Kazunari Minakawa

Wide-range temperature dependences of Brillouin scattering properties in polymer optical fiber

We investigate the temperature dependences of the Brillouin scattering properties in a perfluorinated graded-index (PFGI-) polymer optical fiber (POF) in a wide temperature range from −160 to 125 °C. The temperature dependences of the Brillouin frequency shift, linewidth, and Stokes power are almost linear at lower temperature down to −160 °C; while they show nonlinear dependences at higher temperature. These behaviors appear to originate from the partial glass transition of the polymer material.

Brillouin frequency shift hopping in polymer optical fiber

We investigated the Brillouin gain spectrum dependence on large strain of up to 60% in a polymer optical fiber (POF) at 1.55 μm, and found that the Brillouin frequency shift (BFS) abruptly changes from ∼2.7 GHz to ∼3.2 GHz. We named this phenomenon “BFS hopping,” and found it to originate from the varied acoustic velocity induced by the stepwise change in the core diameter of the POF. This is because of the yielding of the overcladding layer composed of polycarbonate. After the occurrence of BFS hopping phenomenon, the BFS dependence coefficients on strain and temperature in the POF were measured to be −65.6 MHz/% and −4.04 MHz/K respectively. These values indicate that, compared to an unstrained POF, further higher-precision temperature sensing with lower strain sensitivity is feasible.

Thermal Memory Effect in Polymer Optical Fibers

The basic properties of a thermal memory effect of polymer optical fibers (POFs) are experimentally investigated. We measure the thermally induced loss as a function of time at several high temperatures, and find that the loss becomes almost constant after heating for ~200 s. The loss remains unchanged even after the heated section is cooled to room temperature. We subsequently measure the optical time-domain reflectometry traces under three different conditions: 1) before a POF section is heated; 2) shortly after the POF section is heated at high temperature; and 3) after the heated section is cooled to room temperature. The traces measured under 2) and 3) are moderately identical, which indicates that the thermal memory effect can be exploited in developing excess-heat detecting system in future.

Polymer optical fiber tapering without the use of external heat source and its application to refractive index sensing

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.

Temperature dependence of Brillouin frequency shift in polymers controlled by plasticization effect

The temperature-dependence coefficient of Brillouin frequency shift (BFS) in perfluorinated graded-index polymer optical fibers is known to change drastically, because of the glass transition, at a certain critical temperature (Tc), above which the BFS becomes more sensitive to temperature. In this paper, we demonstrate that the Tc value can be adjusted by varying the dopant concentration, which is originally used to form the graded-index profile in the core region. Furthermore, we show that the temperature sensitivity of the BFS is enhanced in the presence of dopant probably because the temperature sensitivity of Youngs modulus is increased. The results indicate a big potential of the temperature sensors based on Brillouin scattering with an extremely high sensitivity in a specific desired temperature range.

Potential Applicability of Brillouin Scattering in Partially Chlorinated Polymer Optical Fibers to High-Precision Temperature Sensing

Compared with other plastic optical fibers (POFs), partially cholorinated graded-index (PCGI-) POFs with high thermal stability are a good candidate for the sensing heads of Brillouin-based distributed temperature sensors. In this work, we estimate the Brillouin frequency shift (BFS) and its temperature dependence in a PCGI-POF using ultrasonic pulse-echo technique. At 1550 nm, the estimated BFS is ~4.43 GHz with its temperature coefficient of approximately -6.9 MHz/K. Its absolute value is even higher than that of perflurointated GI-POF, indicating that the Brillouin scattering in PCGI-POFs can be potentially applied to high-precision temperature sensing.

Simplified optical correlation-domain reflectometry using polymer fiber

We develop a simplified configuration of optical correlation(or coherence-) domain reflectometry (OCDR) using a polymer optical fiber (POF). The conventional reference light path is removed by using as reference light the Fresnel-reflected light caused at the interface between a silica single-mode fiber and the POF. We demonstrate its basic operation and investigate the dependences of the spatial resolution and signal-to-noise ratio (SNR) on the sweep time of the laser modulation frequency. The optimal sweep time is found to be 30ms (corresponding to a repetition rate of 33Hz), at which both the resolution and SNR are maintained at high values.

Dependence of Brillouin frequency shift on temperature in poly(pentafluorostyrene)-based polymer optical fibers estimated by acoustic velocity measurement

Poly(pentafluorostyrene) (PPFS), which can be easily synthesized and has a low optical loss window at 850nm, is a promising alternative for a costly perfluorinated polymer as a base material of polymer optical fibers (POFs). To investigate the potential of a PPFSPOF as a Brillouin-based temperature sensing fiber, the Brillouin frequency shift and its temperature dependence of PPFS were measured using an ultrasonic pulse-echo technique. The temperature coefficient, which determines the sensitivity of the temperature sensing, was approximately !7.1MHz/K independently of the molecular weight and was nearly identical to that in perfluorinated POFs.

Polarization scrambling in Brillouin optical correlation-domain reflectometry using polymer fibers

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.

Clarification of strain-temperature cross-sensitivity effect on Brillouin frequency shift in plastic optical fibers

The strain-temperature cross-sensitivity effect on Brillouin frequency shift (BFS) in plastic optical fibers (POFs) is fully investigated. First, we show that the strain coefficient of the BFS is dependent on temperature. In the strain ranges of 0–1.2% and 4.0–9.0%, the temperature dependence is linear with coefficients of 1.5 MHz/%/°C and −0.4 MHz/%/°C, respectively. We then find that the temperature coefficient of the BFS is linearly dependent on strain with a coefficient of 1.5 MHz/°C/% in the strain range from 0 to 1.2%. For 4.0–9.0% strains, the BFS basically decreases with increasing temperature. These results indicate that temperature (and strain) compensation for the strain (and temperature) sensitivity of the BFS is required to correctly detect the strain and temperature magnitude in POF-based Brillouin sensing. We also show that temperature sensing with no sensitivity to strain is potentially feasible by using POFs pre-strained for >13%.