TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 3
blank size. Westerhoff et al. 16 show a considerable improvement in homogeneity of FK5 glass,
making 0.5 ppm values achievable. The specific glass type decided upon for the polarizer will
need to be evaluated individually.
Stress birefringence
Typical values for optical glass show a birefringence level of not much less than 4 nm/cm after
selection. These values are measured again through the entire length of a blank and give no
information about spatial distribution. However, the Corning fused silica samples of Ladison et
al. 15 were found to exhibit much smaller values, as little as 0.07 nm/cm with an average of 0.26
nm/cm over fifteen samples. Marker et al. 17 studied the effect of annealing on BK7, and were
able to demonstrate a value as low as 0.52 nm/cm with very slow annealing (0.3 K/hr). This
clearly shows that the relatively high published birefringence values for optical glass are subject
to significant reduction, a conclusion hopefully applicable to the glass selected for the polarizing
beamsplitters. The wavelength dependence of the stress-optic coefficient for fused silica has
been studied by Priestley. 18 This variation is an inherent material property not subject to
reduction, hence it will need to be modeled accurately. Birefringence may be manifested as a
wavefront error but also as a polarization error, and the two are not necessarily correlated. The
wavefront error may be correctable through a specially fabricated compensating plate, as has
been demonstrated in the case of CaF 2 lenses. 19 This was shown to remove a low order (radial)
wavefront error resulting in halving of the overall error.
Scatter
At 633 nm, internal scatter (transmission loss) in fused silica has been measured to be just over
10 -5 /cm. 20 This is likely tolerable with appropriate stray light reduction design. The same study
includes some depolarization information, but it is not easily comprehensible. This is primarily
Rayleigh scatter. In optical glasses, bulk scatter can also be caused by bubbles. Special glass
selection can substantially decrease the bubble content. Class “0” glass has less than 0.03 mm 2 of
area per 100 cm 3 of glass occupied by bubbles greater than 50 μm (or ~0–5 individual bubbles).
There are glass types within the LaK/SK group in the 0–1 bubble class.
3.1.6 Coatings
Objective
Coating performance of the TPF-C reflective optics can critically affect the achievement of the
TPF-C goals (ability to detect and characterize terrestrial planets). Some general coating
requirements that TPF-C shares with similar telescopes include high reflectance consistent with
the overall throughput budget, operation over the required bandpass of 500–800 nm (goal 500–
16 T. Westerhoff, K. Knapp, and E. Moersen: “Optical materials for microlithography applications,” SPIE
Proc. 3424, 10-19 (1998).
17 A. J. Marker, III, B. Wang, and R. Klimek: “Effect of residual stress in optical glass on the transmitted
wavefront,” Proc. SPIE 4102, 211-218 (2000).
18 R. Priestley: “Birefringence dispersion in fused silica for DUV lithography,” Proc. SPIE 4346, 1300-
1305 (2001).
19 J.E. Webb: “In search of the optimal objective for 157 nm,” Photonics Spectra, pp. 94-98, Dec. 2003.
20
S. Logunov and S. Kuchinsky: “Light scattering in CaF 2 and fused silica for DUV
applications” presented at SPIE 2003.
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