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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 27 — Dec. 17, 2012
  • pp: 28758–28763
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Simple intradyne PSK system for udWDM-PON

Josep Prat, Victor Polo, Panagiotis Zakynthinos, Ivan Cano, Jeison Tabares, Josep M. Fàbrega, Dimitrios Klonidis, and Ioannis Tomkos  »View Author Affiliations


Optics Express, Vol. 20, Issue 27, pp. 28758-28763 (2012)
http://dx.doi.org/10.1364/OE.20.028758


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Abstract

A homodyne coherent receiver for ultra-dense WDM-PON with off the shelf components is presented. It consists of a conventional DFB, phase switched clock signal, an optical coupler instead of a 90° hybrid, balanced photodetectors and digital signal processing. The phase swing for a DBPSK signal was optimized and the performance was experimentally evaluated in terms of the sensitivity for several laser linewidths. The acceptable frequency offset and clock time delay was also assessed. The results exhibit a sensitivity of −48 dBm at a BER of 10−3 and indicate a high tolerance to phase noise.

© 2012 OSA

1. Introduction

Homodyne/intradyne coherent reception has resurrected as a feasible approach for ultra-dense-WDM transport and access networks. In Fiber-to-the-Home (FTTH) passive optical networks (PONs) it enables compatibility with current splitter-based fiber infrastructure and is a promising candidate to provide high optical bandwidth [1

1. K. Y. Cho, U. H. Hong, and A. Agata, “10-Gb/s, 80-km reach RSOA-based WDM PON employing QPSK signal and self-homodyne receiver,” Proc. OFC'12, paper OW1B.1, (2012)

5

5. B. Schrenk, J. M. Fabrega, C. Kazmierski, J. Lázaro, and J. Prat, “SOA/REAM as vector modulator for QAM upstream,” Proc. OFC'11, paper OThK1 (2011)

]. Heterodyne optical receivers can be a first approach to coherent detection but it presents inherent image frequency interference. As a result, homodyne or intradyne (without optical phase lock) reception is considered a better solution [6

6. L. G. Kazovsky, G. Kalogerakis, and W. Tao, “Homodyne phase-shift-keying systems: past challenges and future opportunities,” J. Lightwave Technol. 12(24), 4876–4884 (2006). [CrossRef]

8

8. J. M. Fabrega and J. Prat, “Experimental investigation of channel crosstalk in a time-switched phase-diversity optical homodyne receiver,” Opt. Lett. 34(4), 452–454 (2009). [CrossRef] [PubMed]

].

A new PSK receiver architecture based on time-switching phase-diversity was experimentally demonstrated in [7

7. J. Prat and J. M. Fabrega, “New homodyne receiver with electronic I&Q differential demodulation,” Proc. ECOC'05, paper We4.P104, (2005)

]. It was a first approach towards a low-cost implementation of a reliable optical homodyne receiver. It showed a 1.8% linewidth/bit-rate ratio tolerance operating in real time, the maximum then reported. For that system, the channel spacing was 3 GHz [7

7. J. Prat and J. M. Fabrega, “New homodyne receiver with electronic I&Q differential demodulation,” Proc. ECOC'05, paper We4.P104, (2005)

, 8

8. J. M. Fabrega and J. Prat, “Experimental investigation of channel crosstalk in a time-switched phase-diversity optical homodyne receiver,” Opt. Lett. 34(4), 452–454 (2009). [CrossRef] [PubMed]

] and all the processing in the receiver was performed with analog hardware requiring tuning and critical adjustments.

2. Network architecture

The network architecture (Fig. 1
Fig. 1 Generic PON architecture.
) is based on a standard PON splitter-based tree. Channel selection is not made in the time-domain (TDM) but in the optical frequency domain (WDM) with ultra-dense GHz channel spacing (udWDM) and electrical channel filtering. Following [10

10. I. Cano, M. C. Santos, V. Polo, and J. Prat, “Dimensioning of OFDMA PON with non-preselected-independent ONUs sources and wavelength control,” Proc. ECOC'11, Tu.E.C.2. (2011)

, 11

11. D. Piehler, “PICs in PONs,” Proc. OFC'12, paper NTu1J.6. (2012)

], the wavelength grid is not kept standard but left flexible for best adaptation to each ONU laser center wavelength to offer the maximum spectral efficiency. A high-resolution spectral analyzer at the OLT monitors and controls the down- and up- stream wavelengths. In this work, we focus on the up-stream direction, as is the most challenging in PONs (the down-stream can use another waveband or a remodulation strategy tolerant to Rayleigh-scattering and reflections [1

1. K. Y. Cho, U. H. Hong, and A. Agata, “10-Gb/s, 80-km reach RSOA-based WDM PON employing QPSK signal and self-homodyne receiver,” Proc. OFC'12, paper OW1B.1, (2012)

].

