Fig. 1. (a) Linearly frequency-swept principle. (b) System schematic diagram. (c) Theoretical model of resonant cavity for generating LFSC.

Fig. 2. LFSC demodulation algorithm flow.

Fig. 3. Simulation results. (a) Interference signal spectrum of LFSC through MZI. (b) Interference signal spectra under different feedback laser powers. (c) Time-domain diagram of interference signal before resampling. (d) Time-domain diagram of interference signal after resampling. (e) Spectra of original interference signal and resampled signal. (f) Resampled signal spectrum after subdivision transformation.

Fig. 4. (a) Schematic of the FSFI ranging system with LFSC demodulation device. ECTL: external cavity tunable laser; PC: polarization controller; VOA: variable optical attenuator; FC: fiber coupler; BPD: balanced photodetector; DAQ: data acquisition. (b) Schematic of LFSC.

Fig. 5. Tuning range and laser line width property of the ECTL. (a) Laser output power signal from ECTL tail fiber. (b) GAC peak signal and wavelength variation during ECTL positive frequency scanning process. (c) GAC peak signal and wavelength variation during ECTL reverse frequency scanning process. (d) Laser line width property of the ECTL.

Fig. 6. Experimental data when the distance from the measurement target is 15.5 m. (a) LFSC interference signal from the MZI. (b) Normalized LFSC interference signal from the MZI. (c) Spectra of LFSC interference signal. (d) Time-domain signal diagram of mixed signals with frequencies of ω1+ω2 and |ω1−ω2|. (e) Spectra of mixed signals with frequencies of ω1+ω2 and |ω1−ω2|. (f) Time-domain plot of resampled signal with frequency 2ω2 after bandpass filtering.

Fig. 7. Measured results. (a), (c) Standard deviation of step displacement at approximately 7.3 m and 15.5 m. (b), (d) Step linear fitting and residual error at approximately 7.3 m and 15.5 m.

Fig. 8. Test results of different measured targets. (a) Surface-oxidized black aluminum plate; (b) 3D printed board made of PLA; (c) sponge; (d) steel nut; (e) cylindrical steel rod; (f) carton; (g) paper; (h) circuit board; (i) plastic bottle.

Fig. 9. (a) Resolution step test. (b) Measurement results of the system with or without environmental disturbances. (c) Demodulation results of LFSC-AM and LFSC-FM. (d) Test results of the resolution.

Fig. 10. (a) Power spectrum of interference signals formed by LFSC and FSI. (b) Measurement results of oxidized black aluminum plate at a distance of 16 m for LFSC and FSI.