Optical Communications, 2nd Edition

10.2 SPATIAL ACQUISITION

Spatial acquisition requires aiming the receiving lens in the direction of the arriving optical field. That is, it must align the normal vector to the aperture area with the arrival angle of the beam. Often alignment is acceptable to within some degree of accuracy. That is, the arrival angle can be within a specified solid angle from the normal vector. This acceptable angle is called the resolution angle (or resolution beam width) of the acquisition procedure, and is denoted Ω_r, in subsequent discussion. The minimal resolution angle is obviously the diffraction-limited field of view, but, in practical design, desired resolution is generally larger. This allows for the possibility of the source blurring into many modes, and for compensating for pointing errors and ambiguities. Although resolution angle must be considered a design specification, its value plays an important role in subsequent analysis.

Acquisition can be divided into one-way and two-way procedures. One-way acquisition is shown in Figure 10.3a. A single transmitter, located at one point, is to transmit to a single receiver located at another point. If satisfactory pointing has been achieved (or equivalently, if the transmitter beamwidth covers the pointing errors), the optical beam will illuminate the receiver point. The receiver knows the transmitter direction to within some uncertainty solid angle Ω_u, defined from the receiver location. The receiver would like to aim its antenna normal to ...