Array waveguide diffraction gratings and photomultiplexing / demultiplexing devices

Abstract

This technology provides the ability to photomultiply and demultiply unpolarized incident light using a single array waveguide diffraction grating. [Solution] The array waveguide diffraction grating 1 formed on the substrate 10 comprises a first slab waveguide 11, a second slab waveguide 12, an array waveguide 13 provided between the first slab waveguide 11 and the second slab waveguide 12, one input waveguide 211 into which wavelength-multiplexed light is input and a plurality of output waveguides 221 into which the wavelength-multiplexed light is demultiplexed and output for each wavelength, connected to the end of the first slab waveguide 11 opposite to the array waveguide 13, and one input waveguide 212 into which wavelength-multiplexed light is input and a plurality of output waveguides 222 into which the wavelength-multiplexed light is demultiplexed and output for each wavelength, connected to the end of the second slab waveguide 12 opposite to the array waveguide 13.

Description

Technical Field

[0001] The present invention relates to an arrayed waveguide grating and an optical multiplexer / demultiplexer device.

Background Art

[0002] An arrayed waveguide grating (AWG) is a device that integrates a plurality of optical waveguides with different optical path lengths on a planar substrate such as silicon or quartz, and multiplexes and demultiplexes light for each wavelength by utilizing the phase difference due to the difference in optical path length. For the purpose of reducing the mounting area of a circuit and achieving miniaturization, an arrayed waveguide grating has been proposed in which a plurality of optical waveguides are formed on a substrate and are stacked and configured (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Silicon chips, in particular, exhibit a strong polarization dependence. Therefore, conventionally, it was necessary to separate unpolarized incident light into TE (Transverse Electric) waves and TM (Transverse Magnetic) waves on the silicon chip and use array waveguide diffraction gratings designed for each polarization. In other words, conventionally, two physically different types of array waveguide diffraction gratings were required. For example, the technology disclosed in Patent Document 1 still fails to solve this problem and requires two array waveguide diffraction gratings.

[0006] This disclosure has been made in view of these circumstances, and its purpose is to provide a technology that can photomultiply and demultiply unpolarized incident light using a single array waveguide diffraction grating. [Means for solving the problem]

[0007] To solve the above problems, an array waveguide diffraction grating according to one aspect of the present invention is an array waveguide diffraction grating formed on a substrate, comprising: a first slab waveguide; a second slab waveguide; an array waveguide provided between the first slab waveguide and the second slab waveguide; one input waveguide connected to the end of the first slab waveguide opposite to the array waveguide, into which wavelength-multiplexed light is input and a plurality of output waveguides that output wavelengths after wavelength-multiplexed light is demultiplexed; and one input waveguide connected to the end of the second slab waveguide opposite to the array waveguide, into which wavelength-multiplexed light is input and a plurality of output waveguides that output wavelengths after wavelength-multiplexed light is demultiplexed.

[0008] Another aspect of the present disclosure is an optical multiplexer / demultiplexer. This device comprises the aforementioned array waveguide diffraction grating and a polarization demultiplexer element. The polarization demultiplexer element has one input port and two output ports, and separates unpolarized light input to the input port into TE-polarized light and TM-polarized light, converts the TM-polarized light into TE-polarized light, and then demultiplexes it into two TE-polarized lights which are output from each of the output ports. The input waveguides of the first slab waveguide and the second slab waveguide are connected to the respective output ports of the polarization demultiplexer element.

[0009] Another aspect of the present invention is a method for depolarizing unpolarized light using an array waveguide diffraction grating. The array waveguide diffraction grating used in this method comprises a first slab waveguide, a second slab waveguide, an array waveguide provided between the first and second slab waveguides, one input waveguide connected to the end of the first slab waveguide opposite the array waveguide for inputting wavelength-multiplexed light and a plurality of output waveguides for outputting wavelength-multiplexed light depolarized at each wavelength, and one input waveguide connected to the end of the second slab waveguide opposite the array waveguide for inputting wavelength-multiplexed light and a plurality of output waveguides for outputting wavelength-multiplexed light depolarized at each wavelength. This method includes the steps of inputting TE-polarized light from unpolarized light into the input waveguide of a first slab waveguide, and inputting light obtained by converting TM-polarized light from unpolarized light to TE-polarized light into the input waveguide of a second slab waveguide.

