Optical waveguide element

Abstract

To provide an optical waveguide element that has a structure allowing easier fabrication than a topological photonic type element and yet has characteristics similar thereto, and has a function equal to or more than that of the topological photonic type element.SOLUTION: In the optical waveguide element, a first area, an intermediate area, and a second area are provided in series in this order in a lengthwise direction of a waveguide core. The waveguide core comprises: a plurality of openings disposed on a virtual line segment drawn at a center in a width direction of the waveguide core in the lengthwise direction of the waveguide core; a center slot coupling the plurality of openings; and a left slot and a right slot disposed in parallel with the center slot at an interval larger than widths of the openings. The first area and the second area are configured by the same shape and size as each other, and the plurality of openings are disposed at a certain period in the first area and the second area. The intermediate area has a different shape from the first area and the second area, and in the first area and the second area, lengths of the openings in the lengthwise direction are less than 1 / 2 of the certain period.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical waveguide element, and more particularly to an optical waveguide element that can be used as an element for detecting a change in refractive index around a waveguide or as an optical wavelength filter. [Background technology]

[0002] In recent years, silicon (Si) photonics has attracted attention as a platform technology for optical waveguide devices. Si photonics features include the compactness and integration of optical waveguides and related optical devices, such as modulators and photodetectors, achieved by utilizing semiconductor manufacturing processes such as CMOS (Complementary Metal Oxide Semiconductor), and high productivity achieved through 200mm or 300mm wafer processes that utilize existing semiconductor manufacturing technologies. Furthermore, Si waveguides, which use Si as the waveguide core and Si oxide (SiO2) cladding, have a relative refractive index difference of up to 40%, providing a high optical confinement effect. In particular, Si nanowire waveguides allow the radius of curvature of bent waveguides and the parallel wiring pitch to be reduced to the order of several microns, enabling the miniaturization of optical circuit layouts.

[0003] One application of Si waveguides is being considered: in devices that detect changes in the refractive index around the waveguide core. When Si is used as the waveguide material, it is possible to reduce the waveguide dimensions, as mentioned above. Furthermore, by using evanescent waves near the waveguide wall, it is possible to detect biological substances adsorbed on the surface of the waveguide core. Devices used to detect biological substances are known as biosensors.

[0004] Devices that use evanescent waves to detect biological materials can be classified into those that use optical resonance, such as ring resonators, and those that use interference, such as Mach-Zehnder types. One-dimensional photonic crystals have been reported to be advantageous for use as optical sensors, such as biosensors. One-dimensional photonic crystals are known to be an excellent technology for realizing narrow-band wavelength filters in applications such as optical communications.

[0005] Furthermore, as photonic crystal elements, elements to which topological photonics technology is applied (topological photonic elements) have recently appeared (see, for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] APL Photonics, Vol.6, p.086105, 2021 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the topological photonic element disclosed in Non-Patent Document 1 is simply an element serving as a resonator, and has not been considered as an element having other functions, such as a function as a biosensor.

[0008] Furthermore, topological photonic elements have a structure in which two or more openings are provided per period, resulting in a very fine structure that is difficult to fabricate.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical waveguide element that has a structure that is easier to fabricate than a topological photonic element, has equivalent characteristics, and has functions equal to or superior to those of a topological photonic element. [Means for solving the problem]

[0010] In order to achieve the above-mentioned object, the optical waveguide element of the present invention, which is formed by providing a waveguide core on a lower clad, is configured by providing a first region, an intermediate region, and a second region in series in this order in the longitudinal direction of the waveguide core.

[0011] The waveguide core includes a plurality of apertures arranged on an imaginary line drawn in the longitudinal direction of the waveguide core at the center of the width direction of the waveguide core, a central slot connecting the plurality of apertures on the imaginary line, and left and right slots arranged parallel to the central slot at intervals greater than the width of the apertures. The first region and the second region have the same shape and size, and the plurality of apertures are arranged at a regular interval in the first region and the second region. The intermediate region has a different shape from the first region and the second region, and the longitudinal length of the apertures in the first region and the second region is less than half of the regular interval.

