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A Directly Modulated Laterally Coupled Distributed Feedback Laser Array Based on Sio2 Planarization Process

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A Directly Modulated Laterally Coupled Distributed Feedback Laser Array Based on Sio2 Planarization Process applied sciences Article A Directly Modulated Laterally Coupled Distributed Feedback Laser Array Based on SiO2 Planarization Process Qichao Wang 1, Jian Wang 1,2,*, Changzheng Sun 1,2 , Bing Xiong 1,2, Yi Luo 1,2,3,*, Zhibiao Hao 1,2, Yanjun Han 1,2,3, Lai Wang 1 , Hongtao Li 1 and Jiadong Yu 1,2,3 1 Beijing National Research Centre for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China; [email protected] (Q.W.); [email protected] (C.S.); [email protected] (B.X.); [email protected] (Z.H.); [email protected] (Y.H.); [email protected] (L.W.); [email protected] (H.L.); [email protected] (J.Y.) 2 Center for Flexible Electronics Technology, Tsinghua University, Beijing 100084, China 3 Flexible Intelligent Optoelectronic Device and Technology Center, Institute of Flexible Electronics Technology of THU, Jiaxing 314006, China * Correspondence: [email protected] (J.W.); [email protected] (Y.L.); Tel.: +86-10-6279-8300 (J.W.); +86-10-6279-4900 (Y.L.) Abstract: Low-cost and high-speed single-mode semiconductor lasers are increasingly required as wide-band access fiber communication expands in recent years. Here, a high-speed laterally coupled distributed feedback (LC-DFB) laser array is achieved based on a SiO2 planarization process. The device exhibits low threshold currents of about 12 mA and high slope efficiencies over 0.26 W/A. Stable single mode operation and high-speed performance are realized with side mode suppression ratios (SMSR) over 45 dB, and 3-dBe bandwidths exceed 14 GHz for all four channels. Such a high-speed and process simple LC-DFB laser array shows great potential to the low-cost fiber communication networks. Keywords: laterally coupled gratings; high-speed modulation; SiO2 Planarization; 1.3 µm; Citation: Wang, Q.; Wang, J.; Sun, C.; distributed feedback laser Xiong, B.; Luo, Y.; Hao, Z.; Han, Y.; Wang, L.; Li, H.; Yu, J. A Directly Modulated Laterally Coupled Distributed Feedback Laser Array 1. Introduction Based on SiO2 Planarization Process. Appl. Sci. 2021, 11, 221. https:// High-speed optical communication develops rapidly with the rapid increase of the doi.org/10.3390/app11010221 amount of data. Low-cost and high-performance laser sources, such as distributed feedback (DFB) laser sources, are urgently needed. Various high-speed DFB lasers were demon- Received: 8 December 2020 strated [1,2], but most of them require at least one epitaxial regrowth step after grating Accepted: 25 December 2020 definition and thus have complicated process and high fabrication costs. Published: 29 December 2020 Laterally coupled (LC) DFB lasers use surface gratings beside the ridge waveguide to select longitudinal mode; thus, they circumvent the complex regrowth step and have the Publisher’s Note: MDPI stays neu- potential for low-cost fiber communication applications. LC-DFB lasers with good static tral with regard to jurisdictional claims performance, e.g., side mode suppression ratio (SMSR), were reported [3–6]. However, in published maps and institutional there are only a few reports about the high frequency performance, e.g., modulation affiliations. bandwidth [7–9]. It is actually a trade-off between the single mode performance and the modulated bandwidth, and it is especially challenging to achieve a high modulation bandwidth for LC-DFB lasers, which often suffer from a low coupling coefficient and thus Copyright: © 2020 by the authors. Li- have a long cavity to select longitudinal mode. censee MDPI, Basel, Switzerland. This In this paper, a high-speed LC-DFB laser array with deep and planar lateral gratings is article is an open access article distributed demonstrated. We have developed a SiO2 planarization process to simplify the fabrication. under the terms and conditions of the The array exhibits low threshold currents of about 12 mA, high slope efficiencies over Creative Commons Attribution (CC BY) 0.26 W/A and high SMSRs over 45 dB. The 3-dBe small-signal modulation bandwidths license (https://creativecommons.org/ exceed 14 GHz for all four channels under injection currents of 100 mA. Such an LC- licenses/by/4.0/). Appl. Sci. 2021, 11, 221. https://doi.org/10.3390/app11010221 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 221 2 of 6 DFB laser array is very promising for applications of high-speed and low-cost optical communication. 