In-process measurement of a keyhole using a low-coherence interferometer with a high repetition rate.

The shape of an instance hole (keyhole) created via a high-power laser was measured using a low-coherence interferometer with the following parameters: repetition rate, 10 MHz; center wavelength, 1550 nm; absolute spatial resolution, 10 µm; and measurement range, 5 mm. The keyhole was created on a 3-mm-thick stainless-steel plate using a high-power laser with 8-kW peak power and 1070-nm center wavelength. The cross-sectional area of the keyhole was measured to be 0.42 mm × 0.78 mm (width × depth) using the interferometer, and its side dimension was 0.46 mm × 0.78 mm (width × depth).

Noise suppression technique for distributed Brillouin sensing with polymer optical fibers.

We develop a new noise suppression technique to perform distributed strain and temperature sensing based on a higher-speed configuration of Brillouin optical correlation-domain reflectometry even when a polymer optical fiber (POF) is used as a sensing fiber. We acquire the spectral difference between with and without reference light, leading to selective observation of the beat signal of Brillouin-scattered light and reference light, which is effective for distributed sensing. After experimentally showing the usefulness of this technique, we demonstrate POF-based distributed temperature sensing and dynamic strain sensing.

Brillouin characterization of slimmed polymer optical fibers for strain sensing with extremely wide dynamic range.

To date, most distributed Brillouin sensors for structural health monitoring have employed glass optical fibers as sensing fibers, but they are inherently fragile and cannot withstand strains of >3%. This means that the maximal detectable strain of glass-fiber-based Brillouin sensors was ~3%, which is far from being sufficient for monitoring the possible distortion caused by big earthquakes. To extend this strain dynamic range, polymer optical fibers (POFs) have been used as sensing fibers. As POFs can generally withstand even ~100% strain, at first, Brillouin scattering in POFs was expected to be useful in measuring such large strain. However, the maximal detectable strain using Brillouin scattering in POFs was found to be merely ~5%, because of a Brillouin-frequency-shift hopping phenomenon accompanied by a slimming effect peculiar to polymer materials. This conventional record of the strain dynamic range (5%) was still far from being sufficient. Here, we have thought of an idea that the strain dynamic range can be further extended by employing a POF with its whole length slimmed in advance and by avoiding the Brillouin-frequency-shift hopping. The experimental results reveal that, by applying 3.0% strain to a slimmed POF beforehand, we can achieve a >25% strain dynamic range, which is >5 times the conventional value and will greatly extend the application fields of fiber-optic Brillouin sensing.

Experimental study on depolarized GAWBS spectrum for optomechanical sensing of liquids outside standard fibers.

We report an experimental study on the spectral dependence of depolarized guided acoustic-wave Brillouin scattering (GAWBS) in a silica single-mode fiber (SMF) on acoustic impedance of external materials. The GAWBS spectrum was measured when the acoustic impedance was changed from 1.51 to 2.00 kg/s·mm2. With increasing acoustic impedance, the linewidth increased; the dependence was almost linear with an acoustic impedance dependence coefficient of 0.16 MHz/kg/s·mm2. Meanwhile, with increasing acoustic impedance, the central frequency linearly decreased with an acoustic impedance dependence coefficient of -0.07 MHz/kg/s·mm2. These characteristics are potentially applicable to acoustic impedance sensing.

Operation of slope-assisted Brillouin optical correlation-domain reflectometry: comparison of system output with actual frequency shift distribution.

Although the proof of concept for slope-assisted (SA-) Brillouin optical correlation-domain reflectometry (BOCDR) has been demonstrated, no reports on its detailed operation have been provided to date. We theoretically and experimentally investigate the relationship between the system output (power-change distribution) of SA-BOCDR and the actual Brillouin frequency shift (BFS) distribution along the sensing fiber and show that these two are not identical. When the strained fiber section is much longer than the nominal spatial resolution, the actual distribution of the BFS (i.e., strain) is well reproduced by the power-change distribution. However, when the length of the strained section is equal to or only a few times the nominal resolution, the correct BFS distribution cannot be directly obtained. Even when the strained section is shorter than the nominal resolution, a shift in the power change can still be observed, which is not the case for standard BOCDR systems. This unique "beyond-nominal-resolution" effect will be of great use in practical applications.

Simplified optical correlation-domain reflectometry without reference path.

We develop a simplified configuration for optical correlation-domain reflectometry (OCDR) without an explicit reference path. Instead, the Fresnel-reflected light generated at the distal open end of the sensing fiber is exploited as a reference light. After the fundamental demonstration, the optimal incident power is found to be approximately 8 dBm. We also show that the loss near the distal end should not be applied, unlike in the case of Brillouin-based OCDR.

Measurement of the optical path length difference in an interferometer using a sinusoidally frequency-modulated light source.

