection waveguide 260 and the 3 × 3 coupler 50 is smaller than that in the case where the connection waveguide 260 is formed as a single mode waveguide, and the difference in the speed of light propagating therethrough is also smaller. small. Therefore, the connection waveguide 260 and the 3 × 3 coupler 50 can be designed with a common length without adjusting the optical path length of the connection waveguide 260. Therefore, the second optical hybrid circuit 200 is easy to design.

  Here, in the second optical hybrid circuit, as shown in FIG. 5, the 1 × 1 coupler 260 may include a first portion 263 and a second portion 264. FIG. 5 is a schematic plan view showing a modification of the second optical hybrid circuit. In FIG. 5, only the optical waveguide core is shown, and the support substrate and the clad are omitted.

  In the optical hybrid circuit 250 according to this modified example, the 1 × 1 coupler 260 includes a first portion 263 and a second portion 264 connected in series in this order. The first portion 263 is formed so as to be further away from the 3 × 3 coupler 50 toward the second portion 264 (that is, toward the output portion 262). Further, the second portion 264 is formed so as to approach the 3 × 3 coupler 50 toward the first output waveguide section 210 (that is, toward the output section 262). As a result, in the optical hybrid circuit 250 according to the modification, a large gap between the 1 × 1 coupler 260 and the 3 × 3 coupler 50 can be secured. Thus, undesired coupling between the light propagating through the 1 × 1 coupler 260 and the light propagating through the 3 × 3 coupler 50 can be prevented.

(Characteristic evaluation)
The inventor performed a simulation for evaluating the characteristics of the second optical hybrid circuit 200 and the optical hybrid circuit 250 according to the modification using the three-dimensional BPM.

  In this simulation, with respect to the second optical hybrid circuit 200 and the optical hybrid circuit 250 according to the modification, light is input from the first input waveguide unit 110 and the second input waveguide unit 120 and the first to fourth output waveguides are input. The intensity of light output from each of the waveguides 210 to 240 was obtained.

  FIG. 6 shows the result of the simulation. FIGS. 6A and 6B are diagrams showing the relationship between the output from each output waveguide and the phase difference in the second optical hybrid circuit 200 and the optical hybrid circuit 250 according to the modification. 6A and 6B, the horizontal axis indicates the phase difference (degree) and the vertical axis indicates the output light intensity. 6A, a curve 501 indicates the output intensity from the first output waveguide unit 210, a curve 502 indicates the output intensity from the second output waveguide unit 220, and a curve 503 indicates the output intensity from the third output waveguide unit 230. And the curve 504 indicates the output intensity from the fourth output waveguide section 240. In FIG. 3B, a curve 511 indicates the output intensity from the first output waveguide unit 210, a curve 512 indicates the output intensity from the second output waveguide unit 220, and a curve 513 indicates the third output waveguide unit. The output intensity from 230 and the curve 514 represent the output intensity from the fourth output waveguide section 240, respectively.

  In the simulation according to FIG. 6A, the second optical hybrid circuit 200 shown in FIG. 4 was assumed. The width of the 1 × 1 coupler 260 was 1 / 1.42 of the 3 × 3 coupler 50 and the length was common to the 3 × 3 coupler 50. Other conditions are the same as in the simulation according to FIG. 3B described above.

  On the other hand, in the simulation according to FIG. 6B, the optical hybrid circuit 250 according to the modification shown in FIG. 5 is assumed. Then, the width of the 1 × 1 coupler 260 was set to 1 / 1.42 of the 3 × 3 coupler 50. In addition, one end of the 1 × 1 coupler 260 connected to the 4 × 4 coupler 40 and one end of the 3 × 3 coupler 50 connected to the 4 × 4 coupler 40 are aligned, and The surface positions of the other end connected to the first output waveguide unit 210 and the other end of the 3 × 3 coupler 50 connected to the second to fourth output waveguide units 220 to 240 were aligned. Further, the first portion 263 and the second portion 264 of the 1 × 1 coupler 260 were designed such that the gap G between the portion closest to the 3 × 3 coupler 50 and the portion farthest from the 3 × 3 coupler 50 was 2 μm.