[0010] Furthermore, any combination of the above components, as well as conversions of the expression of the present invention between methods, apparatus, systems, recording media, computer programs, etc., are also valid embodiments of the present invention. [Effects of the Invention]

[0011] According to the present invention, unpolarized incident light can be photomultiplied and demultiplied using a single array waveguide diffraction grating. [Brief explanation of the drawing]

[0012] [Figure 1]This is a plan view of an array waveguide diffraction grating according to the first embodiment. [Figure 2] This is an enlarged view of the area around the first slab waveguide in Figure 1. [Figure 3] This is an enlarged view of the area around the second slab waveguide in Figure 1. [Figure 4] This figure shows an example of the arrangement of input waveguides and output waveguides connected to the first slab waveguide. [Figure 5] This figure shows an example of the arrangement of input waveguides and output waveguides connected to the first slab waveguide. [Figure 6] This is a plan view of a photomultiplier / demultiplier device according to a second embodiment. [Figure 7] This is a schematic diagram of a polarization separation element for wave descaling. [Figure 8] This is a schematic diagram of a polarization separation element for wave multiplexing. [Figure 9] This is a flowchart showing the processing procedure of the method according to the third embodiment. [Modes for carrying out the invention]

[0013] The present invention will be described below with reference to the drawings, based on preferred embodiments. In embodiments and modifications, the same or equivalent components and parts will be denoted by the same reference numerals, and redundant explanations will be omitted as appropriate. In addition, the dimensions of parts in each drawing will be enlarged or reduced as appropriate to facilitate understanding. Furthermore, some elements that are not important for explaining the embodiments will be omitted in each drawing. In addition, terms including ordinal numbers such as "first," "second," etc., are used to describe various components, but these terms are used only to distinguish one component from others, and the components are not limited by these terms.

[0014] In the following description, all array waveguide gratings are assumed to be designed to operate in accordance with TE-polarized light.

[0015] [First Embodiment] Referring to FIGS. 1 and 2, the configuration of the arrayed waveguide grating according to the first embodiment will be described. FIG. 1 is a plan view of the arrayed waveguide grating 1 according to the first embodiment. The arrayed waveguide grating 1 is formed by integrating a large number of waveguides with different patterns on a substrate 10 such as silicon or quartz. The arrayed waveguide grating 1 includes a first slab waveguide 11, a second slab waveguide 12, an arrayed waveguide 13, one input waveguide 211 connected to the first slab waveguide 11, a plurality of output waveguides 221 connected to the first slab waveguide 11, one input waveguide 212 connected to the second slab waveguide 12, and a plurality of output waveguides 222 connected to the second slab waveguide 12. The arrayed waveguide 13 is provided between the first slab waveguide 11 and the second slab waveguide 12.

[0016] FIG. 2 is an enlarged view of the periphery of the first slab waveguide 11 in FIG. 1. The input waveguide 211 and the plurality of output waveguides 221 are connected to the end of the first slab waveguide 11 on the side opposite to the arrayed waveguide 13. The input waveguide 211 is provided between the output waveguides 221.

[0017] The first slab waveguide 11 is formed by optically coupling one end to the arrayed waveguide 13 and the other end to the input waveguide 211 and the plurality of output waveguides 221. The first slab waveguide 11 has a flat core shape such that the transmitted light spreads radially in the horizontal direction of the substrate 10.

[0018] FIG. 3 is an enlarged view of the periphery of the second slab waveguide 12 in FIG. 1. The input waveguide 21 and the plurality of output waveguides 222 are connected to the end of the second slab waveguide 12 on the side opposite to the arrayed waveguide 13'. The input waveguide 212 is provided between the output waveguides 222.

[0019] The second slab waveguide 12 is formed by optically coupling one end to the arrayed waveguide 13 and the other end to the input waveguide 212 and the plurality of output waveguides 222. The second slab waveguide 12 has a flat core shape such that the transmitted light spreads radially in the horizontal direction of the substrate 10.

[0020] The array waveguide 13 is formed from multiple optical waveguides and connects the first slab waveguide 11 and the second slab waveguide 12. The optical path lengths of each optical waveguide forming the array waveguide 13 generally differ by ΔL. This ΔL is not necessarily limited to a constant value, and in some cases, a correction is applied to ΔL in order to broaden the bandwidth of the demultiplexing characteristics.

