JP2008241937A - Multimode interference optical coupler and mach-zehnder optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multimode interference optical coupler which is capable of increasing an allowable width error while keeping an overall length short. <P>SOLUTION: The multimode interference optical coupler 100 includes the first input/output optical waveguides 110 and 120, at least two second input/output optical waveguides 140 and 150 adjacent to each other, and a multimode optical waveguide 130 provided between the first input/output optical waveguides 110 and 120 and the second input/output optical waveguides 140 and 150 and for propagating multimode guided wave. The multimode optical waveguide 130 included in the multimode interference optical couple 100 includes a first multimode optical waveguide part 131 having the first input/output optical waveguides 110 and 120 connected thereto and a second multimode optical waveguide part 132 which has the second input/output optical waveguides 140 and 150 connected thereto and has a width narrower than that of the first multimode optical waveguide part 131 in a direction perpendicular to the propagation direction of the multimode guided wave. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a multimode interference optical coupler and a Mach-Zehnder optical modulator.

  In recent years, optoelectronics have been dramatically developed, and information transmission technology such as optical communication using light has been remarkably developed. Various optical elements have been developed in this optoelectronics. Among them, an optical coupler that separates propagating light, an optical splitter that couples light, and the like are important as optical elements that support an optical communication network that has various uses and is complicated and diversified.

  This optical coupler is mainly classified into a directional coupler type, an X-branch type, a Y-branch type, and a multimode interference (MMI) type. Among these, the MMI type optical coupler (hereinafter referred to as “MMI optical coupler”) has an advantage that the structure can be simplified, it can be formed in a small size, and the characteristics do not change so much depending on the refractive index and the wavelength of propagating light. Moreover, it has the characteristic that it is hard to be influenced by the deviation of the dimensional accuracy of a manufacturing process.

  Also, a Mach-Zehnder type optical modulator using an optical coupler can have a minute waveguide structure due to its high refractive index, but when manufacturing a Mach-Zehnder type optical modulator, in order to form a fine structure MMI optical couplers are often used rather than directional coupler type optical couplers that are easily affected by the manufacturing process. Also, since two output ports can be prepared, it is possible to select a port that outputs modulated light with little wavelength chirp effect, so that a 2 × 2 type MMI optical coupler is used rather than a 2 × 1 type MMI optical coupler. The use of MMI optical couplers has become more common.

  Many developments have been made to further improve the performance of such MMI optical couplers. For example, in Patent Document 1, in order to reduce the wavelength characteristics of excess loss and loss due to an MMI optical coupler, an invention is formed in which the width of the core of a single mode waveguide is tapered so as to increase as it approaches the multimode optical waveguide. Patent Document 2 discloses an invention in which the waveguide width is narrowed at the center of the multimode optical waveguide in order to shorten the overall length of the MMI optical coupler.

JP 2000-162454 A US Pat. No. 5,689,597

  When manufacturing an MMI optical coupler, a microfabrication technique such as photolithography or etching is used, and a manufacturing error (hereinafter referred to as “width error”, “width error dW”) from an optimum width W due to a deviation in dimensional accuracy of the manufacturing process. "," DW ", etc.). As described above, the MMI optical coupler has characteristics superior to those of other types of optical couplers. However, the change in characteristics of the conventional MMI optical coupler due to the width error dW is still large.

  According to the MMI optical coupler described in Patent Document 1, the change in characteristics due to the width error dW can be reduced by forming the single mode optical waveguide in a tapered shape, but the influence of the width error dW is sufficiently affected. It has not yet been reduced. Further, according to the MMI optical coupler described in Patent Document 1, since the single mode optical waveguide is tapered, the gap between the single mode optical waveguides arranged side by side becomes narrow. This gap is formed by digging by dry etching or the like. However, if this gap is narrow, it becomes difficult for the etching gas to enter, and it becomes difficult to separate reaction products used in the manufacturing process. As a result, there is a problem that the etching rate is extremely deteriorated and it is difficult to manufacture the gap of 2 μm or less.

  Therefore, it is conceivable to set the gap to 2 μm or more. In this case, however, it is necessary to increase the width W of the MMI optical coupler, and accordingly, it is necessary to increase the optimum length L of the MMI optical coupler. There is a problem that the overall size of the MMI optical coupler becomes large.

  As described above, a method for shortening the length L of the MMI optical coupler is disclosed in Patent Document 2, but sufficient shortening efficiency has not been obtained even by the MMI optical coupler described in Patent Document 2. . Further, when the 2 × 2 MMI optical coupler is configured, there is a problem that the distribution ratio when the light is branched is deviated from 1: 1.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is a new and improved that can increase an allowable width error while keeping the entire length short. An object is to provide a multimode interference optical coupler and a Mach-Zehnder type optical modulator.

  In order to solve the above problems, according to an aspect of the present invention, a first input / output optical waveguide, at least two second input / output optical waveguides adjacent to each other, a first input / output optical waveguide, and a second input / output are provided. A multimode interference optical coupler having a multimode optical waveguide provided between the optical waveguide and propagating multimode guided light, wherein the multimode optical waveguide is connected to the first input / output optical waveguide. A multimode optical waveguide section and a second input / output optical waveguide are connected to each other, and a width in a direction perpendicular to the propagation direction of the multimode waveguide light is narrower than the first multimode optical waveguide section. A multi-mode interference optical coupler is provided that is configured. According to this configuration, it is possible to reduce changes in the characteristics of the multimode interference optical coupler due to changes in width error.

  The first input / output optical waveguide may be arranged at a distance of about 1/6 of the width of the multimode optical waveguide from the central axis of the multimode optical waveguide parallel to the propagation direction of the multimode optical waveguide. .

  Further, the side surface of the second multi-mode optical waveguide portion parallel to the propagation direction and the outer surface of each second input / output optical waveguide may be joined without a step.

  In addition, at least one of the first input / output optical waveguide and the second input / output optical waveguide may have a width that expands in a tapered shape as the multimode optical waveguide approaches. According to such a configuration, the spot size of the input light to the multimode optical waveguide can be increased by the width expanding in a tapered shape, and as a result, the efficiency of interference with the excited primary mode can be increased. And loss can be reduced.

  Further, the second input / output optical waveguide is arranged at a distance of 1/6 to 1/5 of the width of the multimode optical waveguide from the central axis of the multimode optical waveguide parallel to the propagation direction of the multimode optical waveguide. May be.

  Further, the width of the second multimode optical waveguide portion may be 50% to 60% of the width of the first multimode optical waveguide portion.

  Further, the length of the second multimode optical waveguide portion may be 2% to 10% of the length of the multimode optical waveguide.

