Han Hainian

Generation of Continuum Extreme-Ultraviolet Radiation by Carrier-Envelope-Phase-Stabilized 5-fs Laser Pulses

Coherent extreme-ultraviolet (XUV) radiation is studied by interaction of carrier-envelope (CE) phase stabilized high energy 5-fs infrared (800 nm) laser pulses with neon gas at a repetition rate of 1 kHz. A broadband continuum XUV spectrum in the cut-off region is demonstrated when the CE phase is shifted to about zero, rather than modulated spectral harmonics when setting of CE phase is nonzero. The results show the generation of isolated attosecond XUV pulses.

Generation of Broadband Spectrum from a Simple Nonlinear-Polarization-Evolution Mode-Locked Yb-Doped Fiber Oscillator *

We demonstrate a nonlinear polarization evolution mode-locked Yb-doped fiber oscillator with the broadband spectrum output operating in a dispersion-managed regime. Pumped by a 976 nm single-mode laser diode, stable mode-locked ultrashort pulses are emitted with an average power of 198 mW at a repetition rate of 124 MHz, corresponding to a pulse energy of 1.6 nJ. The output spectrum spans from 950 nm to 1150 nm so that the transform-limited pulse duration is as short as 23 fs. Due to the imperfect dispersion compensation, we compress the pulses to 32 fs in this experiment.

Prospects for femtosecond ultrahigh intensity laser system towards Exawatt level

The new generation system for ultrahigh-power laser running over Exawatt (EW, 10 18  W) level is emerging recently, which is under a mechanism called Raman backscattering (RBS) in plasma. The main advantage of using plasma is that it can tolerate much higher laser intensities, 10 17  W cm −2 , more than five order of solid-state devices limited, 10 12  W cm −2 . Although Petawatt (PW, 10 15  W) laser pulses have been realized by some groups based on the chirped pulse amplification scheme (CPA), many cutting-edge scientific researches and technical applications, such as inertial confinement fusion (ICF), plasma physics, astrophysics, plasma-based particle accelerators, and X-ray lasers, need even higher laser power than EW level. For these purposes, huge laser projects like the extreme light infrastructure (ELI) have been proposed to offer new paradigm in EW class laser power. However, such an ultrahigh intensity laser system could only be achieved by CPA using very large (beyond 1 m 2 ) and expensive compressor gratings. In addition, to extrapolate CPA to the EW power range, hundreds of such gratings would be required. Even if there is enough budget, the problem of energy restriction on the last grating is still under question. Therefore, it is crucial to find a new medium or new technology for generating femtosecond ultra-intense laser pulses. This paper introduces a solution for generating laser intensities many orders of magnitude higher than currently results. This technology of optical amplification that a process known as Raman backscattering amplification and compression could enable the generation of femtosecond pulses of 20000 times the original seed intensity in a column of plasma just a few millimeters in length and less than a millimeter in width, without stretcher and compressor. Such sufficient high intensity and lower frequency of the pulse amplification indeed have got into super-radiant amplification regime (SRA) or stimulated Raman backscattering (SRBS). It has revealed that an unprecedented large pulse intensity amplification could be realized. The seed pulse will be compressed to 25 fs and its unfocused intensity increased from 1×10 15 to 4×10 17  W cm −2 . Furthermore, it could be increased further to around 1×10 23 W cm −2 by focusing the resulting beam to 1 μm. Therefore, it might be possible to increase this power up to EW in a larger plasma and using more powerful optical pumping.

Time and frequency domains comparisons of carrier-envelope-offset stabilization with two different schemes

We report the locking results of carrier-envelope offset for the two experimental schemes - f-to-2f design and monolithic scheme-and compare them by calculating Allan deviation and the measuring in- and out-of-loop phase noise.

Absolute-frequency measurement with a high stable monolithic optical frequency comb

In recent years the femtosecond optical frequency combs have led to remarkable advances in metrology optical clocks and attosecond generation etc. In general, the photonics crystal fiber is necessary to broaden the laser spectrum for f-2f method implemented in the traditional femtosecond optical frequency comb. The unavoidable instability introduced by the fiber limits the wide use of the frequency comb. However, owing to obviating the photonics crystal fiber and the interferometer replaced by a single PPLN crystal, the locked state of monolithic frequency comb based on Ti: sapphire laser has been significantly extended, typically from about 1 hour to more than 9 hours. Furthermore, the novel design frequency comb has shown its great potential to reach long-term stabilization as long as more advanced technologies for anti-shock and temperature stabilization can be used. We then test the locked state of the comb by recording fceo and frep with universal frequency counters. Besides the long-term locked result, we can also demonstrate that the Allan deviations for the two signals drop as ideal τ+1, respectively. This indicates that the white phase noise is the dominant noise process and then confirms that both fceo and frep controlled in experiment are in phase-locked loop. The reproducibility of locked fceo has been tested by standard statistical method, and the distributions of the locked data are uniform at 95% confidence level.