Measuring Optical Distance at Record-High Speed

Graphic representation of high-speed measurements with a laser beam on a bullet. (Source: C. Grupe, P. Trocha, KIT)

Scientists of Karlsruhe Institute of Tech­nology KIT and École poly­technique fédérale de Lausanne EPFL have demon­strated the fastest distance measure­ment 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 comprise real-time 3D cameras based on highly precise and compact lidar systems.

For decades, distance metro­logy by lidar – laser-based light detec­tion and ranging, has been an established method. 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 size of the optical distance measure­ment systems. A team of researchers headed by Christian Koos at KIT’s Insti­tute of Photonics and Quantum Elec­tronics (IPQ) together with the team of Tobias Kippen­berg at EPFL has started to address this challenge in a joint acti­vity, aiming at a concept for ultra-fast and highly precise lidar system that shall fit into a matchbox one day.

To demon­strate the viability of their approach, the scien­tists used a gun bullet flying at a speed of 150 m/s. “We managed to sample the surface structure of the projec­tile on-the-fly, achieving micro­meter accuracy”, Koos comments, “To this end, we recorded one hundred million distance values per second, corre­sponding to the fastest distance measure­ment so far demon­strated.“ This demon­stration was enabled by a new type of chip-scale light source, gene­rating optical frequency combs. The combs are generated in optical micro­resonators, tiny circular structures, which are 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 regular a pulse train that features a broadband optical spectrum.

The concept crucially relies on high-quality silicon nitride micro­resonsators with ultra-low losses. “We have developed low-loss optical reso­nators, in which extremely high optical inten­sities can be generated – a prere­quisite for soliton frequency combs,” says Kippen­berg, “These Kerr frequency combs have rapidly found their way into new appli­cations over the previous years.“ In their demon­strations, the researchers combined findings from different areas. “In the past years, we have exten­sively studied methods for ultra-fast communi­cations using chip-scale frequency comb sources,” Christian Koos explains. “We now transfer these results to another research area – optical distance measure­ments.”

In 2017, the two teams reported on the potential of chip-scale soliton comb sources in optical telecommu­nications. 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 demon­strated combi­nation of precision and speed in the ranging experiment is an important milestone in itself, the researchers aim at carrying the work further and at elimi­nating the remaining obstacles towards technical appli­cation. For instance, the range of the method is still limited to typical distances of less than 1 meter. 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 fitting into the volume of a matchbox. The silicon-nitride micro­resonators are already commer­cially available by EPFL’s spinoff Ligentec SA that has specia­lized on fabri­cation of silicon nitride-based photonic inte­grated circuits.

The envisaged sensors might serve a wide variety of appli­cations, e.g., for 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 by laborious distance metro­logy. Moreover, the lidar concept might pave the path towards high-perfor­mance 3D cameras in microchip format, which may find wide­spread appli­cations in auto­nomous navigation. (Source: KIT)

Reference: P. Trocha et al.: Ultrafast Optical Ranging Using Microresonator Soliton Frequency Combs, Science 359, 887 (2018); DOI: 10.1126/science.aao3924

Links: Laboratory of Photonics and Quantum Measurements (T. Kippenberg), EPFL, Lausanne, Switzerland • Institute of Photonics and Quantum Electronics (IPQ), Karlsruhe Inst. of Technology KIT, Karlsruhe, Germany

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