On-chip ultrafast laser design
The laser is essentially constructed from a piece of erbium-doped silicon nitride waveguide sandwiched between two Bragg gratings, pumped with (currently) off-chip 1480-nm continuous-wave diodes.
“Using the Mamyshev oscillator concept, the two gratings are deliberately offset in wavelength so continuous-wave lasing can’t be sustained,” explains Qiu. “No single wavelength is reflected by both gratings. Pulses, however, can undergo self-phase modulation within the waveguide, which broadens their spectrum and effectively bridges the wavelength gap between the two gratings. It allows pulsed operation and suppresses ordinary continuous-wave lasing.”
Biggest challenge? “Fabricating such a long cavity on chip, about 42 cm in length, was technically challenging,” says Qiu. “This was made possible, thanks to previous work by generations of members of our lab, as well as the continuous effort and vision of Professor Tobias Kippenberg, in developing low-loss silicon nitride PICs.”
The researchers performed extensive simulations using a custom code for generalized nonlinear Schrödinger equation pulse propagation, along with an original self-consistent iteration approach for the erbium-doped gain. “These simulations were essential for mapping out the feasible parameter regime and guiding the design,” Qiu says. “We’re making the code publicly available.”
As far as projects go, this one went remarkably smoothly—it took only about eight months from coming up with the design to observing the first on-chip modelocking.
And the backstory behind the idea to use Mamyshev oscillators is a good one. “I’d been thinking about how to build a modelocked laser using erbium-doped waveguides, but simulations showed the high nonlinearity of tightly confined waveguides could easily lead to pulse breaking and I couldn’t find a good solution,” says Qiu. “Then, during a walk to lunch, I described this struggle to Zhongshu Liu, an intern student from Tsinghua (now at Harvard) that I was mentoring. He was working on an unrelated project but had prior experience with fiber-based Mamyshev oscillators. We immediately realized this concept solved exactly this problem for fiber-based lasers and could be a solution to our problem as well—and moved forward from there. It’s a nice reminder that informal and international scientific exchanges can spark ideas in completely unexpected ways.”