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Sensors and Actuators B 155 (2011) 258–263

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

iniature interferometric humidity sensors based on silica/polymer microfibernot resonators

u Wu ∗, TianHu Zhang, YunJiang Rao, Yuan Gongey Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, Sichuan 610054, China

r t i c l e i n f o

rticle history:eceived 25 August 2010eceived in revised form5 December 2010

a b s t r a c t

In this paper, two fiber-optic interferometric humidity sensors based on silica/polymer microfiber knotresonators (SMKR/PMKR) are reported. These tiny humidity sensors are directly fabricated by using sil-ica/polymer microfibers without any humidity-sensitive coating. The silica microfiber knot resonatorsensor has a humidity sensitivity of ∼12 pm/10%-RH within a linearity range from 15%-RH to 60%-RH,

ccepted 19 December 2010vailable online 27 December 2010

eywords:ptical fiberumidity sensors

while the polymer microfiber knot resonators sensor has a humidity sensitivity of ∼88 pm/10%-RH, witha linearity range from 17%-RH to 95%-RH. The temporal response of the PMKR sensor is <0.5 s. Such typesof humidity sensors have advantages of easy fabrication, fast response, extremely compact size, stableand low cost, they would find potential applications in micro-scale humidity sensing.

© 2010 Elsevier B.V. All rights reserved.

ilica/polymer microfibersnot resonators

. Introduction

Monitoring and controlling humidity is required in many fields,uch as food processing, weather forecasting, pharmaceutical andemiconductor industries. In the past few years, different schemesave been investigated for humidity sensing, e.g., capacitanceumidity sensors, resistive humidity sensors, optical humidity sen-ors and so on [1–4]. Optical fiber humidity sensors offer thedvantages of small size, immunity to electromagnetic interfer-nce, corrosion resistant, remote operation, and the ability ofultiplexing the information from many sensors into a single opti-

al fiber [5]. So far, there have been considerable research efforts ineveloping optical humidity sensors. Baririan fabricated an opticalber humidity sensor based on a tapered fiber with agarose gel,easuring the humidity by the output light power changes [6–8].uto et al. presented a plastic optical fiber sensor for real-time

umidity monitoring which the fiber itself is sensitive to humidity.n 2006, Maria and co-workers proposed an optical fiber long-eriod grating humidity sensor with poly (ethylene oxide)/cobalthloride coating [9–13]. Recently, Bhola et al. reported an inte-rated optical micro-ring resonator humidity sensor based on

ol–gel and the wavelength shift measurement [14–16]. Most ofhe humidity sensors needed to be coated with humidity sensi-ive materials, and were based on the measurement of intensityhanges [7–12]. However, coating often makes the fabrication pro-

∗ Corresponding author.E-mail address: wuyuzju08@gmail.com (Y. Wu).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.12.030

cess complex, and the measurement accuracy is greatly influencedby the fluctuations of the laser power and the additional losses inthe sensing system [6].

Optical microfibers/nanofibers knot, loop and coil resonatorshave been attracting great attentions in many sensing fields, dueto its advantages of small size, high Q-factor, absolute wavelengthmeasurement, low loss, etc. [17–21]. In this paper, the microfiberknot resonators (MKRs) used for humidity sensing are proposedand demonstrated, which show the characteristics of high sensi-tivity, fast response and wide detecting range. As the refractiveindex of the MKR is modified by the humidity, the resonancewavelength will be shifted. The dynamic range, resolution and theresponse time of the MKR humidity sensors made of silica/polymermicrofibers have been investigated by experiment, respectively.Compared with other fiber optic humidity sensors, these MKRhumidity sensors have advantages such as no need for coating, lessefforts to facture, smaller size, faster response time and lower cost,they would find applications in micro-scale humidity sensing.