  According to FIGS. 6A and 6B, in both the second optical hybrid circuit 200 and the optical hybrid circuit 250 according to the modified example, the output light from the first to fourth output waveguide units 210 to 240 is as described above. A desired phase relationship is shown.

(Third optical hybrid circuit)
An optical hybrid circuit according to a third embodiment of the present invention (hereinafter, also referred to as a third optical hybrid circuit) will be described with reference to FIG. FIG. 7 is a schematic plan view showing the third optical hybrid circuit. In FIG. 7, only the optical waveguide core is shown, and the support substrate and the clad are omitted. The third optical hybrid circuit is different from the above-described first and second optical hybrid circuits in that the 1 × 1 coupler as the connection waveguide is formed with a common design with the 3 × 3 coupler. I do. Components common to the first and second optical hybrid circuits are denoted by the same reference numerals, and description thereof is omitted.

  In the third optical hybrid circuit 300, the connection waveguide section 360 is a 1 × 1 coupler configured as an MMI coupler. In the following description, the connection waveguide section 360 may be referred to as a 1 × 1 coupler 360.

  In the 1 × 1 coupler 360, the input unit 361 is arranged on one end side, and the output unit 362 is arranged on the other end side. The input unit 361 is arranged on one end of the 1 × 1 coupler 360 on the side closer to the 3 × 3 coupler 50, and the output unit 362 is arranged on the other end of the 1 × 1 coupler 360 on the side farther from the 3 × 3 coupler 50. , The input unit 361 and the output unit 362 are arranged at positions that are point-symmetric with each other.

  The input section 361 of the 1 × 1 coupler 360 is connected to the first output section 42 a of the 4 × 4 coupler 40. The output section 362 of the 1 × 1 coupler 360 is connected to the first output waveguide section 210. The 1 × 1 coupler 360 sends the light sent from the first output section 42 a of the 4 × 4 coupler 40 to the first output waveguide section 210.

  The 1 × 1 coupler 360 has a common design with the 3 × 3 coupler 50 configured as a so-called mirror image type coupler. The input unit 361 of the 1 × 1 coupler 360 corresponds to the third input unit 51c of the 3 × 3 coupler 50. The output unit 362 of the 1 × 1 coupler 360 corresponds to the first output unit 52a of the 3 × 3 coupler 50. Therefore, the 1 × 1 coupler 360 functions in the same manner as the 3 × 3 coupler 50, and can output the light input from the input unit 361 from the output unit 362 disposed at a point-symmetric position.

  In the third optical hybrid circuit 300, the connection waveguide section 360 is configured by a 1 × 1 coupler formed by a common design with the 3 × 3 coupler 50. For this reason, the connection waveguide portion 360 and the 3 × 3 coupler 50 have the same group refractive index, and the speed of light propagating therethrough is also common. Therefore, the timing at which the light input to the 3 × 3 coupler 50 is output from the 3 × 3 coupler 50 and the light input to the 1 × 1 coupler 360 without adjusting the optical path length of the 1 × 1 coupler 360. Can be associated with the timing output from the 1 × 1 coupler 360. Further, in the third optical hybrid circuit 300, since the 1 × 1 coupler 360 is designed in common with the 3 × 3 coupler 50, the design is easy.

