Directional coupler and manufacturing method thereof
By designing optical waveguides of different widths and gap core layers with low equivalent refractive index on high-top optical waveguides with InP as the substrate, the problems of large size and manufacturing error resistance are solved, and small-scale directional coupling is realized. and high-efficiency optical power conversion.
Patent Information
- Application Number
- CN202280089980.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2022-06-16
- Publication Date
- 2025-01-10
AI Technical Summary
The existing directional coupler is large in size and is not resistant to manufacturing errors in high-top optical waveguides with InP as the substrate, making it difficult to achieve effective optical power conversion, especially when the optical waveguide width is more than 0.5 microns, the application of the prior art is difficult. .
A high table top structure on a semiconductor substrate is designed, and optical waveguides of different widths are formed on both sides of the optical power conversion part, and a design with low equivalent refractive index is considered on the gap core layer to shorten the optical waveguide interval, thereby achieving optical Effective conversion of power and reduction of size.
A small error-resistant directional coupler is built on a high-meter optical waveguide with InP as the substrate, taking into account the shortening of the optical power conversion distance and the improvement of the conversion rate, reaching an optical power conversion rate of about 50%.
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Figure CN119301490A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a directional coupler composed of a high-table optical waveguide with InP as a substrate and a manufacturing method thereof. Background Art
[0002] As materials for optical semiconductor devices, various materials such as silicon (Si), indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN), lithium niobate (LiNbO3), or compound semiconductors based on them are used. In such optical semiconductor devices, optical waveguides are widely used as basic components. Optical waveguides confine light to a certain local area by making the refractive index higher than the surrounding area, and by forming the area into a linear shape, light is propagated in a desired direction.
[0003] In addition, directional couplers are also widely used as one of the basic components of optical semiconductor devices for realizing various functions. Directional couplers optically couple the light propagation modes of two waveguides by bringing two independent optical waveguides close to each other at a distance less than the wavelength of the propagating light. This allows arbitrary power conversion of the propagating light.
[0004] However, in a general directional coupler, in order to sufficiently convert the optical power propagating through one optical waveguide to the other optical waveguide, a distance several tens of times longer than the wavelength of the propagating light is required, which causes a problem of increasing the size of the device.
[0005] In addition, in 3dB couplers and 3dB splitters that are often used in directional couplers and distribute 50% of the optical power to the other, there is also a problem that the allowable error range of the device length to obtain the desired conversion rate is very small, about 1 micron. The small allowable error range of the device length relative to the overall device length means that the directional coupler is a device that is not tolerant to manufacturing errors.
[0006] As a technology for solving the above-mentioned problems, a technology is disclosed in which a so-called sub-wavelength grating (SWG) structure is introduced into a directional coupler composed of a thin line waveguide based on Si, thereby shortening the conversion distance of optical power from one optical waveguide to another optical waveguide (for example, refer to non-patent document 1). The SWG structure is a periodic structure with a period of less than the period of propagating light arranged in a direction perpendicular to the two optical waveguides, in which the waveguide widths of two optical waveguides with a distance close to the wavelength are made different. In this method, the conversion distance of optical power is shortened by introducing an asymmetric waveguide width, and the reduction in the conversion rate of optical power, which becomes a problem at this time, is compensated by the SWG structure. As a result, a 3dB coupler that is a directional coupler and has a shorter device length is successfully realized.
[0007] Non-patent document 1: C.Ye et al., "Ultra-Compact Broadband 2x 23dB PowerSplitter Using a Subwavelength-Grating-Assisted Asymmetric DirectionalCoupler," Journal of Lightwave Technology, vol.38, no.8 (2020)
[0008] In Si fine-wire waveguides, the optical waveguide width is less than 0.5 microns, and it is easy to form a structure in which two optical waveguides are close to each other by less than 0.2 microns. It is difficult to apply the prior art to other than such Si fine-wire waveguides. In particular, the optical waveguide is weakly enclosed in the height direction, so a deeper digging portion is required. On the other hand, in a high-top optical waveguide based on InP, the optical waveguide width is more than 0.5 microns, and a distance between the two optical waveguides is required. Therefore, it is difficult to apply the prior art to a high-top optical waveguide based on InP. Summary of the invention
[0009] The present disclosure has been made to solve the above-mentioned problems, and an object of the present disclosure is to provide a directional coupler that can be applied to a high-top optical waveguide based on InP, has a small device size, and is resistant to manufacturing errors, and a method for manufacturing the same.
