Coherent Beam Combining Through Mutual Injection Locking of Individual Lasers

Beam-combining is a technology for generation of a single high power beam from individual laser modules. Coherent beam-combining has the advantage that the combined beam has a smaller divergence angle and an increased brightness at the lasing wavelength. In this dissertation, coherent beam-combining of multiple continuous-wave individual lasers that are coupled through an external combining section is studied. The mutual coherence among multiple laser beams is achieved through mutual injection-locking, which can be viewed as the formation of the compound-cavity mode. Mutual injection-locking of two individual lasers is demonstrated experimentally. With large difference in the cavity-lengths of individual lasers and low Q-factors for individual lasers, mutual injection-locking is self-sustainable without active stabilization. This is due to the self-formation of the compound-cavity mode. Three individual lasers are experimentally combined through mutual injection-locking in the Michelson interferometer configuration. Under mutual injection-locking, not only a single longitudinal compound-cavity mode is formed with maximum output power, but also a single coherent spatial mode is formed which leads to a smaller divergence in the combined beam. A theoretical model is developed to study mutual injection-locking of two individual lasers, where the combining section is a resonant cavity and the coupling between two lasers is frequency-dependent. Thus evolution of circulation fields in frequency domain is necessary. Full evolution model in three sections is needed when lengths of three cavities are roughly equal. Two reduced models, with combining section much shorter than individual lasers and acting as an etalon, or with individual lasers much shorter than combining section and acting as regenerative amplifiers, are also studied. The lockable detuning ranges is obtained analytically and is determined by the condition of a single maximum in roundtrip net gain profile of the circulation field in the combining section, and numerical simulation in the three cavity-length cases confirm it. The lower the Q-factors of the individual lasers are, the larger the lockable detuning range is. The detuning of the combining section only influences the location of the compound-cavity mode, so that a separate active control of the combining section is not necessary