A laser apparatus comprising: - a first laser source operable to generate a first laser beam having a least a beam component having a first frequency; - a second laser source operable to generate a second laser beam having a least a beam component having a second frequency; - a beam combiner operable to combine the first and second laser beams into a combined initial laser beam comprising at least a frequency component having the first frequency, and a frequency component having the second frequency; - one or more nonlinear frequency mixers operable to perform a frequency mixing process of a frequency component having the first frequency and a frequency component having the second frequency and resulting in at least a frequency component having a third frequency equal to a sum or a difference of the first and second frequencies; wherein the laser apparatus is configured to direct the combined initial laser beam through a first one of the one or more nonlinear frequency mixers resulting in a first frequency-mixed beam, the first frequency-mixed beam comprising a frequency component having the first frequency, a frequency component having the second frequency, and a frequency component having the third frequency; wherein the laser apparatus is further configured to direct the resulting first frequency-mixed beam along an intermediate beam path to a subsequent nonlinear frequency mixer chosen from the first and another one of the one or more nonlinear frequency mixers, resulting in a second frequency-mixed beam comprising at least an output frequency component having at least said third frequency; characterized in that the laser apparatus comprises - an adjustable dispersive element configured to change an optical path length of the intermediate beam path of the first frequency-mixed beam so as to compensate for dispersion along at least the intermediate beam path; wherein the adjustable dispersive element is operable to adjust a relative phase relation between the frequency components having the first, the second, and the third frequencies of the first frequency-mixed beam. - a first laser source operable to generate a first laser beam having a least a beam component having a first frequency; - a second laser source operable to generate a second laser beam having a least a beam component having a second frequency; - a beam combiner operable to combine the first and second laser beams into a combined initial laser beam comprising at least a frequency component having the first frequency, and a frequency component having the second frequency; - one or more nonlinear frequency mixers operable to perform a frequency mixing process of a frequency component having the first frequency and a frequency component having the second frequency and resulting in at least a frequency component having a third frequency equal to a sum or a difference of the first and second frequencies; wherein the laser apparatus is configured to direct the combined initial laser beam through a first one of the one or more nonlinear frequency mixers resulting in a first frequency-mixed beam, the first frequency-mixed beam comprising a frequency component having the first frequency, a frequency component having the second frequency, and a frequency component having the third frequency; wherein the laser apparatus is further configured to direct the resulting first frequency-mixed beam along an intermediate beam path to a subsequent nonlinear frequency mixer chosen from the first and another one of the one or more nonlinear frequency mixers, resulting in a second frequency-mixed beam comprising at least an output frequency component having at least said third frequency; characterized in that the laser apparatus comprises - an adjustable dispersive element configured to change an optical path length of the intermediate beam path of the first frequency-mixed beam so as to compensate for dispersion along at least the intermediate beam path; wherein the adjustable dispersive element is operable to adjust a relative phase relation between the frequency components having the first, the second, and the third frequencies of the first frequency-mixed beam.