[0010] The directional coupler disclosed in the present invention is characterized by comprising: a semiconductor substrate; a first optical waveguide and a second optical waveguide of a high-table structure, which are formed side by side on the semiconductor substrate; and a surrounding cladding, which is formed around the first optical waveguide and the second optical waveguide, wherein the first optical waveguide and the second optical waveguide have: an optical power conversion unit, which distributes light propagating in one of the first optical waveguide and the second optical waveguide to the first optical waveguide and the second optical waveguide at a desired power ratio; a first curved waveguide, which is connected to the input side of the optical power conversion unit and reduces the distance between the first optical waveguide and the second optical waveguide as it approaches the optical power conversion unit; and a second curved waveguide, which is connected to the output side of the optical power conversion unit and reduces the distance as it moves away from the optical power conversion unit. The optical power conversion unit increases the interval between the first optical waveguide and the second optical waveguide. In the optical power conversion unit, the interval between the first optical waveguide and the second optical waveguide is less than the wavelength of the light. The first optical waveguide and the second optical waveguide are respectively high-table structures having a lower cladding, a core layer, and an upper cladding formed in sequence on the semiconductor substrate. The first optical waveguide and the second optical waveguide have different widths. A gap core layer is formed between the core layers of the first optical waveguide and the second optical waveguide of the optical power conversion unit and on the lower cladding. Considering that the equivalent refractive index of the gap core layer is lower than the equivalent refractive index of the core layer of the first optical waveguide and the second optical waveguide when the light leaks in the height direction.
[0011] In the present disclosure, the gap core layer is designed in a manner that takes into account that the equivalent refractive index of the gap core layer when light leaks in the height direction is lower than the equivalent refractive index of the core layer of the optical waveguide. As a result, a directional coupler that can also be applied to a high-top optical waveguide based on InP, has a small device size, and is resistant to manufacturing errors can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Figure 1 It is a top view showing the directional coupler according to the first embodiment.
[0013] Figure 2 This is a perspective view showing an optical power conversion section of the directional coupler according to the first embodiment.
[0014] Figure 3 This is a diagram showing the results of calculating the sharing ratio of the directional coupler for the structure of the first embodiment, the conventional high-mesa waveguide structure, and the conventional buried waveguide structure.
[0015] Figure 4 It means used for Figure 3 A cross-sectional view of the structure of implementation example 1 of the calculation.
[0016] Figure 5 It means used for Figure 3Calculated cross-sectional view of an existing high-mesa waveguide structure.
[0017] Figure 6 It means used for Figure 3 Calculated cross-sectional view of an existing buried waveguide structure.
[0018] Figure 7 Yes Figure 3 FIG. 5 is a graph showing the result of calculating the ratio of the optical powers respectively contained in the two optical waveguides by overlapping integration of the light intensity distribution at each z position.
[0019] Figure 8 1 is a diagram showing the results of calculating the optical power distribution ratio with respect to the position in the longitudinal direction of the directional coupler.
[0020] Fig. 9 1 is a diagram showing the results of calculating the optical power distribution ratio with respect to the position in the longitudinal direction of the directional coupler.
[0021] Fig.10 1 is a diagram showing the results of calculating the optical power distribution ratio with respect to the position in the longitudinal direction of the directional coupler.
[0022] Fig.11 1 is a diagram showing the results of calculating the optical power distribution ratio with respect to the position in the longitudinal direction of the directional coupler.
[0023] Fig.12 It is a perspective view showing a directional coupler according to the second embodiment.
[0024] Fig.13 This is a diagram showing the result of calculating the distribution ratio of the directional coupler in the structure of the second embodiment.
[0025] Fig.14 Yes Fig.13 A graph showing the results of calculating the ratio of the optical powers respectively contained in the first optical waveguide and the second optical waveguide on the input side by overlap integration of the light intensity distribution at each z position.
[0026] Fig.15 It is a perspective view showing a directional coupler according to the third embodiment.
[0027] Fig.16 This is a top view showing a mask used when forming a waveguide.
[0028] Fig.17 It is a perspective view showing a directional coupler according to a fourth embodiment.
[0029] Fig.18 It is a perspective view showing a directional coupler according to the fifth embodiment.
[0030] Fig.19It is a perspective view showing a directional coupler according to a sixth embodiment. DETAILED DESCRIPTION
[0031] A directional coupler and a method for manufacturing the same according to an embodiment will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals and duplicate description may be omitted.
