Study On The Spectroscopic Properties Of Er～(3+) In Er～(3+)(/Yb～(3+)): LiNbO₃ Crystals And The Optical Amplification In Er～(3+)/Yb～(3+): Ti: LiNbO₃ Channel Waveguide

In the first part of this dissertation, spectroscopic properties of as-grown, annealed,and vapor transport equilibration (VTE) treated Er³⁺(/Yb³⁺): LiNbO₃ crystals werestudied in detail. On the basis of measured α-polarized absorption spectra of Z-cutas-grown and annealed Er³⁺/Yb³⁺: LiNbO₃ crystals, the spectroscopic parameters ofEr³⁺ in as-grown and annealed LiNbO₃ crystals were analyzed by using the Judd-Ofelt(J-O) theory. The J-O parameters are determined by using least square method.According to the fitted J-O parameters, the spontaneous emission rates, thefluorescence branch ratios as well as the radiative lifetimes of the excited states werenumerically calculated. In addition, Yb³⁺ codoping and thermal anneal effects on thespectroscopic properties of Er³⁺ were also discussed. The absorption characteristics ofVTE treated crystals are summarized in comparison with those of as-grown crystals.The [Li+ + RE³⁺]/[Nb⁵⁺] (RE³⁺ =Er³⁺ or Er³⁺ + Yb³⁺) ratio in the VTE-treatedEr³⁺(/Yb³⁺): LiNbO₃ crystals were evaluated using the optical absorption edge orRaman linewidth of the 153 cm-1 E(TO) phonon. In the same way, the spectroscopicparameters of Er³⁺ in as-grown and VTE-treated crystals were calculated by using theJ-O theory. Influences of Er³⁺ doping level, type of crystal cut and VTE treatment onthe key spectroscopic parameters are discussed and summarized in comparison withthe results of as-grown crystals. The theoretical result about VTE effect on thelifetime of emission manifold ⁴I_13/2 is in rough agreement with the experimental resultreported earlier. The results of as-grown crystals are also compared with thosereported previously.In the second part of this dissertation, optical amplification of small signal at 1530nm in 980 nm pumped Er³⁺/Yb³⁺: Ti: LiNbO₃ strip waveguides was numericallyanalyzed based upon the rate equation model of Er³⁺-Yb³⁺ system and both pump andsmall signal power propagation equations. The rate equation model establishedinvolves total eight energy levels of Er³⁺-Yb³⁺ system, and simultaneously takes intoaccount excited state absorption and cooperative upconversion processes within Er³⁺as well as possible energy transfer processes between Er³⁺ and Yb³⁺. A comparison ofnumerical results from the eight-level model with those from a highly simplifiedfive-level model has indicated that the ⁴S_3/2 manifold of Er³⁺ must be included in therate equations. Dependence of threshold pump power and 1530 nm amplification gainon initial pump power (for gain only), Yb³⁺ surface concentration and waveguidelength were calculated. The characteristics of pump power evolution along thewaveguide axis, population density and population distribution, as well as theinfluence of 560 nm and 1060 nm fluorescence lifetime on the numerical result werealso studied. The role of Yb3+ codoping is demonstrated. A novel design idea that Yb3+is only incorporated at the end of the waveguide is proposed to reuse the residualpump power and hence render the signal to gain a further amplification.