A preview of this full-text is provided by Optica Publishing Group.
Content available from Optics Express
This content is subject to copyright. Terms and conditions apply.
Temperature sensing up to 1300°C using
suspended-core microstructured optical fibers
Stephen C. Warren-Smith,1,2,* Linh Viet Nguyen,1 Catherine Lang,1 Heike Ebendorff-
Heidepriem,1,4 and Tanya M. Monro1,3,4
1Institute for Photonics and Advanced Sensing (IPAS) and School of Physical Sciences, The University of Adelaide,
Adelaide 5005, Australia
2Currently with Leibniz Institute of Photonic Technology (IPHT Jena), Albert-Einstein-Straße 9, 07745 Jena,
Germany
3University of South Australia, Adelaide, SA 5000, Australia
4ARC Centre of Excellence for Nanoscale Biophotonics, The University of Adelaide, Adelaide, SA 5005, Australia
*stephen.warrensmith@ipht-jena.de
Abstract: We demonstrate a new approach to high temperature sensing
using femtosecond laser ablation gratings within silica suspended-core
microstructured optical fibers. The simple geometry of the suspended-core
fiber allows for femtosecond laser processing directly through the fiber
cladding. Pure silica glass is used, allowing the sensor to be used up to
temperatures as high as 1300°C while still allowing the fibre to be spliced to
conventional fiber. The sensor can also be wavelength division multiplexed,
with three sensors in a single fiber demonstrated.
©2016 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (060.3735) Fiber Bragg gratings; (060.4005)
Microstructured fibers; (280.6780) Temperature.
References and Links
1. B. Culshaw, “Optical fiber sensor technologies: opportunities and-perhaps-pitfalls,” J. Lightwave Technol. 22(1),
39–50 (2004).
2. B. Lee, “Review of the present status of optical fiber sensors,” Opt. Fiber Technol. 9(2), 57–79 (2003).
3. A. Othonos, “Fiber Bragg gratings,” Rev. Sci. Instrum. 68(12), 4309–4341 (1997).
4. T. Habisreuther, T. Elsmann, Z. Pan, A. Graf, R. Willsch, and M. A. Schmidt, “Sapphire fiber Bragg gratings for
high temperature and dynamic temperature diagnostics,” Appl. Therm. Eng. 91, 860–865 (2015).
5. T. Elsmann, T. Habisreuther, A. Graf, M. Rothhardt, and H. Bartelt, “Inscription of first-order sapphire Bragg
gratings using 400 nm femtosecond laser radiation,” Opt. Express 21(4), 4591–4597 (2013).
6. V. I. Kopp and A. Z. Genack, “Chiral fibres: adding twist,” Nat. Photonics 5(8), 470–472 (2011).
7. J. Canning, M. Stevenson, S. Bandyopadhyay, and K. Cook, “Extreme silica optical fibre gratings,” Sensors
(Basel Switzerland) 8(10), 6448–6452 (2008).
8. D. Barrera, V. Finazzi, J. Villatoro, S. Sales, and V. Pruneri, “Packaged optical sensors based on regenerated
fiber Bragg gratings for high temperature applications,” IEEE Sens. J. 12(1), 107–112 (2012).
9. C. R. Liao and D. N. Wang, “Review of femtosecond laser fabricated fiber Bragg gratings for high temperature
sensing,” Photonic Sens. 3(2), 97–101 (2013).
10. Y. Li, M. Yang, D. N. Wang, J. Lu, T. Sun, and K. T. V. Grattan, “Fiber Bragg gratings with enhanced thermal
stability by residual stress relaxation,” Opt. Express 17(22), 19785–19790 (2009).
11. J. Canning, N. Groothoff, K. Cook, C. Martelli, A. Pohl, J. Holdsworth, S. Bandyopadhyay, and M. Stevenson,
“Gratings in structured optical fibres,” Laser Chem. 2008, 239417 (2008).
12. S. J. Mihailov, “Fiber Bragg grating sensors for harsh environments,” Sensors (Basel) 12(12), 1898–1918 (2012).
13. R. Kostecki, H. Ebendorff-Heidepriem, S. C. Warren-Smith, and T. M. Monro, “Predicting the drawing
conditions for microstructured optical fiber fabrication,” Opt. Mater. Express 4(1), 29–40 (2014).
