Bogaerts, Wim

Duan, GH

Fedeli, JM

Grosse, P

Hartmann, JM

Jan, C

Le Liepvre, A

Lelarge, F

Reed, GT

Thomson, D

Van Thourhout, Dries

Virot, L

Vivien, L

English

Ghent University Academic Bibliography

Silicon photonics transceivers with integrated hybrid lasers   J-M Fédéli1, L Virot1,2,3, G.H. Duan5, L Vivien2, D. Thomson6, J-M Hartmann1, C.Jany5,  P.Grosse1, A. Le Liepvre5, W.Bogaerts4, G.Reed6, D.Van Thourhout4, F.Lelarge5,   1CEA, LETI, Minatec Campus, 17 rue des Martyrs, F-38054 Grenoble, France 2Institut d’Electronique Fondamentale (IEF), Univ. Paris-Sud, CNRS, Bât 220, F-91405 Orsay France 3STMicroelectronics, Silicon Technology Development, Crolles, France 4 Photonic Research Group,– Ghent University, Ghent, Belgium 5 III-V Lab, a joint lab of Alcatel-Lucent Bell Labs France, Thales Research and CEA, Avenue A. Fresnel, 91767 Palaiseau, France 6 ECS/ORC, University of Southampton, Southampton, Hampshire, UK Phone: +33 4 3878 6879 E-mail: jean-marc.fedeli@cea.fr   1. Introduction Submicron silicon photonics have generated an increas-ing interest in recent years, mainly for optical telecommuni-cations or for optical interconnects in microelectronic circuits. The rationale of silicon photonics is the reduction of the cost and energy of communications systems through the integra-tion of photonic components and an electronic integrated circuit (IC) on a common chip (telecommunications applica-tions), or the enhancement of IC performances with the in-troduction of optics inside a high performance chip (core to core communications), or low cost sensors. By co-integrating optics and electronics on the same chip, high- functionality, high-performance and highly integrated devices can be fab-ricated with a well-mastered microelectronics fabrication process. The FP7 HELIOS project aims to combine a photonic layer with a CMOS circuit by using microelectronics fabrica-tion processes. A first goal was to develop high performance generic building blocks for a broad range of applications: WDM sources by III-V/Si heterogeneous integration [1], fast modulators [2,3] and detectors [4], passive circuits and packaging. With these building blocks, a transmitter with an InP on Si laser and a 16 channel receiver have been assem-bled. 2. Receiver This receiver is imaged in Figure 1: A 2D surface grating couples the light coming from a single mode fiber SMF fiber into the circuit and separates the two polarizations while transforming the TM polarization into TE. Identical 200GHz 16 channel AWGs receive the two input signals and demultiplex the guided TE modes. The two 16 output waveguides are then connected to 16 Ge photodiodes. We have developed a self-aligned process for the fabrica-tion of the waveguides using two photolithography steps with a 193 nm stepper and two Si dry etching steps for the fabrication of gratings and waveguides on optical SOI substrates from SOITEC (220nm Si on top of 2µm Buried OXide). Then cavities are defined for the selective epitax-ial growth of Germanium. This is achieved by deposition of a silica layer which is etched at the end of waveguides. In order to achieve direct coupling, the silicon floor of the cavities is etched down to 50nm on top of the BOX. Ger-manium was then selectivity grown in the cavities and Chemical Mechanical Polishing used to adjust the thick-ness. A thick layer of silica was then deposited and win-dows opened in it down to the Ge layer underneath. In a self-aligned process, the doped regions (N and P) of the lateral PIN Ge photodetector were then defined by ion implantation of Phosphorus and Boron in the defined openings. The separation of the openings thus defines the width of the Ge intrinsic region. A 1µm thick SiO2 layer was then deposited and planarized before etching 400nm diameter holes in it. These holes were filled with Ti/W and planarized in order to get W vias. A Ti/TiN/AlCu metal stack was deposited on this flat surface. DUV 248nm li-thography together with and Cl2 etching was used to fabricate the metallic pads.  The tests were performed on a 200mm wafer prober. Using basic spirals, the propagation losses were statisti-cally measured on the wafers. For 480nm x 220nm clad-ded waveguides, the losses were found at 2.3 dB/cm.  The 2D grating coupler was adapted from the Gent University design [5] to the self-aligned technology (Figure 2). Using 2x 2D couplers mounted face to face, the Polarization Dependant Loss (PDL), the spectral response could be measured. The optimal efficiency for the 2D grating cou-pler was experimentally found to be 15% (~8dB coupling losses) at 1550nm with a 3dB bandwidth of 55nm. The minimum PDL was measured at ~1dB at ~1550nm.  The 16 channels AWG with 200GHz separation is also adapted from a design from Gent University [5] with the technol-ogy and with the introduction of some absorbing sections. To reduce the crosstalk, an enlarged strip waveguide is used for the array part. So the crosstalk levels were measured at -15 dB, and the minimum center-channel insertion losses around 2.8 dB.  