JP5053301B2 - Waveguide type optical circuit - Google Patents

Description

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

  In a conventional 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 pattern is adjacent. As a result, the supply of glass particles is reduced during the formation of the upper cladding, and the glass particle density is sparse. On the other hand, in the region (non-gap part) that is not sandwiched between the two waveguide cores, the supply of glass fine particles is sufficient, so that the glass fine particle density is different between the gap part and the non-gap part. Stress toward the inside of the gap is generated in the core. As a result, the optical principal axes of the two waveguide cores are inclined, and polarization mode coupling occurs.

WO 2006-075702 (paragraph 0024)

Therefore, in a Mach-Zehnder Interferometer (MZI) circuit using DC or DC, polarization dependent loss (PDL: Polarization Dependent Loss) 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, since 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, 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 such conventional problems, and an object of the present invention is to provide a waveguide type optical circuit in which the polarization mode coupling is reduced and the PDL caused by the polarization mode coupling is reduced. There is.

In order to solve the above-mentioned problems, a first aspect of the present invention is a waveguide type optical circuit including an optical coupler which is an optical branching coupler, wherein the optical coupler is an optical coupler in which waveguide cores are close to each other. And a dummy pattern for suppressing the inclination of the optical principal axis of the waveguide core is formed along both sides of the waveguide core of the optical coupler.
According to this configuration, two waveguide cores of the optical coupler are formed by forming a dummy pattern along both sides of the waveguide core of the optical coupler and providing a structure sandwiched between the dummy pattern and the waveguide core in the non-gap portion. 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 in the gap (gap part). Therefore, by forming the dummy pattern, the glass fine particle density in the non-gap part can be brought close to the glass fine particle density in the gap part. At this time, since the glass particle density on both sides (non-gap part and gap part) of each core is close to each other, the stress of the waveguide core caused by the difference in glass particle density in the upper cladding formation process Occurrence can be suppressed. In other words, by providing a dummy pattern, the optical principal axes of both waveguide cores are tilted by the stress, so that polarization mode coupling in which polarization is coupled between polarizations can be suppressed, and PDL caused by polarization mode coupling can be reduced. Can do. Here, the optical coupler includes an optical coupler that generates polarization mode coupling in a region where the waveguide core is close, such as a DC, a multimode interference (MMI) coupler, and an asymmetric X-type branching device.

The waveguide type optical circuit according to another aspect of the present invention is characterized in that a length of the dummy pattern is equal to or longer than a length of a coupling portion contributing to coupling of the optical coupler.
According to this configuration, since the optical axis of the waveguide core is suppressed from being inclined in the entire region contributing to the coupling, the polarization mode coupling can be suppressed, and the PDL caused by the polarization mode coupling can be reduced. The effect of being able to appear is more appropriate. PDL resulting from the polarization mode coupling can be reduced.

In the waveguide-type optical circuit according to another aspect of the present invention, an interval between the dummy pattern and the waveguide core of the optical coupler is equal to or larger than a gap size between the waveguide cores of the optical coupler. It is the above.
According to this configuration, since the glass particle density becomes sparse depending on the gap size in the gap structure of about several μm, the gap between the dummy pattern and the waveguide core of the optical coupler and the gap between the waveguide core of the optical coupler By matching the sizes, the glass fine particle densities can be matched, and the stress generation can be most effectively suppressed. For this reason, as described above, the optical principal axes of both waveguide cores are suppressed from being inclined, polarization mode coupling in the optical coupler is suppressed, and PDL caused by polarization mode coupling can be reduced. The distance between the dummy pattern and the waveguide core of the optical coupler is best the same as the gap size between the adjacent waveguide cores of the optical coupler. However, if the dummy pattern is too close to the waveguide core, minute coupling will occur between the dummy pattern and the waveguide core, so minute coupling will occur if the gap is made larger than the gap size. Can be suppressed and loss can be suppressed. Furthermore, by providing a dummy pattern, it is possible to suppress polarization mode coupling in which the optical principal axes of both waveguide cores are tilted due to the stress and coupling between polarized waves in an optical coupler, thereby reducing PDL caused by polarization mode coupling. Can do. Thus, loss can be suppressed and PDL caused by polarization mode coupling can be reduced.