[0012] In a preferred embodiment of the optical waveguide element of the present invention, the length of the openings in the first region and the second region is equal to or less than 1 / 4 of the constant period.

[0013] In a further preferred embodiment of the optical waveguide element of this invention, the first region is composed of a plurality of units each consisting of an opening and a central slot, and in each unit, the opening is located on the end side where light is input. The second region is composed of a plurality of units each consisting of an opening and a central slot, and in each unit, the opening is located on the end side where light is output. The intermediate region is composed of two adjacent openings and a central slot between the two openings, and the length of the central slot in the intermediate region is shorter than the length of the central slot that constitutes one unit of the first region and second region.

[0014] In one preferred embodiment of the optical waveguide element of the present invention, the two openings in the intermediate region are the same size as the openings that form one unit of the first and second regions. In another preferred embodiment of the optical waveguide element of the present invention, the area of ​​either one of the two openings in the intermediate region is smaller than the area of ​​the opening that forms one unit of the first and second regions.

[0015] Furthermore, the surface of the waveguide core on the lower cladding may be exposed to the substance to be measured.

[0016] Alternatively, an upper cladding covering the waveguide core may be provided on the lower cladding. [Effects of the Invention]

[0017] The optical waveguide element of the present invention has a structure that is easier to fabricate than a topological photonic element, has equivalent characteristics, and has functions equal to or greater than those of a topological photonic element. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is a schematic diagram illustrating a first optical waveguide element. [Figure 2] FIG. 4 is a schematic diagram illustrating a second optical waveguide element. [Figure 3] FIG. 10 is a diagram showing the results of a simulation performed using three-dimensional FDTD. [Figure 4] FIG. 4 is a diagram showing wavelength characteristics of a first optical waveguide element. [Figure 5] FIG. 10 is a diagram for explaining characteristics when the measurement substance absorbs light. [Figure 6] FIG. 10 is a schematic diagram showing the configuration of a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and positional relationship of each component are merely shown in a schematic manner to enable understanding of the present invention. Furthermore, while preferred configuration examples of the present invention will be described below, the materials and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes and modifications can be made that achieve the effects of the present invention without departing from the scope of the configuration of the present invention.

[0020] (First optical waveguide element) An optical waveguide element (hereinafter also referred to as a first optical waveguide element) according to a first embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a schematic diagram for explaining the first optical waveguide element. Fig. 1(A) is a schematic plan view of the first optical waveguide element, omitting a support substrate and a lower cladding (described later) and showing only the waveguide core. Fig. 1(B) is a diagram showing a cut end surface of the first optical waveguide element taken along line AA. Fig. 1(C) is a diagram showing a cut end surface of the first optical waveguide element taken along line BB.

[0021] The first optical waveguide element includes a lower clad 20 on a support substrate 10 and a waveguide core 30 on the lower clad 20. The first optical waveguide element can be easily manufactured by using, for example, a commercially available SOI (Silicon On Insulator) substrate. The SOI substrate is configured by sequentially stacking a support substrate layer, an SiO2 layer, and a Si layer, and the support substrate layer serves as the support substrate 10. The SiO2 layer serves as the lower clad 20. For example, the Si layer is patterned by dry etching until it reaches the lower clad 20, thereby forming the waveguide core 30.

[0022] To prevent light propagating through the waveguide core 30 from escaping to the support substrate 10, the distance between the support substrate 10 and the optical waveguide element 30, i.e., the thickness of the lower cladding 20, is preferably 1 μm or more. The thickness of the waveguide core 30 is preferably 200 to 400 nm, which is a value that can achieve single-mode conditions in the thickness direction.

[0023] Light propagates according to the planar shape of the waveguide core 30, achieving the desired function of the first optical waveguide element. Here, an example will be described in which the planar shape of the waveguide core 30 is rectangular. Light input to the first optical waveguide element propagates in the longitudinal direction of the waveguide core 30. In the following description, the longitudinal direction may also be referred to as the propagation direction. The direction perpendicular to the upper surface of the lower cladding 20 may also be referred to as the thickness direction, and the direction perpendicular to both the propagation direction and the thickness direction may also be referred to as the width direction. The dimension in the width direction may also be referred to as the width, and the dimension in the longitudinal direction may also be referred to as the length.

