XPM in dispersion-compensated links

Reducing residual dispersion: In the previous sections, XPM was studied in links with uniform dispersion maps, either DSF or SSMF fibre followed by a constant length o f SSMF conversion fibre. In the following experiments, a single span o f 40km SSMF fibre was combined with approximately 8km o f DCF fibre for exact dispersion compensation, followed by 22km o f SSMF conversion fibre, resulting in a similar span length as before. XPM- induced distortion was investigated for pre- and post-compensated links with 13dBm/channel launch power and the results for rrix versus are compared with DSF in Fig. 4.13. The values o f nix were a factor o f >4 lower than for the DSF experiments due to higher w alk-off between channels in the SSMF and DCF, approximately by a factor o f 10 as determined by the difference in fibre dispersion. For the 2.5Gbits/s experiment and 45km span length, the w alk-off increases by 1.8 bits for every A/l=lnm increase in channel spacing. The residual dispersion for both dispersion maps is shown in Fig 4.14. The higher distortion in the pre­ compensated (DCF+SSM F) span, shown in Fig. 4.14(b), was due to the larger positive residual dispersion averaged, Leff, because SSMF following the DCF+SSMF link increased PM-IM conversion. In Fig. 4.14(a), the residual dispersion over is m inimised reducing the subsequent PM-IM conversion o f XPM phase modulation. In summary, these measurements highlight that the combination o f increased walk-off overT<Le,^ and low residual dispersion effectively reduce XPM intensity distortion.

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Fig. 4.13 2.5G bit/s pum p-probe experiments, nix versus A4, lines: split-step Fourier algorithm, test-fibre followed by SSMF converter; ( • ) : SSMF+DCF, ( ^ ) : DCF+SSM F link, ( ■ ) ; DSF

Chapter 4: XPM pump-probe experiments 86 (a) (b ) Q S S M F D C F S S M F c o n v e r te r Q D C F S S M F S S M F c o n v e r te r

F ig . 4 .1 4 R e s id u a l d is p e rs io n D ’res(z), g iv e n b y e q u a tio n (2 .3 3 ), f o r ( a ) p o s t- c o m p e n s a tio n , (b ) p r e ­ c o m p e n s a tio n , fo llo w e d b y 2 2 k m o f S S M F (D co „v = + 3 7 0 p s/n m ). In (a ) th e d is ta n c e -a v e ra g e d r e s id u a l d is p e rs io n o v e r is m in im is e d re d u c in g P M - I M c o n v e r s io n a n d X P M in te n s ity d is to r tio n , in (b ) p r e -c o m p e n s a tio n in c re a s e s th e to ta l r e s id u a l d is p e rs io n

X P M interference: The interference between the XPM from the DCF and SSMF fibre in pre­ compensation was investigated in more detail for different periodic bit-sequences. Fig. 4.15 compares rrix versus AA for 1010...- and 1100...-modulation. The XPM m odulation index is plotted on a logarithmic scale to highlight its oscillation over the full range o f measured AT. For increased channel spacing the channel walk-off along the full DCF section increases by 2bit per nm resulting in re-alignment o f the periodic pattern after the DCF and, since the pow er is attenuated by only 4dB, additional XPM is generated in the subsequent SSMF. The additional PM from the nonlinearity o f the SSMF fibre can interfere constructively with the XPM components created in the DCF resulting in increased distortion. In contrast, the minima o f rrix indicate destructive interference between XPM generated in both fibres. In experiments reported in [HUI99], XPM distortion was shown to oscillate dependent on the bit-rate o f the pump. The oscillation period reduced with increased bit-rate and AT. However, bit-rate and AT both affect the walk-off and, therefore, the measurements shown in Fig. 4.15 are described by the same interference mechanism. In the case o f the 1010...-pattern maximum XPM distortion was measured for AT-0.8nm, 1.2nm and 1.7nm. The pulse width o f the bit-pattem was increased by a factor o f 2 reducing the bit-rate o f the pump channel. The 11001100... bit- sequence decreased the walk-off by 50% for the same AT resulting in higher XPM distortion and reduced oscillation o f m..

C hapter 4: XPM pum p-probe experim ents 87

In conclusion, this experiment shows for the first time the variation o f distortion due to intra­ span interference between XPM generated in two different fibre types. This mechanism is similar to the interference between XPM from different spans in a multi-span link. The extended analysis of the interaction between XPM contributions from different fibre spans will be presented in section 4.2 describing the BT-LEANET network.

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W avelength spacing (nm)

I'ig . 4 .1 5 M e a su re d X P M d isto rtio n /»,. vs. A 4 in D C F + S S M F link, c o m p a ris o n o f d if fe re n t p a tte rn le n g th . 1 3 d B m /c h a n n e l, p ro b e: C W , p u m p at 2 .5 G b it/s: 1 0 1 0 ...-se q u e n c e ( ^ ).

