Scalable data-efficient real-time 4D imaging FMCW LiDAR with dual Mach–Zehnder interferometers

Yi Hao; Qingyang Zhu; Yaqi Han; Zihan Zang; Annan Xia; Connie Chang-Hasnain; H. Y. Fu

doi:10.1364/PRJ.560809

Photonics Research, Volume. 13, Issue 10, 2766(2025)

Scalable data-efficient real-time 4D imaging FMCW LiDAR with dual Mach–Zehnder interferometers

Yi Hao¹, Qingyang Zhu¹, Yaqi Han¹, Zihan Zang¹, Annan Xia¹, Connie Chang-Hasnain^1,2, and H. Y. Fu^1、*

Author Affiliations

¹Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China

²Berxel Photonics Co., Ltd., Shenzhen 518071, China

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Figures & Tables(9)

Fig. 1. Coherent detection principle of the FMCW LiDAR.

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Fig. 2. (a) Principle of the beat frequency down-shifting and the laser frequency sweep of the tunable laser (blue line), path 1 of the MZI (red line), path 2 of the MZI (purple line), path 3 after CIR (yellow lines), path 4 (green lines), and BPD (yellow and green lines). FC, fiber coupler; MZI, Mach–Zehnder interferometer; CIR, circulator; BPD, balanced photodetector. (b) Simulated beat spectra for a stationary target detected by the BPD. Red dashed line: a Butterworth low-pass filter (LPF) with a 300-kHz cutoff frequency. Green line: beat spectrum after applying the LPF. Gray line: beat spectrum without the LPF.

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Fig. 3. (a) Schematic diagram of the dual-MZI-based FMCW LiDAR system. (b) Measured relationship between the output wavelength and the modulation voltage of the MEMS-VCSEL. (c) Interpolated modulation voltage and the corresponding corrected laser frequency sweep. (d) Scanning pattern of the 2D beam scanning structure. AFG, arbitrary function generator; EDFA, erbium-doped fiber amplifier; VOA, variable optical attenuator; OSC, oscilloscope. The red line and black line represent the fibers and cables, respectively.

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Fig. 4. Schematic illustration of the optical frequency sweeps (a) distorted by laser phase noise and (b) approximated as linear within coherence time.

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Fig. 5. High-resolution 3D imaging performance. (a) Schematic diagram of the targets consisting of two cubes, one cylinder, and a flat panel as the background covered with highly reflective sheets. (b) Measured beat signal from BPD 1 and (c) reference beat signal from BPD 2 of one up ramp with the corresponding beat spectra (d) and (e) of two adjacent spectral windows using overlapped short-time Fourier transform (STFT). (f) High-resolution 3D imaging results with $39 \times 1999$ pixels along the $y$ - and $x$ -axes. Depth deviation for (g) the background, (h) plane of the large cube, (i) slopes and edge of the small cube, and (j) surface of the cylinder. (k) Depth profile along the red dash line in (a).

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Fig. 6. Ultra-fast real-time 3D imaging performance. Photo of (a) three carved letters “T”, “H”, “U” and (c) a rotating fan, all covered by highly reflective sheets and fixed on a rotator. (b) and (d) are the imaging results of (a) and (c) with $39 \times 99$ pixels along the $y$ - and $x$ -axes and 10 frames at 200-kHz fast-axis scanning rate and 2-kHz frame rate.

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Fig. 7. 4D imaging performance. (a) Schematic diagram of the targets consisting of a rotator and a flat panel as the background covered with highly reflective sheets. (b) Beat spectra with different $α$ altered by the galvo mirror, where red and blue curves represent the up and down ramps, respectively. (c) 4D imaging results with $2 \times 999$ pixels along the $y$ - and $x$ -axes. (d) Velocity distribution of the upper pixels along the $x$ -axis in (c) with the (e) velocity error and deviation of the middle 100 pixels

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Table 1. Comparative Summary of Beat Frequency Down-Shifting Methods in FMCW LiDAR Systems

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Table 1. Comparative Summary of Beat Frequency Down-Shifting Methods in FMCW LiDAR Systems

Ref.	[21,22]	[23]	[24]^b	This Work^c
Method	AOM	AOD	Electrical frequency mixer	Fiber-based MZI
Model	SGTF40-1550-1P, China Electronics Technology Group	DTSX-400, Photon Lines	–^a	SMF-28
Frequency shift	40 MHz	104 MHz	10 MHz	1.15 GHz
Physical size	Medium	$57.10 mm \times 27.60 mm \times 22.30 mm$	$18.80 mm \times 22.86 mm \times 13.72 mm$	$242 μm \times 242 μm \times 1.60 m$
Cost	High	High	USD $59.33	USD $2.92 per meter

Table 2. Comparison of the Recent FMCW LiDAR Systems^a

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Table 2. Comparison of the Recent FMCW LiDAR Systems^a

Ref.	[8]	[22]	[23]	[24]	[27]	[36]	[37]	This Work
Laser	DFB (1538.46 nm est.)	DFB (1550 nm)	Flutter-wavelength-swept laser (1535 nm)	EO-ECDL (1550 nm)	Stepped-frequency-swept laser (1550 nm)	Akinetic all-semiconductor programmable swept laser (1316 nm)	Bench-top tunable laser (1550 nm)	HCG MEMS-VCSEL (1550 nm)
Sweep rate	1 kHz	10 kHz	100 kHz	5 kHz	26.50 kHz	15.94 kHz	25 kHz	100 kHz
Tuning rate	26 MHz/μs	350.52 MHz/μs	142 MHz/μs	7.10 MHz/μs	33.13 MHz/μs	181.70 GHz/μs	500 MHz/μs	146.88 GHz/μs
Scanning structure	Galvo mirror	Galvo mirror	Acousto-optic deflector and transmission grating	Galvo mirror	Galvo mirror	Galvo mirror and transmission grating	Bulk photonic crystal waveguide and mirror	Transmission grating and galvo mirror
Depth pixels per frame	$40 \times 40$	$51 \times 78$	$600 \times 190$ (2.60 Hz)	–	–	$475 \times 1000$ (16 Hz)	–	$39 \times 1999$ (100 Hz)
Axial resolution	1.15 cm	9.50 mm	21 cm	10.56 cm	11.99 cm	2.82 mm	15 mm	5.10 mm
Whether real-time imaging the dynamic scene	No	No	Yes	No	No	Yes	No	Yes
Fast-axis scanning rate	–	–	2 kHz	–	–	15.94 kHz	–	200 kHz
Frame rate	–	–	10 Hz ( $200 \times 45$ pixels per frame)	–	–	33.20 Hz ( $475 \times 400$ pixels per frame)	–	2 kHz ( $39 \times 99$ pixels per frame)
Velocity resolution	1.54 mm/s	15.50 mm/s	0.15 m/s	–	–	–	19 mm/s	0.16 m/s
Maximum velocity error	2 mm/s	0.09 m/s	0.08 m/s	–	–	–	29 mm/s	0.50 m/s
Maximum velocity standard deviation	–	37 mm/s	0.20 m/s	–	–	–	–	0.18 m/s

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Yi Hao, Qingyang Zhu, Yaqi Han, Zihan Zang, Annan Xia, Connie Chang-Hasnain, H. Y. Fu, "Scalable data-efficient real-time 4D imaging FMCW LiDAR with dual Mach–Zehnder interferometers," Photonics Res. 13, 2766 (2025)

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