Application of COTS high power laser diodes and driver for free space laser communication terminal

Recently, there has been strong interest in the application of commercial-off-the-shelf (COTS) electronic and optoelectronic (O/E) components for free space laser communication application. Besides the space qualified packaging issues, the main problems of using COTS O/E transmitter are: 1) Telecom grade laser transmitters do not have sufficient power to meet the free space laser communication requirements; 2) COTS laser diode transmitter driver circuits have limited peak drive current, usually below 100 mA, which is too low for driving high power laser diodes; and 3) COTS high power laser diodes are usually not used for high data rate applications since the high speed performance of the laser/driver combination is usually inadequate. In this paper we will present our latest study results on the SDL 5430 and SDL 580 high power laser diodes driven by high current laser drivers at data rates from 600 Mb/s to over 1 Gb/s. Several models from the HY6000 family of high current and high speed laser diode drivers from Hytek Microsystems Inc., designed for free space laser communication applications, have been tested with the SDL high power laser diodes. Using direct drive technique with NRZ modulation, average output power over 100 mW at 622 Mb/s were obtained with these low cost Hytek drivers. For data rates over 1 Gb/s, the parasitic associated with the laser diode becomes an important limitation factor. We have measured the capacitance of the SDL 5430 and the new junction up SDL 580 laser diodes, an equivalent circuit model is developed to examine the effect of these parasitics on the speed of the laser diode. The results are consistent with our experimental observations.