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Liquid lenses eye commercial breakthrough

31 Oct 2003

Following a recent cash injection of €2 m, French start-up Varioptic is building a pilot line capable of producing tens of thousands of liquid lenses with a tunable focal length. Michael Hatcher follows the path to commercialization.

From Opto & Laser Europe November 2003

Mobile telephones incorporating cameras are becoming increasingly popular with consumers eager to send each other snaps of themselves and their surroundings.

However, these cameras have a weakness: their zoom and focus function. Attempting to focus on close-up items with small features, such as the text of a newspaper, generally yields a poor-quality image because the lens used is designed to focus on more distant objects. Another problem is that the camera function relies on piezoelectric drivers and a variety of moving parts that are susceptible to wear and tear over time.

In an ideal world, the cameras would incorporate something akin to the human eye - a tunable lens that can change its shape to switch focus between distant and close-up objects in a fraction of a second. It's an ideal that could become a reality by the end of this year, when French start-up company Varioptic plans to have just such lenses rolling off a pilot production line.

Liquid inspiration Varioptic's lenses are plastic devices that contain two liquids. One liquid is based on a water-soluble formulation, while the other is oil-based. The non-polar oil layer is in contact with a positive electrode, whereas the polar water-based formulation is negatively biased. The curved interface between the oil and water layers acts as a lens.

With no applied voltage, the lens focuses on objects at infinity. However, when the voltage applied to the electrodes is altered, the curvature of the liquid-liquid interface changes and with it the focal length of the lens. Such is the versatility of these lenses that they can switch their shape from concave to convex in a matter of milliseconds.

Bruno Berge founded Varioptic in March 2002 following 10 years of development work on tunable liquid lenses, first at France's National Centre for Scientific Research and later at the Ecole Normale Supérieure de Lyon. Since performing this early research, Berge - now president of the company - has subtly changed the design of the lenses to make them more robust. The latest lenses are smaller and thinner than early prototypes, with vastly improved shock resistance. Measuring 8 mm wide and 4-10 mm thick, the devices are small enough to be integrated into miniature camera systems.

Berge says that his latest lenses can switch in 20 ms to focus on objects between infinity and just 4-5 cm distant, with a variation of 30 dioptres possible with a 4.5 mm clear aperture. He believes that miniature cameras using the tunable lens could accurately focus on paper to scan documents, or be used for fingerprint or iris recognition, while still being able to focus on images in the far distance.

A key development over the past few years, according to Berge, was a reduction in the voltage needed to drive the lens. Since 1999 it has fallen from around 200 V to the current level of 40 V, enabling the device to be driven (via a CMOS chip) by the 3 V battery typically used to power a mobile phone. This was achieved by reducing the thickness of the insulating oil-based layer and playing around with the two liquid formulations, the exact contents of which remain a closely-guarded secret. Some clever physical chemistry was also needed to ensure that the two layers stayed in the liquid phase between -30 and 80 °C.

The newer lenses are more robust than their forerunners. In the early prototypes, the two liquid layers had tended to mix when shaken. Drops of liquid crossing over into the wrong layer render the lens useless. However, thanks to the thinner layers used, the latest prototypes can withstand shocks of 500 G. "Mixing of the two layers is no longer a problem," said Berge.

Varioptic's plan is to supply the individual lenses to camera-system manufacturers whose cameras are then integrated into larger devices. In the short term, the mobile telephone market will be targeted. "The main advantages of the Varioptic solution are that it eliminates moving parts and that it can be miniaturized easily," said Berge. "The autofocus system is faster, more compact, more robust and cheaper than motorized systems. Our technology has attracted a large Asian electronics manufacturer, with whom we have just signed a large contract to develop prototypes and produce a pre-series product." Varioptic's target is to start industrial production of lenses for mobile phone cameras next year.

