Method and apparatus for forming a tube-encased bragg grating

Abstract

A method and apparatus for forming a tube-encased fiber grating includes an optical fiber,which is encased within and fused to at least a portion of a glass capillary tube,and a substantially transparent index-matching medium,, such as an optically flat window, having an optically flat surface,adjacent to the tube,. A substantially transparent index-matching intermediate material (e.g., UV transparent oil),is used between the window,and the tube,to substantially eliminate the interface between the tube,and the medium,. A pair of writing beams,are incident on and pass through the medium,, the tube,and intersect and interfere in a region,on the fiber,. Also, the width Wb of the writing beams,may be set to be less than the width Woil of the intermediate material,to eliminate surface damage (ablations) of the tube,. Attentively, the medium,may have a geometry to eliminate surface ablations (e.g., a collar shape) and/or to match the shape of the tube,. More than one grating or pair of gratings may be written in the tube-encased fiber or one or more gratings may be formed in multiple fibers,encased in the tube,. Further, high intensity writing beams,on the fiber,may be achieved by having the medium,have a predetermined thickness T.