Abstract
Dissolved carbon dioxide (dCO2) evaluation is very important in many different fields. In this work, a new, integrated, colorimetric-optical fiber-based system for dCO2 monitoring in aquaculture industry was developed. The sensing chemistry is based on colorimetric changes of the used indicator—poly p-nitrophenol (pNPh)—in contact with CO2. Preliminary tests were done in a laboratory environment (calibration) and in a laboratory Recirculating Aquaculture System (RAS) with controlled CO2 injection. The results have shown the suitability of the new sensor for assessing dCO2 dynamics in RAS and its fast detection of low dCO2 concentrations in an appropriate operation range.

Abstract
Optical microbubble resonators are among the highest sensitivity optical sensors. In the context of its application in the detection of water micro contaminants, in portable systems, their interrogation must be made by tracking the resonant wavelength peak position with the highest accuracy possible, at a reasonable cost. In this work different laser sources and scanning methods were tested and compared, aiming the development of a portable prototype. Each tunable laser source, was evaluated using a C2H2 Gas cell, which provided an absolute wavelength reference. Light transmitted through the cell was recorded using a photodetector and a software controlled feedback loop, enabling locking into selected reference peaks. Three distinct scanning methods were tested and compared for each laser source: large and short-range laser scanning and external waveform dithering, from which minimum standard deviations of 20, 0.18, and 0.07 pm, were obtained, respectively.

Abstract
A sensor based on 2 hollow core microspheres is proposed. Each microsphere was produced separately through fusion splicing and then joined. The resultant structure is a Fabry-Perot interferometer with multiple interferences that can be approximated to a 4-wave interferometer. Strain characterization was attained for a maximum of 1350 mu epsilon, achieving a linear response with a sensitivity of 3.39 +/- 0.04 pm/mu epsilon. The fabrication technique, fast and with no chemical hazards, as opposed to other fabrication techniques, makes the proposed sensor a compelling solution for strain measurements in hash environments.

Abstract
New miniaturized sensors for biological and medical applications must be adapted to the measuring environments and they should provide a high measurement resolution to sense small changes. The Vernier effect is an effective way of magnifying the sensitivity of a device, allowing for higher resolution sensing. We applied this concept to the development of a small-size optical fiber Fabry-Perot interferometer probe that presents more than 60-fold higher sensitivity to temperature than the normal Fabry-Perot interferometer without the Vernier effect. This enables the sensor to reach higher temperature resolutions. The silica Fabry-Perot interferometer is created by focused ion beam milling of the end of a tapered multimode fiber. Multiple Fabry-Perot interferometers with shifted frequencies are generated in the cavity due to the presence of multiple modes. The reflection spectrum shows two main components in the Fast Fourier transform that give rise to the Vernier effect. The superposition of these components presents an enhancement of sensitivity to temperature. The same effect is also obtained by monitoring the reflection spectrum node without any filtering. A temperature sensitivity of -654 pm/degrees C was obtained between 30 degrees C and 120 degrees C, with an experimental resolution of 0.14 degrees C. Stability measurements are also reported.

Abstract
An optical fiber Fabry-Perot (FP) for relative humidity (RH) sensing is proposed. The FP cavity is fabricated by splicing a short length of hollow silica tube in a single mode fiber. The fiber is then coated with a polyvinylidene fluoride (PVDF) thin film to work as a mirror. The fabrication process of the FP interferometer with a dip coating process in a PVDF/dimethyl formamide solution is presented. The pattern fringes of the FP suffer a wavelength shift due to the change in the PVDF's refractive index with the ambient RH variation. A short overview of the cavity's formation and stability is presented. The RH response of the FPI cavity is tested. The sensor presented a sensitivity of 32.54 pm/%RH at constant temperature and -15.2 pm/degrees C for temperature variation.

Abstract
Microfiber knot resonators are widely applied in many different fields of action, of which an important one is the optical sensing. Microfiber knot resonators can easily be used to sense the external medium. The large evanescent field of light increase the interaction of light with the surrounding medium, tuning the resonance conditions of the structure. In some cases, the ability of light to give several turns in the microfiber knot resonator allows for greater interaction with deposited materials, providing an enhancement in the detection capability. So far a wide variety of physical and chemical parameters have been possible to measure using microfiber knot resonators. However, new developments and improvements are still being done in this field. In this chapter, a review on sensing with microfiber knot resonators is presented, with particular emphasis on the application of these structures as temperature and refractive index sensors. A detailed analysis on the properties of these structures and different assembling configurations is presented. An important discussion regarding the sensor stability is presented, as well as alternatives to increase the device robustness. An overview on the recent developments in coated microfiber knot resonators is also addressed. In the end, other microfiber knot configurations are explored and discussed.