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Laser Light Measures Distance

Laser Light Measures Distance
Coherent laser ranging, most importantly frequency-modulated continuous-wave LiDAR, is where the laser is set up to emit linear optical frequency chirps.
Technology Briefing


Light detection and ranging (or LiDAR) are based on an array of techniques using laser light to measure distances by multiplying the time delay between transmitted and received optical signals with the speed of light. Modern 3D LiDAR sensors combine high lateral/vertical and radial resolution. And, they represent the most crucial and expensive components in the ongoing revolution in level 4 and 5 self-driving cars.

Most current LiDAR sensors rely on the time-of-flight operation principle where short pulses or pulse patterns are emitted from the sensor aperture and the power of back-reflected light is detected using a square-law photodetector. However, a different principle is that of coherent laser ranging, most importantly frequency-modulated continuous-wave (or FMCW) LiDAR, where the laser is set up to emit linear optical frequency chirps. Heterodyne mixing with a replica of the emitted laser light maps the target distance to a radiofrequency.

Coherent detection has many inherent advantages such as enhanced distance resolution, direct velocity detection via the Doppler effect, and imperviousness to sunlight glare and interference. But the technical complexity of precisely controlling narrow-linewidth frequency-agile lasers has so far prevented the successful parallelization of FMCW LiDAR.

But, as described in the journal Nature, researchers have found a new way to implement a parallel FMCW LiDAR engine by using integrated nonlinear photonic circuitry. They coupled a single FMCW laser into a silicon-nitride planar micro-resonator, where the continuous wave laser light is converted into a stable optical pulse train due to the double balance of dispersion, nonlinearity, cavity pumping, and loss.

The small size of the micro-resonator means that the comb teeth are spaced 100 GHz apart, which is enough to separate them using standard diffraction optics. Because each comb tooth inherits the linear chirping of the pump laser, it was possible to create up to 30 independent FMCW LiDAR channels in the micro-resonator.

Each channel is simultaneously capable of measuring distance and velocity of a target, while the spectral separation of the different channels makes the device immune to channel crosstalk. And it could improve data acquisition rates by tenfold versus prior coherent FMCW LiDAR in the near future,

Furthermore, the spatial separation of emitted beams and operation in the 1550 nm-wavelength band relaxes the otherwise stringent eye and camera safety limitations.

The concept relies on high-quality silicon-nitride micro-resonators with record-low losses. The silicon-nitride microresonators are already commercially available from EPFL spinoff LiGENTEC SA which specializes in the fabrication of silicon nitride-based photonic integrated circuits.

This work paves a way for the widespread application of coherent LiDAR in autonomous vehicle applications. The researchers are now focused on heterogeneous co-integration of lasers, low-loss nonlinear micro-resonators, and photodetectors into a single and compact photonics package.


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