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Abstract: Laser communication links in space are attractive alternatives to present -day Key Words: laser communications, free space, intersatellite links, space. Also Explore the Seminar Topics Paper on Laser Communications with Abstract or Synopsis, Documentation on Advantages and. LASER Communication - Download as Word Doc .doc /.docx), PDF File .pdf), Text File .txt) or read online.
Laser communication is capable of much higher data rates than RF. The laser beam width can be made as narrow as the diffraction limit of the optic allows. The antennae gain is proportional to the reciprocal of the beam width squared.
To achieve the potential diffraction limited beam width a single mode high beam quality laser source is required; together with very high quality optical components throughout the transmitting sub system. The possible antennae gain is restricted not only by the laser source but also by the any of the optical elements. In order to communicate, adequate power must be received by the detector, to distinguish the signal from the noise.
Laser power, transmitter, optical system losses, pointing system imperfections, transmitter and receiver antennae gains, receiver losses, receiver tracking losses are factors in establishing receiver power. The required optical power is determined by data rate, detector sensitivity, modulation format ,noise and detection methods.
Laser Communications Published on Dec 06, More Seminar Topics: Are you interested in this topic. Fewer onboard consumables are required over the long lifetime because there is less disturbance to the satellite compared with larger and heavier RF systems.
The narrow beam divergence of affords interference-free and secure operation.
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. The source output passes through an optical system into the channel.
The optical system typically includes transfer, beam shaping, and telescope optics. The receiver beam comes in through the optical system and is passed along to detectors and signal processing electronics.
There are also terminal control electronics that must control the gimbals and other steering mechanisms, and servos, to keep the acquisition and tracking system operating in the designed modes of operation.
A block diagram of a typical laser communication terminal. Plots of aperture diameter Vs.
The laser beamwidth can be made as narrow as the diffraction limit of the optics allows. This is given by the beamwidth equal to 1.
This antenna gain is proportional to the reciprocal of the beamwidth squared. The most important point here is that to achieve the potential diffraction-limited beamwidth given by the telescope diameter, a single-mode high-beam-quality laser source is 4.
The beam quality cannot be better than the worst element in the optical chain, so the possible antenna gain will be restricted not only by the laser source itself, but also by any of the optical elements, including the final mirror or telescope primary. Because of the requirement for both high efficiency and high beam quality, many lasers that are suitable in other applications are unsuitable for long distance free-space communication.
In order to communicate, adequate power must be received by the detector to distinguish signal from noise. Laser power, transmitter optical system losses, pointing system imperfections, transmitter and receiver antenna gains, receiver losses, and receiver tracking losses are all factors in establishing receiver power. The required optical power is determined by data rate, detector sensitivity, modulation formats, noise, and detection methods.
Fig 3. There are two distributions: A threshold must be set that maximizes the success rate and minimizes the error rate.
One can see that different types of errors will occur. Even when there is no signal present, the fluctuation of the nonsignal sources will periodically cause the threshold to be exceeded. This is the error of stating that a signal is present when there is no signal present.
The signal distribution may also fall on the other side of the threshold, so errors stating that no signal is present will occur even when a signal is present. For laser communication systems in general, one wants to equalize these two error types. In the acquisition mode, however, no attempt is made to equalize these errors since this would increase acquisition time.
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 — nm.
Two parallel beams are used, one for transmission and one for reception. Figure 4: LSA Photonics 7. Of the three, acquisition is generally the most difficult; angular tracking is usually the easiest. Communications depends on bandwidth or data rate, but is generally easier than acquisition unless very high data rates are required.
Acquisition is the most difficult because laser beams are typically much smaller than the area of uncertainty. Satellites do not know exactly where they are or where the other platform is located, and since everything moves with some degree of uncertainty, they cannot take very long to search or the reference is lost.
Instability of the platforms also causes uncertainty in time. In the ideal acquisition method, shown in Figure 4, the beamwidth of the source is greater than the angle of uncertainty in the location of receiver.
The receiver field of includes the location uncertainty of the transmitter. Unfortunately, this ideal method requires a significant amount of laser power.
This is because a lower pulse rate is needed for acquisition than for tracking and communications. High peak power pulses more easily overcome the receiver set threshold and keep the false alarm rate low.
A low duty cycle transmitter gives high peak power, yet requires less average power, and is thus a suitable source for acquisition. As the uncertainty area becomes less, it becomes more feasible to use a continues source of acquisition, especially if the acquisition time does not have to be short. At optical frequencies noise characteristics are significantly different than those at radio frequencies.
In the RF domain, quantum noise is quite low, while thermal noise predominates and does not vary with frequency in the microwave region. However, as the wavelength gets shorter, quantum noise increases linearly, and in the laser regime thermal noise drops off very rapidly, becoming insignificant at optical wavelengths.
Because there is so little energy in a photon at radio frequencies, it takes many problems to equal the thermal noise. The quantum noise is actually the statistical fluctuations of the photons, which is the limiting sensitivity at optical frequencies.
However, in optical receivers employing direct detection and avalanche photodiodes, the detection process does not approach the quantum limit performance. For this type of optical receiver, the thermal noise due to the preamplifier is usually a significant contributor to the total noise power.
Free space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. Atmospheric turbulence can degrade the performance of free-space optical links, particularly over ranges of the order of 1 km or longer.
Inhomogeneities in the temperature and pressure of the atmosphere lead to variations of the refractive index along the transmission path. These index inhomogeneities can deteriorate the quality of the received image and can cause fluctuations in both the intensity and the phase of the received signal. Aerosol scattering effects caused by rain, snow and fog can also degrade the performance of free-space optical communication systems. The primary background noise is the sun.
The solar spectral radiance extends from the ultraviolet to the infrared, with the peak in the visible portion of the spectrum. A star field is an area of the sky that includes a number of stars. If one were able to look only at an individual star, one would find a brightness similar to that of the sun; but a star field as a whole is composed of small point sources of light, the stars in the field, against a dark area having no background level.
The background is reduced by making both the field of view and the spectral width as narrow as possible. Heterodyne systems will enable further reduction, but with a increase in terminal complexity. First, the beam-width attainable with the laser communication system is narrower than that of the RF system by the same ratio at the same antenna diameters the telescope of the laser communication system is frequently referred as an antenna.
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 to transmit much less power than the RF system for the same receiver power.
However, since it is much harder to point, acquisition of the other satellite terminal is more difficult. Some advantages of laser communications over RF are smaller antenna size, lower weight, lower power and minimal integration impact on the satellite. Laser communication is capable of much higher data rates than RF. Download Seminar Report on Laser communications.
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