We introduce a novel optical propagation delay measurement scheme for distance estimation. It is based on a ring oscillator in which the oscillation signal is replaced by the clock information contained in optical data. A clock-and-data recovery can recover the oscillation signal at the receive end. Correlation of the received pattern with the transmitted pattern and a measurement of the bit duration by a frequency counter allow to determine the distance. The scheme has been realized at 1550 nm wavelength, using an externally modulated laser, a commercial 155.52 Mb/s clock-and-data recovery and a field-programmable gate array. Short-term repeatability is <10 µm at an equivalent free-space distance of 72 m. Measurement interval is 0.1 s. At 3 km distance the relative repeatability is 8·10−8. The readout can be corrected with measured temperature data.
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