We present a detailed analysis of the modal properties, dispersive behavior, and nonlinear mode coupling in graded-index multimode fibers (GIMFs), and lay out a simplified form of a generalized nonlinear Schrodinger equation, which can be used to explore the rich nonlinear dynamics related to the propagation and interaction of light pulses in GIMFs in a tractable manner. We also briefly discuss an application of the presented formalism in the study of four-wave mixing in these fibers. While the reported formalism is fairly general, our presentation is mainly targeted at device applications in which short segments of GIMFs are used.

Pulse Propagation in a Short Nonlinear Graded-Index Multimode Optical Fiber

High sensitivity evanescent-field gas sensor based on modified photonic crystal fiber for gas condensate and air pollution monitoring

Amongst the most important and frequently used fibre devices, fibre Bragg and long-period gratings are conventionally fabricated by low-intensity (I < 107 W cm?2) UV quanta with an energy of about 5 eV, which coincides with the maximum of the absorption band of defects in germanosilicate glass (the usual material of a fibre core). Such a single-quantum photochemical technique produces refractive index changes in the fibre core and not in the fibre cladding. The use of single-quantum excitation with high-energy vacuum UV photons with 157 nm wavelength or two-quantum 193 nm excitation through the real intermediate state results in a higher excitation energy (7.9 and 12.8 eV, respectively) and significantly increases the efficiency of grating inscription. However, neither of these high-energy approaches is free of disadvantages: the 157 nm radiation is strongly absorbed by practically all optical materials and even by air; the application of the second approach is based on the existence of an intermediate state, i.e. presence of absorption at the irradiation wavelength. The new multi-photon high-excitation-energy approach to fibre grating fabrication is based on refractive index change modification by high-intensity (I ~ 1011?1013 W cm?2) femtosecond UV, near-UV or IR laser radiation applied to fibre, which acquires a total excitation energy of about 8?12 eV via two-, three- or even five-photon (through the intermediate virtual state/states) absorption processes. Such a high value of excitation energy exceeds the band-gap energy values for both the fibre core and the cladding, which could result in asymmetric light energy deposition inside the fibre and even inside the fibre core. We will consider the advantages of this novel technique such as grating fabrication in fibres of any content, including photonic crystal ones; the writing of extremely stable gratings with erasing temperatures above 1000 ?C; the point-by-point inscription of Bragg gratings, including non-uniform 'chirped' ones; the creation of fibre gratings with high polarization properties; etc.

http://iopscience.iop.org/article/10.1088/0957-0233/18/1/R01/pdf

Multi-photon high-excitation-energy approach to fibre grating inscription

Binary SiO2/TiO2 and SiO2/Fe2O3 nanoparticle (diameter < 100 nm) aerosols of varying mole ratios of Ti or Fe to Si were generated in a premixed Bunsen-type aerosol flame reactor. The distribution of species within the particles was investigated using transmission electron microscopy, electron energy loss spectrometry, x-ray diffraction, and Fourier transform infrared spectroscopy. Phase segregation was observed to varying degrees in qualitative agreement with segregation expected from binary phase diagrams for the bulk systems. Differences between the SiO2/TiO2 and SiO2/Fe2O3 systems can be explained by considering the variation in the thermodynamically stable liquid-phase solubility and differences in the ability of iron and titanium ions to substitute for silicon ions in the network structure.

T. Izawa

Papers

Optical Fibers: Materials and Fabrication