Silicon-on-insulator polarization splitting and rotating device for polarization diversity circuits

Transcription

1 Downloaded from orbit.dtu.dk on: Ot 19, 2018 Silion-on-insulator polarization splitting and rotating devie for polarization diversity iruits Liu, Liu; Ding, Yunhong; Yvind, Kresten; Hvam, Jørn Marher Published in: Optis Express Link to artile, DOI: /OE Publiation date: 2011 Doument Version Publisher's PDF, also known as Version of reord Link bak to DTU Orbit Citation (APA): Liu, L., Ding, Y., Yvind, K., & Hvam, J. M. (2011). Silion-on-insulator polarization splitting and rotating devie for polarization diversity iruits. Optis Express, 19(13), DOI: /OE General rights Copyright and moral rights for the publiations made aessible in the publi portal are retained by the authors and/or other opyright owners and it is a ondition of aessing publiations that users reognise and abide by the legal requirements assoiated with these rights. Users may download and print one opy of any publiation from the publi portal for the purpose of private study or researh. You may not further distribute the material or use it for any profit-making ativity or ommerial gain You may freely distribute the URL identifying the publiation in the publi portal If you believe that this doument breahes opyright please ontat us providing details, and we will remove aess to the work immediately and investigate your laim.

2 Silion-on-insulator polarization splitting and rotating devie for polarization diversity iruits Liu Liu, 1,2,* Yunhong Ding, 2,3 Kresten Yvind, 2 and Jørn M. Hvam 2 1 Shool for Information and Optoeletroni Siene and Engineering, South China Normal University, Guangzhou, China 2 DTU-Fotonik, Tehnial University of Denmark, Ørsteds Plads Building 343, 2800 Lyngby, Denmark 3 Wuhan National Laboratory for Optoeletronis, Shool of Optoeletronis Siene and Engineering, Huazhong University of Siene and Tehnology, Wuhan, China *realdream.liuliu@gmail.om Abstrat: A ompat and effiient polarization splitting and rotating devie built on the silion-on-insulator platform is introdued, whih an be readily used for the interfae setion of a polarization diversity iruit. The devie is ompat, with a total length of a few tens of mirons. It is also simple, onsisting of only two parallel silion-on-insulator wire waveguides with different widths, and thus requiring no additional and nonstandard fabriation steps. A total insertion loss of 0.6dB and an extintion ratio of 12dB have been obtained experimentally in the whole C-band Optial Soiety of Ameria OCIS odes: ( ) Integrated optis devies; ( ) Polarization-seletive devies. Referenes and links 1. W. Bogaerts, R. Baets, P. Dumon, V. Wiaux, S. Bekx, D. Taillaert, B. Luyssaert, J. Van Campenhout, P. Bienstman, and D. Van Thourhout, Nanophotoni waveguides in silion-on-insulator fabriated with CMOS tehnology, J. Lightwave Tehnol. 23(1), (2005). 2. T. Tsuhizawa, K. Yamada, H. Fukuda, T. Watanabe, J. Takahashi, M. Takahashi, T. Shoji, E. Tamehika, S. Itabashi, and H. Morita, Mirophotonis devies based on silion mirofabriation tehnology, IEEE J. Sel. Top. Quantum Eletron. 11(1), (2005). 3. C. Manolatou, S. G. Johnson, S. Fan, P. R. Villeneuve, H. A. Haus, and J. D. Joannopoulos, High density integrated optis, J. Lightwave Tehnol. 17(9), (1999). 4. T. Barwiz, M. R. Watts, M. A. Popović, P. T. Rakih, L. Soi, F. X. Kärtner, E. P. Ippen, and H. I. Smith, Polarization-transparent mirophotoni devies in the strong onfinement limit, Nat. Photonis 1(1), (2007). 5. W. Bogaerts, D. Taillaert, P. Dumon, D. Van Thourhout, R. Baets, and E. Pluk, A polarization-diversity wavelength duplexer iruit in silion-on-insulator photoni wires, Opt. Express 15(4), (2007), 6. H. Fukuda, K. Yamada, T. Tsuhizawa, T. Watanabe, H. Shinojima, and S. Itabashi, Silion photoni iruit with polarization diversity, Opt. Express 16(7), (2008), 7. H. Fukuda, K. Yamada, T. Tsuhizawa, T. Watanabe, H. Shinojima, and S. Itabashi, Ultrasmall polarization splitter based on silion wire waveguides, Opt. Express 14(25), (2006), 8. L. Liu, Y. Ding, K. Yvind, and J. M. Hvam, Effiient and ompat TE-TM polarization onverter built on silion-on-insulator platform with a simple fabriation proess, Opt. Lett. 36(7), (2011). 9. H. Deng, D. O. Yevik, C. Brooks, and P. E. Jessop, Design rules for slanted-angle polarization rotators, J. Lightwave Tehnol. 23(1), (2005). 10. M. R. Watts, and H. A. Haus, Integrated mode-evolution-based polarization rotators, Opt. Lett. 30(2), (2005). 11. H. Fukuda, K. Yamada, T. Tsuhizawa, T. Watanabe, H. Shinojima, and S. Itabashi, Polarization rotator based on silion wire waveguides, Opt. Express 16(4), (2008), Y. Yue, L. Zhang, M. Song, R. G. Beausoleil, and A. E. Willner, Higher-order-mode assisted silion-oninsulator 90 degree polarization rotator, Opt. Express 17(23), (2009), Z. Wang, and D. Dai, Ultrasmall Si-nanowire-based polarization rotator, J. Opt. So. Am. B 25(5), (2008). 14. K. Bayat, S. K. Chaudhuri, and S. Safavi-Naeini, Ultra-ompat photoni rystal based polarization rotator, Opt. Express 17(9), (2009), # $15.00 USD Reeived 11 Apr 2011; revised 18 May 2011; aepted 30 May 2011; published 15 Jun 2011 (C) 2011 OSA 20 June 2011 / Vol. 19, No. 13 / OPTICS EXPRESS 12646

