JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 22, NOVEMBER 15, 2011 3453

Performance Enhancement of VibrationSensing Employing Multiple Phase-Shifted

Fiber Bragg GratingAsrul Izam Azmi, Deep Sen, Wenjuan Sheng, John Canning, and Gang-Ding Peng

Abstract—Multiple phase-shifted fiber Bragg gratings(MPS-FBGs) are proposed for a novel intensity-type vibra-tion sensing scheme with broadband source based interrogation.In this scheme, the collective and simultaneous operation of thesubchannels of MPS-FBG multiplies the total acquired signalpower change at a particular channel, considerably enhancingsensitivity. Compared to a standard FBG scheme, our experimentscheme attained significant sensitivity enhancement of nearly20 dB using 17 phase-shifted FBG. These experimental resultsagree with the theoretical prediction based on cross-correlationrelations. The sensitivity enhancement while retaining the systemsimplicity would be an attractive option for an economical coarsewavelength division multiplexed (CWDM) vibration sensingsystem based on a single broadband source.

Index Terms—Fiber grating, multiple phase-shifted, multiplesubchannels sensing, vibration sensor.

I. INTRODUCTION

T HE encouraging prospects of optical fiber sensing havedrawn increasing attention from manufacturers and re-

searchers alike. Optical fiber sensors have emerged as an alter-native to their electronic counterparts, with distinct advantages[1]. Among the areas under the research spotlight are vibrationand acoustic sensing, with a number of applications in test bedsnow ready for practical implementation. Interferometric-typesensors [2] are known to achieve the highest sensitivity, butare also associated with problems such as polarization-inducedfading and require complex demodulation algorithms or costlyFaraday rotators in novel Michelson schemes [3], [4]. Forlow cost, simplicity, and robust operation, an intensity-typesensor [5]–[11] is normally preferred. Given their feasibilityin achieving required sensitivity and their ability to transcribea high fidelity vibration wave, intensity-type vibration andacoustic sensors are capable of complex dealings [5], [6].However, their performance characteristics and related issues

Manuscript received June 06, 2011; revised September 04, 2011; acceptedSeptember 18, 2011. Date of publication September 29, 2011; date of currentversion November 16, 2011.A. I. Azmi is with the School of Electrical Engineering and Telecommuni-

cations, The University of New South Wales Sydney, NSW 2052, Australiaand also with Faculty of Electrical Engineering, Universiti Teknologi Malaysia,Skudai, 81310 Johor, Malaysia (e-mail: asrul@unsw.edu.au).D. Sen, W. Sheng, and G.-D. Peng are with the School of Electrical Engi-

neering and Telecommunications, The University of New South Wales Sydney,NSW 2052, Australia.J. Canning is with the Interdisciplinary Photonics Laboratories (iPL), School

of Chemistry, The University of Sydney, Sydney NSW 2006, Australia.Digital Object Identifier 10.1109/JLT.2011.2169940

in implementation have also attracted considerable attention.Fiber Bragg grating (FBG) intensity-type sensors have demon-strated good linearity within a large dynamic range of up to 60dB [7]. The achieved minimum detectable strain is typically inthe order of a tenth [7] to a hundredth [8] nano-strain Hz .Most intensity-type vibration and acoustic sensors employ nar-rowband sources, such as tunable laser diodes, [5]–[10] owingto their excellent sensitivity as well as the flexibility to choosethe lasing wavelength at the availability of FBG wavelength.In contrast, utilization of a broadband source in vibration

and acoustic sensing is motivated by simplicity and low cost,i.e., avoiding expensive tunable laser sources and controlelectronics. Typically, a broadband source, either as amplifiedspontaneous emission (ASE) or light emitting diode (LED),may produce higher total power than a tunable laser. However,it usually has significantly lower power within a spectral rangefor sensing operation (W/nm). This leads to a deficiency inoptical power utilization, and ultimately lower signal-to-noiseratio (SNR). Sensitivity enhancement for a system employingnarrowband source usually relies on increasing output powerand/or increasing steepness of FBG reflectivity [8]–[10]. Sincebroadband sources are normally limited in terms of their totaloutput power and emit wide spectral range of output power,these enhancement techniques are not particularly suitable forsystems employing broadband sources. A gain mechanism maybe introduced into the system, such as insertion of a fiber am-plifier system prior delivery of the filtered power to the sensor[11] or utilization of an electronic amplifier at the receiverend. However, this addition not only contradicts the aim forsimplicity and low cost, but also generates undesired intensityin noise that is described by the amplifiers’ noise figure [12].A novel scheme based on multiple phase shift (MPS) FBG to

