High-Order Distortion in Directly Modulated Semiconductor Lasers in High-Loss Analog Optical Links With Large RF Dynamic Range

A theoretical and experimental investigation of the effects of high-order nonlinear distortion products produced by directly modulated semiconductor lasers on the performance of high-loss analog optical communication links requiring large RF dynamic range is reported. In order to provide sufficient RF dynamic range to support radio services in links with high optical transmission loss, for example in radio over free-space optics (RoFSO), while keeping costs low, it is necessary to use directly modulated lasers. However, in these applications the lasers must be driven to high modulation depths to maximize dynamic range. Simulations show that under these unique conditions the first detectable nonlinear distortion is often the result of dynamic distortion due to the laser being driven near threshold. It is shown that this type of distortion is characterized by a sharp increase in the contribution of high-order (fourth order or greater) nonlinear terms resulting from the influence of laser relaxation oscillations. As a consequence, the third-order spurious-free dynamic range (SFDR) metric no longer accurately reflects the performance of such links as it assumes that third order effects are dominant. An alternative measure of dynamic range called dynamic-distortion-free dynamic range (DDFDR) is proposed. This differs in that the upper limit is defined as the modulating power at which the peak optical modulation index (OMI) reaches unity. At this point the error vector magnitude (EVM) measured for a range of different wireless services starts to increase rapidly due to high order distortion. This makes DDFDR a practical, service-independent metric of dynamic range. For two different wireless services it is observed experimentally that on average the DDFDR upper limit predicts the EVM knee point to within 1.1 dB, while the third-order SFDR predicts it to within 6.2 dB. The DDFDR is thus shown to be a more accurate indicator of real link performance when high-order distortion is dominant.