Fig. 1. Schematic diagram of OFDR

Fig. 2. DFB laser open-loop correction steps

Fig. 3. Overall scheme of optoelectronic phase locked loop system based on FPGA

Fig. 4. Adaptive loop filter structure based on PI control

Fig. 5. z-domain structure of optoelectronic phase-locked loop

Fig. 6. Results of driving DFB laser with original triangular wave signal. (a) Time frequency curve of beat frequency signal; (b) FFT power spectrum of beat frequency signal

Fig. 7. Open loop correction results. (a) Time-frequency curve of the first iteration beat frequency signal; (b) time-frequency curve of the second iteration beat frequency signal; (c) time-frequency curve of the third iteration beat frequency signal; (d) FFT power spectrum of the third iteration beat frequency signal

Fig. 8. Correction results of open loop combined with photoelectric phase-locked loop closed-loop. (a) Time frequency curve of beat frequency signal; (b) FFT power spectrum of beat frequency signal

Fig. 9. FFT power spectra of beat frequency signal of 6 m fiber to be tested. (a) Measurement result using uncorrected light source; (b) measurement result using corrected light source

Fig. 10. Fitting curves for fiber measurement results. (a) Fitting curve of fiber measurement results for uncorrected source; (b) fitting curve of fiber measurement results for corrected source

Fig. 11. Reproducibility measurement results of FUT. (a) Error curves of 20 repeated measurements; (b) error bar curve of FUT with different lengths