We demonstrated a novel on-chip polarization controlling structure, fabricated by standard 0.18-um foundry technology. It achieved polarization rotation with a size of 0.726 um * 5.27 um and can be easily extended into dynamic polarization controllers.
Deep Dive into Experimental demonstration of an ultra-compact on-chip polarization controlling structure.
We demonstrated a novel on-chip polarization controlling structure, fabricated by standard 0.18-um foundry technology. It achieved polarization rotation with a size of 0.726 um * 5.27 um and can be easily extended into dynamic polarization controllers.
Silicon on insulator (SOI) is a prevailing platform for its CMOS compatibility and integration compactness. Simultaneously, strong polarization dependence occurs which makes polarization control essential. A great deal of effort has been made but on-chip polarization control is still on the way mainly due to the fabrication problems, either too small features size (60 nm) [1], stringent fabrication accuracy requirement [2,3], large device size (longer than 100 m) [4] or incompatibility with CMOS fabrication process [5].
In this paper, we propose and experimentally demonstrate a practical polarization controlling structure with compact size, wide bandwidth and low insertion loss. Moreover, the structure directly links dynamic polarization control with phase manipulation and it can be easily fabricated by the mature 0.18-m CMOS technology.
A partially etched slot waveguide is introduced to realize the polarization rotation [Fig. 1 (a)]. H=220 nm, Slot=200 nm and Slab=90 nm are chosen according to the design rules of the fabrication foundry, while W=726 nm is optimized for polarization rotation. As shown in Fig. 1(b), it supports three eigenmodes. The mode with neff =2.222, marked as TE0 slot , is quasi-TE polarized since the major transverse fields are Ex and Hy. The other two modes, EM1 slot and EM2 slot , are hybridized in polarizations. When a fundamental TM mode in strip waveguide (TM0 strip ) is injected into such a partially etched slot waveguide, EM1 slot and EM2 slot will be excited since it does not support TM0 slot mode. The effective refractive index (neff) difference between EM1 slot and EM2 slot modes will cause beat and W has been optimized to achieve polarization rotation at half-beat length / ( 2) 5.27 μm
, as shown in Fig. 2(a). Then, two strip waveguides are utilized to symmetrically decouple the light into two separate TE0 strip modes with a phase difference of . When TE0 strip is injected, it will be converted to TE0 slot due to the field similarity and decoupled into two separate TE0 strip modes with a phase difference of This polarization rotation scheme has been verified with the measured optical images in Fig. 2(b). Inputting TE0 strip and TM0 strip into this structure simultaneously, interference occurs in the two output ports and a wavelength shift of FSR/2 is observed [Fig. 2(c)]. It can be explained as: in a reference waveguide (WGref), TE0 strip and TM0 strip modes will not interfere with each other since they are orthogonal. However, if the injected TM0 strip mode gradually transforms into TE0 strip mode and it has a phase delay with the original TE0 strip mode, interference will take place and result in two complementary output transmission spectra. From this perspective, the observed interference and wavelength shift are also strong evidences of polarization rotation.
Separating the incident optical power in TE0 strip and TM0 strip modes is always wanted. A polarization rotator and splitter (PRS) is proposed by attaching a π/2 phase-shifter and a directional coupler (DC) [Fig. 3(a)]. The measured results in Fig. 3(b) and 3(c) indicates that the insertion loss (IL) is -0.90 dB (-2.28 dB) and the extinction ratio (ER) is 14.56 dB (11.77 dB) at 1550 nm wavelength for TE0 strip (TM0 strip ) incidence. The insets in Fig. 3(b) and 3(c) indicates that the output light is dominated by TE polarization but have not been fully coupled into one waveguide. Therefore, the performance will be better if the DC is optimized properly. Moreover, simulations proved that using strip-slot mode converter [6] can further elevate the performance: the IL is -0.7 dB (-0.9 dB) and the ER is 22 dB (34 dB) for TE0 strip (TM0 strip ) incidence.
As discussed above, the proposed polarization rotation structure associates the phase difference with polarization state: TE0 corresponds to two light beams with phase difference of 0 and TM0 corresponds to π. Here we show an example of dynamic polarization controllers by integrating the structure with phase tuning elements (Fig. 4).
We have proposed and designed an untra-compact polarization rotation structure using partially etched slot waveguide which is just 0.726 m × 5.27 m in size and can be easily fabricated. The function of rotation is experimentally verified with a PR and PRS which are fabricated by standard 0.18-m foundry technology. Moreover, the structure associates polarization states with phase difference which enables on-chip dynamic polarization control by mature phase manipulation in silicon photonics.
This work was supported by the Natural Science Foundation of China (Grant No. 61120106012).
[1] A. V. Velasco, M. L. Calvo, and P. Cheben, et. al., “Ultracompact polarization converter with a dual subwavelength trench built in a silicon-on-insulator waveguide,” Opt. Lett. 37, 365 (2012).
[2] L. Liu, Y. Ding, K. Yvind, and J. Hvam, “Efficient and compact TE-TM polarization converter built on silicon-on-insulator platform with a simple f
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