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Signed chromatic dispersion monitoring of OOK signal by evaluating the distance ratio of delay tap sampling
Optics Communications 334 (2015) 58–62
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Signed chromatic dispersion monitoring of OOK signal by evaluating the distance ratio of delay tap sampling Xiurong Ma a,b,c,n, Xiao Wang a,b,c, Zhaocai Ding a,b,c, Yujun Xu a,b,c a
The Department of Computer and Communication Engineering, Tianjin University of Technology, Tianjin 300384, China Engineering Research Center of Communication Devices and Technology, Ministry of Education, Tianjin 300384, China c Tianjin Key Laboratory of Film Electronic and Communication Devices, Tianjin 300384, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 12 June 2014 Received in revised form 18 July 2014 Accepted 2 August 2014 Available online 20 August 2014
We analyze the residual chromatic dispersion (CD) monitoring of non-return-to-zero on–off keying (NRZ-OOK) signals using delay-tap asynchronous sampling. By evaluating the distance ratio of delay-tap plots and using a 0.5-bit optical delay interferometer (ODI), signed residual CD can be monitored accurately via numerical simulations. This CD monitoring method is independent of optical signal-tonoise ratio (OSNR) and polarization-mode dispersion (PMD). The measuring range was around 7 900 ps/ nm for 10-Gbit/s NRZ-OOK link and 7 60 ps/nm for 40 G link, the monitoring error is smaller than 2 ps/nm. & 2014 Elsevier B.V. All rights reserved.
Keywords: Delay tap sampling Chromatic dispersion monitoring (CD) Distance ratio Optical signal-to-noise ratio (OSNR) Polarization-mode dispersion (PMD)
1. Introduction With the rapid development of high-speed and capacity fiber optical communication networks, optical performance monitoring (OPM) becomes increasingly important, particularly regarding signal quality monitoring such as optical signal-to-noise ratio (OSNR), Q-factor, and dispersion [1]. Chromatic dispersion (CD) proves to be critical item of OPM and one of the main impairments that limit the performance of optical systems [2,3]. A variety of residual CD monitoring schemes has been proposed and verified [4–9]. The majority of them require inserting components into the distributed feedback (DFB) laser, such as pilot tone [4,5], RF modulated ASE noise [6], or optical frequency modulated (FM) signal [7]. The method of inserting tones at the transmitter can realize CD monitoring by measuring the power of a CD-induced RF tone at the receiver. Or, such RF tone can be obtained from clock tones for pulse shapes such as non-return-to-zero (NRZ)-, returnto-zero (RZ)- and carrier-suppressed (CS)–RZ on–off keying (OOK) [8]. Yet, extra pilot tone, ASE noise or optical FM signal would lead to mitigation of system performance, and that, any change to the transmitter will increase the system cost and complexity. The
n
Corresponding author. E-mail addresses: [email protected] (X. Ma), [email protected] (X. Wang). http://dx.doi.org/10.1016/j.optcom.2014.08.008 0030-4018/& 2014 Elsevier B.V. All rights reserved.
scheme of detecting the phase of recovered clock at receiver needs no additional monitoring signal added to the transmitter [9], however, clock recovery and high speed phase comparator are indispensable, which also results in increased system cost and complexity. In some backbone transmission links, NRZ format still presents, moreover, OOK format is widely used in access network, for instance, 10 G NRZ, and later the speed will increase to 40 Gbit/s with NRZ format, under the circumstances, communication quality is greatly affected by the residual CD of fiber link, as one of the most critical items of OPM, CD monitoring for OOK links is quite significant [10]. Jeffrey et al. [11] use artificial neural networks trained with parameters derived from delay-tap asynchronous sampling to monitor CD of NRZ link, it needs no requiring of clock recovery and high-speed components but gets a small range of CD and cannot differentiate the sign. Recently, a single sideband (SSB) spectrum phase difference detection technology [10] is proposed to realize CD measuring of OOK links, this method can easily differentiate the positive and negative residual CD of the fiber link, without any configuration in transmitter, not needing high-speed components, so that it is cost effective but it becomes unreliable in the presence of OSNR and PMD. In this letter, we modeled a CD monitoring method for 10-Gbit/s and 40-Gbit/s NRZ-OOK signal based on delay-tap sampling scheme by evaluating the distance ratio of delay-tap plots. We analyzed plots with 1B delay for CD monitoring [2,3]. Then the distance ratio
X. Ma et al. / Optics Communications 334 (2015) 58–62
is defined which displays the one-to-one relationship with CD and the CD range is confirmed. With an ODI, we can also differentiate positive and negative CD. On the other hand, this scheme shows advantages of independent of OSNR and PMD as well as easy to implement and low error.
