Polarization rotator-splitters in standard active silicon photonics platforms

We demonstrate various silicon-on-insulator polarization management structures based on a polarization rotator-splitter that uses a bi-level taper TM0-TE1 mode converter. The designs are fully compatible with standard active silicon photonics platforms with no new levels required and were implemented in the IME baseline and IME-OpSIS silicon photonics processes. We demonstrate a polarization rotator-splitter with polarization crosstalk < −13 dB over a bandwidth of 50 nm. Then, we improve the crosstalk to < −22 dB over a bandwidth of 80 nm by integrating the polarization rotator-splitter with directional coupler polarization filters. Finally, we demonstrate a polarization controller by integrating the polarization rotator-splitters with directional couplers, thermal tuners, and PIN diode phase shifters. © 2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (230.5440) Polarization-selective devices. References and links 1. Editorial, “Simply silicon,” Nat. Photonics 4, 491 (2010). 2. T. Baehr-Jones, L. Pinguet, T., G.-Q., S. Danziger, D. Prather, and M. Hochberg, “Myths and rumours of silicon photonics,” Nat. Photonics 6, 206–208 (2012). 3. T. Barwicz, M. R. Watts, M. A. Popovic, P. T. Rakich, L. Socci, F. X. Kartner, E. P. Ippen, and H. I. Smith, “Polarization-transparent microphotonic devices in the strong confinement limit,” Nat. Photonics 1, 57–60 (2007). 4. M. R. Watts and H. A. Haus, “Integrated mode-evolution-based polarization rotators,” Opt. Lett. 30, 139–140 (2005). 5. M. R. Watts, H. A. Haus, and E. P. Ippen, “Integrated mode-evolution-based polarization splitter,” Opt. Lett. 30, 967–969 (2005). 6. L. Chen, C. R. Doerr, and Y.-K. Chen, “Compact polarization rotator on silicon for polarization-diversified circuits,” Opt. Lett. 36, 469–471 (2011). 7. H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S. Itabashi, “Silicon photonic circuit with polarization diversity,” Opt. Express 16, 4872–4880 (2008). 8. D. Vermeulen, S. Selvaraja, P. Verheyen, P. Absil, W. Bogaerts, D. Van Thourhout, and G. Roelkens, “Siliconon-insulator polarization rotator based on a symmetry breaking silicon overlay,” IEEE Photon. Technol. Lett. 24, 482–484 (2012). 9. L. Liu, Y. Ding, K. Yvind, and J. M. Hvam, “Efficient and compact TE-TM polarization converter built on silicon-on-insulator platform with a simple fabrication process,” Opt. Lett. 36, 1059–1061 (2011). 10. D. Dai and J. E. Bowers, “Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires,” Opt. Express 19, 10940– 10949 (2011). 11. Y. Ding, H. Ou, and C. Peucheret, “Wideband polarization splitter and rotator with large fabrication tolerance and simple fabrication process,” Opt. Lett. 38, 1227–1229 (2013). #202433 $15.00 USD Received 3 Dec 2013; revised 19 Jan 2014; accepted 20 Jan 2014; published 10 Feb 2014 (C) 2014 OSA 24 February 2014 | Vol. 22, No. 4 | DOI:10.1364/OE.22.003777 | OPTICS EXPRESS 3777 12. T.-Y. Liow, K.-W. Ang, Q. Fang, J.-F. Song, Y.-Z. Xiong, M.-B. Yu, G.-Q. Lo, and D.-L. Kwong, “Silicon modulators and germanium photodetectors on soi: monolithic integration, compatibility, and performance optimization,” IEEE J. Sel. Top. Quantum Electron. 16, 307–315 (2010). 13. T. Baehr-Jones, R. Ding, Y. Liu, A. Ayazi, T. Pinguet, N. C. Harris, M. Streshinsky, P. Lee, Y. Zhang, A. E. Lim, T. Y. Liow, S. H. Teo, G. Q. Lo, and M. Hochberg, “Ultralow drive voltage silicon traveling-wave modulator,” Opt. Express 20, 12014–12020 (2012). 14. www.opsisfoundry.org. 15. W. Yuan, K. Kojima, B. Wang, T. Koike-Akino, K. Parsons, S. Nishikawa, and E. Yagyu, “Mode-evolution-based polarization rotator-splitter design via simple fabrication process,” Opt. Express 20, 10163–10169 (2012). 16. D. Dai, Y. Yang, and J. E. Bowers, “Mode conversion in tapered submicron silicon ridge optical waveguides,” Opt. Express 20, 13425–13439 (2012). 17. W. Sacher, T. Barwicz, and J. K. Poon, “Silicon-on-insulator polarization splitter-rotator based on TM0-TE1 mode conversion in a bi-level taper,” in “Conference on Lasers and Electro-Optics, OSA Technical Digest,” (2013), p. CTu3F.3. 18. N. Walker and G. Walker, “Polarization control for coherent communications,” J. Lightwave Technol. 8, 438–458 (1990). 19. T. Saida, K. Takiguchi, S. Kuwahara, Y. Kisaka, Y. Miyamoto, Y. Hashizume, T. Shibata, and K. Okamoto, “Planar lightwave circuit polarization-mode dispersion compensator,” IEEE Photon. Technol. Lett. 14, 507–509 (2002). 20. C. Doerr and L. Chen, “Monolithic PDM-DQPSK receiver in silicon,” in “Optical Communication (ECOC), 2010 36th European Conference and Exhibition on,” (2010), pp. 1–3. 21. C. Doerr, N. Fontaine, and L. Buhl, “PDM-DQPSK silicon receiver with integrated monitor and minimum number of controls,” IEEE Photon. Technol. Lett. 24, 697–699 (2012).


