Focusing from Infinity to Here, Almost

Hank Hogan

What do cell phone camera users and eyeglass wearers have in common? Both could benefit from recent work by research scientist Hongwen Ren and optics professor Shin-Tson Wu of the University of Central Florida in Orlando, who are developing an adaptive liquid crystal lens that can change focal length quickly on command. They recently produced a 6-mm-aperture lens with a focal length that can be changed from infinity to about 96 cm in 1 s.

A schematic shows the basic structure of a liquid crystal lens that provides a broad range of focal lengths whenever a voltage is applied to the system. Along with voltage level, the choice of material to fill the sag between the glass shell and the substrate upon which it sits determines the range of achievable focal lengths. ITO = indium tin oxide; LC = liquid crystal; d_g = substrate thickness; d_s = maximum sag of the glass shell. Images reproduced with permission of Optics Express.

That is a much wider range than previous results, an improvement Wu attributes to the design of the device. “The key innovation here is to use a glass shell.”

The scientists constructed their liquid crystal lens using a spherical glass shell with a 0.72-mm gap. They sputtered indium tin oxide on the inside of the shell, filled the shell with a transparent material and glued it to a liquid crystal cell with a 25-μm gap. They coated the inner surface of the cell opposite the shell with an ITO film before filling it with liquid crystal material.

When they applied a voltage via the ITO layers, the curved electrode on the inside of the shell generated a varying electric field from edge to center. The result was a corresponding refractive index change in the liquid crystal and a lens with a voltage-dependent focal length.

In the past, such lenses suffered from poor focal length tunability. By adding a glass shell with a filled sag region, however, the researchers improved the tunability without slowing the lens’ response time.

Using a polarized microscope from Olympus, they observed interference rings generated by the device as they increased the voltage from 0 to 140 V_rms. They did this for two lenses, one with a sag filled with air and the other filled with an ultraviolet-curable polymer. They found that the focal length of the version with an air-filled sag could be tuned from infinity to 96 cm; the version with the polymer sag device was, as expected, tunable over a smaller range.

Interference rings generated by the liquid crystal lens system when 140 V_rms were applied show the differences between polymer-filled (a) and air-filled sags (b). In the air-filled system, the focal length range is infinity to ~96 cm.

As for the future, the researchers are reducing the thickness of the liquid crystal substrate and increasing the cell gap as well as using a liquid crystal with higher birefringence. Those changes are expected to lower the operating voltage sevenfold. To resolve the polarization dependency of the device, they are planning to stack two lenses at right angles in terms of liquid crystal orientation.

These improvements involve some compromises. “We should optimize the lens structure so that response time, operating voltage and focal length tunability can be balanced,” Wu noted.

Optics Express, Nov. 13, 2006, pp. 11292-11298.