LightPointe (United States)

Until recently atmospheric dust was not a major concern for free-space optics communication systems, however the heavy pollution and the recent increase in dust-storm events in Asia and particularly in China, has motivated the studies we present in this paper. In this paper we will discuss the impact of dust on free-space optics system operations for transmission wavelengths up to 10μm. We focus our studies on China as dust is normally present in its lower atmospheric regions, and since dust originating in China is transported over most of Asia during heavy dust storms. In general, heavy dust loadings consists of larger dust particles that are lifted from the ground and whose atmospheric concentrations can reach values that are high enough to interrupt the operation of free-space optics communication links. Similar to the characterization of fog conditions, dust storm studies are usually based on visibility data. However, in the case of mineral dust, visibility data are not sufficient to accurately predict the impact of dust on atmospheric transmissions. Transmission coefficients during dusty days cannot be simply calculated from visibility data for the entire range of useful laser wavelengths due to complex wavelength dependencies on mineral dust refractive indices. In order to test laser operating conditions in China, we ran extensive MODTRAN calculations of atmospheric transmissions for different dust conditions in the visible-IR range. Wavelength-dependent optical properties of atmospheric dust, which are required as input to the MODTRAN dust models, were calculated via Mie theory (spherical dust) using representative dust morphological and mineralogical properties such as refractive indices, size distributions and concentrations. We also tested an effect of particle nonsphericity at several wavelengths. We found that the effect of non-sphericity is small for direct beam transmissions, thought, it may become an important factor for diffused forward-scattered radiation. We found that changes in dust particle concentrations and dust particle size distributions during dust-storms storngly affect the transmission in the entire visible and IR wavelength range.

Although relatively unaffected by rain and snow, free space infrared laser optical communication systems are affected by fog and atmospheric turbulence. We have calculated the signal loss from scintillation to predict bit error rates by varying the amount of scintillation, the link distance, and system details. Increasing the number of transmitters and the effective area was predicted to improve scintillation in earlier work in this field. We found that increasing both the number of transmitters and the effective receiver area greatly improved the signal loss caused by scintillation and thus, the bit error rate. For example, under high scintillation (Cn 2 equals 10 -13 m -2/3 ), the four transmitter/receiver system at a link distance of 1 km should experience a bit error rate of better than 10 -10 for a 4.0 dB loss. Under the same conditions, an identical system with only one transmitter and receiver should experience a bit error rate of 10 -1.5 (0.032 bits per second) for the same 4.0 dB loss. Increasing just the number of transmitters or the number of receivers improved the signal, but increasing both improved the signal even more than expected. In general, single beam systems should be limited to link distances less than about 600 m.

12 A free-space laser link closes an otherwise all-fiber SONET ring, demonstrating for the first time the feasibility of using wireless optics as a back-up to fiber in an application demanding the highest levels of statistical availability and sub-50-ms protection-restoral times. This experiment demonstrates that protocol-transparent wireless optical links can be readily internetworked with industry- standard fiber-based protection protocols to achieve SONET restoral times in the event of a fiber cut. By using the wireless optics as a back-up to fiber rather than as the primary link, end-users are normally protected from the unavoidable burst errors and outages that can arise on a wireless optical link in the event of anomalously poor atmospheric visibility or unanticipated line-of-sight obstructions. While an all-fiber SONET ring operating over physically diverse paths is generally preferred, hybrid fiber/air rings operating over physically-diverse paths (fiber as one path and air as the other) will easily meet or exceed existing Bellcore availability standards for SONET rings. The hybrid part-fiber, part-air ring advantageously protects customers from fiber cuts (a.k.a. `backhoe fade') and may be preferable to over service via either an unprotected fiber spur or over a `collapsed' fiber ring made up of fiber segments sharing a common conduit. The experiment is performed at an OC-12 (622 Mbps) data rate in a point-to-consecutive point configuration which demonstrates the use of a relay site to work-around a line- of-sight obstruction.