Research On The Characteristics Of The Mode-locked Yb³⁺-doped Large-mode-area Photonic Crystal Fiber Laser

Intense ultrashort laser pulse generation from diode-pumped rare-earth-doped photonic crystal fiber lasers is a novel approach to achieve low-cost and compact femtosecond laser source, which is a promising work-horse on industry and scientific research. The photonic crystal fiber can scale up the mode area while keeping a single guided mode due to the endless single mode property, which effectively scale down the fiber nonlinearity, and is ideal for high power laser pulse propagation. A high energy mode-locked fiber laser is developed in this thesis. A segment of single polarization Ytterbium doped large mode area photonic crystal fiber laser serves as the gain media, while a piece of semiconductor saturable absorber mirror is used for self-started mode locking operation. The mode locking dynamics under various cavity dispersion is systematically investigated. The thesis can be divided into the following 6 parts:1. The fundamentals of femtosecond generation, the development of femtosecond laser technology as well as the waveguide mechanism of photonic crystal fiber are presented. The theory of nonlinear propagation of ultrashort laser pulses in media is introduced.2. A numerical model of mode-locked photonic crystal fiber laser is built, which is used to simulate the pulse dynamics in the fiber laser. The simulated results fit the experimental results very well.3. Soliton mode-locked photonic crystal fiber laser is experimentally investigated. Laser pulses with 900 mW of average power, 19 nJ of single pulse energy and 570 fs of pulse duration are achieved from the fiber laser. The influence of polarization extinction ratio of photonic crystal fiber on output pulse quality is analyzed. The noise of the laser, including energy fluctuation and timing jitter is also measured.4. Stretched soliton mode-locked photonic crystal fiber laser is experimentally investigated, pulse duration as short as 78 fs is achieved from the laser. The bounded solution pairs are also observed in the fiber laser.5. All normal dispersion mode-locked photonic crystal fiber laser is experimentally investigated, laser pulses with 2 W of average power, 40 nJ of single pulse energy(corresponding to 51 MHz of repetition rate) and 3.6 ps of pulse duration are achieved, which can be dechirped to 410 fs by extra-cavity dispersion compensation. 97 nJ laser pulses at a repetition rate of 11 MHz are achieved by adding a multi-pass cavity to lengthen the cavity path.6. A compact teraherz time domain spectral system is developed based on photonic crystal fiber laser and amplifier, nearly single cycle high power teraherz pulses are generated.