Photon pair generation by intermodal spontaneous four wave mixing in birefringent, weakly guiding optical fibers

TL;DR

This paper investigates photon pair generation via spontaneous four wave mixing in birefringent, few-mode fibers, combining theoretical analysis and experimental measurements to identify processes and characterize the fiber's modal properties.

Contribution

It provides a comprehensive study of SFWM in birefringent fibers, including mode conservation rules and a method to identify specific processes and fiber characteristics using a genetic algorithm.

Findings

Multiple SFWM processes are possible in birefringent fibers.

Conservation of OAM and parity governs viable SFWM processes.

Experimental spectra match theoretical predictions for specific modes.

Abstract

We present a theoretical and experimental study of the generation of photon pairs through the process of spontaneous four wave mixing (SFWM) in a few-mode, birefringent fiber. Under these conditions, multiple SFWM processes are in fact possible, each associated with a different combination of transverse modes for the four waves involved. We show that in the weakly guiding regime, for which the propagation modes may be well approximated by linearly polarized modes, the departure from circular symmetry due to the fiber birefringence translates into conservation rules which retain elements from azimuthal and rectangular symmetries: both OAM and parity must be conserved for a process to be viable. We have implemented a SFWM source based on a "bow-tie" birefringent fiber, and have measured for a collection of pump wavelengths the SFWM spectra of each of the signal and idler photons in coincidence with its partner photon. We have used this information, together with knowledge of the transverse modes into which the signal and idler photons are emitted, as input for a genetic algorithm which accomplishes two tasks: i) the identification of the particular SFWM processes which are present in the source, and ii) the characterization of the fiber used.