Preparation and optical properties of thulium doped lanthanum aluminum silicate glass photonic crystal fiber

Abstract

Lanthanum aluminum silicate glass has excellent optical properties, such as high solubility of rare earth ions, good thermal stability, and excellent physicochemical properties. Some of its physicochemical properties are similar to those of silica glass, so it is easy to combine with silica glass to fabricate special optical fibers. It is considered to be an ideal laser glass matrix material. In this paper, a series of Tm<sup>3+</sup> doped lanthanum aluminum silicate glasses with different concentrations for <i>x</i>Tm<sub>2</sub>O<sub>3</sub>-(70–<i>x</i>)SiO<sub>2</sub>-21Al<sub>2</sub>O<sub>3</sub>-9La<sub>2</sub>O<sub>3</sub> (<i>x</i> = 0.2%, 0.4%, 0.6%, 0.8%, 1%, mole fraction) are successfully developed by the traditional high-temperature melting method. Using thulium-doped lanthanum aluminum silicate glass as the fiber core, thulium-doped double-cladding photonic crystal optical fibers are prepared by the stack-and-draw technique and rod in tube method. The core diameter of the thulium-doped lanthanum aluminosilicate glass double-clad photonic crystal fiber is as long as 21.7 μm, the inner cladding diameter is about 119.3 μm, and the outer diameter is about 236.8 μm. The optical properties of thulium-doped lanthanum aluminum silicate glass and its optical fiber are studied experimentally. Under the excitation of a 793 nm laser, the fluorescence bandwidth of thulium-doped lanthanum aluminum silicate glass reaches 223 nm in a wavelength range of 1550–2050 nm. The fiber laser constructed with thulium-doped lanthanum alumino-silicate glass fiber achieves a laser operating at around 2 μm. The fiber laser resonant cavity consists of a pair of dichroic mirrors. The front dichroic mirror has high transmittance for light at 793 nm and high reflectivity (99.9%) for the light within a wavelength range of 1850–2050 nm. The back dichroic mirror has high reflectivity (99.9%) for light at 793 nm and high transmittance (~ 15%) at 2050 nm. Under the experimental conditions in our laboratory, the laser power reaches 253 mW. The highest slope efficiency is 9.67%, which is close to that of the thulium-doped glass fiber laser reported in the literature. It is also found that the central wavelength of fiber laser is shifted toward the longer wavelength with the increase of the optical fiber's length. These results suggest that thulium-doped lanthanum aluminum silicate glass and thulium-doped photonic crystal fiber are suitable for 2-μm fiber laser.