A free space optical communication system is disclosed whereby the received power at the receive optical fiber is reduced by adjusting the transmitted light beam and the receive optical fiber with respect to each other when a specified power threshold is exceeded so that they intersect not at a point of focus, but at a point of divergence of the transmitted beam. In one embodiment, the transmitted beam may be diverged by moving the transmit optical fiber to a point in front of the focal plane of the transmit telescope along the longitudinal axis of that telescope until at least a portion of the beam is incident upon the receive optical fiber. This causes the transmitted beam to diverge and, accordingly, the cross-sectional area to increase at the point where it is incident upon the receive telescope. As a result, the cross-sectional power per unit area of the transmitted beam at the receive telescope is reduced which correspondingly reduces the power that is incident upon the receive optical fiber.
In another embodiment, instead of moving the optical fiber at the transmit telescope, the optical fiber located at the focal plane of the receive telescope may be moved to either a point in front of or to a point to the rear of the focal plane of the receive telescope along the longitudinal axis of the receive telescope. Since the maximum possible received power is achieved at the focal point within the focal plane, any movement of the receive optical fiber away from the focal point along the longitudinal axis of the receive telescope will result in a reduction of the received power.