  In order to solve the above problems, according to another aspect of the present invention, a first input / output optical waveguide, at least two adjacent second input / output optical waveguides, a first input / output optical waveguide, A multimode optical waveguide that is provided between the two input / output optical waveguides and propagates the multimode guided light, and the multimode optical waveguide is connected to the first input / output optical waveguide. And a second multimode optical waveguide section, to which a second input / output optical waveguide is connected, and a second multimode optical waveguide section whose width in the direction perpendicular to the propagation direction of the multimode waveguide light is narrower than that of the first multimode optical waveguide section. Two mode interference optical couplers are provided, and a second input / output optical waveguide of one multimode interference optical coupler and a second input / output optical waveguide of another multimode interference optical coupler are connected by a connection waveguide. Characterized by Mach Enda optical modulator is provided. According to this configuration, it is possible to reduce changes in the characteristics of the multimode interference optical coupler due to changes in width error.

  As described above, according to the present invention, an allowable width error can be increased while keeping the entire length short.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, the duplicate description is abbreviate | omitted by attaching | subjecting the same code | symbol.

  Before describing an MMI optical coupler according to an embodiment of the present application, first, a conventional MMI optical coupler described in Patent Document 1, for example, will be described with reference to FIG.

<Conventional MMI Optical Coupler 500>
FIG. 8 is an explanatory diagram for explaining a conventional MMI optical coupler 500. 8A shows the configuration of the conventional MMI optical coupler 500, FIG. 8B shows the characteristics of the conventional MMI optical coupler 500, and FIG. 8C shows the configuration of the conventional MMI optical coupler 500. The Mach-Zehnder type optical modulator 700 is shown, and (D) shows the characteristics of the Mach-Zehnder type optical modulator 700.

  The characteristics of the MMI optical coupler are determined by the intensity ratio of the output light when the light is branched by the MMI optical coupler, the change in the intensity of the output light with respect to the width error at the time of manufacture, etc. It also appears in a change in output intensity due to this width error when a modulator is configured. Therefore, hereinafter, the characteristics of the MMI optical coupler and the Mach-Zehnder optical modulator are simply referred to as “characteristics”.

  As shown in FIG. 8A, the conventional MMI optical coupler 500 includes single mode optical waveguides 510, 520, 540, and 550, and a multimode optical waveguide 530. The multimode optical waveguide 530 is disposed between the single mode optical waveguides 510 and 520 and the single mode optical waveguides 540 and 550. Each of the single mode optical waveguides 510, 520, 540, and 550 has a width that expands in a tapered shape as the multimode optical waveguide 530 is approached.

  This conventional MMI optical coupler 500 is a ridge waveguide (not shown) having a three-layer structure of a lower clad, an optical waveguide and an upper clad that are sequentially stacked on a substrate formed of a chemical semiconductor such as n-type InP. Z). The lower cladding is formed of n-type InP or the like, the optical waveguide is formed of InGaAsP, and the upper cladding is formed of p-type InP.

Here, the conventional MMI optical coupler 500 is formed with the following dimensions assuming that light having a wavelength of 1.55 μm is propagated.
The widths of the narrow end portions of the tapered single mode optical waveguides 510, 520, 540, and 550 were 1.8 μm, the width of the wide end portion was 2 μm, and the length was 50 μm. The length of the multimode optical waveguide 530 was 205 μm and the width was 12 μm. At this time, the gap between the single mode optical waveguide 510 and the single mode optical waveguide 520 and the gap between the single mode optical waveguide 540 and the single mode optical waveguide 550 were set to 2 μm. The thickness of the lower cladding was 0.5 μm, the thickness of the optical waveguide layer was 0.2 μm, and the thickness of the upper cladding was 2 μm.

  The characteristics of the conventional MMI optical coupler 500 having such a configuration are calculated by performing simulation using a simulation method such as Beam Propagation Method (hereinafter referred to as “BPM”). Shown in B). The horizontal axis represents the width error dW, which is a width error, and the vertical axis represents the output light intensity when the input light intensity is 1. This simulation method and each axis are common in the graphs described below.

  8B shows the first output light 620 output from the single mode optical waveguide 540 and the single mode optical waveguide when the input light 610 is input to the single mode optical waveguide 510 of the conventional MMI optical coupler 500. FIG. The change with respect to the width | variety error dW with the 2nd output light 630 output from 550 is shown.

  As can be seen from FIG. 8B, the intensity of the first output light 620 increases from about 0.35 to about 0.49 as the width error dW changes from −0.3 μm to −0.1 μm. is doing. When the dW is between −0.1 μm and 0 μm, the intensity of the first output light 620 becomes a substantially constant value. As the dW changes from 0 μm to 0.3 μm, the first output light 620 is about It decreases from 0.49 to around 0.3. On the other hand, the intensity of the second output light 630 is substantially the same as the intensity of the first output light 620 during dW = −0.3 to 0.3 μm.

  That is, the conventional MMI optical coupler 500 can make the intensity ratio of the first output light 320 and the second output light 330 about 1: 1 regardless of the change in dW. However, according to the conventional MMI optical coupler 500, when | dW | increases, the intensity of the first output light 620 and the second output light 630 decreases rapidly, and is approximately 0 in the vicinity of dW = 0.3 μm. It can be seen that it decreases to 3.

  Next, output light when the Mach-Zehnder type optical modulator 700 is configured using two conventional MMI optical couplers 500 will be described. The Mach-Zehnder type optical modulator 700 includes two MMI optical couplers 500 and two coupling waveguides.

  The coupling waveguide includes a curved waveguide 210, a straight waveguide 220, and a curved waveguide 210. One of the coupling waveguides connects the single mode optical waveguide 540 of one MMI optical coupler 500 and the single mode optical waveguide 540 of the other MMI optical coupler 500, and the other one of the coupling waveguides is The single mode optical waveguide 550 of one MMI optical coupler 500 is connected to the single mode optical waveguide 550 of the other MMI optical coupler 500. Each of the connecting waveguides is the same as that used in the MMI optical coupler 100 according to an embodiment of the present invention, and will be described later.

  FIG. 8D shows the result of calculating the characteristics of the conventional Mach-Zehnder type optical modulator 700 having such a configuration by performing simulation using a simulation method such as BPM.

  FIG. 8D shows the third output light 640 output from the other single mode optical waveguide 510 when the input light 610 is input to one single mode optical waveguide 510 of the conventional Mach-Zehnder optical modulator 700. And a change with respect to the width error dW of the fourth output light 650 output from the single mode optical waveguide 520.

  As can be seen from FIG. 8D, the intensity of the third output light 640 is suppressed to a small value of 0.1 or less with respect to the change in dW. On the other hand, the intensity of the fourth output light 650 is 0.5 when dW = −0.3 μm, but increases as dW increases, exceeds 0.6 when dW = −0.23 μm, and dW = It is 0.6 or more up to about 0.12 μm. The fourth output light 650 is 0.6 or less when dW is 0.12 μm or more.