2. Principle of chromatic dispersion measurement The model of the NRZ signal transmission system is illustrated in Fig. 1. Fiber link consists of single mode fiber (SMF) and dispersion compensation fiber (DCF). PMD emulator is used to simulate PMD values. An erbium-doped fiber amplifier (EDFA)
with a variable optical attenuator (VOA) in front is to adjust the received OSNR. The OSNR is fixed to be 21 dB when measuring CD. An 0.5-bit optical delay interferometer (ODI) can differentiate the polarity of CD. After balanced detection, the delayed and nondelayed signal are sampled as two measurements (X and Y), separated by a fixed time corresponding to the delay length τ. The obtained sample points are from the same pulse or the adjacent pulses, they can reflect the pulse shape information which has a strong relationship with the impairments. The samples are plotted as (X,Y) pairs, producing a two-tap plot. Fig. 2 displays two-delay-tap plots with 1B delay and CD¼450 ps/nm. As is known in Refs. [2,3], when the delay equals to 1B, with the increase of residual CD, the main diagonal curvature in delay-tap plots is gradually serious. Therefore, a dispersion parameter distance ratio (DR) is defined to quantify the distinct features and measure the amount of accumulated CD in OOK signals. As shown in Fig. 2, distance ratio is defined as follows: Distance ratio ¼ d1=d2
Fig. 1. Setup for CD monitoring using delay-tap sampling.
59
ð1Þ
Where, d1 is the distance between point A and C, d2 is the distance between point O and C. A represents the intersected point of midcourt line X ¼ maxðXÞ=2 and Y ¼ maxðYÞ=2. O is the original point, while line OA crossing with the main diagonal of the delaytap sampling plot (inner side) forms point C. To ensure high precision, we compute all the points in the circle with point C as the center and r ¼0.01 as the radius [12]. In fact, the eye diagram can reflect the distortion of waveform obviously [12–14], thereby, we can also study the distortion of eye diagram caused by residual CD. Fig. 3(a)–(c) display the eye diagrams of OOK signal under different residual CD (0 ps/nm, þ450 ps/nm and 450 ps/nm). Where, the delay in one arm of the ODI is set to be 50 ps (0.5-bit). A characteristic can be achieved that eye diagram is symmetric against peak point when there is no CD, and eye diagram will lean to the right side with negative CD and to the left side with positive CD. We can also know that the degree of the slant will increase as the amount of residual CD becomes larger. We can make use of the asymmetric trait of eye diagrams of OOK signal induced by positive or negative residual CD together with the delay tap sampling technique to differentiate positive and negative residual CD.
3. Simulation results and discussions
Fig. 2. Two-tap plots with 1B delay and CD¼ 450 ps/nm of 10 G link for feature extraction.
Consider a transmission system as shown in Fig. 1, 10-Gb/s or 40-Gb/s NRZ-OOK signal is generated in the transmitter, and then sent to fiber link which consists of SMF with nonlinearity and DCF.
Fig. 3. Eye diagram of OOK signal with different values of CD (ps/nm) (a) 0; (b) þ450 and (c) 450.
