We review our recent work on the generation of novel optical solitons arising from different, high orders of dispersion and combinations thereof. By incorporating a spectral pulse-shaper in a mode-locked laser cavity, we can tailor the net-cavity dispersion, allowing us to access a wide range of new operating regimes corresponding to previously unobserved soliton pulses. We demonstrate the generation of solitons arising between self-phase modulation and any pure, negative even order of dispersion, as well as soliton molecules consisting of multiple solitons with different frequencies but that are temporally coincident.
The nonlinear coefficient γ is a key parameter for studying nonlinear pulse propagation, as it relates the power in a waveguide mode to the nonlinear phase shift per unit length. It is well understood in dielectric waveguides, but less so in lossy plasmonic waveguides. A number of methods for calculating γ have been proposed, each producing different expressions. Here, we comprehensively compare these methods for a surface plasmon polariton propagating at a gold-air interface, obtaining new insights into the nonlinear response of lossy waveguides.
We report a fibre laser incorporating an intracavity pulse-shaper that induces a large anomalous net cavity quartic dispersion in which the mode-locked pulses propagate as pure-quartic solitons. We characterize the laser output pulses using a set of spectral and temporal phase resolved measurements and resonant dispersive wave analysis, and find that the results are in excellent agreement with analytic predictions.
Butt-coupling of light into a surface plasmon is a simple and compact coupling method with a range of potential uses in photonic circuitry. Although butt-coupling has been successfully implemented in many coupling configurations, the coupling effectiveness is not fully understood. Here, we present a semi-analytical study which models the coupling efficiency of an incident beam into a surface plasmon on silver in the presence of loss using an projection method in one dimension. We find that the coupling efficiencies for silver between the wavelengths of 0:38 - 1:6 μm reach 77 - 88% with optimum incident beam parameters.
Many nonlinear systems exhibiting wave propagation, support solitons, nonlinear excitations that propagate unchanged,
due to a balance of nonlinearity and dispersion. Of particular interest, both as a subject within photonics, as well as a
topic of basic research, is their interaction with periodic structures, such as photonic crystals or gratings. Optical fibers
and fiber gratings are rich experimental environments for nonlinear physics. The propagation of light in such a fiber is
described approximately by the nonlinear Schrödinger equation. Here we demonstrate, both in experiment and
simulation, that the process of soliton excitation, which is inherently discrete, profoundly changes the high power
transmission properties of pulses through a Fiber Bragg grating for frequencies close to the band-edge. The quantization
manifests itself in a characteristic staircase shape of the transmission spectrum at high powers. This behaviour is
analyzed by a systematic study of the temporally resolved transmission spectra, which allows us to identify gap solitons
as causing the transmission quantization. They act as discrete, self-induced transmission channels, because only solitons
are able to propagate through the otherwise "forbidden" band-gap.
We study numerically nonlinear pulse propagation in a phase-shifted Bragg grating with a π phase-shift. The phase-shift
acts as a cavity, accumulating the field inside the grating, and hence improving the switching efficiency. Due to material
nonlinearity such cavity can operate in a bistable regime, enabling all-optical switching between high and low
transmission states. We give optimization criteria for grating design that reduce the switching threshold and minimize
the response time of the device. We demonstrate that if the grating and the pulse parameters are chosen carefully, a
temporal reshaping of the transmitted pulse occurs. An asymmetric shape of the output pulse is an indication of the pulse
self-switching between the two states of a bistable Bragg cavity.
We demonstrate a method of local spectral enhancement of an ultrafast soliton pulse. We use an in-line acoustic long-period
grating (LPG), a periodic structure modifying both the phase and the loss of the propagating light, and which is
readily tuned by simple adjustment of an applied electrical signal. The soliton perturbed by this narrow-band filter
evolves with nonlinear propagation into an intense localised spectral peak. Our setup consists of creation of a red-shifted
optical soliton by propagation of pulses from a fibre laser in standard single-mode optical fibre, followed by imposition
of a spectrally narrow LPG near to the soliton peak, and then continuing propagation. The wavelength and the peak value
of the resulting local enhancement can be tuned by adjustment of the applied acoustic frequency and amplitude. The physics of the observed local spectral enhancement will be discussed in detail here.
We consider coupling between free-space and rod-type photonic crystals using semi-analytic 2D methods, and find that for frequencies in the second and third bands this coupling is almost perfect over a large range of angles. We explain this remarkable property in terms of the scattering resonances of the individual inclusions and then confirm the presence of this effect in fully 3D FDTD calculations.
Many of the applications of photonic crystals and photonic crystal fibers require the periodic structure to have some type of defect. In photonic crystal fibers a point defect defines the fiber core, whereas in photonic crystals a line defect acts as a waveguide, and point defects act as cavities. The modeling of these defects usually either makes use of periodic boundary conditions, by which the defect is replicated periodically, or models a photonic crystal of finite extent. However, some applications, for example the cut-off behavior of a defect mode where the field extends very widely, require methods that can model a defect in an otherwise infinite and perfectly periodic structure. Here we present such a method. It combines the method of fictitious sources with averaging over the Brillouin zone, and we apply it to study the long-wavelength behavior of the fundamental mode of photonic crystal fibers.
In solid core Microstructured Optical Fibers (MOFs), guidance of light is due to a finite number of layers of holes surrounding a solid core. Because the potential barrier is finite, all modes are leaky, blurring the distinction between guided and non-guided modes. Through simulations using a multipole formulation, we clarify the definition of modal cutoff in MOFs. We establish that the fundamental mode of MOFs undergoes a transition between modal confinement and non-confinement similar to modal cutoff. An asymptotic analysis gives us a better understanding of mode properties on each side of the cutoff but also near cutoff and leads us to define a cutoff point and a cutoff region for the fundamental mode. Three operation regimes with very different mode properties can be distinguished. Only two of these are of practical interest, one with strong mode confinement and another with broader field distributions. The former is of interest for single-mode guidance with strong confinement, whereas the latter, the cutoff region, is where highly adjustable chromatic dispersion can be achieved. We provide a map of the parameter space (MOF "phase diagram") summarizing the operating regimes of MOFs, and show for a few examples how this map can be used for deterministic MOF design.
We describe a multipole theory of photonic crystal or more generally microstructured optical fibers (MOF). We review basic MOF properties such-as losses and number of modes-obtained with our method and expose considerations and results on dispersion management taking into account the losses.
Microstructured optical fibres (MOFs) have aroused great interest in recent years because of their unusual optical properties. These include their ability to be effectively single moded over a very large range of wavelengths, tailorisable dispersion, high or low non-linearity(depending on the hole design) and large core single mode fibres. We have recently fabricated the first Microstructured Polymer Optical Fibres (MPOFs), which further extend the range of possibilities in MOFs. The properties of polymers can be tailored to specific applications (eg:made highly non-linear or having gain) in a way that is not possible in glass. Further, the large range of fabrication methods available in polymers, including casting and extrusion, mean that the structures that can be obtained are very difficult to make by capillary stacking- the method used in glass MOFs. Here we present the latest results from our group using MPOFs, including single mode fibre and Bragg fibres.
We report on the fabrication f comb filters which operate on the principle of linear polarization rotation through the twisting of the birefringent axes of photosensitive elliptical-core fibers by UV radiation. The polarization coupling spectrum of these filters consist of a series of narrow resonances which are roughly equally spaced in frequency and which extend over the entire single-mode frequency range of the fiber. Some applications of these structures are also discussed.
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