14. S. C. Warren-Smith, R. Kostecki, L. V. Nguyen, and T. M. Monro, “Fabrication, splicing, Bragg grating writing,
and polyelectrolyte functionalization of exposed-core microstructured optical fibers,” Opt. Express 22(24),
29493–29504 (2014).
15. S. C. Warren-Smith and T. M. Monro, “Exposed core microstructured optical fiber Bragg gratings: refractive
index sensing,” Opt. Express 22(2), 1480–1489 (2014).
16. Heraeus, “Thermal properties,” http://heraeus-
quarzglas.com/en/quarzglas/thermalproperties/Thermal_properties.aspx (Accessed May 2015).
17. Z. Borisova, Glassy Semiconductors (Springer US, 1981).
#254937
Received 1 Dec 2015; revised 27 Jan 2016; accepted 28 Jan 2016; published 12 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003714 | OPTICS EXPRESS 3714
1. Introduction
Fiber Bragg gratings (FBGs) are well known for their use in sensing physical parameters such
as temperature, strain, and pressure [1, 2]. An attractive feature is that FBGs with different
pitches can be written at different points along the length of the optical fiber, with each FBG
producing a discrete narrowband reflection that can be wavelength division multiplexed.
Depending on the particular design and application, FBG sensors also offer high accuracy and
precision, high speed interrogation, and reflection mode operation.
Fiber Bragg gratings fabricated with the traditional technique of ultra-violet (UV)
inscription in photosensitive silica-based fibers have a maximum operating temperature limit
of approximately 500°C due to thermal annealing out of the refractive index modifications
above this temperature [2, 3]. Developing grating-based sensors that can measure high
temperatures, such as 1000°C and beyond, is an area of active research. Sapphire crystal
fibers with femto-second laser written Bragg gratings are perhaps the leading technology with
sensing up to 1900°C demonstrated [4, 5]. However, such fibres are highly multimode with
broad Bragg reflections. It is also not possible to splice sapphire crystal fibers to conventional
silica fiber. For applications up to 1300°C a number of fabrication techniques have been
developed to inscribe gratings in silica-based fibers, such as chiral gratings [6], regenerated
fiber Bragg gratings [7, 8], and femto-second laser written gratings [9, 10].
Chiral gratings are fabricated by rotating and translating an elliptical core fiber through a
short heat zone [6]. These fibers have been demonstrated as temperature sensors up to 1000°C
and are commercially available. Alternatively, regenerated fiber Bragg gratings are fabricated
by thermally annealing a “seed” grating, such as a standard type I UV inscribed grating. These
gratings have been shown to survive to temperatures up to 1100°C [8]. In one particular
example a regenerated FBG was shown to survive up to 1295°C, however the fiber itself was
shown to become extremely brittle after reaching such temperatures [7]. It should also be
noted that the fabrication of such gratings requires treating the fiber with high pressure
hydrogen for time periods on the order of 24 hours and gratings prepared in this manner have
poorer reflectivity compared to the seed gratings [8].
A promising approach to fabricate optical fiber temperature sensors up to temperatures the
order of 1300°C is the use of undoped single-material (e.g. pure fused silica) optical fibers,
such as microstructured optical fibers, which avoid the potential for dopant diffusion at high
temperatures [9]. To create an FBG in an undoped fiber requires techniques that do not rely
on glass photosensitivity. To create a high temperature stable grating a technique known as
femtosecond laser ablation can be used where by using sufficiently short duration pulses,
multiphoton absorption can lead to ionization of electrons and the physical removal (ablation)
of material [11, 12]. However, a particular challenge of microstructured optical fibers is that
the complex cross section distorts and scatters the inscription beam [11].
In this paper we demonstrate the inscription of fiber Bragg gratings on the core of pure-
silica suspended-core microstructured optical fibers (SCFs) using femtosecond laser ablation.
The simple geometry of the SCF (three large holes) allows the direct writing of FBGs as the
focused femtosecond laser beam is not significantly distorted when passing through the fiber
cladding. We demonstrate that these can survive up to 1300°C and can be wavelength
division multiplexed. Here we present three sensing elements operating in a single device.
