A broadband polarization beam splitter based on directional coupling

Abstract

The present invention relates to a broadband polarization beam splitter based on directional coupling, belonging to the field of photon integration technology, comprising a silicon substrate, a silicon waveguide core layer and a silicon dioxide cladding layer; the silicon waveguide core layer is located inside the silicon dioxide cladding layer; the silicon dioxide cladding layer is located on the upper surface of the silicon substrate; the silicon waveguide core layer comprises an input waveguide, a first section of a directional coupler curved waveguide, a second section of a directional coupler curved waveguide, a third section of a directional coupler curved waveguide, a first section of a filter curved waveguide, a second section of a filter curved waveguide, a TE polarization output waveguide and a TM polarization output waveguide. The present invention introduces a sub-wavelength grating structure into the coupler, so that the TE polarization state is in a reflection state in the coupling region, reducing its coupling efficiency, and the TM polarization state has almost no effect, thereby ensuring a high TM coupling efficiency and improving the extinction ratio of the TE polarization state; the invention can be prepared by an integrated process, the process is simple and the tolerance is large, and the invention has the advantages of large bandwidth, high polarization extinction ratio, small insertion loss and small size.

Description

Technical Field

[0001] The invention relates to a broadband polarization beam splitter based on directional coupling, belonging to the technical field of photon integration. Background Art

[0002] There is a high refractive index difference between the cladding and core of the silicon-on-insulator waveguide, which can effectively confine the light field. However, the high refractive index difference can also lead to problems such as polarization mode dispersion and polarization-related loss. In optical communication technology, since the optical signal transmitted through the optical fiber is randomly polarized, the device is required to be insensitive to polarization. Polarization diversity schemes are usually used to solve the polarization problem of photonic devices. Polarization beam splitters can split or combine two orthogonal polarization modes of light and are important key devices for eliminating polarization sensitivity in polarization diversity schemes.

[0003] In recent years, people have developed polarization beam splitter structures based on directional couplers, photonic crystals, multimode interference couplers (MMI), hybrid plasma silicon waveguides and other structures. The polarization beam splitter structure based on photonic crystals is relatively complex, has large insertion loss, and is not easy to prepare. The polarization beam splitter structure based on multimode interference couplers is very large. Although the size can be reduced through quasi-static imaging and cascade structures, the size is still in the mm range. The polarization beam splitter structure based on Mach-Zehnder interferometer (MZI) is relatively easy to make, but the size is often relatively large and the bandwidth is relatively narrow. Hybrid plasma waveguides can enhance coupling and shorten the device length, but this waveguide suffers additional losses due to metal absorption.

[0004] Directional coupling polarization beam splitters have the advantages of simple design and manufacturing, small size, easy integration, and good performance. In recent years, they have attracted widespread attention from researchers. However, ordinary directional coupling polarization beam splitters have a small working bandwidth and high crosstalk. Ordinary grating couplers have low coupling efficiency due to the energy leakage of light in the grating. Summary of the invention

[0005] The object of the present invention is to provide a broadband polarization beam splitter based on directional coupling type, which realizes small size, low insertion loss, high extinction ratio characteristic and broadband transmission characteristic.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A broadband polarization beam splitter based on directional coupling type comprises a silicon substrate, a silicon waveguide core layer and a silicon dioxide cladding layer; the silicon waveguide core layer is located inside the silicon dioxide cladding layer; the silicon dioxide cladding layer is located on the upper surface of the silicon substrate;

[0008] The silicon waveguide core layer comprises an input waveguide, a first section of a curved waveguide of a directional coupler, a second section of a curved waveguide of a directional coupler, a third section of a curved waveguide of a directional coupler, a first section of a curved waveguide of a filter, a second section of a curved waveguide of a filter, a TE polarization output waveguide and a TM polarization output waveguide; the input waveguide, the first section of a curved waveguide of a directional coupler, the second section of a curved waveguide of a filter and the TE polarization output waveguide are connected in sequence; the third section of a curved waveguide of a directional coupler is connected to the TM polarization output waveguide;

[0009] The first section of the directional coupler curved waveguide, the second section of the directional coupler curved waveguide, and the third section of the directional coupler curved waveguide are arranged in sequence from top to bottom and have the same center.

