Laser communications offer a viable alternative to RF communications for intersatellite links and other applications where high-performance links are necessary. High data rate, small antenna size, narrow beam divergence, and a narrow field of view are characteristics of laser communication that offer a number of potential advantages for system design. The high data rate and large information throughput available with laser communications are many times greater than in radio frequency (RF) systems. The small antenna size requires only a small increase in the weight and volume of host vehicle. In addition, this feature substantially reduces blockage of fields of view of the most desirable areas on satellites. The smaller antennas, with diameters typically less than 30cm, create less momentum disturbance to any sensitive satellite sensors. The narrow beam divergence of affords interference-free and secure operation.
Lasers have been considered for space communications since their realization in 1960. 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. Advances in system architecture, data formatting, and component technology over the past three decades have made laser communications in space not only a viable but also a attractive approach to intersatellite link applications. The high data rate and large information throughput available with laser communications are many times greater than in radio frequency (RF) systems. The small antenna size requires only a small increase in the weight and volume of host vehicle. In addition, this feature substantially reduces blockage of fields of view of the most desirable areas on satellites. The smaller antennas, with diameters typically less than 30cm, create less momentum disturbance to any sensitive satellite sensors. Fewer onboard consumables are required over the long lifetime because there are fewer disturbances to the satellite compared with larger and heavier RF systems. The narrow beam divergence of affords interference-free and secure operation.
1.1 FEATURES OF LASER COMMUNICATIONS SYSTEM
A block diagram of typical terminal is illustrated in Fig 1. 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.
The extremely high antenna gain made possible by the narrow beams enables small telescope apertures to be used. Plots of aperture diameter vs. data rate for millimetre and optical waves are shown in Fig 2. A laser communications system operating at 1 GB/s requires an aperture of approximately 30 cm. In contrast, a 1 GB/s millimetre wave system requires a significantly larger aperture, 2-2.75 m. The laser beam width can be made as narrow as the diffraction limit of the optics allows. This is given by the beam width equal to 1.22 times the wavelength of the light, divided by the radius of the output beam aperture. This antenna gain is proportional to the reciprocal of the beam width squared. The most important point here is that to achieve the potential diffraction-limited beam width given by the telescope diameter, a single-mode high-beam-quality laser source is required, together with very high-quality optical components throughout the transmitting subsystem. 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.
When the receiver is detecting signals, it is actually making decisions as to the nature of the signal (when digital signal are being sent it distinguishes between ones and zeros). Fig 3. shows the probability of detection vs. measured photocurrent in a decision time. There are two distributions: one when a signal is present (including the amount of photocurrent due to background and dark current in the detector), and one when there is no signal present (including only the non signal current sources). 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 non signal 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.
Free space laser communications systems are wireless connections through the atmosphere. They work similar to fibre 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.
Figure 1.4: MAGNUM 45 High-Speed Laser-Communication Systems (Source:LSA Photonics)
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.
2.1 LINK 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 fibre 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. Semiconductor lasers have been in development for three decades and have only recently
(within the past 7 years) demonstrated the levels of performance needed for the reliable operation as direct sources .typically operating in the 800-900 nm range(gallium arsenide/gallium aluminium arsenide)their inherently high efficiency(50%)and small size made this technology attractive. The key issues have been the life times, asymmetric beam shapes, output power. Solid state lasers have offered higher power levels and the ability to operate in high peak power modes for the acquisition. When laser diodes are used to optically pump the lasing media graceful degradation and higher overall reliability is achieved. A variety of materials have been proposed for laser transmitters: neodyminium doped yttrium aluminium garnet (Nd: YAG) is the most widely used. Operating at 1064 nm, these lasers require an external modulator leading to a slight increase in the complexity and reliability. With the rapid development of terrestrial fibre communications, a wide array of components is available for the potential applications in space. These include detectors, lasers, multiplexers, amplifiers, optical pre amplifiers etc. Operating at 1550nm erbium doped fibre amplifiers have been developed for commercial optical fibre communications that offer levels of performance consistent with many free space communications applications. There are three basic link types: acquisition, tracking and communications. The major differences between the link types are reflected in the required signal criterion for each. For acquisition the criterion is acquisition time, false alarm rate, probability of detection. For the tracking link the key considerations are the amount of error induced in the signal circuitry. This angle error is referred to as the noise effective angle. For the communications link, the required data and the bit error rates are of prime importance.
2.2 TRANSMITTER PARAMETERS
The transmitter parameter consists of certain key laser characteristics, losses incurred in the transmitter optical path, transmit antennae gain, and 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. Transmit optical path loss is made up of optical transmission losses and the loss due to the wave front quality of the transmitting optics. The wave front error loss is analogous to the surface roughness loss associated with the RF antennas. The optic transmit antenna gain is analogous to the antenna gain in the 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 laser sources suitable for the free space communications tend to exhibit a Gaussian intensity distribution in the main lobe. The reduction in the far field signal strength due to the transmitter pointing is the transmitter pointing losses. The pointing error is composed of bias (slowly varying) and random (rapidly varying) components.
2.3 CHANNEL PARAMETERS
The channel parameters for an optical intersatellite link(ISL) consist 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 platform in metres.