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X. Ma et al. / Optics Communications 334 (2015) 58–62
The positive chromatic dispersion is added to the signal using SMF, the negative chromatic dispersion is added using DCF. The signal with CD is then sent to PMD emulator. EDFA is used to add ASE noise to the signal and a variable optical attenuator (VOA) is adopted in front to change OSNR among 15–30 dB. The OSNR is maintained at level of 21 dB for all CD measurements. The fiber simulates the residual CD in the range of 1000–1000 ps/nm, after that, the signal is filtered by an optical bandpass filter (OBPF). The 0.5-bit ODI is followed by balanced detectors. After O/E conversion, the electrical signal is sampled in two parts and a variable delay is introduced in one path so as to set the delay value to one symbol period ðτ ¼ BÞ. Then signal is sampled asynchronously and delay-tap scatter plots are obtained to finish the measurement of residual CD. Fig. 4(a)–(c) illustrates plots with different residual CD (0 ps/nm, 450 ps/nm and 900 ps/nm) of 10 G link, we can see that the distance ratio will increase with the increasing of residual CD. Similar results can be derived for 40 G link as shown in Fig. 4(d)–(f). In Fig. 5(a), the distance ratio is plotted as a function of residual CD which ranges from 0.06 to 0.57 as CD changes from 900 ps/ nm to þ900 ps/nm in 10 G link. The measurement is symmetric with respect to the origin. Similarly, the range of residual CD of 40 G link is obtained in Fig. 5(b) which represents the distance ratio vs residual CD, ranging from 0.16 to 0.54 as CD changes from
60 ps/nm to 60 ps/nm. The level is limited by diagonal distortion at 7900 ps/nm or 760 ps/nm at which point the measurement ceases to increase monotonically with the increase of accumulated dispersion. The horizontal error bars represent the accuracy with which the DCF could be adjusted ( 72 ps/nm). The vertical error bars show the standard deviation of DR assessed over 50 measurements. In 10 G link, Deviation of DR is 6.47e-002 for 0 ps/nm, 7.15e-002 for 7450 ps/nm, and 4.25e-002 for 7900 ps/nm. In 40 G link, deviation of DR is 1.86e-002 for 0 ps/ nm, 2.30e-002 for 730 ps/nm, and 2.80e-002 for 760 ps/nm. We can also see that CD range decreases when the transmission rate increases which matches the dispersion tolerance with different rates [15]. The distance ratio increases with CD monotonously which indicates that this method can be used to measure residual CD. Fig. 6(a) shows the dependence of accuracy expressed as the standard deviation of DR on OSNR for CD of 0, 450, and 900 ps/nm, accuracy better than 2 ps/nm can be achieved for all values of CD, and the accuracy can be lower than 1 ps/nm for OSNR larger than 23 dB. So, OSNR is insensitive to the accuracy of the method. In Fig. 6(b), when PMD changes among 0 30 ps, the accuracy kept below 1 ps/nm for all values of CD, and the accuracy is not influenced by PMD. Similar conclusions can be obtained for 40 G as shown in Fig. 7(a) and (b).
Fig. 4. Simulated delay-tap plots with different residual CD (a–c) 10 G and (d–f) 40 G link.
Fig. 5. Distance ratio as a function of CD for (a) 10 G link and (b) 40 G link.
X. Ma et al. / Optics Communications 334 (2015) 58–62
61
Fig. 6. Dependence of accuracy of 10 G link on (a) OSNR and (b) PMD.
Fig. 7. Dependence of accuracy of 40 G link on (a) OSNR and (b) PMD.
Fig. 8. Impact of OSNR on CD measurement for NRZ link (a) 10 G and (b) 40 G.
Fig. 9. Impact of PMD on CD measurement for NRZ link (a) 10 G and (b) 40 G.
4. Impact of OSNR and PMD on monitoring results To investigate the impact of OSNR and PMD on CD monitoring by using this technique, a variety of simulations are operated.
