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Low Loss Polycarbonate Polymer Optical Fiber for High Temperature FBG Humidity
Sensing
Woyessa, Getinet; Fasano, Andrea; Markos, Christos; Rasmussen, Henrik K.; Bang, Ole
Published in:
I E E E Photonics Technology Letters
Link to article, DOI:
10.1109/LPT.2017.2668524
Publication date:
2017
Document Version
Peer reviewed version
Link back to DTU Orbit
Citation (APA):
Woyessa, G., Fasano, A., Markos, C., Rasmussen, H. K., & Bang, O. (2017). Low Loss Polycarbonate Polymer
Optical Fiber for High Temperature FBG Humidity Sensing. I E E E Photonics Technology Letters, 29(7), 575-
578. https://doi.org/10.1109/LPT.2017.2668524
IEEE PHOTONICS TECHNOLOGY LETTERS
1
Abstract— We report the fabrication and characterization of a
polycarbonate (PC) microstructured polymer optical fiber
(mPOF) Bragg grating (FBG) humidity sensor that can operate
beyond 100ºC. The PC preform, from which the fiber was
drawn, was produced using an improved casting approach to
reduce the attenuation of the fiber. The fiber loss was found
reduced by a factor of two compared to the latest reported PC
mPOF [20], holding the low loss record in PC based fibers.
PC mPOFBG was characterized to humidity and temperature,
and a relative humidity (RH) sensitivity of 7.31±0.13 pm/% RH
in the range 10-90% RH at 100ºC and a temperature sensitivity
of 25.86±0.63 pm/ºC in the range 20-100 ºC at 90% RH were
measured.
Index Terms— Annealing, Fiber gratings, Humidity
measurement, Optical fiber sensors, Plastic optical fiber,
Temperature measurement.
I. INTRODUCTION
HE interest in polymer optical fiber (POF) sensors is
steadily increasing because of their low processing
temperature, high flexibility in bending, high fracture
toughness, ease of handling, and non-brittle nature, which are
properties that glass fibers do not have [1]. In addition, POFs
have a high elastic strain limit with low Young’s modulus and
they are biocompatible, which makes them advantageous for a
range of strain and bio-sensing applications [2-11]. Some
polymers, such as PMMA, are humidity sensitive and strongly
absorb water [12-15], while others, such as Topas and Zeonex,
have been reported to be insensitive to humidity [16-19].
Therefore, one of the key characteristics of PMMA based
POFs is their ability to highly absorb moisture. The moisture
absorption leads to a change in the refractive index and size of
the fiber, which consequently change the Bragg wavelength
[12]. Therefore, PMMA based fiber Bragg gratings (FBGs)
Manuscript received xxxx; revised xxxx; accepted xxxx. Date of
publication xxxx, 2016; date of current version xxxx. This work was
supported by People Programme (Marie Curie Actions) of the European
Union's Seventh Framework Programme (608382); Danish Research Council
(FTP) (4184-00359B); Carslberg Foundation (CF14-0825) and Innovation
Fund Denmark ( ShapeOCT) (4107-00011A).
Getinet Woyessa, Christos Markos, and Ole Bang are with the Department
of Photonics Engineering (DTU Fotonik), Technical University of Denmark,
DK-2800 Kgs. Lyngby, Denmark (e-mail: gewoy@fotonik.dtu.dk,
chmar@fotonik.dtu.dk and oban@fotonik.dtu.dk).
Andrea Fasano and Henrik K. Rasmussen are with the Department of
Mechanical Engineering, Technical University of Denmark, DK-2800 Kgs.
Lyngby, Denmark (e-mail: andfas@mek.dtu.dk and hkra@mek.dtu.dk).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org. Digital Object Identifier xxxx
are considered as potential candidates for developing humidity
sensors [12-13].
