Distributed fiber optic shape sensing with simultaneous interrogation of multiple fibers based on Rayleigh-signature domain multiplexing.

We propose a method for shape sensing that employs Rayleigh-signature domain multiplexing to simultaneously probe the fibers or cores of a shape sensing setup with a single optical frequency-domain reflectometry scan. The technique enables incrementing the measurement speed by a factor equal to the number of multiplexed fibers at the expense of an increased noise floor in accordance with the Cramér-Rao lower bound. Nonetheless, we verify that the shape reconstruction performance of the proposed method is in very good agreement with that of conventional sequential core interrogation.

Influence of birefringence- and core ellipticity-induced mode coupling on the gain statistics of forward-pumped Raman amplified few-mode fiber links.

The equations describing light propagation in a few-mode fiber for space-division multiplexing are derived under the presence of linear mode coupling and both Kerr- and Raman-induced nonlinearity. By considering physical models of stress birefringence and core ellipticity, the effect of such fiber imperfections on the gain of a forward-pumped Raman-amplified link is assessed through numerical simulations. The average gain and the variation of signal power at the output of the amplified fiber span is numerically evaluated for different levels of coupling strength in fibers supporting 2 and 4 groups of LP modes, identifying three main propagation regimes and assessing the effect of coupling between different groups of degenerate modes.

Rayleigh-Based Distributed Optical Fiber Sensing.

Distributed optical fiber sensing is a unique technology that offers unprecedented advantages and performance, especially in those experimental fields where requirements such as high spatial resolution, the large spatial extension of the monitored area, and the harshness of the environment limit the applicability of standard sensors. In this paper, we focus on one of the scattering mechanisms, which take place in fibers, upon which distributed sensing may rely, i.e., the Rayleigh scattering. One of the main advantages of Rayleigh scattering is its higher efficiency, which leads to higher SNR in the measurement; this enables measurements on long ranges, higher spatial resolution, and, most importantly, relatively high measurement rates. The first part of the paper describes a comprehensive theoretical model of Rayleigh scattering, accounting for both multimode propagation and double scattering. The second part reviews the main application of this class of sensors.

The Characterization of Optical Fibers for Distributed Cryogenic Temperature Monitoring.

Thanks to their characteristics, optical fiber sensors are an ideal solution for sensing applications at cryogenic temperatures, such as the monitoring of superconducting devices. Their applicability at such temperatures, however, is not immediate as optical fibers exhibit a non-linear thermal response which becomes rapidly negligible below 50 K. A thorough analysis of such a response down to cryogenic temperatures then becomes necessary to correctly translate the optical interrogation readings into the actual fiber temperature. Moreover, to increase the fiber sensitivity down to a few kelvin, special coatings can be used. In this manuscript we described the thermal responses experimental characterization of four commercially available optical fiber samples with different polymeric coatings in the temperature range from 5 K to 300 K: two with acrylate coatings of different thickness, one with a polyimide coating and one with a polyether-ether-ketone (PEEK) coating. Multiple thermal cycles were performed consecutively to guarantee the quality of the results and a proper estimate of the sensitivity of the various samples. Finally, we experimentally validated the quality of the measured thermal responses by monitoring the cool down of a dummy superconducting link from room temperature to approximately 50 K using two fibers coated, respectively, in acrylate and PEEK. The temperatures measured with the fibers agreed and matched those obtained by standard electronic sensors, providing, at the same time, further insight in to the cool-down evolution along the cryostat.

Machine Learning Estimation of the Phase at the Fading Points of an OFDR-Based Distributed Sensor.

The paper reports a machine learning approach for estimating the phase in a distributed acoustic sensor implemented using optical frequency domain reflectometry, with enhanced robustness at the fading points. A neural network configuration was trained using a simulated set of optical signals that were modeled after the Rayleigh scattering pattern of a perturbed fiber. Firstly, the performance of the network was verified using another set of numerically generated scattering profiles to compare the achieved accuracy levels with the standard homodyne detection method. Then, the proposed method was tested on real experimental measurements, which indicated a detection improvement of at least 5.1 dB with respect to the standard approach.

