On-chip integrated polarization beam splitter

Abstract

The invention relates to an on-chip integrated polarization beam splitter which comprises a first multi-mode interference coupler, a second multi-mode interference coupler, a first waveguide and a second waveguide. The input end of the first multimode interference coupler is connected with the input waveguide, the first output end is connected with the first end of the first waveguide, and the second output end is connected with the first end of the second waveguide; the first input end of the second multimode interference coupler is connected with the second end of the first waveguide, the second input end is connected with the second end of the second waveguide, the first output end is connected with the first output waveguide, and the second output end is connected with the second output waveguide; the first waveguide and the second waveguide are the same in length and different in width, the first waveguide and the second waveguide are each provided with a thermo-optical phase shifter, and the thermo-optical phase shifters are used for conducting thermo-optical phase modulation so that the phase difference between TE polarization and TM polarization can be integer multiples of pi. According to the invention, the polarization extinction ratio can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon optical devices, and in particular to an on-chip integrated polarization beam splitter. Background Art

[0002] Photonic integration technology based on SOI platform is widely used in fields such as communication and sensing due to its series of advantages such as low loss, compact structure and compatibility with CMOS process. These advantages also make it possible to use electronic devices to manufacture photonic integrated circuits (PICs). Researchers have successfully developed waveguides for high refractive index contrast, and the submicron and 220nm SOI platforms have the advantage of ultra-high birefringence, which makes the on-chip polarization control devices have very compact size. However, for micron-level silicon photonic platforms, especially 3-micron silicon photonic platforms, they face the challenge of low polarization dependence. Unlike submicron waveguides that exhibit strong polarization dependence, 3-micron thick waveguides have the characteristics of low propagation loss, small process tolerance and small polarization dependence. Although 3-micron thick SOI materials have the advantage of polarization insensitivity, as one of the core components, polarization beam splitters play an important role in realizing polarization-insensitive PICs. In addition, polarization beam splitters are also key devices for many applications, such as coherent optical communications, which can improve spectral efficiency and thus increase the capacity of communication systems.

[0003] The current common mode is to design the waveguide width and length of the asymmetric arm to achieve different optical path differences between TE and TM polarized light in the asymmetric arm. However, the 3-micron-thick waveguide itself has the characteristic of low polarization dependence, which leads to low tolerance of polarization beam splitter parameters and high processing precision requirements. It is difficult to accumulate different optical path differences in the asymmetric arm to reach an integer multiple of π, resulting in reduced bandwidth. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide an on-chip integrated polarization beam splitter capable of improving the polarization extinction ratio.

[0005] The technical solution adopted by the present invention to solve its technical problems is: providing an on-chip integrated polarization beam splitter, including a first multimode interference coupler, a second multimode interference coupler, a first waveguide and a second waveguide; the input end of the first multimode interference coupler is connected to the input waveguide, the first output end is connected to the first end of the first waveguide, and the second output end is connected to the first end of the second waveguide; the first input end of the second multimode interference coupler is connected to the second end of the first waveguide, the second input end is connected to the second end of the second waveguide, the first output end is connected to the first output waveguide, and the second output end is connected to the second output waveguide; the first waveguide and the second waveguide have the same length and different widths; the first waveguide and the second waveguide are both provided with a thermo-optical phase shifter, and the thermo-optical phase shifter is used to perform thermo-optical phase modulation so that the phase difference between TE polarization and TM polarization is an integer multiple of π.

[0006] The width of the second waveguide is greater than the width of the first waveguide.

[0007] The width ranges of the first waveguide and the second waveguide are obtained by:

[0008] The objective function is constructed with the maximum effective refractive index difference of TE polarization and the maximum effective refractive index difference of TM polarization as the goal, and the phase difference of two TE polarized light beams is 0 and the phase difference of two TM polarized light beams is π as the constraints;

[0009] Solving the objective function to obtain a TE polarization effective refractive index range and a TM polarization effective refractive index range of the first waveguide and the second waveguide;

[0010] According to the TE polarization effective refractive index range and the TM polarization effective refractive index range of the first waveguide and the second waveguide, combined with the relationship between the effective refractive index and the waveguide width, the width range of the first waveguide and the second waveguide is obtained.

