Periodic Reporting for period 1 - COMBCHIP (Highly-Efficient Seeded Frequency Comb Generation on a Chip)
Période du rapport: 2023-09-01 au 2025-02-28
Generating new colors of light through nonlinear optical processes is a fundamental driver of modern optical technologies. One key outcome is the optical frequency comb—a spectrally diverse, coherent light source with transformative applications in precision spectroscopy, energy-efficient communications, and optical atomic clocks. However, current comb systems are bulky and power-intensive, restricting their use to laboratory settings. The push to reduce the size, weight, and power (SWaP) of these sources has spurred progress in microcomb technologies, yet generating octave-spanning combs at microwave repetition rates using on-chip lasers remains a significant challenge. The COMBCHIP project addresses this by combining highly nonlinear AlGaAs waveguides with a novel seeded pumping scheme to develop ultra-efficient, broadband comb generators. This advancement will potentially enable compact, low-power comb systems for real-world applications such as satellite-based atomic clocks and mid-infrared spectroscopy.
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.
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.
We have successfully achieved the primary objective of this project: bridging the octave-spanning frequency comb with integrated laser technology. The prototype AlGaAs waveguides demonstrated broad spectral coverage exceeding one octave when pumped with picosecond pulses, which are compatible with integrated mode-locked lasers. Our exploration of the seeded pumping approach led to a significant reduction in the required pump power compared to conventional approaches – marking a major step toward practical, chip-scale comb systems. Under the CW-seeded pumping, the required peak power for generating dispersive waves was reduced to just a few watt, while the pulsed-seeded pumping brought the threshold down to sub-watt-level – well within the power range of integrated laser sources. Unlike conventional SCG, which typically requires high-peak-power femtosecond pulses, our seeded pumping scheme is independent of the pump pulse width, substantially relaxing the requirements on the pump laser and enhancing compatibility with chip-scale systems.