3. Receiver scheme

The proposed diversity receiver, shown in Fig. 2(a)
Fig. 2 (a) Receiver module schematics, (b) BER against clock delay with respect to Tb
, has two main parts: the first consists of a coherent photo-receiver with an added clock-synchronous optical phase swing (0-~90°). The second part is digital post-processing which performs the signal demodulation and synchronous combination of the orthogonal I&Q components. The local laser does not need to be phase-coherent with the incoming optical carrier, although an automatic wavelength controller is required to maintain the two wavelengths close to each other. Also, a polarization controller is used to compensate signal fluctuations due to state of polarization (SOP) changes, although in a practical deployment a polarization diversity scheme or a fast polarization controller has to be included at the OLT [6

6. L. G. Kazovsky, G. Kalogerakis, and W. Tao, “Homodyne phase-shift-keying systems: past challenges and future opportunities,” J. Lightwave Technol. 12(24), 4876–4884 (2006). [CrossRef]

].

The coherent photo-receiver mixes the incoming signal optical field with the scrambled local laser carrier in the balanced photo-detector stage. The local laser is externally modulated with a phase modulator controlled by the filtered data clock, producing a phase change to beat the I and Q signal energy at the first and second half of each bit time (Tb) respectively.

After the conventional balanced photodetector an electrical filter is placed to reduce the noise and reject the interference from adjacent WDM channels. Due to the synchronous 0-90° phase scrambling, at this point the signal has twice the bandwidth of the low-pass equivalent data power spectrum. It is important to note that the extracted data clock is relevant in this operation, as synchronizing the optical phase swing. This is noted in Fig. 2(b) where a small delay in the clock signal with respect to the bit reduces the bit error ratio (BER).

The two techniques were tested with data obtained experimentally. White noise was added to the samples to compare their performance in terms of the BER for several signal to noise ratios (SNR). The results are plotted in Fig. 3(c). Both techniques perform identical, however method 1 requires less components and is simpler to implement. Hence we employed it for the rest of the experiments. Furthermore, a similar opto-electronic function has been integrated in an InP substrate revealing its potential low-cost production [12

12. A. Ramaswamy, L. A. Johansson, J. Klamkin, C. Sheldon, H. F. Chou, M. J. Rodwell, L. A. Coldren, and J. E. Bowers, “Coherent receiver based on a broadband optical phase-lock loop,” Proc. OFC’07, paper PDP3, (2007)

].

4. Experimental setup

5. System performance

The system performance was tested measuring the BER by counting the errors in the received bits, as a function of the input power to the receiver. As the overall operation is affected by the specific waveform [7

7. J. Prat and J. M. Fabrega, “New homodyne receiver with electronic I&Q differential demodulation,” Proc. ECOC'05, paper We4.P104, (2005)

, 9

9. J. M. Fabrega and J. Prat, “Homodyne receiver prototype with time-switching phase diversity and feedforward analog processing,” Opt. Lett. 32(5), 463–465 (2007). [CrossRef] [PubMed]

] several phase swings values were considered for optimization. The calibration was performed using a narrow linewidth (100 kHz) external-cavity-laser (ECL). The results are shown in Fig. 5(a)
Fig. 5 BER against phase swing for several received optical powers, (a) experimental results, (b) simulation results with ideal (solid) and offset (dotted) clock signal with 100kHz laser linewidth
for several received optical powers. Likewise, in Fig. 5(b) are plotted the results when using ideal components in a simulation environment. The difference in the sensitivity values between the experimental and the simulated curves are because of the ideal components considered in the second case.

The experimental phase swing for a DFB laser is also presented in Fig. 5(a). In this case, the optimum appears at about 90° phase swing. However, the curve is very flat, showing a small penalty when compared with the 66 degrees. This is because of the BER floor obtained for such a laser.

