Focus on Disruptively Innovative Optical Fibre-Based Sensors

We demonstrate the capability for the identification of single particles, via a neural network, directly from the backscattered light collected by a 30-core optical fibre, when particles are illuminated using a single mode fibre-coupled laser light source. The neural network was shown to be able to determine the specific species of pollen with ∼97% accuracy, along with the distance between the end of the 30-core sensing fibre and the particles, with an associated error of ±6 μm. The ability to be able to classify particles directly from backscattered light using an optical fibre has potential in environments in which transmission imaging is neither possible nor suitable, such as sensing over opaque media, in the deep sea or outer space.

Quasi-distributed measurement of strain and/or temperature is often implemented using arrays of weakly reflecting fiber Bragg gratings (FBGs) whose reflection peaks are centered at the same nominal wavelength. The signals are obtained by measuring the phase difference between the reflections of consecutive FBGs. Typically, in such a system, the spatial resolution of the interrogator must be compatible with the spatial separation between consecutive FBGs. Insufficient resolution leads to an overlap of reflection peaks, a decrease in the differential-phase signal and poor sensitivity. In this paper, we study the use of two different sparsity based methodologies for improving the sensitivity of such quasi distributed acoustic sensing systems in the case where traditional signal processing approaches do not provide sufficient spatial resolution. These methods enable relaxing the requirements regarding the interrogator or, alternatively, reducing the needed separation between reflectors. Experimentally, these techniques were used to measure 1 kHz dynamic strain induced in a fiber segment between two discrete reflectors, located at the end of a 4 km long fiber. The separation between the reflectors was 18 m while the pulse (spatial) width was intentionally chosen bigger than that. It yielded approximately 5 dB increase in the measured signal compared to the traditional processing approach and an order of magnitude improvement in the sensitivity, $\sim 0.9\tfrac{\mu {\rm{rad}}}{\sqrt{\mathrm{Hz}}}$.

The potential of discrete and distributed fiber-based sensors exploiting the Rayleigh scattering signature of doped amorphous silica is investigated for the real time monitoring of molecular hydrogen (H₂) detection. We showed that the impact of the refractive index changes induced by the H₂ diffusion into the silica host matrix can be used to detect and quantify this gas presence through two approaches: first via the related fiber length variation and second through the observed spectral shift. Comparing the obtained results with H₂ diffusion calculations, we can estimate the sensor sensitivity thresholds to be ∼10¹⁶ n_molecule cm⁻³ for the distributed measurements (spatial resolution better than 1 mm) and below ∼10¹⁹ n_molecule cm⁻³ for the discrete-one. The presented architecture of the sensor is well adapted to the monitoring of slowly evolving H₂ concentrations such as the ones expected in nuclear waste repositories as the time response of the sensor remains limited by the diffusion of the gas within the optical fiber. These threshold values and time responses can be easily improved by optimizing the length, the composition and/or the geometry of the sensing fiber.

A new concept of an all optical, dual-fiber-based Pirani thermal vacuum gauge is proposed and demonstrated. The configuration utilizes two fibers: one that produces heat, and one that responds to the resultant thermal exchange. The temperature of the latter fiber is a function of the heat transfer through the gas in which it resides. The active heat-generating fiber is a luminescence-quenched, heavily Yb³⁺-doped optical fiber that efficiently produces thermal energy when optically pumped. The temperature sensor is implemented with a conventional commercial fiber Bragg grating. Both fibers are inserted into a custom vacuum chamber whose internal pressure can be carefully controlled. Performance of the system is characterized with pressures ranging from 20 mTorr to Standard Pressure. The proposed system may also be used as a sensitive flow rate sensor.

