TABLE OF CONTENTS A signal undergoes an E/O transformation at a fiber optic transmitter and a corresponding O/E transformation at the receiver. This chapter discusses the O/E process, with emphasis on the first two blocks in that flow: the semiconductor optical detector, or photodiode, and preamplifier. The two together are referred to as the receiver s front-end. With the growing need for commercial systems operating at 10 and even 40 Gb/s, the design of front-ends has become more important and technically challenging.
A complete optical receiver is more complicated than an optical transmitter. The signal from the fiber will be low in power and distorted because of the source and fiber characteristics. The electrical circuitry following the O/E transformation attempts to overcome some of these problems. In Chapter 4, the electrical circuits used to drive the optical source were not discussed. It was assumed that preceding the LED or ILD were drive circuits of sufficient bandwidth and output. Their design and realization is not overly troublesome because signal levels are high, and there are few noise or distortion problems.
In contrast, the circuitry that the semiconductor detector feeds directly into is very important in determining overall system performance. These two components, detector and preamplifier, are the dominant source of noise added to the signal in the receiver. As a result, the design of the front-end is critical in determining receiver performance. Front-end sensitivity is defined as the minimum detectable optical power in dBm. The sensitivity is determined by the required system signal-to-noise...
