Intermodal Four-Wave-Mixing and Parametric Amplification in km-long Fibers

We theoretically and numerically investigate intermodal four-wave-mixing in km-long fibers, where random birefringence fluctuations are present along the fiber length. We identify several distinct regimes that depend on the relative magnitude between the length scale of the random fluctuations and the beat-lengths of the interacting quasi-degenerate modes. In addition, we analyze the impact of polarization mode-dispersion and we demonstrate that random variations of the core radius, which are typically encountered during the drawing stage of the fiber, can represent the major source of bandwidth impairment. These results set a boundary on the limits of validity of the classical Manakov model and may be useful for the design of multimode parametric amplifiers and wavelength converters, as well as for the analysis of nonlinear impairments in long-haul spatial division multiplexed transmission

Nonlinear Dynamics in Multimode Optical Fibers

Multimode optical fibers have recently reemerged as a viable platform for addressing a number of long-standing issues associated with information bandwidth requirements and power-handling capabilities. The complex nature of heavily multimoded systems can be effectively exploited to observe altogether novel physical effects arising from spatiotemporal and intermodal linear and nonlinear processes. Here, we have studied nonlinear dynamics in multimode optical fibers (MMFs) in both the normal and anomalous dispersion regimes. In the anomalous dispersion regime, the nonlinearity leads to a formation of spatiotemporal 3-D solitons. Unlike in single-mode fibers, these solitons are not unique and their properties can be modified through the additional degrees of freedom offered by these multimoded settings. In addition, soliton related processes such as soliton fission and dispersive wave generation will be also drastically altered in such multimode systems. Our theoretical work unravels some of the complexities of the underlying dynamics and helps us better understand these effects. The nonlinear dynamics in such multimode systems can be accelerated through a judicious fiber design. A cancelation of Raman self-frequency shifts and Blue-shifting multimode solitons were observed in such settings as a result of an acceleration of intermodal oscillations. Spatiotemporal instabilities in parabolic-index multimode fibers will also be discussed. In the normal dispersion regime, this effect can be exploited to generate an ultrabroad and uniform supercontinuum that extends more than 2.5 octaves. To do so, the unstable spectral regions are pushed away from the pump, thus sweeping the entire spectrum. Multimode parabolic pulses were also predicted and observed in passive normally dispersive tapered MMFs. These setting can obviate the harsh bandwidth limitation present in single-mode system imposed by gain medium and be effectively used for realizing high power multimode fiber lasers. Finally, an instant and efficient second-harmonic generation was observed in the multimode optical fibers. Through a modification of initial conditions, the efficiency of this process could be enhanced to a record high of %6.5

An accurate envelope equation for light propagation in photonic nanowires: new nonlinear effects

We derive a new unidirectional evolution equation for photonic nanowires made of silica. Contrary to previous approaches, our formulation simultaneously takes into account both the vector nature of the electromagnetic field and the full variations of the effective modal profiles with wavelength. This leads to the discovery of new, previously unexplored nonlinear effects which have the potential to affect soliton propagation considerably. We specialize our theoretical considerations to the case of perfectly circular silica strands in air, and we support our analysis with detailed numerical simulations.Comment: 5 figures. The normalization of the fields is now more appropriate (orthonormal). Figure concerning dispersion of gamma0 has been eliminated. New figures for nonlinear coefficients and pulse propagation for the corrected envelope functio

Effects of Raman scattering and attenuation in silica fiber-based parametric frequency conversion

Four-wave mixing in the form of Bragg scattering (BS) has been predicted to enable quantum noise less frequency conversion by analytic quantum approaches. Using a semi-classical description of quantum noise that accounts for loss and stimulated and spontaneous Raman scattering, which are not currently described in existing quantum approaches, we quantify the impacts of these effects on the conversion efficiency and on the quantum noise properties of BS in terms of an induced noise figure (NF). We give an approximate closed-form expression for the BS conversion efficiency that includes loss and stimulated Raman scattering, and we derive explicit expressions for the Raman-induced NF from the semi-classical approach used here.Comment: 14 single col pages, 11 figure

Study and Control of Nonlinearity in Large-Mode-Area Fibers.

Practical advantages and high power of fiber lasers make them important in many scientific and industrial applications. However, relatively small mode-area and long propagation-length in an optical fiber also enhances the nonlinear interactions, posing certain limits on achievable average and peak powers in fiber lasers. In this dissertation, we explore such nonlinear effects and their control in CCC fibers, a practically important type of large-core effectively-single-mode fibers. Many applications require short wavelengths. We study use of four-wave-mixing (FWM) for wavelength conversion in CCC fibers. Our theoretical analysis shows that under proper conditions CCC fibers can be used for efficient and high-power wavelength conversion from ~1µm to yellow-red visible wavelengths. We study use of spectral filtering properties of CCC fibers for suppressing stimulated Raman scattering (SRS). SRS suppression has been experimentally achieved in two types of spectrally-tailored CCC fibers, demonstrating an additional degree of design freedom, combining core-size scalability and SRS suppression. Average powers in large-core amplifying fibers are limited by the thermally induced transverse mode instability (TMI). We show that TMI is essentially a two-beam coupling process, causing stimulated scattering from the fundamental to higher-order modes. We show that increasing higher-order mode suppression in CCC fibers increases TMI threshold power. CCC fibers are low-birefringence fibers, in which fiber coiling and twisting produces externally induced linear and circular birefringence. Presence of the later complicates nonlinear polarization evolution (NPE) at high peak powers, which can degrade polarization preservation at the amplifier or laser output. Our experimental and theoretical analysis shows that with proper signal excitation and fiber packaging conditions linear output polarization can be maintained under a wide range of output peak powers. Additionally, this dissertation also includes a study of some design aspects of large-core polygonal-CCC fibers, directly related to fiber modal properties used in controlling nonlinear interactions. Results of this work are important for using CCC, as well as other types of flexible (i.e. non-rod type) effectively-single-mode fibers, in high power and energy fiber lasers.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116668/1/andrehu_1.pd