Xinru Ji — Page 2 | Search arXiv

arXiv2022

A photonic integrated circuit based erbium-doped amplifier

Erbium-doped fiber amplifiers have revolutionized long-haul optical communications and laser technology. Erbium ions could equally provide a basis for efficient optical amplification in photonic integrated circuits. However, this approach has thus far remained impractical due to insufficient output power. Here, we demonstrate a photonic integrated circuit based erbium amplifier reaching 145 mW output power and more than 30 dB small-signal gain -- on par with commercial fiber amplifiers and beyond state-of-the-art III-V heterogeneously integrated semiconductor amplifiers. We achieve this by applying ion implantation to recently emerged ultralow-loss Si3N4 photonic integrated circuits with meter-scale-length waveguides. We utilize the device to increase by 100-fold the output power of soliton microcombs, required for low-noise photonic microwave generation or as a source for wavelength-division multiplexed optical communications. Endowing Si3N4 photonic integrated circuits with gain enables the miniaturization of a wide range of fiber-based devices such as high-pulse-energy femtosecond mode-locked lasers.

physics.optics↗

arXiv2021

Compact, spatial-mode-interaction-free, ultralow-loss, nonlinear photonic integrated circuits

Nonlinear photonics based on integrated circuits has enabled applications such as parametric amplifiers, soliton frequency combs, supercontinua, and non-reciprocal devices. Ultralow optical loss and the capability for dispersion engineering are essential, which necessitate the use of multi-mode waveguides. Despite that rich interaction among different spatial waveguide eigenmodes can give rise to novel nonlinear phenomena, spatial mode interaction is typically undesired as it increases optical loss, perturbs local dispersion profile, and impedes soliton formation. Adiabatic bends, such as Euler bends whose curvature varies linearly with their path length, have been favoured to suppress spatial mode interaction. Adiabatic bends can essentially connect any two waveguide segments with adiabatic mode conversion, thus efficiently avoid mode mixing due to mode mismatch. However, previous works lack quantitative measurement data and analysis to fairly evaluate the adiabaticity, and are not based on photonic integrated circuits with tight optical confinement and optical losses of few dB/m. Here, we adapt, optimize, and implement Euler bends to build compact racetrack microresonators based on ultralow-loss, multi-mode, silicon nitride photonic integrated circuits. The racetrack microresonators feature a small footprint of only 0.21 mm^2 for 19.8 GHz FSR. We quantitatively investigate the suppression of spatial mode interaction in the racetrack microresonators with Euler bends. We show that the optical loss rate (15.5 MHz) is preserved, on par with the mode interaction strength (25 MHz). This results in an unperturbed microresonator dispersion profile. We further demonstrate single soliton of 19.8 GHz repetition rate. The optimized Euler bends and racetrack microresonators can be key building blocks for integrated nonlinear photonic systems, programmable processors and photonic quantum computing.

physics.optics↗