JP4361030B2 - Mode splitter and optical circuit - Google Patents

Description

  The present invention relates to an optical circuit using an embedded optical waveguide formed on a substrate, and more specifically, a high-order propagation light propagating in an optical waveguide is adiabatically transitioned to another waveguide, and has low loss and wavelength dependence. An optical circuit capable of branching an optical signal and an application circuit thereof are provided.

  The development of optical communication systems has been promoted globally, and its spread has expanded from the importance and convenience to general consumers. Accordingly, further expansion of communication capacity is required. Under such circumstances, with the maturation of the wavelength division multiplexing communication technology, there is a current demand for the development of technology that can achieve both cost reduction and high performance of the optical device. Among optical devices, various optical circuits using embedded optical waveguides such as wavelength multiplexers / demultiplexers, splitters, and optical switches have been put into practical use, and currently play an important role in optical communication systems. The present invention relates to a mode splitter capable of changing the output destination of a signal propagating through a waveguide, depending on the order of the propagation mode, among optical circuits using embedded optical waveguides.

  FIG. 1 is a diagram showing an asymmetric Y-branch structure adopted as a basic structure of a conventional mode splitter. (See Non-Patent Document 1 below). The waveguide is composed of a core and a clad formed on the substrate. In the drawings described below, the waveguide is shown only by the shape of the core. The input waveguide 1 is a two-mode waveguide capable of propagating two modes, and two output waveguides 2 having different waveguide widths are coupled to the input waveguide 1 in a Y shape. Such an optical circuit is generally called an asymmetric Y branch circuit.

  The two modes propagating through the input waveguide 1 have different propagation constants, and the fundamental mode (0th order mode) has a larger propagation constant than the first order mode. On the other hand, since the two output waveguides 2a and 2b have different waveguide widths, the propagation constants of the fundamental modes thereof are different. When such output waveguides 2a and 2b are connected to the input waveguide 1 in a Y-branch shape, light having a large propagation constant among the light of two propagation modes input to the input waveguide 1 The (basic mode) is output to the output waveguide 2a side where the propagation constant of the basic mode is large. Conversely, light having a small propagation constant is coupled to the other output waveguide 2b side. Based on this principle, the two modes of light input to the input waveguide 1 are output to the separate output waveguides 2a and 2b, respectively, and the asymmetric Y branch circuit functions as a mode splitter.

W.K.Burns et.al., “An Analytic Solution, for Mode Coupling in Optical Waveguide Branches”, IEEE Journal of Quantum Electronics, Vol. QE-16, No. 4, 1980

  However, the conventional asymmetric Y-branch circuit has a problem that a lot of loss occurs at a portion where the input waveguide 1 and the two output waveguides 2a and 2b branch. As can be seen from FIG. 1, in the portion where the output waveguides 2a and 2b branch, the two output waveguides are very close to each other. For this reason, when the cladding material is deposited after the core processing, the cladding material must be filled in a very narrow gap portion (indicated by a in FIG. 1). However, when the upper clad is embedded after the core processing, it is difficult for the upper clad material to enter such a narrow gap portion. As a result, cavities and voids are generated in the narrow gap portion, and a large loss occurs due to the cavities and voids. In addition, the following problems were encountered when forming the core portion.

  FIG. 2 is a diagram for explaining a branch portion of a conventional mode splitter. Even when the core portion is formed, there is a problem that the core of the Y branch portion is actually connected as shown in FIG. In general, the core portion of the optical waveguide is formed by processing the deposited core film by photolithography and etching techniques, but the two output waveguides 2a and 2b are very close to each other at the Y branch portion. Narrow gaps cannot be formed due to limitations. Of the light incident from the input waveguide 1, the light propagating through this “connected portion” becomes leaked light without being coupled to the output waveguides 2 a and 2 b, causing a large loss. If the angle formed by the output waveguides 2a and 2b is increased in order to avoid the connection of the Y branch portions, the mode splitter does not operate. Such an increase in loss due to limitations on the processing technology of the waveguide has been a big problem in the conventional splitter.

  The present invention has been made in view of such problems, and an object thereof is to provide a low-loss mode splitter and a low-loss optical circuit using the mode splitter.

In order to achieve such an object, according to the first aspect of the present invention , at least two types of propagation modes having different propagation orders are guided in an optical circuit composed of a waveguide composed of a core and a clad on a substrate. A main waveguide that can be waved, a tapered portion that is arranged with a certain gap from the main waveguide, and the width of the waveguide is changed, and a waveguide that is continuously connected to the tapered portion and has a waveguide width different from that of the main waveguide. One input light having an output waveguide having a waveguide width and a sub-waveguide in which at least one of the propagation modes having different propagation orders is adiabatically transitioned among the at least two types of propagation modes having different propagation orders. converting a mode splitter which separates into two output light is optically coupled to the output of the main waveguide of the mode splitter, a part or all of the optical signal to the high-order mode from the fundamental mode , And a reflective mode converter configured to reflect the converted light, the reflected light signal, characterized in that output from the secondary waveguide of the mode splitter.
The invention according to claim 2 is the optical circuit according to claim 1, wherein the reflective mode conversion element is a slant grating .

The invention according to claim 3 is characterized in that the output portion waveguide includes a curved waveguide continuously connected to the tapered portion.

According to a fourth aspect of the present invention, the output portion waveguide includes a straight waveguide continuously connected to the tapered portion and a curved waveguide continuously connected to the straight waveguide. Features.

The invention described in claim 5 is characterized in that a trapezoidal taper portion in which the tip of the taper portion is cut along a plane perpendicular to the main waveguide is characterized.

The invention described in claim 6 is characterized in that an optical termination section for absorbing or diverging output light is provided at the termination section of the output section waveguide of the sub-waveguide.

The invention according to claim 7 is characterized in that the gap between the tapered portion and the main waveguide is 0.2 μm or more and 5 μm or less.

The invention according to claim 8 is characterized in that the tapered portion is a linear taper whose width increases linearly, and the taper angle is 0.004 degrees or more and 1 degree or less.

The invention according to claim 9 is a fundamental mode in which propagation modes capable of propagating through the main waveguide are only a first propagation mode and a second propagation mode having different propagation orders, and propagate through the output section of the sub-waveguide. The effective refractive index of is smaller than the effective refractive index of the first propagation mode and larger than the effective refractive index of the second propagation mode.