The temperature and relative humidity (RH) operational
limits and stability of polymer optical fiber Bragg gratings
(POFBGs) strongly dependent on two parameters: the glass
transition temperature (T
g
) and moisture absorbing capability
of the fiber material. The temperature operational limit of
POFBGs produced from humidity insensitive polymers, such
as Topas and Zeonex, relies exclusively upon their T
g
and thus
they can reliably operate 15-20 ºC below their T
g
regardless
of the surrounding RH level [16-19]. However, this is not the
case for other polymers such as PMMA which have high
affinity to water. The temperature operational limit of PMMA
POFBGs is strongly dependent on the surrounding RH level
and vice versa. At ambient or lower RH level, they can operate
15-20 ºC below their T
g
. It has been reported that PMMA
POFBGs can operate up to 90 ºC at ambient RH [20].
However, when the surrounding RH is higher than the ambient
RH, the temperature operational limit decreases significantly.
For instance, when PMMA POFBGs humidity sensors are
operated up to 90% RH, the maximum operational
temperature is 75 ºC [15]. This is attributed to the fact that
glass transition temperature of PMMA decreases with
increasing humidity [21]. So far, characterization for
temperature measurement of POFBGs has been carried out
using a hot plate or a conventional oven with no control on
relative humidity [20, 22]. It is known that as the temperature
of the hot plate or oven increases the corresponding
surrounding relative humidity decreases dramatically. To
verify this behavior we performed a systematic investigation
using an environmental controlled chamber. The chamber was
first programmed to have a fixed RH of 90% and ambient
temperature. Releasing the RH of the chamber and by
increasing the temperature up to 80 ºC, it can be clearly seen
from Fig. 1 how the RH of the chamber significantly and
rapidly decreases and reaching an equilibrium at 1% RH in
less than 3 hours. Based on this response, we can conclude
that similar behavior occurs in a humidity uncontrolled
environment such as for example in an open space hot element
or oven. At 1% RH, T
g
of PMMA is expected to be higher
than the one at ambient relative humidity as the water uptake
capability will be lower and water acts as a plasticizer for
PMMA [21]. Therefore, humidity is a limiting factor in the
maximum operating temperature of POFBGs which have
strong affinity to water. Similarly, when PMMA POFBGs are
used as humidity sensors, the range of operation is highly
dependent on the environmental temperature. PMMA
mPOFBGs have been operated in the range 10-90% RH at
Low loss polycarbonate polymer optical fiber
for high temperature FBG humidity sensing
Getinet Woyessa, Andrea Fasano, Christos Markos, Henrik K. Rasmussen, and Ole Bang
T
This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at http://dx.doi.org/10.1109/LPT.2017.2668524
Copyright (c) 2017 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
IEEE PHOTONICS TECHNOLOGY LETTERS
2
maximum limiting temperature of 75 ºC with no hysteresis
[15]. At 90% RH and temperature beyond 80ºC, the grating is
experiencing a significant degradation and it is unable to
operate. Therefore, PMMA based POFBGs can operate
beyond 75 ºC provided the corresponding RH level is lower
than 90%.
Thus, it is extremely important for different applications to
develop POFBG humidity sensors which can fulfill and
operate at both high temperatures beyond the operational limit
of PMMA POFBGs, and also wide range of relative humidity
operation. In this work, we demonstrate for the first time a
record low loss mPOFBGs humidity sensor that can operate
beyond 90% RH and 100 ºC using PC as the fiber material.
Recently, it has been demonstrated that PC mPOFBG can
operate up to 125 ºC at ambient relative humidity [23]. The
glass transition temperature and water absorption (saturation
value) at 23ºC for different polymer used for the fabrication of
mPOFs is listed in Table I.
II. EXPERIMENTS AND RESULTS
The microstructured fiber used in this report was fabricated in-
house at DTU Fotonik using a drill and draw method
described in [23]. However, to reduce the loss of the fiber we
used an improved casting approach for the preform
production. One of the most crucial factors which define the
final fiber loss is perhaps the quality of the cast preform. The
casting procedure was optimized with regard to two aspects:
drying phase and melting phase. A better water removal was
achieved by using an oven with enhanced air circulation. This
made it easier for the water trapped inside the plastic pellets to
diffuse out during the drying, thereby improving the overall
quality of the casting process. The melting phase was
extended to remove any possible micro-sized residuals of
crystals left in the final preform. Indeed, incomplete melting
due to insufficient melting time might lead to the presence of
micro-crystals in the core of the cast preform, which would
result in higher scattering loss. This can be seen in the visible
region in Fig. 2(b), where the gap between old and new fiber
transmission losses increases with decreasing wavelength.
