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This paper presents an overview of optical fiber sensor networks for remote sensing. Firstly, the state of the art of remote fiber sensor systems has been considered. We have summarized the great evolution of these systems in recent years; this progress confirms that fiber-optic remote sensing is a promising technology with a wide field of practical applications. Afterwards, the most representative remote fiber-optic sensor systems are briefly explained, discussing their schemes, challenges, pros and cons. Finally, a synopsis of the main factors to take into consideration in the design of a remote sensor system is gathered. ; This work was supported by the Spanish Government project TEC2010-20224-C02-01.

A review on all-fiber lasers based on photonic crystal fibers is presented. Photonic crystal fibers present improved features beyond what conventional optical fibers can offer. Due to their geometric versatility, photonic crystal fibers can present special properties and abilities which can lead to enhanced lasing structures. A brief description of photonic crystal fibers and fiber laser's properties is presented. All-fiber laser structures developed using photonic crystal fibers are described and divided in two groups, depending on the cavity topology: ring cavity fiber lasers and linear cavity fiber lasers. All-fiber lasers applications in the photonic crystal fiber related sensing field are described. ; The authors are grateful to the Spanish government project TEC2010-20224-C02-01 and the European project ECOAL-MGT – SUDOE Program.

This work experimentally proposes a new interrogation method which consists on the combination of the information given by both domains: time and frequency. On the one hand, the sensor information is obtained in the frequency domain as usual and, on the other hand, the detection of the different pulses generated through mode locking allows the identification of each FBG. It has been proved that FBGs spaced less than 0.15 nm can be interrogated. The sensor network based on simple ring Erbium doped fiber laser using the FBGs, located in a serial configuration, as wavelength selective reflectors for the lasing structure, acting them also as the sensing transducers elements. ; This work was supported in part by the Spanish Government projects TEC2013-47264-C2-2-R, innocampus and FEDER funds.

We propose and demonstrate two ultra-long range fiber Bragg grating (FBG) sensor interrogation systems. In the first approach four FBGs are located 200 km from the monitoring station and a signal to noise ratio of 20 dB is obtained. The second improved version is able to detect the four multiplexed FBGs placed 250 km away, offering a signal to noise ratio of 6-8 dB. Consequently, this last system represents the longest range FBG sensor system reported so far that includes fiber sensor multiplexing capability. Both simple systems are based on a wavelength swept laser to scan the reflection spectra of the FBGs, and they are composed by two identical-lengths optical paths: the first one intended to launch the amplified laser signal by means of Raman amplification and the other one is employed to guide the reflection signal to the reception system. ; This work was supported by the Spanish Government project TEC2010-20224-C02-01.

A characterization of a modulated random mirror laser has been experimentally carried out. Unlike conventional internally modulated fiber lasers, no distortion of the modulating frequency or self-mode-locking effects were measured. The behavior of the laser using pulsed and analog modulation up to 12 GHz is shown. ; This work was supported by the Spanish Government project TEC2010-20224-C02-01.

In this work, two all polarization-maintaining (PM) highbirefringence (Hi-Bi) fiber loop mirrors (FLM) which are immune to external polarization perturbations are validated both theoretically and experimentally. Simplified and stable versions of classical FLMs were attained using a PM-coupler and by fusing the different Hi-Bi fiber sections with an adequate rotation angle between them. Since the polarization states are fixed along the whole fiber loop, no polarization controllers are needed. This simplifies the operation and increases the stability of the systems, which were also validated as ultra-high resolution sensors, experimentally obtaining a resolution of 6.2·10−4 °C without averaging. ; This work was supported by the Spanish Government project TEC2013-47264-C2-2-R and the FEDER funds.

The ability to operate despite failure will become increasingly important as the use of optical sensor networks grows, and the amount of sensing information to be handled by a sensor network is increasing, especially for safety and security applications. In this review, the four categories of protection to allow service to be reestablished after a failure (dedicated/shared and line/path) are thoroughly discussed. This paper also presents an overview of the most representative robust fiber-optic sensor systems, discussing their schemes, pros and cons. ; The authors are grateful to the Spanish Government project TEC2010-20224-C02-01.

The authors present the experimental results showing how the external mirror reflectivity affects the polarization properties of a vertical cavity surface emitting laser subject to optical feedback from an extremely short external cavity. The amplitude of modulation of the polarization switching current with the external cavity length is found to be proportional to the external mirror reflectivity, confirming its key role in achieving polarization control and stabilization of such lasers using optical feedback. Numerical simulations presented here show good agreement with experiments. ; The authors acknowledge the financial support of the Public University of Navarra, IAP "Photon" network of the Belgian government, FWO-Flanders, GOA and OZR of the VUB, and Project TEC-2004-05936-C02-01 of the MEC Spain.

