Optical Inter-Satellite Links (OISLs)

1. Light ModulationDigital data from the satellite's processor is converted into pulses of infrared light using semiconductor laser diodes. Most space optical systems use wavelengths around 1,550 nanometers (nm)—the same optical band used in terrestrial fiber-optic networks—allowing space hardware to leverage mature ground-based fiber technology.2. Pointing, Acquisition, and Tracking (PAT)Because laser beams do not spread out like radio waves, hitting a receiving satellite thousands of kilometers away while traveling at orbital speeds ($7\text{ to }8 \text{ km/s}$) requires extreme precision:Coarse Acquisition: Satellites exchange orbital location coordinates and point their optical gimbals using star trackers and onboard GPS.Fine Tracking: Fast steering mirrors and position-sensing detectors make thousands of micro-adjustments per second to compensate for spacecraft vibration (jitter) and hold the beam centered on the target aperture with micro-radian accuracy.3. Optical ReceptionThe receiving satellite collects incoming photons using a compact optical telescope, directing the light onto high-sensitivity avalanche photodiodes or coherent optical receivers. These detectors convert the light pulses back into electrical signals for onboard processing or further relaying. Why Laser Comms Are Replacing Radio Frequency (RF)Massively Higher ThroughputData capacity scales with carrier frequency. While radio systems operate in the gigahertz range ($\text{GHz}$), infrared lasers operate in the terahertz range ($\sim 193 \text{ THz}$)—over 10,000 times higher in frequency. This allows a single optical link to achieve data rates from 10 Gbps to over 100 Gbps, compared to typical RF inter-satellite links that top out around 1 to 2 Gbps.Reduced SWaP (Size, Weight, and Power)Radio waves spread rapidly, requiring large parabolic dish antennas (1–3 meters wide) and high transmitter power to deliver usable signal strength over long distances. In contrast, an optical terminal uses