Polarisation selective integrated silicon photonic devices

The main objective of this thesis was the development of polarisation selective gratings in silicon-on-insulator (SOI) technology. These devices can find numerous applications in the design of highly performing optical filters and, more in general, in all those devices that require on-chip manipulation of the polarisation state. The development of these devices was preceded by the optimisation of several fabrication processes, such as lithography and dry etching, and the re-design of a number of key components such as inverse polymer tapers and metallic heaters for thermal tuning. This activity culminated into a very robust process flow for SOI devices, with repeatable propagation losses in the order of 1 dB/cm, heaters with a very high tuning efficiency of 12 mW per π phase shift and 2dB and 1dB waveguide-to-optical fibre coupling losses for the TE and TM polarised mode, respectively. The grating designs developed in this thesis consisted of periodic holes etched onto the top surface of the silicon optical waveguide. Such geometry overlaps strongly with the TM polarised mode only and does not introduce additional losses to the TE mode. The benefit and the additional functionalities provided by the top grating geometry was assessed on two different polarisation sensitive devices. The first consisted in a microring resonator with integrated gratings for the emission of optical vortex beams, for which the top gratings provided a route to engineer the polarisation state of the emitted beam. The second device was a Bragg grating filter, where the top grating allowed the demonstration of extinction ratio values as high as 60dB by filtering the residual TM mode generated by the strong polarisation scattering

High Extinction Ratio Polarization Selective TE/TM Bragg Gratings Filters on Silicon-on-insulator

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High-extinction-ratio TE/TM selective Bragg grating filters on silicon-on-insulator

We report on the design and fabrication of TE and TM polarization selective Bragg grating filters in the form of sinusoidal perturbations on the waveguide sidewall and etched holes on the top of the waveguide, respectively. Combining the two geometries on a silicon-on-insulator waveguide resulted in Bragg grating filters with high extinction ratios of approximately 60 dB

High-extinction-ratio TE/TM selective Bragg grating filters on silicon-on-insulator

We report on the design and fabrication of TE and TM polarization selective Bragg grating filters in the form of sinusoidal perturbations on the waveguide sidewall and etched holes on the top of the waveguide, respectively. Combining the two geometries on a silicon-on-insulator waveguide resulted in Bragg grating filters with high extinction ratios of approximately 60 dB

Trimming of silicon-on-insulator ring-resonators via localized laser annealing

We propose a post-fabrication trimming method for the silicon-on-insulator photonic platform based on localised laser annealing of hydrogen silsesquioxane (HSQ) cladding. The technique is fast, does not degrade the device performance, does not require additional fabrication steps, and can therefore be implemented at minimal cost. Here we experimentally demonstrated how the spectrum of a ring resonator can be shifted by over 1 nm by annealing a section of the device as short as 30 µm, corresponding to a change in the effective refractive index of ∼10−2. Modifications of both the HSQ refractive index and its chemical structure as a function of the annealing temperature are also discussed. Trimming of multi-ring resonators indicate that this technique can be effectively used for post-fabrication reconfiguration of complex photonic circuits or to compensate for the fabrication tolerances of a typical CMOS process

Integrated microrings for on-chip filtering and efficient FWM generation

A silicon micro-ring device with tunable four-wave mixing efficiency is presented. The micro-ring is monolithically integrated with a dual polarisation filter based on a cascaded three ring resonator geometry. Four-wave mixing is demonstrated with signal and idler transmitted through the filter with only 1.8 dB of insertion loss. The pump light is rejected with over 62 dB extinction for both TE and TM guided modes

High accuracy transfer printing of single-mode membrane silicon photonic devices

A transfer printing (TP) method is presented for the micro-assembly of integrated photonic devices from suspended membrane components. Ultra thin membranes with thickness of 150nm are directly printed without the use of mechanical support and adhesion layers. By using a correlation alignment scheme vertical integration of single-mode silicon waveguides is achieved with an average placement accuracy of 100±70nm. Silicon (Si) μ-ring resonators are also fabricated and show controllable optical coupling by varying the lateral absolute position to an underlying Si bus waveguide

Orbital Angular Momentum Mode Multiplexer Based on Multimode Micro-Ring Resonator with Angular Gratings

We demonstrate silicon photonic orbital angular momentum multiplexing devices based on mulitmode microring resonator. Up to four optical beams carrying different orbital angular momentum states can be selectively excited from different input ports

Silicon photonic filters with high rejection of both TE and TM modes for on-chip four wave mixing applications

Wavelength selective filters represent one of the key elements for photonic integrated circuits (PIC) and many of their applications in linear and non-linear optics. In devices optimised for single polarisation operation, cross-polarisation scattering can significantly limit the achievable filter rejection. An on-chip filter consisting of elements to filter both TE and TM polarisations is demonstrated, based on a cascaded ring resonator geometry, which exhibits a high total optical rejection of over 60 dB. Monolithic integration of a cascaded ring filter with a four-wave mixing micro-ring device is also experimentally demonstrated with a FWM efficiency of -22dB and pump filter extinction of 62dB

Transfer Printing of Photonic Nanostructures to Silicon Integrated Circuits

Optical systems require the integration of technologies fabricated on different materials. We use a transfer printing technique to integrate pre-processed III-V, polymer and silicon membrane devices onto passive optical circuits with nano-metric positional accuracy