Model for a partially coherent Gaussian beam in atmospheric turbulence with application in Lasercom

Abstract

Analytic expressions for the mutual coherence function (MCF) and the scintillation index of a partially coherent lowest order Gaussian beam wave propagating through the atmosphere (based on Kolmogorov spectrum model) are developed for the pupil plane of a receiving system. Partial coherence of the beam is modeled as a thin (complex) phase screen with Gaussian spectrum (Rytov theory and ABCD ray matrices are applied). The relation between the second-and fourth-order statistics for a beam with any degree of coherence in the atmosphere is introduced with the help of ''effective'' beam parameters, deduced from the free-space MCF. In particular, the scintillation (in weak and strong atmospheric conditions), based on these parameters, is studied as a function of the diffuser's strength and that of the atmosphere. The model is applied for the calculation of the SNR and bit error rates (OOK modulation) of the communication link with diffuser at the transmitter and slow detection system. The improvement of bit error rates is observed in weak and strong atmospheric turbulence. In the weak regime, the optimal diffuser can be found. © 2004 Society of Photo-Optical Instrumentation Engineers.

Scintillation Index of a Gaussian Schell-Model Beam on Slant Atmospheric Turbulence

“…Here ρ T is the coherence length of a spherical wave propagating on the slant turbulent atmosphere, and C 2 n (ξL) is the model for the refractive-index structure constant in the atmosphere. This effective wave radius for the partially coherent GSM beam through atmospheric turbulences agrees with that by Korotkova [33], where…”

“…There is a small error a = ΛLξ/k in Equation (3.55) in [33]. Using Equations (7), (8), the evaluation of Equation (14) leads to…”

“…It is noted that Ref. [33] introduces a thin random phase screen to model the diffuser in front of the laser transmitter and characterize the phase screen by a power spectrum function rather than by a correlation function as in the Gaussian Schell-model. It should be noted that W 1 is Gaussian beam spot radius in receiver plane not partially coherent beam because the effects of partially coherent beam are accounted by q c .…”

Suppression of intensity fluctuations in free space high-speed optical communication based on spectral encoding of a partially coherent beam

“…demonstrated. [4][5][6] Combining partially coherent beams with a time-averaging photodetector leads to a significant scintillation reduction with the corresponding improvement of the BER by several orders of magnitude. 6 Unfortunately, the time-averaging method cannot be applied to gigabit rate communication.…”

Scintillation index for two Gaussian laser beams with different wavelengths in weak atmospheric turbulence

“…More recently is was shown theoretically [11] and experimentally [12] that the effective propagation distance measuring beam spreading in a turbulent medium relative to free-space spreading is larger for spatially partially coherent beams than for fully coherent ones. Other works demonstrated the improved performance obtained by using spatially partially coherent beams in terms of the normalized intensity distribution [13], the mean squared beam width [14,15], power in the bucket and Strehl ratio [15], the scintillation index [16], the bit error rate (BER) [16,17,18] and the average signal to noise ratio (SNR) [16].…”