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s optical soliton transmission link under the impact of chirp and TOD
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Optik 121 (2010) 800–807 www.elsevier.de/ijleo
Performance analysis of NRZ, RZ, CRZ and CSRZ data formats in 10 Gb/s optical soliton transmission link under the impact of chirp and TOD Jagjit Singh Malhotra, Manoj Kumar DAV Institute of Engineering & Technology, Jalandhar, Punjab, India Received 8 April 2008; accepted 17 August 2008
Abstract This paper presents the performance analysis of non-return-to-zero (NRZ), return-to-zero (RZ), chirped return-tozero (CRZ) and carrier suppressed return-to-zero (CSRZ) data formats in optical soliton transmission link under the impact of chirp and third-order dispersion (TOD). The performance of these data formats has been analyzed on the basis of certain performance metrics, viz, bit error rate (BER), Q2 (dB), OSNR, eye opening, etc. It has been reported here that the performance of CRZ and CSRZ modulation format is better as compared to NRZ and RZ in a soliton transmission link. Further, CSRZ modulation format has been found to deliver optimum performance on the basis of performance evaluation metrics reported in this paper. In case of NRZ and CSRZ, comparatively narrow power spectrum has been observed. Best eye opening, highest value of Q2 (dB) of 18 dB and lowest value of BER of the order of 1016 has been reported in case of CSRZ among the considered data formats. The results have been obtained by varying noise figure from 3.0 to 9.0. No considerable effect of noise was observed. It was observed that at very narrow and ultra short pulse width, OSNR value suffers heavily and reduced to even negative values in dB, thus inducing a high degree of OSNR power penalty. The results were obtained by varying chirp factor from 0.6 to +0.6. Negative chirp resulted in improved OSNR as compared to positive chirp. RZ data format yielded a broader optical spectrum, comparatively low spectral efficiency and poor OSNR thus it was found that RZ format is not suitable for optical soliton transmission under the impact of chirp and TOD. r 2009 Elsevier GmbH. All rights reserved. Keywords: Soliton; Q-factor; BER; Data format; Eye diagram
1. Introduction Owing to the rapid growth of capacity requirement for long-haul lightwave communication links, system Corresponding author. Tel.: +91 9872201740; fax: +91 181 2207650. E-mail addresses: [email protected] (J.S. Malhotra), [email protected] (M. Kumar).
0030-4026/$ - see front matter r 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2008.08.010
engineering is focused on maximizing the system capacity and minimizing the performance degradation caused by transmission impairments [1]. Signal modulation format is a key issue, which determines transmission quality and spectral efficiency. In order to maximize optical transmission link capacity, system design and optimization have to take into account all the contributing facts, such as channel data rate, transmission distance, signal optical power,
ARTICLE IN PRESS J.S. Malhotra, M. Kumar / Optik 121 (2010) 800–807
amplifier noise figure, channel wavelength spacing, optical amplifier spacing, fiber dispersion and non-linear parameters, dispersion management strategy, receiver bandwidth and so on [2–4]. One of the most important facts in the system, which affects the choices of all other system parameters, is the signal optical modulation format. In fact, signal optical spectral bandwidth, tolerance to chromatic dispersion, resistance to nonlinear effects, susceptibility to accumulated noise and other system performance measures are directly related to the optical modulation format [5–7]. For high-speed optical soliton communication, the data transmission reliability is degraded by the system impairments like GVD and fiber non-linearities. Until not a long time ago, non-return-to-zero (NRZ) had been the dominant optical modulation format in fiber–optic systems. The demand of high transmission capacity and better system reliability has fostered the recent research on advanced optical modulation formats [8]. Further, the performance of 1st and 2nd order pathaveraged soliton long haul transmission link has been investigated in [9] including the impact of third-order dispersion (TOD) at varied chirp. The observations establish that the pulse width (fwhm) remains within the optimal range without and up to certain discrete values of the chirp factor. The comparative investigation and suitability of various data formats for optical soliton transmission links at 10 Gb/s for different values of positive and negative chirps up to 0.7 has been reported for certain data formats viz. NRZ, return-tozero (RZ) soliton, RZ raised cosine and RZ super Gaussian in [10]. From system designer’s point of view, impairments in optical transmission need to be addressed. Moreover, how these affect the performance of the transmission link has to be investigated and ways to improve it have to be suggested. Therefore, it is important to investigate the robustness of various existing and new data formats on the performance of optical soliton transmission link. In this paper, we have analyzed the performance of NRZ, RZ, chirped return-to-zero (CRZ) and carrier suppressed return-to-zero (CSRZ) data formats in 10 Gb/s optical soliton transmission link under the impact of chirp and TOD. Modulation formats other than the conventional NRZ such as RZ, CRZ and CSRZ have been investigated for certain performance measures viz. Q factor, bit error rate (BER), eye opening, OSNR, etc. Optical soliton transmission link has been employed to further enhance the system capability as optical solitons have inherent capability of using fiber non-linearity to its advantage. The different data formats used by us present some notable differences in the context of data-rate/distance tradeoffs and other device-related parameters that affect transmission performance.
