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After characterisation o f the UNI in isolation all-optical regeneration was investigated in 40Gbit/s transmission experiments. In this section, multiple 40Gbit/s optical 3R data regeneration in a transmission link using a SOA-based nonlinear interferometer and electronic clock recovery is discussed. The cascadability, jitter tolerance and amplitude noise suppression o f the regenerator were investigated and the error-free transmission was increased to distances greater than 2000km. The experimental set-up shown in Fig 6.29 combines the loop described in Fig. 6.7. The UNI-based 40Gbit/s data regeneration was investigated before and after loop transmission and after each recirculation.

The transmitter consisted o f a lOGHz external cavity mode-locked laser (EC-MLL #1) operating at 1549.6nm, which produced 7ps pulses after nonlinear compression in 100 meters o f dispersion decreasing fibre (DDF), to remove residual chirp. The pulse width was similar to the value used in section 6.1 and optimised for transmission. As before, lOGHz RZ pulses were modulated by a 2^-1 PRBS data sequence using a lithium niobate modulator, and passively multiplexed to 40Gbit/s using a fibre interleaver. The output o f the last fibre span (DSF 3) was used to drive the regenerator in case the UNI was operated inside the loop.

40Gbit/s transmitter demux (a) UNI before loop (b) UNI after loop 40G bit/s M U X EC-M LL#]

XL

data _{Recirculating loop}

lOGbit/s BERT 2^1 AOM-1 20:80 AOM-2 DSF 2 DSF 3 DSF 1 (c) UNI inside loop

Fig. 6.29 40G bit/s recirculating loop experim ent investigating o f all-optical regenerators. DSF 1-3: L ;,=33km each, EC-M LL #1 : external cavity m ode-locked laser used as transmitter, (a), (b), (c) indicate the configurations investigated in UNI transmission experiments, EC-M LL #2 (clock for UNI) and the clock recovery unit are included in the UNI symbol

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The degraded 40Gbit/s data signal at T, was split using a 3dB coupler before entering the UNI. One output was used as the switching signal to drive the SOA-based optical gate. The second output o f the coupler was fed into an electronic clock recovery unit, which is based on an electronic phase-locked loop discussed in section 6.1.4. The recovered lOGHz clock signal at the output was subsequently used to drive a second RZ source (EC-MLL #2) producing TfWHM=^V^ pulses at T2=1550.5nm. This laser was optimised for short pulse width and operation with the UNI. This lOGHz optical pulse stream o f EC-M LL #2 was then passively interleaved to 40GHz, and used as a high-quality clock signal to the UNI. The regenerated signal with 2-2=Ti was (re-) launched into the loop or directly detected. For detection, the loop output signal was demultiplexed to lOGbit/s using an electro-absorption modulator, and then analysed with the BERT.

Loop transmission without UNI: The BER o f the 40Gbit/s PRBS pattern was measured as a function o f transmission distance using the recirculating loop. For loop operation without the regenerator, error-free transmission at 40Gbit/s could be achieved up to 1200km when using the EC-MLL #1 at T=1549.6nm. As described in section 6.1.6 the main limiting factor was timing jitter due to the residual dispersion totalling +0.2ps/(km*nm) at the operating wavelength.

Configuration (a) - UNI before loop: Shortening the pulse width o f the transmitted pulses from Tf w h m =7ps to 4.5ps, either by locating the regenerator directly after the transmitter, or

exchanging the pulse source with EC-MLL #2, reduced the error-free distance to 200km. A simulation o f the transmission link as discussed in section 6.1.6 predicts a jitter-lim ited transmission distance o f approximately 2000km at T=1550.5nm launching 4ps pulses. This is a reduction o f approximately 20% in distance compared to a system using 7ps pulses. However, the significant difference between the calculated transmission distance and the measurements using the EC-MLL #2 confirms the strong dependence o f the achievable transmission distance on the light source and show that this distance depends critically on the properties o f the probe source such as pulse width and jitter. In the following, the EC-M LL #2 was chosen as the local source supplying the RZ probe pulses for the UNI. In this case, the results o f this measurement, approximately 200km, determine the maximum transmission distance between two regenerator stages.

Configuration (b) - UNI after loop: W hen the regenerator was placed at the output o f the loop, the error-free transmission distance was reduced from 1100km to 700km. This was due to the finite switching window o f approximately A r=8ps o f the UNI estimated by the flight

Chapter 6: OTDM experim ents 184

time difference o f 15ps determined by the physical dimensions o f the SOA, and the pulse widths of the two interacting pulses. In the case of transmission without UNI the switching window was given by the demultiplexing EAM and corresponds to half the bit-period, I2.5ps for 40Gbit/s transmission. The degree of timing jitter of the incoming data bits tolerable for the interaction of the clock and signal pulse is determined by the width o f the switching window A ra n d determines the error-free transmission distance. According to [AGR95] the jitter-limited distance scales as A r'^ with the switching window o f the UNI. Therefore, with the UNI connected to the loop output, the expected transmission distance was approximately 800km, in good agreement with the BER measurement.

1E-05 1E-06 - 2 5p s A n / 1E-07 - a: 1E-08 - LU CO 1E-09 -

o o o o o o o o

Fig. 6.30 Bit-error-rate (BER) vs. distance measurement for 40Gbit/s signal (2^-1 PRBS).

