Designing microstructured polymer optical fibers for cascaded quadratic soliton compression of femtosecond pulses

TL;DR

This paper explores the design of microstructured polymer optical fibers for efficient cascaded quadratic soliton compression of femtosecond pulses, demonstrating effective compression with standard fiber designs.

Contribution

It investigates fiber design parameters for cascaded quadratic nonlinearities, showing that standard designs can achieve efficient soliton compression without minimizing group-velocity mismatch.

Findings

Effective quadratic nonlinearity of 5 pm/V or more is needed for soliton formation.

Large relative hole size and pitch near the pump wavelength optimize group-velocity matching.

Standard fiber designs can produce excellent pulse compression without specialized group-velocity mismatch minimization.

Abstract

The dispersion of index-guiding microstructured polymer optical fibers is calculated for second-harmonic generation. The quadratic nonlinearity is assumed to come from poling of the polymer, which in this study is chosen to be the cyclic olefin copolymer Topas. We found a very large phase mismatch between the pump and the second-harmonic waves. Therefore the potential for cascaded quadratic second-harmonic generation is investigated in particular for soliton compression of fs pulses. We found that excitation of temporal solitons from cascaded quadratic nonlinearities requires an effective quadratic nonlinearity of 5 pm/V or more. This might be reduced if a polymer with a low Kerr nonlinear refractive index is used. We also found that the group-velocity mismatch could be minimized if the design parameters of the microstructured fiber are chosen so the relative hole size is large and the hole pitch is on the order of the pump wavelength. Almost all design-parameter combinations resulted in cascaded effects in the stationary regime, where efficient and clean soliton compression can be found. We therefore did not see any benefit from choosing a fiber design where the group-velocity mismatch was minimized. Instead numerical simulations showed excellent compression of $\lambda=800$ nm 120 fs pulses with nJ pulse energy to few-cycle duration using a standard endlessly single-mode design with a relative hole size of 0.4.