boost the measured total optical power change (which detectedas an AC voltage by photodetector) is therefore proposed. As aresult of the phase shifts, or resonances, a MPS-FBG has mul-tiple narrow bandwidth subchannels within its reflection band-width. They may work collectively to produce multiplied signalpower since the total power is not only proportional the reso-nance quality, , but also the number of resonances, , i.e.,. More importantly, the resulting signal power change at par-

ticular wavelength shift may be improved with the synchronoussubchannels operation. This correspondingly enhances the sen-sitivity (in terms of SNR) in comparison to the typical system,which comprises only single subchannel FBG. Hence, by usingMPS-FBG we may exploit broad features of the source for per-formance enhancement, while retaining the same simple inter-

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rogation as a normal FBG scheme. Previously, multiple phase-shifted grating devices have been reported for telecommunica-tion application as wavelength demultiplexers [13] and multi-channel dispersion compensators [14]. An alternative methoduses multiple phase shifts adjusted in phase to superpose spec-trally the cavity quality leading to extraordinarily enhanced res-onance directly at one specific wavelength which is ideal forultra low detection of parameters such as magneto-optic shiftsin silica fibers where the magneto-optic coefficient is very weak[15]. However, whilst it has clear advantages in potential denseWDM measurements where ultra-sensitivity is required, it re-quires a stabilized tunable narrow linewidth source or sophis-ticated dispersion measurements for clear detection of signalchanges [16], or resonance shifts. While the Vernier effect inmultiple subchannel shifts has been commonly reported for con-tinuous wavelength tuning of laser [17], [18] applications, andfew reported in other applications such as strain sensing [19]and microwave photonic filter [20].This paper establishes an MPS-FBG sensing model to un-

derstand the underlying optimum sensing condition, and sub-sequently to examine the potential of MPS-FBG in this sensingscheme. Experimental work has been carried out to validate thesensitivity enhancement expected from theoretical analysis.

II. THEORY

A. Multiple Subchannels Sensing Concept and ModelFormulation

The setup and sensing concept based on multiple subchan-nels FBG are shown in Fig. 1(a) and (b) respectively. TwoMPS-FBGs with identical spectral shapes are used (i.e., MPS-FBGswith similar number of phase shifts). The wavelength of refer-ence MPS-FBG is biased at a point that is slightly offset relativeto the sensing MPS-FBG, such that any change of wavelengthwill result in maximum power change. Utilization of non-iden-tical spectral shapes is also possible, however, the implementa-tion will be more difficult due to design, fabrication and spec-tral adjustment complexities. The reference MPS-FBG, whichis isolated from the vibration wave, filters the broadband sourceand then delivers it to the sensing MPS-FBG via a circulator.As the sensors experienced dynamic strain, the wavelength shiftchanges the optical power received by the photodetector ac-cording to the amplitude and frequency of the vibration wave.The proposed sensing concept is shown in Fig. 1(b) is based

on synchronous operation of locally formed multiple subchan-nels. Hence, the total optical power change is a multiple of thatof the single subchannel systems [9], [21]. Guard bands are suf-ficiently allocated between the adjacent subchannels. This en-sures the avoidance of overlapping between the lower sensingsubchannels and the upper reference subchannels, which couldimpair operation. The conceptual transmission spectra shownin Fig. 1(b) would be ideal in terms of performance scalability.However, in practice such subchannel allocation could proveextremely difficult to meet using appropriate writing methods,such as superimposed FBG [22]. Therefore, an approach basedon identical MPS-FBG has been opted for. In the MPS-FBGpair, sufficient guard band is created by using a transmissionmode at the reference MPS-FBG that is much narrower than the

Fig. 1. (a) Setup of the intensity-type vibration sensor using a broadband sourceand a FBG pair; and (b) the conceptual transmission spectra of reference andsensing gratings of the proposed scheme.

reflection property of the sensing MPS-FBG. Provided a typ-ical broadband source has a built-in isolator, the back-reflec-tion from the reference MPS-FBG can be eliminated withoutinserting an isolator to the output of ASE source. It is necessaryto eliminate back-reflection to ensure intensity stability of thesource. The FBG pair based sensing system is capable of com-pensating temperature effects by appropriate positioning of theFBG pair, as has been demonstrated in the related works [9],[21].A general sensing model was established to systematically

analyze the effect of MPS-FBG spectrum characteristics on sen-sitivity. Based on the operational framework of the circuit, thepower spectral density, arriving at the photodetector can bewritten as

