We report on a laser frequency sweep linearization method by iterative learning pre-distortion for frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) systems. A pre-distorted laser drive voltage waveform that results in a linear frequency sweep is obtained by an iterative learning controller, and then applied to the FMCW LiDAR system. We have also derived a fundamental figure of merit for the maximum residual nonlinearity needed to achieve the transform-limited range resolution. This method is experimentally tested using a commercial vertical cavity surface-emitting laser (VCSEL) and a distributed feedback (DFB) laser, achieving less than 0.005% relative residual nonlinearity of frequency sweep. With the proposed method, high-performance FMCW LiDAR systems can be realized without expensive linear lasers, complex linearization setups, or heavy post-processing.

Laser frequency sweep linearization by iterative learning pre-distortion for FMCW LiDAR.

We propose and demonstrate a novel linearizing method of optical frequency-sweep of a laser diode for frequency-modulated continuous-wave (FMCW) reflectometry. In order to linearly sweep the optical frequency, we adopt a reference interferometer and an electric phase comparator. The interference beat signal of the reference interferometer is phase-compared with an external reference rectangular signal having a fixed frequency near the interference beat signal frequency by a lock-in amplifier. The error signal from the lock-in amplifier is fed back to the modulating signal of the injection current of the laser. Thus, a phase-locked loop composed of optical and electric circuits can be established, and the beat signal frequency is locked to the frequency of the reference signal. The optical frequency of the laser diode is, therefore, excellently linearly swept in time. In order to experimentally confirm the linearity of the proposed method, we apply this frequency-swept laser diode to the FMCW reflectometry. Resultingly, the improvement of the linearity is estimated to be about 10 dB. And the theoretically limited spatial resolution of the FMCW reflectometry is achieved. The backscattered light in optical waveguide devices is measured by the FMCW reflectometry using the proposed light source, and the propagation loss of a single-mode glass waveguide is successfully evaluated.

Linearizing Optical Frequency Sweep of a Laser Diode for FMCW Reflectometry : Realization by a Digital Filter

Simultaneous measurements of velocity and distance via a dual-path FMCW lidar system

A microwave photonic approach is proposed to achieve a high resolution enhanced by a factor of 2 for the range and velocity measurement. In the transmitter, a V-shaped linear-frequency-modulated (LFM) microwave signal is generated through the frequency doubling enabled by photonics, with large time-bandwidth product for improving the range resolution, whereas in the receiver, the round-trip duration and Doppler frequency shift (DFS) between the transmitted LFM signal and the received echo signal are mapped into two doubled frequency spacings between two pairs of target sidebands by incorporating two cascaded electrooptic modulators. Thus, the distance and velocity of the moving target can be measured from the beat note of the target sidebands, showing a high resolution improved by a factor of 2. Experiments have been implemented and demonstrated in X-band with a bandwidth of 2 GHz. The distance measurement indicates an absolute error less than 3 mm. In addition, the performance of the proposed system in terms of low power level is discussed.

High-Resolution Range and Velocity Measurement Based on Photonic LFM Microwave Signal Generation and Detection

We propose and demonstrate a photonics-based method for simultaneously measuring distance and velocity by adopting multi-band linear-frequency-modulated microwave signals with opposite chirps as transmitted signals. A dual-drive Mach-Zehnder modulator and photodetector (PD) are used to mix transmitted and echo signals. By analyzing the electrical spectra of the electrical signals generated by the PD, distance and the magnitude and direction of velocity can be derived simultaneously, which is significant for real-time radar applications. Proof-of-concept experiments using the proposed approach are also presented. The measured relative errors for distance and velocity are less than 0.005% and 0.59%, respectively.

Photonics-based simultaneous measurement of distance and velocity using multi-band LFM microwave signals with opposite chirps.

