Distributed-Feedback (DFB) laser stabilization using a ring resonator in silicon on insulator

As integrated photonics becomes more widely adopted for communication, sensing, and signal processing applications, stabilizing on-chip laser sources remains a critical challenge, especially in systems where isolators are not desired. Distributed Feedback (DFB) lasers are commonly used due to their low-cost and compact form factor. Still, their sensitivity to back-reflections from grating couplers, waveguide facets, and packaging interfaces can cause performance degradation when co-integrated with silicon photonic chips, including linewidth broadening and mode hopping. This thesis presents an experimental and modelling-based study on stabilizing a hybrid-integrated DFB laser using a single silicon ring resonator with a moderate quality factor (Q ≈ 34000). The laser is coupled to a silicon photonic chip via photonic wire bonding (PWB), and the external feedback path includes a ring resonator, a Mach-Zehnder interferometer (MZI) for feedback strength control, and a thermo-optic phase shifter for phase tuning. All components are fabricated in a standard SOI process without requiring high-Q structures or off-chip isolators. A time-delayed rate-equation model is developed to describe the effects of resonant optical feedback and feedback phase delay on laser dynamics. The model includes the impact of amplitude and phase feedback using an effective reflectivity formulation derived from the ring resonator’s response. Experimental validation includes optical and RF spectral analysis, relative intensity noise (RIN) measurements, and linewidth extraction using a delayed self-heterodyne method fitted with a Voigt profile. The measurements show that optical feedback from the ring reduces the laser linewidth by more than 5× under optimized biasing and phase conditions. Using phase control extends the locking range and improves tolerance to parasitic reflections introduced using an on-chip MZI. The combined feedback and phase tuning allow single-mode operation up to approximately –5 dB of on-chip parasitic reflected power without an isolator. The approach demonstrates a practical, fabrication-compatible path towards isolator-free laser stabilization using moderate-Q ring resonators. It is well suited for integration in silicon photonic systems where space and process constraints limit traditional high-Q or off-chip stabilization methods.