In Fig. 8, the distance ratio is plotted as a function of residual CD for OSNR of 18 dB, 25and 30 dB.The distance ratio changes averagely 1.63e-003/1 dB of OSNR and 8.35e-004/1 dB of OSNR for 10 G and 40 G links respectively.
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X. Ma et al. / Optics Communications 334 (2015) 58–62
In a similar way, Fig. 9 indicates the distance ratio changes 1.21e-003 and 1.14e-003 as PMD varies with 1 ps for 10 G and 40 G links respectively. An conclusion can be drawn that this method is independent of OSNR and PMD both for 10 G and 40 G links.
[4] [5] [6]
5. Conclusions
[7] [8]
In this letter, we analyzed the CD monitoring of NRZ-OOK signal using delay-tap asynchronous sampling. By evaluating the distance ratio of delay-tap plots and using a 0.5-bit ODI, signed residual CD monitoring was realized. This method was insensitive to OSNR and PMD. The measuring range was around 7 900 ps/nm for 10-Gbit/s NRZ-OOK link and 7 60 ps/nm for 40 G link. References [1] D. Kilper, R. Bach, D. Blumenthal, D. Einstein, T. Landolsi, L. Ostar, M. Preiss, A.E. Willner, J. Lightwave Technol. 22 (2004) 294. [2] S.D. Dods, T.B. Anderson, Proceedings of the Optical Fiber Communication Conference (OFC), Anaheim, CA, 2006, Paper OThP5. [3] T.B. Anderson, S.D. Dods E. Wong, P.M. Farrell, Asynchronous measurement of chromatic dispersion from waveform distortion, in: Proceedings of the Optical
[9]
[10] [11]
[12] [13] [14] [15]
Fiber Communication Conference and the 2006 National Fiber Optic Engineers Conference, OFC 2006. N. Liu, W.D. Zhong, Y.J. Wen, Z. Li, Opt. Express 15 (3) (2007) 839. K.J. Park, C.J. Youn, J.H. Lee, Y.C. Chung, IEEE Photonics Technol. Lett. 15 (6) (2003) 873. G.J. Pendock, X. Yi, C. Yu, W. Shieh, IEEE Photonics Technol. Lett. 20 (10) (2008) 821. S.K. Ibrahim, S. Bhandare, D. Sandel, A. Hidayat, A. Fauzi, R. Noé, IEE Proc. Optoelectron. 153 (5) (2006) 235. Z. Pan, Y. Xie, S.A. Havstad, Q. Yub, A.E. Willner, V. Grubsky, D.S. Starodubovc, J. Feinberg, Opt. Commun. 230 (2003) 145. H. Kawakami, E. Yoshida, H. Hubota, Y. Miyamoto, Novel signed chromatic dispersion monitoring technique based on asymmetric waveform distortion in DQPSK receiver, in: Proceedings of the OECC, 2008, Paper WeK-3. Bin Li, Fengguang Luo, Weilin Zhou, Zhihua Yu, Ming Tian, Lu Shi, Opt. Express 19 (25) (2011) 25584. Jeffrey A. Jargon, Xiaoxia Wu, Alan E. Willner, Optical performance monitoring by use of artificial neural networks trained with parameters derived from delay-tap asynchronous sampling, in: Proceedings of the CLEO/QELS, Paper OThH1, San Jose, California, 2009. Z. Li, Z. Jian, L. Cheng, Y. Yang, C. Lu, A.P. Lau, C. Yu, H.Y. Tam, P.K. Wai, Opt. Express 18 (3) (2010) 3149. J. Zhao, Z. Li, D. Liu, L. Cheng, C. Lu, H.Y. Tam, J. Lightwave Technol. 27 (23) (2009) 5295. Z. Li, J. Zhao, L. Cheng, Y. Yang, C. Lu, A.P.T. Lau, H.Y. Tam, P.K.A. Wai, OECC, FW7, 2009. Govind P. Agrawal, Fiber-Optic Communications Systems, 3rd Edition, John Wiley & Sons., The Institute of Optics University of Rochester, NY, 2002.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Signed chromatic dispersion monitoring of OOK signal by evaluating the distance ratio of delay tap sampling Xiurong Ma a,b,c,n, Xiao Wang a,b,c, Zhaocai Ding a,b,c, Yujun Xu a,b,c a
The Department of Computer and Communication Engineering, Tianjin University of Technology, Tianjin 300384, China Engineering Research Center of Communication Devices and Technology, Ministry of Education, Tianjin 300384, China c Tianjin Key Laboratory of Film Electronic and Communication Devices, Tianjin 300384, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 12 June 2014 Received in revised form 18 July 2014 Accepted 2 August 2014 Available online 20 August 2014
