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Fiber Bragg gratings (FBGs) have been extensively used for single-point and multi-point measurements, mostly inscribed in single-mode fibers. However, it is feasible to inscribe FBGs in multimode fibers, which resist bending and can perform discriminative sensing of multiple physical parameters. When using a simple experimental setup to measure the temperature dependence of the dip in the transmission spectrum, significant fluctuations in its spectral power can be observed. Therefore, this study shows that the temperature-dependent spectral power fluctuations in multimode FBGs can be mitigated using a reflectometric configuration with suppressed modal interference, leading to higher-reliability temperature sensing.

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 demonstrate that fiber Bragg gratings in polymer optical fibers can lead to reflection peaks in any wavelength range when exciting high-order propagation modes, which can enhance the design of sensing systems for specific applications.

We present a method for measuring the transmission loss of an optical fiber using simplified optical correlation-domain reflectometry. By intentionally lowering the spatial resolution of the system, we observe the Rayleigh scattered signal for the first time without the need for a frequency shifter. Using this method, we simultaneously measure the transmission loss, location, and loss of faulty connections along a 10 km long fiber under test by performing distributed reflected-power measurements of Rayleigh scattering and Fresnel reflection.

A simple, inexpensive, and high-sensitivity temperature and strain sensor based on a single-mode–multimode–single-mode (SMS) structure with core offset is developed and experimentally characterized. This sensor does not require specialty fibers and can be fabricated using a standard fiber fusion splicer. The dependencies of the temperature and strain sensitivities on the core-offset amplitudes at the input and output single-mode/multimode fiber boundaries are investigated. The results indicate that the maximum temperature and strain sensitivities are two times and eight times higher than those of the standard SMS structure, respectively. The limit of the sensitivity enhancement by core offset is also revealed.

Compressed sensing (CS) is proposed in Brillouin optical correlation-domain reflectometry to enhance the effective repetition rate at any single position along an optical fiber. Through the random selection of modulation frequencies applied to the laser source in each epoch that the entire fiber is traversed and the reconstruction of the under-sampled data, a picture of the distributed and time-varying Brillouin frequency shift values can be achieved. Dynamic strains at frequencies of 20 Hz and 40 Hz are measured with CS at different under-sampling rates. The effective repetition rate is enhanced by 10 times with an improved temporal resolution.

Modal interference in a multimode fiber (MMF) has been utilized to develop simple sensors for various physical parameters. Herein, first, we investigate the dependences of the core diameter and the numerical aperture (NA) of the MMF on the performance of multimode-interference-based strain and temperature sensing. We find that a larger core diameter leads to higher temperature sensitivity but lower strain sensitivity (absolute value) and that higher NA does not influence the temperature sensitivity but results in higher strain sensitivity (absolute value). Subsequently, using the obtained low strain sensitivity, we demonstrate strain-insensitive temperature sensing with a sensitivity of approximately 6.1 pm °C−1.

In simplified optical correlation-domain reflectometry (OCDR) without the use of an electrical spectrum analyzer (ESA), the noise floor of the reflectivity distribution is distorted in long-range measurement, which sometimes causes significant errors in the distributed reflectivity measurement. Herein, we experimentally analyze the distorted noise floor by high-speed ESA-free OCDR and clarify the origin of the noise floor. Then we show that this noise floor can be compensated by appropriate filtering. Consequently, distributed reflectivity measurement over a 1.1 km long sensing fiber is properly performed at a repetition rate of 100 Hz.

We characterize fiber gratings inscribed line-by-line in perfluorinated graded-index polymer optical fibers (PFGI-POFs) for tactile sensing. First, we measure the dependence of the wideband transmitted light spectrum on the distance between the fiber grating and an object composed of metal or rubber, clarifying that the spectral power is significantly dependent on the object position in a wide wavelength range. Subsequently, we experimentally show that this behavior is caused by the light scattering at the fiber grating, indicating that PFGI-POF gratings are potentially applicable to simple low-cost tactile sensors or high-sensitivity short-range displacement sensors with no need for additional supportive tools.

Various types of fiber-optic temperature sensors have been developed on the basis of modal interference in multimode fibers, which include not only glass fibers but also polymer optical fibers (POFs). Herein, we investigate the spectral patterns of the modal interference in multi-core POFs (originally developed for imaging) and observe their unique temperature dependencies with no clear frequency shift or critical wavelength. We then show that, by machine learning, the modal interference in the multi-core POFs can be potentially used for highly accurate temperature sensing with an error of ∼0.3 °C.

