Humidity-insensitive optical fibers for distributed sensing applications.

Humidity is a critical environmental factor in various applications, and its temperature dependence must be considered when developing thermo-hygrometer fiber sensors. The optical fibers that constitute the sensor must have a temperature reference, which should be resistant to humidity to avoid cross-sensitivities. This paper presents two innovative optical fibers insensitive to humidity over temperatures ranging from -20∘ C to 55°C. To the best of our knowledge, the novel standard size optical fibers coated with acrylate and silicone are tested under controlled conditions using an optical time-domain reflectometer sensor based on Rayleigh scattering. The sensor achieves meter-range resolution over kilometers of length with a response time of few minutes.

Distributed measurement of mode group effective refractive index difference in a few mode optical fibers.

The possibility to perform distributed measurements of the effective refractive index difference between distinct modes in few mode optical fibers is demonstrated using phase sensitive optical time domain reflectometry. Effective refractive index differences between LP02, LP21a and LP21b modes are measured with for a spatial resolution of 24m.

Closed-loop technique based on gain balancing for real-time Brillouin optical time-domain analysis.

A closed-loop servo control based on balancing the gain of two probing frequencies is proposed for real-time Brillouin optical time-domain analysis (BOTDA) without post-processing. With the most basic BOTDA hardware setup, the system can perform measurement in 150 ms and track a sudden Brillouin frequency shift (BFS) change in excess of 300 MHz (corresponding to a temperature change of more than 250°C) over ∼5 km of fiber with a spatial resolution of 2 m. Moreover, the feedback loop is independent of the loss experienced by the probe and pump, with no requirement on the BFS uniformity along the fiber. All these advantages make the proposed system suitable for field applications in harsh environments.

Long-distance distributed pressure sensing based on frequency-scanned phase-sensitive optical time-domain reflectometry.

In this paper, a long-distance distributed pressure sensing system based on a special fiber and using frequency-scanned phase-sensitive optical time-domain reflectometry is proposed. The fiber shows high pressure sensitivity (159 MHz/bar) and low loss (3 dB/km) owing to its simple structure made of two large air holes in the cladding. The pressure response of the two orthogonal polarization axes of the fiber is explored distinctively. Distributed pressure sensing over a long sensing range (720 m) and high spatial resolution (5 cm) is demonstrated, resulting in 14,400 resolved sensing points with uncertainty on pressure of 0.49 bar. Discrimination between the temperature/strain and pressure responses is demonstrated, taking advantage of the different pressure and temperature sensitivities of the two polarization axes. In addition, the temperature response of the fiber is studied and the simulation results show the possibility of scaling the temperature sensitivity by adjusting the size of the core. The sensing distance limit due to crosstalk between the polarization axes is also discussed.

Impact of optical noises on unipolar-coded Brillouin optical time-domain analyzers.

Noise models for both single-pulse and coded Brillouin optical time-domain analyzers (BOTDA) are established to quantify the actual signal-to-noise ratio (SNR) enhancement provided by pulse coding at any fiber position and in any operating condition. Simulation and experimental results show that the polarization noise and spontaneous Brillouin scattering (SpBS) to signal beating noise could highly penalize the performance of coded-BOTDA, depending on the code type and the interrogated fiber position. The models also serve as a useful tool to optimize the SNR improvement by trading off the accumulated Brillouin gain and optical noises.

Study on the signal-to-noise ratio of Brillouin optical-time domain analyzers.

The signal-to-noise ratio (SNR) of Brillouin optical time-domain analyzers (BOTDA) is modelled and experimentally validated, using direct detection with and without the use of optical pre-amplification. The behavior of SNR as a function of the Brillouin gain and the probe power reaching the photo detection is analyzed in depth using this developed model and checked using two photodetectors with different specifications. It proves that a pre-amplification associated to a good-quality photodetector and a well-matched post-processing filtering can secure the highest SNR for direct-detection BOTDA. Such an optimal SNR presents only a 2.3 dB penalty compared to the ideal shot-noise-limited case that can only be reached using rather sophisticated configurations. In addition, the model here established predicts the SNR at any fiber position in any given experimental condition.

Short spatial resolution retrieval from a long pulse Brillouin optical time-domain analysis trace.