In order to determine the sensitivity of the system and to confirm the optimum phase swing, BER data was plotted varying the Rx input power for several phase swings using the 100 kHz linewidth ECL. Figure 6
Fig. 6 Receiver sensitivity for several phase swings.
shows these results, giving a sensitivity of −48 dBm at a BER of 10−3 and −46 dBm at BER of 10−6. These results improve the ones reported in [9

9. J. M. Fabrega and J. Prat, “Homodyne receiver prototype with time-switching phase diversity and feedforward analog processing,” Opt. Lett. 32(5), 463–465 (2007). [CrossRef] [PubMed]

] by about 4 dB.

The laser linewidth tolerance of the system was also evaluated. Different lasers were used as light sources with datasheet specified linewidths going from 100 kHz (ECL) up to 15 MHz (DFB). The BER measures for several lasers are plotted in Fig. 7(a)
Fig. 7 (a) Receiver sensitivity for several linewidths, (b) Sensitivity penalty against frequency offset between Tx and Rx lasers
. The legends consider the total linewidth with the Tx and Rx lasers together. For a BER of 10−3 there is a penalty of almost 1 dB when increasing the total linewidth from 200 kHz to 2 MHz, and further enlarges to 2 dB and almost 5 dB for a 4 MHz and 30MHz laser respectively. For a BER of 10−6, the penalties are of 2.8 dB and 4 dB for 2 MHz and 4 MHz linewidths compared with the one of 100 kHz.

In practice, the Rx LO would be different than the Tx laser, hence frequency offset between the two light-sources would limit the performance of the Rx. Figure 7(b) plots the sensitivity penalty for a FEC target BER of 10−3against the frequency offset obtained by simulations. It can be noticed that a 1dB penalty is achieved already at 70MHz, very close to the one observed in [9

9. J. M. Fabrega and J. Prat, “Homodyne receiver prototype with time-switching phase diversity and feedforward analog processing,” Opt. Lett. 32(5), 463–465 (2007). [CrossRef] [PubMed]

]. This indicates that the receiver is very sensitive to wavelength fluctuations.

6. Conclusion

We have proposed and demonstrated a PSK homodyne architecture based on a simpler phase-diversity set-up using digital processing. High linewidth tolerance was experimentally achieved, enabling the use of DFB with a combined Tx/Rx linewidth of 30MHz, representing a tolerance of 2.4% of the bit rate in an udWDM-PON scenario. Besides, the use of DSP enhances the sensitivity in about 4dB compared with an analog implementation.

This new architecture is implementable using commercially available semiconductor lasers (i.e. DFB lasers) and low-cost standard components, either optical or electrical. Thus, it constitutes an enabling technique towards ultra-dense-WDM PONs, featuring few GHz spacing wavelengths with electrical channel filtering, simple tuning and improved sensitivity.

Acknowledgments

This work was supported in part by the European FP7 COCONUT, Conacyt grant 185291, and project TEC2011-25215 (ROMULA) of the Spanish Ministry for Science and Innovation

References and links

1.

K. Y. Cho, U. H. Hong, and A. Agata, “10-Gb/s, 80-km reach RSOA-based WDM PON employing QPSK signal and self-homodyne receiver,” Proc. OFC'12, paper OW1B.1, (2012)

2.

R. Rodes, N. Cheng, J. B. Jensen, and I. Tafur, “10 Gb/s real-time all-VCSEL low complexity coherent scheme for PONs,” Proc. OFC’12, paper OTh4G.2 (2012)

3.

D. Lavery, R. Maher, D. Millar, B. C. Thomsen, P. Bayvel, and S. Savory, “Demonstration of 10~Gbit/s colorless coherent PON incorporating tunable DS-DBR lasers and low-complexity parallel DSP,” Proc. OFC’12, paper PDP5B.10 (2012)

4.

H. Rohde, S. Smolorz, S. Wey, and E. Gottwald, “Coherent optical access networks,” Proc. OFC'11, paper OTuB1 (2011)

5.

B. Schrenk, J. M. Fabrega, C. Kazmierski, J. Lázaro, and J. Prat, “SOA/REAM as vector modulator for QAM upstream,” Proc. OFC'11, paper OThK1 (2011)

6.

L. G. Kazovsky, G. Kalogerakis, and W. Tao, “Homodyne phase-shift-keying systems: past challenges and future opportunities,” J. Lightwave Technol. 12(24), 4876–4884 (2006). [CrossRef]

7.

J. Prat and J. M. Fabrega, “New homodyne receiver with electronic I&Q differential demodulation,” Proc. ECOC'05, paper We4.P104, (2005)

8.