Introduction
Silicon-on-insulator (SOI) is becoming a common platform for integrated photonic circuits because the high refractive index contrast enables compact devices and the growing availability of foundry services allows complex silicon photonic chips to be fabricated at low costs [1,2].However, high index contrast waveguides also possesses a large birefringence.Because the polarization of the input light to a chip from an optical fiber is not usually fixed, polarization transparent photonic devices and circuits are needed at the receiver and along an optical communication link.To this end, polarization diversity can be implemented [3][4][5].Essential elements in polarization diverse photonic circuits are polarization splitters and polarization rotators.A challenge for polarization splitters and rotators in SOI is that they often require high aspect ratio features, extra layers, or an air cladding [3][4][5][6][7][8][9][10][11], which are not compatible with common foundry processes.
In this work, we combine the splitter and rotator functionalities into a polarization rotatorsplitter (PRS) that is fully compatible with standard foundry processes.The PRS requires only a single silicon (Si) material layer with top and bottom SiO 2 cladding.This Si layer must be patterned with both a full and partially-etched level; no high aspect ratio features are required.The compatibility with standard foundry processes enables us to demonstrate the PRS and an active polarization controller using the IME baseline and IME-OpSIS silicon photonics processes [12][13][14].The PRS uses a bi-level taper that converts a fundamental TM mode (TM0) input into a first-order TE mode (TE1) output, as proposed in [15,16].Our work is the first demonstration of PRSs using this mode conversion in bi-level tapers, and we presented a preliminary report in [17].Although TM0-TE1 mode conversion in bi-level tapers was demonstrated in [16], a full PRS was not demonstrated.Overall, our work paves the way for polarization diversity, polarization controllers, and polarization-multiplexed transmitters and receivers in standard active SOI photonic platforms.
The paper is organized as follows: we describe our adiabatic bi-level taper PRS design and demonstration in Section 2; then, we show the PRS integrated with directional coupler polarization filters for improved polarization crosstalk in Section 3; finally, we apply the PRS to an active polarization controller in Section 4.

Adiabatic bi-level taper polarization rotator-splitter
Our PRS design is shown in Fig. 1(a), where the red regions represent the full height of the Si and the purple regions represent the partially-etched level of Si.We designed the PRS for the IME baseline and OpSIS silicon photonics processes, which have a top silicon thickness of 220 nm and a partially-etched thickness of 90 nm.Our PRS uses a bi-level taper for TM0-TE1 mode conversion and symmetric SiO 2 cladding.After the bi-level taper, the TE0 and TE1 modes are separated into two waveguides using an adiabatic coupler instead of the directional coupler proposed in [10], the Y-branch proposed in [15], or the Y-branch and multi-mode interferometer in [11].Overall, our PRS design is entirely adiabatic.This is a key distinction from the earlier PRS designs in [10,11,15], which have non-adiabatic elements that limit the bandwidth, increase the senstivity to variations in waveguide dimensions, and increase the insertion loss.The total length of the PRS design is about 475 μm.The remainder of this section is organized as follows: in Section 2.1, we provide a detailed description of the PRS design and operating principles, and in Section 2.2, we describe our experimental demonstration of a bi-level taper PRS.