A novel, to the best of our knowledge, postprocessing technique is proposed to extract with a flexible and variable spatial resolution the information from Brillouin optical time-domain analyzers, obtained using a pulse longer than the acoustic settling time. The negative impact of the acoustic transient effect is suppressed, enabling a Brillouin response proportional to the spatial resolution and a Brillouin gain spectrum keeping its natural linewidth. This leads to a better overall sensing performance, in particular for submetric spatial resolutions, with no compromises on sensing range and measurement time.

Distributed and dynamic strain sensing with high spatial resolution and large measurable strain range.

A distributed and dynamic strain sensing system based on frequency-scanning phase-sensitive optical time domain reflectometry is proposed and demonstrated. By utilizing an RF pulse scheme with a fast arbitrary waveform generator, a train of optical pulses covering a large range of different optical frequencies, short pulse width, and high extinction ratio is generated. Also, a Rayleigh-enhanced fiber is used to eliminate the need for averaging, allowing single-shot operation. Using direct detection and harnessing a dedicated least mean square algorithm, the method exhibits a record high spatial resolution of 20 cm, concurrently with a large measurable strain range (80µÎµ, 60 demonstrated), a fast sampling rate of 27.8 kHz (almost the maximum possible for a 55 m long fiber and 60 frequency steps), and low strain noise floor (<1.8nε/Hz for vibrations below 700 Hz and <0.7nε/Hz for higher frequencies).

Observation of Stimulated Brillouin Scattering in Silicon Nitride Integrated Waveguides.

Silicon nitride (Si_{3}N_{4}) has emerged as a promising material for integrated nonlinear photonics and has been used for broadband soliton microcombs and low-pulse-energy supercontinuum generation. Therefore, understanding all nonlinear optical properties of Si_{3}N_{4} is important. So far, only stimulated Brillouin scattering (SBS) has not yet been reported. Here we observe, for the first time, backward SBS in fully cladded Si_{3}N_{4} waveguides. The Brillouin gain spectrum exhibits an unusual multipeak structure resulting from hybridization with high-overtone bulk acoustic resonances of the silica cladding. The reported intrinsic Si_{3}N_{4} Brillouin gain at 25 GHz is estimated as 4×10^{-13} m/W. Moreover, the magnitude of the Si_{3}N_{4} photoelastic constant is estimated as |p_{12}|=0.047±0.004, which is nearly 6 times smaller than for silica. Since SBS imposes an optical power limitation for waveguides, our results explain the capability of Si_{3}N_{4} to handle high optical power, central for integrated nonlinear photonics.

High-resolution distributed shape sensing using phase-sensitive optical time-domain reflectometry and multicore fibers.

In this paper, a highly-sensitive distributed shape sensor based on a multicore fiber (MCF) and phase-sensitive optical time-domain reflectometry (φ-OTDR) is proposed and experimentally demonstrated. The implemented system features a high strain sensitivity (down to ∼0.3 µÉ›) over a 24 m-long MCF with a spatial resolution of 10 cm. The results demonstrate good repeatability of the relative fiber curvature and bend orientation measurements. Changes in the fiber shape are successfully retrieved, showing detectable displacements of the free moving fiber end as small as 50 µm over a 60 cm-long fiber. In addition, the proposed technique overcomes cross-sensitivity issues between strain and temperature. To the best of our knowledge, the results presented in this work provide the first demonstration of distributed shape sensing based on φ-OTDR using MCFs. This high-sensitivity technique proves to be a promising approach for a wide range of new applications such as dynamic, long distance and three-dimensional distributed shape sensing.

Fiber Bragg grating-based thermometer for drill bit temperature monitoring.

The temperature measurement of a drill bit during an implantology drilling process is proposed by using a fiber Bragg grating fitted inside the drill bit. Due to the rotational nature of the drilling process, a free-space fiber-optic rotary joint is used for interrogating the fiber Bragg grating. Due to mechanical clearances and interferometric noise induced at this rotary joint, signal integrity is strongly deteriorated and is not workable without adequate measures. These measures involve a proper fiber lensing and a signal processing in order to remove the interferometric noise. Finally, a heating measurement on an implantology drill bit is performed and discussed for drilling several holes on a pork jaw sample.

Design rules for optimizing unipolar coded Brillouin optical time-domain analyzers.

