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s WDM signals over NZDSFs
Optical Fiber Technology 9 (2003) 107–118 www.elsevier.com/locate/yofte
Performance verification of tera-bit transmissions using 30 ch × 40 Gb/s WDM signals over NZDSFs Yonghoon Kim,a Jaehoon Lee,a Yonggyoo Kim,a O. Mizuhara,b and Jichai Jeong a,∗ a Department of Radio Engineering, Korea University, 1,5 Ka, Anam-dong, Sungbuk-ku,
Seoul 136-701, South Korea b Agere Systems Inc., 9999 Hamilton Blud., Breinigsville, PA 1031, USA
Received 30 May 2002; revised 17 October 2002
Abstract We investigate the performance of Tbps transmissions using wavelength-division multiplexing (WDM) technologies based on the 40 Gbps line rate. To find the optimum conditions, the transmission performance is theoretically evaluated for various fiber types. To confirm modeling, the transmission performance is experimentally measured for TWF transmissions over 85 and 342 km. Bit error rates (BERs) and eye-diagrams have been calculated and measured to investigate the system performance. All the channels have characteristics of error free transmission. The calculated BERs and eye-diagrams have a good agreement with the experimental results. 2003 Elsevier Science (USA). All rights reserved.
1. Introduction There has been a strong interest in high capacity lightwave transmission systems and networks in recent. To increase data transmission capacities, several methods are investigated by adding more channels in the wavelength division multiplexing (WDM) system and enforcing the bit rate of existing channels using the time division multiplexing (TDM) [1,2]. Experimental demonstration of ultra high capacity systems has employed optical amplifiers with wide bandwidths and/or the optical duo-binary modulation, while using data rates of 10 to 40 Gb/s for each WDM channel [3,4]. A lot of works have already been done like 40 Gb/s soliton transmissions using a dispersion shifted fiber (DSF) [5], and TDM/WDM transmissions using SMF [6]. * Corresponding author.
E-mail address: [email protected] (J. Jeong). 1068-5200/03/$ – see front matter 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S1068-5200(03)00002-6
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As the data capacity is increased, the fiber type is an important issue due to the chromatic dispersion, polarization-mode dispersion, and the nonlinear effects, such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS). The effects of various fiber types have been investigated for the chromatic dispersion and the nonlinear effects in 40 Gb/s systems [7]. The problems and solutions of the polarization mode dispersion (PMD) of 40 Gb/s optical transmission systems were studied [8]. The analysis of design and effects was performed in the dispersion management [9]. Over the world, much of fibers installed are the conventional SMF, and the demand for capacities in the short haul networks such a metropolitan area network is as great as the long haul network. Having a large core area, compared to other fibers, the SMF is very tolerant of the nonlinear effect of fibers. However, due to high dispersion at 1.55 µm windows, it is not moderate in high data rate transmissions, such as more than 20 Gb/s, without dispersion compensation. On the other hand, the nonzero dispersion shifted fiber (NZDSF) is attractive in high data rate systems due to a low dispersion value. Since NZDSF has a small core effective area, however, it is weak to the nonlinear effects of fibers. In new terrestrial and transoceanic fiber installation, the fiber tolerance for fiber nonlinear effects, the chromatic dispersion and PMD are important factors with cost efficiency in new fiber selection. Then the comparison of two fibers, SMF and NZDSF, is essential in 10 Gb/s and higher data rate WDM systems. In this paper, we theoretically investigate the effects of dispersion and nonlinear characteristics for various fiber types to compare transmission performances using SMF and NZDSF in 40 Gb/s WDM systems. First, we modeled the fiber to determine transmission characteristics. Using the modeled fiber including the chromatic dispersion and the nonlinear effects including SRS effect, the transmission performances are evaluated as a function of dispersion compensation methods. BERs and eye-diagrams are calculated to estimate the system performance using modeled fibers over 320 km SMF or NZDSF in 4 ch × 40 Gb/s WDM systems. We report the transmission performance of 30-ch transmission over 85 km Truewave fiber (TWF), using an 40 Gb/s data rate for each channel. In the same conditions, the system performance of 25 ch × 40 Gb/s WDM systems over 342 km transmission was evaluated, and the simulation results are compared to the experimental results. Section 2 describes the modeling of fiber characteristics, transmitters, receivers, and BER characteristics. The dispersion compensation methods are investigated for various fiber types. In Section 3, the simulation and the experiment is presented using TWF in 40 Gb/s WDM system. The results of experiments and simulation are investigated and compared from the calculated BER and eye-diagram. Finally, the conclusions are presented in Section 4.
