This project investigates a novel seeded pumping scheme for supercontinuum generation (SCG) in highly nonlinear AlGaAs waveguides. The work began with the design of AlGaAs waveguides to support broadband frequency comb generation. To extend the bandwidth of the SCG-based frequency comb, the dispersion of the nonlinear AlGaAs waveguide was carefully engineered to enable two dispersive wave phase-matched frequencies, spanning over an octave. This was accomplished through numerical mode simulation, where the sub-micron cross-sectional dimensions of the waveguide are tailored to control its dispersion properties. By mapping the group velocity dispersion (GVD) and effective mode area versus the waveguide geometries and analyzing the integrated dispersion over about 400 THz frequency range (assuming pumping within the telecom bands), waveguide designs that support dual dispersive wave generation across over an octave were identified. To get efficient dispersive wave generation, we compared the integrated dispersion profiles of the candidate designs and selected those with lowest dispersion barriers between the pump and dispersive wave frequencies for device fabrication.
AlGaAs waveguide samples were fabricated as part of this project. The fabrication process involved multiple steps including epitaxy growth of AlGaAs wafers, wafer bonding, substrate removal, electron-beam lithography and dry etching. Given the strong dependence of waveguide dispersion on device geometries - especially in high-index-contrast AlGaAs structures - the epitaxy growth and patterning processes were carefully optimized to achieve dimensional control with nanometer-scale precision. The etching process was also refined to produce smooth and vertical waveguide sidewalls, which are critical for minimizing linear losses and maintaining precise dispersion characteristics.
Proof of concept experiments were carried out using low-power, picosecond pulses to pump the fabricated waveguides. Dispersive wave frequencies were initially identified using conventional pulsed pumping with sub-picosecond pulses with high peak power. Following this, both continuous-wave (CW) and pulsed-seeded pumping schemes were evaluated on waveguides that support two dispersive wave generation. Under the CW-seeded pumping scheme, we achieved an octave-spanning SCG through dispersive wave generation with a tenfold reduction in the required peak compared with conventional pumping methods. The pulsed-seeded pumping scheme further reduced the threshold to sub-watt levels, well within the power range of integrated mode-locked lasers.
