JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 37, NO. 4, FEBRUARY 15, 2019 1241

Femtosecond Laser Microprinting of a FiberWhispering Gallery Mode Resonator for

Highly-Sensitive Temperature MeasurementsZhengyong Li , Changrui Liao , Jia Wang, Ziliang Li, Peng Zhou, Ying Wang ,

and Yiping Wang , Senior Member, IEEE, Senior Member, OSA

Abstract—We demonstrate a novel method for producing whis-pering gallery mode (WGM) resonators on a microfiber using fem-tosecond laser two-photon polymerization technology. The typicalresonance was observed by coupling the microresonator with atapered fiber in a region where they were physically separated.Unlike conventional WGM devices, this photosensitive resin-basedresonator was deposited on a microfiber, whereby the structuralintegration was improved. Resonance was experimentally demon-strated using two types of ring structures, i.e., circle and racetrack.The devices were successfully used for temperature measurements,achieving a maximum temperature sensitivity of 1.68 nm/ °C anda Q-factor of 1.9 × 103.

Index Terms—Femtosecond lase microprint, optical fiber de-vices, temperature measurements, whispering gallery mode.

I. INTRODUCTION

THE physical description of whispering gallery modes(WGMs), first introduced by Rayleigh, relied on acous-

tic ray interpretation. This included extensive investigations ofmultiple internal reflections along the inner gallery boundary[1]. Mie [2] and Debye [3] provided the first in-depth theoret-ical analysis of optical WGM resonators, based on Maxwell’sequations, at the turn of the century. After several decades ofdevelopment, WGM theory has become more systematic andaccurate. The unique spectral properties of WGMs, includingnarrow line widths, tunability, high Q-factors, and high stabilityunder environmental conditions [4], [5] have attracted signifi-cant interest in recent years. WGM resonators have the ability toconfine and couple light using total internal reflection. As such,

Manuscript received November 18, 2018; revised December 24, 2018; ac-cepted December 30, 2018. Date of publication January 4, 2019; date of cur-rent version February 22, 2019. This work was supported in part by the Na-tional Natural Science Foundation of China (61575128, 61635007), in part bythe Natural Science Foundation of Guangdong Province (2018B030306003),in part by the Science and Technology Innovation Commission of Shenzhen(KQJSCX20170727101953680), and in part by the Development and ReformCommission of Shenzhen Municipality Foundation. (Corresponding author:Changrui Liao.)

The authors are with the Key Laboratory of Optoelectronic Devices andSystems of Ministry of Education and Guangdong Province, College ofOptoelectronic Engineering, Shenzhen University, Shenzhen 518060, China(e-mail:, lizhengy88@126.com; cliao@szu.edu.cn; wangjia2016@email.szu.edu.cn; liziliang2016@email.szu.edu.cn; gavinzhoupeng@163.com; yingwang@szu.edu.cn; ypwang@szu.edu.cn).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2019.2890991

a robust and efficient coupling methodology is critical for opti-mizing resonator performance. Various coupling methods havebeen reported in recent years, such as prism coupling [6], angle-polished coupling [7], and fiber taper coupling [8]. Among these,fiber taper coupling is reported to have the highest efficiency(∼90%) [9]. However, the coupling structures are physicallyseparated from the resonators, which is not compact enough.Additionally, a variety of WGM resonators have been proposed,such as microspheres [10]–[12], microdisks [13], [14], micror-ings [15], microcylinders [16], [17], and microbubbles [18]. Acylinder resonator cavity with a Q-factor of 1.44 × 103 wasfabricated in a D-fiber using femtosecond (Fs) laser microma-chining [19]. However, this process often produces rough res-onator surfaces, which can increase scattering loss. A polymermicrospheres is encapsulated into the capillary of a mercedes-type microstructured optical fiber, directly contacting with theguiding core, which can excite WGM resonance [20]. However,the diameter of the capillary limits the size and quantity of themicrospheres, which make this method lack of flexibility.