J. M. Fabrega and J. Prat, “Experimental investigation of channel crosstalk in a time-switched phase-diversity optical homodyne receiver,” Opt. Lett. 34(4), 452–454 (2009). [CrossRef] [PubMed]

9.

J. M. Fabrega and J. Prat, “Homodyne receiver prototype with time-switching phase diversity and feedforward analog processing,” Opt. Lett. 32(5), 463–465 (2007). [CrossRef] [PubMed]

10.

I. Cano, M. C. Santos, V. Polo, and J. Prat, “Dimensioning of OFDMA PON with non-preselected-independent ONUs sources and wavelength control,” Proc. ECOC'11, Tu.E.C.2. (2011)

11.

D. Piehler, “PICs in PONs,” Proc. OFC'12, paper NTu1J.6. (2012)

12.

A. Ramaswamy, L. A. Johansson, J. Klamkin, C. Sheldon, H. F. Chou, M. J. Rodwell, L. A. Coldren, and J. E. Bowers, “Coherent receiver based on a broadband optical phase-lock loop,” Proc. OFC’07, paper PDP3, (2007)

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.1660) Fiber optics and optical communications : Coherent communications

ToC Category:
Access Networks and LAN

History
Original Manuscript: October 2, 2012
Revised Manuscript: November 9, 2012
Manuscript Accepted: November 12, 2012
Published: December 12, 2012

Virtual Issues
European Conference on Optical Communication 2012 (2012) Optics Express

Citation
Josep Prat, Victor Polo, Panagiotis Zakynthinos, Ivan Cano, Jeison Tabares, Josep M. Fàbrega, Dimitrios Klonidis, and Ioannis Tomkos, "Simple intradyne PSK system for udWDM-PON," Opt. Express 20, 28758-28763 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-27-28758


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References

  1. K. Y. Cho, U. H. Hong, and A. Agata, “10-Gb/s, 80-km reach RSOA-based WDM PON employing QPSK signal and self-homodyne receiver,” Proc. OFC'12, paper OW1B.1, (2012)
  2. R. Rodes, N. Cheng, J. B. Jensen, and I. Tafur, “10 Gb/s real-time all-VCSEL low complexity coherent scheme for PONs,” Proc. OFC’12, paper OTh4G.2 (2012)
  3. D. Lavery, R. Maher, D. Millar, B. C. Thomsen, P. Bayvel, and S. Savory, “Demonstration of 10~Gbit/s colorless coherent PON incorporating tunable DS-DBR lasers and low-complexity parallel DSP,” Proc. OFC’12, paper PDP5B.10 (2012)
  4. H. Rohde, S. Smolorz, S. Wey, and E. Gottwald, “Coherent optical access networks,” Proc. OFC'11, paper OTuB1 (2011)
  5. B. Schrenk, J. M. Fabrega, C. Kazmierski, J. Lázaro, and J. Prat, “SOA/REAM as vector modulator for QAM upstream,” Proc. OFC'11, paper OThK1 (2011)
  6. L. G. Kazovsky, G. Kalogerakis, and W. Tao, “Homodyne phase-shift-keying systems: past challenges and future opportunities,” J. Lightwave Technol.12(24), 4876–4884 (2006). [CrossRef]
  7. J. Prat and J. M. Fabrega, “New homodyne receiver with electronic I&Q differential demodulation,” Proc. ECOC'05, paper We4.P104, (2005)
  8. J. M. Fabrega and J. Prat, “Experimental investigation of channel crosstalk in a time-switched phase-diversity optical homodyne receiver,” Opt. Lett.34(4), 452–454 (2009). [CrossRef] [PubMed]
  9. J. M. Fabrega and J. Prat, “Homodyne receiver prototype with time-switching phase diversity and feedforward analog processing,” Opt. Lett.32(5), 463–465 (2007). [CrossRef] [PubMed]
  10. I. Cano, M. C. Santos, V. Polo, and J. Prat, “Dimensioning of OFDMA PON with non-preselected-independent ONUs sources and wavelength control,” Proc. ECOC'11, Tu.E.C.2. (2011)
  11. D. Piehler, “PICs in PONs,” Proc. OFC'12, paper NTu1J.6. (2012)
  12. A. Ramaswamy, L. A. Johansson, J. Klamkin, C. Sheldon, H. F. Chou, M. J. Rodwell, L. A. Coldren, and J. E. Bowers, “Coherent receiver based on a broadband optical phase-lock loop,” Proc. OFC’07, paper PDP3, (2007)

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