Gain and Bandwidth Enhancement of Directly Modulated Analog Fiber Optic Links Using Injection-Locked Gain-Coupled DF'B Lasers

Sub-carrier-multiplexed (SCM) fiber optic systems have received much attention for local access networks, fiber radios, and cable television distributions [l]. With SCM, multiple channels of analog videos and digital telephony/data can all be simultaneously transmitted by a single optical carrier and detected by the same photodetector. Direct modulation of semiconductor lasers is a simple, low-cost approach for the SCM systems. However, its performance is often limited by the modulation bandwidth and the slope efficiency of the lasers. At present, the modulation bandwidths of typical commercial semiconductor lasers are in the range of several gigahertz, and the slope efficiencies vary from 0.1 to 0.4 W/A. Injection locking has been widely studied to improve the dynamics of semiconductor lasers since 1982. Recently, strong injection locking has been shown to be a very effective way to enhance the modulation bandwidth of index-coupled distributed feedback (IC-DFB) laser [2]-[5]. The nonlinear distortion is also reduced by strong optical injection locking [6]. Gain-coupled DFB (GC-DFB) lasers offer many advantages over conventional IC-DFB lasers for direct modulations [7]. They have higher single mode yield, lower chirp, and better resistance to external reflections. In this paper, we report on the experimental demonstration of directly modulated analog fiber optic links employing injection-locked gain-coupled DFB (GCDFB) lasers. Strong optical injection locking enables the link to operate up to 18 GHz, which is much higher than the relaxation oscillation frequency (7 GHz). In addition, the RF modulation efficiency has also been enhanced by 8 dB.