  As described above, according to the present invention, a mode splitter with low loss and less wavelength dependency can be realized. Further, by adopting the mode splitter having this structure for the input waveguide of the arrayed waveguide diffraction grating, it is possible to reduce the center wavelength error and improve the transmission characteristics. Furthermore, the wavelength add-drop function can be easily realized by incorporating this mode splitter into the wavelength-dependent mode conversion element.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, the operation principle of the mode splitter of the present invention will be described.

  FIG. 3 is a diagram showing the configuration of the mode splitter according to the present invention. This mode splitter includes a main waveguide 3 made of a straight waveguide, and a sub-waveguide 4 made of a taper portion 5 and an output portion waveguide 6. The tapered portion 5 has a tapered shape in which the waveguide width gradually increases from the input end side (left side in FIG. 3) of the main waveguide 3 toward the output end side. The taper portion 5 is smoothly and continuously connected to the output portion waveguide 6. Further, the tapered portion 5 is arranged with a gap at a constant interval from the main waveguide 3. This gap will be described later with reference to FIG.

  The main waveguide 3 is a waveguide through which at least two propagation modes having different orders can propagate. The mode splitter has a function of outputting at least one of the at least two propagation modes from the sub waveguide 4 and propagating the remaining propagation modes through the main waveguide 3 as they are. The waveguide widths of the main waveguide 3 and the sub-waveguide 4 are different. As the waveguide widths are different, the effective refractive indexes of the main waveguide 3 and the sub-waveguide 4 can be made different as will be described in detail later. This is because the propagation mode conversion can be realized. Normally, the 0th-order mode signal light is propagated through the main waveguide 3 as it is, so that the waveguide width of the sub-waveguide 4 is narrower than the waveguide width of the main waveguide 3. However, the sub waveguide 4 may have a wide waveguide width.

  Although the form in particular is not restrict | limited, the output part waveguide 6 is preferably comprised with the waveguide containing a curved waveguide. That is, by making the output waveguide 6 connected to the tapered portion 5 a curved waveguide, the sub waveguide 4 can be gently separated from the main waveguide 3 and the overall circuit size can be reduced. Therefore, it becomes the best form. However, it may be a straight waveguide or a combination thereof.

  FIG. 4 shows the relationship between the normalized frequency (v) and the effective refractive index (neff) in a rectangular waveguide having a square cross-sectional shape and having the same refractive index of the upper and lower claddings. In general, the number of propagation modes that can propagate through a waveguide and the effective refractive index of each propagation mode are determined by the core shape and size of the waveguide, the refractive indexes of the core and the cladding, and the like. The normalized frequency (v) is approximately

It can be expressed. (Although it is not an accurate normalized frequency of the three-dimensional optical waveguide, an analytical expression used in an optical fiber or the like is used for simplification of explanation.) However, λ is the wavelength, n1 is the refractive index of the core, and n0 The refractive index of the cladding, a, is the width of the waveguide and the height of the waveguide.

  When the waveguide size, refractive index, and wavelength used are determined, the normalized frequency is obtained. As shown in FIG. 4, when the normalized frequency of the main waveguide 3 is determined, the number of modes that can propagate through the main waveguide 3 and the effective refractive index of each mode are determined. However, in the case of the mode splitter of the present invention, as described above, the main waveguide 3 needs to have at least two propagation modes. For such a main waveguide 3, the sub-waveguide 4 has a different normalized frequency. The example shown in FIG. 4 will be described. The sub-waveguide 4 has two propagation modes (B0, B1). When the main waveguide 3 and the sub-waveguide 4 are adjacent to each other and the energy transfer between modes is adiabatic, a part of the propagation mode of the main waveguide 3 transitions to the sub-waveguide 4 without loss. To do. The propagation modes (A 0, A 1, A 2, A 3) propagated through the main waveguide 3 are converted into a propagation mode including the main waveguide 3 and the sub waveguide 4 in the vicinity of the vicinity of the sub waveguide 4. Here, the mode (local normal mode) including the main waveguide 3 and the sub-waveguide 4 is (A0, B0, A1, B1, A2, A3) from the lower order, that is, from the higher effective refractive index. It becomes. Therefore, the propagation mode of the main waveguide 3 is converted in order from the lower order of the local normal mode. That is, A0 is converted to the A0 mode, which is the lowest order mode among the local normal modes, the A1 mode is changed to the B0 mode, the A2 mode is changed to the A1 mode, and A3 is changed to B1.

  FIG. 5 is a diagram for explaining an example of the mode conversion described above. That is, when the normalized frequency v value as shown in FIG. 4 is set, the 0th-order mode (A0) and the second-order mode (A2) among the modes propagating through the main waveguide 3 as shown in FIG. The first-order mode (A1) and the third-order mode (A3) are output to the sub-waveguide 4. As described above, by appropriately setting the normalized frequency v of the main waveguide 3 and the sub-waveguide 4, a desired propagation mode among the modes propagating through the main waveguide 3 can be branched to the sub-waveguide 4.

  FIG. 6 is a diagram schematically showing the electric field intensity distribution of the mode splitter. As an example, the state of the electric field intensity distribution of the mode splitter in which the fundamental mode and the primary mode are incident on the main waveguide 3 and only the primary mode is branched to the sub waveguide 4 is schematically shown. In each of the cross sections at the broken line portion of the mode splitter of the circuit layout shown in FIG. 6A, (b) the refractive index of the cross section, (c) the fundamental mode electric field intensity distribution, and (d) the primary mode electric field intensity distribution. It is shown.

  As described above, in the conventional mode splitter as shown in FIG. 1, an increase in loss due to processing technology limitations becomes a problem. Therefore, the present invention proposes a mode splitter having a structure as shown in FIG. In the mode splitter according to the present invention, the sub waveguide 4 provided with the taper portion 5 and the output portion 6 is disposed adjacent to the linear main waveguide 3. By adopting such a shape, it is possible to obtain a structure that does not have an acute angle portion and a narrow portion that are difficult to form by core processing.

  FIG. 7 is a diagram for further explaining the tapered portion and the gap of the mode splitter according to the present invention. The tapered portion 5 of the main waveguide 3 and the sub-waveguide 4 has a gap 7 of a certain distance, and this distance is a distance that can be formed by core processing. Further, this gap distance needs to be a distance at which the upper cladding layer can be deposited after the core processing is formed. Furthermore, by making this gap distance constant, it is possible to adiabatic transition of the higher-order mode propagating light propagating through the main waveguide 3 to the sub-waveguide 4 without loss. However, if the gap 7 is made too wide, the length of the sub-waveguide 4 necessary for the higher-order mode to transition to the sub-waveguide 4 increases. At the same time, excess losses increase.

