Enhanced all-optical cavity-tuning using graphene.

All-optical tuning of the resonance of an optical cavity is used to realise optical signal-processing including modulation, switching, and signal-routing. The tuning of optical resonance is dictated by the two primary effects induced by optical absorption: charge-carrier-generation and heat-generation. Since these two effects shift the resonance in opposite directions in a pure silicon-on-insulator (SOI) micro-ring resonator as well as in a graphene-on-SOI system, the efficiency and the dynamic range of all-optical resonance-tuning is limited. In this work, in a graphene-oxide-silicon waveguide system, we demonstrate an exceptional resonance-tuning-efficiency of 300 p m/m W (0.055 π/m W), with a large dynamic range of 1.2 n m (0.22 π) from linear resonance to optical bistability. The dynamics of the resonance-tuning indicates that the superior resonance-tuning is due to large linear-absorption-induced thermo-optic effect. Competing free-carrier dispersion is suppressed as a result of the large separation between graphene and the silicon core. This work reveals new ways to improve the performance of graphene-on-waveguide systems in all-optical cavity-tuning, low-frequency all-optical modulation, and switching.

Broadly tunable wideband optical single sideband generation using self-coupled silicon resonator.

We propose and demonstrate a tunable broadband optical single sideband generation using self-coupled silicon micro-ring resonator. We exploit self-coupling in a ring cavity to generate tunable resonance splitting. Using the proposed device, single sideband with carrier signal is generated from a double sideband signal. Experimental verification of power fading free transmission through a 43 km signal mode optical fiber is achieved for an RF frequency range of 1-20 GHz, extendible to higher frequencies. We also achieved a spurious free dynamic range of > 99.9±1.05 dB.Hz2/3 over the demonstrated frequency range. Furthermore, error-free data transmission of 1-12 Gbps over a 43 km fiber is also demonstrated with a detailed analysis of bit error rate as well.

Carbon-nanotube-on-waveguide thermo-optic tuners.

We demonstrate on-waveguide thermo-optic tuners based on solution-processed metallic carbon nanotubes (CNTs) on silicon-on-insulator (SOI) and silicon nitride (SiN) microring resonators operating around 1550 nm. On SOI microring resonators using planarized wire waveguides, a thermo-optic power efficiency of 29 mW/FSR and a thermal transient of 1.3 µs are achieved. The heater is shown to be more power-efficient than conventional metal heaters and has lower thermal transient than both metal heaters and graphene-based heaters. On SiN microring resonators using rib waveguides, improvement in power efficiency with an increase in coverage of CNTs is demonstrated, indicating localized heating using the CNTs; this is favorable for low thermal cross-talk. An optimal power efficiency of 142 mW/FSR and a thermal transient of 5.8 µs are achieved.

On-chip optical transduction scheme for graphene nano-electro-mechanical systems in silicon-photonic platform.

We present a scheme for on-chip optical transduction of strain and displacement of graphene-based nano-electro-mechanical systems (NEMS). A detailed numerical study on the feasibility of three silicon-photonic integrated circuit configurations is presented: the Mach-Zehnder interferometer (MZI), the micro-ring resonator, and the ring-loaded MZI. An index sensing based technique using an MZI loaded with a ring resonator with a moderate Q-factor of 2400 can yield a sensitivity of 28 fm/Hz and 6.5×10-6%/Hz for displacement and strain, respectively. Though any phase-sensitive integrated-photonic device could be used for optical transduction, here we show that optimal sensitivity is achievable by combining resonance with phase sensitivity.

Nonlinear properties of and nonlinear processing in hydrogenated amorphous silicon waveguides.

We propose hydrogenated amorphous silicon nanowires as a platform for nonlinear optics in the telecommunication wavelength range. Extraction of the nonlinear parameter of these photonic nanowires reveals a figure of merit larger than 2. It is observed that the nonlinear optical properties of these waveguides degrade with time, but that this degradation can be reversed by annealing the samples. A four wave mixing conversion efficiency of + 12 dB is demonstrated in a 320 Gbit/s serial optical waveform data sampling experiment in a 4 mm long photonic nanowire.

Generation of correlated photons in hydrogenated amorphous-silicon waveguides.

We report the first (to our knowledge) observation of correlated photon emission in hydrogenated amorphous-silicon waveguides. We compare this to photon generation in crystalline silicon waveguides with the same geometry. In particular, we show that amorphous silicon has a higher nonlinearity and competes with crystalline silicon in spite of higher loss.

High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible silicon-on-insulator platform.

A new generation of Silicon-on-Insulator fiber-to-chip grating couplers which use a silicon overlay to enhance the directionality and thereby the coupling efficiency is presented. Devices are realized on a 200 mm wafer in a CMOS pilot line. The fabricated fiber couplers show a coupling efficiency of -1.6 dB and a 3 dB bandwidth of 80 nm.

Bridging the gap between nanophotonic waveguide circuits and single mode optical fibers using diffractive grating structures.

In this paper, the use of diffractive grating structures to efficiently interface between a single mode fiber and a high index contrast waveguide circuit is outlined. We show that high index contrast grating structures allow for broadband and high efficiency coupling. Since no polished facet is required on the photonic integrated circuit to interface with the optical fiber, fiber-to-chip grating couplers enable wafer-scale testing, reducing the cost for testing large scale integrated optical circuits. We show that two-dimensional grating structures can solve the problem of the huge polarization dependence of high index contrast photonic integrated circuits. Finally, an optical probe is presented, which allows testing individual components of a photonic integrated circuit, analogous to the electrical probes used in micro-electronics.