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APL Photonics LETTER scitation.org/journal/app
Frequency scanned phase sensitive optical
time-domain reflectometry interrogation
in multimode optical fibers
Cite as: APL Photon. 5, 031302 (2020); doi: 10.1063/1.5138728
Submitted: 15 November 2019 •Accepted: 24 February 2020 •
Published Online: 16 March 2020
K. Markiewicz,1,2,a) J. Kaczorowski,1,2,3 Z. Yang,1L. Szostkiewicz,2,4,5 A. Dominguez-Lopez,2K. Wilczynski,2
M. Napierala,2T. Nasilowski,2and L. Thévenaz1
AFFILIATIONS
1EPFL Swiss Federal Institute of Technology, Institute of Electrical Engineering, SCI STI LT, Station 11,
CH-1015 Lausanne, Switzerland
2InPhoTech Sp. z o.o., 400A Poznanska St., Ozarow Mazowiecki 05-850, Poland
3Institute of Micromechanics and Photonics, Warsaw University of Technology, ´
Sw. A. Boboli 8 St., 02-525 Warsaw, Poland
4Polish Centre for Photonics and Fibre Optics, Racławickie St. 8/12, 20-037 Lublin, Poland
5Faculty of Physics, Warsaw University of Technology, Warsaw 00-662, Poland
a)Author to whom correspondence should be addressed: kmarkiewicz@inphotech.pl
ABSTRACT
Standard multimode optical fibers normally support transmission over some 100 modes. Large differences in the propagation constant and
the spatial distribution of distinct modes degrade the performance of phase-sensitive optical time-domain reflectometry measurements. In
this work, we present a new realization of a coherent time-domain interrogation technique using single-mode operation in multimode fibers.
We demonstrate effectively distributed strain sensing on three different multimode optical fibers. Up to 4 km of multimode fiber has been
correctly interrogated, featuring a spatial resolution of 20 cm.
©2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5138728
., s
I. INTRODUCTION
Distributed optical fiber sensors (DOFS) based on Raman,
Brillouin, or Rayleigh scattering become ubiquitous in situations
where the measurement of physical quantities, such as vibration,
strain, or temperature, induced by multiple events over long dis-
tances is required.1–6 Research on DOFS based on Rayleigh scatter-
ing has naturally focused on single-mode fibers due to the absence
of intermodal effects. A major interest of DOFS lies in the oppor-
tunity to interrogate the already deployed fibers. However, a sig-
nificant proportion of those is multimode, so it would be bene-
ficial to take advantage of the existing multimode fiber networks.
On top of that, aiming at overcoming the limitations of the cur-
rent optical networks, a great investment in the development of few
and multimode optical fibers for mode division multiplexing7–9 has
been carried out, leading to an increase in the ratio of deployed
multimode to single-mode fibers. It is, thus, reasonable to expect that
in the foreseeable future, the demand for DOFS utilizing multimode
optical fibers will also grow. Coherent techniques based on Rayleigh
scattering, in which the phase of the scattered signal is ana-
lyzed, need a more sophisticated setup when the fiber under
test is multimode rather than single-mode. In fact, in the litera-
ture, demonstrations of detection of single or multiple speckles of
scattered light in multimode links can be found.10–14 To enable
proper coherent Rayleigh-scattering-based interrogation in multi-
mode fibers, techniques developed for mode division multiplexing
have been utilized, namely, selective mode excitation. In this paper,
we report on direct strain measurements using phase-sensitive
optical time-domain reflectometry [ϕ-OTDR (optical time-domain
reflectometry)] based on single-mode operation in several few and
multimode optical fibers. In addition to the above-mentioned
advantages, this method also diminishes modal dispersion,15 which
can lower the spatial resolution by tens of centimeters at a dis-
tance of 1 km. These results may facilitate the introduction and
APL Photon. 5, 031302 (2020); doi: 10.1063/1.5138728 5, 031302-1
© Author(s) 2020
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adoption of not only Rayleigh-scattering-based DOFS in multimode
optical fibers that are already deployed for telecommunication but
also Raman-scattering-based sensors.
II. METHODOLOGY
In order to assess the strain response of the multimode fibers
under test, we developed a high-resolution ϕ-OTDR setup simi-
lar to the one presented by Hartog,16 but combined with an addi-
tional higher-order mode filter (HOMF),15 as shown in Fig. 1. A
distributed feedback (DFB) laser at a wavelength of 1550 nm was
used as the light source. By tuning the driving current of the laser,
the laser frequency can be scanned to characterize the frequency-
dependent response of the backscattered signal. In this experiment,
a 29 GHz scanning range was obtained with steps of 92 MHz, which
results in 315 measured fiber responses. The frequency step size was
chosen to finely retrieve the signal intensity as a function of the laser
central frequency for each point along the fiber under test. The laser
tuning accuracy was 10 MHz. An electro-optic intensity modulator
(EOM) driven by a pulse generator was used to shape 2 ns square
pulses (which render a 20 cm spatial resolution), launched with a
repetition rate of 1 kHz. The expected FWHM of the peaks in the
pattern is equal to 300 MHz for the used spatial resolution.17 To
secure an extinction ratio over 60 dB, as required in order to achieve
a 20 cm resolution over a 5 km range, a semiconductor optical ampli-
fier (SOA) is inserted after the EOM to perform a second optical
pulse gating, though with longer pulses. To increase the pulse power,
an erbium-doped fiber amplifier (EDFA) is placed after the SOA.
