We study the impact of laser phase noise on ranging precision in a frequency modulated continuous-wave (FMCW) LiDAR system, demonstrating ranging of 384.72 m with ∼15 cm precision at 7× intrinsic laser coherence length

Bayvel, Polina

Killey, Robert I

Liu, Zhixin

Yi, Wenting

Zhou, Zichuan

English

UCL Discovery

Impact of Laser Phase Noise on RangingPrecision Within and Beyond Laser CoherenceLength in FMCW LiDARWenting Yi,1,* Zichuan Zhou,1 Zhixin Liu,1 Polina Bayvel,1 and Robert I. Killey11 Optical Networks Group, Department of Electronic and Electrical Engineering, University College London,London WC1E 7JE, U.K.*w.yi.17@ucl.ac.ukAbstract: We study the impact of laser phase noise on ranging precision in a frequency-modulated continuous-wave (FMCW) LiDAR system, demonstrating ranging of 384.72 mwith ∼15 cm precision at 7× intrinsic laser coherence length. © 2023 The Author(s)1. IntroductionThe booming of the autonomous vehicles market and other sensing applications in recent years have stimulated thedevelopment of light detection and ranging (LiDAR) techniques. In particular, frequency-modulated continuous-wave (FMCW) LiDAR has attracted a lot of interest, as it offers a higher receiver sensitivity, ranging resolutionand precision over a longer distance compared to time-of-flight (TOF) LiDAR [1–3].In FMCW LiDAR systems, the maximum ranging distance is commonly assumed to be limited by the lasercoherence length, as the ranging precision is degraded by incoherent measurement [4]. To enable long-distanceranging of a few hundred meters, a low-linewidth laser is often assumed to be necessary, which potentially in-creases the system cost. A number of studies have explored the possibility of mitigating precision penalties fromlaser phase noise [5]. However, although a certain reduction in ranging precision would be expected in the inco-herent regime, its dependence on ranging distance and laser linewidth has not been thoroughly assessed. If theperformance is acceptable, the use of a relatively high-linewidth laser source might be a compromise solution formany cost-sensitive applications. Therefore, in this paper, through both experiments and simulations, we assessthe penalties arising from laser phase noise on the ranging precision when operating within and beyond the lasercoherence length. We show that, using a 1.2 MHz linewidth laser, a precision of ∼15 cm is still achievable whenoperating at a ranging distance of 384.72 m, corresponding to 7× intrinsic laser coherence length.2. Experiment and simulation setupThe system design used for experiments and simulations is shown in Fig.1a. In order to evaluate the impact oflaser phase noise, lasers with linewidths of 100 kHz and 1.2 MHz were used in the experiment, both with anoutput power of 9.6 dBm. The light was then externally modulated using an in-phase and quadrature modula-tor (IQM), avoiding the impact of modulation nonlinearity or limited tuning speed suffered from using directly-modulated laser sources. The single-sideband (SSB) FMCW signal was generated by independently modulatingthe in-phase (I) and quadrature(Q) arms of the IQM by [6]:VI(t) =VD cos(2π f1t +πBt2Tm)VQ(t) =VD sin(2π f1t +πBt2Tm)(1)where t denotes the time instance, VD is the amplitude of the driving voltage, f1 represents the starting frequencyof the chirp signal, B is the chirping bandwidth and Tm is the chirp period. In both simulation and experiment,f1 =2 GHz, B =5 GHz and Tm =5 µs. The IQM was biased at the null point with a peak-to-peak driving voltageof approximately Vπ . Fig.1b shows the optical spectrum of the transmitted signal at a resolution bandwidth of0.01 nm. Fiber delay lines were used to emulate the time-of-flight. For laser linewidths ∆v of 100 kHz and 1.2 MHzrespectively, the corresponding coherence lengths are approximately 53 m and 637 m in the standard single-modefiber (SSMF), given by Lcoh = cnπ∆v where c is the speed of