[0032] Implementation Method 1
[0033] Figure 1 1 is a top view of a directional coupler according to Embodiment 1. Two optical waveguides 2 and 3 are formed side by side on a semiconductor substrate 1. Optical waveguides 2 and 3 are regions with a higher refractive index than the surrounding regions. Light is locally confined in these regions. Light is allowed to propagate only in a certain direction of optical waveguides 2 and 3. Optical waveguide 2 has optical waveguides 2a to 2e. Optical waveguide 3 has optical waveguides 3a to 3e. Figure 1 In FIG. 1 , arrows schematically show light 4 a input to the directional coupler and output lights 4 b and 4 c distributed by the directional coupler.
[0034] The optical waveguides 2a and 3a on the input side of the directional coupler are arranged side by side. The interval between the optical waveguides 2a and 3a is a sufficient distance that is several times or more of the wavelength of the propagating light. One end of the optical waveguides 2b and 3b is connected to the optical waveguides 2a and 3a, respectively. The shape of the optical waveguides 2b and 3b is a combination of circular arcs, a combination of sine waves and cosine waves, a cycloid curve or a clothoid curve, etc. The optical waveguides 2b and 3b connected to the input side of the optical power conversion unit are curved waveguides that reduce the interval between the optical waveguides 2 and 3 as they approach the optical power conversion unit, so that the interval between the optical waveguides 2 and 3 is approached from several times or more of the wavelength size of the propagating light to the wavelength size without light loss. The length of the optical waveguides 2b and 3b is about 10 times or more of the wavelength of the propagating light.
[0035] One end of the optical waveguide 2c, 3c of the optical power conversion unit is connected to the other end of the optical waveguide 2b, 3b respectively. The optical waveguides 2c, 3c are arranged parallel to each other and close to each other. The interval between the optical waveguides 2c, 3c is the wavelength of the propagating light or less. The optical power conversion unit distributes the light propagating in one side of the optical waveguides 2c, 3c to the optical waveguides 2c, 3c at a desired power ratio.
[0036] One end of the optical waveguide 2d, 3d is connected to the other end of the optical waveguide 2c, 3c, respectively. The shape of the optical waveguide 2d, 3d is a combination of circular arcs, a combination of sine waves and cosine waves, or a cycloid curve. The optical waveguide 2d, 3d connected to the output side of the optical power conversion unit is a curved waveguide that expands the distance between the optical waveguides 2, 3 as it is farther away from the optical power conversion unit, so that the distance between the optical waveguides 2, 3 is expanded from the wavelength of the propagating light to several times the wavelength without light loss. The length of the optical waveguide 2d, 3d is about 10 times or more of the wavelength of the propagating light. The optical waveguides 2e, 3e on the output side of the directional coupler are connected to the other ends of the optical waveguides 2d, 3d, respectively.
[0037] Figure 2 1 is a perspective view showing an optical power conversion unit of a directional coupler according to Embodiment 1. In addition, the x-axis and y-axis are shown in the figure for clarity. Figure 1 and Figure 2 The optical waveguide 2c is a high-top structure having a lower cladding layer 5, a core layer 6a, and an upper cladding layer 7a stacked in sequence on the semiconductor substrate 1. The optical waveguide 3c is a high-top structure having a lower cladding layer 5, a core layer 6b, and an upper cladding layer 7b stacked in sequence on the semiconductor substrate 1.
[0038] The core layers 6a and 6b are made of a material having a higher refractive index than the lower cladding layer 5 and the upper cladding layers 7a and 7b, and are regions for enclosing light. Between the core layers 6a and 6b of the optical waveguides 2c and 3c of the optical power conversion section, a gap core layer 6c is formed on the lower cladding layer 5. The gap core layer 6c is made of the same material as the core layers 6a and 6b, and is hollowed so that the height of the upper surface is lower than that of the core layers 6a and 6b.
[0039] The surrounding cladding 8 is formed around the optical waveguides 2c and 3c and is made of a material such as SiO2 or SiN having a lower refractive index than the lower cladding 5, the core layers 6a and 6b, the upper cladding layers 7a and 7b, and the interstitial core layer 6c.
[0040] The width of the optical waveguide 2c is about 1.5 times the width of the optical waveguide 3c. Similarly, the width of the optical waveguides 2a to 2e is about 1.5 times the width of the optical waveguides 3a to 3e. Therefore, the directional coupler of the present embodiment is an asymmetric directional coupler in which the optical waveguide 2 and the optical waveguide 3 have different widths. In addition, the width of the optical waveguides 2 and 3 is less than the net wavelength of the propagating light, and the overall height is greater than the net wavelength. The core layers 6a, 6b and the gap core layer 6c are formed on the common lower cladding layer 5, so the height of the lower surface of the gap core layer 6c is the same as the height of the lower surface of the core layers 6a and 6b. In addition, the gap core layer 6c and the core layers 6a and 6b are in contact with each other.
