Cost efficient pulse source for return-to-zero differential phase shift keyed transmission systems

It is anticipated that future long-haul lightwave transport networks will be required to exhibit multi-Tb/s capacities. High-speed transmission systems employing advanced modulation formats like RZ DPSK in conjunction with balanced detection have shown superior performance in terms of extended reach (better receiver sensitivity) and by offering better immunity to dispersion and/or nonlinear impairments. Key factors in the acceptance and deployment of such technologies, by service providers, will to a large extent depend on the cost of the entire system and also on the performance of the optical pulse source used. Picosecond pulse sources have been demonstrated using several different techniques such as pulse shaping, mode-locking of fibre ring lasers or semiconductor lasers,and gain-switching, all of which have unappealing attributes for the applications concerned. For example, pulse carving is achieved by gating Continuous Wave (CW) light with a sinusoidally driven Electro-Absorption Modulator (EAM) or Mach-Zehnder Modulator (MZM). While this method offers short pulse generation over a wide wavelength range, it suffers from large insertion losses and requires the use of expensive components including post amplification. Similarly, mode-locking has the ability to generate very short pulses at fixed frequencies, however cavity complexity, wavelength instability and limited repetition rate tunability are major disadvantages associated with this technique. The simplest and most robust pulse generation technique involves gain-switching of Distributed Feed- Back (DFB) lasers. The inherent simplicity of direct modulation, results in the gain-switched pulse source being cost-efficient, which proves to be of great practical significance with regard to market adoption. Unfortunately, while low cost and simplicity are among the numerous advantages of this technique, it does suffer from some drawbacks, such as side mode suppression ratio (SMSR) degradation and relatively large- temporal jitter. These shortcomings have been overcome by self seeding or external light injection. However, the adoption of such techniques not only increases the cost and complexity of the pulse generation technique but also can lead to unstable operation. In this paper, we investigate the gain switching of Discrete Mode (DM) laser diodes and demonstrate the use of the generated picosecond pulses in an RZ DPSK system. We also compare the performance of the Gain Switched DM Laser (GS DML) pulses with that of conventional DFB laser diode pulses in the same RZ DPSK system. Difference in performance is explained by analyzing the phase noise properties of the two transmitters.