JP5019649B2 - Waveguide type optical circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a waveguide type optical circuit with reduced PDL. <P>SOLUTION: A waveguide type optical circuit has an optical coupler which is an optical branch coupler constructed such that waveguide cores are arranged adjacent to each other, and a dummy pattern formed along the both sides of the waveguide cores of the optical coupler and restricting the inclination of an optical main axis of the waveguide cores. It is preferable that the length of the dummy pattern is the same as or exceeds the length of the coupling part contributing to the coupling of the optical coupler. It is preferable that the distance between the dummy pattern and the waveguide cores of the optical coupler is the same as or exceeds the size of the gap between the waveguide cores of the optical coupler. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to a waveguide optical circuit including an optical coupler such as a directional coupler.

  In a conventional directional coupler (Directional Coupler, DC), as described in Patent Document 1, in a region (gap portion) sandwiched between two waveguide cores in a waveguide proximity portion, a waveguide pattern is formed. Due to the proximity, the supply of glass particles is reduced when the upper clad is formed, and the density of the glass particles becomes sparse. On the other hand, in a region (non-gap portion) that is not sandwiched between the two waveguide cores, the glass fine particles are sufficiently supplied. For this reason, the glass fine particle density is different between the gap part and the non-gap part, and stress that tilts inward of the gap part is generated in the two waveguide cores after the formation of the upper clad. As a result, the optical principal axes of the two waveguide cores are inclined, and polarization mode coupling occurs.

International Publication No. WO2006 / 075702 (paragraph 0024)

  Therefore, in a Mach-Zehnder Interferometer (MZI) circuit using DC or DC, polarization dependent loss (PDL) due to polarization mode coupling in DC occurs. As an optical branching coupler such as DC, there is a PLC type optical coupler such as a planar lightwave circuit (PLC) type 2 × 2 coupler. Further, as an MZI circuit using DC, there is a PLC type variable optical attenuator. However, there is a problem that the large PDL prevents the spread of PLC type optical couplers and PLC type variable optical attenuators. In addition, the same phenomenon as DC occurs in optical branch couplers such as multi-mode interference (MMI) couplers and Y-branches that have a structure in which the waveguide cores are close to each other, so MMI couplers, Y-branches, etc. Even in the MZI circuit using the optical branching coupler, PDL caused by polarization mode coupling occurs.

  The present invention has been made in view of the above, and an object thereof is to provide a waveguide-type optical circuit with reduced PDL.

  In order to solve the above-described problems and achieve the object, a waveguide type optical circuit according to the present invention includes an optical coupler that is an optical branching coupler configured by arranging waveguide cores close to each other, and And a dummy pattern which is formed along both sides of the waveguide core of the optical coupler and suppresses the inclination of the optical principal axis of the waveguide core.

  The waveguide type optical circuit according to the present invention is characterized in that, in the above invention, the length of the dummy pattern is equal to or longer than the length of the coupling portion contributing to coupling of the optical coupler. To do.

  In the waveguide type optical circuit according to the present invention, in the above invention, is the gap between the dummy pattern and the waveguide core of the optical coupler equal to the gap size between the waveguide cores of the optical coupler? Or more.

  The waveguide-type optical circuit according to the present invention is characterized in that, in the above-mentioned invention, the width of the dummy pattern is such that the fundamental mode of the waveguide propagation light of the optical coupler is not coupled or more. To do.

  The waveguide type optical circuit according to the present invention is characterized in that, in the above invention, the optical coupler is a directional coupler.

  The waveguide type optical circuit according to the present invention is characterized in that, in the above invention, the length of the dummy pattern is equal to or longer than the length L1 of the coupling portion.

  The waveguide type optical circuit according to the present invention is characterized in that, in the above invention, the optical coupler is a multimode interference coupler.

  In the waveguide type optical circuit according to the present invention, the distance between the dummy pattern and the waveguide core of the optical coupler is such that the polarization mode coupling amount of the optical coupler is −25 dB or less. At least one of the length of the dummy pattern and the width of the dummy pattern is set.

  The waveguide type optical circuit according to the present invention is a Mach-Zehnder interferometer circuit comprising the two optical couplers and two arm waveguides connected between the two optical couplers. It is characterized by providing.

  The waveguide type optical circuit according to the present invention is a planar lightwave circuit having an overcladding formed by a flame deposition method in the above invention.