The waveguide type optical circuit according to another aspect of the present invention is characterized in that the width of the dummy pattern is a width that does not couple a prescribed mode of waveguide propagation light of the optical coupler, or more.
When the width of the dummy pattern is such that the prescribed mode of the waveguide propagation light of the optical coupler is coupled, the light of the prescribed mode (single mode) guided through the waveguide core of the optical coupler is coupled to the dummy pattern. The light is not coupled to the output-side waveguide, resulting in cladding mode light and excessive loss. If the width of the dummy pattern is made wider than the width in which the prescribed mode of the waveguide propagation light is not coupled or more than that, the propagation constant is different, so that the light in the prescribed mode is not coupled to the dummy pattern, and no excessive loss occurs. Thereby, excess loss can be suppressed.

  A waveguide type optical circuit according to another aspect of the present invention is characterized in that the optical coupler is a directional coupler.

  In the waveguide type optical circuit according to another aspect of the present invention, the length of the dummy pattern is the length dL of the strong coupling portion of the directional coupler and the weak coupling that contributes to the coupling of the directional coupler. It is characterized in that it is equal to or longer than the length L1 of the coupling portion, which is the sum of the length of the portion ΔL twice (ΔL × 2). According to this configuration, the length of the dummy pattern contributes to DC coupling, that is, the length dL of the strong coupling portion contributing to DC coupling and the length of the weak coupling portion ΔL contributing to DC coupling. It is set to be equal to or longer than the length L1 of the coupling portion, which is the sum of double (ΔL × 2). This suppresses the tilt of the optical axis of the waveguide core in the entire region contributing to the coupling, so that the polarization mode coupling can be suppressed, and the PDL caused by the polarization mode coupling can be reduced. The effect appears more appropriately.

  The waveguide type optical circuit according to another aspect of the present invention is characterized in that the optical coupler is a multimode interference coupler.

  A waveguide type optical circuit according to another aspect of the present invention provides an interval between the dummy pattern and the waveguide core of the optical coupler, and the dummy so that a polarization mode coupling amount of the optical coupler is −25 dB or less. At least one of the length of the pattern and the width of the dummy pattern is set. Although it is difficult to completely eliminate polarization mode coupling while suppressing excess loss in the coupler, if the polarization mode coupling is -25 dB or less, the coupler is sufficiently small, and a PDL reduction effect is obtained.

A waveguide type optical circuit according to another aspect of the present invention includes the two optical couplers and a Mach-Zehnder interferometer circuit including two arm waveguides connected between the two optical couplers. It is characterized by.
According to this configuration, in the MZI circuit using two optical couplers, it is possible to suppress polarization mode coupling in which coupling occurs between polarizations in each coupler, so that PDL caused by polarization mode coupling can be reduced.

  A waveguide type optical circuit according to another aspect of the present invention is a planar lightwave circuit in which an overcladding is formed by a flame deposition method.

  The waveguide type optical circuit according to another aspect of the present invention is characterized in that the planar lightwave circuit is a PLC type star coupler. According to this configuration, a PLC type star coupler with reduced PDL caused by polarization mode coupling can be realized.

  A waveguide type optical circuit according to another aspect of the present invention is characterized in that the planar lightwave circuit is a PLC type optical coupler such as a PLC type 2 × 2 coupler. According to this configuration, a PLC-type optical coupler such as a PLC-type 2 × 2 coupler with reduced PDL caused by polarization mode coupling can be realized.

In the waveguide type optical circuit according to another aspect of the present invention, the planar lightwave circuit is a PLC type optical variable light that causes a thin film heater formed on at least one of the two arm waveguides to function as a phase shifter. It is an attenuator.
In a PLC type variable optical attenuator, light is extinguished using a region where the coupling efficiency is between 0 and 100. However, the larger the extinction ratio is, the more easily the influence of polarization dependence becomes, and the PDL becomes larger. Is an important device. According to this configuration, it is possible to realize a PLC-type variable optical attenuator with reduced PDL caused by polarization mode coupling.