[0011] The first end of the second waveguide is closer to the first multimode interference coupler than the first end of the first waveguide, and the second end of the first waveguide is closer to the second multimode interference coupler than the second end of the second waveguide.

[0012] The thermo-optical phase shifter comprises:

[0013] a metal layer covering an upper surface of the first waveguide and / or the second waveguide;

[0014] A heating electrode is arranged on the metal layer;

[0015] A metal lead wire, used to connect the heating electrode to a DC source;

[0016] Wherein, the resistivity of the metal layer is greater than the resistivity of the metal lead.

[0017] The shape of the heating electrode is trapezoidal or rectangular.

[0018] The thermo-optical phase shifter comprises:

[0019] A heavily doped region, arranged on both sides of the first waveguide and / or the second waveguide;

[0020] A voltage applicator is used for applying voltage to the heavily doped region.

[0021] The heavily doped region is located at 1.5-3 μm on both sides of the first waveguide and / or the second waveguide, and the length of the heavily doped region is 150-300 μm and the width is 10-15 μm.

[0022] Beneficial Effects

[0023] Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art: the present invention arranges a thermo-optic phase shifter on the asymmetric arm, utilizes the thermo-optic effect to compensate for the refractive index change caused by the waveguide size deviation, and uses thermo-optic phase modulation to make the phase difference between TE polarization and TM polarization an integer multiple of π, thereby achieving a larger polarization extinction ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a schematic structural diagram of an on-chip integrated polarization beam splitter according to an embodiment of the present invention;

[0025] Figure 2 is a graph showing the variation of the effective refractive index of TE polarization and the effective refractive index of TM polarization with the waveguide width;

[0026] Figure 3 It is a schematic diagram of asymmetric arm length scanning;

[0027] Figure 4 It is the wavelength distribution diagram of the splitting ratio formed by using the optimal asymmetric arm length;

[0028] Figure 5 is a schematic diagram of a waveguide structure using sputtered metal in an embodiment of the present invention;

[0029] Figure 6 is a schematic diagram of the electrode structure of the thermo-optical phase shifter in an embodiment of the present invention;

[0030] Figure 7 is a schematic diagram of a heavily doped waveguide structure in an embodiment of the present invention;

[0031] Figure 8 3 is a thermal field transmission diagram during π phase shift in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0033] An embodiment of the present invention relates to an on-chip integrated polarization beam splitter, such as Figure 1 As shown, it includes a first multimode interference coupler 1, a second multimode interference coupler 2, a first waveguide 3 and a second waveguide 4.

[0034] The input end of the first multimode interference coupler 1 is connected to the input waveguide 8, the first output end is connected to the first end of the first waveguide 3, and the second output end is connected to the first end of the second waveguide 4; the first input end of the second multimode interference coupler 2 is connected to the second end of the first waveguide 3, the second input end is connected to the second end of the second waveguide 4, the first output end is connected to the first output waveguide 5, and the second output end is connected to the second output waveguide 6.

[0035] The first waveguide 3 and the second waveguide 4 have the same length and different widths. In this embodiment, the first end of the second waveguide 4 is closer to the first multimode interference coupler 1 than the first end of the first waveguide 3, and the second end of the first waveguide 3 is closer to the second multimode interference coupler 2 than the second end of the second waveguide 4.

[0036] In this embodiment, thermo-optic phase shifters 7 are disposed on both the first waveguide 3 and the second waveguide 4. The thermo-optic phase shifters 7 are used to perform thermo-optical phase modulation so that the phase difference between TE polarization and TM polarization is an integer multiple of π.

[0037] The principle of the on-chip integrated polarization beam splitter in this embodiment to achieve TE and TM splitting is as follows: the phase difference between the two TE polarized light beams is set to 0, the phase difference between the two TM polarized light beams is set to π, and then the phase difference between the TE polarized light and the TM polarized light reaches π.

