RESUMO
In free space optical communication, photodetectors serve not only as communications receivers but also as position sensitive detectors (PSDs) for pointing, tracking, and stabilization. Typically, two separate detectors are utilized to perform these tasks, but recent advances in the fabrication and development of large-area, low-noise avalanche photodiode (APD) arrays have enabled these devices to be used both as PSDs and as communications receivers. This combined functionality allows for more flexibility and simplicity in optical system design without sacrificing the sensitivity and bandwidth performance of smaller, single-element data receivers. This work presents the development of APD arrays rated for bandwidths beyond 1 GHz with measured carrier ionization ratios of approximately 0.2 at moderate APD gains. We discuss the fabrication and characterization of three types of APD arrays along with their performance as high-speed photodetectors.
RESUMO
The irradiance fluctuations imposed on a laser beam that has propagated over horizontal terrestrial paths in the range of 2 to 24 km are compared to lognormal (LN) and gamma-gamma (GG) distributions. For the direct links reported here the irradiance fluctuations follow a LN distribution except in cases of weak turbulence, characterized by a scintillation index of less than 1 and a Fried parameter larger than the receiver aperture, in which case the GG distribution gives an improved fit. In very weak turbulence the difference between the two distributions is insignificant.
RESUMO
Parameters characterizing the atmospheric turbulence in a 16 km maritime optical link were measured for the months of January through June of 2007 on a continuous basis, as conditions allowed. Both the scintillation index sigmaI(2) and the atmospheric structure constant Cn(2) are found to have a strong dependence on the air-minus-water temperature difference. There is no obvious diurnal variation of Cn(2) or of sigmaI(2) akin to the reduction in turbulence level seen in terrestrial links in the hour before sunrise and the hour after sunset. Results from the analysis of these data highlight a need for new approaches to modeling beam propagation in a maritime environment.
RESUMO
BACKGROUND: National Health Service reforms and government directives have placed primary care in the forefront of health care provision. Robust information is needed to inform and direct the district nursing response to these initiatives. AIM: To gain a detailed understanding of the district nursing service in the West Hertfordshire area, determine capacity for development and formulate plans for the 21st century. METHODS: Action research was the supporting methodology. A project group of working professionals led the study, identifying several areas for development, of which two are discussed: workload measurement and caseload profiling. KEY FINDINGS: An understaffed service with unpredictable workload which mitigates against proactive working. The inappropriate use of staff skills prevents efficient, cost-effective care. CONCLUSIONS: Robust caseload management with strict referral and discharge criteria and prioritization of visits offers the possibility of more effective time management. Skill mix review could enable more appropriate use of specialist skills.