FSO Link Budget
Free-space optical link budget from transmitter divergence, receiver aperture, path loss and atmospheric attenuation, with presets and the margin versus range.
TX gain 8/θ²
69.03 dBi
θ = 1.00 mrad half-angle
RX gain (πD/λ)²
106.14 dBi
Free-space path loss
198.18 dB
atmosphere 3.0 dB
Received power
-18.02 dBm
Friis: -18.01 dBm
Margin vs -30 dBm
11.98 dB
Received power vs range
- Received power
- Sensitivity
- Current
Model (Friis transmission)
P_r = P_t + G_t + G_r − 20 log₁₀(4πR/λ) − γ_atm·R − L_misc [dB]
The transmitter is a Gaussian beam of full 1/e² divergence θ, so its on-axis gain is 8/(θ/2)². The receiver collects the plane wave over its aperture: G_r = (πD/λ)². The free-space path loss and the two gains together are the beam spreading: for a small receiver the product equals 2a²/w(R)², the share of a Gaussian beam of radius w = θR/2 that falls on an aperture of radius a. Closer in, or for a large receiver, the card shows the exact capture 1 − exp(−2a²/w(R)²) with w(R) = w₀√(1 + (R/z_R)²). Atmospheric attenuation is per kilometre: take it from the fog, rain or snow calculators. With θ fixed, λ cancels in P_r (G_t·G_r/FSPL = 2D²/(θ²R²) for a small receiver); it matters only through the atmospheric attenuation, which you enter for your wavelength, and the diffraction-limited waist.
Not modelled: turbulence fading, pointing jitter statistics and background noise. The sensitivity is the receiver's power at the target bit error rate.
Model and references
Textbook approximation. A standard approximation from the literature (for example paraxial, plane-wave or Gaussian-beam), valid within the range the page states. About the tiers
Budget table from Friis (G_t = 8/θ², θ half-angle; G_r = (πD/λ)²); the received power is the exact Gaussian capture 1 − exp(−2a²/w²), which Friis matches far out. Atmosphere in dB/km; no turbulence.
- Friis H. T. (1946). A note on a simple transmission formula. Proc. IRE 34(5), 254–256. doi:10.1109/jrproc.1946.234568
- Degnan J. J., Klein B. J. (1974). Optical antenna gain. 2: Receiving antennas. Appl. Opt. 13(10), 2397. doi:10.1364/ao.13.002397
- Kim I. I., Korevaar E. (2001). Availability of free-space optics (FSO) and hybrid FSO/RF systems. Proc. SPIE 4530, 84. doi:10.1117/12.449800
Report a problem
Related calculators
FSO Geometric Loss
Geometric spreading loss of a divergent FSO beam: top-hat footprint from exit aperture and divergence versus the exact Gaussian-beam capture by the receiver aperture.
Lasercom Link Budget
Lasercom link budget with Gaussian-beam transmit and receive gains, free-space path loss, spot size at the receiver, and pointing and atmospheric losses.
Optical Antenna Gain
Telescope transmit gain (Klein–Degnan, clipped and obscured Gaussian beam), receive gain, far-field divergence, beam waist and Rayleigh range, with beam quality M² up to 10 as a Gaussian Schell-model beam.
Point-Ahead Angle
Point-ahead angle 2v⊥/c from the transverse relative velocity, compared with the diffraction half-angle, with the miss distance and light time for LEO and GEO laser links.