**Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval**

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

#### **1. Introduction**

190 Optical Communications Systems

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Nowadays, a skyrocketing growth is observed worldwide in the bit rates of transmitted information, which is associated with development of broadband information transmission types. The annual global internet protocol (IP) traffic will exceed half a Zettabyte in four years. At just under 44 Exabytes per month, the annual run rate of traffic in the late 2012 will be 522 Exabytes per year. Driven by high-definition video and high-speed broadband penetration, the consumer of IP traffic will bolster the overall IP growth rate so that it sustains a steady growth rate through 2012, growing at a compound annual growth rate (CAGR) of 46 percent (see Fig. 1) [Cisco Systems, 2008].

Fig. 1. Global IP Traffic Forecast (2006–2012) [Cisco Systems, 2008].

In turn, to provide high-quality transmission it is necessary to develop the next generation optical networks (NGONs) that would transmit properly huge volumes of information. The optical transmission systems from the very outset have been able to offer new possibilities for solving problems of ever increasing urgency that are dictated by the need for frequency bands and transmission speed. Such networks have become one of the most important

Realization of HDWDM Transmission System with the Minimum Allowable Channel Interval 193

The complexity of a system's design in optical communications can be seen as the result of a large number of components with different parameters and operational states. The description of the interaction between the optical signal and transmission disturbances is a multi-dimensional issue, whose solution depends on the relation between different system parameters. The right approach to the optimization of system settings and derivation of design rules must take into account the interaction of effects which take place in each component. In this section, the system components needed for realization of an HDWDM

The role and realization of an optical transmitter become important with increased channel data rates in the system. While the optical transmitters at lower channel data rates are less complex and easier to realize by direct modulation of a laser diode, the realization becomes more complex with the increasing channel data rate, thus raising the requirements on electrical and optical components of the optical transmitter. The conventional optical transmitter employs the amplitude/intensity modulation (AM, IM) of the laser light (better known as on-off keying (OOK)), because different signal levels for marks and spaces are characterized by the presence of optical power. The amplitude modulation can be realized by direct or external modulation of the laser diode. For the realization of transmission systems with channel data rates larger than 2.5 Gbit/s, the external modulation presents a better solution, because the impact of laser internal chirp on optical signal can be reduced

efficiently, but, on the other hand, the complexity of optical transmitters increases.

Fig. 2. Mach-Zehnder Modulator (MZM) principles: a) structure b) transmission function

External modulation can be realized with a LiNbO3-based Mach-Zehnder modulator (MZM) (see Fig. 2) [Kaminow et al., 2009]. The operational principles of MZMs are based on the electro-optic effect, which is characterized by variation in the applied electrical field causing changes of the refractive index in the modulator arms. The variation of the refractive index in the modulator arms induces a change of material propagation constant *β*, resulting in different phases in both modulator arms. The input optical signal is divided by a 3-dB coupler into two equal parts – in lower and upper arm of the MZM. The external modulator is driven by an electrical signal with corresponding data rate. Depending on the electrical driving signal,

**2.1 Selection of HDWDM main components** 

transmission system are described.

[Kaminow et al., 2009].


components in the telecommunication hierarchy, whose integration with standard network services and applications promotes rapid evolution of fiber optics and its wide implementation into all telecommunication branches (see Table 1 [McGloin & Reid., 2010]).

Table 1*.* Evolution of fiber optics and its wide implementation into all telecommunication branches

Currently, many research topics in the field of optical transmission systems (mostly grounded on novel modulation techniques) are focused on increasing the total data transmission speed of an individual optical fiber [Abbou et al., 2008, Bhamber et al., 2007, Bobrovs et al., 2008]. An alternative − but equally valid − approach to increasing the data transmission is to decrease the wavelength division multiplexing (WDM) channel spacing to high-dense dimensions while keeping the existing data transmission speed for an exact channel [Ozoliņš et al., 2011, Bobrovs et al., 2009].

High performance optical filters make the groundwork for realization of high-speed highdensity WDM (HDWDM) transmission systems [Pfennigbauer & Winzer, 2006]. High channel spacing and data transmission rate set strict requirements for HDWDM filter characteristics, so any imperfections in their parameters, such as amplitude and phase responses, could become critical. The low channel separation from adjacent channels is one of these imperfections in optical filter parameters [Agrawal, 2001, Ozoliņš et al., 2009].
