ECOC Postdeadline Papers © 2011 OSA
A 10G Linear Burst-Mode Receiver Supporting Electronic Dispersion Compensation for Extended-Reach Optical Links
P. Ossieur1, N.A. Quadir1, S. Porto1, M. Rensing1, C. Antony1, W. Han1, P. O’Brien1, Y. Chang2 and P.D. Townsend1 1 Tyndall National Institute, University College Cork, Ireland, email:
[email protected] Vitesse Semiconductor Corporation, Transport Systems Engineering, United States, email:
[email protected]
2
Abstract: We present a novel 10G linear burst-mode receiver with wide (22.7dB) dynamic range. With electronic dispersion compensation, 3400ps/nm (200km) chromatic dispersion can be tolerated at 2dB penalty in bursty ASE-impaired links using C-band electro-absorption modulators. OCIS codes: (060.2360) Fiber optic links and subsystems; (060.4510) Optical communications
1. Introduction Today, optical networks are emerging that transport 10Gb/s bursts over extended-reach (>100km) fibre paths. Examples include hybrid wavelength multiplexed, time division multiple access passive optical networks (DWDMTDMA PONs) [1] and optical burst-switched (OBS) DWDM metro networks [2]. Burst-mode receivers (BMRxs) are required to handle the bursty input signals [3]. As these networks typically operate over the C-band, dispersion compensation is required. Compared to costly optical dispersion compensation, electronic dispersion compensation (EDC) offers the advantages of being adaptive, having no insertion losses and a small physical footprint. EDC requires linear receivers as opposed to limiting receivers [4,5]. While continuous-mode linear receivers are commercially available, to the best of our knowledge no 10Gb/s linear BMRx (LBMRx) has been reported so far. Indeed existing BMRxs include limiting stages [3,6,7], preventing their use in combination with EDC especially close to the overload range of the receiver. Here, we report the first realization of a 10Gb/s LBMRx and demonstrate that the device can enable EDC to extend the range of high (20dB) dynamic range burst-mode optical links. 2. Design and operation of the linear burst-mode receiver
Fig. 1 – Linear BMRx block diagram.
Fig. 2 – The linear BMRx die and submount.
Fig. 1 shows a block diagram of the LBMRx. The anode of a 10GHz PIN photodiode is connected to a high-speed (3dB bandwidth: 8.5GHz) transimpedance amplifier A1 whose gain can be continuously adjusted from 50Ω to 1.8kΩ. The cathode of the photodiode is connected to a second, low-speed (3dB bandwidth: 75MHz) transimpedance amplifier A2. Amplifier A2 has a linear gain of 500Ω over the entire input dynamic range (-20dBm to 0dBm), hence its averaged output swing is proportional to the optical input power. This separation of the highspeed signal path from the amplitude measurement block allows separate optimization of the functions of each path. Next, peak detector PKD1 measures the amplitude of each burst. This peak amplitude is provided to the gain adaptation block AGC1 which quickly (25ns) adjusts the gain of the transimpedance amplifier A1 such that its output swing equals a given reference. The gain adaptation block AGC1 also provides half the peak current to a replica A’1 of the transimpedance amplifier A1, thus creating a reference for the subsequent single-ended to differential conversion using amplifier A3 (gain: 6dB). Additional gain is provided by post-amplifiers A4, whose gain can be continuously adjusted from 4dB to 21dB. Using measurements of the amplitude of the burst with the peak detecting block PKD2, the gain of the post-amplifiers A4 is quickly (25ns) adjusted such that its output swing equals a given reference. The amplifier chain is designed to have a total harmonic distortion (THD) less than 5% (250MHz sinewave with 6dB extinction ratio, 10 harmonics taken into account) across the entire input dynamic range of -20dBm to 0dBm, which translates to 20µA to 1.6mA peak input current). This complies with industry-agreed specifications for linear optical receivers and state-of-the-art continuous-mode linear optical receivers [8,9]. Between bursts, an external reset pulse (width: 10ns) is required to reset the peak detectors thus preparing the LBMRx for a new burst. The unavoidable dc-offsets stemming from mismatch between transistors and resistors are eliminated in a calibration step when the LBMRx is put into first use with the ‘DC-offset compensation block’.
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3. Characterization of the linear burst-mode receiver The LBMRx was fabricated in a 0.25µm SiGe BiCMOS process; the die measures 2.4x2.1mm2 and uses 650mW with 2.5V/3.3V supplies. It was flip-chipped on an AlN substrate and wire-bonded to a 10G PIN photodiode (Fig 2). ‘Soft’ burst DFB generator ‘Loud’ burst DFB generator
Max.+12dBm
VOA EAM
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SSMF: Standard single-mode fibre LBMRx: Linear burst-mode receiver EDC: Electronic Dispersion Compensation CDR: Clock and Data Recovery
Fig. 3 – Experimental setup for characterization of the LBMRx (insets: alternating stream of loud and soft bursts; eye at transmitter output).
