| With the rapid development of fiber mode-locked laser,it has been widely used in precision machining,optical communication,precision measurement and other cutting-edge scientific fields.With the development of the research on fiber laser,the noise problem of fiber mode-locked laser has attracted more and more attention.Especially in the fields of optical clock distribution,high-speed optical data transmission and optical frequency comb,the noise performance of laser seed source is an important factor restricting the performance of the whole system.Therefore,in order to improve the existing low-noise applications and further develop new applications,it is necessary to correctly understand,measure and control the noise of fiber mode-locked lasers.The noise sources of laser can be divided into three main sources:pump noise,environmental noise caused by external factors such as temperature change,mechanical vibration,acoustics vibration,and spontaneous emission noise.The main work of this paper focuses on the research of pump noise suppression methods.Firstly,based on the master equation,the noise source of fiber mode-locked laser is studied in theory.The basic concept of noise,the source of noise,the influence of noise on laser parameters,the measurement method of noise and the mathematical model of fiber mode-locked laser noise are introduced,and the causes of pump noise and spontaneous emission noise are analyzed.Then,a low noise diode laser driver control module based on the principle of P-I-D control is designed to improve the power stability of the pump laser,stabilize the pump wavelength and reduce the noise of the pump laser.Finally,aiming at the coupling process of the pump noise to the phase noise,the influence of the gain process and dispersion on the noise characteristics of the laser is analyzed.The main contents of this paper are as follows:1.The driving circuit of low noise semiconductor laser based on precise PID negative control is introduced.The driving circuit uses STM32 and Precision Op Amp chips to form a negative feedback loop,which completes the precise control of the output optical power and operating temperature of the semiconductor laser.The test results show that the driving circuit has good linearity between pump current and pump power and reliability.In the 10-hour power stability test,the standard deviation of output power is only 9×10-5 w.The phase noise and intensity noise of an erbium-doped fiber mode-locked oscillator based on semiconductor saturable absorption mirror(SESAM)are measured by using the driving circuit.Through the optimization of PID parameters,the influence of different PID design parameters on the noise of the pump laser is analyzed,and the low-noise pump laser driver module is completed.2.The influence of phase noise on the output gain of the fiber laser is studied.The experiment is based on a nonlinear polarization rotation(NPR)mode-locked Yb doped fiber mode-locked laser.In order to study the influence of gain parameters,four kinds of ytterbium doped fibers with different doping concentrations were used as gain fibers,and other experimental conditions and laser parameters remained unchanged.Noise transfer function and power spectral density of single sideband phase noise are introduced to evaluate the characteristics of phase and intensity noise.Finally,it is concluded that the coupling process of pump noise and other ambient noise to the output phase noise can be reduced and the phase noise can be suppressed by using the gain fiber with lower doping concentration.3.The noise characteristics of mode-locked laser with different intracavity dispersion are studied.A nonlinear polarization rotation(NPR)mode-locked Yb doped fiber mode-locked laser is used in the experiment.By adjusting the spacing between grating pairs,the net intracavity dispersion of the mode-locked laser is changed.Through the measurement of the noise transfer function and the power spectral density of the single sideband phase noise,it is concluded that the oscillator has better noise characteristics when the net dispersion in the cavity is near zero. |
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