Research ArticleDistributed Intrusion Sensor Using DFB Laser with OpticalFeedback and Saturable Absorber

Kyoo Nam Choi

Department of Information and Telecommunication Engineering, Incheon National University, Incheon 22012, Republic of Korea

Correspondence should be addressed to Kyoo Nam Choi; knchoi@inu.ac.kr

Received 18 September 2017; Revised 14 December 2017; Accepted 26 December 2017; Published 8 February 2018

Academic Editor: Stefano Stassi

Copyright © 2018 Kyoo Nam Choi. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Characteristics of a distributed intrusion sensor using a coherent DFB laser diode with an external optical feedback andsaturable absorber were experimentally investigated. The stimulus at a location of 2 km using a PZT transducer placed thelocation of a simulated intruder in Φ-OTDR trace after averaging 32 times. Field trials demonstrated the detection of avehicle and a pedestrian crossing above the sensing line and a loop in a burial depth of 50 cm. This distributed intrusionsensor using a coherent DFB laser diode as the light source had the advantages of a simple structure and intruderdetection capability at the underground burial location.

1. Introduction

The intrusion sensors to locate an intruder’s position aredrawing increasing attention in the security industry.However, it requires multiple point sensors to extend thedetection range using conventional point sensors [1]. Thus,it has been desired to detect an intruder with location infor-mation using a single sensor. For this purpose, the distributedsensor became the proper choice [2, 3]. Also, it is necessary toconceal the intrusion sensor in an unexposed location,such as buried underground [4]. The sensitivity of a sensordecreases considerably as the burial depth increases [5]; thus,the sensitivity needs to compensate for the attenuation due toabsorption in soil. In this respect, the coherent fiber opticalsensor is a suitable candidate in terms of sensitivity [6]. How-ever, the resolution of a coherent optical sensor depends onthe coherence length of a light source [7]. Various conceptshave been proposed regarding intrusion sensors. A singleintrusion sensor using a leaky coaxial cable has been demon-strated to provide intrusion location along a perimeter sensorcable with shallow burial [8], and the coupled wave device(CWD) using a perimeter guided radar detection systemhas shown the capability to detect over complex geometryabove ground [9]. However, these wired sensing methodshave a relatively short detection range, below km scale, due

to attenuation in copper wire. In the case of using multimodeoptical fiber, an intrusion sensor using Speckle patternvariation showed moderate sensitivity, but without locationinformation [10]. The single-mode optical fiber intrusionsensor using Mach-Zehnder interferometry [11] or fiberBragg grating (FBG) showed exceptional sensitivity, but alsowithout intruder location information. Thus, multiple sen-sors were required using cascaded or array architecture toprovide intruder location information. The intrusion sensingtechnique using Rayleigh scattering in a single-mode opticalfiber had the advantage of providing distance information;however, the sensitivity in optical time domain reflectometry(OTDR) configuration [12] was not sufficient for under-ground burial application. The phase-sensitive Φ-OTDRhad fully distributed extremely high sensitivity with accuratelocalization and simultaneous multipoint detection capabil-ity in comparison to other optical fiber intrusion sensors.However, it used a fiber laser with an internal active medium[13], which is exposed to ambient air. And the ambientfactors such as airflow, vibration, and temperature inherentlyinfluenced the stability of the coherent laser. Thus, the opticalfiber intrusion sensor with a compact coherent light sourcewith an integrated active medium has been preferred foroutdoor field intrusion sensor application. Our presentinterest is to realize an optical fiber intrusion sensor with a

HindawiJournal of SensorsVolume 2018, Article ID 4848254, 11 pageshttps://doi.org/10.1155/2018/4848254

compact coherent light source, which is immune to ambi-ent factors in comparison to a coherent fiber laser, forfield application.

This paper is about a distributed phase-sensitive Φ-OTDR intrusion sensing system with a compact coherentlight source using a distributed feedback semiconductor laserwith an optical feedback and a saturable absorber.

