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Annual Meeting Optical Society of America

Measurement of coherence time by fourth-order Interference

Z. Y. Ou, E. C. Gage, Brian E. Magill, L. Mandeļ

Published Jan 1, 1988• 0 citations
Publisher

Abstract

By using an intensity correlation, or photon coincidence, technique and a Mach-Zehnder interferometer with artificially applied phase fluctuations in one arm, we measured the coherence time of a thermal light beam. The light was derived from the filtered fluorescence of a rhodamine 6G dye solution. The two-photon coincidence rate was measured as a function of path difference and was found to dip 50% when the two arms of the interferometer are balanced. The coherence time of ~100 fs is determined from the measured width of the dip and is consistent with the passband of the filters. The method does not require the stability needed in second-order interference measurements, and its time resolution can be much shorter than that of the photodetectors. The theory of this type of fourth-order interference technique is discussed.

DOI
10.1364/oam.1988.mx1

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By using an intensity correlation, or photon coincidence, technique and a Mach-Zehnder interferometer with artificially applied phase fluctuations in one arm, we measured the coherence time of a thermal light beam. The light was derived from the filtered fluorescence of a rhodamine 6G dye solution. The two-photon coincidence rate was measured as a function of path difference and was found to dip 50% when the two arms of the interferometer are balanced. The coherence time of ~100 fs is determined from the measured width of the dip and is consistent with the passband of the filters. The method does not require the stability needed in second-order interference measurements, and its time resolution can be much shorter than that of the photodetectors. The theory of this type of fourth-order interference technique is discussed.
A new fourth-order interference experiment has been carried out and analyzed theoretically in classical and in quantum terms. Two photons produced in the process of parametric down-conversion provide the two inputs to a Mach-Zehnder type of interferometer, while two photodetectors coupled to a coincidence counter measure the output. The coincidence rate, after subtraction of accidentals, exhibits a cosine variation with the optical path difference, in agreement with quantum mechanics, but in disagreement with a classical analysis. By contrast, when two coherent light beams from a He:Ne laser are used as inputs to the interferometer, no fourth-order interference is observed.
A new experimental procedure for determining the (second-order) coherence time of a light beam is described that is based on a combination of conventional interferometry with an intensity-correlation technique. It permits measurements of coherence times that are several orders of magnitude shorter than the resolving times of the detectors, which ordinarily limit correlation measurements. The validity of the method is demonstrated by an experiment in which the transverse fluorescent light from a dye laser is measured.
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