Yueliang Wang

Enhanced 2.0 μm emission and gain coefficient of transparent glass ceramic containing BaF 2 : Ho 3+ ,Tm 3+ nanocrystals

Transparent glass ceramic containing BaF(2):Ho(3+),Tm(3+) nanocrystals has been prepared by melt quenching and subsequent thermal treatment. The precipitation of BaF(2) nanocrystals was confirmed by X-ray diffraction and high-resolution transmission electron microscopy. Intense 2.0 microm fluorescence originating from Ho(3+): (5)I(7) --> (5)I(8) transition was achieved upon excitation with 808 nm laser diode. A large ratio of forward Tm(3+) --> Ho(3+) energy transfer constant to that of backward process indicated high efficient energy transfer from Tm(3+)((3)F(4)) to Ho(3+)((5)I(7)), benefited from the reduced ionic distances of Tm(3+)-Tm(3+) and Tm(3+)-Ho(3+) pairs and low phonon energy environment with the incorporation of rare-earth ions into the precipitated BaF(2) nanocrystals. The results indicate that glass ceramic is a promising candidate material for 2.0 microm laser.

Low-repetition-rate ytterbium-doped fiber laser based on a CFBG from large anomalous to large normal dispersion

We report on the long-cavity ytterbium-doped fiber laser based on a chirped fiber Bragg grating (CFBG) to control the dispersion in the cavity. The proposed fiber lasers operating in large anomalous dispersion and large normal dispersion are intensively studied in this paper. To the best of our knowledge, this is the first time for us to give the dispersion map over such a wide range. The experimental results show that, for large anomalous dispersion, the state of soliton rains can be obtained. Nanosecond pulse is obtained in the large normal dispersion regime with spectral filter which is different from the operation mode without spectral filter. This study may pave the way to best apply the fiber laser in many fields.

Laser damage dependence on the size and concentration of precursor defects in KDP crystals: view through differently sized filter pores.

We investigate the laser-induced damage performance at 1064 nm of potassium dihydrogen phosphate (KDP) crystals grown using filters of different pore sizes. The aim is to explore a novel method for understanding laser-matter interactions with regard to physical parameters affecting the ability of damage precursors to initiate damage. By reducing the pore size of filters in continuous filtration growth, we can improve laser damage resistance. Furthermore, we develop a model based on a Gaussian distribution of precursor thresholds and heat transfer to obtain a size distribution of the precursor defects. Smaller size and/or lower concentration of precursor defects could lead to better damage resistance.

Temporal contrast enhancement of picosecond pulses based on phase-conjugate wave generation

A practical technique based on the phase-conjugate wave (PCW) generation is proposed to improve the temporal contrast of the picosecond pulses. Our theory predicts the temporal contrast of the picosecond pulses can be enhanced to about the cube of the temporal contrast of the input pulse via the PCW generation, in which the conversion efficiency from the incident pulse to the PCW is about 25%. In a proof-of-principle experiment, the temporal contrast of picosecond pulses was enhanced from 1.7×10(3) to 8.6×10(8) with the conversion efficiency of 10%. This technique is promising to effectively eliminate the background of the ultrashort and ultraintense laser pulses in the future.

Mitigation of scattering defect and absorption of DKDP crystals by laser conditioning

The variation of scattering and absorption in DKDP crystals by laser conditioning was investigated by combining light scattering technique and on-site transmittance measurement technique. Laser-induced disappearance of scattering defects was observed, and variation of transmittance was achieved. Using Mie theory, a kind of absorbing defects, aside from scattering defect, was discovered. Moreover, the experimental results demonstrated that the absorption of crystal could be mitigated by laser conditioning.

