Aku Antikainen

Dual-pump frequency comb generation in normally dispersive optical fibers

We show that tunable frequency combs can be generated by launching two continuous-wave pumps at slightly different wavelengths into a normally dispersive optical fiber. The dual-pump configuration allows the tuning of comb spacing into the terahertz regime. Different pump powers and frequency separations are explored numerically to determine the effect of the input parameters on frequency comb generation. The relative power of the two pumps is found to be a critical factor through its crucial effect on optical wave breaking. The pump powers and fiber length are shown to have a significant effect on the comb width as well. Unlike in the case of supercontinuum generation, Raman scattering is found to have a negligible or even a slightly detrimental effect. Our findings also help bridge the gap between work done on the propagation of a single pulse and the evolution of dual-pump signals in normally dispersive highly nonlinear fibers.

Dynamics of Soliton Cascades in Fiber Amplifiers

We study numerically the formation of cascading solitons when femtosecond optical pulses are launched into a fiber amplifier with less energy than required to form a soliton of equal duration. As the pulse is amplified, cascaded fundamental solitons are created at different distances, without soliton fission, as each fundamental soliton moves outside the gain bandwidth through the Raman-induced spectral shifts. As a result, each input pulse creates multiple, temporally separated, ultrashort pulses of different wavelengths at the amplifier output. The number of pulses depends not only on the total gain of the amplifier but also on the width of the input pulse.

Determination of modes of elliptical waveguides with ellipse transformation perturbation theory

All optical fibers, including single-mode, multimode, and multicore fibers, exhibit some degree of birefringence, either purposely, such as in polarization-maintaining fibers, or inadvertently due to material or fabrication imperfections. Finding a low-complexity method to accurately calculate the modal characteristics of elliptical fibers has been a long-standing problem. We present a novel accurate perturbative method that avoids the difficulties associated with the traditional Mathieu function treatment. The method is also applicable to a broader class of oscillating systems with elliptical geometry.