CN111007592A - 3dB directional coupler capable of shortening coupling length and manufacturing method thereof - Google Patents
3dB directional coupler capable of shortening coupling length and manufacturing method thereof Download PDFInfo
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- CN111007592A CN111007592A CN201911156789.9A CN201911156789A CN111007592A CN 111007592 A CN111007592 A CN 111007592A CN 201911156789 A CN201911156789 A CN 201911156789A CN 111007592 A CN111007592 A CN 111007592A
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/122—Basic optical elements, e.g. light-guiding paths
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/13—Integrated optical circuits characterised by the manufacturing method
- G02B6/132—Integrated optical circuits characterised by the manufacturing method by deposition of thin films
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/13—Integrated optical circuits characterised by the manufacturing method
- G02B6/136—Integrated optical circuits characterised by the manufacturing method by etching
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/13—Integrated optical circuits characterised by the manufacturing method
- G02B6/138—Integrated optical circuits characterised by the manufacturing method by using polymerisation
Abstract
The invention discloses a 3dB directional coupler capable of shortening coupling length and a manufacturing method thereof, wherein the 3dB directional coupler comprises a substrate, a lower cladding, a middle cladding and an upper cladding are sequentially covered on the substrate from bottom to top, a first coupling waveguide is arranged in the middle cladding, a second coupling waveguide is arranged in the upper cladding, an arc waveguide and a straight waveguide are respectively arranged on the first coupling waveguide and the second coupling waveguide, and the arc waveguide is arranged in the middle of the straight waveguide; the protruding portion of the arc waveguide of the first coupling waveguide corresponds to the protruding portion of the arc waveguide of the second coupling waveguide and constitutes a coupling region. According to the invention, the first coupling waveguide and the second coupling waveguide are not in the same plane, the first coupling waveguide and the second coupling waveguide form a coupling region, and after incident light enters the coupling region, electric signals are coupled in the first coupling waveguide and the second coupling waveguide which are distributed up and down through evanescent fields, so that the coupling length of the directional coupler can be effectively shortened, the insertion loss is reduced, the process tolerance is enlarged, and the coupling efficiency is improved.
Description
Technical Field
The invention belongs to the technical field of communication device processing, and particularly relates to a 3dB directional coupler capable of shortening coupling length and a manufacturing method thereof.
Background
With the advent and rapid development of optical fiber communication technology, the worldwide demand for optical fiber communication technology is increasing. The directional coupler can transmit and distribute optical signals, is an optical passive device which is most used in optical fiber communication, and is an important optical device for controlling an optical path in technical engineering. The design of the directional coupler with small insertion loss, high coupling efficiency and large process tolerance becomes a hot point of research in recent years.
In the waveguide coupler, two coupling waveguides are on the same plane, and optical signals are coupled back and forth at the left and right of a waveguide coupling area, so that the directional coupler of the parallel waveguides is sensitive to process errors of the device size, and the length of the coupling area and the width of the waveguides have obvious influence on the output result of the directional coupler. Due to the existence of process errors, the coupling efficiency of the directional coupler is reduced, and meanwhile, the loss of output power is caused.
Disclosure of Invention
Aiming at the problems of small process tolerance and low coupling efficiency of the existing directional coupler, the invention provides the 3dB directional coupler with the shortened coupling length and the manufacturing method thereof, the coupling length is greatly shortened by utilizing the waveguide coupling principle, and the directional coupler has the characteristic of high tolerance.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows:
a3 dB directional coupler capable of shortening coupling length comprises a substrate, wherein a lower cladding, a middle cladding and an upper cladding are sequentially covered on the substrate from bottom to top, a first coupling waveguide is arranged in the middle cladding, a second coupling waveguide is arranged in the upper cladding, an arc waveguide and a straight waveguide are arranged on the first coupling waveguide and the second coupling waveguide, and the arc waveguide is arranged in the middle of the straight waveguide; the arc-shaped waveguide is connected with the straight waveguide, the protruding part of the arc-shaped waveguide of the first coupling waveguide corresponds to the protruding part of the arc-shaped waveguide of the second coupling waveguide to form a coupling region, and when incident light enters from one side of the first coupling waveguide or the second coupling waveguide, due to interaction of evanescent fields between the waveguides, the waveguides are coupled in the coupling region, so that the coupling length of the waveguides is shortened.