Detailed polarization rotator-splitter design and operation
The PRS operation relies on the principle of mode evolution [4,5].The evolution of the modes with the first and second highest effective indices (i.e., "mode 1" and "mode 2") in the PRS is illustrated in Fig. 1(b).In the first half of the bi-level taper, the Si ridge and partially-etched slab continuously widen, and this is where the TM0-TE1 mode conversion occurs.The partiallyetched slab breaks the vertical symmetry of the waveguide [10,16], which produces a large difference in the effective indices of modes 2 and 3 throughout the structure, as shown in Fig. 1(c).This allows a TM0 input to remain in mode 2 all along the bi-level taper and evolve into, first, a "hybridized" mode with TM0 and TE1 features, and finally, the TE1 mode.A TE0 input simply remains in mode 1 and exits the bi-level taper in the TE0 mode.The second half of the bi-level taper, where the Si ridge continues to widen and the partially-etched slab narrows, is used to provide a fully-etched, wide waveguide as the input to the adiabatic coupler.
The adiabatic coupler follows the bi-level taper and consists entirely of fully-etched Si waveguides with symmetric SiO 2 cladding, which prevents crosstalk between the TE1 and TM0 modes.The mode evolution in the adiabatic coupler can be understood from the mode profiles in Fig. 1(b).Here, TE0 and TE1 refer to supermodes of the composite two-waveguide structure.At the start of the coupler, a "narrow" 200 nm wide waveguide begins with a blunt tip next to a "broad" 850 nm wide waveguide; the gap between the waveguides is 200 nm.The TE0 and TE1 modes are well confined in the broad waveguide and have little overlap with the narrow waveguide.Then, the broad waveguide is narrowed to a 650 nm width and the narrow waveguide is widened to a 500 nm width; the gap is held constant at 200 nm.At this point, the TE0 mode is well confined in the broad waveguide while the TE1 mode is well confined in the narrow waveguide.Finally, the narrow waveguide is bent away from the broad waveguide using an arc with a radius of 450 μm.As the waveguides separate, the TE0 and TE1 supermodes of the adiabatic coupler evolve into the TE0 modes of the isolated top and bottom waveguides, respectively.

Polarization rotator-splitter measurements
PRSs were fabricated in the IME baseline process, and optical micrographs of the PRS are shown in Fig. 2. The PRS inputs and outputs lead to edge couplers with 220 nm wide square tips.We measured the PRS by coupling light from a tunable laser from free-space using objective lenses.Manually-adjustable, free-space, linear polarizers were placed at the input and Figure 3 shows the measured transmission spectra of the two PRS outputs for TE and TM inputs.The transmission spectra have been normalized to the transmission spectra of the edge couplers to extract the spectral characteristics of the PRS only.From Figs. 3(a) and 3(b), the polarization crosstalk at both output ports was less than -13 dB over a wavelength range between 1530 nm and 1580 nm; the crosstalk increased to about -10 dB for wavelengths between 1500 nm and 1530 nm.Due to inaccuracies in aligning the coupling lenses between measurements, the error in the transmission values was about ± 0.5 dB.Other than normalizing out the edge coupler transmission, no post-processing was applied to the data in Figs.3(a) and 3(b).
The extracted insertion loss of the PRS is shown in Figs.3(c) and 3(d).The raw transmission spectra in the black curves overestimate the PRS insertion loss since Fabry-Perot oscillations from the chip facets and measurement apparatus were not fully removed by normalizing the data to the transmission of the edge couplers.The edge coupler loss calibration structures and PRS had different Fabry-Perot oscillations, and the Fabry-Perot oscillations of the measurement setup changed between measurements due to realignments.We post-processed the raw transmission spectra of the PRS and edge couplers to reduce the contribution of the Fabry-Perot oscillations and obtained the more accurate insertion loss data in the red curves.Chip facet Fabry-Perot oscillations were easily identified from the waveguide lengths and group indices, and oscillations that differed little between devices and polarization settings were attributed to the measurement apparatus.From the post-processed data, the insertion loss and polarizationdependent loss (PDL) were less than 1.5 dB and 1.6 dB, respectively, over a wavelength range from 1530 nm to 1580 nm.Our ± 0.5 dB realignment error estimate is evident from the red curves, which have some points with transmission > 0 dB.The large-period oscillations in Fig. 3(d) may be Fabry-Perot oscillations from reflections at the chip facets and the waveguide discontinuity at the beginning of the adiabatic coupler or within the bi-level taper.
Through this first demonstration of a bi-level taper PRS, we can identify four simple design improvements to reduce the crosstalk and increase the bandwidth.First, the blunt-tip at the start of the adiabatic coupler in Fig. 1(a) could be replaced by an arc with a large radius to eliminate any mode coupling caused by the waveguide discontinuity.Second, reducing the waveguide gap in the adiabatic coupler will reduce the crosstalk or the coupler length required to achieve the crosstalk we demonstrated; this is due to an increase in the effective index difference between the TE0 and TE1 modes [4,5].Third, the widths and length of the bi-level taper can be optimized; from Fig. 3(a), the incomplete TM0-TE1 mode conversion in the bi-level taper is a large component of the crosstalk.Finally, the PRS can be cascaded with additional PRSs or other types of polarization clean-up filters [6].This latter approach is demonstrated in Section 3 where the PRS is integrated with directional coupler clean-up filters for reduced crosstalk.

