We quantify, through simulations and experiments at 21.4 GBaud, the effect of in-band crosstalk on several advanced optical modulation formats, showing a 1-dB penalty at a bit-error ratio of 1×10 -3 from a crosstalk of -18 dB, -24 dB, and -32 dB for QPSK, 16-QAM, and 64-QAM, respectively. OCIS codes: (060.2360) Fiber optics links and subsystems; (060.4080) Modulation IntroductionCommercial 100-Gb/s transmission systems available and in development today are largely based on single-carrier 28-Gbaud polarization-division-multiplexed (PDM) quadrature phase-shift keying (QPSK). Such signals can be operated on a 50-GHz frequency grid in wavelength-division-multiplexed (WDM) systems (i.e., at a spectral efficiency, SE, of 2 b/s/Hz), while still providing spectral margin for several optical add/drop nodes. In order to further increase transport capacities, greater SEs can be achieved using higher-level quadrature amplitude modulation (QAM), such as 16-QAM and 64-QAM. The more severe impact of in-band crosstalk on these higherorder formats can represent an important limitation arising, e.g., from imperfect optical add/drop nodes or doubleRayleigh backscatter in Raman amplified systems [1,2]. In addition, in-band crosstalk represents a key limitation in the upcoming class of spatially multiplexed systems, where low-crosstalk multi-core fiber [3,4] is being designed to avoid complicated multiple-input multiple-output (MIMO) processing to counteract crosstalk [5,6]. In all of the above scenarios, it is important to know the crosstalk levels at which higher-order modulation formats start to suffer penalties. In this paper we quantify the increasing crosstalk penalties when increasing the constellation size in single-polarization QAM systems, using simulations and 21.4-GBaud QPSK, 16-QAM, and 64-QAM experiments.

We demonstrate a 222 GBd on-off-keying transmitter in a short-reach intra-datacenter scenario with direct detection after 120 m of standard single mode fiber. The system operates at net-data rates of >200 Gb/s OOK for transmission distances of a few meters, and >177 Gb/s over 120 m, limited by chromatic dispersion in the standard single mode fiber. The high symbol rate transmitter is enabled by a high-bandwidth plasmonic-organic hybrid Mach-Zehnder modulator on the silicon photonic platform that is ribbon-bonded to an InP DHBT 2:1 digital multiplexing selector. Requiring no driving RF amplifiers, the selector directly drives the modulator with a differential output voltage of 622 mV pp measured across a 50 Ω resistor. The transmitter assembly occupies a footprint of less than 1.5 mm × 2.1 mm.

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