Intensity-directed Equalizer for Chirp Compensation Enabling DML-based 56Gb/s PAM4 C-band Delivery over 35.9km SSMF Kuo Zhang(1),(2), Qunbi Zhuge(2),(3),*, Haiyun Xin(1), Mohamed Morsy-Osman(2), Eslam El-Fiky(2), Lilin Yi(1), Weisheng Hu(1), David V. Plant(2) (1) State Key Laboratory of Advanced Optical Communication System and Networks, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China (2) Dept. of ECE, McGill University, Montreal, QC H3A 2A7, Canada, *qunbi.zhuge@mcgill.ca (3) Ciena Corporation, Ottawa, Ontario, K2H 8E9, Canada Abstract We propose an intensity-directed equalizer to address chirp induced eye skewing and inter- symbol interference in DML-DD systems, and experimentally demonstrate a transmission of 56Gb/s PAM4 signals using a 16.8GHz C-band DML over 35.9 km SSMF. low-cost optical laser (DML) can be used Introduction The emerging bandwidth-hungry datacenter and client side applications require high data-rate transceivers. Directly and modulated to implement low-cost transmitters, and it has other advantages such as high output power and small footprint. The main distinction of direct modulation from external modulation such as Mach-Zahnder modulators or electro-absorption lasers is that electrical signal is directly applied to the laser’s gain section. The dependence of refractive index on carrier density makes the directly modulated signal suffer from severe distortions due to the interaction between laser chirp and fiber chromatic dispersion (CD). This limits in C-band transmission, which has advantages such as DWDM compatibility and lower attenuation, and thus most of previous high speed DML-based systems are at O-band1. It is reported that, for C-band DML-based direct detection (DML-DD) transmission without optical CD compensation or other optical processing, the record distance for a 56Gb/s PAM4 signal is 20km2. the application of DMLs In this paper, regarding DMLs’ intrinsic chirp property, we propose and experimentally demonstrate an intensity-directed feedforward and decision feedback equalizer (ID-FFE/ID- DFE) to not only remove the inter-symbol interference (ISI) caused by bandwidth limitation as conventional FFE/DFE does3, but also compensate the chirp-induced eye skewing and ISI. This equalizer applies different coefficients to interfering symbols according to their intensity levels and has comparable computational complexity as conventional FFE/DFE. With this equalizer and a 16.8GHz DML, we demonstrate 56Gb/s PAM4 C-band transmission over 35.9km standard single mode fiber (SSMF) without optical CD compensation or other optical processing, which is a record distance to the best of our knowledge. Fig. 1: Illustration of the interaction between laser chirp and fiber chromatic dispersion for PAM4 signals. is Origin and operation of ID-FFE/ID-DFE Generally, the DML’s chirp can be categorized into adiabatic and transient chirp. In this paper, we mainly consider the adiabatic chirp and the transient chirp is suppressed by a high output power4. The essence of the DML’s adiabatic frequency chirping that different optical intensity levels have different optical frequencies due to carrier density and thus they travel with different velocities due to CD. For a DML with a positive adiabatic chirp, a higher intensity level leads to a larger optical frequency. Hence, the eye diagram of DML-DD PAM4 signals usually manifests a skewing effect after SSMF transmission as shown in Fig. 1, which has also been observed in many previous experiments5. For this reason, PAM4 symbols with different intensity interference contributions to their adjacent symbols. However, the conventional FFE/DFE applies one group of coefficients to mitigate ISI, regardless of the symbol intensity levels. As a result, the ISI caused by the eye skewing effect cannot be removed, and the corresponding issues, such as high bit-error-ratio (BER) of the least significant bit (LSB) and high sensitivity to decision clock jitter, are caused. levels have different To cope with the chirp-related property that symbols with different intensity levels have different interference contributions, we propose an intensity-directed equalizer as depicted in Fig. 2(b). Distinct from the conventional FFE/DFE shown in Fig. 2(a), the proposed ID-FFE/ID-DFE first classifies a symbol into different levels according to its intensity as shown in the grey inset. Then coefficients from different groups are f3f2f1f0f3f2f1f0SSMF