  The waveguide type optical circuit according to the present invention is characterized in that, in the above invention, the planar lightwave circuit is a PLC type optical coupler.

  The waveguide type optical circuit according to the present invention is characterized in that, in the above invention, the planar lightwave circuit is a PLC type star coupler.

  In the waveguide type optical circuit according to the present invention, the planar lightwave circuit is composed of two optical couplers and two arm waveguides connected between the two optical couplers. A PLC-type variable optical attenuator comprising a Mach-Zehnder interferometer circuit and a thin film heater formed on at least one of the two arm waveguides, wherein the thin film heater functions as a phase shifter. To do.

  According to the present invention, there is an effect that a waveguide type optical circuit with reduced PDL can be realized.

FIG. 1 is a plan view showing a schematic configuration of a waveguide optical circuit according to the first embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line XX in FIG. FIG. 3 is a diagram for explaining the length of the dummy pattern in FIG. FIG. 4 is a plan view showing a conventional waveguide type optical circuit having no dummy pattern. FIG. 5 is a cross-sectional view taken along line YY of FIG. FIG. 6 is a diagram for explaining the stress applied to the core and how the core is tilted. FIG. 7 is a plan view showing a schematic configuration of a waveguide optical circuit according to the second embodiment of the present invention. FIG. 8 is a graph showing the coupling efficiency of the waveguide type optical circuit shown in FIG. FIG. 9 is a graph showing the PDL of the waveguide type optical circuit shown in FIG. FIG. 10 is a graph showing excess loss of the waveguide type optical circuit shown in FIG. FIG. 11 is a plan view showing a modification of the waveguide type optical circuit shown in FIG. FIG. 12 is a plan view showing another modification of the waveguide type optical circuit shown in FIG.

  Embodiments of a waveguide type optical circuit according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. Moreover, in each drawing, the same code | symbol is attached | subjected suitably to the same or corresponding element. Furthermore, it should be noted that the drawings are schematic, and the relationship between the thickness and width of each layer, the ratio of each layer, and the like may differ from the actual ones. Even between the drawings, there are cases in which portions having different dimensional relationships and ratios are included.

(First embodiment)
First, the waveguide type optical circuit according to the first embodiment of the present invention will be described.
FIG. 1 is a plan view showing a schematic configuration of a waveguide optical circuit according to the first embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line XX in FIG. FIG. 3 is a diagram for explaining the length of the dummy pattern in FIG.

  The waveguide type optical circuit 10 according to the first embodiment includes one directional coupler 11. The directional coupler 11 is an optical coupler (optical branching coupler) that is configured so that the waveguide cores 11a and 11b are close to each other and face each other. Dummy patterns 21 and 21 are formed along both sides of the waveguide cores 11a and 11b, respectively, to suppress tilting of the optical principal axes of the waveguide cores 11a and 11b. That is, the dummy patterns 21 and 21 are formed on both sides of the waveguide cores 11a and 11b so as to sandwich the adjacent portions of the waveguide cores 11a and 11b. Further, as shown in FIG. 2, the waveguide cores 11 a and 11 b and the dummy patterns 21 and 21 are surrounded by the lower cladding layer 32, the side cladding layer 33, and the upper cladding layer 34.

The dummy patterns 21 and 21 are preferably made of a material having a birefringence index of 1 × 10 ⁻⁴ or more to improve polarization maintaining properties. For example, the dummy patterns 21 and 21 may include waveguide cores 11a and 11b. The same material can be used.

  The length L (see FIG. 1) of each of the dummy patterns 21 and 21 may be equal to or longer than the length L1 (see FIG. 3) of the coupling portion that contributes to the coupling of the directional coupler 11. Here, the coupling portion refers to a region where the coupling efficiency is greater than 0%.

  As shown in FIGS. 1 and 2, the distance B between the dummy pattern 21 and the waveguide core 11 a and the distance B between the dummy pattern 21 and the waveguide core 11 b are the waveguide core 11 a and the waveguide core 11 a close to the directional coupler 11. It may be the same as or larger than the gap size A between 11b.

  Regarding the interval B, in order to reduce the PDL caused by the polarization mode coupling in the directional coupler 11, it is best to set the interval B to the same size as the gap size A. Further, if each dummy pattern 21 is too close to the waveguide cores 11a and 11b of the directional coupler 11, minute optical coupling occurs between the dummy pattern 21 and the waveguide cores 11a and 11b. Is made larger than the gap size A × 1.0, it is possible to reduce the PDL caused by the polarization mode coupling while suppressing the occurrence of minute optical coupling and suppressing the loss.

