micromachines

Communication Multiple Light Coupling and Routing via a Microspherical Resonator Integrated in a T-Shaped Configuration System

Georgia Konstantinou 1,2 , Karolina Milenko 3, Kyriaki Kosma 1 and Stavros Pissadakis 1,* 1 Foundation for Research and Technology-Hellas (FORTH), Institute of Electronic Structure and Laser (IESL), GR-711 10 Heraklion, Greece; georgia.konstantinou@epfl.ch (G.K.); [email protected] (K.K.) 2 EPFL, École polytechnique fédérale de Lausanne, CH-1015 Lausanne, Switzerland 3 Department of Electronic Systems, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; [email protected] * Correspondence: [email protected]; Tel.: +30-2810-391348

 Received: 18 September 2018; Accepted: 9 October 2018; Published: 15 October 2018 

Abstract: We demonstrate a three-port, light guiding and routing T-shaped configuration based on the combination of whispering gallery modes (WGMs) and micro-structured optical fibers (MOFs). This system includes a single mode optical fiber taper (SOFT), a slightly tapered MOF and a BaTiO3 microsphere for efficient light coupling and routing between these two optical fibers. The BaTiO3 microsphere is semi-immersed into one of the hollow capillaries of the MOF taper, while the single mode optical fiber taper is placed perpendicularly to the latter and in contact with the equatorial region of the microsphere. Experimental results are presented for different excitation and reading conditions through the WGM microspherical resonator, namely, through single mode optical fiber taper or the MOF. The experimental results indicate that light coupling between the MOF and the single mode optical fiber taper is facilitated at specific wavelengths, supported by the light localization characteristics of the BaTiO3 glass microsphere, with spectral Q-factors varying between 4.5 × 103 and 6.1 × 103, depending on the port and parity excitation.

Keywords: microstructured optical fibers; whispering gallery modes; light localization

1. Introduction High quality light trapping in dielectric, microspherical cavities can be readily achieved using whispering gallery modes (WGMs) resonation; attracting constant academic and potential industrial interest [1,2]. In the WGM resonation process light is confined at the inner interface of a high refractive index dielectric microcavity, illustrated as closed-loop trajectory where rays circulate through total internal reflection (TIR). Several types of optical geometries and corresponding materials have been employed for demonstrating and implementing WGM resonation in photonic devices. The confinement of light propagation within a closed, spherical symmetry dielectric cavity, leads to a three-fold modal quantization of the permitted localization states of light within the cavity volume [3], denoted with three complementary quantum numbers, per k-vector, being directly dependent upon the optogeometric characteristics of the cavity. The interface mechanism of TIR on the boundary of a curved surface is inherently a low loss light process, leading to high quality factors Q denoting light localization up to ×108, upon materials, excitation protocols and resonator geometries used [4]. A great number of photonic devices based on WGM resonation have been presented including chemical and biological sensors [2,5], lasers [6], wavelength routers and metrological devices [7]. The efficiency of WGMs excitation and related Q-factors of the supported resonances critically depends upon the material properties of the resonator (surface roughness, Rayleigh scattering etc.),

