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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 24 — Nov. 19, 2012
  • pp: 26958–26968
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Rapid and complete hitless defragmentation method using a coherent RX LO with fast wavelength tracking in elastic optical networks

Roberto Proietti, Chuan Qin, Binbin Guan, Yawei Yin, Ryan P. Scott, Runxiang Yu, and S. J. B. Yoo  »View Author Affiliations


Optics Express, Vol. 20, Issue 24, pp. 26958-26968 (2012)
http://dx.doi.org/10.1364/OE.20.026958


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Abstract

This paper demonstrates a rapid and full hitless defragmentation method in elastic optical networks exploiting a new technique for fast wavelength tracking in coherent receivers. This technique can be applied to a single-carrier connection or each of the subcarriers forming a super-channel. A proof-of-concept demonstration shows hitless defragmentation of a 10 Gb/s QPSK single-carrier connection from 1547.75 nm to 1550.1 nm in less than 1 µs. This was obtained using a small (0.625 kB) link-layer transmitter buffer without the need for any additional transponder. We also demonstrated that the proposed defragmentation technique is capable of hopping over an existing connection, i.e. 10 Gb/s OOK at 1548.5 nm, without causing any degradation of its real-time Bit Error Rate (BER) value. The proposed scheme gives advantages in terms of overall network blocking probability reduction up to a factor of 40.

© 2012 OSA

1. Introduction

Elastic optical networking [1

1. M. Jinno, H. Takara, B. Kozicki, Y. Tsukishima, Y. Sone, and S. Matsuoka, “Spectrum-efficient and scalable elastic optical path network: architecture, benefits, and enabling Technologies,ˮ,” IEEE Commun. Mag. 47(11), 66–73 (2009). [CrossRef]

] is a promising approach for achieving efficient spectrum utilization in a network by allocating just-enough bandwidth to each user’s demands. In contrast to traditional WDM networks, the spectrum is divided into arbitrary or smaller frequency units (e.g. 12.5 or 25 GHz, also known as optical subcarriers). This enables various connections (flexpaths) to use an arbitrary number of subcarriers depending on their bandwidth demands. However, this added flexibility also raises new challenges such as spectral fragmentation [2

2. Y. Yin, K. Wen, D. J. Geisler, R. Liu, and S. J. Yoo, “Dynamic on-demand defragmentation in flexible bandwidth elastic optical networks,” Opt. Express 20(2), 1798–1804 (2012). [CrossRef] [PubMed]

]. Since the fragments are neither contiguous in the frequency domain nor aligned along a path, they become stranded bandwidths that can hardly be utilized by new connection requests. Spectral defragmentation, which aims at re-optimizing the spectrum utilization, typically involves rerouting or re-assignment of the spectrum location of existing connections [3

3. A. N. Patel, P. N. Ji, J. P. Jue, and T. Wang, “Defragmentation of transparent flexible optical WDM (FWDM) networks,ˮ (Optical Society of America, 2011), p. OTuI8.

].

Ref [4

4. T. Takagi, H. Hasegawa, K.-i. Sato, Y. Sone, A. Hirano, and M. Jinno, “Disruption minimized spectrum defragmentation in elastic optical path networks that adopt distance adaptive modulation,ˮ (Optical Society of America, 2011), p. Mo.2.K.3.

]. presents a disruption-minimized make-before-break (MbB) technique exploiting the rerouting solution to do defragmentation. Ref [5

5. F. Cugini, M. Secondini, N. Sambo, G. Bottari, G. Bruno, P. Iovanna, and P. Castoldi, “Push-pull technique for defragmentation in flexible optical networks,ˮ in Optical Fiber Conference (2012), p. JTh2A.40.

, 6

6. K. Sone, X. Wang, S. Oda, G. Nakagawa, Y. Aoki, I. Kim, P. Palacharla, T. Hoshida, M. Sekiya, and J. C. Rasmussen, “First demonstration of hitless spectrum defragmentation using real-time coherent receivers in flexible grid optical networks,ˮ in European Conference on Optical Communication (2012), p. Th.3.D.1.

] proposes wavelength sweeping hitless defragmentation methods that leverage the automatic frequency control (AFC) capabilities of coherent receivers. However, the MbB method requires an additional transmitter to provide an extra connection while the wavelength sweeping methods cannot achieve complete defragmentation without disrupting other existing connections.

