Investigations Of The Techniques For Fiber-laser Frequency Comb

As one of the greatest inventions in the 20th century,optical frequency combs produced by frequency-controlled mode-locked lasers have important applications in a wide range of fields,such as optical frequency metrology,time-frequency transfer based on optical clocks,and precise coherent measurement.Fiber-laser-based optical frequency comb has been the subject of considerable research in recnet years,because of its advantages of simple structure,low cost and easy operability property.This dissertation focuses on the key principles and techniques of fiber-laser frequency comb.A series of research work about theories and experiments are developed and the main works and results are as follows:Firstly,based on the theoretical models such as the nonlinear Schrodinger equation(NLSE),and the genearalized nonlinear Schrodinger equation(GNLSE)for pulse propagating in dispersion and nonlinear fibers,the pulse characteristics of the fiber mode-locked laser,chirped pulse amplification method and the technique of supercontinuum generation in a highly nonlinear fiber are respectively investigated here;Then using the perturbation theory based on the master equation,the characteristics of transfer functions of repetion rate and offset frequency are deeply studied;On this basis,the fixed-piont frequency mode is used to research various noise sources that impact the frequency comb performance and analyze the key feedback-control mechanism,then some methods and technological means are put forward for suppressing comb noise.Secondly,a design scheme for the optical frequency comb based on femtosecond fiber laser is proposed.A dispersion-managed soliton mode-locked erbium-doped fiber laser with pulse width of 55 fs and repetition frequency of 210 MHz is designed,and a chirped pulse fiber amplication link is optimized.An octave supercontinuum from 1080 nm to 2320 nm is generated with the highly nonlinear fiber,which makes the signal-to-noise ratio of the detected carrer-envelope offset frequency reach 32 dB by the f-2f autodyne method.A good performance of the long-term phase-locking stabilization for fiber laser optical frequency combs with the improved phase-locking loop using a PI regulator is demonstrated,and based this key point some locking electronic circuits are designed in the experiment.Furtherly,by respectively locking the 4th harmonic wave of repetition rate and the carrier envelope offset frequency to a commercial rubidium atomic clock,an optical frequency comb is locked with high precision.Measurement results show that the standard deviations of the repetition rate and the carrier envelope offset frequency for the comb are 0.65 mHz and 1.76 mHz at 1s counter gate time,corresponding to the Allan deviations of 1.74×10-13 and 1.80 × 10-11 for 100 s timescale,respectively.Thirdly,the phase-locking mechanism and locking results of a frequency comb based on a period-doubling mode-locked(PD-ML)fiber laser are investigated.By optimizing the length of the OFS-980 optical fiber in the laser cavity,a fiber laser that can be switched from fundamental mode locking to PD-ML simply by changing the pump bias current is designed.The characteristics of the output pulses of the two mode-locked states are similar.Investigation results show that the new comb teeth generated in the PD-ML mode are strongly correlated with the original teeth and have a consistent carrier-envelope offset frequency,controlling the pump power and cavity length with the help of a piezoelectric transducer is also suited.Furthermore,by changing the pump bias current only,we achieve an FML-based phase-locked optical comb using the same f-to-2f heterodyne beat system and locking circuit.Hence,we have achieved phase locking of optical combs with two different repetition rates at a same fiber system,and switching between them.The investigation results described above may be very helpful in developing low-noise fiber-laser-based optical frequency combs for satisfying application requirements in fields such as optical freuqency metrology,optical spectroscopy,and microwave generation and so on.