Micromachines 2018, 9, 521; doi:10.3390/mi9100521 www.mdpi.com/journal/micromachines Micromachines 2018, 9, 521 2 of 9 its geometry, real and imaginary refractive index, launching conditions and far field or near field signal collection method [8]. Prisms [9], gratings [10], angle cleaved [11], and tapered optical fibers [12] were used for the WGM evanescent field, light coupling into microspheres, leading to different Q-factors and practical consideration including stability of coupling, simplicity of implementation, integration, and interaction with the ambient environment [13]. In addition to the above, MOFs have also been used for reading or exciting WGM resonation inside microspherical cavities, starting with the work of Francois, et al., where a microsphere was wedged onto the end face of a collapsed core MOF [14]; others have used hollow core optical fibers [15,16]. An alternative and high integration WGM excitation approach was presented by Kosma et al. by placing a microspherical resonator inside the air capillary of a MOF taper, for providing a compact and robust operation [17]; similar implementations were also presented by other groups [18,19]. Generally, the use of the end face of standard, tapered and MOFs has been evolved into an efficient configuration for exciting WGMs in microspheres, offering advantages such as single port operation, good yield in fluorescing signal collection, robustness, and versatility in functionalizing and trapping the micro-spherical cavity. Herein, we present light coupling and routing between the MOF taper and a standard optical fiber taper (SOFT), through a BaTiO3 microsphere semi-immersed inside the capillary of the end face of the MOF taper; where the SOFT is placed perpendicularly to the MOF. This T-shaped light coupling system provides interesting WGMs excitation and collection with all three created fiber ports along with light paths parities. We anticipate that the proposed T-shaped excitation system can lead to numerous devices and applications of WGM resonators for sensing, filtering and frequency stabilization, and can be used in integrated optical devices as an efficient add-drop filter. The use of BaTiO3 microspheres constitutes a base for straightforwardly attaining permanent and/or transient photorefractive tuning of the spectral characteristics of the WGMs using pulsed or continuous wave (CW) laser beams at low pump powers. This type of spectral trimming has already been demonstrated for a variety of materials and WGM resonating geometries [20–22]. Potentially, the photorefractive tuning of the WGM spectral characteristics can also happen for light excitation (for example using 405 nm or 532 nm CW lasers) through an all silica MOF, where the photorefractivity of the MOF itself will be minimum. An additional reason for using BaTiO3 microspheres is the possibility of permanent encapsulation and fixation of the system MOF-BaTiO3-SOFT using standard ultraviolet (UV) glues (see Norland UV adhesives, Norland Products, Inc., Cranbury, NJ, USA) with a refractive index lower than silica, without great compromising of the Q-factors of the excited WGMs [23]. Our investigations include the steps followed for realizing the device presented, and its spectral characterization and discussion for different portal excitations and read-outs.

2. Experimental Section A grapefruit shaped MOF (drawn by ACREO, Stockholm, Sweden), consisting of two concentric cores, a 3.5% germanium doped silica inner core with diameter 8.5 µm and an outer silica core of diameter 16.1 µm, 20.8 µm diameter 5-air capillaries and 125 µm cladding diameter was used (see Figure1a). For facilitating efficient wedging of the microspheres available, as well as increasing the evanescence field tail extending outside the microstructured core, this MOF was adiabatically tapered down to ~55% of its initial size and cleaved at the transition region, resulting in a tip diameter of ~68 µm and air capillaries scaling down to a ~11 µm diameter (see Figure1b,c). For these tapering conditions the fundamental guiding mode is still confined at the Ge doped core of the grapefruit MOF, however, with a more extended modal profile covering the microstructured area. In Figure2, we show the fundamental guiding mode confinement by using a commercial modal solver. Micromachines 2018, 9, 521 3 of 9 Micromachines 2018, 9, x 3 of 9 Micromachines 2018, 9, x 3 of 9

FigureFigure 1. 1. ( a()a )Scanning Scanning electron electron microscope microscope (SEM) (SEM) pict pictureure of of the the un-tapered, un-tapered, grapefruit grapefruit shaped shaped Figuremicro-structured 1. (a) Scanning optical electron fiber (MOF) microscope used, with (SEM) five picture air capillaries of the and un-tapered, germanium grapefruit doped core, shaped (b) micro-structuredmicro-structured optical optical fiber fiber (MOF) (MOF) used, used, with with five five air air capillaries capillaries and and germanium germanium doped doped core, core, (b )( SEMb) SEM image of the attached BaTiO3, microsphere with 25 μm diameter, top fitting in one of the SEM image of the attached BaTiO3, microsphere with 25 μm diameter, top fitting in one of the image of the attached BaTiO3, microsphere with 25 µm diameter, top fitting in one of the capillaries capillariescapillaries (diameter: (diameter: 11.2 11.2 μ μm)m) of of the the tapered tapered MOF. MOF. The The other other empty empty capillaries capillaries can can be be seen seen in in the the (diameter: 11.2 µm) of the tapered MOF. The other empty capillaries can be seen in the background, background,background, ( c()c )optical optical microscope microscope picture picture of of a aside side view view on on the the T-shaped T-shaped light light coupling coupling system system (c) optical microscope picture of a side view on the T-shaped light coupling system (single mode optical (single(single mode mode optical optical fiber fiber taper taper (SOFT) (SOFT) placed placed perpendicular perpendicular to to MOF MOF in in a acontact contact with with the the fiber taper (SOFT) placed perpendicular to MOF in a contact with the microsphere). microsphere).microsphere). Additionally, since the adiabaticity criterion was followed, coupling from the central mode to Additionally,Additionally, since since the the adiabaticity adiabaticity criterion criterion was was followed, followed, coupling coupling from from the the central central mode mode to to modes supported at the extended microstructured core area is rather limited; the last is confirmed by modesmodes supported supported at at the the extended extended microstructured microstructured core core area area is is rather rather limited; limited; the the last last is is confirmed confirmed by by the absence of significant beating features in the transmission spectra of the tapered MOF. thethe absence absence of of significant significant beating beating features features in in the the transmission transmission spectra spectra of of the the tapered tapered MOF. MOF.

