Laser communications offer a viable alternative to RF communications for inter satellite links and other applications where high-performance links are a necessity. High data rate, small antenna size, narrow beam divergence, and a narrow field of view are characteristics of laser communications that offer a number of potential advantages for system design.
SYSTEM CHARACTERISTICS AND DESCRIPTION
The key system characteristics which when quantified, together gives a detailed description of a laser communications system. These are identified and quantified for a particular application. The critical parameters are grouped into five major categories: link, transmitter, channel, receiver, and detector parameters.
The link parameters include the type of laser, wavelength, type of link, and the required signal criterion. Today the lasers typically used in free space laser communications are the semiconductor laser diodes, solid-state lasers, or fiber amplifier lasers. Laser sources are described as operating in either in single or multiple longitudinal modes. In the single longitudinal mode operation the laser emits radiation at a single frequency, while in the multiple longitudinal mode, multiple frequencies are emitted.
The transmitter parameter consists of certain key laser characteristics, losses incurred in the transmit optical path, transmit antennae gain, transmit pointing losses. The key laser characteristics include peak and average optical power, pulse rate and pulse width. In a pulsed configuration the peak laser power and duty cycle are specified, whereas in continuous wave application, the average power is specified
The channel parameters for an optical inter satellite link(ISL) consists of range and associated loss ,background spectral radiance and spectral irradiance. The range loss is directly proportional to the square of wavelength and inversely proportional to the square of the separation between the platforms in meters.
The receiver parameters are the receiver antenna gain, the receive optical path loss, the optical filter bandwidth and the receiver field of view. The receiver antenna gain is proportional to the square of effective receiver diameter in meters and inversely proportional to the square of the wavelength. The receiver optical path loss is simply the optical transmission loss for systems employing the direct detection techniques. However for the lasers employing the coherent optical detection there is an additional loss due to the wavefront error. The preservation of the wave front quality is essential for the optimal mixing of the received signal and the local oscillator fields on the detector surface
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