The remainder of the paper is organized as follows: Section II shows a network scenario with undergoing defragmentation. The analysis shows that our technique can guarantee faster defragmentation time and lower blocking probability compared to what can be achieved with wavelength sweeping techniques [5

5. F. Cugini, M. Secondini, N. Sambo, G. Bottari, G. Bruno, P. Iovanna, and P. Castoldi, “Push-pull technique for defragmentation in flexible optical networks,ˮ in Optical Fiber Conference (2012), p. JTh2A.40.

, 6

6. K. Sone, X. Wang, S. Oda, G. Nakagawa, Y. Aoki, I. Kim, P. Palacharla, T. Hoshida, M. Sekiya, and J. C. Rasmussen, “First demonstration of hitless spectrum defragmentation using real-time coherent receivers in flexible grid optical networks,ˮ in European Conference on Optical Communication (2012), p. Th.3.D.1.

]. Section III discusses the hitless defragmentation testbed. We introduce the coherent RX with fast wavelength tracking, which is at the core of our defragmentation technique, and show a proof-of-concept experimental demonstration of the defragmentation technique using a three-node network testbed. Section IV concludes by highlighting the main results presented in this manuscript and by summarizing future directions.

2. Network performance of different defragmentation techniques

Defragmentation operations are triggered when a group of n connections (n = 30 in this case) depart the network. For comparison, simulations were run under same conditions except for the wavelength sweeping technique, which has to follow the tuning sequence to avoid interference when there is a gap in the spectrogram. We assume the sweep speed is 500 ms/Slot as suggested in [6

6. K. Sone, X. Wang, S. Oda, G. Nakagawa, Y. Aoki, I. Kim, P. Palacharla, T. Hoshida, M. Sekiya, and J. C. Rasmussen, “First demonstration of hitless spectrum defragmentation using real-time coherent receivers in flexible grid optical networks,ˮ in European Conference on Optical Communication (2012), p. Th.3.D.1.

], while in the proposed technique the defragmentation latency is always below 1 µs regardless the target spectral positions.

Figure 2(a)
Fig. 2 Comparison of wavelength sweeping technique and proposed technique in terms of (a) blocking probability vs. offered load, and (b) average network defragmentation time per signal departure vs. offered load.
shows that defragmentation can significantly or marginally reduce the bandwidth blocking probability (BBP) depending on the defragmentation method used. However, there is a factor of 40 improvement (at load of 300 Erlangs) using the proposed technique compared with the sweeping technique. In terms of average time per defragmentation operation, Fig. 2(b) shows that the proposed technique outperforms the sweeping technique by a few orders of magnitude. The sweeping technique (incompletely) conducts defragmentation in 1.2 seconds at load of 300 Erlangs, while the proposed method completes it in 7 µs. Note that the defragmentation time decreases with traffic load for both techniques because the defragmentation algorithm completes in less time when there is less room to tune the connections.

3. Hitless defragmentation experiment

3.1 Network scenario

Figure 3(a)
Fig. 3 (a) 3-node network scenario: EOTN - elastic optical transponder; WSS: wavelength selective switch; NC&M: network control and management. (b) spectrum allocation before (top) and after (bottom) defragmentation. (c). Defragmentation steps with WSS reconfiguration to accept the new spectrum position for connection A.
depicts a simple 3-node network scenario for proof-of-concept demonstration of the proposed defragmentation technique. Each node is equipped with elastic optical transponders (EOTN) and reconfigurable optical add and drop multiplexers (ROADMs), which can be wavelength selective and/or colorless.

3.2 Coherent RX with fast wavelength tracking – architecture and working principle

Note that, in theory, it would be possible to control the defragmentation operation from the NC&M and remove the need of the AWG-based feedback loop shown in Fig. 5. In practice, this would add a lot of complexity in the network control plane. In fact, in order to minimize the defragmentation latency while requiring minimum amount of buffering and no data loss, it is necessary to minimize the amount of time the TX and LO lasers have a frequency offset bigger than ± baud-rate/8 [9

9. R. Maher, D. Millar, S. Savory, and B. Thomsen, “Widely tunable burst mode digital coherent receiver with fast reconfiguration time for 112Gb/s DP-QPSK WDM networks,” J. Lightwave Technol. 99, 1 (2012). [CrossRef]

]. If the defragmentation is entirely controlled by the NC&M, the NC&M would need to send the defragmentation control messages for switching the TX and LO lasers synchronously, calculating exactly the propagation time between NC&M and TX, between NC&M and RX, and between the TX and the RX. For these reasons the wavelength tracking function of Fig. 5 is essential.