(1)

where is the power spectral density (PSD) emitted by thebroadband source. and are reflection and transmis-sion spectra functions of the reference and sensing MPS-FBGs,respectively. In this analysis, and are numericallyacquired from transfer matrix analysis, thus it implies that thespectral characteristics (e.g., and peak reflectivity) are con-stituted in the functions. Subscripts and denote the refer-ence and sensing MPS-FBGs. is the total wavelength shiftof the sensing MPS-FBG with regard to that of the referenceMPS-FBG. In sensing operations, the total wavelength shift in-cludes the initial wavelength offset and the wavelength shift dueto vibration. is the total optical loss accounting for the lossimposed by passive components. The total optical power, in-cident on the photodetector is therefore given by

(2)

where is the wavelength interval between samples, is aninteger such that is an integer denoting numberof data points of the spectrums and is an integer representingthe index notation for overlapped matrix between and

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. Assuming is spectrally flat, (2) can be writtenas

(3)

where the symbol represents the cross-correlation operator.Using this relation, the sensitivity assessment therefore accountsonly for and profiles while and are consideredconstants. The generated photocurrent relies on the photode-tector responsivity, which, in this case, can be assumed con-stant as the sensors are operating within a similar wavelengthrange. The amplitude of AC voltage generated at the receiveris proportional to the rate of optical power change with regardto the wavelength shift. The rate of optical power change ratecan be obtained by differentiating with respect to the

. The relative sensitivity is determined bycomparing between two pairs of designs. Allnumerical analysis were carried out using Matlab software.For individual subchannel sensors, it can be shown that

the optical power change is proportional to the peak reflec-tivity of both FBGs. Consider that the reflectivity function ofreference and sensing FBGs can be represented as

) and , whereand are the peak reflectivity constant, while,

and are the normalized reflectivity functionswith regards to the peak reflectivity. By factoring the peakreflectivity constants out of the calculation (using summationand derivative identities), it can be shown that sensitivity isproportional to the peak reflectivity of both FBGs, that is

(4)

Hence, is crucial that for theMPS-FBG pair to possessed highreflectivity of the stopbands and high transmission factor of thepassbands, for the optimal performances.While the steepness ofthe reflection spectrum, i.e., the change of reflectivity per unitwavelength, determines the transition profile from low to peaksensitivity. This would suggests the clear benefit of high steep-ness and flattened peak reflectivity, i.e., a filter with the rectan-gular transfer function, in both reference and sensing FBGs asthe transition to peak sensitivity will be more drastic.

B. Sensitivity Analysis of MPS-FBG Scheme

Fig. 2 shows the biasing condition of the fabricated normalFBG, which is used as the benchmark performance, and, the5-and 17-phase shifted (PS) FBG, which are used in the testedscheme. With various design issues involved, this section onlyfocuses on sensing performance, while the design approachis discussed in the following subsection. Using the presentedrelation, it has been shown that the peak sensitivity of the singlesubchannel FBG is proportional to reflectivities of the pair.Hence, the FBG pair that is shown in Fig. 2(a) with 100%reflectivity represents the maximum sensitivity that can beachieved by the benchmark operation. For the MPS-FBG underspatially uniform vibration, the extension of phase shift region,

, ( , with being the effective refrac-tive index) is proportional to the extension of grating period ,and therefore the round-trip phase shift, is maintained at

Fig. 2. Biasing conditions of the fabricated normal FBG, 5-PS FBG and 17-PSFBG pairs.