  According to the present invention, it is possible to realize a waveguide type optical circuit in which polarization mode coupling is reduced and PDL caused by polarization mode coupling is reduced.

The top view which shows schematic structure of the waveguide type optical circuit which concerns on 1st Embodiment of this invention. Sectional drawing along the XX line of FIG. The figure explaining the length of the dummy pattern in FIG. The top view which shows schematic structure of the waveguide type optical circuit which concerns on 2nd Embodiment of this invention. The top view which shows the conventional waveguide type optical circuit without a dummy pattern. Sectional drawing along the YY line of FIG. (A), (B) is sectional drawing similar to FIG. 6, and is a figure for demonstrating a mode that the stress concerning a core and a core incline. The graph which shows the coupling efficiency of the waveguide type optical circuit shown in FIG. The graph which shows PDL of the waveguide type optical circuit shown in FIG. The graph which shows the excess loss of the waveguide type optical circuit shown in FIG. The top view which shows the modification of the waveguide type optical circuit shown in FIG. The top view which shows another modification of the waveguide type optical circuit shown in FIG.

Embodiments of a waveguide optical circuit embodying the present invention will be described below with reference to the drawings. In the description of each embodiment, the same parts and parameters are denoted by the same reference numerals, and redundant description is omitted.
(First embodiment)
A schematic configuration of a waveguide optical circuit according to a first embodiment of the present invention will be described with reference to FIGS.
1 shows a schematic configuration of a waveguide type optical circuit according to a first embodiment of the present invention, FIG. 2 is a cross-sectional view taken along line XX of FIG. 1, and FIG. 3 shows a dummy pattern of FIG. It is a figure explaining length.

  A waveguide type optical circuit 10 according to the first embodiment shown in FIG. 1 includes one directional coupler 11. The directional coupler 11 is an optical coupler (optical branching coupler) in which 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 pattern 21 is formed on both sides of the waveguide cores 11a and 11b so as to sandwich the waveguide cores 11a and 11b.

  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, as shown in FIG. 3, the length L1 of the coupling portion is the length dL of the strong coupling portion contributing to the coupling of each directional coupler 11 and the weak coupling portion contributing to the coupling of the directional coupler 11. It is the sum of twice the length of ΔL (ΔL × 2).

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. However, if each dummy pattern 21 is too close to the waveguide cores 11a and 11b of the directional coupler 11, minute coupling occurs between the dummy pattern 21 and the waveguide cores 11a and 11b. By setting the gap size to A or more, it is possible to suppress the loss by suppressing the occurrence of minute coupling.

The width W (see FIGS. 1 and 2) of each of the dummy patterns 21 and 21 may be a width in which the prescribed mode (single mode) of the waveguide propagation light of the directional coupler 11 is not coupled or more. .
When the width W of the dummy pattern 21 is a width that couples the prescribed modes of the waveguide propagation light of the directional coupler 11, the prescribed mode (single mode) that guides the waveguide cores 11 a and 11 b of the directional coupler 11. Mode) light is coupled to the dummy pattern 21, and light is not coupled to the output-side waveguide, resulting in cladding mode light and excessive loss. If the width W of the dummy pattern 21 is wider than the width where the prescribed mode of the waveguide propagation light is not coupled or more than that, the propagation constant is different, so that the light of the prescribed mode is not coupled to the dummy pattern 21 and excessive loss occurs. Does not occur. 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 will be described with reference to FIG.
(1) A silica material (SiO ₂ glass fine particles) to be the lower clad layer 32 and the core layer 33 is deposited on a silicon (Si) substrate 31 by a flame deposition (FHD) method and heated. To melt and clear the glass film.
(2) Thereafter, a desired waveguide is formed by photolithography and reactive ion etching. Here, the dummy patterns 21 and 21 are formed along the waveguide cores 11a and 11b of the directional coupler 11 and the waveguide cores 11a and 11b. Here, if the dummy patterns 21 and 21 are drawn on the photomask on which the waveguide pattern used in photolithography is drawn, the waveguide cores 11a and 11b and the dummy pattern are added to the core layer 33 without adding a manufacturing process. 21 and 21 can be formed simultaneously.
(3) Thereafter, a silica material (SiO ₂ glass fine particles) to be the upper cladding layer 34 is deposited by the FHD method, each waveguide is embedded in the upper cladding layer 34, and heated at a high temperature to melt the glass film. Make it transparent (vitrified).

