External cavity laser with multiple materials micro-ring reflectors

An external cavity laser, comprising:

• a photonic integrated platform comprising a gain chip providing a gain medium and at least one reflector chip;
• a resonant cavity optically coupled to the gain medium and comprising a first and a second micro-ring resonators (MRR) in a Vernier configuration provided in the at least one reflector chip, the first MRR and second MRR being made of different materials and having corresponding different group indices, the first and second MRRs having different radii selected in view of said corresponding different group indices, wherein the at least one reflector chip consists of a monolithic reflector chip integrating first and second functional layers respectively comprising the first and second MRRs;
• a tuning mechanism for tuning a spectral response of at least one of the first and second MRR;
• and
• a waveguide structure defining said resonant cavity, the waveguide structure comprising:
• the first and second MRRs;
• one or more waveguide branches extending in each functional layer;
• and
• one or more optical vias allowing light to travel between the functional layers of the monolithic reflector chip.