Abstract
A method to fabricate chromium-doped lithium niobate substrates in which the active ions are introduced by thermal diffusion from a film is reported. Chromium concentration depth profiles have been obtained by secondary-ion-mass spectrometry and the relevant diffusion parameters have been derived. Fluorescence spectrum and upper laser level lifetime of chromium diffused proton-exchanged and chromium/titanium-diffused lithium niobate waveguides have been measured. A simple model has been used to estimate the performance of such structures as waveguide optical amplifiers and lasers. (C) 1995 American Institute of Physics.

Abstract
We report what is to our knowledge the first laser operation of Nd3+-doped LiNbO3 near 900 nm. An absorbed power threshold of 26 mW was obtained when the device was pumped at 814 nm. The design of the waveguide geometry to favor laser operation at this wavelength is demonstrated. (C) 1997 Optical Society of America.

Abstract
Relations between fabrication conditions and optical characteristics of planar waveguides made by proton exchange in benzoic acid are documented in the literature, but reports on the characterization of waveguide fabrication processes, performed in a systematic way, could not be found, resulting in the need to combine data from several authors. Discrepancies among results from different researches are evident, resulting from different experimental methodologies and calibration of equipment. Therefore, aiming at extracting a consistent data set, optical characterization of the refractive index profile was employed to study series of samples. The objective was to develop a methodology for fabrication of proton-exchanged channel waveguides in LiNbO3 operating in the single-mode regime at several wavelengths, with specific characteristics required to optimize integrated devices. To achieve this, it is necessary to obtain the relations between the optical characteristics of the waveguides and their fabrication conditions, and to introduce models of the waveguide formation process. (c) 2007 Society of Photo-Optical Instrumentation Engineers.

Abstract
The possibility of fabricating efficient optical amplifiers in LiNbO3, realized by localization of the dopant on surface areas was theoretically evaluated and the feasibility of fabricating efficient amplifiers in such doped structures was experimentally verified. The model of amplifiers based on 3-level laser systems realized by local doping incorporates dopant localization, the influence of the interaction length, as well as that of the effective pump area and of the pump power in the amplifier performance. It was verified that localized doping allows optimization of amplifier performance through adjustment of the active region geometry to the mode intensity profile. The experimental results confirmed that the width of the metal stripe deposited on the surface must be optimized so that gain is maximum, for a given value of the pump power. It was experimentally demonstrated that transverse localization of the active ions reduces the threshold pump power, which may be relevant for realization of integrated lasers in Er:LiNbO3.

Abstract
Modelling of laser oscillation at 0.9 mu m in Ti waveguides in LiNbO3 doped with Nd ions is presented. Laser emission at 0.9 mu m in Ti waveguides in Nd:LiNbO3 crystals was recently demonstrated. However, lasing was reported as unstable and lasting only a few seconds, with parasitic lasing at the higher gain transition at 1.08 mu m shown to be a problem. In this work the possibility of obtaining efficient and stable laser oscillation at 0.9 mu m in Ti:LiNbO3 waveguides, fabricated in substrates doped with Nd ions by thermal diffusion of thin metallic stripes or planar thin films, was theoretically evaluated. It was concluded that emission at 0.9 mu m, with complete suppression of the parasitic emission at 1.08 mu m, should be possible by selective increase of the losses at 1.08 Irm, through optimization of waveguide and laser cavity, spatial localization of the Nd ions and the use of the dependence on polarization of the emission cross sections at 0.9 and 1.08 mu m.

Abstract
An investigation on optical amplification in Ti waveguides in LiNbO3 doped with Er ions by thermal diffusion of thin metallic stripes is presented. The possibility of fabricating efficient optical amplifiers in LiNbO3 substrates realized by localization of the dopant on surface areas of the crystals was theoretically evaluated and the feasibility of fabricating efficient amplifiers in such doped structures was experimentally verified. It was concluded that the localized doping technique allows optimization of amplifier performance through adjustment of the active region geometry to the mode intensity profile.