In case of a 12.5 GHz grid, a 400 port AWG and 400 photodetectors (PDs) would be necessary, which is possible but complex. A practical approach, that still has all the advantages summarized in Table1, can be to use the proposed technique on a coarser grid, e.g. 50 GHz. In this way, 100 PDs and 100 port AWG would be sufficient. Our technique could be then used to quickly move the connection as close as possible to the target wavelength, while the techniques in [5

5. F. Cugini, M. Secondini, N. Sambo, G. Bottari, G. Bruno, P. Iovanna, and P. Castoldi, “Push-pull technique for defragmentation in flexible optical networks,ˮ in Optical Fiber Conference (2012), p. JTh2A.40.

, 6

6. K. Sone, X. Wang, S. Oda, G. Nakagawa, Y. Aoki, I. Kim, P. Palacharla, T. Hoshida, M. Sekiya, and J. C. Rasmussen, “First demonstration of hitless spectrum defragmentation using real-time coherent receivers in flexible grid optical networks,ˮ in European Conference on Optical Communication (2012), p. Th.3.D.1.

] could be used for fine adjustment of the connection on a finer grid (12.5 or 25 GHz).

3.3 Experimental results of hitless defragmentation in elastic optical network testbed

The second input of WSS1 is fed with a 10Gb/s OOK connection at λ2,, which represents connection B in the scenario of Fig. 3. Connection B is dropped at output 2 of WSS2 and monitored with a 10 Gb/s error analyzer (EA). A switch on the FPGA board activates the defragmentation operation by switching TLD1 wavelength from λ1 to λ3. The EA monitors the BER of the connection B lying in between λ1 and λ3 during the defragmentation operation. In this experiment, the values of λ1, λ2, and λ3 are 1547.75 nm, 1548,5 nm and 1550.1 nm respectively.

During the switching of TLD1 wavelength, Rocket IOs hold the data transmission from their buffers for 512 ns in order to allow enough time for the coherent RX to lock at the new wavelength λ3. Once locked, Rocket IOs start to transmit the data again. In this way no data is lost, and at the application layer, no interruption would be noticed, which is a necessary requirement to guarantee hitless operation.

Figure 7(a)
Fig. 7 (a) trace showing the time when defragmentation happens. (b) Frequency offset between TX and LO lasers after they switched to lambda3 VS time; (c) BER VS time instant from which DSP processing and BER counting start.
shows a real-time scope trace showing the time when defragmentation happens. The real time scope used for sampling the signal was running at 25 GSample/s. The ~20ns gap in Fig. 7(a) is the time required by the two TLDs (TX and LO) to reach λ3 with an offset much smaller than the photo-detectors bandwidth (3-dB bandwidth of 40 GHz). The meaningful data starts only after ~(512 −20) ns from the end of the 20ns gap. The reason for choosing THOLD = 500 ns can be understood from Fig. 7(b) and Fig. 7(c). In order for the carrier phase recovery to work, the frequency offset needs to be within ± baudrate/8. After that, any slow variation can be tracked. Figure 7(b) shows the frequency offset between TX and LO lasers after they reach λ3 (before DSP carrier frequency/phase estimation). Then, - 0.4 GHz is the frequency difference between the TX and LO frequency after both lasers stabilize. Figure 7(c) shows the BER as function of the time instant from which DSP processing and BER counting start, (after the TLDs reach λ3). At 0.5 µs, right after the useful data start, BER is well below 10−3 even though there are still occasionally some large phase glitches that DSP cannot recover. As we increase the time instant from which DSP and BER counting starts, the BER performance in Fig. 7(c) becomes better because we basically discard more symbols that carrier phase recovery could not retrieve (because of the frequency offset is bigger than ± baudrate/8). However, at 2 µs the lasers are more stable, and the BER reaches below 10−6, which is the lower bound of meaningful BER value. In fact, for each BER point in Fig. 7(c), we used three acquisition of 180 µs, each one corresponding to 4.5 million samples, given 25 GSample/s. Then, the total number of symbols was 900,000 for each acquisition, for a total of 2,700,000 symbols and 5,400,000 bits.

