FMCW laser ranging breaks the coherence length limitation via phase noise cancellation

Phase noise (PN) of a laser source is a limitation on the measurement range for a frequency-modulated continuous-wave (FMCW) laser ranging system. A narrow-linewidth laser is required to achieve long-range ranging. We proposed a FMCW laser ranging system which is able to break the limitation of coherence length via pilot-assisted PN cancellation. The system adds an acousto-optic modulator (AOM) to generate a frequency known pilot signal, from which the PN can be extracted. PN-cancellated signal can be obtained by removing the extracted PN term. The experimental results show the proposed system can recover the coherent peak beyond the 8-times of coherence length and achieve 0.88-cm precision at 861.39 m. This method can be used to achieve long-range of FMCW laser ranging using a wide linewidth laser.