In this example, we compare two types of fibers, Corning’s MetroCor and SMF-28 fibers for metro network applications. MetroCor Fiber has a negative dispersion, whereas SMF-28 has a positive dispersion in the EDFA bands. Dispersion characteristics of these two fibers are shown in Figure 1. For metro applications, directly modulated lasers (DMLs) are preferred because of their low cost, but they have a higher chirp compared to externally modulated lasers and this results in more penalty due to dispersion when standard positive dispersion SMF is used.
Typically, DMLs are rated for 100 km transmission distances over SMF-28 fiber with less than 2 dB dispersion-induced penalty. One option to overcome this effect is to use a fiber with negative dispersion that can take advantage of the positive chirp characteristics of DMLs to enhance transmission distances.
MetroCor fiber has a zero dispersion wavelength near 1630 to 1640 nm. As a result, this fiber has an average dispersion of about -3 ps/nm/km in L-band and about -8 ps/nm/km in C-band. The loss, the dispersion slope and effective area are typical of other conventional NZ-DSF fibers.
Figure 1: Dispersion characteristics of MetroCor and SMF-28 fibers
Modeling Directly Modulated Lasers
We will first investigate DMLs that we are going to use in our simulations. We have modeled two types of DMLs by using our Laser Rate Equations model. One is strongly adiabatic chirp dominated (DML-1) and the other one is strongly transient chirp dominated (DML-2). The model parameters are extracted from the measured parameters of [1]. Laser parameters are given in Figure 2 and Figure 3.
Figure 2: Extracted parameter values of DML-1
Figure 3: Extracted parameter values of DML-2
Transmission Characteristics of DMLs
To understand the chirp characteristics of directly modulated lasers to address the dispersion-induced deformation on the transmitted signals at 2.5 Gbps, we performed a series of simulations. We also compared our simulation results with the results of [1] to validate our model. The project is shown in **figure 4.
This project is found in the Tomkos_JSTQE_May_June2001.osd file. This project contains four different layouts. The first two layouts use DML-1 and the last two layouts use DML-2. Layout 1 and Layout 3 use SMF-28 fiber whereas Layout 2 and Layout 4 use MetroCor fiber. As in [1], attenuations of fibers are disabled to isolate the effect of chirp and dispersion. Rise and fall times of the NRZ electrical pulse are 0.5 bit and exponential shapes are selected.
Figure 4: Project layout to investigate the effect of laser chirp on transmission performance
Figure 5 shows the received eye diagram for the case of an adiabatic chirp dominated transmitter (DML-1) after transmitting over 300 km of SMF-28 fiber. The received eye is deformed but not completely closed.
Figure 6 on the other hand, shows the received eye diagram for the case of an adiabatic chirp dominated transmitter (DML-1) after transmitting over 300 km of MetroCor fiber. In this case, deformation is smaller because MetroCor fiber has a small absolute value of dispersion that is less than half of that for SMF-28.
Figure 7 shows the received eye diagram for the case of a transient chirp dominated transmitter (DML-2) after transmitting over 300 km of SMF-28 fiber. In this case, the received eye pattern is severely closed due to significant intersymbol interference.
Figure 8, on the other hand, shows the received eye diagram for the case of a transient chirp dominated transmitter (DML-2) after transmitting over 300 km of MetroCor fiber. The only noticeable effect in this case is the formation of some peaks on the top of ones.
The received eye is completely open. These simulation results are in perfect agreement with the results of [1].
Figure 5: Eye diagram at the receiver side after 300 km propagation on SMF-28 fiber when DML-1 is used
Figure 6: Eye diagram at the receiver side after 300 km propagation on MetroCor fiber when DML-1 is used
Figure 7: Eye diagram at the receiver side after 300 km propagation on SMF-28 fiber when DML-2 is used
Figure 8: Eye diagram at the receiver side after 300 km propagation on MetroCor fiber when DML-2 is used
Transmission at higher bit rates
In this section, we will show the simulation results at 10 Gbps bit rate. The project is found in Tomkos_JSTQE_May_June2001_10Gbps.osd file. Here we have used the same DML-1 that we used in previous sections for 2.5 Gbps transmission. The signal propagates over 120 km SMF-28 or MetroCor fiber. The extinction ratio of the transmitter is set to about 9 dB, in this case. Figure 9 and Figure 10 show the eye diagrams after 120 km propagation over either SMF-28 or MetroCor fiber, respectively. When MetroCor is used, the eye is still open after 120 km of propagation.
Figure 9: Eye diagram at the receiver side after 120 km propagation at 10 Gbps on SMF-28 fiber
Figure 10: Eye diagram at the receiver side after 120 km propagation at 10 Gbps on MetroCor fiber
32 Channel DWDM system simulation at 2.5 Gbps
In this section, we reproduce the experimental results of [1] for the 32 channel DWDM system at 2.5 Gbps. The project is found in the 32 Channel DWDM Metro.osd file. The 32 channels are between 1533.5 and 1558.2 nm with 100 GHz spacing on the ITU-T and are multiplexed.
In this design, DMLs are used. Transmitted power is about -3 dB/channel. Multiplexed channels are propagated either over 300 km of SMF-28 fiber or 300 km of MetroCor fiber. EDFAs with 18 dB gain and 4 dB noise are inserted after every 100 km of fiber. The received signal is filtered to get channel 21 and channel 30.
The eye diagrams for these channels for MetroCor fiber simulation are shown in Figure 11 and Figure 12. For these channels, Q is about 10 dB as defined 10log(Q).
Figure 13 and Figure 14, on the other hand, show the eye diagrams for these channels for SMF-28 fiber simulation. For these channels, Q is about 5 dB, which is below 9 dB. A Q of 9 dB corresponds to a BER of 10-15. These results are in perfect agreement with the experimental findings of [1].
Figure 11: Eye diagram of channel 21 at the receiver side after 300 km propagation at 2.5 Gbps over MetroCor fiber
Figure 12: Eye diagram of channel 30 at the receiver side after 300 km propagation at 2.5 Gbps over MetroCor fiber
Figure 13: Eye diagram of channel 21 at the receiver side after 300 km propagation at 2.5 Gbps over SMF-28 fiber
Figure 14: Eye diagram of channel 30 at the receiver side after 300 km propagation at 2.5 Gbps over SMF-28 fiber
References:
[1] I. Tomkos et. al., “Demonstration of negative dispersion fibers for DWDM metropolitan networks”, IEEE J. of Select. Top. in Quan. Elec. 7, p.439, 2001.
[2] Chris Kennedy et. al., “The performance of negative dispersion fiber at 10 GBps and significance of externally and directly modulated lasers”, NFOEC’01, 2001.
[3] David Culverhouse et. al., “Corning MetroCor Fiber and its Application in Metropolitan Networks”, Corning’s White Paper WP5078.
[4] G. P. Agrawal, Nonlinear Fiber Optics, Second Edition, Academic Press, 1995.