Py-fmas: A python package for ultrashort optical pulse propagation in terms of forward models for the analytic signal

Abstract

We present a flexible, open-source Python package for the accurate simulation of the z-propagation dynamics of ultrashort optical pulses in nonlinear waveguides, especially valid for few-cycle pulses and their interaction. The simulation approach is based on unidirectional propagation equations for the analytic signal. The provided software allows to account for dispersion, attenuation, four-wave mixing processes including, e.g., third-harmonic generation, and features various models for the Raman response. The propagation equations are solved on a periodic temporal domain. For z-propagation, a selection of pseudospectral methods is available. Propagation scenarios for a custom propagation constant and initial field pulses can either be specified in terms of a HDF5 based input file format or by direct implementation using a python script. We demonstrate the functionality for a test-case for which an exact solution is available, by reproducing exemplary results documented in the scientific literature, and a complex propagation scenario involving multiple pulses. Program summaryProgram Title: py-fmasCPC Library link to program files:https://doi.org/10.17632/7s2cv9kjfs.1Developer's repository link:https://github.com/omelchert/py-fmasCode Ocean capsule:https://codeocean.com/capsule/8221780Licensing provisions: MITProgramming language: Python3Supplementary material: Reference manual, extended user guide, and usage examples are hosted on gitHub pages under https://omelchert.github.io/py-fmas.Nature of problem: Solves for the z-propagation dynamics of spectrally broad ultrashort optical pulses in single mode nonlinear waveguides in terms of propagation models for the analytic signal of the optical field [1–3]. The implemented models include, e.g., third-harmonic generation and the Raman effect.Solution method: The initial real-valued optical field is defined on a periodic one-dimensional temporal grid and converted to the complex-valued analytic signal. z-stepping is performed via spectral methods. The software implements a selection of algorithms with fixed or adaptive stepsize, commonly used in nonlinear optics for solving nonlinear Schrödinger type equations.Additional comments including restrictions and unusual features: The range of applicability of the provided software is equivalent to that of the forward Maxwell equation [4]. For reasonably chosen initial conditions, it can be used beyond the unidirectional approximation as a bidirectional model for a complex field, allowing to describe forward and backward waves coupled through nonlinear interactions [1]. The software implements various models for the Raman response and allows to calculate spectrograms, detailing the time-frequency composition of the analytic signal. Additionally, a convenience class for analyzing propagation constants is provided.