Optical waveguide element

Abstract

To achieve wavelength separation independently of polarized waves using as few elements as possible, without the need for separately preparing an element to separate polarized waves.SOLUTION: An optical waveguide core comprises a rib waveguide extending in the lengthwise direction and having a relatively larger thickness, and a slab waveguide core on both sides of the rib waveguide and having a relatively smaller thickness. A grating asymmetrical to the center axis of the rib waveguide is provided on either side of the rib waveguide. At a point where one side surface of the rib waveguide is not carved, there is a region on one end face side where the slab waveguide is nonexistent, and at a point where one side surface of the rib waveguide is carved, the slab waveguide exists on one end face side. At a point where the other side surface of the rib waveguide is not carved, there is a region on the other end face side where the slab waveguide is nonexistent, and at a point where the other side surface of the rib waveguide is carved, the slab waveguide exists on the other end face side.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical waveguide element, for example, an optical waveguide element applicable to an element that multiplexes and demultiplexes light for transmission using a plurality of different wavelengths through a single optical fiber. [Background technology]

[0002] In recent years, passive optical networks (PONs) have become mainstream as subscriber optical access systems. In a PON, one central office device (OLT: Optical Line Terminal) and multiple subscriber devices (ONUs: Optical Network Units) are connected via optical fibers and star couplers, and multiple ONUs share one OLT. In a PON, the optical signal wavelengths used for downstream communication are different from those used for upstream communication to prevent mutual interference between downstream communication from the OLT to the ONU and upstream communication from the ONU to the OLT.

[0003] Therefore, a multiplexing / demultiplexing element is required to separate and multiplex the optical signals with different wavelengths used for downstream and upstream communications. In general, OLTs and ONUs are configured by spatially coupling optical wavelength filters, photodiodes (PD), and laser diodes (LD) as multiplexing / demultiplexing elements to realize the function of transmitting and receiving optical signals with different wavelengths.

[0004] In order to achieve spatial coupling, alignment work is required to align the optical axes between the optical wavelength filter, PD, and LD. In response to this, optical wavelength filters that are configured using waveguides have been developed to eliminate the need for this optical axis alignment work. In addition, when forming these optical wavelength filters, silicon (Si) waveguides that use silicon-based materials as the waveguide material have attracted attention because of their compactness and ease of mass production (see, for example, Patent Document 1).

[0005] In a Si waveguide, the optical waveguide core, which is essentially the optical transmission path, is formed from Si. The optical waveguide core is then covered with a cladding made of a material with a lower refractive index than Si, such as silica. This configuration makes the difference in refractive index between the optical waveguide core and the cladding extremely large, allowing light to be tightly confined within the optical waveguide core. As a result, a small curved waveguide with a bending radius as small as, for example, 1 μm can be realized. This makes it possible to create optical circuits of the same size as electronic circuits, which is advantageous for miniaturizing the entire optical device.

[0006] In addition, the manufacturing process for semiconductor devices such as CMOS (Complementary Metal Oxide Semiconductor) can be used for Si waveguides, which is expected to lead to the realization of photonics-electronics convergence (silicon photonics), in which electronic and optical functional circuits are formed simultaneously on a chip.

[0007] In a PON that uses wavelength division multiplexing (WDM) technology, a different receiving wavelength is assigned to each ONU. The OLT generates downstream optical signals for each ONU at a transmission wavelength corresponding to the receiving wavelength of the destination ONU, and multiplexes and transmits these. Each ONU selectively receives an optical signal with its assigned receiving wavelength from downstream optical signals multiplexed with multiple wavelengths. In the ONU, optical wavelength filters are used to selectively receive downstream optical signals with each receiving wavelength. Technology has been realized to configure optical wavelength filters using the above-mentioned Si waveguides.

[0008] Optical wavelength filters using Si waveguides include, for example, those using Mach-Zehnder interferometers and those using arrayed waveguide gratings (AWGs). In addition, optical wavelength filters using Si waveguides include ring resonator, grating, and directional coupler type tunable wavelength filters that can tune the output wavelength and have the advantage of being easy to use due to their simple element structure (see, for example, Patent Document 2). However, all of these optical wavelength filters operate only with a specific polarized wave.