A variety of fiber-optic temperature sensors based on inline Mach–Zehnder interferometry (MZI) have been implemented for their ease of fabrication and cost efficiency, but it is difficult to control the length of the sensing area (i.e. the fiber section with sensitivity). Herein, we develop a new temperature sensor based on inline MZI by connecting two single-mode fibers (SMFs) with different mode-field diameters and by applying a load to one of the SMFs. The fiber section between the connector and the load operates as the sensing area, the length of which can be dynamically controlled by changing the load position.

In standard single-end-access Brillouin optical correlation-domain reflectometry (BOCDR), the systematic error caused by the phase difference between amplitude modulation (AM) and frequency modulation (FM) in the light source can be up to tens of megahertz, causing considerable errors in strain and temperature measurement. In this letter, we develop a new concept of two-end-access BOCDR, in which light is injected into each end of a sensing fiber in turn and the measured results of the Brillouin frequency shift distributions are combined. We numerically show that this configuration can suppress the systematic error caused by the AM-FM phase difference by approximately 90%.

Brillouin optical correlation-domain reflectometry (BOCDR) generally suffers from the trade-off relationship between the spatial resolution and the measurement range. In this work, we perform a proof-of-concept demonstration of a chirp modulation scheme to extend the measurement range of BOCDR. In this scheme, optical frequency is modulated using a chirp-shaped waveform. We show that, although the spatial resolution is slightly deteriorated, the measurement range and the range-to-resolution ratio can be relatively largely improved. In our preliminary experiment, the range-to-resolution ratio is 471, which is approximately 3 times higher than that of a standard configuration, proving the effectiveness of this scheme.

We show that 1 °C drift of laser temperature yields a strain error of ?8 ??in Brillouin optical correlation-domain reflectometry. Under the assumption that the strain measurement accuracy is +/-20 ??(severe requirement), +/-2.5 °C drift of the laser temperature can be permitted for proper system operation. We prove this prediction by performing distributed strain measurements at different laser temperatures. Our results indicate that, if we allow a strain measurement accuracy of approximately +/-100 ??(or more), we can even remove the thermal control of the laser, leading to cost reduction and downsizing of the system.

We measure the twist dependence of the reflected spectrum of a fiber Bragg grating (FBG) inscribed in a perfluorinated graded-index (PFGI) polymer optical fiber (POF). With increasing twist, the strain sensitivity decreases with a coefficient of -34.2 pm/%/turns/m, while the temperature sensitivity increases with a coefficient of 0.39 pm/°C/turns/m. At a twist of 150 turns m?1, the temperature sensitivity is 3 times higher than that with no twist. The controllability of the strain and temperature coefficients without considerable cost is beneficial not only to highly sensitive temperature sensing but also discriminative sensing of strain and temperature using two PFGI-POF-FBGs.

We show that infrared thermometry can be used to detect a breakage point in an optical fiber embedded in a structure. Around the breakage point, incident light induces heat, which is detected using an infrared thermometer. By this method, we can obtain position information of a breakage point, including accurate information on where it is located in the actual structure, on a real-time basis. We experimentally show the practical usefulness of this method by detecting a breakage point in an optical fiber embedded in a composite structure comprising carbon fiber-reinforced plastics.

All the previous configurations of optical correlation-domain reflectometry (OCDR) have been based on the direct modulation of laser driving current. However, some lasers are not designed for high-speed large-amplitude modulation, and the modulation amplitude and frequency are mutually dependent. In this work, to mitigate these drawbacks, we propose an external modulation scheme for OCDR. After confirming its basic operation, we show that a non-specially designed laser can be employed. We also clarify that the interdependence of the modulation amplitude and frequency is negligibly small in this scheme, which is not the case for the conventional direct modulation scheme.

Slope-assisted Brillouin optical correlation-domain reflectometry (SA-BOCDR) is a recently developed structural health monitoring technique for measurements of strain, temperature, and loss distributions along optical fibers. Although the basic operational principle of this method has been clarified, no measurements using optical fibers embedded in actual structures have been reported. As a first step towards such practical applications, in this study, we present an example of an SA-BOCDR-based diagnosis using a composite structure with carbon fiber-reinforced plastics. The system's output agrees well with the actual strain distributions. We were also able to detect the breakage of the embedded fiber, thus demonstrating the promise of SA-BOCDR for practical applications.

