A video processing system capable of performing coherence length measurements and characterization of fiber-coupled laser diodes

The increasing interest in fiber optic communication and sensor systems utilizing interferometric detection of optical fields has made accurate determination of the light field’s coherence parameters more important. We have developed a computer based measurement system which allows for video frame grabbing of Michelson interferometer output beams. Key attributes of this system are that it uses a standard non-dedicated microcomputer (Macintosh II) and relatively inexpensive commercially available software and frame acquisition hardware. Video frames are acquired and simply processed allowing fringe contrast to be easily determined, from which coherence length may be found. While the system described here is capable of performing optical contrast measurements of any field illuminating the video sensor array, its primary use in our facility has been to examine coherence of laser diodes that have been coupled into various types of single and multimode optical fiber. In addition, the system has been used to determine laser diode coherence length as the diode was subjected to various biasing conditions. Such measurements will be presented as will a complete description of the entire system.