A dual-sideband (DSB) chirped lidar for simultaneous real-time ranging and velocimetry is proposed and demonstrated, in which a Mach–Zehnder modulator (MZM) is applied to generate the required optical DSB frequency-modulated continuous-wave (FMCW) signal. The inherent opposite frequency chirp and contrary wavelength offset to the optical carrier of the two generated sidebands make it possible to measure the distance and velocity by frequency mixing without complex post digital signal processing, meanwhile, make the measurement of velocity immune to the nonlinearity of the FMCW optical signals. An experiment is carried out, in which an 8–18 GHz saw-tooth FMCW signal is used to drive an MZM, generating a wideband optical DSB FMCW signal. The distance and the velocity are simultaneously derived from the real-time frequency spectra. Accurate velocimetry with a nonlinear FMCW signal is also investigated.

Simultaneous Real-Time Ranging and Velocimetry via a Dual-Sideband Chirped Lidar

The invention discloses a laser measurement method based on double-sideband modulation, which is used for measuring distance information and speed information of an object to be measured simultaneously. The laser measurement method comprises the steps of modulating a linear frequency modulation microwave signal on an optical carrier, and generating a carrier-suppressed double-sideband modulation optical signal; then using the generated double-sideband modulation optical signal as probe light, and transmitting the probe light to the object to be measured; performing frequency beating on an echo signal and a local oscillation optical signal to obtain an electric signal which simultaneously contains time delay information and Doppler frequency shift information of the echo signal, wherein the local oscillation optical signal is obtained by splitting the optical carrier or the double-sideband modulation optical signal; and finally extracting the time delay information and the Doppler frequency shift information of the echo signal from the electric signal, obtaining distance information of the object to be measured according to the time delay information, and obtaining speed information of the object to be measured according to the Doppler frequency shift information. The invention further discloses a laser measurement device based on double-sideband modulation. The laser measurement method and device based on double-sideband modulation can simultaneously realize real-time and accurate measurement for the distance information and the speed information of the object to be measured.

Laser measurement method and device based on double-sideband modulation

A system for simultaneous measurement of micro-wave Doppler frequency shift (DFS) and angle of arrival (AOA) is proposed based on a silicon integrated chip. In the chip, two dual-drive Mach–Zehnder modulators (DDMZMs) with a microring resonator (MRR) inserted between them are integrated. When an external laser is coupled into the chip, it is first modulated by two echo signals via the first DDMZM and then goes through the MRR to introduce an optical phase shift to one of the first-order sidebands that falls into the resonance of the MRR. After being remodulated by the transmitted signal at the other DDMZM, the lightwave is coupled out from the chip and separated into two channels by an optical coupler. In each channel, a bandpass filter (BPF) is employed to select the upper and lower sidebands. By measuring and comparing the powers, frequencies, and phases of the beat signals in the two channels, the DFS (without direction ambiguity) and AOA can be acquired. Based on the proposed method, DFS within ±100 kHz at 10, 15, and 20 GHz is successfully estimated with a maximum error of ±0.028 Hz, and AOA measurement from 0° to 75° with errors less than ±1.53° is also realized.

Simultaneous Measurement of Microwave Doppler Frequency Shift and Angle of Arrival Based on a Silicon Integrated Chip

By integrating cascaded silicon microring resonator modulators (MRMs) and a silicon-germanium photodetector on a single SOI chip, an ultra-compact integrated microwave photonic mixer with a footprint as small as 740 mm × 270 mm is proposed. Experimental results show that the LO/RF bandwidth of the mixer is 6–20 GHz, the IF bandwidth is 0–175 MHz, and its conversion efficiency is high than -12 dB. Our work leads to a significant reduction in device footprint compared to formerly reported discrete microwave photonic mixers or integrated mixers based on Mach-Zehnder modulators.

Liangzun Tang

Papers

Simultaneous Real-Time Ranging and Velocimetry via a Dual-Sideband Chirped Lidar

Laser measurement method and device based on double-sideband modulation

Simultaneous Measurement of Microwave Doppler Frequency Shift and Angle of Arrival Based on a Silicon Integrated Chip

Silicon Integrated Microwave Photonic Mixer