**2.1. Basic principle of WDM**

Wavelength division multiplexing (WDM) is a technology used to combine or retrieve two or more optical signals of different optical center wavelengths in a fiber. It allows fiber capacity to be expanded in the frequency domain from one channel to more than 100 channels. This is accomplished by first converting standard, non – WDM optical signals to signals with unique WDM wavelengths that correspond to the available channel center wavelengths in the WDM multiplexer and demultiplexer. Typically, this is done by replacing non – WDM transceivers with the proper WDM channel transceivers. WDM channels are defined and labeled by their center wavelength or frequency and channel spacing. The WDM channel wavelength assignment is an industry standard defined in ITU – T documentation. Then the different WDM signal wavelengths are combined into one fiber by the WDM multiplexer. In the fiber, the individual signals propagate with little interaction assuming low signal power. For high powers, interchannel interaction can occur. Once the signals reach the fiber link end, the WDM demultiplexer separates the signals by their wavelengths, back to individual fibers that are connected to their respective equipment receivers. Optical receivers have a broad reception spectrum, which includes all of C (1530- 1565 nm) band. Many receivers can also receive signals with wavelengths down to O (1260- 1360) band "in [11]".

234 Optical Communication

Obviously that it is easier to achieve a larger channel's spectral efficiency if for optical signal modulation and coding some of novel modulation formats are used. 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. [7,8]" that using such novel modulation formats as quadrature amplitude modulation (16 – QAM particularly) and orthogonal frequency – division multiplexing(OFDM) together with polarization division multiplexing (PDM) technique it

Study object of this chapter is optimal mixed WDM system configuration that provides lowest in system's channels detected signals BER values. This developed mixed WDM system's model is offered for the future design of backbone optical networks and can be considered under the concept of next generation optical network (NGON). Chosen optical signal modulations formats and per channel bit rates, according to authors' thoughts, are the most appropriate and probable at this moment. It was concluded after careful evaluation of current state of optical telecommunication networks, their possible and the most likely

With the explosive growth in demand for capacity in national, regional, and metropolitan optical networks, high spectral efficiency wavelength division multiplexing transmission becomes essential. Current WDM optical transport systems are primarily based on 10 Gbit/s channels that are modulated with on – off – keying on a 50 GHz channel grid. Capacity upgrade of these systems calls for 40 Gbit/s wavelength channels to be carried in the same

Wavelength division multiplexing (WDM) is a technology used to combine or retrieve two or more optical signals of different optical center wavelengths in a fiber. It allows fiber capacity to be expanded in the frequency domain from one channel to more than 100 channels. This is accomplished by first converting standard, non – WDM optical signals to signals with unique WDM wavelengths that correspond to the available channel center wavelengths in the WDM multiplexer and demultiplexer. Typically, this is done by replacing non – WDM transceivers with the proper WDM channel transceivers. WDM channels are defined and labeled by their center wavelength or frequency and channel spacing. The WDM channel wavelength assignment is an industry standard defined in ITU – T documentation. Then the different WDM signal wavelengths are combined into one fiber by the WDM multiplexer. In the fiber, the individual signals propagate with little interaction assuming low signal power. For high powers, interchannel interaction can occur.

system "in [5,9]". To achieve this, several technical challenges need to be resolved.

can be achieved SE larger than 6 bit/s/Hz and even reaches 7 bit/s/Hz.

development strategy and trends in the future.

**2.1. Basic principle of WDM** 

**2. Necessity of mixed WDM transmission system** 

The process of combining and separating the different wavelengths in the fiber is the basis of WDM technology. As a rough analogy, a WDM demultiplexer can be thought of as a prism separating different colors of light from the incident white light ray. Each color of light can represent a unique WDM signal. The white light is the aggregate of all colors of light and represents all the WDM signals propagating in the fiber. It should be noted that WDM wavelength and WDM channel have different meanings. WDM wavelength refers to the center wavelength of an optical signal or a WDM channel. WDM channel refers to an optical signal communications path that is defined by a center wavelength and a spectral pass band. It can be full duplex, half duplex, or simplex. Most telecommunication systems require full duplex communication channels. All full duplex fiber communication channels require two optical signals, one in each direction, to work properly. For a two fiber full duplex WDM system, the two laser signals (one at both ends of the fiber link on different fibers) are assigned the same wavelength, which is referred to as the channel assignment. However, in cases where only one fiber is used for full duplex communications, both lasers will have different wavelength assignments and therefore communication channel uses two WDM channels. Typically, most WDM systems will use two fibers for a WDM link. Therefore, the WDM wavelength assignment is the same for both lasers for that channel and is referred to as the channel assignment.

The heart of any WDM system is the basic WDM (also referred to as passive WDM). The basic WDM only consist of optical filters without any electronics. Therefore, it is completely passive and highly reliable. The filters are designed to pass a selected light spectral range, referred to as a channel, with low loss and reject or reflect all other spectrum. A combination of these filters results in a multichannel WDM multiplexer or demultiplexer. At the transmit end, the multiplexer combines the unique wavelengths from each channel port into one common fiber and at the receive end the demultiplexer coupler separates the combined signals from the common fiber to their respective channel ports. For some systems, the multiplexer and demultiplexer units are the same and are interchangeable (universal units) "in [11]".

In recent years the dramatic increase of demand for transmission capacity is observed and to secure an appropriate quality of service (QoS) level telecommunications services providers must constantly and continuously develop their transmission systems in use "in references [4,12]". Currently, as a study object of many scientific works have been chosen and focused directly on the fibre total transmission capacity increases, and this can happen in a three different ways. 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. And the third way, total transmission capacity increment, using channel compaction with simultaneous increment of individual channel's transmission bit rate.

It is clear, that none of the proposed fibre's transmission capacity increment solution can be realized immediately, but it requires a certain amount of time and work, as any solution should be implemented gradually in several stages to avoid unnecessary problems.