The waveguide width and the waveguide thickness of the first coupling waveguide and the second coupling waveguide are both 6 μm, and the vertical distance between the first coupling waveguide and the second coupling waveguide is 1.98 μm.
The arc waveguides comprise a first arc waveguide, a second arc waveguide, a third arc waveguide and a fourth arc waveguide, the straight waveguides comprise a first straight waveguide and a second straight waveguide, and the first straight waveguide and the second straight waveguide are symmetrically arranged; one end of the first straight waveguide is connected with one end of the first arc waveguide, the other end of the first arc waveguide is connected with one end of the second arc waveguide, the other end of the second arc waveguide is connected with one end of the third arc waveguide, the other end of the third arc waveguide is connected with one end of the fourth arc waveguide, and the other end of the fourth arc waveguide is connected with one end of the second straight waveguide; the arc formed by connecting the first arc-shaped waveguide and the second arc-shaped waveguide is symmetrical to the arc formed by connecting the third arc-shaped waveguide and the fourth arc-shaped waveguide, and the bending direction of the first arc-shaped waveguide is opposite to that of the second arc-shaped waveguide.
The bending radii of the first arc-shaped waveguide, the second arc-shaped waveguide, the third arc-shaped waveguide and the fourth arc-shaped waveguide are 12000 mu m.
The bending angles of the first arc-shaped waveguide, the second arc-shaped waveguide, the third arc-shaped waveguide and the fourth arc-shaped waveguide are all 2.5 degrees, namely the arc center angles of arcs corresponding to the first arc-shaped waveguide, the second arc-shaped waveguide, the third arc-shaped waveguide and the fourth arc-shaped waveguide are 2.5 degrees.
The substrate is made of monocrystalline silicon or quartz plate.
A method for manufacturing a 3dB directional coupler with shortened coupling length comprises the following steps:
s1, cleaning the surface of the substrate;
s2, forming a lower cladding layer on the substrate by using a thermal oxidation method;
s3, forming a first germanium-doped silicon dioxide waveguide layer on the lower cladding layer by a chemical vapor deposition method;
s4, forming a first polysilicon hard mask layer on the first germanium-doped silicon dioxide waveguide layer by using a chemical vapor deposition method;
s5, coating a first photoresist on the first polysilicon hard mask layer, transferring the pattern of the first coupling waveguide on the photoetching plate I to the first photoresist, and forming a basic pattern of the first coupling waveguide on the first polysilicon hard mask layer according to the pattern on the first photoresist by a photoetching technology;
s6, removing the first photoresist through etching;
s7, etching the first germanium-doped silicon dioxide waveguide layer according to the basic pattern of the first coupling waveguide on the first polysilicon hard mask layer by using an inductive coupling plasma etching method;
s8, removing the first polysilicon hard mask layer through wet cleaning to obtain a first coupling waveguide;
s9, growing an intermediate cladding on the lower cladding and the first coupling waveguide by a low-stress boron-phosphorus-silicon glass doped method;
s10, forming a second germanium-doped silicon dioxide waveguide layer on the middle cladding layer by a chemical vapor deposition method;
s11, forming a second polysilicon hard mask layer on the second germanium-doped silicon dioxide waveguide layer by using a chemical vapor deposition method;
s12, coating a second photoresist on the second polysilicon hard mask layer, transferring the pattern of the second coupling waveguide on the photoetching plate II onto the second photoresist, and forming a basic pattern of the second coupling waveguide on the second polysilicon hard mask layer by the photoetching technology according to the pattern on the second photoresist;
s13, removing the second photoresist through etching;
s14, etching the second germanium-doped silicon dioxide waveguide layer according to the basic pattern of the second coupling waveguide on the second polysilicon hard mask layer by using an inductive coupling plasma etching method;
s15, removing the second polysilicon hard mask layer through wet cleaning to obtain a second coupling waveguide;
s16, forming an upper cladding on the middle cladding and the second coupling waveguide by using a low-stress doped borophosphosilicate glass method;
and S17, annealing the upper cladding, and finishing the manufacture of the 3dB directional coupler after the annealing is finished.