2. Theory of optical transmission over fibers 2.1. Optical transmission over fibers Optical signal transmissions through the optical fiber are considered to be lossy, dispersive, and nonlinear. The evolution of optical signals can be obtained from the nonlinear
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Schrödinger equation [6]. Equation (1) includes the loss, dispersion, and nonlinear effects, such as SPM, XPM, FWM, and SRS, i ∂ 2 Am 1 ∂ 3 Am αm ∂Am ∂Am + β1 + β2 Am − β3 + ∂z ∂t 2 6 2 ∂t 2 ∂t 3 N 2 = iγ |Am | + 2 |An |2 Am n=1, n=m
m−1 N gmn ωm gmn + |An | − |An | Am , 2Aeff ωn 2Aeff n=1
(1)
n=m+1
where Am is a slowly varying amplitude of the propagating waves, gmn is the Raman gain coefficient between channels, β1 is the inverse group velocity, β2 and β3 the first- and second-order group velocity dispersions, a the absorption coefficient, and γ (= N2 ω0 /cAeff ) the nonlinearity coefficient (N2 is the nonlinear coefficient and Aeff is the effective core area). The pulse envelope A is normalized such that |A|2 represents the optical power. SPM, XPM, and FWM are modeled by the nonlinear Schrödinger equation. The left terms on the right-hand side term of Eq. (1) result from the nonlinear refractive index. The first term leads to SPM, and the second term results in XPM. Except for the SRS effect, the general solution of the nonlinear differential equation (1) can be expressed as z −αz (2) exp Am (z, t) = Am 0, t − exp iφm (z, t) , vgm 2 where
1 − e−αz z z 2 0, t − 0, t − A A m m α vgm vgm 2 z −αz z + 2 An 0, t − + dmn z e dz vgm
φm (z, t) = γm
(3)
0
is the phase shift caused by SPM and XPM, and dmn is the walk-off parameter. From Eq. (3), the phase shift of signals caused by SPM and XPM is calculated. The right term on the right-hand side of Eq. (1) describes the SRS effect. In relation to the Raman gain coefficient and channel spacing, the amount power shift is determined among the transmitted signals. The nonlinear Schrödinger equation of Eq. (1) is a nonlinear partial differential equation that does not generally lend itself to analytic solution. This equation can be solved by the split-step Fourier method [10]. To obtain the accurate solution, the step size can be adjusted in relative to a passed optical power through fibers. 2.2. Calculation of BER characteristics To calculate BERs in optical transmission systems, a transmitter based on LiNbO3 modulators was modeled in the raised cosine pulseshape with considering the extinction
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ratio and frequency chirp. The extinction ratio used in our simulation was about 12 dB. We used the chirp parameter (α) expressed by the method proposed by Koyama and Iga [11]. Erbium-doped fiber amplifier (EDFA) preamplifier and PIN receivers are modeled by a very effective way. They can provide to improve the receiver sensitivity by optically amplifying the incoming signal before the signal is incident on a photodetector [12,13]. Including the inter-symbol interference (ISI) effects, the BER characteristics are calculated by averaging the probability of errors per bit. Since the bandwidth of the receiver filter is much smaller than the optical bandwidth, beat noises can be approximated to the Gaussian statistics [14]. Thus, BER characteristics for a single bit can be calculated [15,16]. The bandwidth of optical filter and electrical filter affects the BER characteristics. To obtain the best BER curve, the bandwidth of electrical filter has a bandwidth of 0.6 × bit rate in binary signal [17]. Besides, to reduce the ASE noise, the bandwidth of optical filter is adjusted. 2.3. Estimated transmission performance due to the dispersion compensation method To determine the method of dispersion compensation in the transmission experiment, the following simulation is performed. The simulation setup is shown in Fig. 1. The signals are multiplexed in optical multiplexer (OMUX). The multiplexed signals were modulated at 20 Gb/s with 27 pseudo-random bit sequence (PRBS) using a dual-drive LiNbO3 modulator. The polarization controller was used to align the polarization of each signal. The
Fig. 1. Schematic diagram of SMF and NZDSF (LEAF, TWF, TWF-RS) over 320 km transmission with 4 ch × 40 Gb/s system.
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gain-flattened EDFA was used by booster-amplifiers. The return-to-zero (RZ) converter transformed the modulated signal to the RZ format. 40 Gb/s signals were generated from the combination of RZ signals and delayed RZ signals by 25 × 7 ps. 4 ch × 40 Gb/s signals were transmitted over 320 km SMF and NZDSF, such as large area fiber (LEAF), TWF, and Truewave fiber with reduced slope (TWF-RS). The first channel wavelength was 1550 nm and a span length was 80 km. The channel spacing was 0.8 nm and fiber launching power was 3 or 6 dB m. Two dispersion compensation methods using the post-compensation and the pre-compensation were compared. In SMF case, the dispersion is compensated for each span. In NZDSF case, however, due to low dispersion of NZDSF, the dispersion is compensated after four spans for the cost efficiency. The received signal was demultiplexed with a tunable bandpass filter with a 3 dB bandwidth of 0.3 nm. The transmission performance is estimated by BER characteristics. Table 1 gives the characteristics of the fiber used in the simulation. When the fiber launching power is 3 dB m, the calculated BER is shown in Fig. 2a. In SMF + DCF case, the post-compensation is better transmission performance than the preTable 1 Fiber characteristics for SMF, DCF, and NZDSF (LEAF, TWF, TWF-RS)
SMF (Corning) DCF LEAF (Corning) TWF (Lucent) TWRSF (Lucent)
Attenuation (α: dB/km)
Dispersion at 1550 nm (D: ps/nm km)
Dispersion slope at 1550 nm (D : ps/nm2 km)
Core area (Aeff : µm2 )
0.22 0.59 0.22 0.22 0.22
16.98 −86.59 4.3 3.6 4.3
0.059 −0.135 0.1 0.07 0.045
80 35 72 55 55
Fig. 2. Calculated BER curve in relation to the dispersion compensation type over 320 km transmission with 4 ch × 40 Gb/s system: open symbol, pre-compensation; closed symbol, post-compensation.
