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Journal of the Optical Society of America A

Two-time coherence of pulse trains and the integrated degree of temporal coherence

Rahul Dutta, Ari T. Friberg, Goëry Genty, Jari P. Turunen

Published Aug 7, 2015• 12 citations
Publisher

Abstract

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.

DOI
10.1364/josaa.32.001631

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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.
We consider temporally integrating interferometric measurements and their relation to the coherence properties of nonstationary light. We find that performing such experiments as a function of time delay is equivalent to spectrally resolving the interference patterns, and time-domain coherence information can be obtained from field autocorrelation only if the source is of the Schell-model type. In an analogy to autocorrelation, we introduce field cross-correlation, which can be used to determine the complete complex field of unknown signal pulses if suitable probe pulses are available. We demonstrate our findings with simulated supercontinuum and free-electron laser ensembles, and discuss the prospect of carrying out experiments.
We measure the recently introduced electromagnetic temporal degree of coherence of a stationary, partially polarized, classical optical beam. Instead of recording the visibility of intensity fringes, the spectrum, or the polarization characteristics, we introduce a novel technique based on two-photon absorption. Using a Michelson interferometer equipped with polarizers and a specific GaAs photocount tube, we obtain the two fundamental quantities pertaining to the fluctuations of light: the degree of coherence and the degree of polarization. We also show that the electromagnetic intensity-correlation measurements with two-photon absorption require that the polarization dynamics, i.e., the time evolution of the instantaneous polarization state, is properly taken into account. We apply the technique to unpolarized and polarized sources of amplified spontaneous emission (Gaussian statistics) and to a superposition of two independent, narrow-band laser beams of different mid frequencies (non-Gaussian statistics). For these two sources femtosecond-range coherence times are found that are in good agreement with the traditional spectral measurements. Although previously employed for laser pulses, two-photon absorption provides a new physical principle to study electromagnetic coherence phenomena in classical and quantum continuous-wave light at extremely short time scales.
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