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Compact Silicon Photonics Coherent WDM Transceiver

Final Report Summary - COWDM (Compact Silicon Photonics Coherent WDM Transceiver)

The goal of the COWDM project is to demonstrate a Silicon Photonics based optical transceiver / receiver pair utilizing optical combs generated with nonlinear effects in on-chip devices and combining these with wavelength domain multiplexing (WDM) and coherent detection. Two approaches to generate light on chip have undergone rapid progress in the last few years: (1) hybrid material integration, e.g. integration of III-V materials into an otherwise group IV semiconductor chip, (2) on-chip, nonlinear parametric generation of multiple wavelengths from a single carrier signal. On-chip nonlinear generation has the advantage of being possible with entirely CMOS compatible material systems. Furthermore, optical conversion efficiencies of up to 45% have been demonstrated in ultra high quality factor resonators, making this a power-efficient method – a key metric for the future deployment in Silicon Photonics since a single semiconductor laser needs to be capable of providing the pump for an entire WDM system.

Best-of-class long haul communication systems have moved towards coherent detection. In this detection scheme, a light source with an optical frequency locked to the carrier frequency of the incoming optical signal is generated at the receiver (the local oscillator) and mixed with the incoming signal, resulting in amplification in the optical domain. This way, the noise performance of the receiver can be improved up to the shot noise limit of the incoming signal. However, in order for this detection scheme to work the optical frequency of the local oscillator has to be very well controlled, for example by locking the receive laser with an optical phase locked loop (OPLL) or by optical injection locking. In this context, being able to lock an entire comb generated at the receiver to the frequency comb generated at the transmitter by solely tuning two degrees of freedom (a comb source is fully locked if two of its lines are locked) is extremely powerful.
The goal of the COWDM project is to combine and adapt ongoing research relating to integrated on-chip comb sources with Silicon Photonics (SiP) planar integrated circuits (PICs) in order to demonstrate a receiver / transceiver pair relying on these technologies. More compact, power efficient and spectrally efficient communications is an important objective for sustaining the fast growth of data center infrastructure.

In the COWDM project we have designed and characterized complete transmitter and receiver subsystems based on integrated Silicon Photonics technology. We have developed ring modulators with an analog bandwidth exceeding 30 GHz optimized for the targeted system architecture [1-2], 90 degree hybrids and reconfigurable add-drop multiplexers [3], and combined them in integrated transceivers [3]. Coupling from an external laser to the Silicon Photonics chip is facilitated by the development of novel, alignment tolerant couplers [4-5]. The functionality of the subsystems was tested with semiconductor single section mode locked lasers [3]. Furthermore, we have designed and fabricated a range of monolithically waveguide coupled integrated microtoroids in view of assessing their suitability to the transceiver architectures developed in the project [6].

For further details on the project, please contact:

Prof. Jeremy Witzens
Institute of Integrated Photonics – RWTH Aachen
Sommerfeldstr. 24
D-52074 Germany
or visit our website at

Selected Publications:
[1] J. Müller, F. Merget, S. Sharif Azadeh, J. Hauck, S. Romero García, B. Shen, “Optical Peaking Enhancement in High-Speed Ring Modulators,” Sci. Rep. 4, Art. 6310 (2014).
[2] (Invited) S. Sharif Azadeh, J. Müller, F. Merget, S. Romero-García, B. Shen, J. Witzens, “Advances in silicon photonics segmented electrode Mach-Zehnder modulators and peaking enhanced resonant devices,” Proc. SPIE 9288, Art. 928817 (2014).
[3] (Invited) J. Müller, J. Hauck, B. Shen, S. Romero-García, E. Islamova, S. Saeed Azadeh, S. Joshi, N. Chimot, A. Moscoso-Martír, F. Merget, F. Lelarge, J. Witzens, “Silicon photonics WDM transmitter with single section semiconductor mode-locked laser,” Adv. Opt. Technol. (De Gruyter) 4(2), 119-145 (2015).
[4] S. Romero-García, B. Marzban, F. Merget, B. Shen, J. Witzens, “Edge Couplers with relaxed Alignment Tolerance for Pick-and-Place Hybrid Integration of III-V Lasers with SOI Waveguides,” J. Sel. Top. Quant. Elec. 20(4), 8200611, special issue on Silicon Photonics (2014).
[5] (Invited) S. Romero-García, B. Shen, F. Merget, B. Marzban, J. Witzens, “Alignment Tolerant Couplers for Silicon Photonics,” J. Sel. Top. Quant. Electron. 21(6), 8200214, special issue on Semiconductor Lasers (2015).
[6] (Invited) J. Richter, M. P. Nezhad, J. Witzens, “Monolithically integrated waveguide-coupled silica microtoroids,” Optoelectronics and Communications Conference (OECC), 2015.