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compensation. The difference of BER curve, however, is very small. In pre-compensation case, the signals passed are distorted when it passed through the DCF. These distorted signals enter to SMF and are more affected by SPM in SMF. But the dispersion of the undistorted signal can be compensated by SPM in the post-compensation. In NZDSF + DCF case, the opposite result is occurred due to SPM increased by a small core area of NZDSF. When the fiber launching power is 6 dB m, the calculated BER is shown in Fig. 2b. In SMF + DCF case, the post-compensation is better than the pre-compensation. The reason is the same as the case of 3 dB m launching power. In NZDSF + DCF case, the transmission performances are changed for fiber types of NZDSF. In the LEAF case, the same transmission performance can be estimated. In the case of TWF and TWF-RS, however, the performance is degraded using the pre-compensation due to the strong XPM effect. This is due to the difference of core area in NZDSF. Since the LEAF has a large core area (72 µm2) compared to the other NZDSFs, the influence of XPM is weak compared to the other NZDSFs. Because the effective core area of TWF and TWF-RS is small (55 µm2), the XPM effect is dominant because the XPM effect is inversely proportional to core area. From the above simulation results, we found that the post-compensation is better than the pre-compensation except for the case of NZDSF + DCF with 3 dB m launching power. So, we considered only the post-compensation in our experiments.
3. Experimental and simulation results of Tbps transmissions 3.1. Configuration of transmission systems The experimental setup of a 40 Gb/s based on WDM transmission systems is shown in Fig. 3. The wavelength ranges are from 1534.5 to 1563.5 nm for 85 km transmission and from 1536.7 to 1560.7 nm for 342 km transmission. The signals were multiplexed in optical multiplexer (OMUX). The multiplexed signals were modulated at 20 Gb/s with 223 − 1 PRBS using a dual-drive LiNbO3 modulator. The gain-flattened EDFA was used as a booster-amplifier. The return-to-zero (RZ) converter transformed the modulated signal to the RZ format. 40 Gb/s signals were generated from the combination of RZ signal and delayed RZ signals by 25 × 7 ps. After de-correlating the signals using DCF, the 40 Gb/s signal was transmitted by TWF with 342 km length. The total system is composed of 4 spans (85 km × 2, 86 km × 2). To amplify the signal, the EDFA is inserted in the each span. To obtain a sufficient optical signal-to-noise ratio (OSNR), the fiber launching power was 0 dB m in 85 km transmission and 5 dB m in 342 km transmission per each channel. Each span loss is about 20 dB including connector loss. All in-line EDFAs have the output power of 20 dB m and the gain flatness of 1 dB in signal wavelength region. The dispersion of transmitted signals is compensated by DCF with the negative slope. After amplifying the signal using a gain flattened EDFA, the received signal was demultiplexed with a tunable bandpass filter with the 3 dB bandwidth of 0.3 nm. The 40 Gb/s received signal was demultiplexed optically into 20 Gb/s data, then received by 20 Gb/s 3R receiver. The recovered data at 20 Gb/s were then electronically demultiplexed into two 10 Gb/s signals in the receiver for BER measurements. The polarization controller is used to align the
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Fig. 3. Schematic diagram of TWF (342 km) transmissions with 25 ch × 40 Gb/s system using WDM and TDM method.
polarization of each signal. The polarization multiplexing of even/odd channels is not used for 20 Gb/s channels. 3.2. Transmission performance of 30 ch × 40 Gb/s WDM signals using TWF over 85 km Figure 4 shows the calculated and measured eye-diagrams at 1534.5 and 1563.5 nm wavelength. We found that the eye-diagrams of both side channels were opened clearly. Though the total dispersion was set to zero at the center channel and residual dispersion remained except for the center channel, we found that all the channels have clear open eyes. The calculated eye-diagram has a good agreement with the measured eye-diagram. Figure 5 shows calculated BER curves for all the WDM channels. The BER characteristics of the center channels are worse than the side channels. It means that XPM and FWM effects are stronger than dispersion effect in systems. The error free transmission was obtained for all channels. The receiver sensitivity at 10−9 BER is shown in Fig. 6. The range of measured and calculated receiver sensitivities is from −23 to −25 dB m. The simulation results have a small discrepancy compared to experimental results. Because shorter PRBS length was used and PMD effect was neglected in simulation, the calculated receiver sensitivity has better performance than the measured one in Fig. 6. The experimental points are much
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(a) Fig. 4. Calculated and measured eye-diagrams over TWF (85 km) transmissions with 30 ch × 40 Gb/s system for (a) channel 1 and (b) channel 30.
