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s WDM transmission system
Optics Communications 217 (2003) 221–225 www.elsevier.com/locate/optcom
Impact of optical (de)multiplexers on 40 Gbit/s WDM transmission system Lin Zhu *, Yejin Zhang, Yi Dong, Minghua Chen, Li Xia, Shizhong Xie Department of Electronics Engineering, Tsinghua University, Beijing 100084, China Received 11 June 2002; received in revised form 11 September 2002; accepted 7 January 2003
Abstract The impact of (de)multiplexers on 40 Gbit/s WDM system is investigated for NRZ and RZ coding formats. Performances of (de)multiplexers with different crosstalk, filter bandwidth and channel spacing are systematically analyzed to get the best system performance for 40 Gbit/s WDM transmission system with high spectral efficiency. The limitations imposed by misalignment and cascade of filters are also discussed. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Optical (de)multiplexers; 40 Gbit/s WDM transmission; High spectral efficiency
1. Introduction In recent years, WDM optical systems are continuously migrating towards both higher single channel bit rate and denser wavelength multiplexing in order to use the available bandwidth most efficiently. In previous spectrally efficient laboratory demonstrations of 40 Gbit/s WDM system transmission, high performance optical filters are proving to be very important [1]. (De)multiplexers with high bandwidth can help avoid waveform distortion due to spectral narrowing, but (de)multiplexers with low bandwidth can help suppress interchannel crosstalk from neighboring channels. So there exists an optimal *
Corresponding author. Tel.: +86-10-6278-4784; fax: +86-106278-8161. E-mail address: [email protected] (L. Zhu).
filter bandwidth to achieve the best system performance, which also means that we can adjust filter bandwidth to get higher spectral efficiency. For spectrally efficient system, the appropriate selection of a coding format is also a critical issue: it should have high tolerance against optical nonlinear effects and noise accumulation, and it also should be suitable for (de)multiplexers with specific crosstalk level and filter bandwidth [2]. Previous works have clarified the importance of these issues at 10 Gbit/s or at 40 Gbit/s without taking account of various crosstalk levels and transmission lines [3–5]. In [3–5], the system performance is described by eye-opening-penalty (EOP) or Qfactor penalty (QP). Since EOP and QP focus on the change of system performance after the transmission, the previous results did not effectively evaluate the real performance of NRZ/RZ transmission system.
0030-4018/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0030-4018(03)01120-9
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L. Zhu et al. / Optics Communications 217 (2003) 221–225
In this paper, the effect of filter bandwidth and channel spacing on 40 Gbit/s WDM system incorporated with transmission line is investigated for the first time. The performance of different coding formats such as NRZ and RZ is investigated by use of Q-factor method. For the longdistance transmission system, it is clear that large scale architecture may cause degradation for optical signal, chiefly because of the effects of cascade and misalignment of optical (de)multiplexers [6]. So we also investigate limitations imposed by these two effects of large-scale transmission system.
2. The system model In order to simulate the general situation of the real transmission system, a schematic diagram in Fig. 1 is considered. The channel to be analyzed is centered at 1550 nm whereas, the neighboring channels create interchannel crosstalk. Five 40 Gbit/s transmitters that emit NRZ or RZ signals with 30 dB extinction and chirp-free modulation are arranged for different channels and multiplexed with a MUX filter. 210 1 bit PRBS patterns are generated to simulate the system signal. The NRZ signals are modeled with a cosine edge and a roll-off factor of 0.6. The RZ pulses are of Gaussian shape with 0.5 duty cycle. Random time delays between channels are applied to ensure realistic decorrelation. The average power of the signal is )3 dBm. The signals are launched into a N-span link consisting of dispersion compensation normalized sections. For all investigations in this paper, an amplifier space of 80 km is used. b2 and b3 in each SMF span are fully compensated by dispersion compensation fiber (DCF) in a postcompensation scheme. The parameters of SMF and DCF are given in Table 1. Erbium-doped
Fig. 1. The system model.
Table 1 Fiber parameters
Dispersion (ps/nm/km) Dispersion slope (ps/nm2 /km) Nonlinear coefficient (mW/km) Loss coefficient (dB/km)
SMF
DCF
16.0 0.08 0.0012 0.25
)90 )0.45 0.004 0.50
optical fiber amplifier (EDFA) is used in each section to compensate the fiber loss. The inline EDFA consists of two EDFAs, one before and one after the DCF fiber. The input power in the DCF fiber has been kept smaller than )5 dBm so that the nonlinear effects in the DCF fiber can be neglected. The EDFA is modeled by wavelength independent gain and noise addition. We assume the noise figure (NF) of 5.0 dB for each optical amplifier. At the receiver, the signal is optically filtered, squared and then electrically filtered by a fourth-order 28 GHz low-pass Bessel filter. The calculation of the propagation in the optical fibers is performed using a standard split-step algorithm with adaptive step-size [7]. The system performance is evaluated from the Q-factor. In all simulations, the Q-factor is calculated by use of the techniques described in [8]. The results of Q-factors use linear units.
