The Vacuum Advantage

In optical fiber, the group refractive index ($n \approx 1.46$) slows light down to roughly $200,000\,\text{km/s}$. In the vacuum of space, light travels at its maximum possible speed ($c \approx 299,792\,\text{km/s}$). By routing data through space, we reduce propagation delay by $\sim 31\%$ compared to the most advanced terrestrial fiber.

The APT Challenge

Acquisition, Pointing, and Tracking (APT) is the most difficult aspect of OISL. Imagine trying to hit a penny with a laser pointer from 2,000 kilometers away while both you and the penny are moving at $27,000\,\text{km/h}$.

• Acquisition: Satellites use 'Beacon' lasers to scan and find each other.
• Pointing: High-precision fast-steering mirrors (FSM) adjust the beam with micro-radian accuracy.
• Tracking: The system continuously corrects for orbital perturbations and vibration (jitter).

Coherent Space Comms

Unlike terrestrial fiber where we can easily amplify signals using EDFAs, space links suffer from $1/r^2$ divergence. To extract data from a faint received signal, we use Coherent Detection.

The received signal is mixed with a 'Local Oscillator' (a laser on the receiving satellite) to translate the phase-modulated signal into a detectable electrical waveform. This allows for high-order modulation like QPSK or 16QAM even with minimal received power.

The Free-Space Optical (FSO) Link Budget Equation.

Conclusion

Inter-satellite laser links represent the final step in the virtualization of the global network. By moving the optical backbone into the void of space, we achieve the ultimate speed limit permitted by the laws of physics.