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2. Performance metrics 2.1. Q-factor Q-factor measures the quality of an analog transmission signal in terms of its signal-to-noise ratio (SNR). As such, it takes into account physical impairments to the signal viz. noise, chromatic dispersion and any polarization or non-linear effects which can degrade the signal and ultimately cause bit errors. In other words, the higher the value of Q factor the better the SNR and therefore the lower the probability of bit errors. m m0 (1) Q¼ 1 s1 þ s0
2.2. BER The BER can be estimated from Eq. (1), and requires Q46 for the BER of 109. This BER gives the upper limit for the signal because some degradation occurs at the receiver end. 1 Q expðQ2 =2Þ pffiffiffiffiffiffi (2) BER ¼ erfc pffiffiffi 2 2 Q 2p
2.3. Eye opening Considering only samples at the optimum sampling instant, it is the difference between the minimum value of the samples decided as logical ‘‘1’’ and the maximum value of the samples decided as logical ‘‘0’’. The unit of this measurement is equal to the unit of the electrical input signal (Fig. 1).
3. Modeling of data formats 3.1. NRZ Fig. 2 shows block diagram of optical soliton transmitter using NRZ modulation format. A bit pattern generator drives the electrical signal generator to feed external modulator, where electrical signal is
Fig. 1. Eye diagram.
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Fig. 2. NRZ transmission system.
Fig. 3. RZ transmission system.
modulated using a mode locked laser which converts an OOK electrical signal with a specified data rate into an OOK optical signal of the same data rate. This signal is then launched into an optical fiber after power normalization.
3.2. RZ Fig. 3 shows the optical soliton transmitter module with RZ modulated signal. The inside topology is the same as for NRZ signal except that electrical signal generator is set to RZ raised cosine format and pulse width is defined by parameter duty cycle instead of risefall time. All other parameters are the same as for NRZ.
Fig. 4. CRZ transmission system.
3.3. CRZ Fig. 4 shows the block diagram of an optical soliton transmitter using chirped return-to-zero data format. Signal is processed though two modulators. First modulator is set to amplitude modulation (rather than ‘Mach–Zehnder’ like in case of NRZ and RZ) with driving electrical signal set to RZ raised cosine modulated signal (with duty-cycle parameter defining pulse width). The second modulator is set to phase modulation (PM) and applies chirp to the signal.
3.4. CSRZ Fig. 5 shows the block diagram of an optical soliton transmitter using CSRZ data format. Here in order to create CSRZ signal, first the NRZ signal is generated by Mach–Zehnder modulator and the output is sent to the other MZ modulator block which is driven by sinusoidal signal. The drive frequency is a half of the bit rate and the amplitude is 2 Vp.
4. System description Figs. 2–5 show the block schematic of optical soliton transmitters designed for NRZ, RZ, CRZ and CSRZ data formats, respectively, under the impact of chirp and TOD. The simulation has been carried out using a
Fig. 5. CSRZ transmission system.
commercial package OptSimTM. Set-up has been modeled consisting of re-circulating loop, where each loop consists of 50 km long standard single mode fiber (SMF) and an optical amplifier (EDFA). Soliton pulses travel through total 10 loops or a transmission length of 500 km. SMF offers an attenuation a ¼ 0.2 dB/km with second- and TOD coefficients as 0.02 ps2/km and 0.179 ps3/km, respectively at 1550 nm. The transmitters modeled using various data formats were restricted to a negative chirp factor equal to 0.3 on account of optimum performance of the soliton transmission link. The value of non-linear refractive index is n2 ¼ 2.6 1020 m2/W, area (A) ¼ 60.31 mm2. The dominant non-linear fiber parameter considered during simulation is the Kerr non-linearity coefficient g ¼ n2 o0 =cAeff . Data source generates a binary sequence of data stream. A soliton source is used to generate pulses of ‘‘sech’’ shape with center emission wavelength
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of 1550 nm, peak power ¼ 56.9 mw and pulse width (fwhm) ¼ 25 ps. The pulses are then modulated using MZ modulator at 10 Gb/s bit rate. Dual-arm MZ modulator is used to modulate optical signal of desired format. PIN diode is used to detect the optical signal, i.e. conversion into electrical signal having characteristics quantum efficiency ¼ 0.8; responsivity (at reference frequency) 0.8751 A/W; 3 dB bandwidth 20 GHz; dark current 0.1 nA; reference wavelength 1550 nm and including quantum noise. BER tester and eye diagram analyzer, signal analyzer & spectrum analyzer were configured to observe the output signal and power spectrum.
5. Results and discussion The investigations have been carried out at a bit rate of 10 Gb/s in an optical soliton transmission link designed to yield a performance analysis of data formats viz. NRZ, RZ, CRZ and CSRZ under the impact of chirp and TOD. We have considered a chirp factor C ranging from 0.6 to +0.6. It is important to investigate the performance of various data formats in an optical soliton transmission link at varied chirp factor in the presence of TOD because the initial chirp can be detrimental to soliton propagation simply because it disturbs the exact balance between GVD and SPM. Chirp has generally been avoided in NRZ systems because it increases the optical
Fig. 6. Power spectrum for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ. Table 1.
