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 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.
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.
A method for direct laser beam diagnostics based on fiber array is introduced. The designing principle and configuration of the method are presented. The experimental results show that it can provide laser power density distribution information with a high spatial resolution better than 3mm and with an error of measurement less than 3%. The theoretical analysis by finite element method(FEM) for laser radiation indicates that this method should be used to measure the high energy laser beam with diameter of several hundred millimeters for tens of seconds. In addition, the results of fiber array technique are compared, qualitatively, with the spatial beam profile obtained by a near infrared charge coupled device(NIR-CCD). These two methods yield consistent results.