Research On Ytterbium Doped Ultrafast Fiber Lasers

As an important part of ultrafast lasers,ultrashort fiber lasers have attracted more and more attention for their unique advantages and have been widely used in industrial fine processing,precise spectroscopy,medical imaging and military defense.Mode-locked fiber lasers,Chirped Pulse Amplification(CPA)technology,together with double-clad highly doped gain fiber and large mode area photonic crystal fiber,form the most important tools for achieving high-power ultrashort output.This dissertation focuses on the research of ultrafast ytterbium-doped fiber lasers and frequency conversion.The main work and the innovative results are presented as follows:1.Based on the Nonlinear Polarization Evolution(NPE)principle,an all-normal-dispersion(ANDi)ytterbium-doped fiber laser was constructed and stable self-starting mode locking was achieved.The ultra-short pulse output with repetition rate of 49.5 MHz,average power of 75.4 mW,spectral range of 1063±7.7 nm and pulse width of 4.6 ps was obtained.After the external cavity compression,the pulse width was 80 fs,which is close to the limit of the Fourier transform.2.A dispersion-managed ytterbium-doped fiber laser was built to achieve an ultra-short pulse output with repetition rate of 75 MHz,average power of 50 mW,spectral range of1040±14 nm and pulse width of 1.3 ps.The signal to noise ratio(SNR)of radio frequency(RF)spectral is higher than 70 dB and there is good spectral quality over a wide RF range(-10–460 MHz).Based on this laser,by redesigning the distribution of mechanical components and devices,a corresponding engineering prototype was set up to reduce the overall size to 20×20×10 cm~3.In a variety of humidity,temperature and vibration condition,it can still achieve long-term stable operation,proved the reliability of the prototype.3.An all-fiber amplification system based on CPA technology was developed.The dispersion-managed ytterbium-doped fiber oscillator was used as the front stage.After single-mode fiber broadening,three-stage polarization maintaining fiber amplification,and pulse compression,the output with spectral range of 1042±5.2 nm,repetition rate of 75MHz and pulse width of 170 fs was obtained.The maximum output power of this ultrashort fiber laser is 12 W.Based on this,5.6 W of 521 nm green light and 330 mW of261 nm ultraviolet light could be obtained by frequency conversion of lithium niobate(LBO)and barium metaborate(BBO).The second and quadrupling frequency conversion efficiencies are 46.5%and 12.2%,respectively.The second frequency output jitter is less than 0.16%(RMS)within 4 hours.4.Using large-mode-area ytterbium-doped photonic crystal fiber for power amplification,specially designed chirped fiber Bragg grating(CFBG)for pulse broadening-compression matching,and a frequency selection module for repetition rate control,the pulse energy of the all-fiber polarization maintaining amplification system was scaled up to 32?J.Ultra-short output with a spectral range of 1038±5.5 nm,a pulse width of 153 fs,and a repetition rate of 1 MHz was obtained.This system has a power stability of 0.64%(RMS)over 24 hours,the maximum peak power of 0.21 GW and the M~2 factor of output beam of1.3 and 1.4(horizontal and vertical).The relationship between the polarization extinction ratio,the pulse width and the output power was studied.The variation of the central wavelength and the spectral width at different positions of the system was measured.