[0021] The first slab waveguide 11 and the second slab waveguide 12, which are connected to the array waveguide 13, are basically arranged symmetrically with respect to the center of the array waveguide 13.

[0022] The first slab waveguide 11 and its connected input waveguide 211 and output waveguide 221, the second slab waveguide 12 and its connected input waveguide 212 and output waveguide 222, and the array waveguide 13 are each formed based on multi-beam diffraction interference that exhibits a summing and desaturation function, with a horizontal connection position relative to the substrate 10.

[0023] Next, the demultiplexing operation by the array waveguide diffraction grating 1 will be explained. In the following explanation, we will take the example of a case where a combined light MLn, obtained by wavelength multiplexing n types of light with wavelengths λ1, λ2, ..., λn, is input to the input waveguide 211 of the first slab waveguide 11, and a combined light Lm, obtained by wavelength multiplexing m types of light with wavelengths λ1, λ2, ..., λm, is input to the input waveguide 212 of the second slab waveguide 12. In this case, as shown below, the array waveguide diffraction grating 1 demultiplexes the combined light MLn by wavelength and outputs it from the output waveguide 222 of the second slab waveguide 12, and demultiplexes the combined light Lm by wavelength and outputs it from the output waveguide 221 of the first slab waveguide 11.

[0024] Wavelength-multiplexed multiplexed light MLn is input to the input waveguide 211 of the first slab waveguide 11. The input multiplexed light MLn is output into the first slab waveguide 11. The multiplexed light MLn output into the first slab waveguide 11 spreads radially and is input into each optical waveguide of the array waveguide 13. As the multiplexed light MLn propagates through the array waveguide 13, which is given a predetermined waveguide length difference ΔL, it is emitted into the second slab waveguide 12 with a different phase. The multiplexed light MLn emitted into the second slab waveguide 12 undergoes multi-beam diffraction interference and is input into the output waveguides 222, which are positioned at the focal points of each demultiplexed wavelength. The wavelength-multiplexed multiplexed light MLn in this way is demultiplexed by wavelength and output from the output waveguide 222 of the second slab waveguide 12.

[0025] Similarly, wavelength-multiplexed multiplexed light Lm is input to the input waveguide 212 of the second slab waveguide 12. The input multiplexed light Lm is output into the second slab waveguide 12. The multiplexed light Lm output into the second slab waveguide 12 spreads radially and is input into each optical waveguide of the array waveguide 13. As the multiplexed light Lm propagates through the array waveguide 13, which is given a predetermined waveguide length difference ΔL, it is emitted into the first slab waveguide 11 with a different phase. The multiplexed light Lm emitted into the first slab waveguide 11 undergoes multi-beam diffraction interference and is input into the output waveguides 221 located at the focal points of each demultiplexed wavelength. The wavelength-multiplexed multiplexed light Lm in this way is demultiplexed by wavelength and output from the output waveguide 221 of the first slab waveguide 11.

[0026] Here, we explained using the descaling operation as an example, but by processing in the exact opposite direction, the combined wave operation is also possible.

[0027] As described above, according to this embodiment, a single array waveguide diffraction grating 1 can be used to separate and output two wavelength-based combined light MLn and combined light MLm, wavelength by wavelength.

[0028] Figure 4 shows an example of the arrangement of input waveguides 211 and output waveguides 221 connected to the first slab waveguide 11. In this example, the output waveguides 221 are arranged on both sides of the input waveguide 211. Six types of light—λ1=1530nm, λ2=1534nm, λ3=1538nm, λ4=1542nm, λ5=1546nm, and λ6=1550nm—are output from the output waveguides 221 after being decoupled by wavelength. However, the group of output waveguides 221 on the left side of the input waveguide 211 outputs light decoupled by m-th order diffraction, while the group of output waveguides 221 on the right side of the input waveguide 211 outputs light decoupled by m+1-th order diffraction. Thus, the input waveguide 211 is positioned so as not to overlap with either the group of output waveguides output by light delimited by m-th order diffraction, or the group of output waveguides output by light delimited by m+1-th order diffraction.