Optical fiber sensors of hydrogen gas (H₂) are conventionally based on the reaction of a sensitive material deposited on the surface of a fiber. Long-term applications of H₂ monitoring require more robust configurations, less sensitive to the degradations of the sensitive layer. To overcome this issue, we develop disruptive polarisation-maintaining optical fibers composed of a sensitive material (Palladium, Pd) integrated into the silica cladding. We present the development of two Panda-type optical fibers with or without embedded Pd particles. These fibers have been fabricated for evaluating, through the measurement of the birefringence, the contribution of Pd particles on the detection of H₂ gas. We have specially developed a gas chamber for measuring on-line the detection of H₂ during its diffusion into the fiber. Dynamic comparisons between both fibers demonstrate the contribution of Pd particles resulting in a faster response time (of about 20 h for our experimental conditions). These results pave the way to the realization of robust optical fibers with enhanced sensitivity to H₂ gas for developing sensing systems compatible with long-term hydrogen monitoring applications in extreme and harsh environments, such as radioactive waste repositories.

Fiber Bragg grating (FBG) sensors are typically bonded on the surface of a structure using an adhesive to collect ultrasonic waves for damage detection in structural health monitoring applications. However, the ultrasonic wave transfer from structure to optical fiber suffers signal attenuation due to the adhesive bond layer, which has a significantly different acoustic impedance than the optical fiber. Therefore, this paper develops a systematic procedure to fabricate an aligned carbon nanotube (CNT)-wrapped FBGs for acoustic impedance matching. Specifically, we first develop an automated CNT winding system to fabricate CNT-wrapped FBGs with varying CNT layer thickness, which are bonded to an aluminum plate for ultrasonic sensitivity testing. We demonstrate that CNT wrapped FBGs do not necessarily produce an increased sensitivity as compared to a reference polyimide-coated FBG, however some outliers are observed with a significant improvement. Using a scanning electron microscopy we examine the cross-section of CNT/adhesive layers, identifying a unique CNT/adhesive bonding morphology with a stiff exterior shell and a relatively compliant inner layer. Finite element simulation validates that this two-layered bonding geometry is most likely the source of the increased FBG ultrasonic sensitivity for the outliers.

An overview of truly remote fiber optic sensors is presented in this work. It starts with a brief introduction of fiber optic sensor networks, showing their advantages and multiple applications. Then, the definition of truly remote networks is provided, and their main challenges discussed, such as increasing the sensing distance and the number of sensors interrogated. Several multiplexing techniques have been compared, such as wavelength, time and coherence division multiplexing. In relation to this, the most recent works showing multi wavelength fiber lasers for wavelength division multiplexing have been grouped and their versatility analyzed. Finally, recent and relevant truly remote fiber optic networks have been gathered and some of the most representative schemes explained in detail, comparing their multiplexing capability and the remoteness of the monitored sensors. Random distributed feedback fiber lasers form part of a number of these schemes, proving the suitability of this type of lasers for their use in ultra-long truly remote sensing applications.

In this paper, various types of high temperature fibre Bragg gratings (FBGs) are reviewed, including recent results and advancements in the field. The main motivation of this review is to highlight the potential of fabricating thermally stable refractive index contrasts using femtosecond (fs) near-infrared radiation in fibres fabricated with non-conventional techniques, such as the molten core method. As a demonstration of this, an yttrium aluminosilicate (YAS) core and pure silica cladding glass optical fibre is fabricated and investigated after being irradiated by an fs laser within the Type II regime. The familiar formation of nanogratings inside both core and cladding regions are identified and studied using birefringence measurements and scanning electron microscopy. The thermal stability of the Type II modifications is then investigated through isochronal annealing experiments (up to T = 1100 °C; time steps, Δt = 30 min). For the YAS core composition, the measured birefringence does not decrease when tested up to 1000 °C, while for the SiO₂ cladding under the same conditions, its value decreased by ∼30%. These results suggest that inscription of such 'Type II fs-IR' modifications in YAS fibres could be employed to make FBGs with high thermal stability. This opens the door toward the fabrication of a new range of 'FBG host fibres' suitable for ultra-high temperature operation.