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.
We present an experimental and theoretical study of photon pairs generated by spontaneous four-wave mixing (SFWM), based on birefringent phasematching, in a fiber that supports more than one transverse mode. We present SFWM spectra, obtained through single-channel and coincidence photon counting, which exhibit multiple peaks shown here to be the result of multiple SFWM processes associated with different combinations of transverse modes for the pump, signal, and idler waves.
We have designed and implemented a photon-pair source, based on the spontaneous four wave mixing (SFWM) process in a few-mode fiber, in a geometry which permits multiple, simultaneous SFWM processes, each associated with a distinct combination of transverse modes for the four participating waves. In our source: i) each process is group-velocity-matched so that it is, by design, nearly-factorable, and ii) the spectral separation between neighboring processes is greater than the marginal spectral width of each process. Consequently, there is a direct correspondence between the joint amplitude of each process and each of the Schmidt mode pairs of the overall two-photon state. Our approach permits hybrid entanglement in discrete frequency and in transverse mode, whereby control of the number of supported fiber transverse modes allows scalability to higher dimensions while spectral filtering may be used for straightforward Schmidt mode discrimination.
Optical fibers have been considered an optimal platform for third-order parametric downconversion since they can potentially overcome the weak third-order nonlinearity by their long interaction length. Here we present, in the first part, a theoretical derivation for the conversion rate both in the case of spontaneous generation and in the presence of a seed beam. Then we review three types of optical fibers and we examine their properties in terms of conversion efficiency and practical feasibility.
We explore three different mechanisms designed to controllably tune the joint spectrum of photon pairs produced by the spontaneous four-wave mixing (SFWM) process in optical fibres. The first of these is fibre tapering, which exploits the modified optical dispersion resulting from reducing the core radius. We have presented a theory of SFWM for tapered fibres, as well as experimental results for the SFWM coincidence spectra as a function of the reduction in core radius due to tapering. The other two techniques that we have explored are temperature variation and application of longitudinal stress. While the maximum spectral shift observed with these two techniques is smaller than for fibre tapering, they are considerably simpler to implement and have the important advantage that they are based on the use of a single, suitably controlled, fibre specimen.
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