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.

Abstract
The interrogation of optical fiber sensors (OFS) often relies on complex devices such as optical spectrum analyzers (OSAs) that are expensive with low portability and mainly suited to laboratory measurements or dedicated interrogation systems with limited spectral range. An interrogation unit was designed and fabricated using a photodetector combined with a micro-electromechanical system and a Fabry-Perot interferometer (MEMS-FPI) working as a tunable filter with a response in the range 1350-1650 nm. Deconvolution techniques were applied to mitigate the effect of the broadband response of the tunable filter on the measured signal. The performance of the unit was validated with the interrogation of long-period fiber gratings (LPFGs) as temperature, refractive index (RI), and relative humidity (RH) sensors. For the temperature, a sensitivity of 0.135 +/- 0.007 nm/degrees C was obtained, which showed a 4.9% relative error when compared to the same measurement with an OSA. For the RI, a sensitivity of 147 +/- 11 nm/RIU was obtained, which showed a relative error lower than 1% when compared to the OSA. For the humidity, sensitivities of 0.742 +/- 0.005 and 0.056 +/- 0.006 nm/%RH were obtained, with errors of 2.75% and 6.67%, respectively, when compared to a commercial dedicated interrogation system. The low relative error obtained when compared to commercial alternatives shows the potential of the system to be used in real-time applications that require portability, low cost, energy efficiency, and capacity for integration in dedicated systems.

Abstract
The sensitivity of one-dimensional Bloch surface wave (BSW) sensors to external refractive index variations using Kretschmann's configuration is calculated analytically by employing first-order perturbation theory for both TE and TM modes. This approach is then validated by com- parison with both transfer matrix method simulations and experimental results for a chosen photonic crystal structure. Experimental sensitivities of (8.4 +/- 0.2)x102 and (8.4 +/- 0.4)x102 nm/RIU were obtained for the TE and TM BSW modes, corresponding to errors of 0.02% and 4%, respectively, when comparing with the perturbation the- ory approach. These results provide interesting insights into photonic crystal design for Bloch surface wave sensing by casting light into the important parameters related with sen- sor performance.(c) 2023 Optica Publishing Group

Abstract
Water vapor sorption is a powerful tool for the analysis of cement paste, one of the most used substances by mankind. The monitoring of cementitious materials is fundamental for the improvement of infrastructure resilience, which has a deep impact on the economy, the environment, and on society. In this work, a multimode fiber was embedded in cement paste for real-time monitoring of cement paste water vapor sorption. Changes in the reflected light intensity due to the build-up of water in the cement paste's pores were exploited for this purpose. The sample was 7-day moist cured, and the relative humidity was controlled between 8.9% and 97.6%. Reflected light intensity was converted into a specific surface area of cement paste (133 m(2)/g) and thickness of water through the Brunauer-Emmett-Teller (BET) method and into a pore size distribution through the Barret-Joyner-Halenda (BJH) method. The results achieved through reflected light intensity agree with those found in the literature, validating the usage of this setup for the monitoring of water vapor sorption, breaking away from standard gravimetric measurements.