The offline DSP included standard digital signal processing functions like clock recovery, constant modulus algorithm (CMA) equalizer, and Viterbi-Viterbi algorithm for carrier frequency and phase estimation. We did not include any fiber chromatic dispersion compensation, since the maximum accumulated chromatic dispersion value in this experiment was 17 ps/nm*km × 25km × 0.04nm = 17 ps.

Figure 8
Fig. 8 The BER performance of the elastic optical network. “black dots”: BER curve obtained with 100KHz linewidth laser; “red dots”: static measurement at λ3 with 2MHz lasers (fast TLDs); “blue diamonds”: BER at λ3 during defragmentation operation.
shows BER measurements as function of OSNR during defragmentation of connection A from λ1 to λ3 (in this measurement we started BER counting at 2 μs from the end of the 20 ns gap). We also compared against a laser with 100 kHz linewidth. The penalty is small, indicating that DSP with differential decoding can sustain fast switching burst-mode operation with both TX and LO lasers switching very fast. Note that in our experiment the baud-rate was limited by the FPGA rocket I/O bandwidth. At higher baud-rates, like in [9

9. R. Maher, D. Millar, S. Savory, and B. Thomsen, “Widely tunable burst mode digital coherent receiver with fast reconfiguration time for 112Gb/s DP-QPSK WDM networks,” J. Lightwave Technol. 99, 1 (2012). [CrossRef]

, 10

10. B. C. Thomsen, R. Maher, D. S. Millar, and S. J. Savory, “Burst mode receiver for 112 Gb/s DP-QPSK with parallel DSP,” Opt. Express 19(26), B770–B776 (2011). [CrossRef] [PubMed]

], the time window in which the frequency offset is bigger than ± baudrate/8 will decrease, making the defragmentation operation even faster.

As mentioned above, an EA monitors in real-time the BER of the connection B at λ2 during the defragmentation operation. Figure 9(a)
Fig. 9 (a) Real-time BER monitor of connection B at lambda2 and (b) accumulated errors on connection B under different defragmentation scenario. BER and errors are calculated over one minute. “x”: no defragmentation operation is performed; “red dots”: fast defragmentation with the proposed technique both in case of wavelength selective or colorless ROADM; “blue diamonds”: defragmentation based on wavelength sweeping.
shows the accumulated BER over different time windows of one minute, and Fig. 9(b) shows the corresponding accumulated errors.

The first measurement shows the BER and accumulated errors of connection B when no fragmentation of connection A is performed. The results, as expected, show 0 accumulated errors over one minute, which corresponds to a BER ≤ 10−11. The second measurement shows the case when defragmentation operation is performed using the proposed technique. For this measurement we replaced the WSS with a 3 dB coupler to emulate a colorless ROADM. In fact, for this particular example with the WSS, the λ2 wavelength would be blocked by the WSS itself (see WSS profile in Fig. 6 - insets (a) and (b)). During the one minute time windows, we performed several defragmentation operation from λ1 to λ3 and back to λ1. No errors were recorded. This proves that our technique allows very fast defragmentation without interfering with existing connections. In this experiment we found out that blanking function was not even necessary. In fact, even without TLD blanking, no errors were observed. This can be explained with the fact that during the switching operation from λ1 to λ3, no significant amount of optical power is generated at λ2. Note that even in the worst case where a significant amount of power at λ2 is generated, because the switching is very fast (nanoseconds time scale), the amount of time that the signal under defragmentation will take to go across the signal at λ2 would be in the order of hundreds of picoseconds [11

11. S. J. B. Yoo, H. J. Lee, Z. Pan, J. Cao, Y. Zhang, K. Okamoto, and S. Kamei, “Rapidly switching all-optical packet routing system with optical-label swapping incorporating tunable wavelength conversion and a uniform-loss cyclic frequency AWGR,ˮ Photonics Technol. Letters 14, 1211–1213 (2002).

], which leads to a minimal number of bit errors that FEC can correct. However, we understand that, in a general case, blanking function should be used to avoid any possible burst-error problem, especially in the case of high baud-rate per subcarrier.

Finally, a third measurement shows what would happen to connection B at λ2 if we would use the wavelength sweeping techniques. For this measurement, we replaced the fast TLD1 with a wavelength-sweeping external cavity laser sweeping from λ1 to λ3 at a speed of 0.5 nm/s.