  The minimum width W (see FIGS. 1 and 2) of each of the dummy patterns 21 and 21 is a single mode that can guide the waveguide propagation light of the directional coupler 11 (waveguide cores 11a and 11b can be guided). The width of the mode) may not be combined, or may be larger. Here, “the fundamental mode of the waveguide propagation light is not coupled” means that the coupling efficiency is almost 0%.

  When the width W of the dummy pattern 21 is such that the fundamental mode of the waveguide propagation light of the directional coupler 11 is coupled, the light of the fundamental mode guided through the waveguide cores 11a and 11b of the directional coupler 11 can be obtained. The light is coupled to the dummy pattern 21 and light is not coupled to the output-side waveguide, resulting in excessive loss as clad mode light. If the width W of the dummy pattern 21 is made wider than the width at which the fundamental mode of the waveguide propagation light is not coupled or more than that, the propagation constants of the waveguide cores 11a and 11b and the dummy pattern 21 are different. The fundamental mode light guided through 11b is not coupled to the dummy pattern 21, and no excessive loss occurs. Thereby, excess loss can be suppressed.

  In the waveguide type optical circuit 10, at least the length L of the dummy pattern 21, the distance B, and the width W of the dummy pattern 21 so that the polarization mode coupling amount of the directional coupler 11 is −25 dB or less. It is preferable to set one.

(Production method of the first embodiment)
A method of manufacturing the waveguide type optical circuit 10 having the above configuration (the following steps (1) to (3)) will be described with reference to FIG.

(1) A silica material (SiO 2) serving as a core layer (not shown) for forming a lower clad layer 32, a waveguide and a dummy pattern on a silicon (Si) substrate 31 by a flame deposition (FHD) method. ₂ type glass fine particles) are deposited and heated to melt and clear the glass film.

  (2) Thereafter, desired waveguides and dummy patterns are formed from the core layer by photolithography and reactive ion etching. Here, waveguide cores 11a and 11b of the directional coupler 11 and dummy patterns 21 and 21 along the waveguide cores 11a and 11b are formed. Here, the dummy patterns 21 and 21 are formed of the same material as the waveguide cores 11a and 11b. If the dummy patterns 21 and 21 are drawn on a photomask on which a waveguide pattern used in photolithography is drawn, the waveguide cores 11a and 11b and the dummy patterns 21 and 21 are formed on the core layer without adding a manufacturing process. Can be formed simultaneously.

(3) Thereafter, a silica material (SiO ₂ glass fine particles) to be the side clad layer 33 and the upper clad layer 34 is deposited by the FHD method, and the waveguide cores 11a and 11b and the dummy patterns 21 and 21 are formed. The glass film is melted and transparentized (vitrified) by being filled with the side clad layer 33 and the upper clad layer 34 and heated at a high temperature.

  In this way, the waveguide type optical circuit 10 shown in FIG. 1 is manufactured. By the way, when the glass fine particles to be the side clad layer 33 and the upper clad layer 34 are deposited and vitrified in the step (3) to form the side clad layer 33 and the upper clad layer 34, the waveguide core 11a. , 11b, the interval B is as narrow as several μm, and the glass particles do not enter between the waveguide cores 11a, 11b. For this reason, the glass particle density between the waveguide cores 11a and 11b becomes sparse.

  However, in the waveguide type optical circuit 10 according to the first embodiment, the dummy patterns 21 and 21 are provided on both sides of the directional coupler 11, and the distance B between the dummy pattern 21 and the waveguide cores 11a and 11b is set to the above. It is equal to or larger than the gap size A. For this reason, the glass fine particle density between the waveguide cores 11a and 11b is sparse, but the glass fine particle density is also sparse around the waveguide cores 11a and 11b. Therefore, there is no problem that the waveguide cores 11a and 11b are inclined inward.

  That is, since the glass particle density between the waveguide cores 11a and 11b approaches the glass particle density between the dummy patterns 21 and 21 and each waveguide core, the stress that tends to incline both the waveguide cores 11a and 11b inward. Is suppressed. For this reason, it is suppressed that the optical principal axis of both waveguide core cores 11a and 11b inclines, generation | occurrence | production of polarization | polarized-light crosstalk light is suppressed in the directional coupler 11, and polarization mode coupling | bonding is suppressed. Thereby, PDL resulting from polarization mode coupling can be reduced.