. Consequently, the transmission and reflection factors of allsubchannels are also maintained while shifted, and this allowsthe multiple subchannels to be effectively used for vibrationsensing.For identical subchannels with similar operating condi-

tions are shown in Fig. 1(b), each subchannel will producesimilar , hence the sensitivity is times of thesingle subchannel. However, sensitivity computation of theMPS-FBG within a complex spectrum requires a numericalapproach which has been explained previously. Fig. 3 illus-trates an example of sensitivity calculation of the center peaksof 5-PS FBG. Here, only center peaks are shown for clarity. Itcan be seen that the transmission bands are narrower than thereflection bands and therefore sufficient guard bands are createdbetween adjacent subchannels. From the cross-correlation op-eration, and followed by derivative operation, the biasing pointfor maximum sensitivity operation can be determined. It can beobserved that the transition of is almost linear at points(a) and (b), suggesting that linear response can be attained ifa sensor is biased at this point. Further, peak sensitivity alsooccurs at these linear points, which give additional advantagesto operation at this point. The sensitivity profile centered atpeak sensitivity is non-symmetrical, as distinguished from thenormal FBG. The high reflectivity and high steepness of thespectrums ensure that each subchannel performs at maximumcapability.Fig. 4(a) shows the relative sensitivity of 5-, 9- and 17-PS

FBG in comparison to the benchmark sensitivity of the normalFBG (i.e., 0-PS). The relative sensitivity is evidently greaterthan the normal FBG (e.g., 9.1 dB for 5-PS), while the high sen-sitivity is confined to a small spectral range (e.g., 0.02 nm for5-PS). Since the MPS-FBG was fabricated without introducinga chirp in the phase sampling function, the stopbands and pass

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Fig. 3. Sensitivity calculation in the MPS-FBG scheme. Only the center peaksare shown for clarity.

bands bandwidth become smaller with an increased number ofphase shifts. Consequently, the high sensitivity range becomesnarrower as can be observed in 17-PS FBG. Numerical anal-ysis result suggests that the total reflected power, at thehighest sensitivity point for the 17-PS is actually lower than the5- and 9-PS FBG, which can be explained from the narrowerof 17-PS. However, the achieved is higher

for the 17-PS due to a more drastic change of at par-ticular from the synchronous subchannels operation. There-fore, under this scheme, the sensitivity will be increased as moresubchannels (or phase shifts) are included, but at the expense ofthe dynamic range.Fig. 4(b) shows the theoretical peak sensitivity of the pro-

posed scheme compared to the normal FBG, suggesting thatsubstantial sensitivity enhancement can be attained, as it is upto 20 dB compared to the normal FBG pair. Note that the ex-pected sensitivity is consequential from the adopted design con-straints, which ultimately determine the MPS-FBG parameters.Different relative sensitivity would be attained using differentdesign criteria. The result obtained from experiment is also com-parable to the theoretical expectation and this will be discussedin Section III.

C. Design Approach and Considerations of MPS-FBGVibration Sensor

MPS-FBG with different phase shift numbers was designedin a way that fair sensitivity comparisons can be made betweenMPS-FBG and normal FBG. Since increments in the numberof phase shifts reduce the reflectivity, the grating length hasbeen increased to maintain high reflectivity. It has been shownfor subchannels that the coupling coefficient for MPS-FBGwould need to attain times higher than normal FBGto achieve similar reflectivity [23] with being the Fourierdiffraction efficiency. Nonetheless, the grating extension needsto be minimized as much as possible to avoid excessive sub-channel narrowing. Excessive subchannel narrowing reflectsthe reduction of sensor dynamic range and added complexityin biasing the sensor at the optimum point. In this study, thehighest number of phase shifts incorporated in grating withoutintroducing chirp while attaining satisfactory subchannelsquality was 17. For MPS-FBG with a higher subchannelscount, a suitable chirp rate can be introduced in the phase

Fig. 4. (a) Relative sensitivity of 5-, 7- and 17-PS FBG benchmarked to thenormal FBG, and (b) relative peak sensitivity against increased phase shiftnumber (similar to increase in number of subchannels).

Fig. 5. General schematic of MPS-FBG used in tailoring the subchannels’reflectivity.

sampling function to expand the subchannels bandwidth so thatit will be more practical in application.Fig. 5 shows the schematic of 5-PS FBG used in tailoring the

reflectivity. MPS-FBG was designed using the transfer matrixmethod (TMM) [24] with the objective being to obtain uniformand high reflectivity of stopbands, while enhancing the trans-mission peak of pass bands. The design approach used is sim-ilar to that used in [13]. The phase shifts were symmetricallyspaced; equal distance between adjacent phase shifts, andequal distance of the both sides of outer phase shifts to gratingedge, . The design of MPS-FBG was carried out by varyingand , while maintaining the total length, .Since a similar approach was used to tailor all designs, only

the optimization of 5-PS is presented. To begin analysis,is set to 10 mm while the coupling coefficient, is set withinfabrication capability, which is 600 m . Fig. 6(a) shows the