3. Simulation results 3.1. Results for a single span system In this section, we mainly focus on the simulation results of a single span system. Notice that the crosstalk level would be determined for a specific filter when the filter bandwidth and channel spacing are defined. But for a specific filter with fixed bandwidth, there is no analytic relation between crosstalk level and channel spacing. So in the following discussion, we will calculate the system performance as a function of both crosstalk level and channel spacing in order to demonstrate a clearly physical picture. First we calculate Qfactor as a function of crosstalk level (CL) at different filter bandwidth (FB) in Fig. 2. In the simulation, the channel spacing is varied to change
L. Zhu et al. / Optics Communications 217 (2003) 221–225
223
the bandwidth is small, NRZ signal performances better than RZ signal. This is because that RZ signal will get very strong spectral distortion due to the narrow band filter at 40 Gbit/s; when the bandwidth is large, the spectral distortion effect relieves, so RZ signal shows better system performance because of higher SNR. By comparing Figs. 2(a) and (b), we could find third Butterworth filter can get better system performance. This is due to more ideal passband character of third Butterworth filter, which can more effectively reject the neighboring channel crosstalk. Generally speaking, lower crosstalk level, larger filter bandwidth and more ideal passband character are better for the system performance. Then, we focus on the system performance when the system spectral efficiency is different. In Fig. 3, we calculate Q-factor as a function of
Fig. 2. Q-factor as a function of crosstalk level (CL) for (a) Gaussian filter and (b) Butterworth filter.
the crosstalk level and all the crosstalk from other channels is considered. Two different analytical filters are used: Gaussian filter corresponding to AWG and third-order Butterworth filter corresponding to general dielectric-film (de)multiplexers. The filter functions do not include phase information. In Fig. 2, when the crosstalk level is small, the system performance is good and steady. On the other hand, Q-factor degrades drastically under very large crosstalk level. In order to get good system performance, CL must be kept smaller than )20 dB. Comparing different modulation formats, we find the system performance greatly depends on the filter bandwidth. Notice that RZ signal has higher peak power and broader spectrum bandwidth, which means that RZ will achieve higher signal-noise-ratio (SNR) and undergo stronger spectral distortion. In Fig. 2, when
(a)
(b) Fig. 3. Q-factor as a function of channel spacing for (a) NRZ signal and (b) RZ signal.
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L. Zhu et al. / Optics Communications 217 (2003) 221–225
channel spacing for both NRZ and RZ system. By use of different channel spacing, we can get the system performance with different system spectral efficiency. Taking Q ¼ 10:0 as a tolerable criterion for NRZ signal, we can draw a horizontal dot line in Fig. 3(a), and taking 80 GHz channel spacing as another desirable criterion for NRZ signal, we can draw a vertical dot line in Fig. 3(a). The area both over the horizontal dot line and on the left of the vertical dot line means that the Q-factor of transmission system is larger than 10.0 with 80 GHZ channel spacing. We can find Gaussian filter with 80 GHz bandwidth is suitable for the criterion we define above in Fig. 3(a). In Fig. 3(b), the same Q-factor and channel spacing are set as acceptable criterion for RZ signal. In Fig. 3(b), we find that the situation is more complex. Notice the performance of RZ transmission system is closely connected with filter bandwidth. When the channel spacing is large, the system performance is mainly determined by filter bandwidth; however, the system performance basically depends on the channel spacing, when it is small. The misalignment is defined as the difference between the center frequency of the signal frequency spectrum and the center frequency of the passband of optical (de)multiplexer, which arises from manufacturing tolerances with specific limits. It also causes system performance impairment. In our system model, since b2 and b3 in each SMF span are fully compensated, the transmission line will not cause additional system penalty when misalignment occurs. In Fig. 4, Q is calculated as a function of misalignment for Gaussian filter (FB ¼ 80 GHz). The different crosstalk level is due to different channel spacing. Since RZ system has broader spectrum bandwidth, misalignment will has minor impacts on its system performance. In Fig. 4, it shows a better and steadier system performance when the filter misalignment becomes larger.