[0029] Figure 5 shows the same arrangement of input waveguides 211 and output waveguides 221 as in Figure 4. Here, as in Figure 4, six types of light—λ1=1530nm, λ2=1534nm, λ3=1538nm, λ4=1542nm, λ5=1546nm, and λ6=1550nm—are separated and output according to their wavelengths. However, in this example, the group of output waveguides 221 to the left of input waveguide 211 outputs light separated by m-1 order diffraction, and the group of output waveguides 221 to the right of input waveguide 211 outputs light separated by m order diffraction. Therefore, input waveguide 211 is positioned so as not to overlap with either the group of output waveguides that output light separated by m-1 order diffraction or the group of output waveguides that output light separated by m order diffraction.

[0030] As shown in the examples in Figures 4 and 5, the input waveguide is positioned so as not to overlap with the group of output waveguides output by light delimited by diffraction of the same order. This prevents light output from the output waveguide from entering the input waveguide, thus preventing a decrease in the signal-to-noise ratio of photomultiplication and demultiplication.

[0031] [Second Embodiment] Referring to Figures 6 to 8, the configuration of the photomultiplier / demultiplier according to the second embodiment will be described. Figure 6 is a plan view of the photomultiplier / demultiplier 2 according to the second embodiment. The photomultiplier / demultiplier 2 comprises the array waveguide diffraction grating 1 of the above-described embodiment, one polarization separation element 30 for demultiplier, and a plurality of polarization separation elements 40 for multiplexing.

[0032] The array waveguide diffraction grating 1 is the same as the one described in the first embodiment, so a detailed explanation will be omitted.

[0033] The polarization separation element 30 for depolarization has one input port 31 and two output ports 32. The polarization separation element 30 depolarizes the light containing TE-polarized light and TM-polarized light (hereinafter referred to as "unpolarized light") input to the input port 31 into two TE-polarized lights and outputs them from each of the output ports 32. As will be described later, the TE-polarized light output from one output port 32 originates from the TE-polarized light of the unpolarized light input to the input port 31, and the TE-polarized light output from the other output port 32 originates from the TM-polarized light of the unpolarized light input to the input port 31.

[0034] The polarization separation element 40 for wave multiplexing includes two input ports 41 and one output port 42. The polarization separation element 40 combines the TE-polarized light input to each of the input ports 41 into unpolarized light and outputs it from the output port 42.

[0035] The input waveguide 211 of the first slab waveguide 11 of the array waveguide grating 1 is connected to one output port 32 of the polarization separation element 30. Similarly, the input waveguide 212 of the second slab waveguide 12 is connected to the other output port 32 of the polarization separation element.

[0036] Each output waveguide 221 of the first slab waveguide 11 of the array waveguide grating 1 is connected to one of the input ports 41 of the corresponding multiplexing polarization separation element 40. Each output waveguide 222 of the second slab waveguide 12 is connected to the other of the input ports 41 of the corresponding multiplexing polarization separation element 40.

[0037] Figure 7 is a schematic diagram of the polarization separation element 30. The polarization separation element 30 has one input port 31 and two output ports 32. When unpolarized light (light containing TE-polarized light and TM-polarized light) is input to the input port 31 of the polarization separation element 30, it separates it into two TE-polarized lights and outputs them from the two output ports 32. In other words, the polarization separation element 30 has both the function of converting TM-polarized light to TE-polarized light (i.e., the function of a half-wavelength version that gives a phase difference π to the vertical polarization component) and the function of separating the light beam into two orthogonal polarizations (i.e., the function of a polarization beam splitter). The TE-polarized light output from one of the output ports 32 is the TE-polarized light that was originally included in the unpolarized light (hereinafter referred to as "TE-derived TE-polarized light"). On the other hand, the TE-polarized light output from one of the output ports 32 is light in which TM-polarized light contained in the unpolarized light has been converted into TE-polarized light (hereinafter referred to as "TM-derived TE-polarized light"). In this way, when unpolarized light is input to the input port 31, the polarization separation element 30 outputs TE-derived TM-polarized light from one output port 32 and TM-derived TM-polarized light from the other output port 32. A device having such a function is disclosed, for example, in Non-Patent Document 1.

