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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 1 — Jan. 2, 2012
  • pp: 186–191
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An upstream reach-extender for 10Gb/s PON applications based on an optimized semiconductor amplifier cascade

Stefano Porto, Cleitus Antony, Peter Ossieur, and Paul D. Townsend  »View Author Affiliations


Optics Express, Vol. 20, Issue 1, pp. 186-191 (2012)
http://dx.doi.org/10.1364/OE.20.000186


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Abstract

We present a reach-extender for the upstream transmission path of 10Gb/s passive optical networks based on an optimised cascade of two semiconductor optical amplifiers (SOAs). Through careful optimisation of the bias current of the second stage SOA, over 19dB input dynamic range and up to 12dB compression of the output dynamic range were achieved without any dynamic control. A reach of 70km and split up to 32 were demonstrated experimentally using an ac-coupled, continuous-mode receiver with a reduced 56ns ac-coupling constant.

© 2011 OSA

1. Introduction

Today, there is high interest in new generations of passive optical networks (PONs) with 10Gb/s line-rates for the downstream and upstream directions. For long reach applications [1

1. R. Davey, J. Kani, F. Bourgart, and K. McCammon, “Options for Future Optical Access Networks,” IEEE Commun. Mag. 44(10), 50–56 (2006). [CrossRef]

], this requires the development of mid-span reach-extenders to increase the PON optical budget to support the additional insertion loss of the trunk fibres. Semiconductor optical amplifiers (SOAs) have been shown to be good candidates for reach-extenders in conventional Gigabit PONs [2

2. D. Nesset, D. Payne, R. Davey, and T. Gilfedder, “Demonstration of enhanced reach and split of a GPON system using semiconductor optical amplifier,” in European Conference on Optical Communication (ECOC 2006), paper Mo4.5.1, Cannes, France.

] and several authors have considered their use in 10Gb/s PONs [3

3. S. Pato, R. Meleiro, D. Fonseca, P. André, P. Monteiro, and H. Silva, “All-Optical Burst-Mode Power Equalizer Based on Cascaded SOAs for 10Gbit/s EPONs,” IEEE Photon. Technol. Lett. 20(24), 2078–2080 (2008). [CrossRef]

5

5. C. Antony, G. Talli, and P. D. Townsend, “SOA Based Upstream Packet Equalizer in 10Gb/s Extended-Reach PONs,” in Proceedings of Optical Fiber Communication Conference (OFC 2009), paper OThA5, San Diego, USA.

]. A theme in [3

3. S. Pato, R. Meleiro, D. Fonseca, P. André, P. Monteiro, and H. Silva, “All-Optical Burst-Mode Power Equalizer Based on Cascaded SOAs for 10Gbit/s EPONs,” IEEE Photon. Technol. Lett. 20(24), 2078–2080 (2008). [CrossRef]

5

5. C. Antony, G. Talli, and P. D. Townsend, “SOA Based Upstream Packet Equalizer in 10Gb/s Extended-Reach PONs,” in Proceedings of Optical Fiber Communication Conference (OFC 2009), paper OThA5, San Diego, USA.

] is to also use the SOA to compress the input dynamic range of the upstream channel (which results from differential access loss and variations in the launched power from the optical network unit (ONU)), thus reducing the required dynamic range for the burst-mode receiver (BMRx) located in the optical line termination (OLT). In [3

3. S. Pato, R. Meleiro, D. Fonseca, P. André, P. Monteiro, and H. Silva, “All-Optical Burst-Mode Power Equalizer Based on Cascaded SOAs for 10Gbit/s EPONs,” IEEE Photon. Technol. Lett. 20(24), 2078–2080 (2008). [CrossRef]

], it is proposed to boost the signal and compress the input dynamic range by operating the SOAs in deep saturation for strong input signals. However, with conventional SOAs, operation in deep saturation leads to large overshoots on rising edges in the input signal and eye closure due to gain compression and recovery effects (which have a time constant of a few hundred picoseconds, comparable to the bit period at 10Gb/s). The authors in [3

3. S. Pato, R. Meleiro, D. Fonseca, P. André, P. Monteiro, and H. Silva, “All-Optical Burst-Mode Power Equalizer Based on Cascaded SOAs for 10Gbit/s EPONs,” IEEE Photon. Technol. Lett. 20(24), 2078–2080 (2008). [CrossRef]

] therefore propose to use an SOA with an increased nanosecond gain recovery time and show theoretically that this reduces the signal quality degradation. However, standard SOAs do not exhibit such long gain recovery times. Another solution uses a narrow optical filter at the receiver side to reduce the signal distortion by suppressing the broadening caused by the chirped components of the signal in the frequency domain [4

4. B. Cao and J. E. Mitchell, “Modelling Optical Burst Equalisation in Next Generation Access Networks,” in Proceedings of International Conference on Transparent Optical Networks (ICTON 2010), paper Th.A2.3, Munich, Germany.

