A multi-wavelength distributed Bragg reflector semiconductor laser comprising a waveguide defining an axis of optical propagation, a wavelength selective DBR section, a plurality of DBR heating elements, a gain section, and a laser substrate, wherein: the axis of optical propagation of the waveguide extends between the front and rear facets of the laser diode and passes through the DBR section and the gain section of the laser; the DBR heating elements are positioned over the waveguide in the DBR section and are spaced in succession along the axis of optical propagation by an intra-grating spacing d to define thermally active grating portions in the DBR section and thermally passive grating portions in the DBR section; each thermally active grating portion defines a first temperature dependent Bragg wavelength that is a function of the grating period of the thermally active grating portion, each thermally passive grating portion defines a temperature dependent Bragg wavelength that is different to the first temperature dependent Bragg wavelength and that is a function of the grating period of the thermally passive grating portion, the waveguide and the laser substrate of the wavelength selective DBR section contribute to define the laser chip thickness b; and the intra-grating spacing d is approximately the same order of magnitude as the laser chip thickness b, wherein said DBR heating elements define an interleaved temperature profile that generates multiple distinct reflection peaks corresponding to distinct temperature dependent Bragg wavelengths associated with the temperature profile, such that the multi-wavelength DBR semiconductor laser is configured to emit at least two wavelengths simultaneously. the axis of optical propagation of the waveguide extends between the front and rear facets of the laser diode and passes through the DBR section and the gain section of the laser; the DBR heating elements are positioned over the waveguide in the DBR section and are spaced in succession along the axis of optical propagation by an intra-grating spacing d to define thermally active grating portions in the DBR section and thermally passive grating portions in the DBR section; each thermally active grating portion defines a first temperature dependent Bragg wavelength that is a function of the grating period of the thermally active grating portion, each thermally passive grating portion defines a temperature dependent Bragg wavelength that is different to the first temperature dependent Bragg wavelength and that is a function of the grating period of the thermally passive grating portion, the waveguide and the laser substrate of the wavelength selective DBR section contribute to define the laser chip thickness b; and the intra-grating spacing d is approximately the same order of magnitude as the laser chip thickness b, wherein said DBR heating elements define an interleaved temperature profile that generates multiple distinct reflection peaks corresponding to distinct temperature dependent Bragg wavelengths associated with the temperature profile, such that the multi-wavelength DBR semiconductor laser is configured to emit at least two wavelengths simultaneously.