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Real-time 52 Gbps PAM4 transmission is demonstrated over single mode fiber (SMF) using a directly modulated laser (DML) and a PHY chip. The inner eye optical modulation amplitude (OMA) receiver sensitivities were measured and compared using avalanche photodetector (APD) and PIN photodetector (PD) for the maximum and minimum chromatic dispersions (CDs) of 400GBase-LR8 link. The measured inner eye OMAs were −17.8 dBm and −18.8 dBm for + 10 ps/nm and −58 ps/nm of CDs at the KP4 bit error rate (BER) threshold of 2 × 10−4 using a PIN PD, respectively. The measured inner eye OMA was improved to −21.0 dBm for −58 ps/nm of CD at the KP4 BER threshold using an APD. Negligible OMA penalty (< 0.4 dB) was captured for operating DML at different bias currents of 40 mA and 60 mA using a PIN PD and an APD for both positive and negative CDs at the KP4 BER threshold.
© 2016 Optical Society of America
1. Introduction
To support the explosive bandwidth (BW) demand in data traffic and cloud services, the cost effective, large-scale installation, and low power solutions are essential. To achieve such requirements and high capacity, a combination of higher order modulation formats, such as four-level pulse amplitude modulation (PAM4) and digital signal processing (DSP) techniques, as well as wavelength division multiplexing (WDM) is necessary [1]. Vertical cavity surface emitting laser (VCSEL) based multimode fiber (MMF) system technology provides a low cost and power efficient solution through SR4 and SWDM4 [2]. PAM4 VCSEL based transmission has recently been demonstrated over conventional and wideband OM4 fibers [3,4]. However, this technology suffers from limited reach and capacity due to limited VCSEL and MMF BWs and/or fiber management complexity. PAM4 VCSEL based system also showed poor receiver sensitivity with a complex equalizer over 2 km single mode fiber (SMF) [5]. Moreover, high BW electro-absorption modulated laser (EML) based technologies have higher power consumption, cost, and footprint and lower output optical power in comparison with directly modulated laser (DML) technologies. Thus, PAM4 DML based system is a promising low power and cost effective technology for achieving higher transmission rates and capacity as well as longer reach in WDM systems [6,7]. The IEEE recently proposed 400GBASE-LR8 (8 × 50 Gbps PAM4 modulation format) with a maximum reach of 10 km SMF over 1272.55 nm-1310.19 nm wavelength range [1]. While the receiver sensitivity of a 25 Gbps PAM4 DML based system has been studied for 100GBASE-LR4 over 2 km SMF [8], there is a need for addressing the feasible receiver sensitivity of DML based system for 400GBASE-LR8 over the proposed wavelength range.
In this paper, we investigate the achievable receiver sensitivities of a 52 Gbps PAM4 DML based system over the proposed 400GBASE-LR8 wavelength range and 10 km SMF using an avalanche photodetector (APD) and a PIN photodetector (PD) at RX. To extend the reach and improve the system performance, a 52 Gbps PAM4 commercial chip was used for electronic modulation/demodulation as well as chromatic dispersion (CD) equalization. The optical powers, extinction ratios (ERs), receiver sensitivities are studied for different DML bias currents using a PIN PD. At the KP4 and KR4 bit error rate (BER) thresholds, optical modulation amplitude (OMA) penalties (< 0.1 dB) were observed for DML bias currents ranging from 40 mA to 60 mA using a PIN PD. BER floors were DML bias current dependent. The optical eye diagrams and measured BERs as a function of OMAs are shown for the minimum negative (−58 ps/nm) and maximum positive ( + 10 ps/nm) CD values of 10 km G.652 SMF over the proposed 400GBASE-LR8 wavelength range using both PIN PD and APD at 40 mA and 60 mA DML bias currents. The receiver sensitivities are shown for back-to-back (B2B) as well as + 10 ps/nm and −58 ps/nm of CDs at the KP4 and KR4 BER thresholds using both PIN PD and APD. We show the measured inner eye OMAs range from −17.8 dBm to −18.8 dBm for + 10 ps/nm and −58 ps/nm of CDs at the KP4 BER threshold of 2 × 10−4 using a PIN PD. The measured inner eye OMA was improved to −21.0 dBm for −58 ps/nm at the KP4 BER threshold using an APD. The measured OMAs were better than the proposed IEEE receiver sensitivity for 10 km-400GBASE-LR8 using both PIN PD and APD [9]. OMA penalties were also negligible for operating DML at different bias currents of 40 mA and 60 mA and CD values at the KP4 BER threshold. The worst and best BER floors were 6 × 10−9 (CD = + 10 ps/nm, Ibias = 40 mA) and 3 × 10−11 (B2B, Ibias = 60 mA) using a PIN PD. The DML bias current of 60 mA showed the lowest measured BER floor in comparison with DML bias current of 40 mA for different CD values using both PIN PD and APD. The measured B2B OMA sensitivity was at least 2 dB better than the previous reported receiver sensitivity at the KP4 BER threshold for 52 Gbps PAM4 using a silicon photonics transceiver [10].
