Fiber Optic Sensors and Sensor Networks Using a Time-domain Sensing Scheme

doi:10.4236/opj.2013.32B055

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Optics and Photonics Journal, 2013, 3, 236-239

doi:10.4236/opj.2013.32B055 Published Online June 2013 (http://www.scirp.org/journal/opj)

Fiber Optic Sensors and Sensor Networks Using a

Time-domain Sensing Scheme

Chuji Wang1*, Malik Kaya1, Peeyush Sahay1, Haifa Alali1, Robert Reese2

1Department of Physics & Astronomy, Mississippi State University, Starkville, MS-39759, USA

2Department of Electrical and Computer Engineering, Mississippi State University, Starkville, MS-39759, USA

Email: *[email protected]

Received 2013

ABSTRACT

Fiber loop ringdown (FLRD) has demonstrated to be capable of sensing various quantities, such as chemical species,

pressure, refractive index, strain, temperature, etc.; and it has high potential for the development of a sensor network. In

the present work, we describe design and development of three different types of FLRD sensors for water, cracks, and

temperature sensing in concrete structures. All of the three aforementioned sensors were indigenously developed very

recently in our laboratory and their capabilities of detecting the respective quantities were demonstrated. Later, all of the

sensors were installed in a test grout cube for real-time monitoring. This work presents the results obtained in the labo-

ratory-based experiments as well as the results from the real-time monitoring process in the test cube.

Keywords: Fiber Loop Ringdown; Structural Health Monitoring; Water, Crack, and Temperature Sensing;

Sensor Network

1. Introduction

Fiber loop ringdown (FLRD) is a time domain sensing

technique which utilizes the rate of the decay of a light

pulse trapped inside a closed fiber loop to generate its

sensing signal—“ringdown time”. FLRD can detect

various chemical and physical quantities, such as small

volume of liquids, pressure, temperature, refractive index

(RI), etc [1]. The trapped light pulse traverses inside the

fiber loop many times before it dies out completely and

in each round trip a small amount of the light is trans-

mitted out of the loop to a detector. The temporal profile

of the transmitted part follows a single exponential decay.

The ringdown time is obtained from this exponential de-

cay waveform of the transmitted light intensity. Owing to

different optical losses inside the cavity, the ringdown

time changes. If an optical loss occurred inside the fiber

loop can be related to an event, for example a change in

pressure, temperature, stress, strain, etc., or even a

change in RI around the fiber core, then that particular

event can be sensed by observing a change in the ring-

down time. Details about the FLRD sensing technique

and its various applications, such as sensing physical

parameters, chemical species, biological quantities, etc.,

can be seen in a number of publications [2-9]. However,

it is a very recent progress, in which FLRD-based sen-

sors have been developed and demonstrated for water,

crack and temperature sensing in actual concrete struc-

tures with possibility of sensor networking, an important

requirement in structural health monitoring (SHM) [10,

11]. In this paper, we describe the design and develop-

ment of three FLRD sensors, namely, FLRD water, crack,

and temperature sensors developed in our laboratory, and

discuss the results obtained from the experiments con-

ducted in laboratory conditions and from the real-time

monitoring process in the concrete structures.

2. Experimental Set-up

Figure 1 shows schematic of a typical FLRD sensor and

its configuration. The figure depicts how all of the three

types of sensors, i.e. water sensors, crack sensors, and

temperature sensors, were connected to a fiber loop. The

part marked as A, B, or C in the figure represents three

different sensor heads for the three different sensing

functions. The sensors were controlled by the electronics,

depicted in the upper part of Figure 1, named as the sensor

control system. In this particular experiment, a single

mode fiber (SMF) (Corning Inc.) was used to construct a

fiber loop of 120-m long, as shown in the figure. A diode

laser (NTT Electronics) operating at 1550 nm was used

to obtain laser pulses which were injected into the fiber

loop through a fiber coupler (Opneti Communication Co)

with a coupling ratio of 0.1/99.9. The transmitted intensi-

ties were collected using a photodiode detector (Thorlabs

Inc.). The ringdown signals were monitored by an oscil-

*Corresponding author.

C. J. WANG ET AL. 237

loscope; subsequently, changes in ringdown time were

recorded by a computer. A brief discussion on the in-

strumentation process of the three sensors is described in

their respective sections later.

3. Results and Discussion

The first-hand experiments with all three sensors were

conducted in our laboratory. Subsequently, all of the

three types of sensors were deployed in a test grout cube

with dimensions of 10 ft × 10 ft × 8 ft, in the designated

test site of the US Department of Energy, located in

Miami, Florida, for real-time monitoring. First, the sen-

sors were installed in an installation panel and the panel

was then loaded inside the test cube that later was en-

tombed by wet grout. Upon drying of the grout, the sen-

sors remain embedded in the grout cube. The extended

arms of the fiber cable, from the sensor unit, coming out

of the grout cube were connected to the sensor control

system in a similar way as shown in Figure 1. The sen-

sors were switched alternatively one at a time and the

data from all of the sensors were recorded individually,

often for a long duration of time, i.e. a few hours to a few

days.

Some of the results obtained from both the experi-

ments, i.e. in the laboratory environment and in the real-

time monitoring in the test grout site (Miami, FL), for the

three types of the sensors, are presented below.