The thicknesses of the first polysilicon hard mask layer and the second polysilicon hard mask layer are both 1 mu m.
The thickness of the lower cladding is 15 mu m, and the thickness of the middle cladding is 7.98 mu m; the thickness of the upper cladding layer was 10 μm.
In step S17, the annealing temperature during the annealing treatment is in the range of 800-1100 ℃ and the duration time is 3-5 hours.
The invention has the beneficial effects that:
according to the invention, the first coupling waveguide and the second coupling waveguide are not in the same plane, the first coupling waveguide and the second coupling waveguide form a coupling region, and after incident light enters the coupling region, electric signals are coupled in the first coupling waveguide and the second coupling waveguide which are distributed up and down through evanescent fields, so that the coupling length of the directional coupler can be effectively shortened, the insertion loss is reduced, the process tolerance is enlarged, and the coupling efficiency is improved.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a front view of the present invention.
Fig. 2 is a schematic structural diagram of the present invention.
FIG. 3 is a diagram of a model for simulating the present invention in Rosft software.
FIG. 4 is a graphical illustration of the energy coupling of the present invention in the operating state in Rosft.
FIG. 5 is a graph of energy coupling values for the invention in the operating state in Rosft.
FIG. 6 is a graph of output power versus simulated operating conditions in Rosft of the present invention after introducing an error.
In the figure, 1 is a first coupling waveguide, 2 is a second coupling waveguide, 3 is a lower cladding, 4 is an intermediate cladding, 5 is an upper cladding, 6 is a substrate, 7 is a first straight waveguide, 8 is a second straight waveguide, 9 is a first arc waveguide, 10 is a second arc waveguide, 11 is a third arc waveguide, and 12 is a fourth arc waveguide.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without inventive effort based on the embodiments of the present invention, are within the scope of the present invention.
Example 1: a3 dB directional coupler with shortened coupling length is disclosed, as shown in FIG. 1, comprising a substrate 6, wherein the material of the substrate 6 is monocrystalline silicon or quartz plate; the substrate 6 is sequentially covered with a lower cladding 3, a middle cladding 4 and an upper cladding 5 from bottom to top, a first coupling waveguide 1 is arranged in the middle cladding 4, a second coupling waveguide 2 is arranged in the upper cladding 5, and the first coupling waveguide 1 and the second coupling waveguide 2 are arranged in parallel; the first coupling waveguide 1 and the second coupling waveguide 2 are both provided with an arc waveguide and a straight waveguide, the arc waveguide is arranged in the middle of the straight waveguide, and the arc waveguide is connected with the straight waveguide; the protruding part of the arc-shaped waveguide of the first coupling waveguide 1 corresponds to the protruding part of the arc-shaped waveguide of the second coupling waveguide 2 and forms a coupling region, when incident light enters from one side of the first coupling waveguide 1 or the second coupling waveguide 2, due to the interaction of evanescent fields between the waveguides, the waveguides are coupled in the coupling region, and further the coupling length of the waveguides is shortened.
The first coupling waveguide 1 and the second coupling waveguide 2 are both rectangular waveguides, and the waveguide widths and thicknesses of the first coupling waveguide 1 and the second coupling waveguide 2 are both 6 micrometers; the vertical distance between the first coupling waveguide 1 and the second coupling waveguide 2 is 1.98 μm.
