Tran Dai Dung <br/><br/>
Analysis of receiver sensitivity for optically-processed TDMA systems using electrooptic modulators as high-speed time demultiplexer 

 - Bangkok :  Asian Institute of Technology,  1996 
 - 86 p. :  ill. 
 - Research studies project report ;  no. TC-96-9 .
 - Asian Institute of Technology.  Research studies project report ;  no. TC-96-9 .
<br/><br/> A research study submitted in partial fulfillment of the requirements for the degree of Master of Engineering. 
<br/><br/>Research Studies Project Report (M.B.A.) - Asian Institute of Technology, 1996 
<br/><br/> In this research study, a n<?vel scheme is presented to implement ultra-high-speed optically-processed time-division multiple-access (OTDMA) receivers which use a new optical time demultiplexer based on two cascaded electrooptic modulators (EOMs). The proposed demultiplexer has several advantages over the conventional EOM-based optical time demultiplexers. First, it can demultiplex an ultra-high-speed optical pulse stream while being driven by a sinusoidal voltage of relatively low frequency, and therefore can eliminate the requirement for using very-high speed and/or wide-bandwidth electrooptic devices and associated electronic circuits. Second, it has better switching characteristic: it introduces less power loss for demultiplexed optical signal. Third, its time switching windows have smaller width thus can minimize the intersymbol interference (ISi) and crosstalk from adjacent channels. Fourth, and which is the most important, it has significantly higher demultiplexing effectiveness: it can demultiplex "one out of more than two channels" (e.g., 3,4, 5 and higher) whereas the conventional EOM-based demultiplexers can demultiplex "one out of only two channels". AH these advantages contribute to the high performance of OTDMA receivers and OTDMA systems overall. Computer simulation is carried out to study the switching characteristic of time switching window of the proposed demultiplexer. It is observed that the shape of such a switching window is very close to the Gaussian function. We also show that the proposed demultiplexer can effectively demultiplex 4-channel OTDMA signal by only tuning the phase of a RF driving voltage at the second EOM in the receiver. Thus, this is a very cost-effective scheme compared to all-optical time switching schemes based on nonlinear optical loop mirrors, because the latter requires high switching power (by using an optical amplifier) and uses an expensive optical tunable delay line. The proposed ultra-high-speed time demultiplexer can be feasibly implemented by using today's mature optoelectronic integrated-circuit technology. Moreover, we find that the crosstalk at this new demultiplexer can be negligible even if we use commercially available EOMs with extinction ratio about 20 dB. The analysis of receiver sensitivity for the proposed OTDMA systems is presented for the first time. By using a Gaussian approximation to the time-switching window, the timedemultiplexed optical pulse can be easily modeled, and therefore the analysis/computation complexity of the system performance is greatly reduced. OTDMA receivers using both avalanche photodiode (APD) and PIN photodiode are considered, and an equalizer is also assumed to be used for eliminating the intersymbol interference at each receiver. The receiver sensitivity is analyzed by taking account for the effects of photodiode and electronic amplifier noise, channel bit rate, extinction ratio of optical source, modulation index or extinction ratio of EOM (at the receiver), and the number of multiplexed channels. For APD receivers, the optimal average avalanche-gain is also found. The analysis methodology for receiver sensitivity presented in this research is used not only for OTDMA systems with two cascaded EOMs, but also for other OTDMA systems using different optical time-demultiplexing schemes such as the cascaded multi-quantum well modulators and nonlinear optical loop mirrors, respectively, or even optical soliton-based OTDMA systems. This needs only to modify the optical pulse shape at the input of a PIN or APD photodiode. Therefore, this research work opens such new topics for future study. 
<br/><br/><label>Subjects--Topical Terms: </label><br/>Optical communications