2. Theoretical analysis

2.1. Medium’s refractive index changes with absorbing the watermolecules

Refractive index (n) refers to the speed radio of light in a vacuumand light spread in the actual medium. It depends on the mediumcomposition, temperature and wavelength. From the material pointof view, we quote the medium molecular refraction R (R means thesum of the constituent ions’ polarization degree), according to the

Y. Wu et al. / Sensors and Actuators B 155 (2011) 258–263 259

L

R

wwmf

dD

D

n

fiacsadibt

2r

rslr

F

wp

Fig. 2. Theoretical simulation of MKRs for different refractive index values. For

Fig. 1. The relationship between materials’ density and refractive index.

orentz–Lorenz formula which can be expressed as [22]

= n2 − 1n2 + 2

M

�= 4�

3NA� (1)

here n is the medium refractive index, M is the medium moleculareight; � is the medium density; NA is Avogadro constant; � is theedium polarization index. According to Eq. (1), we can get the

unction of the n as follows:

n2 − 1n2 + 2

= 4�NA��

3M(2)

We assume the right side of the above equation as D (short foreterminant). So the refractive index is changed with the variable, which can be described as

= 4�NA��

3M(3)

n2 − 1n2 + 2

= D (4)

Solve Eq. (4), we get

2 = 1 + 2D

1 − D(5)

The above equation (Eq. (5)) relates the refractive index as aunction of D. The calculated results are plotted in Fig. 1. Accord-ng to the figure, clearly can be seen that refractive index changesre consistent with changes in D, in the case of other factors areonstant, D changes and changes in medium density � is also con-istent. So we can get the conclusion: in the case of others factorsre constant, if the medium swelled after absorbing the water, theensity � decreased resulted in the refractive index n decreased,

n the opposite, if the medium shrunk after absorbing the water, orecause the water molecules filled the interstitial gaps of materials,he density � increased resulted in the refractive index n increased.

.2. The resonance wavelength of the MKRs varies with theefractive index change

In this section, we discussed the relationship between theefractive index and the resonance wavelength of the MKRs. Byolving the coupled mode equations, the transmission property ofight propagating along the MKRs can be obtained. The free spectralange (FSR) of MKR can be given as [23]

SR ≈ �2

Ng�D= �2

NgL(6)

here � is the wavelength, Ng is the group index of the moderopagating in the microfiber, and L is the loop length.

clarity, only three characteristic spectra are shown for n1 = 1.4500 (green line),n1 = 1.4495 (red line), and n1 = 1.4490 (blue line). (For interpretation of the refer-ences to color in this figure legend, the reader is referred to the web version of thearticle.)

When the length or index of the microfiber or both of them var-ied, leading to resonant wavelength shifts. In order to obtain theresonant wavelength in a dynamic refractive index and loop lengthfields, we calculated the derivative of Eq. (6) with respect to theabove variables, thus the relationship can be evaluated as below:

��

�= �L

L+ �n

n(7)

where n is the effective index of the mode propagating in themicrofiber, �n is the refractive index variation and �L is the looplength variation.

In our experiment, the length of the microring was very short,about 2.5 mm, resulted in the �L to be negligible, so Eq. (7) is sim-plified as

��

�= �n

n(8)

Since, MKRs can be achieved the high quality factor (Q ∼ 105)devices [24], small resonant shifts, and hence small changes inhumidity can be detected. When the wavelength is about 1540 nm,resonant wavelength shifts with the refractive index changes waspotted in Fig. 2.

3. Experimental and results

3.1. Sensor fabrication

The silica microfibers used in this work were fabricated byflame-heated taper-drawing of a single-mode fiber (SMF-28, Corn-ing) [25], while polymer (poly-methyl methacrylate, PMMA)microfibers were fabricated by direct drawing of solvated poly-mers that have been reported elsewhere [26]. These microfiberswith minimum diameter of <200 nm and length up to millimetersshowed smooth outer surface morphology without pronouncedbending or obvious structural defects. They can be bent withmuch smaller radius of curvature than that of standard opticalfibers to form much more compact optical waveguide structures[17]. Fig. 3(a) and (b) showed the SEM (scan electron microscope)images of a 500 × 10−6-m diameter SMKR with 1.2 × 10−6-m diam-eter silica microfiber and a 500 × 10−6-m diameter PMKR with2.1 × 10−6-m diameter PMMA microfiber.