3 15. J. Zhang, M. Yu, G. Lo, and D. L. Kwong, Silion waveguide based mode-evolution polarization rotator, IEEE J. Sel. Top. Quantum Eletron. 16(1), (2010). 16. FIMMWAVE/FIMMPROP, Photon Design Ltd, 1. Introdution Silion-on-insulator (SOI) has been onsidered reently as a promising platform for photoni iruits, largely driven by the CMOS-ompatible fabriation tehnology and the high refrative-index ontrast of the waveguide struture [1,2]. High-density integration and mass prodution of devies are therefore made possible. On the other hand, this high index ontrast also indues a large polarization dependent dispersion or loss for normal omponents [3], and makes SOI inonvenient to integrate with other polarization insensitive platforms, like optial fiber networks. Instead of pursuing diffiult polarization-independent devies on SOI, a polarization diversity sheme ould be employed [4 6]. In this ase, the orthogonal polarization omponents of the input light are first split into two different waveguides by using a polarization splitter [7]. A polarization rotator is then employed in one of the waveguides to rotate the polarization 90 [8 15]. Therefore, for the rest of the photoni hip only one polarization has to be proessed. At the output, a opy of the polarization rotator and splitter an be implemented in order to ombine the two polarizations without interferene. We have suessfully demonstrated a ompat and effiient polarization onverter on SOI based on the ross-polarization oupling between two SOI wires waveguide with different widths [8]. As ompared to other polarization onverters [9 15], this struture an be fabriated together with ommon deeply-ethed SOI wire waveguides and devies, and thus requires no additional fabriation steps. In this paper, we further extend the ability of the polarization onverter in [8], and show that suh a single devie struture an be used for polarization splitting and rotating (PSR) simultaneously. Thus, a polarization diversity sheme on SOI an be readily implemented. We theoretially study the transmission properties of the proposed devie. The optimized strutures are also fabriated and haraterized experimentally. (a) TE TE (b) y w 2 g w 1 x h air SiO 2 air Si () x z ross SiO 2 TM TE adiabati taper adiabati taper Fig. 1. Shemati struture and working priniple of the proposed polarization splitter and rotator. (a) three-dimensional model; (b) x-y ross-setion; () x-z ross-setion. 2. Design and simulation input l through The struture of the present PSR devie is skethed in Fig. 1. It onsists of two parallel SOI photoni wire waveguides oupled to eah other. In order to ahieve an effiient rosspolarization oupling, air is employed as the top-ladding. The widths (w 1 and w 2 ) of the two waveguides (waveguide 1 and waveguide 2, respetively) are adjusted so that the effetive index of the fundamental transverse-magneti (TM) mode in waveguide 1 is equal to that of # $15.00 USD Reeived 11 Apr 2011; revised 18 May 2011; aepted 30 May 2011; published 15 Jun 2011 (C) 2011 OSA 20 June 2011 / Vol. 19, No. 13 / OPTICS EXPRESS 12647

