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\begin{document}  % As a general rule, do not put math, special symbols or citations  % in the abstract  \section{Abstract}  We demonstrate the integration of vertical-cavity surface-emitting laser (VCSEL) arrays with silicon photonics chip using flip-chip bonding technique, with bidirectional vertical-coupled grating coupler for light coupling.  \section{Introduction}  One of the most important remaining issues faced by the silicon photonics community is the on-chip laser source. The potential solution falls into two categories. The direct solution for this issue is making lasers based on epitaxial growth on silicon, and the hybrid solutions is realized by the the integration of silicon photonics chips with III-V epitaxy on laser chips. The direct solution has challenges of material limitations, while the hybrid solution can take advantages from the mature fabrication processes for the high quality silicon photonics chips and the VCSELs. Enormous progress has been made in VCSEL technology in the past two decades, with various successful attempts at hybrid integration of VCSELs to various carrier substrates \cite{krishnamoorthy1999vertical, mathine1996reduction, yeh1994integration}. The flip-chip bonding technique has been used to bond both single VCSEL \cite{krishnamoorthy1999vertical} and VCSEL arrays to CMOS chips \cite{krishnamoorthy200016}. However, flip-chip bonding of VCSELs with silicon photonics chips is more challenging than the bonding of VCSELs with CMOS chips. High alignment accuracy is required during the bonding process and the output from the VCSEL need to be aligned to the vertical grating coupler on the silicon photonics chip. In this paper, we demonstrate the integration of 2x4 VCSEL \cite{hofmann20091} array with silicon photonics chip using the flip-chip bonding technique, with custom designed vertical grating couplers as the input/output interfaces. The schematic of the bonding structure is shown in Fig.~\ref{schematic}.  % An example of a floating figure using the graphicx package.  % Note that \label must occur AFTER (or within) \caption.  % For figures, \caption should occur after the \includegraphics.  % Note that IEEEtran v1.7 and later has special internal code that  % is designed to preserve the operation of \label within \caption  % even when the captionsoff option is in effect. However, because  % of issues like this, it may be the safest practice to put all your  % \label just after \caption rather than within \caption{}.  %  % Reminder: the "draftcls" or "draftclsnofoot", not "draft", class  % option should be used if it is desired that the figures are to be  % displayed while in draft mode.  %  \begin{figure}[ht]  \centering  \includegraphics[width=2.5in]{/Users/yunwang/Latex/2015OIC/graphs/Schematic}  \caption{Schematic of the flip-chip bonding for the VCSEL on a silicon photonics chip. The vertical grating coupler results in bi-directional coupling into two waveguides.}  \label{schematic}  \end{figure}  % Note that IEEE typically puts floats only at the top, even when this  % results in a large percentage of a column being occupied by floats.  % An example of a double column floating figure using two subfigures.  % (The subfig.sty package must be loaded for this to work.)  % The subfigure \label commands are set within each subfloat command,  % and the \label for the overall figure must come after \caption.  % \hfil is used as a separator to get equal spacing.  % Watch out that the combined width of all the subfigures on a   % line do not exceed the text width or a line break will occur.  %  %\begin{figure*}[!t]  %\centering  %\subfloat[Case I]{\includegraphics[width=2.5in]{box}%  %\label{fig_first_case}}  %\hfil  %\subfloat[Case II]{\includegraphics[width=2.5in]{box}%  %\label{fig_second_case}}  %\caption{Simulation results.}  %\label{fig_sim}  %\end{figure*}  %  % Note that often IEEE papers with subfigures do not employ subfigure  % captions (using the optional argument to \subfloat[]), but instead will  % reference/describe all of them (a), (b), etc., within the main caption.  % An example of a floating table. Note that, for IEEE style tables, the   % \caption command should come BEFORE the table. Table text will default to  % \footnotesize as IEEE normally uses this smaller font for tables.  % The \label must come after \caption as always.  %  %\begin{table}[!t]  %% increase table row spacing, adjust to taste  %\renewcommand{\arraystretch}{1.3}  % if using array.sty, it might be a good idea to tweak the value of  % \extrarowheight as needed to properly center the text within the cells  %\caption{An Example of a Table}  %\label{table_example}  %\centering  %% Some packages, such as MDW tools, offer better commands for making tables  %% than the plain LaTeX2e tabular which is used here.  %\begin{tabular}{|c||c|}  %\hline  %One & Two\\  %\hline  %Three & Four\\  %\hline  %\end{tabular}  %\end{table}  % Note that IEEE does not put floats in the very first column - or typically  % anywhere on the first page for that matter. Also, in-text middle ("here")  % positioning is not used. Most IEEE journals/conferences use top floats  % exclusively. Note that, LaTeX2e, unlike IEEE journals/conferences, places  % footnotes above bottom floats. This can be corrected via the \fnbelowfloat  % command of the stfloats package.  \section{Vertical Grating Coupler}  A vertical grating coupler is required to couple the light from the VCSEL onto the silicon photonics chip. Such vertical grating couplers have been designed to couple light from optical fibers onto silicon photonics chips \cite{chen2008fabrication}. However, the coupling efficiency of the single-side coupled vertical grating coupler can be further improved, and a coupling efficiency limit of 50$\%$ applies when the structure is symmetric. In addition, the near-field mode size of the output from the VCSEL is about 6 \si{\micro\meter}, smaller than the 9-\si{\micro\meter} mode from an optical fiber. So a smaller grating coupler is required to couple the light from the VCSEL, which makes high-accuracy alignment more difficult to achieve during the bonding process. A bidirectional vertical grating coupler is designed to couple light from the VCSEL as shown in Fig.~\ref{schematic}. The grating coupler is a symmetric structure with uniform gratings. Output from the VCSEL diffracts at the center of the grating coupler and couples equally into the waveguides on both sides of the grating. The top view of a microscope image of the bidirectional grating coupler is shown in Fig.~\ref{VGC}(a). Two adiabatic tapers are used on both sides of the grating coupler to convert the mode from the VCSEL into the fundamental mode of the sub-micron waveguides. The coupled light in the two arms of the vertical grating coupler can be either used separately or recombined, depending on the application. Such bidirectional vertical grating couplers can be also made into arrays to couple light from VCSEL arrays, like shown in Fig.~\ref{VGC}(b).    \begin{figure}[ht]  \centering  \includegraphics[width=2.5in]{/Users/yunwang/Latex/2015OIC/graphs/VGC1.pdf}\\  (a)\\  \includegraphics[width=2.5in]{/Users/yunwang/Latex/2015OIC/graphs/VGC2.pdf}\\  (b)  \caption{Microscopic image of (a) top view of a single bidirectional vertical grating coupler; (b) 2 x 4 bidirectional vertical grating coupler array.}  \label{VGC}  \end{figure}  %Nine grating periods are used in the bidirectional vertical grating coupler to couple the light from the VCSEL, and a grating width of 6 \si {\micro\meter} is used for the grating coupler  \subsection{Fabrication and Measurement}  The designed vertical grating coupler has a grating period of 575 \si{\nano\meter} with a duty cycle of 0.7. The bidirectional vertical grating coupler arrays have been fabricated using electron beam lithography. The fabrication used silicon-on-insulator substrate with 300 \si {\nano\meter} silicon on 2 \si {\micro\meter} buried silicon dioxide layers. The substrates were 25 \si {\milli\meter} squares diced from 150 \si{\milli\meter} wafers. Each arm of the vertical grating coupler is connected to its own output (detuned) grating coupler. Bond pads were designed for the silicon photonics chip matching the dimensions of the VCSEL pads. Metal wires are used on the silicon photonics chip to connect the bond pads to corresponding driving pads far away from the bond pads (Fig.~\ref{VGC}(b)). Therefore, when the VCSEL is flip-bonded to the silicon photonics chip, we can use the driving pads on the silicon photonics chip to drive the VCSEL.  Bonding process goes here.....  The comparison of the V-I and L-I curves of a VCSEL, before and after bonding, are shown in Fig.~\ref{LIV}. It can be seen from Fig.~\ref{LIV}(a) that the driving voltages of the VCSEL increased after bonding to the silicon photonics chip, which is caused primarily by the additional resistance from the pads and metal wires on the silicon photonics chip. The red crosses shown in Fig.