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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 8 — Apr. 22, 2013
  • pp: 10166–10171
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Differential carrier phase recovery for QPSK optical coherent systems with integrated tunable lasers

Irshaad Fatadin, David Ives, and Seb J. Savory  »View Author Affiliations


Optics Express, Vol. 21, Issue 8, pp. 10166-10171 (2013)
http://dx.doi.org/10.1364/OE.21.010166


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Abstract

The performance of a differential carrier phase recovery algorithm is investigated for the quadrature phase shift keying (QPSK) modulation format with an integrated tunable laser. The phase noise of the widely-tunable laser measured using a digital coherent receiver is shown to exhibit significant drift compared to a standard distributed feedback (DFB) laser due to enhanced low frequency noise component. The simulated performance of the differential algorithm is compared to the Viterbi-Viterbi phase estimation at different baud rates using the measured phase noise for the integrated tunable laser.

© 2013 OSA

1. Introduction

Advanced modulation formats in combination with digital coherent receivers are currently receiving significant attention for the next-generation of optical communication systems. In particular, the quadrature phase shift keying (QPSK) modulation format has attracted a lot of research interest to increase data capacity due to its robustness to linear impairments and optical signal-to-noise ratio requirements [1

1. S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1164–1179 (2010) [CrossRef] .

, 2

2. C. Fludger, T. Duthel, D. van den Borne, C. Schulien, E.-D. Schmidt, T. Wuth, J. Geyer, E. De Man, K. Giok-Djan, and H. de Waardt, “Coherent equalization and POLMUX-RZ-DQPSK for robust 100-GE transmission,” J. Lightwave Technol. 26(1), 64–72 (2008) [CrossRef] .

]. With coherent detection, it becomes possible to compensate for various impairments in the digital domain including phase noise contributions from the transmitter laser and the local oscillator (LO). Laser phase noise is known to have a limiting effect on the performance of optical coherent systems by increasing the bit error ratio (BER) and causing cycle slips. Therefore, reliable digital carrier phase estimation is indispensable for QPSK coherent receivers with a free-running LO without an optical phase locked loop.

Several carrier phase estimation algorithms have been proposed in the literature [3

3. E. Ip and J. M. Kahn, “Feedforward carrier recovery for coherent optical communications,” J. Lightwave Technol. 25(9), 2675–2692 (2007) [CrossRef] .

9

9. S. Zhang, P. Y. Kam, C. Yu, and J. Chen, “Laser linewidth tolerance of decision-aided maximum likelihood phase estimation in coherent optical M-ary PSK and QAM systems,” IEEE Photon. Technol. Lett. 21(15), 1075–1077 (2009) [CrossRef] .

] for the QSPK modulation format. The Viterbi and Viterbi phase estimation (VVPE) [10

10. A. J. Viterbi and A. N. Viterbi, “Nonlinear estimation of psk-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory 29(4), 543–551 (1983) [CrossRef] .

] is the most commonly employed technique which raises the QPSK symbols to the power of four to remove the phase encoded data modulation. The VVPE has been demonstrated to be an effective algorithm for coherent systems employing commercially available distributed feedback (DFB) lasers. To add network configurability such as the flexibility to switch between different channels, widely-tunable lasers are also of interest in coherent optical communication systems with robust carrier phase recovery algorithms [11

11. D. Lavery, R. Maher, D. S. Millar, B. C. Thomsen, P. Bayvel, and S. J. Savory, “Digital coherent receivers for long-reach optical access networks,” J. Lightwave Technol. 31(4), 609–620 (2013) [CrossRef] .

]. However, it is noted that the requirements for rapid channel switching may conflict with design approaches that minimize linewidth and FM noise associated with integrated tunable lasers.

