Cascaded recirculating transmission line without bending loss limitations

Abstract

A single-mode optical fiber delay line for filtering at high frequencies utilizes an nth order cascaded recirculating configuration comprised of n+1 couplers. Each coupler has two input fibers and two output fibers. One of the output fibers of each coupler except the last is connected to one of the input fibers of the next coupler in cascade to form a feed-forward path. The other output fiber of each coupler except the first is connected to the other input fiber of the preceding coupler, thus forming a feed-backward path. The feed-forward path and the feed-backward path of adjacent couplers form a delay loop. The delay time in each succeeding delay loop is increased by an incremental delay .tau. equal to the reciprocal of the center frequency of the filter. The delay in the first delay loop is selected to be some multiple m of .tau., where m is an integer selected for a total delay T which permits the use of optical fiber of a length sufficient to avoid bending losses in the optical fiber. The incremental time delay .tau. can be made extremely small, thereby making the operating frequency of the filter extremely high.