hv photonics Communication High-Q-Factor Silica-Based Racetrack Microring Resonators Yue-Xin Yin 1, Xiao-Jie Yin 2,3, Xiao-Pei Zhang 4, Guan-Wen Yan 1, Yue Wang 2, Yuan-Da Wu 2, Jun-Ming An 2, Liang-Liang Wang 2 and Da-Ming Zhang 1,* 1 State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699# Qianjin Street, Changchun 130012, China; [email protected] (Y.-X.Y.); [email protected] (G.-W.Y.) 2 State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China; [email protected] (X.-J.Y.); [email protected] (Y.W.); [email protected] (Y.-D.W.); [email protected] (J.-M.A.); [email protected] (L.-L.W.) 3 Shijia Photons Technology, Hebi 458030, China 4 National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China; [email protected] * Correspondence: [email protected] Abstract: In this paper, ultrahigh-Q factor racetrack microring resonators (MRRs) are demonstrated based on silica planar lightwave circuits (PLCs) platform. A loaded ultrahigh-Q factor Qload of 1.83 × 106 is obtained. The MRRs are packaged with fiber-to-fiber loss of ~5 dB. A notch depth of 3 dB and ~137 pm FSR are observed. These MRRs show great potential in optical communications as filters. Moreover, the devices are suitable used in monolithic integration and hybrid integration with other devices, especially in external cavity lasers (ECLs) to realize ultranarrow linewidths. Keywords: optical devices; resonators; integrated optics; wavelength filtering devices Citation: Yin, Y.-X.; Yin, X.-J.; Zhang, 1. Introduction X.-P.; Yan, G.-W.; Wang, Y.; Wu, Y.-D.; An, J.-M.; Wang, L.-L.; Zhang, D.-M. With the development of ultrahigh-speed optical interconnection, coherent optical High-Q-Factor Silica-Based Racetrack communication [1–3], and coherent detection technology [4–7], more urgent requirements Microring Resonators. Photonics 2021, are put forward for the narrow linewidth, high power, and high stability of laser source. 8, 43. https://doi.org/10.3390/ However, limited to the length of cavity, the spectral linewidth conventional distributed photonics8020043 feedback (DFB) lasers and distributed bragg reflector (DBR) lasers are typically in the order of ~MHz, along with a small tuning range around a few nm [8,9]. Therefore, external Received: 7 January 2021 cavity lasers (ECLs) become the first choice to replace traditional lasers to supply narrow Accepted: 4 February 2021 linewidth of ~sub 100 kHz and wide tuning range. The ECLs based on blazed diffraction Published: 6 February 2021 gratings [4] are of wide tuning range and narrow linewidth, but suffer from huge size, com- plex package, and difficult alignment, which increase the cost and harm the stability of the Publisher’s Note: MDPI stays neutral lasers. On the other hand, hybrid integration between photonics chips and semiconductor with regard to jurisdictional claims in lasers provides an ideal method to fabricated ECLs. These hybrid integration ECLs have published maps and institutional affil- been investigated based on different materials, such as III-V [3], silicon [10–12], silica [6], iations. SiN [13], and polymers [14–16]. Compared with other materials, silica planar lightwave circuits (PLCs) are attractive for ECLs owing to its high mechanical strength, high thermal stability, low loss, and almost the same core geometry with fiber cores, which is suitable for an optical communication system. Besides, silica-based waveguides are widely used to fab- Copyright: © 2021 by the authors. ricate commercial applications, such as optical splitters [17], array waveguide grating [18], Licensee MDPI, Basel, Switzerland. and optical buffer [19]. Therefore, silica based ELCs are promising to realize large scale This article is an open access article integration for optical communication. distributed under the terms and In particular, the microring resonators (MRRs) with compact sizes and ultrahigh- conditions of the Creative Commons Q factors show great potential in lasers [10–14,20], sensors [21–24], and filters [25–28]. Attribution (CC BY) license (https:// Compared with basic MRRs, the racetrack MRRs using lateral coupling, increase the creativecommons.org/licenses/by/ coupling gap size and the fabrication tolerance. Here, in this paper, ultrahigh-Q factor 4.0/). Photonics 2021, 8, 43. https://doi.org/10.3390/photonics8020043 https://www.mdpi.com/journal/photonics Photonics 2021, 8, x FOR PEER REVIEW 2 of 8 Photonics 2021, 8, 43 2 of 9 MRRs are designed, fabricated, and experimentally demonstrated. The proposed race- racetracktrack MRRs MRRs are built are designed, on silica PLCs fabricated, platform. and experimentallyOwing to the ultralow demonstrated. loss of the The waveguide, proposed racetrackultrahigh MRRsQ factors are of built 1.83 on × silica 106 are PLCs acquired. platform. From Owing measured to the transmission, ultralow loss a of notch the waveguide, ultrahigh Q factors of 1.83 × 106 are acquired. From measured transmission, depth of 3 dB and ~137 pm FSR is observed. The MRRs are packaged with fiber-to-fiber a notch depth of 3 dB and ~137 pm FSR is observed. The MRRs are packaged with loss of ~5 dB. These MRRs are suitable used in optical communications as filters. Moreo- fiber-to-fiber loss of ~5 dB. These MRRs are suitable used in optical communications as ver, the devices are potential used in monolithic integration and hybrid integration with filters. Moreover, the devices are potential used in monolithic integration and hybrid other devices, especially used in external cavity lasers (ECLs) to realize ultra-narrow lin- integration with other devices, especially used in external cavity lasers (ECLs) to realize ewidths. ultra-narrow linewidths. 