AZMI et al.: PERFORMANCE ENHANCEMENT OF VIBRATION SENSING 3457

Fig. 6. (a) Peak transmission loss of all subchannels of 5-PS FBG forvarying phase shift spacing, , while total length, , is fixed at 10 mm,and (b) implications of varying at fixed mm mm onsubchannels’ bandwidth and central stopband loss .

analysis result where refers to the peak trans-mission loss of th subchannels. From the calculated stopbandloss, it appears that the optimum spacing ranges from 0.95 mmto 1.05 mm; thus, for 5-PS design, mm mmwas selected for fabrication. Within this range, the transmis-sion loss of all lobes coincides at optimum loss, even thoughand are declining but still within dB loss. Theoreti-

cally, the loss on the left and right side are present in a pair dueto symmetrical properties of the transmission spectrum. Impli-cations of varying can be seen in Fig. 6(b). Increasingresults in an improvement of stopbands loss . However,it also decreases the subchannels bandwidth. It seems that thesetrade-offs can be compromised at mm, and thereforethis length was used in the experiment.For the experiment, MPS-FBGs with the following dimen-

sions were fabricated: 5-PS ( mm, mm,mm) and 17-PS ( mm, mm,

mm). Both MPS-FBGs were fabricated at a similar Bragg’swavelength in order to obtain similar illumination from ASEsource. The MPS-FBGs were written on SMF-28 hydrogenloaded fiber by UV scanning through a phase mask. The phasemask was mounted onto a dither stage, which provides therequired apodization and phase shift. All MPS-FBGs werewritten by a 244 nm frequency doubled Argon ion laser atoutput power of 150 mW and scanning speed of 2 mm/min.The small wavelength offset between the reference and thesensing MPS-FBG is created by the differential strain appliedto fiber during fabrication with system accuracy of g, e.g.,for 5-PS FBG, 2.5 g strain was applied to create wavelengthspacing shown in Fig. 2. Post-annealing may also contributeto wavelength discrepancy by variation of temperature andapplied strain; therefore, extreme care was taken to ensureconsistency of this process. The performance of the fabricatedMPS-FBG was in line with the theoretical expectations interms of channel spacing and average grating strength as indi-cated in Fig. 7. There are mismatched between the theoreticaland experimental transmission spectrums, noticeably at thesidelobes of the 5-PS FBG, while the central subchannels arealmost identical. Through theoretical analysis performed on thefabricated spectrums, it was found that the relative sensitivityof the fabricated spectrums is close to the theoretical spectrum(e.g., only 1 dB difference for the 5-PS FBG). This is due to

Fig. 7. Theoretical and experimental transmission spectrum of 5- and 17-PSFBG.

the most of the light filtering operations are performed at thecentral subchannels. While the sidelobes are also affected thesensing performance but only at minimal extend.

III. EXPERIMENT

A. Setup

In the system setup, a National Instrument PCI-6251 data ac-quisition card was configured using Labview software to per-form data acquisition tasks with the following values: samplingrate of 50 kHz, buffer size of 8192 bits, voltage range ofV, and differential port mode. The setting produced a time res-olution of 20 s and frequency resolution of 6.1 Hz. The re-ceiver gain (Optiphase V-500) was set similarly for all sensorsat 100 kV/W. A composite made of E-glass fiber and vinylesterresin with a size of 20 cm 10 cm and thickness of 4.62 mmmade from vacuum infusion was selected as the material for thetesting.The MPS-FBG sensors were glued to the surface of the

test material in normal orientation to the underneath piezo-electric transducer used to transmit sinusoidal signal from afunction generator (BWD-112). It is assumed that the vibrationimpinging on the sensor through the material was uniformbecause of the normal incidence of the vibration wave andthe relatively longer vibration wavelength compared to theMPS-FBG length. Hence, simultaneous wavelength shifts of allsubchannels were expected when impinged by uniform pres-sure. The gluing conditions were kept similar for all sensors(thickness: 1.5 mm and length: 30 mm) to achieve similaracoustic coupling efficiency to the composite to the sensor.A piezoelectric sensor was placed close to the MPS-FBG

sensor to record the absolute pressure in close proximity tothe MPS-FBG sensor. The spectral power of the C-Band ASEsource (BWC-ASE 1), measured using an optical spectrumanalyzer (Agilent-86140B), showed readings of dBm at1528 nm. If the MPS-FBGs experienced dissimilar temperaturechange, the preset relative wavelength offset will be changed,consequently alters the preset optimum operating point. There-fore, it was necessary to eliminate the spectral drift due tochange of surrounding temperature. This was to ensure thatthe sensitivity measurement was obtained only at a particularoptimum operating point, therefore avoids falsification of thesensitivity measurement. The DC voltage at the receiver wasmeasured periodically to ensure the reference DC voltagelevel was stable, implying a constant optical power was de-tected at receiver, hence, the unchanged of the operating point.

