A method for converting the optical frequency of a single-mode, single-frequency laser source includes generating a linearly-polarized single-frequency seed laser signal at a fundamental frequency; amplifying the seed laser signal in an optical amplifier; coupling the amplified signal into an external cavity formed by at least two mirrors, one of the mirrors being installed on a piezo actuator; enhancing the coupled signal inside the external cavity by locking a resonance frequency of the cavity to the frequency of the seed laser by continuously adjusting the optical length of the cavity with the piezo actuator; and converting the frequency of the enhanced signal using a non-linear crystal placed in a beam path inside the cavity. A method for converting the optical frequency of a single-mode, single-frequency laser source, includes generating a linearly polarized single-frequency seed laser signal at a fundamental frequency, the seed laser being a diode laser configured to adjust the frequency in response to controllable pump current variations; amplifying the seed laser signal in an optical amplifier; coupling the amplified signal into an external cavity formed by at least two mirrors; enhancing the coupled signal inside the external cavity by locking the frequency of the diode seed laser to a resonance frequency of the cavity by continuously adjusting the pump current of the diode seed laser; and converting the frequency of the enhanced signal using a non-linear crystal placed in a beam path inside the cavity.