4 the fundamental transverse-eletri (TE) mode in waveguide 2, i.e., n 1,TM eff n 2,TE eff, whih is usually alled the phase mathing ondition. In this ase, an effiient oupling between these two modes an be ahieved [8]. The effetive index of a single SOI wire waveguide with air top-ladding at different widths is shown in Fig. 2. Here, we hoose w 1 = 600 nm and w 2 = 333 nm. The height h of the silion waveguide layer is 250 nm, whih is also the parameter of the SOI wafer used in the experiments. On the other hand, at the hosen widths the effetive index n of the TE mode in waveguide 1 is very different from that of any guided mode in 1,TE eff waveguide 2 (f. Figure 2). Therefore, aording to the oupling mode theory the TE mode in waveguide 1 is expeted to pass freely aross the devie. In general, suh a struture in Fig. 1 exhibits the abilities to ouple the TM mode from the input waveguide to the adjaent waveguide and simultaneously onvert it into a TE mode, while leave the TE mode untouhed in the input waveguide. This is exatly the harateristi of the interfae setion of a polarization diversity iruit, whih is onventionally onstruted by asading an individual polarization splitter and rotator [4 6]. A similar struture of two SOI waveguides with the same width was used as a polarization splitter [7]. In the proposed struture, an additional funtionality of polarization rotation an be ahieved. Fig. 2. Effetive indies of the fundamental TE and TM modes of an air-ladded SOI wire waveguide with different widths w. The model is shown in the inset. h = 250 nm. The propagation of the optial field in the proposed devie is first studied by a threedimensional finite-differene time-domain (3D-FDTD) method, as shown in Fig. 3. The gap g here is 100 nm, and the rest of the strutural parameters are the same as above. One an learly see that the input TM mode in waveguide 1 is gradually onverted to the TE mode in waveguide 2 along the propagation diretion. There exists an optimal oupling length for the maximal onversion, beyond whih the optial power is oupled bak to waveguide 1. As for the input TE mode, the optial power is all onfined in waveguide 1 aross the whole devie. These results omply with the disussions in the previous paragraph. It is worthwhile to note that the disretization lattie here is set to a rather large value, i.e. 20 nm, whih is not fine enough to desribe the dimensions of the struture aurately. Further refining the grid will, however, make the simulation effort unaffordable with the alulation resoures at hand. Thus, the results shown in Fig. 3 are just qualitative analyses. # $15.00 USD Reeived 11 Apr 2011; revised 18 May 2011; aepted 30 May 2011; published 15 Jun 2011 (C) 2011 OSA 20 June 2011 / Vol. 19, No. 13 / OPTICS EXPRESS 12648

5 Fig. 3. Time snap-shots of the fields at steady state within the x-z plane whih lies at the enter of the SOI wire waveguide in the y diretion. The simulation is done by 3D-FDTD with w 1 = 600 nm, w 2 = 333 nm, g = 100 nm, and h = 250 nm. The wavelength is 1550 nm. (a) and (b) are for the ase of the TM mode input; () and (d) are for the ase of the TE mode input. (a) and () are distributions of the E x fileld; (b) and (d) are distributions of the E y fileld. Fig. 4. Simulated transmission oeffiients between differently polarized modes from the inputport to the ross-port (a) and the through-port (b). The models are shown in the insets respetively. Here, w 1 = 600 nm, w 2 = 333 nm, g = 100 nm, h = 250 nm, and l = 36.8 μm. The transmission oeffiients whih are not presented are well below 35dB. In order to model aurately the transmission properties of the proposed devie, a mode expansion and mathing method [16] is then employed, whih is more suitable to deal with ij - the struture involved here. Figure 4 shows the transmission oeffiients T when the oupling length l = 36.8 μm. Here, the subsript t or indiates the response at the throughport or ross-port of the devie (f., Fig. 1), respetively. The supersript i-j, where i, j an be TE or TM, means the transmission from mode i at the input to mode j at the output. Considering the omplexity, the simulation model is redued orrespondingly as shown in the insets of Fig. 4, where the bending setions are negleted. For the through-port response TE-TE shown in Fig. 4(b), T t remains lose to 1 in the whole wavelength range of simulation. On the other hand, the TM mode from the input port is suppressed at the through port, and a dip is observed in the T t urve at 1550 nm wavelength, whih orresponds to the position where the phase-mathing ondition is fulfilled between the fundamental TM mode in waveguide 1 and the fundamental TE mode in waveguide 2. Aordingly, a peak is observed in the T urve shown in Fig. 4(a), whih reahes 90%, orresponding to an overall insertion loss of 0.46dB. The unonverted TM mode power remains partly in the through-port ( T t ) and the ross-port ( T ). They beome the main soure of rosstalk for the proposed devie. Nevertheless, the extintion ratio is still better than 13dB in the whole C-band (1530 nm 1565 nm). t( ) # $15.00 USD Reeived 11 Apr 2011; revised 18 May 2011; aepted 30 May 2011; published 15 Jun 2011 (C) 2011 OSA 20 June 2011 / Vol. 19, No. 13 / OPTICS EXPRESS 12649

