Using CGH for Testing Aspheric Surfaces Nasrin Ghanbari OPTI 521 Introduction Spherical wavefront from interferometer is incident on CGH Reflected light will have an aspheric phase function CGH cancels the aspheric phase ID: 764846
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Using CGH for Testing Aspheric Surfaces Nasrin Ghanbari OPTI 521
Introduction Spherical wavefront from interferometer is incident on CGH Reflected light will have an aspheric phase function CGH cancels the aspheric phaseEmerging wavefront will be spherical and it goes back to interferometer CGH Aspheric Mirror
Design Process Start with design and optimization of CGH in Zemax: Single pass geometry Phase functionDouble pass geometry Design of CGH in Zemax Alignment CGH Conversion to line pattern Fabrication
Virtual Glass Snell’s law: If n 1 = 0 then sin θ 2 =0Therefore θ2 =0 and the emerging ray is perpendicular to aspheric surface
Single Pass Geometry CGH
Beam Footprint Width of the spot size: The number of waves of tilt needed to separate diffraction orders: [1] [1] Dr. Jim Burge, “Computer Generated Holograms for Optical Testing”
Phase Design Zernike Coefficient Value Zernike Coefficient Value Zernike Coefficient Value A 1 0.00E+00 A 13 3.85E-04 A 25 -9.49E-03 A 2 1.10E+02 A 14 6.89E-05 A 26 0.00E+00 A 3 0.00E+00 A 15 -2.50E+00 A 27 -7.18E-01 A 4 -3.27E+01 A 16 -3.94E-01 A 28 -2.89E-01 A 5 7.00E+01 A 17 -3.07E+00 A 29 -4.89E-05 A 6 -1.74E-01 A 18 -8.31E-05 A 30 -1.80E-05 A 7 -6.57E-02 A 19 -3.30E-05 A 31 7.30E-02 A 8 -2.89E+01 A 20 1.60E+00 A 326.16E-03A 9-4.41E+00A 216.22E-01A 332.35E-05A 10-4.13E-04A 221.06E-04A 342.06E-06A 111.24E+01A 23-3.56E-06A 35-4.81E-03A 126.26E+00A 24-1.76E-01A 365.94E-04
Zernike Fringe Phase M is the diffraction order of the CGH N is the number of Zernike terms; Zemax supports up to 37Zi (ρ,φ) is the ith term in the Zernike polynomialAi is the coefficient of that term in units of waves. A i Z i (ρ,φ) A11 A2ρ cos(φ) A3 ρ sin(φ)A4 2 ρ2 - 1 A5 ρ 2 cos (2 φ) A 6 ρ 2 sin(2 φ ) . . .
Double Pass Geometry The double pass geometry models the physical setup. Check the separation of various diffraction orders Flip the sign of diffraction order for CGH and radius of curvature for the mirror
Diffraction Orders Use multi-configuration editor in Zemax The +1 diffraction order appears in redTo block other orders place an aperture at best focus.
Sources of Error Pattern Distortion: error in the positioning of the fringe lines Misalignment of CGH: alignment marks and cross hairs are placed around the main CGH [2] R. Zehnder , J. Burge and C. Zhao, “Use of computer generated holograms for alignment of complex null correctors”
2D Line Pattern Phase Function Position on Substrate Wavefront Profile [1] Chrome Segment Spacing [1] Dr. Jim Burge, “Computer Generated Holograms for Optical Testing”
Physical Setup CGH *Photos taken at the Mirror Lab
Conclusion P hase function of CGH can be optimized for a particular testing geometry.The process is carried out in three stepsTilt must be added to CGH to separate +1 order from the other diffraction orders.Diffraction efficiency was not discussed; for an amplitude grating it is about 10% for the +1 order For accurate placement of CGH in the testing setup, it is necessary to include the alignment CGH.
Thank You Chunyu Zhao Daewook Kim Javier Del HoyoTodd HorneWenrui CaiWon Hyun Park