General Discussion On Phase Locking Of Broad Stripe Diode Arrays By External Cavity

Laser diode array (LDA) is characterized by its compactness and high efficiency. It has been widely used in free-space optical communications, solid lasers pumping, and material processing. But LDA generally operates in multiple lateral modes, and the outputs of its individual emitters are incoherent because of it's broad stripe emitters, which worsen the output quality of LDA, and therefore LDA's application in other fields has been limited. The phase locking of LDA is a potential method to overcome the shortcoming, and it has become a hot topic in recent years.There have been many theories analyzing the mechanism of phase-locking of LDA by external cavity (EC), and the suitability of these theories has also been confirmed by experiments. But those theories are usually based on EC with specific parameters, and hence they cannot be generally used to analyze the phase locking mechanism in the universal sense. In this thesis, we have made a general discussion on LDAs of broad tripe emitters phase locked with ECs without referring to specific device parameters. For a phase locked LDA based on the diffraction coupling between the nearest neighbor emitters, analytical solutions have been derived for the coupling matrix equation, including the finite reflection of the front facet of the LDA facing the EC. Even though the thresholds of the super modes of anLDA are affected by the residual reflection at the front facet, it can still be proved analytically that the favored mode of the LDA is either the lowest or highest order super mode.A key factor, to some extent, determines the phase-locking of LDA is that the conformidity of the frequencies of the individual emitters. If the frequency differences becomes too large, it is difficult to phase lock the LDA. So the efficiency of heat sink is very important to phase-locking. A heat sink of low efficiency of will lead to large frequency excursions among emitters. In this work, the analytical expression of the temperature rise of the heat sink of an LDA has been obtained by solving the heat conduction equation. The influence of convection heat transfer coefficient on the temperature rise of the sink top and the bottom surface has been analyzed. The results obtained are in agree with that calculated with the finite element method and confirmed by the experiment results.In this work, phase-locking of an LDA has been achieved by using a simple EC consisting of a fast axis collimating lens and a high-reflection external mirror. The quality of the output beam has been improved. The far field pattern consists of 3 lobes, and the divergence of each lobe is about 10 mrad. The spectrum has been narrowed from 2.2nm to 0.3nm and the threshold current decreases from 7.0A to 5.5A with and without feedback respectively.