Research On Ytterbium Fiber-laser Frequency Combs

The proposal of femtosecond optical frequency combs is a significant breakthrough in the field of ultrafast lasers and precision spectroscopy.Acting as precision optical synthesizers,optical frequency combs connect the optical frequency with the microwave frequency tightly and provide powerful tools for precise frequency measurement.Today's optical frequency combs have strong advantages in many technological areas,such as optical clock,absolute distance measurement and precision spectroscopy.This dissertation focuses on the development of ytterbium fiber-laser frequency combs.The main research works and the innovative results are summarized as follows:1.We constructed several femtosecond Yb-doped fiber laser oscillators based on the nonlinear polarization rotation(NPR)mode-locking technique and SESAM mode-locking technique for fiber amplifiers and frequency combs,respectively.Firstly,a 250-MHz repetition rate fiber oscillator based on NPE mode-locking operating at near-zero net-cavity dispersion was realized with 120-mW average power and 45-nm spectrum bandwidth.Secondly,a self-similar NPE-mode-locked fiber laser delivering pulses with 39-mW average power and 20-nm spectrum bandwidth was built at 50-MHz repetition rate.Further,a polarization-maintaining SESAM-mode-locked fiber laser was demonstrated with 69-MHz repetition rate and 37-mW output power.The maximum spectrum bandwidth of the oscillator was 34 nm and the dechirped pulse duration was 64 fs.In addition,an all-fiber design SESAM fiber laser was realized,which provided an ideal seed source for the integration and miniaturization of mode-locked fiber laser systems.2.By using three kinds of highly nonlinear fibers,we studied the progresses of supercontinuum generation in fibers.Firstly,we obtained an octave-spanning spectrum covering 550 nm to 1350 nm in a homed-made tapered single-mode fiber pumped by 250-MHz,74-fs,460-mW pulses.Secondly,we employed photonic crystal fibers in spectrum broadening progress and obtained an octave-spanning spectrum covering 650 nm to 1600 nm.To reduce the incident pulse energy and the length of highly nonlinear fibers,an experiment of tapered photonic crystal fibers was performed.Octave-spanning spectrum covering 520 nm to 1150 nm was obtained using a homemade tapered photonic crystal fiber with 400 pJ low energy injection.Moreover,to improve the optical efficiency of supercontinumm generation,we studied the end-face processing and low-loss fusion splicing of photonic crystal fiber.3.Based on the octave-spanning supercontinumm obtained at previous experiment,a fully phase-locked ytterbium fiber-laser frequency comb was demonstrated in our experiment.Firstly,we locked the frequency comb to a microwave reference.The carrier-envelope offset frequency was generated by a standard f-to-2f interferometer with 40-dB signal-to-noise ratio.The offset frequency and the repetition rate were simultaneously stabilized to the microwave standard for more than 29 hours.Secondly,in order to improve the locking accuracy of the fiber-laser frequency comb,the comb was locked to an ultra-stable optical reference at 972 nm.The line width of the locked beat signal is narrowed to 1.24 mHz and the frequency instability was calculated to be 1.77×10-18/s and 4.4×10-20/1 000 s.In order to improve the stability and practicability of the ytterbium fiber-laser frequency combs,a research on all-fiber design octave-spanning spectrum generation was carried out.By employing a specific photonic crystal fiber for dispersion management and a tapered photonic crystal fiber for spectrum expansion,we realized all-fiber design for pulse compression and octave-spanning spectrum generation in 1 ?m.The output spectrum covered 520 nm to 1250 nm and the output power fluctuation is as low as 0.08%.In addition,using a standard f-2f interferometer,a 37-dB free running fceo signal is detected based on this spectrum,which shows the superior performance of all-fiber design system.4.For the application of long-distance time-frequency transfer,a research on engineered ytterbium fiber-laser frequency combs was carried out.In order to reduce the impact of surrounding environment on combs,we improved the structure of ytterbium fiber-laser frequency comb systems and added external control circuit.Based on the improvement measures,we demonstrate a 500 mW low-power ytterbium fiber-laser frequency comb for the experiment in the lab and a 20 W high-power ytterbium fiber-laser frequency comb based on linear chirped pulse amplification for the experiment outdoors.