[0009] Furthermore, in order to accommodate both TE (Transverse Electric) polarized waves and TM (Transverse Magnetic) polarized waves, there is a structure in which a polarization separation element and a polarization rotation element are provided in front of the optical wavelength filter (see, for example, Non-Patent Document 1). [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Pat. No. 4,860,294 [Patent Document 2] JP 2003-215515 A [Non-patent literature]

[0011] [Non-Patent Document 1] Optics Express vol.20, No.26, p.B493-B500, December 10, 2012 Summary of the Invention [Problem to be solved by the invention]

[0012] As described above, AWGs and tunable wavelength filters have polarization dependence. Therefore, in order to use them in a polarization-independent manner, it is necessary to align the polarization using, for example, the polarization separation element and polarization rotation element described above.

[0013] The present invention has been made in consideration of the problems of the conventional techniques described above, and an object of the present invention is to provide an optical waveguide element that realizes polarization-independent wavelength separation with as few elements as possible, without the need for a separate element for separating polarization. [Means for solving the problem]

[0014] In order to achieve the above object, the optical waveguide element of the present invention comprises a support substrate, a cladding formed on the support substrate, and an optical waveguide core embedded in the cladding and provided parallel to the upper surface of the support substrate. The optical waveguide core comprises a rib waveguide extending in the length direction and having a relatively large thickness, and slab waveguides having a relatively small thickness on both sides of the rib waveguide. Gratings are provided on both sides of the rib waveguide that are antisymmetric with respect to the central axis of the rib waveguide.

[0015] Here, where one side of the rib waveguide is not carved, there is an area on one end face where the slab waveguide is not present, and where one side of the rib waveguide is carved, there is a slab waveguide on one end face, and where the other side of the rib waveguide is not carved, there is an area on the other end face where the slab waveguide is not present, and where the other side of the rib waveguide is carved, there is a slab waveguide on the other end face.

[0016] According to a preferred embodiment of the optical waveguide element described above, the slab waveguide is provided with a grating that is antisymmetric with respect to the central axis of the rib waveguide, and the grating provided in the slab waveguide is provided with the same period as that of the grating provided in the rib waveguide, but shifted by half a period.

[0017] According to a preferred embodiment of the optical waveguide element described above, where one side of the rib waveguide is not carved, an opening is provided in the slab waveguide on one end face side, and where the other side of the rib waveguide is not carved, an opening is provided in the slab waveguide on the other end face side.

[0018] Here, the grating provided on the rib waveguide preferably has a sinusoidal structure. Effect of the Invention

[0019] According to the optical waveguide element of the present invention, polarization-independent wavelength separation can be realized with a small number of elements, without the need for a separate element for separating polarization. [Brief description of the drawings]

[0020] [Figure 1] FIG. 2 is a schematic plan view of a first element. [Diagram 2] FIG. 2 is a schematic cross-sectional end view of a first element. [Diagram 3] FIG. 1 is a diagram for explaining the characteristics of the first element (1). [Figure 4] FIG. 2 is a diagram illustrating the characteristics of the first element. [Diagram 5] FIG. 4 is a schematic diagram of a second element. [Figure 6] FIG. 4 is a diagram for explaining characteristics of a second element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, an embodiment of the present invention will be described with reference to the drawings, but the shape, size and positional relationship of each component are merely shown in a schematic manner to allow the present invention to be understood. In addition, a preferred configuration example of the present invention will be described below, but the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiment, and many changes or modifications can be made that can achieve the effects of the present invention without departing from the scope of the configuration of the present invention. In each figure, hatching may be applied even in a plan view to facilitate understanding. Hatching may be omitted in a view showing a cut end surface.

[0022] In the following explanation and in each figure, an example in which the grating width and period are constant is shown, but this is a schematic representation of the grating configuration. When actually designing and manufacturing an optical wavelength filter, the grating width and period do not necessarily need to be constant.

[0023] (Configuration of the first element) An optical waveguide element (hereinafter also referred to as the first element) according to a first embodiment of the present invention will be described with reference to Figs. 1 and 2. Fig. 1 is a schematic plan view of the first element. Fig. 1 shows the planar shape of an optical waveguide core, with other components omitted. Figs. 2(A) and (B) are schematic cross-sectional end views of the first element. Fig. 2(A) shows a surface cut along line AA in Fig. 1, and Fig. 2(B) shows a surface cut along line BB in Fig. 1.

[0024] In the following description, the direction along the light propagation direction of each component is defined as the length direction, the direction perpendicular to the upper surface of the support substrate is defined as the thickness direction, and the direction perpendicular to the length direction and thickness direction is defined as the width direction.

[0025] The first element is configured to include a support substrate 10, a cladding 20, and an optical waveguide core 30. The first element can be used as an optical wavelength filter.

[0026] The support substrate 10 is formed in the form of a flat plate made of, for example, single crystal Si.

