Optics Communications 285 (2012) 2218–2222

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Optics Communications

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Microscopic multi-point temperature sensing based on microfiberdouble-knot resonators☆,☆☆

Yu Wu ⁎, Lan Jia, Tianhu Zhang, Yunjiang Rao, Yuan GongUniversity of Electronics Science & Technology of China, Key Laboratory of Optical Sensing and Communications (Education Ministry of China), Chengdu 610054, China

☆ This work was supported by Key Laboratory of Mafor Automobile Parts (Chongqing University of Technolo☆☆ The authors arewith the Key Laboratory of Optical Fib(Education Ministry of China), University of Electronic ScChengdu, Sichuan 611731, China.

⁎ Corresponding author. Tel.: +86 28 81630278.E-mail addresses: wuyuzju08@gmail.com, wuyuzj

0030-4018/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.optcom.2011.12.107

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 July 2011Received in revised form 3 November 2011Accepted 30 December 2011Available online 13 January 2012

Keywords:MicrofiberKnot resonatorMultiple-point temperature sensing

A novel fiber-optic sensor structure fabricated by cascading two optical microfiber knot resonators (MKRs) isproposed and demonstrated in this paper. A theoretical model for describing the principle of the sensingstructure is given and its temperature responses are characterized experimentally. Experimental resultsshow that high-precision and simultaneous multi-point temperature sensing in micro-scale can be achieved.Such a sensing structure also has the potential for achieving dual-parameter measurement to eliminate thecross-talk between two parameters in micro-scale.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, microfiber based photonic devices, such as micro-fiber lasers, filters, interferometers and sensors, have been attractingconsiderable attentions and extensive studies. Because of its advan-tages such as strong evanescent wave transmission, efficient evanes-cent coupling, low-bending loss, etc. Microfiber can be used to form avariety of optical structures in the micro-scale, which shows a greatapplication prospect in the field of micro-photonic devices and nearfield optical sensing. High-performance microfiber knot resonatorswere fabricated with an extinction ratio of >20 dB, free spectralrange (FSR) up to 14.9 nm, and quality factor, Q, as high as {10^{6}}[1–5]. The microfiber-knot resonator can be used as optical filters,as well as temperature, humidity, refractive-index sensors [6–9].Nano-thickness materials and technology are widely used now inmany areas, such as material, biology, medicine, chemistry, micro-electronics and micromechanical systems, for temperature character-istics and reliability researches, the high spatial resolution multi-point temperature measurement in the micro-scale has become akey technology. In this paper, we propose a microfiber double-knotresonators (MDKRs) structure for microscopic multiple-point tem-perature sensing. Two MKRs with diameters of ~500 μm are cascadedin series. 2-points temperature sensing in less than one millimeter

nufacture and Test Techniquesgy) Ministry of Education.er Sensing and Communicationsience and Technology of China,

u@163.com (Y. Wu).

rights reserved.

range is realized, with temperature sensitivity of ~17 pm/°C. It is indi-cated by theoretical analysis and experimental results that such astructure can be used to detect the temperature distribution in themicro-scale.

2. Theoretical analysis

Fig. 1 shows the schematic diagram of the MDKRs. This MDKRssensing structure is composed of two microfiber knots, which aredrawn from standard single-mode fibers. As the diameters of themicrofibers are in the sub-wavelength scale, the light could propagateby means of evanescent-wave coupling in the coupling region be-tween these two knots. Series-wound twoMKRs could also be formedby evanescent waves self-coupling [10].

By solving the electromagnetic field equations of each ports of theMDKRs structure, the transmission spectrum of this structure can bedescribed as [11–13]

E4 ¼ exp jβLCð Þ � ja2c2 þa2

2b22 exp jβ2L2ð Þ

1� a2c2 exp jβ2L2ð Þð Þ

!" #

• ja1c1 þa1

2b12 exp jβ1L1ð Þ

1� a1c1 exp jβ1L1ð Þð Þ

" #• E1

ð1Þ

In the equation, β is the propagation constant, n1, n2 is thetemperature-dependent relative refractive index of the two MKRs re-spectively, r,k represent the coupling loss and efficiency of the MKR;ai=(1- ri)0.5,bi=(1-ki)0.5, ci= jki

0.5, L1, L2 denote the loop lengths ofthe two MKRs, the light transmission length between them is LC.

Based on the above theoretical analysis, we built the simulationmodel with parameter values of r1=r2=0.1, k1=0.6, k2=0.4,

Fig. 1. The schematic diagram of the MDKRs structure.

