Fabrication Tolerant Integrated Polarization Rotator Design Using the Jones Calculus

A fabrication tolerant polarization rotator (PR) building block is added to our generic photonic integration technology. Using asymmetric waveguides, we propose a PR with guided modes that do not enclose a 45° angle with the substrate. Rather, we find that polarization performance and loss can be improved substantially by allowing the mode angles to vary along the device. We numerically optimize PR designs, targeting at high extinction and high fabrication tolerance. The optimization makes use of the Jones formalism and the fact that Jones calculus is numerically much faster than beam propagation methods. A comparison of our Jones model and commercial simulation software shows excellent agreement. Fabricated devices show a polarization extinction between 10 and 14 dB across different wafers, at an excess loss of 1 dB. Compared to PR designs without tapers, the loss is 3 dB lower and the polarization extinction is now reproducible from wafer to wafer.

I. Introduction

In photonic integrated circuits (PICs), polarization is probably the hardest parameter to control compared to the other degrees of freedom of light. Even the most advanced PICs steer clear of bringing polarization handling into the chip, relying on off-chip polarization handling [1]–[3]. The applications for polarization handling optical sub systems are vast, however. Polarization sensitive optical coherence tomography (PS OCT), for instance, can greatly improve the achievable contrasts compared to non-PS OCT [4], [5]. Polarization controllers are key building blocks of many optical systems, but they are typically implemented in technologies of LiNbO₃-footprint or bigger [6]. Optical communication systems already make wide use of the polarization division multiplexing (PDM) technique in coherent systems [1], [3], but direct-detection schemes for PDM are also emerging [7], [8]. All those systems could benefit greatly from optical integration.