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Sensors and Actuators A 136 (2007) 238–243

New sensing mechanisms using an optical time domainreflectometry with fiber Bragg gratings

Lo Yu-Lung a,b,∗, Xu Shao-Hung a

a Department of Mechanical Engineering, National Cheng Kung University, Taiwanb Advanced Optoelectronic Technology Center, National Cheng Kung University, Taiwan

Received 25 July 2006; received in revised form 16 November 2006; accepted 21 November 2006Available online 14 December 2006

bstract

A new optical time domain reflectometry (OTDR) sensing mechanism combined with fiber Bragg gratings (FBGs) is presented. The intensity-ased optical fiber sensor is implemented between a pair of FBGs in an OTDR system. Based on the ratio from two reflecting intensities of theeference and the sensing FBGs, the appropriate measurable parameters associated with the intensity variations can be detected. By applying theresnel reflection signals caused by FBGs as the measuring indicator, the dynamic range and signal-to-noise ratio of an OTDR FBGs system are

mproved in comparison to the traditional method. Besides, by adapting the system using a double-light-source scheme, the spatial resolution

imited by the ghost zone in a traditional OTDR system is also resolved.

The sensors in an OTDR FBGs system could be configured for multiplexing and immunity to the variations from the light source and theead-in/out fibers caused by the external loads. The new method using single-light-source and double-light-source schemes has been validated byber bending loss and refractive index measurements.2006 Elsevier B.V. All rights reserved.

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eywords: Fiber Bragg grating (FBG); Optical time domain reflectometry (OT

. Introduction

The OTDR is an effective optical testing technique devel-ped in 1976 [1] which is capable of locating and characterizingber links by detecting the Rayleigh backscattering and com-on reflection signals. The advances of this technique have also

een reviewed by Tateda and Horiguchi [2]. Since the first intro-uction of this technique, many researches have been done tochieve better performance of this measuring system, such asxtending measurable range [3], improving spatial resolution4], and eliminating dead zone effect [5]. The most distinctuperiority of an OTDR over other optical test techniques ishat an OTDR only needs to accesses one end of fiber link. Thisdvantage makes an OTDR especially suitable for multipointonitoring of long length fiber links such as these embedded in

ivil structures or power cables.Traditional OTDR sensors utilize the Rayleigh backscattering

ower level to indicate the sensing condition [6–8]. Knowles et

∗ Corresponding author.E-mail address: loyl@mail.ncku.edu.tw (Y.-L. Lo).

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924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2006.11.024

Fiber bending loss measurement; Refractive index measurement

l. [7] used sawtooth shaped fiber clampers to induce microbend-ng corresponding to applied pressure. But owing to the intrinsiceakness of the backscattering signal, the measured fiber length

s limited and sensitive detectors are required for a good stabil-ty of the measurement. Other OTDR methods [8,9] which usedhe Fresnel reflection as the sensing indicator have also beeneported. Gu et al. [8] used fiber splices to create Fresnel reflec-ion for multipoint reinforced concrete strain monitoring. Yangt al. [9] used right angle prisms to detect the liquid level. How-ver, Fresnel reflections tend to exceed the dynamic range ofTDR system, which may compare its reflectivity of the sens-

ng configuration. In any OTDR configuration, there is a tradeoffetween the spatial resolution and the laser pulse width; there-ore, in the case of multipoint sensing, this issue must be takingccount. Recently, Zhang et al. [10] proposed a new OTDR sys-em that was used to interrogate the multiplexing low-reflectanceragg-grating-based sensors. The interrogation principle of theBG is similar to the edge-filter interrogation method and is

ased on the overlapping integral of an edge spectrum of theTDR source and FBG spectra.

In this study, we integrate a pair of FBGs with an OTDRs a new sensing scheme for multiple intensity-based sensors

Y.-L. Lo, S.-H. Xu / Sensors and Actuators A 136 (2007) 238–243 239

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Fig. 1. Schematic diagram o

ased on the idea proposed by Lo et al. [11]. A pair of FBGs areesigned as reference and sensing indicators instead of designingBG as a sensor introduced by Zhang et al. [10]. Besides, bydapting the double-light-source reference scheme, the spatialesolution limited by the ghost zone in an OTDR system coulde resolved. To demonstrate the feasibility new OTDR FBGsensing methods have been applied to fiber bending loss andefractive index measurements.

