JP6487805B2 - Waveguide type mode converter - Google Patents

Description

  The present invention relates to a mode converter constituting a mode multiplexer / demultiplexer in multimode transmission for optical communication.

  With the spread of smartphones and the like, a communication line capable of transmitting a large amount of traffic is required. In Asia, North America and Europe, an increase in capacity of an optical communication line backbone network is promoted. In order to increase the capacity, multilevel modulation technology and dense wavelength division multiplexing (DWDM) technology have been used, but in recent years, spatial multiplexing transmission (SDM) technology has attracted attention.

  Multi-mode transmission, which is one of such SDM techniques, is a multiplex transmission technique that transmits a signal on each of a plurality of propagation mode optical signals on a single fiber, and uses a plurality of cores. It is often used in combination with multi-core transmission, and is an effective transmission means for increasing the number of spatial multiplexing. In this multimode transmission, a mode multiplexer / demultiplexer is required. In other words, the optical signal received in the basic mode is converted into a higher-order mode, multiplexed, and sent to the FMF (Few Mode Fiber). It is a mode duplexer that splits a signal and converts it to a fundamental mode. In such a mode multiplexer / demultiplexer, a mode converter for converting from the fundamental mode to the higher order mode or from the higher order mode to another higher order mode is used.

  A PLC (planar lightwave circuit) type mode multiplexer / demultiplexer is one of the main candidates for multimode transmission / demultiplexers from the viewpoint of low loss and integration.

The mode multiplexer / demultiplexer described in Non-Patent Document 1 forms four modes of multiplexing / demultiplexing using a directional coupler and a vertical mode converter. In Non-Patent Document 1, the four modes are represented by LP ₀₁ , LP _11a , LP _11b , and LP _21a , which correspond to E ₁₁ , E ₂₁ , E ₁₂ , and E ₂₂ , respectively. . E ₁₁ , E ₂₁ , E ₁₂ , and E ₂₂ represent electric field distributions of light propagating through an optical waveguide having a basically rectangular cross section. When this electric field distribution is generally expressed by E _MN (M and N are integers), E _MN has M as the total number of peaks and valleys of the electric field distribution in the x-axis direction (substrate horizontal direction), y The total number of peaks and valleys of the electric field distribution in the axial direction (substrate vertical) is N. E ₁₂ represents a mode in which the total number of peaks and valleys in the electric field distribution in the x-axis direction is 1, and the total number of peaks and valleys in the y-axis direction is 2. These contents are as described in Non-Patent Document 2 (P27 to P30).

  In general, the x-axis direction represents the horizontal direction of the waveguide substrate, the y-axis direction represents the vertical direction of the waveguide substrate, and the z-axis direction represents the light propagation direction.

  A typical quartz-based PLC chip has a lower clad, a core, and an upper clad laminated on a Si substrate or a quartz substrate. The mode multiplexer / demultiplexer of Non-Patent Document 1 includes a mode converter for a substrate vertical direction having a core 400 shown in FIG. In general, a quartz PLC core is formed in a quadrangular shape, but the core 400 shown in FIG. 11 includes a notch 410 as shown in FIG. The arrow shown in FIG. 11A indicates the symmetry axis of the waveguide. A mode symmetric with respect to the symmetry axis is an eigenmode of light propagating in the waveguide. The eigenmodes at this time are shown in FIGS. 11 (b) and 11 (c).

When the E ₂₁ mode is incident on the waveguide having the core shown in FIG. 11A, the mode shown in FIG. 11B and the mode shown in FIG. 11C are excited, and the respective modes are excited. The modes propagate with different effective refractive indexes and interfere with a beat length inversely proportional to the difference between the effective refractive indexes. By adjusting the propagation length of the light propagating through the waveguide, the two modes shown in FIGS. 11B and 11C overlap with a certain phase difference, and E ₁₂ is generated. That is, mode conversion from E ₂₁ to E ₁₂ is realized.

The mode multiplexer / demultiplexer described in Patent Document 1 multiplexes and demultiplexes six modes from E ₁₁ to E ₃₂ by connecting a quartz PLC chip having a directional coupler tilted by 90 degrees. (First embodiment of the same document).

