Carrier-envelope Phase Changes Research Articles

Selection of electron quantum trajectories in the macroscopic high-order harmonics generated by near-infrared lasers

We theoretically investigate the selection of electron quantum trajectories in the macroscopic high harmonic generation (HHG) with few-cycle, 1200-nm near-infrared lasers by changing gas pressure. The selected continuous HHG spectra are able to support the isolated attosecond pulses (IAPs) in the soft x rays. The time-frequency analysis of high harmonics indicates that the long-trajectory electron emissions are dominant at lower pressure and the short-trajectory emissions are suffered at higher and optimal pressure. The quantitative analysis of phase mismatch uncovers the physical origin of dynamic changes in the harmonics fields after propagation in the gas medium. Furthermore, we check the dependency of IAP on the carrier-envelope phase (CEP) of laser pulses and find that IAP can still be obtained even if the CEP changes.

Carrier-envelope phase changes in the focal region: propagation effects measured by spectral interferometry

Spectral interferometric measurements are presented that show how wave propagation affects the carrier-envelope phase (CEP) of an ultrashort pulse in the focal region and results in variations that are different from the Gouy phase shift. Wavelength-dependent properties of the input beam are investigated and are seen to influence how the CEP is altered. The measured CEP changes show characteristics similar to the variations predicted by theory.

Drift and noise of the carrier–envelope phase in a Ti:sapphire amplifier

We report on the drift and noise measurement of the carrier–envelope phase (CEP) of ultrashort pulses in a three-pass Ti:sapphire-based amplifier. Spectrally and spatially resolved interferometry makes it possible to investigate the absolute CEP changes due exclusively to the amplifier, that is, entirely separated from the incidental phase fluctuations of the oscillator. We found that propagation through the amplifier crystal could result in an increase up to 30 mrad noise depending on the repetition rate, cooling, and pumping conditions. Most of this noise is related to mechanical vibrations and thermal instabilities. The absolute CEP drift of thermal origin can be as large as 11 mrad/°C for each mm of the amplifier crystal, originating from inefficient heat conduction during the absorption of pump pulses. The noise of the thermal CEP drift is inversely proportional to the repetition rate, as was shown experimentally and proven by simulations.

Carrier-envelope-phase insensitivity in high-order harmonic generation driven by few-cycle laser pulses.

We present evidence for self-stabilization of the relative spectral phase of high-order harmonic emission against intensity variations of the driving field. Our results demonstrate that, near the laser focus, phase matching of the harmonic field from a macroscopic target can compensate for the intensity dependence of the intrinsic phase of the harmonics emitted by a single radiator. As a consequence, we show experimentally and theoretically the insensitivity of the harmonic spectra produced at the laser focus against variations of the carrier-envelope phase (CEP) of a sub-two-cycle driving field. In addition, the associated attosecond pulse trains exhibit phase locking against CEP changes of the few-cycle driver.

In situ measurement of nonlinear carrier-envelope phase changes in hollow fiber compression.

We demonstrate a simple and robust single-shot interferometric technique that allows the in situ measurement of intensity-dependent phase changes experienced by ultrashort laser pulses upon nonlinear propagation. The technique is applied to the characterization of carrier-envelope phase noise in hollow fiber compressors both in the pressure gradient and in the static cell configuration. Measurements performed simultaneously with conventional f-to-2f interferometers before and after compression indicate that the noise emerging in the waveguide adds up arithmetically to the phase noise of the amplifier, thus being strongly correlated to the phase noise of the pulses coupled into the compressor.

Long-term carrier-envelope-phase stabilized fiber-bulk hybrid laser with millihertz linewidth and 50 W average power

Long-term carrier-envelope-phase stabilization was demonstrated in high-power large-modearea fiber chirped-pulse spectral fraction amplification at 1030 nm of a broadband femtosecond Ti:sapphire laser. The carrier enveloped phase was stably controlled with the maximum average power up to 50 W by pre-compensating the carrier-envelope phase changes in the high-power fiber amplifiers. The locked in loop and out of loop frequency fluctuation was measured to be 1.44 and 46 mHz within half an hour. The locked carrier-frequency exhibited an in loop and out of loop beat signal linewidth of 1.66 and 2.27 mHz, accordingly accumulated phase jitter was estimated to be 0.22 and 0.73 rad, respectively, with the integration range from 10 mHz to 100 kHz. The phase stabilization was measured to change little as the output power increased, confirming negligible amplitude-to-phase noise conversions in the linear high-power fiber chirped pulse amplification.

