# Optical Transmitters without Driver Amplifiers--Optimal Operation Conditions
## 5 Measurements  
We evaluated the influence of DPreD in a coherent optical link. We show 16 QAM 28- and 56-GBd signals generated using the SRRC pulse shape. The signals were generated with a Micram DAC4 [58] with a sampling rate of 100 GSa/s, a 3-dB bandwidth of 35 GHz, and a frequency-dependent ENOB between 4 and 6 bits. The DAC was connected without driver amplifiers to an LiNbO\({}_{3}\) Oclaro modulator (\(V_{\mathrm{\pi}}\) of 3.5 V, 3-dB bandwidth of 38 GHz). The measurements were done with a coherent receiver with a sampling rate of 160 GSa/s and a 3-dB bandwidth of 63 GHz. The performance was evaluated for different DAC output swings, roll-off factors, and symbol rates. Figure 4: Electrical characterization of the influence of digital pre-distortion (DPreD) on the RMS amplitude of an SRRC signal generated with 100 GSa/s (**a**) without DPreD and (**b**) with DPreD. Both plots were linearly normalized to the same maximum value. The colors indicate the symbol rate (6.25, 12.5, 28, 37.5, 50, 56, 71, 80, 90, and 95 GBd). The signal bandwidth results from the symbol rate and the roll-off factor. woDPreD: without digital pre-distortion; wDPreD: with digital pre-distortion. Figure 3: Characterization of frequency and phase response. (**a**) Amplitude spectrum of transmitted and received coarse non-integer spaced comb with a reduced set of 34 spectral lines. (**b**) Frequency and phase response measured with a comb of 331 spectral lines. Figure 5 summarizes the applied DSP steps and gives an overview of the experimental set-up. In the transmitter (Tx) DSP, binary information was mapped to complex symbols, the desired pulse shape was applied, and, if desired, the DPreD was used to correct the frequency response. The experimental set-up consisted of the Micram DAC4 connected without electrical driver amplifiers to the IQ modulator, which modulated a laser operated at a wavelength of 1550 nm. The optical output of the modulator was amplified and filtered with a 2-nm bandpass filter before it was detected with a coherent receiver. In the receiver (Rx) DSP, DPostD was applied in the first step--if desired. Afterward, timing recovery [59], carrier recovery, least-mean-square equalization, hard decision demapping, and bit error ratio testing (BERT) were applied. For the analysis below, we report the BER we acquired using the BERT and the effective SNR, which we determined by comparing the transmitted symbols with the received symbols after the equalizer. The effective SNR allows modulation format-independent analyses and was, therefore, chosen for the final evaluations. Figure 6a,b show the influence of the DAC output voltage on the bit error ratio (BER) and the effective SNR of a 28-GBd 16 QAM signal. The BER was determined from the ratio of wrongly transmitted bits to the total number of transmitted bits. The DAC output swing could be chosen between 350 and 750 mV\({}_{\text{PP}}\). We evaluated the performance in the low- and the high-SNR regime and show the results for DPreD in blue and DPostD in red. The SNR was adapted to the low- and high-SNR regime by optical noise loading. Due to the higher electrical signal power of signals with DPostD, there is an improvement in the low-SNR, as well as the high-SNR, regime when compared to DPreD. In both SNR regimes, there is an increase of 0.5 dB in effective SNR when changing from DPreD to DPostD. This corresponds to an increase of 12% in optical signal power. Figure 6c shows the BER for a 56-GBd 16 QAM signal measured for different roll-off factors. The performance increases with the roll-off factor. The slope of the improvement changes at an ROF of 0.4, which can be seen due to the given trend lines that are given below and above 0.4. The change in BER slope at a roll-off factor of 0.4 can be attributed to the amplitude of SRRC signals, as discussed in Section 2. We attribute the continued BER improvement above 0.4 to a wider horizontal eye-opening, which gives resilience toward jitter, and thus, a better BER.