Applications of fiber Bragg grating sensors and high-speed interrogation techniques

STRUCTURAL CONTROL AND HEALTH MONITORINGStruct. Control Health Monit. 2005; 12:269–282Published online 6 June 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/stc.70

Applications of fiber Bragg grating sensors andhigh-speed interrogation techniques

Akihito Hongo*,y, Seiji Kojima and Shinji Komatsuzaki

Photoelectronics Research Center, Advanced Technology Laboratories, Engineering and Development Group,

Hitachi Cable, Ltd., Hitachi-shi, Ibaraki-ken, 319-1414, Japan

SUMMARY

We are developing several kinds of fiber Bragg grating (FBG) sensors for application in river surveillancemonitoring, civil structural monitoring, aerospace health monitoring, and so on. In this paper we describehighly precise FBG sensing systems, specifically water-level sensors, as a practical example. We haveachieved sensor accuracy of � 0.1% F.S., i.e. � 1 cm for a full measurement range of 10m in water level.For other applications, FBG interrogation techniques are described for high-frequency vibrationmonitoring or acoustic emission detection. In this FBG interrogation system, optical filters based onplanar lightwave circuit (PLC) technology are used to measure the wavelength of the reflected light fromeach FBG sensor. We discuss two types of the optical filter, a Mach–Zehnder interferometer (MZI) and anarrayed waveguide grating (AWG). The wavelength shift of the FBG sensor due to strain or temperaturechange is determined through direct detection of the light intensity ratio from the output ports of theoptical filter. We have made a FBG interrogation unit which allows four-channel FBGs to be connected inseries to measure strain in the x, y, and z directions as well as temperature. Copyright # 2005 John Wiley& Sons, Ltd.

KEYWORDS: FBG sensors; river surveillance monitoring; structural monitoring; high-speed interrogation;optical filter; PLC technology

1. INTRODUCTION

Fiber Bragg gratings (FBGs) involve a periodic modulation of the refractive index along a givenlength of an optical fiber, fabricated by UV irradiation of the fiber [1,2]. Due to the couplingbetween forward and backward propagating modes, some wavelengths of light are reflectedrather than transmitted through the fiber at the location of the grating. In the telecommunica-tion industry, FBGs have been used extensively for various kinds of optical devices such asoptical add/drop multiplexers, laser stabilizers, and optical amplifier gain flattening filters. Inaddition, FBGs have been widely applied in sensors, since an FBG reflects a specific wavelengthwhich shifts sensitively, depending on the applied strain or temperature [3]. In Figure 1, variousFBG sensor applications and the detection method used (i.e. the FBG interrogation techniques

Received 24 August 2004Revised 9 November 2004

Accepted 10 December 2004Copyright # 2005 John Wiley & Sons, Ltd.

yE-mail: [email protected]

*Correspondence to: Akihito Hongo, Photoelectronics Research Center, Advanced Technology Laboratories,Engineering and Development Group, Hitachi Cable, Ltd., Hitachi-shi, Ibaraki-ken, 319-1414, Japan.

used to measure the wavelength shift) are classified according to their measurement frequency[4]. As FBG interrogation techniques improve, the application fields of FBG sensors areexpanding. Compared with other optical fiber sensors, such as those based on Brillouin opticaltime domain reflectometry (BOTDR), FBG sensors offer highly precise high-speed sensing [5,6].

From among the many possible sensor applications, in the first half of this paper we describea water-level monitoring system as a practical example where highly precise sensing is needed.This sensor does not need particularly high-speed sensing, but high accuracy is essential. Tomeet this requirement, we have optimized the mechanical and optical properties of FBGs usedin this sensor and have compensated for the temperature dependence of the interrogationequipment as well as that of the FBG sensors.

On the other hand, high-speed sensor applications, such as the detection of vibration,acceleration, impact damage, and acoustic emission, are also important subjects. In the secondhalf of this paper we describe FBG interrogation techniques, which use optical filters based onplanar lightwave circuit (PLC) technology [7,8]. When strain or temperature change causes awavelength shift of the FBG, the optical filter converts the wavelength shift into a change in theoutput power from the filter. This approach is especially suitable for high-speed sensing becausethere are no moving parts.

