Long-Range (>100km) Distributed Vibration Sensor based on

Φ-OTDR Technique with Spread Amplification and Detection of

Probe Pulses

David Sanahuja

, Javier Preciado

2,3

, Jesús Subías

, Carlos Heras

, Lucía Hidalgo

, Iñigo Salinas

Pascual Sevillano

, Juan José Martínez

and Asier Villafranca

Departamento de Física Aplicada, Ciencias, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain

Departamento de Ingeniería Electrónica y Comunicaciones, EINA, Universidad de Zaragoza, María de Luna 1,

50018 Zaragoza, Spain

Aragón Photonics Labs (APL), Prado 5, 50009 Zaragoza, Spain

a.villafranca}@aragonphotonics.com

Keywords: Optical Fiber, Fiber-Optic Sensor, Distributed Sensor, Rayleigh Scattering, Coherent Optical Time Domain

Reflectometry (Φ-OTDR), Long-Range Distributed Sensing, Modulation Instability, Fading.

Abstract: This paper presents a set of results to demonstrate a long-range (>100km) distributed vibration sensor (DAS)

based on the Coherent Optical Time Domain Reflectometry (Φ-OTDR) technique using distributed

amplification of the probe pulses and detection of the backscattered traces, which demonstrates great

capability to achieve long-range distances with great sensitivity. In this case, optical amplifiers have been

placed along the sensing optical fiber, each one followed by a detection stage. Results for some traces detected

in each of the spans along the sensing fiber, and some measurements of stimuli produced by a vibration at the

end of each of the sections of the sensing fiber, are showed here. This work, framed in the project SACOH

(Long-range Distributed Vibration Sensing by Coherent Rayleigh Backscattering), has been carried out in

collaboration between the University of Zaragoza and the company APL (Aragón Photonics Labs).

1 INTRODUCTION

Distributed vibration sensing technologies have

greatly expanded their use in the recent years due to

the wide range of applications that they offer (Bao

and Chen, 2012). Among these applications

(monitoring of the integrity of civil engineering

structures and power plants, detection of leaks,

control of railways, traffic control...), it stands out

perimeter surveillance of infrastructures with a large

perimeter to be monitored. Surveillance strategies

based on conventional technologies (video

surveillance or conventional motion sensors…) are

no longer viable after a few kilometers due to the

large increase in their cost, because of the enormous

growth in complexity and in management problems

of monitoring such infrastructures when the length to

be monitored is increased. Therefore, the

development of new long-range detection strategies

such as those based on distributed optical sensing

technologies, in particular distributed vibration

sensing techniques, has great interest.

Distributed optical sensing technologies use

different measurement strategies, and their operation

is based on the use of a wide variety of physical

phenomenologies, mainly Rayleigh, Brillouin and

Raman optical dispersion. Systems based on Raman

and Brillouin scattering are mostly employed in the

monitoring of the integrity of large structures (Barrias

et al., 2016) through temperature and mechanical

stress measurements, while systems based on

Rayleigh scattering are mostly used in dynamic

scenarios typical of perimeter surveillance (Rao et al.,

2008).

Measurement systems based on coherent optical

reflectometry base their operation on Rayleigh

backscattering to detect disturbances due to stimuli

(vibrations or pressure changes) produced on the fiber

environment, by sensing local phase changes

(Muanenda, 2018). A pulsed laser with a highly stable

emission frequency and high coherence is used to

196

Sanahuja, D., Preciado, J., Subías, J., Heras, C., Hidalgo, L., Salinas, I., Sevillano, P., Martínez, J. and Villafranca, A.

Long-Range (>100km) Distributed Vibration Sensor based on F-OTDR Technique with Spread Ampliﬁcation and Detection of Probe Pulses.

