Scientists of Karlsruhe Institute of Technology (KIT) and École Poly-technique Fédérale de Lausanne (EPFL) have reported that they achieved the fastest distance measurement attained so far. The researchers demonstrated on-the-fly sampling of a gun bullet profile with micrometer accuracy. The experiment relied on a soliton frequency comb generated in a chip-based optical microresonator made from silicon nitride. Potential applications include real-time 3D cameras based on highly precise and compact LIDAR systems.
Distance metrology by means of lasers, Laser-Based Light Detection and Ranging (LIDAR), has been an established method for decades. Today, optical distance measurement methods are being applied in a wide variety of emerging applications, such as navigation of autonomous objects, e.g. drones or satellites, or process control in smart factories. These applications are associated with very stringent requirements regarding measurement speed and accuracy, as well as the size of the measurement systems.
The team of researchers has started to address this challenge, developing a concept for an ultra-fast and highly precise LIDAR system that could ultimately fit into a matchbox. To demonstrate the viability of their approach, the scientists used a bullet flying at a speed of 150 m/s. They were able to sample the surface structure of the projectile on-the-fly, achieving micrometer accuracy by recording 100 million distance values per second.
This demonstration was enabled by a new type of chip-scale light source developed at EPFL, which produces optical frequency combs. The combs are generated in optical microresonators, tiny circular structures fed by continuous-wave light from a laser source. Mediated by nonlinear optical processes, the laser light is converted into stable optical pulses — dissipative Kerr solitons — forming a regular pulse train with a broadband optical spectrum. The concept relies on high-quality ultra-low loss silicon nitride microresonators in which extremely high optical intensities can be generated.
In their demonstrations, the researchers combined findings from the field of ultra-fast communications using chip-scale frequency comb sources, with results from the field of optical distance measurements. In 2017, teams from KIT and EPFL published a joint article reporting on the potential of chip-scale soliton comb sources in optical telecommunications. Optical frequency combs consist of light with a multitude of precisely defined wavelengths — the optical spectrum then resembles the teeth of a comb. If the structure of such a comb is known, the inference pattern resulting from superposition of a second frequency comb can be used to determine the distance traveled by the light. The more broadband the frequency combs, the higher is the measurement accuracy. In their experiments, the researchers used two optical microchips to generate a pair of nearly identical frequency combs.
The scientists consider their experiment to be a first demonstration of the measurement technique. Although the combination of precision and speed in the ranging experiment is an important milestone in itself, the researchers aim at carrying the work further to enable practical applications. For instance, the range of the method is still typically limited to distances of less than 1 m. Moreover, today's standard processors do not permit real-time evaluation of the large amount of data generated by the measurement. Future activities will focus on a compact design, enabling highly precise ranging, while shrinking the size to the volume of a matchbox. The silicon-nitride microresonators are already commercially available.
The sensors might be used in a wide variety of applications, e.g., high-throughput in-line control of high-precision mechanical parts in digital factories, replacing state-of-the-art inspection of a small subset of samples that requires laborious distance metrology. Moreover, the LIDAR concept could pave the path towards high-performance 3D cameras in microchip format, which would have widespread applications in autonomous navigation.