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Electro-optical lenses focus lasers in two dimensions

26 Jul 2007

The ability to focus laser light in two dimensions could herald a new era of high-speed optical storage and retrieval.

A novel configuration of electro-optical lenses is helping researchers in the US to focus light in two dimensions. What's more, the optical power of the lens can be continuously tuned from - 129 m-1 to 129 m-1 in both directions by varying the applied voltage. (Applied Physics Letters 90 201106)

"Our research is aimed at increasing data read/write rates and data storage density," Mahesh Krishnamurthi, a researcher at Pennsylvania State University, told optics.org. "The poor response time of devices used to read/write data in current technology acts like a bottleneck in achieving the high speeds that are possible in optical systems."

The ability to tune the optical power of a lens allows for the spatial control of the focal point. "Spatial control over the focal plane is useful in multilayered data retrieval and laser-based surgeries," commented Krishnamurthi. "Tuning optical power at a very high frequency helps in high-speed optical storage and retrieval. In addition, modulating the laser beam diameter has applications in laser micro machining."

Existing dynamic focusing is predominantly achieved using electro-mechanical mirrors and liquid crystals. The Penn State team instead used an electro-optic lens to allow the optical power to be modulated by varying an externally applied voltage, as opposed to a conventional lens where the optical power is fixed.

The lens was based on a ferroelectric material that was fabricated with a stack of cylindrical-shaped domains. "We used lithium tantalate, which is characterized by a high electro-optic coefficient, good stability under high electric fields, and low response time to applied electric fields," commented Krishnamurthi. "This results in high bandwidth and low power consumption."

When an external voltage is applied, the domains within the material act like a cylindrical lens. "They can behave like a concave or a convex lens depending on the direction of the external voltage. Hence, the incident laser light can be focused in one dimension," explained Krishnamurthi.

In order to focus the laser light in two dimensions, a similar device is positioned orthogonal to the first device. By operating both devices simultaneously, the laser light can be focused in two dimensions.

According to Krishnamurthi, the optical power achieved is comparable to conventional lenses - but it could be improved further. "The optical power can be scaled up by cascading more lenslets in the stack and also by decreasing the diameter of the lenslets," he said.

This technique could also be extended to perform two-dimensional scanning. "The laser light can be scanned/ deflected by fabricating prism-shaped domains instead of cylindrical domains," explained Krishnamurthi. "This has applications in reducing seek-time in optical data retrieval and storage and in laser micro machining."

The next goal for the team is to explore the possible applications of two-dimensional focusing in laser micro machining and to work on laser-beam shaping. "The key limitations of this device are the high voltage requirements for commercial applications which can be addressed by fabricating the device on thinner wafers, reducing the radius of curvature of the cylindrical lenslets or by using materials with a higher electro-optic coefficient," concluded Krishnamurthi.

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