2. Sensor fabrication
The optical fiber used was silica (Suprasil F300, Heraeus) suspended-core microstructured
optical fiber [13], with an outer diameter of 160 µm [Fig. 1(a)] and a core diameter of
approximately 10 µm [Fig. 1(b)]. These fibers were then spliced to standard FC/APC
connectorized single-mode fiber (SMF28) using an arc splicer (Fujikura FSM-100P) and the
same technique previously determined for exposed-core microstructured optical fiber (MOF)
[14]. That is, standard SMF-SMF splicer settings but with reduced arc current (12.5 mA
#254937
Received 1 Dec 2015; revised 27 Jan 2016; accepted 28 Jan 2016; published 12 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003714 | OPTICS EXPRESS 3715
compared to 16.5 mA), increased arc duration (3.0 s compared to 2.0 s), and manual
alignment.
Fig. 1. Scanning electron images (SEMs) of the suspended-core fiber with a femtosecond laser
ablation grating. (a) The fiber cross section. (b, c) magnified images of (a).
Bragg gratings were written into the SCFs by adapting a technique previously developed
for exposed-core MOFs [14, 15]. That is, the FBGs were written by focusing 800 nm
femtosecond laser (Hurricane Ti:sapphire) pulses using a 50X long working distance
microscope objective onto the core of the fiber. A repetition rate of 100 Hz was used and the
SCF was translated to yield second order Bragg reflections at the desired wavelength (e.g.
1550 nm). In comparison to the exposed-core MOF, for the SCF the fs laser pulses must
traverse across the fiber cladding and thus some distortion of the pulses was expected. This
can be seen in Figs. 1(b) and 1(c), where the ablation points overlap. However, there was
sufficient periodicity in the gratings that were formed in order to measure a defined Bragg
reflection, as will be seen in the proceeding sections. In addition, there were several technical
considerations that were introduced by using the enclosed structure. Firstly, the pulse energy
required was greater: 400 nJ pulses were used compared to 200-250 nJ for open structures. It
was also difficult to determine the location of the optical fiber core and thus a visible laser
was coupled to the spliced single-mode fiber so that scattering from the ablation spots could
be observed as a confirmation of the femto-second laser focusing position.
3. High temperature sensing
To test the operating temperature limit of the sensors a fiber as prepared in Sec. 2 was first
packaged within a silica tube (approx. 2 mm outer diameter (OD)) for physical protection.
The packaged sensor was then inserted into the graphite resistance furnace of a silica optical
fiber draw tower. The length of the microstructured optical fiber was sufficiently long such
that the spliced single-mode fiber was located well outside of the heat zone of the furnace.
The particular grating used had a pitch of 1080 nm and a length of 20 mm. The resulting
reflected spectrum at room temperature is shown in Fig. 2(a), measured using an Optical
Sensor Interrogator (OSI, National Instruments PXIe-4844). The reflected spectrum shows a
peak reflectivity at 1556 nm, a full-width at half maximum of 48 pm, and a peak reflectivity
greater than 30 dB above the background. The reflected spectrum shows peaks from higher
order modes, as expected, however the fundamental mode reflectivity was approximately 8.5
dB above that of the higher order modes (at room temperature) and can further be improved
by using SMF with a better mode field match to the fundamental mode of the SCF.
To measure the temperature experienced at the grating a B-type thermocouple was located
at the top point of the 20 mm long grating, noting that the spliced connection (i.e. the light
path) was directed through the top of the furnace. The furnace set-point was initially set at
850°C and at this temperature the location of the grating was adjusted so that the top point of
the grating was at the hottest point within the furnace. The furnace was then increased in
temperature in steps of 50°C and then held for five minutes. The spectra recorded at several
points are shown in Figs. 2(b)-2(f). The results in Fig. 2 show the expected shift to longer
wavelengths at higher temperatures. In addition, the reflected spectrum was found to be stable
for the duration of the hold time (5 minutes) up to 1300°C. Once the temperature was
increased beyond this temperature (1350°C) the grating was found to decrease in reflectivity
#254937
Received 1 Dec 2015; revised 27 Jan 2016; accepted 28 Jan 2016; published 12 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003714 | OPTICS EXPRESS 3716
rapidly. The peak reflectivity reduced by 2.0 dB in the first 5 minutes and 8.6 dB after one
hour, in addition to a change in the spectral shape [Fig. 2(f)]. The grating did not recover after
returning to room temperature, indicating permanent damage to the sensor. The temperature at
which the sensor failed (between 1300°C and 1350°C) lies significantly above the annealing
point of fused silica (1100°C [16]), but also well below the softening point (1600°C [16]). At
a temperature of 1300°C fused silica (F300) glass has viscosity of 109.87 Pa.s (calculated using
Eq. (2) in [13]), which is close to the dilatometric softening temperature (1010 Pa.s) [17]. This
is the temperature at which a sample (e.g. a glass rod) with an external load will no longer
expand in length for increasing temperature due to sample dilation and deformation. This
implies that deformation happens on the micron-scale and thus micron-scaled holes such as
those created by femto-second ablation can (at least partially) collapse, destroying the grating.