]. However, this introduces loss and would require the use of tightly wavelength-specified, temperature-controlled lasers in the ONU, which are expensive. In contrast, recent 10Gb/s PON standards specify a wide 20nm wavelength band for the upstream channel to allow the use of uncooled lasers at the ONU. In [5

5. C. Antony, G. Talli, and P. D. Townsend, “SOA Based Upstream Packet Equalizer in 10Gb/s Extended-Reach PONs,” in Proceedings of Optical Fiber Communication Conference (OFC 2009), paper OThA5, San Diego, USA.

], it is proposed to switch the SOA bias current based on the magnitude of the incoming packets. While this allows significant compression of the input dynamic range while avoiding saturation of the SOA, it requires a monitoring photodiode and high-speed electronics to adjust the bias current on a packet basis.

This paper demonstrates the feasibility of an upstream reach-extender based on an optimised cascade of two SOAs that overcomes the disadvantages of the above methods. The scheme gives sufficient dynamic range compression without any dynamic control to enable the use of an ac-coupled, continuous-mode receiver at the optical line termination.

2. Principle of operation, experimental setup and results

Figure 1
Fig. 1 Experimental setup (DML = Directly Modulated Laser, PG = Pattern Generator, PC = Polarisation Controller, VOA = Variable Optical Attenuator, RX = Receiver, CR = Clock Recovery, LA = Limiting Amplifier, ED = Error Detector, DR = Dynamic Range).
shows the experimental setup. The ONU employed a 1310nm DFB laser (as a 1270nm laser was not available for this experiment) which was directly modulated at a line-rate of 10.3125Gb/s (NRZ, 231-1 PRBS, 6.5dB extinction ratio, close to the worst-case 10GEPON specification). An SOA whose bias current was modulated on a packet basis was used to emulate an alternating sequence of loud (corresponding to the maximum expected input power to the reach-extender) and soft (corresponding to the minimum expected input power to the reach-extender) packets thus emulating two ONUs with different path losses to the OLT. For correct emulation, a 1nm wide optical filter was used to suppress the ASE noise from this gated SOA. A variable optical attenuator was used to emulate splitting loss in the PON distribution network. The reach-extender was located between the distribution network (split: 32, reach: 10km) and a 60km trunk fibre. Due to differential access loss, the power of packets at the input of the reach-extender was assumed to range from −25dBm (soft packets) to −6dBm (loud packets) (IEEE 802.3av class PR20 [6

6. IEEE Standard, 802.3 av (2009).

]). The reach-extender consisted of a cascade of two SOAs. The first SOA provided low-noise amplification; the second SOA provided additional compression of the input dynamic range and boosted the signal launched into the trunk fibre. The OLT comprised an SOA preamplifier, a 17nm bandpass filter for out-of-band amplified spontaneous emission (ASE) suppression (a 20nm filter was not available for this experiment, however this does not alter the outcomes of this experiment), and a conventional continuous-mode PIN receiver with a reduced 56ns ac-coupling time constant (560pF coupling capacitor) to make it suitable for burst-mode operation with less than 800ns preamble (10GEPON standard). A variable optical attenuator was added in front of the PIN receiver to measure power penalties. The SOAs used in the experiments had + 21dB small signal gain, 3dB gain saturation output power of + 11dBm, 1dB polarisation dependent gain (PDG) and a 7dB noise figure at 1310nm wavelength and 250mA bias. The polarisation controllers were for experimental characterisation purposes only and would not be required in a real deployed system, where the reach-extender would need to provide sufficient system margin for all input polarisation states, including the worst-case state.