(Waveguide type optical circuit without dummy pattern)
Next, as a comparative example of the first embodiment, a conventional method for producing a waveguide type optical circuit without a dummy pattern (the following steps (1a) to (3a)) will be described with reference to FIGS. To do.

  FIG. 4 is a plan view showing a conventional waveguide-type optical circuit provided with the directional coupler 110. This waveguide type optical circuit is manufactured as follows. 5 and 6 are cross-sectional views taken along line YY of FIG. In particular, FIGS. 6A and 6B are views for explaining the stress applied to the core and how the core is tilted.

(1a) By a FHD method, a silica material (SiO ₂ glass fine particles) serving as a core layer (not shown) for forming a lower cladding layer 320 and a waveguide is deposited on a silicon (Si) substrate 310 (see FIG. 5), the glass film is melted and transparentized by heating.

  (2a) Thereafter, a desired waveguide is formed from the core layer by photolithography and reactive ion etching. Here, waveguides such as the waveguide cores 110a and 110b of the directional coupler 110 are formed.

(3a) Thereafter, a silica material to be used as the side cladding layer 330 and the upper cladding layer 340 is deposited by the FHD method, and the respective waveguide cores 110a and 110b are embedded in the side cladding layer 330 and the upper cladding layer 340. And the glass film is melted and transparentized (vitrified).
In this way, a waveguide type optical circuit including the directional coupler 110 is manufactured.

  By the way, in the step (3a), when the glass fine particles to be the side cladding layer 330 and the upper cladding layer 340 are deposited and vitrified to form the side cladding layer 330 and the upper cladding layer 340, the waveguide core 110a. , 110b is as narrow as 1 μm, and glass particles do not enter much between the waveguide cores 110a, 110b. For this reason, the glass particle density between the waveguide cores 110a and 110b becomes sparse. At this time, a force F (see FIG. 6A) for inclining both the waveguide cores 110a and 110b inward acts to vitrify. Accordingly, in a very extreme case, both the waveguide cores 110a and 110b are tilted inward in a slightly deformed state, and the optical main axes of the waveguide cores 110a and 110b are tilted (see FIG. 6B).

  In such a comparative example, coupling between polarized lights having different polarizations (polarization mode coupling) occurs, and the polarization dependence of the optical circuit increases, so that PDL due to polarization mode coupling occurs.

  According to 1st Embodiment comprised as mentioned above, there exist the following effects.

  (1) Dummy patterns 21 and 21 are formed along both sides of the waveguide cores 11a and 11b of the directional coupler 11, and the dummy pattern 21 is formed in a non-gap portion (a region not sandwiched between two waveguide cores). A structure sandwiched between the waveguide core 11a or the waveguide core 11b is provided. As a result, the glass fine particle density in the non-gap part can be made sparse in the same manner as the glass fine particle density becomes sparse between the two waveguide cores 11a and 11b (gap part). Therefore, by forming the dummy pattern 21, the glass fine particle density in the non-gap portion can be brought close to the glass fine particle density in the gap portion. At this time, since the glass particle density on both sides (non-gap part and gap part) of each of the waveguide cores 11a and 11b is close, the waveguide cores 11a and 11b generated due to the difference in glass particle density in the upper clad layer forming process It is possible to suppress the generation of stress that tends to tilt. That is, the provision of the dummy pattern 21 can suppress polarization mode coupling in which the optical principal axes of the two waveguide cores 11a and 11b are coupled by light of different polarizations in the tilt directional coupler 11 due to the stress. PDL caused by wave mode coupling can be reduced.

  (2) The length L of each of the dummy patterns 21 and 21 is equal to or longer than the length L1 of the coupling portion that contributes to the coupling of the directional coupler 11. As a result, since the optical principal axes of the waveguide cores 11a and 11b are suppressed from being inclined in the entire region contributing to the coupling (the length L1 of the coupling portion), the polarization mode coupling can be suppressed, and the polarization mode can be suppressed. The effect that the PDL caused by the binding can be reduced appears more appropriately. The PDL caused by the polarization mode coupling can be more effectively reduced.

