The manufacture of modern optical components requires the use of high performance
interferometers, usually based on phase-shifting techniques.
However, there is currently no commercial phase-shifting interferometer having the capacity to
measure large parts, such as those found in Inertial Confinement Fusion Lasers (NIF and
Méga-Joule), and other large systems.
Standard interferometers lack the simultaneous qualities of accuracy and spatial resolution
required for the measurement of such components. Indeed, it has been shown that surface
ripples with wavelengths of around 1 to 10 millimeters are extremely dangerous for large ICF
optics, even at low amplitude, because of the process of non-linear ripple amplification present
in high power laser systems.
In order to circumvent the restrictions on size and performance of standard interferometers, we
have designed and built a scanning interferometer, using a standard "small" diameter phase
shifting interferometer. A PC computer is used to control the measurement process, acquire
the interferograms and stitch the measurements together to produce the original large surface.
The measurement sequence is completely automated.
The advantages ofthis technique are low cost, small size, and no loss ofspatial resolution.
One system has been in actual use for the characterization of large size mirrors (approx. 400 x
600 mm) since december 1994.
In this presentation, we shall look over the design of the system, produce actual measurements,
and discuss the technical implications of the stitching process in relation to specifications such
as those currently being derived for ICF large components.
This work is supported by CEA/CEL-V, as part of the Laser MégaJoule Program.