Orlando Frazão

Abstract
The present letter proposes the implementation of Vernier-effect harmonics through the virtualization of different reference cavities. A Fabry-Perot interferometer (FPI), actuated by a piezoelectric transducer (PZT), was employed as the sensing element. Subsequently, the sensitivity of the dynamic range was investigated for both the individual interferometer and the implementation of the Virtual Vernier effect. A sensitivity of (8 +/- 0.05)x10(-3) nm/nm was achieved for the single sensor measurement. Considering the implementation of the Vernier effect, the following sensitivities were obtained: (65.6 +/- 0.08)x10(-3) nm/nm for the fundamental, (132 +/- 1)x10-3 nm/nm for the first harmonic, and (192 +/- 1)x10(-3) nm/nm for the second harmonic. Furthermore, a maximum dynamic range of 11.25 mu m and a maximum resolution of 5 pm were achieved. This study highlights the advantages of simultaneously measuring both a single sensor cavity and a harmonic of the Virtual Vernier effect, in order to achieve large dynamic ranges along with high resolution.

Abstract
Azobenzenes are a class of compounds which allow the writing and erasure of linear birefringence along any desired direction, through their ability to photoisomerize. This property enables applications requiring polarization control, which, despite extensive exploration in the visible spectrum, have yet to be fully capitalized in the infrared region. This study aims to systematically characterize the creation and relaxation of induced linear birefringence dynamics in azopolymers thin films for the 1550 nm region. Maximum birefringence values as high as 6.02 x 10(-2) were attained during the recording phase with a 445 nm pump laser, that stabilized at 5.40 x 10(-2) during the relaxation phase, achieved for a 2.4 mu m sample. In addition, a maximum phase shift of Delta Phi = 0.54 pi stabilizing at Delta Phi = 0.50 pi, was observed for a 9.7 mu m sample with a 532 nm writing laser. Accordingly, this shows the promising potential of azopolymers for many applications.

Abstract
In this letter, we propose a method for utilizing the internal cavities of optical circulator devices-commonly referred to as parasitic cavities-as optical reference cavities. The method involves using an optical circulator operating at 1550 nm, illuminated by a light source at 1330 nm, thereby enhancing the amplitude of the interferometric signals generated by the internal optical cavities. The system was characterized by using both an Optical Spectrum Analyzer (OSA) and the Low-Coherence Interferometry (LCI) technique. Experimental results indicate that the Optical Path Difference (OPD) remains constant with varying aperture sizes, thereby confirming the feasibility of employing the optical circulator as a reference sensor. Finally, its performance as a reference sensor is demonstrated through its integration with an external cavity that functions as a displacement sensor.

Abstract
In this study, different configurations based on linear fiber lasers were proposed and experimentally demonstrated to measure the concentration of liquid solutions. Samples of paracetamol liquid solutions with different concentrations, in the range from 52.61 to 201.33 g/kg, were used as a case-study. The optical gain was provided by a commercial bidirectional Erbium-Doped Fiber Amplifier (EDFA) and the linear cavity was obtained using two commercial Fiber Bragg Gratings (FBGs). The main difference of each configuration was the coupling ratio of the optical coupler used to extract the system signal. The sensing head corresponded to a Single-Mode Fiber (SMF) tip that worked as an intensity sensor. The results reveal that, despite the optical coupler used (50:50, 60:40, 70:30 or 80:20), all the configurations reached the laser condition, however, the concentration sensing was only possible using a laser drive current near to the threshold value. The configurations using a 70:30 and an 80:20 optical coupler allowed paracetamol concentration measurements with a higher sensitivity of (-3.00 +/- 0.24) pW/(g/kg) to be performed. In terms of resolution, the highest value obtained was 1.75 g/kg, when it was extracted at 20% of the output power to the linear cavity fiber laser configuration.

Abstract
This paper presents an approach to enhancing sensitivity in optical sensors by integrating self-image theory and graphene oxide coating. The sensor is specifically engineered to quantitatively assess glucose concentrations in aqueous solutions that simulate the spectrum of glucose levels typically encountered in human saliva. Prior to sensor fabrication, the theoretical self-image points were rigorously validated using Multiphysics COMSOL 6.0 software. Subsequently, the sensor was fabricated to a length corresponding to the second self-image point (29.12 mm) and coated with an 80 mu m/mL graphene oxide film using the Layer-by-Layer technique. The sensor characterization in refractive index demonstrated a wavelength sensitivity of 200 +/- 6 nm/RIU. Comparative evaluations of uncoated and graphene oxide-coated sensors applied to measure glucose in solutions ranging from 25 to 200 mg/dL showed an eightfold sensitivity improvement with one bilayer of Polyethyleneimine/graphene. The final graphene oxide-based sensor exhibited a sensitivity of 10.403 +/- 0.004 pm/(mg/dL) and demonstrated stability with a low standard deviation of 0.46 pm/min and a maximum theoretical resolution of 1.90 mg/dL.