3.1. Water Sensor

After the plastic jacket of the single mode fiber (SMF)

was removed, one section of the fiber was etched in a

48% Hydrofluoric (HF) acid solution for 32 - 33 min to

prepare a FLRD water sensor head. Once a water sensor

head was created, concrete was prepared by mixing ce-

ment-aggregate (Quikrete) and water. After a half of a

Figure 1. A laboratory-based fiber loop ringdown system.

One sensor control system operates multiple FLRD sensor

units, e.g. sensor units A, B, and C.

carton bar, with dimensions of 35 cm × 5 cm × 5 cm, was

filled with the concrete, the etched fiber was placed in-

side the concrete bed and the carton bar were filled com-

pletely. It took about two days for the concrete bar to dry

out completely. The sensor head remained embedded in

the concrete bar, forming one sensor unit. Figure 2

shows FLRD water sensor’s response, for a time duration

of 19 hrs, with the experiment conducted under labora-

tory conditions. After the 1-hr data collection with the

dry concrete bar, 10 ml water was poured on the concrete

bar surface to test the sensor’s response. Whenever the

poured water reached the sensor head, the FLRD sensor

sensed the water in near-real time, resulting a change in

ringdown time. Later, the ringdown time decreased

gradually and returned to the baseline when the concrete

bar dried out again.

Figure 2(b) shows the data collected from the FLRD

water sensor after the sensor was embedded into the test

grout cube. The data shows the FLRD water sensor’s

behavior before, during, and after the grouting process.

The increase in the ringdown time suggests the presence

of water around the sensor head, which dried out in time

later, as exhibited by the decrease in the ringdown time.

Figure 2. (a) Response of the FLRD water sensor; experi-

ment conducted under labora tory conditions. (b) Data from

the FLRD water sensor embedded in the test grout cube

that was located in Miami, FL.

C. J. WANG ET AL.

238

3.2. Crack Sensor

Figure 3(a) shows the results from one of the FLRD

crack sensor units, with the experiment conducted in the

laboratory environment. A sensor unit, with dimensions

of 20 cm × 5 cm × 5 cm was constructed by embedding

an optical fiber cable in a concrete bar; while during the

process of forming the concrete bar, a bare SMF was laid

down as it is, i.e. without any modification, inside the

concrete bed, along the longest axis of the bar, so that,

upon drying it became an integrated unit. The extended

fiber cable, i.e. the small portion coming out from the

two ends of the bar, was spliced into a fiber loop. Cracks

were produced manually, by hitting a nail on the top sur-

face of the bar; subsequently, changes in ringdown time

were observed. In this particular experiment, from the

point of no cracks on the surface to a crack with a surface

crack width (SCW) of 3.5 mm wide, measured on the top

surface of the bar, a significant change in the ringdown

time, e.g. from 15.5 μs to 9.5 μs, was recorded. Experi-

mentally, all of the crack sensors responded to a SCW as

small as 0.5 mm, which was exhibited by a considerable

change in the ringdown time, from 0.5 - 1.0 μs for a 0.5

mm increase in SCW, as shown in Figure 3(a). This re-

sult leads to a theoretical SCW detection limit of 31 μm.

Figure 3. (a) Response of the FLRD crack sensor in the

laboratory environment (Starkville, MS). (b) Response of

the crack sensor installed in test grout cube in Miami, FL.

Figure 3(b) shows real-time monitoring results from the

sensor unit that was installed in the test grout cube. The

data were recorded continuously for 20 days, starting

from the time when the grouting began. Unlike the labo-

ratory experimental results, i.e. in Figure 3(a), the change

in ringdown time in this case was not sharp, instead a

slow change in the ringdown time was observed. De-

crease and increase in the ringdown time indicate a pos-

sible contraction(s) and expansion(s) that might have

happened in the grout volume in the process of drying of

the grout cube.

3.3. Temperature Sensor

FLRD temperature sensors were fabricated by using a

commercially available fiber Brag grating (FBG) with a

central wavelength at 1567 nm. In the FLRD temperature

sensors, the FBG itself acts as a sensor head; therefore, it

does not require any modification of the fiber to con-

struct a sensor head. However, the FBG was covered by

a copper tubing to protect it from being damaged. Two

FLRD temperature sensors were embedded into the grout

cube for testing. Figure 4 shows the remotely collected

(in Starkville, MS) data from one of the two FLRD tem-

perature sensors during a period of 12 days.

4. Conclusions

Three types of fiber loop ringdown (FLRD) sensors were

designed and developed for the purposes of water, crack,

and temperature sensing in concrete structures. The per-

formance of the sensors was first tested with small size

concrete bars in our laboratory at Mississippi State Uni-

versity, Starkville, MS. The laboratory experimental re-

sults established the idea that FLRD technique was in-

deed capable of sensing water, cracks, and temperature in

actual concrete structures. Later, all of the sensors were

Figure 4. Response of the FLRD temperature sensor for a

period of 12 days.

C. J. WANG ET AL.

239

entombed in the test grout cube, with dimensions of 10 ft

× 10 ft × 8 ft, located in Miami, Florida, for evaluating

its performance in real-world scenarios. The sensors are

controlled and data are still being collected, remotely in

our laboratory. The results from both the experiments, i.e.

conducted in the laboratory and from the real-time

monitoring at the test site, have been discussed in this

work. This work represents, for the first time, the study

of FLRD technology in real-world applications.

5. Acknowledgements

The work is supported by the National Science Founda-

tion (grant #CMMI-0927539) and the US Department of

Energy through Savannah Nuclear Solutions (grant

#AC84132N).

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