[0041] The equivalent refractive index of the gap core layer 6c is calculated by taking into account the leakage of light from the gap core layer 6c to the surrounding cladding 8 and the lower cladding 5. The equivalent refractive index of the core layers 6a and 6b is calculated by taking into account the leakage of light from the core layers 6a and 6b to the upper claddings 7a and 7b and the lower cladding 5. In general, the thinner the thickness of the core layer, the more light leaks out of the upper and lower claddings and the lower equivalent refractive index is reduced. Therefore, in this embodiment, the gap core layer 6c is dug lower than the core layers 6a and 6b of the optical waveguides 2c and 3c in a manner that takes into account the equivalent refractive index of the gap core layer 6c when the light leaks in the height direction, which is lower than the equivalent refractive index of the core layers 6a and 6b of the optical waveguides 2c and 3c.
[0042] Specifically, the equivalent refractive index n of the interstitial core layer 6c is taken into consideration when light leaks in the height direction. eff The amount of digging of the gap core layer 6c is adjusted so that the following formula (1) is satisfied with an error of 10% or less.
[0043] n eff =(n core -n clad )×0.09×ln(k0w gap / 4.2)+1.0+n clad (1)
[0044] Here, n core is the refractive index of the core layers 6a, 6b and the interstitial core layer 6c. clad is the refractive index of the surrounding cladding 8. k0 is the wave number of light propagating in a vacuum. gap It is the proximity distance of the optical waveguides 2c and 3c of the optical power conversion unit, that is, the width of the gap core layer 6c.
[0045] Compared with other optical waveguides such as buried waveguides and thin line waveguides, the high mesa waveguide has a stronger light confinement inside the optical waveguide. In addition, the formation of the gap between the two optical waveguides in the optical power conversion section requires deep etching, so the width of the gap w gap It needs to be above a certain value. In such a structure, the light propagation modes in the two optical waveguides will not be fully optically coupled to each other, and it is difficult to fully generate optical power conversion. Therefore, in this embodiment, by leaving a part of the gap core layer 6c in the gap portion, each light propagation mode is intentionally expanded in the direction of the gap portion to enhance optical coupling. In addition, by adopting an asymmetric waveguide width in which the widths of the optical waveguides 2c and 3c of the optical power conversion portion are different, the difference in propagation constants of each light propagation mode becomes larger, thereby shortening the conversion distance of optical power from one optical waveguide to the other optical waveguide. As a result, a directional coupler that takes into account both a shorter optical power conversion distance and a maximum optical power conversion rate of about 50% can be realized.
[0046] Figure 3This is a diagram showing the results of calculating the sharing ratio of the directional coupler for the structure of the first embodiment, the conventional high-mesa waveguide structure, and the conventional buried waveguide structure. Figure 4 It means used for Figure 3 A cross-sectional view of the structure of implementation example 1 of the calculation. Figure 5 It means used for Figure 3 A cross-sectional view of a conventional high-mesa waveguide structure calculated by the method of FIG. Figure 6 It means used for Figure 3 A cross-sectional view of a conventional buried waveguide structure calculated by . In the conventional buried waveguide structure, after the gap is completely dug, the surrounding area is completely filled with an InP layer 9 having a refractive index of 3.17.
[0047] Each structure is an asymmetric directional coupler in which an optical waveguide 2c on the light input side and an optical waveguide 3c on the light output side are close to each other with a gap of 0.6 micrometers in width. The optical waveguides 2c and 3c are surrounded by a surrounding cladding 8 composed of SiO2 with a refractive index of 1.45. The width of the optical waveguide 2c is 0.6 micrometers. The width of the optical waveguide 3c is 0.4 micrometers. The optical waveguide 2c has an upper cladding 7a with a thickness of 1.2 micrometers composed of InP with a refractive index of 3.17, a core layer 6a with a thickness of 0.5 micrometers composed of AlGaInAs multiple quantum wells with an average refractive index of 3.32, and a lower cladding 5 with a thickness of 1.2 micrometers composed of InP with a refractive index of 3.17. The optical waveguide 3c has an upper cladding layer 7b with a thickness of 1.2 microns made of InP with a refractive index of 3.17, a core layer 6b with a thickness of 0.5 microns made of AlGaAsAs multiple quantum wells with an average refractive index of 3.32, and a lower cladding layer 5 with a thickness of 1.2 microns made of InP with a refractive index of 3.17. In the structure of the first embodiment, the equivalent refractive index of the gap core layer 6c is 3.129.