  Here, in general, in order to maintain the product quality of the Mach-Zehnder type optical modulator, it is desired that the intensity of the fourth output light is larger than 0.6 to 0.7. Therefore, in the following, in order to make this intensity larger than about 0.6 to 0.7, the range of the width error dW that can be allowed as an error is referred to as “allowable width error”. That is, the larger the allowable width error, the more the variation in product quality due to the width error dW caused by the manufacturing process deviation can be suppressed.

  The allowable width error of the Mach-Zehnder type optical modulator 700 due to the conventional MMI optical coupler 500 is about −0.2 μm to 0.1 μm, about 0.3 μm, as shown in FIG. Accordingly, the applicant of the present application has focused on the fact that the quality of the MMI optical coupler can be further improved by further widening the allowable width error and suppressing the change in the characteristics of the MMI optical coupler with respect to the change in the width error dW.

Here, the optimum dimensions of such a 2 × 2 MMI optical coupler and a Mach-Zehnder optical modulator are generally when the length of the multimode optical waveguide is L and the width is W,
L = 2nW ² / (3λ) (Formula 1)
It is represented by This n means the refractive index of the waveguide, and λ means the wavelength of the propagating light.

The change in the characteristics of the MMI optical coupler is expressed, for example, when the value of dL / L becomes equal to or greater than a predetermined value.
dL / L = (2 × dW) / W (Expression 2)
It is represented by That is, as dW increases, if dL / L also increases and exceeds a predetermined value, the change in the characteristics of the MMI optical coupler increases, leading to deterioration of product quality. The range of the width error dW in which the dL / L is equal to or greater than a predetermined value and the characteristic change is not large is an allowable width error.

  In order to suppress the change in characteristics due to dW, it is possible to reduce the dW instead of increasing the tolerance error, but for this purpose, it is necessary to increase the accuracy of the manufacturing process. However, it is not easy to improve the processing accuracy in units of several μm. Therefore, by increasing W and reducing the influence of dW, it is possible to increase the allowable width error dW that does not deteriorate the product quality. However, when W is increased, the optimum L is larger than (Equation 1). Therefore, the overall size of the MMI optical coupler is increased.

  Therefore, as a result of intensive studies to improve the product quality as described above and solve the above problems, the applicant of the present application has arrived at the present invention as follows. Hereinafter, an MMI optical coupler according to an embodiment of the present invention will be described in detail.

<MMI Optical Coupler 100 According to the Present Embodiment>
The MMI optical coupler according to the present embodiment exhibits an effect in various applications, but particularly when applied to a Mach-Zehnder optical modulator. Therefore, in the following, first, the configuration of the MMI optical coupler according to the present embodiment will be described with reference to FIGS. 1 to 3, and then the characteristics of the MMI optical coupler and the Mach-Zehnder type light using this MMI optical coupler will be described. The configuration and characteristics of the modulator will be described.

(MMI optical coupler 100)
FIG. 1 is an explanatory diagram illustrating a configuration of an MMI optical coupler 100 according to the present embodiment. FIG. 2 is a cross-sectional view showing a configuration of the MMI optical coupler 100 according to the present embodiment, and is a cross-sectional view taken along the line AA of FIG. FIG. 3 is a plan view showing the configuration of the MMI optical coupler 100 according to this embodiment. FIG. 1 shows an example in which a waveguide of light input / output to / from the MMI optical coupler 100 is integrally formed with the MMI optical coupler 100 and formed on the substrate 10. In FIG. 3, only the MMI optical coupler 100 is shown.

  FIG. 1 shows the MMI optical coupler 100 used for branching propagating light when the MMI optical coupler 100 is used in a Mach-Zehnder optical modulator. In the following, the case where light propagates from the x-axis negative direction in FIG. 1 to the positive direction will be described, but the present invention is not limited to this example, and the light is directed from the x-axis positive direction in FIG. Needless to say, it may be propagated to. That is, in the following configuration shown in FIG. 1, the members of the multimode optical waveguide 130 are inverted with respect to the y-axis, and the single mode optical waveguides 110 and 120 are used as output waveguides of the MMI optical coupler 100. It is also possible to use at least one of 150 as an input waveguide.

The MMI optical coupler 100 according to the present embodiment is formed on a substrate 10 formed of a compound semiconductor such as n-type InP, for example, as shown in FIGS. The MMI optical coupler 100 is formed as a ridge waveguide 20 having a three-layer structure including a lower clad 20A, an optical waveguide layer 20B, and an upper clad 20C. The lower cladding 20A of the thickness of the (z-axis direction of the thickness in FIG. 2) and _{D A,} and the thickness of the optical waveguide layer 20B and _{D B,} the thickness of the upper cladding 20C and _{D C.}

  Here, the lower clad 20 </ b> A may be formed of a compound semiconductor such as n-type InP as with the substrate 10, or may be formed integrally with the substrate 10. The optical waveguide layer 20B may be formed of, for example, InGaAsP, and the upper clad 20C may be formed of, for example, a compound semiconductor such as p-type InP.

  In the MMI optical coupler 100, a lower clad 20A, an optical waveguide layer 20B, and an upper clad 20C are sequentially laminated on the substrate 10 to form a ridge waveguide 20, and then the lower clad 20A and the optical waveguide layer 20B. The upper cladding 20C is formed by dry etching or the like.

  The MMI optical coupler 100 having such a three-layer structure is formed on the substrate 10 with the following configuration. That is, the MMI optical coupler 100 includes single mode optical waveguides 110 and 120, a multimode optical waveguide 130, and optical waveguides 140 and 150.

  The single-mode optical waveguides 110 and 120 are an example of a first input / output optical waveguide that propagates single-mode guided light. Light from the outside is input to the multi-mode optical waveguide 130 or from the multi-mode optical waveguide 130. Output light to the outside.

  These single mode optical waveguides 110 and 120 are the ends of the multimode optical waveguide 130 substantially parallel to the direction in which light propagates in the optical waveguide layer 20B that is a plane parallel to the substrate 10 (hereinafter referred to as “propagation direction”). Connected to each other and formed symmetrically with respect to the central axis 133 of the multimode optical waveguide 130. In the following description, it is assumed that light is input to the single mode optical waveguide 110.

  The single mode optical waveguides 110 and 120 have, for example, a width (y) in a direction orthogonal to the propagation direction (hereinafter referred to as “width direction”) as the multimode optical waveguide 130 (first multimode optical waveguide portion 131) approaches. (Axial length) may be formed to expand in a tapered shape. For example, the single mode optical waveguide 110 may be formed in a symmetric shape with respect to the central axis 111 parallel to the propagation direction, and the single mode optical waveguide 120 is configured with respect to the central axis 121 parallel to the propagation direction. It may be formed in a symmetrical shape. In the following, the length of each member in the width direction (y-axis direction) is simply referred to as “width”, and the length of each member in the propagation direction (x-axis direction) is simply referred to as “length”.

  In this case, as shown in FIG. 3, the width of the narrow ends of the single mode optical waveguides 110 and 120 is Wss1, and the width of the wide end is Wsl1. The length of the single mode optical waveguides 110 and 120 is Ls1. The characteristics of the MMI optical coupler 100 can be improved by widening the width Wsl1 of the wide end portion.

