Optical interferometers are typically categorized by their source type into incoherent (white-light) and coherent (laser). Both approaches provide adequate solutions for many measurement applications, offer unique advantages, and suffer distinct limitations. A novel interferometry method, spectrally controlled interferometry, is presented, which successfully merges many advantages from both categories while bypassing some of the limitations. The relationship between measurement accuracy and fringe stability as a function of fundamental control parameters is explored. Surface measurements of common optical components are presented, and method specific noise sources and measurement accuracy are assessed as well.
Spectrally controlled interferometry (SCI) is a method which presents a host of advantages over traditional coherent and white light interferometry. As its name suggests, the source spectrum is precisely controlled to produce localized fringes whose location and phase are tunable. The approach has been demonstrated to produce accurate interferometric measurements of planar and spherical optics in the presence of detrimental back reflections over a large range of cavity sizes. Phase shifted measurements of single surfaces can be done without any means of mechanical phase shifting. Additionally, existing systems can be converted to be SCI compatible as the method is implemented entirely at the source level of the instrument. Previous demonstrations of this method have applied temporal phase shifting, but use of the SCI method does not preclude the use of alternative measurement techniques. While traditionally, SCI measurements are acquired by shifting the phase of the spectrum modulation function, here we present an alternative method for phase shifting via mean wavelength shift. It is a convenient extension of SCI because typically source parameters are already controlled electronically and shifting mean wavelength of the source adds no additional complication or modification to the existing hardware. By utilizing wavelength shifting novel architectures for instantaneous measurements become possible. In this paper we present two methods of instantaneous surface measurements: using carrier fringe approach and simultaneous PSI by mean wavelength shift. Various phase measurements of multiple surface cavities via both methods are presented to demonstrate the capability. Comparisons are made to traditional SCI and standard coherent phase shifting measurements. Limitations and sources of noise are addressed as well.
Conventional interferometry is widely used to measure spherical and flat surfaces with nanometer level precision but is plagued by back reflections. We describe a new method of isolating the measurement surface by controlling spectral properties of the source (Spectrally Controlled Interferometry-SCI). Using spectral modulation of the interferometer's source enables formation of localized fringes where the optical path difference is non-zero. As a consequence it becomes possible to form white-light like fringes in common path interferometers, such as the Fizeau. The proposed setup does not require mechanical phase shifting, resulting in simpler instruments and the ability to upgrade existing interferometers. Furthermore, it allows absolute measurement of distance, including radius of curvature of lenses in a single setup with possibility of improving the throughput and removing some modes of failure.
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