Abstract: An optical switch for switching an optical path by changing the refractive index of the circumference of a light coupling region of a directional coupler is disclosed. Such a change is accomplished by a photochromic compound in the circumferential medium of the light coupling region. Such a photochromic compound when beamed with light of a specific wavelength undergoes a photochromic reaction which changes the refractive index of the circumferential medium of the light coupling region. Such a reaction is reversible when beamed with light of another specific wavelength.

Abstract: An optical fiber comprising a glass optical fiber and a rigid protective layer which is coated directly on the glass fiber wherein a constricting force F generated by the protective layer is from -0.05 kg to +0.05 kg when the force F is defined by the following equation: ##EQU1## which has improved transmission performances.

Abstract: Optical-transmission fibers serve to transmit signal over some distance. Said fibers have traditionally been constructed of a glass fiber; consisting of a central core and an outside clad, a surrounding soft layer around said fiber and a hard layer around the soft layer. Because of the material construction, said fibers had high dynamic losses. By altering the materials used around the glass fiber, the present invention has successfully dramatically reduced the dynamic loss and therefore greatly improved the possibilities for signal transmission. The organic material used to enhance transmission ability may vary but the effect is such that the dynamic loss remains low as compared to any previous art.