. OTDR FBGs sensing methods

.1. Single-light-source method for bending losseasurement

The schematic diagram of a single-light-source OTDR FBGsensing method is illustrated in Fig. 1. Two FBGs withragg wavelengths 1549 nm and 1550 nm used, respectively,s reference and sensing indicators are connected with a radius-ontrollable fiber ring that simulates a sensor. Assuming thatlaser having the same peak power at 1549 nm and 1550 nm

s launched by an OTDR into this fiber link, the power ratio η

etween the powers reflected by two FBG indicators is given by

= R155010a/5

R1549(1)

prom

Fig. 2. Schematic diagram of a d

single-light-source method.

here R1549 is the reflectivity of the 1549 nm FBG, R1550 theeflectivity of the 1550 nm FBG, and a is the attenuation in dBaused by the bending loss of fiber ring, which has been thor-ughly analyzed by Marakami et al. [12]. From Eq. (1), we canee that the peak power from the laser fluctuations can be elim-nated. The fluctuations from the lead-in/out fiber due to thexternal loads also can be compensated.

As discussed in [10], the power reflected by a FBG sensorn an OTDR system is changed due to temperature variations.bviously in Fig. 1, the power from two FBG indicators could

lso be affected by temperature variations. Therefore, in ordero compensate for the power fluctuation caused by temperatureariations, two FBG indicators with the same Bragg wave-ength are highly recommended under the strain-free condition.t should be noticed that two FBG indicators with the same Braggavelength encounter the same temperature variation.

.2. Double-light-source method for bending losseasurement

In an OTDR measurement, there is a tradeoff between thepatial resolution and the width of the laser pulse. For exam-

le, as the laser pulse widens, a cross-talk between two FBGeflection peaks may occur if the FBGs are closed by spaced. Inrder to overcome this cross-talk problem, a double-light-sourceethod is proposed and is illustrated in Fig. 2. In this method,

ouble-light-source method.

240 Y.-L. Lo, S.-H. Xu / Sensors and Act

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ig. 3. Elimination of the cross-talk effect between two close spaced FBG peakssing the double-light-source method.

second light source at 1310 nm is employed and the 1549 nmBG replaced by a FBG at 1310 nm. Two FBG reflection peaksre generated separately by two laser sources at 1550 nm and310 nm, therefore the overlap of the signals accomplished bywo different traces as illustrated in Fig. 3. As illustrated in Fig. 3,he cross-talk effect is eliminated and the spatial resolution in

double-light-source OTDR FBGs system could be greatlymproved. Consequently, the distance between two FBG indica-ors in the sensing region is not limited by the ghost zones in aonventional OTDR. However, it must be noticed that this non-host-zone condition is only suitable for the attenuation of theayleigh backscattering as the sensing mechanism. If the Fres-el reflection is used as the sensing mechanism, the ghost zonetill degrades the spatial resolution in the double-light-sourceTDR FBGs system.

Similarly, taking the ratio between the reflected powers fromBGs, the power ratio can be expressed as

= P1550R155010a/5

P1310R1310(2)

here P1550 and P1310 are the peak powers corresponding toouble light sources in an OTDR and R1550 and R1310 are theeflectivities of FBGs at 1550 nm and 1310 nm. Different fromq. (1), the peak powers in P1550 and P1310 still remain in Eq.

2), which means that power fluctuations from the light sourcesannot be compensated and the so-called source-to-source errorrises in the measurement. The fluctuations from the lead-in/outber due to the external loads can still be compensated. However

he loss from scattering, absorption, and bending are all related tohe wavelength of the light traveling in the fiber [13], the fluctua-ions cannot be completely compensated by the two-wavelengthompensation scheme.

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able 1pecifications of a POFC SMF130V single mode fiber

iber radius (�m) Refractive index

ore Cladding Coating Core

.6 62.5 125 1.463

uators A 136 (2007) 238–243

In Fig. 2, the power from two FBG indicators could alsoe affected by temperature variations. Therefore, in order toompensate for the power fluctuation caused by temperatureariations, two FBG indicators with an athermal package [14,15]re highly recommended under the strain-free condition. It is dif-erent from the temperature compensation scheme for the setupn Fig. 1 because two light sources in an OTDR system used inig. 2 have different light spectrum.

The related power budget analysis in multiplexing for theingle-light-source and double-light-source methods can beeferred to Zhang et al. [10]. The difference between the newroposed system and Zhang et al. [10] is the purpose of FBGsmplemented in an OTDR system. The new proposed systemses FBGs as indicators, however Zhang et al. [10] uses FBGss sensors. In determining the multiplexing capability, the dataariations and signal-to-noise ratio in the measuring system alsolay the important roles. On the other hand, in order to enhancehe signal-to-noise ratio of the FBG signals reflected from thend part of transmitted line, the higher reflectivity of FBGs coulde chosen.

