Experimental demonstration of chiral spiral waveguides without waveguide crossings

Spiral waveguides are important building blocks both for telecom and sensing applications [1], [2]. We have recently demonstrated that VTT micron-scale silicon photonics platform has several advantages, compared to other platforms, when looking for a trade-off between compactness and propagation loss [3]. Our previous work has been focused on non-chiral spirals, where the light enters the spiral, let’s say, clockwise and exits counter-clockwise. On the other hand, some sensing applications like gyroscopes require non-zero chirality [4], which implies as many waveguide crossings as the spiral rounds. Light passes in each crossing twice, first in one direction and afterwards in the orthogonal one. Here we propose and demonstrate experimentally a novel configuration that doesn’t require any waveguide crossing. It is based on MMI splitters designed to couple light completely in the cross port. The concept is sketched in Fig. 1(a). Nested loops are coupled to each other through the MMI splitters, which effectively work as waveguide crossings with collinear waveguides, at variance with the orthogonal waveguides of usual crossings. We have designed and fabricated three chiral spirals on our platform, based on 3 µm thick silicon strip waveguides. They have 7, 13 and 49 loops respectively, corresponding to the effective lengths 6.6 cm, 12.5 cm and 47.9 cm, which include the 1.58 mm MMI length counted twice (see Fig. 1(a)).