The sweep takes about 7 seconds and it starts at the 20th second in each of the one-minute time windows. As we can see the result is catastrophic and the BER goes from 10−11 to ~10−2 (inset in Fig. 9(b) shows the eye diagram acquired over 1 minute).

4. Conclusions

We proposed and demonstrated a hitless defragmentation method that leads to complete defragmentation of the spectral map in an elastic optical network without relying on sequential tuning of wavelengths. The method utilizes a transmitter with a fast tunable laser and a burst-mode coherent receiver with fast wavelength tracking at the node. By additional blanking and buffering capability, the defragmentation can hop across existing live connections without interrupting each other’s services at the application layer. We experimentally demonstrate a hitless defragmentation scheme in the elastic optical network testbed. The fast auto-tracking technique involves an athermal AWG with a detector array sensing a change in the TX wavelength, without the need for end-to-end bi-directional coordination between transmitters and receivers. The experiment conducted here for a single 10 Gb/s QPSK channel achieved defragmentation in less than 1 µs, leading to the requirement for ~0.625 kB memory, and can scale to 1 Tb/s for 62.5 kB memory without significantly increasing the defragmentation time since the tuning of individual wavelengths of multiple transmitters can take place in parallel. In this case, the RX will need to receive specific information from the NC&M in order to differentiate multiple wavelength changes simultaneously. This method can also be used in a modular operation to defragment a Nyquist-WDM superchannel by moving one subcarrier at a time, and can be used in Pol-Muxed Elastic Optical Networks when polarization modulated or polarization multiplexed transmitters and receivers are used.

Acknowledgments

We acknowledge Nistica for the loan of the WSSs. We would like to acknowledge also Dr. Nicolas Fontaine and Dr. Benn Thomsen for useful discussions on digital signal processing (DSP), and Mr. Mingyang Zhang and Prof. Zuqing Zhu for their help on the networking simulations. This work was supported in part by CISCO URP and Ericsson POPCORN project.

References and links

1.

M. Jinno, H. Takara, B. Kozicki, Y. Tsukishima, Y. Sone, and S. Matsuoka, “Spectrum-efficient and scalable elastic optical path network: architecture, benefits, and enabling Technologies,ˮ,” IEEE Commun. Mag. 47(11), 66–73 (2009). [CrossRef]

2.

Y. Yin, K. Wen, D. J. Geisler, R. Liu, and S. J. Yoo, “Dynamic on-demand defragmentation in flexible bandwidth elastic optical networks,” Opt. Express 20(2), 1798–1804 (2012). [CrossRef] [PubMed]

3.

A. N. Patel, P. N. Ji, J. P. Jue, and T. Wang, “Defragmentation of transparent flexible optical WDM (FWDM) networks,ˮ (Optical Society of America, 2011), p. OTuI8.

4.

T. Takagi, H. Hasegawa, K.-i. Sato, Y. Sone, A. Hirano, and M. Jinno, “Disruption minimized spectrum defragmentation in elastic optical path networks that adopt distance adaptive modulation,ˮ (Optical Society of America, 2011), p. Mo.2.K.3.

5.

F. Cugini, M. Secondini, N. Sambo, G. Bottari, G. Bruno, P. Iovanna, and P. Castoldi, “Push-pull technique for defragmentation in flexible optical networks,ˮ in Optical Fiber Conference (2012), p. JTh2A.40.

6.

K. Sone, X. Wang, S. Oda, G. Nakagawa, Y. Aoki, I. Kim, P. Palacharla, T. Hoshida, M. Sekiya, and J. C. Rasmussen, “First demonstration of hitless spectrum defragmentation using real-time coherent receivers in flexible grid optical networks,ˮ in European Conference on Optical Communication (2012), p. Th.3.D.1.

7.

G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of Nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM subcarriers,ˮ,” J. Lightwave Technol. 29(1), 53–61 (2011). [CrossRef]

8.

L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: A tutorial,ˮ,” J. Lightwave Technol. 22(1), 193–202 (2004). [CrossRef]

9.

R. Maher, D. Millar, S. Savory, and B. Thomsen, “Widely tunable burst mode digital coherent receiver with fast reconfiguration time for 112Gb/s DP-QPSK WDM networks,” J. Lightwave Technol. 99, 1 (2012). [CrossRef]

10.

