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Multi-span optical systems using Hybrid Fiber Amplifiers are optimized for the lowest under dispersion management for a 2000 Km link operating at 42.7 Gbit/s. The best span configuration including optimum Raman/Erbium gain balance and ideal dispersion compensation degree is found. It is shown that hybrid amplifier configurations behave better than EDFA-only and Raman-only systems, accumulating less noise and nonlinearities.

We present an accurate method for evaluating the bit error rate in optically preamplified direct-detection, by accounting for both the intersymbol-interference and the exact Laguerre photon-count statistics. A quantum-limited sensitivity of 33.9 photons/bit is derived.

We analyse the prospects of dispersion-tolerant transmission techniques such as electronic distortion compensation and optical duobinary modulation. Our investigation of both technical and economical aspects shows that these techniques are predominantly beneficial for Metro DWDM transmission.

We present an optical phase locked loop based on sub-carrier modulation and designed using commercial optical components. In principle the output of a continuous-wave laser is subcarrier modulated by a common LiNbO3 MachZe-hender driven by an electrical voltage controlled oscillator. The proposed architecture is used as optical receiver for homodyne coherent detection of ultra-dense WDM 2.5 Gbps 2-PSK signals with 6.25 GHz spacing. This method offers the potential for providing many closely spaced multigigabit channels and enables coherent lightwave technology to become commercially viable.

Influence of the filter shape on the performance of a single-wavelength combined FSK/IM scheme is investigated by simulations and experiments. For 156 Mbit/s FSK label and 10 Gbit/s IM payload, central wavelength misalignment of the signal and the optical filter can be tolerated up to 30 GHz without noticeable penalties. Results of the simulation agree very well with the experiments.

Numerical optimization of the ultra high dense WDM RZ-DPSK transmission has been performed by means of fine tuning the duty cycle and shape, carrier reshaping, and pre- and post-filtering parameters.

We analyze 20-Gb/s RZ-DQPSK transmission in a 33-GHz spaced 64-channel DWDM system over a 2000-km laboratory straight line. In particular filtering impact on DQPSK modulation format due to optical filter and a de-interleaver at the receiver and pre-filter at the transmitter are experimentally evaluated and commented.

Symbol DWDM transmission: every wavelength is symbol, carrying LogN bits, where N — number of wavelengths; one wavelength in fiber every moment. Combined transmission: several wavelengths (symbols) in fiber every moment. Application: ultra long haul.

We report a new quantum key distribution scheme using direct-modulation method associated with single sideband detection (SSB). Experiments were carried out at 850 nm using standard electronic and optical components.

We successfully demonstrated superimposing DPSK over inverse-RZ optical pulses using SOA-XGM for 2-bit per symbol modulation/demodulation at 20-Gbit/s to simply double spectral efficiency. Error free operation less than 10 was achieved for both RZ and DPSK signals.