Yongming Nie

Generation of dark hollow femtosecond pulsed beam by phase-only liquid crystal spatial light modulator

Based on the refractive laser beam shaping system, the dark hollow femtosecond pulse beam shaping technique with a phase-only liquid crystal spatial light modulator (LC-SLM) is demonstrated. The phase distribution of the LC-SLM is derived by the energy conservation and constant optical path principle. The effects of the shaping system on the temporal properties, including spectral phase distribution and bandwidth of the femtosecond pulse, are analyzed in detail. Experimental results show that the hollow intensity distribution of the output pulsed beam can be maintained much at more than 1200 mm. The spectral phase of the pulse is changed, and the pulse width is expanded from 199 to 230 fs, which is caused by the spatial-temporal coupling effect. The coupling effect mainly depends on the phase-only LC-SLM itself, not on its loaded phase distribution. The experimental results indicate that the proposed shaping setup can generate a dark hollow femtosecond pulsed beam effectively, because the temporal Gaussian waveform is unchanged.

Generation and analysis of both in-phase and out-phase radially polarized femtosecond-pulse beam

Abstract. Both in-phase and out-phase radially polarized femtosecond-pulse (RPFP) beams have been generated with one phase-only liquid crystal spatial light modulator, which effectively modulates the phase retardation distributions of a pulse beam wavefront by two reflections. The intensity distributions and polarizing properties of both in-phase and out-phase RPFP beams are detected, and the temporal properties of in-phase RPFP beams are investigated in detail. Experimental results indicate that we effectively produce an RPFP beam. And the temporal duration of the output in-phase RPFP beam is 183 fs about 14 fs shorter than the input Gaussian femtosecond-pulse beam. The temporal durations of arbitrary polarized components of an in-phase RPFP beam vary less than 3.5%.

Double-slit interference of radially polarized vortex beams

Abstract. Both radially polarized (RP) and radially polarized vortex (RPV) beams are generated by an experimental setup with one phase-only liquid crystal spatial light modulator which efficiently modulates the phase retardation distributions of input beam by twice reflections. The polarizing properties and double-slit interference of both RP and RPV beams are investigated in detail. Misplacement and tilt appear in double-slit interference fringes of both RP beams and RPV beams in simulations and experiments. The fringe tilt number F in the intermediate region is proportional to the topological charge l of RPV beams with the approximate relation Fs(l)=0.8125l in simulations and Fe(l)=0.8182l in experiments. The double-slit interference method can be utilized to determine and analyze the topological charge of the beams.

Unambiguous demonstration of soliton evolution in slow-light silicon photonic crystal waveguides with SFG-XFROG.

We demonstrate the temporal and spectral evolution of picosecond soliton in the slow light silicon photonic crystal waveguides (PhCWs) by sum frequency generation cross-correlation frequency resolved optical grating (SFG-XFROG) and nonlinear Schrödinger equation (NLSE) modeling. The reference pulses for the SFG-XFROG measurements are unambiguously pre-characterized by the second harmonic generation frequency resolved optical gating (SHG-FROG) assisted with the combination of NLSE simulations and optical spectrum analyzer (OSA) measurements. Regardless of the inevitable nonlinear two photon absorption, high order soliton compressions have been observed remarkably owing to the slow light enhanced nonlinear effects in the silicon PhCWs. Both the measurements and the further numerical analyses of the pulse dynamics indicate that, the free carrier dispersion (FCD) enhanced by the slow light effects is mainly responsible for the compression, the acceleration, and the spectral blue shift of the soliton.

Analysis of femtosecond optical vortex beam generated by direct wave-front modulation

Abstract. The generation of femtosecond optical vortex beam based on direct wave-front modulation with phase-only liquid crystal spatial light modulator is demonstrated. The spatial and temporal properties of the generated femtosecond vortices are investigated in detail. The experimental results show remarkable agreement with the results of the theoretical analysis and simulations, and indicate that the method we utilized can efficiently generate femtosecond optical vortex beam of arbitrary topological charge. The temporal and spectral properties of the femtosecond pulsed beam are hardly affected by the phase dislocation imposed on the wave-front.

Generation of radially polarized femtosecond pulse beam with a polarization plates array

With both ultrafast optical properties of femtosecond pulse and cylindrically symmetric polarization properties of radially polarized light, the radially polarized femtosecond pulse beam has significant applications in super-high density optical storage and ultra-intense lasers. A scheme for generating radially polarized femtosecond pulse beam by a polarization plates array is proposed, in which a phase-only liquid crystal spatial light modulator (LC-SLM) is used to load different phase retardation distribution in transverse into linearly polarized femtosecond pulse beam. Associated with a quarter wave plate, the input linearly polarized femtosecond pulse beam will be converted to radially polarized femtosecond pulse beam at the back of the polarization plates array. The experimental results indicate that the scheme can be well used to generate radially polarized light, and more effective results can be obtained with the increase of sectored polarization plates.

Generation of flat temporal phase distribution of optical pulse by photonic crystal waveguides

We generate a flat temporal-phase distribution optical pulse by 1.3-mm-long photonic crystal waveguide. The effect of coupled pulse energy on the temporal-phase distribution of the output pulse is analyzed by numeral simulating. Simulation results indicate that the root mean square of the output pulse phase decreases to 0.0095 with the optimum coupled pulse energy, which is about 30 pJ, and the narrowest output pulse width is 418 fs. The generation of a flat temporal-phase distribution optical pulse on-chip scale results in potential application prospect in optical communication, pulse compression, pulse shaping and other nonlinear optical application fields.

Flat-top temporal and spatial profiles femtosecond pulse beam generated by phase only modulating

The method for generating temporal flat-top waveform and spatial flat-top profile femtosecond pulse beam by phase and polarization controlling is proposed and demonstrated. Based on direct wave front phase modulating, flat-top spatial intensity distribution can be obtained. Combining a folded 4f zero-dispersion system with a polarization controlling setup, the temporal flat-top waveform is generated. Experimental results indicate that for the input both temporal and spatial Gaussian pulse beam with 363 fs temporal width and 1.5 mm beam waist, the temporal width of the output shaped pulse beam is 1.2 ps and 1.9mm beam waist, and the rms variation is about 9.2%, which prove that the temporal flat-top and spatial flat-top femtosecond pulse beam can be generated effectively.

Operation speed limited by the electric properties of the photorefractive spatial light modulator

With excellent photo-electric and electro-optical effects, the photorefractive spatial light modulators can work as optically addressed modulators in optical information processing systems and parallel optical computing systems. The photo-induced current pulses of the photorefractive spatial light modulator observed in the experiments are analyzed to conclude the characteristics, and to find the relationship between the properties of the current pulse and the structural parameters. Furthermore, the origin of the photo-induced current pulse is analyzed, and the methods to improve the operation speed of the spatial light modulators are proposed. The research results will be significant for extending the applications of the photorefractive spatial light modulators, and pushing ahead the research of all-optical modulation materials and devices for optical supercomputing.

Temporal shaping of the femtosecond pulse by micro-gratings array

A temporal femtosecond pulse shaping device, based on all-diffractive method, is designed for arbitrary waveform generation. The key components of the device are micro-gratings arranged in line. By changing the period and phase pattern of each grating, the diffraction angle, phase and amplitude of the first order diffraction light can be modulated. Experimental results are consistent well with simulation results, which indicate that arbitrary temporal waveforms can be gained by using micro-gratings array. Additionally, the configuration of the device allows for multiple outputs and can operate over a large wavelength range from ultraviolet to infrared pulse.