Real Time Metrology Using Heterodyne Interferometry

The Air Force Weapons Laboratory (AFWL) located at Albuquerque, NM has developed a digital heterodyne interferometer capable of real-time, closed loop analysis and control of adaptive optics. The device uses independent phase modulation of two orthogonal polarizations of an argon ion laser to produce a temporally phase modulated interferogram of the test object in a Twyman-Green interferometer. Differential phase detection under the control of a Data General minicomputer helps reconstruct the phase front without noise effects from amplitude modulation in the optical train. The system consists of the interferometer optics, phase detection circuitry, and the minicomputer, allowing for complete software control of the process. The software has been unified into a powerful package that performs automatic data acquisition, OPD reconstruction, and Zernike analysis of the resulting wavefront. The minicomputer has the capability to control external devices so that closed loop analysis and control is possible. New software under development will provide a framework of data acquisition, display, and storage packages which can be integrated with analysis and control packages customized to the user's needs. Preliminary measurements with the system show that it is noise limited by laser beam phase quality and vibration of the optics. Active measures are necessary to reduce the impact of these noise sources.