Fig. 3 shows the setup used to characterize the LBMRx. The outputs of two DFB lasers (wavelengths: 1550nm, linewidth broadened using a 2kHz, 3% modulation depth tone for suppression of stimulated brillouin scattering) were non-return to zero (NRZ) modulated with 10Gb/s data using electro-absorption modulators (EAMs). The EAM bias voltages were fixed at 0.9V for all experiments. Next, a stream of alternating ‘loud’ and ‘soft’ bursts was generated using variable optical attenuators and two semiconductor amplifiers (SOAs), whose bias currents were switched on or off in alternating fashion, and hence acted either as boosters or shutters. Each packet was 3.27µs long and consisted of a 150ns preamble with sequences of 1s and 0s for settling of the LBMRx gain according to the strength of the burst, followed by 231-1 PRBS data (on which bit-error rates (BER) were measured). The extinction ratio on the ‘soft’ channel was 9dB, on the ‘loud’ channel it was 7dB. The packets were separated with 25.6ns guard bands. For the transmission experiments, two gain-clamped EDFAs [10] were used to provide sufficient launch power for two spans of standard (ITU-T G.652) single-mode fibre. The launch powers were always kept below +12dBm for the ‘loud’ burst. A final EDFA before the LBMRx was used as an optical pre-amplifier and a means to control the optical signal to noise ratio (OSNR) of the signal coupled to the LBMRx. The signal was filtered using an optical filter (0.5nm) and coupled to the LBMRx. The output of the LBMRx is ac-coupled with 560pF capacitors to the EDC chip. The EDC chip consists of a 9-tap feedforward equalizer (FFE) and a 4-tap decision feedback equalizer (DFE). A clock-and-data recovery module was used to provide a recovered clock for the error detector.
Fig. 4a Input and output traces.
Fig. 4b BER vs. input power on the photodiode.
Fig. 4c. BER versus OSNR.
Fig. 4a shows the response of the LBMRx to bursts with 15dB dynamic range. The inset shows the clear open output eye of the -15dBm burst. First, the ‘back-to-back’ (no fibre, no EDFAs) performance of the LBMRx (no EDC) is evaluated as a function of the power on the photodiode, see Fig. 4b. When all bursts have equal power (called the ‘static’ case), the sensitivity is -23.2dBm at a BER of 1.1x10-3 (the threshold for RS(255,223) forward error correction, see e.g. IEEE 802.3av 10GEPON). Next, the BER during the loud burst was measured to test the overload of the LBMRx; see Fig. 4b. The shape of the BER curve is attributed to the fact that for input powers above -5dBm, the bias voltage across the PIN photodiode started to drop which decreased the photodiode bandwidth, and increased its capacitance. This results in eye closure and deterministic bit errors, which will be removed in a new design. The power coupled onto the photodiode was limited to 0dBm due to experimental setup limitations. Finally, we measured the sensitivity penalty due to a preceding loud burst of 0dBm (‘dynamic’ case) and found it to be 0.5dB at a BER of 1.1x10-3. Hence, a dynamic range of at least 22.7dB can be supported, which exceeds typical requirements for access or OBS networks. Next, we measured the LBMRx performance for an ASE noise impaired signal by adding an EDFA before the LBMRx. All optical signal-to-noise ratios (OSNRs) were measured in a 0.1nm reference bandwidth. First, the BER vs. OSNR for a given input power onto the photodiode was measured in the ‘static’ case. Indeed unlike conventional optically pre-amplified receivers, which operate far away from the thermal noise limited region, this is not necessarily possible for optically pre-amplified BMRxs as these need to support large dynamic ranges, whereby the upper limit of the dynamic range may be limited by the BMRx overload or the maximum output power of the optical amplifier. Hence an OSNR characterization
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1.5
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parameterized vs. input power (on the photodiode) is required: see Fig. 4c. The error floor for the -20dBm case is due to the thermal noise of the LBMRx. The BER curves in the case where the burst under consideration is preceded by a burst at 0dBm signal power are also shown. The OSNR penalty due to the preceding loud packet is shown in Fig. 5a: 1.4dB penalty can be seen for an input signal power of -20dBm, which falls to 0.4dB for an input power of -16dBm. The OSNR penalty as a function of input power due to a preceding loud burst (0dBm) is shown in Fig. 5a: it becomes negligible for input signal powers above -12dBm, which is attributed to the fact that any ‘memory’ from the loud burst is negligibly small once the input power of the burst under consideration is larger than -12dBm. P = -20dB m P = -15dB m P = 0dB m
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Fig. 5a OSNR penalty due to ‘loud’ packet.
