Effect of Linewidth on the Relative Intensity Noise in Random Distributed Feedback Raman Fiber Lasers.

We experimentally explore the relation between spectral linewidth and RIN transfer in half-open cavity random distributed feedback Raman lasers, demonstrating for the first time the possibility of adjusting the pump-to-signal RIN transfer intensity and cut-off frequency by using spectral filtering in the reflector section. We apply this approach to a 50-km laser system, operating in the C-Band, reliant on a standard single-mode fiber. We obtained a minimum bandwidth of 13 pm, which translates into a visible RIN cut-off at 800 MHz.

A Dual-Wavelength Fiber Laser Sensor with Temperature and Strain Discrimination.

This work presents a dual-wavelength C-band erbium-doped fiber laser assisted by an artificial backscatter reflector. This fiber-based reflector, inscribed by femtosecond laser direct writing, was fabricated into a single mode fiber with a length of 32 mm. The dual-wavelength laser obtained, centered at 1527.7 nm and 1530.81 nm, showed an optical signal-to-noise ratio over 46 dB when pumped at 150 mW. Another feature of this laser was that the power difference between the two channels was just 0.02 dB, regardless of the pump power, resulting in a dual emission laser with high equalization. On the other hand, an output power level and a central wavelength instability as low as 0.3 dB and 0.01 nm were measured, in this order for both channels. Moreover, the threshold pump power was 40 mW. Finally, the performance of this dual-wavelength fiber laser enhanced with a random reflector for sensing applications was studied, achieving the simultaneous measurement of strain and temperature with sensitivities around 1 pm/µÎµ and 9.29 pm/°C, respectively.

Liquid level sensor based on dynamic Fabry-Perot interferometers in processed capillary fiber.

In this work, a novel optical fiber sensor capable of measuring both the liquid level and its refractive index is designed, manufactured and demonstrated through simulations and experimentally. For this, a silica capillary hollow-core fiber is used. The fiber, with a sensing length of 1.55 mm, has been processed with a femtosecond laser, so that it incorporates four holes in its structure. In this way, the liquid enters the air core, and it is possible to perform the sensing through the Fabry-Perot cavities that the liquid generates. The detection mode is in reflection. With a resolution of 4 µm (liquid level), it is in the state of the art of this type of sensor. The system is designed so that in the future it will be capable of measuring the level of immiscible liquids, that is, liquids that form stratified layers. It can be useful to determine the presence of impurities in tanks.

Anomalous relative intensity noise transfer in ultralong random fiber lasers.

We present, for the first time, an experimental demonstration of RIN noise transfer dampening at low frequencies in random distributed feedback ultralong Raman fibre lasers based on conventional telecommunication fibres. Furthermore, we present a thorough theoretical description of the phenomenon and demonstrate how our model can be used to predict the observed behaviour, identifying the general requirements for system improvement through RIN transfer reduction.

Gamma Radiation-Induced Effects over an Optical Fiber Laser: Towards New Sensing Applications.

In the present work, the effect of gamma radiation on the performance of different types of erbium-doped fibers (EDFs) when they are used in a fiber ring cavity (FRC) configuration is studied. Several pieces of commercial EDF are gamma-ray irradiated with different doses to evaluate the output power variations over time. The influence of different doses, from 150 Gy to 1000 Gy, over the output power level measurement and their amplified spontaneous emission (ASE) are experimentally evaluated both in the C and L bands. By using an FRC configuration we can detect the presence of gamma radiation. We can also estimate the irradiation doses applied to EDFs by measuring the slope of the short-term emission power.

Quasi-distributed vibration sensing using OFDR and weak reflectors.

We proposed a quasi-distributed vibration sensing technique using in-line weak reflectors and optical frequency domain reflectometry (OFDR). As a result, we achieved an 8 kHz measurable vibration frequency with a 15-20 cm spatial resolution employing a low repetition rate (∼8 Hz). Moreover, a measurable frequency of 30 kHz was achieved for a 1.5 m spatial resolution. The ability of the system to determine the frequency and amplitude of several sections vibrating simultaneously is evaluated for different configurations. Because of the simple arrangement, high detectable frequency, and high sensitivity, this approach is expected to be especially well suited for mechanical vibration sensing applications, particularly in medium-sized structures.

Spectral shadowing suppression technique in phase-OTDR sensing based on weak fiber Bragg grating array.

A postprocessing procedure is presented to suppress spectral shadowing in phase-OTDR sensing systems based on a weak fiber Bragg grating array. A complete theoretical analysis of the interfering signals has been carried out to identify a compensation method. The proposed approach has been applied to simulated and experimental phase-OTDR in the context of vibration measurements. Fast Fourier transform has been employed to analyze the obtained results, which has verified the validity of the proposed method to suppress spectral shadowing.

Virtual FBGs Using Saturable Absorbers for Sensing with Fiber Lasers.

The spectral narrowing of Fiber Bragg Gratings (FBGs) introduced by unpumped Er-doped fiber (EDF) was analyzed for fiber lasers (FL). Owing to spatial hole burning (SHB), the spectral response of a virtual FBG can be employed for narrowing the band pass filter employed to determine the lasing wavelength of laser cavities. A common FL was mounted to analyze the spectral stability of the method, and a FL sensor for strain and temperature measurements was experimentally characterized to determine the stability of the narrowing effect achieved by the unpumped EDF, which acts as a virtual FBG. The results exhibited remarkably good narrowing effects of the spectral response of uniform FBGs.