In this study, we experimentally demonstrate a photosensi-tive resin-based WGM resonator with a Q-factor of 1.9 × 103.The ring resonator was printed on a microfiber using Fs lasertwo-photon polymerization (TPP) technology. The resulting mi-crofiber exhibited stronger evanescent fields, which coupledlight into the ring resonator. The light was confined by con-tinuous total internal reflection (TIR), resulting from the highrefractive index (RI) of the photosensitive resin compared withthe surrounding media. This TIR process excited WGMs in theresonator cavity. Two conventional types of ring resonators, i.e.,circle and racetrack, were experimentally demonstrated for gen-erating WGM resonance. This device was successfully appliedto high-precision temperature measurements, achieving the sen-sitivity of 1.68 nm/°C, which improves one order of magnitudesthan PDMS microsphere [21], glass microsphere [10], [12].

II. WGM OPERATING PRINCIPLES

Fundamentally, the generations of WGMs in optical res-onators rely on TIR at the external interface. It is helpful toimprove propagating losses via stronger limitation of the WGMsbecause of larger RI contrasts. Higher contrast between the res-onator and surrounding material can increase achievable Q fac-tors. Conversely, a low RI gradient can improve extension of the

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1242 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 37, NO. 4, FEBRUARY 15, 2019

Fig. 1. A schematic diagram of the polymer ring WGM resonator.

mode profile beyond the limitations of the resonator mediumand into the sensing domain, which can potentially lead to in-creased sensitivity. In addition, material losses (absorption andsurface scattering) are also important in determining mode linewidth. The choice of resonator material is a critical factor in sen-sor design, motivating the search for novel materials to improvesensor performance and reduce costs.

When the SMF is tapered to several micrometers, the diame-ter of the guiding mode is larger than the tapered region, whichcan let out a part of light propagating along the surface of themicrofiber. As such, microfibers are more suitable for couplinglight into resonators. As shown in Fig. 1, a circular ring wasdeposited on the microfiber. Input light propagates along thefiber core and leaked light enters the circular ring, inducingresonance. Finally, a fraction of this light recouples into the mi-crofiber. The resulting WGM resonance spectrum was measuredusing an optical spectrum analyzer (OSA). In order to guaranteethe light resonating in the ring cavity the phase change for thelight should be an integer multiple of 2π:

2πnr = mλ, (1)

where n and r are the RI and radius of the ring resonator, respec-tively. Rearranging this equation gives the free spectral range(FSR), which can be written as

FSR =λ2

2πnr. (2)

This result indicates the FSR is inversely proportional to theradius of the ring cavity for a given material at the identicalwavelength. As such, the FSR can be optimized by adjustingthe radius of the circular ring resonator.

III. EXPERIMENTAL CONFIGURATION

The whole fabrication process includes two primary steps:drawing the microfiber and fabricating the ring resonators usingFs laser TPP technology. The SMF (Corning, SMF-28) usedin these experiments has the core and cladding diameters of8 and 125 μm, respectively. We designed and constructed afiber-tapped system capable of producing submicron-diametermicrofibers. Detailed information regarding this process can befound in a study by Fu et al. [22]. In this experiment, we drawthe microfiber with diameters of 2–4 μm and a homogeneousarea length of ∼1 cm. The microfibers were then immobilizedon magnesium fluoride glass instead of silica glass. Becauseits RI is lower than the fiber, thereby limiting light propagatingalong the surface of the microfiber.

Fig. 2. (a) A schematic diagram of the Fs laser TPP micromachining sys-tem. (b) The microfiber fixing method used during two-photon polymerizationfabrication.

The second step is fabrication of the ring resonator, which re-quires an Fs laser micromachining system as shown in Fig 2(a).This system utilizes an Fs laser and highly precise 3D transla-tional stages. The Fs laser (Spectra-Physics, Solstice) exhibiteda wavelength of 800 nm, a pulse duration of 120 fs, and a repe-tition rate of 1 kHz. A seed mode Fs laser with a high repetitionrate of 80 MHz was selected as the laser source for inducingTPP. An expander was implemented to increase the size of laserbeam. Finally, the beam was focused on the surface of the mi-crofiber immersed in a photosensitive resin by an oil-immersionmicroscope objective (MO) with an NA value of 1.35. A tunableattenuator, consisting of a λ/2 wave plate (W) and a Glan prism(P), was used to adjust the output laser power. Irradiation timewas controlled by a PC-driven mechanical shutter. A mountedCCD camera was used to monitor the fabrication process.