We analyze the residual chromatic dispersion (CD) monitoring of non-return-to-zero on–off keying (NRZ-OOK) signals using delay-tap asynchronous sampling. By evaluating the distance ratio of delay-tap plots and using a 0.5-bit optical delay interferometer (ODI), signed residual CD can be monitored accurately via numerical simulations. This CD monitoring method is independent of optical signal-tonoise ratio (OSNR) and polarization-mode dispersion (PMD). The measuring range was around 7 900 ps/ nm for 10-Gbit/s NRZ-OOK link and 7 60 ps/nm for 40 G link, the monitoring error is smaller than 2 ps/nm. & 2014 Elsevier B.V. All rights reserved.
Keywords: Delay tap sampling Chromatic dispersion monitoring (CD) Distance ratio Optical signal-to-noise ratio (OSNR) Polarization-mode dispersion (PMD)
1. Introduction With the rapid development of high-speed and capacity fiber optical communication networks, optical performance monitoring (OPM) becomes increasingly important, particularly regarding signal quality monitoring such as optical signal-to-noise ratio (OSNR), Q-factor, and dispersion [1]. Chromatic dispersion (CD) proves to be critical item of OPM and one of the main impairments that limit the performance of optical systems [2,3]. A variety of residual CD monitoring schemes has been proposed and verified [4–9]. The majority of them require inserting components into the distributed feedback (DFB) laser, such as pilot tone [4,5], RF modulated ASE noise [6], or optical frequency modulated (FM) signal [7]. The method of inserting tones at the transmitter can realize CD monitoring by measuring the power of a CD-induced RF tone at the receiver. Or, such RF tone can be obtained from clock tones for pulse shapes such as non-return-to-zero (NRZ)-, returnto-zero (RZ)- and carrier-suppressed (CS)–RZ on–off keying (OOK) [8]. Yet, extra pilot tone, ASE noise or optical FM signal would lead to mitigation of system performance, and that, any change to the transmitter will increase the system cost and complexity. The
n
Corresponding author. E-mail addresses: [email protected] (X. Ma), [email protected] (X. Wang). http://dx.doi.org/10.1016/j.optcom.2014.08.008 0030-4018/& 2014 Elsevier B.V. All rights reserved.
scheme of detecting the phase of recovered clock at receiver needs no additional monitoring signal added to the transmitter [9], however, clock recovery and high speed phase comparator are indispensable, which also results in increased system cost and complexity. In some backbone transmission links, NRZ format still presents, moreover, OOK format is widely used in access network, for instance, 10 G NRZ, and later the speed will increase to 40 Gbit/s with NRZ format, under the circumstances, communication quality is greatly affected by the residual CD of fiber link, as one of the most critical items of OPM, CD monitoring for OOK links is quite significant [10]. Jeffrey et al. [11] use artificial neural networks trained with parameters derived from delay-tap asynchronous sampling to monitor CD of NRZ link, it needs no requiring of clock recovery and high-speed components but gets a small range of CD and cannot differentiate the sign. Recently, a single sideband (SSB) spectrum phase difference detection technology [10] is proposed to realize CD measuring of OOK links, this method can easily differentiate the positive and negative residual CD of the fiber link, without any configuration in transmitter, not needing high-speed components, so that it is cost effective but it becomes unreliable in the presence of OSNR and PMD. In this letter, we modeled a CD monitoring method for 10-Gbit/s and 40-Gbit/s NRZ-OOK signal based on delay-tap sampling scheme by evaluating the distance ratio of delay-tap plots. We analyzed plots with 1B delay for CD monitoring [2,3]. Then the distance ratio
X. Ma et al. / Optics Communications 334 (2015) 58–62
is defined which displays the one-to-one relationship with CD and the CD range is confirmed. With an ODI, we can also differentiate positive and negative CD. On the other hand, this scheme shows advantages of independent of OSNR and PMD as well as easy to implement and low error.