  For example, the single mode optical waveguides 110 and 120 each have a width (in the y-axis direction in FIG. 1) from the central axis 133 of the multimode optical waveguide 130 that is parallel to the light propagation direction. The multimode optical waveguide 130 may be connected in the vicinity of a position separated by a distance of about 1/6 of the (length).

  That is, when the width of the multimode optical waveguide 130 is Wm, the distance P1 from the central axis 133 to the central axes 111 and 121 of the single mode optical waveguides 110 and 120 is (1/6) × Wm. By connecting the single mode optical waveguides 110 and 120 to this position, when the length L in the propagation direction of the multimode optical waveguide 130 deviates from the optimum value, the light output direction deviates greatly from the optimum case. The MMI optical coupler 100 can maintain the characteristic of outputting light in the propagation direction.

  Here, if the gap between the single mode optical waveguide 110 and the single mode optical waveguide 120 is Wg1, Wg1 may be set to 2 μm or more, for example. By setting the gap Wg1 in this way, it is possible to suppress the influence on the product quality due to the accuracy of the manufacturing process by dry etching or the like.

  The MMI optical coupler 100 has been described with respect to the case where the first input / output optical waveguide includes the two single mode optical waveguides 110 and 120 as illustrated in FIG. 1, but may include the single single mode optical waveguide 110, Three or more single mode optical waveguides may be provided. When a plurality of single mode optical waveguides are provided, the first input / output optical waveguides are arranged in parallel in the width direction.

  The multi-mode optical waveguide 130 excites the multi-mode guided light by causing the input single-mode guided light to interfere, and propagates the multi-mode guided light. The multimode optical waveguide 130 is disposed between the single mode optical waveguides 110 and 120 and the optical waveguides 140 and 150.

  The width of the multimode optical waveguide 130 is narrowed at the end (the end on the x-axis positive direction side) where the optical waveguides 140 and 150 are connected. That is, the multimode optical waveguide 130 is wider than the first multimode optical waveguide portion 131 to which the first multimode optical waveguide portion 131 to which the single mode optical waveguides 110 and 120 are connected and the optical waveguides 140 and 150 are connected to each other. And a narrow second multimode optical waveguide section 132.

  The width of the first multimode optical waveguide portion 131 is Wm, and the width of the second multimode optical waveguide portion 132 is Wm2. In this case, the width of the multimode optical waveguide 130 means the width Wm of the first multimode optical waveguide portion 131.

  The first multimode optical waveguide section 131 is a waveguide for exciting the multimode waveguide light by causing the input single mode waveguide light to interfere with each other and propagating the multimode waveguide light. Single mode optical waveguides 110 and 120 are connected to the first multimode optical waveguide portion 131.

  The first multimode optical waveguide section 131 is formed with a width wider than the second multimode optical waveguide section 132 in the width direction and longer than the second multimode optical waveguide section 132 in the propagation direction. The length of the first multimode optical waveguide portion 131 is Lm1, and the width is Wm.

  The second multi-mode optical waveguide section 132 propagates the multi-mode waveguide light by expanding the mode gap of the multi-mode waveguide light by causing the propagating light to interfere in a different interference state from the first multi-mode optical waveguide section 131. It is a waveguide. The second multimode optical waveguide section 132 connects the first multimode optical waveguide section 131 and the optical waveguides 140 and 150.

  The second multimode optical waveguide section 132 is formed with a width narrower than that of the first multimode optical waveguide section 131 in the width direction and shorter than the first multimode optical waveguide section 131 in the propagation direction. That is, when the width of the second multimode optical waveguide section 132 is Wm2 and the length is Lm2, Wm2 <Wm and Lm2 <Lm1.

  The width Wm2 is preferably 50 to 60% of the width Wm of the multimode optical waveguide 130, for example. If this Wm2 is made smaller than 50% of Wm, the output intensity when the Mach-Zehnder type optical modulator is configured will be lowered as a whole. Further, if Wm2 is larger than 60%, the allowable width error is narrowed, and the change in characteristics due to the change in the width error dW is increased.

  The length Lm2 is preferably 2% to 10% of the length L of the multimode optical waveguide 130, for example. When this Lm2 is less than 2% of L, the allowable width error becomes narrow, and the change in characteristics due to the change in the width error dW becomes large. Further, if Lm2 exceeds 10% of L, the output intensity when the Mach-Zehnder type optical modulator is configured is lowered as a whole, and the characteristics are deteriorated.

  Note that the first multimode optical waveguide section 131 may have a tapered portion whose width becomes narrower as the second multimode optical waveguide section 132 approaches. However, the shorter the length of the tapered portion, the better the characteristics of the MMI optical coupler 100. When this length is 5 μm or more, the characteristics deteriorate. Therefore, it is preferable that the length of the tapered portion in the propagation direction is, for example, less than 5 μm.

  As described above, the multimode optical waveguide 130 includes the first multimode optical waveguide portion 131 having a large width and the second multimode optical waveguide portion 132 having a small width. A step is formed on a side surface parallel to the direction. Conventionally, it is known that when such a step is formed, the characteristics of the MMI optical coupler are deteriorated. In the MMI optical coupler 100 according to the present embodiment, such characteristics are not deteriorated. We have succeeded in further improving the characteristics.

  The optical waveguides 140 and 150 are examples of a second input / output optical waveguide that propagates single-mode guided light, and outputs light from the multi-mode optical waveguide 130 to the outside, or transmits external light to the multi-mode optical waveguide. Input to 130.

  Here, it is assumed that light is output from the optical waveguides 140 and 150 to the outside as described above. The optical waveguides 140 and 150 are juxtaposed adjacent to each other in the width direction, and are connected to the end of the second multimode optical waveguide portion 132 substantially parallel to the propagation direction.

  Moreover, the optical waveguides 140 and 150 may be formed so that the width increases in a tapered shape as the second multimode optical waveguide portion 132 of the multimode optical waveguide 130 is approached, for example. For example, the optical waveguide 140 may be formed in a symmetric shape with respect to the central axis 141 parallel to the propagation direction, and the optical waveguide 150 is symmetric with respect to the central axis 151 parallel to the propagation direction. It may be formed in a shape.

  Here, as shown in FIG. 3, the width of the narrow end of the optical waveguides 140 and 150 is Wss2, and the width of the wide end is Wsl2. The length of the optical waveguides 140 and 150 is Ls2.

  The connection position of the optical waveguides 140 and 150 depends on the width Wm2 of the second multimode optical waveguide section 132 and the width Wsl2 of the optical waveguides 140 and 150 at the connection end. However, the optical waveguides 140 and 150 are each, for example, 1/6 to 1 of the width of the multimode optical waveguide 130 (the length in the y-axis direction in FIG. 1) from the central axis 133 of the multimode optical waveguide 130. It is preferable to connect to the second multimode optical waveguide section 132 in the vicinity of a position separated by a distance of / 5.

