Discussion on the improvement of opto-RF link properties by using a bipolar cascade laser source

This paper studies the possible improvement in RF link properties that can stem from electrically cascading several laser sources and combining the light from each source into a single information-carrying light beam. The effect of carrier recycling is first studied within a discrete architecture consisting of n individual laser diodes macroscopically connected in series. An RF link gain improvement proportional to n², and a link noise figure improvement proportional to n is found. The model is validated by experimental data. The architecture nonetheless carries some drawbacks, including the need for a zero-loss optical combining device to benefit from the RF link gain improvement, and some bandwidth shortcomings. The effect of carrier recycling within an integrated laser device, a so-called bipolar cascade laser, is then studied. In order to push back the limitations of the discrete architecture, the device consists of n active regions integrated into a single laser cavity. A rate equation model is applied to this promising structure and it is found that, in good agreement with previously published results, the external efficiency is expected to increase by a factor of n, leading to a possible RF link gain improvement by a factor of n². However, because the laser noise is dominated by the photon corpuscular noise, a weak influence of electrically cascading active junctions into a single laser cavity on the laser intensity noise, and thus on the link noise figure, is expected