No regeneration: ( X ) : loop transmission without regenerator using EC-MLL #1, ( ♦ ) : loop transmission using EC-MLL #2 as transmitter,

R egeneration: (A ): regeneration after transmission using EC-MLL #1 for loop transmission, (■): transmission o f regenerated signal from EC-MLL #2, (O): transmission with 20 cascaded regenerators (BER <10'^)

C onfiguration (c) - UNI inside loop: The UNI was placed inside the loop giving an effective regenerator spacing o f 100 km. Error-free transmission was extended to greater than 2000km, an order of magnitude greater than that achieved with the direct transmission o f EC-MLL #2 and almost twice as far as in case o f optimised transmission at T=I549.6nm using EC-MLL #1. This fact clearly demonstrates the cascadablility of the regenerator, and the associated reduction in jitter accumulation since the maximum jitter is only determined by the fibre span between two regenerators. Fig. 6.31 shows the results of lOGbit/s BER measurements made on the 40Gbit/s input signal, the 40Gbit/s signal after a single regeneration, and the 40Gbit/s regenerated signal after 2000km transmission through 20 regenerators.

Chapter 6: OTDM experiments 185 - 5 " -6” UJ m w o _i -8" - 9 " -1 0" -20 -12 -10 -22 -1 8 -16 -14 Power, dBm

Fig. 6.31 BER measurem ent vs. received power for ( ♦ ) : 40G bit/s input back-to-back, (A ): 40Gbit/s signal after a single regeneration, (+): 2000km transm ission using 20 cascaded regenerators

The penalty o f the 40Gbit/s signal after a single regeneration compared with the 40Gbit/s input back-to-back was only 1.4dB, lower than the value 2.2dB obtained in the experiments described in section 6.2.2. This is due to the shorter pattern length o f 2^-1 and the different clock source (ECL-MLL #2). By adding 20 regenerative stages the system still operated error-free and the penalty was only increased by a further 1.9dB, corresponding to a penalty o f only O.ldB per stage.

Chapter 6; OTDM experiments 186

6.3 Summary

In summary, all-optical 3R regeneration was investigated using a semiconductor ultra-fast nonlinear interferometer (UNI). Initially, 40Gbit/s all-optical data switching, without wavelength conversion, was demonstrated using a pattern length o f 2^’-l. By varying the delay o f the locally generated clock with respect to the incoming data stream, 40Gbit/s to lOGbit/s regenerative demultiplexing was also achieved. The penalty o f the UNI due to varying polarisation o f the switching signal was < ld B and can be further reduced by polarisation insensitive SOA devices. However, the long-term stability o f the UNI depends critically on the polarisation fluctuations o f the RZ pulses providing the clock pulses for the UNI. The performance o f the UNI can be further improved by minimising the spurious reflections from the SOA facets, causing in 1.3dB penalty when operating in counter­ propagation. The temporal switching window o f the UNI must be decreased to enable operation at bit-rates beyond 40Gbits/s. However, the reduced switching window requires to reduce timing jitter o f the incoming signal, e.g. by reduced regenerator spacing.

The UNI was also investigated in 40Gbit/s transmission experiments. W hilst the short optical pulses required for correct operation o f the regenerator resulted in a non optimised transmission performance, it was demonstrated that cascading regenerators significantly increased the error-free transmission distance to greater than 2000km. Transmission involving 20 cascaded regenerative units was successfully demonstrated, confirming the jitter suppression anticipated for these devices. Experiments involving regeneration after loop transmission indicate that timing jitter o f the transmission system can be a limiting factor for the further increase o f the spacing between regenerators.

C om parison W D M - O T D M : In this thesis nonlinearities in W DM and OTDM transmission were investigated. A typical WDM transmission system investigated in chapter 4 and 5 consisted o f two or more lOGbit/s channels using the NRZ format. In contrast, the OTDM system described in chapter 6 used a single channel at 40Gbit/s transmitting a RZ signal. The different transmission formats are compared on the basis o f the experiments described in this thesis.

Clearly, in single channel RZ transmission SPM can be used to counteract the linear pulse broadening due to dispersion, increasing the error-free transmission distance. However, the OTDM system required an accurate control o f dispersion map, power and wavelength. The wavelength tolerance for transmission over more than 1000km was shown to be less than Inm. The generation o f short pulses also required a more complex transmitter set-up than in a WDM experiment with NRZ pulses. However, due to the digital nature o f single channel RZ

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transmission, all-optical regeneration could be used to overcome pulse distortion and timing jitter. The concept o f regeneration reduced the sensitivity o f OTDM to the dispersion map, power and wavelength. Although the regeneration and cascadability o f the UNI was demonstrated at 40Gbit/s, the successful operation required a careful and constant control o f the signal polarisation. In addition, the short pulses required for the operation o f the UNI resulted in non-optimised transmission performance limiting the distance between two subsequent regenerators. Increasing the capacity o f the single channel RZ transmission, all- optical regeneration based on the UNI will become more complex, since all channels must be demultiplexed before regeneration. The use o f W DM for RZ-based systems must also ensure that dispersion and nonlinearity are balanced for all wavelengths requiring the use o f dispersion slope compensation.

Relatively simple NRZ-based W DM transmission used the total transmission bandwidth by adding channels at different wavelengths. However, in this thesis multi-channel nonlinear effects were shown to degrade the transmission performance significantly for narrow channel spacing and high launch power. Unlike RZ pulses in OTDM, the NRZ pulses were distorted by XPM and dispersion in a non-uniform way, and the distortion could not be cancelled but only minimised by reducing the residual dispersion o f the link. In particular long sequences o f ‘ I ’-bits experienced XPM -induced intensity distortion whilst RZ pulses in the single channel experiments were mainly degraded by timing jitter.

Chapter 7: Summary and conclusions 188

Chapter 7