light, and n is the refractive index of the SSMF [5].The fiber delay length L was varied from 43.68 m to 384.72 m to evaluate the impact of laser phase noise on theranging precision.The receiver comprised a 3-dB coupler combining the local oscillator (LO) and the delayed SSB FMCW signal,followed by a single-ended photodiode, previously shown to be a strong candidate for low-cost and low-complexity(a)1550.6 1550.8 1551 1551.2 1551.4 1551.6Wavelength (nm)-60-50-40-30-20-100Power (dBm)(b)Fig. 1: (a) System setup of single-photodiode based coherent FMCW LiDAR. IQ: in-phase and quadrature; DAC:digital-to-analog converter; EDFA: Erbium-doped fiber amplifier; SSMF: standard single-mode fiber; VOA: vari-able optical attenuator; LO: local oscillator; Rx: received signal; PC: polarization controller; PD: photodiode; RF:radio frequency; FFT: fast Fourier transform. (b) Optical spectrum of transmitted SSB FMCW signal.1.1 1.2 1.3 1.4 1.5Frequency (GHz)-70-60-50-40-30-20-10010Normalized power (dB)1.2 MHz100 kHz1.18 1.19 1.2Frequency (GHz)-40-200Normalized power (dB)(a)237.8 238 238.2 238.4 238.6Delay line  L (m)00.20.40.60.8Probability1.2 MHz100 kHz100 kHz  2 cm1.2 MHz  13 cm(b)Fig. 2: At a delay line L = 238.28 m, comparing 1.2 MHz and 100 kHz laser source: (a) normalized frequency-domain power spectra of detected beat signals; (b) distributions of estimated distance over 100 traces.coherent LiDAR systems [6]. The received FMCW signal was first aligned with the LO using a polarizationcontroller. The LO power was set to 13.4 dBm, and the Rx power was set to −21 dBm across all the measurementsto ensure that the beat signal could always be discerned from the noise floor. The output of the single-endedphotodiode, which had a responsivity of 0.6 A/W, was sampled by an oscilloscope at 50 GSa/s, each measurementcapturing 25,000 points, corresponding to one chirp period. One hundred traces were saved at each distance. Thedetected beat frequency fb was identified by a peak search after applying a fast Fourier transform (FFT) to thereceived signal waveform. The estimated distance was calculated from fb through L=fbcTmnB . The ranging precisionwas quantified by the standard deviation σ of the estimated distance over 100 measurements.The same system was assessed through simulations using MATLAB. The laser phase noise was modeled asa Wiener process, with laser linewidth ∆v varying from 100 kHz up to 1.2 MHz. The SSMF delay lines L werevaried from 10 m up to 490 m, the maximum ranging distance being limited by the discontinuity of the frequencychirp sawtooth waveform. Monte Carlo simulations were carried out to assess the ranging precision with 1000measurements performed at each delay length and laser linewidth.3. Results and discussionFig.2a shows the frequency-domain power spectra at a delay line L = 238.28 m, exhibiting a beat frequencyof 1.19 GHz. With a laser linewidth of 100 kHz, 238.28 m is still within the coherence length ( LLcoh ≈ 0.37).The laser phase noise from the LO and the delayed copy are almost correlated, and close to canceling eachother, resulting in a relatively narrow beat tone. In the case of the 1.2 MHz laser source, however, the incoherentmeasurement ( LLcoh ≈ 4.5) causes spectral broadening in the frequency domain as can be seen in Fig.2a. This leadsto increased uncertainty in identifying the center frequency of the beat tone, degrading the ranging precision.Fig.2b shows the distribution of estimated distances over 100 measurements in the experiment. In the case of(a)0 100 200 300 400 500Delay line  L (m)051015Precision (cm)1200 kHz1000 kHz800 kHz500 kHz300 kHz100 kHz(b)Fig. 3: (a) Ranging precision versus LLcoh with 1.2 MHz and 100 kHz laser source, for delay lines from 43.68 m to384.72 m. (b) Numerical sweep of ranging precision with laser linewidth varying from 100 kHz to 1.2 MHz.1.2 MHz laser linewidth, the spectral broadening results in a variation of estimated