2. Device Structure, Design and Fabrication Methods Figure1a depicts the schematic of the proposed LC-DFB laser array in which laterally coupled gratings are along both sides of the ridge waveguide to provide both lateral optical Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 6 confinement and longitudinal feedback. A 2-µm-thick SiO2 dielectric layer supports the planar electrodes and passivates the side wall of ridge waveguides. A p-type electrode pad with a diameter of 100LC-DFBµm introduces laser array is a very pad promising capacitance for applicationsC of 0.18 of pF. high-speed and low-cost optical The intrinsic modulationcommunication. speed of DMLs is limited by the relaxation oscillation fre- quency ( f ). The f is given by [10]. r r 2. Device Structure, Design and Fabrication Methods Figures 1a depicts thep schematic of the proposed LC-DFB laser array in which laterally coupled gratingsGnga arehi along( bothI − Isidesth) of theq ridge waveguide to provide both lateral op- fr = = D (I − Ith) (1) tical confinementqV and longitudinal2p feedback. A 2-μm-thick SiO2 dielectric layer supports the planar electrodes and passivates the side wall of ridge waveguides. A p-type electrode pad with a diameter of 100 μm introduces a pad capacitance C of 0.18= pF. where G is the active regionThe optical intrinsic confinement modulation factor, speed ofng DMLsis the is group limited velocity, by the relaxationa ¶g /oscillation¶N [11] fre- is the differential gain, quencyhi is the ( internal). The is quantum given by [10]. efficiency, and I and Ith are the bias current and the threshold current, respectively. q is the elementary charge,( ) and V is the active volume. q = ( ) =( ) (1) The D factor D = 1/2p Gngahi /(qV) describes the slope of fr on the square root of the bias current above the thresholdwhere is [the12 ].active region optical confinement factor, is the group velocity, = ⁄ Strained multi-quantum [11] wells is the (MQWs)differential weregain, designed is the internal for quantum high speed efficiency, modulations and and are the bias current and the threshold current, respectively. is the elementary charge, which considered theand improving is the active of the volume. differential The D factor gain =1a [13⁄ ]. 2 The( epitaxial)()⁄ describes structure the isslope the same as Li, A.K., whichof on consists the square of root five of periods the bias current of 5-nm-thick above the threshold InGaAlAs [12]. quantum-well and 8.5-nm-thick InGaAlAsStrained barrier multi-quantum [14]. The optical wells (MQWs) confinement were designed factor for of high the activespeed modulations region which considered the improving of the differential gain [13]. The epitaxial structure is G is also an important parameter for fr. The relationship of the coupling coefficient k the same as Li, A.K., which consists of five periods of 5-nm-thick InGaAlAs quantum-well and G versus the ridgeand width 8.5-nm-thick are calculated InGaAlAs bybarrier the [14]. finite The elementoptical confinement analysis factor (FEA) of methodthe active re- as shown in Figure1b.gion The opticalis also an confinement important parameter factor for G.increases The relationship with of the the coupling width ofcoefficient the waveguide, yet a narrowκ and waveguide versus the isridge needed width forare calculated an LC-DFB by the to finite increase element the analysis grating (FEA) coupling coefficient k.method A narrow as shown waveguide in Figure 1b. would The optical result confinement in more power factor leakage increases out with of the the width of the waveguide, yet a narrow waveguide is needed for an LC-DFB to increase the grating waveguides, thus decreasingcouplingG coefficient. However, κ. A itnarrow enhances waveguide the optical would result field in in more the power grating leakage region out of and increases k. The strengththe waveguides, of the thus grating’s decreasing feedback . However, is it described enhances the by optical the field normalized in the grating coupling coefficient (kregionL). The and cavityincreases length . The strength is inverse of the to grating’sk when feedbackkL is is a described constant. by the The normal-fr is proportional to theizedk and couplingG by coefficient Equation (κL). (1). The cavity Theridge length is width inverse is to designedκ when κL is as a constant. 1.2 µm The is proportional to the and by Equation (1). The ridge width is designed as 1.2 μm considering a compromiseconsidering between a compromisek and G. between Figure1c and shows. Figure the variation 1c shows the of variationk versus of dutyκ versus cycle γ assuming a ridgeduty widthcycle γ assuming of 1.2 µ ma ridge and width a grating of 1.2 μ orderm and ofa grating 3. A dutyorder of cycle 3. A ofduty 0.9 cycle is of designed to obtain a higher0.9 is designedk taking to obtain the etching a higher broadening taking the etching effect broadening into account. effect into account. (a) (b) (c) FigureFigure 1. 1.( a(a)) SchematicSchematic of the of thestructure structure of the laterally of the laterallycoupled distributed coupled feedback distributed (LC-DFB) feedback laser array (LC-DFB) with deep laserand planar lateral gratings. (b) κ (blue line) and (red line) versus the ridge width with the third order gratings and a duty arrayfactor with of 0.5.
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