[0010] The first section of the curved waveguide of the filter is located above the second section of the curved waveguide of the filter, and the front end of the curved waveguide of the first section of the curved waveguide of the filter has the same center as that of the second section of the curved waveguide of the filter;

[0011] The second section of the curved waveguide of the directional coupler is a sub-wavelength grating structure, and the grating structure is connected by a connecting bridge. The effective optical path of the TM polarization of the first section of the curved waveguide of the directional coupler, the second section of the curved waveguide of the directional coupler, and the third section of the curved waveguide of the directional coupler are the same, so as to couple the TM polarization. The TE polarization of the second section of the curved waveguide of the directional coupler is in a reflection state, so as to filter the TE polarization.

[0012] A further improvement of the technical solution of the present invention is that: the input waveguide has a height of 220 nm, a width of 550 nm, a bending angle of 28.5°, and a bending radius of 8 μm.

[0013] A further improvement of the technical solution of the present invention is that the first section of the curved waveguide of the directional coupler has a height of 220 nm, a width of 550 nm, a bending angle of 28.5°, and a bending radius of 18.5 μm.

[0014] The further improvement of the technical solution of the present invention is that the height of the second section of the curved waveguide of the directional coupler is 220nm, the grating period is 500nm, the duty cycle is 0.8, the nano island width is 490nm, the connecting bridge width is 15nm, the bending angle is 28.5°, and the bending radius is 19.1μm.

[0015] A further improvement of the technical solution of the present invention is that the third section of the curved waveguide of the directional coupler has a height of 220 nm, a width of 390 nm, a bending angle of 28.5°, and a bending radius of 19.65 μm.

[0016] A further improvement of the technical solution of the present invention is that the coupling spacing between the first curved waveguide section of the directional coupler and the second curved waveguide section of the directional coupler is 80nm; the coupling spacing between the second curved waveguide section of the directional coupler and the third curved waveguide section of the directional coupler is 110nm.

[0017] A further improvement of the technical solution of the present invention is that the first section of the curved waveguide of the directional coupler, the second section of the curved waveguide of the directional coupler and the third section of the curved waveguide of the directional coupler meet the phase matching condition of the TM polarization state.

[0018] A further improvement of the technical solution of the present invention is that: the first section of the curved waveguide of the filter has a height of 220 nm, a width of 340 nm, a bending angle of 30°, and a bending radius of 8.7 μm.

[0019] A further improvement of the technical solution of the present invention is that the second section of the curved waveguide of the filter has a height of 220 nm, a width of 550 nm, a bending angle of 30°, and a bending radius of 8 μm.

[0020] A further improvement of the technical solution of the present invention is that the first section of the curved waveguide of the filter and the second section of the curved waveguide of the filter meet the phase matching condition of the TM polarization state.

[0021] Due to the adoption of the above technical solution, the technical effects achieved by the present invention are as follows:

[0022] The present invention introduces a sub-wavelength grating structure into the coupler, so that the TE polarization state is in a reflective state in the coupling region, reducing its coupling efficiency, while the TM polarization state has almost no effect, thus greatly improving the extinction ratio of the TE polarization state under the premise of ensuring high TM coupling efficiency. The formation principle of the coupler is based on the phase matching principle. By changing the width of the three-segment waveguide of the directional coupler, the effective optical path of the TE polarization state in the waveguide is different, while the effective optical path of the TM polarization state is the same, which leads to TM phase matching and effective coupling to the TM output waveguide.

