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Fiber optic time-of-flight radar with a submeter spatial resolution for the measurement of integral strain

This paper describes a fibre-optic interrogation device based on a pulsed time-of-flight (TOF) technique for the measurement of integral strain. The precision of the measurement system is 100 μm (1 ps) and it has a spatial resolution of less than 0.50 m (5 ns), achieved by the use of ultra-short probe pulses of about 500 ps, a GHz band receiver channel and a custom-made time-to-digital converter (TDC) implemented in a standard CMOS process. The TDC can simultaneously measure the distance to 9 reflectors (e.g., Bragg gratings) in the fibre core using the same optical pulse. Combined with a common receiver channel and an ultra-fast timing discriminator, this capability makes the system fast and stable, thus enabling both long-term and dynamic measurements. Potential application areas of the system include measurement of integral strain and its derivatives, especially in large civil engineering structures and composite materials. Pull tests with bare optical fibres have demonstrated that the obtained results are in good agreement with those of a reference sensor.