OPTICAL SIGNAL TRANSMISSION IN NEW GENERATION FIBRES USING ADVANCED MODULATION FORMATS AND VCSEL TRANSMITER

Abstract

In the current society, the data transmitted using the internet is tremendous and has been occasioned by the rapid broadening of communication infrastructure. The use of traditional NRZ format is becoming less applicable due to its bandwidth limitations. To address this bandwidth limitation there is need of advanced modulation formats that has high transmission capacity. A VCSEL transmitter was used because of its low operational cost occasioned by its low power. The work started with the implementation of the traditional Non Return to Zero (NRZ) modulation and phase shift keying (PSK) modulation systems as they form the basic building blocks in the design of higher modulation formats. Receiver sensitivity for 10 Gbps NRZ and PSK signals were obtained to be -18.60 dBm and -10.72 dBm respectively. NRZ gave a better receiver sensitivity of -7.88 dBm in comparison to PSK. A three level Pulse Amplitude Modulation (2-PAM) format was also implemented. However, due to high power requirement for 2-PAM, it could not be able to cross the BER threshold of 10-9. This makes it unsuitable for signal transmissions. A four level pulse amplitude modulation (4-PAM) that doubles the data rate per channel from 10 Gbps to 20 Gbps by transmitting more information in the amplitude of the carrier signal was implemented. In 4-PAM, two streams of electrical data each having a bit rate of 10 Gbps were combined so as to give a 20 Gbps multilevel system with a receiver sensitivity of -6.04dBm. However, 4-PAM had a higher penalty compared to NRZ and PSK. The possibility of using a single photodiode in demodulating the multilevel signal reduces the cost of implementing the scheme since no complex circuitry is required for both transmission and reception. Moreover, 4-PAM when integrated in a Dense Wavelength Division Multiplexing (DWDM) system will achieve even higher data rate per channel thus saving on the cost of capital investment and hence an efficient utilization of the available bandwidth. VCSEL and DFB lasers were also compared at transmission distance of 3.21 km. VCSEL showed very low sensitivity of -3.86 dBm as compared to -12.6 dBm for DFB. VCSEL being a low power laser has a low sensitivity. 4-PAM format therefore with VCSEL as a transmitter is suitable for implementing in access (Municipal) networks hence creating jobs by ensuring that businesses have fast, affordable and a very reliable Internet access. This will improve the productivity of existing businesses and attract new businesses to communities. By so doing, it also allows individuals to work from home more effectively and efficiently, Support the use of advanced healthcare and security systems and also it will strengthen long term social investments in the form of better-linked schools and libraries. This will eventually improve the economy of the country.