The reach-extender was first characterised in continuous-mode and in a back-to-back (B2B) configuration without the fibres and the OLT preamplifier SOA. A mid-stage optical filter, which is typically used to remove out-of-band ASE noise from the first SOA, was intentionally omitted here. Note that in order to satisfy the specifications in [6

6. IEEE Standard, 802.3 av (2009).

] for a 10GEPON system (which defines the upstream wavelength window from 1260 to 1280nm), the bandwidth of any mid-stage filter needs to be at least 20nm wide. As is evident from the eye diagrams shown in Fig. 2a
Fig. 2 (a): Soft packet eye diagrams at the reach-extender output (back-to-back), (b): gain of the 2nd stage SOA vs. input power (solid line) and Pinsat for different bias currents (dots), (c): input saturation power of the 2nd stage SOA vs. bias current.
(measured at the output of the reach-extender for soft packets, bias current of the 2nd stage SOA: 250mA), significant reduction of the eye-closure induced by patterning can be achieved by omitting such a filter. Patterning occurs when the input signals at a bit rate comparable to the gain recovery time are sufficiently strong to saturate the SOA, leading to carrier depletion in the gain medium and consequently reducing the optical gain. The signal-dependent gain of the saturated SOA leads to distortions at the bit level, which results in a signal-dependent extinction ratio degradation and intersymbol interference (ISI) [7

7. K. Inoue, “Waveform distortion in a gain-saturated semiconductor optical amplifier for NRZ and Manchester formats,” IEE Proc., Optoelectron. 144(6), 433–437 (1997). [CrossRef]

,8

8. A. Ghazisaeidi, F. Vacondio, A. Bononi, and L. A. Rusch, “Bit Patterning in SOAs: Statistical Characterization through Multicanonical Monte Carlo Simulations,” IEEE J. Quantum Electron. 46(4), 570–578 (2010). [CrossRef]

]. The observed reduction in patterning can be explained by noting that for soft packets the input to the 2nd stage SOA mainly consists of ASE noise (−4.5dBm signal power, +2.7dBm ASE power). The total power is sufficiently high to saturate the 2nd stage SOA. The ASE noise acts as a continuous-wave (CW) ‘holding’ beam, which is known to speed-up the gain recovery in the SOA and reduces the patterning [9

9. R. J. Manning and D. A. O. Davies, “Three-wavelength device for all-optical signal processing,” Opt. Lett. 19(12), 889–991 (1994). [CrossRef] [PubMed]

]. No similar improvement was observed for loud packets, which is consistent with the above explanation as the input to the 2nd stage SOA is then dominated by the signal rather than the ASE from the first SOA. Hence, a mid-stage filter is unnecessary for loud packets and its omission is shown to enhance the performance in the soft packets case, besides being a more cost-effective solution.

In principle, SOA gain saturation can be used to reduce the dynamic range that must be supported by the OLT receiver if the concomitant patterning effect can be ameliorated. We demonstrate here that this is achievable by reduction of the bias current of the 2nd stage SOA. This allows significant reduction of patterning induced penalties stemming from deep saturation of the 2nd stage SOA, while at the same time maintaining essentially the same amount of input dynamic range compression. An explanation for this is provided hereafter. The large-signal gain of an SOA can be written as [10

10. G. P. Agrawal, “Fiber-Optic Communication Systems,” 3rd ed. (John Wiley & Sons, Inc, 1997).

]:
G=G0exp[(G1)(Pin/Psat)],
(1)
where G represents the amplifier gain, G0 its unsaturated value, Pin the input power of the signal being amplified, and Psat the saturation power which depends on the gain-medium properties. The input saturation power Pinsat of an amplifier can be defined as the input power for which the gain G is reduced by half (or by 3dB) from its unsaturated value G0, and can be derived from Eq. (1) by using G = G0 / 2:
Pinsat=2ln(2)[G0(Ibias)2]Psat(Ibias),
(2)
where it is indicated explicitly that G0 and Psat are both dependant on the amplifier bias settings Ibias. Both Eq. (1) and (2) agree with the experimental data shown respectively in Fig. 2b and 2c, where G0 and Psat were measured for different values of the 2nd stage SOA bias current (15, 17.5, 20, 25, 30, 40, 50, 100, 150, 250mA). Figure 2b shows the well-known relation between the gain of the amplifier and its input power. The input saturation powers Pinsat are emphasised with a circular marker for each bias current. From this set of curves one can see that Pinsat rapidly decreases with increasing SOA bias current. This is especially clear when plotting Pinsat as a function of bias current as shown in Fig. 2c. For the maximum bias current of 250mA Pinsat is −6dBm, while for a bias current of 15mA Pinsat increases up to + 4dBm. In other words, if the bias current is reduced from 250 to 15mA, the 2nd stage SOA is able to tolerate a 10dB stronger input signal before entering the saturation regime. Reducing the bias current of the 2nd stage SOA may degrade the overall noise figure, however here such degradation is negligible. Indeed the noise figure NFtot of the cascaded SOA equals to [11