2. Distributed Optical Fiber Sensor

2.1. Coherent Φ-OTDR. The architecture of coherent Φ-OTDR is similar to conventional OTDR for locating faultpoints of communication optical fiber. The difference is thatthe coherent Φ-OTDR requires a light source with a longercoherence length and narrower linewidth. The light from acoherent light source is modulated using an external modula-tor, and it is converted to continuous short optical pulsetrains. Then, these light pulse trains are optically amplifiedto transmit through a long-haul optical fiber sensing line.The narrow light pulse from an optical fiber amplifier isdivided by a 3 dB optical fiber coupler to transmit an opticalprobe pulse and to receive the returned sensing signalsimultaneously. The optical sensing signal returned byRayleigh backscattering is converted into an electricalwaveform in a PIN photodetector. The single-mode opticalfiber for optical communication is used to sense an intruder.Then, the electrical signal is amplified and filtered to enhancethe signal-to-noise ratio. Furthermore, the signal is averagedto reduce jagged waveform and processed to deduce theintruder location information. The concept of coherentΦ-OTDR to discriminate the presence of an intruder isshown in Figure 1.

In case where the light pulse travels the distance throughan optical fiber core, a part of the light is scattered by volumehaving a refractive index difference in comparison to therefractive index of the optical fiber core. If the incident lightintensity is Ii, then the total scattered light intensity Is, whichis reflected by j number of scattering objects, can be estimated[14, 15]. The ratio, Rbs, of scattered light power, ΔP, perincident light power, P(z), at distance z in a single-mode fibercan be approximately estimated, as shown in (1), because theattenuation of optical fiber is mostly due to Rayleigh scat-tering. In (1), F represents the ratio of backscattered lightwhen incident optical power has the Gaussian, and α isthe attenuation coefficient [16].

Rbs ≈ F ⋅ΔPP z

= F ⋅ α ⋅ Δz 1

The distance, L, to the intruder location is calculated bythe time difference between transmitted, TT, and received,TR, light pulses using group velocity, vg, of light inside thesensing fiber core, as shown in (2). Also, the distance resolu-tion, Δz, depends on pulse width, Tpw, of the transmitted lightpulse, as shown in (3). The received power, Pbs, by backscat-tering at distance L from the transmitter, can be expressed asshown in (4), when the incident optical power is Pi. Thus, thesensitivity of a distributed optical fiber sensor is proportionalto transmitted optical power and pulse width. If we decrease

the pulse width to increase the distance resolution, it willresult in decreased signal reception. Thus, an optical poweramplifier is essential in a distributed intrusion sensor.

L = vg ⋅TR − TT

2 , 2

Δz ≈Tpwvg2 , 3

Pbs ≈12 ⋅ F ⋅ α ⋅ vg ⋅ Tpw ⋅ Pi ⋅ e

−2αL 4

The coherent Φ-OTDR has a significant advantage overconventional OTDR in terms of sensitivity; however, itrequires the light source to have a narrow linewidth as wellas low frequency drift. The backscattered light wave in coher-ent Φ-OTDR forms a random state of polarization; thus, thereception signal shows a jagged waveform. The intruderinduces the phase modulation to backscattered signal insidethe optical fiber core, and this phase modulated signal canbe expressed as a nonstationary autocorrelation function,Ryy, as shown in

Ryy t, t − T = E yi t ⋅ yj t − T 5

The envelop of a phase modulated signal shows Rayleighdistribution, because y(t) is a narrowband noise process [17].The jagged waveform due to Rayleigh distribution andfading can be reduced by filtering and averaging whenthe variation is within a tolerable range. Ambient factors,such as vibration and instant temperature variation due toairflow, influence phase stability of the light source, becausethey modulate the optical phase inside the optical fiber core.The performance of an intrusion sensor using a Φ-OTDRconfiguration requires not only longer coherence length butalso immunity to ambient factors, and those are directlyrelated to measurement precision.

Sensingopticalfiber

Photodetector

3dBcoupler

PZT

Indexmatching

fluid

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Externalmodulator

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Display

Figure 1: The block diagram of coherent Φ-OTDR distributedsensor.