[0038] Figure 8 is a schematic diagram of the polarization separation element 40 for multiplexing. The polarization separation element 40 has two input ports 41 and one output port 42. When TE-polarized light is input to each of the input ports 41, the polarization separation element 40 separates it into unpolarized light and outputs it from the output port 42. The polarization separation element 40 can use the same elements as the polarization separation element 30 for depolarization. In other words, the polarization separation element 40 may be the same as the polarization separation element 30 for depolarization connected in the reverse direction.

[0039] Next, with reference to Figure 6, the demultiplexing operation by the optical multiplexer 2 will be explained. In the following explanation, we will take the example of a case where unpolarized light (combined light) ML3, which is obtained by wavelength multiplexing three types of light with wavelengths λ1, λ2, and λ3, is input to the input port 31 of the polarization separation element 30 for demultiplexing.

[0040] The unpolarized light ML3 input to the input port 31 of the polarization separation element 30 is separated by the polarization separation element 30. As a result, TE-polarized light originating from TE is output from one output port 32, and TE-polarized light originating from TM is output from the other output port 32.

[0041] The TE-polarized light originating from one output port 32 is input to the input waveguide 211 of the first slab waveguide 11 of the array waveguide diffraction grating 1. As described in the first embodiment, this TE-polarized light (combined light) originating from the TE passes through the array waveguide 13 and then through the second slab waveguide 12, where it is decoupled by wavelength and output from the output waveguide 222 of the second slab waveguide 12. That is, decoupled light of wavelengths λ1, λ2, and λ3 is output from the three second slab waveguides 12, respectively.

[0042] The demultiplexed light signals λ1, λ2, and λ3 are each input to one of the corresponding input ports 41 of the polarization separation element 40 through the waveguide.

[0043] Now, the TE-polarized light originating from the TM output from the other output port 32 is input to the input waveguide 212 of the second slab waveguide 12 of the array waveguide diffraction grating 1. This TE-polarized light (combined light) originating from the TM passes through the array waveguide 13 and then through the first slab waveguide 11, where it is decoupled by wavelength and output from the output waveguide 221 of the first slab waveguide 11. In other words, decoupled light of wavelengths λ1, λ2, and λ3 is output from the three first slab waveguides 11, respectively.

[0044] The demultiplexed light signals λ1, λ2, and λ3 are each input to the other input port 41 of the corresponding polarization separation element 40 through the waveguide.

[0045] Each of the polarization separation elements 40 for wave combining combines the TE-polarized light (deselected light) originating from the TE and the TE-polarized light (deselected light) originating from the TM that are input to the input port 41, and outputs unpolarized light from the output port 42.

[0046] Here, we explained using the descaling operation as an example, but by processing in the exact opposite direction, the combined wave operation is also possible.

[0047] As described above, according to this embodiment, wavelength-multiplexed unpolarized light can be separated wavelength by wavelength using an optical multiplexing / demultiplexing device equipped with a single array waveguide diffraction grating.

[0048] In the above embodiment, the optical multiplexer / demultiplexer 2 was equipped with both a polarization separation element 30 for demultiplexing and a polarization separation element 40 for multiplexing. However, if it is sufficient to convert wavelength-multiplexed unpolarized light into two TE-polarized lights and then wavelength-separate each of the TE-polarized lights (i.e., it is not necessary to convert the two wavelength-separated TE-polarized lights back into unpolarized light), then the optical multiplexer / demultiplexer 2 does not need to be equipped with a polarization separation element 40 for multiplexing.

[0049] Preferably, the distance from the output waveguide 221 of the first slab waveguide 11 to the polarization separation element 40 for multiplexing is equal to the distance from the corresponding output waveguide 222 of the second slab waveguide 12 to the polarization separation element 40 for multiplexing. By adopting this configuration, when the TE-polarized light originating from TE output from output waveguide 221 and the TE-polarized light originating from TM output from output waveguide 222 reach the polarization separation element 40 for multiplexing, their phases are aligned, thus preventing a decrease in the signal-to-noise ratio of the optical multiplexing and demultiplexing.

[0050] To make the distance from the output waveguide 221 of the first slab waveguide 11 to the polarization separation element 40 equal to the distance from the corresponding output waveguide 222 of the second slab waveguide 12 to the polarization separation element 40, one of the connections may be made into meander connections, for example, as shown in Figure 6. However, the method is not limited to this, and any suitable connection method may be used to make the distances equal.