  Here, the excess loss means an unexpected (undesirable) loss when an optical circuit or the like is manufactured. Distinguish between “coupling loss”, which is a loss that occurs at the coupling with the fiber, and “propagation loss” (waveguide loss), which is a loss that occurs due to fluctuations in the waveguide, processing errors, etc. Use for.

  The optimum value of the gap distance described above varies depending on the refractive index and size of the core, but in a waveguide with a relative refractive index difference of about 0.1% to 5% of the silica-based glass waveguide, the gap distance is 0.1 μm or more and 5 μm. The following is preferable, and 0.1 μm or more and 3 μm or less is optimal. Even when the gap distance is 0.1 μm or more and 10 μm or less, the function as a mode splitter can be realized.

  Further, in order to realize a low-loss mode splitter, the sub-waveguide 4 is provided with a tapered portion 5 as shown in FIG. In the case where the sub waveguide 4 is configured by only a straight waveguide or a curved waveguide without the taper portion 5, the light propagating through the main waveguide 3 suddenly feels the sub waveguide 4. For this reason, excess loss occurs due to light being reflected or disturbed at the start of the sub-waveguide 4. When the tapered portion 5 of the sub-waveguide 4 is a straight waveguide, a part of light propagating through the main waveguide 3 regardless of the order of the propagation mode, as is generally seen in a directional coupler. May cause the same direction coupling in the sub-waveguide 4. Therefore, not only the function as a mode splitter is deteriorated, but also a large wavelength dependency occurs in the branching ratio of the mode.

  Therefore, the spatial refractive index change can be moderated by providing the sub-waveguide 4 with the tapered portion 5 as in the mode splitter according to the present invention. Furthermore, since the same direction coupling can be avoided, a low-loss mode splitter can be realized, and a branching ratio with less wavelength dependency can be realized. As described above, if the taper portion 5 is too long, the circuit size increases, which is not preferable.

  The optimum value for the taper angle 8 of the taper portion 5 shown in FIG. 7 varies greatly depending on the width of the main waveguide 3, the width of the sub-waveguide 4, the gap distance, and the like, but the higher-order mode of the main waveguide 3 is 0.1 dB or less. It is necessary to determine the taper angle 8 to such an extent that adiabatic transition to the sub-waveguide 4 occurs due to loss. Further, it is desirable that the taper angle is 8 so that the crosstalk amount can be ensured to be -30 dB or more. Here, the amount of crosstalk is the ratio between the light of the higher-order mode that has transitioned to the sub-waveguide and the light that the higher-order mode cannot adiabatically transition to the sub-waveguide 4 and propagates through the main waveguide 3 as it is. To express.

  8 and 9 are diagrams showing examples of calculation results of the relationship between the taper angle and excess loss. As described above, the optimum value of the taper angle 8 varies depending on the width of the main waveguide 3, the width of the sub waveguide 4, the refractive index, the gap 7, and the like. 8 and 9, the mode splitter gap is 2 μm, the core height of each waveguide is 6 μm, the waveguide width is 3 μm in the sub-waveguide 4, and the main waveguide 3 is changed to 6 μm, 7 μm, and 8 μm. The case is calculated.

  The taper angle 8 is determined so as to reduce excess loss according to the waveguide width, gap, and the like. For example, when the width of the main waveguide 3 is 6 μm, the optimum taper angle 8 that can secure an excess loss of 0.1 dB or less and a crosstalk of 30 dB or more is optimally 0.4 degrees or less. However, when the width of the main waveguide 3 is 6 μm, if the taper angle 8 is set to 0.02 degrees or less, an increase in the entire circuit length becomes a problem. Therefore, it is preferable to set an appropriate taper angle 8 in consideration of the above relationship. For example, the minimum value of the taper angle can be 0.004 degrees.

  From the calculation results of FIGS. 8 and 9, the specific taper angle 8 for realizing a low-loss mode splitter in an optical circuit having a relative refractive index difference of about 0.5% to 5% is as follows. 0.02 degree to 2.0 degree is preferable, and 0.05 degree to 1.0 degree is optimal. However, even if the taper angle 8 is in the range of 0.01 to 2.0 degrees, the function as a mode splitter can be realized.

  FIG. 10 is a diagram showing a mode splitter according to the present invention in which the waveguide width of the tapered portion changes nonlinearly. It is composed of a main waveguide 10 and a sub waveguide 9. The tapered portion is not necessarily in the form of a linear taper. As shown in FIG. 9, it may be a sub-waveguide 9 in the form of a taper in which the waveguide width changes nonlinearly.

  FIG. 11 is a diagram showing another configuration of the mode splitter according to the present invention. As shown in FIG. 11, in the sub-waveguide 12 of the mode splitter, a mode splitter with a lower loss is possible by including a straight portion 14 formed by a straight waveguide between the output portion waveguide 15 and the tapered portion 13. It becomes. When the taper part 13 and the output part waveguide 15 composed of a curved waveguide are connected, the light branched from the main waveguide 11 to the sub-waveguide 12 remains affected by meandering immediately after branching. For this reason, a loss may occur at the coupling portion between the tapered portion 13 and the output portion waveguide 15. In such a case, the light can be smoothly propagated to the output portion by a method such as shifting the central axes of the tapered portion 13 and the output portion waveguide 15. However, the operation of light meandering immediately after branching varies depending on the manufactured chip, and when a large number of chips are formed on a substrate, excessive loss varies between the chips.

  Therefore, as shown in FIG. 11, the effect of meandering or the like can be prevented by inserting a straight line portion 14 constituted by a straight waveguide after the taper portion 13. As a result, excessive loss can be reduced. The length of the straight line portion 14 is preferably 1 cm or less, and most preferably 1 mm or less, due to the influence of circuit size limitations and the like. The output waveguide 15 connected to the straight portion 14 can be gently separated from the main waveguide 11 by starting with a curved waveguide, and the overall circuit size can be reduced. Therefore, it becomes the best form. However, it is not limited to a curved waveguide. Details will be described later in the first embodiment.

  The basic configuration and operation of the mode splitter according to the present invention have been described above. In the following part, application examples of the mode splitter according to the present invention to various optical circuits will be described.