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Fig. 8. Comparison of signal spectrums at frequency 5.6 kHz at a similar pres-sure for all tested sensors.

Room temperature was monitored throughout the experiment,and gave a constant reading of 20 C. The sensor operatingpoint was set close to the optimum point during the fabricationprocess. This operating point was further fine-tuned, maintainedor calibrated during experiment by submerging the referenceMPS-FBG underwater with a controlled temperature. Thistechnique produced uniform temperature and quasi-static straindistributions on the MPS-FBG, hence avoiding anomalies thatcould distort the spectrum. The fact that room temperature wasconstant has allowed continuous measurement was taken overextended period of time. The fine tuning also equalized theintensity noise level of all sensors.

B. Sensitivity Comparison

Fig. 8 shows the instantaneous root mean squared (RMS)spectrums of all the sensors at frequency 5.6 kHz when a si-nusoidal signal was imparted at a reference pressure. The lowerlimit of detection was limited by intensity noise to 83 dB re1 V. Observation of the signal enhancement can be directlymade by comparing the peak voltage excursion to the noisefloor. From the instantaneous spectra, it is shown that 5- and17-PS FBG produced excess sensitivity over the normal FBGby 8.7 dB and 20.2 dB respectively. Apparently, no harmonicsappeared in the waveforms, hinting that the sensor would pro-duce a linear response. To quantify the relative sensitivity, mea-surements were taken between 600 Hz and 16 kHz with RMSaveraging mode . The frequency responses between 600Hz to 16 kHz for 5 and 17 PS-FBG used to quantify the averagesensitivity enhancement is shown in Fig. 9, given that the sen-sitivity enhancements compared with the normal FBG are 7.4dB and 18.5 dB for 5-PS and 17-PS design, respectively. Theseexperimental values are plotted in Fig. 4(b). The responses ofthe sensors are non-uniform, varying nearly dB from theaverage response. The slight discrepancy between experimentaland theoretical results is believed due to a small offset from theoptimum biasing point. Better control of biasing points as wellas the improvement of grating transmission and reflection fac-tors could further enhance the performance.

Fig. 9. Frequency responses of 5- and 17-PS FBG.

Fig. 10. Dynamic range tested for 5-PS FBG in comparison to the normal FBGat frequency 5.6 kHz.

The dynamic range test was performed at the selected fre-quency of 5.6 kHz. A continuous sinusoidal signal was pro-duced within signal generator allowable driving voltage, whichcovered 30 dB detectable pressure range (0 dB = 158 dB re1 Pa). Fig. 10 illustrates the detected signal magnitude as afunction of pressure for the normal FBG and the 5-PS FBG. Asexpected for such small pressure range, the 5-PS FBG exhib-ited linear response. This is corresponding to the result shown inFig. 4(a), whereby within limited pressure range, the sensitivitycurve is almost linear. RMS time averaging mode (20 times)was applied and achieved 8 dB intensity noise suppression,thus improving the minimum lower limit of detection. Since theresponse slope is also dependent on the biasing point, fine tuningof the operating point could achieve a higher response slope.

C. Response to Simulated Vibration Source

The normal FBG, 5-PS FBG and 17-PS FBG were also testedwith simulated acoustic emissions [25], produced by 2 mmStaedtler HB pencil leads breaking. With two available setups,two sensors were able to be tested simultaneously. The sourcepoint was located 10 cm longitudinally from the central sensors.The recorded waveforms of this test are shown in Fig. 11. Themeasured amplitude noise level was equal for all sensors at2 mV within 25 kHz. The similar arrival time of the voltagebursts give indication of similar distance of the tested sensors.In the first test between normal FBG and 5-PS FBG, the 5-PSFBG (with V) produced an improvement in SNR

AZMI et al.: PERFORMANCE ENHANCEMENT OF VIBRATION SENSING 3459

Fig. 11. Comparison of waveforms from pencil lead break test (a) Test I:normal FBG versus 5-PS FBG, and (b) Test II: 5-PS FBG versus 17-PS FBG.

