packaged silica microsphere-taper coupling system for robust thermal sensing application

Packaged silica microsphere-tapercoupling system for robust thermal

sensing application

Ying-Zhan Yan,1 Chang-Ling Zou,2 Shu-Bin Yan,1 Fang-Wen Sun,2

Zhe Ji,1 Jun Liu,1 Yu-Guang Zhang,1 Li Wang,1 Chen-Yang Xue,1

Wen-Dong Zhang,1 Zheng-Fu Han,2 and Ji-Jun Xiong1,∗1Key Laboratory of Instrumentation Science and Dynamic Measurement (North University of

China), Ministry of Education, Taiyuan 030051, China2Key Lab of Quantum Information, University of Science and Technology of China, Hefei

230026, China

*[email protected]

Abstract: We propose and realize a novel packaged microsphere-tapercoupling structure (PMTCS) with a high quality factor (Q) up to 5× 106

by using the low refractive index (RI) ultraviolet (UV) glue as the coatingmaterial. The optical loss of the PMTCS is analyzed experimentally andtheoretically, which indicate that the Q is limited by the glue absorption andthe radiation loss. Moreover, to verify the practicability of the PMTCS, ther-mal sensing experiments are carried out, showing the excellent convenienceand anti-jamming ability of the PMTCS with a high temperature resolutionof 1.1× 10−3◦C. The experiments also demonstrate that the PMTCS holdspredominant advantages, such as the robustness, mobility, isolation, and thePMTCS can maintain the high Q for a long time. The above advantagesmake the PMTCS strikingly attractive and potential in the fiber-integratedsensors and laser.

© 2011 Optical Society of America

OCIS codes: (280.4788) Optical sensing and sensors; (230.5750) Resonators; (230.3990)Micro-optical devices.

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#139929 - $15.00 USD Received 20 Dec 2010; revised 24 Feb 2011; accepted 24 Feb 2011; published 14 Mar 2011(C) 2011 OSA 28 March 2011 / Vol. 19, No. 7 / OPTICS EXPRESS 5753

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1. Introduction

Whispering gallery mode (WGM) optical microcavities [1–3] with high quality factors (Q)have attracted more and more attention in sensing research [4–9]. In traditional microcavitysensors, the resonant spectrum shift is detected to estimate the change of the refractive index(RI) of the cavity or the environment, and the sensing resolution depends on the Q greatly. Typi-cally, ultrahigh Q microspheres, coupled by the fiber taper with nearly ideal coupling efficiency[10–12], are popularly used. However, there are some limitations of the microsphere-taper sys-tem when promoting the sensing research to practical application. First, the effective couplingcan be affected, and even be broken when loading the vibration on the taper or the microsphere.Second, WGMs are very sensitive to surroundings, which is an advantage for sensors but makesthe structure invalid in high RI materials or non-uniform dielectrics. Third, the exposure of thetraditional coupling system makes the Q-maintenance challenging greatly, because the waterand the dust in the air could spoil the Q drastically [13]. Finally, high-resolution 3D translationstages are necessary in the traditional coupling system [10], however, the stages are expensiveand bulky, limiting the mobility in applications.

In this paper, for the first time, we propose and realize a novel packaged microsphere-tapercoupling structure (PMTCS) experimentally. We demonstrate that the PMTCS can avoid theproblems addressed above. The PMTCS is stable without 3D translation stages, robust againstthe vibration and free to move. Moreover, as a protective layer, the package body isolates themicrosphere-taper from the surroundings, and maintains the Q above 106 for a long time. In


addition, by using the PMTCS, we realize a high sensitive and robust fiber-integrated thermalsensor which is potential for the distributed sensor network.

Fig. 1. (a)-(d) Illustration of the packaging process. (e) A micrograph of a typical PMTCSstructure.

2. Fabrication

In experiments, individual microspheres are fabricated with the diameter (D) ranging from180μm to 650μm by melting the end of the optical fiber. WGMs are excited by a tunable laser(1550nm wavelength band, linewidth < 300kHz) through a fiber taper. And the gap between mi-crosphere and taper is controlled by electromechanical 3D X-Y-Z stages with 20nm resolution,as shown in Fig. 1(a).

To package the microsphere-taper coupling system, ultraviolet (UV) glue is used. The glueis fibre coating material with low RI (ng ≈ 1.35), which is made from special silicone acrylatewith the product specification KD-310. The packaging process is comprised of four steps, asshown in Fig. 1(a)-(d). First of all, optimal coupling in the air with desirable resonant dips(Fig. 2(a)) needs to be achieved, as shown in Fig. 1(a). And then the UV glue is coated onthe microsphere-taper by using a glue spreading machine in a dropping manner, as shown inFig. 1(b). Afterward, the package is solidify through 10 minutes exposure under a UV lamp, asshown in Fig. 1(c). In the last step, the microsphere stem, mounted on the 3D stages, is truncatedby using a heat burning manner. The final PMTCS is independent of the 3D stages and can bemoved freely, as shown in Fig. 1(d). The whole procedure is monitored by two microscopes,horizontally and vertically. It is worth noting that we need to re-adjust the coupling before thesolidification, because the initial coupling is affected by the glue dropping which changes theenvironment and the surface tension around the microsphere as well as the taper. The resonantspectra fluctuate randomly at the first moment of the glue dropping, and gradually stabilizewhen the glue inosculates with the coupling structure after a few minutes. Particular attentionhas also to be paid to avoid the taper cracking, especially at its fragile taper waist.

