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The challenge of upgrading a previously installed standard fiber base to operate at single channel bit rates of 40 Gb/s is attracting considerable attention. Use of the return-to-zero (RZ) modulation format together with strong dispersion management has emerged as a promising solution. In strongly pulse-overlapped RZ transmission systems, the intrachannel impairments dominate the interchannel ones.

Intrachannel nonlinearities – cross-phase modulation (IXPM), and four-wave mixing (IFWM) – are responsible for the time shifts and energy exchange between interacting pulses. The energy exchange due to the intrachannel FWM leads to the generation of ghost pulses.

To generate the 40 Gb/s optical signal we have used a CW laser source, Mach-Zehnder modulators driven in a push-pull configuration to get a chirp free transmission, a NRZ pulse pattern generator, and a sinusoidal electrical signal generator. The NRZ format is generated by means of the MZM modulator, which is driven by a 40 Gb/s NRZ electrical signal and modulates the optical signal generated by the CW laser.

The RZ signal is generated using two MZM modulators (with normal bias) concatenated. The first modulator generates an NRZ optical signal. A sinusoidal electrical signal with the frequency of 40 GHz is applied to both electrodes in the second MZM to generate the RZ signal.

The CSRZ signal is generated in a similar way to the RZ format, however, the frequency of the sinusoidal electrical signal applied in the second MZM has half the bit rate, 20 GHz. The second MZM was biased at extinction in order to provide alternating optical phases between 0 and for the neighbouring time slots. To generate the MDRZ, a NRZ duo-binary signal was created using a delay-and-subtract circuit. This signal drove the first MZM (biased at extinction). The second modulator (with normal bias) was driven by a sinusoidal electrical signal with the frequency of 40 GHz. In order to avoid recursive decoding in the receiver, a duobinary precoder was used. The duobinary precoder was composed of an exclusive-or gate with a delayed feedback path.

Results – DWDM Network Design Software
The signal spectrums obtained for the different modulation formats are illustrated below.

Fig 1. NRZ

Fig 2. RZ

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Fig 3. CSRZ

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Fig 4. MDRZ

CSRZ has smaller bandwidth than that of RZ. It suppresses the unmodulated carrier, but adds tones at NB/2 frequencies away from the carrier frequency (N is the odd integer #, and B is the bit rate). A very important property of this format is the introduction of the 180° phase-shift in the consecutive bits.

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Fig 5. CSRZ

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Fig 6. MDRZ

Analysis – DWDM Network Design Software

In a comparison of the dispersion properties of RZ and CSRZ, it is expected that CSRZ would have the larger resistance to dispersion due to its smaller bandwidth. Because of the alternate bit-phase reversal, which reduces the intrachannel IXPM and IFWM, CSRZ could be expected to have greater nonlinear tolerance in comparison with RZ.

The enhanced tolerance for nonlinearity of CSRZ with respect to RZ has already been experimentally demonstrated.
DRZ and MDRZ have the smallest bandwidths compared with other discussed formats. Reduced and reshaped power spectral densities will alleviate the impact of the chromatic dispersion. Suppression of the tones at NB/2 frequencies (away from the carrier frequency) will reduce the impact of the ghost pulses and therefore the intrachannel FWM. A critical property of the MDRZ format is the introduction of the 180° phase-shift in the consecutive bits containing “one”. It is believed that this feature of MDRZ is the reason for the substantial reduction in the growth of the ghost pulses.

The images below describe the dependence of the Q-factor on Averaged Power and Amount of Prechirping. The X-axis is the fraction of total SMF length preceding the DCF. X=0 corresponds to pre-compensation and X=1 corresponds to post-compensation.

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