Isfahan University of Technology
Department of Electrical and Computer Engineering
Issue date: Auguest 1, 2013
Contributor: Mahdi Malekanehrad
Supervisor: Dr. Asghar Gholami
Counselor: Dr. Abolghasem Zeidabadi Nejad
Keywords: Optical Receiver, Photodiode, Limiting Amplifier, Sensitivity, BER
Increasing demand of high speed digital services, increases the use of optical communication systems. Optical transmitter, optical fiber as communication channel and optical receiver are three main parts of these systems. Short haul and long haul optical communication are as application area. Local Area Network (LAN) is one of the short haul applications that provides the capability of high speed connection between different parts of the network. With increasing the data rate, Gigabit Ethernet (GbE) network protocol has been introduced and its primary standard has adjusted by IEEE for optical fiber networks. 1000Base-Sx part describes the characteristics of optical fiber transceiver based on Multimode Optical Fiber (MMF) at 770 to 860nm wavelength for 220 to 550m distances.
In this thesis, design and implementation of optical receiver based on 1000Base-SX part of IEEE802.3Z standard of GbE networks is described. Because of low cost optical elements at 850nm, the use of this wavelength is common.
The use of Vertical Cavity Surface Emitting Laser (VCSEL) is common as light source in the transmitter of short haul optical communication systems. VCSEL driver modulates the digital signal on the VCSEL output optical power. So, the input digital signal is transmitted as the VCSEL light intensity fluctuations. Optical signal is transmitted by MMF and reaches to the optical receiver. Photodiode converts the optical signal to the electrical current signal. Due to losses in the channel and connectors, the received current intensity is low and can be strongly affected by noise. Amplifiers amplify and convert the current to the voltage signal.
In the implemented system, appropriate high frequency transmission lines are needed to transmit high speed signals. Scattering parameters of transmission lines have simulated by Advanced Design System (ADS) software and their dimensions are optimized for minimum reflection and good impedance matching between input and output ports.
The measurement results of implemented optical receiver show that at 4.25 Gb/s bit rate and received optical power less than -17.7 dBm, output signal eye diagram have good quality and pass the GbE eye diagram mask. Bit Error Rate (BER) test at 4.25 Gb/s bit rate, 1000m fiber length, -9.3 dBm transmitter power and -13.66 dBm received power is done to determines the receiver sensitivity. The implemented optical receiver has the sensitivity better than -13.66 dBm with the BER less than 10-12