Terrestrial Free Space Optics

Aside from optical communication through the glass fiber there is growing interest in free space optics (FSO) communication, i. e., transmission of information by a laser beam through air.

Advantages of FSO

  • No costly installation of fiber cables
  • Fast installation of the equipment
  • No wavelength regulation as for RF links

Fields of applications of FSO

  • Fast connectivity solutions (e. g. disaster recovery)
  • ‘Last-Mile’ network solutions
  • Temporary network provision
  • Cable free connectivity in the finance sector
  • Microcell connectivity for mobile communication

Typical application which have already been installed are city links between roof tops with distances up to several kilometres and bitrates in the Gb/s range.

Main challenges for FSO

1. Enough transmitter power has to be seen by the optical receiver This means the laser must be pointed correctly so that the beam hits the receiver and the free space path must have small loss.

FSO transmission is hindered by atmospheric turbulence, fog, building movement, inteference of sunlight.

2. Transmitter power must obey the eye safety requirements which limits maximum transmitter power.

State of the art

A number of FSO products exist offering bitrates of a few Gb/s requirements over up to 3 km distance. Typical performance bitrate x distance = 3 Gb/s km.

Our goal

Develop a FSO system with 10 Gb/s over 10 km, i.e increase of performance by a factor of 30.

How can this be achieved?

  • Choice of the wavelength 1550 nm yields a factor 50 better eye safety than for the common 850 nm FSO wavelength
  • Deployment of components, subsystems and transmission concepts from fiber technology
  • Wavelength division multiplexing (WDM) for capacity increase
  • Choice of optimum modulation format for FSO
  • Implementation of forward error correction (FEC) and digital signal processing (DSP) to increase reliability

Technical Background

At Fraunhofer HHI technical expertise notably covers

  • Design, fabrication and characterization of components
  • Numerical optimization of transmitters and receivers
  • Setup and characterization of optical transmitters and receivers
  • Numerical simulation of the optical WDM FSO link
  • Experimental validation of the whole design