Pulse source requirements for OTDM systems

A simulation model for investigating the impact of incoherent crosstalk due to pulse tail overlapping is proposed. Requirements to pulse width and Pulse Tail Extinction Ratio introducing a maximum of 1 dB penalty is extracted. Introduction Optical Time Division Multiplexing (OTDM) is an attractive technique to increase the overall capacity of optical communication systems, either as ultra-high bit rates at a single wavelength HI or as a combination of Wavelength Division Multiplexing (WDM) and OTDM W. In both cases, the modulatin format is Return-to-Zero (RZ). The objective of this paper is to extract the requirements for the RZ pulses in terms of Full Width Half Maximum (FWHM) and Pulse Tail Extinction Ratio (PTER). Theory In a basic OTDM system, the pulse source is characterised by the pulse shape, pulse width and repetition rate B. The emitted pulse train is split into 'N branches, each containing a modulator and a specific delay, which enables the possibility to intedeave the bits from each branch. The OTDM signal will have an aggregated bit rate of NxB. In the receiver the OTDM signal is demultiplexed to the ' N individual channels, before O/E converted and processed electronically. If the electrical fields from the pulses are overlapping, &her due to the pulse width or due to a finite extinction ratio, the neighbouring channels, upon OIE conversion, can deteriorate the demultiplexed channel. The noise terms in the receiver due to this process can be shown to consist of Intersymbol Interference (El) and interferometric crosstalk 134. The interferometric crosstalk terms are dependent on the coherence time of the pulse source in the OTDM system. If the delay between the pulses is larger than the coherence time, the interferometric cmstalk terms will vary fast, and can be regarded as noise, and is denoted incoherent crosstalk 134. Model The impact of multiplexing the pulses to an OTDM signal has been evaluated by implementing a simulation model. 'N identical pulse trains are implemented, each pulse train defined by the pulse shape, the pulse FWHM width, repetitiin rate and the PTER. see Fig. 1. Each pulse is assigned a random phase, evenly distributed between 0 and 2rr. to simulate the impact of the incoherent crosstalk. The pulse trains are modulated wilh a 2'-l Pseudo Random Bit Sequence (PRES), delayed and multiplexed. After multiplexing, the OTDM signal is demultiplexed using an ideal square shaped window with infinite extinction ratio before O/E conversion and evaluation using a BER module with optimised threshold level and decision time. From the BER values the power penalty tor each channel is calculated. To determine the average penalties for all the channels, the entire system can be recalculatsd up to 1000 times. Results First the impact of multiplexing pulse trains based on ideal pulses, i.e. pulses with no additional pedestal (see Fig. l ) , is investigated. As the amplitude of the electrical fields are approaching zero outside the designated time slot, it would be expected that a specific demuniplexed channel would only be affected by the two immediate neighbouring Figutn I : Illustration of a pulse train based on ideal channels, and consequently not limited by the number of pulses and based on pulses wah an additional added OTDM channels. This is confirmed in Fig. 2 where the pedestal, defining VI0 PTER. power penalty for 4x40, 8x40, 16x40 and 32x40 Gbiffs versus the FWHWimeslot is illustrated with no significant difference between the introduced power penalties. AS opposed lo the results in Fig. 2, the same simulations are canied out for pulse trains based on ideal pulses with an addnional added pedestal (see Fig. 1). 0 & ',f ptk PRa c__ TIN (%""an mer' 0-7803-788&1/W$17.00@2003 IEEE 302 Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 24,2010 at 08:12:50 EST from IEEE Xplore. Restrictions apply.