  (3) The interval B is equal to or larger than the gap size A. As a result, in the gap structure of about several μm, the glass fine particle density becomes sparse depending on the gap size A. Therefore, by matching the gap B with the gap size A, both sides of the waveguide cores 11a and 11b (non-gap) The glass fine particle density in the portion and the gap portion can be matched, and the generation of the stress can be most effectively suppressed. For this reason, it is suppressed that the optical principal axis of both waveguide cores 11a and 11b inclines, polarization mode coupling in the directional coupler 11 is suppressed, and PDL resulting from polarization mode coupling can be reduced. The interval B is best the same as the gap size A. However, if the dummy pattern 21 is too close to the waveguide cores 11a and 11b, minute optical coupling occurs between the dummy pattern 21 and the waveguide cores 11a and 11b. Thus, it is possible to suppress the loss by suppressing the occurrence of minute optical coupling. Furthermore, by providing the dummy pattern 21, polarization mode coupling in which the optical principal axes of the two waveguide cores 11a and 11b are coupled to each other in the directional coupler 11 due to the stress can be suppressed. Caused PDL can be reduced. Thus, loss can be suppressed and PDL caused by polarization mode coupling can be reduced.

  (4) The width W of the dummy pattern 21 is set to a width that does not couple the fundamental mode of the waveguide propagation light of the directional coupler 11 or more. When the width W of the dummy pattern 21 is such that the fundamental mode of the waveguide propagation light of the directional coupler 11 is coupled to the dummy pattern 21, the light of the fundamental mode that guides the waveguide core of the directional coupler 11. Is coupled to the dummy pattern 21, and light is not coupled to the output-side waveguide, resulting in excessive loss as clad mode light. If the width W of the dummy pattern 21 is made wider than the width at which the fundamental mode of the waveguide propagation light is not coupled or more than that, the propagation constants of the waveguide cores 11a and 11b and the dummy pattern 21 will be different. As a result, the fundamental mode light guided through the waveguide cores 11a and 11b is not coupled to the dummy pattern 21, and excess loss does not occur. Thereby, excess loss can be suppressed.

  (5) At least one of the length L of the dummy pattern 21, the interval B, and the width W of the dummy pattern 21 is set so that the polarization mode coupling amount of the directional coupler 11 is −25 dB or less. . Thereby, the polarization mode coupling can be set to a sufficiently small value of −25 dB or less while suppressing excess loss in the directional coupler 11, and a PDL reduction effect can be obtained.

  (6) According to the manufacturing method of the waveguide type optical circuit 10 described above, the manufacturing method is almost the same as the conventional method, and the waveguide type optical circuit 10 can be manufactured without adding a special manufacturing process. .

(Second Embodiment)
FIG. 7 is a plan view showing a schematic configuration of a waveguide type optical circuit 10A according to the second embodiment of the present invention. The waveguide type optical circuit according to the second embodiment shown in FIG. 7 is configured as a PLC type variable optical attenuator (PLC-VOA) 10A.

  The PLC-VOA 10A includes two directional couplers 11 and 12, two arm waveguides 13 and 14 connected between the two directional couplers 11 and 12, and a thin film provided on the upper portion of the arm waveguide 14. A Mach-Zehnder interferometer (MZI) circuit 20 including a heater 15 is provided. Here, a PLC-VOA composed of one MZI circuit will be described. However, since the present invention is applied to an optical coupler in the MZI circuit, a PLC- constructed by connecting MZI circuits in multiple stages. The present invention is also applicable to VOA.

  In the PLC-VOA 10A, by applying electric power to the thin film heater 15 from the outside, the arm waveguide 14 is heated, and the effective refractive index of the arm waveguide 14 is changed through a thermo-optic effect corresponding to the amount of heat generated. Can do. The change in the effective refractive index of the arm waveguide 14 corresponds to the change in the optical path length for the propagating signal light. That is, the optical path length difference between the arm waveguides 13 and 14 can be set by changing the voltage applied to the thin film heater 15.

