Experimental demonstration of a “pin-like” low-divergence beam in a 1-Gbit/s OOK FSO link using a limited-size receiver aperture at various propagation distances

In free-space optical (FSO) communications, there are scenarios (e.g., from a ground station to a drone/airplane) in which: (i) the transmitter (Tx) can have a relatively large aperture whereas the receiver (Rx) aperture should preferably be much smaller, and (ii) the distance between the Tx and Rx can vary such that beam divergence will cause a variation in the recovered signal power. In such cases, transmission using a fundamental Gaussian beam can be significantly degraded due to beam truncation caused by a limited-size Rx aperture. Here, we experimentally demonstrate a 1-Gbit/s on-off keying (OOK) FSO transmission link using a structured “pin-like” beam with a limited-size Rx aperture at various distances. The pin-like beam is generated by passing a Gaussian beam through an “Airy-type” phase pattern in the radial direction. When propagating, this structured beam first narrows and then tends to maintain its narrow beam size over a fairly wide range of distances. In comparison to its Gaussian counterpart, our experimental results show that the pin-like beam has ∼13 to 8 dB less power loss at distances ranging from 0.45 m to 0.8 m with an Rx aperture diameter of 1 mm. Moreover, we simulate the propagation of the pin-like beam and show its relatively lower power loss for a wide distance variation in a longer link (e.g., ∼1 km) with a limited-size Rx aperture. Furthermore, our results show that the pin-like beam can have a wider range of low-power-loss distances than a Gaussian beam that is focused to a given distance.