Coarse‐Frequency‐Comb Multiple‐Beam Interferometry: Phase Assessment Using Common Phase Shifting Procedures
- Conference date: 16–21 May 2010
- Location: Monte Verita (Ascona)
Real wedge interferometers of the Fizeau‐type do not allow for fringes in case of a spectral broad band source ‐or in short: for white light fringes. Here, the use of a suitable frequency comb source will help to overcome this limitation on the one hand and on the other it offers the capability for enhanced phase sensitivity in high precision measurements of surface deviations. Frequency combs can be produced by passive filtering of the light emitted by a broad band source as a superlum‐diode or a fs‐laser. The frequency comb produced by a common fs‐laser is extremely fine, i.e., the frequency difference of consecutive peaks is very small or the distance of consecutive pulses of the pulse train might be of the order of 1 m. Therefore, the coarse pulse train produced by passive filtering of a broad band source is better adapted to the needs of surface testing interferometers. White light fringes are either applied for the profiling of discontinuous surfaces or can serve as an indication for the correct choice of the subdivision of the inter order distance for multiple beam fringes. During the last decennium it became more and more clear that spatially incoherent sources provide better measuring accuracy in surface measurements due to the reduced influence of dust diffraction patterns. The advantage of laser illumination can nevertheless be maintained if the laser light is made spatially incoherent through moving scatterers in the light path. Here, we will discuss the application of spatially incoherent broad band light frequency filtered through a Fabry‐Perot filter. The main applications are in the following fields: (1) surface profiling applications using two‐beam Fizeau interferometers, (2) selection of single cavities out of a series of interlaced cavities, and (3) sensitivity enhancement for multi‐beam interferometers for planeness or sphericity measurements. The emphasis of the presented work will be on the sensitivity enhancement provided by multiple beam interferometry in combination with frequency comb illumination. Through the use of a set of properly selected wavelengths the resulting interference pattern will become on the one hand more and more cosine‐type with increasing enhancement factors and on the other hand it will be shown how the nonlinear relationship of the intensity distribution on the phase in the multiple beam interferometer can be overcome. Typical systematic errors show a periodicity with 4‐times the fringe frequency of the interference pattern. By using the averaging of the measuring results of two measurements having a phase offset of π/4 it is possible to reduce this error by at least one order of magnitude. The impact of the nonlinear intensity profile of multiple beam fringes in transmitted light can in addition be reduced through inverted intensity values in the common phase shifting equations. It will be shown that in this way repeatability values can be obtained of 1.2 Å peak to valley and 0.12 Å rms.
- Frequency combs
- Surface measurements
- Light diffraction
- Light scattering
- Surface patterning
- Testing procedures
- Visible spectra
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Y. K. Semertzidis , M. Aoki , M. Auzinsh , V. Balakin , A. Bazhan , G. W. Bennett , R. M. Carey , P. Cushman , P. T. Debevec , A. Dudnikov , F. J. M. Farley , D. W. Hertzog , M. Iwasaki , K. Jungmann , D. Kawall , B. Khazin , I. B. Khriplovich , B. Kirk , Y. Kuno , D. M. Lazarus , L. B. Leipuner , V. Logashenko , K. R. Lynch , W. J. Marciano , R. McNabb , W. Meng , J. P. Miller , W. M. Morse , C. J. G. Onderwater , Y. F. Orlov , C. S. Ozben , R. Prigl , S. Rescia , B. L. Roberts , N. Shafer‐Ray , A. Silenko , E. J. Stephenson , K. Yoshimura and EDM Collaboration
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