###### Abstract  
In this work, we systematically analyze the impact of three kinds of Mach-Zehnder modulator (MZM) imbalances, including bias deviation, amplitude mismatch, and differential time skew in intensity-modulation direct-detection (IM-DD) links. It is shown that, for power fading limited transmission, the imbalances can be utilized as advantages rather than impairments. Specifically, the bias deviation with single-arm driven mode and amplitude mismatch with differential driven mode can increase the available bandwidth by shifting the frequency of fading notches. Meanwhile, time skew provides another way to avoid fading by shaping the double sideband (DSB) signal into a vestigial sideband (VSB) with an asymmetrical transfer function. In the transmission experiment, 34 Gbaud Nyquist 6/8-ary pulse amplitude modulation (PAM-6/8) signals are used for investigation in a 20 km dispersion-uncompensated standard single-mode fiber (SSMF) link. With the help of a Volterra nonlinear equalizer, all three kinds of imbalances can achieve bit-error rates (BERs) below the 7% and 20% hard-decision forward error correction (HD-FEC) thresholds for PAM-6 and PAM-8 signals, respectively. The received power sensitivity is also compared at the back-to-back (BTB) case and after fiber transmission. Both numerical simulation and experimental demonstration confirm that the dispersion-induced power fading can be effectively suppressed with bias, amplitude, or skew imbalance, providing a feasible solution for transmission distance extension of C-band DD links. M 2021 10 2866 2021  
systems by introducing notches in the signal spectrum [8]. Therefore, it is worth studying the methods of suppressing the power fading and extending the reach of DD optical links. In recent years, extensive efforts have been undertaken to cope with power fading. The first approach is to perform optical dispersion compensation. For passive optical network (PON) and mobile fronthaul (MFH) scenarios, O-band transmission is still preferred [9]. In Reference [10], 100 Gb/s/\(\lambda\) PON downstream is demonstrated with a 29 dB power budget by using a 10 G-class directly-modulated laser (DML) and a semiconductor optical amplifier (SOA). However, the transmission distance is limited by the fiber loss, and the capacity scalability is restricted by the uneven gain characterization of the optical amplifier and dispersion slope in the O-band. An alternative choice is to employ an optical approach, such as a dispersion-compensating fiber (DCF) [11] and a tunable dispersion compensation module [12] with C-band transmission. By matching the accumulated dispersion values between DCF and the transmission fiber link, the first 400 G real-time inter-DCI is reported based on 8 \(\times\) 50 Gb/s PAM-4 [11]. Secondly, vestigial sideband (VSB) or single sideband (SSB) modulation can overcome power fading by avoiding the superposition of two dispersed sidebands. In Reference [13], 100 Gb/s VSB PAM-4 is successfully delivered over 45 km SSMF with an optical band-pass filter (OBPF). However, precise wavelength control is needed to align with the optical filter. For SSB signaling, digital/analog Hilbert transform [14; 15; 16; 17], virtual carrier [18], and optical carrier [19] based schemes have been fully investigated, at the cost of doubled digital-to-analog convertors (DACs) or additional electrical components. Thirdly, as the frequency nulls cannot be efficiently mitigated by linear equalizers, transmitter-side Tomlinson-Harashima precoding (THP) [20; 21] and receiver-side decision feedback equalizer (DFE) [22; 23] are proposed. The former algorithm requires channel information through feedback, while the latter suffers from error propagation. Moreover, dispersion pre-compensation [24; 25] can be performed by the inverse dispersion transfer function, if complex-valued modulation is allowed.