  In the PLC-VOA 10A, the signal light input from the input port P1 of the MZI circuit 20 is branched by the directional coupler 11 on the input side, propagates independently through the two arm waveguides 13 and 14, and then the desired optical path. The output signal is recombined by the directional coupler 12 on the output side with a long difference and output from the output port P3. At this time, the coupling efficiency of the MZI circuit 20 is maximum (1) when the optical path length difference between the arm waveguides 13 and 14 is 0, and is minimum (0) when the optical path length difference is equal to one half of the signal light wavelength. ) When the optical path length difference is between these, the coupling efficiency changes continuously from 1 to 0. That is, by setting the optical path length difference in a timely manner, a desired coupling efficiency can be obtained, and it can be operated as a variable optical attenuator.

  In the PLC-VOA 10 </ b> A, dummy patterns 21 are formed on both sides of the directional coupler 11 to prevent the optical principal axes of the waveguide cores 11 a and 11 b from being inclined, and the waveguides are formed on both sides of the directional coupler 12. A dummy pattern 22 is formed to suppress tilting of the optical principal axes of the cores 12a and 12b. Note that the parameters such as the interval B, the gap size A, the length L of the dummy patterns 21 and 22 and the width W of the dummy patterns 21 and 22 in the PLC-VOA 10A shown in FIG. The parameters are the same as those in the circuit 10. In the PLC-VOA 10A, the length L of the dummy patterns 21, 22 and the interval B and the width of the dummy patterns 21, 22 are set so that the polarization mode coupling amount of the directional couplers 11 and 12 is -25 dB or less. It is preferable to set at least one of W.

(Production Method of Second Embodiment)
A manufacturing method of PLC-VOA 10A having the above configuration (the following steps (1) to (4)) will be described based on the manufacturing method of the first embodiment described above and FIG.

(1) Deposit a silica material (SiO ₂ -based glass fine particles) on the silicon (Si) substrate and form a core layer for forming the lower cladding layer, waveguide core and dummy pattern on the silicon (Si) substrate, and heat it. To melt and clear the glass film.

  (2) Thereafter, desired waveguides and dummy patterns are formed from the core layer by photolithography and reactive ion etching. Here, waveguide cores 11a and 11b of the directional coupler 11, two arm waveguides 13 and 14, and waveguide cores 12a and 12b of the directional coupler 12, and the waveguide cores 11a and 11a, Dummy patterns 21 and 22 along 11b, 12a, and 12b are formed. Here, if a dummy pattern is drawn on a photomask on which a waveguide pattern used in photolithography is drawn, the waveguide and the dummy pattern can be simultaneously formed on the core layer without adding a manufacturing process.

(3) Thereafter, a silica material (SiO ₂ glass fine particles) to be the side cladding layer and the upper cladding layer is deposited by the FHD method, and each waveguide and dummy pattern is formed on the side cladding layer and the upper cladding layer. It is embedded and heated at a high temperature to melt and transparentize (vitrify) the glass film.

  (4) Subsequently, a heater and a wiring electrode are formed on the upper clad layer.

  In this way, the PLC-VOA 10A is manufactured. By the way, when the glass fine particles to be the side cladding layer and the upper cladding layer are deposited and vitrified in the step (3) to form the side cladding layer and the upper cladding layer, the distance between the waveguide cores 11a and 11b. The distance B between B and the waveguide cores 12a and 12b is as narrow as several μm, and glass particles do not enter much between the waveguide cores 11a and 11b and between the waveguide cores 12a and 12b. For this reason, the glass particle density between the waveguide cores 11a and 11b and between the waveguide cores 12a and 12b becomes sparse.

  However, in the PLC-VOA 10A according to this embodiment, the dummy patterns 21 and 22 are provided on both sides of the directional couplers 11 and 12, respectively, and the interval B between the dummy pattern 21 and the waveguide cores 11a and 11b and the dummy pattern 22 are provided. The gap B between the waveguide cores 12a and 12b is equal to or larger than the gap size A. Therefore, the glass particle density between the waveguide cores 11a and 11b and between the waveguide cores 12a and 12b is sparse, but the glass particle density is also sparse around the waveguide cores 11a and 11b. There is no problem that 12a and 12b are inclined inward.

  That is, since the glass particle density between the waveguide cores 11a and 11b and between the waveguide cores 12a and 12b approaches the glass particle density between the dummy patterns 21 and 22 and each waveguide core, both the waveguide cores 11a and 11b. And the generation of stress that tends to tilt 12a and 12b inward is suppressed. For this reason, tilting of the optical principal axes of both waveguide cores 11a, 11b and 12a, 12b is suppressed, and polarization mode coupling in which coupling between lights having different polarizations in each directional coupler 11, 12 is suppressed. PDL caused by polarization mode coupling can be reduced.