6 Similar to a ommon diretional oupler on the SOI platform, the proposed PSR devie also has a tight tolerane to the variations in devie dimensions, e.g., waveguide widths. A slight deviation from the designed value will make the phase-mathing ondition fail, whih will mainly result in a derease in the effiieny of the ross-polarization oupling at the desired wavelength. We further study the influene of the variation in w 1 on the devie performane. Figure 5(a) shows the ross-port responses with w 1 = 332 nm, 333 nm, and 334 nm. One finds that the oupling wavelength peaking T shifts with a rate of about 15 nm/nm with respet to the variation of w 1. Figure 5(b) shows the relation between w 1 and w 2 in order to maintain the phase-mathing ondition at 1550 nm. It is almost a linear urve with a slop of about 12, whih implies that the devie performane is muh less sensitive to the variation of w 2. This sensitivity differene an be explained by the loal derivatives of the two urves shown in Fig. 2 at the working point. We also find that employing larger waveguide widths will improve the extintion ratio at the output and derease the sensitivity to the dimension variations. On the other hand, this will weaken the strength of the rosspolarization oupling and inrease the devie length. A large waveguide ross-setion will also make the effetive indies of the high-order modes more lose to those of the working fundamental modes (f., Fig. 2). This will introdue unwanted oupling to those modes. Fig. 5. (a) Simulated transmission oeffiients. Here, w 2 = 332 nm, 333 nm, 334 nm along the solid arrow diretion, and the rest of the parameters are the same as those in Fig. 4. (b) Relation between w 1 and w 2 in order to maintain the phase-mathing ondition at 1550 nm. 10mm Fig. 6. Sanning eletron mirosope piture of a fabriated devie. Arrows indiate propagation diretions of the light. 3. Experiment and measurement We fabriated the designed devies on a ommerial SOI wafer using E-beam lithography (JEOL JBX-9300FS) and dry ething tehnologies. A piture of a finished devie is shown in Fig. 6. Adiabati tapers were used at the input and two output ports for onneting with the standard single-mode SOI waveguide of 450 nm wide in the rest of the iruit [f., Fig. 1()]. The fabriated samples were leaved for haraterization. The waveguides were tapered up to a width of 4 μm at the leaved faets for better oupling with lensed fibers. The measurement # $15.00 USD Reeived 11 Apr 2011; revised 18 May 2011; aepted 30 May 2011; published 15 Jun 2011 (C) 2011 OSA 20 June 2011 / Vol. 19, No. 13 / OPTICS EXPRESS 12650

7 transmission T (db) results are illustrated in Fig. 7. We refer to Ref [8]. for the detailed measurement setup and alibration proedure. In general, the measured transmission urves are mathing well those from the simulations in Fig. 4. The overall insertion loss and extintion ratio in the whole C- band is about 0.6dB and 12dB, respetively, slightly worse than the simulation results. Judged by the period, the fast osillations observed in, e.g., the T urve is likely due to a slight error in alibrating the measurement setup [7,8]. The fabriation repeatability of the proposed devie was also studied. Figure 8 shows the measured T urves for two PSR devies with the same designed parameters and on the same die. A good onsisteny an be found. Further investigation on improving the ross-die stability is also undertaken. Fig. 7. Measured transmission oeffiients between different polarized modes from the inputport to the ross-port (a) and the through-port (b) Conlusion wavelength (nm) Fig. 8. Measured transmission oeffiient parameters on one SOI die. T of two PSR devies with the same We have introdued a ompat SOI-based PSR devie used for the interfae setion of a polarization diversity iruit. The devie is based on the ross-polarization oupling effet between two parallel SOI wire waveguides with air top-ladding, whih an be fabriated in one lithography and ething step as needed for making the rest of the SOI iruit. We have demonstrated that suh a single devie struture an at as an effiient polarization splitter and rotator simultaneously. A total insertion loss of 0.6dB and an extintion ratio of 12dB have been obtained experimentally for a fabriated devie. # $15.00 USD Reeived 11 Apr 2011; revised 18 May 2011; aepted 30 May 2011; published 15 Jun 2011 (C) 2011 OSA 20 June 2011 / Vol. 19, No. 13 / OPTICS EXPRESS 12651