Adaptive Vibration-Resistant Techniques For Phase-Shifting Interferometer

Phase-shifting interferometry (PSI) is a highly sensitive method employed for non-contact precision optical testing. However, the external micro-vibrations are often the dominant error sources in phase-shifting interferometers since its sensitivity. Much effort has been invested in understanding how vibration affects PSI metrology and in methods to reduce this sensitivity. Here, some vibration-insensitive methods for PSI are concisely introduced firstly in this paper. After that, a novel vibration-resistant method is described which uses an acousto-optical modulator (AOM) as optical frequency-shifter to compose a heterodyne vibration-measuring system built into a phase-shifting interferometer, two high-speed avalanche photodiodes (APD) are used in optical heterodyne detecting system and output two analog heterodyne signals up to 40MHz. The phase angle between the signals is directly measured by an integrate RF/IF phase detector, thus, the phase angle's variation, caused by external vibration, between reference wave-front and test wave-front is determined. Based on the phase angle departure from preset phase, a DSP device is used to predict the phase variation, and then feedback signal is generated from it to compensate the vibration. A least mean square (LMS) algorithm for adaptive signal processing is programmed in the DSP. However, the LMS algorithm is sign modified by quadrant-identified feedback method, which is put forward for the first time, to adaptive different preset phase, so the wave-front phase can be locked on any value that means the phase-shifting capability of this interferometer. Acting as vibration compensator, a sort of pre-pressing PZT (piezoelectric transducer) phase-shifter is designed with high frequency-response characteristic. Furthermore, a phenomenon of sub-processional motion of this PZT phase-shifter is explored for the first time. Lastly, optical tests are carried out on this adaptive vibration-resistant phase-shifting interferometer in the presence of vibration no more than 100 waves Hz , a series of phase-shifting interferograms are correctly sampled and then phase-error map is generated, in conclusion, this adaptive interferometer works well in a micro-vibrant environment.