Research On Mode Control Technology Of Diode Laser Fiber Combining

The development of high-energy industrial processing,optical fiber communication,and military weapons requires high-quality fundamental mode diode lasers.High quality means high stability,high power density,and high mode purity.A single diode laser device has defects such as low unit power and large divergence angle.It is necessary to use photosynthetic beam and mode control technology to improve the power and quality of diode laser.In response to this difficulty,diode laser beam combining technology based on fiber beam combining devices came into being.As one of the core devices of this technology,the photonic lantern can realize low-loss coupling between single-mode and multi-mode systems.At the same time,the single-channel input light is easy to control,and it is easy to realize coherent beam combining to obtain fundamental mode diode laser output.However,with the increase of the combined beam power,the accumulation of heat load in the high-power core diameter will lead to two kinds of harmful effects in the fiber.One is the energy transfer between the low order mode and the high order mode in the fiber,resulting in the oscillation of the output transverse mode.The second is the contraction of the effective mode field area.The thermal effect of fiber will lead to the increase of beam combining loss and transverse mode oscillation effect,which makes the beam quality worse.Therefore,it is necessary to apply more sensitive dynamic control to the input diode laser to suppress its influence.As an application branch of adaptive optics technology,the active mode control system that can realize multi-parameter regulation is very suitable for mode regulation in the process of photonic lantern photosynthetic beam.It can monitor the power of the fixed position of the output light field in real time,continuously adjust the parameters such as the phase,amplitude and polarization of the input light,and maintain the phase matching of the multi-channel input light based on the negative feedback adjustment,so as to realize the high-speed dynamic modulation of the beam combining mode,suppress the influence of the transverse mode oscillation effect,realize the low-loss diode laser coherent beam combining,and obtain the high-power,high fundamental mode composition ratio and good stability high-quality diode laser output.Therefore,this paper conducts theoretical,simulation and experimental research on mode control in diode laser fiber combining technology based on photonic lanterns.The specific contents and innovations are as follows:In the first chapter,this paper introduces the research level of mode control technology of diode laser fiber beam combining at home and abroad.It is pointed out that although the existing beam combining technology can achieve high power output,the influence of transverse mode oscillation effect on the output light quality still exists,and the further improvement of output light power is hindered.At the same time,there is also a lack of evaluation methods for the mode transition process in diode laser fiber combining technology,and it is impossible to further suppress the transverse mode oscillation effect theoretically.Based on this,the research content of this paper is proposed : the active mode control system based on high-speed modulation realizes the mode control of the combined light in the photonic lantern,and realizes stable highquality diode laser output;a method for analyzing the mode control fluctuation of photonic lanterns is proposed.Based on the gray matrix extraction algorithm,the mode control performance of photonic lanterns on the combined beam is analyzed when the mode oscillation exists,which provides theoretical and experimental basis for further improving the stability of the combined beam.In the second chapter,the theory of mode control technology for diode laser beam combining is introduced.Several types of technology and application advantages and disadvantages of mode control in diode laser beam combining process are analyzed.The control algorithm,theory and parameter optimization direction of active mode control system are mainly introduced.The stochastic parallel gradient descent(SPGD)algorithm is optimized and used as the core of the control system.At the same time,several key indicators for evaluating the regulation effect of the mode are compared,and the shortcomings of the existing technology are pointed out.The Gaussian fitting degree and the variance of the angular power distribution are selected to evaluate the fundamental mode composition ratio and stability of the light field,respectively,as the main evaluation indicators for subsequent experiments.In the third chapter,a gray matrix extraction algorithm is innovatively proposed to measure and evaluate the change of the mode control ability of the photonic lantern to the combined beam when the mode oscillation is strong and the beam stability is poor,so as to evaluate the mode control performance of the photonic lantern and facilitate the next step to improve the mode stability of the combined beam.Its reliability is verified from the theoretical basis and the numerical accuracy of the gray matrix.At the same time,the advantages of the gray matrix extraction algorithm are compared with the previous mode control performance evaluation methods of photosynthetic beam devices.The algorithm can achieve real-time performance measurement,and the measurement method is simple.The reliability of image analysis is as high as 98 %.Finally,the algorithm is used to evaluate the effect of mode control in the process of diode laser beam combining,which explains the fluctuation of output light quality well,indicating that the algorithm is stable and reliable.In the fourth chapter,the optimized active mode control system is used to control the mode of diode laser beam combining in photonic lanterns to achieve stable fundamental mode profile beam combining output.Firstly,the diode laser beam combining process in the photonic lantern is simulated,and the mode-controlled photosynthetic beam experiment based on the photonic lantern is carried out.Based on the Finite Difference Beam Propagation Method(FD-BPM),the optical coupling process of 3 × 1 non-mode-selected photonic lanterns is simulated.The ratio of the beam combining loss to the fundamental mode component of the output light under different tapers is compared,and the parameters are optimized according to the results.A photonic lantern structure with a taper of 0.12 is selected for diode laser beam combining.The transmission loss is 0.45 d B,and the output optical fundamental mode power ratio is 30 %.The experimental device of diode laser beam combining based on photonic lantern is designed and built.The optimized SPGD algorithm is used as the control core,and the active mode control is used to realize the three subgrade mode diode laser beam combining and mode control.The power fluctuation of the output light before and after the control system is turned on is compared.The experimental results show that the optimized active mode control system can realize the mode control of the diode laser beam combining process in the photonic lantern,and the stability and quality of the output beam combining light are greatly improved.Considering that the photonic lantern itself has mode loss and insertion loss,the actual measured beam combining loss of the whole optical path is less than 2.7 d B.The far-field spot profile of the output beam is a stable fundamental mode shape,the Gaussian fitting degree is 0.92,and the variance fluctuation of the angular power distribution of the optical field is less than 7 %.