<title>500 MHz Fiber Optic Communication Link Using An Injection Laser Diode</title>

Abstract

AbstractThe microwave terminals, satellite ground stations, and CATV industries demand wideband, high- frequency communicationlinks. A very -low -mesa- stripe Gai_XAlXAs- Gai_yAlyAs double -heterostructure (DH) injection laser diode (ILD), and asilicon avalanche photodetector (APD) have been utilized in a 500 -MHz fiber -optic system to demonstrate high- frequency feasibility. The ILD, capable of being modulated up to 700 MHz with 1.25 -dB /octave rolloff, coupled 4.9 mW into a singleCorning low -loss fiber with microlens formed at the input end. Above 700 MHz, the distortion becomes severe due to relaxa- tion oscillation. The frequency- dependent nonlinearity of ILD, which can be compensated by a special circuit design, hasbeen analyzed by Volterra series. Using a gain equalization circuit in both the ILD transmitter and APD receiver, the systemhas very flat response up to 270 MHz and usable response to 500 MHz (S /N = 56 dB). Above 270 MHz, the gain is reduced due to the characteristics of the APD. With two tones (90 and 125 MHz), the system has an S/N (each tone) of 58 dB in 100 -kHzbandwidth measurements. Without nonlinear compensation, the second- and third -order intermodulation products (IMP'S) are24 dB below the carrier. Presently, with special compensation network, IMP's of 33 dB below the carriers are obtained.IntroductionSignificant progress of long -lived, room -temperature, CW heterojunction injection laser diodes ( ILD's) has made them anexcellent contender for fiber -optic communication, high- density information processing, wideband film recorder, and manyother applications. The ILD's are particularly attractive to fiber -optic and future space communication because of their highefficiency, small size, ease of direct modulation, low intramodal dispersion, and ease of coupling into single fiber due to thesmall emitting area and low emission angle. Recently, the fiber -optic applications in microwave terminals, satellite groundstations, and CATV industries demanded wideband, high- frequency analog links. This paper describes a 500 -MHz fiber -opticsystem utilizing a very -low- mesa -stripe Gai_XAl As- Gai_yAIyAs double -heterostructure (DH) ILD, a silicon avalanchephotodetector (APD), and a single Corning low -loss fiber with microlens formed at the input end.This paper is intended primarily to present the developments, the design techniques, the problem areas, and the first -cutexperimental results. Many performance parameters such as signal -to -noise (S /N) ratio and intermodulation products (IMP's)still can be improved further.