Fig. 2. Reflected spectra measured from the SCF Bragg grating at increasing temperatures
within a silica fiber draw tower. The spectra shown are at temperatures (a) 20°C, (b) 500°C, (c)
1000°C, (d) 1200°C, (e) 1300°C, and (f) 1350°C. For temperatures of 400°C and above the
temperature was recorded with a co-located B-type thermocouple.
This experiment has successfully demonstrated the ability of femtosecond laser ablation
gratings in suspended-core microstructured optical fibers to measure temperature up to
1300°C. A limitation of this experiment is that the furnace used has a short hot zone that
varies by approximately 100°C over the length of the grating (20 mm) at 1300°C [13]. This
leads to the broadening of the reflected spectrum that can be seen in Fig. 2 and a reduction in
the maximum reflectivity. Experiments using either a furnace with a more even temperature
distribution or by using gratings of shorter length will allow the performance limits of these
devices to be explored more fully.
4. Calibration and multiplexed sensing
The results in Fig. 2 are insufficient to form a calibration curve due to the uneven temperature
distribution of the silica draw tower furnace used for these tests. In order to create a
calibration curve a second sensor was inserted centrally into a tube furnace (ModuTemp) and
a K-type thermocouple was located at the center of the Bragg grating. The temperature of the
tube furnace was raised to 1000°C and held there for several hours until both the
thermocouple and the FBG showed stable readings, at which point the temperature profile of
the furnace was deemed stable. The temperature was then reduced in steps of 100°C and held
for at least one hour to ensure the furnace temperature was stable before a spectrum was
recorded. The shift in the position of the fundamental mode reflection relative to room
temperature is shown in Fig. 3. A quadratic function was used to form the calibration curve
due to the nonlinear thermo-optic response of the sensor. This is likely due to a combination
of the silica thermo-optic coefficient and the mode confinement within the suspended-core
[11]. Note that the calibration curve is valid for a particular set of parameters such as fiber
#254937
Received 1 Dec 2015; revised 27 Jan 2016; accepted 28 Jan 2016; published 12 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003714 | OPTICS EXPRESS 3717
geometry, material, and grating structure. In the following experiment we have assumed the
impact of grating pitch, and thus the Bragg wavelength, to be negligible for this proof-of-
concept demonstration. This is an approximation as material and waveguide dispersion will
cause a slight change to the calibration curve at different wavelengths.
Fig. 3. Calibration curve measured using a SCF Bragg grating in a tube furnace. The shift in
temperature (ΔT) and shift in wavelength (Δλ) are relative to values at room temperature. The
quadratic fit was set to zero at the origin.
A wavelength division multiplexed sensor was fabricated by writing three physically
separated gratings into a suspended-core fiber using the same technique described in Sec. 2.
In this case the gratings were made 10 mm long and three different pitches were used (Λ =
1055.7 nm, 1069.6 nm, 1083.5 nm). The gap between the gratings was 25 mm and 17 mm,
respectively. The fiber was then spliced to conventional single mode fiber and packaged
within a 2.0 mm OD silica tube. The packaged sensor was inserted into a soft glass fiber draw
tower (graphite induction furnace), with the spliced connection passed through the bottom of
the furnace. The reflected spectrum recorded at room temperature is shown in Fig. 4(a). Note
that the lower grating had the strongest reflection, indicating optical loss across the gratings.
Fig. 4. (a) Reflected spectrum from the multiplexed sensor at room temperature. *Indicates the
peaks that were tracked. (b) Change in temperature recorded by the FBGs, corresponding to the
peaks indicated in (a) and calibrated using the results in Fig. 3.
