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Frequency scanned phase sensitive optical time-domain reflectometry interrogation in multimode optical fibers


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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.
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APL Photonics 5, 031302 (2020); 5, 031302
© 2020 Author(s).
Frequency scanned phase sensitive optical
time-domain reflectometry interrogation in
multimode optical fibers
Cite as: APL Photonics 5, 031302 (2020);
Submitted: 15 November 2019 • Accepted: 24 February 2020 • Published Online: 16 March 2020
K. Markiewicz, J. Kaczorowski, Z. Yang, et al.
Distributed acoustic sensing for seismic activity monitoring
APL Photonics 5, 030901 (2020);
Distributed optical fiber sensing: Review and perspective
Applied Physics Reviews 6, 041302 (2019);
Big data on the horizon from a new generation of distributed optical fiber sensors
APL Photonics 5, 020401 (2020);
APL Photonics LETTER
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
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:
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
., s
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
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APL Photonics LETTER
adoption of not only Rayleigh-scattering-based DOFS in multimode
optical fibers that are already deployed for telecommunication but
also Raman-scattering-based sensors.
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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APL Photonics LETTER
commonly deployed in buildings and short-range telecommunica-
tion links.
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
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APL Photonics LETTER
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
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
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APL Photonics LETTER
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
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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... Fiber optik multi-mode telah dipelajari sebagai platform yang kuat dan praktis untuk aplikasi optik nonlinier dan optik kuantum. Fiber optik multi-mode standar biasanya mendukung transmisi lebih dari 100 mode (Markiewicz et al., 2020). Keunggulan fiber multimode adalah tingkat masukan cahaya yang lebih tinggi dan peningkatan efisiensi penangkapan (Murray et al., 2018). ...
Full-text available
Many types of fiber optics have been developed and used by researchers to create best fiber. This study aimed to provide an overview of the the type of fiber optics based on the number of mode and refractive index. Also the study provide a breif comparison of these types with their strengths and weakness. A total 30 article published (from the year 2016 to 2020) in main journals wew investigated thoroughly through document analysis method. The study reveals some type of fiber optic. However, each types has some advantages as well as disadvantages over the others that should be kept in mind in their usages. A carefull user, such as researcher or others would be aware of the types and select the most effective one for his/her purposes.Keywords: Fiber optics, number of mode, refractive indexAbstrak: Banyak jenis fiber optik telah dikembangkan dan digunakan oleh para peneliti untuk menghasilkan jenis terbaik. Penelitian ini bertujuan untuk memberikan gambaran tentang jenis fiber optik berdasarkan jumlah mode dan indeks biasnya. Studi ini juga memberikan perbandingan singkat dari jenis-jenis ini dengan kelebihan dan kekurangannya. Sebanyak 30 artikel yang diterbitkan (dari tahun 2016 hingga 2020) di jurnal-jurnal utama yang telah diteliti secara menyeluruh melalui metode analisis dokumen. Studi ini mengungkapkan kelbihan dan kekurangan setiap jenis fiber optik. Namun, masing-masing jenis memiliki beberapa kelebihan dan kekurangan dibandingkan yang lain yang harus diingat dalam penggunaannya. Pengguna yang berhati-hati, seperti peneliti atau orang lain akan menyadari jenisnya dan memilih yang paling efektif sesuai dengan tujuan penggunaan.Kata-kata kunci: Fiber optic, jumlah mode, dan indeks bias
... Ces différences entre les constantes de propagation est par exemple problématique pour interroger les fibres multimodes à l'aide de méthodes OTDR (Optical Time-Domain Reflectometry), où la connaissance de la vitesse de propagation du signal est cruciale pour remonter à la position d'un défaut. Néanmoins, éliminer les modes d'ordre élevé à l'aide d'un filtre adapté [113] permet de réaliser de telles mesures. Dans un modèle à rayons, le diamètre de coeur des fibres multimodes implique également des variations de l'angle d'incidence sur le dioptre à l'extrémité de la fibre. ...