[0024] When the first optical waveguide element is used as an optical sensor, the region used to detect changes in refractive index is referred to as the sensing region 90. In this case, a substance to be measured, such as a biological substance, is placed around the sensing waveguide 32, which is the waveguide core 30 of the sensing region 90, and changes in refractive index caused by the substance to be measured are detected.

[0025] An input waveguide 34 and an output waveguide 36 are connected to the sensing waveguide 32, also as the waveguide core 30. Light sent to the sensing waveguide 32 via the input waveguide 34 passes through the sensing waveguide 32 and is output from the output waveguide 36.

[0026] The sensing region 90 includes a first region 91 and a second region 92 arranged in series along the propagation direction. The first region 91 is provided on the input waveguide 34 side, and the second region 92 is provided on the output waveguide 36 side. An intermediate region 93 is provided between the first region 91 and the second region 92.

[0027] The sensing waveguide 32 has a plurality of apertures 50 on an imaginary line segment 42 drawn along the longitudinal direction at the center in the width direction. Also, a central slot 52 connecting adjacent apertures 50 in the longitudinal direction is formed on the imaginary line segment 42. Furthermore, slots are provided on line segments 44 and 46 parallel to the imaginary line segment 42 at positions sandwiching the plurality of apertures 50. Here, the slot provided on the imaginary line segment 44 is referred to as a left slot 54, and the slot provided on the imaginary line segment 46 is referred to as a right slot 56. The left slot 54 and the right slot 56 are arranged at an interval greater than the width of the apertures 50, and are spaced apart from the plurality of apertures 50.

[0028] A plurality of openings 50, a center slot 52, a left slot 54 and a right slot 56 are formed in the waveguide core 30 as through holes exposing the lower cladding 20.

[0029] In the first region 91, the plurality of openings 50 are periodically provided at a constant period Λ. The second region 92 has a structure similar to that of the first region 91. That is, in the second region 92, the plurality of openings 50 are periodically provided at a constant period Λ. The intermediate region 93 has a structure different from that of the first region 91 and the second region 92. The length of the openings 50 should be less than ½ of the period Λ, preferably ¼ or less.

[0030] In the first region 91, one opening 50 and a central slot 52 connected to this opening 50 form one unit 94. A plurality of units 94 are connected in series. In the opening 50 and central slot 52 that form one unit 94, the opening 50 is arranged on the input waveguide 34 side.

[0031] As in the first region 91, in the second region 92, one opening 50 and a central slot 52 connected to this opening 50 form one unit 94. A plurality of units 94 are connected in series. In the opening 50 and central slot 52 that form one unit 94, the opening 50 is arranged on the output waveguide 36 side.

[0032] In the intermediate region 93, two openings 50 adjacent to each other in the longitudinal direction and a central slot 52 between the two openings 50 form one unit. Only one unit is disposed in the intermediate region 93. The central slot 52 in the intermediate region 93 has a different length from the central slots 52 in the first region 91 and the second region 92, and in this example, is configured to be shorter. In this case, the distance between two openings 50 adjacent to each other in the longitudinal direction in the intermediate region 93 is narrower than the distance in the first region 91 and the second region 92.

[0033] As a result, in the first optical waveguide element, the sensing waveguide 32 has periodic openings 50 formed therein as a whole, and the positions of the openings 50 in the longitudinal direction are shifted in the intermediate region 93.

[0034] In this way, the intermediate region 93, which is the boundary between the first region 91 and the second region 92, has a different structure from the other regions, and therefore a resonant structure is generated there. The input light input from the input waveguide 34 to the sensing waveguide 32 in the sensing region 90 is diffracted because it is within the bandgap of the opening 50, and a small amount reaches the intermediate region 93, which is the boundary between the first region 91 and the second region 92. The light that resonates near the intermediate region 93, which is the boundary between the first region 91 and the second region 92, is output via the output waveguide 36.

