Chromatic dispersion and optical fibre disturbance, such as temperature and vibration, degrade the high spatial resolution of OFDR, especially in remote interrogation. The proposed compensation technique achieves 40. m from over 30 mm. Additionally, we demonstrate its application for interrogating a remote optical device. (c) 2025 The Author(s)

We propose an evaluation method for overhead cable sag that utilizes distributed acoustic sensing to draw a catenary curve. Spatiotemporal analysis for dynamic strain distribution locates a single cable span from successive cable spans and reveals the strain propagation speed along a single cable span through the two-dimensional Fourier transform. The propagation speed then characterizes the catenary curve.

The complex impulse responses of two types of coupled 2-core fibers, with homogeneous and heterogeneous cores, were measured by using a measurement system based on linear optical sampling. The measurements were performed over a bandwidth of 20 nm (∼2.5 THz), and the spatial-mode dispersion (SMD) was analyzed. Several lengths of both homogeneous and heterogeneous core fibers were prepared to investigate the length dependence of SMD. The SMDs for divided frequency bands were examined by numerically analyzing the measured broadband impulse response. Consequently, the power impulse response at each subband was obtained, and the variation in the SMDs was examined. In addition, the statistical distribution of the SMD was revealed to be different for each type of the 2-core fiber. The SMD observed at a specific band is handled by the receiver in space-division multiplexing transmission systems. Finally, we present a measurement of mode dependent loss (MDL) of a 2-core fiber, and discuss the impact of time axis inaccuracy to the MDL measurement. These analyses performed are expected to be beneficial for designing such systems.

Distributed acoustic sensing (DAS) measures local strains along a sensing fiber using as an array of dense virtual strain sensors. One related application of DAS is a seismic wave sensing. Due to the densely deployed virtual sensors, DAS can detect seismic wave propagation along the sensing fiber, which is useful for underground structure imaging. Thus, DAS has been used as an alternative to the conventional sparsely deployed seismometers for investigating geophysical phenomena. DAS for seismic wave sensing uses optical time domain reflectometry (OTDR), which measures a differential phase caused by elongation between two locations. The bandwidth of a probe pulse light defines the gauge length and allowable differential phase or strain rate. Although OTDR has shown to be effective, the nature of the log-scale amplitude of earthquakes may require a broad amplitude dynamic range for DAS. To broaden the bandwidth, we utilize optical frequency domain reflectometry (OFDR), which launches a frequency-swept probe light and measures optical frequency shift proportional to the strain. Using an existing optical fiber in a telecommunication network as a sensing fiber, we demonstrate sensing for two seismic waves with amplitude scales of 0 and 3. Because the spatial resolution is higher than that of OTDR, we can finely visualize underground facilities along the existing telecommunication optical fiber. We also discuss an inherent limitation of OFDR that becomes evident when measuring a long wavelength vibration such as a seismic wave.

Transmission length dependency of complex impulse responses of coupled 2-core fibers are investigated using coherent sampling with picosecond time resolution over 20-nm bandwidth. Spectrally decomposed analysis is accomplished to observe the statistical nature.

We review recent advances in distributed fiber optic sensing (DFOS) and their applications. The scattering mechanisms in glass, which are exploited for reflectometry-based DFOS, are Rayleigh, Brillouin, and Raman scatterings. These are sensitive to either strain and/or temperature, allowing optical fiber cables to monitor their ambient environment in addition to their conventional role as a medium for telecommunications. Recently, DFOS leveraged technologies developed for telecommunications, such as coherent detection, digital signal processing, coding, and spatial/frequency diversity, to achieve improved performance in terms of measurand resolution, reach, spatial resolution, and bandwidth. We review the theory and architecture of commonly used DFOS methods. We provide recent experimental and field trial results where DFOS was used in wide-ranging applications, such as geohazard monitoring, seismic monitoring, traffic monitoring, and infrastructure health monitoring. Events of interest often have unique signatures either in the spatial, temporal, frequency, or wavenumber domains. Based on the temperature and strain raw data obtained from DFOS, downstream postprocessing allows the detection, classification, and localization of events. Combining DFOS with machine learning methods, it is possible to realize complete sensor systems that are compact, low cost, and can operate in harsh environments and difficult-to-access locations, facilitating increased public safety and smarter cities.

Distributed vibration sensing (DVS) utilizes an optical fiber as an array of virtual strain sensors. It can reveal dynamic strain distributions such as seismic events and car traffic, thus giving a new function to existing optical communication fibers. In this work, we use optical frequency domain reflectometry (OFDR) to investigate the dynamic strain distribution along an optical fiber implemented in a deployed aerial optical fiber cable that vibrates with the wind. The OFDR setup is customized for distant location interrogation with a high repetition rate of the probe light and overcomes the OFDR drawbacks of short measurement range and low repetition rate, thus enabling application to the investigation of dynamic strain along a range of telecommunications outside a plant. Sub-1-m spatial resolution measurements revealed that dynamic strain along the optical fiber in an aerial cable depends on the deployment condition. It also provides the deployed optical telecommunication fiber with a function for visualizing the deployment conditions of the telecommunications outside plants.

