Alternative fiber-optic backreflectance sensor for measurement of the effective focal lengths of optical elements and distances

An alternative and simple fiber-optic backreflectance sensor method for accurate measurement of either effective focal lengths of optical elements and short distances using a single-mode optical fiber and cw laser is presented. The method is based on the intensity sensing of focused backreflectance laser emission and the spatial location of three specific points: the focal point of the focusing optical element and two object points. The single-mode fiber is a key optical element and serves simultaneously as a point laser source for testing, an object for projecting and a highly sensitive point receiver of the focused backreflectance emission. The experimental and analytical results demonstrate the potential of the method for locating the spatial points and determining the effective focal length with accuracies exceeding 1 μm and 0.5%, respectively, as well as for short distance measurement with submicron accuracy.