The power injected into each fiber was chosen separately such that
it is the maximum power for which the measurement quality does
not drop due to modulation instability. The maximum peak power
that we could achieve with the described setup was 26 dBm. After
the EDFA, the pulse is sent through a circulator to the multimode
fiber under test. At the junction between the single and the multi-
mode fibers, multiple modes are excited. A HOMF, manufactured by
InPhoTech, is inserted to effectively filter out all higher-order modes
propagating in the optical fiber while introducing additional 2 dB
losses on the launch in the setup. These losses vary no more than
0.2 dB for the different multimode fibers used in the experiment.
To verify that the HOMF ensures single-mode operation, M2tests
were performed. For every fiber under test, the M2value obtained
at the far end of the fiber was below 1.1. This value indicates that in
the worst possible scenario, the power in the fundamental mode is
at least 10 times higher than in all the other modes combined. In the
absence of strong perturbations applied to the fiber, e.g., a bad splice,
during propagation along the multimode fiber, the power coupling
occurs mainly between modes within one mode group, while cou-
pling between different mode groups is negligible.18 At each point,
part of the light is Rayleigh-backscattered into all modes propagat-
ing inside the fiber under test.19 The returning signal is once again
filtered by the HOMF, and the retrieved fundamental mode signal is
then amplified by a second EDFA to increase the signal to noise ratio.
A tunable filter with a bandwidth of 1 nm is inserted to reject most
of the amplified spontaneous emission (ASE) from the EDFA. The
signal is then detected by a DC-coupled photodiode with a 1 GHz
bandwidth, which is sufficient to properly retrieve the targeted spa-
tial resolution. Finally, the electrical signal from the photodiode is
digitized by means of a 4 GHz oscilloscope. The measured signal
was averaged 100 times in order to increase the signal to noise ratio.
The total time for a single scan was around 1 h due to communica-
tion between the devices used in the setup not being optimized. With
appropriate optimization, the time can be reduced to below 5 min.
In order to apply a known strain, the fiber was fixed to a micro-
metric translation stage. Strain measurements were carried out by
performing two scans of the laser frequency, i.e., before and after
applying the strain. For each point along the fiber, the cross cor-
relation of intensity vs laser frequency was calculated. From the
maximum of the calculated cross correlation, a frequency shift is
obtained, which is a linear function of the induced strain. Three
different optical fibers have been tested: the first one is a multi-
mode OM4 fiber manufactured by Draka, which supports 34 LP
mode groups at a wavelength of 1550 nm. The second one is a 6 LP
mode graded-index fiber also manufactured by Draka. The third
one is a 4 LP mode graded-index fiber manufactured by InPhoTech.
All of these fibers show a parabolic refractive index profile in the
core. This choice is designed to test single-mode operation both for
fibers with few-mode groups and for highly multimode optical fibers
FIG. 1. Experimental setup used for strain measurements in
multimode optical fibers with ϕ-OTDR. PC, polarization con-
troller; EOM, electro-optic modulator; SOA, semiconductor
optical amplifier; EDFA, erbium doped fiber amplifier; PD,
photodiode; SMF, single mode fiber; MMF, multimode fiber;
and FUT, fiber under test. For testing, three different optical
fibers were used: 4 km of OM4, 1 km of 6 LP Draka, and
100 m of 4 LP InPhoTech.
APL Photon. 5, 031302 (2020); doi: 10.1063/1.5138728 5, 031302-2
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commonly deployed in buildings and short-range telecommunica-
tion links.
III. MEASUREMENT RESULTS
For proper ϕ-OTDR measurements in a single-mode fiber, a
high trace visibility (of at least 0.75) for every section of the fiber is
expected, defined as
Visibility =Imax −Imin
Imax +Imin
, (1)
where Imax and Imin are the maximum and minimum intensities
of the fiber segment, respectively, for which visibility is calculated.
To assess the performance of the system, visibilities observed for
a single-mode fiber (SMF), the OM4 Draka, the 6 LP Draka, and
the 4 LP InPhoTech optical fibers were calculated over every 20 m
long segment from the acquired trace, as shown in Fig. 2. An SMF
was characterized without a HOMF to verify the performance of
the measurement setup in a known situation. The visibility values
obtained for the SMF were lower than those reported in the state-
of-the-art as a result of a limited extinction ratio. For each segment,
the visibilities for few and multimode optical fibers are slightly lower
than those obtained for the single-mode fiber, but all of them are still
in excess of 0.8, confirming the proper single-mode operation. Vis-
ibility values lower than those for the SMF are probably an effect of
the signal from other modes going through the HOMF. It turns out
that the visibility is lower over the first 20 m in the 4 LP InPhoTech
fiber, which is an artifact caused by a strong spurious reflection and
the partial saturation of the detector.
