Absolute distances were measured using two femtosecond lasers of different pulse repetition rates by revisiting the dual-comb interferometric method proposed by Coddington et al (2009 Nature Photon. 3 351–6). The apparatus built for experiments was designed to eliminate the dead zones in the measurement range by separating the measurement pulses from the reference pulses using orthogonal polarization. In addition, the pulse repetition rate of the signal laser was made tunable in order to extend the non-ambiguity range (NAR) by adaptively adjusting the synthetic wavelength in consideration of the de facto measurement stability in the air. Actual tests performed in the open air proved that a target distance of 69.3 m is measured without interruptions at a 200 µs update rate in the presence of a ∼170 µm drift of the optical path length caused by the fluctuation of the refractive index of air. The proposed hardware system design for effective NAR extension will facilitate the use of dual-comb interferometry for various terrestrial applications.
Ultrashort pulse lasers are emerging as an advanced tool of distance
measurement, with their unique temporal and spectral characteristics
being extended to diverse principles of absolute ranging and
instrumentation. Here, a systematic methodology is presented for
absolute ranging by means of the time-of-flight measurement of
ultrashort light pulses using dual-comb asynchronous optical sampling.
Based on an elaborate uncertainty analysis, influencing system
parameters such as the pulse duration, repetition rate, and averaging
time are optimized to achieve a sub-µm measurement accuracy. The
absolute ranging system developed in this study demonstrates a
combined standard uncertainty of 0.986 µm for a 0.5 ms averaging over
a distance range of 3.0 m, with a further reduction to 0.056 µm when
the averaging time is increased to 0.5 s. The outstanding performance
leads to unprecedented multitarget applications: machine feed control
with thermal error compensation in real time as well as the
nondestructive inspection of multilens assembly in a production
line.
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