Figure 1(e) shows a typical micrograph (side view) of a PMTCS (D = 340μm), in whichthe package body is asymmetric due to the gravity. As the backbone, the tapered fiber tra-verses across the package body to hold the PMTCS. At the package boundary (marked withblue triangles), there are two contact points which cause extra scattering loss (less than 20%).In the package experiments, the scattering loss can be reduced through using a shorter taperand encapsulating it completely. This is also an effective way to improve the robustness whichis determined by the un-stretched single-model fiber after packaging while depending on thefragile taper for an unpackaged system. In addition, the evanescent decay length (d) of WGMs(d = λ/(2π

√n2

s −n2)1001[3],ns and n are the RI of the microsphere and the surroundings,


respectively.) increases from 0.1516λ to 0.3008λ , which causes a larger field overlap betweenWGMs and the taper. This makes the critical coupling easier to fulfill for a thicker taper. Be-sides, the coating layer with the minimum thickness about 30μm (� d) ensures the completeisolation of WGMs from the outside, as shown in Fig. 1(e).

3. Test results and analysis

By comparing the spectra before and after the packaging, a red shift to the longer wavelength(S) has been observed, which can be estimated through S = λ 2

πD (1√

n2s−n2

g− 1√

n2s−n2

air

) [3], where

nair is the air RI. For a microsphere with D≈ 421μm, the S we observed is 2.35nm, which agreeswell with theoretical prediction of 2.3nm, as shown in Fig. 2(a) and Fig. 2(b). The package canalso eliminate high radial order WGMs and offer much more regular spectra. Because after thepackage, the relative RI (ns/n) decreases from 1.44 to 1.07, corresponding to an increase of thetotal reflection critical angle from 43.62 to 70.92 degree, which results in much larger leakagefor high radial order WGMs [2].

Fig. 2. (a) and (b) are contrast resonant spectra for a microsphere (D = 421μm) before andafter the packaging, respectively. (c) Tested and fitted Q versus microsphere D.

As the most important parameter of the microsphere, the intrinsic Q (Qtot) of WGMs can beexpressed as

Q−1tot = Q−1

abs +Q−1sca +Q−1

rad , (1)

where Qabs, Qsca and Qrad are related to the absorption, surface scattering and radiation loss,respectively. In our experiments, the microspheres are made of the commercial fiber, wherehigh purity silica is used. This ensures Qabs greater than 1010 in the air. Qsca originates fromthe surface rayleigh scattering, and is mainly determined by the surface smoothness. For amicrosphere induced by the surface-tension, the surface roughness is nanometer-sized (1nm−10nm), which indicates the Qsca above 108 [14]. The radiation loss of WGMs strongly dependson the size and RI of the cavity. At 1.55μm waveband, Qrad > 108 can be achieved in the airfor silica microspheres with D larger than 25μm [13, 14]. In Fig. 2(c), we plot the measuredQtot against D for the unpackaged microspheres by using white squares, where the recorded


maximum Qtot is around 108. Here, D is larger than 100μm, indicating that the Qtot is limitedby the surface scattering loss, which agrees well with the fitted Qsca.

However, compared with the unpackaged structure, Qtot decreases apparently in the PMTCS.As shown by red circles in Fig.2(c), the measured Qtot decreases sharply when the diameter isless than 200μm, and Qtot is always smaller than 107 for larger microspheres. The similarphenomenon has been reported in microtoroids embedded in the water by Armani et al [15].Former experiments have indicated that a coating on the microcavity surface can greatly reducethe Qsca [16]. Thus, the loss in the PMTCS mainly originates from the radiation loss and theglue absorption. We fit the Qtot (blue line) with ng = 1.351+ 5×10−6i by analytically solv-ing the WGMs in microsphere at λ = 1550nm (the formulas can be found in Ref. [12] andRef. [13]). The results agree with the measurements greatly. Here, the imaginary part of ng iscorresponding to the amount of the absorption loss when the light propagates in the glue. Witha real ng=1.351, we obtain the Qrad (green line) which decreases exponentially with the reduc-tion in the diameter. In addition, particular attention should be paid to prevent contaminatingthe UV glue, because the contaminants attached to the microcavity or the taper can increase theoverall scattering loss. On the contrary, mixing nano-particles at a certain concentration in theglue provides a feasible way to control the backscattering [17,18]. It is worth noting that the Qof the PMTCS is still much higher than the highest Q (2× 105) of a packaged microfiber coilresonator [19].