Prior to the experiment we determined which reflections corresponded to each grating by
locally heating the gratings separately and observing which reflection peaks shift in
wavelength, which is indicated by the colored bands in Fig. 4(a). For the temperature sensing
experiment only the fundamental mode reflections were tracked, which are indicated with
asterisks. The furnace set-point was then raised in steps of 100°C up to 800°C and, using the
calibration shown in Fig. 3, the results are shown in Fig. 4(b). Note that the temperatures
measured by the gratings (up to 525°C) were less than 800°C because the furnace’s
thermocouple is located in a different physical location (within the susceptor) compared to the
FBG sensor (center line of the susceptor). The measured difference of approximately 275°C
between the susceptor’s center line (FBG sensor) and the furnace’s thermocouple agrees with
measurements we have previously made using a thermocouple within a silica tube at the
center line of the susceptor.
#254937
Received 1 Dec 2015; revised 27 Jan 2016; accepted 28 Jan 2016; published 12 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003714 | OPTICS EXPRESS 3718
While the maximum temperature tested in this particular experiment was only 525°C, the
results of Fig. 4 demonstrate that a suspended-core microstructured optical fiber femtosecond
laser ablation temperature sensor can be successfully multiplexed with up to three elements.
5. Discussion and conclusions
We have demonstrated temperature sensing up to 1300°C using femtosecond laser ablation
gratings within silica suspended-core microstructured optical fiber. This relatively simple
fiber geometry has been used as the femtosecond laser beam is not significantly distorted
when passing through the cladding structure. The use of silica glass allows for direct splicing
to conventional single-mode fiber, thus easy integration with commercial interrogators, and
provides the possibility of long length sensors due to low propagation loss. The maximum
operating temperature of the sensor was found to lie between 1300°C and 1350°C. Further
research needs to be conducted to determine the exact maximum operating temperature limit
of the sensor. The long term stability of the sensor at these temperatures also needs to be
investigated further as the current study was limited to five minute intervals for each
temperature measured.
Wavelength division multiplexed (WDM) sensing has been demonstrated, with three
temperature sensing elements shown here. One limitation in the number of WDM sensors is
the multi-mode nature of the suspended-core fiber used for this work, leading to several peak
reflections per grating. The wavelength separation of the gratings must be large enough to
accommodate the modes (approximately 12 nm) plus the shift over the desired wavelength
range (14 nm for 1000°C). For an interrogator bandwidth of 80 nm this limits the number of
sensors to three. The multiplexing capability can be improved through further designing the
suspended-core MOF to reduce the number of propagating modes. The most obvious way of
doing this is by reducing the core diameter. Improvements in the grating inscription process,
such as the use of optimized laser pulse energy and filling the MOF air hole with near index-
matching liquid to allow the writing beam to focus better in the core region, should also help
reduce the optical loss observed in our multiplexed sensing experiment.
An additional parameter that requires further optimization is the grating length. Profiling
of short-length furnaces, such as in fiber drawing towers, would benefit from better spatial
resolution than can be achieved using the 10-20 mm gratings used here. To reduce grating
length whilst maintaining reflectivity a stronger index modulation is required. One way to
achieve this could be to, again, reduce the core diameter in order to increase the overlap of the
propagating modes with the ablation points. However, careful design optimization is required
as smaller core diameters may lead to increased FBG inscription and splicing difficulty.
Acknowledgments
The authors acknowledge Ben Johnston from Macquarie University for assistance in using the
femtosecond laser facility at Macquarie University and Alastair Dowler, Peter Henry, Roman
Kostecki, and Anthony Leggatt from the University of Adelaide for their contribution to the
silica fiber fabrication. This work was performed in part at the OptoFab node of the
Australian National Fabrication Facility utilizing Commonwealth, and South Australian and
New South Wales State Government funding. Stephen Warren-Smith and Linh Nguyen
acknowledge the support of a Photonics Catalyst Grant supported by the South Australian
State Government and SJ Cheesman (Port Pirie, South Australia). Stephen Warren-Smith and
Catherine Lang acknowledge the support of a Denis Harwood Innovation Grant. Stephen
Warren-Smith is currently supported by the European Commission through the Seventh
Framework Programme (FP7), PIIF-GA-2013-623248. Tanya Monro acknowledges the
support of an ARC Georgina Sweet Laureate Fellowship.
#254937
Received 1 Dec 2015; revised 27 Jan 2016; accepted 28 Jan 2016; published 12 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003714 | OPTICS EXPRESS 3719