[0048] Figure 3 The horizontal axis represents the width direction of the directional coupler, corresponding to Figure 1 , Figure 2 The x-axis in . Figure 3 The vertical axis represents the length direction of the directional coupler, corresponding to Figure 1 , Figure 2 The z-axis in . Figure 3 The multiple curves overlap to show the energy distribution of light every 0.5 micrometers in the z direction, and are illustrated in a way that makes it easy to visually know the x position of the light. Figure 3 The vicinity of x=-0.6 micrometer is where the optical waveguide 2c is provided. Figure 3 The vicinity of x=0.4 micrometer is where the optical waveguide 3c is provided.
[0049] Figure 7 Yes Figure 3 The light intensity distribution at each z position is a graph showing the result of calculating the ratio of the optical powers respectively contained in the two optical waveguides by overlapping integral. According to the above calculation results, it can be seen that in the existing buried waveguide structure, a length of 25 microns is required to reach a light distribution rate of 50%. It can be seen that in the existing high-table waveguide structure, the optical coupling between the two optical waveguides is too weak, and no matter how long the directional coupler is, there is almost no conversion of optical power. On the other hand, it can be seen that in the structure of embodiment 1, when the length of the directional coupler is about 8 microns, the optical power distribution rate is 50%. Compared with the existing structure, it is possible to achieve a balance between shortening the directional coupler by about 1 / 3 and a power conversion rate of 50%.
[0050] Next, the effect of formula (1) is described. Figures 8 to 11 : is a graph showing the result of calculating the optical power distribution ratio with respect to the position in the length direction of the directional coupler. The optical power distribution ratio is the ratio of the optical power contained in each of the two optical waveguides. Figure 4 The height of the gap core layer is changed in the structure so that the equivalent refractive index of the gap part changes from the refractive index of SiO2 in the surrounding cladding layer, 1.45, to the refractive index of the multi-quantum well of the core layer, 3.21. Figures 8 to 11 is the gap width w between the two optical waveguides gap The cases are set to 0.2 micron, 0.4 micron, 0.6 micron and 0.8 micron respectively.
[0051] The equivalent refractive index conditions indicated by bold lines in each figure are the conditions closest to formula (1). According to each figure, it can be seen that the optical power distribution rate at the peak position of the curve graph is about 50% when the condition of formula (1) is satisfied. The optical power distribution rate is 50% at the peak position of the curve graph where the slope of the curve graph decreases, that is, the area where the change in the optical power distribution ratio is the smallest. Therefore, the range of the z position where the optical power distribution rate is approximately 50% becomes wider, and a reliable directional coupler can be realized for manufacturing errors. At this time, for example, if the error of the optical power distribution rate is set to ±10%, the condition of formula (1) allows an error of about 10% from the optimal condition.
[0052] Based on the above, in this embodiment, the gap core layer 6c is provided in such a manner that the equivalent refractive index of the gap core layer 6c is lower than the equivalent refractive index of the core layers 6a and 6b of the optical waveguides 2c and 3c when light is leaked in the height direction. Specifically, the equivalent refractive index n of the gap core layer 6c is set to be lower than the equivalent refractive index of the core layers 6a and 6b of the optical waveguides 2c and 3c when light is leaked in the height direction. eff The amount of the gap core layer 6c is adjusted so that the error is less than 10% to satisfy the formula (1). Thus, a directional coupler that can be applied to a high-top optical waveguide based on InP, has a small device size, and is resistant to manufacturing errors can be obtained.
[0053] In addition, the shape, raw materials, and positional relationship of the directional coupler are not limited to the present embodiment. For example, the positions of the optical waveguide on the input side and the optical waveguide on the output side can also be replaced. The material of the semiconductor substrate 1 can also be other materials such as Si, GaAs, SiO2, SiN or LiNbO3. The core layer does not have to be a multiple quantum well, as long as it is a material with a higher refractive index than the cladding, for example, it can be SiO2 doped with Ge or the like on the SiO2 cladding. In this case, since the refractive index of the device is reduced as a whole, it is not conducive to the enlargement of the device size, but the optical loss of the propagating light is reduced, so a low-loss device can be achieved.
[0054] Implementation Method 2
[0055] Fig.12 : is a perspective view showing a directional coupler according to Embodiment 2. The gap core layer 6c is made of the same material as the core layers 6a and 6b, a part of which is hollowed out so that its height is lower than the core layers 6a and 6b, and has a SWG (Sub-wavelength Grating) structure in which the first region 10a and the second region 10b having different heights are periodically repeated at a pitch less than the length of the net wavelength of light propagating in the optical waveguides 2c and 3c.
