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Laser communications are key to ensuring fast and precise transmission of data from spacecraft systems, which offers the unique advantages over radio frequency (RF) systems, including size, mass, low power consumption, reduced noise, larger bandwidth, compactness, reduced complex frequency planning and RF interface issues.
Laser crosslinks will allow the transfer of data between satellites at rates compatible with ground fiber networks. This is an exciting era for satellite laser communications.
Laser communications will revolutionize the return of data from destinations beyond the low Earth orbit, increase outreach opportunities from outer space and improve the quality of life of astronauts during long space missions. As we strive to put humans on Mars for the first time, it is imperative that we develop a communication system to support these activities at the highest possible level.
Don Cornwell, NASA
NASA is working to forever change the way astronauts communicate to and from space using an advanced laser communications system called Laser-Enhanced Mission and Navigation Operational Services (LEMNOS), which will allow faster connections than ever before.
Simply imagine being able astronauts picking up a cell phone and video-conferencing with their family and friends from 34 million miles away, just the same as they might on Earth. LEMNOS may make these capabilities and more a reality in the near future.
In the most satellite communication system, the laser is preferred as a light source due to the long communication distance. At these distances are associated high levels of reduction and only lasers having the ability to establish efficient links because of their specific characteristics: the emission of monochromatic radiation (well-defined wavelength), highly directive and narrow light beam.
The exploration of space laser communications involves the development of more efficient and cost-effective space communications equipment. Because radio frequency wavelengths are longer, the size of their transmission beam covers an area of about a hundred miles; therefore, it required large collecting antennas for RF data transmission.
The laser wavelengths are 10,000 times shorter, allowing data to be transmitted over narrower, tighter beams. The smaller wavelengths of laser-based communications are safer, less degradation, delivering the same amount of signal power to much smaller collecting antennas.
Because of its characteristics, more compact design and greater energy efficiency in converting electricity supplied into light energy, making it possible to establish links at distances greater than 40 000 km. Currently, semiconductor lasers and gas lasers are the most used in solid-state lasers.
The type of laser is chosen according to the characteristics of the link that is implemented, such as distance, altitude, the environmental conditions and the power level required in the receiver. It also depends on the wavelength chosen for the link as well as the modulation format used. Table 1 shows some examples of semiconductor lasers used in satellite communication.
Table 1. Examples of solid-state lasers used in satellite communication.
|Type of Laser
Requires elaborate modulation equipment, diode or solar pumping.
Reliable, small, rugged, compact, directly and easily modulation, easy to combine into arrays.
Requires external modulation, power limited, uses gas tube.
|CO2 (gas laser)
IR range, detectors are poor, uses a discharge tube, modulation difficult.
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