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.
0 comments:
Post a Comment