1.College of Electronic and Optical Engineering & College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing 210023, China 2.Bell Honors School, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
Abstract:An ultra-compact 1 × 2 demultiplexer based on multimode interference (MMI) waveguide is proposed to separate the 1310 nm and 1550 nm wavelengths, in which Si3N4/SiNx/Si3N4 sandwiched structure is used to realize polarization insensitivity. Firstly, how to use Si3N4/SiNx/Si3N4 sandwich structure to achieve polarization-independent is discussed. Keeping the width of MMI waveguide WMMI unchanged, the beat lengths of two orthogonal polarization states at same wavelength versus refractive indexes of SiNx are calculated. Similar simulation curves with different WMMI values and wavelengths are also provided. The result shows that there are crossing points in the beat length curves. It means that the beat lengths for the two orthogonal polarization states at the same wavelength can be identical by choosing the proper refractive index of the SiNx. More importantly, under exactly the same premise, for the two wavelengths, their crossing points are almost identical. Then, how to realize the function of wavelength separation is studied. A variable called the beat length ratio is given, which is defined as the beat length ratio of two working wavelengths under the same polarization state. When the beat length ratio equals an even number divided by an odd number, one wavelength is even multiple of beat length and the other wavelength is odd multiple of beat length, and vice versa, that is to say, a single image and a mirror image for the two working wavelengths are formed respectively. Therefore, the two working wavelengths will output from different output ports, therefore the two wavelengths are successfully separated from each other. The demultiplexer based on Si3N4/SiO2 platform has a compact structure, easy integration and good tolerance. Three-dimensional finite-difference time-domain method is used for simulation, and the results show that the size of the MMI waveguide is 4.6 μm × 227.7 μm; the insertion loss and crosstalk are as low as 0.18 dB and –25.7 dB respectively; a broad 3-dB bandwidth of 60 nm is achieved. Moreover, the fabrication deviation of the key structural parameters about the device is discussed in detail, and the insertion loss and crosstalk are considered. To demonstrate the transmission characteristics of the demultiplexer, the evolution of the excited fundamental mode in the demultiplexer is also given. The novel demultiplexer is polarization independent and can work at wavelengths of 1310 nm and 1550 nm simultaneously. It has potential application value in future integrated optical circuits. Keywords:multimode interference/ Si3N4/SiO2 platform/ polarization-insensitive/ sandwiched structure
为了便于后续波长分离时提供相关参数, 需要进一步探讨器件满足偏振无关条件时, WMMI与n(SiNx)的变化关系如图4所示, 随着WMMI增大, n(SiNx)值单调递减. 图5(a)给出了对应于图4中每一组WMMI和n(SiNx), 所求出的不同波长不同偏振态所对应的拍长. 很明显: 对于同一波长的两个正交偏振态所对应的拍长而言, 两者差别较小. 为了进一步刻画它们间的差异, 引入不同偏振态间的拍长比$M = {L_{\text{π}}}(\lambda, {\rm{TE}})/{L_{\text{π}}}(\lambda, {\rm{TM}})$进行描述, 图5(b) 给出了M随WMMI的变化关系, 这些离散的点基本围绕在1附近分布, 进一步验证了其满足偏振无关条件. 图 4 器件满足偏振无关条件时, n(SiNx)随WMMI的变化关系 Figure4.n(SiNx) as functions of WMMI when the demultiplexer is polarization-insensitive.
图 5 器件满足偏振无关条件时 (a) ${L_{\text{π}}}$和(b) M随WMMI的变化 Figure5. (a) ${L_{\text{π}}}$ and (b) M as functions of WMMI when the demultiplexer is polarization-insensitive.