In this paper a technique to measure the distributed birefringence profile along optical fibers is proposed and experimentally validated. The method is based on the spectral correlation between two sets of orthogonally-polarized measurements acquired using a phase-sensitive optical time-domain reflectometer (ϕOTDR). The correlation between the two measured spectra gives a resonance (correlation) peak at a frequency detuning that is proportional to the local refractive index difference between the two orthogonal polarization axes of the fiber. In this way the method enables local phase birefringence measurements at any position along optical fibers, so that any longitudinal fluctuation can be precisely evaluated with metric spatial resolution. The method has been experimentally validated by measuring fibers with low and high birefringence, such as standard single-mode fibers as well as conventional polarization-maintaining fibers. The technique has potential applications in the characterization of optical fibers for telecommunications as well as in distributed optical fiber sensing.

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 με, respectively. These values agree very well with the theoretically expected measurand resolutions.

Chemical sensing is of great importance in many application fields, such as medicine, environmental monitoring, and industrial process control. Distributed fibre-optic sensing received significant attention because of its unique feature to make spatially resolved measurements along the entire fibre. Distributed chemical sensing (DCS) is the combination of these two techniques and offers potential solutions to real-world applications that require spatially dense chemical measurements covering large length scales. This paper presents a review of the working principles, current status, and the emerging trends within DCS.

The thermomechanical behavior of a standard single mode fiber with different coating materials is theoretically analyzed under different temperature conditions. Results show that the thermal expansion/shrinkage of the fiber coating introduces an extra strain on the optical fiber and can modify its thermal response. Distributed fiber sensors based on coherent Rayleigh and Brillouin scatterings are employed to characterize the impact of different coatings on the temperature sensitivity. The standard coating with dual-layer demonstrates little influence on the thermal response at room temperature due to the softness of primary coating, but it increases the temperature sensitivity by some 50 % at ~ 220 K as the primary coating becomes stiffer at low temperature. Optical fibers with aluminum (Al) and Ormocer® coatings are also experimentally tested. All the measured results agree well with the theoretical analysis.

Phase-sensitive optical time domain reflectometry (φOTDR) is an excellent distributed fiber sensing technique and has been applied in various areas. Its noise is however never been comprehensively studied to the best of our knowledge. The different detection noise sources in such a sensing system are thoroughly investigated. The impacts of thermal noise, shot noise and the beat between signal and the amplified spontaneous emission from a pre-amplifier have been theoretically and experimentally demonstrated. Due to the random nature of the φOTDR signal, the detection noise demonstrates distinct features at different fiber positions in a single measurement. The theoretical analysis and the experimental result explicitly affirm most of the fiber sections, and the difference at some positions may be explained by ambient noise.

The phase errors resulting from in-phase/quadrature (I/Q) demodulation in phase-sensitive optical time-domain reflectometry (φOTDR) are investigated theoretically, numerically, and experimentally for two phase recovery schemes: one employing Kramers-Kronig (KK)-based coherent detection, and a second one based on a direct-detection scheme using an imbalanced Mach- Zehnder interferometer, a 3 × 3 coupler and 3 photodetectors. Mathematical models for the estimation of the differential phase variance are proposed and experimentally validated for these two schemes. The probability density function of the phase variance verifies an uneven longitudinal distribution of the phase errors in the two cases. Results point out that KK-based φOTDR sensors have no special advantages in terms of noise performance compared to standard full I/Q coherent detection, so that large phase errors can be verified at fading locations, leading to severe phase unwrapping problems. On the contrary, the direct-detection scheme based on interferometer is highly dependent on the signal difference between photodetectors, showing higher robustness against fading even if its probability density function is wider than the KK-based case. Experimental results also show the advantages of selecting data points with high in-phase and quadrature amplitudes to achieve reliable vibration measurements, especially in KK-based φOTDR sensors.