  In the third optical hybrid circuit 300, the output unit 362 of the 1 × 1 coupler 360 and the third optical hybrid circuit 300 are different from the first optical hybrid circuit 100 and the second optical hybrid circuit 200. The distance from one output unit 52a is increased. Therefore, by using the output port section 212 of the first output waveguide section 210 and the output port section 222 of the second output waveguide section 220 as a routing waveguide, the output sections 362 and 3 × 3 couplers of the 1 × 1 coupler 360 are used. The output light from the first output unit 52a can be sent to the common first balanced light receiver 80. At this time, the output port section 212 of the first output waveguide section 210 and the output port section 222 of the second output waveguide section 220, and the output port section 232 and the fourth output waveguide section of the third output waveguide section 230 By arranging the optical path lengths of the output port units 242 of the 240, it is possible to adjust the arrival timing of light to each of the first balanced light receiver 80 and the second balanced light receiver 90.

(Production method)
Each of the optical hybrid circuits described above can be easily manufactured by using, for example, an SOI (Silicon On Insulator) substrate. Hereinafter, an example of a method for manufacturing an optical hybrid circuit will be described.

First, an SOI substrate formed by sequentially laminating a support substrate layer, a SiO ₂ layer, and a Si layer is prepared. Next, an optical waveguide core is formed by performing, for example, dry etching and patterning the Si layer. Next, for example, by using a CVD (Chemical Vapor Deposition) method, SiO ₂ is formed on the SiO ₂ layer by covering the optical waveguide core. As a result, the optical waveguide core is included by the cladding of the SiO ₂ layer, and an optical hybrid circuit is obtained.

10: Supporting substrate 20: Cladding 30: Optical waveguide core 40: 4 × 4 coupler 50: 3 × 3 coupler 60, 260, 360: Connection waveguide section 61: Tapered section 62: First arm waveguide section 63: Linear conduction Waveguide portion 64: second arm waveguide portion 80: first balanced light receiver 90: second balanced light receiver 100: first optical hybrid circuit 110: first input waveguide portions 111 and 121: input port portions 112 and 122. : Input taper section 120: second input waveguide section 200, 250: second optical hybrid circuit 210: first output waveguide section 211, 221, 231, 241: output taper section 212, 222, 232, 242: output Port section 220: second output waveguide section 230: third output waveguide section 240: fourth output waveguide section 300: third optical hybrid circuit

Claims (8)

