Yanzhi Wang

Thermal-dynamical analysis of blister formation in chirped mirror irradiated by single femtosecond lasers

The laser-induced damage behaviors of chirped mirrors (CMs) are studied by single 800 nm, 38 fs lasers. The CMs provide group delay dispersion of around -60  fs² and average reflectivity of about 99.4% with bandwidth range of 200-300 nm at a central wavelength of 800 nm. Interestingly, a circular blister feature appears in the CMs at a wide range of laser fluence. An optical microscope, atomic force microscope, scanning electron microscope, and surface profiler are applied to describe the blister characteristics. An adiabatic expansion model of ideal gas is adopted to illustrate the formation dynamics of blisters. The evolution of blisters can be explained by partial evaporation of the film and a subsequent gas expansion, driving the bulging of the film stack up to the stress limit, where the blister fractures. According to this model, the energy absorption ratio of blisters increases monotonously with increasing laser fluence before the occurrence of the focal spot confinement effect.

Ultrashort laser-driven stable-buckling of blisters in chirped mirror

Multilayer dielectric mirrors (niobium pentoxide/silica) were irradiated by single 800 nm-38fs laser pulses. A circular blister feature appeared in the chirped mirrors at a wide range of laser fluence, while only existed above a few percent of the damage threshold for the high-reflector. Absorptive source of laser energy can be determined by combining theoretical simulation with focused ion beam technology. Mechanical analysis with a linear-elastic buckling model reveals the blister evolution belonging to a stable-buckling process until the blister radius confined by limited laser spot size.

High dispersive mirrors for erbium-doped fiber chirped pulse amplification system

We report on the development of near-infrared high dispersive mirrors (HDM) with a group delay dispersion (GDD) of -2000 fs2. A HDM pair based on one optimized result at two reference wavelengths (1550 nm and 1560 nm) can reduce the total oscillation of the GDD effectively in the wavelength range of 1530-1575 nm. This HDM pair is designed and fabricated in a single coating run by means of the nonuniformity in film deposition. For the first time, near-infrared HDMs with two different reference wavelengths have been successfully applied in an erbium-doped fiber chirped pulse amplification system for the compression of 4.73 ps laser pulses to 380 fs.

Design and analysis of different types of dispersion mirrors

Three types of dispersion mirrors are designed and discussed. The first type is the complementary chirpedmirror pair used for providing smooth group delay dispersion (GDD) in the wavelength range of 550−1050 nm. Such mirrors are obtained by shifting the GDD oscillation period. The second type of mirror combines the characteristics of chirped mirrors and Gires-Tournois interferometer mirrors. It provides a high dispersion compensation of about −800-fs GDD in the range of 780−830 nm and about −1150-fs GDD in the range of 1020−1045 nm. The third type is a protected silver mirror with high reflectivity and low dispersion in the range of 650−1000 nm at 45◦. OCIS codes: 310.1620, 320.5520, 320.7160. doi: 10.3788/COL201008S1.0018.

Reducing electric-field-enhancement in metal-dielectric grating by designing grating with asymmetric ridge

Diffraction gratings are an essential optical component of high-power, short-pulse lasers. The maximum output of high-power pulsed lasers is always determined by laser resistance of gratings and this resistance is strongly dependent on the local near electric field intensity in the grating structure. We presented a novel method of reducing electric-field-enhancement in metal-dielectric grating by designing asymmetric grating ridge while maintaining high diffraction performance. Compared with the common isosceles trapezoidal grating, the grating with asymmetric ridge got a 0.04% reduction of diffraction efficiency in TE polarization at 1053 nm incident wavelength but a 21.3% reduction of maximal electric-field-enhancement in grating structure. This method can be applied to any surface-relief gratings to reduce the electric-field-enhancement for improving the laser induced damage threshold (LIDT) of grating and supporting the grating-based chirped pulse amplification (CPA) system to develop into higher peak-power levels.

Effects of substrate etching on laser-induced damage of 355nm space coatings

355nm UV lidars are of high spectral resolution and are demanded in space-borne applications. As a result of the vulnerability of the coatings in the laser systems and coatings irradiated by 355nm UV laser are more prone to be damaged than visible or infrared lasers. Thus, the study of the processing on substrate surface will contribute to the improvement of the performance of optical coatings. The dual ion beam sputtering can provide compact and stable layers for space applications. Thus, Al2O3/SiO2 high reflection(HR) and anti-reflection(AR) coatings deposited on acid etched substrate were prepared by dual ion beam sputtering. Damage characteristics on the surface were analyzed after 355nm laser irradiation on the coatings. The damage morphology measured by focused ion beam-field emission scanning electron microscopy (FIB-FESEM) and depth measurement of damage pits hint that the damage of HR coatings occurs in the coating layers rather than the substrate. The HR and AR coatings deposited on acid etched substrate are of larger changes on laser-induced damage thresholds(LIDT) than these on the non-etched substrate. The damage mechanisms of coatings are attributed to thermal absorption and the damage area is a molten pool surrounded by mechanical ejection. The etched area with nicks caused the damage on the HR coatings deposited on etched substrate. And due to the elimination of defects on the subsurface of the AR coatings by etching method, the LIDT of AR coatings was improved.

Design and fabrication of thin-film spatial filter

The combination of lens and pinhole limits the enhancement of the laser output power in the high-power laser system. Low-pass spatial filter without focusing can surmount the drawbacks of the pinhole filters. The low-pass spatial filters based on multilayer dielectric film are analyzed and their filtering performances are validated. The non-focusing low-pass spatial filter is successfully explored to substitute for the focusing one. The design method is based on phase-shifted Rugate thin-film spatial filter, narrow bandpass filter and the combined device of long-wave-pass and short-wave-pass cutoff filters, and the angular spectrum bandwidth of bandpass filter are up to submillimeter radians. We mainly discuss three design methods and point out their advantages and disadvantages to find out the best one. The experimental results show that the effects of random and system error during depositing the filter is mainly responsible for the deviation of the designed and measured values.

Design and Preparation of Chirped Mirrors Used in Ti:sapphire Lasers

The designed chirped-mirror (CM), around -60fs2 group delay dispersion in the wavelength 700-900nm, has been prepared by ion beam sputtering. By balancing the intra-cavity dispersion with manufactured non-pairs CMs, 9.5fs pulse is obtained.