or KDP, is an important electro-optic material. It is currently
used for frequency conversion of the OMEGA laser at LLE, and it
will be used for the same purpose in the National
Ignition Facility (NIF) currently under construction at LLNL.
It is commonly used in electro-optic devices such as Pockels cells.
KDP is representative of a class of optical crystals which are difficult
to polish because they are soft and water-soluble.
point diamond turning (SPDT) is the current state-of-the-art process
for finishing KDP. Surfaces with an rms roughness of 1 to 3 nm are
achieved for flat plates up to 30 cm in diameter. SPDT is done by
showering mineral oil over the KDP plate and cutting the surface
with a stiff, multi-million dollar CNC machine. The oil, a flammability
hazard, is removed with toluene or xylene.
ability to readily change the MRF carrier fluid from water to a
non-aqueous liquid opens up the possibility for processing soft,
water-sensitive parts. A search for a carrier fluid evaluated candidates
for compatibility with KDP and the MRF process. A number of water-miscible
fluids tested were found to leave a "fog" on the surface
of KDP. Even just a few minutes contact with 200-proof ethanol transferred
enough moisture from the air to leave visible defects. A dicarboxylic
acid ester was eventually identified as a successful carrier fluid.
It has a very low vapor pressure, a low viscosity, it does not evaporate,
and it is easily cleaned out of an MRF machine. Single crystal plates
of potassium dihydrogen phosphate (KDP) have been successfully polished,
demonstrating that MRF can smooth to the nm level. It also removes
the tool marks left from SPDT.
of Tool Marks Left from Single Point Diamond Turning of a
D. Jacobs, “Manipulating mechanics and chemistry in precision optics
finishing,” Science and Technology of Advanced Materials
8 153-157 (2007).
R. Arrasmith, I. A. Kozhinova, L. L. Gregg, H. J. Romanofsky, A.
B. Shorey, S. D. Jacobs, D. Golini, W. I. Kordonski, P. Dumas, and
S. Hogan, "Details of the Polishing Spot in Magnetorheological
Finishing (MRF)," in Optical Manufacturing and Testing III,
edited by H. Stahl (SPIE, Bellingham, WA, 1999), Vol. 3782, pp.