Effect of Water and Aerosols Absorption on Laser Beam Propagation in Moist Atmosphere at Eye-Safe Wavelength of 1.57 μm

Abstract

: To realize optical wireless power transmission, atmospheric propagation of eye-safe wavelength (1.57µm) laser beams was theoretically investigated. Laser beams are affected by the presence of water vapor and aerosols which absorb and scatter the laser energy. The scattering coefficients of water molecules and aerosols were estimated to be about 6.3 × 10 -7 and 5.6 × 10 -5 m -1 , respectively, at wavelength (λ 0 ) of 1.57µm. Furthermore, the absorption coefficients of moist air at 30% relative humidity and aerosols were estimated to be about 6.16 × 10 -3 and 2.52 × 10 -5 m -1 , respectively, at λ 0 = 1.57µm. Then simulation of laser beam propagation in the moist atmosphere at λ 0 = 1.57µm was performed using these coefficients. Under the condition of no wind, the beam intensity decreases rapidly with increasing the length z and the rate of decrease slows down as the beam radius (ω) increases. When z h is defined as the z where the normalized intensity is halved, the z h (= 25 m) at ω = 20 mm when input power P = 10 W is about three times longer than that (= 8 m) when P = 100 W. This result indicates that the thermal distortion of laser beams due to accumulated heat around the z axis becomes more conspicuous as the optical power increases. The effect of this thermal beam distortion can be weakened when the laser beam is subject to crosswinds. Under the condition of gentle uniform wind with wind velocity v = 5 m/s, propagation of laser beams with ω = 20 mm was studied when P = 100 W. The z h (= 105 m) when v = 5 m/s is about 13 times longer than that (= 8 m) when v = 0 m/s. Thus, under conditions of v = 5 m/s and 30% relative humidity, laser beams with P = 100 W and ω = 20 mm can propagate over 100 m without damaging the initial beam shape at λ 0 = 1.57µm.