between free-space optical links on one hand and glass fiber systems and microwave Key Words: laser communications, free space, intersatellite links, space. Free-Space Laser Communication ————————. Wave Optics Simulation of Pseudo-Partially Coherent Beam Propagation Through Turbulence: Application. Explore Free Space Laser Communications with Free Download of Seminar Report and PPT in PDF and DOC Format. Also Explore the.
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pdf. FREE SPACE LASER COMMUNICATIONS SEMINAR REPORT . 6. soundofheaven.info OPERATION Free space laser communications systems . Article (PDF Available) in IEEE Communications Magazine 38(8) - of free space laser communications to address needs and. PDF | On Aug 1, , Arun K. Majumdar and others published Free-Space Laser Communications IX.
For laser communication systems in general, one wants to equalize these two error types. Figure 4: Features, Application, Advantages. 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. Free space laser communications systems are wireless connections through the atmosphere. Free-space laser communications with adaptive optics: A semiconductor laser diode beam combiner is assumed for the transmitter source employing four lasers at mW each.
Skip to main content Skip to table of contents. Advertisement Hide. Front Matter Pages i-xi. Pages Atmospheric channel effects on free-space laser communication.
Jennifer C. Ricklin, Stephen M. Hammel, Frank D. Eaton, Svetlana L. Free-space laser communication performance in the atmospheric channel. Laser communication transmitter and receiver design. Free-space laser communications with adaptive optics: Atmospheric compensation experiments. Optical networks, last mile access and applications. 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. However, some systems can be signal-quantum-noise- limited, rather than background-limited, without having to resort to heterodyne detection.
Key system characteristics are identified and subsequently quantified for a particular application. In the first part of this section we identify the key parameters that make up a link table listing. This low data rate is only used as an example and gives a point of reference for RF systems of similar performance. Key system characteristics or parameters must be identified and quantified to fully describe the system.
Critical parameters can be grouped in to five major categories: 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. 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.
Multimode sources are employed in direct detection systems and typically have spectral widths from 1. Semiconductor lasers have been in development for the three decades and have only recently within last five years demonstrated the levels of performance needed for reliable operation as direct sources. However key issues have been the lifetimes, asymmetric beam shape, and output power. Research into integrated phased arrays proved to be more challenging than first anticipated, forcing the use of single emitters and output powers in the mW range.
Inherent beam combiners employing wavelength-division multiplex or other techniques were employed for those application requiring greater power. When diode lasers are used to optically pump the lasing media graceful degradation and higher overall reliability compared to lamp pumped systems is achieved.
A variety of materials have been proposed for laser transmitters; however, neodymium doped yttrium aluminum garnet Nd: YAG is the most widely developed. Operating at nm, these lasers require an external modulator, leading to a slight increase in complexity and reliability. The modulator must be capable of operating at required pulse rates as well as handling the power levels from the laser.
With the rapid development of terrestrial fiber communications, a wide array of components are available for potential application in space. These include detectors, lasers, multiplexers, amplifiers, drive electronics, optical preamplifiers, and others. Operating at nm, erbium doped fiber amplifiers EDFA have been developed for commercial optical fiber communications that offer levels of performance consistent with many free-space laser communications applications mW range.
Issues here revolved around the space qualification of terrestrial components and the desire to achieve as much performance i. There are three basic link types: The major differences between the link types are reflected in the required signal criteria for each.
For acquisition, the criteria are typically the acquisition time, false alarm rate, probability of detection, and, if a multiple detection scheme is used, how many detections m of the total number possible, n are required.
For the tracking link, the key consideration is the amount of angle error induced by the receiver circuitry. For the communications link, the key considerations are the required data and bit error rates. Also of prime importance, once a laser type is selected, is the modulation format used to impress information on the laser carrier. A brief description of the required signal calculations for each of the three major link types is given laser in this section.
Figure 6. Photo of 1. 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 cycles are specified, while in continues-wave applications the average power is specified. In continues-wave applications, such as coherent communication employing frequency shift keying FSK or phase shift keying PSK , the pulse rate and width describe the symbol rate and symbol duration of the data impressed on the laser carrier.
Transmit optical path loss is made up of optical transmission losses and loss due to the wave-front quality of the transmitting optics, degrading the theoretical far-field on-axis gain. The wave front error loss is analogous to the surface roughness loss associated with RF antennas. The optical transmit antenna gain is exactly analogous to the antenna gain in RF systems, and describes the on-axis gain relative to an isotropic radiator with the distribution of the transmitted laser radiation defining the transmit antenna gain.
The reduction in the far-field signal strength due to transmitter mispointing is the transmitter pointing loss. For each link in a laser system, a pointing budget must be determined. The pointing budget is typically composed of bias slowly varying and random more rapidly varying components. The bias components are the alignment and detector gain mismatch errors; the random components are the NEA and residual error due to base motion disturbances.
When pointing error is a combination of bias and random terms, a somewhat more complex expression must be evaluated. The point to stress here is that once the pointing error is determined, the system beamwidth must be sized appropriately. Since this article deals with ISLs, losses due to the atmosphere are not considered. These losses can be quite large and mitigation of the effects complex. The background level depends on the relative altitudes of the platforms, the time of the year, and the wavelength selected.
The receiver optical path loss is simply the optical transmission loss for systems employing direct detection techniques. However, for laser systems employing coherent optical detection either homodyne or heterodyne there is an additional loss due to wavefront error. The preservation of the wavefront quality is essential for optical mixing of the received signal and local oscillator fields on the detector surface.
To first order, the loss expression is the same as that previously defined for the transmit wavefront error. The optical filter bandwidth specifies the spectral width of the narrow-bandpass filter employed in optical intersatellite links.
Optical filter reduce the amount of unwanted background entering the system. The optical width of the filter must be compatible with the spectral width of the laser source. In addition to source considerations, the minimum width also be determined by the acceptable transmission level of the filter; typically the transmission of the filter decreases with spectral width.