[0056] Considering the equivalent refractive index n of the first region 10a when the front light leaks in the height direction gap1 and the equivalent refractive index n of the second region 10b gap2 The following formula (2) is satisfied with an error of 10% or less.
[0057]
[0058] Here, n gap1 and n gap2 are the equivalent refractive indices of the first and second regions 10a and 10b, respectively, and f is a filling factor representing the ratio of the length of the first region 10a to the length of the gap core layer 6c in the light propagation direction. If the length of the first region 10a is set to a [um] and the length w of the second region 10b is set to b [um], then f = a / (a+b). The other structures are the same as those in Embodiment 1.
[0059] In the first embodiment, the equivalent refractive index, i.e., the height of the gap core layer 6c has to be uniquely determined according to the gap width and the refractive index of the material of the optical waveguide. However, in the present embodiment, the equivalent refractive index of the gap core layer 6c of the light propagating in the optical waveguide is the average refractive index of the first and second regions 10a and 10b of different heights determined by the filling factor. Therefore, by introducing the SWG structure, a directional coupler having an optimal optical power distribution ratio can be realized by combining the two heights of the first and second regions 10a and 10b and the filling factor regardless of the gap width and the refractive index of the material of the optical waveguide.
[0060] Fig.13 : is a graph showing the result of calculating the distribution ratio of the directional coupler in the structure of Embodiment 2. The equivalent refractive index n of the first region 10a in the height direction is gap1 is 3.055, and the equivalent refractive index n of the second region 10b is gap2 is 3.205, the period of the SWG structure is 0.22 microns, and the filling factor is 0.5. Other conditions and Figure 3 The situation is the same. In the figure, the vicinity of x=-0.6 micrometers is the position where the optical waveguide 2c is provided, and the vicinity of x=0.4 micrometers is the position where the optical waveguide 3c is provided.
[0061] Fig.14 Yes Fig.13 The light intensity distribution at each z position is a graph showing the result of calculating the ratio of the optical power contained in each of the first optical waveguide and the second optical waveguide on the input side by overlapping integral. Due to the constraints of the calculation program, the equivalent refractive index is about 10% smaller than the target value. Therefore, compared with the calculation results of embodiment 1, the conversion rate of optical power becomes smaller, but the characteristics of the optical power conversion rate of about 50% in accordance with the target can be obtained. According to this result, in order to suppress the distribution ratio of optical power to 50% ± 10%, the allowable error from the optimal condition of formula (2) is about 10% as in embodiment 1.
[0062] Implementation 3
[0063] Fig.15 3 is a perspective view showing a directional coupler according to Embodiment 3. As in Embodiment 2, the gap core layer 6c is a SWG structure in which the first and second regions 10a and 10b are periodically repeated at a pitch less than the length of the net wavelength of light propagating in the optical waveguides 2c and 3c. The height of the second region 10b is lower than that of the first region 10a.
[0064] In this embodiment, on the mutually facing side surfaces of the upper cladding layers 7a and 7b of the optical waveguides 2c and 3c, hollowed portions 11a and 11b having a width less than the width of the optical waveguides 2c and 3c and a depth not reaching the core layers 6a and 6b are formed at the same period and the same filling factor as the first and second regions 10a and 10b. In the light propagation direction (z direction), the positions of the hollowed portions 11a and 11b coincide with the position of the second region 10b. In the light propagation direction, the positions of the non-hollowed portions 12a and 12b of the side surfaces coincide with the position of the first region 10a.
[0065] Fig.16 This is a top view of a mask used when forming a waveguide. The opening 13a of the mask 13 has three-way walls and a small opening ratio. The dug-in portions 11a and 11b are formed by etching at the opening 13a. The opening 13b has two-way walls and a small opening ratio. A shallower first region 10a is formed by etching at the opening 13b. The opening 13c has no walls and a large opening ratio. A deeper second region 10b is formed by etching at the opening 13c. In this way, the optical waveguides 2c and 3c can be formed by a single deep etching by utilizing the micro-load effect in which the etching depth depends on the opening ratio of the mask. In addition, the actual opening shape of the mask depends largely on the etching device or conditions, so Fig.16 The mask shape is just an example.