BER Q2 (dB)
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bandwidth and hence the effect of GVD. However, recently PM prior to launch, which is similar to chirp, has been used as a counter measure against the deleterious effects of fiber non-linearity [9]. Fig. 6(a)–(d) presents the optical power spectrum obtained for the data formats viz. NRZ, RZ, CRZ and CSRZ. Peak power in all the cases has been observed at central wavelength, i.e. 1550 nm. The comparatively narrow spectrum has been observed in the case of NRZ and CSRZ. It was investigated that SPM effect results in spectral narrowing of optical spectrum for negatively chirped pulses. Thus the results for Q factor and BER were obtained at a chirp factor C ¼ 0.3. The resulting BER and Q2 values in respect of the data formats viz. NRZ, RZ, CRZ and CSRZ have been depicted in Table 1. Fig. 7(a)–(d) presents the eye diagrams obtained for the data formats viz. NRZ, RZ, CRZ and CSRZ. The eye tends to close in case of RZ whereas reasonably good eye opening has been observed in case of NRZ, but it is best in case of CRZ and CSRZ. Fig. 8(a)–(d) presents the plots of LAMP v/s BER at varied fwhm over a span of 50 km. For ultra short pulse width of fwhm ¼ 5 ps, BER increases exponentially with the increase in LAMP for all the data formats. In case of NRZ and RZ, almost constant BER of the order of 1010 and 106 has been observed at fwhm ¼ 25 and 45 ps, respectively. In case of CRZ, though lowest BER of the order of 1012 has been observed at fwhm ¼ 25 ps initially, but it steeply rises to 107 after 25 km. Although slightly high BER of 1010 has been observed at fwhm ¼ 45 ps, but it remains constant throughout the span of 50 km. In case of CSRZ, BER of 1014 and 1011 has been observed for fwhm ¼ 25 and 45 ps, respectively. Fig. 9(a)–(d) presents LAMP v/s Q2 at varied fwhm. The evident observation from the above plots can be seen that at fwhm ¼ 5 ps, gain fall exponentially for all the data formats. Almost constant Q2 (dB) of 16 and 14 has been observed for fwhm ¼ 25 and 45 ps, respectively in case of NRZ. In case of RZ, at fwhm ¼ 25 ps, initially Q2 (dB) value of 17 was observed which gradually drooped to 14 after 30 km span. However, it remained constant at 16 for fwhm ¼ 45 ps. For CRZ, Q2 (dB) values reported are 16 and 14, respectively, at fwhm ¼ 25 and 45 ps, respectively. Highest value of Q2 (dB), i.e. 18 among the considered formats has been observed in case of CSRZ at 25 ps and it is 16 at fwhm ¼ 45 ps.
BER and Q2 (dB) value for optical soliton transmission link with different data formats. NRZ
RZ
CRZ
CSRZ
3.5040e008 15
2.9052e006 14
3.6467e015 16
1.4350e016 17
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Fig. 7. Eye diagram for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 8. BER v/s LAMP plot at varied fwhm for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Since noise is inadvertently present in all the systems, dependence of BER on LAMP with variable noise for the aforesaid data formats has been plotted in Fig. 10(a)–(d). The results have been obtained by varying noise figure from 3.0 to 9.0. Similarly, effect of noise on Q2 has been studied by using Q2 v/s LAMP plot as shown in Fig. 11(a)–(d). No considerable effect of noise was observed up to 25 km; however, a slight increase in BER was observed after 25 km except in case of RZ which remained unaffected by noise. Similarly Q2 (dB) dropped by 1–3 dB with increase in noise figure from 3.0 to 9.0; however, NRZ had minimal effect of noise in this case. This indicates the appreciable noise immunity of the designed system.
Fig. 9. Q2 v/s LAMP plot at varied fwhm for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 10. BER v/s LAMP plot at varied noise figure for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 12(a)–(d) shows variation of OSNR during one span length at different values of pulse width. It can be seen from the above plots that OSNR varies between 30 and 15 dB over the span of 50 km in case of fwhm ¼ 45 ps except for RZ data format where the OSNR falls exponentially and even plunges below zero dB. As the pulse width of the optical soliton pulse is further narrowed to 25 ps and ultra short pulse width of 5 ps, OSNR value suffers heavily and reduces to even negative values in dB, thus inducing a high degree of OSNR power penalty. In order to investigate the variation in OSNR during one amplifier spacing LAMP under the impact of chirp, the results for the considered data formats have been
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Fig. 11. Q2 v/s LAMP plot for at varied noise figure for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 13. OSNR v/s LAMP plot at varied chirp for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 12. OSNR v/s LAMP plot at varied fwhm for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 14. OSNR v/s fwhm plot at varied chirp for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
plotted in Fig. 13(a)–(d) at varied chirps. Here the performance of various data formats in the optical soliton transmission link at various chirp factors has been investigated keeping in view that all practical sources of short optical pulses have a frequency chirp imposed on them. It has been investigated here that in general OSNR value decreases linearly over the span length for all data formats. Our results reveal that comparatively high value of OSNR is maintained at zero and negative values of chirp factor, i.e. Cp0. It was also established from our findings as observed from Fig. 13(b) that chirp has no effect on RZ data format, i.e. OSNR value remains same at all chirp factors throughout the span length. SPM effect on the rising and falling edges of a pulse results in spectral broadening for all values of induced
chirp [9]. Therefore, variation of OSNR at different values of pulse width under the influence of variable chirp for the aforesaid data formats has been shown in Fig. 14(a)–(d). The results have been obtained by varying chirp factor from 0.6 to +0.6. It is evident from the plots of Figs. 13(a)–(d) and 14(a)–(d) that negative chirp has resulted in improved OSNR as compared to positive chirp. At negative values of chirp, there is an increase in OSNR in respect of all mentioned data formats except RZ, when the pulse width is increased from ultra short pulse width of 5–45 ps. Our results reveal that the increase in pulse width in case of RZ does not result in increase of OSNR at all values of chirp, i.e. Cp0 and C40 as observed in case of other data formats such as NRZ, CRZ and CSRZ.