. Experimental setups and results

Experiments have been conducted to characterize the OTDRBGs systems for bending loss and refractive index measure-ents. The conventional OTDR using the backscattering light

evel as the sensing mechanism have also been conducted foromparison. Anritsu MW9060 OTDR with MW0944 plug-innit is applied in this study for experiments.

.1. Bending loss measurement

The experimental setup and the corresponding OTDR tracesing the single-light-source OTDR FBGs method are illustratedn Fig. 4. The fiber radius controller is connected between twoBGs that their reflectivity is limited as low as 10% so as not

o exceed the dynamic range of an OTDR system. Because anTDR system utilizes a pulsed laser as the light source and

he power of each pulse fluctuates within a certain range, theverage time is set to 120 s in order to suppress this fluctuationnd obtain more stable signal. The cross-talk problem in theingle-light-source method could be easily solved by increasinghe distance between two FBGs. Also, using the narrower pulseidth of a light source could improve the cross-talk problem. In

his case, the pulse width in 10 ns is chosen for the bending loss

easurement.When the bending loss is induced by a fiber radius controller,

he reflection peak level from FBG and backscattering level bothecrease downward. In order to make a comparison between the

Cutoff wavelength (�m)

Cladding Coating

1.458 1.508 1.253

Y.-L. Lo, S.-H. Xu / Sensors and Actuators A 136 (2007) 238–243 241

R trac

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Fig. 4. Experimental setup and the OTD

ew proposed sensing method and the traditional backscatter-ng level sensing method, the power level difference between thewo FBG reflection peaks and backscattering levels in extracted.ll the parameters of a POFC SMF130V single mode fiber are

isted in Table 1, and the experimental results along with theheoretical prediction analyzed by a bending loss model [16] arellustrated in Fig. 5. When the radius of the fiber ring is smallerhan 8 mm, the average data points of backscattering level dif-

erence start to deviate from the theoretical curve, however theverage data points of FBG peak difference still tend to followhe theoretical curve. This explained that the intrinsic weaknessf the backscattered signal limits the sensing range even the

Fig. 5. Experimental results in comparison with theoretical prediction.

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e using the single-light-source method.

ider pulse width in 100 ns is chosen, however the FBG reflec-ion peaks still can be clearly distinguished. Therefore, usingBG reflection as an indicator exhibits a larger dynamic rangend shows better signal-to-noise ratio as illustrated in Fig. 4.

Fig. 6 illustrates the standard deviations of the two methodsollowing Fig. 5 under different radii of the fiber ring. It can beeen that when the radius of the fiber ring is larger than 8 mm,he standard deviation of the backscattering level difference isarger than that of the FBG peak difference, which means that therst method exhibits larger fluctuation than the second. Besides,

s the radii of the fiber ring are smaller than 8 mm, the standardeviation of the backscattering level difference rises rapidly thatndicates the backscatter level sensing down to the noise level.n Fig. 6, it is found that the average standard deviations in

ig. 6. Standard deviation from FBG reflection and backscattered level methods.

242 Y.-L. Lo, S.-H. Xu / Sensors and Actuators A 136 (2007) 238–243

d the

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Fig. 7. Experimental setup with an optical attenuator an

n OTDR FBGs system and a traditional OTDR are 0.036 dBnd 0.25 dB, respectively, for bending loss tests from 8.1 mm to.5 mm radius. It is concluded that an OTDR FBGs system hasider dynamic range, less standard deviation and better signal-

o-noise ratio.

.2. Refractive index measurement

The OTDR system has been designed for sensing variousarameters in many fields. We apply the proposed methodo the refractive index measurement for demonstration. Thexperimental setup and the correspondent OTDR trace usinghe double-light-source method are illustrated in Fig. 7. Aber gap filled with salty water is used as refractive indexensor. An optical attenuator is installed in the lead-in/outber to demonstrate the compensation scheme in the double-

ight-source method. Instead of employing a single-light-sourceethod, a double-light-source method is used not only to demon-

trate the refractive index measurement but also to discuss somemportant issues. It is expected that the Frensel reflection fromhe fiber gap easily saturates the OTDR signal, as illustrated inig. 7; however by adjusting the FBG reflectivity, two Frenseleflections from the two FBG indicators are still in the measuringange. On the other hand, it can be noticed the spatial resolutions not greatly improved. This is because the sensing mechanismn the fiber gap is the Frensel reflection not backscattering. Thus,he distance between FBG and fiber gap is still limited by thehost zone in an OTDR system.

