Tunable twin beams generated in hollow-core photonic crystal fibers

Photon-number correlated twin beams can be efficiently generated either by parametric down-conversion in a nonlinear crystal or by four-wave mixing in an optical fiber. Here, we investigate a novel system which uses noble-gas-filled hollow-core kagomé-style photonic crystal fibers (PCFs) for the generation of femtosecond pulsed, quantum-correlated twin beams in the modulational instability (MI) regime. This system eliminates one detrimental source of uncorrelated photons compared to solid-core fiber-based systems, which is Raman scattering. Moreover, it increases the flexibility of fiber twin-beam sources by providing tunable signal and idler wavelengths and an adjustable spectral mode structure in a single spatial mode. We study first the chromatic dispersion of kagomé PCFs. For this, we explore experimentally and numerically the validity of the commonly used capillary approximation for the dispersion of these hollow-core PCFs. We introduce an heuristic extension of this model in order to improve the dispersion approximation in the wavelength range above 1 μm. The kagomé PCFs are subsequently filled with high-pressure argon gas and used for the generation of twin beams in the MI regime. The pressure-tunable dispersion landscape makes it possible to tune the signal and idler spectrum over ~80 THz per sideband and the source manifests nonclassical photon-number correlations of 35% below the shot-noise level. The broadband (~50 THz) signal and idler beams show quantum correlations up to a record brightness of 2500 photons/mode containing only a single spatial mode as well as 4.3 effective frequency modes. Further, we explore the spectral photon-number correlations. We show that it is possible to retrieve the shapes and weights of time-frequency Schmidt modes of bright twin beams from a spectral covariance measurement. Additionally, we demonstrate two new approaches to shape the joint spectral correlations by tuning the pump-pulse chirp and the filling-gas pressure of the fiber, which enable us to reduce the effective number of modes from 4.3 to 1.3. We use these handles to also observe the influence of modified spectral cross-correlations on fluctuations in MI. Last, we find experimentally that the number of time-frequency Schmidt modes is not a constant but depends on the parametric gain in the system