NTT Makes '1,000x Faster' Variable Focus Lens

Mar 31, 2009
Yousuke Ogasawara, Nikkei Electronics
A diagram showing the principle of NTT's variable focus lens that uses KTN (potassium tantalate niobate), a kind of "electro-optic crystals," whose refractive indices change depending on applied voltage
A diagram showing the principle of NTT's variable focus lens that uses KTN (potassium tantalate niobate), a kind of "electro-optic crystals," whose refractive indices change depending on applied voltage
[Click to enlarge image]
The KTN-based variable focus lens element. A single unit works as a cylindrical lens. It looks opaque because the transparency of KTN varies with the temperature. And the photo was taken at a temperature at which it does not become transparent.
The KTN-based variable focus lens element. A single unit works as a cylindrical lens. It looks opaque because the transparency of KTN varies with the temperature. And the photo was taken at a temperature at which it does not become transparent.
[Click to enlarge image]
A comparison of the sizes of a single KTN crystal and the KTN-based variable focus lens element. According to NTT, it can stably manufacture crystals several times larger than the single crystal on the right.
A comparison of the sizes of a single KTN crystal and the KTN-based variable focus lens element. According to NTT, it can stably manufacture crystals several times larger than the single crystal on the right.
[Click to enlarge image]
The KTN-based variable focus lens module. It uses a Peltier device for temperature control.
The KTN-based variable focus lens module. It uses a Peltier device for temperature control.
[Click to enlarge image]

Nippon Telegraph and Telephone Corp (NTT) developed a variable focus lens that uses KTN (potassium tantalate niobate, KTa1-xNbxO3), a kind of "electro-optic crystals," whose refractive indices change depending on applied voltage.

It takes only 1μs for the lens to adjust its focal length. This is 1,000 times faster than the existing variable focus lenses using PZT (piezoelectric zirconate titanate), etc, NTT said. For example, if the lens is combined with a KTN-based optical beam scanner, three-dimensional (3D) beam scanning becomes possible, the company said.

NTT separately located two electrodes in parallel on each of two opposite faces of a KTN crystal cut into a cube. When the positive electrodes on one side and the negative electrodes on the other side are electrically charged, the electric field leaks in between the positive and negative electrodes. Because of the distribution of this electric field, the value of the refractive index continuously changes between those electrodes and makes the crystal work as a convex lens.

The focal length varies in proportion to the square of applied voltage. So the focal length can be adjusted by using this property. As a single lens works as a cylindrical lens that focuses light passing through it into a line, two lenses can be combined and used as a lens unit that focuses light onto a point. The focal shift is 4cm when used with a lens (f=25cm) and a voltage of 1kV.

NTT developed a technology to stably grow KTN crystal in 2003. And, in May 2006, the company developed a beam scanner taking advantage of the fact that, when an electric current is run through a KTN crystal, the electric field is inclined in the crystal and the value of the refractive index inside it shows continuously varying distribution. Unlike this scanner, the variable focus lens announced this time does not run a current in a crystal.

The lens element exhibited at a presentation meeting March 26, 2009, looked opaque, but it was because the transparency of KTN changes in accordance with temperature.

The temperature at which KTN becomes most transparent can be set in a range between slightly above 0K and 400K by arranging the proportions of Ta and Nb in KTN, according to NTT. Therefore, it is necessary to maintain the temperature at the set level when the crystal is used as a lens. The company confirmed a product life of up to 10,000 hours for the lens at the current state.

NTT will announce this achievement March 31, 2009, at the JSAP (Japan Society of Applied Physics) 56th Spring Meeting, 2009, which will take place at Tsukuba University from March 30 to April 2, 2009.