  In various optical circuits, higher-order modes of light propagating through a waveguide may deteriorate circuit performance. In such a case, by using the mode splitter according to the present invention, it is possible to extract the light of the higher order mode and improve the circuit performance. At this time, the higher-order mode light extracted by the mode splitter must be appropriately terminated. Therefore, by providing a substance or waveguide structure that diverges or absorbs propagating light at the terminal end of the sub-waveguide, it is possible to prevent performance degradation of the optical circuit.

  Specifically, when diverging light, a method of making the tip of the sub-waveguide spherical or a method of making it triangular is preferable. Alternatively, a method of simply cutting or disconnecting the waveguide and then directing the waveguide in a direction that does not affect other optical circuit portions has a sufficient effect. In the case of absorbing light, it is also possible to put a light absorber such as carbon in the terminal portion of the sub-waveguide. These will be described later in the second embodiment.

  In the mode splitter described so far, a general design method in the case where there are two or more modes propagating through the main waveguide has been described. Hereinafter, a mode splitter in which only the fundamental mode and the primary mode can propagate in the main waveguide and only the fundamental mode can propagate in the sub-waveguide will be described.

  A mode splitter that can propagate only two modes in the main waveguide and only one mode in the sub-waveguide has the narrowest design conditions. Therefore, a mode splitter that is most easily designed and operates at high performance is obtained. It becomes possible. Further, it is also a form that is most easily applied to an actual circuit.

  In general, in order to realize a high-performance mode splitter, it is important to appropriately set an effective refractive index of a mode capable of propagating through the main waveguide and the sub-waveguide (see the explanation part of FIG. 4). In this mode splitter in which only two modes can propagate to the main waveguide and only one mode can propagate to the sub-waveguide, it is important that the primary mode of the main waveguide efficiently makes adiabatic transition to the sub-waveguide with low loss. It is. For this purpose, the effective refractive index of the sub-waveguide is preferably 30% to 70% of the sum of the effective refractive index of the fundamental mode and the primary mode of the main waveguide, and is preferably 45% to 55%. Is optimal. However, even when it is between 20% and 80%, the operation as a mode splitter is possible.

  In various optical circuits, circuit design is often performed on the assumption that only the fundamental mode propagates in a waveguide on the circuit. In such an optical circuit, undesired optical circuit performance degradation may occur when light of a higher-order mode generated in the optical circuit or outside the optical circuit propagates through the optical circuit.

  FIG. 12 is a diagram showing an example in which the mode splitter according to the present invention is applied to the arrayed waveguide diffraction grating 37. In a wavelength multiplexer / demultiplexer such as the arrayed waveguide diffraction grating 37, distortion in the shape of transmission characteristics occurs due to the generation of higher-order modes, and variations in the center wavelength occur. In such a case, the mode splitter 36 according to the present invention is inserted into the input waveguide 33 or the output waveguide 34 connected to the slab waveguides 30 and 32 at both ends of the arrayed waveguide 31, as described above. Problems caused by higher order modes can be solved. For example, as shown in FIG. 12, the problem caused by the higher-order mode can be solved by inserting the mode splitter 36 according to the present invention into the input waveguide 33.

  FIG. 13 is a diagram showing another example in which the mode splitter according to the present invention is applied to the arrayed waveguide diffraction grating 37. As shown in FIG. 13, by inserting the mode splitter 20 according to the present invention into the output-side waveguide 34 of the arrayed-waveguide diffraction grating 31, a higher-order mode that propagates to the output side of the arrayed-waveguide diffraction grating 37 is fundamental. It is possible to eliminate crosstalk degradation, center wavelength shift, and the like that occur when coupled to a mode. Details will be described later in the third embodiment.

  Problems due to the influence of higher-order modes in optical circuits, such as the distortion of the shape of transmission characteristics and crosstalk degradation described above, can be avoided by blocking higher-order modes. However, when the higher-order mode is cut off, the higher-order mode light diverges and becomes leaked light that cannot be controlled in the circuit. As a result, the circuit performance is degraded. On the other hand, when the mode splitter according to the present invention is used, a higher-order mode can be extracted as propagating light to the sub-waveguide. Therefore, the higher-order mode does not become leaked light, and processing such as termination processing can be appropriately performed. Further, by taking out from the optical circuit, it is possible to monitor whether or not the higher order mode has occurred.

  FIG. 14 is a diagram schematically showing an example in which the mode splitter according to the present invention is applied to a wavelength add / drop circuit. Using the mode splitter according to the present invention, it is possible to realize a wavelength Add / Drop circuit that can extract or add only signal light of a specific wavelength from the waveguide. The wavelength add / drop circuit shown in FIG. 14 has a configuration in which a wavelength dependent mode conversion element 44 and a mode splitter 45 are connected. Among the signal lights (λ1 to λ5) having a plurality of wavelengths input to the input terminal 41, the signal having the specific wavelength is converted by the wavelength-dependent mode conversion element 44 that can convert only the signal light having the specific wavelength into a higher-order mode. Only light can be converted. For example, only the signal light having the wavelength λ3 is converted into the higher order mode and separated by the mode splitter 45, so that only the signal light having the specific wavelength λ3 can be separated and taken out from the output terminal 42. Conversely, by reversing the input / output directions of all signal lights, only signal light of a specific wavelength can be multiplexed.

  FIG. 15 is a diagram schematically showing another example in which the mode splitter according to the present invention is applied to a wavelength add / drop circuit. Unlike the configuration of FIG. 14 described above, a mode splitter 45 and a reflective wavelength dependent mode conversion element 46 are connected. By using the reflective wavelength-dependent mode conversion element 46, a wavelength separator can be realized. That is, the reflection wavelength-dependent mode conversion element 46 that can convert only a specific wavelength of signal light (λ1 to λ5) input to the input terminal 47 into a higher-order mode is specified. Mode conversion is possible only for signal light of a wavelength of. For example, only the signal of wavelength λ3 is reflected and converted into a higher order mode, and then separated by the mode splitter 45, so that only the signal of specific wavelength λ3 can be separated and taken out from the output terminal 48. Similarly, in the configuration of FIG. 15, by reversing the input / output direction, it is possible to multiplex only the signal light having the characteristic wavelength. By connecting the circuits shown in FIG. 14 and FIG. 15 in multiple layers, a multi-wavelength add / drop device can be realized. Details will be described later in the fourth embodiment.

  For the mode conversion elements in the two application examples described in FIGS. 14 and 15, a long period grating can be used as the wavelength dependent mode conversion element 44. Further, a slant grating can be used as the reflective wavelength-dependent mode conversion element 46.

