# 47.5 GHz Membrane-III-V-on-Si Directly Modulated Laser for Sub-pJ/bit 100-Gbps Transmission
## 3 Results
### Evaluation of 60 GBaud PAM-4 Transmission  
We evaluated the suitability of our DML for future 100 Gbps/lane systems by performing bit-error rate (BER) measurements of 60 GBaud PAM-4 signals at back-to-back (BTB) and after 2-km SSMF transmissions. A schematic of the experimental setup used for these measurements is illustrated in Figure 5a. The 60 GBaud PAM-4 signals were generated by an arbitrary waveform generator (AWG) at a peak-to-peak modulation bias of 75 mV and with a sampling rate of 120 GSa/s. A root-raised cosine (RRC) filter with a roll-off factor of 22% was used for pulse shaping. Then the RF signal was amplified by a linear RF amplifier with a gain of 22 dB. The fiber coupling was performed by butt-coupling an HNAF, spliced together with a SSMF pigtail, to the chip's facet. The modulated spectra at 25 \({}^{\circ}\)C and 50 \({}^{\circ}\)C are shown in Figure 3d,f, respectively. In these figures, the optical amplification effect due to the PPR is observed on the long-wavelength side of the spectrum.  
At the receiver side, the opto-electronic conversion was performed by a commercial _p-i-n_ photodiode (PIN), which was integrated together with a trans-impedance amplifier (TIA). The PD's 3-dB bandwidth was around 32-GHz. Note that no optical amplifier was used in this transmission setup. After the photodetection, the signals were stored by a digital sampling oscilloscope (DSO) at a sampling rate of 160 GSa/s for semi-real time offline processing. In the receiver digital signal processing, the signals were first downconverted to 120 GSa/s (2x times 60 GBaud) and correlation-based time synchronization was performed. Then, match filtering was performed by a similar 22% roll-off RRC filter, followed by least-means square adaptive equalization. The digital equalizer was a low-complexity nonlinear filter [23], with 71 linear taps and 51 nonlinear taps, composed of a second and third-order polynomial filter with the addition of only 10 second-order cross-beating terms. The equalized PAM-4 eye diagrams obtained after the 2-km SSMF transmissions, are shown in Figure 5c for 25 \({}^{\circ}\)C operation and Figure 5d for 50 \({}^{\circ}\)C operation. Distortion-free PAM-4 eye diagrams are observed in both cases.  
A summary of the BER performance for different received optical powers (ROPs) is shown in Figure 5b. The illustrated data contain results for both BTB and 2-km SSMF transmissions, and both at 25 \({}^{\circ}\)C and 50 \({}^{\circ}\)C operations. These measurements were obtained by placing a variable optical attenuator prior to the PIN (not shown in Figure 5a). For all cases a BER below the 400G Ethernet's standard KP4-FEC threshold [19] was achieved.  
Figure 4: Normalized laser electro-optic (EO) responses: **(a)** Simulated using a bias current of 9.1 mA (25 \({}^{\circ}\)C operation is assumed). Solid line represents the data of the proposed DML structure, while the dashed line represents the data with the same directly-modulated lasers (DML) structure without including the DBR-2 section; **(b)** Experimental data at 25 \({}^{\circ}\)C and 50 \({}^{\circ}\)C, using bias currents of 9.1 mA for the 25 \({}^{\circ}\)C operation and 15.1 mA for the 50 \({}^{\circ}\)C operation.  
Considering a 5.8% overhead for such a KP4-FEC implementation, the net rate was around 113.42 Gbps (gross rate: 120 Gbps). Based on this net rate value and the power DC consumptions obtained in the previous subsection, the energy/bit was estimated to be 0.17 pJ/bit at 25 \({}^{\circ}\)C and 0.34 pJ/bit at 50 \({}^{\circ}\)C case. As a reference, the 7% overhead hard-decision FEC (HD-FEC) threshold [24] is also depicted in Figure 5b. For this HD-FEC threshold, a net rate of around 112.15 Gbps could be achieved with an excess power margin of around 6 dB. This margin can accommodate excess losses caused by WDM/SDM (de-)multiplexers in an integrated multichannel transceiver.