Optical fibers, which have a hetero-core structure, that is, a junction of optical fibers with different core diameters, provide a sensing function to the transmission lines themselves. This simple structure is fabricated by inserting and fusing optical fibers with different core diameters into a fiber transmission line. The measurement principle is based on changes in the transmission light intensity. It does not require any temperature compensation, allowing for the construction of a measurement system with high sensitivity and low cost among optical fiber measurement technologies. Since hetero-core optical fiber sensors were invented, research, development, and industrial applications have been carried out in various fields. In this special issue, we introduce the measurement principles, the latest research and development results, and practical applications in industry.

View full abstract

This commentary describes a bird’s-eye-view of the hetero core technique’s invention, its first patent story, the originality and advantage over conventional sensors and the fist academic publication, together with untold stories behind the days of 25-year’s development. With the broad applications which will be shown in the following chapters, you will realize the prospective way to the social implementation as a new industrial business scene for the purposes of the infrastructure inspection and disaster prevention named “outdoor security”. This commentary as the special issue of the Review of Laser Engineering is followed by five more chapters in which very attractive applications will be presented with showing the diversity, scalability and durability of the hetro-core sensors as a potential industrial growth. The established small business company, Core System Japan Co., Ltd, is developing by getting through, so called the Devil River, the Valley of Death, and toward the Darwinian Sea.

View full abstract

Hetero-core optical fiber sensors and their sensing systems offer attractive advantages for industrial applications due to their diverse sensing mechanisms and a cost-effective, low-power measuring scheme. This paper, which presents the development of hetero-core fiber optic sensing systems, and discusses our challenges in industrializing infrastructural monitoring and disaster management and our focus on “outdoor security”.

View full abstract

Early damage detection in large structures requires monitoring of their dynamic behaviors, and measuring low-frequency vibrations is essential. We propose a hetero-core fiber-optic accelerometer designed to measure low-frequency vibrations to meet this requirement. This accelerometer has an excellent cost-performance measurement system based on light intensity and durability that resists electromagnetic interference, corrosion, and temperature changes. Furthermore, hetero-core fiber-optic accelerometers based on beam structure, specifically a cantilever and a semicircular curved beam, have an advantage of being constructed solely with optical fibers. Hence, the optical fiber itself functions as a beam structure, facilitating the miniaturization of the device’s housing. Therefore, this paper focuses on sensors with cantilevered and semicircular curved beam structures and presents their performances, including resonant frequency, sensitivity, and transverse sensitivity.

View full abstract

Wearable sensors, which help monitor biological and body movement information in daily life, are increasingly required to be unconscious and unrestrained in the human body. Hetero-core optical fiber sensors have the advantages of small diameter, lightweight, and flexibility, and they can be embedded in flexible materials such as cloth and tape. We have developed flexible sensors incorporating hetero-core optical fibers that can be useful in healthcare and welfare applications. In this paper, we first describe flexible sensors incorporating hetero-core optical fibers. Next, blood pressure estimation using tape-type sensors is presented. Finally, we show that the cloth-type sensor can be attached to clothing to detect body movements and used for action recognition and nursing care training.

View full abstract

Hetero-core optical fiber can detect various chemical phenomena on the sensing part where evanescent waves are generated. These waves interact with the chemicals on the surface, propagating light spectra changes. When dye is adsorbed, the propagating light spectra resemble the transmitted spectra of the chemicals. In addition, the interaction between evanescent waves and metal thin film or metal nanoparticles prepared on the sensing part respectively causes a surface plasmon resonance and a localized surface plasmon resonance. These sensors detect the refractive index changes caused by a binding reaction on the surface. This review describes chemical sensors that utilize hetero-core optical fibers.

View full abstract

A novel chemical sensor based on a hetero-core optical fiber structure is reported where lipid films coat a cylindrical cladding. Two lipids were applied to the fiber surface: oleic acid and phosphoric acid di(2-ethylhexyl) ester (PAEE). An Au/lipids-coated sensor showed different shifts of the surface plasmon resonance (SPR) peak wavelength and changes in the spectral width for different chemical solutions (i.e., quinine, hydroquinidine, gramine, and caffeine) whose refractive indices are close to each other. The oleic acid- and PAEE-direct-coated sensors induced optical losses of approximately 0.4 and 0.2 dB, respectively, for a refractive index difference of 0.0008 between water and quinine. Compared with Au/ lipid-coated sensors, the lipid monolayer showed the different sensitivities to chemical substances and detected a wide range of chemical substances, including quinine. Changing the type of lipid in the film controls the selectivity of chemical substances.

View full abstract