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URL : - EN : Refractive index measurement is a way to obtain information on a medium, such as the elements composing it, its temperature or its pressure. The so-called Fresnel sensor, based on the measurement of the reflected power at the end of a single-mode optical fiber, is a simple way to perform refractive index measurements, and is suitable for in-situ measurements of curing or water content in composite structures. Nevertheless, this point sensor only allows measurements on a small area at the end of the fiber, making it very sensitive to local disturbances. This thesis investigates the possibility of using special fiber structures as Fresnel sensors, from a theoretical and experimental point of view, in order to widen the field of possible measurements. Polarization maintaining optical fibers are first considered, with the objective to realize optical anisotropy measurements. Multimode fibers are then studied in order to significantly increase the measurement volume thanks to their large core diameter, and to validate the use of low cost plastic fibers as Fresnel sensors. Finally, the use of spectroscopy on a Fresnel sensor is considered, with the objective of using the possible absorption bands in a medium to obtain additional information about it, or to take into account the scattering of light by particles located at the end of the fiber. FR : La mesure de l'indice optique d’un milieu est un moyen d'obtenir des informations comme les éléments chimiques qui le composent, sa température ou sa pression. Le capteur dit de Fresnel, basé sur la mesure de la puissance réfléchie à l’extrémité d’une fibre optique monomode, est un moyen simple de réaliser des mesures d’indice optique, et est adapté à des mesures in-situ de réticulation ou de teneur en eau dans des structures composites. Néanmoins, ce capteur ponctuel ne permet de faire des mesures que sur une petite zone à l'extrémité de la fibre, le rendant très sensible aux perturbations locales. Cette thèse étudie la possibilité d’utiliser des structures de fibres spéciales en tant que capteur de Fresnel, d’un point de vue théorique et expérimental, afin d’élargir le champ de mesures possibles. Les fibres optiques biréfringentes sont d’abord considérées, avec l'objectif de réaliser des mesures d'anisotropie optique. Les fibres multimodes sont ensuite étudiées afin d'augmenter sensiblement le volume de mesure grâce à leur diamètre de coeur important, et de valider l’emploi de fibres plastiques à bas coût en tant que capteur de Fresnel. Enfin, l'utilisation de la spectroscopie sur un capteur de Fresnel est envisagée, avec l'objectif d'utiliser les possibles bandes d'absorption dans un milieu pour obtenir des informations supplémentaires sur ce dernier, ou bien de prendre en compte la diffusion de la lumière par des particules situées à l'extrémité de la fibre.
... The derived model is valid for any system for which the peak value of a resonance is evaluated through quadratic least-square fitting. In the case of coherent Rayleigh-based DOFS, for instance in direct-detection frequency-scanned φ-OTDR systems, the most widely and commonly-used method to estimate the relative value of the FS between reference and measurement signals is cross-correlation [18][19][20]. Cross-correlation is a standard method utilised for delay estimation in sonar and radar systems [21][22][23], and is also adopted in other coherent Rayleigh-based DOFS [24]. The presence of unavoidable additive noise in the traces being correlated fundamentally limits the performance of the cross-correlation estimator and leads to uncertainty in the estimated FS. Besides, other experimental parameters, such as spatial resolution, can also influence the accuracy of estimation. ...
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The phase change of back-scattered light due to external perturbations is retrieved in coherent Rayleigh-based distributed sensors by estimating the frequency shift (FS) between the traces of different measurements. The uncertainty associated with the estimator, due to the presence of system noises, can lead to an inaccurate evaluation of the FS. Additionally, in coherent Rayleigh-based sensors, the calculation of the signal-to-noise ratio (SNR) from the jagged back-scattered intensity trace using the statistical estimators can cause an erroneous determination of the absolute value of the SNR. In this work, a method to accurately evaluate the non-uniform SNR caused by the stochastic variation of the back-scattered light intensity along the fibre is presented and validated. Furthermore, an analytical expression to evaluate the uncertainty in the FS estimation using one of the standard estimators, namely cross-correlation, is presented. A direct-detection frequency-scanned phase-sensitive optical time-domain reflectometer (φ-OTDR) is employed for the experimental verification of the expression as a function of two crucial system parameters: the SNR and the spatial resolution. The performance of various distributed sensing system configurations utilising cross-correlation for determining the FS occurring due to the external perturbations can be properly predicted hereafter with the aid of the analytical expression presented in this study.
