A TUNABLE, DOUBLE-WAVELENGTH HETERODYNE-DETECTION INTERFEROMETER WITHFREQUENCY-LOCKED DIODE-PUMPED ND-YAG SOURCES FOR ABSOLUTE MEASUREMENTS

A double heterodyne interferometric instrument using a tunable synthet ic wavelength for the absolute measurements of distance and position i s presented. The optical synthetic wavelength is generated by a pair o f PZT-tunable diode-pumped Nd:YAG lasers operating at 1.064 mu m. Base d on a closed-loop scheme, a suitable electronic circuit has been deve loped to implement the frequency locking of the two lasers. A digital frequency comparator provides an error signal, used to control the sla ve laser, by comparing the laser beat frequency to a reference oscilla tor. Demodulation of the superheterodyne signals is obtained by a rf d etector followed by low-pass filtering. Distance measurements are obta ined by a digital phase meter gauging the phase difference between the demodulated signals from a measuring interferometer and from a refere nce interferometer. The paper presents the optical and the electronic layouts of the instrument as well as experimental results from a labor atory prototype. (C) 1995 American Institute of Physics.