I am reading the book: Kaushal, H., Jain, V.K. and Kar, S., 2017, Free space optical communication, New Delhi: Springer India.
The operating wavelengths for beacon and data transmission were discussed. Beacon wavelength window is 780 to 1064 nm, for data transmission it is 1520 to 1560 (1600) nm.
The following explanations are given for the data transmission wavelength window:
The 1550 nm wavelength is commonly used as data operating wavelength due to following reasons:
(i) Reduced background noise and Rayleigh scattering: The absorption coefficient of the Rayleigh scattering has functional dependence with the wavelength as 4: Consequently, there is almost negligible attenuation at higher operating wavelengths as compared to those at the visible range.
(ii) High transmitter power: At 1550 nm a much higher power level (almost 50 times) than at lower wavelengths is available to overcome various losses due to attenuation.
(iii) Eye-safe wavelength: The maximum permissible exposure (MPE) for eye is much higher at 1550 nm wavelength than at 850 nm. This difference can b explained by the fact that at 850 nm, approximately 50 % of the signal can reach the retina whereas at 1550 nm, the signal is almost completely absorbed by cornea itself. And therefore the signal received at the retina is negligibly small.
Why 780 to 1064 nm is chosen for beacon transmission wasn't explained.
Could someone explain why beacon signal operates in lower range?
What are reasons the difference of operating wavelengths?
Beacon signal sends information as well: "Where are you?", "hello I am here" and we send data via data link. What are the atmosphere effects on the beacon signal?