Study On Power Amplification And Phase Noise Suppression Of High-repetition-rate Ultra-short Pulses

Power amplification and its phase noise suppression of high-repetition-rate ultra-short pulses is a key topic for achieving high-power near-infrared frequency comb as well as other applications including phase-coherent pulse synthesis, high-precisidn spectroscopy, UV optical comb, mid-infrared optical comb and optical metrology. To study the power-amplification induced phase noise of ultra-short pulses, my works are focused on high-repetition-rate ultra-short pulse generation, fiber-based power amplification and precise control and phase noise suppression of high-power pulses. In this dissertation, a feed-forward carrier-envelope offset frequency stabilization scheme was demonstrated to precisely control and to compensate phase noise induced by the ytterbiumd-doped fiber laser oscillator and the power amplifiers with a fast acoustic-optical frequency shifter. Consequently, the CE offset frequency of the high-power pulses was successfully suppressed within a frequency linewidth of～1.4mHz. Furthermore, based on the precise control of pulse CE offset frequency and repetition rate, the near-infrared comb was achieved with an average power up to7W and a pulse width of130fs.The works demonstrated in the dissertation include:ultra-short pulse generation, passive pulse synchronization, high-repetition rate pulse generation with harmonic mode-locking, high-power fiber amplification and precise control of pulse carrier-phase offset frequency. The details are summarized as follows:1. High-repetition-rate ultra-short fiber lasers were established with output pulse width around hundreds fs, which would be employed as optical sources for achieving near-infrared frequency combs.1) Ultra-short pulses with pulse width of87fs were produced by a mode-locked Er-doped fiber laser with nonlinear polarization rotation and intra-cavity dispersion compensation. Meanwhile, stretched-pulse Yb-doped fiber laser has been realized with extra-cavity dechirped output pulse width of～55fs.2) The stretched-pulse Yb-doped fiber laser was harmonically mode-locked to produce pulses at a repetition rate of558MHz, corresponding to its12-order harmonics of the fundamental repetition rate. The high-repetition-rate pulses were further boosted to20W by Yb-doped double-clad fiber amplifiers and then compressed to180fs. Furthermore, the impact of passive harmonic mode-locking on laser output properties were experimentally investigated. The pulse widths as well as the offset frequency linewidths were narrowed in the harmonic mode-locking states.2. To further obtain high-power laser pulses, Yb-doped double-clad fiber amplification system has been realized to boost pulse power up to hundreds Watts.1) By using a two-stage Yb-doped fiber pre-amplifier and a two-stage double-clad fiber power amplifier in cascade, the synchronized ns pulses were amplified to131W of average power, corresponding to single pulse energy of0.55mJ. And based on this synchronization amplification system, the time-phase noise of high-power pulses was carefully investigated.2) All-fiber chirped pulse amplification system based on the Yb-doped double-clad fiber amplifiers has been developed to obtain high-power ultra-short pulses with average output power of～200W and compressed pulse width of-300fs.3. Successful phase-noise suppression and precise synchronization control of high-power ultra-short pulses was achieved.1) By using cross-absorption modulation, passive synchronization between femtosecond Ti:S laser and square nanosecond Yb-doped and Eb-doped fiber laser has been achieved with cavity mismatch tolerance of8.2and3.6cm, respectively. This is the highest tolerable cavity-length difference ever been obtained for synchronized mode-locked lasers, which supports a reliable synchronized laser source for OPCPA.2) A self-referenced feed-forward scheme based on an acousto-optic frequency shifter was used to compensate the CE phase noise of high-power ultra-short pulses. As a result, the phase noise and timing jitter of pulse CE frequency were measured to be370mrad and120as, respectively.3) The carrier-envelope phase stabilized Yb-fiber laser system with an average output power of～7W and pulse width of130fs has been achieved. The offset frequency of the high-power pulses was locked to the repetition rate of～64.5MHz with a relative linewidth of～1.4mHz. This near-infrared laser comb would be an ideal source for high-precision optical measurements.