Determination of single-mode fiber coupler design parameters from loss measurements

A method is presented for determining the minimum spacing between the fiber core centers in a single-mode fiber coupler which consists of two substrate blocks each provided with a fiber fixed in a curved groove. Immersion liquids with known refractive indices are placed on the top surface of one coupler block and the introduced power loss of the light guided within the fiber is measured. By fitting these results to calculated curves the minimum spacing may be determined with an error of less than 0.2 μm.