more random than the calculated points. This may result from the instability of CDR in receiver to sample and extract 10 Gb/s signals form incoming 20 Gb/s signals. However, the difference is a very small and the trend of receiver sensitivity is very similar in between the measured and calculated results. 3.3. Transmission performance of 25 ch × 40 Gb/s WDM signals using TWF + DCF over 342 km Figure 7 shows the calculated BER curves for all channels. Due to the residual dispersion and nonlinear effects of fibers, the receiver sensitivity is changed from −26.5 to −23 dB m. Since the side channel has a high-accumulated dispersion unlikely the 85 km transmission due to the increased TWF length, the transmission performance of the side channels is worse than the center channels. The measured BER characteristics are shown Fig. 8. The measured BERs are similar to the calculated ones. In 342 km transmission, all the channels have characteristics of error free transmissions both the calculated and measured BERs.
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(b) Fig. 4. (Continued.)
Fig. 5. Calculated BER characteristics of TWF (85 km) transmissions with 30 ch × 40 Gb/s system.
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Fig. 6. Calculated and measured receiver sensitivities (BER = 10−9 ) of TWF (85 km) transmission with 30 ch × 40 Gb/s system.
Fig. 7. Calculated BER characteristics of TWF (342 km) transmissions with 25 ch × 40 Gb/s system.
4. Conclusions We investigated the transmission performance depending on various fiber types in 40 Gb/s transmission system. From the modeling of transmitter, fiber, and BER characteristics, with considering the frequency chirp, extinction ratio, ISI, and nonlinear effects in fiber, the effect of dispersion compensation was calculated. The post-compensation has a better performance than the pre-compensation except for 3 dB m fiber launching power
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Fig. 8. Measured BER characteristics of TWF (342 km) with 25 ch × 40 Gb/s transmissions system.
in NZDSF. NZDSF has better performance due to maintaining high OSNR caused by less EDFAs required for the same transmission distance in 40 Gb/s transmission, compared to SMF. We calculated and measured the transmission performance of 30 ch × 40 Gb/s data rate over 85 km and 25 ch × 40 Gb/s over 342 km of TWF using WDM and TDM method. The simulated eye-diagrams and BER characteristics had a good agreement with the experimental ones. The experimental and simulated BER data for each WDM channel was shown no sign of an error floor.
Acknowledgment This work was supported in part by the National Research Laboratory Program and the BK21 program in South Korea.
References [1] G.E. Keiser, A review of WDM technology and applications, Optical Fiber Technol. 5 (1) (1999) 3–39. [2] Y. Miyamoto, T. Kataoka, M. Tomizawa, Y. Yamane, 40-Gbit/s ETDM channel technologies for optical transport network, Optical Fiber Technol. 7 (4) (2001) 289–311. [3] S. Aisawa, T. Sakamoto, M. Fukui, J. Kani, M. Jinno, K. Oguchi, Ultra-wideband, long distance WDM demonstration of 1 Tbit/s (50/spl times/20 Gbit/s), 600 km transmission using 1550 and 1580 nm wavelength bands, Electron. Lett. 34 (1998) 1127–1129. [4] M. Murakami, K. Suzuki, H. Maeda, T. Takahashi, A. Naka, N. Ohkawa, M. Aiki, High-speed TDM-WDM techniques for long-haul submarine optical amplifier systems, Optical Fiber Technol. 3 (4) (1997) 320–338. [5] K. Suzuki, H. Kubota, A. Sahara, M. Nakazawa, 40 Gbit/s single channel optical solition transmission over 70,000 km using in-line modulation and optical filtering, Electron. Lett. 34 (1) (1998) 98–99. [6] W. Pieper, R. Ludwig, C.M. Weinert, B. Kuhlow, G. Przyrembel, M. Ferstl, E. Pawlowski, H.G. Weber, 4-channel × 40 Gb/s unrepeatered OTDM transmission over 100-km standard fiber, IEEE Photon. Technol. Lett. 10 (1998) 451–453.