FigureFigureFigure 2. 2. (2.a () a( Fundamental)a )Fundamental Fundamental modemode mode forfor for thethe the MOFMOF MOF in in a a2D 2D cross-section cross-section cross-section image imageimage by by adopting adopting COMSOL COMSOL COMSOL (Multiphysics(Multiphysics(Multiphysics 3.5, 3.5, 3.5, COMSOL COMSOL COMSOL Inc., Inc., Inc., Stockholm,Stockholm, Stockholm, Sweden)Sweden) Sweden) simulation. simulation. simulation. ( b (b) )Fundamental Fundamental mode mode mode in in ina a2D a2D 2D cross-sectioncross-sectioncross-section image image image for for for a a smaller asmaller smaller claddingcladding cladding sizesize size after after the thethe tapering taperingtapering process processprocess (68 (68 μ μmµm diameter), diameter), and and and accordinglyaccordinglyaccordingly smaller smaller smaller core core core and and and air air air capillaries.capillaries. capillaries. Increased Increased spreading spreadingspreading of of the the mode mode is is isobserved observed observed due due due to to to reducedreducedreduced spatial spatial spatial confinement. confinement. confinement.

Barium titanate (BaTiO3) glass microspheres of poly-disperse sizes and typical eccentricities of BariumBarium titanate titanate (BaTiO (BaTiO3)3) glass glass microspheresmicrospheres of poly-disperse sizes sizes and and ty typicalpical eccentricities eccentricities of of thethethe order order order of of 10%of 10% 10% (Mo-Sci (Mo-Sci (Mo-Sci Corporation) Corporation) Corporation) werewere were used.used. used. Th The The erefractive refractive refractive index indexindex of of these these microspheres microspheres microspheres varies varies varies between 1.9 and 2.1, depending on the stoichiometric ratio between Ba and Ti. A BaTiO3 betweenbetween 1.9 1.9 and and 2.1, depending2.1, depending on the on stoichiometric the stoichiometric ratio between ratio between Ba and Ti.Ba Aand BaTiO Ti.3 microsphereA BaTiO3 microsphere with a diameter 25 μm was attached to the end face of the MOF taper tip while being withmicrosphere a diameter with 25 µ am diameter was attached 25 μm to was the attached end face to of the the end MOF face taper of the tip MOF while taper being tip semi-immersed while being semi-immersed inside one of its air capillaries (Figure 1b). Air suction was used to ensure stable insidesemi-immersed one of its airinside capillaries one of (Figureits air capillaries1b). Air suction(Figure was1b). Air used suction to ensure was stableused to positioning ensure stable and positioning and attachment of the microsphere. Figure 1 presents scanning electron microscopy attachmentpositioning of and the attachment microsphere. of the Figure microsphere.1 presents Fi scanninggure 1 presents electron scanning microscopy electron (SEM) microscopy images of (SEM) images of the employed MOF and the BaTiO3 microsphere. Additionally, a single mode (SEM) images of the employed MOF and the BaTiO3 microsphere. Additionally, a single mode the employed MOF and the BaTiO3 microsphere. Additionally, a single mode optical fiber (SMF-28, Corning Inc., Corning, NY, USA) was tapered adiabatically down to 0.9 µm final waist diameter to