An optical waveguide core;
And a clad including the optical waveguide core,
The optical waveguide core includes:
On one end side, there are first to fourth input units arranged in parallel in this order, and on the other end side, there are first to fourth output units arranged in parallel in this order, and a first input unit and a first output unit are provided. Oppose each other, the second input unit and the second output unit oppose each other, the third input unit and the third output unit oppose each other, and the fourth input unit and the fourth output unit oppose each other. × 4 coupler,
On one end side, there are first to third input units arranged in parallel in this order, and on the other end side, there are first to third output units arranged in parallel in this order. Opposing each other, a second input unit and a second output unit opposing each other, and a third input unit and a third output unit opposing each other.
Including a connection waveguide section having an input section and an output section,
A first output unit of the 4 × 4 coupler is connected to an input unit of the connection waveguide unit;
A second output of the 4 × 4 coupler is connected to a first input of the 3 × 3 coupler;
A third output of the 4 × 4 coupler is connected to a second input of the 3 × 3 coupler;
A fourth output of the 4 × 4 coupler is connected to a third input of the 3 × 3 coupler;
A first light is input to a first input of the 4 × 4 coupler;
A second light is input to a third input of the 4 × 4 coupler;
The 4 × 4 coupler is configured as a multi-mode interference coupler, and outputs the interference light between the light input to the first input unit and the light input to the third input unit to the first output unit and the fourth output unit. The interference light between the light output from the unit and input to the first input unit and the light obtained by applying a phase rotation of π / 2 to the light input to the third input unit is output to the second output unit and the second output unit. 3 Output from the output unit,
The 3 × 3 coupler is configured as a multi-mode interference coupler. The light input to the first input unit is output from the third output unit, the light input to the second input unit is output from the second output unit, Lights input to the three input units are output from the first output unit, respectively.
The connection waveguide section outputs light input to the input section from the output section,
The timing at which the light input to the 3 × 3 coupler is output from the 3 × 3 coupler and the timing at which the light input to the connection waveguide is output from the connection waveguide are aligned. An optical hybrid circuit characterized by the following. The optical hybrid circuit according to claim 1, wherein the connection waveguide is a single mode waveguide. A first input waveguide connected to a first input of the 4 × 4 coupler;
A second input waveguide connected to a third input of the 4 × 4 coupler;
A first output waveguide unit connected to an output unit of the connection waveguide unit;
A second output waveguide connected to a first output of the 3 × 3 coupler;
A third output waveguide connected to a second output of the 3 × 3 coupler;
A fourth output waveguide connected to a third output of the 3 × 3 coupler,
The first input waveguide section includes an input taper section whose width continuously increases toward an end connected to the 4 × 4 coupler,
The second input waveguide section includes an input taper section whose width continuously increases toward an end connected to the 4 × 4 coupler,
The second output waveguide section includes an output taper section whose width continuously decreases in a direction away from the end connected to the 3 × 3 coupler,
The third output waveguide section includes an output taper section whose width continuously decreases in a direction away from an end connected to the 3 × 3 coupler,
The fourth output waveguide section includes an output taper section whose width continuously decreases in a direction away from the end connected to the 3 × 3 coupler,
The optical hybrid circuit according to claim 2, wherein the connection waveguide portion includes a tapered portion whose width continuously decreases in a direction away from an end connected to the 4x4 coupler. The connection waveguide unit is a 1 × 1 coupler configured as a multi-mode interference coupler,
An input section of the connection waveguide section is arranged on one end side of the connection waveguide section, and an output section of the connection waveguide section is arranged on the other end side of the connection waveguide section,
The optical hybrid circuit according to claim 1, wherein an input unit and an output unit of the connection waveguide unit are arranged to face each other. The connection waveguide portion includes a first portion and a second portion connected in series in this order,
The first portion is formed so as to be separated from the 3 × 3 coupler toward an output portion of the connection waveguide portion,
5. The optical hybrid circuit according to claim 4, wherein the second portion is formed so as to approach the 3 × 3 coupler toward an output portion of the connection waveguide portion. 6. The connection waveguide unit is a 1 × 1 coupler configured as a multi-mode interference coupler,
An input section of the connection waveguide section is arranged on one end side of the connection waveguide section, and an output section of the connection waveguide section is arranged on the other end side of the connection waveguide section,
The input section and the output section of the connection waveguide section are arranged at positions that are point-symmetric with each other,
2. The optical hybrid circuit according to claim 1, wherein the connection waveguide has a common design with the 3 × 3 coupler. 3. A first input waveguide connected to a first input of the 4 × 4 coupler;
A second input waveguide connected to a third input of the 4 × 4 coupler;
A first output waveguide unit connected to an output unit of the connection waveguide unit;
A second output waveguide connected to a first output of the 3 × 3 coupler;
A third output waveguide connected to a second output of the 3 × 3 coupler;
A fourth output waveguide connected to a third output of the 3 × 3 coupler;
Further comprising
The first input waveguide section includes an input taper section whose width continuously increases toward an end connected to the 4 × 4 coupler,
The second input waveguide section includes an input taper section whose width continuously increases toward an end connected to the 4 × 4 coupler,
The first output waveguide section includes an output taper section whose width continuously decreases in a direction away from an end connected to the connection waveguide section,
The second output waveguide section includes an output taper section whose width continuously decreases in a direction away from the end connected to the 3 × 3 coupler,
The third output waveguide section includes an output taper section whose width continuously decreases in a direction away from an end connected to the 3 × 3 coupler,
The said 4th output waveguide part contains the output taper part whose width | variety reduces continuously in the direction away from the end connected with the 3x3 coupler, The one in any one of Claims 4-6 characterized by the above-mentioned. An optical hybrid circuit according to claim 1. The optical hybrid circuit according to claim 1, wherein the optical waveguide core is formed using silicon as a material.

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