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 upper clad layer 34 are deposited and vitrified in the step (3) to form the upper clad layer 34, the interval B between the waveguide cores 11a and 11b and the waveguide cores 12a and 12b. The distance B of the gap is as narrow as several μm, and glass particles do not enter so 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 waveguide type optical circuit 10 according to the present embodiment, the dummy patterns 21 and 21 are provided on both sides of the directional coupler 11, and the interval B between the dummy pattern 21 and the waveguide cores 11a and 11b is set to the gap size. Same or higher than A. For this reason, the density of the glass particles between the waveguide cores 11a and 11b is sparse, but the density of the glass particles is also sparse around the waveguide cores 11a and 11b, so that there are no problems such as the waveguide cores 11a and 11b falling inward.

  That is, the glass particle density between the waveguide cores 11a and 11b approaches the glass particle density between each of the dummy patterns 21 and 21 and each of the waveguide cores, and the stress of trying to incline both the waveguide cores 11a and 11b inward. Occurrence is suppressed. For this reason, tilting of the optical principal axes of both waveguide core cores 11a and 11b is suppressed, polarization mode coupling in which coupling between polarized waves in the directional coupler 11 is suppressed, and PDL caused by polarization mode coupling is reduced. can do.

(Waveguide type optical circuit without dummy pattern)
Next, as a comparative example of the first embodiment, a conventional method for manufacturing a waveguide type optical circuit without a dummy pattern will be described with reference to FIGS.
FIG. 5 shows a conventional waveguide type optical circuit provided with a directional coupler 110. This waveguide type optical circuit is manufactured as follows. 6 and 7 are cross-sectional views taken along line YY in FIG. 5, and particularly FIGS. 7A and 7B are views for explaining the stress applied to the core and the state in which the core is inclined.

(1a) A silica material (SiO ₂ glass fine particles) to be the lower cladding layer 320 and the core layer 330 is deposited on the silicon (Si) substrate 310 by the FHD method (see FIG. 6), and heated to form a glass film. To make it transparent.
(2a) Thereafter, a desired waveguide is formed 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 the upper clad layer 340 is deposited by the FHD method, each waveguide is filled with the upper clad layer 340, and heated at a high temperature to melt and clear (vitrify) the glass film.
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 upper clad layer 340 are deposited and vitrified to form the upper clad layer 340, the interval between the waveguide cores 110a and 110b is as narrow as 1 μm, There is not so much between the waveguide cores 110a and 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. 7A) for tilting 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 with some deformation, and the optical principal axes of the waveguide cores 11a and 11b are inclined (see FIG. 7B).
In such a state, coupling between polarized waves (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 cores 11a and 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 fine particle density on both sides (non-gap part and gap part) of each waveguide core 11a, 11b is close, the waveguide cores 11a, 11b generated by the difference in glass fine particle density in the upper clad formation process are inward of the gap. It is possible to suppress the generation of stress that attempts to collapse. That is, by providing the dummy pattern 21, it is possible to suppress the polarization mode coupling in which the optical principal axes of the two waveguide cores 11 a and 11 b are coupled with each other in the directional coupler 11 due to the stress. Caused PDL 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. Here, the length L1 of the coupling portion is twice the length dL of the strong coupling portion contributing to the coupling of each directional coupler 11 and the length of the weak coupling portion ΔL contributing to the coupling of the directional coupler 11. It is the sum of (ΔL × 2). Thereby, since the optical principal axes of the waveguide cores 11a and 11b are prevented 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. PDL resulting from the polarization mode coupling can be 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 stress generation 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 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 bonds. 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 prescribed mode (single 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 prescribed mode of the propagation light of the waveguide of the directional coupler 11 is coupled, the prescribed mode (single mode) for guiding the waveguide core of the directional coupler 11 is obtained. The light is coupled to the dummy pattern 21, and the 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 wider than the width where the prescribed mode of the waveguide propagation light is not coupled or more than that, the propagation constant is different, so that the light of the prescribed mode is not coupled to the dummy pattern 21 and excessive loss occurs. 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. . As a result, it is difficult to completely eliminate the polarization mode coupling while suppressing excess loss in the directional coupler 11, but if the polarization mode coupling is -25 dB or less, the directional coupler 11 is sufficiently small, and a PDL reduction effect is obtained. It is done.