  According to 2nd Embodiment comprised as mentioned above, there exist an effect similar to the said 2nd Embodiment and the further effect in addition to this as follows.

  (1) Since the dummy patterns 21 and 22 are provided on both sides of the directional couplers 11 and 12, respectively, polarization mode coupling is performed in the directional couplers 11 and 12, as in the case of the first embodiment. Thus, PDL caused by polarization mode coupling can be reduced.

  (2) In the PLC-VOA 10A, the light is extinguished using the region where the coupling efficiency is between 0 and 100. However, the larger the extinction ratio (that is, the smaller the coupling efficiency), the more easily the influence of the polarization dependence occurs. Since it becomes large, it is an important device to reduce PDL. According to the present embodiment, it is possible to realize the PLC-VOA 10A in which PDL caused by polarization mode coupling is reduced.

  (3) When the coupling efficiency from the port P1 (input port) to the port P3 (output port) of the MZI circuit 20 is set to about 4% in the PLC-VOA 10A shown in FIG. Measured coupling efficiency in the case of 1 (μm) which is the same size as 3 or 3 (μm), 5 (μm) and 10 (μm) larger than the gap size A, and no dummy pattern for comparison , PDL, and excess loss are shown in FIGS. 8, 9, and 10, respectively.

  From FIG. 8, when the distance B is set to 1 (μm), 3 (μm), 5 (μmm), and 10 (μm), the above coupling efficiency can be obtained to the same extent as when there is no dummy pattern. I understand.

  In FIG. 9, the PDL is the smallest when the interval B = 1 (μm), increases in the order of the interval B = 1, 3, 5 (μm), and B = 10 (μm) B = 5 (μm) or more. Then it is almost the same. In addition, the PDL is smaller in each of the intervals B = 1, 3, 5, 10 (μm) than in the case without the dummy pattern. Therefore, it can be seen that the PDL is reduced by forming the dummy pattern, and the effect of the present invention is obtained. Further, in the range of B ≦ 5 (μm), the PDL becomes smaller depending on the gap width, and when the interval B is set to 1 (μm) which is the same size as the gap size A, the PDL becomes the smallest. That is, it can be seen that when the distance B is the same as the gap size A, the PDL reduction effect of the present invention is most highly obtained. In addition, when B ≧ 5 (μm), the PDL is almost the same value as in the case of B = 5 (μm), and the PDL has almost no dependency on the interval B, and the PDL is smaller than that without the dummy pattern. That is, if the interval B ≧ 5 (μm) is used, a constant PDL reduction effect can be obtained without being affected by the B manufacturing error.

  On the other hand, FIG. 10 shows that the excess loss is high only when B = 1 (μm). This is because if the dummy patterns 21 and 22 are too close to the waveguide cores of the directional couplers 11 and 12, minute optical coupling occurs. That is, when B = 1 (μm), the PDL reduction effect is the highest, but excess loss occurs. Therefore, if the interval B is made larger than the gap size A and, for example, B = 3, 5, 10 (μm) is used, the PDL reduction effect can be obtained while suppressing the excessive loss. The excess loss shown in FIG. 10 includes coupling loss.

  (4) According to the manufacturing method of the PLC-VOA 10A described above, the manufacturing method is almost the same as the conventional method, and the PLC-VOA 10A can be manufactured without adding a special manufacturing process.

In addition, this invention can also be changed as follows and can also be set as a reference example .
In the PLC-VOA 10A according to the second embodiment, instead of the directional couplers 11 and 12 described as an example of the optical coupler (optical branching coupler) in which the waveguide cores are close to each other, the MMI as shown in FIG. A waveguide, such as a PLC-VOA, using two couplers 40 and having a plurality of MZI circuits each composed of these two MMI couplers 40 and two arm waveguides connected between the two MMI couplers 40 Type optical circuit can be configured . The MMI coupler 40 includes two waveguide cores 41a and 41b on the input side close to each other, and two waveguide cores 42a and 42b on the output side close to each other. Dummy patterns 43 and 43 are formed on both sides of the waveguide cores 41a and 41b, and dummy patterns 44 and 44 are formed on both sides of the waveguide cores 42a and 42b, respectively. In FIG. 11, reference numeral “45” denotes a slab waveguide for generating multimode optical interference.

