Gain Optimization by Modulator-Bias Control in Radio-Over-Fiber Links

The authors propose a method to optimize the RF gain in narrowband radio-over-fiber links employing a Mach-Zehnder modulator followed by an erbium-doped fiber amplifier (EDFA) for amplification. Optimization is achieved by control of the modulator bias in order to improve the signal optical-modulation depth (OMD). Thus, for a given modulation amplitude, the optical signal has a reduced mean optical power and can access the small signal gain of the EDFA. This unsaturated gain is higher than the saturated one, thereby significantly increasing the RF gain of the link. Simultaneous optimization of OMD is also desirable to reduce detector saturation and fiber-induced nonlinear effects. They derive an analytical expression to describe optimum operating conditions for the modulator bias and validate their results through numerical simulation and experimental work. The proposed optimum modulator operating point is experimentally proven to be applicable to multicarrier signals like those used in 802.11a/g protocols