Although a handful of other organizations, including Bell Labs, US, are now investigating the commercial possibilities of tunable lenses based on liquids, Varioptic is by far the closest to commercial realization. The company currently employs 10 staff at its headquarters in Lyon, and Berge contends that it holds a strong position thanks to its intellectual property. In September, the company's finances received a massive boost when international venture-capital group Sofinnova Partners poured €2 m into Varioptic's coffers.

Berge told Opto & Laser Europe that his firm is finishing off a pilot production line that will allow it to manufacture up to 50,000 lenses every month. "We don't want to be just a laboratory," he said. "Our philosophy is to go to the pilot production level." This is a key point - mobile phone manufacturers churn out tens of millions of units per year and will want to see evidence of the mass-manufacturability of components before incorporating them into their products.

The Varioptic pilot line is currently being qualified and Berge and colleagues are working to ensure the reliability of their lenses. At the moment, only custom products are being developed but Berge hopes "off-the-shelf" lenses will be available from around the middle of next year.

Berge is also keen to point out that mobile phone cameras are not the only potential application for the lenses. He is developing a miniature zoom lens for digital cameras, and says that the medical and automotive industries could both benefit from the technology. Compact optical systems built around the tunable lens, for instance, could be of interest to endoscope manufacturers such as Zeiss and Storz.

Berge says that the miniature zoom lens could also be used in more complex applications such as confocal microscopy systems and tumour detection. Following the recent funding round, Varioptic now has the resources to approach key medical instrumentation companies with the lens.

Liquid lenses could also be useful for in-car telemetry - for instance, they could monitor the distance to other vehicles or to stationary hazards using the lens zoom. The speed at which it can switch between focal distances would enable the lens to take measurements from a moving vehicle.

Powerful patents According to Berge there is little direct competition in the field of liquid-lens technology, mainly owing to two patents held by Varioptic. However, at Bell Labs in the US, Tom Krupenkin and his colleagues have been looking into microlenses that are tunable in all three spatial dimensions. As well as being able to switch focal length, the microlenses can move laterally.

These lenses are based on a similar principle to the Varioptic technology, with a droplet of transparent, conductive liquid (potassium sulphate solution) positioned on top of a smooth dielectric substrate. The droplet sits on a ground electrode and is surrounded by a number of other control electrodes. The focus of the lens is altered by increasing or decreasing the bias on all of the control electrodes simultaneously, while the position of the droplet can be shifted by increasing the bias to just one of the control electrodes.

So far, Bell Labs is keeping any further developments under wraps, with Krupenkin only prepared to say that the liquid lens team has made "lots of progress" over the past year. "We are currently actively pursuing research and development of the tunable liquid microlens technology with a view towards its commercialization," he told Opto & Laser Europe.

One topic Krupenkin was happy to discuss was the development of lenses that are tunable when wet but can be "locked" into a specific focus, as they are made from a photopolymerizable material. The focal length of the Bell Labs lens can be altered by up to around 30% and Krupenkin says that the scheme could be extended to incorporate a number of microlenses for more complex photonic functions.

Meanwhile, on the west coast of the US at the University of California's San Diego campus, De-Ying Zhang and team have been developing liquid lenses inspired by the animal kingdom. Zhang's lenses switch focal length between 41 and 172 mm as the pressure inside the lens membrane is decreased. At the same time, the numerical aperture changes from 0.24 to 0.058. Zhang's work forms part of the US military's bio-optic synthetic systems project, which aims to produce a controllable-index lens with a dynamic field of view by 2006.

Interesting though these developments are, it is clear that Varioptic is leading the path to the commercialization of liquid lenses. While his potential competitors are still working on fundamental aspects of their technology, Berge is refining the reliability and manufacturability of his lenses. A key recent achievement has been a centring mechanism that ensures the stability of the optical axis along which the lens focuses - an issue that Berge says other developers have yet to address, and which highlights the progress that Varioptic is making. Your next mobile phone may well feature a bit of French ingenuity.

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