AbstractsComputer Science

Efficient optical front-end for data communications

by Meer Sakib

Institution: McGill University
Department: Department of Electrical and Computer Engineering
Degree: PhD
Year: 2015
Keywords: Engineering - Electronics and Electrical
Record ID: 2060612
Full text PDF: http://digitool.library.mcgill.ca/thesisfile130430.pdf


In this thesis, a low complexity implementation of a soft-decision circuit is proposed for the decoding of low density parity check (LDPC) codes in conventional direct detection optical systems. A certain amount of the optical power prior to the optical receiver is used for determining the confidence levels of the soft-decision receiver. The proposed scheme requires no modification of the current hard decision optical infrastructure while benefiting from soft-decision decoding approach. A low complexity soft-decision circuit from low cost off-the-shelf components is experimentally demonstrated and performance is evaluated in terms of post forward error correction (FEC) bit error rate (BER) versus pre-FEC Q-factor. The optimized threshold levels are initially investigated. It was found that the performance of the hardware is optimum if the difference between two confidence level is 45 mV for an input peak to peak voltage level of 320 mV where threshold spacing ratio between mark-confidence/hard-decision-bit and space-confidence/ hard-decision-bit is ~1.3. The optimum optical coupling ratio for the soft-decision circuit was also studied. It is shown that, for the coupling ratio of 80:20 a coding gain of 2.75 and 6.73 dB at an output BER of 10e-4 and 10e-9 is achieved, respectively when compared to hard-decision uncoded systems for LDPC(32000,29759) code. The system suffers a Q-factor penalty of 0.35 dB. It is found that the net coding gain at 45 Gb/s 7.06 and 9.62 dB for post-FEC BER of 10e-7 and 10e-12 (extrapolated), respectively, for long block length LDPC(32768,26803) code suitable for single-mode transmission links. It is also shown that at a lower bit rate of 15 Gb/s using RS(255,239), LDPC(672,336), (672, 504), (672, 588), and (1440, 1344), an optical power saving of 3, 5,7, 9.5 and 10.5 dB, respectively, is achieved for a multimode link with a 6-tap finite impulse response (FIR) equalizer. In the proposed front-end, fewer components are required as compared to other soft-decision front-end solution, and potentially consumes less power. The optical front-end consumes 2.71 and 4.95 W at 32 GS/s and 45 GS/s, respectively.A monolithic optical front-end for using soft-decision codes in coherent receivers is designed and fabricated using CMOS compatible 220 nm Si on 2 µm SiO2 based SOI technology. The optical hybrid is based on a 4x4 MMI and has a bandwidth of higher than 20 nm. The directional coupler is also designed for obtaining the desired coupling ratio. Balanced photodetectors with on-chip biasing capacitors exhibits a bandwidth of 16.2 GHz at -4.5V reverse bias. The on-chip components make the device compact and suitable for monolithic integration. The front-end is packaged with RF connectors using a high frequency ceramic substrate. The soft-decision front-end is experimentally demonstrated and performance is evaluated in terms of post forward error correction (FEC) bit error rate (BER) versus pre-FEC Q-factor. It is found that the net coding gain at 25 Gb/s 6.2 dB for post-FEC BER of 10e-7, for long block…