Comparison between Different Structures of Suspended-Core Microstructured Optical Fibers for Volatiles Sensing.

In this paper, different core structures of microstructured optical fibers (MOFs) for low-finesse Fabry⁻Pérot (FP) sensors are experimentally compared to get the highest sensitivity. These devices are designed for volatile organic compounds (VOCs) measurements. Indium tin oxide (ITO) thin films were deposited by sputtering on the MOFs and different optical fast Fourier transform (FFT) phase responses from the FP were measured for saturated atmospheres of ethanol. It has been demonstrated that the sensitivities of the developed sensors depend strongly on the geometry and the dimensions of the MOF-cores. The sensors show recovery times shorter than 100 s and the baselines are fully recovered after every exposure to ethanol vapors.

Study of Optical Fiber Sensors for Cryogenic Temperature Measurements.

In this work, the performance of five different fiber optic sensors at cryogenic temperatures has been analyzed. A photonic crystal fiber Fabry-Pérot interferometer, two Sagnac interferometers, a commercial fiber Bragg grating (FBG), and a π-phase shifted fiber Bragg grating interrogated in a random distributed feedback fiber laser have been studied. Their sensitivities and resolutions as sensors for cryogenic temperatures have been compared regarding their advantages and disadvantages. Additionally, the results have been compared with the given by a commercial optical backscatter reflectometer that allowed for distributed temperature measurements of a single mode fiber.

High resolution polarization-independent high-birefringence fiber loop mirror sensor.

In this work, two all polarization-maintaining (PM) high-birefringence (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.

Experimental and numerical characterization of a hybrid Fabry-Pérot cavity for temperature sensing.

A hybrid Fabry-Pérot cavity sensing head based on a four-bridge microstructured fiber is characterized for temperature sensing. The characterization of this cavity is performed numerically and experimentally in the L-band. The sensing head output signal presents a linear variation with temperature changes, showing a sensitivity of 12.5 pm/°C. Moreover, this Fabry-Pérot cavity exhibits good sensitivity to polarization changes and high stability over time.

Vertically coupled microring resonators using one epitaxial growth step and single-side lithography.

A new concept for the fabrication of integrated microring resonators, requiring only one single epitaxial growth and two single-side lithographic steps, is proposed in what is the simplest fabrication scheme for vertical microrings published to date. The approach is based on two vertically stacked phase matched core layers. The effect of bus waveguide, coupling region and ring structure parameters is theoretically analyzed. Numerical calculations predict high performance devices with quality factors of over 10000. The scheme can feature both active and passive regions, allowing the fabrication of microring lasers.

Comparative study of ring and random cavities for fiber lasers.

An experimental comparison of three fiber laser structures with the same Raman gain medium is presented in order to establish the main pros and cons of each basic scheme. The first fiber laser is based on a hybrid ring-random fiber laser, the second one is a pure ring fiber laser, and the last one is a random fiber laser. Several aspects have been taken into account in the study. First, from the optical point of view, the parameters of interest compared are output power, lasing threshold, slope efficiency, power fluctuations, and the longitudinal modes have been analyzed. Second, the possible utilization of fiber lasers in digital modulated optical communication systems is also studied.

Application of remote power-by-light switching in a simplified BOTDA sensor network.

We propose and demonstrate the use of spatial multiplexing as a means to reduce the costs of distributed sensing networks. We propose a new scheme in which remote power-by-light switching is deployed to scan multiple branches of a distributed sensing network based on Brillouin Optical Time Domain Analysis (BOTDA) sensors. A proof-of-concept system is assembled with two 5-km sensor fiber branches that are alternatively monitored using a fast remotely controlled and optically powered optical switch. The multiplexed distributed sensor fibers were located 10 km away from the interrogation unit and a Raman pump is used to remotely power the switch. Furthermore, the deployed BOTDA unit uses an alternative configuration that can lead to simplified setups.

Highly-efficient fully resonant vertical couplers for InP active-passive monolithic integration using vertically phase matched waveguides.

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.

An in-reflection strain sensing head based on a Hi-Bi photonic crystal fiber.

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.

Multiplexing of six micro-displacement suspended-core Sagnac interferometer sensors with a Raman-Erbium fiber laser.

This work experimentally demonstrates a long-range optical fiber sensing network for the multiplexing of fiber sensors based on photonic crystal fibers. Specifically, six photonic crystal fiber sensors which are based on a Sagnac interferometer that includes a suspended-core fiber have been used. These sensors offer a high sensitivity for micro-displacement measurements. The fiber sensor network presents a ladder structure and its operation mode is based on a fiber ring laser which combines Raman and Erbium doped fiber amplification. Thus, we show the first demonstration of photonic crystal fiber sensors for remote measurement applications up to 75 km.

Micro-displacement sensor based on a hollow-core photonic crystal fiber.

A sensing head based on a hollow-core photonic crystal fiber for in-reflection measurement of micro-displacements is presented. The sensing structure takes advantage of the multimodal behavior of a short segment of hollow-core photonic crystal fiber in-reflection, being spliced to a single mode fiber at its other end. A modal interferometer is obtained when the sensing head is close to a mirror, through which displacement is measured.

Optical fiber networks for remote fiber optic sensors.

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.