A self-driving car has a hard time recognizing the difference between a toddler and a brown bag that suddenly appears because of limitations in how it senses objects using Light Detection and Ranging (LiDAR). The autonomous vehicle industry is exploring frequency modulated continuous wave (FMCW) LiDAR to solve this problem.
Researchers have built a way that this type of LiDAR could achieve higher-resolution detection of nearby fast-moving objects through mechanical control and modulation of light on a silicon chip. FMCW LiDAR detects objects by scanning laser light from the top of an autonomous vehicle. A single laser beam splits into a comb of other wavelengths, called a microcomb, to scan an area. Light bounces off of an object and goes to the detector through an optical isolator or circulator, which ensures all reflected light ends up at the detector array. The new method uses acoustic waves to enable faster tuning of these components, which could bring higher-resolution FMCW LiDAR detection of nearby objects.
The technology integrates microelectromechanical systems (MEMS) transducers made of aluminum nitride to modulate the microcomb at high frequencies ranging from megahertz to gigahertz. An array of phased MEMS transducers, also used in cellphones to discern cellular bands, stirs light at gigahertz frequencies by launching a corkscrew-like stress wave into a silicon chip. The stirring motion modulates light such that it can only travel in one direction.
Other transducers in the same technology excite an acoustic wave that shakes the chip at megahertz frequencies, demonstrating sub-microsecond control and tuning of the laser pulse microcomb or soliton. This light modulation technique not only integrates mechanics with optics but also the fabrication processes involved, making the technology more commercially viable. The MEMS transducers are simply fabricated on top of the silicon nitride photonics wafer with minimal processing.
The new technology could provide the impetus for micro-comb applications in power-critical systems such as in space, data centers, and portable atomic clocks or in extreme environments such as those with cryogenic temperatures.