Free-space optics (FSO)

Free-space optics (FSO) refers to any optical system where light propagates through air, vacuum, or any other unconfined medium between source and detector, as opposed to being guided through an optical fiber, waveguide, or other confined geometry. The term encompasses two principal application areas:

• FSO communications: optical wireless data links (free-space laser communications), used for satellite-to-satellite, satellite-to-ground, building-to-building, and deep-space links
• General laboratory free-space optics: any laser-source-plus-optics setup where the beam propagates in air; this includes the vast majority of laboratory optical experiments

Why FSO over fiber.

FSO communications link design.

A typical FSO comms link consists of:

1. Transmitter: laser source (typically 1550 nm DFB or fiber laser), beam-forming optics (collimator producing a Gaussian beam with controlled divergence), pointing/tracking system
2. Free-space propagation channel: atmosphere (with turbulence, scattering, beam wander) or vacuum
3. Receiver: telescope to collect a fraction of the diverging beam, photodetector (often PIN or APD), data recovery electronics

The link budget is:

$$P_{\text{rx}}=P_{\text{tx}}\cdot \frac{A_{\text{rx}}}{\pi (\theta L{)}^2}\cdot L_{\text{atm}}\cdot L_{\text{optics}},$$

where $\theta$ is the half-angle beam divergence, $L$ is the propagation distance, $A_{\text{rx}}$ is the receiver aperture area, $L_{\text{atm}}$ is atmospheric loss, and $L_{\text{optics}}$ is the receiver/optics loss.

For a typical ground-to-ground building link at 1 km:

Atmospheric impairments.

Atmospheric windows. Best atmospheric transmission occurs in specific wavelength bands:

Eye safety. FSO transmitters at 1550 nm benefit from the cornea and lens absorbing this wavelength before light reaches the retina, allowing higher transmit power (Class 1 limits to $\sim 10$ mW). Visible-wavelength FSO requires much lower transmit power ($<1$ mW) for eye safety, limiting practical operation.

Standard FSO use cases.

• Last-mile connectivity: rooftop-to-rooftop links bypassing telecom infrastructure
• Disaster recovery and military: rapid-deployment fiber-equivalent links
• Datacenter interconnect: ultra-high-bandwidth between adjacent buildings without underground fiber
• Satellite-to-ground: GEO relay and lunar links (ESA EDRS, NASA LCRD, NASA LLCD)
• Satellite-to-satellite: ISL (inter-satellite link) constellations; Starlink uses laser ISL
• Lunar / planetary: lunar laser communications (NASA LLCD on LADEE, 2013); deep-space optical communications (NASA DSOC on Psyche, 2023)

Standard performance milestones.

The link budget for free-space propagation, geometric spreading loss, pointing, atmospheric attenuation, and scintillation, with worked terrestrial, inter-satellite, and deep-space examples, is in The free-space optical link budget.

References: H. Kaushal and G. Kaddoum, "Optical communication in space: challenges and mitigation techniques," IEEE Commun. Surveys Tuts. 19, 57 (2017); Andrews & Phillips, Laser Beam Propagation through Random Media (2nd ed., SPIE Press, 2005) for the comprehensive atmospheric-turbulence treatment.