works
The dynamic aspects of wavelength routed WDM optical networks are related to variations of the power at each wavelength or in the number of wavelengths of the network. The investigation of the dynamic phenomena is related to the network performance analysis. In optical networks, the EDFAs are operating in gain saturation. The gain saturation can be obtained, for constant pump power, by changing the total input power. Thus, when the input power varies, either by changes in the number of channels or by variations in channel input power, variation of the output power of all amplified channels occurs until re-balance can be reached between the absorption and emission processes. The time between the previous balance and new balance is specified by a transient period and dynamic variation in optical power, and is referred to as an optical power transient. The rate at which the EDFA gain changes during the transient period is determined by both the spontaneous lifetime of the metastable state and the total amount of saturated output power. The transient periods of gain saturation and recovery were found to be, in the early period of EDFA development, a few hundred µs [56, 100]. The effective transient period of multichannel WDM optical networks, which are characterised by high output powers, are specified by tens of µs [101]. One of the major advantages of an EDFA is the absence of inter-channel crosstalk among WDM channels by the signal modulation. Thus, the gain of the EDFA is not affected by the signal modulation in high speed data transmission rates over a few Mbit/s. This is the reason for using steady state EDFA models to describe high speed data transmission systems [102].
Research and development has been concerned with the optical power transient effects since the early stages of EDFA applications in optical networks [103]. In point-to-point links of the network, the power transient can be as high as a hundred milliwatts. When the system has been switched-on (including a pump source) for a while, if the input signal is instantaneously applied to the link, then a high transient power occurs which may damage electrical components in the receiver or optical components on the link. Transient power also occurs at system restart after a failure of the link cable.
The power transient phenomena have been characterised in the International Telecom- munication Union (ITU) Recommendation G.663. This document specifies the minimiza- tion methods of the optical power transient or surge phenomena in the network link using EDFAs, such as the pump power must be reduced or switched-off when any signal failure is observed, and that after the signal channel become available, the restart of the pump laser must be performed gradually [104]. Transient phenomena can be very significant in wide- area optical networks such as long-haul submarine systems. Therefore, a lot of research has been done to mitigate the effect of the transient phenomena in wavelength routed WDM net- works using EDFAs, a brief review of protection schemes of WDM networks is presented in Section 1.9.
Regarding the dynamic behaviour of the EDFA in an optical wavelength routed network, the optical power traversing the cascaded EDFAs in the link will vary according to system management requirements. For example, in channel reconfiguration, adding a new wave- length during network expansion or dropping of wavelengths during link failure. These events affect the total power input to the link because, in wavelength routed WDM networks, the wavelengths from each input link are distributed by OXC and OADM (as discussed in Chapter 1) to different output links according to the routing information. Power transients in a network link with cascaded EDFAs will induce transients in the remaining channels in the link [105]. Analysis shows that the speed of evolution of the power transients at the output of cascaded EDFAs is a multiple of the output of the single EDFA. Computations for an optical link with cascaded EDFAs showed that for the loss of a large number of channels, the power of surviving channels can alter by 1 dB within a time duration of 100 ns [106].
Moreover, in packet-switched optical networks, if the packet traffic is directly fed into the specified wavelengths, then this can result in variation in the total power applied to each
fibre link. Thus, cross-gain saturation can induce effects in channels similar to the transient phenomena of circuit switched networks resulted from channel addition or dropping [107– 109]. The numerical and statistical analysis of bursty traffic is the main research topic of this thesis. Services of the optical network, such as gigabit Ethernet local area networks, multimedia, applications of variable bit rate video have been found to be associated with variable traffic behaviour [110]. The investigation has indicated that the variation increases substantially with packet interval times [111, 112], and that the traffic characteristic is self- similar in these networks [112]. Increased variability produces self-similarity, thus longer intervals of higher or lower input power are more likely to happen, this gives time for an EDFA to achieve very different gain values or cause output power swings [113, 114]. The power swing at the output of each surviving channel, due to cross-saturation effects in the amplifiers, deteriorates the quality performance of the channels through four conditions or mechanisms when channel loading varies in the network links.
First, dropping of wavelengths during link failure, increases the power in the optical link. This power increase in the link, introduces problems of optical nonlinearity. There are five main optical nonlinear effects that can deteriorate the propagating signals in the optical networks: SBS, SRS, SPM, XPM and FWM. These five non-linear effects are discussed in more detail in Section 1.7.7. The impact of these five nonlinear effects on the WDM optical network performance manifest themselves in four different ways.
Second, network reconfigurations when system expansion is demanded may require the addition of some channels, the optical power at the receiver can be reduced during the tran- sient period which results from channel addition, which would increase the noise level. If the signal power at the receiver is lower than the receiver sensitivity, bit-error rate would strongly deteriorate [52].
Third, the power swings during the transient period may cause deterioration of optical SNR due to the change of inversion level, and also the change in gain spectrum during transient phenomena [52].
Fourth, the power swings during the transient period at the receiver may require fast optimization techniques for the receiver threshold power, this might be problematic for par- ticular receivers [52].
mance, it is essential to implement the dynamic models (2.11), and (2.21) mentioned in Sec- tions 2.3, and 2.4. The numerical simulation techniques used for accurate implementation of the gain dynamic models are discussed in Section 2.9.