Abstract
The design and characterization of a slow-wave series push-pull traveling wave silicon photonic modulator is presented. At 2 V and 4 V reverse bias, the measured -3 dB electro-optic bandwidth of the modulator with an active length of 4 mm are 38 GHz and 41 GHz, respectively. Open eye diagrams are observed up to bitrates of 60 Gbps without any form of signal processing, and up to 70 Gbps with passive signal processing to compensate for the test equipment. With the use of multi-level amplitude modulation formats and digital-signal-processing, the modulator is shown to operate below a hard-decision forward error-correction threshold of 3.8×10-3 at bitrates up to 112 Gbps over 2 km of single mode optical fiber using PAM-4, and over 5 km of optical fiber with PAM-8. Energy consumed solely by the modulator is also estimated for different modulation cases.
Highlights
Silicon photonics (SiP) is of immense interest for short-reach optical interconnects because of its CMOS compatibility, high yield and accurate fabrication resulting in lower fabrication costs for high-volume production, and large index contrast allowing dense optical integration
In order to maximize the electro-optic (EO) bandwidth of a traveling wave Mach-Zehnder modulator (TWMZM), three main aspects need to be considered [3]: (1) the microwave attenuation should be as low as possible; (2) the microwave group velocity must match the optical group velocity; and (3) the impedance of the transmission line should be matched to the driver and termination impedance to reduce reflections that can contribute to inter-symbol-interference (ISI) and to obtain the highest possible voltage drop across the modulator
With a series push-pull (SPP) modulator, the p-n junction loaded index of the transmission line is estimated to be lower than the optical group index resulting in the optical and microwave packets traveling through the modulator at different velocities
Summary
Silicon photonics (SiP) is of immense interest for short-reach optical interconnects because of its CMOS compatibility, high yield and accurate fabrication resulting in lower fabrication costs for high-volume production, and large index contrast allowing dense optical integration. The carrier-depletion effect, onto which relies the electro-optical modulation in silicon is relatively weak and long phase shifting regions for low drive voltages are necessary This requires traveling wave modulator designs as opposed to shorter lumped modulators. With a series push-pull (SPP) modulator, the p-n junction loaded index of the transmission line is estimated to be lower than the optical group index resulting in the optical and microwave packets traveling through the modulator at different velocities. The design of silicon TWMZMs are highly dependent on the p-n junction resistance and capacitance and the bias voltages This is not the case for LiNbO3 or other modulators based on electro-optics effects and transmission line design is simplified [21]. Energy consumed by the modulator for these different driving configurations are calculated
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