1.2. FORMULACIÓN DEL PROBLEMA
2.2.5. Estados Financieros
The performance of dispersion managed standard fibre by the use o f dispersion correcting fibre has been explored. It transpires that positioning the negative dispersion fibre immediately after the amplifier, preceding the positive dispersion fibre, gives better perform ance in terms of eye-penalty and, in the presence of noise, higher Q. A lthough the D CF has higher nonlinearity due to its smaller effective area, and in terms o f minimising the nonlinearity w ould therefore be best positioned at a point o f low est pow er, i.e. after the SM F, the effect o f its high dispersion and its high loss conspire to reduce the impact of SPM on an intensity modulated channel. As the D CF dispersion is negative the SPM acts to broaden the linearly dispersing pulses. The dispersion and loss w ork to broaden the pulses and reduce their peak power in a shorter distance than the distance over which SPM maximally chirps the pulses. The dispersed and chirped signal enters the SM F and experiences a relatively smaller but positive dispersion which com presses the pulses to widths narrow er than at input. Repeated at each span the pulses are further narrow ed and the eye-diagram opened. A lOOps (lOGb/s data rate), 4m W peak pow er pulse over a num ber compensated spans was studied. The narrow ing of a pulse corresponds to an increasing spectral width which eventually becomes distorted by the finite width o f the optical filter. Increasing the filter can alleviate some of the eye-closure: w idening the filter above a FWHM of 40G H z is possible in a single channel, relaxing the constraints for an optical filter. This has two potential draw backs. The noise pow er passed to the receiver w ould increase from both the ASE noise from amplifiers and spurious pow er from any neighbouring channels. This highlights the need for larger channel spacing in lOGb/s W D M system. W ith smaller bit-rates the constraints arising from spectral broadening are reduced and it was found that capacity o f the fibre, in terms o f transm ission-bit-rate product, was greatest for a 5Gb/s channel: halving the bit-rate to 2.5G b/s does not produce a doubling in the transm ission distance nor doubling the bit-rate to lOGb/s produce less than half the transm ission distance. This “optimum” bit-rate for a single channel performance will be examined in the context of multi-channel performance in the next chapter.
Returning to the lOGb/s channel propagation and comparing it’s performance against the tw o other dispersion managed 50km-span schemes suggests that the D C F+SM F fibre configuration out-performs both the -/+DSF and the DSF+SM F schemes. In term s o f the effect o f nonlinearity and dispersion this is indeed the case. Examining the loss o f the D CF+SM F and the -/+DSF spans reveals that there is 15.6dB and 10.5 dB loss in each span, respectively. The D CF+SM F fibre com bination has ju st over 3 times the loss o f the D SF span. The loss of the D SF fibre is 0.21dB/km. By increasing the D SF span length to 74.3km the total loss matches that of the D CF+SM F span. The path averaged pow er [3],
which takes into account the attenuation of the fibre, is given as P ^ = P J , , where is the ratio of effective length to span length, L^ffU^span- the 50km span o f D SF 0.38 but equals 0.27 in 74.3km of DSF. This is the value of D C F+SM F. If the span length of DSF is increased to 74.3km (i.e. 37.15km D SF with -4ps/nm /km follow ed by the same length o f positive GVD) the resulting 3dB distance against pow er relationship is plotted in Figure 5.23, alongside the 50km -span and the D C F+SM F line. The 3dB distance can be sum m arised as
~ 16573? (74.3km span DSF) (5.7)
The slope o f the fitted line is 1.39 times larger than that for the 50km -span, equation (5.2), which is the approximately the same factor as the ratio o f for the tw o span lengths, 0.377 to 0.270. Thus with equal path average pow ers in 50km of D SF or D C F+ SM F spans the transm ission distance for equivalent eye-penalty is greater in the -/+D SF management scheme than for the com pensated standard fibre schem e. Equivalently, for the same input pow er, the 50km -span D SF can be extended by just under one half to 74.3km , giving the same span loss and path averaged pow er as the D C F+SM F span, and longer a transmission distance can be obtained. Applying similar analysis to D SF+SM F gives a of 0.343 which again is more than in D C F+ SM F value of 0.270. Rescaling the path average pow er increases the slope o f equation (5.3) to
15244km /m W - within 50 (km) of the slope o f D C F+SM F, equation (5.4).
10000 0» u c 5C/3
5
15I
Qi w PQ B 1000 --- ''"s ... ...Vs ^ ^ ... 1.1 h . — ^ f i I ^ ■ 75 km span □ 50 km span . 1 ■Ss > 1, 1 k ' l l 2 3 4 Pow er (m W ) 6 7 8 9 10Figure 5.23 Comparison of 3dB transmission distances against peak input power for the dispersion managed -/+DSF scheme (4ps/nm/km) with span lengths of 50 and 75 km
In single channel operation dispersion managed dispersion shifted fibre provides the best perform ance in the presence o f self-phase modulation and dispersion. This management schem e w ould be suited to the construction o f new netw orks where the fibre parameters can be specified at the design stage. U pgrading the existing standard fibre netw ork is possible with Dispersion Compensating Fibre w hich can improve transm ission perform ance close to that of D SF even though the D CF has a higher nonlinear coefficient. The perform ance of the DCF-kSMF arrangement is very susceptible to inexact matching of the fibre dispersions due to their large dispersion values w hich quickly accumulate with each span if left uncom pensated. If inexact compensation cannot be avoided it has been
show n that under-com pensation is preferable to over-com pensation, i.e. shorter
com pensating fibre than the optimum. With the behaviour o f a single channel in the presence of SPM and GVD understood the addition o f other channels within such system s will now be addressed.