A photonic integrated circuit (100, 200), comprising: a first waveguide (102, 202) on a first waveguide substrate (104, 204), a second waveguide (112, 212) on a second waveguide substrate (114, 214), a resonator device, comprising: a first mirror (120, 220), comprising a first layered structure (122, 222) on a first mirror substrate (124, 224), wherein the first mirror substrate (124, 224) is provided by or fixed to the first waveguide substrate (104, 204) or the second waveguide substrate (114, 214), a second mirror (130, 230), comprising a second layered structure (132, 232) on a second mirror substrate (134, 234), wherein the second mirror substrate (134, 234) is provided by or fixed to the first waveguide substrate (104, 204) or the second waveguide substrate (114, 214); wherein the first mirror (120, 220) and the second mirror (130, 230) form end mirrors of an optical resonator, and wherein at least a portion (106, 206) of the first waveguide (102, 202) is within the optical resonator; wherein the first waveguide substrate (104, 204) is continuous with the second waveguide substrate (114, 214), and wherein the first waveguide (102, 202) at a first end (108, 208) of the first waveguide is aligned with the second waveguide (112, 212) at a second end (118, 218) of the second waveguide (112, 212) characterised in that the optical resonator and the second waveguide (112, 212) are aligned with respect to one another, and in that a portion of the first layered structure (122, 222) or the second layered structure (132, 232) is embedded within a continuous body formed by the first waveguide substrate (104, 204) and the second waveguide substrate (114, 214). a first waveguide (102, 202) on a first waveguide substrate (104, 204), a second waveguide (112, 212) on a second waveguide substrate (114, 214), a resonator device, comprising: a first mirror (120, 220), comprising a first layered structure (122, 222) on a first mirror substrate (124, 224), wherein the first mirror substrate (124, 224) is provided by or fixed to the first waveguide substrate (104, 204) or the second waveguide substrate (114, 214), a second mirror (130, 230), comprising a second layered structure (132, 232) on a second mirror substrate (134, 234), wherein the second mirror substrate (134, 234) is provided by or fixed to the first waveguide substrate (104, 204) or the second waveguide substrate (114, 214); wherein the first mirror (120, 220) and the second mirror (130, 230) form end mirrors of an optical resonator, and wherein at least a portion (106, 206) of the first waveguide (102, 202) is within the optical resonator; wherein the first waveguide substrate (104, 204) is continuous with the second waveguide substrate (114, 214), and wherein the first waveguide (102, 202) at a first end (108, 208) of the first waveguide is aligned with the second waveguide (112, 212) at a second end (118, 218) of the second waveguide (112, 212) characterised in that the optical resonator and the second waveguide (112, 212) are aligned with respect to one another, and in that a portion of the first layered structure (122, 222) or the second layered structure (132, 232) is embedded within a continuous body formed by the first waveguide substrate (104, 204) and the second waveguide substrate (114, 214). a first mirror (120, 220), comprising a first layered structure (122, 222) on a first mirror substrate (124, 224), wherein the first mirror substrate (124, 224) is provided by or fixed to the first waveguide substrate (104, 204) or the second waveguide substrate (114, 214), a second mirror (130, 230), comprising a second layered structure (132, 232) on a second mirror substrate (134, 234), wherein the second mirror substrate (134, 234) is provided by or fixed to the first waveguide substrate (104, 204) or the second waveguide substrate (114, 214); wherein the first mirror (120, 220) and the second mirror (130, 230) form end mirrors of an optical resonator, and wherein at least a portion (106, 206) of the first waveguide (102, 202) is within the optical resonator; wherein the first waveguide substrate (104, 204) is continuous with the second waveguide substrate (114, 214), and wherein the first waveguide (102, 202) at a first end (108, 208) of the first waveguide is aligned with the second waveguide (112, 212) at a second end (118, 218) of the second waveguide (112, 212) characterised in that the optical resonator and the second waveguide (112, 212) are aligned with respect to one another, and in that a portion of the first layered structure (122, 222) or the second layered structure (132, 232) is embedded within a continuous body formed by the first waveguide substrate (104, 204) and the second waveguide substrate (114, 214).