Using second-order coherence theory of nonstationary light we show that supercontinuum can be split into quasi-coherent and quasi-stationary parts. Our results opens the route for a universal definition of the coherence degree for broadband sources.
We measure experimentally the second-order coherence properties of supercontinuum generated in a photonic crystal fiber. Our approach is based on measuring separately the quasicoherent and quasistationary contributions to the cross-spectral density and mutual coherence functions using a combination of interferometric and nonlinear gating techniques. This allows us to introduce two-dimensional coherence spectrograms which provide a direct characterization and convenient visualization of the spectrotemporal coherence properties. The measured second-order coherence functions are in very good agreement with numerical simulations based on the generalized nonlinear Schrödinger equation. Our results pave the way towards the full experimental characterization of supercontinuum coherence properties. More generally, they provide a generic approach for the complete experimental measurement of the coherence of broadband sources.
We examine the temporal coherence properties of trains of nonidentical short optical pulses in the framework of the second-order coherence theory of nonstationary light. Considering Michelson's interferometric measurement of temporal coherence, we demonstrate that time-resolved interferograms reveal the full two-time temporal coherence function of the partially coherent pulse train. We also show that the result given by the time-integrated Michelson interferogram equals the true degree of temporal coherence only when the pulse train is quasi-stationary, i.e., the coherence time is a small fraction of the pulse duration. True two-time and integrated coherence functions produced by specific models representing perturbed trains of mode-locked pulses and supercontinuum pulse trains produced in nonlinear fibers are illustrated.
The work presented in this thesis deals with the coherence of optical pulse trains in time, frequency, and space domains.This thesis, based on the second-order coherence theory of nonstationary light, primarily consists of theoretical investigations which result in useful insights in the coherence characterization of pulsed optical sources.Analytical models that physically represent supercontinuum pulse trains, broadband axicon fields, and slightly disturbed mode-locked pulse trains are proposed.Numerically simulated realizations of supercontinuum pulse trains are also considered in this study.In addition, an experimental setup with corresponding analytical analysis is presented to illustrate the evaluation of the two-time coherence function of pulse trains and it is shown that for that one needs to perform time-resolved measurements of the interference fringes.Moreover, information conveyed by the timeintegrated measurements of the interference fringes produced by supercontinuum light and slightly perturbed mode-locked laser is explained through analytical and numerical investigations.An experimental demonstration of the time-integrated measurements is reported for an almost fully coherent femtosecond pulse train.
A supercontinuum (SC) laser pulse, which is generated through third-order nonlinear process, is valuable light for various applications such as optical coherence tomography (OCT), and spectral measurement. Coherence of a SC pulse is one of the most important properties because the accuracy and sensitivity of these measurements are determined by its noise property. Recently, the second-order coherence property has been revealed by some numerical simulations and experiments [1, 2]. According to these previous reports, the coherence of the SC pulse has been almost lost through the spectral broadening process. However, the depletion of the coherence has been studied only in two bases: time and frequency. Thus, we experimentally study the coherent basis from noisy modes of the SC pulse by utilizing multimode quantum theory [3, 4].