Spectral properties of photon pairs generated by spontaneous four-wave mixing in inhomogeneous photonic crystal fibers

Abstract

The photonic crystal fiber (PCF) is one of the excellent media for generating photon pairs via spontaneous four-wave mixing. Here we study how the inhomogeneity of PCFs affect the spectral properties of photon pairs from both the theoretical and experimental aspects. The theoretical model shows that the photon pairs born in different places of the inhomogeneous PCF are coherently superposed, and a modulation in the broadened spectrum of phase-matching function will appear, which prevents the realization of spectral factorable photon pairs. In particular, the inhomogeneity induced modulation can be examined by measuring the spectrum of the individual signal or idler field when the asymmetric group-velocity matching condition is approximately fulfilled. Our experiments are performed by tailoring the spectrum of pulsed pump to satisfy the specified phase-matching condition. The observed spectra of individual signal photons, which are produced from different segments of the 1.9 m inhomogeneous PCF, agree with the theoretical predictions. The investigations are not only useful for fiber-based quantum state engineering, but also provide a dependable method to test the homogeneity of PCF.

Generation of frequency de-correlated photon pairs by using photonic crystal fiber

“…One sees that the measured results of g (2) are less than the theoretical predictions due to the inhomogeneity of PCF. 20 However, it is obvious that the measured g (2) increases with the decrease of pump pulse duration ∆T p , indicating the changing tendency observed in experiment agrees with the theoretical analysis. In order to verify the influence of the high order dispersion, we then measure g (2) of the signal photons by fixing the FWHM of incident pump at 6.9 nm and increasing the average pump power P a .…”

Minimizing the frequency correlation of photon pairs in photonic crystal fibers

“…One sees that the measured results of g (2) are less than the theoretical predictions. Considering that the Raman scattering at such a large detuning is negligible [16,32], we believe that the observed reduction of g (2) is caused by the inhomogeneity of PCF [25]. However, it is obvious that the measured g (2) increases with a decrease of pump pulse duration T p , indicating that the changing tendency observed in experiment agrees with the theoretical analysis.…”

“…Indeed, there is a simple relation between g (2) and the Schmidt number K : g (2) = 1 + 1/K [24]. However, for the determination of K we need to know detailed information about the JSA function, whose precise description is difficult to obtain because of various kinds of imperfections in realistic experiments, such as the quantity of pump chirp and the unexpected variation of dispersion along the fiber [25]. Therefore, measuring g (2) seems to be more practical for reliably testing the mutual spectral correlation of photon pairs.…”

“…We note that, for each set of experimental data, there is a small peak sitting next to the main peak of coincidence, which can be ten times greater than that of the theoretically predicted sinc oscillation. Since the coincidence patterns measured under the AGVM condition reflect the shape of the phase matching function [25], and the calculation is performed by assuming that the PCF has homogeneous dispersion, we believe that the observation of the theoretically unexpected small peak is because the dispersion of the PCF used in experiment is not homogeneous.…”

Characterisation of fabrication inhomogeneities in Ti:LiNbO3 waveguides

“…While waveguiding systems offer many advantages, these systems also generally exhibit higher losses and are much more sensitive to device imperfections, when compared to their bulk counterparts. It is known that small device imperfections can dramatically reduce device performance [9][10][11][12][13][14][15][16]. For this reason, it is critical to assess the limits of the current fabrication technology to identify the classes of nonlinear processes physically achievable and to devise strategies to overcome such limitations.…”