The output power of a laser beam is increased without decreasing the beam quality, whilst still maintaining polarization.
For this several single lasers, mostly laser diodes, are arranged in external laser resonators which share the final section in front of the outcoupling mirror. By wisely choosing spectral filtering elements, all lasers oscillate on interlaced spectral combs of longitudinal laser modes. The result is an increase in output power by a factor of about 1.8 per coupling stage.
The initial beam has basically unchanged beam parameters (diameter, divergence, beam quality, beam propagation factor M²). Despite internal polarization coupling, the resulting beam is polarized at the exit mirror.
An array of laser resonators is built so that they share the outcoupling mirror. One or more Lyot-filters separate different frequency components into the different channels which are each driven by a separate gain element.
From this arrangement of filters every diode laser can only oscillate on the wavelength that it is assigned to by the filter.