Apparatus for combining laser radiation (1) 5 which apparatus comprises a seed laser (2), a splitter (3), a plurality of amplifier chains (4), a reference amplifier chain (7), detection means (5). demodulator means (6), and phase control means (12), wherein each of the amplifier chains (4) comprises at least one optical amplifier (11), optical radiation (17) emitted from the seed laser (2) is split into the plurality of amplifier chains (4) by the splitter (3). amplified by the plurality of amplifier chains (4), interfered, detected by the detection means (5), demodulated by the demodulator means (6), and a signal (27) indicative of path length imbalance (14) fed back to the phase control means (12), and wherein the apparatus is characterized in that the output power emitted by each amplifier chain (4) is at least 50 W, the bandwidth (8) of the seed laser (2) is at least 1 GHz, and the path length difference (14) between each amplifier chain (4) and the reference amplifier chain (7) is less than the coherence length (10) of the seed laser (2).