The MKRs were assembled by manipulating with two fiber-

tapers under a microscope. A microfiber taper, which was usedas the collecting fiber, was arranged adjacent to the freestand-ing end of SMKR to form a coupler. The two microfiber taperscan attract tightly via Van Der Waals and electrostatic attractiveforce in the coupling region. For PMKR, it needed two microfiber

260 Y. Wu et al. / Sensors and Actuators B 155 (2011) 258–263

Fp∼

tTbwb

3

fimso0

p

ig. 3. The SEM pictures of SMKR and PMKR (insets shows details of the cou-ling area): (a) silica knot with diameter ∼500 �m; (b) PMMA knot with diameter500 �m.

apers, each connected to one of the free standing ends of the PMKR.he microfiber tapers served as the launching and collecting fibersy evanescent wave coupling. Finally, the SMKR/PMKR structuresere placed on an MgF2 slab between two stages fixed on a plastic

ase due to its low refractive index (∼1.37), as show in Fig. 4.

.2. Measurement procedure and results

To investigate the performance of these MKRs humidity sensors,rstly we tested the output spectrum of the MKRs. in the experi-ents, we used Si720 (MICRON OPTICS, USA) as the input light

ource and optical spectrum analyzer (OSA). The band range of theutput spectrum is 1510–1590 nm and light detection resolution is.2 pm. It combined output light source and detection equipment.

Fig. 5 showed the typical transmission spectra of the silica andolymer microfiber knots with diameters of 2.4 and 2.8 mm, respec-

Fig. 4. Schematic diagram of the MKRs humidity sensing structures.

Fig. 5. Transmission spectra of the silica and polymer microfiber knots: SMKR (blackline) and PMKR (red line). (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of the article.)

tively. The 2.4 mm diameter knot, assembled with 1.2 × 10−6-mdiameter silica microfiber (refractive index ∼1.45), had a Q factor of∼15,000 and a FSR of ∼0.22 nm. The 2.8 mm diameter knot, assem-bled with 2.1 × 10−6-m diameter polymer microfiber (refractiveindex ∼1.49), showed a Q factor of ∼20,000 and a FSR of ∼0.17 nm.The Q factors obtained from these knot resonators were closed toor higher than those reported for microfiber loop resonators [27].

In order to demonstrate the humidity sensing properties ofthe MKRs, the sensor device was placed in a humidity controller(Blueangel company’s products), in which the humidity can be con-trolled by varying the amount of wet air with a dehumidifier andthe environmental temperature can be kept constant. We still usedSi720 as the input source and spectrum analyzer, as showed inFig. 6. At the beginning, the humidity controller was full of wetair and at a very high humidity (almost 100%), then we began topump the wet air out of the humidity controller. During the process,the temperature was fixed at the room temperature at 23 ◦C andwe recorded the wavelength data in different humidity. The reso-nance spectrum at different humidity was showed in Fig. 7(a). Fromthe figure, we can get as the humidity decrease, a significant shift(21 pm for humidity increase from 14% to 37%) of the resonancewavelength (decreasing of the wavelength) was observed. Whichcan be explained as the porous matrix of the silicon traps watermolecules on its interior surface. This trapped water increased theaverage density of the silicon resulted in the increasing of refractiveindex. The change in the refractive index changed the resonancewavelength. Fig. 7(b) illustrated the relationship between humid-ity and resonance wavelength drift over the humidity range from14%-RH to 96%-RH. Within the range from 20%-RH to 60%-RH, theresult shows the linearity between humidity and resonant wave-length shift and the square regression coefficient (R2) is 0.9849. The

sensor exhibited high sensitivity (12 pm wavelength shift for 10%relative humidity change) as a function of RH for approximately270 pm change of the resonance wavelength in the range from 20%-RH to 94%-RH. In addition, it can be seen that at high RH level (e.g.,

Fig. 6. Schematic diagram of the experimental setup.

