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.

Reason	Application
No physical infrastructure required	Disaster recovery, military deployment, remote sensing
Higher data rates over short ranges	Optical wireless backhaul, last-mile connectivity
No spectrum licensing	Unlicensed; no regulatory burden vs RF
Deep-space communications	Fiber is impossible at planetary distances
Higher beam directivity (security)	Difficult to intercept; secure data transmission
Vacuum operation possible	Satellite, lunar, Mars communications

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:

Parameter	Typical value
Transmit power	100 mW
Beam divergence	1 mrad full-angle
Receive aperture	100 mm diameter
Propagation distance	1 km
Free-space loss (geometric)	$\sim 27$ dB
Atmospheric attenuation (clear)	$\sim 1$ dB
Receiver collection efficiency	$\sim 80$%
Received power	$-10$ to $-15$ dBm
Achievable data rate	1 – 10 Gb/s

Atmospheric impairments.

Effect	Mechanism	Mitigation
Atmospheric attenuation	Molecular absorption + aerosol scattering	Wavelength choice (avoid water absorption lines); higher transmit power
Turbulence-induced scintillation	Refractive-index variations in air; intensity fluctuations at receiver	Adaptive optics; spatial diversity; aperture averaging
Beam wander	Large-scale turbulence eddies steer the beam	Pointing-and-tracking systems; beam expansion
Beam spreading	Turbulence broadens the beam beyond diffraction limit	Larger transmit aperture
Fog and haze	Mie scattering by water droplets	Multi-wavelength systems; mm-wave fallback
Cloud blockage	Total signal loss during cloud transit	Multiple ground stations; high-altitude relay

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

Window	Wavelength range	Notable feature
Visible	400 – 700 nm	Hindered by ambient sunlight (in daytime); used for short-range applications
Near-IR	800 – 1100 nm	Used in older FSO systems; affected by water absorption
Telecom S/C/L band	1280 – 1625 nm	Eye-safe at higher power; mature components from telecom infrastructure
Short-wave IR	1500 – 1800 nm	Best penetration through atmospheric haze and fog
Long-wave IR	8 – 14 μm	Best penetration through fog and clouds (atmospheric window); requires HgCdTe or QCL detectors

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: deep-space links (Mars Reconnaissance Orbiter LCRD, ESA EDRS, NASA LCRD)
• Satellite-to-satellite: ISL (inter-satellite link) constellations; Starlink uses laser ISL
• Lunar / planetary: lunar laser communications (NASA LADEE 2013, LCRD 2021)

Standard performance milestones.

System	Year	Data rate	Distance
Bell Labs first FSO (visible)	1880	speech	200 m
OSI FSO commercial system	2000	1.25 Gb/s	$< 2$ km
NASA LADEE (lunar)	2013	622 Mb/s	400,000 km
OICETS / ARTEMIS GEO	2008	50 Mb/s	45,000 km
ESA EDRS	2016	1.8 Gb/s	45,000 km
NASA LCRD	2021 – ongoing	1.244 Gb/s	45,000 km
DSOC Psyche (deep-space)	2023 – 2026	25 Mb/s	$\sim$ 0.4 AU = 60M km

References: Saleh & Teich, Fundamentals of Photonics, Ch. 25 for FSO transmission fundamentals; Andrews & Phillips, Laser Beam Propagation through Random Media (2nd ed., SPIE Press, 2005) for the comprehensive atmospheric-turbulence treatment.