The Study On The Real-time Distributed Temerature Measurement And Optimization Of Gain Fiber In Fiber Lasers

Thermal effect in the gain fiber is one of the main factors which limit the power improvement of high power fiber amplifiers.With the growing power of fiber lasers,the temperature in the gain fiber core increases sharply due to the quantum loss and background loss in the fiber.However,the excessive temperature can lead to the destruction of the fiber material and mode instability(MI)in fiber lasers,which restrict the further power improvement seriously.In order to improve the output power and protect the gain fiber,it is necessary to measure the distributed core temperature of the gain fiber in real time,which is also beneficial to understand the mechanism of MI.At present,thermal resistance,thermocouple and thermal imager are mostly used to measure the surface temperature of the gain fiber in fiber lasers.Using these conventional measurement methods,one can only get the surface temperature of the gain fiber while the core temperature cannot be obtained,let alone to detect the distributed temperature along the fiber.So it is difficult to meet the need of the temperature measurement in high power fiber lasers.Based on this,in this paper,the optical frequency domain reflectometry(OFDR)is proposed to measure the core temperature of the gain fiber,and on this basis,the optimization of gain fiber temperature distribution is studied preliminarily.The main contents are as follows:Firstly,the advantages and disadvantages of the existing distributed fiber sensing methods are analyzed comprehensively.Several possible distributed methods to measure the core temperature of the gain fiber in fiber lasers are introduced.By comparing the temperature measurement requirement in fiber lasers with various measurement methods,the OFDR is selected to measure the core temperature of the gain fiber in fiber lasers finally.Secondly,the theory of using OFDR to measure temperature in the gain fiber is studied in detail.According to the basic principle of OFDR,the typical factors influencing the measurement process and the disturbance of the special characteristics of the gain fiber to OFDR are analyzed.It is pointed out that the accuracy of OFDR can be ensured by increasing the bending radius and choosing the appropriate fiber numerical aperture.According to rate equations,thermal conduction models and the principle of OFDR,the temperature of the gain fiber in fiber amplifiers and the measured temperature calculated by OFDR model is simulated.The original theoretical temperature is compared with the theoretical measured temperature,and the results can provide a reference for the experimental temperature measurement of the gain fiber in high power fiber lasers.Thirdly,based on the OFDR technique,the gain fiber temperature in fiber amplifier is measured experimentally.First,the temperature distribution in the active fiber is measured without laser amplification and the measurement results is calibrated.A fiber amplifier based on the master oscillator power amplifier(MOPA)is built and the temperature of the gain fiber is measured in different output power with or without the amplified spontaneous emission(ASE).The temperature measurement accuracy of OFDR in the gain fiber is verified when the fiber laser is in operation,which can be used as a reference for temperature monitoring and measurement in high power fiber lasers.Finally,the optimization of the gain fiber temperature distribution is studied preliminarily.The maximum temperature and the temperature distribution in the gradient doping fiber is investigated and its suppression to the nonlinear effect is discussed.The results show that the gradient doping fiber can reduce the maximum temperature and optimize the temperature distribution of the gain fiber without changing the laser output power,thus reducing the nonlinear effects in the fiber,such as the stimulated Brillouin scattering(SBS),revealing that gradient doping fiber is a feasible method to improve the output power of fiber lasers in the future.In this paper,a real-time distributed gain fiber temperature measurement method based on OFDR is studied theoretically and experimentally,and on this basis,the optimization of gain fiber temperature distribution is explored preliminarily.Results show that the OFDR technique can measure the distributed temperature of the fiber core in fiber lasers accurately in real time,and the longitudinal gradient doping of the fiber is a feasible method to improve the output power of fiber lasers.These researches can provide reference for the further study of the temperature measurement of the gain fiber,mechanism of nonlinear effects and MI in high power fiber lasers.