We report the generation of optical pulses with an energy of 0.55 m J and duration of 1.6-cycle (4.4 fs) at repetition rate of 1 kHz using a differentially pumped hollow fiber and chirped mirrors. Compared to the statically gasfilled scheme, the differentially pumping hollow fiber is demonstrated to support more energy output with higher transmission efficiency, and to increase the spectral broadening due to a reduction of ionization defocusing in plasma at the fiber entrance. The differentially pumping technique is proved to be an effective way to obtain optical pulses with mono-cycle and higher energy.

https://iopscience.iop.org/article/10.1088/0256-307X/27/5/054211/pdf

Generation of Sub-2 Cycle Optical Pulses with a Differentially Pumped Hollow Fiber

This paper investigates the effects of walk-off among optical pulses on cross-phase modulation induced modulation instability in the normal dispersion region of an optical fibre with high-order dispersion. The results indicate that, in the case of high-order dispersion, the walk-off effect takes on new characteristics and will influence considerably the shape, position and especially the number of the spectral regions of the gain spectra of modulation instability. Not only the group-velocity mismatch, but also the difference of the third-order dispersion of two optical waves will alter the gain spectra of modulation instability but in different ways. Depending on the values of the walk-off parameters, the number of the spectral regions may increase from two to at most four, and the spectral shape and position may change too.

Effects of walk-off on cross-phase modulation induced modulation instability in an optical fibre with high-order dispersion

Starting from the extended nonlinear Schrodinger equation in which the self-steepening effect is included, the evolution and the splitting processes of continuous optical wave whose amplitude is perturbed into time related ultra-short optical pulse trains in an optical fibre are numerically simulated by adopting the split-step Fourier algorithm. The results show that the self-steepening effect can cause the characteristic of the pulse trains to vary with time, which is different from the self-steepening-free case where the generated pulse trains consist of single pulses which are identical in width, intensity, and interval, namely when pulses move a certain distance, they turn into the pulse trains within a certain time range. Moreover, each single pulse may split into several sub-pulses. And as time goes on, the number of the sub-pulses will decrease gradually and the pulse width and the pulse intensity will change too. With the increase of the self-steepening parameter, the distance needed to generate time-dependent pulse trains will shorten. In addition, for a large self-steepening parameter and at the distance where more sub-pulses appear, the corresponding frequency spectra of pulse trains are also wider.

Generation of time-dependent ultra-short optical pulse trains in the presence of self-steepening effect

Experiments and simulations of the spectral phase of few-cycle fs-laser pulses during supercontinuum generation in a neon gas filled hollow core fiber

We present an experimental investigation of a filamentation-assisted fourth-order nonlinear optical process in KTP crystals pumped by intense 1.53 eV (807 nm) femtosecond laser pulses. Femtosecond light pulses at 2.58 eV (480 nm) are generated by the fourth-order nonlinear polarization (P(4) (ω2) = χ(4) (ω2, ω, ω, ω, – ω1) E3 (ω) E* (ω1), where E(ω) corresponds to the pump frequency and E(ω1) to the supercontinuum generated through filamentation). If the system is seeded by a laser beam at ω1 or ω2 and there are spatial and temporal overlaps with the pump beam, E(ω1) and E(ω2) are simultaneously amplified. When the intensity of the seed laser beam exceeds a certain intensity threshold, the contribution of P(4) (ω) = χ(4) (ω, ω1, ω2, – ω, – ω) E (ω1) E (ω2) (E* (ω))2 becomes non-negligible, and the amplification weakens. The conversion efficiency from the pump to the signal at 2.58 eV (480 nm) attains to 0.1%.

Filamentation-assisted fourth-order nonlinear process in KTP crystal

We propose a novel technique for generating intense few to mono-cycle femtosecond pulses. The simulation demonstrate that for the temperature difference of 300K, the spectrum of the output pulses is increased by 67% and the transform limited pulse width is reduced almost by half, compared with those obtained with hollow fibres in uniform temperature.

Simulation of Femtosecond Pulse Propagation through Hollow Fibre Filled with Noble Gases of Gradient Temperature

The spectrum broadening and pulse compression of the ultrashort pulse propagatio n in cascaded hollow fibers,by solving the generalized nonlinear Schrdinger eq uation with the split_step Fourier Method,are discussed.The simulation shows tha t the technique using cascaded hollow fibers is a reliable route to achieve few to mono-cycle pulses of high energy.

Spectrum broadening of ultrashort pulse propagation in cascaded hollow fibers and pulse compression

By increasing the local temperature at different positions along the argon gas-filled tube, the difference in filament and spectrum broadening was compared in this paper. The experimental results show that the filament can be controlled by increasing the local temperature of the gas-filled tube. The best way to realise the maximum of such a influence is to heat at the focus point. Spectrum broadening will be narrow with the increase of the local temperature.

Difference in filament and spectrum broadening induced by femtosecond pulses in argon gas with a temperature gradient at different positions

Temperature controlled filamentation is experimentally demonstrated in a temperature gradient gas-filled tube. The proper position of the tube is heated by a furnace and two ends of the tube are cooled by air. The experimental results show that multiple filaments are shrunken into a single filament or no filament only by increasing the temperature at the beginning of the filament. This technique offers another degree of freedom of controlling the filamentation and opens a new way for intense monocycle pulse generation through gradient temperature in a noble gas.

Temperature controlled filamentation in argon gas

The well-known Fraunhofer multi-slit diffraction is described as the multi-slit interference modulated by the single-slit diffraction, namely the multiplication between the single-slit diffraction factor and the multi-slit interference factor. By considering the simplified argument we show that the multi-slit diffraction of evanescent waves which are in the near-field region also has the interference and diffraction effects, and that this two-fold effect can be expressed as the convolution of the diffraction factor and the interference factor. Our conclusion could be helpful to understand the contribution of evanescent waves to the optical responses of sub-wavelength structures such as slits and grooves.

http://iopscience.iop.org/article/10.1088/0256-307X/25/6/030/pdf

Song Zhen-Ming

Papers

Simulation of Femtosecond Pulse Propagation through Hollow Fibre Filled with Noble Gases of Gradient Temperature

Spectrum broadening of ultrashort pulse propagation in cascaded hollow fibers and pulse compression

Difference in filament and spectrum broadening induced by femtosecond pulses in argon gas with a temperature gradient at different positions

Temperature controlled filamentation in argon gas

Multi-Slit Diffraction of Evanescent Electromagnetic Waves