The bulk material loss of the PC polymer and the PC mPOFs
losses are shown in Fig. 2(a) and 2(b), respectively. The
minimum fiber propagation loss was found to be ~ 4.06 dB /m
at 819 nm and at this wavelength the material loss is ~ 3.58
dB/m. It should be emphasized that the fiber loss was reduced
by a factor of two compared to the first fabricated PC mPOF
[23], holding thus the record in PC POFs loss. Both the cane
and the fiber have been drawn at 170°C and 10.5 MPa
drawing stress. The final core and cladding diameter of the
fabricated fiber are 10 μm and 125 μm, respectively. The
average size of the holes diameter and the pitch size are 2.5
μm and 6.25 μm, respectively. The hole to pitch ratio is 0.4
ensuring that the fiber is endlessly single mode [28]. A
microscope image of the PC mPOF end facet, which was
cleaved with a custom made cleaver at a temperature of 80°C
of both blade and fiber [29], is shown as an inset in Fig. 2(b).
A fiber Bragg grating was first inscribed in the fabricated
PC mPOF. The phase mask writing technique was used for the
FBG inscription while the detailed experimental setup can be
found in [30]. The phase mask used for grating inscription has
a uniform period of 572.4 nm and the writing laser was a 325
nm HeCd CW UV laser. We used only 5 mW for the
Fig. 2 (a) Bulk material optical loss for the improved PC solid rod. (b) Measured transmission loss of PC mPOF from both old and
improved casting methods. Inset: Optical microscope image of the fabricated PC mPOF.
Fig.1 The humidity response of an oven as the
temperature is increasing.
TABLE I
POLYMERS GLASS TRANSITION TEMPERATURE AND WATER
ABSORPTION (SATURATION VALUE) AT 23°C
Polymer types
Glass
transition
temperature
Water absorption
(saturation value) at
23ºC
PMMA [24]
106ºC
2.1%
PC [25]
145ºC
0.3%
TOPAS5013S-04 [26]
134ºC
<0.01%
Zeonex 480R [27]
138ºC
<0.01%
This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at http://dx.doi.org/10.1109/LPT.2017.2668524
Copyright (c) 2017 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
IEEE PHOTONICS TECHNOLOGY LETTERS
3
inscription of the grating. The grating had a length of 2 mm
and the Bragg wavelength was located at 892.24 nm with
reflection strength of 30 dB and a full width half maximum of
0.92 nm. The PC mPOFBG was then annealed at 125 ºC for
36 hours in a conventional oven without humidity control. The
new Bragg wavelength after annealing was blue shifted to
880.19 nm. To be sure that the PC mPOFBG was properly
annealed for humidity sensing operation up to 90% RH and at
high temperatures, we further annealed it in the climate
chamber at 90 % RH and 100 ºC for 6 hours, as high humidity
has been shown to strongly assist the annealing of PMMA
POFBGs [15]. For this, the PC mPOFBG was connectorized
[31], and placed in a climate chamber (CLIMACELL, MMM
Group). A supercontinuum source (SuperK Compact, NKT
Photonics) has been used as the broadband light source and a
spectrometer (CCS175 - Compact Spectrometer, Thorlabs) has
been used to continuously track and record the grating during
annealing in the climate chamber. This additional annealing
did not lead to any further permanent blue shift, indicating that
the POFBG was indeed properly annealed for operation at
temperatures and relative humidity levels of 100 ºC and 90%
RH, respectively. The normalized Bragg reflection spectrum
of the PC mPOFBG at 90 % RH and 100°C is shown as an
inset in Fig. 3(b).
