A photonic integrated circuit , comprising: a first waveguide on a first waveguide substrate , a second waveguide on a second waveguide substrate , a resonator device, comprising: a first mirror , comprising a first layered structure on a first mirror substrate , wherein the first mirror substrate is provided by or fixed to the first waveguide substrate or the second waveguide substrate , a second mirror , comprising a second layered structure on a second mirror substrate , wherein the second mirror substrate is provided by or fixed to the first waveguide substrate or the second waveguide substrate ; wherein the first mirror and the second mirror form end mirrors of an optical resonator, and wherein at least a portion of the first waveguide is within the optical resonator; wherein the first waveguide substrate is continuous with the second waveguide substrate , and wherein the first waveguide at a first end of the first waveguide is aligned with the second waveguide at a second end of the second waveguide characterised in that the optical resonator and the second waveguide are aligned with respect to one another, and in that a portion of the first layered structure or the second layered structure is embedded within a continuous body formed by the first waveguide substrate and the second waveguide substrate . a first waveguide on a first waveguide substrate , a second waveguide on a second waveguide substrate , a resonator device, comprising: a first mirror , comprising a first layered structure on a first mirror substrate , wherein the first mirror substrate is provided by or fixed to the first waveguide substrate or the second waveguide substrate , a second mirror , comprising a second layered structure on a second mirror substrate , wherein the second mirror substrate is provided by or fixed to the first waveguide substrate or the second waveguide substrate ; wherein the first mirror and the second mirror form end mirrors of an optical resonator, and wherein at least a portion of the first waveguide is within the optical resonator; wherein the first waveguide substrate is continuous with the second waveguide substrate , and wherein the first waveguide at a first end of the first waveguide is aligned with the second waveguide at a second end of the second waveguide characterised in that the optical resonator and the second waveguide are aligned with respect to one another, and in that a portion of the first layered structure or the second layered structure is embedded within a continuous body formed by the first waveguide substrate and the second waveguide substrate . a first mirror , comprising a first layered structure on a first mirror substrate , wherein the first mirror substrate is provided by or fixed to the first waveguide substrate or the second waveguide substrate , a second mirror , comprising a second layered structure on a second mirror substrate , wherein the second mirror substrate is provided by or fixed to the first waveguide substrate or the second waveguide substrate ; wherein the first mirror and the second mirror form end mirrors of an optical resonator, and wherein at least a portion of the first waveguide is within the optical resonator; wherein the first waveguide substrate is continuous with the second waveguide substrate , and wherein the first waveguide at a first end of the first waveguide is aligned with the second waveguide at a second end of the second waveguide characterised in that the optical resonator and the second waveguide are aligned with respect to one another, and in that a portion of the first layered structure or the second layered structure is embedded within a continuous body formed by the first waveguide substrate and the second waveguide substrate . <EOS>