A photonic device (100, 200, 300, 400) comprising: a coupler (110, 210, 320, 420, 514) having two inputs (112, 114, 212, 214) and two outputs (116, 118, 216, 218, 322, 324, 422, 424) making a total of four inputs (112, 114, 212, 214) and outputs (116, 118, 216, 218, 322, 324, 422, 424); a plurality of photodetectors (132, 134, 136, 232, 232a, 234, 236, 362, 364, 366, 462, 464) coupled to three of the four inputs (112, 114, 212, 214) and outputs (116, 118, 216, 218, 322, 324, 422, 424) and configured to provide signals indicative of power of light at each of the three of the four inputs (112, 114, 212, 214) and outputs (116, 118, 216, 218, 322, 324, 422, 424); and a processing section (140, 240, 350, 450) configured to receive the signals provided by the plurality of photodetectors (132, 134, 136, 232, 232a, 234, 236, 362, 364, 366, 462, 464); characterized in that the photonic device (100, 200, 300, 400) further comprises: a further coupler (340, 440) connected in series with the coupler (110, 210, 320, 420, 514) and having two further inputs and two further outputs (442, 444) for a total of four further inputs and outputs (442, 444); and a set of one or more further photodetectors (342, 466, 468) coupled to a respective one or more of the four further inputs and outputs (442, 444) of the further coupler (340, 440) and configured to provide signals indicative of power of light at each of the respective one or more of the four further inputs and outputs (442, 444) of the further coupler (340, 440), wherein signals provided by the set of further photodetectors (342, 466, 468) are provided to the processing section (140, 240, 350, 450). a coupler (110, 210, 320, 420, 514) having two inputs (112, 114, 212, 214) and two outputs (116, 118, 216, 218, 322, 324, 422, 424) making a total of four inputs (112, 114, 212, 214) and outputs (116, 118, 216, 218, 322, 324, 422, 424); a plurality of photodetectors (132, 134, 136, 232, 232a, 234, 236, 362, 364, 366, 462, 464) coupled to three of the four inputs (112, 114, 212, 214) and outputs (116, 118, 216, 218, 322, 324, 422, 424) and configured to provide signals indicative of power of light at each of the three of the four inputs (112, 114, 212, 214) and outputs (116, 118, 216, 218, 322, 324, 422, 424); and a processing section (140, 240, 350, 450) configured to receive the signals provided by the plurality of photodetectors (132, 134, 136, 232, 232a, 234, 236, 362, 364, 366, 462, 464); characterized in that the photonic device (100, 200, 300, 400) further comprises: a further coupler (340, 440) connected in series with the coupler (110, 210, 320, 420, 514) and having two further inputs and two further outputs (442, 444) for a total of four further inputs and outputs (442, 444); and a set of one or more further photodetectors (342, 466, 468) coupled to a respective one or more of the four further inputs and outputs (442, 444) of the further coupler (340, 440) and configured to provide signals indicative of power of light at each of the respective one or more of the four further inputs and outputs (442, 444) of the further coupler (340, 440), wherein signals provided by the set of further photodetectors (342, 466, 468) are provided to the processing section (140, 240, 350, 450). a further coupler (340, 440) connected in series with the coupler (110, 210, 320, 420, 514) and having two further inputs and two further outputs (442, 444) for a total of four further inputs and outputs (442, 444); and a set of one or more further photodetectors (342, 466, 468) coupled to a respective one or more of the four further inputs and outputs (442, 444) of the further coupler (340, 440) and configured to provide signals indicative of power of light at each of the respective one or more of the four further inputs and outputs (442, 444) of the further coupler (340, 440), wherein signals provided by the set of further photodetectors (342, 466, 468) are provided to the processing section (140, 240, 350, 450).