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2.2. Coherent Light Source. Spectral linewidth is an importantparameter of a fiber-optic distributed intrusion sensor. Thelinewidth requirement for coherent optical communicationis less than 500 kHz [18] and tens of kHz for fiber-optic gyroapplication [19]. The fiber-optic intrusion sensor showeddistributed sensing capability using a fiber laser with a<3 kHz spectral linewidth [7]. The experimental spectrallinewidth value of an AlGaAs/GaAs CSP laser is ~38MHzat 3mW [20]. A distributed feedback (DFB) laser has anactive region with a periodic structure as a diffraction grating,and this grating behaves not only as a wavelength selectionelement but also as a resonator to produce a single longitudi-nal mode, in contrast to a conventional Fabry-Perot laser. Aconventional single-frequency tunable DFB laser diode has aspectral linewidth of ~2MHz, and this is regarded as toobroad for fiber-optic distributed sensor application. A longerlaser cavity and higher facet reflectivity are needed fornarrow linewidth [21]; however, this leads to laser modehopping and multilongitudinal mode oscillation in case of aFabry-Perot cavity. The linewidth Δν of the tunable DFBlaser diode is approximate as shown in (6), where α0 is thesum of absorption loss and scattering loss, κ is the couplingcoefficient, and L is the cavity length [22]. This shows thatthe higher κ and longer L are needed to shorten spectrallinewidth. Also, the external optical feedback to a DFB laserdiode affects the linewidth of the laser [23]. The absorptioncoefficient varies according to the absorbed light quantity ina saturable absorber [24], and this can affect the spectrallinewidth in the lasing cavity.

Δν∝ α0κ2L3

6

A coherent light source is fabricated using a DFB laserdiode, antireflection coated on one facet side and high-reflection on the other facet side to form an externaloptical cavity between an external fiber Bragg gratingand DFB laser diode. An optical feedback loop is formedby using an optical circulator. The amount of optical feed-back is adjusted by using a variable optical attenuator, andit is monitored using an optical fiber coupler. The centerwavelength of the external fiber Bragg grating is selectedto match the lasing wavelength of the DFB laser diode.Furthermore, the lasing wavelength of the DFB laser diodeis precisely adjusted to the center wavelength of the exter-nal fiber Bragg grating by adjusting the temperature of theDFB laser diode. The unconnected fiber end is immersedin index matching fluid to avoid unwanted reflection.Two saturable absorbers are inserted in the lasing cavityto further reduce the spectral linewidth, as shown inFigure 2. The erbium-doped optical fiber is used as thesaturable absorber.

2.3. Spectral Linewidth Measurement. The wavelengthresolution of a conventional optical spectrum analyzer is~0.1 nm; thus, a self-heterodyne measurement setup isneeded to measure the extremely narrow linewidth [25].The resolution of the self-heterodyne measurement setupis determined by optical fiber delay line length, L, wherethe delay time, τdelay =ng/c, is ~5 μs/km×Lkm. In theequation, c is the speed of light, and ng is the group refractiveindex. The free spectral range (FSR), ΔνFSR, is represented asc/(ngLc), where Lc is the coherence length. Thus, the spectralwidth exceeding ΔνFSR can be measured within the resolu-tion limit. The −3 dB spectral width, ΔλFWHM, of a lightsource can be calculated from coherence length usingλo

2/(πLc) [26]. An optical modulator can be used to shiftthe center frequency to display the spectral waveform ona RF spectrum analyzer.

AR coatedfacet

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Temperaturecontroller

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attenuator

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Opticalpowermeter

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fluid

Figure 2: The block diagram of coherent light source.

DFB LD

10/90 coupler

Saturable absorber

3dB coupler Current controlOptical circulatorFBG

Variable attenuator Saturable absorber

Figure 3: Top view of coherent light source.

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3. Experiment

3.1. Coherent Light Source. The DFB laser diode withλc = 1530.618 nm center wavelength and Δλ = 0.02 nmwavelength width was used as the pumping source. AnFBG with λc = 1530.32 nm center wavelength at 23.5°Cand Δλ=0.99nm wavelength width was used as the reflec-tor to form an external Fabry-Perot cavity. The lasingwavelength of the DFB laser diode was adjusted to thecenter wavelength of the FBG by controlling the temperatureof the DFB laser diode. The exact matching of two centerwavelengths is essential to the performance of a coherentlight source. Two erbium-doped fibers, each 1-inch long,having −7.5 dB/m loss at 1530 nm, were used as saturableabsorbers. Also, a variable optical attenuator with anattenuation range of 0~60 dB was used to provide opticalfeedback. The optical power of the coherent light source withan external Fabry-Perot cavity was reduced to ~5% of theDFB laser diode alone. The assembled coherent light sourceis shown in Figure 3.