2. Principle of chromatic dispersion measurement The model of the NRZ signal transmission system is illustrated in Fig. 1. Fiber link consists of single mode fiber (SMF) and dispersion compensation fiber (DCF). PMD emulator is used to simulate PMD values. An erbium-doped fiber amplifier (EDFA)
with a variable optical attenuator (VOA) in front is to adjust the received OSNR. The OSNR is fixed to be 21 dB when measuring CD. An 0.5-bit optical delay interferometer (ODI) can differentiate the polarity of CD. After balanced detection, the delayed and nondelayed signal are sampled as two measurements (X and Y), separated by a fixed time corresponding to the delay length τ. The obtained sample points are from the same pulse or the adjacent pulses, they can reflect the pulse shape information which has a strong relationship with the impairments. The samples are plotted as (X,Y) pairs, producing a two-tap plot. Fig. 2 displays two-delay-tap plots with 1B delay and CD¼450 ps/nm. As is known in Refs. [2,3], when the delay equals to 1B, with the increase of residual CD, the main diagonal curvature in delay-tap plots is gradually serious. Therefore, a dispersion parameter distance ratio (DR) is defined to quantify the distinct features and measure the amount of accumulated CD in OOK signals. As shown in Fig. 2, distance ratio is defined as follows: Distance ratio ¼ d1=d2
Fig. 1. Setup for CD monitoring using delay-tap sampling.
59
ð1Þ
Where, d1 is the distance between point A and C, d2 is the distance between point O and C. A represents the intersected point of midcourt line X ¼ maxðXÞ=2 and Y ¼ maxðYÞ=2. O is the original point, while line OA crossing with the main diagonal of the delaytap sampling plot (inner side) forms point C. To ensure high precision, we compute all the points in the circle with point C as the center and r ¼0.01 as the radius [12]. In fact, the eye diagram can reflect the distortion of waveform obviously [12–14], thereby, we can also study the distortion of eye diagram caused by residual CD. Fig. 3(a)–(c) display the eye diagrams of OOK signal under different residual CD (0 ps/nm, þ450 ps/nm and 450 ps/nm). Where, the delay in one arm of the ODI is set to be 50 ps (0.5-bit). A characteristic can be achieved that eye diagram is symmetric against peak point when there is no CD, and eye diagram will lean to the right side with negative CD and to the left side with positive CD. We can also know that the degree of the slant will increase as the amount of residual CD becomes larger. We can make use of the asymmetric trait of eye diagrams of OOK signal induced by positive or negative residual CD together with the delay tap sampling technique to differentiate positive and negative residual CD.
3. Simulation results and discussions
Fig. 2. Two-tap plots with 1B delay and CD¼ 450 ps/nm of 10 G link for feature extraction.
Consider a transmission system as shown in Fig. 1, 10-Gb/s or 40-Gb/s NRZ-OOK signal is generated in the transmitter, and then sent to fiber link which consists of SMF with nonlinearity and DCF.
Fig. 3. Eye diagram of OOK signal with different values of CD (ps/nm) (a) 0; (b) þ450 and (c) 450.
