Q: How much divergence are we talking about?Ī: A beam with a typical 1 milliradian (0.057°) spread diverges to one meter over a distance of one kilometer, Figure. Depending on the specific optical emitter and the lens arrangement, the laser beam will spread to some extent. Of course, for a laser with its coherent output, this divergence is significantly less. To overcome these difficulties, the “negative” issue of beam divergence is turned to an advantage.Ī: Although popular literature usually portrays the laser beam as a thin beam which retains that thinness over the entire travel distance, in reality, the laser beam spreads or diverges the same as a regular non-laser light does. There are issues of structure swaying, vibration, temporary and even semi-permanent shifts due to thermally induced effects, minor seismic events, traffic-induced micro-motion, nearby mass-transit vibrations, and more. Q: What are some possible “mechanical” impairments?Ī: Buildings and towers, even if firmly attached to Earth, are not without motion. To facilitate aiming, many units come with a built-in telescopic imaging system similar to a spotting scope on a rifle and coupled with a video camera and display, which the installation crew uses to align and adjust the mounting angle. However, we are talking about an ultra-precise straight line over kilometers in many cases. Q: How do you aim the optical transmitter at the receiver?Ī: Since the receiver is, by definition, visible to the transmitter, it may seem easy enough to simply look and aim. This part looks at some practical issues associated with their implementation. The first part of this article established the basic operating principles and rationale for free-space optical links. Free-space optical links provide a reliable, cost-effective, and quick way to link two fixed communication nodes located a few kilometers apart.
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