[0051] [Third Embodiment] Referring to Figure 9, a method for depolarizing unpolarized light using an array waveguide diffraction grating according to a third embodiment will be described. Figure 3 is a flowchart showing the processing steps of the method according to the third embodiment. This method includes steps S1 and S2.

[0052] The array waveguide diffraction grating used in this method is the same as the array waveguide diffraction grating 1 described in the first embodiment. That is, this array waveguide diffraction grating comprises a first slab waveguide, a second slab waveguide, an array waveguide provided between the first slab waveguide and the second slab waveguide, one input waveguide connected to the end of the first slab waveguide opposite the array waveguide for inputting wavelength-multiplexed light and a plurality of output waveguides for outputting wavelength-multiplexed light after it has been demultiplexed, and one input waveguide connected to the end of the second slab waveguide opposite the array waveguide for inputting wavelength-multiplexed light and a plurality of output waveguides for outputting wavelength-multiplexed light after it has been demultiplexed.

[0053] In this method, in step S1, TE-polarized light from unpolarized light is input to the input waveguide of the first slab waveguide.

[0054] In this method, in step S2, light obtained by converting TM-polarized light within unpolarized light to TE-polarized light is input to the input waveguide of the second slab waveguide.

[0055] To convert TM-polarized light within unpolarized light into TE-polarized light, the polarization separation element 30 for depolarization shown in Figure 7 may be used.

[0056] The present invention has been described above based on embodiments. The embodiments are illustrative, and it will be understood by those skilled in the art that various modifications are possible in combinations of their components and processing processes, and that such modifications also fall within the scope of the present invention. [Explanation of Symbols]

[0057] 1...Array waveguide diffraction grating, 10...Substrate, 11...First slab waveguide, 12...Second slab waveguide, 13...Array waveguide, 211...Input waveguide, 212...Input waveguide, 221...Output waveguide, 222...Output waveguide, 30...Deselector polarization separator, 31...Input port, 32...Output port, 40...Combiner polarization separator, 41...Input port, 42...Output port, S1...Step of inputting TE-polarized light from unpolarized light into the input waveguide of the first slab waveguide, S2...Step of inputting light obtained by converting TM-polarized light from unpolarized light to TE-polarized light into the input waveguide of the first slab waveguide.

Claims

1. An array waveguide diffraction grating formed on a substrate, The first slab waveguide and The second slab waveguide, An array waveguide provided between the first slab waveguide and the second slab waveguide, A single input waveguide into which wavelength-multiplexed light is input, and a plurality of output waveguides into which wavelength-multiplexed light is separated and output for each wavelength, are connected to the end of the first slab waveguide opposite to the array waveguide. Connected to the end of the second slab waveguide opposite to the array waveguide, one input waveguide into which wavelength-multiplexed light is input, and a plurality of output waveguides into which wavelength-multiplexed light is demultiplexed and output for each wavelength, An array waveguide diffraction grating characterized by comprising the following features.

2. The array waveguide diffraction grating according to claim 1, characterized in that the input waveguide is provided at a position that does not overlap with the group of output waveguides output by light separated by diffraction of the same order.

3. The array waveguide diffraction grating according to claim 1 or 2, One polarization separation element for descaling, A photomultiplier / demultiplier device equipped with, The aforementioned polarization separation element for depolarization comprises one input port and two output ports, and depolarizes unpolarized light input to the input port into two TE-polarized lights and outputs them from each of the output ports. The photomultiplier / demultiplier device is characterized in that the input waveguides of the first slab waveguide and the second slab waveguide are each connected to the respective output ports of the polarization separation element for demultiplier.

4. It further comprises multiple polarization separation elements for multiplexing, The polarization separation element for wave multiplexing comprises two input ports and one output port, and combines the TE-polarized light input to each of the input ports into unpolarized light and outputs it from the output port. The optical multiplexer / demultiplexer according to claim 3, characterized in that the output waveguides of the first slab waveguide and the second slab waveguide are each connected to the corresponding input ports of the polarization separation element for multiplexing.

5. The optical multiplexing and demultiplexing device according to claim 4, characterized in that the distance from the output waveguide of the first slab waveguide to the polarization separation element for multiplexing is equal to the distance from the corresponding output waveguide of the second slab waveguide to the polarization separation element for multiplexing.

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