The performance of unipolar unicolor coded Brillouin optical time-domain analysis (BOTDA) is evaluated based on both Simplex and Golay codes. Four major detrimental factors that limit the system performance, including decoded-gain trace distortion, coding pulse power non-uniformity, polarization pulling and higher-order non-local effects, are thoroughly investigated. Through theoretical analysis and an experimental validations, solutions and optimal design conditions for unipolar unicolor coded BOTDA are clearly established. First, a logarithmic normalization approach is proposed to resolve the linear accumulated Brillouin amplification without distortion. Then it is found out that Simplex codes are more robust to pulse power non-uniformity compared to Golay codes; whilst the use of a polarization scrambler must be preferred in comparison to a polarization switch to mitigate uncompensated fading induced by polarization pulling in the decoded traces. These optimal conditions enables the sensing performance only limited by higher-order non-local effects. To secure systematic errors below 1.3 MHz on the Brillouin frequency estimation, while simultaneously reaching the maximum signal-to-noise ratio (SNR), a mathematical model is established to trade-off the key parameters in the design, i.e., the single-pulse Brillouin amplification, code length and probe power. It turns out that the optimal SNR performance depends in inverse proportion on the value of maximum single-pulse Brillouin amplification, which is ultimately determined by the spatial resolution. The analysis here presented is expected to serve as a quantitative guideline to design a distortion-free coded BOTDA system operating at maximum SNR.

Performance analysis of frequency shift estimation techniques in Brillouin distributed fiber sensors.

The performance of post-processing techniques carried out on the Brillouin gain spectrum to estimate the Brillouin frequency shift (BFS) in standard Brillouin distributed sensors is evaluated. Curve fitting methods with standard functions such as polynomial and Lorentzian, as well as correlation techniques such as Lorentzian Cross-correlation and Cross Reference Plot Analysis (CRPA), are considered for the analysis. The fitting procedures and key parameters for each technique are optimized, and the performance in terms of BFS uncertainty, BFS offset error and processing time is compared by numerical simulations and through controlled experiments. Such a quantitative comparison is performed in varying conditions including signal-to-noise ratio (SNR), frequency measurement step, and BGS truncation. It is demonstrated that the Lorentzian cross-correlation technique results in the largest BFS offset error due to truncation, while exhibiting the smallest BFS uncertainty and the shortest processing time. A novel approach is proposed to compensate such a BFS offset error, which enables the Lorentzian cross-correlation technique to completely outperform other fitting methods.

Forward Brillouin scattering acoustic impedance sensor using thin polyimide-coated fiber.

The standard single-mode fiber has been demonstrated as an optomechanical sensor recently to measure the acoustic impedances of surrounding liquids by means of the generation and detection of forward-stimulated Brillouin scattering (FSBS). FSBS allows the mechanical properties of an external material to be probed directly through the interaction of guided light and transverse sound waves that occurs entirely inside the fiber structure. In this technique, having a low-loss interface between the fiber bulk and the external medium is essential for precise measurement; however, it leads to the necessary but impractical removal of the thick polymer fiber coating in most reported methods. Here, we use a commercially available 80-µm-diameter optical fiber coated with a 8-µm-thick polyimide coating layer to measure the acoustic impedances of the surrounding liquids, showing accurate measurement results while retaining the mechanical strength of the fiber.

Hybrid Golay-coded Brillouin optical time-domain analysis based on differential pulses.

Different approaches to implement unipolar Golay coding in Brillouin optical time-domain analysis based on a differential pulse pair (DPP) are investigated. The analysis points out that dedicated post-processing procedures must be followed to secure the sharp spatial resolution associated with the DPP method. Moreover, a novel hybrid Golay-DPP coding scheme is proposed, offering 1.5 dB signal-to-noise ratio improvement with respect to traditional unipolar Golay coding, while halving the measurement time, constituting a 3 dB overall coding gain enhancement. Proof-of-concept experiments validate the proposed technique, demonstrating a 50 cm spatial resolution over a 10.164 km long sensing fiber with a frequency uncertainty of 1.4 MHz.

Local activation of surface and hybrid acoustic waves in optical microwires.

Elastic vibrations in subwavelength structures have gained importance recently in fundamental light-matter studies and various optoacoustic applications. Existing techniques have revealed the presence of distinct acoustic resonances inside silica microwires yet remain unable to individually localize them. Here, we locally activate distinct classes of acoustic resonances inside a tapered fiber using a phase-correlation distributed Brillouin method. Experimental results verify the presence of surface and hybrid acoustic waves at distinct fiber locations and demonstrate, to the best of our knowledge, the first distributed surface acoustic wave measurement. This technique is important for understanding properties of optoacoustic interactions and enabling designs of novel optomechanical devices.