[0035] In the sensing region 90, the periphery of the sensing waveguide 32 and the inside of the opening 50 are exposed to a liquid containing the biological substance to be measured, and a change in refractive index caused by the substance to be measured is detected.

[0036] Here, when detecting with high sensitivity a change in refractive index due to the adsorption of a biological substance in the sensing region 90, it is preferable not to provide an upper cladding that covers the sensing waveguide 32. In this case, the liquid containing the biological substance to be measured functions as the upper cladding.

[0037] On the other hand, the propagation loss of light propagating through the waveguide core 30 tends to be lower when an upper cladding made of the same material as the lower cladding 20 is present around the waveguide core 30. Therefore, in the region of the waveguide core 30 where the input waveguide 34 and output waveguide 36 are formed other than the sensing waveguide 32 provided in the sensing region, it is preferable to provide an upper cladding covering the waveguide core 30 made of SiO2, the same material as the lower cladding 20.

[0038] Furthermore, when the first optical waveguide element is used as a wavelength filter, it is preferable to provide an upper cladding covering the waveguide core 30 over the region in which the waveguide core 30 is formed, made of SiO2, the same material as the lower cladding 20. Covering the entire periphery of the waveguide core 30 with the lower cladding 20 and upper cladding can reduce loss at the interface between the waveguide core 30 and its surroundings.

[0039] (Second optical waveguide element) An optical waveguide element according to a second embodiment of the present invention (hereinafter also referred to as a second optical waveguide element) will be described with reference to Fig. 2. Fig. 2 is a schematic diagram for explaining the second optical waveguide element. Fig. 2(A) is a schematic plan view of a sensing waveguide of the second optical waveguide element. Fig. 2(B) is a schematic plan view showing an enlarged view of the opening near the intermediate region and the central slot. Fig. 2(C) is a schematic plan view showing an enlarged view of the opening near the intermediate region and the central slot of another configuration example of the second optical waveguide element.

[0040] The second optical waveguide element differs from the first optical waveguide element in the planar shape of the waveguide core 30 in the intermediate region 97. Other configurations are the same as those of the first optical waveguide element, so redundant explanations may be omitted.

[0041] 2A and 2B, in the intermediate region 97 of the second optical waveguide element, one unit is formed by two longitudinally adjacent openings and a central slot between the two openings. Only one unit is disposed in the intermediate region 97. The central slot 52 in the intermediate region 97 has a different length from the central slots 52 in the first region 91 and the second region 92, and in this example, is configured to be shorter. For example, the length of the central slot 52 in the intermediate region 97 is half the length of the central slot 52 in the first region 91 and the second region 92. Thus, the distance between two longitudinally adjacent openings in the intermediate region 91 is narrower than the distance in the first region 91 and the second region 92.

[0042] Furthermore, one of the two openings in the intermediate region 97, in this example, opening 57 on the second region side, is formed with an area smaller than the other openings 50. The length of opening 57 on the second region side is configured to be half the length of the other openings 50, for example.

[0043] In this second optical waveguide element, similarly to the first optical waveguide element, an intermediate region 97, which is the boundary between the first region 91 and the second region 92, has a structure different from that of the other regions, and therefore a resonant structure is generated here.

[0044] 2A and 2B, the length of the opening 57 on the second region side of the intermediate region 97 is shorter than the openings 50 provided in other positions. If the length of the opening 57 is short, it may be difficult to form the opening 57.

[0045] Therefore, in another configuration example of the second optical waveguide element shown in FIG. 2(C), the opening 58 on the second region side of the intermediate region 98 is configured differently from the openings 50 provided in the first region 91 and the second region 92.

[0046] For example, when the length and width of the opening 50 provided in the first region 91 are L and W, respectively, the length and width of the opening 57 in the second optical waveguide element shown in Fig. 2(B) are L / 2 and W, respectively. In contrast, in another configuration example of the second optical waveguide element shown in Fig. 2(C), the length and width of one of the two openings in the intermediate region 98, in this example, the opening 58 on the second region 92 side, are, for example, L and W / 2. With this configuration, the length of the opening 58 is the same as the other openings 50, which makes it possible to avoid difficulties in creating the openings.

