Editorial for the Special Issue: “Advanced Optical-Fiber-Related Technologies”

An optical fiber is a flexible glass or plastic fiber that can transmit light from one end to the other. An optical fiber consists of three basic concentric elements: a core surrounded by cladding, which is then surrounded by an outer coating [1,2]. The core is usually made of glass or plastic, although other materials are sometimes used, depending on the transmission spectrum desired. The core and cladding form a waveguide that is essential for transmitting light. The cladding is usually made of the same material as the core, but with a slightly lower index of refraction. This difference in index causes total internal reflection, which is responsible for transmitting light along the fiber without it escaping through the sidewalls. Optical fibers do not need a grounding connection. They are immune to electromagnetic interference. They emit no radiation themselves that might cause interference. Fiber-optic transmission systems are superior to metallic conductor-based systems in many applications. Copper wire is about 13 times heavier than fibers. Fibers are also easier to install and require less duct space. Contrary to metallic conductors, the bandwidth of optical fibers permits them to transmit signals that contain considerably more information. In addition, optical fibers have low power loss. Optical fibers have numerous applications. They have gained wide-spread acceptance in communications including telecommunications, local area networks, industrial control systems, avionic systems, and military command and control systems. The fibers themselves can be used as a distributed sensor to measure a number of environmental effects, such as temperature, strain, and acoustic signals. They can deliver light from a remote source to a detector to obtain pressure, temperature, or spectral information. Optical fibers can deliver high levels of power for tasks such as laser cutting, welding, marking, and drilling. They can illuminate areas that are difficult to reach. Optical fibers are widely used as the active gain medium in a special form of solid-state lasers known as fiber lasers.

This Special Issue presents the latest advanced optical-fiber-related technologies and their applications. It includes 10 contributions that collectively offer insights into the actions and strategies of current fiber-optics. Here, I will briefly summarize the content of the contributions to this Special Issue.

• Brief Review of Recent Developments in Fiber Lasers [contribution 1].

This paper reviews the recent achievements in high-power rare earth (RE)-doped fiber lasers, Raman fiber lasers, and Brillouin fiber lasers. RE-doped fiber lasers operate in several wavelength ranges including 1050–1120 nm (ytterbium-doped fiber lasers), 1530–1590 nm (erbium- and erbium–ytterbium-doped fiber lasers), and 1900–2100 nm (thulium- and holmium-doped fiber lasers). White spaces in the wavelength spectrum, where no RE-doped fiber lasers are available, can be covered by Raman lasers.

2.

Advances in High–Speed, High–Power Photodiodes: From Fundamentals to Applications [contribution 2].

This paper reviews the fundamentals of high-speed, high-power photodiodes, mirror-reflected photodiodes, microstructure photodiodes, photodiode-integrated devices, and related equivalent circuits and design considerations. These characteristics of photodiodes and the related photonic-based devices are analyzed and reviewed in detail, and the devices are compared, providing a new path for the application of these in short-range wireless communications in 6G and beyond.

3.

Water Pipeline Leakage Detection Based on Coherent φ-OTDR and Deep Learning Technology [contribution 3].

Leakage in water supply pipelines remains a significant challenge. It leads to resource and economic waste. In this paper, the authors have developed several leak detection methods, including the use of embedded sensors and pressure prediction.

4.

Optical Cable Lifespan Prediction Method Based on Autoformer [contribution 4].

In this paper, a novel method for predicting the service life of optical cables based on the Autoformer model combined with the calculation method has been proposed. The method developed in the paper validates the superiority and stability of the Autoformer model in predicting cable lifespan, which can offer a more reliable approach for ensuring cable technology reliability and the management of associated industries.

5.

A Multi-Format, Multi-Wavelength Erbium-Doped Fiber Ring Laser Using a Tunable Delay Line Interferometer [contribution 5].

This work demonstrates the use of an erbium-doped fiber amplifier (EDFA), a tunable bandpass filter (TBF), and a tunable delay line interferometer (TDLI) to form a ring laser that produces multi-format, multi-wavelength laser beams. The proposed system enables dual-wavelength spacing ranging from 0.3 nm to 3.35 nm, with a switchable wavelength position at approximately 1527 nm to 1535 nm, providing flexible tunability.

6.

Wavelength-Dependent Bragg Grating Sensors Cascade an Interferometer Sensor to Enhance Sensing Capacity and Diversification through the Deep Belief Network [contribution 6].

This paper used machine learning to enhance the number of fiber-optic sensing placement points and promote the cost-effectiveness and diversity of fiber-optic sensing applications. The framework adopted is the FBG cascading an interferometer, and a deep belief network (DBN) is used to demodulate the wavelength of the sampled complex spectrum.

7.

A Hybrid GAN-Inception Deep Learning Approach for Enhanced Coordinate-Based Acoustic Emission Source Localization [contribution 7].

In this paper, a novel approach to coordinate-based acoustic emission (AE) source localization has been proposed to address the challenges of limited and imbalanced data from fiber-optic AE sensors used for structural health monitoring (SHM). A hybrid deep learning model has been developed, combining four generative adversarial network (GAN) variants for data augmentation with an adapted inception neural network for regression-based prediction.

8.

Simultaneous Vibration and Temperature Real-Time Monitoring Using Single Fiber Bragg Grating and Free Space Optics [contribution 8].

This work showcases simultaneous temperature and vibration measurement using a single-fiber Bragg grating (FBG) sensor. The concurrent interrogation of vibration and temperature by the FBG sensing system can be integrated with free space optics (FSO), which reduces the costs associated with fiber-optic cables and overcomes terrain barriers.

9.

Side-Illuminating Optical Fiber for High-Power-Density-Mediated Intraluminal Photoacoustic Imaging [contribution 9].

In this work, the design, method of fabrication, and characterization of a new compact, side-fire optical fiber that can deliver high-energy laser pulses for PA imaging have been described. Side-fire illuminators were fabricated using UV laser ablation to create windows on the side of a 1.5 mm diameter single-core, multi-mode optical fiber with a reflective silver coating and a beveled end. Devices with 10 mm, 20 mm, and 30 mm window lengths were fabricated and their beam profiles were characterized.

10.

Novel Fiber Bragg Grating Sensing Structure for Simultaneous Measurement of Inclination and Water Level [contribution 10].

This paper presents a pair of fiber Bragg grating (FBG) subsidence sensor systems designed to simultaneously measure tilt and water levels and explore the system’s potential to detect temperature variations. The configuration of the FBG subsidence sensor is intentionally skewed to enhance measurement sensitivity. The system is capable of concurrently detecting a 0.5 cm variation in water level and a 0.424° change in tilt, with tilt measurements spanning from −1.696° to 1.696°.

I believe that these works will collectively help to clarify the current state of optical-fiber-related technologies.