In this paper, we first characterize the phase noise of an integrated tunable laser and a standard DFB laser using a digital coherent receiver. The low frequency noise components for the two lasers are extracted from the frequency modulation (FM) noise spectra. The performance of a differential carrier phase recovery algorithm is then investigated and compared to the VVPE to compensate for the laser phase noise associated with the integrated tunable laser at different baud rates for a QPSK coherent system in the back-to-back configuration. We show that for transmission speeds less than 5 Gbaud, the differential algorithm can give comparable performance to the VVPE with lower implementation complexity.

2. Laser phase noise characterization

Fig. 1 Experimental setup to measure the phase noise characteristics of the lasers under test. (PD: Photodiode)

Fig. 2 Measured phase noise fluctuations for (a) integrated tunable laser and (b) DFB laser using coherent detection.
Fig. 3 FM-noise spectra for the lasers under test with analytical fit to extract the low-frequency noise components.

Table 1. Extracted Parameters for lasers under test

table-icon
View This Table

3. Simulation results

The compensation of the phase noise fluctuation associated with the integrated tunable laser is discussed below for a QPSK coherent system. The performance of the VVPE is compared to the differential algorithm at different baud rates by re-sampling the measured phase noise. The estimated phase, θest, which is common to all the samples in the block, was obtained using [6

6. D.-S. Ly-Gagnon, S. Tsukamoto, K. Katoh, and K. Kikuchi, “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” J. Lightwave Technol. 24(1), 12–21 (2006) [CrossRef] .

]
θest=14argk=1Nrk4,
(4)
where N is the averaging block size for the VVPE algorithm. The recovered QPSK constellation was then differentially decoded to solve the four-fold phase ambiguity.

Differential carrier phase recovery is an alternative algorithm to the VVPE that can be used to recover the transmitted bits from the phase difference between consecutive QPSK symbols. The differential algorithm can thus be achieved using
Δθ=arg(rk)arg(rk1),
(5)
followed by the mod(Δθ, 2π) operation. Equation (5) is a direct differential decoding on the optical field measured by a coherent receiver and, in contrast to the VVPE, the algorithm does not require the power of four operation to remove the data modulation, averaging or phase unwrapping for carrier phase recovery. For hardware implementation on FPGAs, for example, the arg(.) operation can effectively be implemented using look-up tables.

The VVPE was found to consistently outperform the differential algorithm at different baud rates for a QPSK coherent system with the measured phase noise of the DFB laser shown in Fig. 2(b). However, the performance of the VVPE was significantly different for the integrated tunable laser depending on the baud rate and the chosen value of N as seen in Fig. 4. The BER floor at 5 Gbaud was close to a BER of 10−3 with N = 4. For lower baud rates, a smaller block size was required by the VVPE to successfully track the large phase noise fluctuation associated with the integrated tunable laser.

Fig. 4 Performance of the differential carrier phase recovery algorithm compared to VVPE for the integrated tunable laser at different baud rates.

Figure 5 shows the SNR required to achieve a target BER of 10−2 and 10−3 assuming hard decision forward error correction with coding overhead of 20% and 7%, respectively [16

16. B. Li, K. Larsen, D. Zibar, and I. T. Monroy, “Over 10 dB net coding gain based on 20% overhead hard decision forward error correction in 100G optical communication systems,” in Proceedings of ECOC 2011, paper Tu.6.A.3 (2011)

]. For transmission speeds higher than 10 Gbaud, the VVPE was found to outperform the differential algorithm by at least 0.7 dB for the results shown in Fig. 5. However, for transmission speeds lower than 5 Gbaud, the performance of the differential algorithm can be seen to be comparable with the VVPE. Carrier phase recovery can thus be achieved effectively using the differential algorithm with widely-tunable and fast-switching DS-DBR lasers currently of interest in coherent-enabled 10 Gbit/s passive optical networks [11

11. D. Lavery, R. Maher, D. S. Millar, B. C. Thomsen, P. Bayvel, and S. J. Savory, “Digital coherent receivers for long-reach optical access networks,” J. Lightwave Technol. 31(4), 609–620 (2013) [CrossRef] .

].

Fig. 5 Required SNR to achieve a target BER of (a) 10−2 and (b) 10−3 for the differential algorithm and the VVPE.

