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.

Model: Textbook approximation

Inputs

TX power (dBm)

TX aperture diameter (cm)

RX aperture diameter (cm)

Wavelength (nm)

Range (km)

TX antenna efficiency

RX antenna efficiency

Pointing loss (dB)

Atmospheric loss (dB)

Results

TX gain99.15 dBi

RX gain104.59 dBi

Free-space path loss252.16 dB

Equivalent Gaussian spot radius (1/e²)15.6 m

same on-axis gain as G_t (η_tx included)

Received power-33.42 dBm

Friis: P_r = P_t G_t G_r (λ / 4πR)² · L_point · L_atm

Gains: G = (π D / λ)² η, with η the antenna efficiency (Gaussian illumination, truncation, obscuration, optics)

FSPL: 20 log₁₀(4πR/λ). It isn't a loss on its own: the gains cancel it, and together they count the beam spreading once.

Spot radius: w = θR with θ = √(8/G_t), the Gaussian beam with the same on-axis gain (η_tx included). Above η_tx = 0.8145 (Klein–Degnan Gaussian optimum) you need a flat-top beam.

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

Friis P_r = P_t G_t G_r (λ/4πR)² with G = (πD/λ)² η at each end; FSPL and gains count the spreading once. Capped at P_t η_t η_r; flags R < 2D_t²/λ. Pointing and atmosphere as fixed dB.

Friis H. T. (1946). A note on a simple transmission formula. Proc. IRE 34(5), 254–256. doi:10.1109/jrproc.1946.234568
Klein B. J., Degnan J. J. (1974). Optical antenna gain. 1: Transmitting antennas. Appl. Opt. 13(9), 2134. doi:10.1364/ao.13.002134
Kaushal H., Kaddoum G. (2017). Optical communication in space: challenges and mitigation techniques. IEEE Commun. Surv. Tutor. 19(1), 57–96. doi:10.1109/comst.2016.2603518

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