New laser helps LiDAR for driverless cars

Lasers are ubiquitous in science and technology – they heal eyes, allow for optical communications at high speed, create new states of matter, and can even detect gravitational waves. Lasers are also used in applications like 3D sensors for autonomous driving to spectroscopic detection of greenhouse gases, they have opened a whole new realm of material processing, they can manipulate atoms and ions in the quantum regime, and can be used to create the most precise atomic clocks.

The innovation potential created by lasers cannot be overstated; hardly any modern technology is as pervasive as coherent laser light. However, the technology and principles of lasers have changed little over the past 50 years, and the majority of commercially used lasers are still based on legacy technical schemes. At the same time, the technology base of photonics has changed fundamentally over the last decade, driven by the advent of photonic integration platforms such as silicon nitride that have now reached industrial maturity.

Photonic integration allows us to build compact systems with unprecedented scalability. But so far, it has been challenging to miniaturize narrow-linewidth laser sources. While integrated lasers have been studied for well over a decade, they fail to achieve a similar phase noise performance as state-of-the-art, fiber lasers.

A conflicting requirement for any laser source is to simultaneously achieve frequency agility; that is, a laser frequency can be actuated with high speed. Frequency agility of narrow linewidth laser sources is key to many applications in frequency metrology as well as for the generation of triangular chirp signals as required in frequency modulated continuous wave (FMCW LiDAR) ranging – one of the most coveted technologies that concern long-range LiDAR.

The most stable lasers to date lack the ability for fast laser tuning and are not compatible with wafer-scale manufacturing – a critical requirement for applications that address mass markets. Attaining both narrow linewidth and frequency agility has been an outstanding challenge and has neither been achieved in bulk lasers nor any integrated platform to date.

Now, a collaboration between EPFL and Purdue University has made significant progress in the field of hybrid integrated tunable lasers. The group managed to demonstrate a compact laser featuring simultaneously ultra-low noise and fast tuning speed. The achievement, published in Nature Communications, significantly lowers the cost of production, and allows new laser sources for optical ranging. It could have the potential to eliminate the continued trade-off between fiber lasers and diode lasers and provide a one-size-fits-all solution that is compatible with wafer-scale manufacturing.

By combining manufacturing of established MEMS processing (Purdue) with photonic circuits based on silicon nitride, fabricated in the EPFL center of MicroNanoTechnology (CMi), the researchers were able to attain for the first time low phase noise that is on par with a fiber laser, while simultaneously achieving an unprecedentedly fast tuning. Their approach is based on the laser self-injection locking technique, which allows for reducing the free-running laser linewidth by at least four orders of magnitude.

To illustrate the full potential, the authors utilize the laser to perform an optical ranging experiment. Coherent laser ranging is a next generation LIDAR technology in which there is major commercial interest. Yet, no integrated laser source has met the stringent requirements of tuning linearity and speed. The authors demonstrate an impressive coherent FMCW LiDAR in a lab environment with 12 cm distance resolution and 100 kHz tuning speed, reconstructing the scene 10 m away from the laser.

“What is remarkable about the result is the laser allows optical ranging without any additional complex linearization procedures,” says Tobias J. Kippenberg, Professor of Physics at EPFL’s School of Basic Sciences.

The work is a remarkable example of integrated photonics not only offering a compact, wafer-scale manufacturing method but also achieving performance metrics that cannot be attained with conventional bulk lasers that have been developed, optimized, and perfected for many decades. It underscores the tremendous potential that hybrid ultralow-loss integrated photonics can have at developing compact ultra-narrow linewidth lasers for the next generation of LiDAR and coherent communications in the long haul.

The chip samples were fabricated in the EPFL center of MicroNanoTechnology (CMi), and in the Birck Nanotechnology Center at Purdue University.