Chengquan Li

Carrier-envelope phase shift caused by variation of grating separation

The effects of variation of the grating separation in a stretcher on the carrier-envelope (CE) phase of amplified pulses are investigated. By translating one of the telescope mirrors in the stretcher with a piezoelectric transducer, it is found that a 1 mum change of the distance causes a 3.7+/-1.2 rad shift of the CE phase, which is consistent with theoretical estimations. The results indicate that optical mounts used for gratings and telescope mirrors must be interferometrically stable; otherwise their vibration and thermal drift will cause significant phase error. The CE phase drift was corrected by feedback controlling the grating separation.

Precision control of carrier-envelope phase in grating based chirped pulse amplifiers.

It is demonstrated that the carrier-envelope (CE) phase of pulses from a high power ultrafast laser system with a grating-based stretcher and compressor can be stabilized to a root mean square (rms) value of 180 mrad over almost 2 hours, excluding a brief re-locking period. The stabilization was accomplished via feedback control of the grating separation in the stretcher. It shows that the long term CE phase stability of a grating based chirped pulse amplification system can be as good as that of lasers using a glass-block stretcher and a prism pair compressor. Moreover, by adjusting the grating separation to preset values, the relative CE phase could be locked to an arbitrary value in the range of 2pi. This method is better than using a pair of wedge plates to adjust the phase after the hollow-core fiber compressor. The CE phase stabilization after a hollow-core fiber compressor was confirmed by a CE-phase meter based on the measurement of the left-to-right asymmetry of electrons produced by above-threshold ionization.

Carrier-envelope phase stabilized 5.6fs, 1.2mJ pulses

The authors report the generation of 1.2mJ pulses with a duration of 5.6fs from a neon filled hollow-core fiber seeded with carrier-envelope phase stabilized 2.2mJ, 25fs pulses. The carrier-envelope phase after the fiber was measured by a second, out-loop f-to-2f interferometer. With seed pulse power locked, the carrier-envelope phase of the two-cycle pulses is controlled to a standard deviation of 370mrad. The peak power of the carrier-envelope phase stabilized pulses, 0.2TW, is twice that previously generated. The significance of seed pulse energy stability for carrier-envelope phase stabilization of few-cycle laser pulses is demonstrated.

Determining the phase-energy coupling coefficient in carrier-envelope phase measurements

By using two f-to-2f interferometers, we measured the phase-energy coupling coefficient for the first time. The results reveal that a 1% in-loop laser energy change causes a 160 mrad carrier-envelope phase shift to output pulses.

Reduction of fast carrier-envelope phase jitter in femtosecond laser amplifiers.

The phase noise of the f-to-2f interferometer used for stabilizing the carrier-envelope phase of a femtosecond laser oscillator was studied by adding a He-Ne laser beam co-propagating with the short pulse laser beam. The noise was reduced to ~60 mrad by stabilizing the optical path length difference of the interferometer. This suppressed the fast jitter of the carrier-envelope phase of the amplified laser pulses from 79 to 48 mrad.

A low-loss, robust setup for double optical gating of high harmonic generation

Previously, a second harmonic field was added to a polarization gating field by a Mach–Zehnder interferometer to gate the high harmonic generation process in argon gas. To reduce the losses of the interferometer, we developed a collinear setup consisting of only two quartz plates and a barium borate crystal. The high intensity at the focus allowed the double optical gating to be performed on neon gas. As a result, a supercontinuous spectrum was produced capable of supporting 130as. There is no delay jitter between the second harmonic field and the polarization gating field associated in this setup, which is necessary for generating stable single isolated attosecond pulses.

Coupling between energy and phase in hollow-core fiber based f-to-2f interferometers

The dependence of the carrier-envelope (CE) phase of the pulses from a hollow-core fiber on the input laser energy was studied using two f-to- 2f interferometers. The CE phase in the in-loop f-to-2f interferometer was measured with the octave spanning white-light spectrum from the hollow-core fiber, whereas the out-of-loop interferometer was based on a sapphire plate. By modulating the input power of the in-loop interferometer and measuring the out-of-loop CE phase at the same time, the coupling coefficient between the measured CE phase and the laser energy for the hollow-core fiber was determined to be 128 mrad per 1% energy change .

Mechanism of phase-energy coupling in f-to-2f interferometry.

White-light generation has been used widely in single-shot f-to-2f interferometers for stabilizing the carrier-envelope (CE) phase of laser amplifiers. The accuracy of the relative phase values measured by such an interferometer is affected by fluctuations in the laser pulse energy. A simple two-step model is proposed to explain the mechanism that couples the laser energy and the CE phase. The model explains the experimentally observed dependence of the group delay between the f and the 2f pulses on the laser energy, as well as the CE phase shift caused by the pulse energy variation.

Generation of 0.5 mJ, few-cycle laser pulses by an adaptive phase modulator

Previously, pulses shorter than 4 fs were generated by compressing white light from gas-filled hollow-core fibers with adaptive phase modulators; however, the energy of the few-cycle pulses was limited to 15 microJ. Here, we report the generation of 550 microJ, 5 fs pulses by using a liquid crystal spatial light modulator in a grating-based 4f system. The high pulse energy was obtained by improving the throughput of the phase modulator and by increasing the input laser energy. When the pulses were used in high harmonic generation, it was found that the harmonic spectra depend strongly on the high order spectral phases of the driving laser fields.

Carrier-envelope phase stabilization by controlling compressor grating separation

Previously, we demonstrated the stabilization of the carrier-envelope (CE) phase of the laser pulses from a chirped pulse amplifier by controlling the effective grating separation in the stretcher. In this work, we show that the CE phase can also be stabilized to ∼230mrad by controlling the grating separation in a compressor. The cutoff frequency of the piezoelectric transducer mounted grating for the phase stabilization system was found to be higher than 60Hz.