We develop a technique for measuring the optical path length difference (OPLD) in an interferometer using a frequency-modulated light source. Compared with conventional methods, this technique offers a high sampling rate, high precision, and cost efficiency, and is capable of determining which of the two optical paths is longer. In addition, we show that this technique works properly even when the OPLD is significantly longer than the coherence length of the light source.

Ultrahigh-speed distributed Brillouin reflectometry.

Optical fibre sensors based on Brillouin scattering have been vigorously studied in the context of structural health monitoring on account of their capacity for distributed strain and temperature measurements. However, real-time distributed strain measurement has been achieved only for two-end-access systems; such systems reduce the degree of freedom in embedding the sensors into structures, and furthermore render the measurement no longer feasible when extremely high loss or breakage occurs at a point along the sensing fibre. Here, we demonstrate real-time distributed measurement with an intrinsically one-end-access reflectometry configuration by using a correlation-domain technique. In this method, the Brillouin gain spectrum is obtained at high speed using a voltage-controlled oscillator, and the Brillouin frequency shift is converted into a phase delay of a synchronous sinusoidal waveform; the phase delay is subsequently converted into a voltage, which can be directly measured. When a single-point measurement is performed at an arbitrary position, a strain sampling rate of up to 100 kHz is experimentally verified by detecting locally applied dynamic strain at 1 kHz. When distributed measurements are performed at 100 points with 10 times averaging, a repetition rate of 100 Hz is verified by tracking a mechanical wave propagating along the fibre. Some drawbacks of this ultrahigh-speed configuration, including the reduced measurement accuracy, lowered spatial resolution and limited strain dynamic range, are also discussed.

Brillouin scattering in multi-core optical fibers for sensing applications.

We measure the Brillouin gain spectra in two cores (the central core and one of the outer cores) of a ~3-m-long, silica-based, 7-core multi-core fiber (MCF) with incident light of 1.55 µm wavelength, and investigate the Brillouin frequency shift (BFS) and its dependence on strain and temperature. The BFSs of both the cores are ~10.92 GHz, and the strain- and temperature-dependence coefficients of the BFS in the central core are 484.8 MHz/% and 1.08 MHz/°C, respectively, whereas those in the outer core are 516.9 MHz/% and 1.03 MHz/°C. All of these values are not largely different from those in a silica single-mode fiber, which is expected because the cores are basically composed of the same material (silica). We then analyze the difference in structural deformation between the two cores when strain is applied to the fiber, and show that it does not explain the difference in the BFS dependence of strain in this case. The future prospect on distributed strain and temperature sensing based on Brillouin scattering in MCFs is finally presented.

Propagation mechanism of polymer optical fiber fuse.

A fiber fuse phenomenon in polymer optical fibers (POFs) has recently been observed, and its unique properties such as slow propagation, low threshold power density, and the formation of a black oscillatory damage curve, have been reported. However, its characterization is still insufficient to well understand the mechanism and to avoid the destruction of POFs. Here, we present detailed experimental and theoretical analyses of the POF fuse propagation. First, we clarify that the bright spot is not a plasma but an optical discharge, the temperature of which is ~3600 K. We then elucidate the reasons for the oscillation of the damage curve along with the formation of newly-observed gas bubbles as well as for the low threshold power density. We also present the idea that the POF fuse can potentially be exploited to offer a long photoelectric interaction length.

Brillouin scattering signal in polymer optical fiber enhanced by exploiting pulsed pump with multimode-fiber-assisted coupling technique.

A cost-effective technique for coupling a polymer optical fiber (POF) with 50 µm core diameter to a silica single-mode fiber (SMF) with 8 µm core diameter is proposed, which can, by exploiting a multimode fiber with 50 µm core diameter, avoid the damage or burning at the butt-coupled POF/SMF interface. Using this coupling technique, we also show that the Brillouin signal in a POF can be enhanced by combined use of pulsed pump and an erbium-doped fiber amplifier. When the pulsed pump with average optical power of 18 dBm (63 mW), duty ratio of 15%, and pulse period of 2 µs is launched into a 200 m-long POF, 4 dB enhancement of the Stokes power is obtained compared to that with 18 dBm continuous wave pump. The relatively small enhancement is probably caused by the high Brillouin threshold of POFs. The Stokes power dependence on duty ratio is nonmonotonic, which might originate from a longer phonon lifetime in POFs than that in silica SMFs.

Brillouin gain spectrum dependence on large strain in perfluorinated graded-index polymer optical fiber.

We investigate the dependence of Brillouin gain spectra on large strain of > 20% in a perfluorinated graded-index polymer optical fiber, and prove, for the first time, that the dependence of Brillouin frequency shift (BFS) is highly non-monotonic. We predict that temperature sensors even with zero strain sensitivity can be implemented by use of this non-monotonic nature. Meanwhile, the Stokes power decreases rapidly when the applied strain is > ~10%. This behavior seems to originate from the propagation loss dependence on large strain. By exploiting the Stokes power dependence, we can probably solve the problem of how to identify the applied strain, when the identification is difficult only by BFS because of its non-monotonic nature.