Towards 'smart lasers': self-optimisation of an ultrafast pulse source using a genetic algorithm

Short-pulse fibre lasers are a complex dynamical system possessing a broad space of operating states that can be accessed through control of cavity parameters. Determination of target regimes is a multi-parameter global optimisation problem. Here, we report the implementation of a genetic algorithm to intelligently locate optimum parameters for stable single-pulse mode-locking in a Figure-8 fibre laser, and fully automate the system turn-on procedure. Stable ultrashort pulses are repeatably achieved by employing a compound fitness function that monitors both temporal and spectral output properties of the laser. Our method of encoding photonics expertise into an algorithm and applying machine-learning principles paves the way to self-optimising `smart' optical technologies

Investigation of defect cavities formed in three-dimensional woodpile photonic crystals

We report the optimisation of optical properties of single defects in three-dimensional (3D) face-centred-cubic (FCC) woodpile photonic crystal (PC) cavities by using plane-wave expansion (PWE) and finite-difference time-domain (FDTD) methods. By optimising the dimensions of a 3D woodpile PC, wide photonic band gaps (PBG) are created. Optical cavities with resonances in the bandgap arise when point defects are introduced in the crystal. Three types of single defects are investigated in high refractive index contrast (Gallium Phosphide-Air) woodpile structures and Q-factors and mode volumes ($V_{eff}$) of the resonant cavity modes are calculated. We show that, by introducing an air buffer around a single defect, smaller mode volumes can be obtained. We demonstrate high Q-factors up to 700000 and cavity volumes down to $V_{eff}<0.2(\lambda/n)^3$. The estimates of $Q$ and $V_{eff}$ are then used to quantify the enhancement of spontaneous emission and the possibility of achieving strong coupling with nitrogen-vacancy (NV) colour centres in diamond.Comment: 12 pages, 11 figure

Novel sources of near- and mid-infrared femtosecond pulses for applications in gas sensing, pulse shaping and material processing

In this thesis the design, construction process and the performance of two femtosecond optical parametric oscillators and one second–harmonic generation femtosecond pulse shaper is described. One oscillator was applied to gas sensing while potential applications of other devices are outlined. ATi:sapphire oscillator was used to pump a periodically–poled lithium niobate– based optical parametric oscillator. This signal–resonant device was configured to produce broadband idler pulses tunable in the range of 2.7–3.4 μm. This wavelength coverage was matched to the ν3 optical absorption band of methane, and Fourier–transform spectroscopy of a CH4:N2 mixture was implemented by employing a mid–IR silica photonic bandgap fibre simultaneously as a gas cell and an optical waveguide. Methane sensing below a 1% concentration was demonstrated and the main limiting factors were identified and improvements suggested. Another optical parametric oscillator was demonstrated which was pumped by a commercial Yb:fibre master oscillator/power amplifier system and was based on a periodically–poled lithium niobate crystal. The signal was tunable between 1.42–1.57 μm and was intended as a source for a subsequent project for waveguide writing in silicon. The oscillator was a novel long–cavity device operating at 15 MHz. The 130 nJ pump pulse energies allowed for 21 nJ signal pulses at a pump power of 2 W. The performance of the oscillator was characterised via temporal and spectral measurements and the next steps of its development are outlined. Finally a pulse shaper based on second harmonic generation in a grating– engineered periodically–poled lithium niobate crystal was demonstrated. Pulses from a 1.53 μm femtosecond Er:fibre laser were compressed and then used as the input to the shaper. The performance of the shaper was tested by performing cross–correlation frequency–resolved optical gating measurements on the output second harmonic pulses and this confirmed the successful creation of multiple pulses and other tailored shapes including square and chirped pulses, agreeing well with theoretical calculations

Ultra-High Q-Factor Silicon Resonator for High Frequency Oscillators

The thesis focuses on the investigation and characterisation of ultra-high Q-factor low loss Silicon resonators with transverse electric (TE)-like electromagnetic band-gap determined by two dimensional periodic structure made of a Silicon slab having a triangular lattice of air cylinders. A band-gap is observed where no energy is propagated through the slab, however engineering defects are created and optimised within the lattice producing resonant cavities and waveguides. The structure being excited with the fundamental TE10 mode can be coupled to external circuits via waveguides and its respective transitions in co-planar waveguide transmission line used to convey the millimetre-wave frequency signals. The ultimate goal is to investigate and characterise the promising low loss and high frequency Silicon resonators suitable for millimetre-wave communications such as used in low phase noise oscillator application and band pass filters. The results clearly show that electromagnetic band-gap structures or photonic crystals (PC) can be utilized for application in high frequency oscillators directly in fundamental mode with great benefits in obtaining ultra-high Q-factor and therefore low phase noise; and with better performance than alternative state-of-art technologies such as crystal oscillators in combination with frequency multiplication or frequency synthesis causing an increase in the overall phase noise by 20 log rule. By successfully demonstrating the experiment of using electromagnetic band-gap structures with oscillators, it is a great contribution towards the solution of the problem of high phase noise affecting high frequency oscillators operating at millimetre-wave band

