High-efficiency Coupling Technology For High-power Fiber Laser Shutters

With the development of optoelectronic technology,the output power of fiber laser is increasing.Fiber laser has been widely used in industrial processing,and the increasing output power also provides more possibilities for laser processing.The high power fiber laser is expensive.In practical application,a single fiber laser can be used as a light source for many processing equipments.Fiber to fiber optic switch(FFS)is one of the key components of the fiber laser beam transmission system.The laser beam output from a single fiber is coupled to different fibers for output by lens coupling,so that to realize the time-sharing multiplexing and increase the utilization of laser energy.In the study of FFS system,how to ensure the coupling efficiency of laser beam is a difficult problem.The high power laser is coupled into different output fibers through the inside optical system.The loss of coupling efficiency will not only cause the output power reduction,but also lead to a large amount of laser energy leaking into the inner and damage the device.Since the fiber core area is small,in order to efficiently couple the laser into the core,it is necessary to make beam parameters at the front surface of output fiber satisfy the coupling condition,and control the coupling error effectively to ensure the accurate matching of the fiber and beam's relative position.How to design an optical path system with numerical aperture and focal spot characteristics,which meets the requirements of high efficiency coupling,and when the internal structure of the optical gate is closed in the process of online use,how to monitor the coupling error in real time so as to effectively improve the coupling efficiency are the key issues in the study of FFS.According to the optical fiber transmission theory and coupling theory,the design of FFS optical system for time division multiplexing is proposed.Aiming at solving the parameters of laser beam transformed by optical system,using the characteristics of Gauss beam propagation and combining with the geometric optics theory,the transformation of laser beam through the optical system is analyzed,and the lens focal length ratio(M)is proposed.The influence of focal length ratio M on the parameters of laser beam is studied,which provides a convenient method for the parameters calculating of beam and lens.Finally,based on the above analysis,a coupling optical system with focal ratio M=1 and high coupling efficiency is designed.In order to reduce the coupling error reasonably,the field intensity distribution of Gauss beam near the front surface of output fiber is simulated,and the effect of different coupling error on coupling efficiency is studied.According to the variation of coupling efficiency with different coupling errors,a three-dimensional adjusting structure is designed to adjust the coupling error.Aiming at the problem of real-time enhance for coupling efficiency in FFS application,the principle of backward light wave produced by output fiber is studied,and the relationship between the strength of backward light wave and coupling error is obtained.A method to control the coupling error in real time is presented by monitoring the intensity of backward scattering light around the rear surface of output fiber.Set up experimental device for FFS,and experiment are carried out.The input fiber used in the experiment is LMA-GDF-20/400-M,the MM-S50/360-22 FA fiber is used as the output fiber.The focal length of the collimating and focusing optical systems is 50 mm.Adjusting the coupling error by mechanism,the relationship between coupling efficiency and different coupling errors is measured,which is consistent with the simulation results.The experiment of backscattering light measurement is designed.By measuring the curve of the scattering light intensity with the coupling error,it is proved that the method of reducing the coupling error by monitoring the scattering light intensity is feasible.When the maximum input laser power is 1009 W,the coupling efficiency can reach 95.1% by adjusting the coupling system using the method of monitoring scattering light.