Abstract
An interference filter is constructed from a pair of front-surface two-way mirrors. These partially reflecting mirrors are clamped at both ends with a small spacer placed at one end, thus forming a thin wedge of air between their face-toface mirrored surfaces. A series of thin film interference spectra are recorded by moving this air wedge apparatus in incremental steps across a beam of white light. The mirror separation as a function of position along the wedge is obtained from analysis of the transmitted spectra. As the separation increases, temporal coherence is lost for successively longer wavelengths until interference effects are no longer observed. We define coherence length measured in this way as twice the wedge thickness where the interference fringe pattern is reduced to the noise level of the transmitted spectrum. Using this definition and technique, a value of 224±20μm is obtained using a white light source. This value is much larger than one would expect for the coherence length of white light measured by more standard techniques, e.g. Michelson interferometry. The implication of our experiment is that coherence length is a property of the light reaching the detector, which in our case is the light transmitted by the interference filter.
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