Multispatial Mode Enabled PAT and AO Terminal Architecture for Free-Space Optical Communications

Free-space optical communication systems transmit data in an outdoor atmosphere by using an invisible beam of light. There’s always been a need for an optimized alignment of the transmission and receiving apertures to facilitate efficient light coupling to the detector. The conventional approach was the pointing, acquisition, and tracking (PAT) systems that measured the tilt for this alignment but at the cost of reducing the signal-to-noise ratio (SNR) and sensitivity. The major problem with the traditional approach lies in its diversion of energy from the communications receiver to a photodiode array for tilt measurement in the focal plane. This splitting of energy adversely affected the performance of the communication system, leading to a significant drop in SNR and sensitivity. Consequently, the challenge is to find a PAT technology that could track tilt without compromising on energy or signal quality.

Technology Description

This technology involves a pointing, acquisition, and tracking (PAT) terminal used in free-space optical communication systems. Unlike conventional technology, which reduces signal-to-noise ratio and sensitivity by drawing energy for tracking purposes, this PAT terminal estimates the tilt angle without any such diversion. It employs a passive-mode converter, like a photonic lantern, mapping power in each spatial mode at the reception aperture to unique single-mode output. Photodetectors are then used to convert the received light into electrical signals. What sets this technology apart is its ability to ensure more efficient coupling of received light to a detector while maintaining the energy needed for communications. Conventional PAT systems diminish their overall performance by diverting energy toward tilt measurement in the focal plane. By recording the distribution of power across different spatial modes for PAT and converting it into tilt information, this methodology eliminates the need for energy diversion, resulting in robust communications with better signal quality.