... Recently, multimode fiber has been reported to be used in φ-OTDR systems [19][20][21][22]. A typical multimode fiber can usually support hundreds of spatial modes, which cannot be easily and separately demodulated with standard photodetectors in conventional φ-OTDR systems. ...
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We propose and experimentally demonstrate a novel interference fading suppression method for phase-sensitive optical time domain reflectometry (Phi-OTDR) using space-division multiplexed (SDM) pulse probes in few-mode fiber. The SDM probes consist of multiple different modes, and three spatial modes (LP01, LP11a and LP11b) are used in this work for proof of concept. Firstly, the Rayleigh backscattering light of different modes is experimentally characterized, and it turns out that the waveforms of Phi-OTDR traces of distinct modes are all different from each other. Thanks to the spatial difference of fading positions of distinct modes, multiple probes from spatially multiplexed modes can be used to suppress the interference fading in Phi-OTDR. Then, the performances of the Phi-OTDR systems using single probe and multiple probes are evaluated and compared. Specifically, statistical analysis shows that both fading probabilities over fiber length and time are reduced significantly by using multiple SDM probes, which verifies the significant performance improvement on fading suppression. The proposed novel interference fading suppression method does not require complicated frequency or phase modulation, which has the advantages of simplicity, good effectiveness and high reliability.
We discuss the possible advantages of using few-mode and multimode fibers in coherent optical time-domain reflectometry sensing systems. We show that the application of few-mode and multimode fibers decreases nonlinear effects and impedes modulation instability. In comparison to standard single-mode optical fiber, few-mode and multimode fibers may help to increase signal power at the receiver. We also show that few-mode fibers may potentially increase sensing distance.
We demonstrate the highest, to the best of our knowledge, capacity obtained for an OM4 fiber reaching 1.6 Tb/s for 3.5 km of OM4 fiber. The result is obtained using a highly selective modal multiplexer exciting the LP 01 mode and a state-of-the-art coherent transmission setup operating in the C-band. The results indicate that there is no need to replace legacy optical fibers to increase their capacity, which may be an extremely important factor when considering the costs of upgrading existing networks.
The possibility to perform distributed measurements of the effective refractive index difference between distinct modes in few mode optical fibers is demonstrated using phase sensitive optical time domain reflectometry. Effective refractive index differences between LP02, LP21a and LP21b modes are measured with for a spatial resolution of 24m.
We propose and experimentally demonstrate a novel interference fading suppression method for phase-sensitive optical time domain reflectometry (φ-OTDR) using space-division multiplexed (SDM) pulse probes in a few-mode fiber. The SDM probes consist of multiple different modes, and three spatial modes (LP01, LP11a, and LP11b) are used in this work for the proof of concept. Firstly, the Rayleigh backscattering light of different modes is experimentally characterized, and it turns out that the waveforms of the φ-OTDR traces for distinct modes are all different and independent. Thanks to the spatial difference of the fading positions for distinct modes, multiple probes from spatially multiplexed modes can be used to suppress the interference fading in φ-OTDR. Then, the performances of the φ-OTDR systems using a single probe and multiple probes are evaluated and compared. Specifically, the statistical analysis shows that the fading probabilities over both the fiber length and the time scale are reduced significantly by using multiple SDM probes, which verifies the significant performance improvement on fading suppression. By introducing the concept of SDM to φ-OTDR, the proposed novel interference fading suppression method avoids the complicated frequency or phase modulation, which has the advantages of simplicity, good effectiveness and high reliability.
