Research On Low Loss Pump Bundle Technology

As the essential components of high-power fiber lasers, the pump combiner not only has to endure high-power pump laser and signal laser, but also to achieve low coupling loss of pump laser and signal laser. The loss will reduce the coupling efficiency and increase the thermolysis burthen of the pump combiner when loading high-power pump laser or signal laser. In turn it will affect the performance of the pump combiner and even the whole laser system. This dissertation studies the technologies of low-loss pump combiner with details as below:1. The loss origins of the pump laser and signal laser in pump combiners are studied. We obtained the the expressions of the area mismatch loss, the axis mismatch loss, the angle mismatch loss and the numerical aperture mismatch loss, which are all related to the loss of pump laser. In the signal aspect, the transmission loss and the mode field mismatch loss are analyzed. The applications of thermally diffused expanded core (TEC) technology to reduce the model filed mismatch loss are also discussed.2. The numerical and experimental studies of fiber tapering with a moveable hydroxygen flame have been performed. The relations of the taper profile and parameters such as flame width, the ratio of flame velocity and the tapering velocity, and the taper length are discussed. We propose a method to optimize the transition region profile of the tapering zone by moving the flame back and forth once and give the prerequisite of flame width and moving distances. In the experiment, the length of the transition region was enlarged by using this method. As a result, the transmission loss of the input laser can be reduced.3.The signal losses in four end-pumped (N+1)×1type combiners with different tapering ratios are theoretically studied. The relations between the transition region length and the transmission loss of the fundamental mode are obtained through adiabatic approximation. We used eigen mode expansion method (EEM) to calculate the mode field mismatch loss of the fundamental mode and obtained the relations between the tapering ratio and the mode field mismatch loss. The transmission field distribution and loss of the LP11mode in tapering profile are discussed. The applications of TEC technology to reduce the mode field mismatch loss of the fundamental mode are studied and the relations of the mode mismatch loss and the heating time are obtained in four end-pumped (N+1)×1type combiners.4. We establish a model for side-pumped (N+1)×1type combiners. Under the frame of geometrical optics, we used Matlab to calculate the relations of the pump coupling efficiency and the tapering angel, the tapering length and the melting depth of the pump laser. This set up the guide for experiments.5. We propose a multiple-step method to fabricate end-pumped combiner and have fabricated a7×1type combiner. The combiner has a single arm endure power large than170W and average pump-coupling loss of0.07dB. The high order modes aroused at the melt point are absorbed by the polymer layer. When the loading power was712W, the temperature of the polymer layer raised to108℃. More work are deserved to improve the thermolysis performance and promote the endure power limit of the combiner.