04-09-2014, 12:43 PM
FREE SPACE LASER COMMUNICATION
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INTRODUCTION
Free space laser communications systems are wireless connections through the atmosphere. They work similar to fiber optic cable systems except the beam is transmitted through open space.
The carrier used for the transmission of this signal is generated by either a high power LED or a laser diode. The laser systems operate in the near infrared region of the spectrum. The laser light across the link is at a wavelength of between 780 – 920 nm. Two parallel beams are used, one for transmission and one for reception.
MAGNUM 45 High-Speed Laser-Communication System
REQUIREMENTS
The requirements are data transmitters and receivers as well as on optical antennas and pointing, acquisition and tracking mechanisms are discussed.
Typical application scenarios are outlined, experimental systems and their technologies are cited.
LITERATURE SURVEY
Free-space laser communications is a very flexible means to connect end users to a high-bandwidth data network via ground-based terminals on top of buildings or to bring a variety of data services to remote locations via satellite terminals in space.
External influences on the optical link due to atmospheric turbulence and vibrations in the transmitter's environment require some method of beam control to stabilize the optical link and maintain a high transmission rate.
Liquid crystal (LC) optics can provide a compact and low-power solution to beam control in laser communications systems
DATA TRANSMITTER
The main parameters characterizing the optical source are wavelength, output power, transverse mode, polarization, linewidth, and modulation capability
The output power will have to be in the range of 100mW and 1 W, depending on the link distance and data rate. It should be available in a single transverse mode to achieve maximum on-axis antenna gain, and in a single longitudinal mode to obtain optimum spectral efficiency. For coherent reception, phase noise is detrimental and thus a narrow linewidth of both the transmitter laser and the local laser oscillator in the receiver is required
ANTENNAS
The transmit antenna is essentially a telescope which magnifies the diameter of the beam emitted by the laser
This beam is generally well modelled by a Gaussian intensity distribution.
The main specifications of the optical antenna are:
diameter of primary mirror (or lens), magnification, aberrations, wavelength dependence of throughput, sensitivity to temperature changes and gradients, and stray light level.
POINTING, ACQUISITION, AND TRACKING
To establish an optical link in space, a sophisticated spatial pointing and acquisition procedure must be initiated. Information on the position of the two space terminals has to be available.
During data transmission, the angle between the line-of-sight and the transmit beam axis must be kept to within a fraction of the transmit beamwidth θ which may be as small as a few μrad. To maintain sufficient alignment of the transmit and receive antennas despite platform vibrations, both terminals have to be equipped with a tracking servo loop. To ensure short acquisition time and adequate tracking accuracy, sufficient optical power for the acquisition and the tracking process must be received.
ADVANTAGES
Laser communication systems offer many advantages over radio frequency (RF) systems.
Most of the differences between laser communication and RF arise from the very large difference in the wavelengths.
For a given transmitter power level, the laser beam is brighter at the receiver by the square of this ratio due to the very narrow beam that exits the transmit telescope.
Taking advantage of this brighter beam or higher gain, permits the laser communication designer to come up with a system that has a much smaller antenna than the RF system and further, need transmit much less power than the RF system for the same receiver power.
The laser beam width can be made as narrow as the diffraction limit of the optic allows.
The required optical power is determined by data rate, detector sensitivity, modulation format ,noise and detection methods.
CONCLUSION
Last but not least it should be noted, that a microwave link designer has – within limits – the possibility to reduce the influence of the propagation effects and to optimize the desired link quality and its costs by choosing the frequency, antenna diameters, diversity protection, etc.
A free space laser communication system has to be taken as it is