  That is, when the distance from the central axis 133 of the multimode optical waveguide 130 to the central axes 141 and 151 of the optical waveguides 140 and 150 is P2, P2 is, for example, 1/6 to the width Wm of the multimode optical waveguide 130. It is preferable to set to 1/5. When P2 exceeds 1/5 of Wm, when the MMI optical coupler 100 constitutes a Mach-Zehnder optical modulator, the allowable width error becomes narrow, and the change in characteristics due to the change in the width error dW becomes large. If P2 is set to be smaller than 1/6, the gap Wg2 between the optical waveguide 140 and the optical waveguide 150 becomes small, and it becomes impossible to suppress the influence on the product quality due to the accuracy of the manufacturing process such as dry etching. In order to suppress this influence, the gap Wg2 is preferably set to 2 μm or more, for example.

  For example, as shown in FIG. 3, the outer side surface 140a in the width direction of the optical waveguide 140 is formed continuously with one side surface 132a in the width direction of the second multimode optical waveguide part 132 without having a step. . The outer side surface 150b in the width direction of the optical waveguide 150 is formed continuously with one side surface 132b in the width direction of the second multimode optical waveguide part 132 without having a step.

  In this way, the MMI optical coupler is configured by the MMI optical coupler by forming the optical waveguides 140 and 150 continuously without any step with respect to the side surfaces 132a and 132b of the second multimode optical waveguide section 132. The characteristics can be improved. That is, when a Mach-Zehnder optical modulator is configured by the MMI optical coupler 100 configured as described above, it is possible to reduce the intensity difference of the output light when the width error dW of the multimode optical waveguide 130 increases or decreases.

  However, the present invention is not limited to this example, and a step may be formed between the outer side surface 140a of the optical waveguide 140 and the side surface 132a of the second multimode optical waveguide unit 132 without being continuously formed. In addition, a step may be formed between the outer surface 150b of the optical waveguide 150 and the side surface 132b of the second multimode optical waveguide portion 132 without being continuously formed.

  As described above with reference to FIG. 1, the case where the single mode optical waveguides 110 and 120 and the optical waveguides 140 and 150 are each formed in a tapered shape whose width increases as approaching the multimode optical waveguide 130 will be described. did. By forming the single mode optical waveguides 110 and 120 and the optical waveguides 140 and 150 in such a shape, at the start point of the multimode optical waveguide 130 (for example, an end portion to which the single mode optical waveguides 110 and 120 are connected). The width of the light excited by the multimode waveguide light is wider than the width of the multimode optical waveguide 130. As a result, only the low-order mode can be excited, and the change in phase difference between modes due to the width error can be reduced. As a result, it is possible to increase the ratio of dL / L, which is an allowable value of the change in the characteristic of the MMI optical coupler 100 in (Equation 2), and to suppress the change in the characteristic of the MMI optical coupler 100 with respect to the width error. .

  The configuration of the MMI optical coupler 100 has been described above. In the case where a Mach-Zehnder type optical modulator or the like is configured using the MMI optical coupler 100, the optical waveguides input and output to the single mode optical waveguides 110 and 120 and the optical waveguides 140 and 150 are, for example, They can be formed on the same substrate 10. As an example of the optical waveguide, a curved waveguide 210 is shown in FIG.

  As shown in FIG. 1, the curved waveguide 210 may be formed integrally with the optical waveguide 140 or the optical waveguide 150, for example. As described above, the input / output optical waveguides are formed integrally with the single mode optical waveguides 110 and 120, the optical waveguides 140 and 150, and the like, so that each single mode optical waveguide 110 and 120 having a tapered shape is formed. , And the optical waveguides 140 and 150 can be configured as a part of the waveguide of input and output light, and the size of the entire optical element can be reduced. However, the present invention is not limited to such an example, and the curved waveguide 210 may be formed separately from the optical waveguide 140 or the optical waveguide 150.

  In FIG. 1, the case where the input light 310 of the propagating light is input to the single mode optical waveguide 110 has been described. In this case, the light is separated into two modes of light by interference in the first multimode optical waveguide section 131. Then, the separated light enters an interference state different from that of the first multimode optical waveguide section 131 in the second multimode optical waveguide section 132, thereby widening the gap between the modes of the two separated light beams. By widening the gap of the separated light, a gap Wg2 between the optical waveguide 140 and the optical waveguide 150 is secured, and the two separated lights are branched into the optical waveguide 140 and the optical waveguide 150 and output. These lights are output as the first output light 320 from the curved waveguide 210 via the optical waveguide 140 and as the second output light 330 from the curved waveguide 210 via the optical waveguide 150, respectively.

  Here, it is assumed that the input light 310 of the propagating light is input to the single mode optical waveguide 110. However, the present invention is not limited to this example. For example, the propagating light is guided through the curved waveguides 210. 140 and the optical waveguide 150 may be input. In this case, the light is coupled in the multimode optical waveguide 130 and output from the single mode optical waveguide 110.

(Characteristics of the MMI optical coupler 100 according to the present embodiment)
The configuration of the MMI optical coupler 100 according to this embodiment has been described above. Hereinafter, the characteristics of the MMI optical coupler 100 when light having a wavelength of about 1.55 μm used in optical communication or the like is propagated to the MMI optical coupler 100 having such a configuration will be described with reference to FIG. To do. However, the dimensions of each component of the MMI optical coupler 100 shown below are examples of this case, and it goes without saying that the following dimensions can be changed as appropriate in the case of propagating light of other wavelengths. Absent.

  FIG. 4 is an explanatory diagram for explaining the characteristics of the MMI optical coupler 100 according to the present embodiment. 4A shows the dimensions of the MMI optical coupler 100, and FIG. 4B shows changes in the first output light 320 and the second output light 330 of the MMI optical coupler 100 with respect to the width error dW in BPM or the like. The calculated result is shown. (C) shows the configuration and dimensions of the Mach-Zehnder type optical modulator 400 configured by the MMI optical coupler 100, and (D) shows the third output light 340 of the Mach-Zehnder type optical modulator 400 with respect to the width error dW, It is a graph which shows the result of having calculated the change of 4 output light 350 by BPM etc. FIG.

  First, an example of dimensions of each component of the MMI optical coupler 100 in this case will be described with reference to FIG. The substrate 10 and the lower clad 20A were formed of n-type InP, and the thickness of the lower clad 20A was 0.5 μm. The optical waveguide layer 20B is formed of InGaAsP and has a thickness of 0.2 μm. Then, the upper clad 20C was formed of p-type InP, and the thickness was 2 μm.