distances from 237.8 m to238.6 m, with a standard deviation σ ≈13 cm. For 100 kHz laser linewidth, the distance estimation is more certain,with 77% of measurements giving an estimated distance of 238.28 m, and the standard deviation is ∼2 cm.Fig.3a compares the ranging precision with delay lines L varying from 43.68 m to 384.72 m when using 1.2 MHzand 100 kHz laser linewidth respectively. The top and bottom x-axes represent the delay lengths L normalized tothe intrinsic laser coherence length Lcoh. For the 100 kHz laser linewidth, all the delay lines used in the experimentwere within the laser coherence length with maximum LLcoh ≈ 0.6. As the distance was increased, the rangingprecision increased from 1 cm and plateaued at 2.5 cm. In the case of 1.2 MHz linewidth, at L =43.68 m, cor-responding to LLcoh ≈ 0.83, the distance is approaching the coherence length, and ∼5 cm ranging precision wasmeasured. Beyond the coherence length, the ranging precision starts to deteriorate rapidly. However, at LLcoh ≈ 7(delay line L =384.72 m), ∼15 cm ranging precision is still achievable.Simulations were performed next, investigating in detail how the ranging precision varies with laser linewidthand ranging distance as shown in Fig.3b. Good agreement between simulation and experimental results presentedabove are achieved. At L <120 m, the ranging precision could be sub-centimeter, while 1 cm precision is observedin the experiment in the case of 100 kHz laser source. This discrepancy might be due to additional noise andtiming jitter in the experiments. Beyond the coherence length, the ranging precision degrades and then plateausat a certain level for each laser linewidth. With delay lines extending to 490 m which is 9× intrinsic coherencelength of 1.2 MHz laser source, ∼15 cm ranging precision is still shown to be possible.4. ConclusionWe studied, through both experiments and simulations, the impact of laser phase noise on the ranging precision ofFMCW LiDAR systems when operating within and beyond the laser coherence length. We showed that, althoughthe ranging precision deteriorates when operating beyond the coherence length, ∼15 cm ranging precision is stillachievable at a distance of 384.72 m which is about 7× intrinsic laser coherence length.Acknowledgement This work was supported by the UK EPSRC TRANSNET program (EP/R035342/1).References1. S. Royo and M. Ballesta-Garcia, “An overview of lidar imaging systems for autonomous vehicles,” Applied Sciences, vol. 9, no. 19,p. 4093, 2019.2. Z. Xu, K. Chen, H. Zhang, and S. Pan, “Multifunction lidar system based on polarization-division multiplexing,” Journal of LightwaveTechnology, vol. 37, no. 9, pp. 2000–2007, 2019.3. T. Liao, M. Hameed, and R. Hui, “Bandwidth efficient coherent lidar based on phase-diversity detection,” Applied Optics, vol. 54,no. 11, pp. 3157–3161, 2015.4. T. Hariyama, P. A. Sandborn, M. Watanabe, and M. C. Wu, “High-accuracy range-sensing system based on FMCW using low-costVCSEL,” Optics express, vol. 26, no. 7, pp. 9285–9297, 2018.5. T. Kim, P. Bhargava, and V. Stojanović, “Overcoming the coherence distance barrier in long-range FMCW LIDAR,” 2018 Conferenceon Lasers and Electro-Optics (CLEO), p. STh3L.7, 2018.6. W. Yi, Z. Li, Z. Zhou, E. Sillekens, T. Gerard, C. Deakin, F. M. Ferreira, L. Galdino, Z. Liu, P. Bayvel et al., “Frequency-modulatedchirp signals for single-photodiode based coherent LiDAR system,” Journal of Lightwave Technology, vol. 39, no. 14, pp. 4661–4670,2021.

Impact of Laser Phase Noise on Ranging Precision Within and Beyond Laser Coherence Length in FMCW LiDAR

https://discovery.ucl.ac.uk/10178040/1/Yi_Impact%20of%20Laser%20Phase%20Noise%20on%20Ranging%20Precision%20Within%20and%20Beyond%20Laser%20Coherence%20Length%20in%20FMCW%20LiDAR.pdf

Impact of Laser Phase Noise on Ranging Precision Within and Beyond Laser Coherence Length in FMCW LiDAR

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