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[7] D. Breuer, H.J. Ehrke, F. Küppers, R. Ludwig, K. Petermann, H.G. Weber, K. Weich, Unrepeated 40-Gb/s RZ single-channel transmission at 1.55 µm using various fiber types, IEEE Photon. Technol. Lett. 10 (1998) 822–824. [8] D. Sandol, M. Yoshida-Dierolf, R. Noé, A. Schöpflin, E. Gottwald, G. Fischer, Automatic polarization mode dispersion compensation in 40 Gbit/s optical transmission system, Electron. Lett. 34 (23) (1998) 2258– 2259. [9] S.K. Turitsyn, E.G. Shapiro, Variational approach to the design of optical communication systems with dispersion management, Optical Fiber Technol. 4 (2) (1998) 151–188. [10] G.P. Agrawal, Nonlinear Fiber Optics, Academic Press, New York, 1989. [11] F. Koyama, K. Iga, Frequency chirping in external modulators, J. Lightwave Technol. 6 (1988) 87–92. [12] C.R. Gilles, E. Desurvire, J.L. Zyskind, J.R. Simpson, Noise performance of Er-doped fiber amplifier pumped at 1.49 um and application to signal preamplification at 1.8 Gb/s, IEEE Photon. Technol. Lett. 1 (1989) 367–369. [13] S.K. Kim, J. Jeong, Transmission performance on frequency response of receivers and chirping shape of transmitters for 10 Gb/s LiNbO3 modulator based lightwave systems, Optics Comm. 175 (2000) 109–123. [14] R.A.A. Lima, M.C.R. Carvalho, L.F.M. Conrado, On the simulation of digital optical links with EDFA’s: an accurate method for estimating BER through Gaussian approximation, IEEE J. Select. Topics Quantum Electron. 3 (1997) 1037–1044. [15] J. Jeong, Y.K. Park, S.K. Kim, T.V. Nguyen, O. Mizuhara, T. Oh, 10-Gb/s transmission performance for positive- and negative-chirped transmission with the self-phase modulation effect, IEEE Photon. Technol. Lett. 10 (1998) 1307–1309. [16] Y.K. Park, S.W. Granlund, Optical preamplifier receivers: application to long-haul digital transmission, Optical Fiber Technol. 1 (1994) 59–71. [17] X. Zheng, F. Liu, P. Jeppesen, Receiver optimization for 40-Gb/s optical duobinary signal, IEEE Photon. Technol. Lett. 13 (2001) 744–746.
Performance verification of tera-bit transmissions using 30 ch × 40 Gb/s WDM signals over NZDSFs Yonghoon Kim,a Jaehoon Lee,a Yonggyoo Kim,a O. Mizuhara,b and Jichai Jeong a,∗ a Department of Radio Engineering, Korea University, 1,5 Ka, Anam-dong, Sungbuk-ku,
Seoul 136-701, South Korea b Agere Systems Inc., 9999 Hamilton Blud., Breinigsville, PA 1031, USA
Received 30 May 2002; revised 17 October 2002
Abstract We investigate the performance of Tbps transmissions using wavelength-division multiplexing (WDM) technologies based on the 40 Gbps line rate. To find the optimum conditions, the transmission performance is theoretically evaluated for various fiber types. To confirm modeling, the transmission performance is experimentally measured for TWF transmissions over 85 and 342 km. Bit error rates (BERs) and eye-diagrams have been calculated and measured to investigate the system performance. All the channels have characteristics of error free transmission. The calculated BERs and eye-diagrams have a good agreement with the experimental results. 2003 Elsevier Science (USA). All rights reserved.
1. Introduction There has been a strong interest in high capacity lightwave transmission systems and networks in recent. To increase data transmission capacities, several methods are investigated by adding more channels in the wavelength division multiplexing (WDM) system and enforcing the bit rate of existing channels using the time division multiplexing (TDM) [1,2]. Experimental demonstration of ultra high capacity systems has employed optical amplifiers with wide bandwidths and/or the optical duo-binary modulation, while using data rates of 10 to 40 Gb/s for each WDM channel [3,4]. A lot of works have already been done like 40 Gb/s soliton transmissions using a dispersion shifted fiber (DSF) [5], and TDM/WDM transmissions using SMF [6]. * Corresponding author.
