Within the FEMTOCHIP project, we have developed a thick-waveguide SiN-based integration platform for photonic integrated circuits to enable the fabrication of complex circuitry based on silicon nitride. The new platform developed uses 400 nm thick SiN, named AN400. Components were for characterising this platform for quality and reliability purposes have been developed. On this new platform, spot size converters, grating couplers, multimode interferometers and directional couplers have been developed. This platform has been released and transferred to a 200 mm line where a 350 nm SiN thickness has been adopted (platform name AN350). On the 200 nm line several novel integrated photonic circuits were developed, most importantly a chip-sized sub-100 fs laser with intracavity pulse energy > 100 pJ and pulse timing jitter < 1 fs, with 100 mW output average power. Such a laser would be an exceptionally powerful platform for a multitude of applications. It can serve as a seed laser to build up complex high-power femtosecond laser sources. The coin-sized laser could act as the seed laser which is amplified to the required output power or energy. Being a laser with extremely low jitter, the laser can act as a clock and sampler for photonic-based analog-to-digital converters. This might bring timing and positioning systems to the next level. Also, lidar or photonic based transmission systems can be an application for the chip-scale laser source, which enables femtosecond laser frequency combs to be integrated on a chip. Although a functional on-chip femtosecond laser could not be demonstrated, several breakthrough developments towards the integrated mode locked laser were achieved. We have demonstrated the concept of large mode area (LMA) waveguides in integrated photonics-based Tm-doped Al2O3-waveguides leading to integrated amplifiers producing as much as 2 W output power directly from a chip. With similar amplifiers we demonstrated on chip femtosecond pulse amplification and on-chip Q-switched lasers producing high energy pulses similar to those from fiber lasers. We also fabricated the complete integrated femtosecond laser including high quality apodized chirped Bragg gratings for the necessary dispersion compensation up to the gain deposition step, which unfortunately was not available during the final period of the program, prohibiting the main demonstration planned for the project. In addition, a six stage on chip interleaver, based on the ultralow-loss SiN-process developed at EPFL, was demonstrated. Pulse repetition rate multiplication from 217 MHz to 13.9 GHz was demonstrated experimentally with very good spurious mode suppression.
Key takeaways:
· Aluminum oxide and Silicon nitride are excellent platforms for realizing miniaturized on-chip high-power lasers
· Important to focus on the most important building blocks to realize a demonstrator
· Plan enough time for machine downtime and/or second sources
· Mode-locked lasers are the most demanding class since all parameters have to be aligned to each other to realize stable lasing
· Quality of the production is elemental for realizing mode-locked lasers: low back reflection and low scattering losses are crucial
Forward-looking recommendations:
· The mode-locked laser is a basic building block for more innovations like on-chip frequency combs and other systems.
· Production scalability might be possible due to wafer scale processes with 200 mm wafers in the near future
The work within FEMTOCHIP will be continued in an EIC Transition project called femto-iCOMB. Please visit www.femto-icomb.eu for further information.
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