An investigation of the temporal coherence length of light

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 spacerplaced at one end, thus forming a thin wedge of air between their face-to-facemirrored surfaces. A series of thin film interference spectra are recorded bymoving this air wedge apparatus in incremental steps across a beam of whitelight. The mirror separation as a function of position along the wedgeis obtained from analysis of the transmitted spectra. As the separationincreases, temporal coherence is lost for successively longer wavelengths untilinterference effects are no longer observed. We define coherence length measuredin this way as twice the wedge thickness where the interference fringepattern is reduced to the noise level of the transmitted spectrum. Usingthis definition and technique, a value of 224 ± 20 µmis obtained using a white light source. This value is much larger than onewould expect for the coherence length of white light measured by morestandard techniques, e.g. Michelson interferometry. The implication of ourexperiment is that coherence length is a property of the light reaching thedetector, which in our case is the light transmitted by the interference filter.