The photosensitive resin (PP-1, Zhichu Optics Co., Ltd.,Shenzhen, China) used in this experiment contains photo-initiators (IGR-369, Ciba-Geigy) and styrene monomers(SR444, SR368, Sartomer) for polymerization [23]. The pro-cessing microfiber was immersed in the photosensitive resin,which was interposed between the cover and slide glass as shownin Fig. 2(b). Maximum cover thickness was ∼110 μm, whichis smaller than the working distance of the oil-immersion MO.The assembled sample was then secured to the 3D translationalstage (Newport, XMS50/XMS50/GTS30V). A single layer oftape with a thickness of 70 μm was attached to the edge of theslide glass to support the microfiber and ensure sufficient spacefor laser inscription. This also ensured a uniform distribution ofphotosensitive resin on the microfiber.

Fs laser TTP is a process in which a monomer is convertedinto a polymer network and cross-linking of polymer chains is

LI et al.: FEMTOSECOND LASER MICROPRINTING OF A FIBER WHISPERING GALLERY MODE RESONATOR 1243

Fig. 3. SEM images of the polymeric ring resonators: (a) a circular ring witha diameter of 20 μm and (b) a racetrack ring with a major axis of 35 μm and aminor axis of 15 μm.

introduced by free radical polymerization. The exposed regionis then insoluble in the solvent and the microstructures can beprinted on the substrate. During polymerization, fluorescencecan be observed at the focal point of the objective lens due to itsnonlinear characteristics [24]. The RI of the photosensitive resinincreased from 1.51 to 1.53 near 1550 nm after laser exposure.

The Fs laser beam is firstly focused on the microfiber and thenshifted to the edge of microfiber. In order to ensure the polymer-ized resonator solidified on the microfiber, the initial laser focalpoint is shifted by 0.5–1 μm towards the microfiber. The powerdensity used in the experiments is 20 mW/μm2. Then the mi-crofiber is translated according to designed track and velocity.After returning to the initial point, the focal point is shifted to-wards the center of resonator and then repeats the former processuntil the line width of the ring resonator satisfies the designedrequirement. When the laser fabrication is completed, the coverslide above the microfiber is carefully taken away and redundantresin can then be rinsed using isopropanol. A scanning electronmicroscope (SEM) was used to image the polymer structures,as shown in Fig. 3. The circular and racetrack rings printedon the microfiber are evident. The line width of the ring res-onator was approximately 2 μm as determined from the Fs laserintensity.

IV. RESULTS AND DISCUSSION

We fabricated two different microresonators using TTP, in-cluding circular and racetrack ring. In order to verify their spec-tral character, the structures were connected with a broadbandlight source and an optical spectrum analyzer. Fig. 4(a) showsthe transmission spectra of a circular ring resonator with a di-ameter of 20 μm. A series of resonance peaks are clearly presentat long wavelengths. The corresponding FSR was measured tobe 25.4 nm. According to the Eq. (2), the theoretical FSR wascalculated to be 25.2 nm, which is in excellent agreement withexperimental results. The transmission spectrum for a racetrack

Fig. 4. The normalized transmission spectra of the ring resonators as a functionof wavelength: (a) the circular ring with a diameter of 20 μm and (b) the racetrackring with a major axis of 35 μm and a minor axis of 15 μm.

ring resonator with a major axis of 35 μm and a minor axis of15 μm is shown in Fig. 4(b). The spectrum is consisted of su-perposition by modal interference (generate five big envelopes)and WGM (small peaks). This result demonstrates the racetrackring is capable of producing WGM resonance with a measuredFSR of 14.5 nm. The Q-factor is defined as

Q =λ

Δλ, (3)

where Δλ is the 3 dB linewidth of the resonant peak. The Q-factor of circular and racetrack ring resonators were calculatedto be 77 and 1.9 × 103 at 1540 nm, respectively.