2219Y. Wu et al. / Optics Communications 285 (2012) 2218–2222

L1=L2=400 μm. According to Eq. (1), the interference phenomenoncan be observed when a microfiber touches itself and causes self-coupling of propagating light. Under certain phase conditions, strongself-coupling of a microfiber knot with small losses can generatehigh-Q resonances. We can obtain the interference spectra of theMDKRs plotted in Fig. 2(a) (blue line). When the two micro-knotshave the same loop length, the interference spectrum shows the reg-ular resonance. If one of the loop lengths is changes, it will modulatethe effective transmission length of light. In this situation, the trans-mission spectrum can be calculated from the Eq. (1). As the phasematching condition of this MDKRs structure is varied, a new phasematching generate the second-order resonance peak (yellow line)between the primary resonance peaks (first-order resonance), asshown in Fig. 2(a). The corresponding phase is plotted in Fig. 2(b).

In order to investigate the sensing characteristics of this double-knots structure, a temperature experiment is set up. According tothe thermo-optical effects, the changes of the ambient temperaturearound the microfiber will result in the variations of the micro-knot's effective refractive index. For the SiO2 microfiber formedMKR, the relationship of the resonance wavelength shift of the

Fig. 2. (a) Theoretical transmission spectra and (b) the phase of the MDKRs with sameand different loop lengths.

interference spectrum and the temperature variations can beexpressed as [14]

Δλ=λ ¼ ΔL=Lþ Δn=nð ÞTemp: ¼ α þ γð ÞΔT ð2Þ

where α is the coefficient of thermal expansion (CTE) of themicrofiber, γ=1/n(dn/dT) is the thermal-optical coefficient (TOC) ofthe microfiber. n is the effective index of the mode propagating inthe microfiber, and Δn is temperature-dependent refractive-indexvariation. According to Eq. (2), it is assumed that the resonancepeak shift Δλ is small compared with wavelength λ, so Δλ could beconsidered to change linearly with the temperature variation. For aSiO2 α=0.05×10−5/°C, γ=1×10−5/°C.

Based on the above analysis, temperature sensing and transmissionproperties are simulated firstly. On the initial state, we assumed twomicro-knots have the same loop length (L1=L2=400 μm). Then, thetemperature changes will modulate both of the refractive index andloop length. As shown in Fig. 3(a), while the ambient temperature ofthe second micro-knot keeps constant, the first micro-knot's tempera-ture increased from t1−1 t1−2 to t1−3. The simulated results showthat the first-order resonance peak shifts with the variation of temper-ature, but the second-order resonance peaks keep their positionsunchanged. Similar results are obtained when the temperature of thesecond micro-knot changes while the first micro-knot's temperaturekeeps constant. From Fig. 3(b), it can be seen that the first-order reso-nance peak was fixed with no moving, but the second-order resonancepeak shifts in accordancewith the temperature. Fig. 3(c) shows the firstand the second-order resonant wavelengths shift when the ambienttemperature of both micro-knots changes simultaneously. These simu-lated results demonstrated that by measuring the peak wavelength-shifts, we can obtain the temperature changes of the two micro-knots,it could realize themulti-point temperature sensing undersize of 1 mm.

3. Experiment

The experimental setup of the temperature sensing was con-structed based on MDKRs structure, as shown in Fig. 4. The microfiberwas fabricated by flame-heated taper-drawing of a single-mode fiber.The diameter is about 2.3 μm and the length is about 80 mm. The mi-crofiber and the standard single-mode optical fiber were connectedvia a fiber taper fabricated by flame-heated taper-drawing. The twomicro-knots with diameters of 506 μm and 500 μm were manufac-tured by manipulating using two fiber-tapers under an optical micro-scope. The distance between the two knots was achieved flexibly byadjusting the length of coupling region, which was about 15 mm inthe experiment. We used an optical spectrum analyzer (ModelSi720, Micron-Optics Inc., USA) to monitor the transmission spectra.

For heating the microfiber knot, two good thermal conductivitywires with the diameter of approximately 350 μm were threadedthrough the microfiber knots respectively, the other ends of thewires were fixed on two electrically controlled hot plates, as shownin Fig. 4. As the transient temperature fields around the heated wireare attenuated dramatically, the heat effects which applied to anothermicrofiber-knot can be ignored. Firstly, we kept the temperature ofthe first micro-knot constant and heated the wire which crossingthe second microfiber knot. Fig. 5(a) shows the transmission spec-trum of the MDKRs structure at second knot's temperature rangesfrom 30.0 °C to 56.5 °C, we can observe the second-order resonancepeak shift about 110 pm during this process. Fig. 5(b) illustrates thelinear results about the two resonance peak shifts over the temperaturerange from 20 °C to 60 °C. When the temperature increased from 20 °Cto 60 °C, we can see that the first-order resonance peak with extinctionratio of more than 10 dB (red line) was unchanged and the second-order resonance peak has (blue line) shiftedwith temperature changes.Experimental results illuminate that these twomicrofiber-knots can bemodulated by temperature separately in this way.