[0023] In order to further improve the extinction ratio of the TM polarization state, the present invention uses a polarization filter connected in series to the TE output port of the coupler, which satisfies the phase matching condition for the TM polarization state. The polarization filter at the TE output port of the coupler can filter out the TM polarization state that is not fully coupled to the grating waveguide, thereby improving the working bandwidth of the device.

[0024] The device of the present invention can be prepared by using an integrated process with a simple process and a large tolerance, a small size, and can achieve a high extinction ratio in a wide wavelength range. It is easy to integrate with other devices and has important research and application value in the fields of polarization control in photon integration and polarization multiplexing in long-distance transmission of optical communications. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a top view of the silicon waveguide core layer of the present invention;

[0026] Figure 2 is a top view of the second section of the curved waveguide of the directional coupler of the present invention;

[0027] Figure 3 It is a schematic diagram of the relationship between the transmittance of the TM and TE modes input into the second section of the curved waveguide of the directional coupler of the present invention and the grating period;

[0028] Figure 4 It is the electric field diagram when the polarization beam splitter of the present invention inputs the TM mode and the TE mode;

[0029] Figure 5 It is a schematic diagram of the relationship between the polarization extinction ratio and insertion loss of the TM and TE modes input to the polarization beam splitter of the present invention and the input wavelength;

[0030] Figure 6 It is a schematic diagram of the relationship between the polarization extinction ratio and insertion loss of the TM and TE modes and the input wavelength when the waveguide width of the polarization beam splitter of the present invention is transformed to ±20nm;

[0031] Among them, 1. input waveguide, 2. first section of curved waveguide of directional coupler, 3. second section of curved waveguide of directional coupler, 4. third section of curved waveguide of directional coupler, 5. first section of curved waveguide of filter, 6. second section of curved waveguide of filter, 7. TE polarization output waveguide, 8. TM polarization output waveguide. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0033] The accompanying drawings show various structural schematic diagrams according to embodiments of the present disclosure. These drawings are not drawn to scale, and some details are magnified and some details may be omitted for the purpose of clear expression. The various regions, shapes, and relative sizes and positional relationships shown in the drawings are only exemplary and may deviate in practice due to manufacturing tolerances or technical limitations.

[0034] A broadband polarization beam splitter based on directional coupling type comprises a silicon substrate, a silicon waveguide core layer and a silicon dioxide cladding layer; the silicon waveguide core layer is located inside the silicon dioxide cladding layer; the silicon dioxide cladding layer is located on the upper surface of the silicon substrate.

[0035] like Figure 1 As shown, the silicon waveguide core layer includes an input waveguide 1, a first section of a directional coupler curved waveguide 2, a second section of a directional coupler curved waveguide 3, a third section of a directional coupler curved waveguide 4, a first section of a filter curved waveguide 5, a second section of a filter curved waveguide 6, a TE polarization output waveguide 7 and a TM polarization output waveguide 8; the input waveguide 1, the first section of the directional coupler curved waveguide 2, the second section of the filter curved waveguide 6 and the TE polarization output waveguide 7 are connected in sequence; the third section of the directional coupler curved waveguide 4 is connected to the TM polarization output waveguide 8.

[0036] The directional coupler is composed of three curved silicon waveguides of different widths. The second curved waveguide 3 (intermediate waveguide) of the directional coupler is designed as a sub-wavelength grating structure. By reasonably designing various parameters of the waveguide, the TE polarized light does not meet the phase matching condition in the coupling region. Therefore, the TE polarized light cannot be coupled between the waveguides, but is directly output from the through end; the optical path of the TM polarized light in the waveguide of the coupling region is the same, that is, the phase matching equation (1) is satisfied, and cross-coupling output is achieved.

[0037] OPL=N1R1K0θ0=N2R2K0θ0=N3R3K0θ0 (1)

[0038] Among them, N1, N2, and N3 represent the effective refractive index of polarized light in the curved waveguide with bending radius R1, R2, and R3, respectively, K0 is the wave vector number in vacuum, and θ0 is the bending angle of the curved waveguide. Since the wave vector number K0 is the same and the bending angles are approximately equal when polarized light is transmitted in the three curved waveguides, the phase matching condition is mainly determined by the product of the bending radius R of the waveguide and the effective refractive index when the mode is transmitted in the waveguide.