Polarization splitter-rotator with improved crosstalk
One approach to improving the performance of the PRS is to cascade it with polarization cleanup filters.This is demonstrated here using directional coupler clean-up filters [6,7] placed in front of the PRS as shown in Fig. 4. In this configuration, it is more appropriate to refer to this structure as a polarization splitter-rotator (PSR) than a polarization rotator-splitter (PRS).In contrast to the PRS by itself, which converts the TM polarization to the TE1 mode before splitting, here, the polarizations are first split with a directional coupler before TM is rotated into TE.This device was fabricated with the IME baseline process as well.
The detailed operation of the PSR can be understood from the annotated micrograph in Fig. for polarization rotation and then two additional clean-up filters.The unused ports of the PRS and directional couplers are terminated with waveguide tapers leading to 200 nm wide blunt tips.The PRS is nominally identical to the PRS demonstrated in Section 2. The inputs and outputs of the whole PSR lead to edge couplers with 220 nm wide tips.The PSR was measured using the same method as Section 2.2 (i.e., free-space coupling with linear polarizers at the input and output of the chip).Figures 5(a) and 5(b) show the measured transmission spectra of the two PSR outputs normalized to the transmission spectra of the edge couplers for TE and TM inputs.The polarization crosstalk at both outputs was less than -22 dB over a wavelength range from 1500 nm to 1580 nm, which was an improvement of 9 dB over the PRS in Section 2.2.In principle, the clean-up filters should have provided a significantly lower crosstalk, but the measurements may have been limited by the accuracy of the input and output polarizers.
The insertion loss of the PSR is shown in Figs.5(c) and 5(d).The black curves are raw data and the red curves have been post-processed to remove Fabry-Perot oscillations from the chip facets and the measurement apparatus, as explained in Section 2.2.From the post-processed data, the insertion loss and PDL of the PSR were less than 2.3 dB and 1.9 dB over a wavelength range from 1530 nm to 1580 nm; the error in the insertion loss was roughly ± 0.5 dB due to realignment error of the coupling lenses.Compared to the PRS in Section 2.2, the insertion fixed at 1570 nm and the input was chosen to be TE-polarized for simplicity.This simple measurement setup did not allow for the extraction of the phase between the output TE and TM polarization components.The polarization controller insertion loss was < 2.5 dB.By driving the top-left and top-right thermal tuners, we generated TM-polarized, -45 • linearly-polarized, and circularly-polarized outputs, which is evident from the output power as a function of the output polarizer angle in Fig. 7(b).0 • corresponds to a horizontal (TE) polarization axis; the uncertainty in the angle was about ±2 • .Crosstalk in the PRSs and non-ideal 3-dB directional couplers limited the extinction for the TM and -45 • measurements.The red curve corresponds to a circularly-polarized output since the power only fluctuates by about 0.2 dB over all output polarizer angles.It was achieved with powers of 15 mW and 12 mW dissipated in the top-left and top-right thermal tuners, respectively.
Next, as a simple demonstration of switching between TM and TE-polarized outputs, we input TE light into the polarization controller and applied voltages only to the top-left thermal tuner [Fig.7(c)] or the top-left PIN diode [Fig.7(d)]; the other tuning elements were not driven.The applied voltage was swept with the output polarizer fixed to pass either TE or TM (marked With a TE-polarized input, bias conditions were chosen to obtain a TM-polarized output (black curve), a -45 • linearly-polarized output (blue curve), and a circularly-polarized output (red curve).(c) Normalized output power as the top-left thermal tuner power was swept.(d) Normalized output power as the top-left PIN diode current was swept.In (c) and (d), the output polarizer was set to pass either TE or TM or removed from the optical path ("Total out").The optical output power curves were normalized to the maximum value in each plot.The magenta labels and dashed lines indicate points where a TM or TE output was generated from the TE input (marked "TE→TM" and "TE→TE", respectively)."TE out" and "TM out" in the plots) or with the output polarizer removed from the optical path (marked "Total out" in the plots).Increasing the voltage on the thermal tuner or the PIN diode shifted the output polarization between TM and TE.At PIN diode currents beyond 10 mA, the optical loss increased substantially, which imbalanced the MZI and increased the total insertion loss of the polarization controller.
The polarization controller presented here is intended to show the full compatability of the PRS with a standard Si photonics platform.Its simple design limits its optical bandwidth and the polarization states that it can create.A complete polarization controller can be achieved with two simple design modifications.First, the optical bandwidth can be extended by compensating for the group delay differences between the TE0 and TM0/TE1 modes in the PRS.This compensation should be applied to both PRSs and can be implemented as an extra length of straight waveguide at one of the PRS outputs.Second, an additional set of phase-shifters should be included before the 2 × 2 MZI to create the full range of relative weights between the TE and TM components required to convert any arbitrary input polarization to an arbitrary output polarization.An endless polarization controller, which can be used in polarization-division multiplexed receivers for polarization-tracking, would further require two extra sets of phase-shifters and two extra directional couplers [18][19][20][21].

