A hybrid beam combining system or method combines a plurality of coherent and incoherent light beams into a composite high power diffraction limited beam. N oscillators each transmit light at one of N different wavelengths and each wavelength is split into M constituent beams. M beams in each of N groups are phase locked by a phase modulator using phase correction signals. The phase locked beams are amplified and coupled into an M×N fiber array. Beams emerging from the array are collimated and incident on a diffractive optical element operating as a beam combiner combining the M outputs at each N wavelength into a single beam. The N single beams are incident and spectrally combined on a grating which outputs a composite beam at a nominal 100% fill factor. A low power sample beam, taken from the N beams emerging from the diffractive optical element, is measured for phase deviations from which the phase correction signals are derived and fed back to the phase modulators. The diffractive optical element may include a weak periodic grating for diffracting the low power sample. The diffractive optical element may also be combined with the spectral combining grating into a single optical element.