Fig. 3. (a) Theoretical transmission spectra when changing the first micro-knot's temperature only; (b) change the second micro-knot's temperature only; (c) change both twoknots's temperature simultaneously.

2220 Y. Wu et al. / Optics Communications 285 (2012) 2218–2222

On the contrary, we kept the wire's temperature in the secondmicro-knot invariable and heated the first micro-knot. Fig. 6(a)shows the transmission spectra of the first micro-knot with tempera-ture of 25 °C, 35 °C, 45 °C and 55 °C. We can see the resonance peakwavelength linearly varied with temperature. Only the first-order

resonance peak drifted corresponding to temperature increasing canbe observed, and the second-order resonance peak keep stationary.

To investigate the differential measurement characteristics of thisMDKRs structure, both two wires were heated simultaneously. Thefirst and second-order resonant wavelengths shift at the same time.

Microfiber kont

Metal wire

Hot plate

Micro-motion stage

Fig. 4. Schematic diagram of the experimental temperature sensing system and themicroscopic image of the MDKRs structure.

Fig. 6. (a) Transmission spectra of MDKRs at temperature of 25 °C, 35 °C, 45 °C and55 °C. (only the first micro-knot heated); (b) temperature responses of the MDKRsover the temperature range between 20 °C and 60 °C.

2221Y. Wu et al. / Optics Communications 285 (2012) 2218–2222

Form the output interference spectrums, we can observe that thetemperature-dependent shift of the first-order resonance peak wasmodulated by the temperature changes of first micro-knot. Similarly,the second-order resonance peak shift depends on the temperature

Fig. 5. (a) Transmission spectra of MDKRs at temperatures of 30 °C and 56.5 °C (onlythe second micro-knot heated), (b) temperature response of the MDKRs over thetemperature range between 20 °C and 60 °C (only the second micro-knot heated).

Fig. 7. Transmission spectra of MDKRs (a) with equal temperature changes; and (b) withdifferent temperature changes.

2222 Y. Wu et al. / Optics Communications 285 (2012) 2218–2222

variations of second micro-knot. Fig. 7(a) shows the transmissionspectrum with the same temperature changes which at 35 °C 38 °Cand 41 °C for both two knots. When the temperature changes equally(ΔT1=ΔT2), the drift of first and second-order resonance peaks wereboth the same, which could achieve the temperature sensitivity of~17 pm/°C. This experimental results also agree well with the simula-tions. In the samemethod, we heat the two wires changes with differ-ent temperature (ΔT1≠ΔT2) respectively, the shifts of two peakswere not identical. The difference between the two peaks shifts varylinearly with temperature changes difference between the twomicro-knots. Hence, the different temperature changes can be distin-guished by this structure.

As shown in Fig. 7(b), the first micro-knot's temperature changesΔT1=10 °C and the second micro-knot's temperature changesΔT2=20 °C resulted in the offsets for both two order resonancepeaks. Due to the different temperature modulations, there areabout 172 pm shift for the first-order resonance peak and 341 pmshift for the second-order resonance peak, respectively.

4. Conclusion

In this paper, a MDKRs based sensing structure usingmicrofiber hasbeen investigated both theoretically and experimentally. Theoreticalanalysis and temperature sensing experiments demonstrated thatmulti-point temperature measurement in a very small scale couldbe realized by measuring the shifting difference between the first andsecond-order resonance peaks. Such a structure could also be appliedfor simultaneous measurement of temperature and other parameterssuch as strain, refractive index undersize ofmillimeter scales.Moreover,the peak wavelength differential measurement method could avoid thecross-talk between dual parameters during the measurement process.In summary, this MDKRs sensing structure has the advantages ofcompact size, low cost, easy integration with other optical systems. It

could find potential applications in certain areas where small scale,multi-point, high precision and multi-parameters measurement areneeded.

Acknowledgment

This work was supported by the Fundamental Research Funds forthe Central Universities of China (ZYGX2010J005) and National NaturalScience Foundation of China under Grant 61107072.

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18142.

Yuta

Wu got his doctor degree at the State Key Laboratory of Modern Optical Instrumen-tion, Zhejiang University in 2008. Now he is doing his researches at the Key Lab of

Optical Fiber sensing & Communications Technology (Education Ministry of China),University of Electronic Science & Technology of China. His researching interests includingthe microfiber sensors, photonics sensors and MOEMS devices.