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Chapter 5:
A Laser Reference Beacon as a Key Element of an Adaptive Optics System
In the preceding chapters on estimating the efficiency of adaptive systems, we analyzed models of a turbulent atmosphere and considered methods for solving the wave equation that describes propagation of optical radiation in the atmosphere. The next important problem is the development of a combined model of an adaptive system using a reference source as one of its key elements. The importance of research into the efficiency of adaptive optics systems with an artificial reference source was understood by the late 1970s. In that period (and even earlier), the main principles of current adaptive electro-optics systems were formulated. According to these principles, the reference source is an element that helps to obtain information on the distribution of fluctuations in the propagation channel of optical radiation. The method of formation of the reference source affects the structure of the system as a whole. If the reciprocity principle is the basis for an adaptive system, then the most appropriate adaptive scheme is one with an independent reference source generating a beam propagating in direction opposite to the direction of the radiation to be corrected. For practical implementation of the emitting optical system, the atmosphere should be included in the feedback loop; i.e., backward scattering should be taken into account, with radiation reflected from an object or inhomogeneities of the atmosphere. In this way, an artificial (virtual) reference source is formed. In the early 1980s, artificial reference sources were named laser guide stars in adaptive astronomy. Such stars became possible through the use of Rayleigh backscattering or elastic scattering of atmospheric aerosols at the altitudes from 8 to 20 km or stimulated emission in clouds of atomic metals (for example, sodium). In the first case, a laser guide star is referred to as a Rayleigh guide star and in the second as a sodium guide star. The requirements for a laser source that forms a Rayleigh star are loose. On the other hand, the requirements for the wavelength, bandwidth, and power of a laser source forming a sodium star are quite strict. This is due to the selectivity and saturation of absorption at stimulated emissions in sodium vapor at altitudes from 85 to 100 km.
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