Jose Darío Sarmiento-Merenguel

Design of optical metamaterial waveguide structures (Conference Presentation)

Subwavelength gratings (SWGs) are periodic structures with a pitch (Λ) smaller than the wavelength of the propagating wave (λ), so that diffraction effects are suppressed. These structures thus behave as artificial metamaterials where the refractive index and the dispersion profile can be controlled with a proper design of the geometry of the structure. SWG waveguides have found extensive applications in the field of integrated optics, such as efficient fiber-chip couplers, broadband multimode interference (MMI) couplers, polarization beam splitters or evanescent field sensors, among others. From the point of view of nano-fabrication, the subwavelength condition (Λ << λ) is much easier to meet for long, mid-infrared wavelengths than for the comparatively short near-infrared wavelengths. Since most of the integrated devices based on SWGs have been proposed for the near-infrared, the true potential of subwavelength structures has not yet been completely exploited. In this talk we summarize some valuable guidelines for the design of high performance SWG integrated devices. We will start describing some practical aspects of the design, such as the range of application of semi-analytical methods, the rigorous electromagnetic simulation of Floquet modes, the relevance of substrate leakage losses and the effects of the random jitter, inherent to any fabrication process, on the performance of SWG structures. Finally, we will show the possibilities of the design of SWG structures with two different state-of-the-art applications: i) ultra-broadband MMI beam splitters with an operation bandwidth greater than 300nm for telecom wavelengths and ii) a set of suspended waveguides with SWG lateral cladding for mid-infrared applications, including low loss waveguides, MMI couplers and Mach-Zehnder interferometers.

Integrated polarization controllers

Polarization management is a key functionality in many photonic applications including telecommunications, polarization diversity circuits and sensing, to name a few. Developing integrated circuits capable of reliably controlling polarization state would result in compact and low cost circuits with improved stability, compared with fiber or bulk optics solutions. Polarization rotators are a key building block of these circuits. Unfortunately, stringent fabrication tolerances make the integration of polarization rotators highly challenging. The main limitation arises from the need to tightly control the profile of the hybrid modes in the rotator waveguide as well as their relative phase shift during propagation. Both values are very sensitive to waveguide geometry variations, seriously hindering their practical application. We have developed a technology independent scheme that enables fabrication error compensation, substantially relaxing device tolerances. In our scheme, three polarization rotation waveguides are interconnected with two tunable phase shifters to correct geometry deviations. Interestingly, these phase shifters also enable dynamic wavelength tuning and output polarization extinction ratio selection. We also showed that, by adding an output phase shifter, we can control the relative phase. Hence, we can yield any desired output state of polarization. We have implemented this scheme in the silicon-on-insulator platform using simple waveguide heaters as tunable phase shifters. We experimentally demonstrated an unprecedented polarization extinction range of 40 dB (±20 dB). Furthermore, the device showed a 98% coverage of the Poincaré sphere with a tunability range covering the complete C-band. These results prove the potential of our scheme to alleviate the, otherwise, very stringent fabrication tolerances, overcoming the major limitation of current integrated polarization managing devices.

Subwavelength structures for nanophotonic couplers, colourless splitters, polarization control and mid-infrared waveguides

We report our recent advances in development of subwavelength engineered dielectric metamaterial structures for integrated photonics. By locally engineering the refractive index of silicon by forming a pattern of holes at the subwavelength scale it is possible to manipulate the flow of light in silicon nanophotonic waveguides [1], [2], [3], [4], [5], [6]. Specifically, we present a broadband fibre-chip edge coupler with a coupling efficiency exceeding 90% for direct coupling between cleaved SMF-28 optical fibres and silicon nanophotonic waveguides [7], as well as the first implementation of a subwavelength nanostructure for laser-to-SOI chip coupling experiments with an InGaAsP/InP buried heterostructure laser. Furthermore, we report on dispersion engineered colourless multimode interference coupler operating over an ultra-broad wavelength range 1260 nm – 1675 nm, exceeding the O, E, S, C, L and U optical communication bands. We also present an integrated polarization controller with largely relaxed fabrication tolerances and tuneable over the complete C-band, with a measured polarization extinction range of 40 dB [8]. Implementations of subwavelength grating engineered structures for evanescent field sensing and mid-infrared silicon photonic circuits [9] with waveguide propagation losses < 0.9 dB/cm at 3.88 µm will also be discussed.