[0066] When the cutouts 11a and 11b are not formed on the side surfaces of the upper claddings 7a and 7b of the optical waveguides 2c and 3c as in the second embodiment, the mask opening area ratio of the regions where the first and second regions 10a and 10b are etched becomes a constant value according to the filling factor. In contrast, in the present embodiment, the cutouts 11a and 11b are allowed to be formed on the mutually facing side surfaces of the upper claddings 7a and 7b of the optical waveguides 2c and 3c. As a result, the mask opening corresponding to the second region 10b etched more deeply can be expanded in the width direction (x direction), so the aperture ratio during etching can be freely designed. By freely designing the aperture ratio and the height of the gap core layer 6c, a structure in which the cutouts 11a and 11b can be formed at the same time by the micro-loading effect can be realized. In addition, by making the depth of the cutouts 11a and 11b for adjusting the aperture ratio formed on the opposite side surfaces of the optical waveguide less than the core layers 6a and 6b, the influence on the optical mode propagating in the optical waveguide can be suppressed to be slight. Therefore, the condition of formula (2) in the second embodiment can be used as it is.
[0067] Implementation 4
[0068] Fig.17: is a perspective view showing a directional coupler of Embodiment 4. The gap core layer 6c is hollowed out so that its height is lower than that of the core layers 6a and 6b, and the first region 10a and the second region 10b having the same height and different refractive index are periodically repeated at a pitch less than the length of the net wavelength of the light propagating in the optical waveguides 2c and 3c. The equivalent refractive index n of the first region 10a is taken into consideration when the front light leaks in the height direction. gap1 and the equivalent refractive index n of the second region 10b gap2 The height of the gap core layer 6c, the refractive index of the first and second regions 10a and 10b, the filling factor, etc. are set in a manner that satisfies formula (2) with an error of less than 10%. The first and second regions 10a and 10b can be formed by doping with impurities, etc. For example, for materials such as LiNbO3 that are difficult to perform fine etching processing, the structure of this embodiment can be applied by alternative means. Other structures and effects are the same as those of embodiment 1.
[0069] Implementation method 5
[0070] Fig.18 6c is a perspective view showing a directional coupler according to Embodiment 5. The height of the gap core layer 6c is the same as that of the core layers 6a and 6b, and it is made of a material with a lower refractive index than the core layers 6a and 6b. The equivalent refractive index n of the gap core layer 6c is taken into consideration when light leaks in the height direction. eff The refractive index of the gap core layer 6c is set in a manner that satisfies formula (1) with an error of less than 10%. The gap core layer 6c with a low refractive index can be formed by doping with impurities, etc. For example, for materials such as LiNbO3 that are difficult to perform fine etching, the structure of this embodiment can be applied using alternative means. In addition, the structure of this embodiment can also be formed by embedding other materials, expanding the options of the manufacturing method. Other structures and effects are the same as those of embodiment 1.
[0071] Implementation 6
[0072] Fig.19 : is a perspective view showing a directional coupler of Embodiment 6. The gap core layer 6c is a SWG structure in which the first and second regions 10a and 10b having different refractive indices are periodically repeated at a pitch less than the length of the net wavelength of light propagating in the optical waveguides 2c and 3c. The height of the first and second regions 10a and 10b is the same as that of the core layers 6a and 6b, and is made of a material with a lower refractive index than that of the core layers 6a and 6b. The equivalent refractive index n of the first region 10a is taken into consideration when the front light leaks in the height direction. gap1 and the equivalent refractive index n of the second region 10b gap2The refractive index, filling factor, etc. of the first and second regions 10a, 10b are set in a manner that satisfies formula (2) with an error of less than 10%. The first and second regions 10a, 10b of the gap core layer 6c can be formed by doping with impurities, etc. For example, for materials that are difficult to perform fine etching processing such as LiNbO3, the structure of this embodiment can be applied using alternative means. In addition, the structure of this embodiment can also be formed by embedding other materials, expanding the options of the manufacturing method. Other structures and effects are the same as those of embodiment 1.
[0073] Description of Reference Numerals
[0074] 1…semiconductor substrate; 2…optical waveguide (first optical waveguide); 3…optical waveguide (second optical waveguide); 2b, 3b…optical waveguide (first curved waveguide); 2c, 3c…optical waveguide (optical power conversion portion); 2d, 3d…optical waveguide (second curved waveguide); 5…lower cladding; 6a, 6b…core layer; 6c…gap core layer; 7a, 7b…upper cladding; 8…surrounding cladding; 10a…first region; 10b…second region; 11a, 11b…dig-in portion; 12a, 12b…non-dig-in portion; 13…mask.