[0047] The area of ​​the opening may be different from the other openings, preferably smaller, and the length and width of the opening are not limited to the above example. For example, in another configuration example of the second optical waveguide element, the length and width of opening 58 on the second region side of intermediate region 98 may both be smaller than the other openings, and the area of ​​opening 58 may be, for example, approximately L·W / 2. The length and width of opening 58 may be set as appropriate depending on the ease of manufacturing, etc.

[0048] (operation) 3 shows the results of a simulation performed using three-dimensional FDTD (Finite Difference Time Domain). In this simulation, an element without a center slot, a left slot, or a right slot, which is the basis of the first waveguide element described with reference to FIG. 1, is used.

[0049] The total width and thickness of the waveguide core 30 were 800 nm and 220 nm, respectively. The opening width was 210 nm, and 20 openings were arranged at a period of 390 nm. The materials of the lower cladding and the waveguide core 30 were SiO2 and Si, respectively, and the upper cladding was made of a material with a refractive index of 1.44. In Figure 3, the horizontal axis represents the length L (unit: nm) of the opening, and the vertical axis represents the Q value, which indicates the sharpness of the resonance peak.

[0050] As shown in Figure 3, the shorter the aperture length, the higher the Q factor. Furthermore, the shift amount of the resonant wavelength relative to the refractive index change, which indicates the sensitivity of the resonant wavelength to the refractive index change (wavelength change sensitivity), decreases as the aperture length decreases. For example, when the aperture length L is 90 nm and 130 nm, the shift amount is 180 nm / RIU (Refractive Index Unit), and when the aperture length L is 200 nm and 270 nm, the shift amount is 300 nm / RIU (Refractive Index Unit).

[0051] In the first optical waveguide element, slots are provided to achieve both a high Q value and improved wavelength change sensitivity. When a center slot is provided as one slot in an element without a center slot, left slot, and right slot, 300 nm / RIU is obtained when the aperture length L is 90 nm and 130 nm. When a left slot and a right slot are provided as two slots, 450 nm / RIU is obtained when the aperture length L is 90 nm and 130 nm. When a center slot, a left slot, and a right slot are provided as three slots, 600 nm / RIU is obtained when the aperture length L is 90 nm and 130 nm. Thus, providing slots improves wavelength change sensitivity.

[0052] Figure 4 shows the wavelength characteristics by 3D FDTD of an element with three slots: a center slot, a left slot, and a right slot. The widths of the center slot, left slot, and right slot are all 80 nm. Apertures, each 270 nm wide, are arranged 90 times. Figure 4(A) shows the case where the aperture length L is 130 nm, and Figure 4(B) shows the case where the aperture length L is 90 nm. In Figures 4(A) and (B), the horizontal axis represents wavelength (unit: μm) and the vertical axis represents optical intensity (unit: dB).

[0053] 4(A) and (B), curve I indicates the light that passes through input waveguide 34 and is input to sensing waveguide 32, curve II indicates the light that is reflected by input waveguide 34, and curve III indicates the light that is output from output waveguide 36. In both cases of Figures 4(A) and (B), a peak wavelength is obtained in the output light (curve III), but the peak intensity at the peak wavelength is higher in Figure 4(B). In other words, the shorter the length L of the aperture, for example, ¼ or less of the aperture period Λ, the lower the optical loss.

[0054] The characteristics when the measured substance absorbs light will be described with reference to FIGS. 5 and 6. FIG. 5 is a diagram for explaining the characteristics when the measured substance absorbs light. In FIG. 5, the horizontal axis represents the wavelength peak height (loss) (unit: dB), and the vertical axis represents the figure of merit. The figure of merit is obtained by multiplying the Q value when there is loss by the rate of change of the peak wavelength with respect to the change in the refractive index of the measured substance. The size of the circle represents the rate of change of the peak wavelength with respect to the change in the refractive index of the measured substance. The loss was evaluated using 3D FDTD at a value where the imaginary part of the refractive index was 0.001. FIG. 6 is a schematic diagram showing an example configuration of a comparative example.

