Sub-kHz-Linewidth External-Cavity Laser (ECL) With Si₃N₄ Resonator Used as a Tunable Pump for a Kerr Frequency Comb

Abstract

Combining optical gain in direct-bandgap III-V materials with tunable optical feedback offered by advanced photonic integrated circuits is key to chip-scale external-cavity lasers (ECL), offering wideband tunability along with low optical linewidths. External feedback circuits can be efficiently implemented using low-loss silicon nitride (Si3N4) waveguides, which do not suffer from two-photon absorption and can thus handle much higher power levels than conventional silicon photonics. However, co-integrating III-V-based gain elements with tunable external feedback circuits in chip-scale modules still represents a challenge, requiring either technologically demanding heterogeneous integration techniques or costly high-precision multi-chip assembly, often based on active alignment. In this work, we demonstrate Si3N4-based hybrid integrated ECL that exploit 3D-printed structures such as intra-cavity photonic wire bonds and facet-attached microlenses for low-loss optical coupling with relaxed alignment tolerances, thereby overcoming the need for active alignment while maintaining the full flexibility of multi-chip integration techniques. In a proof-of-concept experiment, we demonstrate an ECL offering a 90 nm tuning range (1480 nm-1570 nm) with on-chip output powers above 12 dBm and side-mode suppression ratios of up to 59 dB in the center of the tuning range. We achieve an intrinsic linewidth of 979 Hz, which is among the lowest values reported for comparable feedback architectures. The optical loss of the intra-cavity photonic wire bond between the III-V gain element and the Si3N4-based tunable feedback circuit amounts to approximately (1.6 ± 0.2) dB. We use the ECL as a tunable pump laser to generate a dissipative Kerr soliton frequency comb. To the best of our knowledge, our experiments represent the first demonstration of a single-soliton Kerr comb generated with a pump that is derived from a hybrid ECL.

Turnkey locking of quantum-dot lasers directly grown on Si

“…It used to be a challenge to reduce the QW laser linewidth to the hertz level with a low-Q external cavity, due to the weak coherent feedback that it offers. In general, the coherence of QW external-cavity lasers is more than one order of magnitude inferior to the high-Q SIL lasers [32][33][34] . Such a performance gap is attributed to the coherence collapse of the QW laser under strong external feedback.…”

Low loss silicon nitride photonics for short wavelengths

“…In this way, flexible Vernier ring lasers can be achieved with high side-mode suppression. [7][8][9][10][11][12].…”

Review of 1.55 μm Waveband Integrated External Cavity Tunable Diode Lasers

“…In 2023, Pascal Maier et al [31] reported an ultra-narrow-linewidth tunable external cavity diode laser based on MZI and MRR using a Si 3 N 4 waveguide combined with 3Dprinted structures to reduce the feedback and coupling losses of the device. The laser could be continuously tuned to 90 nm with a linewidth as low as 979 Hz.…”