After the annealing process, the humidity response of the
PC mPOFBG sensor was measured at three different
temperatures: 25 ºC, 50 ºC and 100 ºC, in the interval of 10-90
% RH. For each temperature level, the humidity measurement
has been done first by increasing the RH from 10% to 90%,
with step of 10% and then decreasing it from 90% to 10%
with 10% step. For both cases, the chamber was programmed
to change the RH in a minute and then to maintain stable the
environmental conditions for 45 mins. The response of the PC
mPOFBG, for both increasing and decreasing relative
humidity at 100ºC, is shown in Fig. 3(a). Fig. 3(b) shows the
humidity response at 100ºC where each measurement point
was taken at the end of the 45 mins stabilization period. The
humidity sensitivity at 100ºC was 7.31±0.13 pm/% RH (R-
squared of 0.998), for both increasing and decreasing relative
humidity. The corresponding humidity sensitivities at 25 ºC
and 50 ºC were measured to be 7.35±0.05 pm/% RH and
7.19±0.11 pm/% RH, respectively. These sensitivity figures
confirm that the humidity response of PC mPOFBG is
unaffected by temperature, and this is due to the fact that the
grating was adequately annealed.
We have also measured the temperature sensitivity at two
different RH levels: 50 % and 90 % RH in the range from 20
to 100 ºC. For each RH level, the temperature measurement
was first performed from 20 ºC up to 100 ºC, with a 10 ºC
step and then decreasing it back to 20 ºC with the same step.
For both cases, the chamber was programmed to change the
temperature in 10 mins and then to maintain the environmental
conditions stable for 45 mins. The response of the PC
mPOFBG for both increasing and decreasing temperature at
90% RH is shown in Fig. 4(a). Fig. 4(b) shows the
temperature response at 90% RH where each measurement
Fig. 4. (a) Measured temperature response at 90 % RH of the PC mPOFBG. (b) Corresponding stabilized temperature
response of the PC mPOFBGs at 90 % RH.
Fig. 3. (a) Measured humidity response at 100 ºC of the PC mPOFBG. (b) Corresponding stabilized humidity
response of the PC mPOFBGs at 100 ºC. Inset: Normalized Bragg reflection spectrum of the PC mPOFBG.
This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at http://dx.doi.org/10.1109/LPT.2017.2668524
Copyright (c) 2017 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
IEEE PHOTONICS TECHNOLOGY LETTERS
4
point was taken at the end of the 45 mins for the stabilization
period. The temperature sensitivity at 90 % RH was
25.86±0.63 pm/ºC. The corresponding sensitivity at 50% RH
was 25.62±0.56 pm/ºC. No hysteresis was also observed
during the temperature characterization, further confirming the
fact that the grating was properly annealed.
III. CONCLUSION
We have developed and characterized a polycarbonate
based mPOFBG humidity sensor that can operate beyond
100 ºC in the relative humidity range 10-90%. The mPOF
preform was made by using an improved casting method and
the measured loss was found to be two times smaller than the
hitherto. The sensor gave a RH sensitivity of 7.31±0.13 pm/%
RH in the range 10-90% RH at 100 ºC and a temperature
sensitivity of 25.86±0.63 pm/ºC in the range 20-100 ºC at 90
% RH.
The humidity sensitivities of our PMMA mPOFBGs and
TOPAS step index POFBGs at 850nm are 45 pm/%RH and
0.45 pm/%RH, respectively. Thus the humidity sensitivity of
PC mPOFBGs is 6 times smaller than PMMA mPOFBG and
16 times larger than TOPAS POFBGs which are basically
humidity insensitive. However, PC mPOFBG humidity
sensors can operate up to 90 % RH at a temperature 25 ºC
higher than the maximum operational limit of PMMA
mPOFBGs. The temperature sensitivity of PC mPOFBGs is
more than a factor of two larger than that of PMMA
mPOFBGs. At ambient relative humidity PC mPOFBGs can
operate up to 125 ºC while PMMA mPOFBG can only be
operated up to 90 ºC. Thus, PC mPOFBGs humidity sensors
can be used in several different applications areas where
humidity measurement at high temperature is required such as
in industry for ceramic driers, in domestic electric appliance
for microwave oven and in agriculture for thermo-hygrostatic
chamber [32].
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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at http://dx.doi.org/10.1109/LPT.2017.2668524
Copyright (c) 2017 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.