As a second step, for each tested optical fiber, we investigated
the frequency shift induced by an applied strain by performing a
frequency scan for three different strain values. Each measurement
was performed twice to confirm that the results are stable if the
strain is unchanged. The measured frequency change between the
two measurements for the same applied strain turns out to be less
than half of the frequency step, which is the minimum detectable
change, confirming the correctness and repeatability of the tests.
Strain noise estimated as the mean measured value for the whole
fiber in a static situation was 0, which indicates that it is smaller than
0.3 με.Figure 3 shows the calculated cross correlation as a function
of position in the fiber and frequency shift for the OM4 Draka, the
6 LP Draka, and the 4 LP InPhoTech fibers. It is clearly visible that a
shift in the maximum of the cross correlation is observed only in the
section where strain is applied for all these three types of fibers. For
the 6 LP Draka optical fiber, there is a visible additional high peak
in the cross correlation spectrum. Such false peaks are an inherent
property of the measurement technique used, as they are the results
of the statistical properties of Rayleigh scattering.20 By measuring the
frequency shift for three different strain values and performing a lin-
ear fitting over the measurement data for each fiber, we were able to
estimate the strain sensitivity of the fundamental mode for all of the
tested fibers (Fig. 4). There are two main sources of error during this
characterization: a minor one, which is related to the frequency step
while scanning the laser frequency, and the dominating one, which is
due to the micrometric translation stage. Estimating changes smaller
than half of the frequency step have limited reliability. Although
there are methods to estimate such small changes, using them will
not have much impact on the results due to the major source of
uncertainty rendered by the use of the translation stage. Since strain
was applied through a manually set translation, the level of accu-
racy for the applied strain is around 6 με. Under these uncertain-
ties, the calculated sensitivities based on the experimental results are
138 ±16 MHz/με, 133 ±17 MHz/με, and 152 ±16 MHz/με for the
OM4 Draka, the 6 LP Draka, and the 4 LP InPhoTech optical fibers,
FIG. 2. Measured trace (blue line) and
calculated visibility of the response over
20 m segments (green line) for (a) SMF,
(b) OM4 Draka, (c) 6 LP Draka, and (d)
4 LP InPhoTech optical fibers. Visibilities
of the measured response for few- and
multi-mode fibers are similar to those
obtained for a single mode fiber.
APL Photon. 5, 031302 (2020); doi: 10.1063/1.5138728 5, 031302-3
© Author(s) 2020
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FIG. 3. (a) Calculated cross correlation in the frequency domain for each point of
the OM4 Draka optical fiber. Other graphs represent zoomed-in sections where
strain was applied for (b) OM4 Draka, (c) 6 LP Draka, and (d) 4 LP InPhoTech.
FIG. 4. Measured frequency shift as a function of the induced strain together with
the fitted linear function for (a) OM4 Draka, (b) 6 LP Draka, and (c) 4 LP InPhoTech.
The determined strain sensitivities for the fundamental modes of the tested optical
fibers based on the linear fitting are equal to 138 ±16 MHz/με, 133 ±17 MHz/με,
and 152 ±16 MHz/με, respectively. Note that the fit was forced to cross the origin
to be performed over three points.
respectively. Within this error range, the obtained values of strain
sensitivity can be safely claimed to be close to a single mode fiber
(150 MHZ/με). As expected, the obtained values were similar due to
the fact that most of the effect of strain sensitivity comes from elon-
gation of the fiber and not from the change of refractive index of
glass.21
IV. SUMMARY
In this work, we have demonstrated a new method for enabling
ϕ-OTDR strain measurements in multimode optical fibers. This was
APL Photon. 5, 031302 (2020); doi: 10.1063/1.5138728 5, 031302-4
© Author(s) 2020
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carried out by selectively exciting and detecting a signal from a given
mode of a multimode fiber, thus enhancing ϕ-OTDR measurements
in existing and future multimode links. The solution presented here
solves the problem of modal dispersion for this kind of measure-
ment and is easily applicable to existing DOFS based on Rayleigh
scattering.
ACKNOWLEDGMENTS
This work was financially supported within the “NODUS”
project carried out within the TEAMTECH programme of the Foun-
dation for Polish Science co-financed by the European Union under
the European Regional Development Fund and was also supported
by the National Centre for Research and Development within the
research project TECHMATSTRATEG1/348438/16/NCBR/2018.
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APL Photon. 5, 031302 (2020); doi: 10.1063/1.5138728 5, 031302-5
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