  (6) According to the manufacturing method of the waveguide type optical circuit 10 described above, the manufacturing method is the same as the conventional method, and the present invention can manufacture the waveguide type optical circuit 10 without adding a manufacturing method.

(Second Embodiment)
FIG. 4 shows 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. 4 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 one arm waveguide of the MZI circuit 20. 14 comprises a Mach-Zehnder interferometer (MZI) circuit 20 comprising a thin film heater 15 provided in the upper part. Here, a PLC-VOA composed of one MZI will be described. However, since the present invention is applied to an optical coupler in the MZI, the PLC-VOA composed of MZIs connected in multiple stages is also described. The present invention is applicable.

  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 first directional coupler 11, propagated independently through the two arm waveguides 13 and 14, and then desired. Recombined by the second directional coupler 12 with the optical path length difference and output from the output port P3. At this time, the coupling efficiency of the MZI circuit 20 is maximum when the optical path length difference between the arm waveguides 13 and 14 is zero, and is minimum 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 the optical variable attenuator can be operated.

  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 method for manufacturing the PLC-VOA 10A having such a configuration will be described with reference to FIG.

(1) A silica material (SiO ₂ glass fine particles) to be the lower clad layer 32 and the core layer 33 is deposited on a silicon (Si) substrate 31 by a flame deposition (FHD) method and heated. To melt and clear the glass film.

  (2) Thereafter, a desired waveguide is formed 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 are formed along 11b, 12a and 12b. Here, if a dummy pattern is drawn on a photomask on which a waveguide pattern used in photolithography is drawn, the waveguide core and the dummy pattern can be simultaneously formed on the core layer 33 without adding a manufacturing process.

(3) Thereafter, a silica material (SiO ₂ glass fine particles) to be the upper cladding layer 34 is deposited by the FHD method, each waveguide is embedded in the upper cladding layer 34, and heated at a high temperature to melt the glass film. Make it transparent (vitrified).

(4) Subsequently, a heater and a wiring electrode are formed on the upper clad.
In this way, the PLC-VOA 10A is manufactured. By the way, when the glass fine particles to be the upper clad layer 34 are deposited and vitrified in the step (3) to form the upper clad layer 34, the interval B between the waveguide cores 11a and 11b and the waveguide cores 12a and 12b. The distance B of the gap is as narrow as several μm, and glass particles do not enter so 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. Problems such as 12a and 12b falling inward do not occur.

That is, 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 each of the dummy patterns 21 and 22 and each of the waveguide cores, and both the waveguide cores 11a and 11b and Generation | occurrence | production of the stress which tries to incline 12a, 12b inside is suppressed. For this reason, tilting of the optical principal axes of both waveguide core cores 11a, 11b and 12a, 12b is suppressed, and polarization mode coupling in which polarization is coupled between the directional couplers 11 and 12 is suppressed. PDL caused by mode coupling can be reduced.
According to 2nd Embodiment comprised as mentioned above, in addition to the effect which the said 1st Embodiment show | plays, there exist the following effects.

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

  (2) In the PLC-VOA 10A, light is extinguished using a region where the coupling efficiency is between 0 and 100. However, the larger the extinction ratio is, the more easily the influence of polarization dependence occurs, and the PDL increases. Is an important device. 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 P1 port (input port) of the MZI 20 to the P3 port (output port) is set to about 4% in the PLC-VOA 10A shown in FIG. Measured coupling efficiency and PDL when the size is 1 (μm) or 3 (μm), 5 (μm) and 10 (μm) larger than the gap size A, and when there is no dummy pattern for comparison. The excess loss is shown in FIGS. 8, 9, and 10, respectively.