  Furthermore, by using the mode splitter of the present invention, it is possible to produce an optical tap circuit having no wavelength dependency. Unlike the above-described mode conversion element that acts only on a specific wavelength, mode conversion without wavelength dependency is possible, and can be easily realized by using an offset connection of the waveguide.

  FIG. 16A is a diagram showing an example in which the mode splitter according to the present invention is applied to an optical tap circuit. FIG. 16B shows an enlarged view of the waveguide offset 50 in the main waveguide 21. The input side of the main waveguide 21 of the mode splitter is provided with a waveguide offset 50 in which the waveguide axis is shifted. The deviation amount of the central axis of the waveguide is defined as an offset length 51 as shown in the enlarged view of FIG. By adopting the waveguide offset 50 in the main waveguide 21, a part of the light propagating through the main waveguide 21 is converted from the fundamental mode to the higher order mode. However, the use of the waveguide offset 50 enables mode conversion with very little wavelength dependency. Thereby, a part of the light propagating through the main waveguide 21 can be extracted to the sub-waveguide 22. The ratio of the light to be extracted, that is, the tap amount of the optical tap circuit can be adjusted by the amount of offset of the waveguide (offset length 51). Therefore, there is an advantage that the tap amount can be easily adjusted by configuring the offset 50 so that the offset length 51 becomes the desired tap amount. Details will be described later in the fifth embodiment.

  FIG. 17 is a diagram showing another example in which the mode splitter according to the present invention is applied to an optical tap circuit. As shown in FIG. 17, variation in the branching ratio can be suppressed by inserting the mode splitter of the present invention into the input waveguide portion of the symmetric Y branch circuit. When a higher-order mode occurs in the input waveguide, the higher-order mode is branched asymmetrically even if the fundamental mode is branched with equal intensity. As a result, the branching ratio becomes asymmetric. However, by using the mode splitter according to the present invention, the higher-order mode is removed, the influence of the higher-order mode is eliminated, and the variation in the branching ratio is suppressed. Details will be described later in Example 6.

  As described above, the mode splitter according to the embodiment of the present invention has been overviewed. Next, a more detailed example will be described. In each example described below, circuit fabrication was performed under the following fabrication conditions.

  Circuit fabrication was performed by a fabrication method represented by a quartz glass-based planar lightwave circuit. A quartz glass containing boron and phosphorus was deposited as a lower cladding on a Si substrate by a flame deposition method, and then germanium-doped glass was deposited as a core layer. Next, the waveguide was patterned by a photolithography technique and a reactive ion etching technique. Thereafter, quartz glass was deposited as an upper clad by a flame deposition method. Through the above steps, a buried waveguide made of quartz glass was produced.

[Example 1]
Hereinafter, embodiments will be described with reference to the drawings. In addition, the concrete numerical value and the preparation method in the following Examples are examples of implementation, and it does not specifically limit to these.

  FIG. 18 is a diagram showing a basic configuration of the manufactured mode splitter according to the present invention. In the configuration of the manufactured mode splitter, the width of the main waveguide 21 is 8.5 μm, the constant gap between the main waveguide 21 and the sub-waveguide 22 is 1.5 μm, and the relative refractive index difference between the core and the clad of the waveguide is 0.75%. It is.

  Further, a curved waveguide having a curvature with a radius R of 10 mm was used for the output portion of the sub waveguide 22. At the same time, the conventional mode splitter shown in FIG. 1 was also produced. The mode splitter of the present invention can reduce the excess loss quality by about 2 dB as compared with the conventional type due to the effects of the gap and the tapered portion. The reason for this difference is that, as described above, it is difficult to produce a branching portion with a conventional mode splitter, and a good waveguide branching portion cannot be produced. In the waveguide branching portion of the conventional mode splitter, the cladding material of the upper cladding has not been satisfactorily embedded. The insertion loss and crosstalk of the mode splitter were measured by changing the waveguide width of the output portion of the sub-waveguide 22 and the taper angle of the taper portion (see FIG. 18). The results are shown in FIG. 19 and FIG. Here, crosstalk refers to the ratio at which the fundamental mode propagating through the main waveguide 21 is output to the sub-waveguide 22 side.

  FIG. 19 is a diagram showing the relationship between insertion loss and crosstalk at a wavelength of 1550 nm when the sub-waveguide width is changed. Actually, considering the simplicity of the experiment, the taper length is fixed to 2 mm, and the sub-waveguide width and the taper angle are changed simultaneously. Even if the width of the sub-waveguide and the taper angle are changed, the insertion loss does not change significantly, and a mode splitter with a low loss can always be realized. However, in order to obtain a crosstalk of 30 dB or more, it is understood that the width of the sub waveguide is optimally 4 μm or less.

  FIG. 20 is a diagram illustrating the relationship between insertion loss and crosstalk when the taper angle of the tapered portion is changed. Actually, considering the simplicity of the experiment, the taper length and the taper angle are simultaneously changed with the sub-waveguide width fixed at 3.5 μm. It can be seen that the taper angle is optimally 0.25 degrees or less in order to obtain a crosstalk of 30 dB or more.

  FIG. 21 is a diagram showing the wavelength dependence of insertion loss. The taper length was 1 mm, the main waveguide width was 7 μm, and the sub-waveguide width was 3 μm. The taper angle at this time is 0.17 degrees. It can be seen that a mode splitter having both high crosstalk of 30 dB or more and low loss can be realized in a very wide wavelength range of 1400 to 1700 nm.

  In addition, since the waveguide width is very small at the tip of the taper portion, when a large number of the mode splitters are manufactured on the substrate, the shape of the most advanced portion may vary depending on the chip. In such a case, as shown in FIG. 22, the tip portion 28 of the sub-waveguide 22 is cut along a plane perpendicular to the main waveguide so as not to generate a new excess loss, thereby forming a waveguide shape. Can be stabilized.

  FIG. 23 is a diagram illustrating the wavelength dependence of crosstalk of an optical circuit including a 500 μm linear waveguide between the taper portion and the output portion waveguide. The gap width was 1.5 μm, the sub-waveguide width was 3 μm, the main waveguide width was 7 μm, the taper length was 1 mm, and the taper angle was 0.17 degrees. As can be seen from FIG. 23, the crosstalk is greatly improved in the wavelength band of 1600 nm. The effect which inserted the linear part 14 after the taper part 13 demonstrated in FIG. 11 can be confirmed.

[Example 2]
A mode splitter further provided with an optical terminal was manufactured by the same manufacturing method as in Example 1. In many cases, the light branched to the sub-waveguide by the mode splitter becomes unnecessary light. In such a case, it is necessary to appropriately terminate the light propagating through the sub-waveguide.

