In-fiber Bragg grating tap

In many network and sensor applications, small amounts of light need to be coupled out at many points from a transmission bus fiber. In-line devices with some wavelength selectivity are particularly desirable, especially if light radiated from the fiber can be efficiently coupled into another fiber. This paper describes a new method developed for the noninvasive production of wavelength selective taps in germanosilicate optical fiber. The technique is based on a recent discovery that Bragg gratings can be formed in the core of a fiber by exposing it through the side of the cladding to a coherent UV interference pattern at a wavelength which lies in the oxygen vacancy defect band of germania.1-3 The grating period and tilt of the Bragg planes are selected to couple coherently a guided mode into a narrow radiated beam which propagates through the side of the cladding. The same arrangement is also useful for coupling light into the fiber.