**5. Results and discussions**

In summary it can be concluded, that the mixed WDM solution allows combining channels with a variety of modulation formats, which are used for optical signal modulation, in one single transmission system, preserving a previously used channel spacing values. We would like to point out one more time, that such mixed solution of transmission system is being offered as the transition state form traditionally used NRZ – OOK modulation format to the alternative modulation formats, such as NRZ – DPSK or 2 – POLSK, which use provides a number of superior properties due to their abilities of providing greater protection from interchannel crosstalk, less exposed to expression of non-linear effects and better exposed to channel filtration, as well as less exposed to chromatic dispersion effect. By gradually introducing new system channels, can be increased the total transmission capacity of fibre, thus avoiding of core networks bottleneck effect and in the same time minimize growth of non-linear optical effect influences, because the alternative modulation formats are able to provide the same BER levels as traditionally used NRZ – OOK, but only at lower input power levels.

The suitability of different chromatic dispersion compensation methods for the most efficient high – speed mixed HDWDM systems, with a 50 km of SSMF span length and following system's configuration: [1st channel: NRZ – OOK, 40 Gbit/s, 193.025 THz] – [2nd channel: 2 – POLSK, 40 Gbit/s, 193.100 THz] – [3rd channel: NRZ – DPSK, 10 Gbit/s, 193.175 THz], has been evaluated. This configuration represents channel's number, modulation format, per-channel data rate and channels' central frequencies at 75 GHz channel spacing. The following recommendations and conclusions are summarized below for the next generation of WDM transmission systems.

 The use of both CD pre- and post-compensation modules together, which might contain DCF of different length, allow to achieve better system's BER values comparing with precompensation or post-compensation schemes used separately. We have found, that the best CD compensation and lowest channels BER results for investigated mixed HDWDM systemwith mixed data rates , mentioned above, can be achieved using 5 km of DCF in pre- and post-compensation modules. The similar results can be achieved using 7 km of DCF in pre-compensation module and 3 km in post-compensation module.


Investigating coherence between BER values and channels' central frequencies position in Cband, it have been stated several facts. Firstly, obtained channels central frequency values for each system channel the worst and the best position in C-band, that provide the highest and the lowest possible detected signals BER values. Secondly, 2 – POLSK modulated signals are not suitable for transmission over NZ – DSF fiber in investigated mixed WDM system, because these signals are sufficiently distorted at appropriate channel central frequency. If mixed system's channels are allocated around nominal of 195.750 THz, then obtained average detected signals BER value is about 10-14 and it is the worst possible case. Whereas the lowest possible average BER value (5×10-25) can be reached anchored mixed systems channels frequency grid to 192.825 THz. Thirdly, NRZ – DPSK modulated optical signals are well suitable for transmission over NZ – DSF.

## **Author details**

262 Optical Communication

Characteristics BER 2nd channel

MIN average BER/ SSMF 3×10-23 195.750

MAx average BER/ SSMF 6×10-17 192.375

MIN average BER/ NZ-DSF 5×10-25 192.825

MAx average BER/ NZ-DSF 1×10-14 195.750

**5. Results and discussions** 

generation of WDM transmission systems.

**Table 6.** Minimal and maximal in system's 1st-3rd channel detected signals BER values

In summary it can be concluded, that the mixed WDM solution allows combining channels with a variety of modulation formats, which are used for optical signal modulation, in one single transmission system, preserving a previously used channel spacing values. We would like to point out one more time, that such mixed solution of transmission system is being offered as the transition state form traditionally used NRZ – OOK modulation format to the alternative modulation formats, such as NRZ – DPSK or 2 – POLSK, which use provides a number of superior properties due to their abilities of providing greater protection from interchannel crosstalk, less exposed to expression of non-linear effects and better exposed to channel filtration, as well as less exposed to chromatic dispersion effect. By gradually introducing new system channels, can be increased the total transmission capacity of fibre, thus avoiding of core networks bottleneck effect and in the same time minimize growth of non-linear optical effect influences, because the alternative modulation formats are able to provide the

same BER levels as traditionally used NRZ – OOK, but only at lower input power levels.

The suitability of different chromatic dispersion compensation methods for the most efficient high – speed mixed HDWDM systems, with a 50 km of SSMF span length and following system's configuration: [1st channel: NRZ – OOK, 40 Gbit/s, 193.025 THz] – [2nd channel: 2 – POLSK, 40 Gbit/s, 193.100 THz] – [3rd channel: NRZ – DPSK, 10 Gbit/s, 193.175 THz], has been evaluated. This configuration represents channel's number, modulation format, per-channel data rate and channels' central frequencies at 75 GHz channel spacing. The following recommendations and conclusions are summarized below for the next

 The use of both CD pre- and post-compensation modules together, which might contain DCF of different length, allow to achieve better system's BER values comparing with pre-

fc (THz) 1st-3rd channel

8×10-26 2×10-24 8×10-23

2×10-16 7×10-28 7×10-19

1×10-31 1×10-24 1×10-40

1×10-27 3×10-14 1×10-40

> Vjaceslavs Bobrovs and Girts Ivanovs *Riga Technical University, Institute of Telecommunications, Latvia*

## **6. References**


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