2. Experimental setup and results
The experimental setup comprised a TOSA, a ROSA, a variable optical attenuator (VOA), and two special SMF spools with measured + 10 ps/nm and −58 ps/nm CD values at 1310 nm, and a PHY chip as shown in Fig. 1(a). The chip performed the main functions, such as 52 Gbps PAM4 clock and data recovery, pulse shaping at the transmitter, adaptive equalization at the receiver, and real-time BER measurement. A buried-hetero distributed feedback (BH DFB) laser was fabricated at 1309.9 nm with 25 GHz BW and used to study PAM4 DML based system performance for 400GBASE-LR8 link. The receiver sensitivities were measured for two different fabricated ROSAs including a PIN PD and an APD with linear trans-impedance amplifiers (TIAs). The responsivities were 0.8 A/W and 0.55 × gain A/W for both PIN PD and APD, respectively. The APD gain was 6 (12) for 16.0 V (17.5 V) bias voltage. The 3-dB BWs of the PIN-PD and APD ROSAs (including TIA) were 22.5 GHz and 21.5 GHz, respectively.
51.56 Gbps PAM4 optical stream was generated by directly and differentially driving the BH DFB laser using 25.78-Gbaud scrambled pseudo-random bit sequences (PRBS) of length 231-1 produced by integrated DACs, with ~0.8 Vpp electrical signal. 25.78 Gbaud PAM4 electrical eye (shown in Fig. 1(b)) was captured using a digital communication analyzer (DCA) with 25 GHz BW. The DML was biased at different currents from 35 mA to 60 mA and the pre-emphasis compensator was optimized in the chip at TX. The measured modulation current amplitude (Ipp) was ~40 mA at the bias current of 50 mA. TOSA modulation responses are shown in Fig. 2(a) for different bias currents. The 3-dB BWs increased from 17.5 GHz to 26.0 GHz when the bias current increased from 30 mA to 60 mA. Figure 2(b) shows the measured average optical powers (AOPs) and ERs of DML as a function of bias currents. When the DML bias current was increased from 35 mA to 60 mA, AOP changed from −1 dBm to + 1 dBm. ER was decreased from 5.5 dB to 3.4 dB by 25 mA increase of DML bias current. The AOPs and ERs were measured at the output of TOSA. The inner eye OMAs were ~-5.4 dBm for all six different bias currents (35-60 mA). Figure 3 shows the captured PAM4 optical eye diagrams for different DML bias currents using a DCA with 25 GHz BW. At the lower DML bias currents, more eye closure and skew were observed in PAM4 optical eyes due to lower TOSA BW and nonlinearity. Nonlinear dynamics of the DML cause the rising edge of the output to be faster than the falling edge of the output as explained in [11]. Thus, different PAM4 eye levels experience different rise and fall times and skews. The nonlinearity increases when the bias current decreases.
Fig. 2 (a) TOSA modulation responses for different bias currents. (b) AOP and ER as a function of DML bias currents.
To observe the effect of bias current on the PAM4 DML based system performance, BERs were measured at different bias currents as a function of inner eye OMAs (Fig. 4) for B2B using a PIN PD. The pre-emphasis compensator was optimized in the chip at TX. The detected signals were connected to the chip through ROSA TIA differential outputs and automatically equalized at RX. Better receiver sensitivity and BER floor were captured for higher DML bias currents. Lower BER floors were observed due to higher TOSA BW, lower RIN, and operating in the linear region of DML at higher bias currents. The inner eye OMA was degraded ~0.6 dB by decreasing the current from 60 mA to 35 mA at the KP4 BER threshold. The BER floor increased from 2 × 10−11 to 1 × 10−6 when the DML bias current changed from 60 mA to 35 mA. Similar receiver sensitivities at the KP4 and KR4 BER thresholds were observed for the DML bias currents in the range of 40-60 mA.
The CD value of 10 km G.652 SMF ranges between −58 ps/nm and + 10 ps/nm over the proposed 400GBASE-LR8 wavelength window [1]. To investigate 52 Gbps PAM4 DML based receiver sensitivities for 400GBASE-LR8, two spools of special fibers were used with −58 ps/nm and + 10 ps/nm at 1310 nm. Figures 5(a)-5(d) show the measured PAM4 optical eyes at the output of two fiber spools for different DML bias currents. The top and bottom rows show the DML optical eyes at 40 mA and 60 mA bias currents. Different columns show the DML optical eyes at the output of two fiber spools with −58 ps/nm and + 10 ps/nm of CDs. Open PAM4 optical eye (Fig. 5(c)) was observed through −58 ps/nm of CD for 60 mA DML bias current. However, overshoots were captured in PAM4 optical eye (Fig. 5(a)) for −58 ps/nm at the lower DML bias current of 40 mA. The received PAM4 optical eyes showed eye closure for + 10 ps/nm of CD (Figs. 5(b) and 5(d)). At + 10 ps/nm of CD and DML bias current of 40 mA, more eye closure was observed due to a lower TOSA BW caused by a lower DML bias current. At a lower bias current, lower TOSA BW and nonlinearity also caused the captured overshoots in Fig. 5(a) for the negative dispersion. However, the positive dispersion acts as a low pass filter after OE conversion and removes the overshoots as seen in Fig. 5(b).