3.2. Spectral Linewidth Measurement. In self-heterodynemeasurement setup, the phase and frequency fluctuation ofan incident optical signal is converted to an amplitude signal,and it is measured using a RF spectrum analyzer. In theMach-Zehnder interferometer configuration of a self-heterodyne measurement setup, the incident optical signalmeets its identical signal through a fiber-optic delay line. Atthis time, two signals interfere with each other and are con-verted from linewidth information to RF power spectrum.The carrier frequency ωc, 100 kHz in this experiment, wasapplied to one side of the fiber-optic line to shift the centerfrequency of the interference signal for viewing in a RF spec-trum analyzer. In the self-heterodyne measurement setup, ifthe length of a fiber-optic delay line is longer, the wavelengthresolution becomes better. At the same time, the opticalsignal power through the fiber-optic delay line needs to beabove the noise equivalent power (NEP) value of an opticalreceiver. When the length of a fiber-optic delay line is equalor shorter than the linewidth of a light source, the phase

of the interference signal becomes partially correlated.The fiber-optic delay line length of the self-heterodynemeasurement setup was selected as 63 km, which is equiva-lent to a ~0.315ms time delay.

In case of no optical feedback, the linewidth will beidentical to the linewith of the long cavity DFB laser diode.In the case of excess optical feedback, instability will beincreased due to spatial hole burning (SHB) and modehopping. Thus, it is necessary to adjust the amount of opticalfeedback to achieve the minimum linewidth. In the firstexperiment, the optical feedback, PFB, was adjusted to−27 dB of optical output power, and the RF spectrum of theself-heterodyne measurement setup is shown in Figure 4. Inthe second experiment, the amount of optical feedback, PFB,was increased to −17 dB of optical output power withoutthe two saturable absorbers. In this case, the linewidth wasenhanced, as shown in Figure 5. In the third experiment,two saturable absorbers were added to the second case,as shown in Figure 2, and then the measured linewidthshowed better enhancement, as shown in Figure 6.

The coherent light source with two saturable absorbershad the most narrow linewidth among the three measure-ments. The measured −3 dB linewidth in Figure 6 is~3 kHz, which is the resolution limit of the self-heterodynemeasurement setup. The actual linewidth is considered tobe below 3 kHz. The 3 kHz linewidth is the FSR, ΔνFSR =c/(ngLc), which is equivalent to ~1× 10−8 nm usingΔλFWHM =λo

2/(πLc).

3.3. Coherent OTDR. The coherent light source was encasedto isolate it from an outside airflow effect. It was double-wrapped with cotton wool and sponge in a styrofoam case.The light from the coherent light source was amplified30 dB using an optical amplifier. Two cascaded opticalsingle-mode fiber spools, each 2 km and 10 km length, wereused as simulated sensing lines. The optical fiber spools wereencased in similar shock absorbing and temperature-stablecases. The dimensions of the case were L60×D60×H60 cm.The PZT (lead zirconate titanate) was used to simulate theintruder and located between 2 km fiber spool and 10 km

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Figure 4: RF spectrum of self-heterodyne setup, when PFB =−27 dB.

4 Journal of Sensors

fiber spool. The phase modulation of the PZT was adjusted to2/3 level of maximum output. The 3 dB optical fiber couplerwas used to transmit and receive optical signals simulta-neously. The signal received was amplified 33 dB using low-noise amplifier (LNA) and filtered by using low-pass filter(LPF) with variable cutoff frequency capability. Finally, thesignal was averaged to minimize signal fluctuation. Theexperimental setup for measurement is shown in Figure 7.

The peak signal at the 2 km location from the startingpoint in the 12 km-long sensor line showed the sensingof the simulated intruder by the PZT transducer. In theΦ-OTDR trace, as shown in Figure 8, the 40 μs point isthe starting location, and the 60 μs point is the 2 km location.The elapsed time of 20 μs is equivalent to the round-trip timethrough the 2 km fiber-optic sensor line. Vehicle trafficoutside the building and pedestrians walking inside thebuilding both behaved as environmental noise, even thoughthe optical fiber spools were contained in shock absorbingboxes. Thus, the measurement was performed at midnightwhen there was no vehicle traffic or pedestrians walking.The noise-like waveform in Figure 8(b) is due partly to thefrequency instability of the light source and partly to envi-ronmental noise sources.

3.4. Intrusion Sensor. A field test was conducted to evaluatethe performance of the intrusion sensor. A single-modeoptical fiber of optical communication grade was used asthe sensing line. The sensing line, with a total length of100m with a 3mm diameter jacket, 44m as buried and56m as an outside connecting line, was inserted at a location

Frequency (5 kHz/div)

Linewidth measurement (FB = −17 dB) withsaturable absorber

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put A

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75 80 85 90 95 100 105 110 115 120 125

Figure 6: RF spectrum of self-heterodyne setup, with saturable absorbers, when PFB =−17 dB.