2. FBG SENSOR APPLICATIONS2.1. FBGs for sensors

The Bragg wavelength shift DlB of the FBG in response to strain and temperature is given by

DlBlB¼ ð1� PeÞDeþ zDT ð1Þ

f

Measurementfrequency

(Response time)

0 1Hz 100Hz 1kHz 1MHz

1s 10ms 1ms

of high buildings,Vibration monitoring

bridgesof power cables, trolleycable, pipelines,etc.

Vibration monitoringSeismographAcceleration monitoring

Applicationfields

1ns

monitoring for largestructures

Strain, temperature,etc.

Surveillance monitoringfor infrastructures(E.g. rivers, roads)

(E.g. buildings,tunnels, dams)

Conversion of wavelength shift into optical power through optical filteringwithout mechanical moving parts

spectroscopy for non-destructiveevaluation

Acoustic emissionHydrophonesImpact damage detection

Detectionmethods

FBG interrogationequipment

Wavelength sweeping by mechanicalmoving parts

Fabry-Perottunable filter driven bypiezoelectric actuator

by stepping motors(Conventional OSA)

power meterTunable laser &

Scanning gratings driven

Figure 1. FBG sensor applications classified by measurement frequency.

Copyright # 2005 John Wiley & Sons, Ltd. Struct. Control Health Monit. 2005; 12:269–282

A. HONGO, S. KOJIMA AND S. KOMATSUZAKI270

where Pe is the effective photo-elastic coefficient for silica fiber, De is the applied longitudinalstrain, z is the thermo-optics coefficient, and DT is the change in temperature. Assuming thattemperature variation is negligible and the effective photo-elastic coefficient Pe ¼ 0:22; typicalvalues for the sensitivity to an applied axial strain is calculated at 1.17 pm/me for 1500 nm inwavelength.

For high-precision sensing systems, we have used polyimide-coated optical fiber as an FBGstrain sensor [9]. The fabrication process is as follows. The polyimide coating layer is partiallyremoved by chemical etching, which is an important process to preserve the mechanical strengthof the fiber. The FBG is inscribed at the stripped part by irradiation from a KrF excimer laseremitting 248-nm light into the optical fiber. After the grating is inscribed, the stripped part is re-coated and the FBG is subjected to high temperature heat treatment at 3008C for 30min; thisaccelerates aging to enhance long-term reliability. That is, the accelerated aging is necessary tosuppress the optical property change of the FBGs over a long period of time. Thermal decay ofthe photo-induced index change is characterized by a power law function of time. The higher theatmospheric temperature, the higher the decay rate of the photo-induced index change will be.The photo-induced index is initially decayed through the high-temperature aging following theFBG inscription, and after that the optical characteristics are stable enough for practical use.Even after such high-temperature treatment, the FBG retained sufficient reflectivity of over90%.

Figure 2 shows typical transmission and reflection spectra of the FBG. We adopted Gaussianshape apodization to suppress the interfering side lobes in the reflection spectrum. Theapodization profile, i.e. the photo-induced refractive-index along the fiber, is modulated throughprecise control of the scanning speed of the excimer laser beam in the longitudinal direction. Theapodization technique was mainly developed for narrow-band wavelength division multiplexingoptical filters [10]. For sensing applications as well, it is important to achieve highly precisesensing. The strain applied to the FBG sensor can be determined from the center wavelengthshift. To measure the center wavelength with sufficient accuracy, the FBG was designed so thatthe 3-dB bandwidth was 0.2 nm or less.

In addition to the advantage of high heat resistance, the polyimide coating prevents slippageat the border between the coating layer and glass region under a high applied strain. Figure 3shows the measured center wavelength changes of FBGs which were coated with UV-curable

-20

0

1545 1547 1549 1551 1553 155Wavelength (nm)

-5-20

0

1555 1545 1547 1549 1551 1553 155Wavelength (nm)

1555

-5

-10

Los

s (d

B)

Los

s (d

B)

-15

-60

-50

-40

-30

-10

0

(a) (b)

Figure 2. Typical transmission and reflection spectra of the FBG: (a) Transmission; and (b) Reflection.


FIBER BRAGG SENSORS FOR MONITORING 271

and polyimide resin. We applied a strain of 400 me two times for each FBG. The external appliedstrain can be precisely transferred to the FBG with polyimide resin coating and the centerwavelength shifted with the rate of change being 1.18 pm/me. Whereas the UV-curable resincoated FBG was inferior in precision and repeatability due to slippage at the border between theUV-curable resin layer and glass region.

