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
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61571237), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20151509), and the NUPTSF of China (Grant No. NY217047)
Received Date:21 November 2020
Accepted Date:12 January 2021
Available Online:07 June 2021
Published Online:20 June 2021
Abstract:Ring resonator fabricated on a silicon-on-insulator is versatile in optical integration, which can be used to realize filters, modulators and switches. However, silicon-on-insulator is difficult to control the polarization dependence, and thus its application is greatly limited. The polarization dependence of the ring resonator is caused mainly by two factors: the coupling coefficients of the coupling region at the same wavelength for the two orthogonal polarization modes are different, and the birefringence effect of curved waveguide results in the different resonant wavelengths of TE and TM polarization modes. When the coupling region polarization independence and the resonant wavelength polarization independence are simultaneously satisfied, the polarization independence of the ring resonator can be realized. In this paper, a new type of polarization-insensitive ring resonator on a silicon-on-insulator is designed based on subwavelength grating and sandwiched structure. Firstly, by adjusting the duty cycle of the subwavelength grating and the refractive index of SiNx in the coupling region, polarization independence of the coupling region is achieved. Secondly, the refractive index of SiNx in curved waveguides is designed to make the resonance wavelengths for orthogonal polarization modes equal. Thirdly, the parameters of the coupling region are optimized to reduce the insertion loss. The three-dimensional finite-difference time-domain method is used for simulation. The results show that the radius of the ring is only 10 μm, the 3-dB bandwidth of the device is less than 0.8 nm, and the insertion loss is lower than 0.8 dB. It has potential applications in the future dense wavelength division multiplexing systems. Keywords:microring resonators/ polarization-insensitive/ subwavelength grating/ sandwiched structure
如上文所述, 当g和n1(SiNx)确定时, 可以通过调节W2得到k2(λ, TE) = k2(λ, TM) = k2, 使得耦合区偏振无关. 为了便于后续器件性能优化时提供参数需要, 进一步探讨耦合区满足偏振无关时, k2受g和n1(SiNx)的影响. 如图4(a)所示. k2随n1(SiNx)的减小而增大, 当n1(SiNx)固定时, k2随g的增大而减小. 图4(b)给出了图4(a)中每一组g和n1(SiNx)实现耦合区偏振无关时所对应的W2. 图 4 (a) 耦合区满足偏振无关时, 不同n1(SiNx)情况下k2随g的变化; (b) 每一组g和n1(SiNx)实现耦合区偏振无关时其对应的W2 Figure4. (a) k2 as a function of g under different n1(SiNx) when the light intensity is polarization-insensitive; (b) W2 as a function of g under different n1(SiNx) when the light intensity is polarization-insensitive
其中Pd为输出端的输出光功率峰值, Pin为输入端的输入光功率. 3-dB带宽指的是当输出功率下降到峰值的一半时的带宽, 在信道间隔为0.8 nm的密集波分复用器中一般要求3-dB带宽小于0.8 nm[11]. 由(4)式可知, 在弯曲波导半径和器件其他波导长度、宽度不变的情况下, 输出端光强度仅与k2有关. 而由定义可知: IL和3-dB带宽均与输出端光强度有关, 因此k2的变化会对IL与3-dB带宽产生直接的影响. 保持结构参数R = 10 μm, Λ = 0.2 μm, W2 = 0.06 μm, n1(SiNx) = 1.9, n2(SiNx) = 1.894不变, 仅通过改变g调整k2的大小. 图6给出了k2与g的变化关系, 由图可见, 随着g的增加, k2从0.35单调递减至0.05. 图 6k2随g的变化 Figure6.k2 of download port as a function of g.
图7(a)展示了k2对IL的影响, 结果表明: IL随着k2的增加而减小, 并且在k2 > 0.3时, IL接近0. 为了得到较小的IL, 希望k2的取值尽量偏大. 图7(b)展示了k2对3-dB带宽的影响. 结果表明: 在k2从0.05增加至0.35的过程中, 3-dB带宽也从0.3单调递增至1.7. 考虑到3-dB带宽小于等于0.8 nm时, 可应用于信道间隔为0.8 nm的密集波分复用器. 因此, 此处可以选择k2 ≤ 0.2, 结合图7(a)所示, 为了得到较小的IL, 希望k2的取值尽量偏大, 最终确定k2 = 0.2. 图 7 (a) k2变化对IL的影响; (b) k2变化对3-dB带宽的影响 Figure7. (a) IL of download port as a function of k2; (b) 3-dB bandwidth as a function of k2.
k2 = 0.2时, 由前文涉及的图4可知: 存在多种n1(SiNx), g与W2的组合, 能够令器件的耦合区满足偏振无关. 而不同的n1(SiNx), g与W2的组合, 会令器件IL发生改变. 为此, 进一步作参数优化. 图8展示了IL与n1(SiNx)的关系: 随着n1(SiNx)的增加IL先减小后增加, 在n1(SiNx) = 2.8时IL达到最小值, 此时微环耦合区各参数分别为g = 0.24 μm, Λ = 0.2 μm, n1(SiNx) = 2.8, W2 = 0.044 μm. 因为耦合区参数发生变化, 所以需要重新调整n2(SiNx)以实现谐振波长的偏振无关. 采用上文的方法进行调整, 当n2(SiNx) = 2.853时有Δλ = 0. 图 8 微环谐振器偏振无关且k2 = 0.2时n1 (SiNx)变化对IL的影响 Figure8. IL as a function of n1(SiNx) when the ring resonator is polarization-insensitive and k2 = 0.2.
此时, TE和TM偏振模时的输出端透过率谱线如图9所示: 在谐振波长1552.26 nm附近, 两者完全重合, 实现了偏振无关. 图 9 TE和TM偏振模时的输出端透过率谱线 Figure9. Measured transmission spectra with TE- and TM-polarized light inputs for the out port.
表1偏振无关微环谐振器的性能参数 Table1.Performances of the polarization-insensitive ring resonator.
此外, 需要考虑工艺制作偏差对器件光学性能的影响. 图10(a)—(d)讨论了器件参数W1, W2的变化ΔW1, ΔW2分别对器件IL和3-dB带宽的影响. 由图10(a)和图10(b)可看出, 器件对W1的容差性较好, 当ΔW1在–8—8 nm间变化时, IL < 1 dB, 3-dB带宽小于0.8 nm. W2主要影响微环谐振器的偏振相关性, 如图10(c)和图10(d)所示, 当ΔW2在–5—5 nm间变化时, IL < 2 dB, 3-dB带宽小于0.8 nm, 此时器件性能仍然较为良好. 图 10 器件性能随结构参数的变化 (a) IL随ΔW1的变化; (b) 3-dB带宽随ΔW1的变化; (c) IL随ΔW2的变化; (d) 3-dB带宽随ΔW2的变化 Figure10. Performances as functions of structural parameters: (a) IL as a function of ΔW1; (b) 3-dB bandwidth as a function of ΔW1; (c) IL as a function of ΔW2; (d) 3-dB bandwidth as a function of ΔW2.