A system comprising: an integrated photonic circuit, PIC, comprising a tunable light source, and a wavelength locker comprising an asymmetric Mach-Zehnder interferometer, AMZI, (110, 200, 300) with an output coupler (112, 206, 306) having a plurality of output ports and, placed at the plurality of output ports, a plurality of respective photodetectors for measuring respective optical interference signals exiting the plurality of output ports when light is coupled from the light source into the AMZI, wherein the output coupler and the plurality of photodetectors are configured as a coherent receiver in which relative phase shifts imparted by the output coupler between two signals being interfered to form the optical interference signals differ between at least two of the output ports by a value that is not a multiple of 180°; memory storing one or more target filter parameters associated with a specified locking frequency of the light source; and electronic processing circuitry configured to compute one or more filter parameters from the measured optical interference signals and tune a frequency of the light source until the one or more computed filter parameters match the one or more target filter parameters. an integrated photonic circuit, PIC, comprising a tunable light source, and a wavelength locker comprising an asymmetric Mach-Zehnder interferometer, AMZI, (110, 200, 300) with an output coupler (112, 206, 306) having a plurality of output ports and, placed at the plurality of output ports, a plurality of respective photodetectors for measuring respective optical interference signals exiting the plurality of output ports when light is coupled from the light source into the AMZI, wherein the output coupler and the plurality of photodetectors are configured as a coherent receiver in which relative phase shifts imparted by the output coupler between two signals being interfered to form the optical interference signals differ between at least two of the output ports by a value that is not a multiple of 180°; a tunable light source, and a wavelength locker comprising an asymmetric Mach-Zehnder interferometer, AMZI, (110, 200, 300) with an output coupler (112, 206, 306) having a plurality of output ports and, placed at the plurality of output ports, a plurality of respective photodetectors for measuring respective optical interference signals exiting the plurality of output ports when light is coupled from the light source into the AMZI, wherein the output coupler and the plurality of photodetectors are configured as a coherent receiver in which relative phase shifts imparted by the output coupler between two signals being interfered to form the optical interference signals differ between at least two of the output ports by a value that is not a multiple of 180°; memory storing one or more target filter parameters associated with a specified locking frequency of the light source; and electronic processing circuitry configured to compute one or more filter parameters from the measured optical interference signals and tune a frequency of the light source until the one or more computed filter parameters match the one or more target filter parameters.