Performance Evaluation of Vertical Cavity Surface Emitting and Distributed Feedback Lasers

Abstract

Optical fibers are not only used in the telecommunication but also used in the Internet and Local Area Networks (LAN) to achieve high signalling rates. Optical networks, based on the emergence of the optical layer in transport networks, provide higher capacity and reduced costs for new applications such as the Internet, video and multimedia interaction, and advanced digital services. The data rate through fiber largely depends upon the transmitter in which key component is laser which is used as optical source. The objective of the thesis work is to optimization of parameters for vertical cavity surface emitting laser (VCSEL) and Distributed Feedback Laser (DFB). The effect of change in ambient temperature is studied for VCSEL with the help of output power evaluation. It is observed that output power decrease with increase in ambient temperature which degrades its performance at high temperature. If a laser is designed with lower threshold voltage the effect of decrease in power due to increase in ambient temperature can be compensated. Propagation of VCSEL signal from single mode and multimode fibers is studied. A VCSEL output signal directly modulated can travel 125 m through multimode fiber and up to 3.7km on single mode fiber. Advantage of direct modulations is that it is easier to implement but it limits the data rate. For higher data rates data should be directly modulated. By using DFB laser which is drived at different values of driving current. Light current characteristics shows that output power increase with increase in driving current up to certain value and after that it gets saturated. But bit error rate first decrease and after certain value it again starts increasing. Driving current is optimized for minimum bit error rate. Comparison between various modulation coding formats combined with modulation techniques is made at the optimized value of the driving current. The coding formats analysed are NRZ: Rectangular, NRZ: Raised cosine, RZ: Rectangular and RZ: Raised cosine. Modulation techniques used are amplitude and phase modulation. After analysing the simulation results it is observed that NRZ rectangular coding format combined with phase modulator give best results with acceptable BER up to longer distance.