2.2. Optical water-level sensor using FBGs

In this section, we describe an optical water-level sensor as a practical example of using FBGs.Figure 4 shows the structure of the sensor head, which consists of a diaphragm, a customizedBourdon tube, and two FBGs. One FBG is used to measure the tensile strain converted fromwater pressure through the diaphragm and the Bourdon tube. The other FBG is used fortemperature compensation. Water pressure proportional to the water level is transmitted to theBourdon tube through the diaphragm. The FBG between the tip and the base of the Bourdontube is then strained in proportion to the water level. The FBG to compensate for thetemperature dependence is connected in series with the FBG for tensile measurement andaccommodated in the sensor case so that it is free of any strain. Air pressure in the sensor case isbalanced with the outer atmospheric pressure through an air induction pipe which contains anoptical fiber approach cable. The strain applicable to the tensile-measuring FBG was set to be0.3% at the maximum water level, which corresponded to about 3.6 nm center wavelength shiftof the FBG.

Figure 5 is a schematic diagram of the water-level measurement system. The broadband lightfrom an ASE light source within a wavelength band from 1530 to 1570 nm is launched into theoptical fiber via a three-port circulator. The reflected light from one FBG sensor comes backtoward the Fabry–Perot tunable filter which acts as a spectroscopic device [11] driven by apiezoelectric transducer (PZT). The light of other wavelengths passes through the FBG and is

0.44

0.445

0.45

0.455

0.46

0.465

0.47

0.475

0.48

Wav

elen

gth

chan

ge (

nm)

00 50 100 150 200

0.05

0.10

time (s)

Polyimide resin coated FBG

UV-curable resin coated FBG

Applying strain of 400 µε

1st measurement

2nd measurement

1st measurement2nd measurement

UV-curable resin coated FBG

ying

1st measurement

2nd measurement

1st measurement2nd measurement

Figure 3. Measured center wavelength changes of FBGs with UV-curable and polyimide resin coating.



transmitted to the adjacent FBG sensors where the light of different wavelength is reflected.Thus, it is possible to achieve multi-point measurement with one optical fiber when several FBGsensors, each with a different Bragg wavelength, are connected in series. The number of FBGsensors that can be made in one optical fiber is limited depending on the maximum wavelength

Diaphragm

Optical fiber

Bourdon tube

FBG for temperaturecompensation

FBG for tensilemeasurement

Sensor case

Water pressure

tensile

Figure 4. Structure of the optical water-level sensor head.

ASELight Source

I/Am

A/D Computer

TemperatureController

ReferenceFBG

Fabry-PerotTunable Filter

3-portcirculator

PZT

D/

OpticalSwitch

Water Level Sensors

Optical Fiber

3

WaterLevel

Displ

ASELight Source

I/VAm

A/D Computer

TemperatureControllern

D/A

OpticalSwitch

1 2

WaterLevel

Display

Figure 5. Schematic diagram of the water-level measurement system.



shift of each FBG and the ASE wavelength band range. Using an optical switch, however,enables the system to accommodate more sensors and to measure different water levels with asingle piece of interrogation equipment. The temperature dependence of the interrogationequipment is also compensated for with a reference FBG having a constant Bragg wavelength,which has a temperature controller and is installed inside the equipment.

Figure 6 shows the relationship between the applied pressure measured by a pressure gaugeand the detected wavelength from the FBG sensor. We obtained good linearity over the fullrange of pressure up to a water level of 10m.

Figure 7 shows the measurement error for each water level at an operating temperature of 0,20, or 408C, as obtained from a linear fitting line calculated from the relationship between thepressure and detected wavelength shown in Figure 6. We achieved sensor accuracy of � 0.1%F.S., i.e. � 1 cm for the full measurement range of 10m in water level.

1542

1543

1544

1545

1546

1547

0 100 200 300 400 500 600 700 800 900 1000

Pressure (cmH2O)

Cen

ter

Wav

elen

gth

Ref

lect

edfr

om F

BG

(nm

)

Figure 6. Relationship between the applied pressure measured by a pressure gauge and the detectedwavelength from the FBG sensor.

-30 1 2 3 4 5 6 7 8 9 10

-2

-1

0

1

2

3

40

020202020

Pressurized Water Level (m)

Err

or f

rom

Fitt

ing

(cm

)

20202040 ˚C

0 ˚C

20 ˚C

Figure 7. Measurement error for each water level at an operating temperature of 0, 20, or 408C.



