Research On Fiber Optical Combs For Simultaneous Locking Multiple Lasers Of Yb Optical Clock

Optical frequency comb connecting optical and microwave frequencies,is applied in optical frequency measurement and low-noise microwave frequency generation.It has gradually transitioned to fiber combs,which have a more compact size,more stable operation,and lower cost.With the help of optical comb,the atomic clock provides a high-precision time and frequency standard based on its ultra-narrow optical transition.In the beginning,the development of atomic clocks is introduced in this thesis.The domestic and international research of optical combs are introduced in detail.We then analyze the advantages and disadvantages of different mode-locking methods.All-fiber mode-locked erbium-doped fiber lasers for the femtosecond fiber optical combs are developed,which are based on the nonlinear polarization rotation technique in the experiment.Then we theoretically derive the equations of the laser transmitting in the fiber based on nonlinear Schr?dinger,elaborate the characteristics of the laser pulse.We build two sets of all-fiber erbium-doped fiber optical combs（Comb-1 and Comb-2）and present a detailed analysis including oscillators,the characteristics of the output seed laser,the erbium-doped fiber ampilifier（EDFA）,supercontinuum（SC）generation and other contents.The pulsed lasers of the combs differ slightly,with the erbium-doped fiber laser of Comb-1 having the pump power of 143 m W,the repetition frequency of 66.83 MHz with SNR of more than 70 d B（RBW=1 MHz）,and the spectral width of 42 nm.Comb-2 has the pump power of 104 m W,the repetition frequency of66.80 MHz,and the spectral width of 38 nm.Then,the SC generated is experimentally investigated on Comb-1 which is detected the carrier envelope offset frequency f₀ with the common method of f-2f self-referencing technique.Firstly,the amplified pulses are broadened by the uniform dispersive highly nonlinear fiber（HNLF）,and the spectra are optimized in the parameters such as dispersion of the pulse value,pulse polarization within the fiber,and the pump power of EDFA.The SC covering the range of 1020～2230 nm at the 20 d B bandwidth is achieved in the HNLF with D=4.205 ps/nm/km.We splice two segments of HNLF with different dispersions at 1550 nm as the hybrid HNLF to generate the SC.It’s found that the spectrum generated by the hybrid HNLF with positive and zero dispersions is better and the range of spectrum covers almost an octave at a bandwidth of 20 d B.The optical comb from the Menlo Systems is locked to the ultra-stable 578 nm laser which is frequency doubled from the 1156 nm laser stabilized on the 30-cm FP cavity.For this comb,the broadened spectrum of 550-1050 nm is obtained by using the photonic crystal fiber.And the laser at 399 nm（doubled from the fundamental source at 798 nm）which runs for the first-stage cooling,lattice laser at 759 nm and repump laser at 649&770 nm involved in the ytterbium atomic clock laser system are locked simultaneously.In addition,the supercontinuum technique is also used to broaden the1560 nm pulse from the Menlo comb oscillator as the seed laser.After EDFA and HNLF,the broadened spectrum covering 1389 nm and 1695 nm can be obtained and successful in frequency stabilization simultaneously.Almost lasers of the ytterbium atomic clock system are referenced to the same source.Comb-2 which is used for frequency locking of lasers at different wavelengths is locked to an ultra-stable laser at 1156 nm without detecting f₀.A broadened spectrum covering over an octave is generated,and the comb is attempted to lock on the ultra-stable laser at 1156 nm.Multiple lasers referenced to the same source are of great significance to the improvement of the stability,the realization of miniaturization and the ultimate realization of portability of the ytterbium atomic clock system.