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Commonly, the frequency shift of back-reflection spectra is the key parameter to measure quantitatively local temperature or strain changes in frequency-scanned Rayleigh-based distributed fiber sensors. Cross-correlation is the most common method to estimate the frequency shift; however, large errors may take place, particularly when the frequency shift introduced by the temperature or strain change applied to the fiber is beyond the spectral width of the main correlation peak. This fact substantially limits the reliability of the system, and therefore requires careful analysis and possible solutions. In this paper, an analytical model is proposed to thoroughly describe the probability of large errors. This model shows that the cross-correlation intrinsically and inevitably leads to large errors when the sampled signal distribution is finite, even under perfect signal-to-noise ratio. As an alternative solution to overcome such a problem, least mean squares is employed to estimate the frequency shift. In addition to reducing the probability of large errors, the proposed method only requires to measure a narrow spectrum, significantly reducing the measurement time compared to state-of-the-art implementations. Both the model and the solution are experimentally verified using a frequency-scanned phase-sensitive optical time-domain reflectometry ( $\varphi$ -OTDR) system, achieving a spatial resolution of 5 cm, with a sensing range of 860 m and an acquisition time below 15 s, over a measurable temperature range of more than 100 K with a repeatability of 20 mK, corresponding to a temperature dynamic range of 5000 resolved points.
The diversity of spatial modes present within a multimode fiber has been exploited for a wide variety of imaging and sensing applications. Here, we show that this diversity of modes can also be used to perform quantitative strain sensing by measuring the amplitude of the Rayleigh backscattered speckle pattern in a multimode fiber. While most Rayleigh based fiber sensors use single mode fiber, multimode fiber has the potential to provide lower noise due to the higher capture fraction of Rayleigh scattered light, higher non-linear thresholds, and the ability to avoid signal fading by measuring many spatial modes simultaneously. Moreover, while amplitude measuring single mode fiber based Rayleigh sensors cannot provide quantitative strain information, the backscattered speckle pattern formed in a multimode fiber contains enough information to extract a linear strain response. Here, we show that by tracking the evolution of the backscattered speckle pattern, the sensor provides a linear strain response and is immune to signal fading. The sensor has a noise floor of 2.9 pɛ/√Hz, a dynamic range of 74 dB at 1 kHz, and bandwidth of 20 kHz. This work paves the way for a new class of fiber optic sensors with a simplified design and enhanced performance.
Considering middle-term predictions of the need for commercial 10-Tb/s optical interfaces working in 1-P b/s optical transport systems by 2024 and recalling the introduction of the optical bypass when entering the wavelength-abundant era in the early 2000s, we re-evaluate the values of hierarchical optical network architecture in light of the forthcoming massive spatial division multiplexing (SDM) era. We introduce a spatial channel (SCh) network (SCN) architecture, where the SDM layer is explicitly defined as a new networking layer that supports the new multiplexing technology of SDM. In an SCN, optical channels (OChs) accommodated in an express SCh bypass the overlying wavelength cross-connects (WXCs) using spatial cross-connects (SXCs) on the route. As one challenge that SCNs will present, we point out that an excessively large SXC insertion loss reduces the optical reach for spectrally groomed OChs. We show that the optical reach of spectrally groomed OChs can be maintained at almost the same level as that for an OCh transported through a conventional single-layer WDM network thanks to the low-loss features of commercially available spatial switches and foreseeable low-loss SDM multiplexers and demultiplexers. As another challenge, we discuss how to achieve growable, reliable, and cost-effective SXCs. We propose two SXC architectures based on sub-matrix switches and core-selective switches. Simple cost assessment shows that the proposed architectures are more cost-effective than the full-size matrix switch-based architecture with 1+1 equipment protection and conventional stacked WXC architecture.