  From FIG. 8, when the distance B is set to 1 (μm), 3 (μm), 5 (μm), 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 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 set to be equal to or larger than the gap size A, for example, B = 3, 5, 10 is used, the PDL reduction effect can be obtained while suppressing the 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 the same as the conventional method, and the present invention can manufacture the PLC-VOA 10A without adding a manufacturing method.
In addition, this invention can also be changed and embodied as follows.

  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, as shown in FIG. Using two MMI couplers 40, a PLC-VOA or the like having a plurality of MZI circuits 20 each composed of these two MMI couplers 40 and two arm waveguides connected between the two MMI couplers 40, etc. The present invention is also applicable to a waveguide type optical circuit. 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. 10, reference numeral “45” denotes a slab waveguide.

Further, in the PLC-VOA 10A according to the second embodiment, two MMI couplers 40 similar to those in FIG. 10 are used 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. When used, dummy patterns 46 and 47 may be formed on both sides of the MMI coupler 40 as shown in FIG. The dummy pattern 46 includes dummy patterns 46a and 46b positioned outside the waveguide cores 41a and 42a, respectively, and a portion 46c connecting the dummy patterns 46a and 46b. Similarly, the dummy pattern 47 has dummy patterns 47a and 47b located outside the waveguide cores 41b and 42b, respectively, and a portion 47c connecting the dummy patterns 47a and 47b.
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 The present invention is also applicable to a waveguide type optical circuit such as a 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, the waveguide type configured as a PLC type optical coupler such as a PLC type star coupler. The present invention is also applicable to optical circuits.
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 is also applicable to a delay demodulation device used in an optical transmission system using the (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.

DESCRIPTION OF SYMBOLS 10 ... Waveguide type optical circuit 10A ... PLC type variable optical attenuator (PLC-VOA) as a waveguide type optical circuit
DESCRIPTION OF SYMBOLS 11, 12 ... Directional couplers 11a, 11b as optical couplers ... Waveguide cores 12a, 12b ... Waveguide cores 13, 14 ... Arm waveguide 15 ... Thin film heater 20 ... Mach-Zehnder interferometer (MZI) circuits 21, 22 ... Dummy pattern A ... Gap B between waveguide cores ... Interval L ... Dummy pattern length W ... Dummy pattern width

Claims (7)

A planar lightwave circuit in which an overcladding is formed by a flame deposition method, and in a waveguide type optical circuit having an optical coupler that is an optical branching coupler,
The optical coupler is a directional coupler in which waveguide cores are close to each other,
Dummy patterns are formed along both sides of the waveguide core of the optical coupler to prevent the optical axis of the waveguide core from being inclined,
The length of the dummy pattern is equal to or greater than the length of the coupling portion where the coupling efficiency of the optical coupler is greater than 0%,
The dummy pattern extends to the bent waveguide portion formed so that the interval between the waveguide cores gradually widens from the narrowest portion,
A waveguide type optical circuit, wherein the gap between the dummy pattern and the waveguide core of the optical coupler is the same as the gap size between the waveguide cores of the optical coupler.   2. The waveguide type optical circuit according to claim 1, wherein the width of the dummy pattern is a width at 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.   4. The apparatus according to claim 1, further comprising: the two optical couplers; and a Mach-Zehnder interferometer circuit including two arm waveguides connected between the two optical couplers. The waveguide type optical circuit described. The planar lightwave circuit waveguide type optical circuit as claimed in any one of claims 1 to 4, characterized in that a PLC type star coupler. The planar lightwave circuit waveguide type optical circuit as claimed in any one of claims 1 to 4, characterized in that a PLC type optical coupler. 5. The waveguide variable optical attenuator according to claim 4, wherein the planar lightwave circuit is a PLC-type variable optical attenuator that causes a thin film heater formed on at least one of the two arm waveguides to function as a phase shifter. Waveguide type optical circuit.

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