Abstract: An active multimode optical fiber consisting of a first core section (11), a thin barrier layer (12) material having a thickness (d2) and a lower refractive index than that of the first core section by an index difference (?n), a second core section (13) having a refrective index equal or higher than that of the first core section, and a cladding (14) having an index lower than that of the first core section. Said index difference and said thickness are selected so that a fundamental core mode couples less strongly with said cladding modes than higher order core modes. A scheme of changing the symmetry of the fiber for reduced sensitivity of the fundamental mode of the first core section to resonance effects.

Abstract: The object of the invention is a photon or particle detector which comprises a transmission dynode situated in a vacuum. The detector comprises a monolithically fabricated semiconductor structure in which electrons are arranged so as to travel from the semiconductor into a vacuum. At least a part of the multiplication region is formed into a layered structure incorporating at least one doped semiconductor transmission dynode and at least one vacuum space.

Abstract: The invention relates to a method of growing a GaAs film on the surface of a Si or GaAs substrate by exposing the growing surface of the substrate in a vacuum to at least one vapor beam containing the Ga elementary component of the GaAs compound, and to at least one vapor beam containing the As elementary component of the GaAs compound. The method is characterized by the steps of (A) growing a GaAs buffer layer by alternately applying the elements of the GaAs compound to the surface of a substrate heated to a first temperature one atom layer at a time, whereby in the formation of each atom layer the growing surface is exposed to a vapour beam containing one elementary component of the GaAs compound only; and (B) heating the substrate to a second temperature higher than the first temperature, and growing another GaAs layer on the buffer layer by applying both of the elementary components of the GaAs compound simultaneously.