As shown in fig. 2, the arc waveguides include a first arc waveguide 9, a second arc waveguide 10, a third arc waveguide 11 and a fourth arc waveguide 12, the straight waveguides include a first straight waveguide 7 and a second straight waveguide 8, and the first straight waveguide 7 and the second straight waveguide 8 are symmetrically arranged; one end of the first straight waveguide 7 is connected with one end of a first arc-shaped waveguide 9, the other end of the first arc-shaped waveguide 9 is connected with one end of a second arc-shaped waveguide 10, the other end of the second arc-shaped waveguide 10 is connected with one end of a third arc-shaped waveguide 11, the other end of the third arc-shaped waveguide 11 is connected with one end of a fourth arc-shaped waveguide 12, and the other end of the fourth arc-shaped waveguide 12 is connected with one end of a second straight waveguide 8; the arc formed by connecting the first arc-shaped waveguide 9 and the second arc-shaped waveguide 10 is symmetrical to the arc formed by connecting the third arc-shaped waveguide 11 and the fourth arc-shaped waveguide 12, and the bending direction of the first arc-shaped waveguide 9 is opposite to that of the second arc-shaped waveguide 10.
The bending radii of the first arc-shaped waveguide 9, the second arc-shaped waveguide 10, the third arc-shaped waveguide 11 and the fourth arc-shaped waveguide 12 are 12000 mu m.
The bending processing angles of the first arc-shaped waveguide 9, the second arc-shaped waveguide 10, the third arc-shaped waveguide 11 and the fourth arc-shaped waveguide 12 are all 2.5 degrees, namely the arc center angles of arcs corresponding to the first arc-shaped waveguide 9, the second arc-shaped waveguide 10, the third arc-shaped waveguide 11 and the fourth arc-shaped waveguide 12 are 2.5 degrees; the centers of the arcs of the first and fourth arc- shaped waveguides 9 and 12 are arranged at the inner sides of the first and second coupling waveguides 1 and 2.
A method for manufacturing a 3dB directional coupler with shortened coupling length comprises the following steps:
s1, the surface of the substrate 6 is cleaned using the cleaning solution.
S2, a lower cladding layer 3 is formed on the surface of the substrate 6 by thermal oxidation, in this embodiment, the thickness of the lower cladding layer 3 is 15 μm.
S3, forming a first germanium-doped silica waveguide layer on the lower cladding layer 3 by chemical vapor deposition.
S4, forming a first polysilicon hard mask layer on the first germanium-doped silicon dioxide waveguide layer by a chemical vapor deposition method, wherein the thickness of the first polysilicon hard mask layer is 1 μm; the first polysilicon hard mask layer covers the lower cladding layer 3 and the first germanium-doped silicon dioxide waveguide layer so as to etch the first coupling waveguide 1 on the first germanium-doped silicon dioxide waveguide layer; in the existing etching technology, because the mask layer has certain loss in the process of etching the waveguide layer, if the etching depth is deeper, certain requirements are made on the thickness of the mask layer, so if a photoresist is directly used as the mask layer, the photoresist with very high thickness is needed, and if polysilicon is used as a hard mask layer, the loss in the etching process is very small, and if the photoresist is used, the temperature has great change in the etching process, the colloid can be burnt at high temperature; therefore, the arrangement of the first polysilicon hard mask layer can enable the etching process to be more stable and consumption-resistant.
S5, coating a first photoresist on the first polysilicon hard mask layer, transferring the pattern of the first coupling waveguide 1 on the reticle I pre-fabricated in the L-Edit software onto the first photoresist by the techniques of glue coating (positive glue), exposure, development, and fixation, and forming the basic pattern of the first coupling waveguide 1 on the first polysilicon hard mask layer by the photolithography technique according to the pattern on the first photoresist.
And S6, removing the first photoresist by an etching method.
And S7, etching the first germanium-doped silicon dioxide waveguide layer according to the basic pattern of the first coupling waveguide 1 on the first polysilicon hard mask layer by using an inductive coupling plasma etching method.