over the normal FBG (with V) by 6.3 dB. Inthe second test, the 17-PS FBG (with V) furtherimproved the SNR over the 5-PS FBG (with V)by 11.7 dB, which was comparable to the predicted sensitivity.This test has sufficiently manifested the improved signal

strength compared to the normal FBG within 25 kHz band-width. This scheme offers an economical hardware system thatis highly feasible for coarse wavelength division multiplexing(CWDM) based on a single broadband source. The potentialpractical applications are centered on low bandwidth appli-cations, such as ground traffic monitoring, intrusion warningsystem and hydrophone. Vibration detection in the time domainwill deteriorate at high bandwidths (over 50 kHz), as thenoise become dominant. For applications that demand higherfrequency range, such as in structural failure monitoring,improvement of the signal power and effective suppression ofnoise are still required. This can possibly be done by increasingthe number of subchannels to improve the signal power, using amore effective method to control operating point; for exampleby using a piezoelectric controller, reducing the optical lossoriginated from connectors and fiber bending, and utilizationof digital signal processing techniques for noise suppression.

IV. CONCLUSION

This paper proposes a novel intensity-type vibration sensorbased on a multiple subchannels sensing concept realizedwith MPS-FBG. Under the MPS-FBG scheme, the vibrationsensitivity has been significantly enhanced over a normalFBG by synchronous sensing of multiple subchannels, whichtranslates into substantial total intensity change. Theoreticalmodels with related analysis, experimental works, potential

issues, and the related counteractive dealings also have beendiscussed. The proposed scheme would provide an econom-ical CWDM scheme using simple intensity-type sensing withsignificantly improved performance. The MPS-FBG sensingscheme could have several distinctive merits: i) compact andlocalized structure which can be used to detect a localizedvibration, ii) dense, uniform subchannels reflectivity and band-width, while having narrower passbands than the stopbands,which permits realization of multiple subchannels scheme, andiii) relatively high manufacturing repeatability. This schemewould be highly desirable to be deployed in any applicationsthat operate within low frequency range, such as ground trafficmonitoring, intrusion warning system and hydrophone.

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Asrul Izam Azmi received the B.Eng. and M.Eng. in electrical engineeringin 2001 and 2004, respectively, from Universiti Teknologi Malaysia (UTM),Skudai.He is currently pursuing the Ph.D. degree at the University of New South

Wales, Sydney, Australia. He joined UTM in 2002 as a tutor and is currently alecturer in the same university. His research interests include development offiber grating-based sensing techniques and application of fiber grating sensingtechnology in mechanical engineering.He joined UTM in 2002 as an academic staff member. His research interests

include development and application of fiber grating-based sensing techniques.

Deep Sen received the B.E. and Ph.D. degrees in electrical engineering fromthe University of New South Wales, Sydney, Australia, in 1989 and 1995,respectively.From 1994 to 2002 he was a senior technical staff member at AT&T (Bell)

Laboratories Research in New Jersey, USA. He is currently principal audio sci-entist at SRS Laboratories, USA, and also teaching and research academic atthe University of New South Wales. His research interests include three-dimen-sional immersive audio recording/representation/synthesis, blind source sepa-ration, speech and music signal processing, cochlear modeling, and prosthetica.Dr Sen is a member of the Acoustical Society of America, Audio Engineering

Society and Association for Research in Otolaryngology.

Wenjuan Sheng, photograph and biography unavailable at time of publication.

John Canning, photograph and biography unavailable at time of publication.

Gang-Ding Peng received the B.Sc. degree in physics from Fudan University,China, in 1982, the M.Sc. degree in applied physics, and the Ph.D. degree inelectronic engineering from Shanghai Jiao Tong University, Shanghai, China,in 1984 and 1987, respectively.From 1987 through 1988 he was a lecturer of the Jiao Tong University. He

was a postdoctoral research fellow in the Optical Sciences Centre of the Aus-tralian National University, Canberra, from 1988-1991. He has been workingwith University of NSW in Sydney, Australia since 1991, was a Queen ElizabethII Fellow from 1992-1996 and is currently a Professor in the same university.He is a fellow and life member of OSA and a life member of SPIE. His researchinterests include optical fiber and waveguide devices, silica and polymer opticalfibers, optical fiber sensors and nonlinear optics.