Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-7-9740
Timestamp: 2019-04-19 00:23:59+00:00

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We demonstrate 20-Gb/s 4-level pulse amplitude modulation (PAM-4) signal generation using a silicon Mach-Zehnder modulator (MZM) in the O-band. The modulator is driven by two independent binary streams, and the PAM-4 signal is thus generated directly on the chip, avoiding the use of power-hungry digital-to-analog converters (DACs). With optimized amplitude levels of the binary signals applied to the two arms of the MZM, a pre-forward error correction (FEC) bit-error rate (BER) as low as 7.6 × 10−7 is obtained. In comparison with a commercially available LiNbO3 modulator, the penalty is only 2 dB at the KP4 FEC threshold of 2.2 × 10−4.
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Fig. 1 Schematic cross section of the phase-shifter. Doped regions are used to achieve phase modulation by carrier depletion in the rib waveguide.
Fig. 2 MZM top view. Coplanar electrodes are used to drive independently the 2 arms, while heaters are used to control the operating point.
Fig. 3 Reached PAM-4 levels for different ratios of AC voltages applied to each arm: (a) a ratio of 2, (b) a ratio of 1.74.
Fig. 4 Experimental setup for PAM-4 eye diagrams and BER measurements. Amp.: electrical amplifiers, BERT: BER tester, PD: photodiode, PDFA: praseodymium-doped fiber amplifier, PRBS: pseudo-random binary sequence (source), TDL: tunable delay line.
Fig. 5 Simulated transmission levels (a) (c) (e) and corresponding measured eye diagrams (b) (d) (f) for different applied voltages.
Fig. 6 (a) 20-Gb/s BER versus average received power for the Si MZM, compared to a LiNbO3 MZM. (b) Eye diagram of the Si MZM. (c) Eye diagram of the LiNbO3 MZM.

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