The ultrafast laser inscription of photonic devices for integrated optical applications

A study of some key areas in which ultrafast laser inscription may usefully be employed is presented. The thesis includes waveguide inscription in a variety of substrates including passive glass, doped glass and a nonlinear crystal. The work contained can be split into three studies, with some overlap between them. Firstly fused silica glass is used, both in planar substrates and as flat fibre, for the inscription of two sensing elements. The planar substrate is used for a device similar in design to a side-polished fibre and the flat fibre is used for the fabrication of a Bragg grating waveguide array. In the second study, waveguides are inscribed in the nonlinear crystal monoclinic bismuth borate, and used for guided mode second harmonic generation. A novel waveguide design is employed to increase overlap between the pump and second harmonic waveguide modes. The remainder of the thesis investigates the applicability of ultrafast laser inscription to the fabrication of compact modelocked lasers. Lasing is demonstrated, both continuous wave and modelocked, using a laser inscribed erbium doped bismuthate glass waveguide as the gain element. A study is then undertaken into methods of integrating carbon nanotubes, used as saturable absorbers to modelock lasers, into laser inscribed waveguides

Optical nonlinear effects and photon pair production in modulated materials

According to quantum field theory an oscillation of boundary conditions in the vacuum can generate real photon pairs, which pop into existence from the zero-point energy. This surprising effect is known as the dynamical Casimir effect (DCE). In this thesis, we focus our attention on using the DCE as a route to experimentally study the quantum vacuum. Initially we explore experimental approaches that rely on an oscillation of vacuum boundary conditions. These require a direct temporal modulation of the boundary condition, which we achieve by temporally modulating the refractive index through the second order nonlinearity of the medium. We show that the key condition for this approach is that the medium is subwavelength in thickness. This leads to a temporal modulation of the refractive index. We use a mechanically exfoliated MoS2 monolayer. We present a detailed study of the second order nonlinearity of 2D (mono-atomic layer) dichalcogenide MoS2, and test its potential for spontaneous parametric down-conversion (SPDC) i.e. amplification of vacuum fluctuations mediated by optical nonlinearity. We present a model of SPDC in MoS2 monolayers and show that our data are compatible with theoretical estimates. We show exciting indications that SPDC may be possible in this material by performing polarization and lifetime measurements. We also discuss and characterise a new photo-luminescence emission around 1500 nm which is enhanced at the edges of multi-layer MoS2. In chapter three we present experimental approaches using spatially modulated third order nonlinear media along the propagation axis. We show that under certain conditions the spatial modulation of the boundary conditions along the nonliner media can be considered as a temporal modulation. Modulated waveguides provide an intensity variation of the pulse along the waveguide. Two different materials are used as substrate material for the waveguides; gallium lanthanum sulphide (GLS), which has a high third order nonlinear susceptibility and fused silica. We present numerical simulation results for photon generation from the modulated waveguides in GLS sample. We also characterise the nonlinear response and effects of the waveguides. In both substrates, we show that the lack of nonlinearity and different sources of losses prevents the creation of photon pairs. Finally, we present an experimental method in which the group velocity dispersion (GVD) is modulated along a photonic crystal fiber. Measurements of the temporal correlations between the newly generated frequencies is presented with a coincident to accidental ratio (CAR) of 7.5: yielding proof of a quantum correlation between the generated photon pairs

Wavelength extension in speciality fibres

Since the invention of the laser and its first application, there has been an almost continuous stream of new applications - many of which require specific laser sources. These applications often require a laser source with a specific power, pulse duration, energy and wavelength. In some cases these demands are easily met, whilst in others they have proven rather more difficult to achieve. Traditionally, wavelength versatility has been limited to the regions for which rare earth or gas gain media are available. These lasers themselves can be used to generate other wavelengths through the nonlinear processes of second and third harmonic generation, as well as sum frequency generation. Despite all of this, there still exists a significant section of the visible and infrared spectrum for which no convenient sources exist. This thesis is concerned with the development of sources in these regions along with broadband sources covering significant portions of the spectrum by themselves. These new wavelengths are generated in a variety of speciality fibres using either nonlinear processes or new gain media doped into standard silica fibres. Three types of speciality fibre are used: low concentration bismuth doped fibre which provides gain in the 1.0-1.4 μm region; photonic crystal fibres; and very high (75%) concentration germanium fibres to generate a laser source at 2.1 μm based upon stimulated Raman scattering. Photonic crystal fibres provide high nonlinearities and controllable dispersion which enables the generation of broadband supercontinuum sources based upon the interaction of many nonlinear effects. Each source will be described in depth, with particular attention given to the underlying physics that gives rise to the source. Previous and current limitations will be examined and an outlook of the future development of such sources will be discussed