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Micromachines 2018, 9, 521 4 of 9 optical fiber (SMF-28, Corning Inc., Corning, NY, USA) was tapered adiabatically down to 0.9 μm final waist diameter to achieve a broad evanescent field. All optical fibers used were thermally achievetapered awith broad the evanescent use of a field.Vytran All GPX-3000 optical fibers (Tho usedrlabs, were Inc., thermally Newton, taperedNY, USA) with optical the use fiber of a Vytranprocessing GPX-3000 equipment. (Thorlabs, Inc., Newton, NY, USA) optical fiber processing equipment. The MOF taper with the attached microsphere was aligned perpendicular to the waist of the tapered fiber,fiber, so thatthat thethe SOFTSOFT waistwaist waswas inin aa contactcontact withwith thethe equatorialequatorial regionregion ofof thethe BaTiOBaTiO33 microsphere (Figure(Figure3 ).3). In In this this way way a threea three channel channe devicel device was was implemented, implemented, with with routing routing point point at the at BaTiOthe BaTiO3 microsphere3 microsphere semi-immersed semi-immersed into theinto end the face end of face the MOF;of the namely MOF; CH1namely and CH1 CH2 and represent CH2 therepresent two portal the two ends portal of theends SOFT of the and SOFT CH3 and the CH MOF3 the port. MOF Moreover, port. Moreover, this three-port this three-port optical optical fiber configurationfiber configuration was characterized was characterized with respect with respect to the orientation to the orientation of the line of the defined line bydefined the MOF by the core MOF and thecore BaTiO and the3 microsphere BaTiO3 microsphere with the axis with of the SOFTaxis of (Figure the SOFT3). In (Figure the parallel 3). In orientation the parallel (State orientation A) the MOF(State centralA) the coreMOF and central the centercore and of thetheBaTiO center3 ofmicrosphere the BaTiO3 are microsphere in line with are the in axis line of with the SOFT,the axis and of the k-vectorsSOFT, and of the light k-vectors propagating of light into propagating the MOF core into and the the MOF circumference core and the of thecircumference BaTiO3 along of the planeBaTiO of3 along the SOFT the excitationplane of restthe alongSOFT theexcitation same axis rest of along propagation. the same In theaxis perpendicular of propagation. orientation In the (Stateperpendicular B) the SOFT orientation excites the(State BaTiO B) 3themicrosphere, SOFT excites however the BaTiO the axis3 microsphere, between the however MOF central the coreaxis ◦ andbetween the BaTiO the MOF3 microsphere central core is positioned and the BaTiO at 90 3 withmicrosphere respect to is the positioned axis of the atSOFT, 90° with thus respect the k-vectors to the ofaxis the of circumferentialthe SOFT, thus the WGMs k-vectors without of the polar circ componentsumferential WGMs excited without do not laypolar on components the same axis excited with thedo not MOF lay core. on the In same the Baxis State with orientation, the MOF core. the position In the B of State the orientation, MOF taper the core position was below of the and MOF far fromtaper thecore SOFT was horizontalbelow and plane, far from eliminating the SOFT any horizontal potential plane, coupling eliminating due to lensing any potential and total coupling internal reflectiondue to lensing cancellation. and total Transmission internal reflection spectra cancella weretion. obtained Transmission for both State spectra A andwere B obtained of this three for port,both opticalState A T-junction.and B of this three port, optical T-junction.