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6. Summary of results The major results obtained from this research work are summarized as follows: The optical power spectrums have been obtained for the data formats viz. NRZ, RZ, CRZ and CSRZ. Peak power in all the cases has been observed at central wavelength, i.e. 1550 nm. The comparatively narrow spectrum has been observed in the case of NRZ and CSRZ. It was investigated that SPM effect results in spectral narrowing of optical spectrum for negatively chirped pulses. Thus the results for Q factor and BER were obtained at a chirp factor C ¼ 0.3. The resulting BER and Q2 values with respect to the data formats, viz, NRZ, RZ, CRZ and CSRZ are of the order of 108, 106, 1015 and 1016 and 15, 14, 16 and 17 dB, respectively. The BER obtained for the data formats, viz, NRZ, RZ, CRZ and CSRZ has been verified using eye diagrams. The eye tends to close in case of RZ whereas reasonably good eye opening has been observed in case of NRZ, but it is best in case of CRZ and CSRZ. For ultra short pulse width of fwhm ¼ 5 ps, BER increases exponentially with the increase in LAMP for all the considered data formats. In case of NRZ and RZ, almost a constant BER of the order of 1010 and 106 has been observed at fwhm ¼ 25 and 45 ps, respectively. In case of CRZ, a low BER of the order of 1012 has been observed at fwhm ¼ 25 ps initially, but it steeply rises to 107 after 25 km. Slightly high BER of 1010 has been observed at fwhm ¼ 45 ps, but it remains constant throughout the span of 50 km. In case of CSRZ, BER of 1014 and 1011 has been observed for fwhm ¼ 25 and 45 ps, respectively. The gain falls exponentially for all the data formats at fwhm ¼ 5 ps. Almost constant Q2 (dB) of 16 and 14 has been observed for fwhm ¼ 25 and 45 ps, respectively in case of NRZ. In case of RZ, at fwhm ¼ 25 ps, initial value of Q2 (dB) was of 17, which gradually droped to 14 after 30 km span. However it remained constant at 16 for fwhm ¼ 45 ps. For CRZ, Q2 (dB) values reported are 16 and 14, respectively at fwhm ¼ 25 and 45 ps, respectively. Highest value of Q2 (dB), i.e. 18 among the considered formats has been observed in case of CSRZ at 25 ps. The effect of noise on these data formats has been investigated by varying noise figure from 3.0 to 9.0. No considerable effect of noise figure was observed upto 25 km, however a slight increase in BER was observed after 25 km except in case of RZ, which remained unaffected by noise. Similarly Q2 (dB) dropped by 1–3 dB with increase in noise figure from 3.0 to 9.0, however NRZ had minimal effect of noise in this case. OSNR varied between 30 and 15 dB over the span of 50 km with fwhm ¼ 45 ps except for RZ data format where the OSNR decreased exponentially and even plunged below zero dB. As the width of the optical soliton pulse was further narrowed to 25 ps and to an
ultra short pulse width of 5 ps, OSNR value suffered heavily and reduced to even negative values in dB, thus inducing a high degree of OSNR power penalty. The performance of various data formats in the optical soliton transmission link at various chirp factors was investigated keeping in view that all practical sources of short optical pulses have a frequency chirp imposed on them. It was observed that in general OSNR value decreased linearly over the span length for all data formats. Our results have revealed that comparatively high value of OSNR is maintained at zero and negative values of chirp factor, i.e. Cp0, however, no effect of chirp on RZ data format was seen, i.e. OSNR value remained same at all chirp factors throughout the span length. The variation of OSNR at different values of pulse width under the influence of variable chirp for the aforesaid data formats was investigated by varying chirp factor from 0.6 to +0.6. The negative chirp resulted in improved OSNR as compared to positive chirp. At negative values of chirp, an increase in OSNR with increase in pulse width for all mentioned data formats was observed except in case of RZ.
References [1] A.M. Odlyzko, Internet traffic growth: sources and implications, in: Proceedings of the Optical Transmission Systems and Equipment for WDM Networking II, Orlando, FL, USA, September 2003, doi:10.1117/12.512942. [2] P.J. Winzer, R.-J. Essiambre, Advanced optical modulation formats, In: Proceedings of the IEEE 94 (5) (2006) 952–985. [3] A.S. Siddiqui, S.G. Edirisinghe, J.J. Lepley, J.G. Ellison, S.D. Walker, Dispersion tolerant transmission using a duobinary encoded polarization shift keying modulation format, in: Optical Communication, ECOC’01, 27th European Conference, vol. 4, 30 September–4 October 2001, pp. 534–535. [4] I. Neokosmidis, T. Kamalakis, T. Sphicopoulos, Nonlinearity tolerance of optical modulation formats in nonzero dispersion fibers, Photonics Technology Letters, IEEE 17 (12) (2005) 2760–2762. [5] M. Jaworski, Optical modulation formats for high-speed DWDM systems, in: Transparent Optical Networks, Proceedings of 2003 Fifth International Conference, vol. 2, 29 June–3 July 2003, pp. 162–165. [6] G. Charlet, Progress in optical modulation formats for high-bit rate WDM transmissions, Selected Topics in Quantum Electronics, IEEE Journal 12 (4) (2006) 469–483. [7] S. Zhang, R. Hui, Impact of optical modulation formats on SPM-induced limitation in dispersion-managed optical systems-a simplified model, in: Advanced Modulation Formats, IEEE/LEOS Workshop, 2004, pp. 31–32. [8] M. Jaworski, Performance evaluation of multilevel optical modulation formats, in: Transparent Optical Networks, Proceedings of 2005 Seventh International Conference, vol. 2, 3–7 July 2005, pp. 389–392.
