Primary intracranial hypotension: pathogenetic and neuroradiological considerations.

Primary or spontaneous intracranial hypotension (SIH) is a rare syndrome which causes postural headache associated with spinal fluid hypotension. We report three cases of SIH, characterised on magnetic resonance imaging (MRI) by pachymeningeal enhancement not only at cerebral level, but also in the cervical spinal cord, which subsequently resolved completely and spontaneously. We discuss the possible pathogenetic mechanisms of the dural alterations and underline the radiological aspects.

Feedback effects in optical communication systems: characteristic curve for single-mode InGaAsP lasers.

An experimental analysis of InGaAsP injection lasers shows an unexpected decrease of the differential quantum efficiency as a function of injected current when optical power is fed back into the active cavity of a diode inserted into a long transmission line. To investigate the response of laser diodes to optical feedback, we base our analysis on a microscopic model, resulting in a set of coupled equations that include the microscopic parameters that characterize the material and the device. This description takes into account the nonlinear dependence of the interband carrier lifetime on the level of optical feedback. Good agreement between the analytical description and experimental data is obtained for threshold current and differential quantum efficiency as functions of the feedback ratio.