The electronic instrumentation for atmospheric laser propagation studies

Abstract

The use of laser radiation in atmospheric communication systems is revealing new forms of signal degradation not previously encountered with lower frequency transmitting sources. The coherence and directivity of the laser which is responsible for its increased signal capacity is degraded by the effect of atmospheric distortion. Random fluctuations in the index of refraction (turbulence), Rayleigh scattering, aerosol scattering and other atmospheric effects create fluctuations (boiling, breathing, dancing, attenuation, transit-time dispersion, etc.) within the beam cross section, which when received produce random fluctuations in the receiver output signal. The two systems which are described, are designed to investigate the effect of atmospheric distortions on the amplitude and phase (or transit time) of the laser beam. The Scintillation System was designed for real-time determination of amplitude scintillation statistics. The system will be incorporated into a multi-wavelength, multi-range experiment for comparison of theoretical and experimental meteorological and optical data. The system includes a laser transmitter, an atmospheric path over which the beam is allowed to diverge to a three-foot cross section, a pair of photomultipliers with variable spacing and small sampling apertures, an analog "scintillation computer," and a recording device. The scintillation computer is a real time, special purpose, analog computer designed to compute the variance and covariance of log amplitude, the probability distribution of log amplitude, and the frequency spectrum of the amplitude scintillation. The system permits the recording of these data as a function of transmitter and receiver aperture, and time. The Short Pulse Distortion System is designed to detect pulse distortion due to transit-time dispersion of the laser beam. The system includes a mode-locked laser (i.e., the optical short pulse generator), a large transmitting aperture (a Cassegrainian telescope), the atmospheric path, a wideband receiver, a sampling oscilloscope, the "Pulse Distortion Analyzer," and a high-quality tape recorder. The Pulse Distortion Analyzer is designed to compare the transmitted pulses to recorded reference pulses and to detect differential area due to time dispersion. The analyzer includes a novel, feed forward, automatic gain control circuit which utilizes a linear photo detector and an analog divider to smooth a 300Hz signal with a 100 microsecond response time. This is necessary to smooth out the signal amplitude fluctuations due to atmospheric turbulence effects. Both systems have been set up and tested on a one-mile path between the Oregon Graduate Center and Skyline Drive in Portland Oregon, using a 6328Å He-Ne Laser. The ultimate use of