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[9] M. Kumar, et al., Significance of prechirping on long-haul path-averaged soliton impulse in re-circulating loop at 10 and 20 Gb/s with TOD, International Journal for Light and Electron Optics (2007), doi:10.1016/j.ijleo.2007.06.015.
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[10] M. Kumar, et al., Comparative investigation and suitability of various data formats for 10 Gb/s optical soliton transmission links at different, International Journal for Light and Electron Optics (2007), doi:10.1016/j.ijleo.2007.09.008.
Optik
Optics
Optik 121 (2010) 800–807 www.elsevier.de/ijleo
Performance analysis of NRZ, RZ, CRZ and CSRZ data formats in 10 Gb/s optical soliton transmission link under the impact of chirp and TOD Jagjit Singh Malhotra, Manoj Kumar DAV Institute of Engineering & Technology, Jalandhar, Punjab, India Received 8 April 2008; accepted 17 August 2008
Abstract This paper presents the performance analysis of non-return-to-zero (NRZ), return-to-zero (RZ), chirped return-tozero (CRZ) and carrier suppressed return-to-zero (CSRZ) data formats in optical soliton transmission link under the impact of chirp and third-order dispersion (TOD). The performance of these data formats has been analyzed on the basis of certain performance metrics, viz, bit error rate (BER), Q2 (dB), OSNR, eye opening, etc. It has been reported here that the performance of CRZ and CSRZ modulation format is better as compared to NRZ and RZ in a soliton transmission link. Further, CSRZ modulation format has been found to deliver optimum performance on the basis of performance evaluation metrics reported in this paper. In case of NRZ and CSRZ, comparatively narrow power spectrum has been observed. Best eye opening, highest value of Q2 (dB) of 18 dB and lowest value of BER of the order of 1016 has been reported in case of CSRZ among the considered data formats. The results have been obtained by varying noise figure from 3.0 to 9.0. No considerable effect of noise was observed. It was observed that at very narrow and ultra short pulse width, OSNR value suffers heavily and reduced to even negative values in dB, thus inducing a high degree of OSNR power penalty. The results were obtained by varying chirp factor from 0.6 to +0.6. Negative chirp resulted in improved OSNR as compared to positive chirp. RZ data format yielded a broader optical spectrum, comparatively low spectral efficiency and poor OSNR thus it was found that RZ format is not suitable for optical soliton transmission under the impact of chirp and TOD. r 2009 Elsevier GmbH. All rights reserved. Keywords: Soliton; Q-factor; BER; Data format; Eye diagram
1. Introduction Owing to the rapid growth of capacity requirement for long-haul lightwave communication links, system Corresponding author. Tel.: +91 9872201740; fax: +91 181 2207650. E-mail addresses: [email protected] (J.S. Malhotra), [email protected] (M. Kumar).
0030-4026/$ - see front matter r 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2008.08.010
engineering is focused on maximizing the system capacity and minimizing the performance degradation caused by transmission impairments [1]. Signal modulation format is a key issue, which determines transmission quality and spectral efficiency. In order to maximize optical transmission link capacity, system design and optimization have to take into account all the contributing facts, such as channel data rate, transmission distance, signal optical power,
ARTICLE IN PRESS J.S. Malhotra, M. Kumar / Optik 121 (2010) 800–807
amplifier noise figure, channel wavelength spacing, optical amplifier spacing, fiber dispersion and non-linear parameters, dispersion management strategy, receiver bandwidth and so on [2–4]. One of the most important facts in the system, which affects the choices of all other system parameters, is the signal optical modulation format. In fact, signal optical spectral bandwidth, tolerance to chromatic dispersion, resistance to nonlinear effects, susceptibility to accumulated noise and other system performance measures are directly related to the optical modulation format [5–7]. For high-speed optical soliton communication, the data transmission reliability is degraded by the system impairments like GVD and fiber non-linearities. Until not a long time ago, non-return-to-zero (NRZ) had been the dominant optical modulation format in fiber–optic systems. The demand of high transmission capacity and better system reliability has fostered the recent research on advanced optical modulation formats [8]. Further, the performance of 1st and 2nd order pathaveraged soliton long haul transmission link has been investigated in [9] including the impact of third-order dispersion (TOD) at varied chirp. The observations establish that the pulse width (fwhm) remains within the optimal range without and up to certain discrete values of the chirp factor. The comparative investigation and suitability of various data formats for optical soliton transmission links at 10 Gb/s for different values of positive and negative chirps up to 0.7 has been reported for certain data formats viz. NRZ, return-tozero (RZ) soliton, RZ raised cosine and RZ super Gaussian in [10]. From system designer’s point of view, impairments in optical transmission need to be addressed. Moreover, how these affect the performance of the transmission link has to be investigated and ways to improve it have to be suggested. Therefore, it is important to investigate the robustness of various existing and new data formats on the performance of optical soliton transmission link. In this paper, we have analyzed the performance of NRZ, RZ, chirped return-to-zero (CRZ) and carrier suppressed return-to-zero (CSRZ) data formats in 10 Gb/s optical soliton transmission link under the impact of chirp and TOD. Modulation formats other than the conventional NRZ such as RZ, CRZ and CSRZ have been investigated for certain performance measures viz. Q factor, bit error rate (BER), eye opening, OSNR, etc. Optical soliton transmission link has been employed to further enhance the system capability as optical solitons have inherent capability of using fiber non-linearity to its advantage. The different data formats used by us present some notable differences in the context of data-rate/distance tradeoffs and other device-related parameters that affect transmission performance.