The evanescent electromagnetic fields on the surface of thering resonator are highly sensitive to changes in the surroundingenvironment. As a result, small temperature variations can bedetected using a WGM resonator. The circular ring resonator,with a diameter of 20 μm, was used for temperature sensingexperiments. It was placed in an electric oven and the temper-ature was gradually increased from 30 °C to 40 °C, with a stepof 2 °C. The transmission spectra of the circular ring resonatorwere recorded by an OSA with a resolution of 0.02 nm, as shownin Fig. 5(a). A red shift was clearly observed as the temperatureincreased. Variation in the resonance wavelength is plotted inFig. 5(b), where a good linear relationship was observed and atemperature sensitivity of 1.68 nm/°C was obtained. However,the circular ring printed on a microfiber exhibits several disad-vantages. The ring is nearly tangential to the microfiber, which

1244 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 37, NO. 4, FEBRUARY 15, 2019

Fig. 5. (a) WGM resonance spectra for the circular ring with a diameter of20 μm as a function of temperature, varying from 30 °C to 40 °C. (b) Dipwavelength plotted as a linear function.

results in a small interface and a weak fastness. This limits tofabricate large size of circular ring resonators, which results inthe wide line width of the resonance peak and low Q-factor.

To solve these issues, we used a racetrack ring as the resonatorinstead of a circular ring. The connection between the racetrackring and the microfiber is much stronger because of the increasedsurface interface (see Fig. 3(b)). As a result, it is more suitable forfabricating larger resonators. The racetrack ring was fabricatedwith a major axis of 35 μm and a minor axis of 15 μm. It wasthen used for subsequent temperature sensing measurements.The dip wavelength exhibited a red shift as the temperatureincreased from 30 °C to 48 °C with a step of 3 °C, as shown inFig. 6(a). After data processing, a good linear relationship wasobtained and the temperature sensitivity was measured to be1.09 nm/°C, as shown in Fig. 6(b). This type of ring resonatorcould be a valuable new tool for high-sensitivity temperaturesensing.

Generally, temperature-induced changes in RI and microres-onator dimension significantly affect WGM resonance wave-lengths. This can be described by the following equation [11]

Δλ = λ0

(1n

dn

dTΔT +

1D

dD

dTΔT

), (4)

where dn/dT is a thermo-optic coefficient for the photosensitiveresin. It is on the order of −10−4 K−1 and causes a blue shift inWGMs. The term D−1dD/dT is a thermal expansion coefficient(>0) and D is the diameter of the ring resonator, which causesa red shift. Normally, response time for thermo-optic effects

Fig. 6. (a) WGM resonance spectra for the racetrack ring with a major axisof 35 μm and a minor axis of 15 μm as a function of temperature, varying from30 °C to 48 °C. (b) Dip wavelength plotted as a linear function.

are tens of microseconds, while the response time for thermalexpansion is much longer than tens of milliseconds [21]. Ifthe transient regime time is lower than 10 ms, thermo-opticeffects are dominant because of insufficient response time forthermal expansion. But in our experiment, each test point wasmaintained for 5 min. As such, there was sufficient time to reachthermal equilibrium. According to Fig. 5(a), the spectra exhibitan obvious red shift due to the effects of both thermo-opticand thermal expansion. However, thermal expansion becomesthe leading function because of large positive absolute thermalexpansion coefficient.

V. CONCLUSION

In summary, we demonstrated an effective method for mi-croprinting WGM resonators using Fs laser TPP technology.WGM resonance was observed as a polymerized ring resonatordeposited on a microfiber, which increased the mechanical sta-bility of the structure and compared with typical WGM res-onators. This method can implement the structural integrationand enhance the flexibility of the resonator size. Two conven-tional types of polymerized WGM ring resonators, i.e., circleand racetrack, were investigated in the experiments. The sizeand the material the resonators could influence the Q-factor. Theadvantage of Fs laser TTP technology is great fabrication flex-ibility. Higher Q-factor can be achieved by increasing the sizeof racetrack or using the photosensitive resin with more match-ing RI. The devices were successfully used for temperaturesensing and clearly exhibited linearity, achieving a maximum

LI et al.: FEMTOSECOND LASER MICROPRINTING OF A FIBER WHISPERING GALLERY MODE RESONATOR 1245

temperature sensitivity of 1.68 nm/°C over a range of 30 °C to40 °C, which has potential applications in precise temperaturemonitoring. In future studies, it could be possible to integrate abatch of WGM resonators on the microfiber.