2.2. DeviceDevice DesignDesign andand SimulationSimulation TheThe MRRMRR proposed is fabricated fabricated on on a a silica silica-based-based PLC PLC platform. platform. The The cross-section cross-section of ofthe the silica silica waveguide waveguide is shown is shown in the in theinset inset of Figure of Figure 1a.1 Ina. order In order to realize to realize compact compact foot- footprint,print, the therefractive refractive index index difference difference between between the the core core and and cladding cladding is is 2% with differentdifferent doping.doping. TheThe refractive refractive indices indices of of the the cladding cladding and and the the core core are 1.4448are 1.4448 and 1.4737(@1550and 1.4737(@1550 nm) respectively.nm) respectively. The single-mode The single-mode condition condition is calculated is calculated by MATLAB by MATLAB based on based the eigenvalue on the ei- equationsgenvalue equations [29], and [29], the thicknessand the thickness of the core of the is core selected is selected to be 4toµ bem 4 according μm according to the to simulationthe simulation results results in Figure in Figure1a. As 1a. Figure As Figu1b shows,re 1b shows, the mode theprofile mode ofprofile the fundamental of the funda- transversemental transverse electric (TE)electric mode (TE) is simulatedmode is simulated through beam through propagation beam propagation method (BPM) method and the(BPM) effective and the refractive effective index refractive is 1.4615. index is 1.4615. FigureFigure 1.1. (a) Relations between between the the core core thickness thickness and and th thee effective effective refractive refractive indices indices at at1550 1550 nm. nm. TheThe insetinset showsshows thethe cross-sectioncross-section ofof waveguide.waveguide. (b) The mode profile profile of the waveguide simulated simulated throughthrough BPMBPM method.method. TheThe structurestructure ofof racetrackracetrack MRRsMRRs withwith directionaldirectional couplerscouplers (DCs)(DCs) isis shownshown inin FigureFigure2 2.. UsingUsing thethe coupledcoupled modemode theorytheory (CMT)(CMT) andand thethe transfertransfer matrixmatrix techniquetechnique (TMT)(TMT) [[30,31],30,31], wewe derivederive thethe transmissiontransmission powerpowerP Pt1t1 inin thethe outputoutput waveguide,waveguide, whichwhich is,is, 2ααθ2 +−tt22 2cos() ==2 2 a + jtj − 2ajtj cos(q) Pt1 =PEjtt11Et1j = 2 (1)(1) 1 +12cos()+−aααθ2j2tjtt2 − 2ajtj cos(q) wherewherea αis theis the loss loss coefficient coefficient of the of the ring, ring, and andt and t andk are κ theare couplerthe coupler parameters parameters with with the 2 2 relationthe relation of k of +κ 22t += =t 1.1 The. The phase phase shift shift in in the the ring ring is, is, q =θ L= bβ (2) cirLcir (2) =+π wherewhere Lcir isis the circumference ofof thethe racetrackracetrack which which is is given given by byL cirLRLcir= 222pR + 2L,, RR beingbeing thethe radiusradius ofof thethe ringring measuredmeasured fromfrom thethe centercenter ofof thethe ringring toto thethe centercenter ofof thethe waveguide,waveguide, LL beingbeing thethe couplingcoupling length.length. TheThe propagationpropagation constantconstant b βis, is, 2p2π b =β =⋅· nen f f (3) l λ eff (3) where l is the wavelength, ne f f is the effective refractive index. Photonics 2021, 8, x FOR PEER REVIEW 3 of 8 λ where is the wavelength, neff is the effective refractive index. Figure 3 shows the relations between transmission efficiency and bend radius. While the bend radius is larger than 1460 μm, bend loss could be lower than 1 dB/cm. From the resonance equation of MRRs, we can derive the expression of the ring radius R and the free spectral range (FSR) as mλ λ n RFSR==0 , 0 eff π (4) 2 nmneff g where m is the resonator order of the MRRs, λ0 is the central resonant wavelength, neff is =−λ λ the effective refractive index, and nngeffeff dnd is the group refractive index. Ac- cording to Figure 4, with a decrease in R, FSR will increase dramatically. However, the bend loss will also increase. Low bend loss means high Q factor, but small FSR will limit Photonics 2021, 8, x FOR PEER REVIEWthe applications—for example, the port number of wavelength division multiplex (WDM)3 of 8 systems and working range of sensors.