Considering the growing interest in adopting distributed fiber optic sensing (DFOS) technology for telecom infrastructure monitoring and protection, ITU-T SG15 recently approved the G.681 Recommendation to address the demand. This standard represents a major milestone and the first step toward building a viable DFOS ecosystem for the telecom industry. It specifies the optical interface parameters for utilizing DFOS for in-service terrestrial networks. This paper discusses DFOS application examples, provides an overview of the G.681 Recommendation, and offers readers insights and rationale behind the development of the standard.

Signal fading creates points along the fiber where phase cannot be extracted, so they are conventionally discarded. Instead, we propose a Kalman-based solution for ϕ-OTDR full-fiber monitoring. Experiments demonstrate phase and temperature estimation with approximately 15 times better spatial density than if removing these points.

We present a theoretical, algorithmic, and experimental study of temperature sensing using ϕ-OTDR with coherent detection. A physics-based model is developed to relate the measured Rayleigh backscattered signal to temperature variations along the fiber, showing that the phase evolution encodes the cumulative temperature change between the interrogator and the sensing location, while the amplitude exhibits only local sensitivity. Based on this insight, we propose robust algorithms for temperature-event detection and temperature-profile reconstruction. Experimental results demonstrate reliable recovery of temperature-induced perturbations in standard single-mode fibers using coherently detected ϕ-OTDR.

An algorithm to convert the recovered optical phase to temperature change is derived and verified in coherent $\phi$-OTDR. Experimental results show a maximum error of 1.5°C compared to thermometer-based reference measurements. © 2024 The Author(s)

CC-OTDR signal envelope shaping is introduced to reduce the impact of non-linear signal interactions on a neighboring wavelength data channel when co-propagating the probing signal with the data signal. Joint co-directional acoustic sensing and 200 Gbps transmission are demonstrated over a 50 km link.

An extension of phase-sensitive optical time domain reflectometry utilizing the transmission of code sequences and correlation is presented. This method enhances the spatial resolution while maintaining the same sensing reach. Different application examples are presented.

We present the findings of a field trial conducted in a power utility infrastructure by probing the operational telecom cable for sensing. We have successfully detected intrusion event like jumping, and door closer. In addition, traffic flow and health of electrical cable were effectively monitored. (c) 2025 The Author(s)

We demonstrate an experimental phase optical time-domain reflectometry (OTDR) system capable of simultaneous detection and classification of various environmental events, such as wind-induced fiber movement, vehicle movement, and audio signatures, with real-time visualization.

A method to reduce laser phase noise by external compensation is presented and numerically evaluated. In this work, a theoretical framework and simulation results are provided and the feasibility of the method is shown.

Hybrid pumping schemes allow for equalizing transmission performance across the cores of a multi-core fiber based transmission system in the presence of fiber loss variations between the cores and core-dependent gain of the involved multi-mode pumped optical amplifiers. Power equalization by distributed Raman amplification comes with improved noise characteristics, but leads to reduced signal-to-interference ratio due to increased crosstalk. In an experimental setup, compensating loss in the outer cores of a 50km long trench-assisted 7-core fiber with this approach is shown to lead to an OSNR penalty increase by around 30% in the center core.

A dither-free bias control technique for correcting bias drifts in electro-optic in-phase and quadrature modulators is presented and experimentally verified. The technique eliminates the need to apply a dither signal to the bias electrodes, and instead relies on the effects of the electrical modulation signals on the optical power. It is suited for applications where the modulation signals are of low frequency and would spectrally overlap with a conventional dither signal.

Wavelength-swept interrogation of two concatenated fiber Bragg grating arrays, consisting of 2500 gratings with 2 cm spacing and 1905 gratings with 5 cm spacing, is demonstrated at different temperatures using coherent correlation OTDR combined with accelerated hybrid laser tuning, which incorporates thermal and piezo tuning. (c) 2025 The Author(s)

Distributed fiber sensing based on correlation-aided phase-sensitive optical time domain reflectometry is presented. The focus is on correlation as an enabler for high spatial resolution. Results from different applications are presented.

A deployed fiber with in-house and underground sections is interrogated with a coherent correlation OTDR. The origin and propagation speed of a hammer-generated pressure wave in the underground section is detected and acoustic signals are monitored.

Pairing coherent correlation OTDR with low-complexity analysis methods, we investigate the detection of fast temperature changes and vibrations in optical fibers. A localization accuracy of ~2 m and extraction of vibration amplitudes and frequencies is demonstrated.

Fiber optic sensing is becoming an important means to physically secure today’s network infrastructure. However, a network-wide deployment of the monitors will require cost reduction of the interrogator system, which can only be achieved by highly integrated system components. In this contribution, we report on the use of an in-house designed single-chip coherent transceiver for acoustic fiber sensing. The transceiver on the basis of silicon photonics contains a high-speed dual-polarization IQ-modulator as well as a coherent receiver with balanced photodiodes and trans-impedance amplifiers, as defined by the OIF integrated coherent transmit-receive optical sub assembly (IC-TROSA) implementation agreement. The laser, used for transmission and as local oscillator, is provided external to the photonic integrated circuit and can be chosen according to the line-width requirements of the sensing system. The acoustic sensing demonstration is using a correlation-based optical time domain reflectometry with coherent detection. This method is able to detect, besides the amplitude information, the phase of the back-scattered signal, which has a significantly higher sensitivity to environmental effects on the fiber, like temperature and strain. As a proof of concept, sensing of an acoustic signal after a fiber span of 20 km is demonstrated by evaluating the obtained phase information, providing information on external dynamic events up to a frequency of 1.75 kHz.

Superimposed temperature variations and dynamic strain applied through a 400 Hz acoustic signal on a 195 m single-mode fiber section are successfully measured using a coherent correlation optical time domain reflectometry as an interrogator.

We report on methods to monitor the transmission path in optical networks using a correlation-based OTDR technique with direct and coherent detection. A high probing symbol rate can provide picosecond-accuracy of the fiber propagation delay, while a sensitive phase detection with a high repetition rate allows the monitoring of dynamic effects in the vicinity of the fiber. We discuss various approaches to evaluate the measured traces and show the results of a few monitoring applications.

A coherent optical subassembly (COSA) is evaluated for coherent-correlation optical time domain reflectometry (CC-OTDR) based fibre sensing. Even though the COSA was originally designed for digital communication applications, acoustic signals with frequencies up to 360 Hz can be detected after 50 km of transmission.

We review the application of correlation for fiber optical sensing using direct or coherent detection. The Correlation-OTDR with direct detection provides a group delay measurement resolution in the order of a few picoseconds, while coherent detection enables dynamic event evaluation by analyzing the phase information.