3. HIGH-SPEED INTERROGATION TECHNIQUES FOR FBG SENSORS

3.1. Interrogation concepts for high-speed detection

The FBG interrogation equipment used in the water-level measurement system contains ademodulator based on a Fabry–Perot tunable filter driven by a PZT. Although the Fabry–Perottunable filter has very high optical resolution, the measurement frequency is restricted to lessthan 100Hz because it contains moving mechanical parts.

For high-speed applications with a measurement frequency of over 1 kHz, FBG interrogationshould be based on a different approach. The basic principle is a conversion of the Braggwavelength shifts into a particular type of optical intensity modulation. Figure 8 illustrates aninterrogation concept based on use of an optical spectral filter whose transmission changes withwavelength [12]. The optical filter transforms Bragg wavelength variations into correspondingoptical intensity changes. As shown in Figure 8, a chirped grating with linear transmission canbe used for such an optical filter.

Although the FBG interrogator shown in Figure 8 is a simple configuration, some technicalproblems remain to be solved. The reflected light from the FBG sensor is divided into the signaloptical power which depends on the wavelength through the optical filter and the referenceoptical power which is wavelength independent. The ideal coupler should have a constantdividing ratio. In practice, though, the dividing ratio of the coupler may change depending ontemperature, wavelength, or polarization. In addition, since the two optical paths after thecoupler are spatially separated from each other, an unstable transmission loss difference mayoccur because of vibration or lack of temperature uniformity.

020

406080

100

Wavelength

Tra

nsm

issi

on(%

)

Transmission of lineardiscri mination grati ng

BroadbandLight Source

3dB Coupler orOptical Circulator

3dB Coupler

Optical Filter(Chirped FBG)

Photo Detector

020

406080

100

Wavelength

Tra

nsm

issi

on(%

)

Transmission of lineardiscri mination grati ng

0

200

406080

100

Wavelength

Tra

nsm

issi

on(%

)

Transmission of lineardiscrimination grating

BroadbandLight Source FBG Sensor

Vsen(λ B)

Vref(λ B)

Figure 8. Interrogation of FBG sensors using the optical spectral filter.



3.2. FBG interrogation using PLC technologies

To solve these technical problems, we have developed an FBG interrogator using PLCtechnologies [7,8]. Since PLC technologies allow superior integration of optical filters on a smallPLC chip, this approach is more suitable for multi-point measurement systems than anapproach using the sort of fiber-type filters shown in Figure 8.

We examined two types of PLC-type optical filter. One was a Mach–Zehnder interferometer(MZI) and the other was an arrayed waveguide grating (AWG). First, we describe the FBGinterrogator, which uses integrated MZI optical filters and a coarse wavelength divisionmultiplexing demultiplexer (CWDM Demux). A schematic diagram and the transmission lossspectra of the four-channel CWDM Demux and the integrated MZI optical filters are shown inFigures 9 and 10, respectively. This FBG interrogation unit can measure strain in the x, y, and zdirections as well as temperature. The CWDM Demux is commercially available and widelyused in metropolitan communication networks.

Detector+

SignalProcessing

FBG Sensors

λ1±∆λ1 λ2±λ2 λ3±∆λ3 λ4±∆λ4

Broad-BandLight Source

Output

IntegratedMZI optical filter

Circulator

4chCWDMDemux

Detector+

SignalProcessing

Broad-BandLight Source

4chCWDMDemux

Figure 9. Schematic diagram of the FBG interrogation unit for a four-point measurement system using thefour-channel CWDM Demux and the integrated MZI optical filters.

0

5

10

15

20

25

30

Los

s (d

B)

4ch CWDM Demux

MZI optical filter

Wavelength

λ1

λ1±∆λ1 λ2±∆λ2 λ3±∆λ3 λ4±∆λ4

λ2 λ3 λ4

Figure 10. Transmission loss spectra of the four-channel CWDM Demuxand the integrated MZI optical filters.



We integrated four identical MZI optical filters into one silica-based 5� 23mm PLC chip.Each MZI consists of two optical arms and two multi-mode interferometer (MMI) 3-dBcouplers. Since the two optical arms of each MZI were formed very closely to each other in thesame PLC chip, there was little fluctuation in the optical characteristics of the MZI optical filtercaused by any temperature or vibration difference between the two optical arms. Figure 11shows the experimental transmission spectrum of each MZI optical filter. The total insertionloss of the MZI was about 0.7 dB. The MZI optical filter has one input port and two outputports and the loss of each channel changes periodically as a function of wavelength. As thepower from one channel rises, that from the other channel falls. In this way, the MZI opticalfilter transforms the Bragg wavelength shift into a corresponding optical intensity change.Taking detection sensitivity and dynamic range into consideration, the initial Bragg wavelengthof the FBG sensor was set close to the intersection of two loss curves where the optical powerthrough the MZI optical filter was equally divided. Table I summarizes the specifications of theMZI optical filter used in the experiment.