Claims
1. A directional coupler, characterized in that: have: Semiconductor substrates; A first optical waveguide and a second optical waveguide of a high-mesa structure, which are formed side by side on the semiconductor substrate; and a surrounding cladding layer formed around the first optical waveguide and the second optical waveguide, The first optical waveguide and the second optical waveguide have: an optical power conversion unit that distributes the light propagating through one of the first optical waveguide and the second optical waveguide to the first optical waveguide and the second optical waveguide at a desired power ratio; a first curved waveguide connected to the input side of the optical power conversion unit, wherein the distance between the first optical waveguide and the second optical waveguide is reduced toward the optical power conversion unit; as well as a second curved waveguide connected to the output side of the optical power conversion unit, wherein the distance between the first optical waveguide and the second optical waveguide increases as the distance from the optical power conversion unit increases; In the optical power conversion unit, the interval between the first optical waveguide and the second optical waveguide is less than the wavelength of the light. The first optical waveguide and the second optical waveguide each have a high-mesa structure including a lower cladding layer, a core layer, and an upper cladding layer sequentially formed on the semiconductor substrate. The first optical waveguide and the second optical waveguide have different widths, A gap core layer is formed between the core layers of the first optical waveguide and the second optical waveguide of the optical power conversion unit and on the lower cladding layer. The equivalent refractive index of the gap core layer is lower than the equivalent refractive index of the core layers of the first optical waveguide and the second optical waveguide in consideration of the light leaking in the height direction.
2. The directional coupler according to claim 1, characterized in that n core is the refractive index of the core layer and the interstitial core layer, n clad is the refractive index of the surrounding cladding, k0 is the wave number of light propagating in vacuum, w gap is the proximity distance between the first optical waveguide and the second optical waveguide of the optical power conversion unit, taking into account the equivalent refractive index n of the gap core layer when the light leaks in the height direction. eff The following formula is satisfied with an error of less than 10%: n eff =(n core -n clad )×0.09×ln(k0w gap / 4.2)+1.0+n clad 。 3. The directional coupler according to claim 2, characterized in that: The interstitial core layer is made of the same material as the core layer, and is dug so that its height is lower than that of the core layer.
4. The directional coupler according to claim 3, characterized in that: The interstitial core layer is a structure in which first regions and second regions having different heights are periodically repeated at a pitch less than the length of the wavelength of the light. f is a filling factor indicating the ratio of the length of the first region to the entire length of the interstitial core layer in the light propagation direction, and takes into account the equivalent refractive index n of the first region when the light leaks in the height direction. gap1 and the equivalent refractive index n of the second region gap2 The following formula is satisfied with an error of less than 10%:
5. The directional coupler according to claim 4, characterized in that: On mutually facing sides of the upper cladding layers of the first optical waveguide and the second optical waveguide, there are formed, with the same period and the same filling factor as the first region and the second region, recessed portions having a width less than the width of the first optical waveguide and the second optical waveguide and a depth that does not reach the core layer, The height of the second area is lower than that of the first area, In the light propagation direction, the position of the hollowed-in portion of the side surface is consistent with the position of the second area, In the light propagation direction, the position of the non-digging portion of the side surface is consistent with the position of the first area.
6. The directional coupler according to claim 2, characterized in that: The interstitial core layer is hollowed out so that its height is lower than that of the core layer, and has a structure in which the first region and the second region having the same height and different refractive index are periodically repeated at a pitch less than the length of the wavelength of the light. f is a filling factor indicating the ratio of the length of the first region to the entire length of the interstitial core layer in the light propagation direction, and takes into account the equivalent refractive index n of the first region when the light leaks in the height direction. gap1 and the equivalent refractive index n of the second region gap2 The following formula is satisfied with an error of less than 10%:
7. The directional coupler according to claim 2, characterized in that: The interstitial core layer is composed of a material having the same height as that of the core layer and a lower refractive index than that of the core layer.
8. The directional coupler according to claim 7, characterized in that: The interstitial core layer is a structure in which first regions and second regions having different refractive indices are periodically repeated at a pitch less than the length of the wavelength of the light. f is a filling factor indicating the ratio of the length of the first region to the entire length of the interstitial core layer in the light propagation direction, and takes into account the equivalent refractive index n of the first region when the light leaks in the height direction. gap1 and the equivalent refractive index n of the second region gap2 The following formula is satisfied with an error of less than 10%:
9. A method for manufacturing a directional coupler, which is a method for forming the directional coupler according to claim 5, characterized in that: The first optical waveguide and the second optical waveguide are formed by one deep etching process using a micro-loading effect in which the etching depth depends on the aperture ratio of the mask.