801
2. Performance metrics 2.1. Q-factor Q-factor measures the quality of an analog transmission signal in terms of its signal-to-noise ratio (SNR). As such, it takes into account physical impairments to the signal viz. noise, chromatic dispersion and any polarization or non-linear effects which can degrade the signal and ultimately cause bit errors. In other words, the higher the value of Q factor the better the SNR and therefore the lower the probability of bit errors. m m0 (1) Q¼ 1 s1 þ s0
2.2. BER The BER can be estimated from Eq. (1), and requires Q46 for the BER of 109. This BER gives the upper limit for the signal because some degradation occurs at the receiver end. 1 Q expðQ2 =2Þ pffiffiffiffiffiffi (2) BER ¼ erfc pffiffiffi 2 2 Q 2p
2.3. Eye opening Considering only samples at the optimum sampling instant, it is the difference between the minimum value of the samples decided as logical ‘‘1’’ and the maximum value of the samples decided as logical ‘‘0’’. The unit of this measurement is equal to the unit of the electrical input signal (Fig. 1).
3. Modeling of data formats 3.1. NRZ Fig. 2 shows block diagram of optical soliton transmitter using NRZ modulation format. A bit pattern generator drives the electrical signal generator to feed external modulator, where electrical signal is
Fig. 1. Eye diagram.
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Fig. 2. NRZ transmission system.
Fig. 3. RZ transmission system.
modulated using a mode locked laser which converts an OOK electrical signal with a specified data rate into an OOK optical signal of the same data rate. This signal is then launched into an optical fiber after power normalization.
3.2. RZ Fig. 3 shows the optical soliton transmitter module with RZ modulated signal. The inside topology is the same as for NRZ signal except that electrical signal generator is set to RZ raised cosine format and pulse width is defined by parameter duty cycle instead of risefall time. All other parameters are the same as for NRZ.
Fig. 4. CRZ transmission system.
3.3. CRZ Fig. 4 shows the block diagram of an optical soliton transmitter using chirped return-to-zero data format. Signal is processed though two modulators. First modulator is set to amplitude modulation (rather than ‘Mach–Zehnder’ like in case of NRZ and RZ) with driving electrical signal set to RZ raised cosine modulated signal (with duty-cycle parameter defining pulse width). The second modulator is set to phase modulation (PM) and applies chirp to the signal.
3.4. CSRZ Fig. 5 shows the block diagram of an optical soliton transmitter using CSRZ data format. Here in order to create CSRZ signal, first the NRZ signal is generated by Mach–Zehnder modulator and the output is sent to the other MZ modulator block which is driven by sinusoidal signal. The drive frequency is a half of the bit rate and the amplitude is 2 Vp.
4. System description Figs. 2–5 show the block schematic of optical soliton transmitters designed for NRZ, RZ, CRZ and CSRZ data formats, respectively, under the impact of chirp and TOD. The simulation has been carried out using a
Fig. 5. CSRZ transmission system.
commercial package OptSimTM. Set-up has been modeled consisting of re-circulating loop, where each loop consists of 50 km long standard single mode fiber (SMF) and an optical amplifier (EDFA). Soliton pulses travel through total 10 loops or a transmission length of 500 km. SMF offers an attenuation a ¼ 0.2 dB/km with second- and TOD coefficients as 0.02 ps2/km and 0.179 ps3/km, respectively at 1550 nm. The transmitters modeled using various data formats were restricted to a negative chirp factor equal to 0.3 on account of optimum performance of the soliton transmission link. The value of non-linear refractive index is n2 ¼ 2.6 1020 m2/W, area (A) ¼ 60.31 mm2. The dominant non-linear fiber parameter considered during simulation is the Kerr non-linearity coefficient g ¼ n2 o0 =cAeff . Data source generates a binary sequence of data stream. A soliton source is used to generate pulses of ‘‘sech’’ shape with center emission wavelength
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of 1550 nm, peak power ¼ 56.9 mw and pulse width (fwhm) ¼ 25 ps. The pulses are then modulated using MZ modulator at 10 Gb/s bit rate. Dual-arm MZ modulator is used to modulate optical signal of desired format. PIN diode is used to detect the optical signal, i.e. conversion into electrical signal having characteristics quantum efficiency ¼ 0.8; responsivity (at reference frequency) 0.8751 A/W; 3 dB bandwidth 20 GHz; dark current 0.1 nA; reference wavelength 1550 nm and including quantum noise. BER tester and eye diagram analyzer, signal analyzer & spectrum analyzer were configured to observe the output signal and power spectrum.