DOI: 10.5220/0007386401960200

In Proceedings of the 7th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2019), pages 196-200

ISBN: 978-989-758-364-3

inject pulses that will produce Rayleigh

backscattering as they propagate along the optical

fiber. Interference patterns are detected, due to the

coherent sum of the backscattered wave fronts

throughout the pulse time, which is dependent on the

relative phase between the different backscattered

waves within the pulse itself. Variations in the phase

relationships between the elements that remain inside

the pulse at a point of the fiber, caused by some

disturbance at that point, have a direct effect on the

detected interference pattern and, therefore, on the

instantaneous optical power corresponding to that

position. This allows the localized detection of the

stimulus with a high sensitivity and resolution,

allowing a dynamic localized detection of the

presence of intruders even with a buried fiber optic

cable (Lu et al., 2010), being possible to use dark

buried fibers (greatly reducing the costs of the system

implementation), and in addition, isolating the cable

from external noise and protecting the cable from

environmental deterioration or manipulation by

intruders.

Figure 1: Direct-detecting Φ-OTDR measurement system.

According to literature, modulation instability

(MI) is one of the main phenomenons that limit the

maximum pump power that can be injected into the

sensing fiber (Martins et al., 2013), reducing,

therefore, the maximum distance at which the

systems based on the Φ-OTDR technique can locate

a stimulus in a distributed manner. High intensity

optical pulses are a way to achieve better

measurement conditions with a Φ-OTDR sensor:

improved resolution, more dynamic range and a high

quotient between signal and noise (SNR-Signal-to-

Noise-Ratio). MI is a non-linear phenomenon

resulting from an anomalous dispersion and the Kerr

effect that depends on the peak power of the injected

pulses and the optical noise generated when the

pulsed probe is amplified. The effect of MI appears

when injected optical intensity goes beyond certain

threshold, producing a fading at some regions of the

measured interferences and decreasing its visibility

hence, which implies a decrease in the sensitivity of

the detector system at corresponding positions in the

sensed optical fiber.

Fading effects, including the one induced by MI,

are one of the most limiting factors of the sensing

capability of systems based on coherent reflectometry

(Healey, 1984a; Healey, 1984b). Reduction of such

phenomenon would make it possible to greatly

increase the sensitivity and range of distributed

sensing systems based on Φ-OTDR technique, so

study and development of new strategies to reduce the

effects of MI is desirable. Since MI sets an upper

constraint on the optical intensity at the input of the

sensing fiber, amplifying the injected pulses in a

distributed way along the fiber, should be a reliable

strategy. In this work results are presented of

straightforward technique by introducing amplifiers

every certain distance.

2 EXPERIMENTAL SETUP

The general objective of this work is the validation of

a prototype measurement equipment for long-range

distributed vibration sensing based on direct-

detecting Φ-OTDR with a measuring range higher

than 100km, which will represent a clear advance on

the current state of technology (Liu et al., 2016; Liu

et al., 2018).

Figure 2: Experimental setup. Direct-detecting Φ-OTDR system with the addition of the distributed sensing system.

Long-Range (>100km) Distributed Vibration Sensor based on F-OTDR Technique with Spread Ampliﬁcation and Detection of Probe Pulses

197

The system to be validated is based on the use of

the conventional direct-detecting Φ-OTDR system

shown in Figure 1, with the addition of a new

architecture for the measuring set-up that allows to

maintain optical probe intensity below MI threshold

along the sensing fiber. The signal detection

infrastructure is distributed along the sensing optical

fiber to improve the range of the system without

impairing the performance and detection capabilities

of the system. To increase the range of the system,

three optical amplifiers (the first one of them inside

of the emission module) separated one from each

other along three spans (two optical fiber coils with a

length of 37km, and the third one with a length of

35km) are used, with each one of the optical

amplifiers followed by a detection optical amplifier

and a detector, so that the detection system is also

distributed along the sensing line.

Figure 3: 50m coil above the mechanical vibrator used to

generate the stimulus.

The basic scheme of the proposed new

architecture is shown in Figure 2.

This architecture allows to improve the range of

the system due to the optical amplifiers distributed

along the sensing fiber and without losing the quality

of the sensed signal thanks to the distributed

detection. As no section of the sensing system

exceeds 37km, the dynamic range of each detector

does not limit the maximum detection distance. In

addition, the pulse repetition period (PRP) is not

limited by the maximum distance of the sensing line,

allowing to have a high PRP rate, as the length of fiber

that the injected pulse has to propagate through in

each span is shorter, and therefore, the number of

captured traces is higher for a shorter acquisition

time, which can be translated as an improvement in

the sensitivity of the system. The proposed setup uses

a single laser source as emitter and a single optical

modulation element, which simplifies the system and

reduces its cost.