We measured a seismic event with OFDR. OFDR together with the unique feature of adjusting the measurement performance was used to analyse the event from different perspectives, and it could also be useful for revealing the features of infrequent seismic events.

We investigate the statistical property of Rayleigh backscattered light to confirm the tolerance to vibration-induced beat frequency offset, which forces us to interrogate an unintentionally-positioned sensor. A long sensor is capable of measuring vibrations correctly.

We demonstrate 40 μm spatial resolution over a few kilometres by optical frequency domain reflectometry (OFDR). The measurement setup overcomes short distance measurement drawback and interrogates distant locations with high resolution. Application of an optical splitter in long-range high-resolution reflectometry is also investigated.

We measure vibration waveforms along an installed aerial optical fiber cable and identify a sagging cable section. This is the first demonstration of telecom optical fiber cable maintenance by distributed vibration sensing.

We investigate the impact of the frequency modulation induced by vibration occurring at a preceding distance in distributed acoustic sensing (DAS) based on optical frequency domain reflectometry (OFDR), which analyzes a vibration waveform by calculating the spectrum shift of Rayleigh backscattered light in the same manner as a fiber Bragg grating. When the measurement time of a beat signal is shorter than the period of the vibration at the preceding distance, the vibration-induced frequency modulation can be treated as a frequency offset for the backscattered light. The frequency offset becomes a distance offset through the distance beat frequency allocation of OFDR, and the distance offset forces us to interrogate the Rayleigh backscattered light spectra at unintentional distances for each measurement since the distance offset is time-varying. Since the spectra at unintentional distances are uncorrelated, the spectrum shift calculated with an uncorrelated spectrum results in a spurious vibration, which is a measured waveform that is different from the actual vibration waveform and that constitutes a measurement error in a DAS measurement. We propose a technique to compensate for the spurious vibration. The technique estimates the vibration-induced distance offset by calculating the cross-correlation between Rayleigh backscattered light waveforms and shifts the spectrum analysis range by the estimated offset in order to track the consistent spectrum. We perform a DAS measurement on a sensing fiber that experiences simultaneous vibrations at different distances and confirm the validity of the proposed technique.

We describe a novel optical correlation domain reflectometry technique based on coherence synchronization between probe and local lights. Coherence synchronization is realized by using artificially generated low coherence lights, which are controlled by arbitrary waveform generators. Thanks to the flexibility of the electronics, we can easily design the coherence characteristics of the artificially generated low coherence lights to yield a correlation gating that filters an autocorrelation of an original laser at an arbitrary delay. The correlation gating realized by coherence synchronization is effective as long as the measurement length is less than the coherence length of the original laser. We demonstrate the measurement of the Fresnel reflection at the end of an optical fiber by using coherence-synchronized optical frequency combs that have only one correlation peak in the fiber under test (FUT). The FUT is 100 m long, and the repetition rate of the optical frequency comb generated by cascaded electro-optic modulators is 9.5 GHz, which corresponds to a 10.5 mm spacing between the periodic correlation peaks. The proposed technique removes the restriction imposed by the periodic correlation peaks present in correlation-based measurement using an optical frequency comb.

We present a robust interferometer-based laser phase noise reduction technique that can easily be tuned to apply phase corrections at frequencies at which the interferometer is less affected by environmental disturbances. The technique operates by measuring the in-phase and quadrature components of the interference pattern along the circular Lissajous figure to compute the short-term phase error and reject long-term interferometer drift, and then uses that phase error to correct the laser phase noise. We successfully demonstrate phase noise reduction for an external cavity semiconductor laser without the need for an interferometer stabilization technique. The proposed technique realizes a frequency noise reduction about 20 dB between 100 Hz and 3 MHz. We also investigate the imbalance of the circular Lissajous figure and show that the performance degradation caused by an amplitude imbalance and a phase deviation from 90° can be compensated for by amplifying the extracted phase noise fluctuation.

A novel optical correlation domain reflectometry technique utilizing artificially-generated low-coherence light is proposed for long-range measurement with high resolution. We successfully demonstrate the selection of one correlation peak of a laser at an arbitrary delay for a fibre under test.

We describe our recent progress on a linear optical sampling technique. Thanks to broad bandwidth and direct complex amplitude accessibility, the technique enables WDM signal demultiplexing by digital signal processing and laser phase noise characterization.