The characterization of the laser linewidth and laser frequency stability is critically important for the evaluation of a
metrology system performance when the working principle is based in interferometric processes. In particular, the
midterm stability range, corresponding to noise in the hundreds of hertz to kilohertz bandwidth, affects strongly the
measurement final accuracy when working at measurement rates at the ksample/s level. In this case, it is of crucial
importance to know the uncertainty associated to the measurement of the laser instantaneous frequency and what is the
variance of this value within the measurement period.
In this paper we present a simple method to measure the frequency noise and obtain the Allan variance statistics for an
External Cavity Diode Laser (ECDL) used in a Frequency Sweeping Interferometry (FSI) scheme for long distance high
accuracy measurements. For this type of lasers, the main contributors affecting the midterm stability are the current and
technical noise, including thermal and mechanical fluctuations, optical feedback, as well as the feedback stabilization
techniques employed to reduce acoustic disturbances. The proposed method is based in the principle of delayed
interferometry, where the variation of the laser center frequency is characterized for measurement conditions in the
kilohertz range. The final accuracy of the metrology system is evaluated in accordance with the laser stability
characteristics obtained by this method.