JP 2013-152272 A

N. Hanzawa, K. Saitoh, T. Sakamoto, T. Matsui, K. Tsujikawa, T. Uematsu, and F. Yamamoto, “Four-mode PLC-based mode multi / demultiplexer with LP11 mode rotator on one chip for MDM transmission ”, ECOC 2014 (Cannes) technical digest, We.1.1.1 Katsuaki Okamoto, “Basics of Optical Waveguide”, Corona, 1992, p.27-30

The mode converter for the substrate vertical direction described in Non-Patent Document 1 has a function of mode conversion from E ₁₂ to E ₂₁ and is used in combination with a mode converter (directional coupler) for the substrate horizontal direction. , E ₁₁ to E ₁₂ , E ₂₁ and E ₂₂ mode conversion is performed. However, in this mode converter, it is not possible to carry out the mode conversion to the E _13, the number of modes that can be converted is limited. Therefore, a large communication capacity cannot be realized.

  In the mode converter described in Patent Document 1, the PLC chip and the PLC chip are connected with an inclination of 90 degrees for mode conversion in the substrate vertical direction. However, downsizing of the mode converter is restricted by connecting the PLC chip and the PLC chip with an inclination of 90 degrees.

  An invention for solving the above-mentioned problems is a waveguide mode converter formed on a substrate, the first waveguide having a cladding layer and a core layer having a higher refractive index than the cladding layer, The core layer and the clad layer are alternately stacked in a direction perpendicular to the substrate, the core layer has a three-layer structure, and the multimode waveguide has the core layer and the clad layer. The first waveguide, the second waveguide, and the multimode waveguide are each connected in this order.

When E ₁₁ mode light is incident on the first waveguide, at least two of the eigenpropagation modes of the second waveguide are excited and the at least 2 modes are excited in the second waveguide. One of the modes interfere, in the multi-mode waveguide may be configured to emit light of higher E ₁₃ modes by the interference.

  There may be two modes excited in the second waveguide. In the second waveguide, the three-layer structure of the core layer may include an upper layer, an intermediate layer, and a lower layer, and the upper layer and the lower layer may be symmetrical with respect to the intermediate layer.

The length of the second waveguide may be set in the light propagation direction so that the conversion efficiency from the E ₁₁ mode to the E ₁₃ mode is maximized.

  According to the present invention, since it is possible to convert to a higher order mode with respect to the vertical direction of the substrate, an increase in communication capacity can be realized. In addition, according to the present invention, since each of the waveguides is connected, the size can be reduced.

It is a schematic diagram which shows an example of the core structure of each waveguide of the mode converter in embodiment. It is a schematic diagram which shows an example of the structure of the mode converter containing a board | substrate and a clad. It is a figure which shows the example of a calculation result of the electric field amplitude distribution obtained by BPM simulation. It is a diagram showing a calculation result example of the overlap integral of the electric field distribution of the mode to be field distribution and finally obtained in each propagation position (E ₁₃ modes on the exit side of the waveguide). In a mode converter of an embodiment, it is a figure showing electric field distribution of a natural propagation mode of a lamination waveguide. It is a figure which shows the electric field distribution of the y-axis direction in the mode excited in the laminated waveguide. It is a figure showing the electric field distribution of the y-axis direction which weighted and added or subtracted Fig.6 (a) and FIG.6 (b). (A) shows the calculation results of the electric field distribution in the y-axis direction in the E ₁₁ mode of the incident side of the waveguide shows the calculation results of the electric field distribution in the y-axis direction in (b) E ₁₃ modes on the exit side of the waveguide FIG. It is a figure for demonstrating 1 dB deterioration tolerance and conversion efficiency at the time of changing the relative refractive index difference of the intermediate | middle layer core of a laminated waveguide. It is a perspective view which shows an example of the mode converter which provided the spot size converter in the incident part of the waveguide. In the conventional example, (a) is a diagram showing a mode converter, specifically a diagram showing a core of a waveguide section, and (b) and (c) a diagram showing a natural propagation mode of a waveguide section.