Spontaneous depolarized guided acoustic-wave Brillouin scattering (GAWBS) was experimentally observed in one of the side cores of an uncoated multicore fiber (MCF). The frequency bandwidth in the side core was up to ?400 MHz, which is 0.5 times that in the central core. The GAWBS spectrum of the side core of the MCF included intrinsic peaks, which had different acoustic resonance frequencies from those of the central core. In addition, the spontaneous depolarized GAWBS in the central/side core was unaffected by that in the other core. These results will lead to the development of polarization/phase modulators using an MCF.

We experimentally investigate the pressure dependence of the Brillouin frequency shift (BFS) in a polymer optical fiber. The BFS dependence on pressure shows a hysteresis, but after several cycles of increasing/decreasing pressure, the hysteresis is mitigated. The pressure dependence coefficient at this state is +4.3 MHz/MPa, the absolute value of which is 5.8 times as large as that of bare silica fibers (the sign is opposite). The reason for this unique behavior is discussed. This result indicates that, by using plastic optical fibers instead of silica fibers, distributed pressure sensing with a higher sensitivity is potentially feasible.

We perform a pilot trial of highly convenient taper fabrication for polymer optical fibers (POFs) using hot water. A ?380-mm-long POF taper is successfully fabricated, and its ?150-mm-long waist has a uniform outer diameter of ?230 µm. The shape is in good agreement with the theoretical prediction. The optical loss dependence on the strain applied to the waist shows an interesting behavior exhibiting three regimes, the origins of which are inferred by microscopic observations. We then discuss the controllability of the taper length.

We demonstrate power-based refractive index (RI) sensing using an ultrasonically crushed polymer optical fiber (POF). This structure can be easily and cost-effectively fabricated within a short time (i.e., ?1 s) without the need to employ external heat sources or chemicals. The only requirement is to simply press a horn connected to an ultrasonic transducer against part of the POF. The RI dependence of the transmitted power shows linear trends in RI ranges of ?1.32 to ?1.36 [coefficient: ?62 dB/RIU (RI unit)] and ?1.40 to ?1.44 (coefficient: ?257 dB/RIU). The temperature dependence of the transmitted power is also investigated.

To achieve distributed reflectivity measurements along an optical fiber, we developed a simplified cost-effective configuration for optical correlation- (or coherence-) domain reflectometry based on a synthesized optical coherence function using sinusoidal modulation. By excluding the conventional optical heterodyne detection that accompanies the frequency shift (without using an acousto-optic modulator) and by exploiting the foot of the electrical Fresnel reflection spectrum, the electrical bandwidth required for signal processing is lowered to several megahertz. We evaluate the basic system performance and demonstrate its high-speed operation (10 ms for one scan) by tracking a moving reflection point in real time.

We perform a pilot trial of the highly convenient taper fabrication of perfluorinated graded-index polymer optical fibers. Instead of conventional external heating, we utilize internal heating caused by high-power propagating light (500 mW in this experiment). An approximately 4-mm-long section of a polymer fiber is tapered, and the outer diameter of the ?2-mm-long waist around its midpoint is approximately 200 µm, which is quite uniform with a standard deviation of 4.3 µm. The polymer fiber taper fabricated by this technique is shown to be capable of generating evanescent waves and thus measuring the refractive indices of liquids from 1.333 to 1.410.

It has been reported that temperature sensors based on modal interference in perfluorinated graded-index polymer optical fibers show extremely high temperature sensitivity at room temperature. In this work, we confirm that the temperature sensitivity (absolute value) is significantly enhanced when the temperature increases toward ?70 °C, which is close to the glass-transition temperature of the core polymer. When the core diameter is 62.5 µm, the sensitivity at 72 °C at 1300 nm is 202 nm/°C/m, which is approximately 26 times the value obtained at room temperature and >7000 times the highest value previously reported using a silica multimode fiber.

We evaluate whether the measurement stability of Brillouin optical correlation-domain reflectometry (BOCDR) using polymer optical fibers (POFs) can be enhanced by polarization scrambling. In this study, two major factors that affect the signal-to-noise ratio in BOCDR, specifically, the spatial resolution and incident power, are varied, and their influences on distributed measurements with polarization scrambling are experimentally investigated. We thus confirm that in POF-based BOCDR, unlike in BOCDR using standard silica glass fibers, polarization scrambling is an effective means of enhancing the measurement stability only when the spatial resolution is sufficiently low or when the incident power is sufficiently high.

In Brillouin optical correlation-domain reflectometry (BOCDR), an undesired "ghost" correlation peak is automatically generated at the open end of a sensing fiber in addition to the desired correlation peak used for distributed measurement. The Brillouin spectrum of the light scattered from this ghost peak, which has a fixed position, overlaps that from the desired peak, inducing noise and lowering the signal-to-noise ratio. The importance of ghost-peak suppression is experimentally shown by demonstrating that a distributed strain measurement can only be successful when a bending loss of 6 dB or larger is applied near the open end.

