Optical Frequency Comb And Optical Frequency Locking Research

Femtosecond optical frequency combs cover a broad spectrum with extremely narrow single spectral lines and,with the aid of optical frequency locking technology,can improve the accuracy of frequency measurement to the order of 10-19,which significantly improves the human ability to explore the microscopic world and scientific laws.The optical frequency locking technology based on the optical frequency comb inherits the high stability of the optical frequency comb itself.It provides high precision,high accuracy,and high-reliability locking means and measurement tools for realizing various scientific research fields such as stable frequency laser,multi-wavelength laser synchronization,precision spectroscopy,precision distance measurement,precision imaging,and precision time-frequency distribution.The purpose of this thesis is to provide a simple,highly flexible,highprecision,and highly robust optical frequency locking system based on an optical frequency comb.Nowadays,there are two main limitations in the locking performance of the synchronous system.The first one is the output pulse trains stability of the optical frequency comb,which directly determines the stability of the reference source.Secondly,the structural design and algorithm optimization of the locking system mainly determines the rejection capability for noise during the locking process.To solve these problems,effective solutions are proposed around the optimization of the compensation algorithm of the digital locking system,the fabrication of optical frequency combs with highly stable repetitive frequency locking,the design of a highly flexible and simple multi-wavelength laser synchronization system,and the design of passive locking dual-comb seed source,which has achieved more outstanding and innovative research results.The main research results of this paper are as follows:1.The long short-term memory network(LSTM)assisted frequency locking system.For the nonlinear errors in the proportional-integral-derivative(PID)compensation algorithm in the digital locking system,this paper propose to combine the advantages of nonlinear model fitting of LSTM to compensate for the errors introduced in the locking process due to nonlinearities and thus achieve more than 1-fold improvement in locking performance.The optical frequency comb repetition frequency locking system based on the optimized locking algorithm combined with the homemade large dynamic range optical frequency comb seed source achieved a locking result with root mean square error of 16.4 mHz within 13 minutes of measurement time,which is more than 4 times the locking performance compared to the PID algorithm In the single frequency laser bias frequency locking,a locking performance improvement of a l-fold decrease in locking standard deviation after locking was achieved in the system.It is also verified in the actual link transmission,and the locking performance improvement is still maintained.2.A simple laser synchronization system based on optical frequency combs.For the problems of poor flexibility and narrow control bandwidth of the analog locking system,this paper propose a laser synchronization system based on the root-mean-square RF detector and acoustic-optical frequency shifter with a hybrid "analog+digital" locking system.In the synchronization locking experiment of a single laser,the absolute frequency jitter standard deviation obtained in a measurement time of 16000 s is 0.28 Hz.In the multi-wavelength synchronization locking experiments with multiple lasers,the standard deviations of the absolute frequency jitter obtained from the measurements at the bias frequencies of 1.2 GHz,2.5 GHz,and 10.8 GHz during the test time of 16000 s are 0.798 Hz,3.986 Hz,and 164.137 Hz,respectively.3.Large repetition frequency difference,tunable passive dual-comb seed source.To address the needs of large repetition frequency difference,tunable,polarization crosstalk-free,and independently controllable provided by the passive locked fiber-type dual optical comb seed source required in time-frequency transfer,this paper propose a high repetition frequency difference bidirectional dual-comb fiber laser based on an all-polarization-maintaining cavity configuration and a semiconductor saturable absorption mirror with single polarization output.The proposed comb laser has a standard deviation of 69 Hz and an Allan deviation of 1.17 × 10-7 at τ=1s under different repetition frequencies of 12.815 MHz.Owing to the passive common-mode noise rejection capability of dual-comb laser,after passing an 84 km fiber link,the frequency stability of the repetition frequency difference signal is improved by two orders of magnitude than the repetition frequency signal at the receiver side.In summary,this paper presents a systematic analysis and targeted optimization of two limitations of optical frequency locking systems based on optical frequency combs.The algorithm optimization of the digital locking system and a simple,large-bandwidth,flexible hybrid "analog+digital" locking system are given,and the dual-comb seed source structure and noise suppression capability for time-frequency transfer are investigated.This paper’s results help solve the multi-wavelength frequency locking problem and achieve highly stable fiber optic time-frequency transfer.