Disorder effects in subwavelength grating metamaterial waveguides.

Alejandro Ortega-Moñux,Jiří Čtyroký,Shurui Wang,Jens H Schmid,Pavel Cheben,Robert Halir,Íñigo Molina-Fernández

doi:10.1364/oe.25.012222

Abstract

Subwavelength grating (SWG) waveguides are integrated photonic structures with a pitch substantially smaller than wavelength for which they are designed, so that diffraction effects are suppressed. SWG operates as an artificial metamaterial with an equivalent refractive index which depends on the geometry of the structure and the polarization of the propagating wave. SWG waveguides have been advantageously used in silicon photonics, resulting in significant performance improvements for many practical devices, including highly efficient fiber-chip couplers, waveguide crossings, broadband multimode interference (MMI) couplers, evanescent field sensors and polarization beam splitters, to name a few. Here we present a theoretical and experimental study of the influence of disorder effects in SWG waveguides. We demonstrate via electromagnetic simulations and experimental measurements that even a comparatively small jitter (~5 nm) in the position and size of the SWG segments may cause a dramatic reduction in the transmittance for wide (multimode) SWG waveguides, while for narrow (single mode) waveguides this effect is negligible. Our study shows that the impact of the jitter on SWG waveguide performance is directly related to the modal confinement.

Highlights

An optical waveguide core comprising a grating with a period smaller than one half of the wavelength for which it is designed to operate, behaves as an artificial metamaterial with an equivalent refractive index which depends on the geometry of the structure and the polarization of the incident wave [1,2,3]
Subwavelength grating (SWG) operates as an artificial metamaterial with an equivalent refractive index which depends on the geometry of the structure and the polarization of the propagating wave
SWG waveguides have been advantageously used in silicon photonics, resulting in significant performance improvements for many practical devices, including highly efficient fiber-chip couplers, waveguide crossings, broadband multimode interference (MMI) couplers, evanescent field sensors and polarization beam splitters, to name a few

Summary

Introduction

An optical waveguide core comprising a grating with a period (pitch) smaller than one half of the wavelength for which it is designed to operate, behaves as an artificial metamaterial with an equivalent refractive index which depends on the geometry of the structure and the polarization of the incident wave [1,2,3]. We report a detailed study of the effects of fluctuations in waveguide geometry [see Fig. 1(b)], called jitter, inherent to any fabrication process, on the performance of SWG waveguide structures. By using electromagnetic simulations and experimental measurements, we find that a jitter as small as 5 nm (2.5% of a typical SWG pitch Λ = 200 nm) in a 3 μm wide SWG waveguide can induce a propagation loss penalty of ∼3 dB per 100 μm as well as significant back-reflections.

Definitions of the test structures and parameters

Simulation results

Discussion

Conclusion