Spontaneous Symmetry Breaking in a Coherently Driven Nanophotonic Bose-Hubbard Dimer.

We report on the first experimental observation of spontaneous mirror symmetry breaking (SSB) in coherently driven-dissipative coupled optical cavities. SSB is observed as the breaking of the spatial or mirror Z_{2} symmetry between two symmetrically pumped and evanescently coupled photonic crystal nanocavities, and manifests itself as random intensity localization in one of the two cavities. We show that, in a system featuring repulsive boson interactions (U>0), the observation of a pure pitchfork bifurcation requires negative photon hopping energies (J<0), which we have realized in our photonic crystal molecule. SSB is observed over a wide range of the two-dimensional parameter space of driving intensity and detuning, where we also find a region that exhibits bistable symmetric behavior. Our results pave the way for the experimental study of limit cycles and deterministic chaos arising from SSB, as well as the study of nonclassical photon correlations close to SSB transitions.

Method for pulse transformations using dispersion varying optical fibre tapers.

I introduce the problem of transforming one optical pulse into another via nonlinear propagation in a length of dispersion varying optical fibre. Then using a genetic algorithm to design the dispersion profiles, I show that the problem can be solved leading to high quality pulse transforms that are significantly better than what has been published previously. Finally I suggestion further work and other applications for this method.

Improved method for estimating the minimum length of modal filters fabricated for stellar interferometry.

We present an improved theoretical model to estimate the minimum fiber length required for achieving a desired degree of wavefront filtering in stellar interferometry. The proposed model is based on modal analysis of the fiber and is compared with numerical results obtained through the beam propagation method as well as with reported experimental observations. We also study the effect of introducing a spatial filter at the output end of the fiber and show that the required fiber length can be reduced significantly by introducing a circular aperture of optimum radius after the fiber.

Optical snakes and ladders: dispersion and nonlinearity in microcoil resonators.

Microcoil resonators are a radical new geometry for high Q resonators with unique linear features. In this paper I briefly summarise their linear properties before extending the analysis to nonlinear interactions in microcoil resonators. As expected such nonlinear resonators are bistable and exhibit hysteresis. Finally I discuss possible applications and extensions to such resonators.

The effect of periodicity on the defect modes of large mode area microstructured fibers.

It is well known that periodic variations in refractive index can be used to create guidance in an optical fiber via photonic bandgap effects. It has also been shown that periodic structure in index-guiding microstructured fibers can lead to the guidance of additional leaky higher-order modes due to bandgap effects. Here we demonstrate that this additional guidance mechanism can have important practical implications in large mode area silica microstructured fibers. We also demonstrate that similar modes can exist when a bandgap is not present and attribute this guidance to a low density of states. Excellent agreement between theoretical predictions and experimental observations is demonstrated. We explore the impact of these additional modes on the practical operation of these fibers and explore ways of minimizing their effects via the fiber geometry.

Control of surface modes in low loss hollow-core photonic bandgap fibers.

We report on the fabrication and characterization of hollow-core photonic bandgap fibers that do not suffer from surface mode coupling within the photonic bandgap of the cladding. This enables low attenuation over the full spectral width of the bandgap--we measured a minimum loss of 15 dB/km and less than 50 dB/km over 300 nm for a fiber operating at 1550 nm. As a result of the increased bandwidth, the fiber has reduced dispersion and dispersion slope--by a factor of almost 2 compared to previous fibers. These features are important for several applications in high-power ultrashort pulse compression and delivery. Realizing these advances has been possible due to development of a modified fabrication process which makes the production of low-loss hollow-core fibers both simpler and quicker than previously.

Design of 7 and 19 cells core air-guiding photonic crystal fibers for low-loss, wide bandwidth and dispersion controlled operation.

We study the modal properties of feasible hollow-core photonic bandgap fibers (HC-PBGFs) with cores formed by omitting either 7 or 19 central unit-cells. Firstly, we analyze fibers with thin core surrounds and demonstrate that even for large cores the proposed structures are optimum for broad-band transmission. We compare these optimized structures with fibers which incorporate antiresonant core surrounds which are known to have low-loss. Trade-offs between loss and useful bandwidth are presented. Finally, we study the effects that small modifications to the core surround have on the fiber's group velocity dispersion, showing the possibility of engineering the dispersion in hollow-core photonic bandgap fibers.

Optimizing the usable bandwidth and loss through core design in realistic hollow-core photonic bandgap fibers.

The operational bandwidth of hollow-core photonic bandgap fibers (PBGFs) is drastically affected by interactions between the fundamental core mode and surface modes guided at the core-cladding interface. By systematically studying realistic hollow-core PBGFs we identify a new design regime robust in eliminating the presence of surface modes. We present new fiber designs with a fundamental core mode free of anticrossings with surface modes at all wavelengths within the bandgap, allowing for a low-loss operational bandwidth of ~17% of the central gap wavelength.

The effect of core asymmetries on the polarization properties of hollow core photonic bandgap fibers.

We present the results of numerical simulations of the modal properties of Photonic Band Gap Fibers (PBGFs) in which a structural distortion of the silica ring surrounding the air core is gradually introduced. We demonstrate that surface modes supported within such fibers are very sensitive to structural distortions, and that any asymmetric change in the structure can break their degeneracy resulting in associated changes in the anticrossing behavior of the orthogonally polarized core modes, and the development of polarization dependent transmission properties. Our results provide insight into recent experimental observations of wavelength dependent PDL and birefringence in PBGFs.