Experimental characterization of group and phase delays induced by bending and twisting in multi-core fibers.

The local variations of group and phase propagation delays induced by bending and twisting a coupled core three-core fiber are experimentally characterized, for the first time, to the best of our knowledge, along the fiber length, with millimeter-scale spatial resolution. The measurements are performed by means of spectral correlation analysis on the fiber's Rayleigh backscattered signal, enabling for a distributed measurement of the perturbation effects along the fiber length. A mathematical model validating the experimental results is also reported.

High-frequency high-resolution distributed acoustic sensing by optical frequency domain reflectometry.

A data analysis algorithm for OFDR-based distributed acoustic sensing (DAS) is proposed, which achieves high acoustic bandwidths of tens of kilohertz with sharp spatial resolutions in the order of centimeters. The non-idealities of the setup as well as the phase noise affecting the measurement are analyzed and a method to compensate them is experimentally demonstrated. The performance of the sensor is evaluated by extensive experimental tests, showing the viability of the proposed technique to achieve high frequency and high spatial resolution distributed acoustic sensing.

Analysis of modal coupling due to birefringence and ellipticity in strongly guiding ring-core OAM fibers.

After briefly recalling the issue of OAM mode purity in strongly-guiding ring-core fibers, this paper provides a methodology to calculate the coupling strength between OAM mode groups due to fiber perturbations. The cases of stress birefringence and core ellipticity are theoretically and numerically investigated. It is found that both perturbations produce the same coupling pattern among mode groups, although with different intensities. The consequence is that birefringence causes the highest modal crosstalk because it strongly couples groups with a lower propagation-constant mismatch. The power coupling to parasitic TE and TM modes is also quantified for both perturbations and is found to be non-negligible. Approximate modal crosstalk formulas valid for weakly-guiding multi-core fibers, but whose parameters are adapted to the present case of strongly guiding OAM fibers, are found to provide a reasonable fit to numerical results. Finally, the effect that modal coupling has on OAM transmission is assessed in terms of SNR penalty.

Statistical analysis of nonlinear coupling in a WDM system over a two mode fiber.

We study the effect of nonlinear coupling in a WDM configuration over a two-mode fiber. A statistical analysis is presented that takes into account the effect of the random phase-sensitive amplification or depletion. Our results show high nonlinear coupling between the modes. We have quantified the channel power fluctuations, due to the wave phase random variations, at the output of the fiber. We also investigate the effect of random linear mode coupling on the nonlinear mode coupling.

Distributed optical fibre sensing for early detection of shallow landslides triggering.

A distributed optical fibre sensing system is used to measure landslide-induced strains on an optical fibre buried in a large scale physical model of a slope. The fibre sensing cable is deployed at the predefined failure surface and interrogated by means of optical frequency domain reflectometry. The strain evolution is measured with centimetre spatial resolution until the occurrence of the slope failure. Standard legacy sensors measuring soil moisture and pore water pressure are installed at different depths and positions along the slope for comparison and validation. The evolution of the strain field is related to landslide dynamics with unprecedented resolution and insight. In fact, the results of the experiment clearly identify several phases within the evolution of the landslide and show that optical fibres can detect precursory signs of failure well before the collapse, paving the way for the development of more effective early warning systems.

Cryogenic-temperature profiling of high-power superconducting lines using local and distributed optical-fiber sensors.

This contribution presents distributed and multipoint fiber-optic monitoring of cryogenic temperatures along a superconducting power transmission line down to 30 K and over 20 m distance. Multipoint measurements were conducted using fiber Bragg gratings sensors coated with two different functional overlays (epoxy and poly methyl methacrylate (PMMA)) demonstrating cryogenic operation in the range 300-4.2 K. Distributed measurements exploited optical frequency-domain reflectometry to analyze the Rayleigh scattering along two concatenated fibers with different coatings (acrylate and polyimide). The integrated system has been placed along the 20 m long cryostat of a superconducting power transmission line, which is currently being tested at the European Organization for Nuclear Research (CERN). Cool-down events from 300-30 K have been successfully measured in space and time, confirming the viability of these approaches to the monitoring of cryogenic temperatures along a superconducting transmission line.