Fig.5b. OSNR vs. reach (LBMRx+EDC).
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Fig. 5c. OSNR vs. reach (conventional BMRx).
Next, the linearity of the LBMRx is demonstrated by coupling it to an EDC chip and performing transmission tests with the fibre spans and EDFAs. As no EDC chip was available whereby the taps could be adjusted on a burst basis (which is necessary as the up to 20dB dynamic range in launched burst power results in differing amounts of self-phase modulation, and hence impairment levels for ‘soft’ and ‘loud’ bursts), we considered the static case. The required OSNR for a BER of 1.1x10-3 is shown in Fig. 5b versus reach for signal powers (incident on the LBMRx photodiode) of -20dBm, -15dBm and 0dBm. LBMRx output eyes for the -20dBm case at 0km and 240km are shown at their required OSNRs. The 1.5dB higher OSNR for -20dBm compared to the -15dBm and 0dBm cases (at 0km) is attributed to the impact of thermal noise which is significant when the LBMRx is operated close to its sensitivity. Assuming 2dB path penalty at each input power and RS(223,255) FEC, at least 200km reach (3400ps/nm, 17ps/nm/km) is achieved across the input dynamic range of the LBMRx. Finally, the performance of the LBMRx with EDC is compared against a conventional limiting BMRx and no EDC (note that using EDC with a limiting receiver does not make sense as the limiting action erases all amplitude information needed to compensate the intersymbol interference due to chromatic dispersion and self-phase modulation). As no suitable conventional BMRx was available, the output of the LBMRx was provided to a 10G limiting amplifier, thus converting the LBMRx into a conventional BMRx. The required OSNR is shown in Fig. 5c. No data could be obtained for the conventional BMRx at 0dBm, and the error detector lost synchronization after 127km in the -20dBm case (as a BER less than 10-3 could not be achieved), hence only data at -15dBm is shown. It can be seen how after 210km (240km), the conventional BMRx has a 4dB (9dB) higher required OSNR compared to the LBMRx+EDC. These results confirm the significant advantage of the LBMRx over a conventional BMRx in extended-reach applications. 4. Conclusion We have demonstrated a novel 10Gb/s linear BMRx that can support a 22.7dB dynamic range with 0.5dB penalty. It is shown how the linearity of the LBMRx can be used together with electronic dispersion compensation to achieve significantly reduced transmission penalties (up to 9dB at 240km) compared to conventional limiting BMRx’s. We gratefully acknowledge financial support of Science Foundation Ireland (grants 06/IN/I969 and PiFAS).
5. References [1] P. Ossieur et. al. “A 135km 8192-split carrier distributed DWDM-TDMA PON with 2x 32 x 10Gb/s capacity, ” J. Lightwave Technol., vol. 29, pp. 463-474, Feb. 2011. [2] D. Chiaroni et. al., “Demonstration of the interconnection of two optical packet rings with a hybrid optoelectronic packet router, ” in Proc. ECOC’2010, Postdeadline paper PD3.5. [3] P. Ossieur et. al., “A 10Gb/s burst-mode receiver with automatic reset generation and burst detection for extended reach PONs” in Proc. Optical Fiber Conference, Paper OWH3, March 2009. [4] K. Azadet et. al., “Equalization and FEC techniques for optical transceivers, ” J. Solid-State Circuits, vol. 37, pp. 317-327, March 2002. [5] Y. Chang, “Recent Progress of EDC Commercialization in Addressing Datacom and Telecom Challenges to Enable High-speed Optical Enterprise, Metro and Long-haul Networks, ” in Proc. Optical Fiber Conference, Paper NWA2, March 2007. [6] K. Hara et.al., “New AC-coupled burst-mode optical receiver using transient-phenomena cancellation techniques for 10Gbit/s class highspeed TDM-PON systems, ” J. Lightwave Technol., vol. 28, pp. 2775-2782, Oct. 2010. [7] J. Nakagawa, “Demonstration of 10G-EPON and GE-PON coexisting system employing dual-rate burst-mode 3R receiver, ” IEEE Photon. Technol. Lett., vol. 22, pages 1841-1843, Dec. 2010. [8] C. Knochenhauer et.al., “40Gbit/s TIA with high linearity range in 0.13µm SiGe BiCMOS, ” IET Elec. Lett., vol. 47, pp. 605-606, May 2011. [9] “Implementation agreement for integrated dual polarization intradyne coherent receivers, ” OIF, IA OIF-DPC-Rx-01.0, April 2010. [10] H. G. Krimmel et. al., “Hybrid electro-optical feedback gain-stabilized EDFAs” in Proc. ECOC’2009, vol. 2, pp. 693-694 (2009).
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