Spectrally controlled source for interferometric measurements of multiple surface cavities

Abstract

We present a new light source capable of locating interference fringes at an adjustable distance from the interferometer. The spectrum is electronically controlled in such a way that the fringes are limited to only one of the surfaces of the optics under test. With the new source it is straightforward, for example, to measure the parallel surfaces of thin glass plates and multiple surface cavities. Existing interferometers, as well as older systems, can be upgraded with this source. Traditional methods of interferometry are widely used and accepted for simple measurement configurations, but measurement accuracy can decrease rapidly with increasing measurement complexity. For example, coherent interferometry struggles to achieve accurate and repeatable results with the presence of any additional feedback surface in the measurement cavity due to temporally coherent back reflections. Conversely, incoherent interferometers can isolate single surfaces for measurement but require more complex interferometer system designs. As a result, many of these systems are limited in their dynamic range of measurable cavity sizes and present considerable difficulties in the alignment process, increasing total measurement time. Both methods are inherently restricted by the intrinsic properties of their respective source. Spectrally controlled interferometry (SCI) is a source driven method which inherits many advantages from both coherent and incoherent interferometry while evading typical limitations. The sources spectral properties are manipulated to produce a tunable coherence function in measurement space which allows control over the coherence envelope width, the fringe location, and the fringe phase. With this source realization, a host of measurement advantages which simplify measurement complexity and reduce total measurement time becomes available. One major application is the extinction of extraneous surface back reflections. Without any mechanical translation, realignment, or traditional piezoelectric transducers, front and back surfaces of planar optics can be isolated independently and complete phase shifting interferometric (PSI) measurements can be taken. Furthermore, because all control parameters are implemented at the source level, the spectrally controlled source is a good candidate for upgrading existing interferometer systems. In this paper, we present the theoretical background for this source and the implications of the method. Additionally, a multiple surface cavity measurement is provided as a means of demonstrating the spectrally controlled sources capability to isolate individual cavities from detrimental back reflections across a large dynamic range of measurable cavity sizes without mechanical realignment. A discussion of the implementation benefits and practical details will be included. Limitations and comparisons to alternative methods will be addressed, as well.