Intermodulation Distortion in High Dynamic Range Microwave Fiber-optic Links with Linearized Modulators

A number of techniques have recently been described to linearize integrated optic intensity modulators, and thus significantly reduce the two-tone intermodulation distortion. The resulting intermodulation distortion produced by these extended linearity modulators then varies as the input power to the fifth-order or higher. When these modulators are included in a fiber-optic link system, the overall intermodulation product is a combination of third-order and higher order terms. In this paper we determine the dynamic range of a cascaded microwave network consisting of a preamplifier, a high dynamic range fiberoptic link with a highly linear modulator, and a post-amplifier. An expression is found that relates the intermodulation power at the output to the relative suppression from the signal level. As an example, a hypothetical 10 GHz low distortion fiber-optic link that has a dynamic range of 125 dB in a bandwidth of 1 Hz is cascaded with various preamplifiers, and it is shown that the dynamic range of the system is reduced by as much as 20 dB, depending upon the third-order intercept of the amplifier.