Fig. 4. Q-factor as a function of filter misalignment for Gaussian filter.
ists one pair of optical mux/demux. In our system model, the system performance mainly depends on optical (de)multiplexers because the system nonlinearity is effectively controlled. In Fig. 5, Q-factor is calculated as a function of the number of system spans. We also consider the impact of misalignment on multispan system performance. More cascaded system spans will cause worse system performance. Due to spectral narrow effects, the performance of RZ system deteriorates more quickly than NRZ system with the incensement of system span number. In Fig. 5, it is found that the combined effects of cascade and misalignment could be effectively controlled when the misalignment is small. Accordant with the previ-
3.2. Results for the multispan system In this section, we mainly concentrate on simulation results of multispan system. For large scale transmission systems, the (de)multiplexers in the multispan system would cause further system degradation. In each transmission span, there ex-
Fig. 5. Q as a function of the number of system spans, filter bandwidth is 80 GHz.
L. Zhu et al. / Optics Communications 217 (2003) 221–225
ous result, we also find the impact of misalignment on cascaded RZ system is less than NRZ system.
4. Conclusion For the first time, the effect of filter bandwidth and channel spacing on 40 Gbit/s WDM system incorporated with transmission lines is investigated. In the dispersion-managed system, we mainly discuss the system performance for NRZ and RZ transmission scheme by use of Q-factor technique. The limitations imposed by misalignment and cascade of filters are also discussed.
225
References [1] Yann Frignac, Sebastien Bigo, J.P. Hamaide, in: ECOCÕ01, TH.B.1. [2] D. Breuer, K. Petermann, IEEE Phot. Techn. Lett. 11 (8) (1999). [3] H. Miyata, H. Onaka, K. Otasuka, T. Chikama, in: OFCÕ97, TUE3. [4] Harald Bock, Georg Mohs, et al., in: ECOCÕ00, Vol. 3, pp. 225–226. [5] Gerardo Castanon, Olga Vassilieva et al., in: ECOCÕ01, MO.F.3. [6] N.N. Khrais, A.F. Elrefaie, et al., IEEE Phot. Techn. Lett. 8 (8) (1996). [7] G.P. Agrawal, Nonlinear Fiber Optics, Academic, San Diego, CA, 1995. [8] C.J. Anderson, J.A. Lyle, Electron. Lett. 30 (1) (1994).
Impact of optical (de)multiplexers on 40 Gbit/s WDM transmission system Lin Zhu *, Yejin Zhang, Yi Dong, Minghua Chen, Li Xia, Shizhong Xie Department of Electronics Engineering, Tsinghua University, Beijing 100084, China Received 11 June 2002; received in revised form 11 September 2002; accepted 7 January 2003
Abstract The impact of (de)multiplexers on 40 Gbit/s WDM system is investigated for NRZ and RZ coding formats. Performances of (de)multiplexers with different crosstalk, filter bandwidth and channel spacing are systematically analyzed to get the best system performance for 40 Gbit/s WDM transmission system with high spectral efficiency. The limitations imposed by misalignment and cascade of filters are also discussed. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Optical (de)multiplexers; 40 Gbit/s WDM transmission; High spectral efficiency
1. Introduction In recent years, WDM optical systems are continuously migrating towards both higher single channel bit rate and denser wavelength multiplexing in order to use the available bandwidth most efficiently. In previous spectrally efficient laboratory demonstrations of 40 Gbit/s WDM system transmission, high performance optical filters are proving to be very important [1]. (De)multiplexers with high bandwidth can help avoid waveform distortion due to spectral narrowing, but (de)multiplexers with low bandwidth can help suppress interchannel crosstalk from neighboring channels. So there exists an optimal *
Corresponding author. Tel.: +86-10-6278-4784; fax: +86-106278-8161. E-mail address: [email protected] (L. Zhu).
filter bandwidth to achieve the best system performance, which also means that we can adjust filter bandwidth to get higher spectral efficiency. For spectrally efficient system, the appropriate selection of a coding format is also a critical issue: it should have high tolerance against optical nonlinear effects and noise accumulation, and it also should be suitable for (de)multiplexers with specific crosstalk level and filter bandwidth [2]. Previous works have clarified the importance of these issues at 10 Gbit/s or at 40 Gbit/s without taking account of various crosstalk levels and transmission lines [3–5]. In [3–5], the system performance is described by eye-opening-penalty (EOP) or Qfactor penalty (QP). Since EOP and QP focus on the change of system performance after the transmission, the previous results did not effectively evaluate the real performance of NRZ/RZ transmission system.
