Spectrally controlled source for interferometric measurements of multiple surface cavities Jan Posthumus 1,2

Transcription

1 Spectrally controlled source for interferometric measurements of multiple surface cavities Jan Posthumus 1,2 Chase Salsbury 1,3, Artur Olszak 1 [1] Äpre Instruments, LLC [2] Laser Peak, GmbH [3] College of Optical Sciences, University of Arizona POM 2018

2 Motivation Multiple back reflections Wavefronts coherently add at the measurement plane Fringe ambiguity degrades measurement accuracy or prevents measurement all together Workarounds Blue tape, Vaseline, AR coating, induced wedge 2

3 Modalities of Interferometry Coherent/Laser Interferometry Incoherent/ White-light Interferometry Light Source Laser Broadband source Fringe Localization Unlocalized Localized at defined plane Secondary Back Reflections Temporally coherent contributions (L coh >L cav ) Do not contribute if outside of nominal coherence envelope Difficulty of Alignment Low difficulty Can be high difficulty Common Path/Aberration Cancellation Most common configuration: Fizeau Most common configuration: Michelson 3

4 Modalities of Interferometry Coherent/Laser Interferometry Incoherent/ White-light Interferometry Light Source Laser Broadband source Fringe Localization Unlocalized Localized at defined plane Secondary Back Reflections Temporally coherent contributions (L coh >L cav ) Do not contribute if outside of nominal coherence envelope Difficulty of Alignment Low difficulty Can be high difficulty Common Path/Aberration Cancellation Most common configuration: Fizeau Most common configuration: Michelson 4

5 Spectrally Controlled Interferometry (SCI) Fourier Spectroscopy SCI Contrast envelope form source spectrum Fourier spectroscopy Source spectrum contrast envelope form Spectrally controlled interferometry (SCI) 5

6 Spectrally Controlled Interferometry Sinusoidal modulation produces sideband fringes at non-zero OPD locations* Removes requirement for mechanical path length matching *U.S./International Patents Pending Chase Salsbury and Artur G. Olszak, "Spectrally controlled interferometry," Appl. Opt. 56, (2017) 6

7 Spectrally Controlled Interferometry Controlled Parameters a.coherence envelope width source bandwidth b.fringe location modulation frequency c.phase of fringes modulation function phase a a b b *U.S./International Patents Pending Chase Salsbury and Artur G. Olszak, "Spectrally controlled interferometry," Appl. Opt. 56, (2017) 7

8 Spectrally Controlled Interferometry Click here for the video demo: 8

9 Implementation of Spectrally Controlled Source Electronic control over spectral distribution No mechanical movement required for Cavity selection (time scale frame rate) Phase shifting (flexible to any phase algorithms) Bandwidth (nominal coherence length) tunable Laser mode for simultaneous alignment of multiple surfaces Incoherent mode for measurement and acquisition Min coherence envelope for surface isolation: 50um OPD Fiber coupled source Integration into standard Fizeau 9

10 Multiple Surface Cavity Measurements 10

11 Plano Optic Measurements Measurements Surface Figure Transmitted Wavefront Homogeneity (refractive index variation) Test Piece Fused Silica window 5mm thick, 50mm diameter 11

12 Source control for surface isolation Transmission Flat Spectrally Controlled Source Standard Fizeau Interferometer Optic Under Test Return Flat P SCI Source Distribution Interferogram ν 12

13 Source control for surface isolation Transmission Flat Spectrally Controlled Source Standard Fizeau Interferometer Optic Under Test Return Flat P SCI Source Distribution Interferogram ν 13

14 Source control for surface isolation Transmission Flat Spectrally Controlled Source Standard Fizeau Interferometer Optic Under Test Return Flat P SCI Source Distribution Interferogram ν 14

15 Source control for surface isolation Transmission Flat Spectrally Controlled Source Standard Fizeau Interferometer Optic Under Test Return Flat P SCI Source Distribution Interferogram ν 15

16 Source control for surface isolation Transmission Flat Spectrally Controlled Source Standard Fizeau Interferometer Optic Under Test Return Flat P SCI Source Distribution Interferogram ν Internal cavities accessible for measurement with Spectrally Controlled Source! 16

17 Source control for surface isolation Transmission Flat Spectrally Controlled Source Standard Fizeau Interferometer Optic Under Test Return Flat P SCI Source Distribution Interferogram Phase Shifted Spectral Distribution ν Phase Shifted Interferogram 17

18 Source control for surface isolation Transmission Flat Spectrally Controlled Source Standard Fizeau Interferometer Optic Under Test Return Flat P SCI Source Distribution Interferogram Phase Shifted Spectral Distribution ν Phase Shifted Interferogram 18

19 Measurement Results: Surface Figure Surface 1: Tilt Removed Low Order Removed 19

20 Measurement Results: Surface Figure Surface 2: Tilt Removed Low Order Removed 20

21 Measurement Results: Transmitted Wavefront Transmission Flat/Ref Flat (Through Optic): Tilt Removed Low Order Removed 21

22 Measurement Results: Homogeneity Traditional Homogeneity Measurements: 1. First Surface 2. Second Surface 3. Transmitted Wavefront 4. Empty Cavity(not shown) 22

23 Advantages of Spectrally Controlled Source Surface figure, TWF, Homogeneity measurements all accessible in a single configuration, alignment process Removes impact of parasitic back reflections in multiple surface cavities Implemented in Fizeau interferometer with no changes to existing hardware 23