[0027] The cladding 20 is formed on the supporting substrate 10 so as to cover the upper surface of the supporting substrate 10 . The cladding 20 is formed from, for example, silicon oxide (SiO2).

[0028] The optical waveguide core 30 is embedded in the cladding 20 parallel to the upper surface of the support substrate 10. The optical waveguide core 30 is formed of a material such as silicon (Si) that has a higher refractive index (3.5) than the refractive index (1.45) of the cladding 20 made of SiO2. As a result, the optical waveguide core 30 functions as an optical transmission path, and light input to the optical waveguide core 30 propagates in a propagation direction according to the planar shape of the optical waveguide core 30.

[0029] The thickness of the optical waveguide core 30 is desirably 200 to 400 nm, which is a value that can achieve a single mode condition in the depth direction.

[0030] Here, in order to prevent the light propagating through the optical waveguide core 30 from escaping to the supporting substrate 10, the optical waveguide core 30 is preferably formed at a distance of at least 1 μm from the supporting substrate 10.

[0031] This first element can be easily manufactured by using, for example, an SOI (Silicon On Insulator) substrate. An example of a method for manufacturing the first element shown in Figs.

[0032] First, an SOI substrate is prepared in which a support substrate layer, a SiO2 layer, and a Si layer are sequentially stacked. Next, for example, dry etching is performed to pattern the Si layer. As a result, a structure can be obtained in which a SiO2 layer is stacked on a support substrate layer as a support substrate 10, and an optical waveguide core 30 is formed on the SiO2 layer. As described later, in this first element, the cross-sectional shape of the optical waveguide core 30 has a rib structure. That is, the optical waveguide core 30 has a thick portion and a thin portion. In this case, the above-mentioned dry etching can be performed in two stages. The first dry etching is completed in a state where the Si layer remains. Then, the second dry etching is completed when the Si layer is removed and the SiO2 layer is reached.

[0033] Although an example of a Si waveguide has been described here, it is also possible to realize the waveguide using a compound semiconductor.

[0034] The first element includes a rib waveguide 100 and a slab waveguide 200 as part of the optical waveguide core 30. That is, the optical waveguide core 30 is a waveguide with a rib structure.

[0035] Here, the thicker portion of the waveguide is referred to as the rib waveguide 100, and the thinner portion of the waveguide is referred to as the slab waveguide 200. The rib waveguide 100 and the slab waveguide 200 extend in the length direction. The slab waveguides 200 are provided on both sides of the rib waveguide 100. When viewed in a plane perpendicular to the length direction, the rib waveguide 100 is sandwiched between the slab waveguides 200.

[0036] Gratings are formed on both side surfaces of the rib waveguide 100. The gratings provided on both side surfaces of the rib waveguide 100 have the same period and have an antisymmetric structure with respect to an imaginary central axis I extending in the length direction. The central axis I is located at the center of the rib waveguide 100 in the width direction.

[0037] A location 142 on the other side 112 side across the central axis I of a location 131 where one side 111 of the rib waveguide 100 is carved is not carved, and a location 141 on the other side 112 side across the central axis I of a location 132 where one side 111 is not carved is carved.

[0038] Gratings are formed on both side surfaces of the slab waveguide 200, similar to the rib waveguide 100. The gratings provided on both side surfaces of the slab waveguide 200 have the same period and have an antisymmetric structure with respect to the central axis I. That is, a location 241 on the other side surface side across the central axis I of a location 231 where one side surface side of the slab waveguide 200 is carved is not carved, and a location 242 on the other side surface side across the central axis I of a location 232 where one side surface side of the slab waveguide 200 is not carved is carved.

[0039] Moreover, the grating provided in the slab waveguide 200 and the grating provided in the rib waveguide 100 have the same period.

[0040] Furthermore, at a location 131 where one side 111 of the rib waveguide 100 is carved, a location 232 on the same side of the slab waveguide 200 is not carved, and at a location 132 where one side 111 of the rib waveguide 100 is not carved, a location 231 on the same side of the slab waveguide 200 is carved.

[0041] Therefore, in this first element, the grating provided in the rib waveguide 100 and the grating provided in the slab waveguide 200 are both antisymmetric with respect to the central axis I, and on the same side, the grating provided in the rib waveguide 100 and the grating provided in the slab waveguide 200 are shifted inversely in period.

[0042] As a result, at locations 101 where one side 111 of the rib waveguide 100 is carved, a slab waveguide 200 is present, but at locations 102 where one side 111 of the rib waveguide 100 is not carved, a slab waveguide is not present.