Hereinafter, a waveguide type mode converter (hereinafter simply referred to as “mode converter”) 1 according to an embodiment of the present invention will be described. The mode converter 1 is a silica-based PLC type mode converter that receives light of a fundamental mode E ₁₁ and emits light of a higher-order mode E ₁₃ . E ₁₁ represents a mode in which the total number of peaks and valleys of the electric field distribution in the horizontal direction and the vertical direction of the substrate is 1, and E ₁₃ represents a total number of peaks and valleys of the electric field distribution in the horizontal direction of the substrate of 1. And a mode in which the total number of peaks and valleys of the electric field distribution in the direction perpendicular to the substrate is 3.

[Configuration of Mode Converter 1]
FIG. 1 is a schematic diagram illustrating an example of a core structure of each of the waveguides 100, 200, and 300 of the mode converter 1 in the present embodiment. FIG. 2 is a schematic diagram illustrating a configuration example of the mode converter 1 including a substrate and a clad.

  As shown in FIG. 1, the mode converter 1 includes three waveguides having different core structures, that is, a waveguide (first waveguide) 100, a laminated waveguide (second waveguide) 200, and a waveguide ( A multimode waveguide) 300. These waveguides 100, 200, 300 are connected in that order.

  In FIG. 1, the x axis indicates the substrate horizontal direction, the y axis indicates the substrate vertical direction, and the z axis indicates the light propagation direction. In FIG. 1, only the core having a higher refractive index than the clad described later is illustrated in order to clarify the structural difference between the waveguides 100 to 300. Both the clad and the core of this embodiment are made of quartz glass.

  The waveguide 100 has a core size of 6.0 μm × 1.5 μm (width × thickness) and a relative refractive index difference of 2.5%. In the following description, the relative refractive index difference is expressed in% as a value given by {(core refractive index) − (cladding refractive index)} / (core refractive index).

  The laminated waveguide 200 has a three-layer core structure. This core structure includes an upper layer 210, an intermediate layer 220 and a lower layer 230. In this embodiment, the relative refractive index difference between the upper layer 210 and the lower layer 230 is 2.0 (%), and the relative refractive index difference between the intermediate layer 220 is 1.5 (%). The thickness of each layer 210, 220, 230 is 1.8 μm. The width of each layer is 6.0 μm.

  A clad (not shown) is formed between the upper layer 210 and the intermediate layer 220, and a clad (not shown) is also formed between the intermediate layer 220 and the lower layer 230. The distance between the upper surface of the upper layer 210 and the lower surface of the lower layer 230 is 8.0 μm.

  The waveguide 300 is configured to be multimode in each of the substrate vertical direction and the substrate horizontal direction. In this embodiment, the core size of the waveguide 300 is 6.0 μm × 8.0 μm (width × thickness), and the relative refractive index difference is 2.0%. By calculating a normalized frequency from the relative refractive index difference and the core size, it can be seen that the waveguide 300 has a multimode waveguide configuration.

  As shown in FIG. 2, the clad 101 of the waveguide 100 is formed on the substrate 102, the clad 102 of the laminated waveguide 200 is formed on the substrate 202, and the clad 301 of the waveguide 300 is formed on the substrate 302. It is formed. The substrates 102, 202, 302 are Si substrates, quartz substrates, or the like.

[Method of Manufacturing Mode Converter 1]
Hereinafter, a method for manufacturing the mode converter 1 will be described with reference to FIG. 1 again.

  The waveguides 100 and 300 are manufactured by the same manufacturing method as that of a general quartz PLC. That is, a clad glass (under clad) is deposited on a Si substrate or a quartz substrate, and a core glass is deposited thereon. Then, using photolithography and RIE, the core pattern shown in FIG. 1 is processed, and a clad glass (over clad) is deposited thereon.

  The laminated waveguide 200 is deposited in the order of the lower layer 230, the cladding layer, the intermediate layer 220, the cladding layer, and the upper layer 210 after depositing cladding glass (under cladding) on the Si substrate. Then, similarly to the waveguides 100 and 300, the core pattern shown in FIG. 1 is processed, and a cladding glass (over cladding) is further deposited.

  After the deposition of the clad glass is completed, the waveguides 100 and 300 are cut out by dicing. Further, the laminated waveguide 200 is similarly cut out by dicing. Thereafter, the waveguide 100, the laminated waveguide 200, and the waveguide 300 are optically aligned by, for example, a fine adjustment aligning table, and then connected to each other using an ultraviolet curable adhesive. Thereby, the mode converter 1 can be produced.