The experimental results for the static and dynamic performance of the FBG interrogationunit using the MZI optical filters are shown in Figures 12 and 13, respectively. In the staticperformance test (Figure 12), the FBG was strained directly with a micrometer. We comparedthe measured strain, i.e. the output value of the FBG interrogation unit, with the applied strain,and found that the scale error was less than 1%. In the dynamic performance test (Figure 13),the FBG was set on a vibration table. The input wave of the vibration table and the output wave

Table I. MZI specifications.

Loss period 16 nmInsertion loss 0.7 dBOperating wavelength range 1500–1600 nmChip size 5� 23 nm

Ch.1 Ch .2

FBGWav elengthL

osss

(dB

)

Wavelength (nm)

40

35

30

25

20

15

10

5

0

Ch.1 Ch.2

FBGWavelength

1540 1544 1548 1552 1556 1560

Figure 11. Experimental transmission loss spectra of each MZI optical filter.



of the FBG interrogation unit were observed by using an oscilloscope when the vibrationfrequency was 1 kHz.

Next, we describe another FBG interrogator using an AWG optical filter. A schematicdiagram of this FBG interrogation unit is shown in Figure 14. The AWG chip was fabricated

FBG

Experimental Setup

Micrometer

0

500

1000

1500

Mea

sure

d St

rain

(µε

)

Scale Error: 0.89%

Applied Strain (µε)

High-Speed FBG Interrogation Unit

0

500

1500

Scale Error: 0.89%

0 500 1000


Figure 12. Static performance of the FBG interrogation unit using the MZI optical filters.


Oscilloscop

Vibration(1kHz)

Vibration Table

Weight

Experimental Setup

Measured wave

Input wave

1 ms (1kHz)


Oscilloscope

FBG

Vibration Table

Measured wave

Input wave

1 ms (1kHz)

Figure 13. Dynamic performance of the FBG interrogation unit using the MZI optical filters.



using the same PLC technology as the MZI chip described above. The reflected light from eachFBG is launched into the AWG via a circulator, and the AWG divides the incoming light intomultiple channels of different wavelengths.

Figure 15 shows the reflected power from the FBG and the transmitted power for channels ofthe AWG with 100-GHz channel spacing. The Bragg wavelength of the FBG was set close to themiddle of the wavelength between the center wavelengths of two adjacent channels of the AWG.The 3-dB bandwidth of the AWG with 100-GHz channel spacing was 0.45 nm. When the Braggwavelength shift of the FBG was 10 pm, the transmitted power change for each of the adjacentAWG channels was � 0.7 dB. Table II summarizes the specifications of the AWG optical filterused in the experiment. This AWG has 16 channels with narrow channel spacing. That is whythe FBG interrogator using this AWG is suitable for high-sensitivity multi-point measurement,although the dynamic range of strain measurement is smaller than that of the FBG interrogatorusing the MZI shown in Figure 9.

Using such a high-speed, highly sensitive interrogation unit, we have developed FBG/PZThybrid sensing system. Figure 16 shows an experimental setup, which consists of a FBGinterrogator, a FBG sensor, a reference PZT sensor, and a PZT actuator. The interrogator

Opt

ical

Pow

er (

dBm

/0.1

nm)

1539 1540 1541Wavelength (nm)

1542 1543 1544

-10

-15

-20

-25

-30

-35

-40

-45

-50

-55

-60

Incoming light powerFBG

AWG AWG AWG AWG

Incoming light power

AWG AWG AWG

Figure 15. Reflected power of the FBG and transmitted power of the AWG with 100-GHz channel spacing.

FBG sensors

Broad-Band Light Source

Circulator

Detector+

SignalProcessing

Output

Broad-Band Light Source

at

Detector+

SignalProcessing

λ1± ∆λ1 λ2± λ2 λ3±∆λ3 λ4± ∆λ4AWG

Figure 14. Schematic diagram of the FBG interrogation unit using the AWG.



detected the wavelength shift of the FBG sensor caused by elastic wave which was generated bythe PZT actuator and propagated in the composite materials. We mounted the FBG sensor andthe PZT actuator on the specimen made of CFRP laminated plate 240� 240� 1mm in size.Another PZT as a reference sensor was also mounted near the FBG sensor. The length of theFBG was 1mm, which was shorter than the span of the elastic wave in order to suppressdeformation of the spectrum shape of the FBG.

