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.