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Abstract:
A method and apparatus for performing refractive index, birefringence and optical activity measurements of a material such as a solid, liquid, gas or thin film is disclosed. The method and apparatus can also be used to measure the properties of a reflecting surface. The disclosed apparatus has an optical ring-resonator in the form of a fiber-loop resonator, or a race-track resonator, or any waveguide-ring or other structure with a closed optical path that constitutes a cavity. A sample is introduced into the optical path of the resonator such that the light in the resonator is transmitted through the sample and relative and/or absolute shifts of the resonance frequencies or changes of the characteristics of the transmission spectrum are observed. A change in the transfer characteristics of the resonant ring, such as a shift of the resonance frequency, is related to a sample's refractive index (refractive indices) and/or change thereof. In the case of birefringence measurements, rings that have modes with two (quasi)-orthogonal (linear or circular) polarization states are used to observe the relative shifts of the resonance frequencies. A reflecting surface may be introduced in a ring resonator. The reflecting surface can be raster-scanned for the purpose of height-profiling surface features. A surface plasmon resonance may be excited and phase changes of resonant light due to binding of analytes to the reflecting surface can be determined in the frequency domain.