Figure 17 shows the input signal to the PZT actuator. The input signal to the PZT isthree cycles of sine wave 300 kHz in frequency, which was filtered through Hamming window.Figures 18 and 19 show the output signals from the reference PZT and from the FBG sensorthrough the AWG filter, respectively. We measured the output powers from the two adjacentchannels of the AWG, which depends on the center wavelength shift of the FBG. By calculatingthe ratio of the output powers, it was converted into the center wavelength shift based on thespectrum data in Figure 15. In this conversion, we assumed that the spectrum shape of the FBGhad not changed by the elastic wave. Comparing two results in Figures 18 and 19, we found thatthe FBG sensors could detect elastic wave propagating in the composite materials by using ourinterrogation unit.

We are now developing a new monitoring system for detecting internal defects in compositematerials. In the composite materials, we embed small-diameter FBG sensors which wedeveloped for health-monitoring in aviation components [13]. These FBG sensors have such asmall diameter that they do not cause any mechanical deterioration when they are embedded.The potential range of such applications suggests that FBG sensing systems will be usedextensively.

Table II. AWG specifications.

Channel spacing 0.8 nm (100GHz)Insertion loss 4.0 dBOperating wavelength range 1540–1553 nmChip size 20� 40mm

Circulator

CFRP Laminated PlateFBG Sensor

PZT Actuator

Input Signal

Broadband Light Sourc Amplifier Signal

Generator

Temperature Controller

Reference PZT Sensor

FBG interrogator

Digital Oscilloscope

Broadband Light Source Amplifier Signal

Generator

Temperature Controller

AWGO/EConverter

PC

Figure 16. Experimental setup of the FBG/PZT hybrid sensing system.



4. CONCLUSION

In this paper, we discuss two topics of our recent studies regarding FBG sensor applications.The first topic is a water-level monitoring system, which is a practical example of surveillancemonitoring. Although the water-level measurement system need not be capable of high-speedsensing, high accuracy is required. We have achieved the sensor accuracy of � 0.1% F.S., i.e.� 1 cm for a full measurement range of 10m in water level. The second topic is FBGinterrogation using PLC technology. We described two types of optical filters, an MZI and anAWG, which convert the Bragg wavelength shift of the FBG sensor into a corresponding optical

-8-6-4-202468

-10 0 10 20 30 40 50 60 70 80 90 100

Time [µs]

Vol

tage

[V

]

Figure 17. Input signal to the PZT actuator.

-150

-100-50

050

100150

-10 10 20 30 40 50 60 70 80 90 100

Time [µs]

Vol

tage

[m

V]

0

Figure 18. Output signal from the reference PZT.

543210

-1-2-3-4-5-10 10 20 30 40 50 60 70 80 90 100

Time [µs]

Wav

elen

gth

shif

t [pm

]

0

Figure 19. Output signal from the FBG sensor through the AWG filter.



intensity change. This technology is promising for various types of high-speed application suchas detection of vibration, acceleration, impact damage, or acoustic emission, because there areno moving parts. We expect such systems to meet the application requirements of high responsespeed, sensitivity, accuracy, sensor multiplexibility, and cost effectiveness.

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9. Satori K, Ikeda Y, Kurosawa Y, Hongo A, Takeda N. Development of small-diameter optical fiber sensors fordamage detection in composite laminates. Proceedings of SPIE 2000; 3986:104–111.

10. Malo B, Teriault S, Johnson DC, Bilodeau F, Albert J, Hill KO. Apodised in-fibre Bragg grating reflectorsphotoimprinted using a phase mask. Electronics Letters 1995; 31:223–225.

11. Kersey AD, Berkoff TA, Morey WW. Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry–Perotwavelength filter. Optics Letters 1993; 18:1370–1372.

12. Schulz WL, Conte JP, Udd E, Kunzler M. Real-time damage assessment of civil structures using fiber gratingsensors and modal analysis. Proceedings of SPIE 2002; 4696:228–237.

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Applications of fiber Bragg grating sensors and high-speed interrogation techniques