  A calculation result of the electric field distribution of the yz section realized by the mode converter 1 will be described with reference to FIG. FIG. 3 is a diagram showing the calculation result obtained by simulation by BPM. BPM (Beam Propagation Method) is an abbreviation for beam propagation method.

  In FIG. 3, the wavelength of light is 1.3 μm, and the polarization of input light is calculated by TE (Transverse Electric). Even if the polarization is TM (Transverse Magnetic), the same result as in FIG. 3 is obtained.

First, the fundamental mode E ₁₁ is incident on the waveguide 100 of FIG. In the laminated waveguide 200 of FIG. 3, it can be seen that a plurality of modes are excited by the fundamental mode E ₁₁ of the waveguide 100 and beat (interfere) at about 170 μm. In the example of FIG. 3, since the length of the laminated waveguide 200 is optimized (for example, 876 μm) and connected to the waveguide 300, almost no beat (interference) occurs in the waveguide 300. This indicates that the E ₁₁ mode of the waveguide 100 can be converted to the E ₁₃ mode of the waveguide 300 with high efficiency.

FIG. 4 shows the propagation distance dependence of the overlap integral between the electric field distribution calculated at each propagation position and the E ₁₃ mode electric field distribution. E ₁₃ modes are E ₁₃ mode of the waveguide 300. In FIG. 4, the horizontal axis represents the propagation distance, and the vertical axis represents the overlap integral between the electric field distribution and the E ₁₃ mode electric field distribution at each propagation distance.

The definition of overlap integral will be described. For example, when two electric field vectors are represented by E ₁ and E ₂ , respectively, the overlap integral (POI: Power Overlap Integral) of the electric field E ₁ and the electric field E ₂ is represented by the following formula (1).

POI represents the power coupling efficiency of the electric field E ₁ and the electric field E ₂ . The overlap integral with E ₁₃ shown in FIG. 4 means the coupling efficiency from the electric field distribution at each propagation position to the electric field distribution of the E ₁₃ mode, that is, the conversion efficiency from the incident E ₁₁ mode to the E ₁₃ mode. From the overlap integral shown in FIG. 4, it can be seen that the conversion efficiency from the E ₁₁ mode to the E ₁₃ mode periodically changes in relation to the propagation distance of light.

  From this, it can be seen that the conversion efficiency can be maximized by optimizing the length of the laminated waveguide 200. In the example of FIG. 4, since the propagation distance when the peak of the overlap integral is sixth is 876 μm, in the mode converter 1 of this embodiment, the laminated waveguide 200 has a length in the z-axis direction of 876 μm. The laminated waveguide 200 and the waveguide 300 are connected to each other. The reason why the length is 876 μm, which is approximately 1 mm, is to facilitate interconnection of the waveguide 100, the laminated waveguide 200, and the waveguide 300.

In the example of FIG. 4, the conversion efficiency from the E ₁₁ mode to the E ₁₃ mode in the waveguide 300 is 0.95676 at a propagation distance of 2000 μm, so the conversion loss is 0.19 dB. Further, in the waveguide 300 at a propagation distance of 2000 μm, a calculation result was obtained that the power efficiency of light propagating in the waveguide cross-sectional direction was 0.9590, and the mode extinction ratio was 26 dB.

From such a result, mode converter 1 shown in Figure 1, it can be seen that conversion with high efficiency in higher E ₁₃ modes in the direction perpendicular to the substrate.

In the mode converter 1 of the present embodiment, as shown in FIG. 3, since the beat has a constant period (about 170 μm), it is considered that two modes are mainly excited. In this case, the propagation constants of the two modes are different, and it is considered that the beat length is proportional to the reciprocal of the difference between the propagation constants. Hereinafter, the mode for generating the beat will be discussed with reference to FIGS. 5A to 5D are diagrams showing the electric field distribution of the eigenmode in the laminated waveguide 200. FIG. In (a) to (d) of FIG. 5, black and white color bands indicate electric field values at the same level. Since the E ₁₁ mode incident on the waveguide 100 is evenly symmetric in the xy direction, in the laminated waveguide 200, the mode of FIG. 5B that is oddly symmetric in the y direction and FIG. The mode of d) is not excited. FIG. 6 is a diagram showing the electric field distribution in the y-axis direction for the mode excited in the laminated waveguide 200, where (a) shows the electric field distribution in the y-axis direction corresponding to FIG. 5 (a), and (b). Shows the electric field distribution in the y-axis direction corresponding to FIG.

