Silicon integrated multi-mode ring resonator

Ring resonator is an essential element in silicon integrated circuit, it is widely used as filter, wavelength multiplexer and switch in single-mode operation regime. As the rapid development of mode division multiplexing (MDM) technique, ring resonator that can process multi-mode signals simultaneously and uniformly is highly desired. However, the severe modal dispersion makes identical transmission for different modes very hard. In this paper, by breaking through the limitation of conventional multi-mode manipulation design with evanescent coupling or mode interference, we propose and demonstrate a multi-mode ring resonator (MMRR) inspired by the free space geometric optics. Arbitrary number of supporting modes can be achieved by simply widening the waveguide width. For proof-of-concept demonstration, an MMRR supporting four modes is fabricated with uniform transmittance. Furthermore, architecture of cascaded four MMRRs are also demonstrated experimentally.

The structure of normal modes in parallel ideal optical fibers with strong coupling

In this paper, we studied an effect of strong evanescent coupling on the structure of normal modes in a system of parallel ideal multimode optical fibers. Using the formalism of the degenerate perturbation theory and a scalar waveguide equation for this system, analytical expressions of higher-order supermodes and their propagation constants have been determined. We have shown that the structure of modes in the case of strong evanescent coupling coincides with the structure of normal modes for weakly coupled parallel fibers. We have demonstrated that in the presence of strong coupling, expressions for corrections to the scalar propagation constant are modified, deducing them analytically.

Addressing the Fabrication Difficulties of Femtosecond Laser Written Surface Waveguides for Enhanced Evanescent Coupling

In this work, the fabrication of optical waveguides embedded in fused silica (Suprasil1) and boro-aluminosilicate glass (Eagle2000) is demonstrated with femtosecond laser direct writing, as well as their suitability to be brought to the surface, through wet etching, for enhanced evanescent coupling with the external dielectric medium. Fused silica demonstrated to be inappropriate in this particular application, as the guiding region is at the bottom of the induced modification, creating a barrier between the guided mode and the substrate’s boundary. Furthermore, the existence of nanogratings meant that, upon contact of the top of the induced modification with the substrate’s boundary, the waveguide is quickly etched. Eagle2000 demonstrated to be superior to fused silica due to its characteristic modification cross-section and absence of nanogratings, which allowed the placement of the guiding region as close to the substrate’s surface as required. However, surface roughness arising from the creation of insoluble products in the HF solution was found. The addition of HCl to dissolve these products was implemented.