Investigation of Allowed Channel Spacing for Differently Modulated Optical Signals in Combined HDWDM Systems

Realization of Mixed WDM Transmission System

“…The first one, the existing 10 Gbit/s Dense WDM system upgrade, but in fact it is the substitution of existing system with 40 Gbit/s DWDM system or faster, because the only 10 Gbit/s system components, which can be used in new 40 Gbit/s system, are fibre, boosters and some external modulated lasers, but all transmitter and receiver electrical parts with bandpass filters must be changed to a new one. The second one, channels compaction by location them closer to each other using smaller channel spacing between them, in that way increasing the number of channel in available transmission frequency spectrum "in [2,15]". In this case, the total transmission capacity increment is achieved only because of increasing the number of channels, as the individual transmission rate in each channel remains unchanged.…”

“…For the system's second channel on -off keying method and NRZ encoding is used, despite the fact, that NRZ -OOK modulation format is not well situated for high density WDM systems with a large number of transmission channels and high transmission rate and, as consequence of that, a high total transmission capacity. This modulation format can be used as a good foundation and reference point for comparison of different modulation formats, because it's traditionally used modulation format in optical transmission systems, due to its relatively simple realization and historical domination "in [2]". As modulation format for the system's third channel binary polarization shift keying (2 -POLSK) was chosen.…”

“…Channel's spectral efficiency can be increased in three different ways. The first one, the reduction of used system's channel spacing "in [2]". This means that a larger number of transmission channels can be allocated in available frequency band.…”

“…This means that a larger number of transmission channels can be allocated in available frequency band. The second one, the increase of per channel bit rate maintaining previously used channel spacing values for separation of transmission channels "in [10]". And finally the third one is the combination of pervious two ways.…”

“…This novel (or advanced) modulation formats provide narrower optical signals spectrum or multilevel encoding schemes that ensure more bits per one symbol than it is in traditional modulation formats, for example, on -off keying (OOK) with non -return to zero (NRZ) encoding format (NRZ -OOK) "in reference [10]". Maximal spectral efficiency, which can be obtained with traditional OOK modulation formats, is about 0.4 bit/s/Hz "in [2]". "It has been reported in ref.…”