And S8, removing the first polysilicon hard mask layer by wet cleaning to obtain the first coupling waveguide 1.
S9, growing an intermediate cladding 4 on the lower cladding 3 and the first coupling waveguide 1 by low stress doped borophosphosilicate glass, wherein the thickness of the intermediate cladding 4 is 7.98 μm in this embodiment.
S10, forming a second ge-doped silica waveguide layer on the intermediate cladding layer 4 by chemical vapor deposition.
S11, forming a second polysilicon hard mask layer on the second germanium-doped silicon dioxide waveguide layer by a chemical vapor deposition method, wherein the thickness of the second polysilicon hard mask layer is 1 μm; the second polysilicon hard mask layer covers the middle cladding layer 4 and the second germanium-doped silicon dioxide waveguide layer so as to etch the second coupling waveguide 2 on the second germanium-doped silicon dioxide waveguide layer; the second polysilicon hard mask layer is arranged, so that the etching process is more stable and consumption-resistant.
And S12, coating a second photoresist on the second polysilicon hard mask layer, transferring the pattern of the second coupling waveguide 2 on the prefabricated photoetching plate II onto the second photoresist through the photoetching technology, and forming the basic pattern of the second coupling waveguide 2 on the second polysilicon hard mask layer through the photoetching technology according to the pattern on the second photoresist.
And S13, removing the second photoresist by etching.
And S14, etching the second germanium-doped silicon dioxide waveguide layer according to the basic pattern of the second coupling waveguide 2 on the second polysilicon hard mask layer by using an inductive coupling plasma etching method.
And S15, removing the second polysilicon hard mask layer by wet cleaning to obtain the second coupling waveguide 2.
And S16, forming an upper cladding 5 on the intermediate cladding 4 and the second coupling waveguide 2 by a low-stress boron-phosphorus-silicon glass doping method, wherein the thickness of the upper cladding 5 is 10 μm in the embodiment.
S17, annealing the upper cladding layer at 800-1100 deg.C for 3-5 h, and finishing the manufacture of 3dB directional coupler.
The following simulation of the operating state of the 3dB directional coupler of this embodiment is performed by using rsoft software to specifically illustrate the influence of the waveguide structure of the 3dB directional coupler on the coupling efficiency.
As shown in fig. 3, a 3dB directional coupler was modeled in the rsoft software, and the refractive indexes of the upper cladding 5, the middle cladding 4 and the lower cladding 3 were set to be 1.4447, the refractive indexes of the first coupling waveguide 1 and the second coupling waveguide 2 were 1.4556 (i.e., the refractive index difference was 0.0108), the cross-sectional dimensions of the second coupling waveguide 2 and the first coupling waveguide 1 were 6 μm × 6 μm, the vertical distance between the coupling regions of the second coupling waveguide 2 and the first coupling waveguide 1 was 1.98 μm, the length of the rectangle formed by the first coupling waveguide 1 and the second coupling waveguide 2 was set to be 4000 μm, and the wavelength of the incident light was 1550 nm.
The energy change condition of the 3dB directional coupler in this embodiment in the simulated operating state is shown in fig. 4 and 5, light is input from the port of the first coupling waveguide 1, light energy is output after being coupled and output through the coupling regions of the first coupling waveguide 1 and the second coupling waveguide 2, and the output result is almost equal to the average distribution; compared with the coupling length of the optical energy in the coupling process of the traditional 3dB directional coupler, the coupling requirement can be met by coupling once in the coupling area, the coupling times are reduced, and the coupling length of the light is shortened, so that the loss of the output power is effectively reduced, and the coupling result is very ideal.
Example 2: the present embodiment is different from embodiment 1 in that a waveguide width error and a coupling region vertical distance error are introduced in the present embodiment.