Figure 3. Schematic representation of the experimental setup setup showing showing the the T-shape T-shape excitation excitation system. system. CH1, CH2CH2 and and CH3 CH3 represent represent ports ports used used to excite to ex andcite measure and measure WGMs. WGMs. Two possible Two WGMspossible excitation WGMs configurationsexcitation configurations are demonstrated are demonstrated in the cross in th sectione cross of section MOF taperof MOF tip taper (yellow tip and(yellow red and arrows). red Thearrows). yellow The arrows yellow are arrows referred are to referred the transmission to the tran spectrumsmission (CH1 spectrum to CH2 (CH1 and to vice CH2 versa) and whereas vice versa) the redwhereas arrows the are red chosen arrows for are the chosen scattering for the spectrum scatteri (CH1/CH2ng spectrum to (CH1/CH2 CH3 and vice to CH3 versa). and Two vice different versa). states,Two different in terms states, of orientation in terms betweenof orientation the axis between of the SOFT the axis with of thethe axisSOFT formed with the by theaxis MOF formed taper by core the andMOF the taper semi-immersed core and the microsphere,semi-immersed were microsphere, studied and were are presented studied and as State are presented A and State as B.State In State A and A theState aforementioned B. In State A the two aforementioned axes are parallel two to axes each are other parallel whereas to each in the other State whereas B they arein the perpendicular. State B they are perpendicular.

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All spectral measurements were made using thethe amplifiedamplified spontaneous emission (ASE) of an erbium doped fiberfiber amplifieramplifier with a spectralspectral range 1518–15801518–1580 nm, whereas an opticaloptical spectrumspectrum analyzer (OSA) with a maximum spectral resolution of 0.01 nm was used as a detector. Polarization resolved measurements were obtained using a 5 m length of polarizing optical fiberfiber (providing a polarization resolving ration of ~30 dB) attached to the end of the corresponding reading port (only CH1 or CH2) and connected directly to the OSA, while being supported in a rotating v-groove for being aligned with the coordination systemsystem ofof thethe microsphere.microsphere.

3. Results and Discussion Transmission spectraspectra of of the the T-junction T-junction for differentfor diffe polarizationrent polarization states, states, port excitation port excitation and reading, and forreading, State Afor and State State A and B are State presented B are presented in Figure 4in. Figure 4.

Figure 4. Both statesstates AA andandB B for for transmission transmission and and scattering scattering are are measured measured and and plotted plotted at firstat first with with an SMF28an SMF28 without without using using polarizing polarizing optical optical fiber. fiber. The couplingThe coupling between between the SOFT the SOFT and the and core the ofcore the of MOF the taperMOF intaper State in A State (a) is A obvious (a) is byobvious observing by observing the elimination the elimination of clear and of sharp clear resonancesand sharp whereasresonances the geometrywhereas the of Stategeometry B (b) of allows State usB ( tob) obtainallows more us to significantobtain more resonances significant in resonances the scattering in the spectra. scattering spectra. The transmission spectra of the SOFT in contact with the BaTiO3 microsphere show WGMs behaviourThe transmission (Figure4, CH1 spectra to CH2 of and the vice SOFT versa) in andcontact are typicallywith the obtainedBaTiO3 microsphere for microspherical show cavities.WGMs Thebehaviour azimuthal (Figurel and 4, radialCH1 qtomodal CH2 and orders vice (TE, versa) TM( qand, l)) are have typically been estimated obtained assuming for microspherical a nominal BaTiOcavities.3 microsphere The azimuthal diameter l and radial 25 µm q (as modal measured orders using (TE, SEMTM(q scans), l)) have and been a refractive estimated index assuming of 1.9 [3 a]. Anominal particular BaTiO feature3 microsphere of the WGM diameter spectra 25 of μ them (as State measured A is a modal using distortionSEM scans) by and means a refractive of broadening index observedof 1.9 [3]. forA particular particular feature WGM orders;of the WGM this is spectra related toof thethe perturbationState A is a modal resulted distortion from the by wedging means of thebroadening BaTiO3 miscosphere observed for into particular the MOF WGM capillary orders; end this face. is related Typical to Q-factors the perturbation for the spectra resulted of State from A the of wedging of the BaTiO3 miscosphere into the MOF capillary end face. Typical Q-factors for the spectra of State A of Figure 4 are ~700, well below values reported for BaTiO3 microspheres of similar

