of k/DE/2 is required. For DE = 10 m and k = 0.8 mm, k/DE/2 = 40 nanoradians. By using adaptive optics, such accuracy appears feasible in the near future. The same conclusion applies to the large relay mirrors, based on the used of segmented phased mirror technology 2. Ref. 3 provides further details about these aspects. Laser systems in space Today, based on efficiency, technology and power criteria, four types of lasers can be considered for use in these networks: • laser diode pumped solid-state lasers, • phased arrays of semiconductor lasers, • solid-state solar pumped lasers, • free electron lasers (FEL). This order also corresponds to the most likely timetable for using these lasers in space applications. In the very short term, laser diode-pumped solid-state lasers are the only lasers that can emit a near diffraction limited beam, to an average power of about one kilowatt, (although with weak overall efficiency). Moreover, the emission wavelength (about 1 pm) requires the use of photovoltaic cells specially designed to obtain good conversion efficiency (compound cells). The state of the art in the domain of high power semiconductor lasers arrays is still not sufficiently advanced to enable energy transmission in the short term. Indeed, these arrays must be phase- matched in order to obtain a beam in fundamental space mode. Today, many laboratories are working on these questions; the best reported performance to date is 16 W in pulsed mode 4. If the phased-array laser diode technology succeeds in the near future, and if significant power levels are obtained on the fundamental mode, this type of laser would replace laser diode-pumped solid state media. Total efficiency could be multiplied by six or more and the system complexity would be reduced in consequence. With directly solar pumped lasers, the conversion efficiency must be greater since there is a direct photon-photon conversion process. This laser technology should therefore take on more importance in space applications. Nevertheless, the efficiency obtained with these lasers today is rather weak, due to the fact that absorption spectra of the amplifier media studied are not well matched to the solar spectrum: sharp lines or badly centered compared with this spectrum. For example, the use factor of sun energy is only 3% for iodine laser (t-C4F9I gas) at 1.3 pm. With solid state materials like Nd:YAG, the very numerous absorption lines distributed in the visible spectrum would give good overall efficiency; the absorp-
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