Research On High Repetition Rate Passively Mode-locked Er-doped Fiber Laser And Its Noise Characteristics

Due to the narrow pulse duration,low timing jitter,and compact,robust structure,the passively mode-locked fiber laser plays an important role in many applications,such as optical frequency comb,photonics-based radar,low noise microwave signal extracting,high speed optical sampling,optical arbitrary waveform generation,and so on.In the past decades,due to the insufficient gain coefficient of active fiber,the pulse repetition rate of the passively mode-locked fiber laser remains at a level of 100 MHz.Recently,with the development of rare-earth doped fiber and saturable absorber,the repetition rate of the passively mode-locked fiber laser could be greatly increased,which attracts special attentions of researchers in fiber laser field.The noise of the passively mode-locked fiber laser,as the basic instrument in low noise systems,e.g.the optical frequency comb or photonics-based radar,has a direct impact on the performance of the systems.The investigation on the noise of these lasers has an important academic value and application prospect.In this thesis,different kinds of passively mode-locked fiber lasers are investigated theoretically and experimentally,and stable femtosecond pulse trains with GHz-level repetition rate are obtained.The noise characteristics of these lasers are detailed studied,the origins of the noises are analyzed,and the results provide approaches to further optimize these low noise systems.The main research content and the innovation of this thesis are presented below:1.The GHz repetition rate passively mode-locked Er-doped fiber laser system is studied based on non-PM components,the noise optimization method of this laser system is proposed.By adopting high gain fiber with small effective mode area and low output ratio,the pulse repetition rate of the Er-doped fiber oscillator is increased to GHz-level from 100 MHz,the mode-locked fiber lasers with 1 GHz,2.68 GHz,and 3.7 GHz repetition rate are made.The outputted pulses from the 2.68 GHz oscillator are amplified and compressed,and finally the femtosecond pulse train with 19.2 nm spectral bandwidth,244 fs pulse duration and 403 mW average power is obtained.The noise characteristics of the output pulses from the oscillator and amplifier are measured and compared,the noise deterioration in the amplifier is caused by the excess ASE noise,which can be improved by optimizing the gain fiber length and the pump power.2.The high repetition rate passively mode-locked fiber laser based on all-PM components is made,which generates GHz-level femtosecond pulse train with high degree of porization;a method to compensate the intracavity dispersion is proposed,which is achieved by adjuting the pump power to optimize the central wavelength.The output pulses have a 1.013 GHz repetition rate,599 fs pulse duration,7.9 nm spectral bandwidth,70 dB signal-noise-ratio,and a degree of polarization of 0.9994.By adjusting the pump power to make the wavelength of the laser at 1553.9 nm,the net cavity dispersion approaches to zero,which can optimizing the noise performance of the laser.The timing jitter at the 7th harmonic of the laser is measured to be 11.7 fs,which exhibits the low noise characteristic of the laser.3.The L-band high repetition rate passively mode-locked fiber laser is studied,and the dynamic evolution model of the intracavity pulses is built by numerical simulation.Usually the Er-doped fiber laser need long gain fiber or intracavity filter to make the wavelength shift to L-band,which are not suitable for high repetition rate fiber laser due to its short cavity.The laser here realizes mode-locking at 1608.8 nm through low cavity loss and high gain fiber,with 1.013 GHz repetition rate,229 fs pulse duration,and 13.6 nm spectral bandwidth.The numerical model of the high repetition rate fiber laser is built based on the parameters of this laser and the nonlinear Schr?dinger equation,the dynamic evolution of the pulses from a random noise signal to stable mode-locked signal is studied,which reveals the formation mechanism of the high repetition rate fiber laser.4.The noise characteristics of high repetition rate passively mode-locked fiber laser are studied theoretically and experimentally.The timing jitters and phase noises at different harmonics of a home-made 586 MHz fiber laser are measured,the results show that the timing jitter decreases with the increasing harmonic order,which is consistent with the theoretical prediction.The variation of phase noises at different harmonics is analyzed,the results show that the relative-intensity-noise-coupled jitters gradually disappear with the increasing harmonic order,the ASE noise is the dominant contributor to the timing jitters at high harmonics.The analysis indicates that the noise performance of optically generated radio-frequency signal can be improved by suppressing the ASE noise of the mode-locked laser.