Temperature-strain discrimination in distributed optical fiber sensing using phase-sensitive optical time-domain reflectometry.

A method based on coherent Rayleigh scattering distinctly evaluating temperature and strain is proposed and experimentally demonstrated for distributed optical fiber sensing. Combining conventional phase-sensitive optical time-domain domain reflectometry (ϕOTDR) and ϕOTDR-based birefringence measurements, independent distributed temperature and strain profiles are obtained along a polarization-maintaining fiber. A theoretical analysis, supported by experimental data, indicates that the proposed system for temperature-strain discrimination is intrinsically better conditioned than an equivalent existing approach that combines classical Brillouin sensing with Brillouin dynamic gratings. This is due to the higher sensitivity of coherent Rayleigh scatting compared to Brillouin scattering, thus offering better performance and lower temperature-strain uncertainties in the discrimination. Compared to the Brillouin-based approach, the ϕOTDR-based system here proposed requires access to only one fiber-end, and a much simpler experimental layout. Experimental results validate the full discrimination of temperature and strain along a 100 m-long elliptical-core polarization-maintaining fiber with measurement uncertainties of ~40 mK and ~0.5 µÎµ, respectively. These values agree very well with the theoretically expected measurand resolutions.

Distributed photothermal spectroscopy in microstructured optical fibers: towards high-resolution mapping of gas presence over long distances.

Chemical sensing using optical fibers is often challenging, as it is generally difficult to achieve strong interaction between the guided light and the analyte at the wavelength of interest for performing the detection. Despite this difficulty, many schemes exist (and can be found in the literature) for point chemical fiber sensors. However, the challenge increases even further when it comes to performing fully distributed chemical sensing. In this case, the optical signal which interacts with the analyte is typically also the signal that has to travel to and from the interrogator: for a good sensitivity, the light should interact strongly with the analyte, leading inevitably to an increased loss and a reduced range. Few works in the literature actually provide demonstrations of truly distributed chemical sensing and, although there have been several attempts to realize these sensors (e.g. based on special fiber coatings), the vast majority of these attempts has failed to reach widespread use due to several reasons, among them: lack of sensitivity or selectivity, lack of range or resolution, cross sensitivity to temperature or strain, or need to work at specific wavelengths where fiber instrumentation becomes extremely expensive or unavailable. In this work we provide a preliminary demonstration of the possibility of achieving distributed detection of gas presence with spectroscopic selectivity, high spatial resolution, potential for long range measurements and feasibility of having most of the interrogator system working at conventional telecom wavelengths. For a full exploitation of this concept, new fibers (or more likely, fiber bundles) should be developed capable of guiding specific wavelengths in the IR (corresponding to gas absorption wavelengths) with good overlap with the analyte while also having a solid core with good transmission behavior at 1.55 µm, and good thermal coupling between the two guiding structures.

Resolving 1 million sensing points in an optimized differential time-domain Brillouin sensor.

A differential pulse-width pair (DPP) Brillouin distributed fiber sensor is implemented to achieve centimetric spatial resolution over distances of several kilometers. The presented scheme uses a scanning method in which the spectral separation between the two probe sidebands is kept constant, while the optical frequency of the pump is swept to scan the Brillouin spectral response. Experimental results show that this method avoids detrimental temporal distortions of the pump pulses, which in a standard implementation prevent the DPP method from operating over mid-to-long distances. Such a novel scanning procedure allows the resolving, for the first time in pure time-domain Brillouin sensors, of 1,000,000 sensing points, i.e., 1 cm spatial resolution over 10 km in a conventional acquisition time.

All-optical flip-flops based on dynamic Brillouin gratings in fibers.

A method to generate an all-optical flip-flop is proposed and experimentally demonstrated based on dynamic Brillouin gratings (DBGs) in polarization maintaining fibers. In a fiber with sufficiently uniform birefringence, this flip-flop can provide extremely long storage times and ultra-wide bandwidth. The experimental results demonstrate an all-optical flip-flop operation using phase-modulated pulses of 300 ps and a 1 m long DBG. This has led to a time-bandwidth product of ∼30, being in this proof-of-concept setup mainly limited by the relatively low bandwidth of the used pulses and the short fiber length.