make the DML more adiabatic chirp dominant. After SSMF transmission, the optical signal is first attenuated by a variable optical attenuator (VOA), and then enters into a PIN-TIA. A real- time oscilloscope (RTO) with an 80GSa/s sampling rate the received signal for offline processing, in which we utilize 23-tap half-symbol-spaced (K=2 in Fig. 2) FFE or ID-FFE and 15-tap DFE or ID-DFE. The coefficients of all the equalizers are determined by a minimum mean square error (MMSE) estimation. is employed to digitize Illustration of chirp compensation To illustrate the effectiveness of the proposed equalizer in chirp compensation, we investigate the eye diagrams after 20km transmission. We only adopt FFE and ID-FFE, since their eye diagrams can be easily obtained by inserting zeros between adjacent taps. ID-FFE In Table 1, we show the eye diagrams with four DSP schemes: (i) FFE, (ii) ID-FFE, (iii) FFE+FFE, where a two-stage FFE is employed, and (iv) FFE+ID-FFE, where a FFE followed by a is employed. The peak-to-peak voltage (Vpp) is set as 1.3V, the output power of the DML is 11dBm, and the received power of the PIN-TIA is -2dBm. As per Table 1, by comparing (i) and (ii) it is seen that the proposed ID-FFE is able to effectively alleviate the chirp induced eye skewing problem. In (iv), the first stage FFE can improve the intensity decision accuracy in the following ID-FFE. Therefore, it is is almost observed completely removed compared to (iii). The second observation is that the ID-FFE presents a broader eye diagram than the FFE. the eye skewing that Tab. 1: Eye diagrams after 20km transmission. (iv) (iii) (i) (ii) FFE ID-FFE FFE+ FFE FFE+ ID-FFE Next, since a larger Vpp produces a higher extinction ratio (ER) resulting in a larger chirp4, we show the BER performance and the eye diagrams with different Vpp in Fig. 4. It is obvious that, when the chirp gets larger as Vpp increases, the eye diagram after FFE shows a severer skewing effect. With Vpp increasing from 0.9V to 1.6V, the BER increases quickly from 3.5×10-3 is mainly because that larger frequency chirp leads to larger velocity differences between intensity levels. In contrast, when ID-FFE is adopted, the eye skewing is effectively corrected, and a significant reduction of BER is achieved. to 3.8×10-2. This (a) (b) Fig. 2: (a) Conventional FFE/DFE. (b) ID-FFE/ID-DFE. Th1, Th2 and Th3 are the three thresholds. Lv1, Lv2, Lv3 and Lv4 are the four levels after decision. applied accordingly. Since a PAM4 symbol has four different intensity levels, we have four groups of coefficients, selected based on three thresholds {Th1, Th2, Th3} for ID-FFE or four levels {Lv0, Lv1, Lv2, Lv3} for ID-DFE. After classification, the desired tap coefficient from {wL,0 wL,1 wL,2 wL,3} or {vQ,0 vQ,1 vQ,2 vQ,3} is used. The following procedures are the same as the conventional FFE/DFE. In terms of complexity, the proposed equalizer only adds a few simple decision circuits. Experimental setup Figure 3 plots the experimental setup. A digital- to-analog converter (DAC) together with an FPGA board is used to generate the PAM4 signals. The DAC is operated with one sample per symbol (1sps) at 28GSa/s. After an electrical amplifier (EA), the electrical signal is applied to modulate a commercial DML with 16.8GHz 3dB bandwidth and ~13.5dBm saturation power. A more than 10dBm output power is applied to Fig. 3. Experimental setup Time (s)Amplitude (AU)In-phase Signal00.0050.010.0150.020.025-505Time (s)Amplitude (AU)In-phase Signal00.0050.010.0150.020.025-505Time (s)Amplitude (AU)In-phase Signal00.0050.010.0150.020.025-505Time (s)Amplitude (AU)In-phase Signal00.0050.010.0150.020.025-505DMLDAC: 1spsFPGAVOAPIN-TIARTOOfflineSSMFEAResamplingEqualization: 2spsSymbol decisionError countingD/Kw0w2wP...x(n)x(n-1)x(n-P)DD...v1vQD/Ky(m-1)y(m-Q)y(m)Coefficient classificationD/KD/K...x(n)Coefficient classificationx(n-1)x(n-Q)DD...v1,0,1,2,3()0()1()2()3QQQQQvifymQLvvifymQLvvvifymQLvvifymQLvy(m)y(m-1)y(m-Q),0,1,2,3()11()22()3()3QQQQQwifxnQThwifThxnQThwwifThxnQThwifxnQThw1w0

