Brillouin scattering properties in rare-earth-doped fibers, including Nd3+-doped, Tm3+-doped, Sm3+-doped, and
Ho3+/Tm3+ co-doped fibers, can potentially be controlled at high speed by pumping, but there has been no report on their
detailed investigations. In this study, the Brillouin gain spectra (BGS) in such rare-earth-doped single-mode fibers are
measured, for the first time to the best of our knowledge, at 1.55 μm without pumping, and the Brillouin frequency shift
(BFS) and its dependences on strain and temperature are investigated. Clear BGS was observed for the Nd3+-doped and
Tm3+-doped fibers, but BGS was not detected for the Sm3+-doped and Ho3+/Tm3+ co-doped fibers probably because of
their extremely high propagation losses at 1.55 μm and small Brillouin gain coefficients. The BFS of the Nd3+-doped
fiber was ~10.82 GHz, and its strain and temperature coefficients were 466 MHz/% and 0.726 MHz/K, respectively. As
for the Tm3+-doped fiber, the BFS was ~10.90 GHz, and its strain and temperature coefficients were 433 MHz/% and
0.903 MHz/K, respectively. These measurement results are compared with those of silica fibers.
Brillouin scattering in perfluorinated graded-index polymer optical fibers (PFGI-POFs) is potentially useful in
developing high-accuracy distributed temperature sensors with reduced strain sensitivity. In this study, we investigate,
both experimentally and theoretically, the influence of the fiber core diameter and length on the Brillouin gain spectra
(BGS) in PFGI-POFs. First, we show that smaller core diameter drastically enhances the Stokes power using PFGI-POFs
with 62.5-μm and 120-μm core diameters, and discuss the Brillouin threshold power. Then, we demonstrate that the
PFGI-POF length has little influence on the BGS when the length is longer than 50 m. We also predict that, at 1.55-μm
wavelength, it is difficult to reduce the Brillouin threshold power of PFGI-POFs below that of long silica single-mode
fibers even if their core diameter is sufficiently reduced to satisfy the single-mode condition. Finally, making use of the
enhanced Stokes signal, we confirm the Brillouin linewidth narrowing effect.
Brillouin scattering properties of optical fibers should be precisely investigated in advance for their applications to
distributed strain/temperature sensing. Especially when the fibers are short, high-loss, or multi-mode, as in the case of
polymer optical fibers, high-power pump is required, but commercially-available high-power lasers or optical amplifiers
are expensive. In this paper, we propose a new method to enhance Brillouin scattering signals with a relatively low-cost
low-power erbium-doped fiber amplifier by using pulsed pump light. When pulsed pump light with average optical
power of 20 mW, duty ratio of 20%, and pulse period of 2 μs, was injected into a silica single-mode fiber, the Brillouin
signal was enhanced by 25 dB compared to that with continuous-wave pump light having the same average power.
We report on the measurement of sound pressure in water utilizing the modulation of the optical reflectivity at the end of
an optical fiber. First, we develop a new experimental setup comprising a low-coherent light source to suppress the
interference noise. Then, we formulate the relation between the sound pressure and the modulation in the reflected light
intensity, and theoretically analyze the performance of this method with emphasis on the directivity and the sensitivity.
We investigated the dependences of Brillouin frequency shift (BFS) on strain and temperature in a perfluorinated gradedindex
polymer optical fiber (PFGI-POF) at 1.55-μm wavelength. They showed negative dependences with coefficients of
-121.8 MHz/% and -4.09 MHz/K, respectively, which are -0.2 and -3.5 times as large as those in silica fibers. These
unique BFS dependences indicate that the Brillouin scattering in PFGI-POFs has a big potential for strain-insensitive
high-accuracy temperature sensing.
We have demonstrated a double-modulation scheme to enlarge the measurement range of Brillouin optical correlationdomain
reflectometry (BOCDR) for fiber-optic distributed strain sensing. In this scheme, the frequency of the laser
output is simultaneously modulated with two different frequencies. A resolution of 27 cm was achieved with 1.5-km
measurement range when a noise-floor compensation technique was used.
A distributed strain measurement with millimeter-order spatial resolution was demonstrated in optical fibers based on
Brillouin optical correlation-domain reflectometry (BOCDR) employing a tellurite glass fiber as the fiber under test.
First, the dependences of Brillouin frequency shifts (BFSs) on strain and temperature in the tellurite fiber were
investigated, and they showed negative dependences with coefficients of -0.023 MHz/με and -1.14 MHz/K, respectively.
Then, using this tellurite fiber, the BFSs around 1-cm strain-applied section were successfully measured based on
BOCDR with a nominal spatial resolution of 6 mm.
We propose Brillouin optical correlation-domain reflectometry (BOCDR), which can measure the strain and/or
temperature distribution along an optical fiber by controlling the interference of continuous lightwaves. In pulse-based
conventional Brillouin optical time-domain reflectometry (BOTDR), it is difficult to achieve a spatial resolution less than
1 m in principle, and the measurement time is as long as 5-10 minutes. On the contrary, the continuous-wave-based
BOCDR can exceed the limit of 1-m resolution, and realize much faster measurement and random access to measuring
position. A 40-cm spatial resolution was experimentally demonstrated with 50-Hz sampling rate.
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