Index and gain gratings in Nd-YVO4 - Applications to speckle vibrometry and photoacoustic detection

Abstract

We perform wave mixing and dynamic holography by exploiting gain saturation in diode pumped laser media like Nd-YVO4. Such crystals offer the possibility of performing adaptive gain interferometers with a sub ms response time, typically between 100- 200μs. This temporal response is of great interest for applications such as speckle interferometry and acousto-optic detection for tissue imaging in biophotonics. In this work we operate in new conditions of high sensitivity with a linear response due to a refractive index contribution in the gain media. It originates from the wavelength mismatch of about 0.1 nm between the incident Nd-YAG probe laser and laser transition in diode pumped Nd-YVO4 amplifier. This permits the recording of two types of volume holograms respectively a gain and a phase volume holograms. It is the index grating which provides a linear response to incident low phase modulation at frequencies higher than 10 kHz. Experimental results confirm this behavior for adaptive interferometry and speckle vibrometry. In addition, the fast response makes the two wave mixing interaction in Nd-YVO4 very interesting for the optical detection of large bandwith photoacoustic waves excited by a pulsed ns Q-Switch laser at 532 nm. Temporal photoacoustic signals and images of small objects behind a scattering media will be shown.