Proceedings Paper • newMulti-wavelength large optics wavefront error measurement in the SWIR range (Conference Presentation)
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 4 inches in the SWIR range. Depending on the coating transmittance and reflectance, the measurements were done at different wavelengths from 1100 nm to 1650 nm. Fizeau interferometry is a standard technique 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. Moreover, for measurements at different wavelengths, a dedicated interferometer has to be built for each different wavelength. The chosen solution is based on a quadriwave lateral shearing interferometer (QWLSI) wavefront sensor. QWLSI is an achromatic technique, meaning that it measures optical path differences 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, an achieved accuracy below 30 nm 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 aperture of the SWIR wavefront sensor. Different sources between 1100 nm and 1650 nm were integrated.