  FIG. 24 is an explanatory diagram of a mode splitter provided with an optical terminal. As shown in FIG. 24A, a termination processing unit 29 is connected to the end of the termination unit of the sub-waveguide 22 of the mode splitter. As shown in FIG. 24B, the termination processing unit 29 performs groove processing on the terminal portion of the sub-waveguide 22, and applies and solidifies a silicon adhesive in which carbon is dissolved in the processed groove. Consists of things. The processing groove of the termination processing unit 29 was configured such that the length of the side parallel to the sub waveguide 22 was 300 μm, and the length of the side perpendicular to the sub waveguide 22 was 200 μm (depth ** μm). The termination processing unit 29 can eliminate noise generated by the leaked light from the sub-waveguide 22 and confirm the reduction of crosstalk from the main waveguide 21 output. Crosstalk in the absence of the optical termination portion 29 was −30 to −40 dB, whereas in the case where the optical termination portion 29 was provided, it was −60 dB or less. It is also possible to use materials other than carbon as the light shielding agent. In addition, the same crosstalk can be reduced by inserting a wavelength cut filter or processing the tip of the waveguide into a triangular shape.

[Example 3]
FIG. 25 is a diagram showing an embodiment of an optical circuit in which the mode splitter according to the present invention is applied to an arrayed waveguide grating (AWG). The arrayed waveguide diffraction grating (AWG) has a configuration in which an input-side slab waveguide 30 and an output-side slab waveguide 32 are respectively connected to both ends of an arrayed waveguide 31 formed on a substrate. An input waveguide 33 is connected to the input side slab waveguide 30, and an output waveguide 34 is connected to the output side slab waveguide 32. The mode splitter of the present invention is disposed in the input waveguide 33 of this optical circuit.

  In the fabricated circuit, the relative refractive index difference between the core and the clad of the waveguide is 1.5%, the waveguide width of the AWG is 5 μm, the width of the main waveguide of the mode splitter is 5 μm, the width of the sub waveguide is 2 μm, and the gap is The taper length is 1.5 mm, the output portion radius R is 20 mm. The AWG was manufactured under the conditions that the center wavelength was about 1550 nm, the channel interval was 100 GHz, and the number of channels was 16 ch.

  As shown in FIG. 25, in this arrayed waveguide diffraction grating, in order to confirm the effect of the mode splitter, the input waveguide 33 with the mode splitter and the input waveguide without the mode splitter are used as the AWG input waveguide 33. It is the composition which has both. In FIG. 25, the input side has two input waveguides and the output side has three output waveguides. However, the mode splitter according to the present invention has a different number of input / output waveguides. Needless to say, this configuration can also be applied.

  FIG. 26 shows the characteristics obtained by comparing the transmission spectrum characteristics of the arrayed waveguide grating with and without the mode splitter. As shown in FIG. 26, the light incident from the input waveguide having the mode splitter has a flat transmission characteristic which is an ideal operation required for the AWG. However, when light is incident from an input waveguide without a mode splitter, the transmittance characteristic in the passband is not flat due to the influence of higher-order modes of the input waveguide, and transmission is performed as shown in FIG. The slope of the right-hand side occurred on the long wavelength side of the shape of the rate characteristic. In particular, the shape of the transmittance characteristic (transmission spectrum) changes depending on the positional relationship between the optical fiber for inputting light into the optical circuit and the input waveguide 33, the transmittance decreases, the deviation from the design characteristic of the transmission spectrum, Degradation of transmission characteristics such as center wavelength deviation was also observed. However, when a signal is incident from an input waveguide with a mode splitter, such deterioration of the transmission characteristics was not recognized.

  Furthermore, as shown in FIG. 13, even when the mode splitter is provided on the output side, the performance degradation due to the higher order mode of the AWG output could be suppressed.

  As described above, by inserting the mode splitter according to the present invention into the input waveguide or the output waveguide of the AWG, it is possible to prevent the shape deterioration of the transmittance characteristics of the AWG.

[Example 4]
FIG. 27 is a diagram showing an example in which the mode splitter according to the present invention is applied to an optical Drop circuit. The optical drop circuit is a circuit that can extract only light of a specific wavelength. As shown in FIG. 27, the present optical Drop circuit includes a main waveguide 21 having an input end A and an output end C, a sub-waveguide 22 having an output end B, and a coupling portion of the main waveguide 21 with the sub-waveguide 22. The slant grating 55 is also provided on the output end C side. The optical signal is input from the input end A of the main waveguide 21 and output from the output end C. However, a signal having a specific wavelength to be extracted is output from the output terminal B.

  As shown in FIG. 27, a slant grating 55 manufactured using an ultraviolet laser of 193 nm is provided on the coupling portion between the main waveguide 21 and the sub-waveguide 22, that is, on the output end C side of the mode splitter according to the present invention. ing. The slant grating 55 reflects only light of a specific wavelength backward (input end A side). At the same time, the reflected light is converted from a fundamental mode to a higher order mode. Therefore, the light of a specific wavelength that is reflected and converted into a higher-order mode is branched to the sub-waveguide 22 by the action of the mode splitter of the present invention, and is output only from the output end B.

  This optical drop circuit was prepared under the conditions of a relative refractive index difference of 0.75% between the core and clad of the waveguide, a waveguide width of 7 μm, a gap of 2 μm, and a taper length of 1 mm. The reflection wavelength of the slant grating was set around 1547.5 nm.

  28 and 29 are diagrams showing the transmission characteristics between the input and output terminals of the optical drop circuit described above. FIG. 28 shows the transmission characteristic of the signal path from the input end A to the output end B, and it can be seen that only light of a desired wavelength (wavelength is 1547.5 nm) is blocked. FIG. 29 shows the transmission characteristics of the signal path from the input end A to the output end B, and it can be seen that the light that has been cut off the signal path of the main waveguide is output to the output end B. As described above, by using the mode splitter according to the present invention, an optical drop circuit capable of extracting only a signal having a specific wavelength can be manufactured.

  FIG. 30 is a diagram showing an example of an Add / Drop circuit using two mode splitters according to the present invention. As shown in FIG. 30, the Add / Drop circuit includes a main waveguide 21 having an input end In and an output end Out, one slant grating 55 provided at an intermediate point of the main waveguide 21, and the input from the perspective of the slant grating 55. The sub-waveguides 22a and 22b are arranged symmetrically on both the end In side and the output end Out side. By placing the mode splitter according to the present invention opposite to one slant grating 55, a signal of a specific wavelength is taken out from the In-Out signal path to the Drop terminal side and added from the Add terminal. Confirmed that it was possible.