  In this case, the refractive index of the substrate 10, the lower clad 20A, and the upper clad 20C is 3.17, and the refractive index of the optical waveguide layer 20B is 3.32. Further, a polymer layer having a refractive index of about 1.5 such as polyimide may be formed on the surface of the substrate 10 where the waveguide such as the MMI optical coupler 100 is not formed. In this case, when two-dimensionalization is performed by the equivalent refractive index method, the refractive index of the waveguide portion can be set to 3.2, and the refractive indexes of the other portions can be set to 1.5.

  As shown in FIG. 4A, the width Wss1 of the narrow ends of the single-mode optical waveguides 110 and 120 is 1.8 μm, the width Wsl1 of the wide end is 2.6 μm, and the length Ls1 is set. Was 50 μm. The gap Wg1 between the single mode optical waveguide 110 and the single mode optical waveguide 120 was set to 2 μm.

  The length L of the multimode optical waveguide 130 was 210 μm, and the width Wm was 12 μm. The length Lm1 of the first multimode optical waveguide part 131 was set to 200 μm, and the length Lm2 of the second multimode optical waveguide part 132 was set to 10 μm. At this time, Lm2 is about 4.8% of L. The width Wm2 of the second multimode optical waveguide portion 132 was set to 6.8 μm. This Wm2 is about 57% of Wm.

  The width Wss2 of the narrow ends of the optical waveguides 140 and 150 is 1.8 μm, the width Wsl2 of the wide end is 2.4 μm, and the length Ls2 is 50 μm. The gap Wg2 between the optical waveguide 140 and the optical waveguide 150 was set to 2 μm.

  The reason why the widths Wss1 and Wss2 of the narrow ends of the single mode optical waveguides 110 and 120 and the optical waveguides 140 and 150 are set to 1.8 μm is to make the light propagating through these positions into a single mode.

  When the input light 310 is input to the MMI optical coupler 100 configured as described above, the first output light 320 output from the optical waveguide 140 and the second output light 330 output from the optical waveguide 150 A change with respect to the width error dW will be described with reference to FIG.

  As can be seen from FIG. 4B, the intensity of the first output light 320 increases from about 0.4 to about 0.47 as the width error dW changes from −0.3 μm to 0 μm. . As the dW changes from 0 μm to 0.3 μm, the first output light 320 decreases from about 0.47 to about 0.4. On the other hand, the intensity of the second output light 330 is substantially the same as the intensity of the first output light 320 during dW = −0.3 to 0.3 μm.

  That is, the MMI optical coupler 100 according to the present embodiment can set the intensity ratio of the first output light 320 and the second output light 330 to about 1: 1 regardless of the change in dW. According to the MMI optical coupler 100, even when the dW magnitude | dW | increases, the intensities of the first output light 320 and the second output light 330 are maintained at a high value of about 0.4 or more. ing.

  Next, the output light when the Mach-Zehnder optical modulator 400 is configured using the two MMI optical couplers 100 as shown in FIG. 4C will be described. The Mach-Zehnder optical modulator 400 includes two coupled waveguides in addition to the two MMI optical couplers 100.

  One of the coupling waveguides connects the optical waveguide 140 of one MMI optical coupler 100 and the optical waveguide 140 of the other MMI optical coupler 100, and the other one of the coupling waveguides is one MMI optical coupler. The optical waveguide 150 of 100 and the optical waveguide 150 of the other MMI optical coupler 100 are connected.

  This connection waveguide may be constituted by a curved waveguide 210 and a straight waveguide 220, for example. In this case, the curved waveguide 210 and the straight waveguide 220 may be integrally formed with the MMI optical coupler 100 on a substrate, for example.

  The width of the curved waveguide 210 and the straight waveguide 220 was 1.8 μm. This is to make the propagating light a single mode. The length of the curved waveguide 210 was 100 μm and the length of the straight waveguide 220 was 300 μm, so that the length of the optical path between each MMI optical coupler 100 was 500 μm. And the distance between each linear waveguide 220 was 15 micrometers. The refractive index of the waveguide layer of the curved waveguide 210 and the straight waveguide 220 is 3.2, and the outer refractive index thereof is 1.5.

  Next, FIG. 4D shows a result calculated by simulating the characteristics of the Mach-Zehnder optical modulator 400 according to the present embodiment having such a configuration by a simulation method such as BPM.

  4D shows a third output light 340 that is output from the other single mode optical waveguide 110 when the input light 310 is input to one single mode optical waveguide 110 of the Mach-Zehnder optical modulator 400. The change with respect to the width | variety error dW with the 4th output light 350 output from the single mode optical waveguide 120 is shown.

  In this case, the input light 310 input from the single mode optical waveguide 110 is branched by one MMI optical coupler 100, and the branched light is passed through the curved waveguide 210 and the straight waveguide 220 to the other MMI optical coupler. Propagate to 100. The propagating light is coupled by the other MMI optical coupler 100 and output from the single mode optical waveguide 110 or the single mode optical waveguide 120.

  As can be seen from FIG. 4D, the intensity of the third output light 340 is suppressed to a small value of 0.1 or less with respect to the change in dW. On the other hand, the intensity of the fourth output light 350 is 0.6 or more even when dW = −0.3 μm, but further increases and decreases as dW increases, and reaches 0 until dW = 0.28 μm. .6 or more. The fourth output light 350 is 0.6 or less when the dW is 0.28 μm or more. That is, according to the MMI optical coupler 100, it is understood that the allowable width error is about −0.3 to 0.28 μm and about 0.6 μm.

(Characteristic comparison of MMI optical coupler)
When the MMI optical coupler 100 is compared with the conventional MMI optical coupler 500 described above with respect to the change in the intensity of the output light with respect to the width error dW, according to the conventional MMI optical coupler 500, when | dW | While the first output light 620 and the second output light 630 are reduced to about 0.3, according to the MMI optical coupler 100 according to the present embodiment, the first output light is increased even when | dW | is increased. It can be seen that the intensity of 320 and the second output light 330 can be maintained at a high value of about 0.4 or more.

  Further, in the conventional Mach-Zehnder type optical modulator 700, the allowable width error is only about 0.3 μm of about −0.2 to 0.1 μm, but according to the Mach-Zehnder type optical modulator 400 according to the present embodiment, It can be seen that this allowable width error can be expanded to about 0.6 μm, which is about −0.3 to 0.28 μm, which is about twice the conventional allowable width error. In other words, according to the present embodiment, the allowable width error can be increased by 80% or more.

  In order to achieve such a wide tolerance error by the conventional Mach-Zehnder type optical modulator 700, it is necessary to increase the width of the multimode optical waveguide 530, and accordingly, it is necessary to increase the length to about 400 μm. There is. Since the length L of the multimode optical waveguide 130 according to the present embodiment is 210 μm, the length can be reduced to about half according to the present embodiment.

(Effect of this embodiment)
As described above, according to the MMI optical coupler 100 according to the present embodiment, it is possible to prevent the intensity distribution characteristic of the output light from being changed due to an error that occurs during manufacturing, particularly the width error dW. Furthermore, according to the Mach-Zehnder optical modulator 400 according to the present embodiment, the allowable width error dW can be increased to about twice. That is, an allowable width error of about ± 0.3 μm sufficient for practical use can be ensured. In other words, the length of the MMI optical coupler 100 can be shortened to about half that of the conventional MMI optical coupler 500 having the allowable error range.

