Stability Of Ultra-fast Pulses From Fiber Lasers

Fiber combs link the optical frequency together with the microwave frequency which has been known well for several decades. It offered a method to measure the optical frequency precisely. People made great revolutionary progress in the measurement of time and frequency with the fiber combs. Stable seed pulses with low noise from the fiber laser determine the long-term reliable operation of the whole fiber comb system. In this dissertation, relative work has been studied to control of the ultra-short pulse in stable operation. Firstly, the instability of the single-mode fiber laser was analyzed. And a robust all polarization-maintained (PM) fiber laser was established as a passive method to stablize the fiber laser. Secondly, a smart fiber laser based on automatic mode locking research was studied as an active approach to quickly recover the mode-locked state from an unmode-locked state. Based on the smart fiber laser, we demonstate a fiber laser system with divided pulse amplification. The laser system carried out 120-fs and 600-mW pulses which cenetered at 1560 nm with a repetition rate of 80.8 MHz. Pulses were further doubled into 780 nm with 240-mW average power which was comparable to the pulses from a traditional Ti:sapphire laser. Finally, pulses group velocity inside the fiber laser cavity were controlled by an electronic polarization controller (EPC). To demonstrate that, the repetition rate of the fiber laser was stablized well by the EPC device. And the carrier-envelope phase (CEP) offset frequency had a linear relationship with the EPC driving voltage.The detailed works in this dissertation include:1. Works about a passively stablized PM fiber laser were carried out in a standing-wave cavity. The combination of the PM fiber and a Faraday rotation mirror ensured not only that pulses transmitted in the fiber cavity were strictly polarized along the slow axis of the PM fiber which maintained polarization state of the laser incident along the principle axis, but also the compensation of the random change of the fiber birefringence induced from the environment. The fiber laser was robust against environment disturbance. Based on the nonlinear polarization rotation (NPR) mechanism, stable mode-locked pulses were generated. The maximum output average power reached 20 mW. The repetition rate could be tuned from 1 to 8 MHz by adjusting the length of the cavity.6-ns pulses could be attained at a center wavelenth of 1030 nm.2. Works about an actively stablized single-mode fiber laser were carried out based on a smart polarization controller device. The characteristics of the mode-locking in a fiber laser was studied. A simple algorithm for mode-locking judgement was used through the smart polarization controller device to realize smart self-started mode-locking.1) By precisely adjusting the driving voltage of the smart polarization controller device inserted in the fiber laser cavity, the NPR effect could be tuned slightly. Spectral broadening and pulse-width change of pulses were observed with the increasing driving voltage. The change of the pulse width and the spectrum were 0.78 ps and 32 nm, respectively..2) The repetition rate of the fiber laser was constantly monitored. The repetition rate of pulses from a stable mode-locked fiber laser was fixed. Based on the mode-locking judgement algorithm, the mode-locked states could be attained during scanning the driving voltage. All of the driving voltages for mode-locked states were sampled in a voltage libariary. The change of the drving voltages distribution with the temperature was also studied. And by controlling the temperature of the fiber laser, the smart self-started fiber laser saved much time during the mode-locking searching based on the libariary.3) Based on the smart self-started Er-doped fiber laser, a fiber laser system with divided pulse amplification was demonstrated. Pulses were amplified up to 600 mW at a center wavelength of 1560 nm. The repetition rate was 80.8 MHz. Furthermore, the amplified pulses were frequency doubled into 780 nm with an average output power of 240 mW. The pulse width was compressed to 100 fs.3. Works about pulses group velocity control inside the fiber laser cavity was learned with the smart polarization controller device. By tuning the polarization state of pulses inside the cavity and the fiber index squeezed by the squeezer, pulses group velocity could be controlled well. To demonstrate it, the repetition rate of the fiber laser was locked to a standard reference frequency. The phase locking was operated for 72 hours. And the standard deviation of the repetition rate was about 1.4 mHz. And the linewidth of the repetition rate was 1.7 mHz. It also paved the way to phase lock the carrier-envelope phase offset by the smart polarization controller device. The relationship between the driving voltage and the carrier-envelope phase offset was also studied.