i) Optical Communication through the Atmosphere

Compared to radiowave or microwave systems optical communication systems through the at- mosphere are characterized by a higher transmission bandwidth and a much higher direction- ality of the radiation. But generally they also suffer from stronger damping losses and turbulence effects in the atmosphere. Therefore, optical systems are favorably applied for high data transmission over shorter point-to- point connections, e.g., between urbanic computer centers or factory buildings, and on the other side they have proven for high-performance data transfer between satellite links in space. The fundamentals of Optical Communications are composed in these lecture notes:

Set-Up of an Optical Communication System

The basic components of an optical communication system can be represented by this block- diagram. The transmitter is a cw or pulsed laser, dependent on the applied modulation type, and is con- trolled by an optical modulator or an analog-digital converter to transfer an input information on the laser carrier frequency. For the further transmission and detection similar components are used as for a range finder. Within their master theses students were developing and testing different optical communica- tion systems, mainly using pulsed semiconductor lasers in combination with pulse-gated-binary- modulation. These systems were successfully designed and applied for medium bandwidths with parallel transmission channels and tested over transmission distances up to 5 km.

Propagation of Visible and Infrared Radiation through the Atmosphere

For longer distances not only transmission losses due to absorption but also Rayleigh- and Mie- scattering as well as turbulence effects have to be considered, which are strongly dependent on the laser wavelength as well as on the environmental and weather conditions. In addition, scattering and turbulence effects have a direct influence on the long-term signal qua- lity and error rate. To quantify the size of all these effects, extensive calculations for different configurations and modulation techniques were performed. Radiation losses on the laser wavelength due to molecular absorption, scattering and diffraction are simulated with our program platform MolExplorer (see next section), which uses the HITRAN- or GEISA-database with almost 6 Mio spectral lines of all relevant components and trace gases in the atmosphere, and which directly calculates the transmitted power from the transmitter with cross-sectional area A S to the receiver with area A E . Dependent on the propagation direction and path length through the atmosphere the program accounts for the changing molecular density and temperature with altitude above ground. The upper Figure shows an example for an Earth-Satellite data transmission to the geo-stationary orbit in 36,000 km height. A secure data exchange is only possible within the optical windows a- round 0.85 µm, 1.1 µm, 1.24 µm and 1.6 µm. Outside these windows the molecular absorption is strongly increasing, primarily caused by water vapor. Also the attenuation in the different windows is increasing with shorter wavelength due to scat- tering and turbulence, but partially compensated by lower diffraction losses. For this simulation a transmitting and receiving area of A S = A E = 0.03 m 2 (cross-section = 20 cm) is assumed, which al- most causes the same diffraction as the beam expansion by turbulences. Under these conditions free-space damping and turbulence with -62 dB at a wavelength of 1.06 µm gives the main contribution to the transmission losses, while scattering at medium visibility (V = 10 km) causes additional -16 dB. With a Nd + :YAG laser at wavelength of 1.064 µm, an average output power of 4 W, a pulse rate of 1 GBit/s and using pulse-code-modulation, under clear sky conditions a signal/noise ratio of 20 dB is achievable and still allows communication even at cirrostratus overcast in 15 km altitude, but at stronger cloudiness, e.g. altostratus with an attenuation of -20dB/km completely breaks down.

References

H. Harde Optical Communications Script of Advanced Course, Helmut-Schmidt-University Hamburg, 2008 H. Harde Laser Communication through the Atmosphere and Optical Fibers Lecture, Carl-Cranz-Gesellschaft, Hamburg, 1988 H. Harde Transmitters for Optical Communications Lecture, Carl-Cranz-Gesellschaft, Hamburg, 8. März 1990 H. Harde Laser Transmission from GEO-Sat to Earth Research Study and Lecture, Helmut-Schmidt-University, funded by EADS Astrium, 01.Feb. 2011 H. Harde, J. Pfuhl MolExplorer: A Tool for Computation and Display of Molecular Spectra from the HITRAN and GEISA Database Helmut-Schmidt-University, 2018