4. Conclusions

A digital coherent receiver has been used to extract the phase noise characteristics of an integrated tunable laser. The performance of the differential carrier phase recovery algorithm has been investigated and compared to the VVPE at different baud rates. The differential algorithm is shown to give comparable performance to the VVPE for transmission speeds less than 5 Gbaud with lower implementation complexity.

Acknowledgments

This work was supported by the Department for Business Innovation and Skills at NPL and the Piano+ CRITICAL project. The authors would like to thank Dr. R. Griffin at Oclaro for measurement support on the phase noise characterization of the lasers.

References and links

1.

S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1164–1179 (2010) [CrossRef] .

2.

C. Fludger, T. Duthel, D. van den Borne, C. Schulien, E.-D. Schmidt, T. Wuth, J. Geyer, E. De Man, K. Giok-Djan, and H. de Waardt, “Coherent equalization and POLMUX-RZ-DQPSK for robust 100-GE transmission,” J. Lightwave Technol. 26(1), 64–72 (2008) [CrossRef] .

3.

E. Ip and J. M. Kahn, “Feedforward carrier recovery for coherent optical communications,” J. Lightwave Technol. 25(9), 2675–2692 (2007) [CrossRef] .

4.

M. G. Taylor, “Phase estimation methods for optical coherent detection using digital signal processing,” J. Light-wave Technol. 27(7), 901–914 (2009) [CrossRef] .

5.

T. Pfau, S. Hoffmann, and R. Noe, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol. 27(8), 989–999 (2009) [CrossRef] .

6.

D.-S. Ly-Gagnon, S. Tsukamoto, K. Katoh, and K. Kikuchi, “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” J. Lightwave Technol. 24(1), 12–21 (2006) [CrossRef] .

7.

A. Leven, N. Kaneda, U.-V. Koc, and Y.-K. Chen, “Coherent receivers for practical optical communication systems,” in Proceedings of OFC 2007, paper OThK4 (2007).

8.

M. Seimetz, “Laser linewidth limitations for optical systems with high-order modulation employing feed forward digital carrier phase estimation,” in Proceedings of OFC 2008, paper OTuM2 (2008).

9.

S. Zhang, P. Y. Kam, C. Yu, and J. Chen, “Laser linewidth tolerance of decision-aided maximum likelihood phase estimation in coherent optical M-ary PSK and QAM systems,” IEEE Photon. Technol. Lett. 21(15), 1075–1077 (2009) [CrossRef] .

10.

A. J. Viterbi and A. N. Viterbi, “Nonlinear estimation of psk-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory 29(4), 543–551 (1983) [CrossRef] .

11.

D. Lavery, R. Maher, D. S. Millar, B. C. Thomsen, P. Bayvel, and S. J. Savory, “Digital coherent receivers for long-reach optical access networks,” J. Lightwave Technol. 31(4), 609–620 (2013) [CrossRef] .

12.

K. Kikuchi, “Characterization of semiconductor-laser phase noise and estimation of bit-error rate performance with low-speed offline digital coherent receivers,” Opt. Express 20(5), 5291–5302 (2012) [CrossRef] [PubMed] .

13.

R. Maher and B. Thomsen, “Dynamic linewidth measurement technique using digital intradyne coherent receivers,” Opt. Express 19(26), B313–B322 (2011) [CrossRef] .

14.

T. Duthel, G. Clarici, C. R. S. Fludger, J. C. Geyer, C. Schulien, and S. Wiese, “Laser linewidth estimation by means of coherent detection,” IEEE Photon. Technol. Lett. 21(20), 1568–1570 (2009) [CrossRef] .

15.

K. Kikuchi, “Impact of 1/f-type FM noise on coherent optical communications,” Electron. Lett. 23, 885–887 (1987) [CrossRef] .

16.

