Polarization power summing in laser diode communication systems

Free-space optical communication systems using laser diode transmitters exhibit data rate limitations imposed by the diode's relatively low average output power. Power summing of multiple laser diodes through polarization beam combining is an effective method for increasing the on-axis far-field intensity of a laser communication transmitter. When using such a combining scheme, laser diodes having the highest degree of polarization purity should be used in order to obtain the maximum output (combined) power. Therefore to examine the applicability of polarization combining systems incorporating laser diodes the polarization characteristics of the devices must be determined. Polarization purity - total output power relationships for two types of single-stripe laser diodes are presented as are similar measurement results for two types of phase locked laser diode arrays (each of these devices has been individually analyzed to determine its candidacy as a free-space communication system transmitter). A simple combining system using the single-stripe diodes or the phase locked laser diode arrays is described with respect to its use in a free-space optical communication system. A polarization beam combiner is described and analytical expressions for the total transmitted power and signal extinction ratio are developed.