Ultra highspeed variable focus liquid lenses for use in laser processing and imaging applications

The ability to rapidly change the focal plane of an optical system or rapidly change the intensity distribution in a given focal plane has many important applications in real-world systems ranging from high powered laser processing to detailed high-resolution microscopy.  For all these practical uses, a fast, tunable optical element with a low distortion and high transmission coefficient is necessary yet few viable systems exist. Dynamic focal scanning can be performed by mechanical translation of an ensemble of optical elements, by employing reconfigurable liquid crystal based modulators, or membrane mirrors. Additionally, adaptive liquid lenses based on changing the curvature of a surface or interface offer an interesting solution for varifocal applications, particularly for small scale devices. Although these technological options continue to be improved, they have a number of limitations in terms of damage threshold, response time, and/or pixilation.  Faster alternatives such as electro-optic lenses and lenses based on acousto-optic crystals have been reported to operate at several hundreds of kHz. As a main drawback, electro-optic lenses are polarization sensitive and acousto-optic crystals have weak transmission efficiency due to the need for multiple crystals for focusing a beam.  Here we present an alternative adaptive lens technology called the tunable acoustic gradient index of refraction (TAG) lens, which has the ability to provide rapidly tunable focal lengths at MHz rates while maintaining a high damage threshold.  The TAG lens is a resonant cylindrical cavity for acoustic waves that modulate the density of the filling fluid, thereby creating a gradient in index of refraction.  Fundamental characterization of the TAG lens is presented including the experimental determination of its response time and temperature dependence with particular emphasis on high power laser processing.  We discuss the use of the lens in high-speed varifocal applications for micromachining as well as in 3-d microscopy for novel biomedical applications providing a detailed quantification of the lens scanning range and accuracy.