1 1 0 0 1 1 0 0 ...-s e q u e n c e ( ■ ). 2 .5 G b it/s

4.1.6 Inlluence o f bit-rate and pulse shape on XPM

An important parameter for XPM is the pulse width increasing the channel w alk-off at higher bit-rates. The pulse transition time decreases for high bit-rates and, therefore, the nonlinear chirp generated by each of these pulse edges in the pump also increases. Unlike other investigations [HU199] this section describes a new approach separating the effect o f pulse width and transition time on XPM-induced distortion. First, XPM was investigated for 65km of SSMF and AT=0.4nm channel spacing. The PPG allowed to vary the bit-rate whilst maintaining a constant 10-90% pulse rise time of A/=56ps. Fig. 4.16 shows a comparison o f the experimental waveform at the EAM output for 2.5Gbit/s and I OGbit/s pump modulation indicating, constant rise time at all bit-rates.

Chapter 4: XPM pump-probe experiments

•t

I

4— ► lOOps

time

Fig. 4.16 Pump channel in back-to-back configuration, 1 OGbit/s (— ) and 2.5Gbit/s (---), the dip at 2.5Gbit/s is due to electrical reflections o f the EAM connectors

B it-rate: Initially, the bit duration was varied from ]^=1000ps (1 Gbit/s) to 7i)=100ps (1 OGbit/s) whilst maintaining a constant amount o f XPM chirp induced by each pulse edge. The short pulses at high bit-rates resulted in high distortion as shown in Fig 4.17. Close spacing o f the pulse transitions at high bit-rates and the fibre dispersion cause interference between neighbouring peaks affecting the shape o f the XPM distortion and increasing its magnitude by approximately a factor o f 2. This overlapping o f individual components occurred for >5Gbit/s whilst the distortion rrix remained relatively constant at lower bit-rates. The shape o f XPM distortion is characteristic only for the amount o f walk-off experienced during the nonlinear interaction. For low bit-rates (1 Gbit/s) and w alk-off (<1 bit), the shape o f the XPM intensity distortion in SSMF fibre is very similar to the DSF results shown in Fig. 4.7. The variation expected for rrix would be even greater in practical systems because at bit- rates lower than 1 OGbit/s the bandwidth is normally reduced and, therefore, the pulse rise time would typically be longer than 56ps used throughout the experiments.

Pulse shape: The influence o f the pulse rise time At on XiPM distortion was calculated numerically by a variation o f the pulse shape for the SSMF link, and the results are shown Fig. 4.18. The pump pulses were modelled by filtering square pulses with a sixth order Bessel filter. For a given bit rate, the distortion increases for sharper pulse edges due to increased chirp introduced by each transition. At low bit-rates, the pulse shape has the strongest influence on rrix increasing the distortion from mx=0.\ at A/=100ps to mx>0.2 for A/=20ps. A comparison with Fig. 4.17 reveals that at bit rates below 5Gbit/s the peaks due to XPM distortion are still isolated and, therefore, rrix is only determined by the amount o f distortion due to the isolated transitions. A t A/=56ps, rrix versus bit-rate for the experimental waveform distortion can be obtained as shown in Fig. 4.18.

Chapter 4: XPM pump-probe experiments 89 Experiment Simulation m Ê o z 0.5 8 0 0 1000 0

200

Fig. 4.17 D ependence o f XPM distortion in probe channel on bit-rate, pump; 1-1 OGbit/s, 1010...- sequence, single SSMF span (Z,=65km), 13dBm/channel, A/1-0.4nm, sim ulation: split-step Fourier algorithm, pulse rise time: A/=56ps

This measurement highlights the interaction o f non linearity and dispersion at bit-rates up to I OGbit/s. The XPM efficiency in equation (2.31), an indicator for the amount o f PM generated, decreases at high bit-rates due to increasing channel walk-off. However, this effect is reduced by fibre dispersion leading to constructive interference between adjacent distortions. The effect o f distortion is enhanced by shorter pulse transitions at 1 OGbit/s increasing the amount o f chirp generated by each transition. These results [THI98] were

Chapter 4; XPM pump-probe experiments 90

confirmed by the work o f [HUI99] which reported increased XPM distortion with bit-rate. In this section the worst case o f XPM distortion in a single span o f a longer system is described, since for multiple spans would no longer experience a continuous increase with bit-rate, but would be more complex due to interference o f XPM generated in different spans. This multi-span interference is described in section 4.2 for the BT-LEANET link.

0.3 At=56ps (experiment) 0.25

^

Fig. 4.18 Distortion o f probe calculated for L=65km o f SSM F in dependence o f pulse rise time At

and bit-rate, pump: 1010... bit-sequence, A4=0.4nm channel spacing, (■ ): 1.25Gbit/s, ( • ) : 2.5Gbit/s, ( ^ ) : 5Gbit/s, (▼): 1 OGbit/s