Y. Wu et al. / Sensors and Actuators B 155 (2011) 258–263 261

Fig. 7. (a) Spectra of SMKR at humidity of 14%-RH (black line), 37%-RH (red line),awit

Rbc

tmtottoltsbiaopou

TT

nd 46%-RH (green line) (inset shows a single resonance peak). (b) Shift of resonanceavelength of the humidity sensor (SMKR) with different levels of humidity. (For

nterpretation of the references to color in this figure legend, the reader is referredo the web version of the article.)

H >50%) the transmittance increased in a steeper way, which cane explained as aggregation of water molecules and formation oflusters on the SMKR [28].

For the PMKR, as showed in Fig. 8(a), from 34%-RH to 47%-RH,he total shift of the resonance wavelength was 114 pm and the

aximum extinction ratio was more than 8 dB. Fig. 8(b) illustratedhe relationship between humidity and resonance wavelength driftver the humidity range from 17%-RH to 98%-RH. The experimen-al measured sensitivity of 88 pm/10%-RH at the range from 17%o 90%, the result shows the linearity between humidity and res-nant wavelength shift and the R2 is 0.9953. As the PMMA hadarger molecule and the gap between molecules were also largerhan silica, resulted in more hydrophilic that explained the PMSKs’ensitivity was much higher than that of SMKRs. In addition, it cane seen that at high RH level (e.g., RH >90%) the shift of resonance

ncreased, which can be explained as the enhanced molecular state

dsorption of water on PMMA Surface. In this process, the retentionf water on the surface of the PMMA occurs through a process ofhysical adsorption without possible swelling effects being ruledut, probably. In these circumstances, the water will keep its molec-lar state. The molecular state will always remain both in solid,

able 1he characteristics of these two kinds of humidity sensors.

Type Diameters of themicrofibers and theknots

Number of points fromthe calibration curve

Linear ran

Silica MKR 1.2 �m/2.4 mm 9 20–60%-RPMMA MKR 2.1 �m/2.8 mm 14 17–90%-R

Fig. 8. (a) Spectra of PMKR at humidity of 34%-RH (black line), 41%-RH (red line),and 47%-RH (green line). (b) Shift of resonance wavelength of the humidity sensor(PMKR) with different levels of humidity. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of the article.)

liquid or vapor, and there will only be a change in the hydrogenbonds between molecules. [28].

To estimate the response time and the reproducibility of thesensor, the PMKR humidity sensor was placed in the humidity con-troller with rapid change of the RH (70%-RH to 83%-RH). A tunablelaser (Agilent 8164A) was tuned to 1572.2 nm, which correspondsto a steep resonance region near the slope is maximum. First, wemaintained the RH in the controller at 70%, and then the doorof the controller was opened suddenly to expose the sensor tothe environmental RH of 83%. Typical time-dependent responseof the sensor was showed in Fig. 9(a). The estimated responsetime (baseline to 90% signal saturation) of the sensor was about500 ms when RH jumped from 70% to 83% which were faster thanother types of RH sensors relying on conventional optical fibersor films (usually on an order of seconds) [4–16]. We believed thatthe fast response of the sensor could be attributed to the smalldiameter of the microfiber and the compact structure which allows

rapid diffusion (or evaporation) of water molecules [6]. In addi-tion, the reproducibility of the sensor was tested by alternatelycycling between 70%-RH and 83%-RH air inside the controller. Theoutput light power was changed from −34 to −39.5 dB accordingto the humidity changed from 70%-RH to 83%-RH and the typical

ge Sensitivity Resolutions (wavelengthresolution of the OSA wasset at 2 pm)

