This article offers a comprehensive survey of results obtained for solitons and complex nonlinear wave patterns supported by nonlinear lattices (NLs), which represent a spatially periodic modulation of the local strength and sign of the nonlinearity, and their combinations with linear lattices. A majority of the results obtained, thus far, in this field and reviewed in this article are theoretical. Nevertheless, relevant experimental settings are also surveyed, with emphasis on perspectives for implementation of the theoretical predictions in the experiment. Physical systems discussed in the review belong to the realms of nonlinear optics (including artificial optical media, such as photonic crystals, and plasmonics) and Bose-Einstein condensation. The solitons are considered in one, two, and three dimensions. Basic properties of the solitons presented in the review are their existence, stability, and mobility. Although the field is still far from completion, general conclusions can be drawn. In particular, a novel fundamental property of one-dimensional solitons, which does not occur in the absence of NLs, is a finite threshold value of the soliton norm, necessary for their existence. In multidimensional settings, the stability of solitons supported by the spatial modulation of the nonlinearity is a truly challenging problem, for theoretical and experimental studies alike. In both the one-dimensional and two-dimensional cases, the mechanism that creates solitons in NLs in principle is different from its counterpart in linear lattices, as the solitons are created directly, rather than bifurcating from Bloch modes of linear lattices.

Solitons in nonlinear lattices

Photonic crystal fibres (PCFs) offer greatly enhanced design freedom compared to standard optical fibres. For example, they allow precise control of the chromatic dispersion (CD) profile—the frequency dependence of propagation speed—over a broad wavelength range. This permits studies of nonlinear pulse propagation in previously inaccessible parameter regimes. Here we report on spectral broadening of 100-fs pulses in PCFs with anomalously flat CD profiles. Maps of the spectral and spatio-temporal behaviour as a function of power show that dramatic conversion (to both longer and shorter wavelengths) can occur in remarkably short lengths of fibre, depending on the magnitude and shape of the CD profile. Because the PCFs used are single-mode at all wavelengths, the light always emerges in a fundamental guided mode. Excellent agreement is obtained between the experimental results and numerical solutions of the nonlinear wave equation, indicating that the underlying processes can be reliably modelled. These results show how, through appropriate choice of CD, nonlinearities can be efficiently harnessed to generate laser light at new wavelengths.

Transformation and control of ultra-short pulses in dispersion-engineered photonic crystal fibres

The development of highly-sensitive miniaturized sensors that allow real-time quantification of analytes is highly desirable in medical diagnostics, veterinary testing, food safety, and environmental monitoring. Photonic Crystal Fiber Surface Plasmon Resonance (PCF SPR) has emerged as a highly-sensitive portable sensing technology for testing chemical and biological analytes. PCF SPR sensing combines the advantages of PCF technology and plasmonics to accurately control the evanescent field and light propagation properties in single or multimode configurations. This review discusses fundamentals and fabrication of fiber optic technologies incorporating plasmonic coatings to rationally design, optimize and construct PCF SPR sensors as compared to conventional SPR sensing. PCF SPR sensors with selective metal coatings of fibers, silver nanowires, slotted patterns, and D-shaped structures for internal and external microfluidic flows are reviewed. This review also includes potential applications of PCF SPR sensors, identifies perceived limitations, challenges to scaling up, and provides future directions for their commercial realization.

/pdf/photonic-crystal-fiber-based-plasmonic-sensors-wyb2yrls3t.pdf

Photonic crystal fiber based plasmonic sensors

Wavelength-scale periodic microstructuring dramatically alters the optical properties of materials. An example is glass photonic crystal fibre1 (PCF), which guides light by means of a lattice of hollow micro/nanochannels running axially along its length. In this letter, we explore stimulated Brillouin scattering in PCFs with subwavelength-scale solid silica glass cores. The large refractive-index difference between air and glass allows much tighter confinement of light than is possible in all-solid single-mode glass optical fibres made using conventional techniques. When the silica-air PCF has a core diameter of around 70% of the vacuum wavelength of the launched laser light, we find that the spontaneous Brillouin signal develops a highly unusual multi-peaked spectrum with Stokes frequency shifts in the 10-GHz range. We attribute these peaks to several families of guided acoustic modes each with different proportions of longitudinal and shear strain, strongly localized to the core2,3. At the same time, the threshold power for stimulated Brillouin scattering4 increases fivefold. The results show that Brillouin scattering is strongly affected by nanoscale microstructuring, opening new opportunities for controlling light—sound interactions in optical fibres.

/pdf/stimulated-brillouin-scattering-from-multi-ghz-guided-xs8qlp534l.pdf

Stimulated Brillouin scattering from multi-GHz-guided acoustic phonons in nanostructured photonic crystal fibres

We investigate strong light confinement in high core-cladding index contrast waveguides with dimensions comparable to and smaller than the wavelength of incident light. We consider oval and rectangular cross sections and demonstrate that an optimal core size exists that maximizes the effective nonlinearity. We also determine that waveguides with asymmetrical cross sections provide the maximum possible nonlinearity, although only a small improvement over the symmetric case. Furthermore, we find that for a specified waveguide shape the largest nonlinearity occurs for nearly the same core area in all cases. Calculations of the dispersion for the optimal-size waveguide at a particular wavelength indicate that the group-velocity dispersion is normal. Ultimately, such designs could be used to develop low-power all-optical devices and to produce waveguides for ultra-low threshold nonlinear frequency generation such as supercontinuum generation.

Optimal waveguide dimensions for nonlinear interactions.

Photonic crystal fibers having a complex microstructure in the transverse plane constitute a new and promising class of optical fibers. Such fibers can either guide light through total internal reflection or the photonic bandgap effect, In this paper, we review the different types and applications of photonic crystal fibers with particular emphasis on recent advances in the field.

http://ieeexplore.ieee.org/iel5/8799/27883/01244868.pdf

Photonic crystal fibers

The use of higher-order radially symmetric (Bessel) modes in large-core optical fibers to provide intramodal phase matching for four-wave mixing in parametric oscillators pumped at 1030 nm is investigated numerically. It is shown that a suitable choice of core diameter in a 4 cm fiber piece can lead to broad tunability of signal/idler bands in the range of 1120–1500 nm (signal) and 780–950 nm (idler), only by adjusting the recirculation timing, while keeping the pump wavelength fixed. Power scaling to the MW level appears feasible, ultimately limited by nonlinear intermodal coupling. The oscillator performance is insensitive to the exact method of fiber recirculation and back-coupling, but highly sensitive to the purity of pump mode excitation. 

J. Laegsgaard

Papers

Photonic crystal fibers

Power scaling of fiber-optic parametric oscillators using higher-order modes