A tunable laser device comprising: a tunable laser section configured to generate light at a lasing wavelength, wherein the tunable laser section comprises an optical cavity for tuning the lasing wavelength within a tuning range spanning from a first wavelength to a second wavelength, wherein the second wavelength is longer than the first wavelength; a tunable transmission optical filter disposed outside of the optical cavity and downstream of the tunable laser section , wherein the tunable transmission optical filter comprises: a passband configured to transmit light at the lasing wavelength, and a stopband configured to attenuate light at a sidelobe wavelength of the tunable laser section , wherein the sidelobe wavelength is different from the lasing wavelength, and wherein the lasing wavelength and the sidelobe wavelength are within the tuning range; a semiconductor optical amplifier, SOA, section optically coupled to and downstream of the tunable transmission optical filter , wherein the SOA section has an amplification band that comprises the tuning range; and characterised in that the tunable transmission optical filter comprises an asymmetric Mach-Zehnder waveguide interferometer tunable to have a transmission maximum at the lasing wavelength, and the centre wavelength of the stopband is close to the sidelobe wavelength. a tunable laser section configured to generate light at a lasing wavelength, wherein the tunable laser section comprises an optical cavity for tuning the lasing wavelength within a tuning range spanning from a first wavelength to a second wavelength, wherein the second wavelength is longer than the first wavelength; a tunable transmission optical filter disposed outside of the optical cavity and downstream of the tunable laser section , wherein the tunable transmission optical filter comprises: a passband configured to transmit light at the lasing wavelength, and a stopband configured to attenuate light at a sidelobe wavelength of the tunable laser section , wherein the sidelobe wavelength is different from the lasing wavelength, and wherein the lasing wavelength and the sidelobe wavelength are within the tuning range; a semiconductor optical amplifier, SOA, section optically coupled to and downstream of the tunable transmission optical filter , wherein the SOA section has an amplification band that comprises the tuning range; and characterised in that the tunable transmission optical filter comprises an asymmetric Mach-Zehnder waveguide interferometer tunable to have a transmission maximum at the lasing wavelength, and the centre wavelength of the stopband is close to the sidelobe wavelength. a passband configured to transmit light at the lasing wavelength, and a stopband configured to attenuate light at a sidelobe wavelength of the tunable laser section , wherein the sidelobe wavelength is different from the lasing wavelength, and wherein the lasing wavelength and the sidelobe wavelength are within the tuning range; a semiconductor optical amplifier, SOA, section optically coupled to and downstream of the tunable transmission optical filter , wherein the SOA section has an amplification band that comprises the tuning range; and characterised in that the tunable transmission optical filter comprises an asymmetric Mach-Zehnder waveguide interferometer tunable to have a transmission maximum at the lasing wavelength, and the centre wavelength of the stopband is close to the sidelobe wavelength. <EOS>