Fig. 4: Comparison of BER performance for the conventional FFE and the proposed ID-FFE with different chirp levels. Transmission performance In the transmission, the output power of the DML and the Vpp of the PAM4 signal are optimized for each distance2. The output powers are 10, 11, 12.5 and 13dBm and Vpp are 1.6, 1.0, 0.9 and 0.8V, respectively, for 0, 20, 30 and 35.9km distances. Four DSP schemes, including FFE, FFE+DFE, ID-FFE, and ID-FFE+ID-DFE, are compared. The BER performances under various transmission distances are shown in Fig. 5. Eye diagrams after FFE and ID-FFE are also plotted. It can be seen that as the transmission distance increases stronger ISI and eye skewing effect are observed after the conventional FFE. for transmission, This is because of the larger velocity difference caused by a larger CD. In the back-to-back (BtB) case, a good performance is obtained and no BER floor or eye skewing is observed. The proposed equalizer slightly outperforms the conventional equalizer mainly due the to mitigation of some transceiver distortion. After 35.9km the conventional FFE/DFE, because of the severe eye skewing a strong ISI is resulted, and the BER floor is above 10-2. The DFE does not reduce the BER in this case. In contrast, by adopting the proposed ID-FFE, the BER can be reduced to 6.4×10-3 at -1dBm received power. Moreover, by adding ID-DFE, the BER can be further reduced to 3.6×10-3, which is below the 3.8×10-3 HD-FEC BER limit. The improvement of ID-DFE is mainly attributed to the fact that there exists residual eye skewing after ID-FFE as observed in Fig. 5 and it can be addressed by the ID-DFE. In our experiment as per Fig. 5, the proposed equalizer extends the distance from 20km, which is similar to the previous record distance2, to 35.9km. the Conclusions We propose an intensity-directed equalizer to address the large chirp of DMLs in C-band transmission. Regarding interference difference due to chirp-related eye skewing, the equalizer applies different groups of filter coefficients for FFE/DFE based on the intensity levels of interfering symbols. With this equalizer and a commercial 16.8GHz DML, the distance of 56Gb/s PAM4 signals is extended from the state-of-the-art 20km to 35.9km. Acknowledgements This work was supported by NSFC (61431009, 61371082, 61521062), National Science and Technology Major Project of the Ministry of Science China (2015ZX03001021), CSC (201606230160). Technology and of References [1] Y. Matsui et al., “112-Gb/s WDM link using two directly modulated Al-MQW BH DFB lasers at 56 Gb/s,” Proc. OFC2015, paper Th5B.6. [2] M. Kim et al., "Transmission of 56-Gb/s PAM-4 Signal over 20 km of SSMF Using a 1.55-μm Directly- Modulated Laser," Proc. OFC2017, Tu2D.6. [3] J. C. Rasmussen et al., “Digital Signal Processing for Short Reach Optical Links,” Proc. ECOC2014, Tu.1.3.3. [4] D. Mahgerefteh et al., “Chirp Managed Laser and Applications,” J. Sel. Top. Quantum Electron., Vol. 15, no. 5, p. 1126 (2010). [5] N. Eiselt et al., “Experimental demonstration of 56 Gbit/s PAM-4 over 15 km and 84 Gbit/s PAM-4 over 1 km SSMF at 1525 nm using a 25G VCSEL,” Proc. Fig. 5: Transmission performance for (a) BtB, (b) 20km, (c) 30km and (d) 35.9km distances. The inset eye diagrams are after FFE and ID-FFE. ECOC2016, Th.1.C.1. Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-5051.01.51E-41E-30.01BERVpp (V) FFE ID-FFETime (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505(a)(d)Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505-9-6-301E-51E-41E-30.010.11BERReceived Power (dBm) w/o Equalizer FFE FFE+DFE ID-FFE ID-FFE+ID-DFE-9-6-301E-41E-30.010.11BERReceived Power (dBm) w/o Equalizer FFE FFE+DFE ID-FFE ID-FFE+ID-DFETime (s)Amplitude (AU)In-phase Signal024681012x 10-3-505Time (s)Amplitude (AU)In-phase Signal024681012x 10-3-505(b)-9-6-301E-41E-30.010.11BERReceived Power (dBm) w/o Equalizer FFE FFE+DFE ID-FFE ID-FFE+ID-DFE(c)-9-6-301E-41E-30.010.11BERReceived Power (dBm) w/o Equalizer FFE FFE+DFE ID-FFE ID-FFE+ID-DFE