Conclusion
In summary, we have demonstrated the first polarization rotator-splitter using a TM0-TE1 mode converter based on an adiabatic bi-level taper and extended the concept to a polarization controller and a polarization splitter-rotator with improved crosstalk using directional coupler clean-up filters.The main advantage of the designs in this work is that they are fully compatible with standard silicon photonic foundry processes and do not require specialty high aspect ratio features, extra layers, or an air cladding.Although the adiabatic transitions make the polarization rotator-splitter long, the design is inherently broadband and tolerant to dimensional variations.

Fig. 1 .
Fig. 1.(a) Schematic of the polarization rotator-splitter (PRS).Widths are labeled in red and purple; lengths use green labels.(b) Schematic showing the profiles of the modes with the first and second highest effective indices (i.e., "mode 1" and "mode 2") at different points along the PRS.In the adiabatic coupler, "mode 1" and "mode 2" refer to supermodes of the composite waveguide.(c) Effective indices (n e f f ) along the first half of the bi-level taper for modes 1 to 3 at a wavelength of 1550 nm.

Fig. 2 .Fig. 3 .
Fig. 2. (a) An optical micrograph of the polarization rotator-splitter fabricated in the IME baseline process.Magnified optical micrographs are shown for (b), the bi-level taper, and (c), the end of the adiabatic coupler.

Fig. 7 .
Fig. 7. Polarization controller measurement data.(a) Current-voltage characteristics of the top-left thermal tuner and PIN diode.(b) Normalized output power as the output polarizer was rotated.With a TE-polarized input, bias conditions were chosen to obtain a TM-polarized output (black curve), a -45 • linearly-polarized output (blue curve), and a circularly-polarized output (red curve).(c) Normalized output power as the top-left thermal tuner power was swept.(d) Normalized output power as the top-left PIN diode current was swept.In (c) and (d), the output polarizer was set to pass either TE or TM or removed from the optical path ("Total out").The optical output power curves were normalized to the maximum value in each plot.The magenta labels and dashed lines indicate points where a TM or TE output was generated from the TE input (marked "TE→TM" and "TE→TE", respectively).