**6. Conclusions**

206 Optical Communications Systems

2.5 Gbit/s 2.5 Gbit/s 2.5 Gbit/s


Time [ns] Due to Kerr effects

NRZ signal BER = 7.3e-22

0 0.2 0.4 0.6 0.8

0

4e-005 3e-005 2e-005 1e-005

Signal [a.u.]

Total output spectrum Power [dBm] **12.5 GHz** channel interval

**2.5 Gbit/s bit rate NRZ-Duobinary-NRZ mixed HDWDM system**

NRZ signal BER = 1.24e-21

0 0.2 0.4 0.6 0.8 Time [ns]

0

4e-005 3e-005 2e-005 1e-005

Signal [a.u.]

NRZ NRZ Duo

Frequency [THz]

Duobinary signal BER = 6.4e-10

0 0.2 0.4 0.6 0.8 Time [ns]

Fig. 15. A 2.5 Gbit/s mixed HDWDM communication system with NRZ-Duobinary-NRZ signals and a 12.5 GHz frequency interval. The output spectrum of the optical signal and the

eye diagrams of the received electrical signal are shown.

0

4e-005 3e-005 2e-005 1e-005

Signal [a.u.]

192.987 193.0 193.0125 193.025 193.035

Our results have proved once more that HDWDM is a powerful technique for increasing the capacity of fiber optics transmission systems. It may be crucial for enabling technology of ultra-high capacity on-chip optical interconnects, as well as chip-to-chip optical interconnects in massively parallel different optical systems. It has been shown that the BER and eye-diagram technique is a good means for evaluating the system performance that allows HDWDM system to be optimized for different parameters.

In contrast to the conventional high speed approach of increasing WDM transmission capacity, we have demonstrated the minimal allowed channel spacing in HDWDM systems, and provided we are able to provide recommendations for future HDWDM solutions.

In the measurements, different optical filter FWHM values (from 0.15 nm to 0.7 nm) were used. The best results were obtained for 0.15 nm, when the eye pattern was opened wider. For evaluation of the signal quality a visual method was employed, in which the eye pattern was evaluated visually in the electric signal analyser varying the quasi-rectangular optical filter FWHM value.

At reducing the channel interval to 18.75 GHz the Kerr effects (self-phase modulation, crossphase modulation, and four-wave mixing) degrades the 2.5 Gbit/s HDWDM system. The signal eye-pattern overlaps with the mask, which means that the signal quality does not

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ensure in this case the BER value of 10-9. From the measurement results it follows that the 25 GHz channel interval ensures a good signal quality and that the signal eye-pattern does not overlap with the mask.

At 10 Gbit/s HDWDM transmission the channel interval should be 37.5 GHz to ensure the signal quality with the BER value of 10-9, which fits well the previous simulation results.

It is established that the operators of telecommunication networks, when creating the HDWDM communication systems, raise the total transmission speed step-by-step in response to the increased request for the data volume. As a result, a mixed HDWDM system is formed, with different transmission speeds (2.5 Gbit/s or 10 Gbit/s), coding formats (NRZ, RZ or Duobinary) and frequency intervals (12.5 GHz, 25 GHz, 50 GHz, 100 GHz). Therefore, in order to ensure stabile functioning (i.e. BER < 10-9 for each signal) of a mixed HDWDM system the channel interval should exceed 25 GHz at a 2.5 Gbit/s transmission speed per channel. In turn, for stabile operation of a mixed 10 Gbit/s WDM system the frequency interval should be raised to 50 GHz.

The Duobinary technique for signal coding ensures a better protection of the transmitted signals against Kerr effects as compared with the RZ coding. This allows a highly compact NRZ-Duobinary-NRZ system to be formed with the 12.5 GHz frequency interval and 2.5 Gbit/s transmission speed per channel. In turn, in the case of a 10 Gbit/s transmission speed per channel it is possible to use an 18.75 GHz frequency interval.