3 RESULTS

Below, there is a series of captures with some of the

obtained results with the aim of validating the Φ-

OTDR-based architecture proposed in this work.

Measurements have been made with a

conventional direct-detecting Φ-OTDR system

without the new architecture (Figure 1): traces along

the length of the three coils in a single phase, and a

measurement of the MI-induced fading by increasing

the pump power of the first optical amplifier (Figures

4 and 6).

Afterward, two more measurements were carried

out to validate the Φ-OTDR system with the new

architecture (Figure 2): traces detected in each span

(Figure 5) and sensitivity measurements (Figure 7) at

the end of each span (a mechanical vibration at a

specific point of sensing fiber, with well-controlled

displacement amplitude and frequency, around the

µm and 100Hz respectively, was used as a stimulus

(Figure 3)).

Figure 4 shows the interferences in a trace

corresponding to pulses with a pulse-width of 800ns

obtained by a conventional direct-detecting Φ-OTDR

system. As can be seen in the figure, interferences in

the trace are lost along the first coil until completely

disappear at 37km.

Figure 4: Detected interferences in a trace along the length (109km) of the three coils in a single phase.

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

198

Figure 5: Interferences recovered in each span by means of the distributed amplification and distributed detection system.

To prevent interferences from being lost along the

first 37km, the pump power was increased to increase

the power of the pulses which are injected into the

sensing fiber. Figure 6 shows how the phenomenon

of the MI-induced fading appears in the trace when

the input power in the fiber is increased, preventing it

from rising enough to obtain interference along the

109km of sensing fiber.

Due to the appearance of the MI-induced fading,

the detection range of the system is limited because

MI represents a limit to the increase in the pump

power of the optical amplifier until, as can be seen in

the figure below, the system ends up acting like a

conventional OTDR.

Figure 6: MI-induced fading in a trace.

Figure 5 shows how the interferences in the traces

in each of the three spans are recovered by means of

the distributed amplification and distributed detection

system.

When the pulses (in this case with a width of

800ns) are amplified in a distributed manner along the

sensing fiber and when the backscattered light is

detected in a distributed manner, it is possible to

reduce the pump power to avoid the negative effects

of the MI and it is possible to recover the traces in

each span of the distributed sensing system, as the

length of the fiber that each injected pulse has to

propagate through is not greater than 37km because

no section of the sensing system exceeds that length.

Because the sensing system with distributed

amplification and distributed detection allows to

recover the traces in each span, the quality of the

detected signal is recovered along the entire sensing

fiber, and it is possible to extend the detection

distance and detect a disturbance due to a stimulus at

greater distance.

As can be seen in Figure 7, it is possible to clearly

see the disturbance caused by a stimulus (in this case

a mechanical vibration) at 37km, 74km and 109km

(at the end of each span of the sensing line).

4 CONCLUSIONS

An analysis of the possibilities of using a distributed

detection architecture to overcome the limit in the

measurement range imposed by the modulation

instability in conventional Φ-OTDR systems has been

carried out.

Comparative results are presented between a

conventional architecture and a three-stage

distributed sensor scheme that includes probe pulses

amplifiers at the beginning of each span and

corresponding detection modules.

The capability of the distributed detection

architecture to measure disturbances at distances

which are higher than 100km with good sensitivity

and resolution is demonstrated, with the advantage of

being simpler than other architectures with

distributed Raman (Wang et al., 2014a) or Brillouin

amplification (Wang et al., 2014b), with offering

excellent expectations for future applications in the

field of long-range distributed sensing.

Figure 7: Mechanical vibration (100Hz, 3Vpp) at 37km, 74km and 109km.

Long-Range (>100km) Distributed Vibration Sensor based on F-OTDR Technique with Spread Ampliﬁcation and Detection of Probe Pulses

199

ACKNOWLEDGEMENTS

This work has been funded in part by the Ministerio

de Economía y Competitividad through the project

RTC-2016-5212-8, and in part, by the Government of

Aragón/European Regional Development Fund.

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