B. Li, K. Larsen, D. Zibar, and I. T. Monroy, “Over 10 dB net coding gain based on 20% overhead hard decision forward error correction in 100G optical communication systems,” in Proceedings of ECOC 2011, paper Tu.6.A.3 (2011)

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.2330) Fiber optics and optical communications : Fiber optics communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: February 14, 2013
Revised Manuscript: March 11, 2013
Manuscript Accepted: April 8, 2013
Published: April 16, 2013

Citation
Irshaad Fatadin, David Ives, and Seb J. Savory, "Differential carrier phase recovery for QPSK optical coherent systems with integrated tunable lasers," Opt. Express 21, 10166-10171 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-8-10166


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References

  1. S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron.16(5), 1164–1179 (2010). [CrossRef]
  2. C. Fludger, T. Duthel, D. van den Borne, C. Schulien, E.-D. Schmidt, T. Wuth, J. Geyer, E. De Man, K. Giok-Djan, and H. de Waardt, “Coherent equalization and POLMUX-RZ-DQPSK for robust 100-GE transmission,” J. Lightwave Technol.26(1), 64–72 (2008). [CrossRef]
  3. E. Ip and J. M. Kahn, “Feedforward carrier recovery for coherent optical communications,” J. Lightwave Technol.25(9), 2675–2692 (2007). [CrossRef]
  4. M. G. Taylor, “Phase estimation methods for optical coherent detection using digital signal processing,” J. Light-wave Technol.27(7), 901–914 (2009). [CrossRef]
  5. T. Pfau, S. Hoffmann, and R. Noe, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol.27(8), 989–999 (2009). [CrossRef]
  6. D.-S. Ly-Gagnon, S. Tsukamoto, K. Katoh, and K. Kikuchi, “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” J. Lightwave Technol.24(1), 12–21 (2006). [CrossRef]
  7. A. Leven, N. Kaneda, U.-V. Koc, and Y.-K. Chen, “Coherent receivers for practical optical communication systems,” in Proceedings of OFC 2007, paper OThK4 (2007).
  8. M. Seimetz, “Laser linewidth limitations for optical systems with high-order modulation employing feed forward digital carrier phase estimation,” in Proceedings of OFC 2008, paper OTuM2 (2008).
  9. S. Zhang, P. Y. Kam, C. Yu, and J. Chen, “Laser linewidth tolerance of decision-aided maximum likelihood phase estimation in coherent optical M-ary PSK and QAM systems,” IEEE Photon. Technol. Lett.21(15), 1075–1077 (2009). [CrossRef]
  10. A. J. Viterbi and A. N. Viterbi, “Nonlinear estimation of psk-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory29(4), 543–551 (1983). [CrossRef]
  11. D. Lavery, R. Maher, D. S. Millar, B. C. Thomsen, P. Bayvel, and S. J. Savory, “Digital coherent receivers for long-reach optical access networks,” J. Lightwave Technol.31(4), 609–620 (2013). [CrossRef]
  12. K. Kikuchi, “Characterization of semiconductor-laser phase noise and estimation of bit-error rate performance with low-speed offline digital coherent receivers,” Opt. Express20(5), 5291–5302 (2012). [CrossRef] [PubMed]
  13. R. Maher and B. Thomsen, “Dynamic linewidth measurement technique using digital intradyne coherent receivers,” Opt. Express19(26), B313–B322 (2011). [CrossRef]
  14. T. Duthel, G. Clarici, C. R. S. Fludger, J. C. Geyer, C. Schulien, and S. Wiese, “Laser linewidth estimation by means of coherent detection,” IEEE Photon. Technol. Lett.21(20), 1568–1570 (2009). [CrossRef]
  15. K. Kikuchi, “Impact of 1/f-type FM noise on coherent optical communications,” Electron. Lett.23, 885–887 (1987). [CrossRef]
  16. B. Li, K. Larsen, D. Zibar, and I. T. Monroy, “Over 10 dB net coding gain based on 20% overhead hard decision forward error correction in 100G optical communication systems,” in Proceedings of ECOC 2011, paper Tu.6.A.3 (2011)

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