126
The 12th International Conference on Polymer Optical Fiber
Fluorescent plastic optical fibers for temperature monitoring
R. M. Ribeiro,
*
L. A. Marques-Filho and M. M. Werneck
Laboratório de Instrumentação e Fotônica/COPPE
Universidade Federal do Rio de Janeiro
21.945-970 Rio de Janeiro/RJ Brasil
Monitoring of temperature is useful for the
electrical power industry and physiological
applications. We report preliminary results on
the characterization measurements of
commercial blue-green fluorescent plastic optical
fibers (FPOFs) when pumped with a blue (465
nm) ultra-bright light-emitting diode (LED) for
temperature sensing. Intensity, wavelength shift
and fluorescence decay-time as modulation
techniques are investigated. Temperature
measurement resolution of ± 0.3
0
C was achieved
but with an intensity-based sensor. A
fluorescence decay-time faster than 100 ns was
measured. High-temperature plastic optical fibers
(POFs) issue is also briefly addressed because of
the imposed limitations.
* Also with the Departamento de Engenharia
Eletrônica & Telecomunicações of Universidade
Católica de Petrópolis (UCP) and Universidade
do Estado do Rio de Janeiro (UERJ).
1) Introduction
Fluorescent plastic optical fibers
(FPOFs) are made up of a cladding and of a
higher refractive index polystyrene core doped
during manufacturing with fluorescent dyes
where light may be guided. Provided it has
correctly pumped with high energy photons, the
fiber produces the excitation of the dyes. The de-
excitation of the dopants induces accordingly an
emission of photon with a longer wavelength. A
fraction of this fluorescence light remains
trapped in the fiber and is guided toward each
end. The fluorescent fibers are sensitive to the
visible light (for instance blue) and emit light of
higher wavelength than the incident light. They
are also called wavelength shifter fibers.
FPOFs and scintillating fibers have
been used as radiation detectors in high-energy
physics [1], ambient lightning determination,
partial discharge detection, optical potentiometer
(position measurement), gauge determination or
position of an object, intrusion detection, high
energy X-ray detection [2], water quality
monitoring [3] and much more.
For the power industry, optical fiber
sensor offers a large number of advantages over
conventional sensor. Most important is the high
immunity to electromagnetic interference, a
strong requirement for sensing in electromagnetic
contaminated environments, e.g. RF-field and in
power lines. Since these sensors are inherently
electrically insulated system and external power
is not required for their operation, they can work
at high electrical potentials and in potentially
explosive environments. Fiber optic sensor can
be made as small and compact devices.
D. Persegol and co-workers [4] describe
a POF-based temperature extrinsic sensor in the
range –20
0
C to +120
0
C with an accuracy of ± 2
0
C for early detection of faults in medium-
voltage (36 kV) substations. However they used
heavily-doped ruby powder packaged in the high-
T POF end as fluorescent material pumped with a
green LED. The fluorescence peaking at 694 nm
wavelength features a long-decay time of 2-4 ms.
Grattam and Kalyminios describe [5] a
number of possible applications of FPOFs
including mean ambient lighting, detection of
partial discharges providing a mechanism for
monitoring faults in electrical circuits and
switches and temperature sensing. However they
describe difficulties in the use of FPOFs for
temperature sensing because of its fast relaxation
time (< 100 ns) of fluorescence.
Despite of such non-encouraging
prescription, we decided to investigate FPOFs as
temperature sensors because of the following
reasons:
1 - The fluorescence based sensor offer the
advantage of a near-zero background, because
the wavelength of the emitted light is always
larger than that of the excitation light, which
makes then in principle much more sensitive and
error immune than those that change only the
absorption when the temperature varies [6].
2 – Large numerical aperture (> 0.50).