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The positive chromatic dispersion is added to the signal using SMF, the negative chromatic dispersion is added using DCF. The signal with CD is then sent to PMD emulator. EDFA is used to add ASE noise to the signal and a variable optical attenuator (VOA) is adopted in front to change OSNR among 15–30 dB. The OSNR is maintained at level of 21 dB for all CD measurements. The fiber simulates the residual CD in the range of 1000–1000 ps/nm, after that, the signal is filtered by an optical bandpass filter (OBPF). The 0.5-bit ODI is followed by balanced detectors. After O/E conversion, the electrical signal is sampled in two parts and a variable delay is introduced in one path so as to set the delay value to one symbol period ðτ ¼ BÞ. Then signal is sampled asynchronously and delay-tap scatter plots are obtained to finish the measurement of residual CD. Fig. 4(a)–(c) illustrates plots with different residual CD (0 ps/nm, 450 ps/nm and 900 ps/nm) of 10 G link, we can see that the distance ratio will increase with the increasing of residual CD. Similar results can be derived for 40 G link as shown in Fig. 4(d)–(f). In Fig. 5(a), the distance ratio is plotted as a function of residual CD which ranges from 0.06 to 0.57 as CD changes from 900 ps/ nm to þ900 ps/nm in 10 G link. The measurement is symmetric with respect to the origin. Similarly, the range of residual CD of 40 G link is obtained in Fig. 5(b) which represents the distance ratio vs residual CD, ranging from 0.16 to 0.54 as CD changes from
60 ps/nm to 60 ps/nm. The level is limited by diagonal distortion at 7900 ps/nm or 760 ps/nm at which point the measurement ceases to increase monotonically with the increase of accumulated dispersion. The horizontal error bars represent the accuracy with which the DCF could be adjusted ( 72 ps/nm). The vertical error bars show the standard deviation of DR assessed over 50 measurements. In 10 G link, Deviation of DR is 6.47e-002 for 0 ps/nm, 7.15e-002 for 7450 ps/nm, and 4.25e-002 for 7900 ps/nm. In 40 G link, deviation of DR is 1.86e-002 for 0 ps/ nm, 2.30e-002 for 730 ps/nm, and 2.80e-002 for 760 ps/nm. We can also see that CD range decreases when the transmission rate increases which matches the dispersion tolerance with different rates [15]. The distance ratio increases with CD monotonously which indicates that this method can be used to measure residual CD. Fig. 6(a) shows the dependence of accuracy expressed as the standard deviation of DR on OSNR for CD of 0, 450, and 900 ps/nm, accuracy better than 2 ps/nm can be achieved for all values of CD, and the accuracy can be lower than 1 ps/nm for OSNR larger than 23 dB. So, OSNR is insensitive to the accuracy of the method. In Fig. 6(b), when PMD changes among 0 30 ps, the accuracy kept below 1 ps/nm for all values of CD, and the accuracy is not influenced by PMD. Similar conclusions can be obtained for 40 G as shown in Fig. 7(a) and (b).
Fig. 4. Simulated delay-tap plots with different residual CD (a–c) 10 G and (d–f) 40 G link.
Fig. 5. Distance ratio as a function of CD for (a) 10 G link and (b) 40 G link.
X. Ma et al. / Optics Communications 334 (2015) 58–62
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Fig. 6. Dependence of accuracy of 10 G link on (a) OSNR and (b) PMD.
Fig. 7. Dependence of accuracy of 40 G link on (a) OSNR and (b) PMD.
Fig. 8. Impact of OSNR on CD measurement for NRZ link (a) 10 G and (b) 40 G.
Fig. 9. Impact of PMD on CD measurement for NRZ link (a) 10 G and (b) 40 G.
4. Impact of OSNR and PMD on monitoring results To investigate the impact of OSNR and PMD on CD monitoring by using this technique, a variety of simulations are operated.
