22 April 1996 Terrestrial laser communication link at 1.3 μm with quadrature amplitude modulation
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In this paper, we describe the architecture and performance of a unidirectional, terrestrial laser communication link designed to operate over a range of 100 m to 1 km. The optical carrier is provided by a 1.3 micrometer laser which is modulated using an electro-optic modulator. Quadrature amplitude modulation (QAM) is used to modulate 150 Mbps input binary data at a subcarrier frequency of 140 MHz. At the receive side of the link, the optical signal is detected with a InGaAs p-i-n photodetector. Because the pointing of our low-cost link is passive, atmospheric fluctuations will induce amplitude fading of the optical carrier. The bit error rate performance of the QAM format is particularly sensitive to amplitude fading. Therefore, the performance of our link is sensitive to the effects of atmospheric turbulence. We discuss how these effects can be minimized by an appropriate choice of the beam diameter with respect to the receiver diameter and by incorporating an automatic gain control circuit. We present experimental results which demonstrate the operation of the link with 16-QAM and 256-QAM over a 100 m path with an atmospheric structure constant (Cn2) of 3 multiplied by 10-13 m-2/3.
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Pierre R. Barbier, Penelope Polak-Dingels, David W. Rush, Daniel M. Rosser, Geoffrey L. Burdge, Robert W. Barnett, "Terrestrial laser communication link at 1.3 μm with quadrature amplitude modulation", Proc. SPIE 2699, Free-Space Laser Communication Technologies VIII, (22 April 1996); doi: 10.1117/12.238407; https://doi.org/10.1117/12.238407

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