Enhancing the sensitivity to small phase changes in double-exposure stroboscopic television holography

We present a new technique for enhancing the sensitivity of double-exposure stroboscopic television holography (TVH) to detect and measure vibrations of small amplitude. The technique is based in the modulation of the phase of the reference beam in synchronism with the vibration of the measurand and derives from a former technique that we originally contrived for phase evaluation. We propose two variants, characterized by the demodulation process used to generate the secondary correlograms, with different behaviors in terms of the sensitivity to the sign of the measurand and of the ease in detecting the presence and shape of the vibration. We have implemented this new technique in an electronic speckle-pattern interferometer and compared its performance with standard TVH techniques; vibrations with amplitudes as small as 8 nm have been observed with this setup.