Mapping of vibration amplitudes by time average holography in Bi12SiO20 crystals

We report the performance of a holographic set-up with a Bi₁₂SiO₂₀ (BSO) photorefractive crystal for measuring vibration amplitudes in a time average holography system. The volume hologram is recorded with the 514.5 nm wavelength of an Ar⁺ laser. In this set-up the reference beam is sinusoidally phase-modulated with the same frequency of the vibrating object. By increasing the value of the phase modulation, the fringes of the time average pattern run along the object surface, so that the vibration amplitude mapping is carried out. This technique allows the improvement of the measurement sensitivity compared with conventional time average holography, even for interferograms with poor visibility.