The Ge photodiode is a butt coupled PIN lateral type of 10µm length. The photodiode sensitivity ~ 0.8 A/W. Ca-pacitance is in the 10 fF range and dark current of the order of 20nA (-0.5V) as seen on figure 4. The typical photodiode was connected to a 2D grating coupler and the polarization of the incoming was randomly changed. For this demonstrator, a separation of 1µm was selected and the measured bandwidth is around 20GHz which is com-fortable for 10GB/s operation and should be also enough for 25Gbit/s operation in new receivers. The spectral characteristic of the receiver is shown in figure 2. With the losses and sensitivity of the basic blocks, the overall receiver sensitivity is in the order of 0.08 A/W with a channel separation of 1.6nm, corresponding to 200GHz. The 2D grating coupler is responsible for the highest loss of sensitivity and should be optimized. Other solutions such as inverse taper or 1D grating coupler with separation of TE and TM coupled to a polarization rotator could be more efficient 3. Transmitter An integrated tunable laser and MZM (ITLMZ) chip which consists of a single mode hybrid III-V/silicon laser [1], a sili-con Mach-Zehnder (MZ) modulator and an optical output coupler have been designed and characterized (figure 3). The single-mode hybrid laser includes an InP waveguide provid-ing light amplification, and a ring resonator allowing to achieve a single mode operation. Two Bragg reflectors etched on silicon waveguides form the laser cavity. The MZ modu-lator is based on 220nm depletion type [2] and allows to modulate the output light emitted from the hybrid laser. The fabrication process begins with 200mm SOI wafers in-corporating a 400 nm thick silicon waveguide layer on a 2µm BOX. DUV 193nm lithography and HBR etching of 180 nm silicon, allow the definition of rib waveguides for the coupling between the bonded III-V and silicon waveguides. By etching 120 nm silicon layer in the 220 nm defined level, rib wave-guides for modulators are fabricated. The third step is the lithography and etching of 50 nm silicon layer, necessary for 220 nm stripe waveguides for Bragg gratings and output couplers. Then different ion implantation steps are carried out in order to make ,p++,p , n and n++ doping for the mod-ulators. An HDP oxide deposition on the wafers and a CMP are used to planarize the wafers. InP samples with the het-erostructure are directly bonded to the planarized SOI wafer after the preparation of the surfaces. Then InP lasers are then processed, and metallization steps are performed for contacting the modulators, the heaters above ring resonators and the hybrid III-V/Si lasers. A NiCr material is deposited and etched above the ring resonator for the fabrication of heaters which thermally tune the resonance frequency of the ring resonator.  The laser threshold CW current is around 41 mA at 20°C and the output power coupled to the silicon waveguide is of around 1.8 mW for an injection current of 100 mA. The maximum output power is around 3 mW at 20°C, and the output power is higher than 0.5 mW at 60°C. Single mode operation with SMSR larger than 35 dB is achieved With the increase of current, the temperature of the silicon waveguide and the surrounding materials increase, leading to increased mode effective index and hence a shift of the resonance wavelength. As a result, the selected cavity mode will jump to another one with the lowest threshold. Figure 4 plots the lasing wavelength as a function of the heating power and a wavelength tuning range of 7 nm was achieved. The ITLMZ is tested by modulating one arm, at 10 Gb/s using a peuso-random binary sequence (PRBS) of a length of 27 – 1. The BER measurement is performed for 8 different wave-lengths by changing the injection current into the heater. The extinction ratio of all those wavelengths varies from 6 to 10 dB. Figure 5 shows the BER curves for all the wave-lengths and also a reference curve for a directly modulated laser, measured using a high sensitive receiver including an avalanche photodiode. One can see that several channels can achieve error free operation with BER < 10-9. The power level difference to achieve the same BER among all channels is around 4 dB  Acknowledgements    The research leading to these results has received funding from European Community’s under grant agreement n°224312 HELIOS. References [1] M. Lamponi &al, “Low-threshold heterogeneously integrated InP/SOI laser with a double adiabatic taper coupler”, IEEE Photonics Technology Letters, Vol: 24, P: 76 –78, 2012. [2] D. J. Thomson, &al, "High contrast 40Gbit/s optical modulation in silicon," Opt. Express 19, 11507-11516 (2011) [3] M.Ziebell &al, “40 Gbit/s low-loss silicon optical modulator based on a pipin diode”, Optics Express Vol. 20, Issue 10, pp. 10591-10596 (2012) [4] L.Vivien & al, “Zero-bias 40Gbit/s germanium waveguide photodetector on silicon," Opt. Express 20, 1096-1101 (2012) [5] W. Bogaerts & al, “A polarization-diversity wavelength duplexer circuit in silicon-on-insulator photonic wires.,” Optics express, vol. 15, no. 4, pp. 1567-78, Feb. 2007.    Figure 1. Image of the receiver   Figure 2. Photodiodes measurement after AWG.      Figure 3. Image of the integrated tunable transmitter, incorporating a hybrid III-V/Si laser with wavelength tunability, and a silicon Mach-Zehnder modulator.   Figure 4. Lasing wavelength as a function of the heating power   Figure 5. Bit error rate of the transmitter for different wavelength   -2-3-4-5-6-7-8-9-10-32 -30 -28 -26 -24 -22 -20 -18 -16Log(BER)Received Power (dBm)Reference12345678

Silicon photonics transceivers with integrated hybrid lasers

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