5. Results and discussion The investigations have been carried out at a bit rate of 10 Gb/s in an optical soliton transmission link designed to yield a performance analysis of data formats viz. NRZ, RZ, CRZ and CSRZ under the impact of chirp and TOD. We have considered a chirp factor C ranging from 0.6 to +0.6. It is important to investigate the performance of various data formats in an optical soliton transmission link at varied chirp factor in the presence of TOD because the initial chirp can be detrimental to soliton propagation simply because it disturbs the exact balance between GVD and SPM. Chirp has generally been avoided in NRZ systems because it increases the optical
Fig. 6. Power spectrum for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ. Table 1.
BER Q2 (dB)
803
bandwidth and hence the effect of GVD. However, recently PM prior to launch, which is similar to chirp, has been used as a counter measure against the deleterious effects of fiber non-linearity [9]. Fig. 6(a)–(d) presents the optical power spectrum obtained for the data formats viz. NRZ, RZ, CRZ and CSRZ. Peak power in all the cases has been observed at central wavelength, i.e. 1550 nm. The comparatively narrow spectrum has been observed in the case of NRZ and CSRZ. It was investigated that SPM effect results in spectral narrowing of optical spectrum for negatively chirped pulses. Thus the results for Q factor and BER were obtained at a chirp factor C ¼ 0.3. The resulting BER and Q2 values in respect of the data formats viz. NRZ, RZ, CRZ and CSRZ have been depicted in Table 1. Fig. 7(a)–(d) presents the eye diagrams obtained for the data formats viz. NRZ, RZ, CRZ and CSRZ. The eye tends to close in case of RZ whereas reasonably good eye opening has been observed in case of NRZ, but it is best in case of CRZ and CSRZ. Fig. 8(a)–(d) presents the plots of LAMP v/s BER at varied fwhm over a span of 50 km. For ultra short pulse width of fwhm ¼ 5 ps, BER increases exponentially with the increase in LAMP for all the data formats. In case of NRZ and RZ, almost constant BER of the order of 1010 and 106 has been observed at fwhm ¼ 25 and 45 ps, respectively. In case of CRZ, though lowest BER of the order of 1012 has been observed at fwhm ¼ 25 ps initially, but it steeply rises to 107 after 25 km. Although slightly high BER of 1010 has been observed at fwhm ¼ 45 ps, but it remains constant throughout the span of 50 km. In case of CSRZ, BER of 1014 and 1011 has been observed for fwhm ¼ 25 and 45 ps, respectively. Fig. 9(a)–(d) presents LAMP v/s Q2 at varied fwhm. The evident observation from the above plots can be seen that at fwhm ¼ 5 ps, gain fall exponentially for all the data formats. Almost constant Q2 (dB) of 16 and 14 has been observed for fwhm ¼ 25 and 45 ps, respectively in case of NRZ. In case of RZ, at fwhm ¼ 25 ps, initially Q2 (dB) value of 17 was observed which gradually drooped to 14 after 30 km span. However, it remained constant at 16 for fwhm ¼ 45 ps. For CRZ, Q2 (dB) values reported are 16 and 14, respectively, at fwhm ¼ 25 and 45 ps, respectively. Highest value of Q2 (dB), i.e. 18 among the considered formats has been observed in case of CSRZ at 25 ps and it is 16 at fwhm ¼ 45 ps.
BER and Q2 (dB) value for optical soliton transmission link with different data formats. NRZ
RZ
CRZ
CSRZ
3.5040e008 15
2.9052e006 14
3.6467e015 16
1.4350e016 17
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Fig. 7. Eye diagram for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 8. BER v/s LAMP plot at varied fwhm for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Since noise is inadvertently present in all the systems, dependence of BER on LAMP with variable noise for the aforesaid data formats has been plotted in Fig. 10(a)–(d). The results have been obtained by varying noise figure from 3.0 to 9.0. Similarly, effect of noise on Q2 has been studied by using Q2 v/s LAMP plot as shown in Fig. 11(a)–(d). No considerable effect of noise was observed up to 25 km; however, a slight increase in BER was observed after 25 km except in case of RZ which remained unaffected by noise. Similarly Q2 (dB) dropped by 1–3 dB with increase in noise figure from 3.0 to 9.0; however, NRZ had minimal effect of noise in this case. This indicates the appreciable noise immunity of the designed system.
Fig. 9. Q2 v/s LAMP plot at varied fwhm for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 10. BER v/s LAMP plot at varied noise figure for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 12(a)–(d) shows variation of OSNR during one span length at different values of pulse width. It can be seen from the above plots that OSNR varies between 30 and 15 dB over the span of 50 km in case of fwhm ¼ 45 ps except for RZ data format where the OSNR falls exponentially and even plunges below zero dB. As the pulse width of the optical soliton pulse is further narrowed to 25 ps and ultra short pulse width of 5 ps, OSNR value suffers heavily and reduces to even negative values in dB, thus inducing a high degree of OSNR power penalty. In order to investigate the variation in OSNR during one amplifier spacing LAMP under the impact of chirp, the results for the considered data formats have been
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Fig. 11. Q2 v/s LAMP plot for at varied noise figure for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 13. OSNR v/s LAMP plot at varied chirp for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 12. OSNR v/s LAMP plot at varied fwhm for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
Fig. 14. OSNR v/s fwhm plot at varied chirp for (a) NRZ, (b) RZ, (c) CRZ and (d) CSRZ.
