High-repetition-rate, 50-µJ-level, 1064-nm, CPA laser system based on a single-stage double-pass Yb-doped rod-type fiber amplifier.

A 1064-nm femtosecond fiber chirped pulse amplification (FCPA) laser system based on a single-stage double-pass Yb-doped rod-type photonic crystal fiber (PCF) amplifier was demonstrated with a pulse repetition rate of 500 kHz, which was specially designed for expected conversion efficiency enhancement of a 10.8 eV source. With a series of Yb:fiber power amplifiers, the average output power was boosted to approximately 35 W. Further, using a transmission gratings-based pulse compressor, an average output power of 27.5 W was achieved, corresponding to a pulse energy of 55 µJ and a compression efficiency of 78.6%. The shortest pulse duration was optimized to be 204 fs, which was also accompanied by obvious pedestal. A pulse duration of 336 fs was also obtained when the pulse quality was at a top priority. To the best of our knowledge, this is the first demonstration of high-repetition-rate high-pulse-energy 1064-nm, instead of 1035-nm, femtosecond laser, based on commercially available Yb-doped rod-type PCF amplifier.

Consecutive 1015-1105-nm wavelength tunable "figure-of-9" mode-locked Yb:fiber oscillator.

A widely wavelength tunable mode-locked Yb-doped fiber oscillator based on nonlinear amplifier loop mirror (NALM) is reported, in which only a piece of short (∼0.5 m) single-mode polarization-maintaining (PM) Yb-doped fiber is employed, instead of the frequently used long (a few meters) double cladding (DC) fiber in previous papers. Experimentally, the center wavelength can be consecutively tuned from 1015 to 1105 nm by tilting the silver mirror, corresponding to a tuning range of 90 nm. To the best of our knowledge, this is the broadest consecutive tuning range in Yb:fiber mode-locked fiber oscillator. In addition, the mechanism of wavelength tuning is tentatively analyzed and attributed to the combined action of the spatial dispersion induced by a tilting silver mirror and the limited aperture in the system. Specific to the wavelength of 1045 nm, the output pulses with 13-nm spectral bandwidth can be compressed to 154 fs.

Stable 5-GHz fundamental repetition rate passively SESAM mode-locked Er-doped silica fiber lasers.

A stable passively mode-locked Er-doped silica fiber laser with a fundamental repetition rate of up to 5 GHz is demonstrated, which, to the best of our knowledge, is the highest repetition rate for 1.5 µm semiconductor saturable absorber mirror (SESAM) mode-locked Er-doped silica fiber (EDF) lasers. A segment of commercially available EDF with a net gain coefficient of 1 dB/cm is employed as gain medium. The compact Fabry-Pérot (FP) cavity features a fiber mirror, namely multiple-layer dielectric films (DFs) directly coated on end facet of a passive fiber ferrule, enabling a short cavity length of 2 cm configured. The mode-locked oscillator operates at 1561.0 nm with a signal-to-noise ratio (SNR) of 62.1 dB, whose average power is boosted to 27 mW by a single-mode Er-doped fiber amplifier (EDFA) and spectral bandwidth is broadened form 0.69 nm to 1.16 nm with a pulse width of 3.86 ps. The fiber laser shows excellent spectral stability without conspicuous wavelength drifting for 3 hours. Moreover, the basic guidelines of selecting SESAM for high repetition rate passively mode-locked fiber lasers is given.

The general amino acid control pathway regulates mTOR and autophagy during serum/glutamine starvation.

Organisms have evolved elaborate mechanisms to adjust intracellular nutrient levels in response to fluctuating availability of exogenous nutrients. During starvation, cells can enhance amino acid uptake and synthesis through the general amino acid control (GAAC) pathway, whereas nonessential cellular contents are recycled by autophagy. How these two pathways are coordinated in response to starvation is currently unknown. Here we show that the GAAC pathway couples exogenous amino acid availability with autophagy. Starvation caused deactivation of mTOR, which then activated autophagy. In parallel, serum/glutamine starvation activated the GAAC pathway, which up-regulated amino acid transporters, leading to increased amino acid uptake. This elevated the intracellular amino acid level, which in turn reactivated mTOR and suppressed autophagy. Knockdown of activating transcription factor 4, the major transcription factor in the GAAC pathway, or of SLC7A5, a leucine transporter, caused impaired mTOR reactivation and much higher levels of autophagy. Thus, the GAAC pathway modulates autophagy by regulating amino acid uptake and mTOR reactivation during serum/glutamine starvation.