The multi-tone amplification for the purpose of mitigating stimulated Brillouin scattering (SBS) effect is investigated. Different wavelengths and power ratio are considered in the simulation model to achieve optimal results. The numerical simulation results indicate that SBS is effectively suppressed in a fiber amplifier with an output from 53.7W to 96.9W by utilizing this approach.
We report on the recent progress on high power pulsed 2.8 μm Er<sup>3+</sup>-doped ZBLAN fiber laser through techniques of passively and actively Q-switching in our research group. In passively Q-switched operation, a diode-cladding-pumped mid-infrared passively Q-switched Er<sup>3+</sup>-doped ZBLAN fiber laser with an average output power of watt-level based on a semiconductor saturable absorber mirror (SESAM) was demonstrated. Stable pulse train was produced at a slope efficient of 17.8% with respect to launched pump power. The maximum average power of 1.01 W at a repetition rate of 146.3 kHz was achieved with a corresponding pulse energy of 6.9 μJ. The maximum peak power was calculated to be 21.9 W. In actively Q-switched operation, a diode-pumped actively Q-switched Er<sup>3+</sup>-doped ZBLAN fiber laser at 2.8 μm with an optical chopper was reported. The maximum laser pulse energy of up to 130 μJ and a pulse width of 127.3 ns at a repetition rate of 10 kHz with an operating wavelength of 2.78 μm was obtained, yielding the maximum peak power of exceeding 1.1 kW.
The discoloration and optical characteristics of the gold plating film under long-time high power laser irradiation are investigated. The fabrication process of gold plating on nickel underplate on rough surface of copper and aluminum alloy substrates is introduced. The measurement results of the diffuse reflectivity for the samples with different surface roughness indicate that roughness of the gold layer surface should be 4μm to obtain the maximum value of diffuse reflectivity. The discoloration and variation of diffuse reflectivity are experimentally studied under 2000W irradiation. The research results show that the discoloration and degrading of reflectivity are caused by the diffusion of Ni to the gold plating surface and forming NiO thin film due to the porosity of the gold film and high temperature treatment. A change of diffuse reflectivity related mechanism is described. Several plating solution recipes are used to eliminate the discoloration and mitigate the degrading of the reflectivity on gold surface.
The output power of a narrow line-width laser is usually limited by the Stimulated Brillouin Scattering effect. In Master Oscillator Power Amplifier structures, multi-point pump could rearrange the gain distribution along the fiber, leading to the suppression of the Stimulated Brillouin Scattering effect with maintained amplification efficiency. A theoretical model concerning 100W-level fiber amplifiers is proposed. Stimulation is performed to analyze the amplification process of the laser signal and Stimulated Brillouin Scattering. The results demonstrate that the power of scattering light decreases from 3.2W to 6.8mW (with two-point pump) indicating the effectiveness of this new technology in Stimulated Brillouin Scattering suppression.
Modeling of Tm-doped fiber lasers pumped with 793 nm, 1.6 μm and 1.9 μm is presented and compared. Output performance of three different pump schemes with active fiber length, pump power and output reflectivity is investigated. Numerical simulation shows that, with 793 nm pump, the cross relaxation process is of vital importance for high efficient operation of Tm-doped fiber laser. And, 1.9 μm pump scheme is more likely to offer even higher output compared with 793 nm pump and 1.6 μm pump.
A compact passively Q-switched and mode-locked erbium-doped fiber laser based on graphene saturable absorber was reported in this paper. A fiber ferrule, which had graphene deposited on the core region, was used as the saturable absorber. The Q-switched operation was initiated with a low pump threshold of about 50 mW at 974 nm and the repetition rate can be widely tuned from 14 kHz to 70 kHz along with the increase of the pump power. Moreover, the mode-locking state working at 1559.7 nm with a 0.4nm spectral bandwidth and about 3 ns pulse duration was also demonstrated in the same ring cavity when the pump power increased to about 150 mW. This is, to the best of our knowledge, the first report of the fiber laser, which had the both Q-switched operation with so wide repetition rate and mode-locking operation in the same ring cavity based on the same graphene saturable absorber.
In this paper, Torrance-Sparrow and Oren-Nayar model is adopt to study diffuse characteristics of laser target board. The model which based on geometric optics, assumes that rough surfaces are made up of a series of symmetric V-groove cavities with different slopes at microscopic level. The distribution of the slopes of the V-grooves are modeled as beckman distribution function, and every microfacet of the V-groove cavity is assumed to behave like a perfect mirror, which means the reflected ray follows Fresnel law at the microfacet. The masking and shadowing effects of rough surface are also taken into account through geometric attenuation factor. Monte Carlo method is used to simulate the diffuse reflectance distribution of the laser target board with different materials and processing technology, and all the calculated results are verified by experiment. It is shown that the profile of bidirectional reflectance distribution curve is lobe-shaped with the maximum lies along the mirror reflection direction. The width of the profile is narrower for a lower roughness value, and broader for a higher roughness value. The refractive index of target material will also influence the intensity and distribution of diffuse reflectance of laser target surface.
Cross relaxation (CR) process in thulium ions is described. Performance of Tm-doped fiber lasers with different dopant concentrations is evaluated numerically with and without CR. Simulation shows that CR process can not only improve the slope efficiency and output of the laser system, but also lower the lasing threshold and extend the growth momentum of the laser performance. Backward LD-clad-pumped Tm-doped fiber lasers are built with Tm-doped fibers of different doping levels. A maximum output of 35.3W around 2μm is obtained with a slope efficiency of 47.2% from the 4.5wt.%- doped fiber laser while a higher slope efficiency of 54.1% was achieved from the 6.8wt.%-doped fiber laser. And, modeling shows that these laser systems are much more efficient than that without CR process.
In the development of high energy laser (HEL) beam diagnostic equipment with optical attenuation by dielectric multilayer films reflector, in order to fulfill the sampling uncertainty request of the system, the transmittance of the attenuator should keep as a constant or only vary in a small range when the incident angle of laser changes. To address this, we analyzed the principle of the conventional dielectric multilayer films reflector and put forward a new and simple offset-central-wavelength multilayer films reflector (OCWMFR) involving no change of the dielectric multilayer films materials and basic fabrication process. Theoretical simulation and experimental results show that the reflector has a good transmittance consistency and can meet the attenuation and sampling requirement of HEL beam diagnosis.