# Scalable Photonic Digital-to-Analog Converters
Furthermore, attaining the output analog signal in these topologies depends on a wide-band opto-electro conversion at the output because each intensity level consists of several signals with different wavelengths. In other words, only an analog electronic signal is attainable at the output, which limits their applicability in case a photonic analog signal is desired. It is worth noting that a PDAC could be also realized by the \(n\)-bit driving of a single modulator (e.g., an electro-absorption or a microring modulator). The downside of this approach is the additional overhead requirement of a bulky and power-hungry electronic DAC (EDAC) for the \(n\)-bit driving of the modulator, making this scheme a less appealing solution as an EDAC alternative. The power consumption of typical 8-bit EDACs ranges from 0.56 to 4.03 W for 60 to 100 GS/sec sampling rates, respectively [23; 24]. Nevertheless, a direct-digital-drive approach could alleviate the need for additional EDACs, to some extent, despite the demand for a digital memory lookup table to deal with non-linearity issues [6]. This topology, loosely classified under both serial and parallel categories, encounters  
Figure 1: State-of-the-art topologies for the implementation of photonic digital-to-analog converters (PDACs) classified under categories of serial [2; 3; 4; 5], incoherent parallel [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17], coherent parallel [18; 19], and serial-parallel [6; 20]. The top schematics illustrate data handling in the 2-bit serial (**left**) and parallel (**right**) PDACs, where each color represents arbitrary 2-bit digital data converted to analog output in electro/optical domain, as depicted by the red dashed line. technical challenges when the PDAC resolution is scaled up because of the trade-off between the inherently finite resonance extinction ratio of microring modulators and the overhead of the scheme dealing with the non-linear power transmission. Parallel PDACs offer an excellent solution to the conversion rate limitations by using multiple parallel channels and allowing for higher conversion rates and higher bit resolutions. The basic idea of parallel PDACs relies on the incoherent or coherent summation of binary switched-weighted signals. The weighting operation is predominantly conducted by splitting the monotone input signal through cascaded symmetric or asymmetric splitters. Past works in the field, such as references [7; 8], are two examples of off-chip fiber-based implementation of the first proposed PDACs. They proposed using a large-area photodetector (PD) and a wide-band incoherent optical input source (i.e., erbium-doped fiber amplifier), respectively, to realize an incoherent summation of signal paths switched by MZMs. Quite similarly in [9], the authors utilized another solution for realizing an on-chip incoherent summation objective, which is using a PD at the end of each weighted signal path and performing the summation in the electronic domain, sacrificing the footprint and bandwidth to avoid coherent summation. Using a polarization multiplexing scheme, the work reported in [10] proposed a 2-bit PDAC switched by a dual-polarization modulator. Furthermore, another class of incoherent parallel PDACs employs spectral weight encoding of the multiwavelength optical signal using nonlinear optical loop mirrors [11], microring resonators (MRRs) [12; 13], MZMs [14], and dual parallel MZMs [15]. The incoherent summation resolution in this class could be improved with a novel quantization scheme enabled by a weight segmentation and balanced detection approach, as introduced in [16; 17]. In this class, not only does the incoherent output summation depends on spectral PD accumulation, but it also demands optical sources at multiple wavelengths that can affect the overall power efficiency of the system, not to mention the requirements of having the same conversion time for each wavelength. In the work of [18], they proposed a coherent scheme based on switching an intensity-weighted optical by employing electro-optic modulators (EOMs).