Coherent Combining Technique Of Fiber Lasers With Common Cavity Coupling Configuration

The laser sources with high brightness and high average power have significant applications in many fields, such as industrial machining, free space communications, laser radar and directed energy. Coherent beam combining of multiple fiber lasers has been expected to be an important technique to obtain a laser source of this kind, and coherent combining of multiple fiber lasers with common cavity coupling configuration is an effective approach by passive self-adjusting process. The passive self-adjusting coherent combining schemes usually own simple configuration and are easy to implement. However, the combined coherent array based on the above mechanism often suffers from the poor stability, combining efficiency and expandability. A novel phase locking scheme of fiber lasers is proposed based on common ring coupled cavity and single mode fiber filtering, which provides a new and effective method to improve the stability, efficiency and expandability of the general passive coherent array. Coherent combining of fiber lasers utilizing Michelson-type coupled cavity is a scheme based on common external cavity coupling, which owns the properties of both common external cavity coupling and collinear interferometric schemes. These two coherent combining schemes of fiber lasers with common cavity coupling configuration are investigated theoretically and experimentally in this paper, and the main achievements are listed as follows:1. The coherent combining scheme of fiber lasers based on the common fiber ring coupled cavity is proposed. Thanks to the all-fiber structure of ring coupled cavity and multiple parallel emitting ports, the stability, efficiency and expandability of combined array are improved evidently. Phase locking of three fiber lasers are demonstrated by using this scheme, and nearly 90% combining efficiency is obtained. The common ring coupled cavity is also employed to lock the phases of two ring cavity lasers and tunable fiber lasers, and the phase locked output with relatively high efficiency is also obtained, which indicates that the proposed phase locking scheme is compatible with ring cavity and tunable fiber lasers.2. The properties of the common fiber ring cavity are analyzed theoretically, and the basic principles when design a suitable common ring coupled cavity are summarized. Some crucial conclusions obtained from the theoretical analysis and experimental study are presented as follows: the optimized coupling ratio of constructing the ring cavity is in the range of 20% ~ 30%, and the small derivation of coupling ratios has little effect on the phase locking results.3. The single-mode fiber filtering technique is introduced into the proposed phase locking scheme with a common cavity, and relatively stable phase relation is formed among the multiple laser emitters. Both the actions of controlling the far-field intensity distribution and improving the phase locking stability are investigated, and the necessity of adopting the spatial filtering and mode selection measures when utilizing the side-by- side combining methods to realize efficient coherent combining are pointed out.4. The Michelson-type coupled cavity with an f‐f?configuration is presented, and its power transmission properties are analyzed theoretically, and the cavity errors which need to be paid more attention to decrease them when adjust the cavity are discovered. Moreover, coherent combining of two fiber lasers using the Michelson-type coupled cavity with an f‐f? configuration is demonstrated under low power condition, and the cavity's functions of enhancing the energy coupling among individual fiber lasers and improving the combining efficiency are also verified in experiment.5. The phase shift properties of the beam splitter in Michelson-type coupled cavity are studied by the eigen-martix analyzing method in Optics, and the general relationship between the phase shifts of two splitted beams used for coherent combining is obtained, and then the experimental phenomenon that when the constructive interference occurs at one side of the beam splitter and the destructive interference has to occur at its opposite side is explained according to its physical mechanism.