Novel Approach to Phase-Sensitive Optical Time-Domain Reflectometry Response Analysis with Machine Learning Methods.

This paper is dedicated to the investigation of the metrological properties of phase-sensitive reflectometric measurement systems, with a particular focus on addressing the non-uniformity of responses along optical fibers. The authors highlight challenges associated with the stochastic distribution of Rayleigh reflectors in fiber optic systems and propose a methodology for assessing response non-uniformity using both cross-correlation algorithms and machine learning approaches, using chirped-reflectometry as an example. The experimental process involves simulating deformation impact by altering the light source's wavelength and utilizing a chirped-reflectometer to estimate response non-uniformity. This paper also includes a comparison of results obtained from cross-correlation and neural network-based algorithms, revealing that the latter offers more than 34% improvement in accuracy when measuring phase differences. In conclusion, the study demonstrates how this methodology effectively evaluates response non-uniformity along different sections of optical fibers.

Optimal defect position in a DFB fiber laser.

Fiber lasers with compact cavity have numerous potential applications in sensing, communications, and medicine. Distributed feedback (DFB) rare-earth doped fiber lasers based on Bragg gratings with a phase shift are the most promising in this aspect. In this paper, we theoretically study such lasers and carry out a complex-frequency analysis of the DFB cavity modes. Our approach is based on the study of poles of open cavity response function and on the laser rate equations. An optimal defect position in the Bragg grating, which maximizes an output power towards one side, was found with this approach. We show that the optimal defect position depends on the pump power. At the pump level close to the lasing threshold, the defect should preferably appear in the middle of the grating to maximize the one-side output power. At higher pumping, the optimal position of the defect becomes asymmetric. We have found specific variables, which allow for determination of optimal defect position for a large variety of DFB laser configurations.

The Sensitivity Improvement Characterization of Distributed Strain Sensors Due to Weak Fiber Bragg Gratings.

Weak fiber Bragg gratings (WFBGs) in a phase-sensitive optical time-domain reflectometer (phi-OTDR) sensor offer opportunities to significantly improve the signal-to-noise ratio (SNR) and sensitivity of the device. Here, we demonstrate the process of the signal and noise components' formation in the device reflectograms for a Rayleigh scattering phi-OTDR and a WFBG-based OTDR. We theoretically calculated the increase in SNR when using the same optical and electrical components under the same external impacts for both setups. The obtained values are confirmed on experimental installations, demonstrating an improvement in the SNR by about 19 dB at frequencies of 20, 100, and 400 Hz. In this way, the minimum recorded impact (at the threshold SNR = 10) can be reduced from 100 nm per 20 m of fiber to less than 5 nm per 20 m of fiber sensor.

Thermal aging of Bragg gratings inscribed in pristine Ge- and N-doped fibers.

In this paper, the thermal aging of temperature-resistant Bragg gratings inscribed in nitrogen-doped silica-core fibers was investigated. The results were compared with their analogues inscribed in pristine weakly germanium-doped fibers. The two-step nature of the nitrogen-doped samples' thermal decay was observed. A study of the changes in the Bragg wavelength during high-temperature annealing was conducted as well.

All-Fiber Highly Sensitive Bragg Grating Bend Sensor.

In this paper, we demonstrated a novel, all-fiber highly sensitive bend sensor based on a four-core fiber rod with a diameter of 2.1 mm. We observed a high resolution of the sensor at a level of 3.6 × 10-3 m-1. Such a sensor design can be used in harsh environments due to the relatively small size and all-fiber configuration, containing no adhesive, nor welded joints.

Method for Determining the Plasmon Resonance Wavelength in Fiber Sensors Based on Tilted Fiber Bragg Gratings.

Surface plasmon resonance-based fiber-optic sensors are of increasing interest in modern sensory research, especially for chemical and biomedical applications. Special attention deserves to be given to sensors based on tilted fiber Bragg gratings, due to their unique spectral properties and potentially high sensitivity and resolution. However, the principal task is to determine the plasmon resonance wavelength based on the spectral characteristics of the sensor and, most importantly, to measure changes in environmental parameters with high resolution, while the existing indirect methods are only useable in a narrow spectral range. In this paper, we present a new approach to solving this problem, based on the original method of determining the plasmon resonance spectral position in the automatic mode by precisely calculating the constriction location on the transmission spectrum of the sensor. We also present an experimental comparison of various data processing methods in both a narrow and a wide range of the refractive indexes. Application of our method resulted in achieving a resolution of up to 3 × 10-6 in terms of the refractive index.

H(2) impact on Bragg gratings written in N-doped silica-core fiber.

The evolution of transmission spectra of Bragg gratings written in an N-doped silica-core fiber in the course of H(2) loading at a pressure of 6 MPa is investigated. It is shown, that penetration of hydrogen molecules in the region of fiber core with written gratings causes irreversible spectrum changes, which do not disappear after subsequent H(2) outcome from the fiber. Bragg gratings' spectra monitoring in the process of H(2) loading is viewed from the angle of photosensitivity mechanisms responsible for formation in N-doped silica-core fibers photoinduced Bragg gratings, capable to operate at very high temperatures.

Response of in-fiber Bragg gratings to hydrogen loading and subsequent heat treatment in H2 ambience.

The evolution of in-fiber Bragg gratings' contrast and resonance wavelengths caused by molecular hydrogen loading and subsequent heat treatment in a hydrogen atmosphere at a pressure of 8 MPa is experimentally studied. Changes in the gratings' transmission spectra brought about by room-temperature hydrogen loading followed by isochronal annealing cycles at temperatures up to 700 degrees C and at invariable hydrogen pressure are recorded in situ. The gratings' thermal decay dynamics in hydrogen ambience and in air are found to be different and dependent on the gratings' type and on the type of glass in the fiber core. In light of the data obtained, the origin of type IIa gratings is anew under discussion.