Distributed measurement of high electric current by means of polarimetric optical fiber sensor.

A novel distributed optical fiber sensor for spatially resolved monitoring of high direct electric current is proposed and analyzed. The sensor exploits Faraday rotation and is based on the polarization analysis of the Rayleigh backscattered light. Preliminary laboratory tests, performed on a section of electric cable for currents up to 2.5 kA, have confirmed the viability of the method.

Editorial: acceptance criteria and editorial procedures for Optics Letters.

Optics Letters Editors strive to provide timely reviews and decisions for authors while bringing top quality papers to the optics community. The purpose of this editorial is to explain Optics Letters' acceptance criteria and editorial procedures. Our hope is that greater transparency concerning the decision-making process will increase understanding as well as acceptance of our criteria and procedures.

Widely time-dispersion-tuned fiber optical oscillator and frequency comb based on multiple nonlinear processes.

An all-fiber optical oscillator based on three nonlinear processes, namely stimulated Raman scattering and broad-band and narrow-band optical parametric amplification, is presented and experimentally characterized. The wavelength tuning is achieved by means of the time-dispersion technique and spans over 160 nm. Through the same technique a fast tunable optical frequency comb has been realized exploiting cascaded four-wave mixing.

Experimental characterization of the counter-propagating Raman polarization attraction.

Recently, fiber Raman amplifiers have proven to be effective in the all-optical control of the state of polarization of signals in single-mode telecommunications optical fibers. Previous works predicted the existence of a quantitative relationship between the achieved degree of polarization and the mean Raman gain. Here, we experimentally validate such a relationship in the case of counter-propagating Raman-based polarization attractors for different pump and signal powers and for different fiber link lengths.

Effects of spin process on birefringence strength of single-mode fibers.

Spin process is the most effective and diffused way to reduce polarization mode dispersion in single-mode optical fibers. All theoretical models adopted so far to describe spun fibers assume that the only effect of spin is to rotate fiber birefringence, without affecting its strength. Yet, experimental analyses of this hypothesis are controversial. In this paper, we report on an extensive experimental characterization of birefringence in spun and unspun fibers. Results indicate that the spinning process has no instantaneous effect on birefringence strength, regardless of the kind of fiber; nevertheless, there might be a small average effect on G.652 fibers.

Limits of applicability of polarization sensitive reflectometry.

We present a detailed theoretical analysis of the measurement limits of polarization sensitive reflectometry, imposed by spatial resolution and measurement accuracy. The limits are conveniently represented in a map of constraints. We also describe and experimentally verify a procedure that allows to measure spin profiles of single-mode fibers with spin rates exceeding the measurable range of the reflectometer. The technique consists in twisting the fiber to locally unwind the spin.

Characterization of a novel dual-core elliptical hollow optical fiber with wavelength decreasing differential group delay.

The modal distribution of a novel elliptical hollow optical fiber is experimentally and numerically characterized. The fiber has a central elliptical air hole surrounded by a germanosilicate lanceolate ring core. Experiments reveal that the fiber behaves like a dual core waveguide and it is found that the differential group delay of each core decreases with wavelength with a PMD coefficient slope of ~10(-2) ps/m/THz. Experimental results are also compared with numerical modeling based on scanning electron microscopy images.

Distributed characterization of bending effects on the birefringence of single-mode optical fibers.

The effects of bending on randomly birefringent, single-mode optical fibers have been analyzed by means of polarization-sensitive optical frequency domain reflectometry. This distributed, pointwise characterization has been performed on ad hoc drawn spun fibers and on a ribbon cable. Experimental results are in agreement with the theoretical predictions and confirm the effectiveness of reflectometry as a means to characterize polarization properties of optical fibers.

Spin-profile characterization in randomly birefringent spun fibers by means of frequency-domain reflectometry.

A novel technique, based on polarization-sensitive, frequency-domain reflectometry, for a full noninvasive characterization of spin profile in randomly birefringent spun fibers is presented. Effective measurements of spin profile in a fiber sample a few tens of meters long are reported, but the technique can be straightforwardly scaled to kilometers-long samples.