[0043] It is preferable that the grating provided in the slab waveguide 200 has a rectangular wave structure, and the grating provided in the rib waveguide 100 has a sinusoidal wave structure. Here, the sinusoidal wave structure can refer to a structure in which the corners are rounded, as opposed to a rectangular wave or triangular wave structure.

[0044] (Operation of the first element) The operation of this first element will now be described.

[0045] Light in the TM fundamental mode input to one end 121 of the rib waveguide 100 is polarization converted to the TE fundamental mode by the gratings of the rib waveguide 100 and the slab waveguide 200, and reflected, with the light having a specific wavelength. Light in the TE fundamental mode input to one end 121 of the rib waveguide 100 is polarization converted to the TM fundamental mode by the gratings of the rib waveguide 100 and the slab waveguide 200, and reflected, with the light having a specific wavelength. The polarization-converted and reflected light is output from one end 121 of the rib waveguide 100. On the other hand, light that is not polarization-converted is output from the other end 122 of the rib waveguide 100.

[0046] In order to convert between different polarizations, it is preferable that the waveguide structure is antisymmetric when viewed from the top and asymmetric in cross section. On the other hand, in a grating with an antisymmetric structure, the TE fundamental mode also converts into the TE first-order mode. In this first element, the conversion between the fundamental mode and the first-order mode in the grating of the rib waveguide 100 is cancelled by the conversion between the fundamental mode and the first-order mode in the grating of the slab waveguide 200, which is shifted by a half period from the grating of the rib waveguide 100.

[0047] (Characteristics evaluation of the first element) A simulation for evaluating the characteristics of the first element, which was performed using three-dimensional finite difference time domain (FDTD), will be described with reference to FIGS.

[0048] In Figures 3(A) and (B) and Figure 4, the horizontal axis indicates wavelength (μm) and the vertical axis indicates output intensity (dB). Figure 3(A) shows a conventional case where the slab waveguide does not include a grating, and Figure 3(B) shows a case where the slab waveguide of this first element includes a grating. Figure 4 shows a case where the grating provided on the rib waveguide has a rectangular wave shape, with the same structure as the first element.

[0049] In Figures 3(A) and (B) and Figure 4, the input light input to one end 121 of the rib waveguide 100 is shown by curve I, the reflected light output from one end 121 of the rib waveguide 100 is shown by curve II, and the transmitted light output from the other end 122 of the rib waveguide 100 is shown by curve III.

[0050] Here, the thickness and width of the rib waveguide 100 are 220 nm and 700 nm, respectively, and the thickness and width of the slab waveguide 200 are 145 nm and 1450 nm, respectively. The grating of the rib waveguide 100 is carved into 150 nm. This grating carving corresponds to the difference in the distance from the central axis I to the end face 111 or 112 on the slab waveguide 200 side, depending on whether or not the carving is present.

[0051] 3(A) and (B) show the results when fundamental mode TE polarized light is input as the input light.

[0052] 3A, a peak of the TM fundamental mode converted from the main TE fundamental mode at 1550 nm wavelength is seen in the reflected light (II) output from one end 121 of the rib waveguide 100. However, a deep dip caused by conversion to the first order mode is seen around 1680 nm on the long wavelength side of the transmitted light (III) exiting the other end 122 of the rib waveguide 100.

[0053] On the other hand, in FIG. 3(B), there is no deep dip on the long wavelength side of the transmitted light (III) exiting from the other end 122 of the rib waveguide 100, and the dip is suppressed to a slight one.

[0054] In this first element, this dip could be easily suppressed by adjusting the thickness of the slab waveguide 200. However, when the grating of the rib waveguide has a rectangular wave structure rather than a sinusoidal wave structure, the conversion to the first-order mode by the grating of the rib waveguide is too strong, making this adjustment difficult.

[0055] As shown in Figure 4, when the grating of the rib waveguide has a rectangular wave structure instead of a sinusoidal wave structure, the conversion to the TE first mode is suppressed, but the required conversion to the fundamental TM mode is also reduced.

[0056] In addition, the effective refractive index of the slab waveguide is reduced due to the presence of gaps in the slab waveguide caused by the grating, which further reduces the equivalent refractive index of the higher order modes. In the conventional technology, as shown in Fig. 3(A), a dip was observed on the short wavelength side of the main peak of the transmitted light (III) exiting from the other end 122 of the rib waveguide 100, but in this first element, as shown in Fig. 3(B), the dip on the short wavelength side has been eliminated.

[0057] If there are dips on the long wavelength side and the short wavelength side of the main peak, the wavelength band that can be used for wavelength multiplexing will be narrowed.