B. C. Thomsen, R. Maher, D. S. Millar, and S. J. Savory, “Burst mode receiver for 112 Gb/s DP-QPSK with parallel DSP,” Opt. Express 19(26), B770–B776 (2011). [CrossRef] [PubMed]

11.

S. J. B. Yoo, H. J. Lee, Z. Pan, J. Cao, Y. Zhang, K. Okamoto, and S. Kamei, “Rapidly switching all-optical packet routing system with optical-label swapping incorporating tunable wavelength conversion and a uniform-loss cyclic frequency AWGR,ˮ Photonics Technol. Letters 14, 1211–1213 (2002).

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.4256) Fiber optics and optical communications : Networks, network optimization
(060.4265) Fiber optics and optical communications : Networks, wavelength routing

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: September 21, 2012
Revised Manuscript: November 6, 2012
Manuscript Accepted: November 6, 2012
Published: November 15, 2012

Citation
Roberto Proietti, Chuan Qin, Binbin Guan, Yawei Yin, Ryan P. Scott, Runxiang Yu, and S. J. B. Yoo, "Rapid and complete hitless defragmentation method using a coherent RX LO with fast wavelength tracking in elastic optical networks," Opt. Express 20, 26958-26968 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-24-26958


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References

  1. M. Jinno, H. Takara, B. Kozicki, Y. Tsukishima, Y. Sone, and S. Matsuoka, “Spectrum-efficient and scalable elastic optical path network: architecture, benefits, and enabling Technologies,ˮ,” IEEE Commun. Mag.47(11), 66–73 (2009). [CrossRef]
  2. Y. Yin, K. Wen, D. J. Geisler, R. Liu, and S. J. Yoo, “Dynamic on-demand defragmentation in flexible bandwidth elastic optical networks,” Opt. Express20(2), 1798–1804 (2012). [CrossRef] [PubMed]
  3. A. N. Patel, P. N. Ji, J. P. Jue, and T. Wang, “Defragmentation of transparent flexible optical WDM (FWDM) networks,ˮ (Optical Society of America, 2011), p. OTuI8.
  4. T. Takagi, H. Hasegawa, K.-i. Sato, Y. Sone, A. Hirano, and M. Jinno, “Disruption minimized spectrum defragmentation in elastic optical path networks that adopt distance adaptive modulation,ˮ (Optical Society of America, 2011), p. Mo.2.K.3.
  5. F. Cugini, M. Secondini, N. Sambo, G. Bottari, G. Bruno, P. Iovanna, and P. Castoldi, “Push-pull technique for defragmentation in flexible optical networks,ˮ in Optical Fiber Conference (2012), p. JTh2A.40.
  6. K. Sone, X. Wang, S. Oda, G. Nakagawa, Y. Aoki, I. Kim, P. Palacharla, T. Hoshida, M. Sekiya, and J. C. Rasmussen, “First demonstration of hitless spectrum defragmentation using real-time coherent receivers in flexible grid optical networks,ˮ in European Conference on Optical Communication (2012), p. Th.3.D.1.
  7. G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of Nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM subcarriers,ˮ,” J. Lightwave Technol.29(1), 53–61 (2011). [CrossRef]
  8. L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: A tutorial,ˮ,” J. Lightwave Technol.22(1), 193–202 (2004). [CrossRef]
  9. R. Maher, D. Millar, S. Savory, and B. Thomsen, “Widely tunable burst mode digital coherent receiver with fast reconfiguration time for 112Gb/s DP-QPSK WDM networks,” J. Lightwave Technol.99, 1 (2012). [CrossRef]
  10. B. C. Thomsen, R. Maher, D. S. Millar, and S. J. Savory, “Burst mode receiver for 112 Gb/s DP-QPSK with parallel DSP,” Opt. Express19(26), B770–B776 (2011). [CrossRef] [PubMed]
  11. S. J. B. Yoo, H. J. Lee, Z. Pan, J. Cao, Y. Zhang, K. Okamoto, and S. Kamei, “Rapidly switching all-optical packet routing system with optical-label swapping incorporating tunable wavelength conversion and a uniform-loss cyclic frequency AWGR,ˮ Photonics Technol. Letters14, 1211–1213 (2002).

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