[0039] Preferably, the input waveguide 1 has a height of 220 nm, a width of 550 nm, a bending angle of 28.5°, and a bending radius of 8 μm.

[0040] Preferably, the first section of the curved waveguide 2 of the directional coupler has a height of 220 nm, a width of 550 nm, a bending angle of 28.5°, and a bending radius of 18.5 μm.

[0041] Preferably, the second section of the curved waveguide 3 of the directional coupler has a height of 220 nm, a grating period of 500 nm, a duty cycle of 0.8, a nano-island width of 490 nm, a connecting bridge width of 15 nm, a bending angle of 28.5°, and a bending radius of 19.1 μm.

[0042] Preferably, the third section of the curved waveguide 4 of the directional coupler has a height of 220 nm, a width of 390 nm, a bending angle of 28.5°, and a bending radius of 19.65 μm.

[0043] The above waveguide parameters are finally selected through calculation and simulation optimization of phase matching equation (1). The first curved waveguide 2 of the directional coupler, the second curved waveguide 3 of the directional coupler, and the third curved waveguide 4 of the directional coupler are arranged from top to bottom and have the same center.

[0044] Preferably, the first section of the curved waveguide 5 of the filter has a height of 220 nm, a width of 340 nm, a bending angle of 30°, and a bending radius of 8.7 μm.

[0045] Preferably, the second section of the curved waveguide 6 of the filter has a height of 220 nm, a width of 550 nm, a bending angle of 30°, and a bending radius of 8 μm.

[0046] It should be noted that the device parameters provided in this embodiment are only typical values ​​for illustrating the principle. When it comes to specific processing technology, other reasonable values ​​may be used, but they must comply with the working principle of the device.

[0047] like Figure 2 As shown in the figure, the second section of the curved waveguide of the directional coupler is connected by a connecting bridge, which makes the structure of the silicon nano-island more solid and convenient for manufacturing. At the same time, the effective refractive index of the grating waveguide can be increased. When the period and width remain unchanged, the duty cycle can be reduced, which is convenient for manufacturing. In order to simplify the manufacturing, the width of the connecting bridge W b It should be as large as possible. At the same time, in order to make the TE polarization reflection efficiency good, W b It should not be too large, so choose W b =0.15μm.

[0048] like Figure 2 As shown, for a sub-wavelength grating, according to the effective medium theory, the sub-wavelength grating waveguide can be equivalent to a uniform medium, and the effective refractive index can be calculated according to formula (2).

[0049]

[0050] Where n1 and n2 are the equivalent refractive indices of the silicon nanoislands and the connecting bridges of the corresponding polarizations, respectively, and f is the duty cycle of the grating waveguide, which is set to 0.8.

[0051] By optimizing the waveguide size and selecting a suitable grating period, the TE polarization can meet the Bragg reflection condition in the communication band, that is, the communication waveband is located in the reflection area, while the TM polarization is transmitted in the waveguide in the Bloch mode.

[0052] The simulation results of the polarization beam splitter disclosed in the present invention are described in detail below according to the diagrams, and the polarization beam splitter structure is simulated using a finite difference time domain method (FDTD).

[0053] In order to select a suitable grating period, the following simulation is performed, fixing the grating parameters: grating width W2 = 0.5 μm, connecting bridge width W b =0.15μm, grating period number N=20, duty cycle f=0.8, grating period ∧ varies from 0.42μm to 0.58μm.

[0054] like Figure 3 As shown in the figure, as the grating period gradually increases, the central wavelength of TM polarization also increases, and the transmittance decreases significantly; the cutoff wavelength of TE polarization increases, which is consistent with Bragg reflection, that is, the central wavelength of reflection is inversely proportional to the grating period. Figure 3 The results shown show that in order to keep the TM polarization unaffected while greatly reducing the coupling efficiency of the TE polarization, the preferred grating period is 500 nm.