E-mail address: [email protected] (J. Jeong). 1068-5200/03/$ – see front matter 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S1068-5200(03)00002-6
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As the data capacity is increased, the fiber type is an important issue due to the chromatic dispersion, polarization-mode dispersion, and the nonlinear effects, such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS). The effects of various fiber types have been investigated for the chromatic dispersion and the nonlinear effects in 40 Gb/s systems [7]. The problems and solutions of the polarization mode dispersion (PMD) of 40 Gb/s optical transmission systems were studied [8]. The analysis of design and effects was performed in the dispersion management [9]. Over the world, much of fibers installed are the conventional SMF, and the demand for capacities in the short haul networks such a metropolitan area network is as great as the long haul network. Having a large core area, compared to other fibers, the SMF is very tolerant of the nonlinear effect of fibers. However, due to high dispersion at 1.55 µm windows, it is not moderate in high data rate transmissions, such as more than 20 Gb/s, without dispersion compensation. On the other hand, the nonzero dispersion shifted fiber (NZDSF) is attractive in high data rate systems due to a low dispersion value. Since NZDSF has a small core effective area, however, it is weak to the nonlinear effects of fibers. In new terrestrial and transoceanic fiber installation, the fiber tolerance for fiber nonlinear effects, the chromatic dispersion and PMD are important factors with cost efficiency in new fiber selection. Then the comparison of two fibers, SMF and NZDSF, is essential in 10 Gb/s and higher data rate WDM systems. In this paper, we theoretically investigate the effects of dispersion and nonlinear characteristics for various fiber types to compare transmission performances using SMF and NZDSF in 40 Gb/s WDM systems. First, we modeled the fiber to determine transmission characteristics. Using the modeled fiber including the chromatic dispersion and the nonlinear effects including SRS effect, the transmission performances are evaluated as a function of dispersion compensation methods. BERs and eye-diagrams are calculated to estimate the system performance using modeled fibers over 320 km SMF or NZDSF in 4 ch × 40 Gb/s WDM systems. We report the transmission performance of 30-ch transmission over 85 km Truewave fiber (TWF), using an 40 Gb/s data rate for each channel. In the same conditions, the system performance of 25 ch × 40 Gb/s WDM systems over 342 km transmission was evaluated, and the simulation results are compared to the experimental results. Section 2 describes the modeling of fiber characteristics, transmitters, receivers, and BER characteristics. The dispersion compensation methods are investigated for various fiber types. In Section 3, the simulation and the experiment is presented using TWF in 40 Gb/s WDM system. The results of experiments and simulation are investigated and compared from the calculated BER and eye-diagram. Finally, the conclusions are presented in Section 4.
2. Theory of optical transmission over fibers 2.1. Optical transmission over fibers Optical signal transmissions through the optical fiber are considered to be lossy, dispersive, and nonlinear. The evolution of optical signals can be obtained from the nonlinear
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Schrödinger equation [6]. Equation (1) includes the loss, dispersion, and nonlinear effects, such as SPM, XPM, FWM, and SRS, i ∂ 2 Am 1 ∂ 3 Am αm ∂Am ∂Am + β1 + β2 Am − β3 + ∂z ∂t 2 6 2 ∂t 2 ∂t 3 N 2 = iγ |Am | + 2 |An |2 Am n=1, n=m
m−1 N gmn ωm gmn + |An | − |An | Am , 2Aeff ωn 2Aeff n=1
(1)
n=m+1
where Am is a slowly varying amplitude of the propagating waves, gmn is the Raman gain coefficient between channels, β1 is the inverse group velocity, β2 and β3 the first- and second-order group velocity dispersions, a the absorption coefficient, and γ (= N2 ω0 /cAeff ) the nonlinearity coefficient (N2 is the nonlinear coefficient and Aeff is the effective core area). The pulse envelope A is normalized such that |A|2 represents the optical power. SPM, XPM, and FWM are modeled by the nonlinear Schrödinger equation. The left terms on the right-hand side term of Eq. (1) result from the nonlinear refractive index. The first term leads to SPM, and the second term results in XPM. Except for the SRS effect, the general solution of the nonlinear differential equation (1) can be expressed as z −αz (2) exp Am (z, t) = Am 0, t − exp iφm (z, t) , vgm 2 where
1 − e−αz z z 2 0, t − 0, t − A A m m α vgm vgm 2 z −αz z + 2 An 0, t − + dmn z e dz vgm
φm (z, t) = γm
(3)
0
is the phase shift caused by SPM and XPM, and dmn is the walk-off parameter. From Eq. (3), the phase shift of signals caused by SPM and XPM is calculated. The right term on the right-hand side of Eq. (1) describes the SRS effect. In relation to the Raman gain coefficient and channel spacing, the amount power shift is determined among the transmitted signals. The nonlinear Schrödinger equation of Eq. (1) is a nonlinear partial differential equation that does not generally lend itself to analytic solution. This equation can be solved by the split-step Fourier method [10]. To obtain the accurate solution, the step size can be adjusted in relative to a passed optical power through fibers. 2.2. Calculation of BER characteristics To calculate BERs in optical transmission systems, a transmitter based on LiNbO3 modulators was modeled in the raised cosine pulseshape with considering the extinction
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ratio and frequency chirp. The extinction ratio used in our simulation was about 12 dB. We used the chirp parameter (α) expressed by the method proposed by Koyama and Iga [11]. Erbium-doped fiber amplifier (EDFA) preamplifier and PIN receivers are modeled by a very effective way. They can provide to improve the receiver sensitivity by optically amplifying the incoming signal before the signal is incident on a photodetector [12,13]. Including the inter-symbol interference (ISI) effects, the BER characteristics are calculated by averaging the probability of errors per bit. Since the bandwidth of the receiver filter is much smaller than the optical bandwidth, beat noises can be approximated to the Gaussian statistics [14]. Thus, BER characteristics for a single bit can be calculated [15,16]. The bandwidth of optical filter and electrical filter affects the BER characteristics. To obtain the best BER curve, the bandwidth of electrical filter has a bandwidth of 0.6 × bit rate in binary signal [17]. Besides, to reduce the ASE noise, the bandwidth of optical filter is adjusted. 2.3. Estimated transmission performance due to the dispersion compensation method To determine the method of dispersion compensation in the transmission experiment, the following simulation is performed. The simulation setup is shown in Fig. 1. The signals are multiplexed in optical multiplexer (OMUX). The multiplexed signals were modulated at 20 Gb/s with 27 pseudo-random bit sequence (PRBS) using a dual-drive LiNbO3 modulator. The polarization controller was used to align the polarization of each signal. The
Fig. 1. Schematic diagram of SMF and NZDSF (LEAF, TWF, TWF-RS) over 320 km transmission with 4 ch × 40 Gb/s system.