As a reference example, in the PLC-VOA 10A according to the second embodiment, instead of the directional couplers 11 and 12 described as an example of the optical coupler in which the waveguide cores are close to each other, the same MMI coupler as in FIG. When two 40 are used, dummy patterns 46 and 46 may be formed on both sides of the MMI coupler 40 as shown in FIG. The dummy patterns 46 and 46 are connected to the outside of the waveguide cores 41a and 42a and the outside of the waveguide cores 41b and 42b, respectively, and are connected to the outside of the MMI coupler 40 to form one dummy pattern. It is what.

Further, in the PLC-VOA 10A according to the second embodiment, instead of the directional couplers 11 and 12, an optical coupler that generates polarization mode coupling in an area where the waveguide core is close, such as an asymmetric X-type branching device (optical It is also possible to construct a waveguide type optical circuit such as PLC-VOA using a branch coupler.

  In the second embodiment, the PLC type variable optical attenuator (PLC-VOA) has been described as an example of the waveguide type optical circuit. However, a PLC type optical coupler such as a PLC type star coupler or a PLC type 2 × 2 coupler is described. The present invention can also be applied to a waveguide type optical circuit configured as follows.

  The present invention also provides a waveguide-type optical circuit comprising an MZI circuit composed of two optical couplers each having a structure in which a waveguide core is close to each other and two arm waveguides connected between the two optical couplers. Therefore, the present invention can also be applied to a delay demodulation device used in an optical transmission system using a DQPSK (Differential Quadrature Phase Shift Keying) method. By applying the present invention to such a delay demodulation device in the same manner as the PLC-VOA 10A, a delay demodulation device with reduced PDL caused by polarization mode coupling can be realized.

10 Waveguide type optical circuit 10A PLC-VOA
11, 12 Directional coupler 11a, 11b, 12a, 12b Waveguide core 13, 14 Arm waveguide 15 Thin film heater 20 MZI circuit 21, 22 Dummy pattern A Gap size B Spacing L, L1 Length P1 Input port P3 Output port W width

Claims (9)

An optical coupler which is an optical branching coupler configured by arranging waveguide cores close to each other;
A dummy pattern that is formed along both sides of the waveguide core of the optical coupler and suppresses the tilt of the optical principal axis of the waveguide core;
With
The optical coupler is a directional coupler ;
Before SL dummy pattern is extended up to the curved waveguide portion distance between the waveguide core is formed so as to expand gradually from the narrowest portion,
The shape of the dummy pattern is such that a side surface of the dummy pattern close to the waveguide core is formed so as to maintain a certain distance from the opposing waveguide core, and the side surfaces of the dummy patterns on both sides far from the waveguide core Are formed so as to be parallel to each other . The length of the dummy pattern is the length of the coupling portion where the coupling efficiency of the optical coupler is greater than 0%.
The waveguide-type optical circuit according to claim 1, wherein the waveguide-type optical circuit is equal to or more than the same.   3. The waveguide type optical circuit according to claim 1, wherein the width of the dummy pattern is a width in which a fundamental mode of waveguide propagation light of the optical coupler is not coupled or more.   At least one of an interval between the dummy pattern and the waveguide core of the optical coupler, a length of the dummy pattern, and a width of the dummy pattern is set so that a polarization mode coupling amount of the optical coupler is −25 dB or less. The waveguide type optical circuit according to claim 1, wherein the waveguide type optical circuit is set. And two of said optical coupler, to claim 1-4, characterized in that it comprises a Mach-Zehnder interferometer circuit composed of the two arm waveguides connected between said two optical couplers The waveguide type optical circuit described. Waveguide-type optical circuit according to any one of claims 1-5, characterized in that the planar lightwave circuit forming the overclad flame deposition method. The waveguide type optical circuit according to claim 6 , wherein the planar lightwave circuit is a PLC type optical coupler. The waveguide type optical circuit according to claim 6 , wherein the planar lightwave circuit is a PLC type star coupler. The planar lightwave circuit includes a Mach-Zehnder interferometer circuit including two optical couplers and two arm waveguides connected between the two optical couplers, and at least one of the two arm waveguides. The waveguide-type optical circuit according to claim 6 , wherein the waveguide-type optical circuit is a PLC-type variable optical attenuator that includes a thin-film heater formed on an upper portion and causes the thin-film heater to function as a phase shifter.

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