Frequency (10 kHz/div)

Linewidth measurement (FB = −17 dB)without saturable absorber

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Figure 5: RF spectrum of self-heterodyne setup, without saturable absorber, when PFB =−17 dB.

Fiber sensor 2 km

Coherentlight source

Fiber sensor 10 km

Figure 7: Experimental setup of distributed intrusion sensor.

5Journal of Sensors

2 km from the light source and 12 km in total length. Thesensing line was buried at a depth of 50 cm and backfilledwith soil. The burial layout of the fiber-optic sensing cableis shown in Figure 9(a), and a photo showing the backfilledsurface is shown in Figure 9(b). A vehicle weighing~1500 kg and a pedestrian weighing ~60 kg were used assample intruders. The test was carried out for two cases:one crossing the whole sensing loop and the other crossingone sensing line only.

In case of crossing one sensing line, the detectionresponse for the vehicle created notches around the sensingpeak, while the response for the pedestrian showed the onedominant peak. The phase-sensitive Φ-OTDR traces forthese two cases, after averaging 32 times, are shown inFigures 10 and 11. By comparing the traces betweenFigures 10(b) and 11(b), the peak output values were found

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Figure 8: Φ-OTDR trace of distributed intrusion sensor. (a) Waveform before detection (dot) and after detection (line). (b) Differencewaveform.

(a) (b)

7 m

8 m

13 m

4 m5 m

Indoor

Sensingarea

Figure 9: Layout of buried sensing cable. (a) Dimension. (b)Surface soil.

6 Journal of Sensors

to be similar. This can be explained as both phase shifts,by vehicle and pedestrian, were more than 2π rad., whichwas enough to be detected. The difference betweenFigures 10(b) and 11(b) indicates that the impact of thevehicle was strong enough to influence both optical fiberspools, which are 12 km long, even though the spools werecontained in shock resistance containers. However, onemomentary movement by the pedestrian located theintruder at the 2 km location. The noise-like waveform inthe other location, as seen in Figure 11(b), is due to fre-quency instability of the light source and environmentalnoise sources.

In the case of crossing the whole sensing loop, the sensingloop started to sense from the vicinity of the loop in bothcases of vehicle and pedestrian. And it influenced the twooptical fiber spools, simulating 12 km-long sensor lines,

although the fiber spools were isolated in shock absorbingboxes. The phase-sensitive Φ-OTDR traces for these casesafter averaging 32 times are shown in Figures 12 and 13.The vehicle driving over the sensing loop showed a broaderpeak signal at the 2 km location due to the averaging effect,as shown in Figure 12(b). However, the continuous walkingmotion by the pedestrian influenced not only the buriedsensing loop, but also the two optical fiber spools, as seen inFigure 13(b). Also, the response by driving the vehicle100m away from the field test bed was observed in a similarpattern as the pedestrian walking over the sensing loop.

The appearance of the Φ-OTDR trace was influenced bynumber of averaging times. Without averaging, it clearlyshowed a full swing variation at the impact location, widein the case of vehicle and narrow in the case of pedestrian.This means phase changes of more than 2π rad. in both cases.

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Figure 10: Phase-sensitive OTDR trace by vehicle in case of crossing one sensing line. (a) Waveform before detection (dot) and afterdetection (line). (b) Difference waveform.

7Journal of Sensors

After averaging 256 times, the trace of the phase-sensitiveΦ-OTDR response converged to a typical OTDR trace.The induced phase change is close to a linear functionof applied weight for a given depth. In the case of thepedestrian weighing 60 kg over a sand box, the phasechange is 6π rad. at a burial depth of 20 cm and 2.5πrad. at a burial depth of 40 cm according to the simulationresults [27]. This implies that the phase changes by bothvehicle and pedestrian are equal to or exceed 2π rad.The sensitivity of this distributed optical fiber intrusionsensor was found to be too high to experimentally measurethe distance resolution unless the entire 12 km sensingoptical fiber was buried at a sufficient depth to cause less thanπ/2 rad. phase shift. However, the distance resolution can beinferred to be ~160m from a 3dB width in Figure 11(b),when the light pulse width is 1.5 μs.