Laser induced defect decrement in DKDP crystals varied with photon energy

In this paper, laser induced reactions of bulk defects in DKDP crystals were real-time detected by ultra-microscopy while high power laser irradiation with different photon energy, and the defect elimination processes were observed. It’s found that there were two kinds of bulk defects that can be eliminated: submicron-scale defect and nanoscale defect clusters. The decrement of submicron-scale defects was related to the laser parameters, such as laser fluence and photon energy. The defect decrement could achieve its maximum value at appropriate laser fluence, while the photon energy was fixed. It indicated that up to ~47% of defects could be eliminated by laser irradiation at 3.50eV (355nm). While the laser fluence was fixed, the amount of defects reduced by laser irradiation at 3.50eV was larger than that at 1.17eV (1064nm). The nanoscale defect clusters were hard to be eliminated by laser irradiation at 1.17eV, while most of them could be reduced by laser irradiation at 3.50eV.

Modeling the effect of nanosecond laser conditioning on the femtosecond laser-induced damage of optical films

The effect of nanosecond laser conditioning on the femtosecond laser-induced damage behaviors of Al2O3, HfO2, SiO2 single layers and Al2O3/SiO2 high reflectors (HR) are explored. During femtosecond laser damage test, negative effects on enhancing the femtosecond laser-induced damage threshold (LIDT) of optical films after the nanosecond laser conditioning is found, which is opposite to the LIDT improvement in the nanosecond range. To explain the mechanism after nanosecond laser conditioning, a theoretical model including multiphoton ionization (MPI), avalanche ionization (AI) and decays of electrons with one defect state is built to simulate the evolution of electron density in the conduction band. A permanent mid-gap defect state resulting from the process of laser conditioning is introduced in our model, which is found to contribute seed electrons to conduction band and hence accelerate the final breakdown. Both the experimental result and theoretical calculation agree very well with each other.

Influence of laser wavelengths and polarizations on absorption of KDP crystal

We measured the absorption of type I doubler KDP crystal at different laser wavelengths (1064, 532 and 355 nm) by using laser induced deflection (LID) technique. We also performed the absorption measurements in the cases where beam polarization was parallel to the principal plane (Pd//Pp) and perpendicular to it (Pd⊥Pp). To account for the experiment results, a model based on crystal dichroism was developed to calculate the absorption coefficients for O ray (αo) and E ray (αe) at different laser wavelengths. It is found that the dichroism is manifested especially clearly at 1064nm, but not clearly at 355nm/532nm. It implies that the absorption at 1064nm is mainly due to lattice absorption, whereas defects absorption is responsible for the absorption at 355nm/532nm.

Characterization of laser induced damage of HR coatings with picosecond pulses

The effect of protective layer on the picosecond laser-induced damage behaviors of HfO2/SiO2 high-reflective (HR) coatings are explored. Two kinds of 1064nm HR coatings with and without protective layer are deposited by electron beam evaporation. Laser-induced damage tests are conducted with 1064nm, 30ps S-polarized and P-polarized pulses with different angle of incidence (AOI) to make the electric fields intensity in the HR coatings discrepantly. Damage morphology and cross section of damage sites were characterized by scanning electron microscope (SEM) and focused ion beam (FIB), respectively. It is found that SiO2 protective layer have a certain degree of improvement on laser induced damage threshold (LIDT) for every AOIs. The onset damage initiated very near to the Max peak of e-field, after which forms ripple-like pits. The damage morphology presents as layer delamination at high fluence. The Laser damage resistance is correspond with the maximum E-intensity in the coating stacks.

Influence of the size and concentration of precursor on laser damage performance in KDP crystal

Laser-induced bulk damage in potassium dihydrogen phosphate (KDP) and its deuterated analog (DKDP) crystals for nanosecond pulses is caused by light-absorbing precursor defects, which are formed during crystal growth. However, current chemical analysis and spectroscopy techniques fail to identify the nature of the responsible precursor defects because of their “invisible” concentration and/or size. In this study, the aim was to explore a novel method for understanding laser–matter interactions with regard to physical parameters, such as size and concentration, affecting the ability of damage precursors to initiate damage. Laser-induced damage performance at 1064 nm of KDP crystals grown using filters of different pore sizes was investigated. By reducing the pore size of filters in continuous filtration growth, laser damage resistance was improved. Furthermore, a model based on a Gaussian distribution of precursor thresholds and heat transfer was developed to obtain a concentration and/or size distribution of the precursor defects. The results revealed that smaller size and/or lower concentration of precursor defects could lead to better damage resistance.