Researchers have created a new ultra-fast tunable laser based on low-loss lithium niobate integrated photonics

A novel ultrafast tunable laser based on low-loss lithium niobate integrated photonics can find applications in technologies such as continuous-wave optical detection and ranging (LiDAR) systems. The device, built by researchers at the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland and IBM Research Europe in Zurich, has a high frequency tuning rate and is superior to previous such lasers in terms of laser linewidth.

Current programmable photonic integrated circuits (PIC) can be unstable and suffer from highlight signal loss -- both of which prevent them from maintaining their programmed state. Lithium niobate, which is commonly used in optical modulators (devices that control the frequency or intensity of transmitted light), has excellent optical and electro-optical properties and provides a possible solution to this problem.

Lithium niobate has recently emerged as an attractive material for PIC substrate, which promises to produce circuits with low optical losses (laser beams can be propagated through them without losing too much power). The material can also support high optical power levels and has a high "Pockels coefficient," meaning that its optical properties can be adjusted using an electric field.

Hybrid laser diode chip

In the new study, detailed in Nature, researchers led by EPFL's Tobias Kippenberg assembled a hybrid device by integrating a distributed feedback laser with a Silicon nitride lithium niobate (Si 3 N 4-LiNbO 3) photonic integrated chip. The latter consists of a thin layer of lithium niobate at the top of the silicon nitride waveguide. While it sounds simple, it took researchers years to master this assembly method because it involved perfectly combining 4-inch-wide LiNbO 3 wafers onto Damascus Si 3 N 4 wafers of the same diameter.

"This configuration allows these circuits with ultra-low losses to be tuned electro-optic," explains team member Viacheslav Snigirev. It also helps to lock laser diode injection into optical microresonators - a technique that enhances laser operation at specific frequencies by using narrow-band back reflections from high quality factor optical microresonators. This makes it possible to narrow the laser linewidth and tune the frequency.

"These properties of lasers make them ideal candidates for frequency modulated continuous wave (FMCW) LiDAR systems," he told Physics World.

Proof of concept FMCW LiDAR system

Through transmission and reflection measurements, the researchers found that their Si 3 N 4 -- LiNbO 3 chip has a low optical propagation loss of 8.5 dB/m, which allows the inherent laser linewidth locked into the laser diode by self-injection to be only 3 kHz. Due to the inherent electro-optical properties of lithium niobate, the device also has electro-optical laser frequency tuning speeds of up to 12 x 10 15 Hz/s while maintaining narrow line width and high tuned linearity. These values are better than previous such devices, allowing the researchers to make a proof-of-concept FMCW LiDAR system with a spatial resolution of 15 cm.

Kippenberg and colleagues say they are now working on more complex photonic architectures for feedback chip circuits. These should further extend the laser's tuning span and increase its output power without affecting its bandwidth.

"We are also committed to improving the fabrication of these circuits by modifying the processing sequence and waveguide geometry to achieve better microresonator quality factor and frequency tuning efficiency," Snigirev said. "Finally, we are eager to extend the range of applications of our new platform - for high-speed modulators and microwave-to-optical interfaces."

Source: Laser Net