Celebrating the 20th anniversary of Optics Express, this paper reviews the evolution of optical fiber communication systems, and through a look at the previous 20 years attempts to extrapolate fiber-optic technology needs and potential solution paths over the coming 20 years. Well aware that 20-year extrapolations are inherently associated with great uncertainties, we still hope that taking a significantly longer-term view than most texts in this field will provide the reader with a broader perspective and will encourage the much needed out-of-the-box thinking to solve the very significant technology scaling problems ahead of us. Focusing on the optical transport and switching layer, we cover aspects of large-scale spatial multiplexing, massive opto-electronic arrays and holistic optics-electronics-DSP integration, as well as optical node architectures for switching and multiplexing of spatial and spectral superchannels.
We propose and demonstrate a method to perform quantitative phase-sensitive optical time domain reflectometry (Φ-OTDR) using multimode fiber. While most Φ-OTDR sensors use single-mode fiber, multimode fiber exhibits higher thresholds for non-linear effects, a larger capture fraction of Rayleigh backscattered light, and the potential to avoid signal fading by detecting many spatial modes in parallel. Previous multimode fiber based OTDR sensors discarded most of the backscattered light and thus failed to take advantage of these noise-reducing factors. Here, we show that by performing off-axis holography with a high-speed camera, we can record the entire Rayleigh backscattered field, maximizing the detected light level and making the sensor immune to fading. The sensor exhibits a high degree of linearity, a minimum phase noise of -80 dB [rel. rad2/Hz], and 20 kHz bandwidth.
A method based on coherent Rayleigh scattering distinctly evaluating temperature and strain is proposed and experimentally demonstrated for distributed optical fiber sensing. Combining conventional phase-sensitive optical time-domain domain reflectometry (φOTDR) and φOTDR-based birefringence measurements, independent distributed temperature and strain profiles are obtained along a polarization-maintaining fiber. A theoretical analysis, supported by experimental data, indicates that the proposed system for temperature-strain discrimination is intrinsically better conditioned than an equivalent existing approach that combines classical Brillouin sensing with Brillouin dynamic gratings. This is due to the higher sensitivity of coherent Rayleigh scatting compared to Brillouin scattering, thus offering better performance and lower temperature-strain uncertainties in the discrimination. Compared to the Brillouin-based approach, the φOTDR-based system here proposed requires access to only one fiber-end, and a much simpler experimental layout. Experimental results validate the full discrimination of temperature and strain along a 100 m-long elliptical-core polarization-maintaining fiber with measurement uncertainties of ~40 mK and ~0.5 με, respectively. These values agree very well with the theoretically expected measurand resolutions.
We propose and evaluate performance of the higher order mode filter for 850 nm multimode fiber transmission. The developed passive component reduces impact of mode dispersion on the systems performance. Excellent operation in the 850 nm transmission experiments is shown.
We investigate the influence of air holes on phase sensitivity in microstructured optical fibers to longitudinal strain. According to the numerical simulations performed, large air holes in close proximity to a fiber core introduce significant compression stress to the core, which results in an increase in the effective refractive index sensitivity to longitudinal strain. The theoretical investigation is verified by an experiment performed on four fibers drawn from the same preform and differentiated by air hole diameter. We show that introducing properly designed air holes can lead to a considerable increase in normalized effective refractive index sensitivity to axial strain from -0.21 e-1 (for traditional single mode fiber) to -0.14 e-1.
Space division multiplexing (SDM) is mainly seen as a means to increase data throughput and handle exponential traffic growth in future optical networks. But its role is certainly more diverse. Research on SDM encourages device integration, brings newfunctionality to network elements, and helps optical networks to evolve. As a result, the number of individual components in future networks will decrease, which in turn will improve overall network reliability and reduce power consumption as well as operational expenditure. After reviewing the state-of-the-art in SDMfiber research and development with a particular focus on weakly coupled single-mode multi-core fibers, we take a look beyond the capabilities of SDM as a means of boosting transmission capacity and discuss ideas and concepts on howto exploit the spatial dimension for improved efficiency and resource sharing in optical networks.