# Scalable Photonic Digital-to-Analog Converters
## 5 Experimental Validation Results  
The conversion rate of the 2-bit PDAC is determined using the above experimental setup, and a PAM-4 eye diagram is obtained on the digital communication analyzer (DCA). An on-chip back-to-back power transmission excluding the DUT demonstrates an approximate 10 dB of loss. Moreover, the transmission at the operating wavelength, located on the slope of the resonance dip, is lower than the peak transmission off the resonance dip. At the operating wavelength, while the ring modulators are biased at 3.5 V reverse bias, the steady state DUT output optical power is only \(-7.2\) dBm. Therefore, the analog output signal is amplified to 1 dBm average power using an EDFA. A tunable optical bandpass filter with a 0.85 nm bandwidth is deployed afterward to filter out the amplified spontaneous emission (ASE) noise of the EDFA. The filtered analog signal is then converted to an analog electrical signal with a photodetector (PD) to be prepared for the DCA. Figure 6a shows the eye diagrams of the 4 Gbps NRZ electrical signal from one of the data channels of the PPG with a resulting Q-factor of 10.4 dB (a). In Figure 6b,c, the PDAC analog output exhibits a Q-factor of 7 dB and 6.5 dB when the MSB and the LSB are modulated, respectively.  
Finally, in Figure 6d, the PDAC analog output shows four distinct levels when both bits are modulated. The eye diagram experimentally validates the successful conversion of the parallel digital signal into an analog optical and later analog electrical signal, which is supported by the effective number of bits (ENOB) value of approximately 2.2 in our repeated measurements. In terms of linearity in the transfer characteristic, to quantify its deviation from an ideal DAC (a linear increase in the output level by 1 LSB for an increase in the input digital code by one), the differential non-linearity (DNL) of the DUT is found to be 0.4 LSB.  
The experimental demonstration of the photonic digital-to-analog conversion at high speed beyond two bits could not be performed because of equipment limitations. By  
Figure 6: (**a**) The eye diagram of the input electrical signal at 4 Gbps, generated by the pseudorandom pattern generator (PPG); the eye diagrams, representing the (**b**) MSB bit and (**c**) LSB bit are modulated with a 4 Gbps on-off keying (OOK) signal; (**d**) 4 GBaud PAM-4 is achieved when both MRMs are modulated by two 4 Gbps data channels.  
applying DC biases to the p-n junctions of the four MRMs, 16 intensity levels were obtained at the output of the 4-bit PDAC (Figure 7). The applied DC forward bias signal shifts the resonances to shorter wavelengths by carrier injection, manifested by the increase in the PDAC signal. Figure 7a illustrates a possible operating wavelength (dashed vertical line at 1551.15 nm) at which different 4-bit sequences produce corresponding intensity levels. In the linear scale, Figure 7b shows the corresponding PDAC output power at the operating wavelength (1551.15 nm). The nonlinear relationship between the bit sequence and output power is explained by the unequal distribution of the input optical power to the MRM branches (an MRM branch receives 50% CW optical power compared to its preceding branch, and the output power is given by the interference of the signals in all branches). The random fluctuations in the spectrum (consequently, in the power levels) are caused by noise in the bias voltages and the MRM driver voltages, as well as room temperature fluctuations (causing resonance drifts), and contribute to a DNL of approximately 0.6 LSB (relatively larger than that of the 2-bit PDAC). In practice, the MRM requires tuning control to stabilize the output optical power.