Highly coherent picosecond pulse generation with sub-ps jitter and high SMSR by gain switching Discrete Mode laser diodes at 10 GHz line rate

Abstract

Abstract:We demonstrate the generation of picosecond pulses with ~800fs jitter and SMSR in excess of 60dB by gain switching a Discrete Mode laser and compare the results with a commercial DFB laser. IntroductionTerabit all-optical communications systems can be realised by reducing the inter channel wavelength spacing in Wavelength Division Multiplexed (WDM) systems or by increasing the per-channel data rate. The latter is extremely attractive as it can be realised through Optical Time Division Multiplexing (OTDM) and implemented in hybrid WDM/OTDM systems [1]. Key components for such OTDM systems are simple low-cost picosecond optical pulse sources that exhibit the required temporal and spectral purity. Picosecond pulse sources have been demonstrated using several different techniques such as pulse carving, mode-locking and gain-switching, all of which have unattractive attributes for the applications concerned. For example, pulse carving is achieved by gating continuous-wave (CW) light with a sinusoidally driven Electroabsorption Modulator (EAM). 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 and limited repetition rate tunability are major disadvantages associated with this technique.The simplest and most robust pulse generation techniques involves gain-switching of single mode DFB laser diodes [2]. 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. While these shortcomings have been overcome by self seeding [3] or external light injection [4], such corrective measures not only increase the cost and complexity of the pulse generation technique but also can lead to unstable operation.In this paper, we investigate gain switching of single mode Discrete Mode laser diodes [5] and demonstrate simple generation of highly coherent picosecond pulses at 10GHz. We also compare the gain switching performance of DM lasers with that of conventional DFB laser diodes. The DM (DFB) laser yielded pulses with measured temporal jitter of ~800 (3ps) and a corresponding SMSR of 62dB (30dB). When the predominantly linear chirp of the DM laser is compensated for by launching the pulse train into a 200m length of commercial dispersion compensation fibre (DCF), near transform limited 10ps pulses at 10GHz are obtained that exhibit temporal jitter ~ 800fs and high SMSR of greater than 60dB.

A Survey of Optical Carrier Generation Techniques for Terabit Capacity Elastic Optical Networks

“…2) OFC Generation using a Gain Switched Single Mode Laser: The first demonstration of this technique was in 2009, where a discrete mode (DM) laser was gain switched, as depicted in Fig. 22 (a), to show an OFC with an FSR of 10 GHz with 13 lines within a 10 dB spectral window [210]. The modulation bandwidth of the DM laser was limited, hence the number of lines and flatness of the generated OFC were poor as shown in Fig.…”