  In this way, the output intensity can be maintained at a high value without changing with respect to the width error dW, so that the uniformity as a product can be improved. Further, by improving the product quality, it is possible to reduce the necessity for fine adjustment in the subsequent use, so that the convenience in use can be improved. That is, when an MMI optical coupler having similar characteristics is manufactured, the accuracy required for a manufacturing apparatus or the like can be reduced, so that the MMI optical coupler can be easily manufactured.

(Change in allowable width error due to the size of each member)
The configurations and effects of the MMI optical coupler 100 and the Mach-Zehnder optical modulator 400 according to the present embodiment have been described above. In the following, when a change in characteristics due to the size of each member of the MMI optical coupler 100, that is, when the size of each member of the MMI optical coupler 100 is changed, the MMI-Zehnder optical modulator 400 using the MMI optical coupler 100 is used. A description will be given of a change in the allowable width error when configured.

  Therefore, hereinafter, a case will be described in which only the dimensions of some of the constituent members are changed based on the MMI optical coupler 100 according to the present embodiment having the above dimensions. Accordingly, only the changed dimensions are shown below, and the other dimensions are the same as those of the MMI optical coupler 100 described above.

(Width Wm2 of the second multimode optical waveguide part 132)
First, with reference to FIG. 5, the change in the allowable width error when the width Wm2 of the second multimode optical waveguide section 132 is changed will be described. FIG. 5 is a diagram showing a change in characteristics with respect to a change in the width Wm2 of the second multimode optical waveguide section 132 in the MMI optical coupler 100 according to the present embodiment.

  (A) shows the MMI optical coupler 100 when Wm2 = 7.3 μm, and the graphs (B) show the intensities of the third output light 340 and the fourth output light 350 in this case. (C) shows the MMI optical coupler 100 when Wm2 = 7.2 μm, and the intensities of the third output light 340 and the fourth output light 350 in this case are shown in the graph of (D). (E) shows the MMI optical coupler 100 when Wm2 = 6.0 μm, and the intensity of the third output light 340 and the fourth output light 350 in this case is shown in the graph of (F).

  As shown in (C), when Wm2 = 7.2 μm, the outer side surfaces 140a and 150b of the optical waveguides 140 and 150 are formed continuously with the side surfaces 132a and 132b of the second multimode optical waveguide part 132, respectively. As described above, the width Wsl2 of the wide end portions of the optical waveguides 140 and 150 is set to 2.6 μm. As shown in (E), when Wm2 = 6.0 μm, the width Wsl2 of the wide end portions of the optical waveguides 140 and 150 is secured in order to secure 2 μm of the gap Wg2 between the optical waveguide 140 and the optical waveguide 150. Was 2.0 μm.

  As shown in (B), the allowable width error when Wm2 = 7.3 μm is about −0.22 to 0.26 μm. This Wm2 = 7.3 μm corresponds to about 61% of the width Wm of the multimode optical waveguide 130, and the distance P2 between the central axes 141, 151 of the optical waveguides 140, 150 and the central axis 133 of the multimode optical waveguide 130 is This is about 1 / 5.5 times the width Wm of the multimode optical waveguide 130. This allowable width error is wider than the allowable width error (about −0.2 to 0.1 μm) of the conventional MMI optical coupler. When Wm2 is longer than 60% of Wm, the allowable width error becomes narrow and approaches the conventional allowable width error.

  As shown in (D), the allowable width error when Wm2 = 7.2 μm is about −0.24 to 0.18 μm. In this case, the distance P2 is about 1 / 5.2 times Wm. This allowable width error is also wider than the conventional allowable width error (about −0.2 to 0.1 μm). When the distance P2 is further increased, the allowable width error approaches the conventional allowable width error. Therefore, when P2 is made larger than about 1/5 times Wm, the allowable width error becomes narrow and approaches the conventional allowable width error.

  As shown in (E), when Wm2 = 6.0 μm, the intensity of the fourth output light 350 changes as a result of a change in the width error dW, and the allowable width error is only around −0.2. . This Wm2 is about 50% of Wm, and the distance P2 is about 1/6 times the width Wm. When the distance P2 is further reduced, the intensity of the fourth output light 350 is further increased. If the intensity of the output light changes in such a manner, the output fluctuation due to the manufacturing error becomes uncertain and the product quality may be deteriorated.

  As can be seen from the above, in this embodiment, the distance P2 between the central axes 141 and 151 of the optical waveguides 140 and 150 and the central axis 133 of the multimode optical waveguide 130 is set to the width Wm of the multimode optical waveguide 130. By setting the frequency to about 1/6 to 1/5, the tolerance error can be widened while suppressing the ripple of the output intensity, and the characteristics of the MMI optical coupler 100 can be improved. Then, by setting the width Wm2 of the second multimode optical waveguide portion 132 to about 50% to 60% of the width Wm of the multimode optical waveguide 130, the allowable width error is widened while suppressing the output intensity ripple. And the characteristics of the MMI optical coupler 100 can be improved.

(Length Lm2 of the second multimode optical waveguide portion 132)
Next, with reference to FIG. 6, a description will be given of a change in the allowable width error when the length Lm2 of the second multimode optical waveguide section 132 is changed as shown in FIG. FIG. 6 is a diagram showing a change in characteristics with respect to a change in the length Lm2 of the second multimode optical waveguide section 132 in the MMI optical coupler 100 according to the present embodiment.

  (B) is a graph showing the intensities of the third output light 340 and the fourth output light 350 when Lm2 = 5 μm, and (C) is the third output light 340 and the fourth output light 340 when Lm2 = 10 μm. (D) is a graph showing the intensity of the third output light 340 and the fourth output light 350 when Lm2 = 15 μm. In addition, since the length L of the multimode optical waveguide 130 is fixed to 210 μm, the length Lm1 of the first multimode optical waveguide portion 131 changes depending on Lm2.

  The case of Lm2 = 10 μm shown in (C) is the case of the MMI optical coupler 100 described above, and the allowable width error is about −0.3 to 0.28 μm. On the other hand, as shown in (B), the tolerance error when Lm2 is shortened to 5 μm is narrowed to about −0.28 to 0.25 μm. However, this allowable width error is also wider than the allowable width error (about −0.2 to 0.1 μm) of the MMI optical coupler 500. In this case, Lm2 corresponds to about 2.3% of L. Further, when this Lm2 is shortened, the allowable width error is further narrowed.

  And as shown in (D), when Lm2 was made longer than 10 μm to 15 μm, the intensity of the fourth output light 350 decreased and changed so as to wave around 0.6. In this case, Lm2 corresponds to about 7.1% of L. Furthermore, when this Lm2 is lengthened, the intensity of the fourth output light 350 decreases. In particular, when Lm2 is about 10% or more of L, the intensity of the fourth output light 350 is further reduced, and the required intensity cannot be obtained.

