Effects of laser diode nonlinearities in hybrid fiber/radio systems

Abstract

With the increasing demand for broadband services, it is expected that hybrid fiber/radio systems may be employed to provide high capacity access networks for both mobile and fixed users. Third generation (3G) mobile systems for example, will operate around the 2.4Ghz band, while fourth generation (4G) systems may operate in the 5.8GHz band or beyond. To make these future generation systems commercially viable it is important to keep costs as low as possible. One method of keeping costs to a minimum is to have a central station (CS) where the radio frequency (RF) data signals are modulated onto an optical carrier and sent to a number of base stations (BS) over optical fiber, before being transmitted over air to the users. This allows the BS complexity to be kept to a minimum. A possible solution for generating the optical RF data signals for distribution over fiber is to directly modulate the RF signal onto an optical carrier using a laser diode. The major problem with this technique is that broadband microwave systems are likely to use frequency division multiplexing for transmitting very high data rates. This will thus involve modulating the laser with electrical data signals at multiple frequencies, which will result in serious interference due to dynamic nonlinearities in standard laser diodes around the electrical transmission frequencies. This distortion, known as intermodulation distortion (IMD), can significantly degrade the performance of optically fed microwave systems for high-speed access networks. This paper examines how this laser nonlinearity degrades the performance of hybrid fiber/radio systems operating in various RF transmission bands, and investigates possible techniques to overcome these problems.