The 3dB directional coupler model of example 1, in which the waveguide has a width and a height of 6 μm, was simulated in rsoft according to the standard model size, and the optical energy output powers of the first coupling waveguide 1 and the second coupling waveguide 2 are represented by dots on the solid line in fig. 6. However, in the actual process, the waveguide size is subject to technical influences to generate certain errors, and the waveguide width fluctuates up and down in the required size of 6 μm. In order to examine the tolerance degree of the present invention, in the present embodiment, the waveguide width of the first coupling waveguide 1 was set to 6.05 μm, and the waveguide width of the second coupling waveguide 2 was set to 5.95 μm, so that the width error of the two coupling waveguides was 0.1 μm, which is a limit condition of the simulation process error. The light energy output power of the introduced waveguide error is represented by a point on a dotted line in fig. 6, and it can be seen that the difference distance between the point on the solid line and the point on the dotted line is very short, that is, after the waveguide width processing error of 0.1 μm is introduced, compared with the waveguide output value without the processing error, the output value curves of the first coupling waveguide 1 and the second coupling waveguide 2 before and after the introduction of the error are nearly coincident, which shows that the waveguide width error has little influence on the output result of the 3dB directional coupler, so the 3dB directional coupler of the present invention has higher process tolerance and stable working coupling output.
And introducing a vertical distance error of the waveguide coupling region, respectively simulating output results of the vertical distance of the waveguide coupling region from 1.94 mu m to 2.06 mu m in the rsoft simulation, and setting other simulation parameters completely consistent with those in the embodiment 1.
Since the height of the intermediate cladding layer 4 directly determines the vertical distance of the coupling region, machining errors are introduced in the intermediate cladding layer 4, i.e. the vertical distance of the waveguide coupling region fluctuates around 1.98 μm under the influence of the process, represented in fig. 6 as points on different abscissas. It can be seen that after the processing error of the intermediate cladding 4 is introduced, the output result of the light energy in the waveguide has a linear change along with the increase of the error, and the output fluctuation caused by the processing error can be controlled at 3.4 dB/mum according to the simulated output value, so that the processing tolerance of the 3dB directional coupler is greatly improved.
In summary, the 3dB directional coupler uses the waveguide coupling principle to distribute the optical paths, and compared with the conventional directional coupler with the waveguide horizontally distributed, the 3dB directional coupler shortens the coupling length, optimizes the coupling efficiency, and improves the waveguide processing tolerance.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (10)
1. A3 dB directional coupler capable of shortening coupling length comprises a substrate (6), and is characterized in that the substrate (6) is sequentially covered with a lower cladding (3), a middle cladding (4) and an upper cladding (5) from bottom to top, a first coupling waveguide (1) is arranged in the middle cladding (4), a second coupling waveguide (2) is arranged in the upper cladding (5), an arc waveguide and a straight waveguide are arranged on the first coupling waveguide (1) and the second coupling waveguide (2), and the arc waveguide is arranged in the middle of the straight waveguide; the protruding part of the arc-shaped waveguide of the first coupling waveguide (1) corresponds to the protruding part of the arc-shaped waveguide of the second coupling waveguide (2) and forms a coupling area.
2. The shortened coupling length 3dB directional coupler according to claim 1, wherein the first coupling waveguide (1) and the second coupling waveguide (2) have waveguide widths and thicknesses of 6 μm, and the vertical distance between the first coupling waveguide (1) and the second coupling waveguide (2) is 1.98 μm.