In Fig. 8, the distance ratio is plotted as a function of residual CD for OSNR of 18 dB, 25and 30 dB.The distance ratio changes averagely 1.63e-003/1 dB of OSNR and 8.35e-004/1 dB of OSNR for 10 G and 40 G links respectively.
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In a similar way, Fig. 9 indicates the distance ratio changes 1.21e-003 and 1.14e-003 as PMD varies with 1 ps for 10 G and 40 G links respectively. An conclusion can be drawn that this method is independent of OSNR and PMD both for 10 G and 40 G links.
[4] [5] [6]
5. Conclusions
[7] [8]
In this letter, we analyzed the CD monitoring of NRZ-OOK signal using delay-tap asynchronous sampling. By evaluating the distance ratio of delay-tap plots and using a 0.5-bit ODI, signed residual CD monitoring was realized. This method was insensitive to OSNR and PMD. The measuring range was around 7 900 ps/nm for 10-Gbit/s NRZ-OOK link and 7 60 ps/nm for 40 G link. References [1] D. Kilper, R. Bach, D. Blumenthal, D. Einstein, T. Landolsi, L. Ostar, M. Preiss, A.E. Willner, J. Lightwave Technol. 22 (2004) 294. [2] S.D. Dods, T.B. Anderson, Proceedings of the Optical Fiber Communication Conference (OFC), Anaheim, CA, 2006, Paper OThP5. [3] T.B. Anderson, S.D. Dods E. Wong, P.M. Farrell, Asynchronous measurement of chromatic dispersion from waveform distortion, in: Proceedings of the Optical
[9]
[10] [11]
[12] [13] [14] [15]
Fiber Communication Conference and the 2006 National Fiber Optic Engineers Conference, OFC 2006. N. Liu, W.D. Zhong, Y.J. Wen, Z. Li, Opt. Express 15 (3) (2007) 839. K.J. Park, C.J. Youn, J.H. Lee, Y.C. Chung, IEEE Photonics Technol. Lett. 15 (6) (2003) 873. G.J. Pendock, X. Yi, C. Yu, W. Shieh, IEEE Photonics Technol. Lett. 20 (10) (2008) 821. S.K. Ibrahim, S. Bhandare, D. Sandel, A. Hidayat, A. Fauzi, R. Noé, IEE Proc. Optoelectron. 153 (5) (2006) 235. Z. Pan, Y. Xie, S.A. Havstad, Q. Yub, A.E. Willner, V. Grubsky, D.S. Starodubovc, J. Feinberg, Opt. Commun. 230 (2003) 145. H. Kawakami, E. Yoshida, H. Hubota, Y. Miyamoto, Novel signed chromatic dispersion monitoring technique based on asymmetric waveform distortion in DQPSK receiver, in: Proceedings of the OECC, 2008, Paper WeK-3. Bin Li, Fengguang Luo, Weilin Zhou, Zhihua Yu, Ming Tian, Lu Shi, Opt. Express 19 (25) (2011) 25584. Jeffrey A. Jargon, Xiaoxia Wu, Alan E. Willner, Optical performance monitoring by use of artificial neural networks trained with parameters derived from delay-tap asynchronous sampling, in: Proceedings of the CLEO/QELS, Paper OThH1, San Jose, California, 2009. Z. Li, Z. Jian, L. Cheng, Y. Yang, C. Lu, A.P. Lau, C. Yu, H.Y. Tam, P.K. Wai, Opt. Express 18 (3) (2010) 3149. J. Zhao, Z. Li, D. Liu, L. Cheng, C. Lu, H.Y. Tam, J. Lightwave Technol. 27 (23) (2009) 5295. Z. Li, J. Zhao, L. Cheng, Y. Yang, C. Lu, A.P.T. Lau, H.Y. Tam, P.K.A. Wai, OECC, FW7, 2009. Govind P. Agrawal, Fiber-Optic Communications Systems, 3rd Edition, John Wiley & Sons., The Institute of Optics University of Rochester, NY, 2002.