Massively parallel FMCW lidar with cm range resolution using an electro-optic frequency comb

Data-compressed FMCW LiDAR with long range and high resolution using TFOC-based receiving

Despite its high range resolution, frequency modulated continuous-wave (FMCW) light detection and ranging (LiDAR) faces challenges from large data volume in long-range detections due to its de-chirping receiving principle. We propose a data-compressed FMCW LiDAR with long range and high resolution, using time-frequency optical comb (TFOC)-based receiving. By replacing the traditional single linear frequency modulating (LFM) reference signal with TFOC, the echo at any distance beats with the nearest LFM comb tooth. This approach breaks the conventional linear relationship between target range and beating frequency, resulting in a substantial decrease in beating frequency, receiver bandwidth, and data volume. An experiment shows a resolution of less than 1.87 cm for targets at equivalent free-space ranges of 0.11-0.50 m, 0.38-179 m, and 1.5-5.2 km. This is realized using a receiver bandwidth of only 125 MHz for echoes with 8 GHz sweeping bandwidth, achieving a high data compression ratio of 64. The proposed approach greatly reduces FMCW LiDAR's demand for large-bandwidth photodetectors and high-performance memory and processors.

A microcomb-empowered Fourier domain mode-locked LIDAR

Light detection and ranging (LIDAR) has emerged as an indispensable tool in autonomous technology. Among its various techniques, frequency-modulated continuous wave (FMCW) LIDAR stands out due to its capability to operate with ultralow return power, immunity to unwanted light, and simultaneous acquisition of distance and velocity. However, achieving a rapid update rate with submicrometer precision remains a challenge for FMCW LIDARs. Here, we present such a LIDAR with a sub-10-nanometer precision and a 24.6-kilohertz update rate by combining a broadband Fourier domain mode-locked (FDML) laser with a silicon nitride soliton microcomb. An ultrahigh-frequency chirp rate up to 320 petahertz per second is linearized by a 50-gigahertz microcomb to reach this performance. Our theoretical analysis also contributes to resolving the challenge of FMCW velocity measurements with nonlinear frequency sweeps and enables us to realize velocity measurement with an uncertainty below 0.4 millimeter per second. Our work shows how microcombs can unlock the potential of ultrafast frequency sweeping lasers.

High-fidelity sub-petabit-per-second self-homodyne fronthaul using broadband electro-optic combs

With the exponential growth in data density and user ends of wireless networks, fronthaul is tasked with supporting aggregate bandwidths exceeding thousands of gigahertz while accommodating high-order modulation formats. However, it must address the bandwidth and noise limitations imposed by optical links and devices in a cost-efficient manner. Here we demonstrate a high-fidelity fronthaul system enabled by self-homodyne digital-analog radio-over-fiber superchannels, using a broadband electro-optic comb and uncoupled multicore fiber. This self-homodyne superchannel architecture not only offers capacity boosting but also supports carrier-recovery-free reception. Our approach achieves a record-breaking 15,000 GHz aggregated wireless bandwidth, corresponding to a 0.879 Pb/s common public radio interface (CPRI) equivalent data rate. Higher-order formats up to 1,048,576 quadrature-amplitude-modulated (QAM) are showcased at a 100 Tb/s class data rate. Furthermore, we employ a packaged on-chip electro-optic comb as the sole optical source to reduce the cost, supporting a data rate of 100.5 Tb/s with the 1024-QAM format. These demonstrations propel fronthaul into the era of Pb/s-level capacity and exhibit the promising potential of integrated-photonics implementation, pushing the boundaries to new heights in terms of capacity, fidelity, and cost.

Thin-film lithium niobate-based electro-optic comb cloning for self-homodyne coherent communication

As the optical communication industry advances, metropolitan area networks (MANs) and radio access networks (RANs) are extensively deployed on a large scale, demanding energy-efficient integrated light sources and simplified digital signal processing (DSP) technologies. The emergence of thin-film lithium niobate (TFLN) has given rise to high-performance, energy-efficient on-chip modulators, making on-chip optical frequency comb (OFC) more appealing. Owing to the phase uniformity and stability of this chip-scale device, it has been possible to eliminate the carrier frequency phase estimation (CPE) in DSP stacks using comb-clone-enabled self-homodyne detection. Here we report the first use, to our knowledge, of a TFLN on-chip electro-optic (EO) frequency comb to realize comb cloning and self-homodyne coherent detection. We transmit three optical pilot tones and eight data channels encoded with 20 Gbaud polarization-multiplexed 16-ary quadrature amplitude modulation (PM-16-QAM) over 10 km and 80 km standard single-mode fibers. The bit error ratios (BERs) of the eight channels reach below 10-3, a result made possible by our on-chip comb. The scalability and mass producibility of on-chip EO combs, combined with the simplified DSP, show potential in our proposed fifth-generation (5G) RAN and MAN transmission scheme.

Ge-polymer bridge waveguide for mode-locked laser pulse generation

A Ge-polymer hybrid waveguide is sandwiched between an indium phosphide (InP) reflective gain chip and a fiber Bragg grating (FBG) to construct a laser system. The hybrid waveguide serves as a bridge between the gain chip and the fiber with tailored mode-field matching at both facets. The 50-nm amorphous Ge (α-Ge) layer shows a nonlinear absorption effect at 1550 nm. The hybrid waveguide is further verified by a femtosecond laser transmission experiment to show the pulse width compression effect. Such waveguide is then integrated inside the laser cavity as a passive saturable absorber to modulate the longitudinal modes for a pulsed output. This polymer-bridged mode-locked laser adopts an InP gain chip for compact assembly and also a FBG with a flexible length to adjust the pulse repetition rate. The mode-locked laser output around the designed 50 MHz repetition rate is demonstrated. The pulse width is measured as 147 ps, and the signal-to-noise ratio is larger than 50 dB. This work introduces a "ternary" mode-locked laser system, taking advantage of discrete photonic components bridged by a polymer-based waveguide. It also proves the feasibility of applying α-Ge films as practical and low-cost saturable absorbers in photonic devices.

Integrated nonlinearity calibration optical-electrical engine for FMCW LiDAR application

We demonstrate an integrated optical-electrical calibration module for improving the nonlinearity of the optical source for frequency-modulated continuous-wave (FMCW) LiDAR applications. The linearity of the light source has a considerable influence on FMCW LiDAR range performance, and calibration is typically necessary. However, a majority of existing calibration techniques are based on separate devices, resulting in high cost and limited integration. Our module is made up of a silicon photonic chip with a long optical delay line, a tunable phase shifter, two balanced photodetectors, and some passive components. For this module, we also built the aided amplification and voltage bias circuits. After packaging this module, we used it with our nonlinearity calibration algorithm to analyze the laser's relative nonlinearity. After nonlinearity calibration, the laser relative nonlinearity 1-r2 could be improved to 10-6∼10-7. In the future, the calibration result of nonlinearity could be enhanced further by increasing the length of the on-chip optical delay line.