Abstract
A method based on equal frequency resampling is proposed to suppress laser nonlinear frequency sweeping for the ultimate spatial resolution in optical frequency domain reflectometry. Estimation inaccuracy of the sweeping frequency distribution caused by the finite sampling rate in the auxiliary interferometer can be efficiently compensated by the equal frequency resampling method. With the sweeping range of 130 nm, a 12.1 µm spatial resolution is experimentally obtained. In addition, the sampling limitation of the auxiliary interferometer-based correction is discussed. With a 200 m optical path delay in the auxiliary interferometer, a 21.3 µm spatial resolution is realised at the 191 m fibre end. By employing the proposed resampling and a drawing tower FBG array to enhance the Rayleigh backscattering, a distributed temperature sensing over a 105 m fibre with a sensing resolution of 1 cm is achieved. The measured temperature uncertainty is limited to ±0.15 °C.
Highlights
High-resolution and long-distance sensing are two essential and trade-off factors in distributed optical fibre sensors (DOFSs) [1,2,3,4,5]
The frequency interval in the zero-crossing resampling is an estimation of the frequency distribution by the zero-crossing points. It is different from the physical process of optical frequency domain reflectometry (OFDR), in which the interference is the sum of all the individual backscattering points along the fibre under test (FUT) and the local oscillator (LO) light
Sweeping ranges of 0.04 nm, 0.4 nm and 4 nm are tested under a 40 nm/s tuning speed, while 20 nm, 50 nm and 130 nm sweeping ranges are tested under a 100 nm/s tuning speed for calculation consumption. This shows that there is little difference between zero-crossing and equal frequency resampling, while the expected spatial resolution is higher than 1 mm, which means that the frequency uncertainty caused by nonlinear sweeping and phase noise is less than the 1 mm frequency resolution
Summary
High-resolution and long-distance sensing are two essential and trade-off factors in distributed optical fibre sensors (DOFSs) [1,2,3,4,5]. Hilbert transformation-based correction provides another straightforward solution to suppressing the nonlinear tuning by obtaining the instantaneous sweep frequency [20]. To further improve phase stability in the optical coherence tomography (OCT), each frequency sweep of the laser is calibrated by using the Hilbert transformation [21]. The realisation of theoretical spatial resolution of OFDR is still challenging due to the inaccurate estimation of frequency sweeping derived from the auxiliary interferometer-based correction, which will further hamper the improvement of sensing accuracy. A ±0.15 ◦C temperature uncertainty obtained in the distributed temperature sensing experimentally demonstrates the efficient suppression of the nonlinear frequency sweeping noise
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