Fig. 5 PAM4 optical eye diagrams after transmitting over (a) −58 ps/nm of CD and (b) + 10 ps/nm of CD at 40 mA DML bias current as well as (c) −58 ps/nm of CD and (d) + 10 ps/nm of CD at 60 mA DML bias current.
To perform real-time BER measurement for PAM4 transmitted signals over special fiber spools using both PIN PD and APD and compare the receiver sensitivities, the detected signals were connected to the chip through ROSA TIA differential outputs. To optimize the receiver sensitivities, both PIN PD and APD TIA gains were manually adjusted. The lowest BER floor was captured at APD bias voltage of 16.0 V. Figures 6(a) and 6(b) show the BER measurements after adaptive equalization as a function of inner eye OMAs using a pre-emphasis at TX. Low latency FECs are preferred for data center applications and IEEE proposed such FECs with BER thresholds of KP4 (2 × 10−4) and KR4 (5 × 10−5) [12]. Thus, the inner eye OMA receiver sensitivities were measured at KP4 and KR4 BER threshold standards for our 52 Gbps PAM4 DML based system over SMF links with + 10 ps/nm and −58 ps/nm of CDs.
Fig. 6 BER vs. inner eye OMA using (a) PIN PD and (b) APD for B2B as well as −58 ps/nm and + 10 ps/nm of CDs at 40 mA and 60 mA DML bias currents (APD bias voltage was 16.0 V). Black solid circles show the inner eye OMA sensitivity of −21.0 dBm at the KP4 threshold and BER floor of 1 × 10−8 when APD bias voltage was set to 17.5 V for −58 ps/nm and 40 mA bias current.
Table 1. shows the measured inner eye OMA receiver sensitivities at these IEEE BER threshold standards. The measured inner eye OMAs (at the KP4 BER threshold) ranged from −17.8 dBm to −18.8 dBm for + 10 ps/nm and −58 ps/nm of CDs as well as B2B using a PIN PD. The best and worst receiver sensitivities at the KP4 BER threshold were captured for PAM4 signals propagating through −58 ps/nm and + 10 ps/nm of CDs using a PIN PD, respectively. This result is consistent with the observed optical eyes at RX (Fig. 5). The measured inner eye OMAs (at the KP4 BER threshold) varied from −18.8 dBm to −20.6 dBm for + 10 ps/nm and −58 ps/nm of CDs as well as B2B using an APD. OMA penalty was negligible (< 0.4 dB) for operating DML at different bias currents of 40 mA and 60 mA using both PIN PD and APD for both positive and negative CDs at the KP4 BER threshold. The BER floors varied from 6 × 10−9 (CD = + 10 ps/nm, Ibias = 40 mA) to 3 × 10−11 (B2B, Ibias = 60 mA) using a PIN PD. The lowest measured BER floor was captured for the DML bias current of 60 mA which was less than measured BER floors at DML bias current of 40 mA for different CD values using a PIN PD/APD. While APD demonstrated better receiver sensitivities in comparison with PIN PD receiver sensitivities, captured BER floors using an APD were higher than BER floors using a PIN PD for both positive and negative CDs. APD showed at least 1.8 dB and 1.0 dB receiver sensitivity improvement over PIN PD for the negative and positive dispersions, respectively. By changing the bias voltage of APD from 16.0 V to 17.5 V at 40 mA DML bias current, the receiver sensitivity at the KP4 BER threshold improved to −21.0 dBm while the BER floor degraded to 1 × 10−8 for −58 ps/nm as shown with black solid circles and arrows in Fig. 6(b). The same behavior was observed for positive and negative CDs and different bias currents.
Table 1. The measured inner eye OMAs for B2B as well as negative and positive CDs for different DML bias currents at KP4 (2 × 10−4) and KR4 (5 × 10−5) using PIN PD and APD.
3. Conclusion
We presented experimental data on 52 Gbps PAM4 DML based system for 10 km 400GBase-LR8 link. The measured receiver sensitivities were better than the proposed IEEE receiver sensitivities for this application using both PIN PD and APD. These results demonstrate the potential realization of 400GBase-LR8 using 52 Gbps PAM4 DML based system for the next generation of data centers with 10 km reach and highly dense switches.
Acknowledgment
We would like to acknowledge Dr. Julie Eng at Finisar. We also thank IC/DSP groups at Broadcom for their support of this research.
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