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Figure4 are ~700, well below values reported for BaTiO 3 microspheres of similar diameter and optical fiber taper excitation [23]; this is related to the great mismatch between the effective indices of the WGMs and both the MOF and SOFT [24]. Accordingly, the resonant wavelengths of WGMs excited with the SOFT and scattered from the microsphere were collected and measured with the MOF (CH2 to CH3, State A), proving the possibility of using the T-shaped system for the light routing. Yet, this spectra (CH3 to CH2 and vice versa-State A) are characterized by a strong background continuous signal emerging from the light coupling from the BaTiO3 microsphere to the two waveguide systems, however not through the WGM resonation, but from a standard total internal reflection cancellation and a standard lensing process. This strong and broad background can also shadow WGM mediated coupling from the MOF to the SOFT. The spectral data for State B excitation presented in Figure4 exhibit specific similarities with those corresponding to State A. Several of the WGM notches for both polarizations spectrally coincide with those of State A, with relative spectral shifts between the two States that are mostly attributed to possible eccentricity of the capillary wedged BaTiO3 microsphere. A point of particular interest is related to the Q-factor of the WGM excited for State B, which appears much greater compared to the one obtained for the same microsphere, while being positioned and excited at State A. For State B the Q factor was calculated to be 1.63 × 103 for the scattering spectra whereas for State A it was calculated to be ~500 [4,15]. This may be attributed to the fact that for State B the positioning of the wedged microsphere with respect to the MOF symmetry is different, with the MOF core not being aligned with the SOFT axis. This affects the excitation conditions of the systems both through the SOFT and the MOF [25,26]. The WGM peaks measured for light routing between the MOF and the SOFT (CH2 to CH3 and vice versa), exhibit a higher signal to noise ratio with respect to those of State A, since the background signal is lowered by almost 30 dB (at wavelength 1530 nm). This significant reduction of the background, broadband signal denotes that light coupling is minimized through lensing effects and total internal reflection cancellation, but rather takes place through the WGMs resonation facilitated at the BaTiO3 microsphere. An interesting point for the State B light routing scheme is that while the MOF core and the SOFT axis do not coincide/align in space, light coupling between the two vertically placed waveguide structures still exist through WGM resonations with polar modal numbers |m| 6= l, where l refers to the azimuthal WGM order [27]. In Figure4 we denote resonances for the two first radial orders along with the azimuthal number for TE and TM modes [4,10]. The free spectral range (FSR) measured in the graphs is in good agreement with the theoretically expected values based on the FSR formula and the corresponding resonant wavelength [1]. In such a light coupling scheme WGMs with |m| 6= l will be delocalized from the azimuthal circumference of the BaTiO3 microsphere defined by the excitation plane of the SOFT [28], possibly allowing modal crossing between modes of high difference between m and l modal order, facilitated by the microsphere wedging perturbation and eccentricity of the microsphere. In another set of measurements, we used the polarization maintaining optical fiber for both States A and B for resolving the two polarization states. In State A because of the position of the sphere with respect to the SOFT, the fiber s-polarization excites mostly TM modes for the microspherical resonator, whereas in State B the fiber s-polarization corresponds mostly to TE modes for the resonator (‘slapping’ and ‘piercing’ polarization). The majority of polarization resolved data obtained appeared quite noisy, hindering WGM resonation features especially in the SOFT to MOF coupling. In general, due to the geometry of the specific WGM resonation scheme, we expect polarization cross-coupling to take place [29], decrementing polarization resolved modal measurements as shown in Figure5. Micromachines 2018, 9, 521 7 of 9 Micromachines 2018, 9, x 7 of 9

