We propose a new method to evaluate coherence properties of wavelength-swept light sources for the optical coherence tomography. The method relies on the depth variation of the noise floor and allows to estimate long coherence length with a limited electrical bandwidth unlike the conventional sensitivity roll-off method. By fitting the theoretically predicted noise-floor variation to the experimental data, we have successfully obtained coherence lengths of 1.6, 0.51, and 0.15m for a microelectromechanically tunable vertical-cavity surface-emitting laser with the sweeping rates of 100, 200, and 400kHz, respectively. The coherence lengths are comparable with those obtained with the roll-off method when the coherence lengths are relatively short.
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