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We present a novel 10G linear burst-mode receiver (LBMRx). Equipped with a PIN photodiode, a high sensitivity of −22.7dBm (bit-error rate: 1.1x10−3) was achieved when handling bursts with a dynamic range of 22.7dB (each −22.7dBm burst was preceded by a 0dBm burst). The LBMRx requires a 150ns preamble for fast gain adjustment at the start of each burst and can handle bursts separated by a guard time as short as 25.6ns. With electronic dispersion compensation, 3400ps/nm (200km) chromatic dispersion can be tolerated at 2dB penalty in ASE-impaired links using C-band electro-absorption modulators.
©2011 Optical Society of America
1. Introduction
Today, optical networks are emerging that transport 10Gb/s non-return to zero modulated (NRZ) bursts over extended-reach (>100km) fibre paths. Examples include hybrid wavelength multiplexed, time division multiple access passive optical networks (DWDM-TDMA PONs) [1] and optical burst-switched (OBS) DWDM metro networks [2]. The traffic transported across such networks consists of bursts (or packets) whose amplitude can vary by over 20dB from one burst to the next. Burst-mode receivers (BMRxs) are required to handle such high dynamic range signals [3]. As these networks typically use C-band wavelengths, dispersion compensation is needed to enable reaches beyond 100km. Compared to costly optical dispersion compensation, electronic dispersion compensation (EDC) offers the advantages of being adaptive, having no optical insertion losses and a small physical footprint. EDC however requires linear receivers as opposed to limiting receivers [4]. While continuous-mode linear receivers are commercially available, most existing BMRxs include limiting stages [3, 5, 6], preventing their use in combination with EDC especially close to the overload range of the receiver. Here, we report the realization of a 10Gb/s linear BMRx (LBMRx) [7] and demonstrate that the device can enable EDC to extend the range of high (20dB) dynamic range optical links.
2. Design and operation of the linear burst-mode receiver
Figure 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, input referred current noise: 726nARMS) transimpedance amplifier (TIA) 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 (−25dBm to 0dBm, corresponding to a peak input current ranging from 5.2μA till 1.6mA at 10dB extinction ratio and a photodiode responsivity of 0.9A/W), hence its averaged output swing is proportional to the optical input power. This separation of the high-speed 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 (within 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 (within 25ns) adjusted such that its output swing equals a given reference. Figure 2(a) shows the values to which the gain of TIA A1, post-amplifier gain A4 and total LBMRx gain are adjusted as a function of the average input optical power (photodiode responsitivity: 0.9A/W, extinction ratio: 10dB), as well as the resulting output amplitude swing. Note how the LBMRx was designed in such a way that the gain of post-amplifier A4 reduces first with increasing input power before the gain of TIA A1. This approach concentrates the gain in the front-end of the LBMRx, thus minimizing penalties due to post-amplifier noise and electrical crosstalk. Note that the full output swing is not realized until the average optical input power exceeds −19dBm, which is acceptable as long as the minimum signal swing exceeds the sensitivity of the subsequent EDC chip. With input sensitivities of commercially available EDC chips as low as 20mV, even for a minimal input power of −25dBm the output swing (100mV) of the LBMRx is sufficiently large.
The LBMRx is designed to have minimum total harmonic distortion (THD) (across the input dynamic range of −25dBm to 0dBm. The simulated THD (250MHz sinewave with 6dB extinction ratio, 10 harmonics taken into account) is shown in Fig. 2(b) (PIN responsivity: 0.9A/W): it remains below 5% for an average optical power ranging from −25dBm to 0dBm. This complies with industry-agreed specifications for linear optical receivers and measured results of continuous-mode linear optical receivers [8,9]. The weak non-linear distortion stems mainly from the non-linearity of the transistor that acts as a voltage controllable feedback resistor in TIA A1, which is required to realize the controllable gain. 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 first put into use with the ‘DC-offset compensation’ block.
3. Experimental setup and electronic dispersion compensation chip
Figure 3(a) 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. The variable optical attenuators before and after this EDFA are used to control the optical signal to noise ratio (OSNR) and power of the signal provided to the LBMRx. The signal was filtered using an optical filter (0.5nm) and coupled to the LBMRx.
Fig. 3 (a) Experimental setup for characterization of the LBMRx (insets: alternating stream of loud and soft bursts; eye at transmitter output). (EAM: electro-absorption modulator, SOA: semiconductor optical amplifier, GC-EDFA: gain-clamped erbium doped fibre amplifier, SSMF: standard single-mode fibre, VOA: variable optical attenuator), (b) block diagram of electronic dispersion compensation chip (VGA: variable gain amplifier, FFE: feedforward equalizer, DFE: decision feedback equalizer, CRU: clock recovery unit).
The output of the LBMRx is ac-coupled with 560pF capacitors to the EDC chip. The EDC chip (see Fig. 3(b)) consists of a 9-tap feedforward equalizer (FFE) and a 4-tap decision feedback equalizer (DFE). A separate clock recovery unit (CRU) extracts a clock which is used in the DFE to perform the final data decision. The FFE and DFE taps are adjusted with an eye monitor and an internal microcontroller such that the eye opening of the received signal is maximized. In the burst-mode experiments the microcontroller was disabled and tap values derived when receiving a continuous signal (no burst-to-burst amplitude variations) were used. Finally, an additional external clock-and-data recovery module was used to provide a 10GHz recovered clock for the error detector.
4. Experimental results
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 onto an AlN substrate and wire-bonded to a 10G PIN photodiode (see Fig. 4(a) ).
Figure 4(b) shows the response of the LBMRx to bursts with 15dB dynamic range. The inset shows the clear open output eye of the −15dBm burst. Figure 5(a) shows the optical-to-electrical transfer curve (S-parameter S21) for various gain settings measured using a vector network analyzer suitably equipped with a Mach-Zehnder modulator. The gain ranges from 85dBΩ down to 47dBΩ. Excellent stability of the frequency response over this gain range can be observed with a 3dB bandwidth ranging from 6.8GHz to 8.8GHz and a maximum peaking of less than 2dB. Next, we performed bit-error rate (BER) measurements. First, the ‘back-to-back’ (no fibre, no EDFAs) BER of the LBMRx (no EDC) is evaluated as a function of the power on the photodiode, see Fig. 5(b). 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. The shape of the BER curve is attributed to the fact that for input powers above −5dBm, the bias voltage across the 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 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 power onto the photodiode was measured in the ‘static’ case. 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 parameterized vs. power (on the photodiode) is required: see Fig. 6(a) . 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. 6(a): 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. 6(b): 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. 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. 7(a) 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 limit. 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. 7(b). 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.
5. 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.
Acknowledgments
We acknowledge financial support of Science Foundation Ireland (grants 06/IN/I969 and 07/SRC/I173).
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