Quantum mode structure of pulsed Kerr squeezing in media with normal, near-zero and anomalous group velocity dispersion

The importance of developing miniature devices that generate non-classical light, such as squeezed states of light, has stimulated research into various materials and schemes for creating such states. We recently reported the generation of squeezed light via Kerr effect in a very short piece of highly nonlinear chalcogenide fiber using a train of femtosecond pulses at 1.56 μm [1]. Despite the large normal dispersion leading to temporal pulse broadening, a significant squeezing of polarization fluctuations was obtained at -3 dB below the shot noise limit. Following this result, we tested a microstructured tellurite fiber using the same experimental setup. The fiber had similar nonlinearity to the chalcogenide fiber, but very low anomalous dispersion at the pump wavelength, providing low pulse distortion. However, very limited squeezing was observed (about -1 dB). Numerical simulations also showed that the squeezing in such a fiber is indeed worse than the squeezing obtained in fibers with larger normal or anomalous dispersion and similar nonlinearity. To explain these results, we numerically investigated the quantum mode structure of the squeezed light. A numerical model and a numerical procedure for reconstructing the quantum Schmidt modes of the output signal were developed according to the approach presented in [2]. The simulations showed that in fibers with significant normal or anomalous dispersion, only a small number of modes are noticeably squeezed, and these modes carry most of the light. In contrast, in the low dispersion regime, there are many populated squeezed modes. Although some modes may be strongly squeezed, the overall squeezing measured for the entire pulse may be noticeably worse. This explains the observed results and highlights the importance of material dispersion for Kerr squeezing. Moreover, a massive set of independently squeezed modes may be a useful quantum resource in itself.