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* 1 , 2
1  College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, China
2  Institute of Semiconductors, Chinese Academy of sciences, Beijing, China


We  propose  and  experimentally  demonstrate  broadband,  low-crosstalk,  and  low-loss  polarization splitter-rotators  (PSRs)  with  optimized  taper structures  at  1550-nm  and  1310-nm  wavelengths, respectively.  The  PSRs  consist  of  particle  swarm  optimization  (PSO)  based  bi-level  tapers  and
shortcuts to adiabaticity (STA) based ridge-waveguide couplers. Ridge waveguides are introduced to increase  the  coupling  coefficient  of  the  STA  based  coupler  and  to  reduce  the  crosstalk  from  TM0 mode. The measured polarization conversion losses (PCLs) and crosstalk (CT) are less than 0.6 dB and  –20 dB,  respectively,  from  1500 nm  to  1600 nm  wavelength  for  the  1550-nm  PSR.  The measured  PCLs  and  CT  are  less  than  1 dB  and  –22 dB, respectively,  from  1260 nm  to  1340 nm wavelength for the 1310-nm PSR.
High-performance  PSRs  are  preferred to  realize  polarization  diversity  for  silicon  photonic  circuits. Various  structures  of  PSR  have  been  proposed, including  asymmetrical  directional  coupler  (ADC) [1,2],  bi-level  taper  plus  ADC  [3],  bi-level  taper  plus  multimode  interference  coupler  [4],  bi-level
taper plus adiabatic coupler [5], and bi-level taper plus asymmetric Y-junction [6]. However, none of them  can  have  a  PCL  value  less  than  1 dB  and  a  CT  value  less  than  –20 dB  within  a  80-nm-wide wavelength range.
The scanning electron microscope (SEM) pictures of the 1550-nm and 1310-nm PSRs are shown in Fig.  1  and  Fig.  2,  respectively.  The  PSR  consists  of  a  TM0-TE1  bi-level  taper  and  a  TE0-TE1 demultiplexer.  The  bi-level  tapers  are  optimized  based  on  the  PSO  method.  The  tapers,  which  are 20-μm  long,  are  divided  into  4  and  10  segments  with  equal  length  for  the  1550-nm  and  1310-nm PSR  respectively.  The  maximum  values  of  the  average  TM0–TE1 conversion efficiencies over  the wavelengths from 1500 nm to 1600 nm and from 1260 nm to 1360 nm are set to the Figure of Merit for the 1550-nm  and  1310-nm  PSRs,  respectively.  The  TE0-TE1  demultiplexer is  optimized  based on  the  STA  method  [7].  The  coupling  length  is  70 μm.  The  minimum  edge-gaps  are  200 nm  and 160 nm for the 1550-nm and 1310-nm PSRs, respectively. The total PSR lengths are about 120 μm.
Fig. 3 shows the measured results of the 1550-nm PSR. The PCLs and CT are less than 0.6 dB and –20 dB,  respectively,  from  1500 nm  to  1600 nm  wavelength.  Fig.  4  shows  the  measured  results  of the 1310-nm PSR. The PCLs and CT are less than 1 dB and –22 dB, respectively, from 1260 nm to
1340 nm  wavelength.  The  measured  broadband  operation  is  limited  by  the  available  bandwidth  of grating couplers.
In  conclusion,  we  have  proposed  and  demonstrated  practical  PSRs  at  1550-nm  and  1310-nm wavelengths, respectively. The PSRs show lower PCL less than 1 dB and lower polarization CT less than –20 dB within a 80-nm-wide wavelength range.

Keywords: polarization splitter rotator, silicon-on-insulator