# Optical Transmitters without Driver Amplifiers--Optimal Operation Conditions
## 1 Introduction  
Low power consumption is one of the major design goals for the next-generation highly integrated datacom systems [1]. The power consumption of an optical transmitter can be significantly reduced by driver-amplifier-less operation [2]. In transmitters without a driver amplifier, the electrical source is directly connected to the electro-optical modulator [3; 4; 5]. This reduces the transmitter power consumption [2], improves the link-noise figure [6], and avoids additional noise and non-linear distortions [7; 8]. Driver-amplifier-less optical transmitters generating multilevel signals were even shown without digital-to-analog converters (DACs) [9; 10]. A key enabler for the driver-amplifier-less optical transmitter technology is an optical modulator with a small \(V_{\pi}\) (voltage to switch the modulator from on to off). In fact, numerous optical modulators that can be operated with driving voltages below 1 V\({}_{\text{PP}}\) (electrical swing) were demonstrated [3; 11; 12; 13; 14; 15] with speeds up to 100 GBd [16]. For the best performance, the frequency response of optical transmitters must be corrected to mitigate distortions and the resulting inter-symbol interference (ISI). However, the compensation of the transmitter frequency response is known to minimize the electrical output signal power of the DAC [17]; this is due to the increase in the peak-to-average power ratio (PAPR) in pre-distorted signals [18; 19; 20]. A loss in the electrical signal power is a particular challenge for driver-amplifier-less optical transmitters, because they already have a reduced margin concerning electrical signal power. Therefore, the correct design of the waveform, and eventually, a wisely chosen clipping [21; 22; 23; 24; 25; 26; 27] are required. State-of-the-art optical transmitters circumvent the penalty of loss in the electrical signal due to predistortion by using electrical driver amplifiers. These amplifiers boost the electrical signal to allow a driving of the modulators in the range of \(V_{\pi}\). Preamplifiers give sufficient margin to apply a digital pre-distortion (DPreD). In fact, digital pre-distortion techniques are more and more deployed in commercial systems. Thus, for instance, a low-cost self-calibration routine to determine the frequency response including the in-phase and quadrature (IQ) skew [28] was developed, and real-time transmission of \(1\) Tb/s with a 28-nm CMOS ASIC digitally compensating for up to 25 dB of loss in electrical-to-optical (EO) and optical-to-electrical (OE) conversion [29] was shown. DPreD can also cancel, in part, the linear and non-linear distortions. The linear pre-distortion methods compensate for the low-pass characteristic of the transmitter, and therefore, eliminate the inter-symbol interference (ISI) of the signal [30; 31; 32; 33; 34; 35; 36; 37]. Non-linear compensation methods can cancel the non-linear transfer function of driver amplifiers, digital-to-analog converters, and Mach-Zehnder modulators. In the case of a dominant non-linear influence on the signal, either Volterra series equalization [38; 39; 40] or pattern-dependent look-up table-based corrections [41; 42] were demonstrated to increase the system performance. Furthermore, the nonlinear Mach-Zehnder modulator transfer function can, for instance, also be linearized by inversion of the latter [40; 43] or by the design of a special modulator [44]. However, because of an increasing demand for transmitters with a reduced power consumption and the emergence of high-speed optical modulators with a small \(V_{\pi}\), it is of interest how to design a waveform that optimizes the optical signal-to-noise ratio (OSNR) generated by a driver-amplifier-less optical transmitter. In this paper, we extend our previous work [45] and show that the correct choice of roll-off factor (ROF) for the signal-pulse shape in combination with clipping and that the digital post-distortion (DPostD), rather than DPreD, enhance the optical signal quality of the driver-amplifier-less transmitters.