Ying Du

Broadband trapeziform multilayer dielectric grating for femtosecond pulse compressor: design, fabrication, and analysis

Rectangular HfO2 grating fabrication is difficult, so a trapeziform multilayer dielectric pulse compression grating (PCG) has been designed using a genetic algorithm and the Fourier mode method. Simulation results show that a greater than 120 nm wavelength range multilayer dielectric grating (MDG) is obtained with a minus-first-order diffraction efficiency (DE) exceeding 95%. The bandwidth is 136 nm with the minus-first-order DE exceeding 90%. The trapeziform structure shows good tolerances for grating fabrication. The multilayer dielectric is deposited by e-beam evaporation, then the MDG is fabricated by reactive ion beam etching. A 128 nm bandwidth of the MDG for the transverse electric polarization with an efficiency exceeding 90% is obtained, and the maximum DE can reach up to 98.3%. The trapeziform MDG is an ideal PCG for chirped pulse amplification systems with high DE, broad bandwidth and good fabrication tolerances. Laser-induced damage thresholds of 0.53 J cm−2 under 40 fs pulses are experimentally reached with very high and uniform efficiency.

Influence of horizontal damage size of grating ridge on the optical properties of multilayer dielectric gratings

Based on typical laser damage morphologies of multilayer dielectric gratings (MDGs), in this paper the influence of horizontal damage size of the grating ridge on optical properties of MDGs is analyzed by numerical calculations using the rigorous coupled wave analysis method. From simulation results, the optical performance of MDGs decreases with the increase in horizontal damage size of the grating ridge. But the maximum electric field value in the grating ridge and the multilayer enhances constantly with the increase in horizontal damage size, thus inducing further laser damage of MDGs. This paper provides the preliminary study for functional damage of MDGs.

Laser-induced damage of multilayer dielectric for broadband pulse compression grating

The multilayer dielectrics (MLDs) for broad bandwidth 800nm pulse compression gratings were fabricated with optimized design by electron beam evaporation using three different kinds of materials (Ta2O5/SiO2/HfO2), which had more than 99% reflectance with bandwidth larger than 160nm around the center wavelength of 800 nm and high transmission at the exposure wavelength of 413nm. Laser-induced damage behaviors of the mirrors were investigated. It was found that the laser-induced damage threshold (LIDT) of the samples could reach 1.0J/cm2 and 2.0J/cm2 in the normal beam (57 degrees, TE mode) at pulse duration of 50fs and 120fs, respectively. The depth information of the damage sites at these two cases was explored by atomic force microscope (AFM). The reason of the sample having so high LIDT was also discussed in this paper. The MLDs provide a solid base for the high laser threshold 800nm pulse compression gratings and may open a new way for broad bandwidth 800nm reflectance coatings used in the ultrashort pulse laser system.

Laser-induced damage properties of antireflective porous glasses

In this paper, porous nanostructures on BK7 glass were manufactured by chemical treatment in order to obtain antireflection (AR) components with improved laser damage resistance. The damage-resistant properties of the samples with nearly 100% transmittances at three pulsed laser wavelength were investigated. The damage tests showed that the BK7 glass with AR nanostructures can achieve the LIDTs of 58J/cm2, 20 J/cm2 and 12 J/cm2 under the irradiation of 12ns 1064nm pulses, 10ns 532nm pulses and 8ns 355nm pulses, respectively. These values are much higher than those of AR coated glasses, but are almost the same level of un-etched substrate. The effect of structural properties on electric field distribution of porous surface was investigated by a three-dimensional (3D) finite difference time-domain (FDTD) model. The simulation results and the morphology of damage site on porous glass are compared to those of un-etched surface, and are discussed to reveal the possible damage mechanism. Finally, some possible solutions to improve the LIDT are proposed.

Femtosecond laser damage of all-dielectric pulse compression gratings

Damage tests of the ultrashort pulse compression gratings were implemented by an 800 nm laser with a pulse width of 40–100 fs. The damaged character of grating ridges demonstrates that laser damage of gratings is closely related to total deposited energy in grating ridges. The average electric field strength in grating ridges is naturally proposed to calculate the theoretical threshold based on the electron production model via photoionization and avalanche ionization, which makes theoretical and experimental results consistent. The results indicate that laser damage resistance of a grating can be evaluated by the average electric field in grating ridges.