A method of using at least a part of a cavity for a first and at least one second laser includes providing a first laser beam to the part of the cavity, splitting the first beam into a first and a second part, splitting the first part into a third and a fourth part, splitting the second part into a fifth and a sixth part, superimposing the third and the fifth part in a way causing extinguishing interference, superimposing the fourth and the sixth part in a way causing cumulative interference to provide a first resulting beam, and performing the above elements with at least one second laser beam provided to the same part of the cavity to provide a second resulting beam.