0030-4018/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0030-4018(03)01120-9
222
L. Zhu et al. / Optics Communications 217 (2003) 221–225
In this paper, the effect of filter bandwidth and channel spacing on 40 Gbit/s WDM system incorporated with transmission line is investigated for the first time. The performance of different coding formats such as NRZ and RZ is investigated by use of Q-factor method. For the longdistance transmission system, it is clear that large scale architecture may cause degradation for optical signal, chiefly because of the effects of cascade and misalignment of optical (de)multiplexers [6]. So we also investigate limitations imposed by these two effects of large-scale transmission system.
2. The system model In order to simulate the general situation of the real transmission system, a schematic diagram in Fig. 1 is considered. The channel to be analyzed is centered at 1550 nm whereas, the neighboring channels create interchannel crosstalk. Five 40 Gbit/s transmitters that emit NRZ or RZ signals with 30 dB extinction and chirp-free modulation are arranged for different channels and multiplexed with a MUX filter. 210 1 bit PRBS patterns are generated to simulate the system signal. The NRZ signals are modeled with a cosine edge and a roll-off factor of 0.6. The RZ pulses are of Gaussian shape with 0.5 duty cycle. Random time delays between channels are applied to ensure realistic decorrelation. The average power of the signal is )3 dBm. The signals are launched into a N-span link consisting of dispersion compensation normalized sections. For all investigations in this paper, an amplifier space of 80 km is used. b2 and b3 in each SMF span are fully compensated by dispersion compensation fiber (DCF) in a postcompensation scheme. The parameters of SMF and DCF are given in Table 1. Erbium-doped
Fig. 1. The system model.
Table 1 Fiber parameters
Dispersion (ps/nm/km) Dispersion slope (ps/nm2 /km) Nonlinear coefficient (mW/km) Loss coefficient (dB/km)
SMF
DCF
16.0 0.08 0.0012 0.25
)90 )0.45 0.004 0.50
optical fiber amplifier (EDFA) is used in each section to compensate the fiber loss. The inline EDFA consists of two EDFAs, one before and one after the DCF fiber. The input power in the DCF fiber has been kept smaller than )5 dBm so that the nonlinear effects in the DCF fiber can be neglected. The EDFA is modeled by wavelength independent gain and noise addition. We assume the noise figure (NF) of 5.0 dB for each optical amplifier. At the receiver, the signal is optically filtered, squared and then electrically filtered by a fourth-order 28 GHz low-pass Bessel filter. The calculation of the propagation in the optical fibers is performed using a standard split-step algorithm with adaptive step-size [7]. The system performance is evaluated from the Q-factor. In all simulations, the Q-factor is calculated by use of the techniques described in [8]. The results of Q-factors use linear units.
3. Simulation results 3.1. Results for a single span system In this section, we mainly focus on the simulation results of a single span system. Notice that the crosstalk level would be determined for a specific filter when the filter bandwidth and channel spacing are defined. But for a specific filter with fixed bandwidth, there is no analytic relation between crosstalk level and channel spacing. So in the following discussion, we will calculate the system performance as a function of both crosstalk level and channel spacing in order to demonstrate a clearly physical picture. First we calculate Qfactor as a function of crosstalk level (CL) at different filter bandwidth (FB) in Fig. 2. In the simulation, the channel spacing is varied to change
L. Zhu et al. / Optics Communications 217 (2003) 221–225
223
the bandwidth is small, NRZ signal performances better than RZ signal. This is because that RZ signal will get very strong spectral distortion due to the narrow band filter at 40 Gbit/s; when the bandwidth is large, the spectral distortion effect relieves, so RZ signal shows better system performance because of higher SNR. By comparing Figs. 2(a) and (b), we could find third Butterworth filter can get better system performance. This is due to more ideal passband character of third Butterworth filter, which can more effectively reject the neighboring channel crosstalk. Generally speaking, lower crosstalk level, larger filter bandwidth and more ideal passband character are better for the system performance. Then, we focus on the system performance when the system spectral efficiency is different. In Fig. 3, we calculate Q-factor as a function of
Fig. 2. Q-factor as a function of crosstalk level (CL) for (a) Gaussian filter and (b) Butterworth filter.
the crosstalk level and all the crosstalk from other channels is considered. Two different analytical filters are used: Gaussian filter corresponding to AWG and third-order Butterworth filter corresponding to general dielectric-film (de)multiplexers. The filter functions do not include phase information. In Fig. 2, when the crosstalk level is small, the system performance is good and steady. On the other hand, Q-factor degrades drastically under very large crosstalk level. In order to get good system performance, CL must be kept smaller than )20 dB. Comparing different modulation formats, we find the system performance greatly depends on the filter bandwidth. Notice that RZ signal has higher peak power and broader spectrum bandwidth, which means that RZ will achieve higher signal-noise-ratio (SNR) and undergo stronger spectral distortion. In Fig. 2, when
(a)
(b) Fig. 3. Q-factor as a function of channel spacing for (a) NRZ signal and (b) RZ signal.
