| The laser frequency comb is considered to have the advantages of high coherence,wide spectrum range,etc.It has great application potential in the field of precision measurement.Although the development of this technology has only been 20 years from the original Ti:sapphire system to today’s optical fiber-based apparatus,each technological advancement has brought tremendous breakthrough for precision measurement.However,the existing fiber-based frequency comb has a lot of noises in a free-running state,and these noises will be transmitted to the measurement result and leads to poor precisions.To suppress these noises and obtain a stable output,the research on the precision frequency locking technology of the fiber femtosecond optical comb has important research value.The input of the laser frequency comb locking system comes from an optical fiber femtosecond laser system built in our laboratory.The system is based on the fiber Kerr effect and is composed of erbium-doped fiber,fiber coupler,polarization controller and single-mode fiber,so as to achieve non-linear polarization rotation mode-locking.After a comprehensive test of the system,it is found that when the operating current of the pump laser exceeds the threshold 480 m A,the system can effectively achieve nonlinear polarization rotation mode locking,and output laser pulse train with a repetition frequency of about 136 MHz.The laser pulse passes through the optical fiber.After attenuation,the detector converts the laser intensity into an electrical signal to achieve frequency locking.The repetition rate stabilization for the fiber laser frequency comb is based on the phase locked loop principle in automatic control technology.To follow the control of the repetition rate,it is necessary to start from the perspective of noise and phase change.In our system,a frequency synthesis module is used to generate a reference signal based on the high-precision clock output.This reference signal together with the repetition rate of the laser comb is discriminated by a phase frequency detector.The error signal is sent to a loop filter and an amplifier for filtering and amplifying.The processed signal is further fed to a piezoelectric ceramic in the resonant cavity after the high voltage amplifying module,and the repetition frequency is stabilized by changing the cavity length of the resonant cavity.In addition,we have designed a stable and reliable power supply for each component of the system,with a ripple amplitude of less than 3m V.The high-quality power supply performance lays the foundation for the system to work reliably for a long time.In order to reduce the interference caused by changes in the external environment,we have developed a temperature control module based on the PID inside the microcontroller STM32.When the system enters the closed-loop state,the temperature control system forms a slow-loop feedback mechanism through real-time acquisition and control of the optical fiber temperature,which offsets the thermal effect of the optical fiber and the influence of the external environmental temperature change.The designed frequency stabilization system is applied to the repetition rate control of the resonant cavity,and good experimental results are obtained.The comparative experiments before and after locking show that the stability of the repetition frequency is significantly improved,and the jitter within 54000 s is less than 10 Hz,achieving the desired effect. |
