Research On High Power And High Stability Yb: Doped Fiber Frequency Comb And Its Noise

Optical frequency comb is an important tool for optical frequency standard and pre-cision spectroscopy.Its applications include attosecond science,low-noise microwave frequency generation,precise distance measurements,extraterrestrial planet detection and so on.With the continuous development of optical frequency comb technology,new applications are emerging,and higher requirements are put forward for the performance of optical frequency comb,such as long-distance time-frequency transfer,and extreme nonlinear spectroscopy.In particular,applications such as long-distance time-frequency transfer and ultraviolet comb generation require not only high frequency stability,but also high average power or peak power.At the same time,in order to meet the future experiments in outdoor or space environments,it is also necessary for the optical fre-quency comb to be as robust and automatic as possible.For this kind of application,a series of research work has been carried out in this paper on the generation and locking of high-power Yb:doped fiber optical frequency comb.The main research contents and innovative achievements of this paper are as follows:1.Based on the principle of NPR,SESAM and NALM mode-locking,we study Yb:doped fiber laser.Firstly,for the application of optical frequency comb,two NPR mode-locked oscillators with a repetition rate of 200 MHz,center wavelength of 1030nm,the output power of 40 m W and spectral width of 56 nm are built.Then,a SESAM mode-locked oscillator with all fiber and all polarization-maintaining is built.The rep-etition frequency of the SESAM oscillator is 76.5 MHz,the central wavelength is 1032nm,the spectral width is 13 nm and the output power is 10 m W.After one stage of Yb:doped fiber amplifier,the output power is increased to 250 m W and the power stabil-ity is 0.046%in 7 hours.In addition,a NALM mode-locked oscillator with a repetition rate of 65 MHz,the central wavelength of 1034 nm,the spectral width of 32 nm and output power of 94 m W is realized.Due to its full polarization-maintaining structure and low noise performance,it is an ideal seed source for optical frequency comb.2.Aiming at the long-distance high-precision time-frequency transfer,we designed and built two high-power engineering optical frequency combs.Based on the 200 MHz NPR mode-locked oscillator,the fiber chirped pulse amplification technology and large mode area photonic crystal fiber is used to reduce the B integral in the amplification process,and the high-order dispersion compensation fiber is used to compensate for the high-order dispersion.The laser output with an output power of 20 W and pulse width of 75 fs is obtained.Then,a piece of tapered photonic crystal fiber is used to obtain the supercontinuum spectrum with high coherence.We detect and lock the CEO signal and beat signal to an ultrastable CW laser reference.The relative frequency stability of the optical comb is 5.5×10^-20/1000 s.As we all know,this is also the first optical frequency comb with the highest stability under high power in the world.3.Based on the optical frequency comb system,we explore the relationship be-tween the pump power fixed point frequency and net cavity dispersion in the Yb:doped fiber frequency comb.By continuously adjusting the distance of the grating pair in the oscillator,we measured the fixed point frequency in different dispersion regions and po-larization states.We found that the pump power fixed point is not always near the carrier frequency,but changes significantly with the variation of net dispersion and polarization states.Especially at the near-zero dispersion point,the fixed point has a local minimum,which is only tens of THz.This is the first time to completely measure the influence of net cavity dispersion and polarization state on the fixed point in the experiment.These laws will guide us to better lock the optical frequency comb.4.Based on the high-power optical frequency comb system,we compared the phase noise and frequency stability of two methods of locking the high-power optical frequency comb to the optical reference.The experimental results show that the low-frequency noise introduced by the non-common-mode optical path will affect the long-term stability of the unlocked branch of the optical frequency comb.The non-common-mode noise can be eliminated by actively locking the corresponding branch.The noise in the amplification and supercontinuum generation will affect the high-frequency fre-quency stability of the amplifier output.This kind of noise can not be eliminated by active feedback control.Compared with the locked oscillator branch,the frequency stability of high-power output in 1000 s is improved by 4 orders of magnitude while locking the amplifier branch.This work quantifies the noise of high power amplifier in the experiment.