An investigation of phase-mask diffraction patterns and fibre Bragg gratings with scanning near-field optical microscopy
In recent years, near-field microscopy has been utilized for assessing the properties of optical wave-guides at an increasing rate. Here, a Scanning Near-field Optical Microscope (SNOM) has been designed and constructed in order to expand this work into an analysis of the optical and structural properties of fibre Bragg gratings, which are used throughout the optical fibre telecommunications network. By imaging the evanescent fields of Bragg gratings, a characterization technique has been developed which has enabled the acquisition of sub-wavelength information about the optical field distribution within a fibre grating and its refractive index structure. Six separate fibre grating samples have been examined, demonstrating the feasibility of the developed scanning technique to become a useful characterization tool. In particular, the study has enabled grating standing wave fringes to be imaged relative to corresponding refractive index fringes, for the first time. The SNOM has also been utilized to map free-space diffraction patterns close to a phase-mask (transmission diffraction grating). The patterns are normally used to create fibre gratings via UV photosensitivity mechanisms. The field distributions have been imaged under various experimental conditions and have revealed some of the technical problems that might occur during the writing of gratings. The measured patterns have also served to confirm existing diffraction grating theory, which has been expanded during the course of this work to produce a new expression for the 'Talbot length', originally formulated by Rayleigh in 1881.