FigureFigure 5. 5. StateState B: B: Transmission Transmission and and corresponding corresponding scatt scatteringering spectra spectra of of the the T-shape T-shape system system obtained obtained withwith the the polarizing polarizing optical optical fiber. fiber. 4. Conclusions 4. Conclusions We communicate results on the investigation of light coupling and routing via a microspherical We communicate results on the investigation of light coupling and routing via a microspherical resonator integrated in a T-shaped optical fiber configuration system, constituted of a BaTiO3 resonator integrated in a T-shaped optical fiber configuration system, constituted of a BaTiO3 microsphere wedged in the capillary of a grapefruit shaped optical fiber while being excited for microsphere wedged in the capillary of a grapefruit shaped optical fiber while being excited for different orientations using a single mode optical fiber taper. The specific optical configuration was different orientations using a single mode optical fiber taper. The specific optical configuration was spectrally characterized in a multi-portal arrangement and the light routing results were discussed spectrally characterized in a multi-portal arrangement and the light routing results were discussed in conjunction with basic WGMs formulation, showing the possibility to rout the light in a 90◦ angle in conjunction with basic WGMs formulation, showing the possibility to rout the light in a 90° angle system from the SOFT to the MOF and vice versa. We intend to continue our investigations of the system from the SOFT to the MOF and vice versa. We intend to continue our investigations of the specific light coupling and routing system for potential use in self-feedback systems for active glass specific light coupling and routing system for potential use in self-feedback systems for active glass microspherical cavities, where the MOF end will be spliced to one end of the SOFT. Another possible microspherical cavities, where the MOF end will be spliced to one end of the SOFT. Another possible application refers to optical routers and microspherical lasers arranged in three dimensions [6], while application refers to optical routers and microspherical lasers arranged in three dimensions [6], being pumped through a single optical fiber taper with specific spectral signatures per MOF port [30]. while being pumped through a single optical fiber taper with specific spectral signatures per MOF Preliminary results in tuning the coupling between the SOFT and the MOF through the WGM spectral port [30]. Preliminary results in tuning the coupling between the SOFT and the MOF through the comb by exploiting the photorefractivity of BaTiO3 microsphere, have resulted in typical spectral shifts WGM spectral comb by exploiting the photorefractivity of BaTiO3 microsphere, have resulted in of the WGMs by ~0.3 nm for a exposure dose of 10.8 J, using a 405 nm CW solid state laser. typical spectral shifts of the WGMs by ~0.3 nm for a exposure dose of 10.8 J, using a 405 nm CW solid stateFunding: laser.This work was partially supported by the projects of ACTPHAST (Grant Agreement No. 619205) and H2020 Laserlab Europe (ECGA 654148). We acknowledge partial support of this work by the project Funding: This work was partially supported by the projects of ACTPHAST (Grant Agreement No. 619205) and “HELLAS-CH” (MIS 5002735) which is implemented under the “Action for Strengthening Research and Innovation H2020Infrastructures”, Laserlab Europe funded by(ECGA the Operational 654148). We Programme acknowledge “Competitiveness, partial support Entrepreneurship of this work andby the Innovation” project “HELLAS-CH”(NSRF 2014–2020). (MIS 5002735) which is implemented under the “Action for Strengthening Research and Innovation Infrastructures”, funded by the Operational Programme “Competitiveness, Entrepreneurship and Acknowledgments: All authors would like to gratefully thank Maria Konstantaki for experimental assistance Innovation”with tapered (NSRF optical 2014–2020). fibers fabrication, and, Mo-Sci Corporation (http://www.mo-sci.com/) for providing the BaTiO microspheres. Warm acknowledgments are also directed to Walter Margulis (ACREO AB, Lund, Sweden) Acknowledgments:3 All authors would like to gratefully thank Maria Konstantaki for experimental assistance for kindly providing the grape-fruit shape MOF used. with tapered optical fibers fabrication, and, Mo-Sci Corporation (http://www.mo-sci.com/) for providing the BaTiOConflicts3 microspheres. of Interest: TheWarm authors acknowledgments declare no conflicts are ofalso interest. directed to Walter Margulis (ACREO AB, Lund, Sweden) for kindly providing the grape-fruit shape MOF used. References Conflicts of Interest: The authors declare no conflicts of interest. 1. Chiasera, A.; Dumeige, Y.; Féron, P.; Ferrari, M.; Jestin, Y.; Nunzi Conti, G.; Pelli, S.; Soria, S.; Righini, G.C. ReferencesSpherical whispering-gallery-mode microresonators. Laser Photonics Rev. 2010, 4, 457–482. [CrossRef] 2. Foreman, M.R.; Swaim, J.D.; Vollmer, F. Whispering gallery mode sensors. Adv. Opt. Photonics 2015, 7, 1. 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