11. H. A. Haus, “The noise figure of optical amplifiers,” IEEE Photon. Technol. Lett. 10(11), 1602–1604 (1998). [CrossRef]

]:
NFtot=NF1+(NF21)/G1NF1,
(3)
where NF1 and G1 are respectively the noise figure and the gain of the first SOA, and NF2 the noise figure of the second SOA. The approximation holds when the gain of the first SOA is sufficiently large and the noise figure of the 2nd SOA reasonably small (which is the case here as the gain of the 1st SOA is greater than 20dB, and the noise figure of the 2nd SOA equals 7dB). In the worst-case a negligible 0.25dB degradation of the overall noise figure was measured for a reduction of the bias current from 250mA to 15mA.

Note that reduction of the bias current reduces the power launched into the trunk fibre, but this is acceptable as long as the input power to the OLT receiver remains higher than its sensitivity. Obviously, this requirement is most stringent for the soft packet. With the setup shown in Fig. 1, in order to support 60km trunk fibre for the soft packet, it was found that a launched power at the output of the cascade of at least 0dBm is sufficient (see below). Figure 3a
Fig. 3 Reach-extender characteristics in B2B, (a): dynamic range and output power for soft packets (SP) and loud packets (LP), (b): soft packets and loud packets power penalties.
shows the output power of the reach-extender for both loud and soft packets. For the loud packets sufficient launch power is always available, however for the soft packets the bias current of the 2nd stage SOA needs to be at least 30mA. Figure 3a also shows the output dynamic range at the reach-extender output for a 19dB input dynamic range. It can be seen how larger dynamic range compression is achieved for higher bias currents. For bias currents higher than 30mA an almost constant 12dB (~19dB-7dB) compression of the input dynamic range can be observed.

3. Conclusion

We have experimentally demonstrated a reach-extender for the upstream direction of 10Gb/s PONs. Up to 12dB compression of a 19dB input dynamic range was achieved. A reduction in patterning induced penalties for soft packets can be achieved by using the broadband ASE from the 1st stage SOA to clamp the gain of the 2nd stage SOA. For loud packets, it is shown that significant reduction in patterning induced penalties can be achieved through careful optimisation of the bias current of the 2nd stage SOA. The reach-extender is shown to be able to support 70km reach for a 32-split 10Gb/s PON.

Acknowledgments

This work was funded by Science Foundation Ireland under Grant 06/IN/I969

References and links

1.

R. Davey, J. Kani, F. Bourgart, and K. McCammon, “Options for Future Optical Access Networks,” IEEE Commun. Mag. 44(10), 50–56 (2006). [CrossRef]

2.

D. Nesset, D. Payne, R. Davey, and T. Gilfedder, “Demonstration of enhanced reach and split of a GPON system using semiconductor optical amplifier,” in European Conference on Optical Communication (ECOC 2006), paper Mo4.5.1, Cannes, France.

3.

S. Pato, R. Meleiro, D. Fonseca, P. André, P. Monteiro, and H. Silva, “All-Optical Burst-Mode Power Equalizer Based on Cascaded SOAs for 10Gbit/s EPONs,” IEEE Photon. Technol. Lett. 20(24), 2078–2080 (2008). [CrossRef]

4.

B. Cao and J. E. Mitchell, “Modelling Optical Burst Equalisation in Next Generation Access Networks,” in Proceedings of International Conference on Transparent Optical Networks (ICTON 2010), paper Th.A2.3, Munich, Germany.

5.

C. Antony, G. Talli, and P. D. Townsend, “SOA Based Upstream Packet Equalizer in 10Gb/s Extended-Reach PONs,” in Proceedings of Optical Fiber Communication Conference (OFC 2009), paper OThA5, San Diego, USA.

6.

IEEE Standard, 802.3 av (2009).

7.

K. Inoue, “Waveform distortion in a gain-saturated semiconductor optical amplifier for NRZ and Manchester formats,” IEE Proc., Optoelectron. 144(6), 433–437 (1997). [CrossRef]

8.

A. Ghazisaeidi, F. Vacondio, A. Bononi, and L. A. Rusch, “Bit Patterning in SOAs: Statistical Characterization through Multicanonical Monte Carlo Simulations,” IEEE J. Quantum Electron. 46(4), 570–578 (2010). [CrossRef]

9.

