Abstract: This invention proposes the fabrication of a polarization-independent fiber optic (in-line) isolator based on a beam aperture method. A micro Grin lens and a small Grin lens are used. They function as the collimators for the forward-going and the backward-going beams from the input and output ends of the isolator, respectively. In addition, the small Grin lens also acts as a beam expander for the backward-going beam. Due to the different apertures of the collimated beams, the optical power of the forward-going beam is almost completely collected by the optical fiber at the output end, whereas only a very small percentage of the optical power of the backward-going beam is collected by the micro Grin lens and the attached input fiber. Because the beam is usually a Gaussian, placing the micro Grin lens at an off axis position will significantly enhance the isolation of the isolator.

Abstract: This invention proposes the fabrication of a polarization-independent air-path isolator based on a beam aperture method. The isolator includes a micro Grin lens (MGL) and three small Grin lenses (SGLs). These lenses focus and collimate the forward and backward beams from the input and output ends of the isolator. In addition, one of the small Grin lenses (SGLs) also acts as a beam expander for the backward beam. Due to the significant difference in widths between the forward and backward collimated beams, the optical power of the forward beam is almost completely collected by the output small Grin lens (SGL) at the output end, while only a very small percentage of the optical power of the backward beam is collected by the small Grin lens (SGL) at the input end. Since a laser beam, in general, has a Gaussian beam-intensity distribution, positioning the micro Grin lens (MGL) at an off axis position will significantly enhance the isolation characteristic of the isolator.