Optical Communications, 2nd Edition

10.1 THE OPTICAL POINTING PROBLEM

Recall that in our discussion of optical apertures and beamwidths in Chapter 1, it was shown that a typical optical beam in a space link could be confined to an angular beam width of less than 1 arcsecond. If this beam is to be detected at a receiver, then this beam must be pointed to within a fraction of this beamwidth. Alternatively, if the beam can be aimed toward a desired receiver (considered as a point) with an accuracy of only, say ± ψ_e radians, then the beam width must be at least 2ψ_e to ensure receiver reception, as shown in Figure 10.1. To emphasize this result numerically, suppose this beam is from a satellite aimed at the Earth from a 22,000-mile altitude. A 50-μrad optical beam from this altitude will illuminate a distance on the Earth of (50 × 10⁻⁶)(22,000) 1 mile. That means the center of the satellite beam must be pointed to within a half mile of the Earth receiver. Contrast this to an RF satellite antenna having a beamwidth of approximately 10° and an Earth coverage of approximately 4000 miles. The RF pointing need be only within 2000 miles, a sizeable reduction in required accuracy.

Figure 10.1. Beamwidths and pointing error in transmitter receiver systems.

The loss due to mispointing depends on the actual shape of the optical beamfront. ...