28 Apr 2005
Laser heated liquid mirrors are a low-cost alternative to mechanically actuated adaptive optics say researchers.
Scientists in Canada have demonstrated a liquid mirror that changes its shape when heated by a laser. According to the Laval University team, the simple and low-cost unit could be used as a wavefront shaping device to compensate for polishing defects in primary mirrors in telescopes. (Applied Optics 44 1595)
Using a patented coating technique, the team creates its liquid mirror by depositing a 150 nm thick metal liquid-like film (MELLF) on to a 5 mm-deep layer of water or oil in a Petri dish. Consisting of a layer of self-assembled colloidal particles, the MELLF provides a smooth optical quality surface better than lambda/20 at 632.8 nm that is ideal for a mirror. The group has made prototype mirrors with a diameter of around 10 cm and does not foresee any upper limit to their size.
To deform the liquid mirror, the scientists heat the surface with a 4.1 mW He-Ne laser emitting at 632.8 nm and detect any change in shape using a Fizeau interferometer. For a silicon oil substrate, the laser heating produced a Gaussian shaped bump on the mirror surface with a peak amplitude of 440 nm and a FWHM of 3.6 mm. The team can tune the deformation by adjusting laser power and spot size, along with the liquid's viscosity.
Monitoring over a two month period, the researchers did not observe any degradation in reflectivity, which is currently around 80%. "With further improvements we should be able to eventually obtain the bulk reflectivity of silver or gold," Professor Ermanno Borra told Optics.org. "Useful wavelengths go from about 450 nm to over 2 microns."