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We present an optical metrology instrument for measuring both transmitted and reflected wavefront error (TWE and RWE) of coated or uncoated optics over a diameter of 5 inches. Depending on the coating transmittance and reflectance, the measurements have to be done at different wavelengths.
Interferometer is a standard instrument to measure the TWE and RWE of uncoated optics. But in the case of coated optics (bandpass filters for example) measurement of TWE is not possible because the optics may not transmit the interferometer laser light.
The chosen solution is based on a quadriwave lateral shearing interferometer (QWLSI) wavefront sensor. QWLSI is an achromatic technique, meaning that it measures OPD at any wavelength without any need for recalibration at specific wavelengths. Consequently, various sources at different wavelengths can be used with the same instrument and metrology bench. In addition, QWLSI measures the derivative of phase contrary to interferometer that measures phase. Therefore, QWLSI has by design a better WFE dynamic range for TWE and RWE measurement. Moreover accuracy (below 15nm RMS) and repeatability (below 2nm RMS) is perfectly adapted to optical metrology measurement.
The optical solution is a standard double pass configuration composed of a collimator and a beam expander to adapt the size of the beam to the wavefront sensor aperture. We use LED sources to avoid any noise due to interferences within the optics, which occur with coherent light. We can use different wavelength between 400nm and 1100nm. We can optimize the longitudinal chromatic aberration by moving a lens from the beam expander.
We characterized the bench according to the ISO 5725 standard for different wavelengths. Its precision was tested with different samples (filters and mirror). The precision on TWE was found to be below 2nm RMS.
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