Squareregressioncoefficient (R2)

H 12 pm/10% RH 0.017 RH 0.9846H 88 pm/10% RH 0.0023 RH 0.9953

262 Y. Wu et al. / Sensors and Actuat

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ig. 9. (a) Typical time-dependent transmittance of the sensor reveals the responseime of about 500 ms when RH jumps from 70% to 83%. (b) Reversible response ofhe sensor tested by alternately cycling 70%-RH and 83%-RH airs.

esponse was given in Fig. 9(b). The practical high reproducibil-ty of these sensor structures could be achieved by improvinghe conformance in microfiber fabricating. Finally, the character-stics of these kinds of humidity sensors were summarized inable 1.

. Conclusion

In conclusion, two fiber-optic interferometric humidity sensorsased on silica/polymer microfiber knot resonators (SMKR/PMKR)ere demonstrated in this paper. The SMKR humidity sensor had a

arge humidity sensitivity at the range from 20%-RH to 96%-RH,hile the PMKR humidity sensor had a humidity sensitivity of88 pm/10% RH, with achieving the range from 17%-RH to 98%-H. The temporal response of the PMKR sensor was less than 0.5 s.hese tiny humidity sensors were fabricated by silica and polymerber directly, and no need to process with sensitive coating. Theylso have advantages of fast response, compact size, good stabilitynd low cost.

cknowledgment

This work is supported by the Key Project of National Naturalcience Foundation of China under Grant 60537040.

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Biographies

Yu Wu got his doctor degree at the State Key Laboratory of Modern Optical Instru-mentation, Zhejiang University. Now he is doing his researches at the Key Lab ofOptical Fiber sensing & Communications Technology (Education Ministry of China),University of Electronic Science & Technology of China. His researching interestsincluding the microfiber sensors, photonics sensors and MOEMS devices. Dr. Wu isa member of the Optical Society of America (OSA).

TianHu Zhang is pursuing a master’s degree in University of Electronic Science andTechnology of China. His research interest is in the area of optical sensing, especiallythe optical micro/nano-fiber sensors.

YunJiang Rao received the ME and PhD degrees in optoelectronic engineeringfrom Chongqing University, China, in 1986 and 1990, respectively, where he leda team to develop the first fully-automatic optical fiber fusion splicing machinein China.He joined the Optoelectronics Division Electric and Electronic Engineer-ing Department Strathclyde University, U.K., as a postdoctoral research fellow in1990. He was with Kent University, U.K., as a Research Fellow and then a SeniorResearch Fellow during 1992–1999, where he made important contributions to

fiber-optic low-coherence interferometry and in-fiber Bragg grating sensors. Dur-ing 1999–2004, he was a Chang-Jiang Chair Professor in Optical Engineering at theDepartment of Optoelectronic Engineering, Chongqing University, China, and estab-lished the Optical Fiber Technology Group with strong support of the Ministry ofEducation of China under the Program of ChangJiang Scholar Professorship. He iscurrently the Head of the Optical Fiber Technology Research Centre and Dean of

ctuat

SSE

YO

Y. Wu et al. / Sensors and A

chool Communication & InformationEngineering at the University of Electroniccience and Technology of China (UESTC) and Chang-Jiang Chair Professor in Opticalngineering.

uan Gong received the PhD degree in Optical Engineering from the Institute ofptics and Electronics, Chinese Academy of Sciences, Chengdu, China, in 2008 and

ors B 155 (2011) 258–263 263

then joined the Key Lab of Optical Sensing and Communications, University of Elec-tronic Science and Technology of China. He has authored or coauthored 30 journaland conference publications, and five issued Chinese patents. His current researchinterests include fiber-optic interferometric sensors, fiber-optic tweezers and cav-ity ring-down technologies. Dr. Gong is a member of the Optical Society of America(OSA).