The spatial resolution of the system is proportional to thepulse width of incident light to optical fiber, as shown in (3),in the case where the backscattered light is sufficient enoughfor detection. And the shorter the length of this distributedsensor is, the larger the backscattered light is. Also, the largerthe output power of the optical fiber amplifier is by cascadingthe other optical fiber amplifier, the larger the backscatteredlight is. Thus, the spatial resolution of the system is inverselyproportional to the length of the sensing line. The minimumspatial resolution was measured as ~40m when the pulsedlight output, with 0.3 μs pulse width, was amplified +20dBby cascading the second optical fiber amplifier for the12 km sensing length.

The sensitivity of the system is a function of sensinglength, burial depth, optical output power, and opticalpulse width. By using the configuration in Figure 1 with

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Figure 11: Phase-sensitive OTDR trace by pedestrian in case of crossing one sensing line. (a) Waveform before detection (dot) and afterdetection (line). (b) Difference waveform.

8 Journal of Sensors

the 12 km sensing line, adequate phase changes wereproduced for burial depths within 50 cm in the experi-ment. To predict the maximum sensing length, an analyt-ical model using the Monte Carlo method was used byassuming the random locations of the scattering centersin the optical fiber. Twelve laser pulses were averaged forthe signal level in each time bin. The number of missedintruders was determined in 100,000 trials. For a missedprobability of 10−4 and a false alarm rate of 10−10, thesensing length was estimated as 50 km at a spatial resolu-tion of 160m by using the same parameter for spatialresolution measurement.

This distributed Φ-OTDR intrusion sensor system issuitable for real-time monitoring of multiple intruders andthe most cost-effective solution for long perimeter intrusiondetection. And the coherent light source using the DFB laser

diode with external optical feedback has the advantage ofsimpler architecture and is immune to environmental noisesin comparison to other coherent light source using a fiberlaser. The degree and stability of coherence length are criticalto the performance of a Φ-OTDR intrusion sensor system.The coherence length variation due to environmental param-eters fluctuates the phase of backscattered light, and thisrequires increasing the averaging number and further signalprocessing to reduce errors, resulting in a delayed response.

4. Conclusion

A distributed intrusion sensor using a DFB laser with exter-nal optical feedback and saturable absorber was fabricatedfor distributed intrusion sensor application. This light sourcehad the advantage of a simple structure in comparison to a

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Figure 12: Phase-sensitive OTDR trace by vehicle in case of crossing whole sensing loop. (a) Waveform before detection (dot) and afterdetection (line). (b) Difference waveform.

9Journal of Sensors

fiber laser, but with longer coherence time than a DFB laseralone. The linewidth of the light source was measured to bebelow the resolution limit of the self-heterodyne measure-ment setup, which is ~3 kHz FSR or ~1× 10−8 nm. Thedistributed intrusion sensor was simulated using twocascaded 2 km and 10 km fiber-optic spools. The intruderlocation was simulated at a location 2 km from the startingpoint. The distributed intrusion sensor in the laboratorysetup showed sensible sensitivity to PZT stimulus. A field testwas carried out by using two cascaded 2 km and 10 km fiber-optic spools. The intruder location was simulated at a loca-tion 2 km from the starting point by inserting a 100m opticalfiber with 3mm diameter jacket. The test was carried out fortwo cases; one crossing the sensing loop and the other cross-ing one line only at a burial depth of 50 cm. Two cases of

intruders, a vehicle and a pedestrian, were attempted in thefield experimental test. A single movement by the pedes-trian across a sensing line or driving across a sensing loopby the vehicle induced the detection peaks at the 2 kmintruder location. The induced phase changes by bothintruders are considered to be above 2π rad. from themeasured responses. However, continuous movement byboth the pedestrian walking and vehicle driving over asensing loop induced multiple peaks around sensing peakdue to the influence of optical fiber spools. The spatial reso-lution was measured as ~40m for the 12 km sensing length,and the usable sensitivity was found to be up to a burial depthof 50 cm. The maximum sensing length was estimated to beup to 50 km at a spatial resolution of 160m according tothe simulation results.

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Figure 13: Phase-sensitive OTDR trace by pedestrian in case of crossing whole sensing loop. (a) Waveform before detection (dot) and afterdetection (line). (b) Difference waveform.

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Conflicts of Interest

The author declares that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

This work was supported by Incheon National UniversityResearch grant in 2016.

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