Fig. 1. Concept of the FMCW Lidar. (a) Schematic of the FMCW Lidar. (b) The compressed sampling of the random single-photon sequence. (c) The spectrum of the beat signal corresponding to the target distance is analyzed by the discrete Fourier transform (DFT). BS, beam splitter; FWHM, full width at half maximum.

Fig. 2. The single-photon FMCW Lidar system. PID, proportional-integral-derivative; PZT, piezoelectric ceramic transducer; DC, direct current; AC, alternating current; FSG, function signal generator; CIR, circulator; PMC, polarization-maintaining coupler; PC, polarization controller; RF, radio frequency signal generator; EOM, electro-optic modulator; SLS cavity, ultra-stable cavity (SLS Company); OSC, oscilloscope; VOA, variable optical attenuator; PD, photoelectric detector; BS, 50/50 beam splitter; SPD, single-photon detector; TIA, time interval analyzer; AMP, amplifier; FSM, fast steering mirror.

Fig. 3. 3D image of target. (a) The target containing multiple objects. (b) The 2D reconstruction. (c) The 3D image according to the frequency mapping distance.

Fig. 4. The spectrum of the beat signal when the laser is running freely and when the closed-loop frequency locking system is employed. The graph is centered on the frequency of 1,620,173 Hz. The black line is the experimental result and the red line is curve fitting. (a) Open-loop operation and classical detection. (b) Closed-loop operation and classical detection. (c) Single-photon detection and closed-loop operation.

Fig. 5. The ranging resolution and accuracy. (a) The FWHM of the spectrum of the beat signal with different target distances and different sweep rates. (b) The spectrum of the beat signal has double peaks because of the frequency shift of the beat signal caused by the Doppler noise of the fiber. (c) The relationship between the measured and theoretical distance increments. The linear fitting slope of the experimental data is 1.02.

Fig. 6. Noise immunity and broadband detection of our proposal. (a) Comparison of the spectra of the beat signals with and without background noise. (b) Test of broadband detection capability of our system from 200 MHz to 3 GHz based on QCS technology.