Three-dimensional classical micro-canonical simulation of nonsequential double ionization with a few-cycle laser pulse

A three-dimensional classical micro-canonical model is used to investigate the ion momentum distributions of Ar atom in nonsequential double ionization with a few-cycle laser pulse. It is found that the momentum distribution has a strong dependence on the carrier-envelope phase of the few-cycle pulse. The centre of gravity of the asymmetry distribution shifts from negative to positive ion momenta with carrier-envelope phase increasing and it is reversed when the carrier-envelope phase changes by π. The momentum distributions of the Ar2+ ion show excellent agreement with the experimental results when we take into account the change in Coulomb potential between the nucleus and electron in the recollision process.

Carrier-envelope phase measurement by all-optical poling with a polar side-chain polymer

Dependence of all-optical poling efficiency on carrier-envelope phase (CEP) could be measured using photoisomerization of dye molecules which are covalently bound to a polymer main chain and have large difference in static dipole moment between the ground state and excited state. Increased chromophore density leads to an order of magnitude reduction in signal-detection time from a dye doped polymer. Analysis of all-optical poling experiments with CEP changes clearly showed the presence of polarization restoring force to zero polarization. This enables resetting of SH activity in the all-optical poling process to be used for fast response loop of CEP stabilization. Phenomenological model could explain well the difference in the growth-and-decay dynamics of poling between sample of dye doped in polymer studied previously and that grafted to a polymer main chain used in the present paper.

Carrier-envelope offset phase-locking with attosecond timing jitter

Inside a femtosecond laser oscillator, no coupling mechanism between the propagation speeds of the carrier and the pulse envelope exists. Therefore, the relative delay between carrier and envelope of a femtosecond oscillator will exhibit irregular fluctuations unless this jitter is actively suppressed. Both intensity and beam pointing fluctuations in the laser can introduce carrier-envelope phase changes. Based on our analysis, we are capable of reducing or avoiding certain mechanisms by proper design of the laser cavity. We use such an optimized cavity to stabilize the carrier envelope-phase to an external reference oscillator with a long-term residual jitter corresponding to only 10 attoseconds in a (100 kHz-0.01 Hz) bandwidth. This is the smallest long-term timing jitter of a femtosecond laser oscillator demonstrated to date. However, it is important to note that this stabilization was obtained with an f-to-2f heterodyne technique using additional external spectral broadening in a microstructure fiber which introduces additional carrier-envelope phase noise. We present a direct heterodyne measurement of this additional carrier-envelope phase noise due to the continuum generation process.

Measurements of carrier-envelope phase changes of 100-Hz amplified laser pulses

We report measurements of the carrier-envelope phase (CEP) changes of 100-Hz amplified laser pulses through observation of the spectral interference between a white-light continuum generated by the self-phase modulation and its second harmonic (SH). The fluctuation of the time delay between the fundamental and the SH, which would be caused by the intensity fluctuation of the pulse, was excluded by deriving the time delay and the phase independently. The CEP slip of the amplified pulses showed a random distribution, which would be mainly due to the fluctuation of the uncontrolled oscillator. Effects of the amplification system on the CEP fluctuation are also discussed.

White-light Continuum Generation for Measurements of Carrier-Envelope Phase Changes of Femtosecond Laser Pulses.

We demonstrated measurement of carrier-envelope phase (CEP) changes of amplified laser pulses by utilizing spectral interferometry between the white-light continuum and its second harmonic. The white-light continuum was generated by self-phase modulation (SPM) in a gas-filled hollow fiber. This method enables us to measure the CEP changes of low-repetition rate laser pulses on a shot-to-shot basis. The CEP-change of the amplified laser pulses exhibits random distribution that is mainly due to the CEP change fluctuation of the seed pulses from a carrier-envelope-offset (CEO) uncontrolled oscillator. We also demonstrated the CEO measurement of a mode-locked Ti:Sapphire oscillator. We utilized the self-referencing method, which measures the temporal beat frequency between the white-light continuum and its second harmonic. The white-light continuum was generated by the SPM in a micro-structure fiber. The temporal evolution of the beat signal showed CEO fluctuation of the uncontrolled oscillator.

Single-shot measurement of carrier-envelope phase changes by spectral interferometry

We demonstrated single-shot measurements of spectral interference between a white-light continuum generated in a hollow-fiber and its second harmonic. The interference has information on the carrier-envelope phase of an input pulse to the fiber and the time delay of the blue wing of the continuum. By analyzing the observed spectral interference, we estimated shot-by-shot changes of the carrier-envelope phase. This method is useful for determining the carrier-envelope phase changes of a low-repetition-rate, high-intensity laser.