Abstract
Coal has been for centuries a central energy source to fulfill industrial and domestic needs. Its large scale extraction produced huge amount of debris that were piled in the neighboring of the mines, quite often going into combustion triggered by events like forest fires or lightning. When in this state it can continue for years, releasing substantial emissions of toxic and greenhouse gases with recognized impact in the environment and, more serious in the short term, in the life quality of the populations located nearby. Continuous monitoring of combustion temperature and emission levels of certain gases opens the possibility to plan corrective actions to minimize their negative impact. Optical fiber technology is wellsuited to this purpose and here it is described the main attributes of a fiber optic sensing system projected to gather data on distributed temperature and gas emission in these harsh environments.

Abstract
Many optical systems based on surface plasmon resonance (SPR) have been developed for working as refractometers, chemical sensors or even for measuring the thickness of metal and dielectric thin films. Sensors based on SPR present very high sensitivity to refractive index (RI) variations when compared to the traditional RI sensors. However, these kinds of systems are usually large, expensive and therefore cannot be used for remote sensing. Optical fibre sensors based on SPR are usually implemented using multimode optical fibres cope with the requirements for remote sensing. In this section a new type of SPR sensor based in a single mode fibre (SMF) is proposed. A section of the SMF was chemically etched by emersion in a 48 % hydrofluoric acid solution, resulting in a tapering effect, with the cladding removing while the core is kept intact. Simulation results are in good agreement with the experimental spectral resonance dip attained around 1550 nm. Sensitivities of 3800 and 5100 nm/RIU were achieved for the reflection and for the transmission modes, respectively, for RI in the 1.33 to 1.37 range.

Abstract
In the present study, we have determined the intrinsic refractive index of anodic aluminium oxide, which is originated by the formation of nanoporous alumina templates. Different templates have been fabricated by the conventional two-step anodization procedure in oxalic acid. Their porosities were modified by chemical wet etching allowing the tuning of their effective refractive indexes (air-filled nanopores + anodic aluminium oxide). By standard spectroscopic light transmission measurements, the effective refractive index for each different template was extracted in the VIS-NIR region. The determination of the intrinsic anodic aluminium oxide refractive index was performed by using the Maxwell-Garnett homogenization theory. The results are coincident for all the fabricated samples. The obtained refractive index (similar to 1.55) is quite lower (similar to 22%) than the commonly used Al2O3 handbook value (similar to 1.75), showing that the amorphous nature of the anodic oxide structure strongly conditions its optical properties. This difference is critical for the correct design and modeling of optical plasmonic metamaterials based on anodic aluminium oxide nanoporous templates.

Abstract
It is proposed the multiplexing of optical fiber-based surface plasmon resonance (SPR) sensors deployed in a ladder topology, addressed in wavelength by combining each sensor with specific fiber Bragg gratings (FBGs) and considering intensity interrogation. In each branch of the fiber layout, the FBGs are located after the sensor and the peak optical power reflected by the FBGs is a function of the relative spectral position between the SPR sensor and the FBG resonances, with the former dependent on the refractive index of the surrounding medium. The concept is tested for the multiplexing of two SPR sensors fabricated in an etched region of a single-mode fiber showing intrinsic refractive index sensitivity up to 5000 nm/RIU, which translates into a sensitivity of similar to 829 dB/RIU from the interrogation approach considered. The obtained refractive index resolution is in the order of 10(-4) RIU, and the crosstalk level between the sensors was found negligible.

Abstract
A sensing system in the near infrared region has been developed for ammonia sensing based on the wavelength modulation spectroscopy (WMS) principle. The WMS is a rather sensitive technique for detecting atomic/molecular species, presenting the advantage that it can be used in the near-infrared region by using the optical telecommunications technology. In this technique, the laser wavelength and intensity were modulated by applying a sine wave signal through the injection current, which allowed the shift of the detection bandwidth to higher frequencies where laser intensity noise was typically lower. Two multi-pass cells based on free space light propagation with 160 cm and 16 cm of optical path length were used, allowing the redundancy operation and technology validation. This system used a diode laser with an emission wavelength at 1512.21 nm, where NH3 has a strong absorption line. The control of the NH3 gas sensing system, as well as acquisition, processing and data presentation was performed.

Abstract
This work presents a fiber cavity ring down (CRD) configuration for the measurement of strain. An optical time-domain reflectometer (OTDR) was used to send impulses down into the fiber loop cavity, inside of which a chirped fiber Bragg grating was placed to act as a strain sensing element. This technique could provide strain results with both conventional CRD-based configuration and the OTDR. The CRD configuration provided a linear response to strain applied in the range 0-2000 epsilon, and a sensitivity of 1.34 ns/epsilon was obtained. For the same operation range, the OTDR interrogation allowed obtaining a periodic behavior due to fiber Bragg grating scanning of the wavelength peaks of the multimode laser source while increasing strain. It is shown that the OTDR interrogation method provide a ring-down time response of about 8 s which is a great improvement when compared to the conventional CRD configuration (23 s). (c) 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1442-1444, 2015