Laser communications

Laser communication systems based on free-space propagation through the atmosphere suffer drawbacks because of factors like atmospheric turbulence and attenuation by rain, snow, haze, or fog. Nevertheless, free-space laser communication systems were developed for many appHcations (89—91). They employ separate components, such as lasers, modulators, collimators, and detectors. Some of the most promising appHcations are for space communications, because the problems of turbulence and opacity in the atmosphere are absent. 

Optoelectronic devices are found in numerous consumer products such as television, compact-disk players, laser communications, laser printers, radar detectors, cellular telephones, direct-broad-cast television, and many others. Many of these applications were developed in Japan and that country is still prominent in the field. 

See also Arsenic (As) Gallium (Ga) Indium (In) Phosphorus (P) InGaAsP semiconductor, in laser communication systems, 22 180 InGaAs semiconductor... 

Further, compound semiconductors are enabling new markets such as fiber optic laser communications, cellular phones, cellular faxes, cellular 2-way video, engine (high temperature) mounted devices, radiation hard devices, medical, and full color displays. We can expect compound semiconductors to contribute greatly to the quality of life over the next several decades. 

It is perhaps worth while to point out that most of the attenuation of infrared radiation in the atmosphere is due to water-vapour absorption bands, the other major contributions coming from carbon dioxide and ozone (Hackforth, i960). The existence of wavelength windows of low absorption is of prime importance in the development of laser communication systems, while the presence of strongly absorbing bands is a major factor in determining the radiation balance of the earth s atmosphere. 

FIGURE 7. Optical unit for laser communication satellite before and after final assembly consisting of low expansion C/C-SiC tube and spider... 

W.K.Pratt Laser Communication Systems (Wiley, New York 1969) pp. 224-229... 

The versatility of the system described above has recently been enhanced by transmitting the exciting microwave radiation as modulation on an infrared laser communications link over a fiber-optic path up to 1 km, before demodulation and harmonic generation produces a mmwave signal at the far end that drives the Fabry-Perot cell. It then acts as a mobile remote analytical spectrometer that can be moved to any convenient location whilst remaining under full control from and returning results continuously to home base. 

Semiconductor lasers have certainly advanced to the stage where in the near future they will replace the more common solid state and gas lasers that have been the workhorses in both the scientific and industrial arenas. Semiconductor lasers will be successfully modelocked and the resultant ultrashort ophcal pulses will be amph-fled to peak power levels approaching the kilowatt region. This will have a tremendous impact on the ultrafast laser community, by providing an inexpensive, efficient, and compact source for ultrafast nonlinear optical studies. In addition, real-time optical signal processing and optical computing will take one step closer to reality with this advancement. 

Vanes were also used to supply propellant for the Near Field InfraRed Experiment (NFIRE) for station keeping. The satellite was launched in 2007 (Btdlweg and Wallrapp, 2012) which carried two payloads a Track Sensor Payload (NFIRE, 2013) to detect and track missiles, and a Laser Communication Terminal (LCT) (Smutny and Lange, 2006) to test laser communication with the German made TerraSar-X satellite. Vanes have also been used in the Iridium constellation (Garrison et al., 1997), the INSAT satellites (Kale et al., 1972 Menon, 1972 Netter and Prasad, 1988), and the Arabsat television satellites (Rollins et al., 1984). 

The authors thank the German Research Community (DFG) for their assistance within their investigation project Beam-Material-Interaction During Laser Beam Machining". 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

In additions to improvements in Si, a variety of devices based on compound semiconductors can be expected. Blue lasers witli high brightness and long operating lifetimes already exist in tlie laboratory. LEDs are likely to be used for all lighting purjDoses. The bandwidtli of optical communications will continue to increase witli ever faster semiconductor lasers. 

At the research level, spectroscopy continues to flourish and is continually developing with occasional quantum leaps. For example, such a leap resulted from the development of lasers. Not all leaps provide suitable material for inclusion in an undergraduate text such as this. Flowever, even in the relatively short period of seven years since the third edition, there have been either new developments or consolidation of rather less recent ones, which are not only of the greatest importance but which can (1 hope ) be communicated at this level. 

The iavention of the laser ia 1958 prompted the beginning of the story of optical fiber communications. This device was capable of produciag a high iatensity, coherent beam of light which could be modulated at a high rate (see Lasers). StiU, no transmission medium of suitable clarity was available. 

GaAs, GaAlAs, and GaP based laser diodes are manufactured using the LPE, MOCVD, and molecular beam epitaxy (MBE) technologies (51). The short wavelength devices are used for compact disc (CD) players, whereas the long wavelength devices, mostly processed by MBE, are used in the communication field and in quantum well stmctures. 

The supplanting of germanium-based semiconductor devices by shicon devices has almost eliminated the use of indium in the related ahoy junction (see Semiconductors). Indium, however, is finding increased use in III—V compound semiconductors such as indium phosphide [22398-80-7] for laser diodes used in fiber optic communication systems (see Electronic materials Fiber optics Light generation). Other important indium-containing semiconductors include indium arsenide [1303-11-3] indium antimonide [1312-41 -0] and copper—indium—diselenide [12018-95-0]. 

Communications. The advent of the laser improved prospects for optical communications enormously. The coherence of the laser meant that techniques developed in the radio portion of the electromagnetic spectmm could be extended to the optical portion of the spectmm. Because lasers operate at frequencies near 10 Hz, they offer a potentially wide bandwidth, equal to about 10 television channels of width (ca 10 Hz). It has not proved possible to take advantage of this full bandwidth because devices such as modulators capable of operating at 10 Hz are not available. 

The most important appHcation of fiber-optic laser-based communication is in long-distance telecommunications (92,93). Fiber-optic systems offer very high capacity, low cost-per-channel, light weight, small size, and immunity to crosstalk and electrical interference. 

In summary, laser-based fiber-optic telecommunications has had a revolutionary impact on long-distance telephone communication and is now expanding into many new appHcations areas. 

The relevance of photonics technology is best measured by its omnipresence. Semiconductor lasers, for example, are found in compact disk players, CD-ROM drives, and bar code scaimers, as well as in data communication systems such as telephone systems. Compound semiconductor-based LEDs utilized in multicolor displays, automobile indicators, and most recendy in traffic lights represent an even bigger market, with approximately 1 biUion in aimual sales. The trend to faster and smaller systems with lower power requirements and lower loss has led toward the development of optical communication and computing systems and thus rapid technological advancement in photonics systems is expected for the future. In this section, compound semiconductor photonics technology is reviewed with a focus on three primary photonic devices LEDs, laser diodes, and detectors. Overviews of other important compound semiconductor-based photonic devices can be found in References 75—78. 

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