[0055] Figure 4 The electric field diagram of the polarization beam splitter when the TE mode and TM mode are input to the embodiment of the present disclosure, and the light with a wavelength of 1550nm is set to be input from the input end of the polarization beam splitter. Figure 4 As shown in the figure, it can be seen that the beam splitting effect of PBS is relatively satisfactory. When TM polarized light is input, the waveguide in the coupling region meets the phase matching condition, and after coupling, it is output from the output end of the waveguide structure 3, that is, the cross port; when TE polarized light is input, the effective optical path in the coupling region is different, and no coupling occurs. The optical signal is directly output from the output end of the waveguide structure 5, that is, the through port.

[0056] Figure 5 The following is a graph showing the relationship between the polarization extinction ratio and insertion loss of the input TM and TE modes and the wavelength (1450nm-1650nm). Specifically, the TM mode polarization extinction ratio can reach 32dB, the extinction ratio in the wavelength range of 1509-1650nm exceeds 20dB, and the insertion loss is less than 1.3dB. This means that the polarizer can guarantee excellent performance in a larger bandwidth range (141nm). The TE mode polarization extinction ratio of the device can reach 30dB, with a high polarization extinction ratio. The polarization extinction ratio in the wavelength range of 1450-1650nm is greater than 23dB, and the insertion loss is less than 0.5dB.

[0057] Figure 6The figure is a schematic diagram of the relationship between the polarization extinction ratio and insertion loss of the TM and TE modes and the input wavelength when the waveguide width of the polarization beam splitter described in the embodiment of the present invention is transformed to ±20nm. When the process error is 20nm, the bandwidth where the PER of TM polarized light is higher than 20dB is 124nm (1514-1638nm), the insertion loss of TM is less than 0.9dB, and the polarization extinction ratio of TE polarized light can still be maintained at more than 20dB in the range of 200nm. When the waveguide width is reduced by 20nm, in the wavelength range of 1516-1650nm, the PER of TM is greater than 20dB, the insertion loss is less than 1.3dB, and the polarization extinction ratio of TE polarized light can maintain a good effect.

[0058] The present invention introduces a grating waveguide structure in the coupler, so that the coupling efficiency of the TE polarization state is greatly reduced, and the TM polarization state is almost not affected, and the coupling length is also reduced. In the coupler, there will be a small amount of TE polarization state coupled to the grating waveguide, but because the Bragg reflection condition is met, it will be reflected back by the grating waveguide, thereby greatly improving the extinction ratio of the TE polarization state. The grating waveguide structure is simple and compact, so that the TM polarization state in the coupler is no longer very sensitive to wavelength and device size deviations, so the device has a large tolerance rate for process errors.

[0059] In order to further improve the extinction ratio of the TM polarization state, a polarization filter is connected in series at the TE output end, which meets the phase matching condition for the TM polarization state and can filter out the TM polarization state that is not fully coupled into the grating waveguide.

[0060] The above simulation results show that the polarization beam splitter of the present invention exhibits good polarization beam splitting characteristics. The polarization beam splitter is not only compact in structure, but also can achieve characteristics such as large process tolerance, low insertion loss, high extinction ratio, and transmission bandwidth. The polarization beam splitter of the present invention has a simple and compact structure and large process tolerance. The present invention has important research and application value in the fields of polarization control in photon integration and polarization multiplexing in long-distance transmission of optical communications.