~\ref{LIV}(b) denote the measured power after bonding. Increasing driving current was applied to the driving pads on the silicon photonics chip and a large area detector was used to measure the output power. A reference loop including a pair of the output grating couplers were measured and the loss from the output grating coupler has been calibrated in Fig.\ref{LIV}(b). The mismatch between L-I curves before and after bonding mainly comes from the bidirectional vertical grating coupler. The fabrication error of the grating coupler is the first source and the offsets between the VCSEL and the vertical grating coupler is another source that may cause the extra loss.   \begin{figure}[ht]  \centering  \includegraphics[width=2.5in]{/Users/yunwang/Latex/2015OIC/graphs/VI-crop.pdf}\\  (a)\\  \includegraphics[width=2.5in]{/Users/yunwang/Latex/2015OIC/graphs/LI-crop.pdf}\\  (b)  \caption{(a) The comparison of the VI curve before and after the VCSEL bonded on the silicon photonics chip; (b) the comparison of the LI curve before and after the VCSEL bonded on the silicon photonics chip.}  \label{LIV}  \end{figure}  %\begin{figure}[ht]  %\centering  %\includegraphics[width=2.5in]{/Users/yunwang/Latex/2015OIC/graphs/measurement.pdf}  %\caption{The LIV curve of the VCSEL after bonding.}  %\label{LIV_after}  %\end{figure}  \section{Conclusion}  We demonstrate the flip-chip bonding of VCSEL with silicon photonics chips. A 2 x 4 VCSEL array has been successfully bonded to a silicon photonics chip with silicon dioxide cladding. A bidirectional vertical grating coupler is designed to couple the light from the VCSEL onto the silicon photonics chip. The bonded VCSEL has been driven using the electrodes on the silicon photonics chip and the corresponding LIV curves have been obtained.  % conference papers do not normally have an appendix  % use section* for acknowledgement  %\section*{Acknowledgment}  %The devices were fabricated Washington Nanofabrication Facility (WNF) at the University of Washington, part of the National Science Foundation’s National Nanotechnology Infrastructure Network (NNIN). We acknowledge Lumerical Solutions, Inc., and Mentor Graphics for the design software.   % trigger a \newpage just before the given reference  % number - used to balance the columns on the last page  % adjust value as needed - may need to be readjusted if  % the document is modified later  %\IEEEtriggeratref{8}  % The "triggered" command can be changed if desired:  %\IEEEtriggercmd{\enlargethispage{-5in}}  % references section  %\bibliographystyle{IEEEtran}  %\bibliography{VCSEL_Bonding}  %  \begin{thebibliography}{1}  %  \bibitem{krishnamoorthy1999vertical}  A.~V.~Krishnamoorthy, L.~M.~F.~Chirovsky, W.~S.~Hobson, R.~E.~Leibenguth, S.~P.~Hui, G.~J.~Zydzik, K.~W.~Goossen, J.~D.~Wynn, B.~J.~Tseng, J.~Lopata \emph{et~al.}, ``Vertical-cavity  surface-emitting lasers flip-chip bonded to gigabit-per-second CMOS  circuits,'' \emph{Photonics Technology Letters, IEEE}, vol.~11, no.~1, pp.  128--130, 1999.  \bibitem{mathine1996reduction}  D.~L.~Mathine, H.~Nejad, D.~R.~Allee, R.~Droopad, and G.~Maracas, ``Reduction of the  thermal impedance of VCSEL's after integration with copper substrates,'' \emph{Applied physics letters},  vol.~69, no.~4, pp. 463--464, 1996.  \bibitem{yeh1994integration}  H.-J.~J. Yeh and J.~S. Smith, ``Integration of GaAs vertical-cavity surface  emitting laser on Si by substrate removal,'' \emph{Applied physics letters},  vol.~64, no.~12, pp. 1466--1468, 1994.  \bibitem{krishnamoorthy200016}  A.~V.~Krishnamoorthy, K.~W.~Goossen, L.~M.~F.~Chirovsky, R.~G.~Rozier, P.~Chandramani, W.~S.~Hobson, S.~P.~Hui, J.~Lopata, J.~A.~Walker, and L.~A~.D'Asaro, ``16 x 16 VCSEL array flip-chip  bonded to CMOS VLSI circuit,'' \emph{Photonics Technology Letters, IEEE},  vol.~12, no.~8, pp. 1073--1075, 2000.  \bibitem{hofmann20091}  W.~Hofmann, M.~Muller, G.~Bohm, M.~Ortsiefer, and M.-C. Amann, ``1.55-um VCSEL  with enhanced modulation bandwidth and temperature range,'' \emph{Photonics  Technology Letters, IEEE}, vol.~21, no.~13, pp. 923--925, 2009.  \bibitem{chen2008fabrication}  X.~Chen, C.~Li, and H.~K. Tsang, ``Fabrication-tolerant waveguide chirped  grating coupler for coupling to a perfectly vertical optical fiber,''  \emph{Photonics Technology Letters, IEEE}, vol.~20, no.~23, pp. 1914--1916,  2008.  \end{thebibliography}  \end{document}