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Fig. 3. PCM "eve pattern" for error rate of 101. ACKNOWLEDGMENTThe authors wish to thank C. A. Burrus for not only providing the electroluminescent diodes but also for much useful information and discussions. The encouragement of S. L. Freeny is also acknowledged. REFERENCES (11 F. P. Kapron. D. B. Keck. and R. D. Maurer. 'Radiation lases in glass optical waveguides," Appl. Phyr. Lett.. vol. 17. pp. 423425, Nov. 1970. [2] G. Whi t e. "A one-gigabit-per-second optical PCM communications system,' Proc. I E E E (Lett.). vol. 58. pp. 1779-1780. Oct. 1970. 131 C. A. Burrus. "Radiance of small-area high-current-density electroluminescent diodes." Proc. I E E E (Lett.). vol. 6 0 , pp. 231-232, Feb. 1972. (41 C. A. Burrus and E. A. Ulmer. Jr.. 'E5irient small-area GaAs-Gal-=Al=As heterostructure electroluminescent diodes coupled to optical fibers." Proc. I E E E [SI J. Stone, .Optical transmission in liquid-core quartz fiben." Appl.AbSfiQCt-It is shown that in the case of a homogeneous and isotropic turbulence the knowledge of the temporal correlation function of the amplitude fluctuations on the laser beam axis allows the estimation of the spatial spectral density of the index of refraction. Under certain conditions this method may be used for atmospheric turbulent flow also, which represents in general a locally homogeneous and isotropic random field.