Abstract
We propose an optical spatial filter (OSF) method to suppress beam wander and spatial noise effects. Signal from random displacements of the focus spot around the optical axis within the constricted area is collected. This method advantageously suppresses fluctuations in signal intensity. The OSF consists of a pinhole and cone reflector. The pinhole produces Fresnel diffraction on the focus spot. The cone reflector provides directed reflectance onto the pinhole for random focus spot displacements due to beam wander. The calculations of signal power are based on fluctuations of signal intensity that are minimized by the circular aperture function of the pinhole and the cosine of the reflectance angle from the cone reflector. The method is applied to free-space optical communications at a wavelength of 1.55 μm with an atmospheric chamber to provide optical propagation media. Based on calculations, the beam wander angles that can be received by the OSF are from 14.0° to 28.0°. Moreover, based on experiment, the OSF with a pinhole diameter of 20.0 μm and cone reflector diameter of 1.5 mm produces signal power of −15.3 dBm. Both calculations and experiment show that the OSF enhances the received signal power in the presence of turbulence.
Highlights
Free-space optical (FSO) communications are a class of prospective telecommunication technologies capable of supporting satellite, terrestrial, high altitude, and mobile terminal-platforms that have been developed worldwide, recently [1,2,3,4]
We propose an optical spatial filter (OSF) as the optical method to suppress beam wander and spatial noise effects before the signal is received by PD
These processes produce turbulent media that affects the optical propagation of the forward-directed signal within the box of turbulence simulator (BTS)
Summary
Free-space optical (FSO) communications are a class of prospective telecommunication technologies capable of supporting satellite-, terrestrial-, high altitude-, and mobile terminal-platforms that have been developed worldwide, recently [1,2,3,4]. Atmospheric turbulence can cause severe problems that degrade FSO performance, such as decreased signal-to-noise ratio (SNR) and increased biterror-rate (BER) [5, 6]. Turbulence causes optical propagation phenomena such as diffraction, scattering, absorption, and beam spreading that attenuate or diffuse the FSO signal through stochastic processes. The FSO experiences spatial noise or random movement of hot spots around the center of the focus spot in the receiver plane [7]. Turbulence produces beam wander that fluctuates the angle-of-arrival on the receiver lens, leading to random movements of the focus spot [8]. Beam wander and spatial noise are turbulence effects on optical propagation that become major factors in fluctuating signal intensity
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