WB1 (Invited) 10:30 - 13:30
An Efficient MOS-Capacitor based Silicon Modulator and CMOS Drivers for Optical Transmitters M. Webster*, P. Gothoskar, V. Patel, D. Piede, S. Anderson, R. Tummidi, D. Adams, C. Appel, P. Metz, S. Sunder, B. Dama, and K. Shastri, Cisco Systems, Inc. Allentown, PA, USA. (*)email:
[email protected] with the gate-oxide centered in the optical mode. A device cross-section and the optical mode are shown in Fig.1(a) with an SEM image of a fabricated device given in Fig.1(b). An advantage of the SISCAP device is that it can be operated in accumulation which provides high charge densities either side of the gate-oxide. With this high charge density region centered within the optical mode, a large perturbation overlap integral[5] is achieved resulting in an efficient modulator that has a VL < 2V.mm at a wavelength of 1310nm. The device C-V curve is shown in Fig.1(c) and the phase modulation as a function of voltage is plotted in Fig.1(d). The SISCAP propagation loss with sufficient dopant levels for 28 Gbps operation is about 6.5 dB/mm. Most of this loss is due to the p-type poly-Si layer. The SISCAP device was fabricated using a 0.13um technology node CMOS process on SOI wafers.
Abstract—We present an efficient MOS-capacitor based silicon modulator. In an MZI configuration, a 9dB extinction ratio at 28 Gbps is achieved from the 1V output of a low-power CMOS inverter driver IC. Keywords— modulator, silicon photonics, MOS, lumpedelement, CMOS driver
I.
INTRODUCTION
Silicon photonics is rapidly developing as a promising technology for many optical communication applications because it can enable low-cost, low-power, and small-size solutions. This is, in part, a result of leveraging the performance and manufacturing capabilities of commercial CMOS fabrication processes. An efficient optical modulator is a key component in any silicon photonics platform. Most silicon photonic modulators utilize the free-carrier dispersion effect [1] to induce a phase change in an optical mode. This can be achieved with device geometries that operate using a MOS-based capacitor [2], a reverse-biased PN-junction [3], and forward injection of a PIN device [4]. Here, we describe the SISCAP (Silicon Insulator Silicon CAPacitor) device which is a MOS-capacitor based silicon photonic modulator. II.
III.
OPERATION AND DRIVE CONFIGURATION
For 1310nm NRZ applications at 28Gbps (such as IEEE 100G LR4 standard), we used an electrical driver IC fabricated using the CMOS 40nm technology node. This allows for lowpower and high-speed CMOS inverters that can provide a 1V output swing. With this drive voltage of 1V driving a SISCAPbased MZI, an RF section length of 400um gives the optimum optical modulation amplitude (OMA) and a resultant extinction ratio of about 9dB at 28 Gbps. A photograph of the MZI section is shown in Fig.2(a).
DEVICE STRUCTURE
The SISCAP device consists of a p-type poly-Si layer, a gate-oxide layer, and a n-type SOI layer. The overlapping poly-Si and SOI layers form a single mode optical waveguide
An advantage of operating the SISCAP structure in accumulation mode is that for voltages greater than about 1V,
Fig. 1: (a) The SISCAP cross-section and optical mode. (b) SEM image of fabricated device. (c) C-V curve. (d) Optical phase modulation versus applied voltage.
978-1-4799-2283-3/14/$31.00 ©2014 IEEE
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Fig. 2: (a) Photograph of MZI structure. (b) Simplified equivalent electrical circuit for driving the SISCAP-based MZI. (c) 28 Gbps PRBS-31 optical eye. (d) 40 Gbps PRBS-31 optical eye.
the phase modulation efficiency is greatly increased, as seen in Fig.1(d). Therefore, with a voltage swing from the driver IC of 1V, it is better to drive the SISCAP voltage between 1.2V and 2.2V, instead of between 0V and 1V. The equivalent electrical circuit of this configuration is shown in Fig.2(b). Driving the SISCAP at higher voltages increases the net capacitance which can impact the RC bandwidth of the modulator. However, under these drive conditions, the SISCAP bandwidth is sufficient to obtain a 28 Gbps PRBS-31 NRZ optical eye with a 50% eye-crossing, > 40% mask-margin, and an extinction ratio of 9dB (at a wavelength of 1310nm). This is shown in Fig.2(c). Driving the modulator at higher datarates is also possible, as shown in Fig.2(d) where a 40 Gbps PRBS-31 NRZ optical eye is obtained, with an extinction ratio of ~8dB.
IV.
REFERENCES [1]
The phase modulation efficiency of the SISCAP device enables a short RF section length (400m) for the MZI to be used. This allows the modulator to be treated as a lumped element RC equivalent circuit, which can simplify the design of the driver circuit. No traveling wave transmission line drivers are required, only a CMOS inverter stage with low output impedance is needed to drive the MZI. However, any inductance between the driver and SISCAP (from wire-bonds and metal routing on optical IC) needs to be modeled and considered in the driver design.
[2]
[3]
[4]
[5]
A typical driver IC design consists of a CMOS inverter chain that connects to the modulator. The power consumption of about 2-3 mW/Gbps is achieved for the complete driver path from input to the IC, through the inverter driver stage, and also power consumed in the MZI. There is also an additional 20mW of DC power required (worst case) to thermo-optically bias the MZI to the nearest quadrature point.
[6]
Finally, we have recently demonstrated the SISCAP device in an IQ modulator configuration for QPSK applications [6].
978-1-4799-2283-3/14/$31.00 ©2014 IEEE
CONCLUSION
We have demonstrated an efficient MOS-capacitor based silicon photonic modulator. In an MZI configuration, the modulator can be treated as a lumped-element RC equivalent load that can be driven directly by a CMOS inverter stage. For a 1V drive voltage, a 28Gbps PRBS-31 NRZ eye was obtained with an extinction ratio of 9dB. This approach to optical modulators should further advance the field of silicon photonics.
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