We demonstrate a fast and cost-effective method of detecting flaws in polymer optical fibers (POFs) using an infrared thermometer. The optical loss dependence of the measured temperature at the flaw is found to be linear with a proportionality constant of approximately 0.74 °C/dB when the propagating light is 24.5 dBm (282 mW) at 1.55 µm. The propagating optical power dependence of the measured temperature at the flaw with a fixed loss also shows a linear behavior, which predicts that a high optical input power is preferable to precise estimation of the loss.

Using the ultrasonic pulse-echo technique, we estimate the Brillouin frequency shift (BFS) and its temperature dependence in partially chlorinated graded-index polymer optical fibers (PCGI-POFs), which have high thermal stability compared with other POFs. At 1550 nm, the estimated BFS is ?4.43 GHz with its temperature coefficient of approximately -6.9 MHz/K, which is ?5.8 times as large as that in silica fibers. Since the fracture strain is ?3.0% (mostly in the elastic region), the BFS dependence on strain in PCGI-POFs cannot be estimated by this technique. These results indicate that Brillouin scattering in PCGI-POFs has a big potential for distributed high-precision temperature sensing.

We propose a new method of enhancing the Brillouin scattering signal with a relatively low-cost low-power erbium-doped fiber amplifier by using pulsed pump light. When a pulsed pump light with an average optical power of 20 mW, a duty ratio of 20%, and a pulse period of 2 µs was injected into a 1-km-long silica single-mode fiber, the Brillouin signal was enhanced by 25 dB relative to that with a continuous-wave pump light having the same average power. We believe that this method can also be used to enhance the weak Brillouin scattering signal in polymer optical fibers for its detailed characterization.

We estimated the dependences of Brillouin frequency shift (BFS) on temperature and strain in poly(methyl methacrylate)-based polymer optical fibers using ultrasonic pulse-echo technique at ?10 MHz. The estimated BFS dependence on temperature was linear with a coefficient of approximately -17 MHz/K at 650 nm pump, -34 times larger than that of silica fibers at 650 nm pump. In contrast, its strain dependence was found to be nonlinear, probably originating from the elastic-to-plastic transition.

The acoustic velocity in a poly(methyl methacrylate) (PMMA) polymer optical fiber (POF) was measured to be 2.8×103m/s using ultrasonic pulse-echo technique, from which its Brillouin frequency shift (BFS) was estimated to be ?13 GHz at 650 nm. The BFS was also predicted to show a negative dependence on temperature with a coefficient of approximately -17.1 MHz/K at 650 nm, -14.5 times larger than that of a silica fiber at 1.55 µm.

The Fresnel reflection (FR) spectra at the ends of a step-index (SI-) multimode fiber (MMF) and a graded-index (GI-) MMF were investigated with heterodyne detection, and compared with that of a standard single-mode fiber (SMF). The FR spectra of the MMFs were found to spread broader than that of the SMF, and the FR spectrum of the SI-MMF was even broader than that of the GI-MMF. It was also demonstrated that the observation of the Brillouin gain spectrum in a polymer optical fiber (POF) is severely influenced by the FR spectrum. We believe that this information will provide a guideline for the design of robust Brillouin sensing systems using MMFs including POFs.

We investigate the temperature-dependences of the Brillouin frequency shift in a tellurite glass fiber and a bismuth-oxide highly-nonlinear fiber using a Brillouin optical correlation-domain reflectometry system for sensing applications. Negative dependences of -1.14 and -0.88 MHz/K are demonstrated, respectively, in contrast to the positive dependence of +1.18 MHz/K in a standard silica fiber.

We performed a stable high-speed entire-length measurement of strain distribution by suppressing the signal fluctuations automatically using polarization scrambling. Strain distribution along the entire length of a 100-m fiber was successfully measured with 40-cm spatial resolution and 19-Hz sampling rate. We also demonstrated a signal-to-noise ratio improvement by compensating the noise-floor of the electrical spectrum analyzer.

The polarization beat length distribution along a single-mode optical fiber (SMF) is measured with a Brillouin optical correlation-domain reflectometry. The beat lengths of SMFs wound on mandrels with radii 10.0, 7.4, and 3.1 cm were measured to be 25.4, 12.1, and 2.56 m, respectively, which agreed well with the theoretical calculations. A distributed measurement was also successfully performed using an SMF comprising two sections with different mandrel radii.