Free Space Laser Communications

Abstract 

     Laser communications offer a viable alternative to RF communications for inter satellite links and other applications where high-performance links are necessity. 

Introduction

      Lasers have been considered for space communications since their realization in 1960. However, it was soon recognized that, although the laser had potential for the transfer of data at extremely high rates, specific advancements were needed in component performance and systems engineering, particularly for space-qualified hardware.

Features Of Laser Communications System

     A block diagram of typical terminal is illustrated in Fig 1. Information, typically in the form of digital data, is input to data electronics that modulates the transmitting laser source. Direct or indirect modulation techniques may be employed depending on the type of laser employed.

Detector Parameters

           The detector parameters are the type of detector, gain of the detector (if any), quantum efficiency, heterodyne mixing efficiency (for coherent detection only), noise due to the detector, noise due to the following preamplifier, and (for track links) angular sensitivity or slope factor of the detector. For optical ISLs based on semiconductor laser diodes or Nd: YAG lasers, the detector of choice is a p-type-intrinsic-n-type (PIN) or an avalanche photodiode (APD). A PIN photodiode can be operated in the photovoltaic or photoconductive mode, and has no internal gain mechanism. An APD is always operated in the photoconductive mode and has internal gain by virtue of the avalanche multiplication process. At shorter wavelengths (810-900 nm) PINs and APDs made of silicon show the best response, but at longer wavelengths (1300-1550 nm) InGaAs and Ge APDs have significantly more excess noise than comparable silicon devices.

Link Parameters

            The link parameters are the type of laser, wavelength, type of link, and required signal criteria. Although virtually every laser type has been considered at one time of another, today the lasers typically used in free space laser communications system are either semiconductor laser diodes, solid state lasers, or fiber amplifiers/lasers. Laser sources are typically described as operating in either single or multiple longitudinal modes. In single longitudinal mode operation the laser emits radiation at a single frequency, while in multiple longitudinal mode operation multiple frequencies are emitted. Single-mode sources are required in coherent detection systems and typically have spectral widths of the order of 10 kHz-10MHz. 

Conclusions

           The system and component technology necessary for successful intersatellite laser communication link exist today. The growing requirements for efficient and secure communications has led to increased interest in the operational deployment of laser crosslinks for commercial and military satellite systems in both low earth and geosynchronous orbits.