  As can be seen from the above, in the present embodiment, by setting the length Lm2 of the second multimode optical waveguide section 132 to about 2 to 10% of the length L of the multimode optical waveguide 130, the allowable width error is widened. And the characteristics of the MMI optical coupler 100 can be improved.

(Width Wsl1 of the narrow end portion of the single mode optical waveguides 110 and 120)
With reference to FIG. 7, the change in the allowable width error when the width Wsl1 of the narrow end portion of the single mode optical waveguides 110 and 120 is changed as shown in FIG. FIG. 7 is a diagram illustrating a change in characteristics with respect to a change in the width Wsl1 of the narrow end portions of the single mode optical waveguides 110 and 120 in the MMI optical coupler 100 according to the present embodiment.

  (B) is a graph showing the intensities of the third output light 340 and the fourth output light 350 when Wsl1 = 2 μm, and (C) is the third output light 340 when Wsl1 = 2.3 μm. And (D) is a graph showing the intensities of the third output light 340 and the fourth output light 350 when Wsl1 = 2.6 μm, and (E). These are graphs showing the intensities of the third output light 340 and the fourth output light 350 when Wsl1 = 2.9 μm.

  As shown in (B), the allowable width error when Wsl1 = 2 μm is about −0.27 to 0.26 μm, and as shown in (C), the allowable width error when Wsl1 = 2.3 μm is about −0.29 to 0.27 μm, and as shown in (D), the allowable width error when Wsl1 = 2.6 μm is about −0.3 to 0.28 μm, and as shown in (E), Wsl1 The allowable width error when = 2.9 μm is about −0.3 to 0.27 μm.

  Therefore, it is understood that the allowable width error can be further widened by setting Wsl1 = 2.6 μm, although the dependency of the allowable width error on Wsl1 is not so large. It can be seen that the overall intensity of the fourth output light 350 is increased by increasing Wsl1.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to the example which concerns. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

  For example, in the above embodiment, the MMI optical coupler 100 includes the two optical waveguides 140 and 150 as the second input / output optical waveguide as shown in FIG. 1, but the present invention is not limited to such an example. For example, the MMI optical coupler 100 may include three or more second input / output optical waveguides. When a plurality of second input / output optical waveguides are provided, the second input / output optical waveguides may be arranged in parallel in the width direction.

It is explanatory drawing which shows the structure of the MMI optical coupler which concerns on one Embodiment of this invention. It is sectional drawing which shows the structure of the MMI optical coupler which concerns on the same embodiment. It is a top view which shows the structure of the MMI optical coupler which concerns on the same embodiment. FIG. 6 is an explanatory diagram for explaining characteristics of the MMI optical coupler according to the embodiment. 4 is a diagram illustrating a change in characteristics with respect to a change in width of a second multimode optical waveguide in the MMI optical coupler according to the embodiment. 6 is a diagram illustrating a change in characteristics with respect to a change in length of a second multimode optical waveguide portion in the MMI optical coupler according to the embodiment. 5 is a diagram showing a change in characteristics with respect to a change in width of an end portion of a single mode optical waveguide having a narrow width in the MMI optical coupler according to the same embodiment. It is explanatory drawing for demonstrating the conventional MMI optical coupler.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Substrate 20 Ridge waveguide 20A Lower clad 20B Optical waveguide layer 20C Upper clad 100 MMI optical coupler 110, 120 Single mode optical waveguide 111, 121 Central axis 130 Multimode optical waveguide 131 First multimode optical waveguide section 132 Second multimode Optical waveguide part 132a, 132b Side surface 133 Central axis 140, 150 Optical waveguide 140a, 150b Outer side surface 141, 151 Central axis 210 Curved waveguide 220 Linear waveguide 310 Input light 320 First output light 330 Second output light 340 Third output Light 350 Fourth output light 400 Mach-Zehnder optical modulator 500 MMI optical coupler 510, 520 Single mode optical waveguide 530 Multimode optical waveguide 540, 550 Single mode optical waveguide 610 Input light 620 First output light 6 0 second output light 640 third output light 650 fourth output light 700 Mach-Zehnder type optical modulator

Claims (8)

A first input / output optical waveguide, at least two second input / output optical waveguides adjacent to each other, and the multimode waveguide light propagated between the first input / output optical waveguide and the second input / output optical waveguide. A multimode interference optical coupler having a multimode optical waveguide,
The multi-mode optical waveguide is
A first multimode optical waveguide section to which the first input / output optical waveguide is connected;
A second multimode optical waveguide section connected to the second input / output optical waveguide and having a width in a direction perpendicular to a propagation direction of the multimode waveguide light that is narrower than the first multimode optical waveguide section;
A multi-mode interference optical coupler comprising:   The first input / output optical waveguide is disposed at a distance of about 1/6 of the width of the multimode optical waveguide from the central axis of the multimode optical waveguide parallel to the propagation direction of the multimode optical waveguide. The multimode interference optical coupler according to claim 1, wherein:   The side surface of the second multimode optical waveguide portion parallel to the propagation direction and the outer surface of each second input / output optical waveguide are joined without any step. Multimode interference optical coupler.   The at least one of the first input / output optical waveguide and the second input / output optical waveguide has a width that expands in a tapered shape as it approaches the multimode optical waveguide. The multimode interference optical coupler according to any one of the above.   The second input / output optical waveguide is separated from the central axis of the multimode optical waveguide parallel to the propagation direction of the multimode optical waveguide by a distance of 1/6 to 1/5 of the width of the multimode optical waveguide. The multimode interference optical coupler according to any one of claims 1 to 4, wherein the multimode interference optical coupler is arranged in a manner described above.   The multimode interference according to any one of claims 1 to 5, wherein the width of the second multimode optical waveguide part is 50% to 60% of the width of the first multimode optical waveguide part. Optical coupler.   The multimode interference light according to claim 1, wherein a length of the second multimode optical waveguide portion is 2% to 10% of a length of the multimode optical waveguide. Coupler. A first input / output optical waveguide, at least two second input / output optical waveguides adjacent to each other, and the multimode waveguide light propagated between the first input / output optical waveguide and the second input / output optical waveguide. A multimode optical waveguide, wherein the multimode optical waveguide is connected to the first multimode optical waveguide portion to which the first input / output optical waveguide is connected, and to the second input / output optical waveguide. Two multimode interference optical couplers each including a second multimode optical waveguide section whose width in the direction perpendicular to the propagation direction of the multimode waveguide light is narrower than one multimode optical waveguide section;
The Mach-Zehnder characterized in that the second input / output optical waveguide of one multimode interference optical coupler and the second input / output optical waveguide of another multimode interference optical coupler are connected by a connection waveguide. Type optical modulator.

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