R. J. Manning and D. A. O. Davies, “Three-wavelength device for all-optical signal processing,” Opt. Lett. 19(12), 889–991 (1994). [CrossRef] [PubMed]

10.

G. P. Agrawal, “Fiber-Optic Communication Systems,” 3rd ed. (John Wiley & Sons, Inc, 1997).

11.

H. A. Haus, “The noise figure of optical amplifiers,” IEEE Photon. Technol. Lett. 10(11), 1602–1604 (1998). [CrossRef]

12.

E. Rotem and D. Sadot, “Performance analysis of AC-coupled burst-mode receiver for fiber-optic burst switching networks,” IEEE Trans. Commun. 53(5), 899–904 (2005). [CrossRef]

13.

Maxim Inc, “NRZ bandwidth – LF cutoff and baseline wander, ” Appl. Note HFAN-09.0.4, available online http://pdfserv.maxim-ic.com/en/an/AN1738.pdf

OCIS Codes
(060.4250) Fiber optics and optical communications : Networks
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Access Networks and LAN

History
Original Manuscript: October 3, 2011
Revised Manuscript: October 28, 2011
Manuscript Accepted: October 29, 2011
Published: December 20, 2011

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
Stefano Porto, Cleitus Antony, Peter Ossieur, and Paul D. Townsend, "An upstream reach-extender for 10Gb/s PON applications based on an optimized semiconductor amplifier cascade," Opt. Express 20, 186-191 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-1-186


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References

  1. R. Davey, J. Kani, F. Bourgart, and K. McCammon, “Options for Future Optical Access Networks,” IEEE Commun. Mag.44(10), 50–56 (2006). [CrossRef]
  2. D. Nesset, D. Payne, R. Davey, and T. Gilfedder, “Demonstration of enhanced reach and split of a GPON system using semiconductor optical amplifier,” in European Conference on Optical Communication (ECOC 2006), paper Mo4.5.1, Cannes, France.
  3. S. Pato, R. Meleiro, D. Fonseca, P. André, P. Monteiro, and H. Silva, “All-Optical Burst-Mode Power Equalizer Based on Cascaded SOAs for 10Gbit/s EPONs,” IEEE Photon. Technol. Lett.20(24), 2078–2080 (2008). [CrossRef]
  4. B. Cao and J. E. Mitchell, “Modelling Optical Burst Equalisation in Next Generation Access Networks,” in Proceedings of International Conference on Transparent Optical Networks (ICTON 2010), paper Th.A2.3, Munich, Germany.
  5. C. Antony, G. Talli, and P. D. Townsend, “SOA Based Upstream Packet Equalizer in 10Gb/s Extended-Reach PONs,” in Proceedings of Optical Fiber Communication Conference (OFC 2009), paper OThA5, San Diego, USA.
  6. IEEE Standard, 802.3 av (2009).
  7. K. Inoue, “Waveform distortion in a gain-saturated semiconductor optical amplifier for NRZ and Manchester formats,” IEE Proc., Optoelectron.144(6), 433–437 (1997). [CrossRef]
  8. A. Ghazisaeidi, F. Vacondio, A. Bononi, and L. A. Rusch, “Bit Patterning in SOAs: Statistical Characterization through Multicanonical Monte Carlo Simulations,” IEEE J. Quantum Electron.46(4), 570–578 (2010). [CrossRef]
  9. R. J. Manning and D. A. O. Davies, “Three-wavelength device for all-optical signal processing,” Opt. Lett.19(12), 889–991 (1994). [CrossRef] [PubMed]
  10. G. P. Agrawal, “Fiber-Optic Communication Systems,” 3rd ed. (John Wiley & Sons, Inc, 1997).
  11. H. A. Haus, “The noise figure of optical amplifiers,” IEEE Photon. Technol. Lett.10(11), 1602–1604 (1998). [CrossRef]
  12. E. Rotem and D. Sadot, “Performance analysis of AC-coupled burst-mode receiver for fiber-optic burst switching networks,” IEEE Trans. Commun.53(5), 899–904 (2005). [CrossRef]
  13. Maxim Inc, “NRZ bandwidth – LF cutoff and baseline wander, ” Appl. Note HFAN-09.0.4, available online http://pdfserv.maxim-ic.com/en/an/AN1738.pdf

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