Amplification from Nd³⁺ doped Si-Ti sol-gel channel waveguides

We report the sol-gel chemistry reaction system and processes used to realize the first grating coupled monolithic solid state optical amplifier based on a Nd³⁺ doped sol-gel tapered rib waveguide. The sol-gel matrix composition is 50SiO₂:50TiO₂:10AlO_1.5:1NdO_1.5. The signal (1064 nm) and pump (803 nm) beams were coupled via grating couplers into and out of the rib waveguide. A photoluminescence lifetime of 138 μs and a pump absorption cross-section of 1.79 × 10^-24 m^-2 were measured. A net internal gain of 3.75 dB/cm was achieved. Theoretical simulations predict that this gain could be further improved. Analysis of the results suggests that the difference between achieved and predicted gain is due to low quantum efficiency caused by two-ion cross-relaxation and up-conversion, as manifestly evident by yellow light emitted from the device. The molecular-level processes during the doped sol-gel fabrication that could lead to these phenomena are discussed, and strategies are suggested to overcome these limitations in order to approach the full potential gain.