It is obvious that the package body isolates the whole coupling system from the surroundings,excluding Q spoiling factors from the dust and water in the air. As shown in Fig. 3(a), whenexposing a microsphere in the air, the Q shows a quick decay due to the water absorptionin a few minutes after its fabrication [13]. When putting it in the smoke, the Q has anotherremarkable decrease due to the scattering by the dust adhering to the surface. By contrast, theQ of PMTCS is much more stable and independent of the surrounding influences. In fact, wehave maintained the Q above 106 for a few months. Thus, the package provides a feasible wayto maintain the Q, paving the way for devices research in practical application.

4. Thermal sensing experiments

To verify the practicability of the PTMCS, thermal sensing experiments are carried out in com-plex dynamic water environment. A beaker containing 600ml water with a stirrer in it is adoptedas the testing environment. The stirrer, not only helps to equalize the temperature, but also sim-ulates a flowing water environment. A thermocouple and a heater are placed in the vicinity ofthe PMTCS, to measure and change the temperature, respectively.

The robustness is confirmed by the undisturbed spectra in the water flow. Furthermore,the completeness of the encapsulation is verified through the unshifted spectra when addingSodium Chloride (NaCl) in the water gradually (keep the same temperature) to change thewater RI about 2× 10−3, which could cause a WGM wavelength shift about 20pm for an un-packaged silica microsphere [20]. Wavelength shifts against the temperature are recorded, insaturated NaCl solution and in pure water, respectively. As shown in Fig. 3(b), wavelengthshifts are not affected by RI changes of the water. The temperature variation leads to changesboth in the size and the RI of the silica microsphere, which subsequently causes resonancewavelength shifts [21–24]. The resonant wavelength shows a red shift about 160.39pm whenthe temperature increases from 14◦C to 26◦C, which indicates a sensitivity of 13.37pm/◦C witha relevant coefficient in linear fitting about 0.998. Taking into account the spectral resolution(Δλmin) of our system, which is 0.015pm, we estimate the resolution (ΔTmin = Δλmin/(dλ/dT ))of the PMTCS microsphere temperature sensor as 1.1×10−3◦C.

This resolution is in the same order of magnitude with the traditional silica microcavity ther-mal sensor [23]. What’s more, the practicability is greatly enhanced in the PMTCS, in which


Fig. 3. (a) Tested Q versus elapsed time of a microsphere coupling system in the air andin the smoke, for un-packaged (in black) and packaged (in red) samples, respectively. (b)WGM wavelength shifts as a function of the surrounding temperature, in NaCl resolutionand in pure water, respectively.

only the temperature change causes the WGM shift, while the change of external RI fails tocause the shift. Besides, the PMTCS thermal sensor shows an excellent anti-jamming abil-ity, and can be used in harsh environments. Furthermore, the PMTCS can provide much morestable performance (for instance, the resolution) due to its superior Q maintenance ability. Ad-ditionally, the bulky translation stages are no longer unnecessary in the PMTCS. The portablestructure makes these sensors easy to move and miniature, which is important in practical ap-plications, especially in microsystem technology research.

Further efforts will be focused on exploring new package materials and improving thepackage technology to enhance the packaged Qtot . The PMTCS could be extended to multi-packaged microsphere-taper system on one fiber to monitor the real-time temperature at differ-ent locations, or package multi-microspheres in a certain point to study the classical analog toelectromagnetic induced transparency (EIT), even its application on gyroscopic [25, 26].

5. Conclusion

In summary, we have packaged silica microsphere-taper coupling systems by using the lowRI UV glue. In our experiments, the (Q) of the packaged structure reaches 5× 106 which islimited by the radiation loss and the glue absorption. The anti-jamming ability and the isolationperformance in PMTCS are verified experimentally. Temperature sensing experiments are alsocarried out. The results show that the PMTCS possesses a high resolution of 1.1× 10−3◦Cwith the remarkable practicability, robustness and convenience. Not only the PMTCS can beapplied to microcavity thermal sensors, but promote the developments of other microcavity-based devices, such as the low threshold laser.

Acknowledgements

Y. Z. Yan and C. L. Zou contributed equally to this work. The work was supported by theNational Basic Research Program of China under Grant No. 2009CB326206, National Science


Foundation of China under Grant Nos. 60707014, 60778029 and 50975266, and the InnovationProject under Grant Nos. 7130907, 9140C1204040909 and 9140C1204040706. Y. Z. Yan wasalso supported by Innovation Project (Grant Nos. 20093076 and 100115122).


packaged silica microsphere-taper coupling system for robust thermal sensing application

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