Towards instantaneous spectrally controlled interferometry

Abstract

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.

Absolute surface metrology of x-ray telescope mirrors through axial shift mapping

“…This measures the optical thickness variation of the substrate, n (t (x, y) − t) (where n = 1.457 is the refractive index of fused silica at λ = 633 nm and t is the thickness of the substrate), from which we calculate the associated TWE = (t (x, y) − t) (n − 1) /n. 22 We compare the SCI extracted TWE to the ASM measured reference surface to determine the reliability of our method.…”

Axial shift mapping: a self-referencing test for measuring the axial figure of near-cylindrical surfaces

“…To compare ASM against another measurement technique, the TWE of the CGH substrate was measured using spectrally controlled interferometry or SCI (Äpre Instruments SpectrÄ source), where we can create interference between the two surfaces of the substrate. This measures the optical thickness variation of the substrate, 𝑛 (𝑡 (𝑥, 𝑦) − t) (where 𝑛 = 1.457 is the refractive index of fused silica at 𝜆 = 633 𝑛𝑚 and 𝑡 is the thickness of the substrate), from which we calculate the associated substrate TWE = (𝑡 (𝑥, 𝑦) − t) (𝑛 − 1) /𝑛 [25]. We compare the SCI-extracted CGH substrate TWE to the ASM measured reference surface to determine the reliability of our method.…”