[0055] In Figure 5, the optical waveguide element (101) of the present invention described with reference to Figure 1 is plotted with, for comparison, the topological photonic crystals (111-112) shown in Figure 6(A), the slotted topological photonic crystals (121-122) shown in Figure 6(B), the photonic crystals (131-133) shown in Figure 6(C), and the slotted photonic crystals (141-143) shown in Figure 6(D).

[0056] In the topological photonic crystals (111-112) shown in Fig. 6(A), two types of openings of different sizes are formed in one period. Fig. 6(B) shows a slotted topological photonic crystal 121 having a center slot, a left slot, and a right slot in one topological photonic crystal 111 shown in Fig. 6(A), and a slotted topological photonic crystal 122 having a center slot, a left slot, and a right slot in the other topological photonic crystal 112 shown in Fig. 6(A).

[0057] 6(C) shows two photonic crystals: a photonic crystal 131 with gradually changing aperture sizes, and a photonic crystal 132 with uniform aperture sizes. For the photonic crystal 132 with uniform aperture sizes, two different sizes are used as comparative examples.

[0058] Figure 6(D) shows slotted photonic crystal 141, which has a central slot in photonic crystal 131 with gradually changing aperture size as shown in Figure 6(C), slotted photonic crystal 142, which has left and right slots in photonic crystal 131 with gradually changing aperture size as shown in Figure 6(C), and slotted photonic crystal 143, which has a central slot, left slot, and right slot in photonic crystal 112 with uniform aperture size as shown in Figure 6(C).

[0059] The smaller the loss or the larger the figure of merit of an optical waveguide element, the better its performance, i.e., the further to the upper left in Figure 5, and the larger the plotted circle, the better its performance. As shown in Figure 5, topological photonic crystals generally exhibit good characteristics, but the optical waveguide element of this invention, which is easy to fabricate, also achieves the best characteristics. [Explanation of symbols]

[0060] 10 Support substrate 20 Lower Cladding 30 Waveguide core 32 Sensing waveguide 34 Input waveguide 36 Output waveguide 50, 57, 58 aperture 52 Center Slot 54 Left Slot 56 Right Slot 91 1st area 92 Second area 93, 97, 98 intermediate area 94 units

Claims

1. An optical waveguide element formed by providing a waveguide core on a lower clad, a first region, an intermediate region, and a second region are provided in series in this order in the longitudinal direction of the waveguide core; The waveguide core is a plurality of openings arranged on an imaginary line drawn in the longitudinal direction of the waveguide core at the center of the waveguide core in the width direction; a central slot on the imaginary line segment connecting the plurality of openings; a left slot and a right slot arranged parallel to the central slot and spaced apart by a distance greater than the width of the opening; Equipped with The first region and the second region have the same shape and size, In the first region and the second region, the plurality of openings are arranged at a constant interval; the intermediate region has a shape different from the first region and the second region; In the first region and the second region, the length of the opening is less than 1 / 2 of the constant period. Optical waveguide element.

2. In the first region and the second region, the length of the opening is equal to or less than ¼ of the constant period. The optical waveguide element according to claim 1 .

3. the first region is configured with a plurality of units when each unit is formed by an opening and a central slot, and in each unit, the opening is arranged on an end side where light is input; the second region is configured by a plurality of units when each unit is formed by an opening and a central slot, and in each unit, the opening is arranged on an end side from which light is output; The intermediate region is composed of two adjacent openings and a central slot between the two openings, and the length of the central slot in the intermediate region is shorter than the length of the central slot that constitutes one unit of the first region and the second region.

3. The optical waveguide element according to claim 1.

4. The two openings in the intermediate region are the same size as the openings constituting one unit of the first and second regions. The optical waveguide element according to claim 3 .

5. The area of ​​any one of the two openings in the intermediate region is smaller than the area of ​​the openings constituting one unit in the first region and the second region. The optical waveguide element according to claim 3 .

6. The surface of the waveguide core on the lower cladding is exposed to a substance to be measured.

3. The optical waveguide element according to claim 1.

7. An upper clad covering the waveguide core is provided on the lower clad.

3. The optical waveguide element according to claim 1.

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