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gain-flattened EDFA was used by booster-amplifiers. The return-to-zero (RZ) converter transformed the modulated signal to the RZ format. 40 Gb/s signals were generated from the combination of RZ signals and delayed RZ signals by 25 × 7 ps. 4 ch × 40 Gb/s signals were transmitted over 320 km SMF and NZDSF, such as large area fiber (LEAF), TWF, and Truewave fiber with reduced slope (TWF-RS). The first channel wavelength was 1550 nm and a span length was 80 km. The channel spacing was 0.8 nm and fiber launching power was 3 or 6 dB m. Two dispersion compensation methods using the post-compensation and the pre-compensation were compared. In SMF case, the dispersion is compensated for each span. In NZDSF case, however, due to low dispersion of NZDSF, the dispersion is compensated after four spans for the cost efficiency. The received signal was demultiplexed with a tunable bandpass filter with a 3 dB bandwidth of 0.3 nm. The transmission performance is estimated by BER characteristics. Table 1 gives the characteristics of the fiber used in the simulation. When the fiber launching power is 3 dB m, the calculated BER is shown in Fig. 2a. In SMF + DCF case, the post-compensation is better transmission performance than the preTable 1 Fiber characteristics for SMF, DCF, and NZDSF (LEAF, TWF, TWF-RS)
SMF (Corning) DCF LEAF (Corning) TWF (Lucent) TWRSF (Lucent)
Attenuation (α: dB/km)
Dispersion at 1550 nm (D: ps/nm km)
Dispersion slope at 1550 nm (D : ps/nm2 km)
Core area (Aeff : µm2 )
0.22 0.59 0.22 0.22 0.22
16.98 −86.59 4.3 3.6 4.3
0.059 −0.135 0.1 0.07 0.045
80 35 72 55 55
Fig. 2. Calculated BER curve in relation to the dispersion compensation type over 320 km transmission with 4 ch × 40 Gb/s system: open symbol, pre-compensation; closed symbol, post-compensation.
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compensation. The difference of BER curve, however, is very small. In pre-compensation case, the signals passed are distorted when it passed through the DCF. These distorted signals enter to SMF and are more affected by SPM in SMF. But the dispersion of the undistorted signal can be compensated by SPM in the post-compensation. In NZDSF + DCF case, the opposite result is occurred due to SPM increased by a small core area of NZDSF. When the fiber launching power is 6 dB m, the calculated BER is shown in Fig. 2b. In SMF + DCF case, the post-compensation is better than the pre-compensation. The reason is the same as the case of 3 dB m launching power. In NZDSF + DCF case, the transmission performances are changed for fiber types of NZDSF. In the LEAF case, the same transmission performance can be estimated. In the case of TWF and TWF-RS, however, the performance is degraded using the pre-compensation due to the strong XPM effect. This is due to the difference of core area in NZDSF. Since the LEAF has a large core area (72 µm2) compared to the other NZDSFs, the influence of XPM is weak compared to the other NZDSFs. Because the effective core area of TWF and TWF-RS is small (55 µm2), the XPM effect is dominant because the XPM effect is inversely proportional to core area. From the above simulation results, we found that the post-compensation is better than the pre-compensation except for the case of NZDSF + DCF with 3 dB m launching power. So, we considered only the post-compensation in our experiments.
