High-performance lasers for fully integrated silicon nitride photonics

Abstract

Silicon nitride (SiN) waveguides with ultra-low optical loss enable integrated photonic applications including low noise, narrow linewidth lasers, chip-scale nonlinear photonics, and microwave photonics. Lasers are key components to SiN photonic integrated circuits (PICs), but are difficult to fully integrate with low-index SiN waveguides due to their large mismatch with the high-index III-V gain materials. The recent demonstration of multilayer heterogeneous integration provides a practical solution and enabled the first-generation of lasers fully integrated with SiN waveguides. However, a laser with high device yield and high output power at telecommunication wavelengths, where photonics applications are clustered, is still missing, hindered by large mode transition loss, non-optimized cavity design, and a complicated fabrication process. Here, we report high-performance lasers on SiN with tens of milliwatts output power through the SiN waveguide and sub-kHz fundamental linewidth, addressing all the aforementioned issues. We also show Hertz-level fundamental linewidth lasers are achievable with the developed integration techniques. These lasers, together with high-Q SiN resonators, mark a milestone towards a fully integrated low-noise silicon nitride photonics platform. This laser should find potential applications in LIDAR, microwave photonics and coherent optical communications.

Chip-Based Laser with 1 Hertz Integrated Linewidth

“…This vision combines active (lasers, modulators, detectors), passive (filters, off-chip coupling), and nonlinear elements (frequency combs, frequency converters) while maintaining small overall volume [16][17][18][19][20][21][22][23][24][25][26][27]. In addition, heterogeneous silicon photonics offers a path towards realizing ultra-stable, high-precision laser performance in a compact and mobile platform and has demonstrated tremendous scalability with 300 mm , which significantly reduces its white frequency noise floor from that of a monolithic III-V DFB [10]. Self-injection locking to a high-Q Si3N4 spiral resonator further suppresses the frequency noise (2), ultimately limited by thermo-refractive noise (TRN) [11,12].…”

“…Silicon nitride (Si 3 N 4 ) photonics adds even more functionality, taking advantage of CMOS compatibility, wide bandgap, and lowloss integrated waveguides [31][32][33]. Si 3 N 4 -based lasers have especially leveraged low-loss [10,11,[34][35][36] and have demonstrated coherence on par with commercial fiber lasers [12]. However, they are ultimately limited by thermo-refractive noise (TRN), which has kept the fractional frequency instability of planar waveguide and solid dielectric resonators above the 10 −13 level typical of quartz oscillators [37].…”

“…Figure 1a shows images of the critical components used for the experiment. The pump source is a fully integrated and electrically driven heterogeneous III-V/Si/Si 3 N 4 laser, fabricated via wafer bonding on a 100 mm wafer [10,46]. With a 20 mm long Si 3 N 4 extended distributed Bragg reflector (E-DBR) fully integrated into the laser cavity, the laser exhibits an instantaneous linewidth of 400 Hz with an on-chip output power over 10 mW [10].…”

“…The pump source is a fully integrated and electrically driven heterogeneous III-V/Si/Si 3 N 4 laser, fabricated via wafer bonding on a 100 mm wafer [10,46]. With a 20 mm long Si 3 N 4 extended distributed Bragg reflector (E-DBR) fully integrated into the laser cavity, the laser exhibits an instantaneous linewidth of 400 Hz with an on-chip output power over 10 mW [10]. Due to the narrow-band feedback of the E-DBR, the instantaneous linewidth corresponding to the white frequency noise floor is significantly reduced (1b-1) compared to that of a solitary gain section laser typical of monolithic III-V DFB lasers [47,48].…”

“…For further noise suppression, the E-DBR laser was edge-coupled and self-injection locked to a high-Q Si 3 N 4 spiral resonator on a separate chip [10,12]. In this scheme, resonant backward Rayleigh scattering is fed back to the laser, which drastically suppresses the frequency noise at offset frequencies within the resonance linewidth (1b-2) [49][50][51].…”