3. The shortened coupling length 3dB directional coupler according to claim 1 or 2, characterized in that the arc-shaped waveguides include a first arc-shaped waveguide (9), a second arc-shaped waveguide (10), a third arc-shaped waveguide (11) and a fourth arc-shaped waveguide (12), the straight waveguides include a first straight waveguide (7) and a second straight waveguide (8), and the first straight waveguide (7) and the second straight waveguide (8) are symmetrically arranged; one end of the first straight waveguide (7) is connected with one end of a first arc-shaped waveguide (9), the other end of the first arc-shaped waveguide (9) is connected with one end of a second arc-shaped waveguide (10), the other end of the second arc-shaped waveguide (10) is connected with one end of a third arc-shaped waveguide (11), the other end of the third arc-shaped waveguide (11) is connected with one end of a fourth arc-shaped waveguide (12), and the other end of the fourth arc-shaped waveguide (12) is connected with one end of a second straight waveguide (8); the arc formed by connecting the first arc-shaped waveguide (9) and the second arc-shaped waveguide (10) is symmetrical to the arc formed by connecting the third arc-shaped waveguide (11) and the fourth arc-shaped waveguide (12), and the bending direction of the first arc-shaped waveguide (9) is opposite to that of the second arc-shaped waveguide (10).
4. The shortened coupling length 3dB directional coupler according to claim 3, characterized in that the first (9), second (10), third (11) and fourth (12) arc waveguides have a bend radius of 12000 μm.
5. The shortened coupling length 3dB directional coupler according to claim 3, characterized in that the bending angles of the first arc-shaped waveguide (9), the second arc-shaped waveguide (10), the third arc-shaped waveguide (11) and the fourth arc-shaped waveguide (12) are all 2.5 °.
6. The shortened coupling length 3dB directional coupler according to claim 3, characterized in that the material of the substrate (6) is a single crystal silicon or quartz plate.
7. A method for manufacturing a 3dB directional coupler with shortened coupling length is characterized by comprising the following steps:
s1, cleaning the surface of the substrate (6);
s2, forming a lower cladding (3) on the substrate (6);
s3, forming a first germanium-doped silicon dioxide waveguide layer on the lower cladding layer (3);
s4, forming a first polysilicon hard mask layer on the first germanium-doped silicon dioxide waveguide layer;
s5, coating a first photoresist on the first polysilicon hard mask layer, transferring the pattern of the first coupling waveguide (1) on the photoetching plate I to the first photoresist, and forming a basic pattern of the first coupling waveguide (1) on the first polysilicon hard mask layer by a photoetching technology according to the pattern on the first photoresist;
s6, removing the first photoresist;
s7, etching the first Ge-doped silica waveguide layer according to the basic pattern of the first coupling waveguide (1) on the first polysilicon hard mask layer;
s8, removing the first polysilicon hard mask layer to obtain a first coupling waveguide (1);
s9, growing an intermediate cladding (4) on the lower cladding (3) and the first coupling waveguide (1);
s10, forming a second germanium-doped silicon dioxide waveguide layer on the intermediate cladding layer (4);
s11, forming a second polysilicon hard mask layer on the second germanium-doped silicon dioxide waveguide layer;
s12, coating a second photoresist on the second polysilicon hard mask layer, transferring the pattern of the second coupling waveguide (2) on the photoetching plate II onto the second photoresist, and forming a basic pattern of the second coupling waveguide (2) on the second polysilicon hard mask layer by the photoetching technology according to the pattern on the second photoresist;
s13, removing the second photoresist;
s14, etching the second germanium-doped silicon dioxide waveguide layer according to the basic pattern of the second coupling waveguide (2) on the second polysilicon hard mask layer;
s15, removing the second polysilicon hard mask layer to obtain a second coupling waveguide (2);
s16, forming an upper cladding (5) on the intermediate cladding (4) and the second coupling waveguide (2);
and S17, annealing the upper cladding, and finishing the manufacture of the 3dB directional coupler after the annealing is finished.
8. The method of claim 7, wherein the first polysilicon hard mask layer and the second polysilicon hard mask layer are both 1 μm thick.
9. The method of manufacturing a 3dB directional coupler with a shortened coupling length as claimed in claim 7 or 8, wherein the thickness of the lower cladding (3) is 15 μm, and the thickness of the middle cladding (4) is 7.98 μm; the thickness of the upper cladding (5) is 10 μm.
10. The method as claimed in claim 9, wherein the annealing temperature is in the range of 800-1100 ℃ for 3-5 hours during the annealing process in step S17.
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