Inverse design and fabrication tolerances of ultra-flattened dispersion holey fibers.

We employ a Genetic Algorithm for the dispersion optimization of a range of holey fibers (HF) with a small number of air holes but good confinement loss. We demonstrate that a dispersion of 0 +/- 0.1 ps/nm/km in the wavelength range between 1.5 and 1.6 microm is achievable for HFs with a range of different transversal structures, and discuss some of the trade-offs in terms of dispersion slope, nonlinearity and confinement loss. We then analyze the sensitivity of the total dispersion to small variations from the optimal value of specific structural parameters, and estimate the fabrication accuracy required for the reliable fabrication of such fibers.

Brillouin characterization of holey optical fibers.

We report the results of detailed measurements on the Brillouin frequency shift (BFS), gain bandwidth, and gain coefficients of several small-core holey optical fibers (HFs) of both uniform and axially varying structural characteristics and compare these with measurements on more conventional fibers. Our measurements show that the BFS of HFs is first-order proportional to the modal index for light propagating along the fiber and is thus extremely sensitive to its precise structural parameters. Our results highlight the possibility of using distributed Brillouin scattering measurements to perform nondestructive structural characterization of HFs, and the possibility of producing Brillouin-suppressed HFs using controlled structural variation along the fiber length.

Cascaded-chi(2)-interaction-based frequency-resolved optical gating in a periodically poled LiNbO3 waveguide.

We demonstrate a novel frequency-resolved optical gating (FROG) configuration based on cascaded second-order nonlinear interactions. Its implementation in a 2.6 cm long quasi-phase-matched LiNbO3 waveguide allowed high-quality retrieval of 2 ps to 80 fJ pulses at 1.56 microm.

Coupled-mode equations for periodic superstructure Bragg gratings.

Periodic superstructure Bragg gratings are gratings in which the parameters vary periodically. Although one can determine their properties by using the standard coupled-mode equations for the electric field envelopes, it is shown that the analysis is simplified if one considers the super envelopes, i.e., the envelopes of the usual envelope functions. These super envelopes satisfy a set of (super) coupled-mode equations, which are formally identical to the standard coupled-mode equations for a uniform grating. This shows that, within certain frequency ranges, any superstructure Bragg grating acts approximately as a uniform grating.

Nonlinear switching in a 20-cm-long fiber Bragg grating.

We report experimental observation of nonlinear all-optical switching of a 20-cm-long fiber Bragg grating. The grating is self-switched owing to the optical Kerr effect and in the nonlinear regime shows a 20-dB increase in transmissivity. This increase is a marked improvement from the switching results obtained from shorter gratings.

All-optical AND gate based on coupled gap-soliton formation in a fiber Bragg grating.

We experimentally demonstrate an all-optical AND gate based on coupled gap-soliton formation in an apodized fiber Bragg grating. A switching contrast of better than 17 dB is obtained with an incident pulse peak power of 2.5 kW.

Nonlinear self-switching and multiple gap-soliton formation in a fiber Bragg grating.

We report, for the first time to our knowledge, the experimental observation of quasi-cw nonlinear switching and multiple gap-soliton formation within the bandgap of a fiber Bragg grating. As many as five gap solitons with 100-500-ps durations were generated from a 2-ns pulse at a launched peak intensity of approximately 27 GW/cm(2). A corresponding increase in the grating transmission from 3% to 40% of the incident pulse energy was observed.

Experimental observation of nonlinear pulse compression in nonuniform Bragg gratings.

We demonstrate a scheme for optical pulse compression by cross-phase modulation that utilizes a nonuniform Bragg grating to work in reflection. Our scheme is similar to the conventional optical pushbroom, which works in transmission. This reflection geometry has the advantage of allowing the compressed signal to be observed easily, as it is spatially separate from the pump. This is to our knowledge the first nonlinear effect to be observed that requires a nonuniform grating.

Nonlinearity in holey optical fibers: measurement and future opportunities.

Holey fibers combine two-dimensional microstructuring with one-dimensional longitudinal propagation, resulting in fibers with tailorable dispersive and nonlinear properties. We measure the effective nonlinearity of a typical holey fiber. The small effective area that is possible in this type of fiber significantly enhances its effective nonlinearity relative to standard fiber.

Holey fibers with random cladding distributions.

We provide what is to our knowledge the first direct confirmation that light can be guided in a holey fiber with randomly distributed air holes in the cladding. We also show that many of the features previously attributed to periodic holey fibers, in particular, single-mode guidance at all wavelengths, can also be obtained with random holey fibers. We provide insight into exactly how sensitive a holey fiber's optical properties are to the details of the cladding profile.

Wavelength-tunable high-power picosecond pulses from a fiber-pumped diode-seeded high-gain parametric amplifier.

We describe a high-gain optical parametric amplifier (OPA) and optical parametric generator (OPG) system pumped by a fiber chirped-pulse amplification source based on novel large-mode-area fiber components. 45-nJ OPG thresholds and 34% pump-signal conversion efficiencies are obtained for ~4-ps pulses. OPA gains in excess of 75 dB and pump-signal conversion efficiencies of ~39% (external) are demonstrated. Furthermore, we show that injection seeding of such high-gain parametric devices by use of a low-power continuous-wave beam results in high-power (>18-kW) picosecond pulses with a time-bandwidth produce of ~0.65, some ten times less than from a free-running OPG. Using such an approach, we obtain 2.7-ps 50-nJ pulses at 1.310 mum .