Impact of Residual Stress in HPFS Fused Silica

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Due to these strong requirements, Corning continues to deliver HPFS ® 7980 fused silica optics with these qualities by improving and developing manufacturing processes to meet new specification requirements. For the optics manufacturer, it is important to understand the relationship between birefringence and wavefront distortion to determine cause and effect. What is considered a “low level of birefringence”? Is birefringence impacting today’s wavefront measurements on fused silica blanks? What can photoelastic theory tell us about the relationships among stress, wavefront distortion, and birefringence? The results of this paper are intended to enhance and expand our existing knowledge of these properties and determine if birefringence is a cause of wavefront distortion in HPFS ® blanks. This knowledge will determine which future actions are pursued to improve refractive index uniformity and reduce wavefront distortions. 2.1 Interferometry Measurements 2. MEASUREMENT TECHNIQUES Corning uses sophisticated interferometer techniques to characterize and quantify optical aberrations. Fifteen samples were measured with a resolution of 1.11mm/pixel at the Corning Canton Plant in Canton, New York. The wavefront distortion, caused by refractive index inhomogeneities, was measured using a Zygo Mark IV GPI phase measuring interferometer with a circularly polarized HeNe laser. The wavefront measurements were then analyzed using Zygo MetroPro software, version 7.3.2. MetroPro is a GUI (Graphic User Interface) for operating Zygo interferometers and analyzing data. The lens blanks are thermally stabilized and made transparent by utilizing index-matching oil. A HeNe laser beam is transmitted through a lens blank. While the laser beam is passing through the glass, the phase of the laser light is being altered due to the index of refraction variations. An interference pattern is formed when the measuring wavefront of the interferometer combines with a reference wavefront. The pattern formed is indicative of the phase variation and, therefore, the index variation. By making measurements of this pattern, the interferometer can determine what the relative index of refraction is for a matrix of points (or pixels) over the aperture of test. 2 The result is a refractive index map of the part which can be analyzed for wavefront distortions. Aberrations can be represented with Zernike polynomials, however for the purposes of this study, only the homogeneity (peak to valley) and RMS (root mean square) values were recorded. 2.2 Birefringence Measurements The same parts measured for homogeneity were shipped to Hinds Instruments, Inc in Portland Oregon for birefringence measurements. A spacing resolution of 5mm was utilized. Hinds Instruments has developed a new birefringence measurement instrument known as the EXICOR TM system. It employs photoelastic modulator (PEM) technology to achieve high sensitivity, repeatability, and fast measurements. The system simultaneously determines both the birefringent magnitude and direction in a sample. 3 The instrument employs a low birefringence photoelastic modulator for modulating the polarization states of a HeNe laser beam. After the modulated laser beam passes through the sample, two detecting channels analyze the polarization change caused by the sample. Hinds’s EXICOR TM software then calculates and analyzes the measurement data. 4 The Exicor TM system determines the residual birefringence in optical components with a sensitivity of 0.005nm for magnitude and
Maximum absolute birefringence values, in units of nm/cm, were recorded for this study. 3. DATA Fifteen HPFS ® 7980 Excimer Grade fused silica samples were measured for birefringence and homogeneity at 632.8nm. The results are summarized in Figure 1. A 90% clear aperture was used to evaluate the data. The homogeneity values ranged from 0.37-0.74 ppm, RMS values ranged from 0.005-0.015 waves, and the maximum absolute birefringence values ranged from 0.07-0.45 nm/cm. Sample # Figure 1: Summary of Corning HPFS ® Fused Silica Homogeneity and Birefringence Measurements Diameter [mm] Thickness [mm] Clear Aperture [mm] Dn [ppm] RMS [waves] Maximum Absolute Birefringence [nm/cm] A 195 51.1 176 0.37 0.005 0.13 B 175 48.2 158 0.56 0.008 0.26 C 195 68.1 176 0.74 0.015 0.17 D 200 55.1 180 0.47 0.007 0.40 E 175 50.3 158 0.50 0.007 0.29 F 180 49.8 162 0.41 0.009 0.19 G 230 45.2 207 0.64 0.010 0.40 H 230 45.1 207 0.72 0.010 0.07 I 220 55.7 198 0.52 0.009 0.40 J 220 54.8 198 0.48 0.009 0.45 K 175 49.5 158 0.63 0.010 0.15 L 220 45.1 198 0.43 0.006 0.30 M 180 50.5 162 0.39 0.005 0.21 N 160 47.3 144 0.39 0.005 0.26 O 175 51.9 158 0.55 0.007 0.29 Average 195 51.2 176 0.52 0.008 0.26 Maximum 230 68.1 207 0.74 0.015 0.45 Minimum 160 45.1 144 0.37 0.005 0.07 Page 3 of 8
Page 1: Achieving Low Wavefront Specificati
Page 5 and 6: The analogous expressions in terms
Page 7 and 8: 4.4. Typical Wavefront and Birefrin

Impact of Residual Stress in HPFS Fused Silica

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