The power ratio η, as illustrated in Fig. 7, between two FBG

ndicators can be expressed as

= P1550R1550ρ2(1 − R)2

P1310R1310(3)

iaar

OTDR trace of the double-light-source salinity sensing.

here ρ is the power coupling coefficient of the fiber gapescribed by Marcuse [17] and R is the power reflection fac-or of a single mode fiber interface [18]. Since ρ is related to theefractive index of the substance filled up with the fiber gap, theower ratio can be related to the refractive index in the fiber gap.ccording to Quan and Fry’s research [19], there is an early lin-

ar relationship between the refractive index and salinity of saltater; therefore, the refractive index measurement in salinityf salt water is applied for demonstration. A 85 �m wide fiberap filled with 0% to 25% (mg/mg) (corresponding to 1.33–1.39efractive index) has been designed in the test. The experimen-al results obtained attenuation with an optical attenuator in theead-in/out fiber along with the theoretical prediction [17,18] arellustrated in Fig. 8. The experimental data are in good agree-

ent with the theoretical prediction for refractive index valuedp to 1.365. Beyond that, the deviation occurs and it could beaused by pollutants accumulated on the end faces of the fiberap, while a decreasing coupling efficiency. The compensationor the attenuation from lead-in/out fiber works efficiently. Then-coincidence of data markers regarding to various attenuationst the same refractive index could be mainly explained by theource-to-source error in Eq. (3).

The average standard deviation in the refractive index mea-urement with the double-light-source method is 0.038 dB. Asompared to that in the bending loss measurement using theingle-light-source method, it is found that using the double-ight-source method has the worse standard deviation. It isecause that power fluctuations from two light sources cannote compensated and the so-called source-to-source error arises

n the measurement. Meanwhile, the bending experiments causeround 9 dB power variation while salinity sensing only causeround 0.4 dB power variation. Therefore, the signal-to-noiseatio of signals obtained in the bending loss measurement is

Y.-L. Lo, S.-H. Xu / Sensors and Act

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ig. 8. Experimental results from the double-light-source OTDR salinity sens-ng.

uch better than that of signals obtained in the refractive indexeasurement.

. Conclusions

In this paper, two new OTDR FBGs sensing methods haveeen developed, with importance of the dynamic range, signal-o-noise ratio, and stability. For example, for the bending lossests, from 8.1 mm to 9.5 mm radius, the average standard devi-tions with the single-light-source method are much improveds 0.036 dB and the dynamic range could be extended less than.1 mm radius. For the refractive index tests, the average stan-ard deviation with the double-light-source method is 0.038 dB.s expected that the deviation in the double-light-source method

s worse than that in the single-light-source one due to poweructuations from two light sources cannot be compensated and

he so-called source-to-source error arises in the measurement.owever, a double-light-source method could enable the minia-

urization of a sensor if a sensor uses Rayleigh backscatterings the sensing mechanism. Furthermore, regardless of exceed-ng the dynamic range of an OTDR system from the Rayleighackscattering or Frensel reflection by a sensor, two new OTDRBGs sensing systems still can work by adjusting the reflectivityf FBGs.

In conclusions, two new OTDR FBGs sensing methods areroved by the bending loss and refractive index measurements,nd they are in good agreement with the theories.

cknowledgements

Funding from the Advanced Optoelectronic Technology Cen-er, National Cheng Kung University under projects from the

inistry of Education and the National Science Council (NSC5-219-M-009-008) of Taiwan is gratefully acknowledged.

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uators A 136 (2007) 238–243 243

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iographies

u-Lung Lo received his BS degree from National Cheng Kung University,ainan, Taiwan, ROC in 1985, and his MS and PhD degrees in mechanical engi-eering from the Smart Materials and Structures Research Center, University ofaryland, College Park, USA in 1992 and 1995, respectively. After graduation,

e joined the Opto-Electronics and Systems Laboratories of the Industrial Tech-ology Research Institute (ITRI), working on fiber optic smart structures. Heas been a member of the Faculty of the Mechanical Engineering Department,ational Cheng Kung University since 1996, where he is now a full professor.is research interests lie in the areas of passive components in optical fiber

ommunications, fiber-optic sensors, optical techniques in precision measure-ents, electronic packaging, and MOEMS. He has authored over 60 technical

ublications and filed several patents. Numerous papers are published in IEEE,SA and SPIE. Dr. Lo is a member of SPIE and SEM.

hao-Hung Xu received his BS degree in 2002 and MS degrees in 2004, all inechanical engineering from National Cheng Kung University, Tainan, Taiwan.