FIG. 7A shows a result obtained by adding weights at the ratio of (a) :( b) = 2: 5 in the modes shown in FIGS. 6A and 6B (FIG. 6). (A) × 0.4 + FIG. 6B). FIG. 7B shows the result of subtracting the mode shown in FIG. 6B from the mode shown in FIG. 6A (FIG. 6A). ) × 0.4—FIG. 6B). Note that FIG. 7 is shown normalized so that the peak or bottom is 1. 8A shows the electric field distribution in the y-axis direction of the fundamental mode E ₁₁ in the waveguide 100, and FIG. 8B shows the electric field distribution in the y-axis direction of the E ₁₃ mode in the waveguide 300. .

Comparing FIG. 7 (a) and FIG. 8 (a), it can be seen that both show values having substantially the same shape. Further, even when FIG. 7B and FIG. 8B are compared, it can be seen that both show values having substantially the same shape. As a result, the ground mode E ₁₁ in the waveguide 100 excites two modes (FIG. 5A and FIG. 5C) in the laminated waveguide 200, and the two modes interfere with each other to cause the waveguide 300. It can be confirmed that _E13 mode is generated at

  In the above description, it has been described that there are two modes to be excited in the laminated waveguide 200. However, even if there are three or more modes, it is sufficient that there are two main modes that contribute to interference. .

Since the E ₁₁ mode in the waveguide 100 is even symmetric with respect to both the x-axis direction and the y-axis direction, the distribution is oddly symmetric with respect to the y-axis direction (FIG. 5B), and with respect to the x-axis direction. And an oddly symmetric distribution (FIG. 5 (d)) cannot be excited.

  The electric field modes shown in FIGS. 5A and 5C are evenly symmetric distributions with respect to the x-axis direction and the y-axis direction, and are excited. That is, since the excited electric field mode has an even symmetric distribution, in the mode converter 1 of the present embodiment, the core structure of the laminated waveguide 200 needs to be at least symmetrical with respect to the y-axis direction. is there. Specifically, both the upper layer 210 and the lower layer 230 need to be arranged at a position equidistant from the intermediate layer 220 and have the same thickness and the same relative refractive index difference. From such a viewpoint, in the mode converter 1 of the present embodiment, the thickness of the clad between the upper layer 210 and the intermediate layer 220 and the thickness of the clad between the intermediate layer 220 and the lower layer 230 are both 1.3. It is set to μm.

FIG. 9 shows a 1 dB degradation tolerance when the relative refractive index difference of the intermediate layer 220 of the laminated waveguide 200 is changed. In FIG. 9, the horizontal axis represents the relative refractive index difference Δ (%) of the intermediate layer 220, and the vertical axis represents the 1 dB degradation tolerance (μm) and the conversion efficiency (dB) from the E ₁₁ mode to the E ₁₃ mode.

  The 1 dB degradation tolerance (μm) on the vertical axis means the light transmission distance (distance in the z-axis direction) when the overlap integral value degrades by 1 dB, as shown in FIG.

  In FIG. 9, the relative refractive index difference between the upper layer 210 and the lower layer 230 is fixed at 2.0 (%).

  In FIG. 9, when the relative refractive index difference of the intermediate layer 220 approaches 1.5 (%) to 2.0 (%) (the relative refractive index of the upper layer 210 and the lower layer 230 = 2.0), the 1 dB deterioration tolerance is It has increased from 54μm to about 90μm. This is because when the difference between the relative refractive index difference of the intermediate layer 220 and the relative refractive index difference (2.0%) of the upper layer 210 and the lower layer 230 is reduced, the two modes excited (FIG. 6A and FIG. 6). This is because the difference in effective refractive index with respect to 6 (c)) is small, that is, the difference in propagation constant is small, and as a result, the beat length (interference interval) becomes long and the width of 1 dB degradation tolerance becomes large.

