Optical communications in the mid-wave IR spectral band |
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Authors: | Narasimha S Prasad |
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Institution: | (1) NASA Langley Research Center, 468/LEOB, 5 North Dryden Street, B1202, Hampton, VA 23681-9403, USA |
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Abstract: | The mid-wave IR (MWIR) spectral band extending from 3 to 5 microns is considered to be a low loss atmospheric window. The
MWIR wavelengths are eye safe and are attractive for several free-space applications including remote sensing of chemical
and biological species, hard target imaging, range finding, target illumination, and free-space communications. Due to the
nature of light-matter interaction characteristics, MWIR wavelength based systems can provide unique advantages over other
spectral bands for these applications. The MWIR wavelengths are found to effectively penetrate natural and anthropogenic
obscurants. Consequently, MWIR systems offer increased range performance at reduced power levels. Free-space, line-of-sight
optical communication links for terrestrial as well as space based platforms using MWIR wavelengths can be designed to operate
under low visibility conditions. Combined with high-bandwidth, eye-safe, covert and jam proof features, a MWIR wavelength
based optical communication link could play a vital role in hostile environments.
A free-space optical communication link basically consists of a transmitter, a receiver and a scheme for directing the beam
towards a target. Coherent radiation in the MWIR spectral band can be generated using various types of lasers and nonlinear
optical devices. Traditional modulation techniques are applicable to these optical sources. Novel detector and other subcomponent
technologies with enhanced characteristics for a MWIR based system are advancing. Depending on the transmitter beam characteristics,
atmospheric conditions may adversely influence the beam propagation and thereby increasing the bit error rate. For satisfactory
transmission over a given range, the influence of atmosphere on beam propagation has to be analyzed. In this chapter, salient
features of atmospheric modeling required for wavelength selection and performance prediction is presented. Potential optical
sources and detectors for building a practical MWIR communication link are surveyed. As an illustration, the design configuration
and experimental results of a recently demonstrated free-space, obscurant penetrating optical data communication link suitable
for battlefield applications is discussed. In this case, the MWIR wavelength was derived using an all solid-state, compact,
optical parametric oscillator device. With this device, weapon codes pertaining to small and large weapon platforms were
transmitted over a range of 5 km. Furthermore, image transmission through light fog, accomplished using this hardware, is
also presented.
Advances in source and detector technologies are contributing to the development of cost effective systems compatible with
various platforms requirements. In coming years, MWIR wavelengths are anticipated to play a vital role in various human endeavors. |
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