[0058] However, in the first element of the present invention, the dips that were previously observed on the long wavelength side and short wavelength side of the main peak are eliminated, making it possible to widen the wavelength range that can be used in wavelength multiplexing.

[0059] (Configuration of the second element) An optical waveguide element (hereinafter also referred to as the second element) according to a second embodiment of the present invention will be described with reference to Fig. 5. Fig. 5(A) is a schematic plan view showing the second element. Fig. 5(B) is a schematic cross-sectional end view of the second element. Fig. 5(A) shows the planar shape of an optical waveguide core, with other components omitted.

[0060] The second element differs from the first element in that an aperture row, rather than a grating, is provided in the slab waveguide 202. Other configurations are the same as those of the first element, so duplicated explanations may be omitted.

[0061] The rows of apertures in the second element are provided on both sides of the rib waveguide 100 along the length direction. The period of the rows of apertures is the same as the period of the grating of the rib waveguide. The apertures 210 are preferably provided at positions in contact with the side surfaces 111 or 112 of the rib waveguide 100. The apertures 210 are provided in places 132 where one side surface 111 of the rib waveguide 100 is not carved, and are not provided in places 131 where one side surface 111 of the rib waveguide 100 is carved.

[0062] As a result, in the vicinity of the rib waveguide 100, where the side of the rib waveguide 100 is carved, a slab waveguide 202 is present, but where the side of the rib waveguide 100 is not carved, a slab waveguide is not present.

[0063] (Characteristics evaluation of the second element) A simulation for evaluating the characteristics of the second element, which was performed using three-dimensional FDTD, will be described with reference to FIG.

[0064] 6, the horizontal axis represents wavelength (μm) and the vertical axis represents output intensity (dB). As in Fig. 3 and Fig. 4, the input light input to one end 121 of the rib waveguide 100 is shown by curve I, the reflected light output from one end 121 of the rib waveguide 100 is shown by curve II, and the transmitted light output from the other end 122 of the rib waveguide 100 is shown by curve III.

[0065] Here, the thickness and width of the rib waveguide are 220 nm and 700 nm, respectively, and the thickness and width of the slab waveguide are 100 nm and 1160 nm, respectively. The grating of the rib waveguide is dug to a depth of 150 nm. The length of the opening in the width direction is 150 nm.

[0066] As with the first element, the grating of the rib waveguide having a sinusoidal wave shape tends to have a higher manufacturing dimensional tolerance.

[0067] As shown in Figure 6, there is no deep dip on the long wavelength side of the main peak, just like the first element, and the dip is suppressed to a slight one. In other words, the same effect as the first element can be obtained. Also, compared to providing a rectangular wave grating in the slab waveguide of the first element, providing an aperture makes it difficult for the rectangular shape to become rounded, making it possible to maintain high diffraction efficiency.

[0068] In this way, the first element and the second element make it possible to realize polarization-independent wavelength separation with a small number of elements, without the need for a separate element for separating polarization. [Explanation of symbols]

[0069] 10 Support substrate 20 Clad 30 Optical waveguide core 100 Rib Waveguide 200 Slab Waveguide 210 Aperture

Claims

1. A support substrate; A cladding formed on the support substrate; an optical waveguide core embedded in the cladding and disposed parallel to an upper surface of the support substrate; The optical waveguide core extends in a longitudinal direction. A rib waveguide having a relatively large thickness and slab waveguides having a relatively small thickness on both sides of the rib waveguide, On both sides of the rib waveguide, gratings are provided that are antisymmetric with respect to a central axis of the rib waveguide; Where one side surface of the rib waveguide is not carved, there is a region on the side of the one end surface where no slab waveguide is present, and where one side surface of the rib waveguide is carved, there is a slab waveguide on the side of the one end surface, In a place where the other side surface of the rib waveguide is not engraved, there is a region where no slab waveguide is present on the other end surface side, and in a place where the other side surface of the rib waveguide is engraved, a slab waveguide is present on the other end surface side. Optical waveguide element.

2. the slab waveguide is provided with a grating that is antisymmetric with respect to the central axis; The grating provided in the slab waveguide has the same period as the grating provided in the rib waveguide, and is shifted by half a period.

2. The optical waveguide element according to claim 1.

3. an opening is provided in the slab waveguide on the one end face side in a place where the one side face of the rib waveguide is not engraved; In the place where the other side surface of the rib waveguide is not engraved, an opening is provided in the slab waveguide on the other end surface side.

2. The optical waveguide element according to claim 1.

4. The grating on the rib waveguide has a sinusoidal structure. The optical waveguide element according to any one of claims 1 to 3.

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