  On the other hand, in FIG. 9, when the relative refractive index difference of the central layer 220 is near 1.5%, the conversion loss is as small as 0.2 dB, but approaches the relative refractive index difference (2.0%) of the upper layer 210 and the lower layer 230. Then, the conversion loss becomes as large as about 3 dB. As the conversion loss increases, the function as a mode converter cannot be achieved. From this result, it can be seen that the relative refractive index difference of the intermediate layer 220 is preferably smaller than the relative refractive index difference of the upper layer 210 and the lower layer 230. .

As described above, according to the mode converter 1 of the present embodiment, the fundamental mode E ₁₁ propagating through the waveguide 100 excites mainly two modes among the plurality of eigenmodes of the laminated waveguide 200. . Then, the two modes propagate with different propagation constants and interfere with each other, so that a higher-order E ₁₃ mode is generated and emitted from the waveguide 300. Conversion to the E ₁₃ mode, than conventional high-capacity communication can be realized.

  In addition, the mode converter 1 is small because it is configured by connecting the three waveguides 100, 200, and 300 without connecting the PLC chip and the PLC chip at an angle of 90 degrees unlike the conventional mode converter. Can be realized.

  For example, if such a mode converter 1 is applied to a mode multiplexer / demultiplexer, more multi-mode transmission is possible, so that communication with a larger capacity than before can be realized.

  As mentioned above, although this embodiment was explained in full detail, the material of a laminated body, an operation | movement wavelength, etc. are not restricted to embodiment, The design change etc. of the range which does not deviate from the summary of this invention are included.

  For example, although the waveguide 100 illustrated in FIG. 1 is illustrated as a multimode waveguide in the x-axis direction, a part of the waveguide 100 may be configured as a single mode waveguide. For example, FIG. 10 shows a configuration example when the spot size converter 105 is attached to the waveguide 100 and the input side of the waveguide 100 is a single mode waveguide. Note that, regardless of the presence or absence of the spot size converter 105, the y-axis direction of the waveguide 100 is a single mode or a pseudo single mode. The spot size converter 105 can connect the waveguide 100 to an existing optical circuit (such as an arrayed waveguide guide (AWG) or a switch) that operates in a single mode or pseudo single mode circuit.

  For example, since the waveguide 300 shown in FIG. 1 is a multimode waveguide, mode coupling (or demultiplexing) or the like is performed by connecting the waveguide 300 to a directional coupler or the like. You can also. For example, after the waveguide 300 is connected to the directional coupler, it may be further connected to an FMF (Few Mode Fiber) to send out the mode multiplexed signal to the fiber. FMF has a small number of modes among multimode fibers and is used for mode multiplexing transmission.

1 mode converter 100 waveguide (first waveguide)
101, 201, 301 Clad 102, 202, 302 Substrate 105 Spot size converter 200 Multilayer waveguide (second waveguide)
210 Upper layer core 220 Middle layer core 230 Lower layer core 300 Waveguide (multimode waveguide)

Claims (5)

A waveguide mode converter formed on a substrate,
A first waveguide having a cladding layer and a core layer having a higher refractive index than the cladding layer;
A second waveguide having a three-layer structure in which the core layer and the cladding layer are alternately stacked in a direction perpendicular to the substrate, and the core layer includes an upper layer, an intermediate layer, and a lower layer;
Including a multimode waveguide having the core layer and the cladding layer;
The first waveguide, the second waveguide, and the multimode waveguide are each connected in this order,
The core layer of the first waveguide is connected only to the intermediate layer of the three-layer structure of the core layer of the second waveguide, and the core layer of the second waveguide is All of the three-layer structures are connected to the core layer of the multimode waveguide. When E ₁₁ mode light is incident on the first waveguide, at least two of the eigenpropagation modes of the second waveguide are excited and the at least 2 modes are excited in the second waveguide. One of the modes interfere, in the multi-mode waveguide, waveguide mode converter according to claim 1, characterized in that it is configured to emit light of higher E ₁₃ modes by the interference .   3. The waveguide type mode converter according to claim 2, wherein the number of modes excited in the second waveguide is two.   4. The waveguide according to claim 2, wherein the upper layer and the lower layer of the three-layer structure of the core layer have a symmetric structure with respect to the intermediate layer. 5. Type mode converter. 5. The waveguide according to claim 4, wherein the second waveguide has a length in a light propagation direction so that conversion efficiency from the E ₁₁ mode to the E ₁₃ mode is maximized. Waveguide mode converter.