a 10g linear burst-mode receiver supporting electronic dispersion compensation for extended-reach...

A 10G linear burst-mode receiver supporting

electronic dispersion compensation for

extended-reach optical links

Peter Ossieur,1,*

Nasir A. Quadir,1 Stefano Porto,

1 Marc Rensing,

1 Cleitus Antony,

1 Wei

Han,1 Peter O’Brien,

1 Y. Chang,

2 and Paul D. Townsend

1

1Photonic Systems Group, Tyndall National Institute, University College Cork, Ireland 2Vitesse Semiconductor Corporation, Transport Systems Engineering, USA

*[email protected]

Abstract: 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

OCIS codes: (060.2360) Fiber optics links and subsystems; (060.4510) Optical

communications.

References and links

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Moodie, A. Poustie, R. Wyatt, B. Harmon, I. Lealman, G. Maxwell, D. Rogers, D. W. Smith, D. Nesset, R. P.

Davey, and P. D. Townsend, “A 135km 8192-split carrier distributed DWDM-TDMA PON with 2x 32 x 10Gb/s

capacity,” J. Lightwave Technol. 29(4), 463–474 (2011).

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Radoaca, “Demonstration of the interconnection of two optical packet rings with a hybrid optoelectronic packet

router,” in Proceedings European Conference on Optical Communication, (2010), Postdeadline paper PD3.5.

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Antony, P. Townsend, and C. Ford, “A 10Gb/s burst-mode receiver with automatic reset generation and burst

detection for extended reach PONs,” in Proceedings Optical Fiber Conference (2009), Paper OWH3.

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“Equalization and FEC techniques for optical transceivers,” IEEE J. Solid-State Circuits 37(3), 317–327 (2002).

5. K. Hara, S. Kimura, H. Nakamura, N. Yoshimoto, and H. Hadama, “New AC-coupled burst-mode optical

receiver using transient-phenomena cancellation techniques for 10Gbit/s class high-speed TDM-PON systems,”

J. Lightwave Technol. 28(19), 2775–2782 (2010).

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GE-PON coexisting system employing dual-rate burst-mode 3R receiver,” IEEE Photon. Technol. Lett. 22(24),

1841–1843 (2010).

7. P. Ossieur, N. A. Quadir, S. Porto, M. Rensing, C. Antony, W. Han, P. O. Brien, Y. Chang, and P. D.

Townsend, “A 10G linear BMRX supporting electronic dispersion compensation for extended-reach optical

links,” in Proceedings European Conference Optical Communication, (2011), Paper Th.13.B.

8. C. Knochenhauer, B. Sedighi, and F. Ellinger, “40Gbit/s transimpedance amplifier with high linearity range in

0.13µm SiGe BiCMOS,” Electron. Lett. 47, 605–606 (2011).

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Rx-01.0, April 2010.

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EDFAs” in Proceedings European Conference on Optical Communication, pp. 693–694 (2009).

#157513 - $15.00 USD Received 2 Nov 2011; revised 17 Nov 2011; accepted 17 Nov 2011; published 1 Dec 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B604

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.

Fig. 1. Linear burst-mode receiver block diagram.

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.

Fig. 2. (a) Simulated gain and output swing vs. optical power, (b) simulated THD vs. optical power.

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)).

Fig. 4. (a) LBMRx mounted on ceramic substrate, (b) LBMRx input and output traces.

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.

Fig. 5. (a) Optical to electrical transfer (S21), (b) Bit-error rate vs input power.

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.

Fig. 6. (a) BER vs OSNR, and (b) OSNR penalty due to preceding loud burst.

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.

Fig. 7. (a) Required OSNR vs. reach (LBMRx + EDC), (b) Required OSNR vs. reach

(conventional BMRx).

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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