plotted in Fig. 13(a)–(d) at varied chirps. Here the performance of various data formats in the optical soliton transmission link at various chirp factors has been investigated keeping in view that all practical sources of short optical pulses have a frequency chirp imposed on them. It has been investigated here that in general OSNR value decreases linearly over the span length for all data formats. Our results reveal that comparatively high value of OSNR is maintained at zero and negative values of chirp factor, i.e. Cp0. It was also established from our findings as observed from Fig. 13(b) that chirp has no effect on RZ data format, i.e. OSNR value remains same at all chirp factors throughout the span length. SPM effect on the rising and falling edges of a pulse results in spectral broadening for all values of induced
chirp [9]. Therefore, variation of OSNR at different values of pulse width under the influence of variable chirp for the aforesaid data formats has been shown in Fig. 14(a)–(d). The results have been obtained by varying chirp factor from 0.6 to +0.6. It is evident from the plots of Figs. 13(a)–(d) and 14(a)–(d) that negative chirp has resulted in improved OSNR as compared to positive chirp. At negative values of chirp, there is an increase in OSNR in respect of all mentioned data formats except RZ, when the pulse width is increased from ultra short pulse width of 5–45 ps. Our results reveal that the increase in pulse width in case of RZ does not result in increase of OSNR at all values of chirp, i.e. Cp0 and C40 as observed in case of other data formats such as NRZ, CRZ and CSRZ.
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6. Summary of results The major results obtained from this research work are summarized as follows: The optical power spectrums have been obtained for the data formats viz. NRZ, RZ, CRZ and CSRZ. Peak power in all the cases has been observed at central wavelength, i.e. 1550 nm. The comparatively narrow spectrum has been observed in the case of NRZ and CSRZ. It was investigated that SPM effect results in spectral narrowing of optical spectrum for negatively chirped pulses. Thus the results for Q factor and BER were obtained at a chirp factor C ¼ 0.3. The resulting BER and Q2 values with respect to the data formats, viz, NRZ, RZ, CRZ and CSRZ are of the order of 108, 106, 1015 and 1016 and 15, 14, 16 and 17 dB, respectively. The BER obtained for the data formats, viz, NRZ, RZ, CRZ and CSRZ has been verified using eye diagrams. The eye tends to close in case of RZ whereas reasonably good eye opening has been observed in case of NRZ, but it is best in case of CRZ and CSRZ. For ultra short pulse width of fwhm ¼ 5 ps, BER increases exponentially with the increase in LAMP for all the considered data formats. In case of NRZ and RZ, almost a constant BER of the order of 1010 and 106 has been observed at fwhm ¼ 25 and 45 ps, respectively. In case of CRZ, a low BER of the order of 1012 has been observed at fwhm ¼ 25 ps initially, but it steeply rises to 107 after 25 km. Slightly high BER of 1010 has been observed at fwhm ¼ 45 ps, but it remains constant throughout the span of 50 km. In case of CSRZ, BER of 1014 and 1011 has been observed for fwhm ¼ 25 and 45 ps, respectively. The gain falls exponentially for all the data formats at fwhm ¼ 5 ps. Almost constant Q2 (dB) of 16 and 14 has been observed for fwhm ¼ 25 and 45 ps, respectively in case of NRZ. In case of RZ, at fwhm ¼ 25 ps, initial value of Q2 (dB) was of 17, which gradually droped to 14 after 30 km span. However it remained constant at 16 for fwhm ¼ 45 ps. For CRZ, Q2 (dB) values reported are 16 and 14, respectively at fwhm ¼ 25 and 45 ps, respectively. Highest value of Q2 (dB), i.e. 18 among the considered formats has been observed in case of CSRZ at 25 ps. The effect of noise on these data formats has been investigated by varying noise figure from 3.0 to 9.0. No considerable effect of noise figure was observed upto 25 km, however a slight increase in BER was observed after 25 km except in case of RZ, which remained unaffected by noise. Similarly Q2 (dB) dropped by 1–3 dB with increase in noise figure from 3.0 to 9.0, however NRZ had minimal effect of noise in this case. OSNR varied between 30 and 15 dB over the span of 50 km with fwhm ¼ 45 ps except for RZ data format where the OSNR decreased exponentially and even plunged below zero dB. As the width of the optical soliton pulse was further narrowed to 25 ps and to an
ultra short pulse width of 5 ps, OSNR value suffered heavily and reduced to even negative values in dB, thus inducing a high degree of OSNR power penalty. The performance of various data formats in the optical soliton transmission link at various chirp factors was investigated keeping in view that all practical sources of short optical pulses have a frequency chirp imposed on them. It was observed that in general OSNR value decreased linearly over the span length for all data formats. Our results have revealed that comparatively high value of OSNR is maintained at zero and negative values of chirp factor, i.e. Cp0, however, no effect of chirp on RZ data format was seen, i.e. OSNR value remained same at all chirp factors throughout the span length. The variation of OSNR at different values of pulse width under the influence of variable chirp for the aforesaid data formats was investigated by varying chirp factor from 0.6 to +0.6. The negative chirp resulted in improved OSNR as compared to positive chirp. At negative values of chirp, an increase in OSNR with increase in pulse width for all mentioned data formats was observed except in case of RZ.
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