[0038] The phase difference can be adjusted by changing the refractive index. The first waveguide and the second waveguide are simulated using the FDE solver to obtain Figure 2 The refractive index shown varies with the waveguide width. In this embodiment, the first waveguide 3 is designed as a narrow arm with a width of W. 1 , the corresponding effective refractive index is n W1 The second waveguide 4 is designed as a wider arm with a width of W 2 , the corresponding effective refractive index is n W2The maximum TE polarization effective refractive index difference and the maximum TM polarization effective refractive index difference are set as the goal, and the phase difference between the two TE polarized light beams is 0 (i.e., Formula 1) and the phase difference between the two TM polarized light beams is π (Formula 2) are set as constraints. The objective function is constructed, and the TE polarization effective refractive index range and TM polarization effective refractive index range of the first waveguide and the second waveguide are obtained by solving the objective function. Figure 2 In this embodiment, the width of the first waveguide 3 can be determined to be in the range of 1.5 μm-2 μm, and the width of the second waveguide 4 can be determined to be in the range of 2.5 μm-3 μm.

[0039]

[0040] Among them, Δφ TE is the phase difference between the two TE polarized beams, Δφ TM is the phase difference between the two TM polarized beams, L is the length of the first waveguide, (n W1 -n W2 ) TE is the effective refractive index difference of TE polarization, (n W1 -n W2 ) TM is the effective refractive index difference for TM polarization, and λ is the wavelength.

[0041] Taking into account the tolerance of the process, the length of the asymmetric arm may be determined to be an appropriate length between 300 μm and 600 μm. Figure 3 , Figure 4 The following are the splitting ratios for the asymmetric arm length and the optimal asymmetric arm length. By further scanning the asymmetric arm length using the EME solver, it was determined that the asymmetric arm length was 350μm, the 3dB length tolerance was 5%, and the width tolerance was low. Further analysis of the simulation results showed that the reason was that the change in waveguide size would change the waveguide refractive index, affecting the accumulated value of the phase difference and thus reducing the device performance.

[0042] In order to compensate for the influence of waveguide size error caused by processing reasons, this embodiment uses a thermo-optic phase shifter to compensate for the refractive index change caused by waveguide size deviation using thermo-optic effect, so that the phase difference between TE polarization and TM polarization can be made π through thermo-optic phase modulation.

[0043] This embodiment adopts two different methods to realize the thermo-optical phase shifter, among which the first method is based on the method of suspended metal heating: by sputtering a metal layer on the upper surface of the first waveguide and / or the second waveguide, and then connecting the heating electrode on the metal layer to the DC source through the lead wire, when the DC source is powered, the heat generated can be transmitted to the inside of the waveguide, so that the refractive index of the first waveguide and / or the second waveguide changes, thereby realizing thermal compensation. The second method is to heavily dope both sides of the first waveguide and / or the second waveguide, by heavily doping ions in the silicon material, so that the material resistance changes, and the electrical energy is converted into thermal energy after the voltage is applied, so that the refractive index of the first waveguide and / or the second waveguide changes, thereby realizing thermal compensation.

[0044] Based on the heating of suspended metal, such as Figure 5 As shown, the thermo-optic phase shifter comprises: a metal layer covering the upper surface of the first waveguide and / or the second waveguide; a heating electrode arranged on the metal layer; and a metal lead wire for connecting the heating electrode to a DC source; wherein the resistivity of the metal layer is greater than the resistivity of the metal lead wire. 2 The top of the layer is covered with a metal layer (such as TiN), which can reduce heat loss, thereby improving the heating efficiency. The metal layer can completely cover the ridge waveguide, or it can be partially covered, but the minimum coverage width cannot be too small when partially covered. For impedance matching, the metal lead in this embodiment can be made of aluminum with a low resistivity, and the overlapped part with TiN is provided with a heating electrode, such as Figure 6 As shown, the heating electrode is designed to be approximately trapezoidal or rectangular.