3. Experimental and simulation results of Tbps transmissions 3.1. Configuration of transmission systems The experimental setup of a 40 Gb/s based on WDM transmission systems is shown in Fig. 3. The wavelength ranges are from 1534.5 to 1563.5 nm for 85 km transmission and from 1536.7 to 1560.7 nm for 342 km transmission. The signals were multiplexed in optical multiplexer (OMUX). The multiplexed signals were modulated at 20 Gb/s with 223 − 1 PRBS using a dual-drive LiNbO3 modulator. The gain-flattened EDFA was used as a booster-amplifier. The return-to-zero (RZ) converter transformed the modulated signal to the RZ format. 40 Gb/s signals were generated from the combination of RZ signal and delayed RZ signals by 25 × 7 ps. After de-correlating the signals using DCF, the 40 Gb/s signal was transmitted by TWF with 342 km length. The total system is composed of 4 spans (85 km × 2, 86 km × 2). To amplify the signal, the EDFA is inserted in the each span. To obtain a sufficient optical signal-to-noise ratio (OSNR), the fiber launching power was 0 dB m in 85 km transmission and 5 dB m in 342 km transmission per each channel. Each span loss is about 20 dB including connector loss. All in-line EDFAs have the output power of 20 dB m and the gain flatness of 1 dB in signal wavelength region. The dispersion of transmitted signals is compensated by DCF with the negative slope. After amplifying the signal using a gain flattened EDFA, the received signal was demultiplexed with a tunable bandpass filter with the 3 dB bandwidth of 0.3 nm. The 40 Gb/s received signal was demultiplexed optically into 20 Gb/s data, then received by 20 Gb/s 3R receiver. The recovered data at 20 Gb/s were then electronically demultiplexed into two 10 Gb/s signals in the receiver for BER measurements. The polarization controller is used to align the
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Fig. 3. Schematic diagram of TWF (342 km) transmissions with 25 ch × 40 Gb/s system using WDM and TDM method.
polarization of each signal. The polarization multiplexing of even/odd channels is not used for 20 Gb/s channels. 3.2. Transmission performance of 30 ch × 40 Gb/s WDM signals using TWF over 85 km Figure 4 shows the calculated and measured eye-diagrams at 1534.5 and 1563.5 nm wavelength. We found that the eye-diagrams of both side channels were opened clearly. Though the total dispersion was set to zero at the center channel and residual dispersion remained except for the center channel, we found that all the channels have clear open eyes. The calculated eye-diagram has a good agreement with the measured eye-diagram. Figure 5 shows calculated BER curves for all the WDM channels. The BER characteristics of the center channels are worse than the side channels. It means that XPM and FWM effects are stronger than dispersion effect in systems. The error free transmission was obtained for all channels. The receiver sensitivity at 10−9 BER is shown in Fig. 6. The range of measured and calculated receiver sensitivities is from −23 to −25 dB m. The simulation results have a small discrepancy compared to experimental results. Because shorter PRBS length was used and PMD effect was neglected in simulation, the calculated receiver sensitivity has better performance than the measured one in Fig. 6. The experimental points are much
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(a) Fig. 4. Calculated and measured eye-diagrams over TWF (85 km) transmissions with 30 ch × 40 Gb/s system for (a) channel 1 and (b) channel 30.
more random than the calculated points. This may result from the instability of CDR in receiver to sample and extract 10 Gb/s signals form incoming 20 Gb/s signals. However, the difference is a very small and the trend of receiver sensitivity is very similar in between the measured and calculated results. 3.3. Transmission performance of 25 ch × 40 Gb/s WDM signals using TWF + DCF over 342 km Figure 7 shows the calculated BER curves for all channels. Due to the residual dispersion and nonlinear effects of fibers, the receiver sensitivity is changed from −26.5 to −23 dB m. Since the side channel has a high-accumulated dispersion unlikely the 85 km transmission due to the increased TWF length, the transmission performance of the side channels is worse than the center channels. The measured BER characteristics are shown Fig. 8. The measured BERs are similar to the calculated ones. In 342 km transmission, all the channels have characteristics of error free transmissions both the calculated and measured BERs.
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(b) Fig. 4. (Continued.)
Fig. 5. Calculated BER characteristics of TWF (85 km) transmissions with 30 ch × 40 Gb/s system.
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Fig. 6. Calculated and measured receiver sensitivities (BER = 10−9 ) of TWF (85 km) transmission with 30 ch × 40 Gb/s system.
Fig. 7. Calculated BER characteristics of TWF (342 km) transmissions with 25 ch × 40 Gb/s system.
4. Conclusions We investigated the transmission performance depending on various fiber types in 40 Gb/s transmission system. From the modeling of transmitter, fiber, and BER characteristics, with considering the frequency chirp, extinction ratio, ISI, and nonlinear effects in fiber, the effect of dispersion compensation was calculated. The post-compensation has a better performance than the pre-compensation except for 3 dB m fiber launching power
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Fig. 8. Measured BER characteristics of TWF (342 km) with 25 ch × 40 Gb/s transmissions system.
in NZDSF. NZDSF has better performance due to maintaining high OSNR caused by less EDFAs required for the same transmission distance in 40 Gb/s transmission, compared to SMF. We calculated and measured the transmission performance of 30 ch × 40 Gb/s data rate over 85 km and 25 ch × 40 Gb/s over 342 km of TWF using WDM and TDM method. The simulated eye-diagrams and BER characteristics had a good agreement with the experimental ones. The experimental and simulated BER data for each WDM channel was shown no sign of an error floor.
Acknowledgment This work was supported in part by the National Research Laboratory Program and the BK21 program in South Korea.
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