Fabrication of Germanosilicate glass optical fibers containing Tm2+ ions and their nonlinear optical properties

Germanosilicate glass optical fibers incorporated with the Tm²⁺ ions were fabricated to enhance optical nonlinearity by providing a strong reduction environment based on the solution doping technique in the modified chemical vapor deposition (MCVD) process. The incorporation of the Tm²⁺ ions into the fiber core was identified by the electron paramagnetic resonance (EPR) spectrum in the fiber preform, and the absorption and emission properties between 350 and 1600 nm of the Tm²⁺ ions in optical fibers and the fiber preform. A strong broad absorption band due to the Tm²⁺ ions appeared from 350 to ∼900 nm, and a broad emission from ∼600 to ∼1050 nm and the other emission from ∼1050 to ∼1300 nm, which were not shown in the Tm³⁺ ions, were found upon Ar-ion laser pumping at 515 nm. Both absorption and emission results confirm that the Tm²⁺ ions in the germanosilicate glass have the 4f-5d energy band from 350 to ∼900 nm and the 4f-4f energy level at ∼1115 nm. Also, the resonant nonlinearity at ∼1310 and ∼1530 nm due to the Tm²⁺ ions in the fiber was measured upon the 515 nm optical pumping by using a long-period fiber grating (LPG) pair method. The nonlinear refractive index n₂ at ∼1310 and ∼1530 nm was found to be ∼4×10^-15 m²/W, where 70% and 30% of the n₂ are attributed to the nonradiative transitions and the radiative transitions, respectively.