The authors experimentally show that isotropic optical feedback from an extremely short external cavity can induce switching between the two fundamental transverse modes with orthogonal linear polarization of otherwise without optical feedback polarization stable vertical cavity surface emitting laser. The switching currents and the hysteresis width can be widely tuned by varying the external cavity length. Moreover, by a proper choice of the optical feedback parameters, the emission can be stabilized in any of the two linearly polarized modes. ; The authors acknowledge the financial support of the Public University of Navarra, IAP 'Photon' network of the Belgian government, FWO-Flanders, GOA and OZR of the VUB, and Project No. TEC-2004-05936-C02-01 of the MEC Spain.

In this work, narrow-band emission lines are generated by means of two random distributed feedback fiber laser schemes. Spectral line-widths as narrow as 3.2 pm have been measured, which significantly improves previous reported results. The laser is analyzed with the aim of obtaining a spectral line-width as narrow as possible. Additionally a variation of this setup for multi-wavelength operation is also validated. Both schemes present a simple topology that use a combination of phase-shifted fiber Bragg gratings and regular fiber Bragg gratings as filtering elements. ; This work was supported by the Spanish Government projects TEC2013-47264-C2-2-R and INNOCAMPUS.

In this work the development of an 'eight-shape' fiber optic sensor for high strain measurement is experimentally demonstrated. This strain sensor is interrogated using a commercial optical time domain reflectometer through a dual configuration that is able to measure backscattering and the transmission of an intensity pulse while reducing unwanted cladding-mode coupling effects. An 'in field' concrete beam bending test was also performed in order to compare the proposed low-cost intensity bending sensor with the mature fiber Bragg grating technology. ; The authors are grateful to the Spanish Government project TEC2010-20224-C02-01.

A new active-passive monolithic integration approach for photonic components based on vertical evanescent coupling is presented. Two vertically stacked waveguides are used in order to provide full resonant power transfer between them and avoiding the need of tapered structures. Light confinement in each waveguide is achieved combining strong lateral asymmetric structures and bent waveguides, both defined during lithography. Low propagation losses for the active waveguide and coupling efficiencies to the passive section as high as 97% have been obtained. ; This work was supported by the Spanish Government project TEC2010-20224-C02-01.

Different multiwavelength Raman fiber lasers based on a hybrid cavity set-up are proposed. The lasing schemes are based in highly birefringent photonic crystal fiber loop mirrors combined with random cavities. The Hi-Bi PCF loop mirrors are characterized by an interferometric output; whereas the random mirrors are created by cooperative Rayleigh scattering due to Raman gain. This configuration allows suppression of Rayleigh associated noise growth, while taking advantage of it as an active part of the laser cavity, enhancing the achievable gain. The proposed fiber lasers present stable operation at room temperature although different output maxima and shapes depending on the fiber loop mirror/random mirror combination. ; The authors are grateful to the Spanish Government project TEC2010-20224-C02-01 and European COST action TD 1001.

In this paper, a theoretical and experimental study of two interferometric sensor multiplexing schemes has been carried out by means of the fast Fourier transform (FFT) analysis. This work addresses one of the main drawbacks of photonic crystal fiber (PCF) sensors, that is, its multiplexing capability. Using a commercial optical interrogator combined with a simple FFT measurement technique, the simultaneous real-time monitoring of several PCF sensors is achieved. A theoretical analysis has been performed where simulations matched with the experimental results. For the experimental verification, highly birefringent (HiBi) fiber sections that operated as sensing elements were multiplexed and tested in two configurations. Due to the FFT analysis, both multiplexing schemes can be properly interrogated by monitoring the FFT phase change at the characteristic spatial-frequency of each sensor. For this purpose a commercial interrogator and a custom Matlab program were used for computing the FFT and for monitoring the FFT phase change in real-time (1 Hz). ; This work was supported by the Spanish Government projects TEC2010-20224-C02-01, TEC2013-47264-C2-2-R, INNOCAMPUS, by the European SUDOE-Interreg Project ECOALMGT and the FEDER funds.

A photonic crystal fiber-based sensing head is proposed for strain measurements. The sensor comprises a Hi-Bi PCF sensing head to measure interferometric signals in-reflection. An experimental background study of the sensing head is conducted through an optical backscatter reflectometer confirming the theoretical predictions, also included. A cost effective setup is proposed where a laser is used as illumination source, which allows accurate high precision strain measurements. Thus, a sensitivity of ~7.96 dB/me was achieved in a linear region of 1,200 μe. ; The authors are grateful to the Spanish government projects TEC2010-20224-C02-01 and Innocampus.