[0045] When using heavy doping design, such as Figure 7 As shown, the thermo-optical phase shifter includes: a heavily doped region, which is arranged on both sides of the first waveguide and / or the second waveguide; and a voltage applicator, which is used to apply voltage to the heavily doped region. In this method, N ions are heavily doped at a position 1.5-3 μm away from the waveguide on both sides of the first waveguide and / or the second waveguide, with a doping concentration of 1×10 20 cm -3 The width of the doped region is 10-15 μm, and the total length of the doped region can be 150-300 μm. Through thermal simulation, the thermal field is then imported into the MODE solver for optical simulation, and finally the following is obtained: Figure 8 The thermal field transmission diagram during π phase shift is shown. At this time, the temperature of the entire thermal phase shifter is lower than 350K, and it will not affect the performance of other devices even in the system. In order to reduce the voltage required to achieve π phase shift, the four-electrode parallel metal wiring method can be used to reduce the resistance.

[0046] It is not difficult to find that the present invention arranges a thermo-optic phase shifter on the asymmetric arm (i.e., the first waveguide and / or the second waveguide), utilizes the thermo-optic effect to compensate for the refractive index change caused by the waveguide size deviation, and uses thermo-optic phase modulation to make the phase difference between TE polarization and TM polarization an integer multiple of π, thereby achieving a larger polarization extinction ratio.

Claims

1. An on-chip integrated polarization beam splitter, characterized in that: It includes a first multimode interference coupler, a second multimode interference coupler, a first waveguide and a second waveguide; the input end of the first multimode interference coupler is connected to the input waveguide, the first output end is connected to the first end of the first waveguide, and the second output end is connected to the first end of the second waveguide; the first input end of the second multimode interference coupler is connected to the second end of the first waveguide, the second input end is connected to the second end of the second waveguide, the first output end is connected to the first output waveguide, and the second output end is connected to the second output waveguide; the first waveguide and the second waveguide have the same length and different width; the first waveguide and the second waveguide are both provided with a thermo-optical phase shifter, and the thermo-optical phase shifter is used to perform thermo-optical phase modulation so that the phase difference between TE polarization and TM polarization is an integer multiple of π.

2. The on-chip integrated polarization beam splitter according to claim 1, characterized in that: The width of the second waveguide is greater than the width of the first waveguide.

3. The on-chip integrated polarization beam splitter according to claim 2, characterized in that: The width ranges of the first waveguide and the second waveguide are obtained by: The objective function is constructed with the maximum effective refractive index difference of TE polarization and the maximum effective refractive index difference of TM polarization as the goal, and the phase difference of two TE polarized light beams is 0 and the phase difference of two TM polarized light beams is π as the constraints; Solving the objective function to obtain a TE polarization effective refractive index range and a TM polarization effective refractive index range of the first waveguide and the second waveguide; According to the TE polarization effective refractive index range and the TM polarization effective refractive index range of the first waveguide and the second waveguide, combined with the relationship between the effective refractive index and the waveguide width, the width range of the first waveguide and the second waveguide is obtained.

4. The on-chip integrated polarization beam splitter according to claim 1, characterized in that: The first end of the second waveguide is closer to the first multimode interference coupler than the first end of the first waveguide, and the second end of the first waveguide is closer to the second multimode interference coupler than the second end of the second waveguide.

5. The on-chip integrated polarization beam splitter according to claim 1, characterized in that: The thermo-optical phase shifter comprises: a metal layer covering an upper surface of the first waveguide and / or the second waveguide; A heating electrode is arranged on the metal layer; A metal lead wire, used to connect the heating electrode to a DC source; Wherein, the resistivity of the metal layer is greater than the resistivity of the metal lead.

6. The on-chip integrated polarization beam splitter according to claim 5, characterized in that: The shape of the heating electrode is trapezoidal or rectangular.

7. The on-chip integrated polarization beam splitter according to claim 1, characterized in that: The thermo-optical phase shifter comprises: A heavily doped region, arranged on both sides of the first waveguide and / or the second waveguide; A voltage applicator is used for applying voltage to the heavily doped region.

8. The on-chip integrated polarization beam splitter according to claim 7, characterized in that: The heavily doped region is located at 1.5-3 μm on both sides of the first waveguide and / or the second waveguide, and the length of the heavily doped region is 150-300 μm and the width is 10-15 μm.

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