Phase Locking Of A Laser Diode Array In An External Cavity

High power diode laser arrays (LDA) have considerable attention in the industrialzed economies for the small volume and high efficiency, and have been widely used in materials processing, laser medicine etc.Phase locking DLA had been developed remarkedly in 1980s. At that time, people had gotten the phase locking DLA through evanescent wave, leaky wave, Y-jont coupling and diffraction coupling. Usually the above mentioned measures are applicable to the traditional DLA with small emitters, and people had not gotten the ideal output power.According to the coupling mode theory and TALBOT cavity theory, we have made an in-depth theoretical analyses on the DLA phase locked in an external cavity (EC) . The phase locking may efficiently improve the radiating properties of the DLA and have good diverging degree, high coherence and scalability. The methods may result in the phase locking on the standard DLA with wide emitters. The integral TALBOT cavity length may not be able to provide sufficient discrimination between the highest and fundamental mode. We may suppress the fundamental mode and choose the highest mode by selecting an external cavity length of a quarter TALBOT distance. The drawback of the 1/4 Talbot cavity is that the LDA generates a double peak far field pattern. The frequency-selective phase-conjugate feedback (FSPCF) system may be used to phase lock the DLA in an EC to overcome the drawback of the double peak farfield in the 1/4 TALBOT cavity and to make full use of the pump sources.An emitter of a DLA positioned in an EC can receive the light emitted from its neighboring elements and that of itself after being reflected at the DLA facet. Expressions describing the received radiations from neighboring elements (C1) and that from itself(S)as soon as from the external cavity(C0)have been deduced. Considering the fact that | C0/S| should be larger than unify if the EC is of effective and | C1/S | should be larger than unify if the phase locking may be established in the EC, requirements on the reflection at the DLA facet have been specified in terms of the cavity length and reflection coefficient of the external mirror.Expressions describing couplings between different emitters of a diode laser array (DLA) , positioned in an external cavity (EC) , have been deduced. The results show that: the increase of the length of the EC reduce the differences between these coupling coefficients, which is favorable to achieving the so called "parallel coupling" at the expense of the effectiveness of the EC; decreasing the reflection coefficient at the front facet of the DLA is favorable to achieving phase locking in EC; the more the parallelly coupled emitters are, the lower the allowed reflection coefficient at the front facet of the DLA is.Through considering the output cavity mirror, fast axis collimation lens, chips welding, cooler designing and so on, we have widely understood the necessary consideration for the phase locking DLA in an EC.The thermal resistance of the DLA with back cooling packaged structure was calculated by three-dimension finite element method. The calculated results indicated that the cooling water channel depth and fin width have optimum values.