Kentaro Nakamura

In recent years, fiber-optic interferometers have attracted considerable attention for airborne acoustic wave detection. Among them, the Sagnac interferometer has been particularly favored due to its strong suppression of low-frequency thermal disturbances and vibrational noise. This paper proposes a phase-bias-enhanced Sagnac interferometric system, which introduces a stable phase bias via a 3×3 fiber coupler, significantly improving acoustic sensitivity. The system eliminates the need for mechanical diaphragms and instead detects sound through refractive index modulation in air, offering higher stability and retaining intrinsic immunity to electromagnetic interference. Its acoustic sensing element adopts a free-space optical path without physical barriers, minimizing disturbance to the acoustic field during measurement. In addition, the system architecture avoids the use of expensive and complex optical components such as electro-optic modulators, resulting in a more compact, simplified, and cost-effective design. Experimental results demonstrate successful detection of airborne audible sound in the 200 Hz – 13 kHz range using an 8 km fiber delay coil. The Sagnac system achieves up to 1.23 × 10⁻² Pa/√Hz sensitivity with an SNR of 43.9 dB at 7700 Hz, demonstrating excellent robustness. This research presents a simple, stable, and cost-efficient solution for high-precision acoustic sensing, showing great potential for applications in sound monitoring, environmental perception, and non-contact acoustic signal acquisition.

Optical fiber sensors based on Brillouin scattering have attracted extensive attention due to their ability to perform distributed strain and temperature measurements. In this study, we explore the Brillouin frequency shift (BFS) characteristics in perfluorinated graded-index polymer optical fibers (POFs) doped with unique materials, focusing on the BFS dependencies on strain and temperature. Our results reveal a strain sensitivity coefficient of –195 MHz/%, which is approximately 1.6 times higher in absolute value than previously reported for other POFs, while the temperature sensitivity (–3.1 MHz K⁻¹) remains comparable to previous findings.

Airborne ultrasound is increasingly recognized as a crucial technology for short-range sonar applications in vehicles and drones, as well as for the development of haptic devices, noncontact manipulation of small objects, medical imaging, and material evaluation. This paper presents a novel approach using a fiber-based Sagnac interferometer to detect acoustically induced modulations of the air's refractive index with a minimal setup. This method offers immunity to low-frequency thermal fluctuations and vibratory disturbances while circumventing the complexities and expenses associated with optical components. By eliminating mechanical moving parts, the system benefits from enhanced robustness and operates without the need for recalibration. Initially, a theoretical analysis of the operating principle is provided, followed by experimental validation, which demonstrates the capability to measure ultrasound frequencies ranging from 26.4 to 96.1 kHz using fiber delay coils spanning 100 to 1000 m. Techniques for assessing sound pressure levels are also discussed. Furthermore, the integration of an electro-optic modulator into the system is discussed, highlighting its significant contributions to improving the system's responsivity, linear dynamic range, and stability.

Scaling down linear actuators is crucial for various industrial applications, yet the efficiency of electromagnetic linear actuators decreases significantly as they are miniaturized. Millimeter-scale miniature ultrasonic motors, on the other hand, maintain high efficiency. This paper describes a new approach to facilitate the miniaturization of traveling wave linear ultrasonic motors by attaching bimorph transducers to the ends of the stator beams. To control the resonance frequency and facilitate the generation of a traveling flexural wave in the beam, grooves are incorporated into the bimorph structure. Mechanical output is improved by amplifying the transverse displacement through the addition of teeth to the beam. Utilizing the Finite Element Method (FEM), a prototype measuring 10 mm × 10 mm × 160 mm was designed, fabricated, and tested. It achieved an output speed of 53.7 mm/s and a thrust of 0.83 N at a peak-to-peak voltage of 300 V and a frequency of 32.7 kHz. The results show that the proposed ultrasonic linear motors with small size, simple structure and overall compactness have promising applications in robotics, precision machining, medical equipment and other fields requiring miniature compact linear motions.

This study explores the impact of a micro dry-etching technique on strain sensing based on multimode interference in a perfluorinated polymer optical fiber (POF). We demonstrate a drastic shift in strain sensitivity, from +34 nm/% in a non-etched POF to −981 nm/% in the etched POF. This notable change may be attributed to the excitation of the axially asymmetrical modes in the etched POF, which is confirmed through simulation on the stress distribution across the core. In addition, a critical wavelength for multimode interference is observed in a POF.

Optical correlation-domain reflectometry (OCDR) is a technique used to detect faulty connections and splices in a fiber under test. Several simple configurations have been developed to reduce the size of the experimental setup. However, the theoretical spatial resolutions calculated by the original theory on standard OCDR do not agree with the experimental values obtained by simplified OCDR. In this paper, to clarify the cause of the inconsistency between the theoretical and measured spatial resolutions in simplified OCDR, we newly establish a comprehensive theory for both standard and simplified OCDR systems using the concept of the “beat spectrum,” which is primarily used in theoretical studies for Brillouin optical correlation-domain reflectometry / analysis (BOCDR / A). In addition, we derive a modified expression for the spatial resolution; this highlights a trade-off relationship between spatial resolution and measurement range, which is a well-known property for BOCDR / A but has not been previously reported for OCDR.

The authors present a streamlined setup for Brillouin scattering observation, which negates the need for an independent reference path by incorporating a variable reflectivity mirror at the terminus of the sensing fibre. The authors’ study uncovers an optimal reflectivity point, specifically at −9 dB, where the signal‐to‐noise ratio (SNR) of the observed Brillouin gain spectrum (BGS) reaches its zenith, approximately doubling the SNR of a conventional simplified setup that utilises Fresnel reflection. Furthermore, the authors elucidate that this setup inherently mitigates the polarisation‐dependent fluctuations of the BGS.

We implement a single-end-access reflectometric configuration for tactile sensing, incorporating a fiber Bragg grating (FBG) inscribed in a polymer optical fiber (POF). The folded architecture of the single-mode-multimode-single-mode structure effectively mitigates the generation of interference patterns, which are commonly induced by multimodal interference in the POF. Our results reveal a significant reduction in spectral power across a broad wavelength range upon object contact with the FBG. Utilizing the transient power changes at a specific wavelength, we successfully demonstrate real-time tactile sensing with this configuration.

We develop a new configuration of distributed strain and temperature sensing technology called double-slope-assisted Brillouin optical correlation-domain reflectometry. Its loss-independent operation is demonstrated through simplified simulation and proof-of-concept experiments using a standard silica single-mode fiber.