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L. Zhu et al. / Optics Communications 217 (2003) 221–225
channel spacing for both NRZ and RZ system. By use of different channel spacing, we can get the system performance with different system spectral efficiency. Taking Q ¼ 10:0 as a tolerable criterion for NRZ signal, we can draw a horizontal dot line in Fig. 3(a), and taking 80 GHz channel spacing as another desirable criterion for NRZ signal, we can draw a vertical dot line in Fig. 3(a). The area both over the horizontal dot line and on the left of the vertical dot line means that the Q-factor of transmission system is larger than 10.0 with 80 GHZ channel spacing. We can find Gaussian filter with 80 GHz bandwidth is suitable for the criterion we define above in Fig. 3(a). In Fig. 3(b), the same Q-factor and channel spacing are set as acceptable criterion for RZ signal. In Fig. 3(b), we find that the situation is more complex. Notice the performance of RZ transmission system is closely connected with filter bandwidth. When the channel spacing is large, the system performance is mainly determined by filter bandwidth; however, the system performance basically depends on the channel spacing, when it is small. The misalignment is defined as the difference between the center frequency of the signal frequency spectrum and the center frequency of the passband of optical (de)multiplexer, which arises from manufacturing tolerances with specific limits. It also causes system performance impairment. In our system model, since b2 and b3 in each SMF span are fully compensated, the transmission line will not cause additional system penalty when misalignment occurs. In Fig. 4, Q is calculated as a function of misalignment for Gaussian filter (FB ¼ 80 GHz). The different crosstalk level is due to different channel spacing. Since RZ system has broader spectrum bandwidth, misalignment will has minor impacts on its system performance. In Fig. 4, it shows a better and steadier system performance when the filter misalignment becomes larger.
Fig. 4. Q-factor as a function of filter misalignment for Gaussian filter.
ists one pair of optical mux/demux. In our system model, the system performance mainly depends on optical (de)multiplexers because the system nonlinearity is effectively controlled. In Fig. 5, Q-factor is calculated as a function of the number of system spans. We also consider the impact of misalignment on multispan system performance. More cascaded system spans will cause worse system performance. Due to spectral narrow effects, the performance of RZ system deteriorates more quickly than NRZ system with the incensement of system span number. In Fig. 5, it is found that the combined effects of cascade and misalignment could be effectively controlled when the misalignment is small. Accordant with the previ-
3.2. Results for the multispan system In this section, we mainly concentrate on simulation results of multispan system. For large scale transmission systems, the (de)multiplexers in the multispan system would cause further system degradation. In each transmission span, there ex-
Fig. 5. Q as a function of the number of system spans, filter bandwidth is 80 GHz.
L. Zhu et al. / Optics Communications 217 (2003) 221–225
ous result, we also find the impact of misalignment on cascaded RZ system is less than NRZ system.
4. Conclusion For the first time, the effect of filter bandwidth and channel spacing on 40 Gbit/s WDM system incorporated with transmission lines is investigated. In the dispersion-managed system, we mainly discuss the system performance for NRZ and RZ transmission scheme by use of Q-factor technique. The limitations imposed by misalignment and cascade of filters are also discussed.
225
References [1] Yann Frignac, Sebastien Bigo, J.P. Hamaide, in: ECOCÕ01, TH.B.1. [2] D. Breuer, K. Petermann, IEEE Phot. Techn. Lett. 11 (8) (1999). [3] H. Miyata, H. Onaka, K. Otasuka, T. Chikama, in: OFCÕ97, TUE3. [4] Harald Bock, Georg Mohs, et al., in: ECOCÕ00, Vol. 3, pp. 225–226. [5] Gerardo Castanon, Olga Vassilieva et al., in: ECOCÕ01, MO.F.3. [6] N.N. Khrais, A.F. Elrefaie, et al., IEEE Phot. Techn. Lett. 8 (8) (1996). [7] G.P. Agrawal, Nonlinear Fiber Optics, Academic, San Diego, CA, 1995. [8] C.J. Anderson, J.A. Lyle, Electron. Lett. 30 (1) (1994).