Under the pump of a diode laser at 790 nm, self-pulsing phenomena of ~2 μm Tm<sup>3+</sup>-doped fiber lasers are investigated.
Laser cavity configuration is changed by using different output-coupling mirrors. When the polished fiber-end (Fresnel
reflection) is used as the output coupler, the cavity is dubbed as "bad" cavity. With this "bad" cavity, regular laser pulse
trains can be obtained near threshold pump level. The pulse width and repetition rate are several micro seconds and several kilohertz, respectively. At higher pump levels, the regular pulse-train shape is broken, and some random pulses occur. At the same time, similar mode-locking phenomenon can be observed. When the output coupler is changed to a T=10% (at 2μm) mirror, regular laser pulse train still can be observed near threshold pump. However, a slight increase of pump leads to randomization of the pulse train. Similar mode-locking phenomenon was not observed with this kind of cavity. When the output coupling is decreased to T=5%, even near the pump threshold, regular pulse train can not be achieved. With increased pump powers, laser output changes from pulse state to continuous wave. Based on the experimental results, a theoretical model is proposed and the origin of self-pulsing is discussed.
Graded-index (GRIN) fiber lens arrays are fabricated from commercial GRIN fibers to collimate a high-power laser diode array. The beam divergence angles are reduced to 4.2 and 14.7 mrad in the fast and slow axes, respectively. The influences of smile and fluctuation in fiber length are discussed. Using an aspherical focal lens system, about 74% power can be launched into a fiber with a numerical aperture (NA) of 0.22 and a core diameter of 400 µm.
Demonstrations of CW lasing in ceramic Cr<sup>2+</sup>:ZnSe are reported. The laser consists of a 1.7-mm
thick ceramic Cr<sup>2+</sup>:ZnSe disk pumped by a double-clad Tm-silica fiber laser at 2050 nm. Using a
concave HR mirror with a radius of curvature of 500 mm as the rear mirror, the laser delivers up to
1030mW of radiation around 2.367 μm.
In order to assess the mutagenic effects of ultraviolet (UV) and solar irradiation on DNA, Raman spectroscopy is used to investigate the structural changes of calf thymus DNA in aqueous solution after ultraviolet radiation. The damage to DNA induced by UV is usually carried out with a germicidal lamp, which mainly covers the region of UVC. For the sake of making a complete investigation of the damage to DNA by ultraviolet radiation, we use different intervals of UV (UVA, UVB and UVC) to conduct our experiments. Such UV radiation is obtained from a solar UV simulator (SS) made by us, which can be conveniently adjusted to different wavelengths and radiation intensities. From the comparison of the Raman spectra of DNA in aqueous solution before and after ultraviolet radiation, it can be inferred that the UV-C has a serious influence on the DNA molecular conformation and damages the hydrogen bonds and bases, UV-B only damages the DNA molecular structure to some extent while UV-A almost does not play an impact on the DNA molecular conformation. For all the three regions of UV, the damage increases with the elongation of irradiation time and is first observed in pyrimidine-dimer bases and deoxyribose. The experimental results also partly support the formation of several types of dimeric lesions between adjacent pyrimidine bases, most notably cyclobutane pyrimidine dimers (CPDs) and the Dewar valence isomers.