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Silicon polarization switch based on symmetric polarization splitter-rotators

Silicon photonic integration is considered to be one of the most promising techniques in realizing high-density photonic integrated circuits because of its compact device size and CMOS compatible fabrication process. However, due to the large index contrast of silicon waveguides, silicon photonic devices are affected by the large polarization mode dispersion, polarization dependent loss and polarization dependent wavelength characteristics. Polarization transparent circuits and polarization diversity schemes, which have polarization rotators and splitters as common key devices, have been proposed to solve this issue[1–3]. To date, various types of passive polarization rotators have been demonstrated[4–8], all of which are static and operation-fixed. However, the dynamic polarization switch is also needed due to its many applications, including polarization modulation[9, 10], coherent communication[11], quantum computing[12], and polarization diversity[2]. Recently, various dynamic polarization controlling devices based on silicon waveguides have been reported[13–15]. Two kinds of devices can be used to achieve a polarization switch in planar silicon waveguide circuits. The first is a phase modulator whose waveguide is illuminated at 45° of its main axis, as shown in Fig. 1(a). The input light is split into two orthogonal polarizations. With a thermal-optical controlling signal, a phase difference between the two orthogonal polarization states is produced, resulting in a changed output polarization orthogonal to its original state. However, the polarizationdependent phase shifter needs to be long enough because the polarization-dependent thermal optical effect is weak in silicon waveguides. The waveguide length is approximate 700 μm for that with SU8 upper cladding and much longer for that with SiO2 upper cladding[13]. The second consists of an ordinary polarization splitter rotator (PSR) followed by a series of 3-dB couplers and phase shifters, and a second PSR[15, 16]. This consists of many elements, including at least two PSRs, two 3-dB couplers and one phase shifter to realize polarization switch, as shown in Fig. 1(b). Here, we propose a new design of polarization switch, which consists of one phase shifter and two symmetric PSRs as shown in Fig. 1(c). The symmetric PSR utilizes the linear combination of TE0 and TM0 as orthogonal bases, while the common PSR directly utilizes the TE0 and TM0 as orthogonal bases[17]. Simulation results show that the device has a TM0-to-TE0 polarization extinction ratio of better than 30 dB over the C band. The simplified device has a shorter length of less than 300 μm compared with those previously reported.