Natasha Vukovic

Ultrafast optical control using the Kerr nonlinearity in hydrogenated amorphous silicon microcylindrical resonators

Microresonators are ideal systems for probing nonlinear phenomena at low thresholds due to their small mode volumes and high quality (Q) factors. As such, they have found use both for fundamental studies of light-matter interactions as well as for applications in areas ranging from telecommunications to medicine. In particular, semiconductor-based resonators with large Kerr nonlinearities have great potential for high speed, low power all-optical processing. Here we present experiments to characterize the size of the Kerr induced resonance wavelength shifting in a hydrogenated amorphous silicon resonator and demonstrate its potential for ultrafast all-optical modulation and switching. Large wavelength shifts are observed for low pump powers due to the high nonlinearity of the amorphous silicon material and the strong mode confinement in the microcylindrical resonator. The threshold energy for switching is less than a picojoule, representing a significant step towards advantageous low power silicon-based photonic technologies.

Novel Method for the Fabrication of Long Optical Fiber Tapers

We report a novel fiber taper facility that allows the fabrication of fiber tapers between a few centimeters and tens of meters in length and that can be used for standard and microstructured optical fibers (MOFs). Efficient reduction of error in diameter variation is achieved using the velocity control feedback loop. The experimental results of the tapering of both the step index fiber and the MOF are presented. The observed outer diameter tracking error over the taper length was within 1%.

Ultra-smooth microcylindrical resonators fabricated from the silicon optical fiber platform

High quality hydrogenated amorphous silicon microcylindrical resonators are fabricated using the recently developed semiconductor optical fiber platform. The resonators are shown to have ultra-smooth surfaces that minimize the effects of scattering losses and exhibit localized whispering gallery modes with loaded quality factors of up to ∼2.8 × 104. Coupled mode theory indicates that the system quality factor is limited by both the bulk material loss of the resonator and the tapered fiber coupling scheme. The large extinction ratios and thermal nonlinearities measured in these microcylindrical resonators suggest that they should find application for low-power all-optical processing.

Guiding properties of large mode area silicon microstructured fibers: a route to effective single mode operation

Numerical simulations are used to investigate the guiding properties of large mode area silicon microstructured fibers. Modal analysis of the isolated high refractive index core and cladding rod inclusions will be applied to show that the guidance mechanism of the composite fiber can be well described via a hybrid of the total internal reflection and antiresonant reflecting optical waveguide models. It will be shown that by selectively filling the cladding holes with silicon, which has been modified to have a slightly raised index, the fiber can be designed to operate in an effectively single-mode regime over an extended wavelength range.

Parabolic Pulse Generation Using Tapered Microstructured Optical Fibres

This paper presents a numerical study of parabolic pulse generation in tapered microstructured optical fibres (MOFs). Based on our results and the algorithms presented, one can determine the linear taper profile (starting and finishing pitch values and taper length) needed to achieve parabolic pulse shaping of an initial Gaussian pulse shape with different widths and powers. We quantify the evolution of the parabolic pulse using the misfit parameter and show that it is possible to reach values significantly better than those obtained by a step index fibre.

Low Birefringence Measurement and Temperature Dependence in Meter-Long Optical Fibers

We propose a relatively simple technique to infer the birefringence on single mode low-birefringence optical fibers based on the use of the Faraday effect. The theoretical model for Faraday rotation in the presence of nonnegligible fiber birefringence and a suitable measurement technique offer a fast and efficient way to determine low values of linear birefringence employing short fiber lengths. Alternative known techniques are not sensitive enough or they are of more difficult implementation. The method is used in only ~1 m-long single-mode optical fibers to obtain the upper-limit birefringence that can be tolerated in order to retain good current sensing sensitivity. The temperature dependence of the Faraday rotation and its causes are also investigated.

Thermal nonlinearity in silicon microcylindrical resonators

We explore the thermally induced nonlinearity in hydrogenated amorphous silicon microcylindrical resonators that are fabricated from the silicon optical fiber platform. In particular, we use a pump-probe technique to experimentally demonstrate thermally induced optical modulation and determine the response time. Through characterization of the thermal properties and the associated resonance wavelength shifts we will show that it is possible to infer the material absorption coefficient for a range of whispering gallery mode resonators.

Tunable continuous wave emission via phase-matched second harmonic generation in a ZnSe microcylindrical resonator

Whispering gallery mode microresonators made from crystalline materials are of great interest for studies of low threshold nonlinear phenomena. Compared to amorphous materials, crystalline structures often exhibit desirable properties such as high indices of refraction, high nonlinearities, and large windows of transparency, making them ideal for use in frequency comb generation, microlasing and all-optical processing. In particular, crystalline materials can also possess a non-centrosymmetric structure which gives rise to the second order nonlinearity, necessary for three photon processes such as frequency doubling and parametric down-conversion. Here we report a novel route to fabricating crystalline zinc selenide microcylindrical resonators from our semiconductor fibre platform and demonstrate their use for tunable, low power continuous wave second harmonic generation. Visible red light is observed when pumped with a telecommunications band source by a process that is phase-matched between different higher order radial modes, possible due to the good spatial overlap between the pump and signal in the small volume resonator. By exploiting the geometrical flexibility offered by the fibre platform together with the ultra-wide 500–22000 nm transmission window of the ZnSe material, we expect these resonators to find use in applications ranging from spectroscopy to quantum information systems.

Birefringence upper limit analysis of low birefringence fibers employed in the Faraday effect current sensors

The theoretical model of the Faraday rotation in the low birefringence optical fiber is proposed to serve as a convenient tool for the determination of the birefringence upper limit allowed to retain current sensor sensitivity. The measurement technique offers a fast and efficient determination of the ultra-low linear birefringence when other techniques are not sensitive enough or they are difficult to implement. A temperature dependence of the Faraday rotation and its causes are investigated.

Fabrication of metre-long fibre tapers

We report a novel fibre taper facility that allows tapering in the intermediate range of lengths of both standard and microstructured optical fibres. In particular, we examine issues related to the diameter fluctuations during tapering.