  FIG. 31 is a diagram showing an example in which a multi-wavelength Add / Drop circuit is configured using the mode splitter according to the present invention. This Add / Drop circuit is composed of a main waveguide 21 having an input end In and an output end Out, and six sub-waveguides 22a, 22b, 22c, 22d, 22e, and 22f that are coupled one after another in a multi-layered structure. Yes. In the main waveguide 21, three slant gratings 55a, 55b, and 55c are arranged at an intermediate point between the input end In and the output end Out. The first-level sub-waveguides 22a and 22b are arranged facing the slant gratings 55a, 55b and 55c, and are coupled to the main waveguide 21 to form a mode splitter. Slant gratings 55d and 55e and slant gratings 55f and 55g are arranged at the output end Drop (λ1) of the sub waveguide 22a and the input end Add (λ1) of the sub waveguide 22b, respectively. Furthermore, the second-level sub-waveguides 22c and 22d are coupled to the first-level sub-waveguides 22a and 22b, respectively, to form a mode splitter. A slant grating 55h and a slant grating 55i are disposed at the output end Drop (λ2) of the sub waveguide 22c and the input end Add (λ2) of the sub waveguide 22d, respectively. Further, the sub-waveguides 22e and 22f in the third layer are coupled to the sub-waveguides 22c and 22d in the second layer to form a mode splitter, and each of them has an output terminal Drop (λ3) and an input terminal. Add (λ3).

  The slant gratings 55a, 55b, and 55c reflect only signals having a wavelength of λ1, λ2, or λ3, respectively. The order of arrangement does not matter. The slant gratings 55d and 55e reflect signals having a wavelength of λ2 or λ3, respectively. The order of arrangement does not matter. Similarly, the slant gratings 55f and 55g reflect signals having wavelengths of λ2 and λ3, respectively. This does not matter the order of arrangement. The slant gratings 55h and 55i reflect a signal having a wavelength of λ3.

  Here, the operation of the Add / Drop circuit will be described. Consider a state in which a plurality of signals including signals of wavelengths λ1, λ2, and λ3 are input from the input end In of the main waveguide 21. The signals of wavelengths λ1, λ2, and λ3 are reflected and converted from the fundamental mode to the higher-order mode by any of the slant gratings 55a, 55b, and 55c, and coupled to the sub waveguide 22a. The signal of wavelength λ1 is output from the output terminal Drop (λ1), but the signals of wavelengths λ2 and λ3 are further reflected and mode-converted by the slant gratings 55d and 55e, respectively. These signals of λ2 and λ3 are coupled to the sub-waveguide 22c, and the signal of wavelength λ2 is output from the output terminal Drop (λ2), but the signal of wavelength λ3 is reflected and mode-converted by the slant grating 55h. . Finally, the signal of wavelength λ3 is output from the output end Drop (λ3) of the sub waveguide 22e. In this way, a signal having a specific wavelength can be sequentially taken out from each Drop terminal. The operation of the Add terminal is the same. As described above, it was confirmed that it is possible to realize not only one wavelength but also a multi-wavelength Add / Drop circuit by configuring the mode splitters in a multi-layered structure.

[Example 5]
FIG. 32 is a diagram showing an optical tap circuit using the mode splitter according to the present invention. The basic configuration of this optical tap circuit is the same as that of the mode splitter according to the first embodiment shown in FIG. 18, and includes a main waveguide 21 and an auxiliary waveguide 22 having an input end and an output end. The manufacturing method of the optical circuit is the same as that of the first embodiment. Each parameter such as the waveguide width of the mode splitter is also the same design as in the first embodiment.

  A characteristic configuration of this embodiment is that an offset connection portion 50 of the waveguide is arranged on the input end side of the main waveguide 21 as shown in FIG. A portion of the light input to the main waveguide 21 is converted into a higher-order mode by the waveguide offset connection portion 50 and is extracted from the output end of the sub-waveguide 22. The branching ratio of the signal light to the output end of the main waveguide 21 and the output end of the sub waveguide 22 can be determined by the offset length 51 shown in FIG.

  FIG. 33 is a diagram showing characteristics of loss (loss) and branching ratio (ratio) of the main waveguide with respect to the offset length of the optical tap circuit shown in FIG. It can be seen that the branching ratio increases as the offset length increases. That is, by changing the offset length, the tap amount can be easily controlled, and a part of the optical signal can be extracted at a desired level.

  FIG. 34 shows transmission spectra of the main waveguide and the sub waveguide of the optical tap circuit shown in FIG. This is a case where the offset length is 4 μm. The wavelength dependence of the transmission characteristics between the input and output of the main waveguide 21 and the transmission characteristics from the input end of the main waveguide 21 to the output end of the sub waveguide 22 is shown. As shown in FIG. 34, it can be seen that very flat transmission characteristics are obtained for both the main waveguide and the sub-waveguide, and a tap circuit having no wavelength dependency can be realized.

[Example 6]
FIG. 35 is a diagram showing an embodiment of a symmetric Y branch circuit using the mode splitter according to the present invention. A symmetrical Y-branch portion is formed continuously from the output side of the main waveguide of the mode splitter shown in the first embodiment through the tapered waveguide portion 29. That is, the main waveguide 24 having the input waveguide 23 forms a sub-waveguide 25 and a mode splitter. On the other hand, the main waveguide 24 is continuously connected to the tapered waveguide portion 29 and further connected to the output waveguide A26 and the output waveguide B27 to form a symmetric Y branch portion. The optical circuit provided with the symmetric Y branch portion was produced by the same method as in Example 1.

  FIG. 36 is a diagram showing a comparison of the branching ratio characteristics of a symmetric Y branch circuit including the mode splitter having the configuration shown in FIG. 35 and a circuit including only a normal symmetric Y branch circuit. The relative refractive index difference between the core and the cladding is 1%, the main waveguide width and the symmetric Y-branch waveguide width are 6 μm, the sub-waveguide width is 3 μm, and the mode splitter gap is 2 μm. In the symmetric Y branch portion, the main waveguide and two symmetric Y branch waveguides are connected by a tapered waveguide. Thirty circuits having the same circuit pattern were produced. Similarly, 30 circuits having only a normal symmetric Y-branch waveguide without a mode splitter were produced, and the variation in the branching ratio was compared and evaluated between them.