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Zhengyong Li received the B.Eng. degree in optical information engineeringfrom Huazhong University of Science and Technology, Wuhan, China, in 2012.He received the M.S. and Doctoral degrees in optical engineering with ShenzhenUniversity in 2015 and 2017, respectively. He is currently working toward thePh.D. degree in optical engineering with Shenzhen University, Shenzhen, China.

Changrui Liao was born in Shandong, China, in 1984. He received the B.S.degree in optical information science and technology and the M.Eng. degree inphysical electronics from the Huazhong University of Science and Technology,Wuhan, China, in 2005 and 2007, respectively, and the Ph.D. degree in opticalengineering from Hong Kong Polytechnic University, Hong Kong, in 2012. Heis currently with Shenzhen University, Shenzhen, China, as an Associate Profes-sor. His current research interests include femtosecond laser micromachining,optical fiber sensors, and optofluidics.

Jia Wang was born in Hubei, China, in 1994. She received the B.Eng. de-gree in optoelectronic technology science, Changchun Polytechnic University,Changchun, China, in 2016. She is currently working toward the M.S. degreewith Shenzhen University, Shenzhen, China.

Ziliang Li was born in Hunan, China, in 1994. He received the B.S. degree fromJiangsu University, Zhenjiang, China, in 2016. He is currently working towardthe Master’s degree with Shenzhen University, Shenzhen, China.

Peng Zhou received the B.Eng. degree in electronic information science andtechnology and the M.S. degree in phase change of femtosecond laser inducedmaterials from Shanghai University, Shanghai, China, in 2008 and 2011, respec-tively. He received the Doctoral degree from City University of Hong Kong,Hong Kong, in 2016. He is currently working toward the Ph.D. degree in opticalengineering with Shenzhen University, Shenzhen, China.

Ying Wang was born in Henan, China, in 1983. He received the B.S. degree inapplied physics and the Ph.D. degree in physical electronics from the HuazhongUniversity of Science and Technology, Wuhan, China, in 2004 and 2010, respec-tively. From 2010 to 2015, he was with the Department of Electrical Engineering,Hong Kong Polytechnic University, Hong Kong, as a Research Associate. Since2015, he has been with the College of Optoelectronic Engineering, ShenzhenUniversity, Shenzhen, China, as a Lecturer. His research interests include opticalfiber sensors and femtosecond laser micromachining.

Yiping Wang (SM’00) was born in Chongqing, China, in 1971. He receivedthe B.Eng. degree in precision instrument engineering from Xi’an Institute ofTechnology, Xi’an, China, in 1995, and the M.S. and Ph.D. degrees in opticalengineering from Chongqing University, Chongqing, China, in 2000 and 2003,respectively. From 2003 to 2005, he was with Shanghai Jiao Tong University,Shanghai, China, as a Postdoctoral Fellow. From 2005 to 2007, he was withthe Hong Kong Polytechnic University as a Postdoctoral Fellow. From 2007 to2009, he was with the Institute of Photonic Technology, Jena, Germany, as aHumboldt Research Fellow. From 2009 to 2011, he was with the OptoelectronicsResearch Centre, University of Southampton, Southampton, U.K., as a MarieCurie Fellow. Since 2012, he has been with Shenzhen University, Shenzhen,China, as a Distinguished Professor. He has authored or co-authored 1 book, 21patent applications, and more than 240 journal and conference papers. His cur-rent research interests include optical fiber sensors, fiber gratings, and photoniccrystal fibers. He is a Senior Member of the Optical Society of America and theChinese Optical Society.