Claims

1. A broadband polarization beam splitter based on directional coupling, characterized in that: It comprises a silicon substrate, a silicon waveguide core layer and a silicon dioxide cladding layer; the silicon waveguide core layer is located inside the silicon dioxide cladding layer; the silicon dioxide cladding layer is located on the upper surface of the silicon substrate; The silicon waveguide core layer comprises an input waveguide (1), a first section of a directional coupler curved waveguide (2), a second section of a directional coupler curved waveguide (3), a third section of a directional coupler curved waveguide (4), a first section of a filter curved waveguide (5), a second section of a filter curved waveguide (6), a TE polarization output waveguide (7) and a TM polarization output waveguide (8); the input waveguide (1), the first section of a directional coupler curved waveguide (2), the second section of a filter curved waveguide (6) and the TE polarization output waveguide (7) are connected in sequence; the third section of a directional coupler curved waveguide (4) is connected to the TM polarization output waveguide (8); The first section of the directional coupler curved waveguide (2), the second section of the directional coupler curved waveguide (3), and the third section of the directional coupler curved waveguide (4) are arranged in sequence from top to bottom and have the same center of the circle; The first section of the curved waveguide (5) of the filter is located above the second section of the curved waveguide (6) of the filter, and the front end of the curved waveguide of the first section of the curved waveguide (5) of the filter has the same center as the second section of the curved waveguide (6) of the filter; The second section of the directional coupler curved waveguide (3) is a sub-wavelength grating structure, the grating structure is connected by a connecting bridge, the effective optical path of the TM polarization of the first section of the directional coupler curved waveguide (2), the second section of the directional coupler curved waveguide (3) and the third section of the directional coupler curved waveguide (4) are the same, so as to couple the TM polarization, and the TE polarization of the second section of the directional coupler curved waveguide (3) is in a reflection state, so as to filter the TE polarization.

2. The broadband polarization beam splitter based on directional coupling according to claim 1, characterized in that: The input waveguide (1) has a height of 220 nm, a width of 550 nm, a bending angle of 28.5°, and a bending radius of 8 μm.

3. The broadband polarization beam splitter based on directional coupling according to claim 1, characterized in that: The first section of the curved waveguide (2) of the directional coupler has a height of 220 nm, a width of 550 nm, a bending angle of 28.5° and a bending radius of 18.5 μm.

4. The broadband polarization beam splitter based on directional coupling according to claim 1, characterized in that: The second section of the directional coupler curved waveguide (3) has a height of 220 nm, a grating period of 500 nm, a duty cycle of 0.8, a nano island width of 490 nm, a connecting bridge width of 15 nm, a bending angle of 28.5°, and a bending radius of 19.1 μm.

5. The broadband polarization beam splitter based on directional coupling according to claim 1, characterized in that: The third section of the curved waveguide (4) of the directional coupler has a height of 220 nm, a width of 390 nm, a bending angle of 28.5°, and a bending radius of 19.65 μm.

6. The broadband polarization beam splitter based on directional coupling according to claim 1, characterized in that: The coupling spacing between the first section of the curved waveguide (2) of the directional coupler and the second section of the curved waveguide (3) of the directional coupler is 80 nm; the coupling spacing between the second section of the curved waveguide (3) of the directional coupler and the third section of the curved waveguide (4) of the directional coupler is 110 nm.

7. The broadband polarization beam splitter based on directional coupling according to claim 1, characterized in that: The first section of the directional coupler curved waveguide (2), the second section of the directional coupler curved waveguide (3) and the third section of the directional coupler curved waveguide (4) meet the phase matching condition of the TM polarization state.

8. The broadband polarization beam splitter based on directional coupling according to claim 1, characterized in that: The first section of the curved waveguide (5) of the filter has a height of 220 nm, a width of 340 nm, a bending angle of 30°, and a bending radius of 8.7 μm.

9. The broadband polarization beam splitter based on directional coupling according to claim 1, characterized in that: The second section of the curved waveguide (6) of the filter has a height of 220 nm, a width of 550 nm, a bending angle of 30° and a bending radius of 8 μm.

10. The broadband polarization beam splitter based on directional coupling according to claim 1, characterized in that: The first section of the filter curved waveguide (5) and the second section of the filter curved waveguide (6) meet the phase matching condition of the TM polarization state.

Images