  In the case of only a normal symmetric Y-branch circuit, the branching ratio of the symmetric Y-branch circuit to the output waveguide A26 and the output waveguide B27 varies due to higher-order modes generated in the preceding circuit of the branch circuit. . However, in the case where the mode splitter according to the present invention is provided, the higher-order mode generated in the preceding circuit of the symmetric Y branch circuit is removed by the mode splitter, so that variation in the branch ratio in the symmetric Y branch circuit is suppressed. You can see that

It is a block diagram of the conventional mode splitter. It is explanatory drawing of the branch part of the conventional mode splitter. It is a conceptual diagram of the mode splitter concerning this invention. It is a conceptual diagram of the principle of operation of the mode splitter concerning this invention. It is an operation | movement conceptual diagram of the mode splitter concerning this invention. It is a figure explaining the mode of propagation of a fundamental mode and a primary mode in a mode splitter concerning the present invention. It is explanatory drawing of the taper part and gap of the mode splitter concerning this invention. It is a figure which shows the calculated value of the relationship of the transmission loss with respect to a taper angle. It is a figure which shows the calculated value of the relationship of the transmission loss with respect to a taper angle. It is a figure which shows the example of the shape of a non-linear taper part. It is a figure which shows the mode splitter provided with the linear waveguide in the subwaveguide. It is a figure which shows the array waveguide diffraction grating provided with the mode splitter concerning this invention in the input waveguide. It is a figure which shows the array waveguide diffraction grating provided with the mode splitter concerning this invention in the output waveguide. It is operation | movement explanatory drawing of the optical drop circuit using a wavelength dependence mode conversion element. It is operation | movement explanatory drawing of the optical drop circuit using a reflection type wavelength dependence mode conversion element. It is an optical tap circuit block diagram using the mode splitter concerning this invention. It is a block diagram of the symmetrical Y branch circuit with a mode splitter concerning this invention. It is a figure which shows the curve waveguide of the mode splitter output part concerning this invention. It is a figure which shows the relationship of the insertion loss with respect to the output waveguide width | variety of the mode splitter concerning this invention, and crosstalk. It is a figure which shows the relationship between the insertion loss with respect to the taper angle of the mode splitter concerning this invention, and crosstalk. It is a figure which shows the wavelength dependence of the insertion loss and crosstalk of the mode splitter concerning this invention. It is a front-end | tip enlarged view of a taper part. It is a comparison figure of the wavelength dependence of crosstalk by the presence or absence of a straight waveguide. It is explanatory drawing of the example of insertion of the light shielding agent provided in the subwaveguide end. It is a figure which shows the structure of the arrayed-waveguide diffraction grating which inserted the mode splitter in the produced input side. It is a figure which shows the comparison of the transmission spectrum characteristic of an array waveguide diffraction grating by the presence or absence of the mode splitter inserted in the input side. It is a conceptual diagram of the optical drop circuit using a slant grating. It is a figure which shows the permeation | transmission characteristic of the produced optical drop circuit. It is a figure which shows the permeation | transmission characteristic of the produced optical drop circuit. It is a multiwavelength Add / Drop circuit block diagram by slant grating. It is a multiwavelength Add / Drop circuit block diagram by slant grating. It is an optical tap circuit block diagram using a mode splitter. It is a figure which shows the offset dependence of the branching ratio and insertion loss in the produced optical tap circuit. It is a figure which shows the transmission spectrum of the produced optical tap circuit. It is a block diagram of the symmetrical Y branch circuit with a mode splitter concerning this invention. It is a figure which compares the variation in the branching ratio of the symmetrical Y branch circuit by the presence or absence of a mode splitter.

Explanation of symbols

1 Input waveguide 2, 2a, 2b, 26, 27, 34 Output waveguide 3, 10, 11, 21 Main waveguide 4, 9, 12, 22, 22a, 22b, 22c, 22d, 22e, 22f, 25 Waveguide 5, 13 Tapered portion 6, 15 Output portion waveguide 7 Gap 8 Taper angle 23, 33 Input waveguide 28 Tip portion 29 Termination processing portion 30 Input slab waveguide 31 Array waveguide 32 Output slab waveguide 55, 55a, 55b , 55c, 55d, 55e, 55f, 55g, 55h, 55i Slant grating

Claims (9)

In an optical circuit composed of a waveguide consisting of a core and a clad on a substrate ,
A main waveguide capable of guiding at least two types of propagation modes having different propagation orders;
A tapered portion that is arranged with a certain gap from the main waveguide and whose waveguide width changes, and an output portion guide that is continuously connected to the tapered portion and has a waveguide width different from the waveguide width of the main waveguide. Including a waveguide,
A mode splitter that separates one input light into two output lights, and has a sub-waveguide in which at least one of the propagation modes of the at least two kinds of propagation modes having different propagation orders is adiabatically transitioned;
A reflective mode conversion element that is optically connected to the output of the main waveguide of the mode splitter, converts part or all of an optical signal from a fundamental mode to a higher-order mode, and reflects the converted light;
The optical circuit, wherein the reflected optical signal is output from a sub-waveguide of the mode splitter . The optical circuit according to claim 2, wherein the reflective mode conversion element is a slant grating . The output waveguide is
The optical circuit according to claim 1, further comprising a curved waveguide continuously connected to the tapered portion . The output waveguide is
A straight waveguide continuously connected to the tapered portion;
A curved waveguide continuously connected to the linear waveguide;
The optical circuit according to claim 1, further comprising: 5. The optical circuit according to claim 1, wherein the tip of the tapered portion is a trapezoidal tapered portion cut by a plane perpendicular to the main waveguide . 6. 5. The optical circuit according to claim 1, further comprising an optical terminal that absorbs or diverges output light at a terminal of the output waveguide of the sub-waveguide . 6. 5. The optical circuit according to claim 1, wherein the gap between the tapered portion and the main waveguide is 0.2 μm or more and 5 μm or less . 5. The optical circuit according to claim 1, wherein the tapered portion is a linear taper whose width increases linearly and has a taper angle of 0.004 degrees or more and 1 degree or less . The propagation modes capable of propagating through the main waveguide are only the first propagation mode and the second propagation mode having different propagation orders, and the effective refractive index of the fundamental mode propagating through the output waveguide of the sub-waveguide is the first propagation mode. The optical circuit according to claim 1, wherein the optical circuit is smaller than an effective refractive index of one propagation mode and larger than an effective refractive index of the second propagation mode .

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